AN INTRODUCTION TO STRUCTURAL BOTANY PART II AN INTRODUCTION TO STRUCTURAL BOTANY PART II FLOWERLESS PLANTS BY DUKINFIELD HENRY SCOTT M.A., LL.D., PH.D., F.R.S., F.LS., F.G.S., F.E.M.S. LATELY HONORARY KEEPER OF THE JODRELL LABORATORY, ROYAL BOTANIC GARDENS, KEW AUTHOR OF "STUDIES IN FOSSIL BOTANY" WITH 120 ILLUSTRATIONS FIFTH EDITION LONDON ADAM AND CHARLES BLACK 1907 First Edition, published . . November, 1896. Second Edition, . . November, 1897. Third Edition, . . January, 1900. Fourth Edition, . . November, 1903. Fifth Edition, ., November, 1907. All Rights Reserved. NOTE TO THE FIFTH EDITION. ONCE more the book has beeii revised throughout, where the progress of the science appeared to render it necessary. I may call attention especially to the notice of Mr. Gwynne-Vaughan's discovery of the true nature of the so-called tracheides of Ferns (p. 47) ; to the recognition of the rnonoecism of Funaria (p. 132), a point to which my attention was first called by the late Prof. Charles Stewart, F.K.S., and to the mention of Mr. V. H. Blackman's observations on the sexuality of the Bust Fungi (p. 263). The concluding chapter, especially, has needed a good deal of revision, and I have here intro- duced a few words on recent remarkable observations on Microcycas, the most cryptogamic of the living Seed- plants. Two fresh illustrations of Bacteria, from Fischer's Vorlesungen, have been introduced (Figs. 109 and 111 A). D. H. SCOTT. October 11, 1907. PREFACE TO PART II OWING to the immense variety of organisation among the Cryptogams, it has been necessary to describe a much larger number of types in the present volume than in Part I. While it was possible to give some idea of the main outlines of structure in Flowering Plants by the full description of three representatives, it has seemed desirable to select no less than twenty - three types for the illustration of Cryptogams, and even then many important groups have been left out. The increased number of types has involved a curtness of treatment, in most cases, which only the relative sim- plicity of many of the forms has rendered possible. It is hoped, however, that the essential morphological points have been brought out, and that a certain continuity has been maintained throughout the book so that the study of the selected examples may serve to give a connected idea however elementary of the great groups of plants. In order to afford a general view of the whole field, the concluding summary has been added. When theoretical points are touched on, the great aim has been to avoid dogmatism, and, so far as space vii viu PREFACE permitted, to put the reader in possession of the evidence as a whole. This applies especially to the question of alternation of generations, as to the nature of which such different views are held. As regards the fundamental homologies between Cryptogams and Phanerogams, an attempt has been made to demonstrate, and not merely to state them. Unless the student be taught to follow the reasoning by which such conclusions are arrived at, morphology loses at once its interest and its educational value. It may be well to state again explicitly that the use of this book requires to be accompanied from the first commencement onwards (1) by the study of living plants in the field, without which all botanical teaching is dull and barren ; and (2) by practical work in the laboratory. The author is indebted to the Trustees of the British Museum for permission to make use of the cuts, re- produced in Figs. 113-117, from Mr. Arthur Lister's Monograph of the Mycetozoa. As in Part I., the figures signed R. S. have been drawn from nature by Mrs. D. H. Scott. Figs. 5 and 44 are the work of Mr. W. C. Worsdell. The source of all figures not original is acknowledged in the descriptions. Special thanks are due to Professor J. Bretland Farmer for his kind help in connection with the Liverworts and the Fucaceae. D. H. SCOTT. October 12, 1896. CONTENTS OF PART II PACK PREFACR vii CHAPTER I THE VASCULAR CRYPTOGAMS . 1 Type IV SELAGINELLA KKAUSSIANA ..... 1 I. External Characters A. Yegetative Organs . . . . .2 B. Reproductive Organs ..... 6 II, Internal Characters of the Vegetative Organs , . 7 a. The Stem ...... 8 b. The Leaves . . . . .12 c. The Rhizophores and Roots . . .13 d. Growing-Points and Mode of Branching a. The Stem ..... 14 P. Rhizophores and Roots . .14 III. Reproduction and Life-History a. The Sporangia and Spores . . .16 6. Germination of the Microspores . . .20 c. Germination of the Megaspores . . .24 d. Fertilisation and Embryology . . .28 Comparison between Selaginella and the Gymnosperms . 31 ix CONTENTS Type V PAGE THE MALE FERN (Aspidium Filix-Mas) . , , ,37 I. External Characters A. Vegetative Organs . . , . 38 B. Reproductive Organs ..... 42 II. Internal Structure of the Sporophyte A. The Vegetative Organs 1. The Stem a. The Vascular System . . ,44 0. Other Tissues of the Stem . . .48 2. The Leaf . ... 48 3. The Root ...... 50 4. The Growing-Points a. The Stem . ... 52 ,8. The Root ..... 53 7. The Leaf ..... 55 B. Reproductive Organs of the Sporophyte . . 55 III. The Oophyte or Sexual Generation A, Development and Structure of the Prothallus . 61 B Development and Structure of the Sexual Organs 1. The Antheridia . . . . .65 2. The Archegonia ..... 67 C. Fertilisation ...... 69 D. Embryology ...... 71 E. Comparison between the Life-History of Ferns and that of the Higher Plants . . .73 Type VI THE FIELD HORSETAIL (Equisetum arvense) . . .73 I. External Characters of the Sporophyte A. Vegetative Organs ... 80 B. Reproductive Organs ..... 82 CONTENTS XI II. Internal Structure and Development of the Sporophyte 1. Vegetative Organs PAO " a. The Stem ...... 83 b. The Leaves ..... 90 c. The Roots ..... 90 d. Growing Points and Branching . . .92 2. Reproductive Organs of the Sporophyte . . 96 III. Development and Structure of the Sexual Generation (Oophyte) 1. The Prothallus . . . . .100 2. The Sexual Organs a. The Antheridia . . . . .101 b. The Archegonia ..... 102 3. The Embryo . . . . . .104 Summary . . . . . , . 106 CHAPTER II THE BRYOPHYTA . . .109 A. THE LIVERWORTS Type VII PELLIA EPIPHTLLA 1. TheThallus ...... 110 2. The Sexual Organs a. The Antheridia . . . . .114 6. The Archegonia . . . . .116 c. Fertilisation . . . . .119 8. The Sporogonium or Fruit a. External Characters . . . . 120 &. Development ..... 121 Summary ...*... 125 xii CONTENTS B. THE MOSSES Type VIII PAGE FUNARIA HYGROMETRICA ...... 126 1. The Leafy Stem a. Structure . . . . . .128 6. Apical Development . . . .132 2. The Sexual Organs .... 132 3. The Sporogonium or Fruit . 137 4. Germination of the Spores .... 142 Summary ....... 144 CHAPTER III THE ALG^ . . ,146 A. THE CHLOROPHYCE.E Type IX (EDOGONIUM 1. Structure , , 147 2. Reproduction a. Asexual ...... 150 b. Sexual .,... 152 Type X ULOTHRIX ZONATA 1. Struetmre . , 159 2. Reproduction . . . . f ,160 Type XI SPIBOGYRA 1. Structure ....*. 168 2. Reproduction . . . . . .170 CONTENTS Xlll Type XII VATTCHERIA . . 1. Structure . 2. Reproduction . Type XIII PLETTEOCOCCUS VTJLGARIS . PAGE 174 175 176 182 B. THE PH.EOPHYCILE Type XIV ECTOCARPTJS SILICULOSUS 1. Structure 2. Reproduction 185 185 Type XV PELVETIA CANALICTJLATA . 1. Structure 2. Reproduction . 189 190 194 C. THE FLORIDE.E Type XVI 201 CALLITHAMNION CORYMBOSTJM 1. Structure 2. Reproduction a. Asexual . b. Sexual 202 204 206 D. THE CYANOPHYCE^ Type XVII NOSTOC 212 CONTENTS CHAPTER IV PAGE THE FUNGI t , .216 Type XVIII PYTHIUM BAKYANUM ...,.* 218 1. Structure . , , . . ,219 2. Reproduction a. Asexual .,,.,, 221 b. Sexual ..,,.. 224 Type XIX PlLOBOLUS CRYSTALLINUS .,,,,, 228 1. Structure ...... 229 2. Reproduction a. Asexual ,,.,., 230 b. Sexual . , . , .233 Type XX SPH^EROTHECA CASTAGNEI 1. Structure . < * c * 235 2. Reproduction , , , , 237 Type XXI PHYSCIA PARIETINA , , , 240 1. Structure and Mode of Life . , . 241 2. Reproduction ...... 247 Type XXII PUCCINIA GRAMINIS ...... 253 Type XXIII THE MUSHROOM (Agaricus campestris) , , , 266 CONTENTS xv CHAPTER V PAQB THE BACTERIA , , ,272 Type XXIV BACILLUS STJBTILIS -..< 273 Type XXV CLADOTHKIX DICHOTOMA ..,,,, 278 CHAPTER VI THE MYXOMYCETES . , -280 Type XXVI BADHAMIA UTRICULARIS 1. The Plasmodium , , . 281 2. The Sporangia and Spores , , , 285 CHAPTER VII CONCLUSION 290 INDEX 309 STRUCTURAL BOTANY part II FLOWERLESS PLANTS CHAPTER I THE VASCULAR CRYPTOGAMS TYPE IV SELAGINELLA KRAUSSIANA Selaginella, from which our first type of Cryptogams is taken, is a large genus, containing between three and four hundred species, most of which inhabit the damp forests of tropical countries. A few are natives of Europe, and one, Selaginella spinosa, grows in our own country, on boggy moors, or in mountainous districts. Some of the tropical species are universally grown in hothouses, and are often popularly called Lycopodium, but the real Lycopodium, or Club Moss, is quite a differ- ent, though an allied, genus. In general appearance the Selaginellas resemble large Mosses, for they have long, usually creeping, stems, thickly clothed with numerous small leaves. With the true 1 2 STRUCTURAL BOTANY Mosses, however, which we shall describe later on, they have nothing whatever to do. Selaginella is chosen as our first flowerless or Cryptogamic type, because in its reproduction and general course of development the genus, perhaps, comes nearer to Flowering Plants than do any other Cryptogams x now living. In other respects, such as its vegetative ana- tomy, the structure of Selaginella is peculiar to itself. We shall therefore pass rapidly over this part of its organisation, and give most of our attention to those reproductive processes which illustrate the relation between Cryptogams and Phanerogams. We will, however, begin by examining the external characters of one or two of the species. I. EXTERNAL CHARACTERS A. VEGETATIVE OBGANS Selaginella Kraussiana, A. Br., 2 a native of S. Africa, Madeira, and the Azores, and the commonest species cultivated in greenhouses, has a creeping stem, which, however, rises a little above the surface of the ground. The main stem is repeatedly forked, and the two branches arising at each bifurcation are alike. From the principal shoots other smaller branches are given off laterally, and these again bear still finer ramifications. The origin 1 The word Cryptogams, constantly used for Flowerless Plants, dates from Linueus, who lived in the eighteenth century. It implies that m these plants the process of fertilisation is hidden, while in Flowering Plants (Phanerogams) it is manifest. This distinction no longer holds good, for, with the help of the microscope, fertilisation is at least as easy to observe in Cryptogams as in Phanerogams. The names, however, are still kept up. 2 Alexander Braun, the authority for the name. THE VASCULAR CRYPTOGAMS 3 of the branches is really the same all through the plant, for all branches are really lateral, but they are formed so near the growing-point that the latter seems to give rise to two equal shoots. In the earlier ramifications, both shoots develop similarly, so that we cannot dis- tinguish between the main axis and the branch. This S&- FIG. 1. SelaginellaKraussiana; general view, r, rliizophores ; s, spikes or cones. (Reduced.) is not the case with the later-formed branches, which are evidently different from the axis which bears them. When a growing-point gives rise to two perfectly equal shoots, the branching is said to be dichotomous. In Selaginella, the branching is not really dichotomous, but it comes very near to being so. The stem bears very numerous small leaves, which STRUCTURAL BOTANY are separated by distinct internodes on the older parts, but are crowded together towards the growing-points. The leaves are arranged in four rows, two of which spring from the lower and two from the upper side of the stern. The leaves on the lower side are much larger than those on the upper (see Pig. 2). The arrangement, if care- fully examined, is found to be in pairs, each pair consisting of one of the large lower leaves and one of the small upper ones, which are exactly opposite each other. Each leaf bears on its upper surface and close to the base, a small membranous out- growth, the ligule, which is best observed on the very young leaves (see Figs. 10 and 16), as it soon withers and disappears. This ligule is characteristic of the whole genus Selaginella, and one other living genus, Isoetes, and, as it seems, is FIG. 2. SelayincllaKraussiana ; young plant, m, megaspore still in connection with plant ; c, two cotyledons ; r, main root ; r', first lateral root. Note the two kinds of leaves. Magnified 6 diameters. (U.S.) unimportant a very ancient character, for it is found in a large family of fossil plants of the coal period (Lcpidodendrecc'). 1 At each ramification of the stem, a root-like organ is given off, which arises at the side of the stem, just below the fork (see Figs. 1 and 3). These organs, the rhizophores, 1 See Studies in Fossil Botany, p. 115. THE VASCULAR CRYPTOGAMS are colourless and destitute of leaves ; they grow straight down to the soil and resemble roots, but have no root- caps. On coming into contact with the ground they branch, giving rise to subterranean rootlets, which have root-caps as usual. If the plant which we examine is fruiting, we shall find that some of the branches, instead of creeping along near the ground, grow straight upwards ; it is these vertical branches which form the terminal spikes or cones. The cones bear the repro- ductive organs ; they differ from the vegetative branches in the fact that all their leaves are of the same size (see Figs. 1 and 3). Other species of Selaginella differ very widely from that just described. Some are minute creeping plants of almost microscopic dimensions, with unbranchecl FlG - s.Selaginclla helvetica, . showing procumbent stem Stems (& Simplex) ; Others have and two fertile spikes, r, climbing stems, which ascend rhizophore ; y, /P ^- Slightly magnified. (After tall trees, and may attain a Dodel-Port.) length of 6 feet (S. exaltata) ; while in others again the stem is stiff and erect, rising vertically to a height of three feet from the ground (S. grandis). A still more important variation is in the arrangement of the leaves. The majority of species agree with S. r 6 STRUCTUKAL BOTANY Kraussiana and S. helvetica (see Fig. 3), in having four rows of leaves, two large and two small, in the vegetative region, while in the spikes all the leaves are alike. In another group, however, to which our native species (S. spinosa) belongs (see Fig. 5), the leaves are all similar, and are arranged spirally, both on the ordinary stem and on the spike. In certain foreign species again, the case is just opposite, for the leaves of the spike, like those of the vegetative stem, are of two kinds. R EEPKODUCTIVE ORGANS The true reproduct- ive organs of Selaginella are the sporangia, con- taining the spores. Each sporangium is a stalked sac, reaching a diameter of about a millimetre (2*5 inch), and is borne in the axil of one of the leaves of the cone (see Figs. 4 and 5). The sporangia are of two kinds : the one kind (the microsporan- gium) contains very numerous small spores 1 (microstores), compar- able in size to pollen- grains. The other FIG. 4. Selaginella helvetica ; part of longitudinal section through spike, showing two sporophylls. ma, niega- sporangium dehiscing ; three out of four megaspores visible ; note abortive mother - cells ; mi, microsporangium with numerous microspores. Magnified about 15 diameters. (After DodeKPort. ) 1 The word spore is applied to any single cell which becomes isolated from the parent plant for reproductive purposes ; cf. Part I. p. 113. THE VASCULAE CRYPTOGAMS kind (the megasporangium or macrosporangium) contains only four spores, megaspores or macrospores, but these spores are so large that the sporan- gium which contains them has to be much larger than that which holds the innumerable microspores (see Fig. 4). Both kinds of sporangia are borne on the same cone ; generally the micro- sporangia are the more numerous, and occupy the axils of all the upper leaves of the cone, while the few megasporangia are found at the base of the cone only. The arrangement, however, varies in different species. mi i -i i r. ,1 Fro. 5. Selaqinella The development and structure of the sninosa ; fertile sporangia will be further described spike. Magnified , , H diameters. below. (Vv. C. W.) II. INTERNAL CHARACTERS OF THE VEGETATIVE ORGANS Among the Flowerless Plants we find a very great variety in characters, which in the Phanerogams remain fairly constant throughout whole Classes. This holds good especially for the internal structure. A description of the anatomy of the Wallflower was sufficient to give a fair general idea of the chief anatomical features of the Dicotyledons generally, and BO it was with our other types of flowering plants. With the Cryptogams the case is quite different. Not only is the anatomy of Selaginella peculiar to that one genus among plants now living, but the variation of 8 STRUCTURAL BOTANY structure among the species is so great that a general description, even of the genus as a whole, is impossible. In an elementary book, we cannot enter into all these variations ; we can only give a short description of two or three forms, which may serve to give some idea of the peculiarities of the genus and of the range of variation among its species. a. The Stem In each of our types of Flowering Plants we found that the stem was traversed by one central cylinder, consisting of the vascular bundles and conjunctive tissue (see Part I. pp. 47, 152, 236). We learnt further that the bundles of the stem are directly continuous with those of the leaves. These facts hold good, with certain exceptions, for the Phanerogams generally. In the Selaginellas the arrangement is totally different. The number of cylinders or steles varies from one up to five or more, not only in different species, but some- times even in different parts of the same plant. The conjunctive tissue is very little developed, and pith is almost always absent, the whole interior of the cylinder being occupied by a solid strand of wood. Consequently it is generally impossible to distinguish the limits of the individual vascular bundles in the stele, or, to be more accurate, the stele in the stem is not differentiated into distinct bundles. Lastly, the vascular system of the stem is not built up entirely of leaf -trace bundles. The greater part of the xylem and phloi ; m can be traced continuously through the whole stem, and only certain portions of the vascular tissue are directly connected with the bundles of the leaves. It will be convenient to begin with a short description THE VASCULAR CRYPTOGAMS 9 of the anatomy of our native species, S. spinosa, which, though exceptional in the genus, illustrates several points of importance. In the upper part of the ascending branches the stem has the structure shown in transverse section in Fig. 6. en. FIG. 6. Selaginella, spinosa ; transverse section of stem, ep epidermis ; en, trabeculce representing endodermis ; c, external cortex ; st, stele ; the seven dark groups are proto- xylem. Magnified about 35 diameters. (After Harvey Gibson.) There is a single central cylinder traversing the middle of the stem. This is surrounded by a wide intercellular space, which is bridged over at intervals by long radiating cells connecting the stele with the cortex. The latter is thick and of ordinary parenchymatous structure, and is bounded externally by a large-celled epidermis without stomata. Now, returning to the stele, we find the structure quite different from anything which we have previously met with in a 10 STRUCTURAL BOTANY stem. There is no pith whatever ; the whole interior of the cylinder is occupied by solid wood, which consists entirely of tracheides. The development of this central mass of wood is also peculiar, for the first-formed elements or protoxylem-groups lie at the outside of the wood ; in this particular case there are seven such groups, and it is from these points that the development of the xylem starts; so we see that in this stem the wood develops centri- petally, just as it does in the root of other plants. This is a very important difference from flowering plants. This centripetal development of the xylem holds good as a general rule 1 for the stems of the Selaginellas and their allies. Surrounding the xylem is a ring of phloem, consisting of parenchyma and sieve-tubes, but with no companion- cells. The sieve-tubes, like those of the Conifers, have their sieve-plates on the lateral walls. The whole stele is bordered by a layer of cells containing starch. Outside this layer is the intercellular space. Each of the cells, which stretch across the space, has a cuticularised band ; these cells represent the endodermis. We see, then, that we have a vascular structure in this plant which differs from anything which we have Been before in stems, as shown by (1) the centripetal xylem ; (2) the absence of pith ; (3) the want of separation between the vascular bundles. This type of stele is a very ancient one : many of the plants of the coal period (Lepidodendron, etc.) had a vascular system almost exactly like that of S. spinosa, though on a much larger scale. This was the case, for example, in stems such as that of which the stump is shown in Part L, Fig. 5. 1 In the trailing part of the stem of S. spinosa it appears that the protoxylem is central. See also p. 13. THE VASCULAR CRYPTOGAMS 11 From each angle of the stele in S. spinosa. where the protoxylem is situated, a slender bundle runs out to a leaf, which it traverses from end to end without branching. Ac we have already mentioned, the structure of this species is exceptional in the genus ; its interest lies chiefly in the resem- l.t.. blance to so many fossil forms, from which we may probably infer that it is a very primitive type of structure. A great many Selaginellas, like S. spinosa, have only a single vascular cylinder, or, in other words, are monostelic ; but most commonly the single stele has a simpler structure. If we now return to the species, S. Kraus- siana, with which we started, we find a total- ly different arrange- ment. In this species, the stem is traversed /.f, FIG. 7. Sclaginella Kraussiana ; dia- grammatic transparent view of stem. st, the two steles, anastomosing at base of branches; l.t, leaf- trace bundles, only shown in upper part. (After Harvey Gibson. ) by two parallel steles, each of which has a single protoxylem-group. The structure of these steles, their course through the stem, and their relation to the leaves, are sufficiently indicated in Figs. 7 and 8. In other species the steles 12 STRUCTURAL BOTANY are more numerous and are sometimes fused together in a complicated manner. The anatomical peculiarities of the stem of the genus Selaginella may be summed up as follows : (1) The stele con- tains no pith. (2) The vascular tissue of the stele is not divided into distinct bundles. (3) The xylem is usually developed FIG. 8. Selaginella Kraussiana ; part of transverse section of stem showing one stele, x, the wood ; px, protoxylem ; ph, phloem ; pe, pericycle ; en, endodermal spec i es there is more cells forming the whole or part of r trabeculse ; c, inner layers of cortex, than 0116 Stele. Magnified about 100 diameters. (After Harvey Gibson.) (4) In many As regards the details of the tissues, it is only necessary to add that the tracheides of the protoxylem are annular or spiral, as is usually the case. The other tracheides usually have long transverse pits, and are hence called scalariform (see Fig. 23, p. 47), from the ladder-like appearance which these pits give to their walls. We shall find this form of tracheide very general among the higher Cryptogams, and shall study it more fully in the Ferns. In one or two species of Selayinella true vessels, arising by cell-fusion, occur in the wood. b. The Leaves The leaves of Selajinella are of excessively simple structure ; each leaf, as we have seen, receives a single THE VASCULAR CRYPTOGAMS 13 vascular bundle from the stem. The bundle traverses the leaf from end to end, forming the midrib ; it has no branches, neither is there any transfusion - tissue, which in Conifers takes the place of the branched veins. The bundle consists of a slender strand of tracheides surrounded by a thin layer of phloum. Around the whole is a bundle-sheath. The rnesophyll of the leaf is very slightly differentiated, the intercellular spaces being a little larger toward the lower surface. The epidermis, like the mesophyll, con- tains chlorophyll ; the chlorophyll bodies in each cell are few and unusually large. The stomata, which have the ordinary structure, are usually found on the under-side of the leaf only, and especi- ally in the neighbourhood of the midrib. The membranous ligule at the base of the leaf on its upper surface has already been mentioned (see Figs. 10 and 16, pp. 18 and 29). This is the simplest type of leaf that we have yet met with. c. The Rhizophores and Roots These organs are generally similar to one another in structure ; the rhizophores in fact may be regarded as roots which have not yet begun to form a root-cap. The anatomical structure is simple, but unlike that in most other roots. There is a single stele, which contains only one group of xylem and one of phloem. This structure, which may be called monarch, is pretty general in Selagindla and its allies. It is a very ancient character, for the rootlets of the fossil relations of Selagindla, which lived in the Carboniferous epoch, had an almost identical structure. The rhizophore, as distinguished from the root, of S. Kraussiana is peculiar in having central protoxylem. 14 STRUCTURAL BOTANY r>ollQ TVa which will form spermatozoids. C'.mature stage ; sp. m, spermatozoid mother-cells, large nucleus, which surrounded by cells of wall of anther- i T, i -i idinm. D, free spermatozoids, each with each cell contains, be- two dlia> ^ magnified 290 diameters ; COmeS Converted into #, magnified 290 diameters ; C, magni- , , -, T , fied 640 diameters ; D, magnified 780 a long, rather club- diameters. (After Belayeff.) shaped body, which has a spiral twist. At the thin end of this body there is a little protoplasm, and at this point two excessively fine protoplasmic threads are attached (see Fig. 12, D). The whole body now constitutes a spermatozoid, and the protoplasmic threads are its cilia. The cell in which a spermatozoid is formed is called its mother-cell. The surrounding cells having completely broken down, the spermatozoid mother-cells are let loose into the water, 22 STRUCTURAL BOTANY for the whole process of the germination of the micro- spores can only go on in water. The microspores are, however, so small that a very little water is sufficient, such as we should find on the surface of the ground after rain or heavy dew. The spermatozoids next become freed from their mother-cells, the walls of which dissolve. As soon as the spermatozoids are at liberty, or even sooner, their cilia begin to lash about in the water, and when free the O ' spermatozoid sets off in active locomotion, exactly like some water-animalcule. The movement is a double one : the spermatozoid travels through the water with its narrow ciliated end foremost, and at the same time it rotates about its own axis. Its motion, in fact, is just like that of a rifle bullet through the air, or that of the screw of a steamer through the water. We must remember that the spermatozoids are of a very minute size ; the body is about ^o l o o of an mcn long, the cilia about twice that length. The spermatozoids are the bodies which perform the act of fertilisation. Each spermatozoid corresponds to one of the generative cells in the pollen-tube of Flower- ing Plants (see Part I. pp. 178, 185, and 269). We know that the generative cell chiefly consists of a very large nucleus, with only a little protoplasm. This is also true of the spermatozoid, which is all nucleus, except the small part at the pointed end and the cilia, which are protoplasmic. The spermatozoid is an actively moving cell, which swims off on its own account, and may eventu- ally find its way to an ovum. It thus differs from the generative cell of the higher plants, which, except in some of the Gymnosperms (see p. 303), is conveyed pas- sively to its destination by the growth of the pollen-tube. THE VASCULAR CRYPTOGAMS 23 It may seem strange to us at first to find a cell belonging to a plant swimming actively about, as if it were an animal. When the first examples of such moving vegetable cells were observed, more than fifty years ago, the discoverer was so much astonished that he thought he had caught the plant at the very moment of its turning into an animal ! Now, we know better. Actively moving cells are produced by most cryptogamic plants ; sometimes they are male cells, as in Selaginella ; sometimes they are sexless spores (see below, p. 151). Movement, in fact, is not specially characteristic of animals as distinguished from plants, for all protoplasm is capable of spontaneous motion. We have seen this already in the case of Elodea (Part I. p. 42), only there the movements go on within a closed cell-wall. Wher- ever movement is of advantage to the plant, we find that its protoplasm can show itself just as active as that of animal cells. In plants, however, owing to their different mode of nutrition, the necessity for locomotion arises less often. We cannot follow the fate of the spermatozoids any further, until we have seen how the megaspores germinate. Before we go on to this, however, we will carry the com- parison between a microspore and a pollen-grain, rather further than we have done hitherto. If we refer back to the account given in Part I. (p. 269) of the germination of the pollen-grain in the Spruce Fir, we shall recall the fact that several coil- divisions take place before the generative cells are formed. In like manner we have found several cell-divisions in the microspore of Selaginella before the spermatozoid mother-cells are formed. The little prothallus-cell, which is first cut off, prob- 24 STRUCTURAL BOTANY ably corresponds to the first two cells cut off in the pollen-grain of Picea (see Part I. Fig. Ill, cells marked 2 and 3, p. 269). In neither case do these cells take any part in the further development. The succeeding divisions are more numerous in Selaginella than in our gymnospermous type. The whole resulting group, including both enveloping and central cells, constitutes an antheridium, the characteristic male organ of the Cryptogams. In the higher Cryptogams, this organ always consists of an enveloping layer of cells enclosing the central group from which the spermatozoids are derived. In the Gyninosperrns the antheridinm is only represented by the " stalk-cell ' and the two genera- tive cells (see Part I. Fig. Ill, 4 and 5, p. 269). The whole is enclosed in the un differentiated vegetative cell, which forms the bulk of the pollen-grain. In Selaginella the vegetative cell ceases to exist ; it is all used up in forming the antheridium, while in Gymno- sperms it persists in order to produce the pollen-tube. We see, then, that microspores and pollen -grains, which agree exactly in their mode of origin, agree also up to a certain point in their mode of germination. The differences between them are connected with the different means by which fertilisation is effected. c. Germination of the Megaspores Unlike the microspores, the megaspores of Selaginella begin to germinate while still in the sporangium. Each of the four megaspores is tetrahedral in shape, like a microspore. It contains at first a single nucleus and abundant protoplasm, in which is a large vacuole containing oil. The nucleus lies near the angle, where the megaspore joins its three sister- cells. We will THE VASCULAR CRYPTOGAMS 25 call this angle the apex of the megaspore. The first sign of germination is the division of the nucleus into two. The divisions are repeated many times, and soon cell-walls begin to appear in the protoplasm, between the daughter - nuclei. The cell - formation is at first limited to the apical part of the spore, but it gradually FIG. 13. Selaginella Martensii; germinating megaspore. c, cellular tissue of prothallus, only complete in upper part of spore ; n, free nuclei ; p, undivided protoplasm of spore ; i, inner, e, outer, layer of cell-wall of spore. Magnified 335 diameters. (After Heinsen.) spreads downwards and inwards. In Fig. 13 a megaspore is shown which is already nearly half-filled with tissue. As a rule, the cell-division extends so far that the tissue fills the whole cavity of the spore. In some cases this process is completed even before the megaspores are set free from the sporangium, while in other cases the lower part of the tissue is developed after the spores have fallen on to the ground. 26 STRUCTURAL BOTANY m. n o The tissue which fills the megaspore is called the prothallus. The prothallus gives rise to the archegonia, or female organs. A cell at the apical end of the prothallus grows larger than the rest, and divides into two by a wall parallel to the outer surface. The upper cell divides by two longitudinal walls, crossing each other at right angles, into four, and each of these four cells divides by a transverse wall into two. Thus a neck is formed, consisting of eight cells arranged in two tiers (see Fig. 14, n, n). (Of course only four of the cells can be seen in longitudinal section.) In the mean time the lower cell, which has so far remained un- divided, forms an outgrowth which penetrates between the cells of FIG. 14. Selagmella ; , . archegonium ready for the neck. IhlS outgrowth IS CUt fertilisation ovum ; O ff as a distinct cell, called the n, cells of neck; m, mucilage in canal. Mag- neck canal -cell. Another smaller nified about 500 dm- u th ventra i canal -cell, is cut meters. (After Pfeffer. ) off below it ; the remaining lower portion of the original central cell is the ovum (cf. Fern- archegonium, p. 6 8, Fig. 35). We see, then, that the devel- opment is just like that in Picea, except that in Selaginella we find two canal-cells instead of one. In Selaginella a real canal is formed, for the canal-cells break down, and an open passage, containing only mucilage, is left between the cells of the neck leading down to the ovum within (see Fig. 14). The first archegonium is sometimes formed before the megaspore is shed. 1 After the dehiscence of the 1 It has occasionally been observed that fertilization takes place and the embryos develop while the megaspores are still retained in their sporangium. THE VASCULAR CRYPTOGAMS 27 megasporangium, which takes place as in the micro- sporangium by a transverse slit, the megaspores are shed onto the ground. The growth of the prothallus con- tinues, the coats of the megaspore are ruptured at its a -4-Ex. FIG. 15. Selaginclla Martensii; longitudinal section through old prothallus showing two embryos, p-^ and p 2 , prothallus ; a, unfertilised archegonium ; r.h, root-hairs ; s, suspensors of embryos ; larger embryo shaded, no cells shown ; r, root ; /, foot ; st, stem ; I, I, cotyledons ; Ex, wall of megaspore. Magnified 165 diameters. (After Pfeti'er.) apical end, and so the upper part of the prothallus becomes exposed, and grows a little beyond the limits of the megaspore (see Fig. 15). Other archegonia are formed around the first one, 28 STRUCTURAL BOTANY and a few root-hairs grow out from the prothallus. The mode of development of the prothallus bears a striking resemblance to that of the endosperm of the Spruce Fir or other Gymnosperms, so that we are justified . in calling both by the one name of prothallus (see Part I. p. 272). The archegonia are formed in the same way in both. The prothallus of Picea is developed within the embryo-sac, that of Selaginella within the megaspore. d. Fertilisation and Embryology The archegonia are fertilised by spermatozoids ; this takes place under water. The spermatozoids, when liberated from the ruptured antheridia, swim actively through the film of water covering the damp earth, and some of them are attracted to the archegonia of any female prothallus which lies near enough. The probable nature of the attraction will be considered when we come to the Ferns (see p. 70). At this time the mucilage formed from the disorganised canal-cells not only fills the canal of the archegonium, but spreads a little beyond its opening (see Fig. 14, ra). The details of fertilisation are not so well known in Selaginella as in the Ferns, but there is no doubt that the spermatozoid becomes caught in the mucilaginous drop and then passes down through the canal to the ovum below. In other plants it has been proved that the spermatozoid unites with the nucleus of the ovum. The really important distinction, then, between the fertilisation of a Cryptogam, such as Selaginella, and that of an ordinary Flowering Plant, consists in the mode in which the male cell is conveyed to the ovum. In the Cryptogams, the journey is accomplished by the active THE VASCULAR CRYPTOGAMS 29 locomotion of the male cell itself; the spermatozoid moves by means of its cilia, and this can only take place under water. In most Phanerogams the generative cell is carried from the pollen-grain to the ovum by the growth of the pollen-tube ; the final result, the union of the nuclei of the two cells, and, no doubt, of certain FIG. 16. Selaginclla, ; advanced embryo in longitudinal section. S, suspensor ; 7?, root ; F, foot ; C, cotyledons (cell- walls omitted) ; L, ligules ; St. apex of stem ; JET, liypocotyl. Magnified 165 diameters. (Alter Pfeffer. ) portions of their protoplasm also, is the same in both Sub- kingdoms. It is now ascertained that spermatozoids are formed in certain Gymnosperms. (See p. 303.) After fertilisation the ovum surrounds itself with a cell-wall of its own, and soon divides by a transverse septum into two cells. The upper cell, i.e. that lying next to the neck of the archegonium, becomes the sus- pensor, which may undergo a few further cell-divisions (see Figs. 15 and 16, S). The lower cell develops into the embryo itself. Owing to the growth of the suspensor in length, the embryo is carried deep down into the 30 STRUCTURAL BOTANY tissue of the prothallus (see Fig. 15). The embryonic cell undergoes segmentation, and very soon the first organs of the embryo are marked out. The apex of the stem lies at the end opposite the suspensor, but not quite in the middle ; the two cotyledons lie on either, side of it. One side of the hypocotyl grows out into a temporary organ, the foot, which grows so rapidly as to force the apex of the stem to one side (see Figs. 1 5 and 16). This organ serves to absorb food for the young plant from the tissues of the prothallus. The first root appears a little later, between the foot and the suspensor. Fig. 15 gives a general idea of the position of these organs, and their relation to the prothallus. In Fig. 16 a slightly more advanced embryo is shown in greater detail. At this stage the young stele, consisting of pro- cambial tissue, can already be traced from the apex of the stem to that of the root. It will be noticed that the cotyledons, like all the other leaves of the plant, possess ligules. The stem begins to branch while the embryo is still enclosed in the prothallus. A young plant of Selaginella Kraussiana after germination is shown in Fig. 2. It is still attached by its foot to the megaspore, or rather to the prothallus inside it. In possessing a suspensor, Selayinella resembles the Gymnosperms and most other Flowering Plants. The position of the organs of the embryo, however, is different. Although the embryo is dicotyledonous, like that of many Gymnosperms as well as of the Dicotyledons, properly so called, the apex of the stem is displaced towards one side, and the first root, though it arises near the suspensor, is not in a line with it (cf. Part I. Figs. 114, F, and 115, pp. 279, 280). The peculiarities THE VASCULAR CRYPTOGAMS 31 of the embryo are connected with the presence of the absorbing organ called the foot, which in Selaginella and most of the higher Cryptogams performs a function (that of absorbing food from the prothallus) which in most Flowering Plants is discharged by the cotyledon or cotyledons. Comparison letiveen Selaginella and the Gymnosperms At first sight the differences between even the highest Flowerless Plants, such as Selaginella, and the Flowering Plants, seem so great that we see little in common between them. In the Cryptogams we find no obvious flowers and no seeds, while fertilisation takes place in a totally different way from that which prevails among typical Phanerogams. If, however, we carefully com- pare the development of Sclaginella with that of a gymnospermous Flowering Plant, we shall find it quite possible to trace the corresponding stages in their life-history ; and, having accomplished this for the lower Flowering Plants and the higher Cryptogams, it will not be impossible to extend the comparison further so as to include the Angiosperms on the one hand, and the simpler Cryptogams on the other. The relations between Gymnosperms and Angiosperms have already been briefly discussed in Part I. Chap. IV. As regards the male organs, the comparison has been drawn above (p. 18). We need only recapitulate the results here. We found that the development and structure of the microsporangium of Selaginella agreed very closely with that of a pollen-sac in the Flowering Plants. The microspores produced in the former are developed precisely in the same way, and have just the same structure as the pollen-grains formed in the latter. 32 STRUCTURAL BOTANY When the microspore germinates it begins by cutting off a little inactive cell (the prothallus-cell), and the pollen- grain of a Gymnosperm does the same. The subsequent divisions lead to the formation of the spermatozoids in the Cryptogam, and of the generative cells in the Gymnosperm. In both cases these are the bodies which effect fertilisation. That the generative cells are homo- logous with spermatozoids has long been recognised. Two Japanese botanists were the first to make the brilliant dis- covery that in some of the Gymnosperms each generative cell actually becomes converted into an active sperma- tozoid resembling those of the Ferns (seep. 303). The antheridium, i.e. the organ in which the male cells are formed, is more complex in Selaginella than in the Gymno- sperms, for in the latter it has almost become reduced to its most essential part, the generative cells themselves. The pollen-tube of the Gymnosperm is not represented in Selaginella, for in the latter the whole contents of the microspore are used up to form the prothallus-cell and antheridium. The comparison of the development gives us then the following chief results : Selaginella. Gymnosperm. 1. Spermatozoids = Generative cells. 2. Prothallus and antheridium = Cell-group in pollen-grain. 3. Microspore = Pollen-grain. 4. Microsporangiu.ru = Pollen-sac. The student, however, must clearly understand that it is quite useless to learn up the names of the equivalent organs, unless he thoroughly grasps the developmental facts on which their comparison is based. We will now compare the female organs in the two Classes. It is best to start with the ovum, which is beyond question the same thing in both. The ovum in Selaginella is produced in an archegonium, which is THE VASCULAR CRYPTOGAMS 33 almost exactly like that of a Gymnosperm, such as Picea, and which develops in just the same way. The arche- gonia of Selaginella are formed from superficial cells of the prothallus, exactly as those of the Fir are formed from superficial cells of the endosperm. The develop- ment, moreover, of the prothallus itself is just like that of the endosperm. In both cases free nuclear division first takes place, then cell- formation begins, and the tissue thus formed goes on growing until it has filled all the available space. We can have no doubt, then, that the female prothallus of Selaginella corresponds to the endosperm of the Fir, which we may, if we like, call by the same name. The only difference is that, in Selaginella, the prothallus grows a little way out from the megaspore, becomes partly green, and forms a few root-hairs. It is, in fact, a more independent structure in the case of the Cryptogam, developing freely on the ground, instead of within the closed tissues of the ovule. Now the cell in which the prothallus of Selaginella develops is the megaspore, while that in which the endosperm of Picea arises is the embryo - sac. We therefore arrive at a new term in the comparison ; the megaspore is the equivalent of the embryo-sac. There are some differences, however, to be dealt with at this point : in normal cases the megaspore is set free and completes its development on the ground, while the embryo-sac remains always enclosed in the ovule or seed. We must remember, however, that the megaspore itself begins its germination while still in the rnegasporangium, and, in exceptional cases, even the embryo-plant may be developed in this position. The thick cuticular- ised wall of the megaspore is obviously a necessity for its protection when it becomes freely exposed. It 3 34 STRUCTURAL BOTANY is interesting, however, to know that in some Gynino- sperms the membrane of the embryo-sac likewise be- comes cuticularised. It has also been found that in some plants of this class (Cycadere) the development of the endosperm is only completed while the seeds are lying on the ground, and in a few cases the endosperm has been observed to burst through the embryo-sac and seed-coats and to become green, just like the prothallus of Selagindla. A more serious difficulty is that there are four megaspores in Selaginella, and only one embryo-sac in the Fir, though there are Gymnosperms which have more than one, as Gnetum. In most Gymnosperms the sister-cells of the embryo-sac become abortive at an early stage of their development, as is the case also with the sister-cells of the fertile niegaspore in some fossil relations of Selaginella, exceptionally in species of Selagin- ella itself, and constantly in the heterosporous Water-Ferns. The organ in which the megaspores are produced is a megasporangiuin ; that in which the embryo - sac develops is the ovule. Both organs arise in the same way from a group of cells near the growing-point. The similarity of their development has already been pointed out (see p. 17). We infer, then, that the megasporangiuin corresponds to the ovule, or more strictly to the nucellus of the ovule, for the megaspor- angiuin has no integument. We have found, however, that the megasporangium and microsporangiuin are just alike in the earlier stages of their growth ; the former, as we have seen, corresponds to the nucellus of an ovule, the latter to a pollen-sac. Hence we must draw the conclusion that a pollen-sac and the nucellus of an ovule are equivalent structures a THE VASCULAR CRYPTOGAMS 35 result which could only have been arrived at by a com- parison with Cryptogams. We may sum up our inferences as to the relations of the female organs in the two types thus : Selaginella. Gymnosperm. 1. Ovuin = 1. Ovum. 2. Archegonium = 2. Archegonium. 3. Prothallus = 3. Endosperm. 4. Megaspore = 4. Embryo-sac. 5. Megasporangium = 5. Nucellus of ovule. If we try to carry this comparison further, and to find the equivalent in the Cryptogam of the carpels and stamens of the Flowering Plant, we meet with some difficulty. In Selaginella neither kind of sporangium is borne actually on a leaf, but in its axil. Similar cases, however, are known among Flowering Plants. We may regard the leaves, in the axils of which the sporangia of Selaginella are situated, as representing stamens or carpels, according as the adjoining sporangium is a micro- or megasporangium. There is, however, no difl'erentiation between carpels and stamens in Selaginella or in any Cryptogam. We may compare the whole spike of Selaginella to a hermaphrodite l flower with no perianth, and with stamens and carpels resembling each other. In some Gymnosperrns also, as in certain members of the family Cycadese, 2 the stamens and carpels are just alike in their vegetative parts. 1 Flowers are called hermaphrodite when, as in those of the Wallflower and the Liiy, the stamens and carpels are both contained in the same flower. 2 As the Cycadese have been referred to more than once, it may be worth while to mention that they are a family of Gymnosperiha, of great geological antiquity, now represented by a few tropical genera, with a palm-like habit. They all have pinnate leaves of great size. The student will find a magnificent collection of living Cycadeae in the Palm- house at Ke\v. 36 STRUCTURAL BOTANY We see, then, that although Cryptogams and Phanero- gams appear to differ so completely from one another, we can yet successfully compare them together and determine the relations between their organs. Organs which resemble each other in their development and their place in the life-history, so that we regard them as morphologically the same organ, are said to be homologous one with another. Organs, on the other hand, which are morphologically different, but are adapted to the same physiological function, are said to be analogous. To go back to our old illustration in the introduction to Part I. (p. 4), the tuber of a potato is homologous with a branch of the stem but analogous with a fleshy root such as that of a carrot. In our comparison between Selaginella and a Gynmosperin we have aimed at establishing the homologies of the various organs ; such comparisons are essential in order to determine the relationships of different groups of plants, for it is only by tracing the homology or morphological equivalence of organs that we can form any idea of the probable modifications which may have taken place during the course of descent. The proof that the reproductive organs in Flowering and in Flowerless Plants are homologous was due to a German botanist named Hofmeister, and is one of the greatest discoveries ever made in morphology. We have every reason to believe that the Flowering Plants are descended from Cryptogams, which resembled Selaginella in having two kinds of spores. Their actual ancestors, however, which, no doubt, have been extinct for millions of years, may very probably have been in other respects quite different from Selaginella (see p. 303) One point remains : we have not yet considered the THE VASCULAR CRYPTOGAMS 37 homologies of the seed. One part of the seed, the testa, is not represented in Selaginella, for the megasporangium is without integuments. The megasporangium itself corresponds to the nucellus of an ovule, as we have already seen, but it never develops into anything of the nature of a seed. This is because the megaspores are normally set free from the megasporangium before fertili- sation takes place, so that the seed-stage is never reached. The megaspore, when it is filled with prothallus and contains an embryo, bears a certain resemblance to a seed, but there is no complete homology ; for, as we have already seen, the development shows that the mega- spore is homologous with the embryo-sac only. The typical seed, such as we find in the higher Flowering Plants, represents a persistent, integurnented megasporan- gium, containing a single megaspore, which produces a prothallus, and, after fertilisation, an embryo, while still in situ. The shedding of the seed thus corresponds to the detachment of the entire megasporangium, together with its integument and contents. Bodies closely analogous with seeds are found in some fossil Lycopods allied to Selaginella, but the true seed of the higher plants appears to have been evolved among the Pteridosperms, Palaeozoic Seed -plants allied to the Ferns. The primitive seeds, however, probably only developed an embryo when ger- mination set in, as sometimes happens in Cycads at the present day. TYPE V THE MALE FERN (Aspidium Filix-Mas, L.) The Ferns are a vast group, enormously outnumbering all the other Vascular Cryptogams put together. The order in the widest sense includes at least sixty genera and three thousand species. In our own native Flora seventeen 38 STRUCTURAL BOTANY genera and about forty species are represented. The limits of both genera and species are, however, very indefinite among the majority of Ferns. The Ferns are the only order of Vascular Cryptogams which has successfully held its own down to the present day, while the other groups are represented by comparatively few surviving forms. It need hardly be said that among this immense family of plants every possible variety of habit is to be found, while in structure the differences are scarcely less great. In size, we find every gradation, from the Tree Ferns of the tropics and New Zealand, which may reach 60 feet in height, down to minute Filmy Ferns hardly larger than Mosses. The main outlines of the life- history are, however, with few exceptions, fairly uniform throughout. The particular Fern which we have chosen as a type serves well to illustrate the chief points in the develop- ment and morphology, but we cannot expect any single representative to give a fair conception of the Order as a whole. I. EXTERNAL CHARACTERS A. VEGETATIVE ORGANS The Male Fern, one of the commonest British Ferns, grows abundantly in woods and hedgerows. The short, stout stem grows obliquely upwards, and rises but little above the surface of the ground. It often reaches a length of about eight inches and a diameter of about one inch ; but it appears much thicker than it really is, because it is completely covered with the bases of the old leaves. The stem at first has the form of a cone with the thin end downwards ; for it grows thicker for a time THE VASCULAR CRYPTOGAMS 39 towards the top, till a constant diameter is attained (Fig. 17). This is commonly the case in plants which a FIG. 17. General view of the Male Fern, a, apex, b, base of stem, which is covered with the remains of old leaves, and bears numerous adventitious roots ; c, c, young leaves, showing circinate vernation. Greatly reduced. have no secondary growth in thickness, and in which, therefore, the increase in bulk must depend entirely on 40 STRUCTURAL BOTANY gradual strengthening of the growing -point. In this respect, though in no other, Ferns resemble the Mono- cotyledons (see Part I. p. 173). The leaves, often called Fronds, are of very large size, one to three feet long, and much subdivided (see Figs. 17 and 18). This is the first example of a com- pound leaf we have had. A compound leaf is one in which thelamina or blade is completely sub- divided, so that its several parts, called leaflets, re- semble distinct leaves. The leaves of the Male Fern are pinnate, that is, the main stalk or rachis, bears two rows of leaf- lets, oxpinnce, one row on each side (see Fig. 18). The pinnae are often subdivided them- selves in the same way, and then the whole leaf is said to be li-pinnate. In the specimen figured, the pinnse are deeply lobecl, but not completely subdivided. Each lobe, like that shown singly in Fig 18, B, may be called a segment. The segment is traversed by - KJ^^M<2^ il ^nfi'i* > ' Fio. 1 8. A, leaf of Hale Fern (much reduced). B, part of a fertile pinna seen from below ; a, rachis ; s, sorus. Magnified. (After Luerssen.) THE VASCULAR CRYPTOGAMS 41 a midrib, springing from the rachis, and giving rise to lateral veins, which fork, and end near the edge of the leaf. This type of venation is a common one in Ferns (see Fig. 18, B). The petiole and rachis, and sometimes also the larger veins of the leaflets, are clothed with brown chaffy hairs, the palece or r amenta t which are characteristic of the order (see Fig. 18). The leaves develop very slowly, arising in the bud two years before they unfold. The growth of the leaf goes on at the apex like that of the stem. Thus the leaf -stalk is the first part to be formed, and is generally the only part developed in the first year of growth. The blade is formed later, and this also grows from the base upwards. The blade of the young leaf is rolled up in such a way that the rachis or midrib forms a spiral like a watch-spring, the apex of the leaf being at the centre of the spiral (see Fig. 17, c). Everyone who has ever watched a Fern coming up in spring must have noticed the form of the young leaves. The curvature is due to the greater growth of the under-side of the leaf, which is external in the bud. Each leaflet is coiled up in a similar way. This mode of folding of the young leaf is called circinate or crosier-like vernation, vernation being a general word for the folding of a leaf in the bud. When the leaf finally expands, the inner side grows more rapidly than the outer, so that the curves become straightened out. Circinate vernation in characteristic of the Ferns generally. The branching of the stem in this Fern is peculiar ; no branches at all are formed at the growing point, but buds arise on the petioles of some of the leaves, springing from their outer sides a little above the 42 STRUCTURAL BOTANY base. These buds, though their first origin takes place very early, only develop into branches at a much later time, and often not until the upper part of the leaf has died off. Few branches are formed in this particular Fern. Ferns vary very much as regards their branching ; in some, as in the Bracken Fern, the stem forks at the apex ; in a few, as in some Filmy Ferns, the branching is axillary like that of flowering plants, while in others, as in some of the Tree Ferns, the stem does not branch at all. The roots which we find on an ordinary full-grown plant are all adventitious, for the original main root of the embryo dies away very early. The adventitious roots, which arise at the bases of the leaves, usually three below each leaf, are very slender and much branched. An old stem is densely clothed with a matted growth of adventitious roots (see Fig. 17). B. KEPRODUCTIVE ORGANS The ordinary Fern plant, such as we have described, is purely asexual. Like Selaginella, it bears, at this stage, sporangia only, but, unlike that genus, its sporangia and the spores which they contain are all of one kind. The sporangia of the Male Fern and of most other Ferns are borne on the lower surface of the ordinary foliage leaves, so that here there is no difference between vegetative leaves and sporophylls. In this respect such Ferns are on a lower level, as regards the physiological division of labour, than any plants which we have yet considered. If we examine one of the fertile leaves in summer, THE VASCULAK CRYPTOGAMS 43 the clusters of sporangia, or sori, as they are called, are very conspicuous on the under-surface. They are usually absent from the basal part of the leaf. On the larger segments, the sori are arranged in two short .rows, one on each side of the midrib (see Fig. 18, B), while on the smaller segments there may be only one or two sori altogether. Each sorus is covered by a kidney-shaped membranous envelope called the indusium, and is seated just over one of the lateral veins. The individual sporangia, which cannot be distinguished without the aid of a lens, are very numerous in each sorus, and every sporangium contains a large number of spores, so that the reproduction of the plant is extremely well provided for. On germination, each spore gives rise to a protkallus, which is a much larger structure here than in Selaginella, and leads quite an independent existence (see Fig. 31, p. 63). The prothallus is a flat, green, heart-shaped body, sometimes as much as half an inch in diameter, attached to the soil by the root - hairs which arise from its under - surface. Prothalli may be found in abundance covering the damp ground where Ferns are growing. In Ferns the same prothallus usually bears both kinds of sexual organs, the antheridia and archegonia. After fertilisation the ovum formed in one of the archegonia becomes an embryo, which eventually grows up to be a new Fern plant. In Ferns, then, we have, in normal cases, a sharp alternation of generations. The Fern plant is the asexual generation, or sporophyte, producing the sporangia, and ultimately the spores. The prothallus is the sexual generation, or o'ophyte, 1 producing the antheridia and archegonia, in which the sexual cells are developed. 1 Al.x, pro- toxylem ; x.p, xy tern-parenchyma. Magnified 200 diameters. (R.S.) repeats the structure of the whole vascular cylinder of the embryonic stem, it is more convenient to describe them as steles or cylinders, but no sharp distinction can be drawn. The wood consists of vessels and parenchyma ; the vessels of Ferns are generally of the kind called sea&m/brm, or ladder-like, from the peculiar structure of their walls, shown in Fig. 23. This structure depends on the form of THE VASCULAR CRYPTOGAMS 47 the pits, which are slightly bordered and much elongated in the transverse direction, so that the thickened ridges between them resemble the rungs of a ladder. Mr. Gwynne-Vaughan has recently found that the elongated elements of these vessels, hitherto called tracheides, are everywhere in open communication with each other, the closing membranes of the pits being absorbed, while the middle lamella of the walls also disappears, leaving only the bare framework of scalariform thick- enings, which may thus be com- pared to the bars of a towel-horse ! In the stele figured (see Fig. 22) there is only one group of protoxylem (px) lying on one side of the wood. In the larger steles of the stem there are usually two or three such groups. Spiral tracheides occur at these points, but usually become destroyed very early as the stem grows in length. Surrounding the wood is a layer of parenchyma containing starch, and then we come to the phloem-zone, consisting of sieve-tubes and paren- chyma. The former have their sieve-plates on the lateral as well as on the oblique terminal walls. They are not unlike those which we observed in Pinus. The phloem again is surrounded by a belt of paren- chyma very rich in starch, beyond which we come to the endoderrnis. The endodermis is really two cells thick, but its inner layer cannot be distinguished from the pericycle except by the fact that its cells fit on exactly FIG. 23. Portions of scala- riform trachere. A, part of wall in surface view. Magnified 187 diameters. Jj, part of wall in section, showing bordered pits. t, torus on closing mem- brane. Magnified 375 (After De 48 STRUCTURAL BOTANY to those of the outer endodermal layer. This outer layer alone has the usual structure of an endodernris (see p. 75, Part I.) and becomes thick- walled. ^. Other Tissues of the Stem The great mass of the ground-tissue, in which the steles are embedded, consists of ordinary parenchyma containing abundant starch. The outer cells have thicker walls, and those nearest the epidermis are narrow and fibrous. They serve to give mechanical stiffness to the stem. The epidermis itself has a thick brown outer wall, and otherwise presents no peculiarities, It bears flat chaffy scales, or ramenta, which are very characteristic of the plant, and indeed of almost all Ferns. They are sometimes of large size, reaching half an inch in length, and consist of a plate of tissue one cell thick, attached to the epidermis at one end ; they arise each from the growth and division of a single epidermal cell. 2. The Leaf As we have already seen, each leaf, at least in the mature plant, receives several steles from the stern (see Figs. 19, 20, and 21). The structure of the petiole is simple enough. The steles (see Fig. 22), as seen in transverse section, are arranged in a horseshoe, embedded in ground-tissue, the outer layers of which consist of very thick-walled cells. The basal part of the petiole is densely clothed with chaffy ramenta, which are more scattered higher up on the leaf. A bundle enters each pinna of the leaf, branching off from one of the two larger steles which are situ- ated near the upper surface of the leaf-stalk. This bundle gives off branches to the right and left, which THE VASCULAR CRYPTOGAMS 49 enter the successive segments of the lamina, and by their further ramifications supply its vascular system (see Fig. 1 8, B). As we trace the bundles into the finer veins of the leaf, we find that the upper part of the phloem gradually dies out, so that the ultimate branches of the bundle system come to be collateral instead of concentric. This is very generally the case in Ferns. If we now endeavour to sum up what we have learnt of the vascular system of the Male Fern, we see that its most striking peculiarity consists in the polystely of the stem, where each strand of wood and bast resembles an entire central cylinder rather than a single vascular bundle. As we follow the leaf- traces outwards, however, we find that the steles assume more and more the character of simple vascular bundles, until in the lamina they have the same collateral structure as in the leaves of flowering plants. It is evident that no sharp line can be drawn between stele and bundle. Eeturning to the lamina of the leaf, we find that its structure is distinctly bifacial. The mesophyll towards the upper surface consists of closely-packed squarish cells, forming a kind of palisade-parenchyma, though the palisade form is not well marked. The lower portion of the mesophyll, on the other hand, is made up of irregularly branched cells, attached to each other by only small parts of their surface, so that large intercellular spaces are left between them. This tissue is thus a typical spongy parenchyma. All the cells of the meso- phyll contain abundant chlorophyll granules (see Fig. 28, A). The epidermis of the lower surface alone bears the stornata, which are very numerous (see Fig. 24). The stomata are characteristic : each pair of guard- 4 50 STRUCTURAL BOTANY cells is half-surrounded by a subsidiary cell, shaped like a horseshoe. The subsidiary cell and guard-cells are ultimately derived from a single mother-cell, which is cut out from one of the epidermal cells by a curved wall. The cells of the epidermis on both surfaces of the leaf have undulating cell-walls fitting closely together. FIG. 24. Part of epidermis from the under-side of leaf of Male Fern. Note the undulating cell-walls and numerous chlorophyll - granules, n, nucleus of epidermal cell ; st, stoma. Magnified 105. (E. S.) The cells contain chlorophyll, as is generally the case iii the epidermis of Ferns, though less usual among flowering plants. 3. The Root The adventitious roots of the Male Fern arise, as we have already seen, at the bases of the leaves, though THE VASCULAR CRYPTOGAMS 51 .--PL they are in direct connection with the principal steles of the stem. The structure of the root in Ferns is, with one or two exceptions, essentially similar to that of the root in flowering plants. In the Male Fern and in many other Ferns the vascular cylinder of the root is diarch (see Fig. 25). The first -formed elements of the wood,protoxylem, lie at the two ends of the xylem-plate, exactly as in the Wallflower (see Part I. p. 73), and the development of the wood advances from these two points in centripetal direction to the middle of the cylinder. The Small first- ^ IG- ^' Transverse section of central part of root of a Fern, showing origin formed tracheides are of rootlet, xy, diarch xylem; ph, Smrallv thickened the l' hl em \pe, pericycle ; en, endodermis ; "y Uj UD c', cortical cells ; a, apical cell of root- larger elements, devel- let ; r.c, root-cap ; p, cortex of rootlet ; nnprl Iflf-Pr nrp Qpnlori jtf, stele of rootlet. Magnified about J > are 150 diameters. (After Van Tieghem.) form. On either side of the xylem-plate, and therefore alternating with the protoxylem-groups, are two strands of phloem. The whole is surrounded by a single layer of pericycle, and this again by the endodermis, which has the usual cuticularised bands on its radial cell-walls. The cortex consists of two zones an inner thick-walled region forming a firm sheath round the cylinder, and an outer portion in which the cells have thinner walls. We often \ 52 STilUCTURAL BOTANY find that the cell - walls of the inner zone are not uniformly thickened ; at the points opposite the two ends of the xylem-plate, the cells remain comparatively thin-walled so as to leave a free passage, through which the water absorbed from the soil can reach the wood,, and thus pass upwards to the stem and leaves. At the exterior of the whole root is the piliferous layer, which bears numerous unicellular root-hairs. We see that, except for minute details, such a root resembles a young root of the Wallflower (see Part I. p. 73), but in the case of the Ferns there is no secondary growth of thickness. When we come to consider the develop- ment, we shall find considerable differences from any of the previous types. 4. The Growing-Points a. The Stem With rare exceptions, the development of both stem and root in Ferns can be referred to a single apical cell, from the divisions of which all tissues and organs arise. This important cell can be easily distinguished from its neighbours, which are derived from it, by its larger size and characteristic form. In the Male Fern and most other members of the class, the apical cell of the stem has the form of an inverted three-sided pyramid or tetrahedron, with its curved base directed outwards (see Fig. 26, which is taken from a simpler Fern-stem, but illustrates the essential features). In longitudinal section, therefore, the cell appears triangular ; its three sides are in contact with the adjacent tissue, while the curved base is free and faces upwards (assuming the stem to be erect). The apical THE VASCULAR CRYPTOGAMS 53 cell divides in regular order by walls successively parallel to each of its three sides. The cells thus cut off are called segments (see Fig. 26). By the growth and repeated subdivision of the three rows of segments all the tissues of the stem are produced. The stem figured is monostelic ; here the first tangential walls formed in the segments mark the limit between the central cylinder and the surrounding FlG - 26. Apex of stem of a T IT Tern (stolon of Ncplirolepis} Cortex. Ill a polystellC Stem, i n longitudinal section, n, SUCh as that of the Male Fern, apical cell; s i} s, segments ,; c, cortex ; p, stele ; r, cell the steles are not marked out from which a root will arise. Magnified 80 diameters. (After Van Tieghem.) until after more numerous divisions have taken place. It is probable that each leaf owes its origin to the out- growth of cells derived from a single segment. 13. The Pioot The root, like the stem, carries on its apical growth by means of a single cell, which here also has the form of an inverted three-sided pyramid. The essential difference between the divisions in the apical cell of the root and in that of the stem, is that, in the former, cell- walls are not only formed parallel to the three sides, but also parallel to the base of the pyramid. The segments thus cut off from the outer end of the apical cell (see Fig. 27.) go to form the root- cap; those cut off laterally build up the tissues of the root itself, in much the same way as in the case of a mom stelic stem. The mode of branching of the root in Ferns differs 54 STRUCTURAL BOTANY in one important respect from that in the higher plants. In Ferns each rootlet arises, not from the pericycle, but from the endodermis, and, in fact, its origin can always be traced to a single endodermal cell, lying opposite one of the groups of protoxylem. The cells destined to give rise to rootlets can be distinguished by their larger size. The cell in question divides up ,r.c. ,-P pc en FIG. 27. Apex of root of a Fern in longitudinal section, showing triangular apical cell, pi, stele ;pe, pericycle; en, endodermis; p, cortex ; r.c. root-cap. The dark lines mark out the cell- groups, each formed from a single segment. Magnified 120 diameters. (After Van Tieghem.) by inclined walls, so as to form at once a pyramidal apical cell, by means of which the further development of the rootlet is carried on (see Fig. 25). The young root, as it makes its way through the tissues of the parent organ, is at first enveloped in a digestive sac, derived from an inner layer of the cortex, and serving to absorb the tissues which have to be penetrated. In the Ferns, the pericycle has nothing to do with the THE VASCULAR CRYPTOGAMS 55 development of the rootlet, beyond forming a pedicel by which it is connected with the vascular tissues of the main root. The origin of the adventitious roots, which play so important a part in the organisation of Ferns, follows the same rule which holds good for the rootlets. Every ad- ventitious root arises from an endodermal cell bordering on one of the steles of the stem. In Fig. 26, for example, the shaded cell marked r is destined to produce a root. We see from this that the first differentiation of the root-forming cells in the stem takes place very early. 7. The Leaf The development of the leaf in Ferns, like that of the stem and root, goes on at the apex, whereas in most flower- ing plants the growth of the leaf chiefly takes place at the base. It is only when still very young that a Fern leaf grows by means of a single apical cell. This cell soon divides up so as to form a row of marginal cells, all of which take equal parts in the subsequent cell-formation. B. EEPRODUCTIVE ORGANS OF THE SPOEOPHYTE We have already seen that the sporangia of the Male Fern are grouped in sori, and that the sori are seated on the back of the leaf, over the lateral veins of a segment or pinnule (see Fig. 18, B). Beneath each sorus is a prominent mass of tissue, which we may call the placenta (see Fig. 28, r). This receives a short branch from the vascular bundle immedi- ately below it. The placenta grows out at its summit into the kidney-shaped indusium, which consists of a membrane, one cell in thickness, attached to the placenta 56 STRUCTURAL BOTANY by a massive stalk (see Fig. 28, i). The sporangia spring from the sides of the placenta, and are all roofed in by the indusium. Each sporangium consists of a long slender stalk, made up of two or three rows of cells, bearing the terminal spore-case or capsule (see Fig. 28, B y C, E). A club-shaped hair, secreting resin, is usually borne on the stalk. The capsule is not spherical but much flattened, resembling the case of a watch in form ; its wall when mature consists of a single layer of cells ; its interior is occupied by the spores, forty-eight to sixty- four in number, which are of a brown colour when ripe. The sides of the capsule are formed of cells with thin membranes, but around its edge runs a single row of larger cells with peculiarly thickened walls of a rich brown colour, forming a very conspicuous feature under the microscope, when the sporangium is ripe. This special row of cells is called the ring or annulus (see Fig. 28, Jj, C, E). The annulus starts from the stalk at one side, passes over the crest of the capsule, and extends about half-way down on the other side. Here it suddenly comes to an end. The cells of the wall immediately below the termination of the annulus are broad and flat ; this is the place where the capsule ultimately opens (see Fig. 28, B and C, st). In the annulus both the inner and the radial cell-w T alls are much thickened ; the free outer walls of the cells, however, remain thin. The function of the annulus is to cause the dehiscence of the sporangium when ripe. Each sporangium arises from a single superficial cell of the placenta ; in this respect it differs from the sporangia hitherto considered, namely, the pollen-sacs and ovules of Flowering Plants, and the two kinds of sporangia in Selaginella. The great majority of Ferns are distin- THE VASCULAR CRYPTOGAMS 57 FIG. 28. A, transverse section of portion of lamina of Male Ferri passing through a sorus ; i, indusium. Magnified 80 diameters. JS, single sporangium in side view ; a, annulus ; st, cells which dehisce ; d, glandular hair. C, sporangium dehiscing at st ; a, annulus. B and C magnified 10(T dia- meters. D, spore mother-cell divided (only three spores visible). Magnified 350 diameters. E, nearly ripe sporan- gium. F, young sporangium ; c, sporogenous cells ; iw, tapetum ; w, wall. G, very young sporangium, showing first divisions. Magnified 260 diameters. (After Luerssen.) 58 STRUCTURAL BOTANY guished by the unicellular origin of their sporangia, A single cell, then, grows out from the surface of the placenta and soon begins to divide. One or two basal cells are often cut off, to begin with, by transverse walls, but they are of no great importance. The terminal cell next undergoes division by inclined walls (see Fig. 28, G), three of which are formed in succession, inclined to each other at an angle of 120, as seen from above. In side view, as shown from the figure, only two of these walls can be seen, and they join each other at an acute angle. The result of these three divisions is to carve out a three-sided pyramidal cell with a free base, quite like the apical cell which we have already described in the stem and root. The next wall formed runs parallel to the free base of this pyramidal cell, so now we have a central cell surrounded on all four sides by the segments which have been cut off from it. The segments undergo a great many more divisions, and form the wall of the capsule, which remains only one cell thick, as all the cell-divisions are at right angles to its surface. The lateral segments also cut off cells below, which go to build up the stalk. In the mean time the pyramidal central cell has itself divided by walls parallel to its four sides, so that it is now surrounded by an inner layer of cells separating it from the wall of the capsule. These intermediate cells undergo further divisions in various directions and form the tapetum, the ultimate destiny of which is to afford food material to the developing spores (see Fig. 28, F, iw). The central cell which remains is the essential part of the whole structure, for this is the archesporium, from which the spores themselves are produced (see Fig. 28, F, c). We see, then, that in this case the archesporium THE VASCULAR CRYPTOGAMS 59 begins as a single cell. It undergoes several cell- divisions (see Fig. 28, F, c). The cells thus produced round themselves off and become the mother-cells of the spores. In many Ferns there are sixteen or more mother-cells in each sporangium, but in the Male Fern there are usually not quite so many. The spore mother-cells are spherical ; as the sporangium has grown more rapidly than they have, they do not fill the whole interior, but float freely in a half-liquid mass derived from the disorganised tapetal cells. Each mother-cell now divides twice so as to form four cells, each of which has at first the shape of a quadrant of a sphere (see Fig. 28, D). These four daughter-cells are the spores. As they ripen they become kidney- shaped, the convex side corresponding to the free outer surface of the mother-cell, while the concave edge of each spore represents the line of junction with its sister-cells. The spore membrane becomes much thickened, and consists of two layers, the outer of which is strongly cuticularised, and assumes a dark-brown colour. We have now seen how the myriads of microscopic dust-like spores which we find on the back of a Fern-frond are produced. It remains for us to learn how they are scattered. It is the annulus which causes the sporangium to open ; dehiscence takes place when the wall of the ripe sporangium has begun to dry up. The cells of the annulus lose water, and consequently contract, the thin outer walls of the cells becoming concave instead of convex (see Fig. 28, C). The final result of this con- traction is that the whole annulus violently straightens itself, and in so doing necessarily tears the sporangium open, the rupture taking place across the broad thin- walled cells at the end of the annulus (see Fig. 28, C, 60 STRUCTURAL BOTANY st). The annulus not only straightens but bends back on itself in the opposite direction. The contraction of the ring and bursting of the sporangium takes place with so much violence as to forcibly eject the spores, which are scattered abroad and may be carried to a great distance by the wind. On the island of Krakatoa (Malay Archipelago), the vegetation of which was completely destroyed by the volcanic eruption of 1883, and which lies about eleven miles from the nearest land, Ferns were among the very first plants to reappear after the catastrophe. It is a very general rule that the dehiscence of sporangia is so contrived as to take place in dry weather ; the advantage of this to the plant is obvious. When the air is dry the spores form a powdery dust, which is easily scattered by the wind, whereas in wet weather they hang together in damp clusters, and could never be properly disseminated. We have now traced the history of the reproductive process in the asexual generation. The most' important points in which the Male Fern differs from Selaginella are the totally different arrangement of the sporangia, the origin of each sporangium from a single cell, and the fact that sporangia and spores are all of one kind. As regards the two latter points, however, all Ferns do not agree with the Male Fern, for in some members of the class the sporangia have a multicellular origin, while others are heterosporous. It now remains for us to follow the germination of the spores, to see how the prothalli are produced from them, to learn how fertilisa- tion is effected, and finally to study the origin of the embryo, which develops once more into the asexual Fern- plant, and thus completes the cycle of life. THE VASCULAR CRYPTOGAMS 61 III. THE OOPHYTE OR SEXUAL GENERATION A. DEVELOPMENT AND STRUCTURE OF THE PROTHALLUS Fern spores can be sown successfully on ordinary garden earth, on peat, on sand, or even on pieces of tile. The last-mentioned material has the advantage that very clean cultures can thus be obtained. It is well to heat the soil, or whatever else is used, up to at least 100 C. (the boiling-point of water) before sowing the spores, so as to destroy the germs of other organisms, which are sure to be present, and which might compete too successfully with the young prothalli. It is important not to sow the spores too thickly, or else when they germinate the prothalli overcrowd each other. The cultures must, of course, be kept moist. It is best to cover them with a bell glass, and to water from below. After about a week, the beginning of germination may be observed ; the spore starts growing and bursts its brown outer membrane. By this time the spores, which in their resting condition are without chlorophyll, will have begun to turn green. Fern spores which contain no chlorophyll when ripe keep their power of germination for a long time. In a few kinds, such as the Royal Fern, Osmunda, the spores are green, and will only germinate if sown at once. The first thing which the germinating spore does is to form a root-hair. An out- growth containing little or no chlorophyll arises from the spore, becomes cut off by a cell-wall, and grows down into the soil ; the remaining larger part of the spore grows out towards the light, and divides at first transversely. A few more transverse walls are formed, the end cell being always the one to divide, so that 62 STRUCTURAL BOTANY -a the prothallus soon takes the form of a short green filament (see Fig. 29), each cell of which may produce a root-hair. In most Ferns the root-hairs of the pro- thallus remain unicellular ; in a few they become multicellular. Soon the transverse divi- sions of the filament cease, an oblique wall FIG. 29. Very young prothallus of an Aspidium. appears in the sp, membrane of spore ; r.h, first root-hair; , ,, a, apical cell. Magnified 210. (R. S.) terminal cell, followed by another at right angles to it, and thus a wedge-shaped apical cell is marked out. This goes on cutting off segments to the right and left, the segments divide up further, and soon the young prothallus becomes converted into a flat cellular plate, which for a time remains only one cell thick (see Fig. 30). As growth goes on, the prothallus tends to become heart-shaped, the growing-point lying at the FIG. 30.-Apex of young prothallus base of a depression between of Aspidium, older > than in Fig. ,, . . 29, seen in surface view, a, apical two lobes. This IS due to cell. Magnified 210. (R. S.) the fact that the apical meristem does not grow so fast as the older tissue which has been produced from it on either side. The single apical cell does not long maintain its in- dependence. It soon divides up into a row of equivalent THE VASCULAR CRYPTOGAMS 63 initial cells, which all take a like share in the subsequent development. The prothallus, which for a short time grows vertically, soon assumes a horizontal position, and henceforth there is a marked difference between the lower side, which is in contact with the soil, and the ; \ A\ST"" 1j ; \-vXiil4Y J ' v ^ N J H* 1 U) T ---i r ^'-1 ' / ^^Mi^P W^^ -^ ^ ^ FIG. 31. Full-grown prothallus seen from below, showing archegonia towards the apex, antheridia and root-hairs towards the base. Magnified about 25 diameters. (After Luerssen. ) free upper surface. It is from the under-side that the new root-hairs arise, and to this side also the sexual organs are limited. A normal full-grown prothallus seen from below is shown in Fig. 31. The middle part, lying just behind 64 STRUCTURAL BOTANY VA V the growing-point, forms a pad or cushion several cells in thickness, while the lateral portions or wings remain one cell thick. The antheridia or male organs arise chiefly on the older basal part of the prothallus and sometimes also on the wings. The female organs or archegonia are limited to the cushion. In ordinary cases the prothallus is monoe- cious, bearing both kinds of sexual organs ; but this is not always so. Male prothalli are not uncommon, and are generally of small size. Sometimes a prothallus at the earliest stage of its development, while still in the form of a short filament, begins to form antheridia. A filamentous prothallus, bearing male organs FIG. 32. Young male prothallus of nn l v -j^ oVmwn in Tier - * i TI ,i * i ''in. y * j. o io 11' .' \v 11 111 .1 i ^ . Male Fern. an, antheridia ; sp, J , c spermatozoids escaping; r.h, root- 32, but Still Smaller hairs. Magnified about 70 diameters. ( After Kny.) Lr - Specimens with arche- gonia only are rarer, and are of the ordinary form. These variations are interesting, because they show how the distinction of sex among the individual prothalli, which has become fixed in Selaginclla and other hetero- eporous forms, appears occasionally as a more or less r.h.--. THE VASCULAR CRYPTOGAMS 65 casual phenomenon even in the homosporous Ferns. Small and ill-nourished prothalli suffice for the pro- duction of antheridia, which quickly fulfil their function and make no great demands on the food supply. On the other hand, archegonia are useless unless provision be made for the nutrition of the embryo after fertilisation ; and so we find female organs on full-grown and well- nourished prothalli only. In the heterosporous Crypto- gams provision is made beforehand, in the spore, for the more abundant nutrition of the female prothallus. R DEVELOPMENT AND STRUCTURE OF THE SEXUAL ORGANS 1. The Antheridia Each antheridium arises from a single cell, the upper part of which grows slightly beyond the general surface of the prothallus, and is cut off by a transverse wall. It then undergoes a few divisions, so that the anther- idium comes to consist of a central cell, surrounded by two ring-shaped cells, one above the other, and covered in on the top by a cap-cell. Some of the stages of development are shown in Fig. 33, A. The central cell divides up repeatedly, and gives rise to the spermatozoid mother-cells, the number of which in each antheridium averages about twenty (see Fig. 33, B). In each mother-cell one spermatozoid is formed. The mature spermatozoid consists of a spirally coiled body like a corkscrew, but thicker at one end than the other. Near the thin end a number of excessively fine cilia (contractile protoplasmic threads) are attached (see Fig. 33, C). The development has been very exactly followed ; it is known that the greater part of the body of the spermato- 5 66 STRUCTURAL BOTANY zoid is formed from the nucleus of the mother-cell ; the cilia, however, and the part of the body to which they are attached, are derived from the protoplasm. In Fig. 33, B the young spermatozoids are shown enclosed in their mother-cells. A P OoO .0000 O FJG. 33. Antheridia of Male Fern. A, 1, 2, and 3. antlieridiaat three successive stages seated on prothallus ; c, central cell ; w, wall. B, older antheridium ; sp, mass of spennatozoid mother-cells; w, Avail. C, a single spermatozoid. Magnified, A and B about 300, C about 700 diameters. (After Kny.) In the ripe antheridium every mother-cell contains its spermatozoid curled up inside it ; as soon as a drop of water comes into contact with the antberidia, they open, by the bursting of their cap - cells (see Fig. 34). The pressure which brings this about is due partly to the swelling of the mother- cells themselves, and partly to that of the ring-cells, which absorb water and press upon the mass of mother-cells, squeezing them out from the antheridium. The whole mass of mother-cells is now set free, but each spermatozoid is still imprisoned within its own mother-cell. The membranes of the latter, how- THE VASCULAR CRYPTOGAMS 67 ever, are soon dissolved, and now the spermatozoids are able to escape, and begin their active career. Each spermatozoid drags with it, attached to the hinder end, a bladder - like sac, which is derived from the inner part of the protoplasm of the mother- cell (see Figs. 33 and 34). The locomotion is very active ; the little spermatozoids go wrig- gling through the water in all directions, always keeping their thin ciliated ends foremost ; they revolve on their axes, and advance at the same time, not in straight lines, but in varying curved paths. Some- i times the little bladders are FIG. 34. Ripe antheriJium, T ,,,,,.-, .. .-, showing spermatozoids lett behind, sometimes they escaping. Magnified 350 hang 011 all the time, until diameters. (After Luers- an archegonium is reached. Before describing the ultimate fate of the spermatozoids, we must now turn our attention to the archegonia. 2. The Archegonia As we have already mentioned, the archegonia do not arise so indiscriminately on different parts of the prothallus as the antheridia do, but are limited to the sides of the thickened cushion. An archegonium, like an antheridium, arises from a single cell, which at first projects only slightly above the level of the neighbouring tissue. It divides by two transverse walls into three cells ; the lowest or basal cell undergoes a few divisions, 68 STRUCTURAL BOTANY but takes no important part in the further development; the middle cell ultimately forms the ovum and the two canal-cells ; while the uppermost of the three grows and divides to form the neck (see Fig. 35). The neck is the only part which projects beyond the surface of the cushion. The neck-cell first divides, by two longitudinal walls at right angles to each other, into four cells placed cross - a.n. a. 'n. n.c. FIG. 35. Development of archegonium of Male Fern. A, very young ; a.n, neck of archegonium ; c, central cell ; b, basal cell. B, rather older ; n.c, neck canal. C, nearly ripe ; n.c, canal cells disorganised. A,B,C, in longitudinal section. D, neck seen from above. Magnified about 250 diameters. (After Kny.) wise, as seen in surface view (see Fig. 35, D). Each of these four cells then divides up repeatedly by approxi- mately transverse walls, so that the neck is finally made up of four rows of cells. While these divisions are going on, the neck is increasing in length, and at the same time the central cell grows up between the four rows of neck-cells x see Fig. 35, A and B), which separate a little to THE YASCULAR CRYPTOGAMS 69 make way for the outgrowth. The projecting part of the central cell is presently cut off by a wall, and forms the canal of the neck. This canal-cell may itself undergo one or two further divisions, but they are usually in- complete, no cell-walls being formed. A second canal- cell is now cut off below the first ; the remaining part of the central cell rounds off its protoplasm, and now constitutes the ovum. The archegonium has by this time reached its complete development. The neck is not straight, but is sharply curved backwards, i.e. towards the basal end of the prothallus (see Figs. 35 and 36). We see that the archegonia are really quite similar to those of Selaginella, and also have much in common with the archegonia of Conifers. C. FERTILISATION In Ferns, as in Cryptogams generally, fertilisation can only take place under water. In nature this happens after rain or heavy dew, when the under-sides of the prothalli are thoroughly wetted. When we are cultivat- ing prothalli it is necessary to sprinkle them with water from above, when the sexual organs are ripe, if we wish to obtain embryos. We have already seen how the antheridia open under water, and how the active spermatozoids are set free. In like manner, the archegonia, when moistened, open to receive them. This happens because the protoplasm of the canal-cells swells up and becomes converted into mucilage, which exercises a pressure on the neck, and causes it to open at the top, the four rows of cells being forced apart. The mucilage now more than fills the canal, and forms a viscid drop at the mouth of the archegonium (see Fig. 36). TO STRUCTURAL BOTANY The spermatozoids swimming through the water are attracted by the archegonia. This remarkable fact, which long remained an absolute mystery, is now so far ex- plained that we have good evidence as to the nature of the substance which attracts them. When a spermatozoid, as it makes its devi- ous way through the water, comes within a short distance of the neck of an open arche- gonium, it turns aside from its course, and makes for the opening. Here it finds the mu- cilaginous drop, and promptly plunges into it. Its movements do not cease, though in the denser fluid they go on more slowly ; the sper- matozoid wriggles its way down the neck, through the mucilage FIG. 36. Archegonium ready for fer- which fills it, and SO at tilisation. o, ovum ; n, neck ; m, , , -, ,-, mucilage extruded from canal ; p, cells ias ^ readies tne OVlim of prothallus. Magnified 350. (After below. Quite a number Strasburger. ) . j i of spermatozoids may be seen swarming around the opening of a ripe archegonium, and several may penetrate down the canal, but probably only one succeeds in uniting with the ovum. Now it has been shown by experiment that the spermatozoids of Ferns are attracted by certain chemical substances, and especially by malic acid. If artificial THE VASCULAR CRYPTOGAMS 71 archegonia are prepared (consisting of tiny capillary glass-tubes) and filled with a mucilage to which a small quantity of this acid has been added, they are found, when placed in water containing fern-spermatozoids, to exercise the same attraction upon them which the real archegonia exercise in nature. The malic acid gradually diffuses out into the water, and the spermatozoids are influenced by it, so that they move in the direction in which the substance is more concentrated, i.e. towards the tube. Although it cannot be proved that the archegonia themselves contain malic acid, as they are too small for a recognisable quantity to be obtained from them, yet this substance is known to be present in the prothallus as a whole ; so there can be little doubt that the natural archegonia owe their attrac- tive influence to the same chemical agent which has proved efficacious in experiment. We see, then, that these minute protoplasmic bodies, the spermatozoids, are not only capable of active move- ment, but also possess a certain power of perception, by which their movements are guided. This is a remark- able illustration of the great fact that the protoplasm of plants and animals is essentially the same, and that the living matter of a plant may show properties usually regarded as belonging especially to animals, whenever such properties are needed. Now that we have learnt how fertilisation is brought about, we will go on to consider its results D. EMBRYOLOGY The first change after fertilisation is the formation of a cell-wall around the protoplasm of the fertilised ovum. 72 STRUCTURAL BOTANY P It now at once begins to grow and divide, becoming the embryo, or young plant, of the sporophyte generation. The embryo of a Fern differs from that of the plants hitherto described, in having no suspensor ; the whole of the fertilised ovum goes to form the embryo. Through- out the Fern-group there is considerable uniformity in the manner of development of the embryo from the ovum. The first wall (called the basal wall) runs nearly par- allel to the axis of the arche- gonium, and at right angles to the axis of the whole prothallus This divides the young embryo into an epibasal FIG. 37. Embryo of a Fern (Pteris) in median an( j a 7 ?? , wo jiW r ,/ section, s, apex of stem ; e, first leaf ; r, root ; L '^ ( /, foot by which embryo is attached to pro- half ; the former thallus ; p, prothallus. Magnified 150. (After f ,, Hofmeister.) a P ex and the latter the base of the whole prothallus. Two more cell-walls then appear, all three being at right angles to each other, so that the embryo is now cut up into eight parts or octants. From the epibasal half the apex of the stem and the first leaf arise, while the hypobasal part produces the apex of the root, and an organ called the foot, which is of a temporary character and serves to attach the young THE VASCULAR CRYPTOGAMS 73 plant to the prothallus and to take up food from it until the embryonic stage is past (see Fig. 37,/). Growth and accompanying cell-division go rapidly on ; the parts which develop quickest are the root and first leaf ; for a long time the stem remains very rudimen- tary. The ventral part of the archegonium becomes much enlarged, to make room for the developing embryo. The root is the first part to break through, whereupon it makes its way down into the soil. It is soon followed by the first leaf, which turns upwards between the lobes of the prothallus, and spreads out its blade to the light. Meanwhile the foot is absorbing the food produced by the prothallus, but this is soon exhausted, and then the embryo becomes an independent plant, which con- tinues its growth, producing fresh leaves and roots. The leaves which are first formed are always of a comparatively simple shape, and it is only gradually that the successive leaves assume the form characteristic of the species. At the same time the stem increases in bulk, and its anatomical structure becomes more complex. We have now traced the normal life-cycle through its complete course, and have got back to the asexual generation, or sporophyte, from which we started. E. COMPARISON BETWEEN THE LIFE-HISTORY OF FERNS AND THAT OF THE HIGHER PLANTS In the Ferns, for the first time, the occurrence of a distinct alternation of generations becomes manifest. In these plants the prothallus, though small, is just as distinct an individual, and leads just as independent a life as does the asexual Fern-plant itself. In fact, we 74 STRUCTURAL BOTANY may even say that the prothallus is the more inde- pendent of the two, for while the young Fern-plant is for a time dependent for its nutrition on the prothallus, the latter is never dependent in any way on the Fern- plant. At any rate we have in normal Ferns two per- fectly definite generations, as distinct as possible from, each other ; one bearing exclusively the sexual, and the other exclusively the asexual organs of reproduction, and in the ordinary course of life these two generations succeed each other in regular alternation. It was in fact from the Ferns that the idea of alternation of generations among plants first arose, though it had been recognised in the animal kingdom long before. Of course the same phenomenon really occurs in Selaginella and even in Flowering Plants, but in all these it is much less conspicuous, because, as we ascend the scale, the sexual generation becomes more and more dependent on the asexual, so that at last the former is reduced to a mere insignificant appendage of the latter, and can scarcely be distinguished from it. We have just seen that, even in such Ferns as our type, when the prothalli happen to be dioecious, the male specimens often remain rudimentary. In Sclaginella, where the difference of sex is fixed, this has gone much further ; the male prothallus is reduced to one little cell, and is so insignificant as to be scarcely recognisable. The female prothallus, which has much more work to do, is much less reduced, but remains almost shut up in the coats of the megaspore, and so does not obviously suggest an independent individual. When we come to the more ancient Flowering Plants the Gymnosperms we find the male prothallus at an equally low level with that of Selaginella, but much modified, in accord- THE VASCULAR CRYPTOGAMS 75 ance with changes in the method of fertilisation. The fact that the pollen-grains are set free and germinate, clearly indicates that they mark the beginning of a new generation, while the production of active sperrnatozoids in some Gymnosperms links them directly with the Cryp- togams. The female prothallus, though not less bulky than that of Selaginella, remains for ever shut up in the megas- pore, which itself never gets free from the sporangium ; so on the female side all trace of an independent existence of the prothallus has been lost, and, except for minute developmental research, we should never have suspected the presence of an oophyte at all. In the Angiosperms matters are still worse for the sexual generation. On the male side, indeed, there is no great change, except that the homologue of the antheridium is harder to recognise, but in the embryo-sac the prothallus is scarcely to be traced, and its very origin has been pushed out of its proper place, so that most of it (the endosperm) has come to be an after- product of fertilisation. In fact, the sexual generation in Angiosperms has become so thoroughly incorporated with the asexual, that it seems almost an affectation here to talk of alternating generations at all, and certainly the existence of such an alternation would never have been discovered except by the comparison with Cryptogams. The clue afforded by the life-history of the Ferns has thus enabled botanists to follow accurately the true course of development in the higher plants, which otherwise we should never have under- stood. The regular alternation of sexual and asexual indi- viduals is often modified in special cases among Ferns. The modification may either result in a lengthening or a 76 STRUCTURAL BOTANY shortening of the ordinary life-cycle. The life-cycle is lengthened when we get vegetative propagation of the Fern-plant, so that the number of asexual generations interposed between two sexual ones is increased. This happens in those Ferns which form buds on their leaves ; the buds become detached and give rise to new plants, as may easily be seen in Asplenium bulbiferum and viviparum, so commonly grown in greenhouses. Everybody must have noticed the minute Fern-plants which are dotted about on the fronds of these Ferns, and which in the form of their little leaves are so different from full-grown specimens. Another way in which the life-cycle may be extended is by vegetative reproduction of the prothallus just the converse of the process already described. In this case a number of additional sexual generations may be introduced into the life-history. This is pretty common among Filmy Ferns, and in some tropical species, in which the prothallus produces little buds from which new prothalli arise, so that the number of sexual individuals may increase indefinitely without the inter- vention of the sporophyte generation. So much for the lengthening of the life-history. In other cases, it is cut short, that is to say, the one genera- tion passes over into the other, without the aid of the regular sexual or asexual reproductive organs. There are two possible cases of this kind; either the sexual generation may give rise directly to the asexual (apoyamy), or conversely the asexual generation may give rise directly to the sexual (apospory). We have no space to go into the details of these exceptional modes of development, but it is necessary to mention them, because it is very important to learn at starting that the distinction between THE VASCULAR CRYPTOGAMS 77 the two generations is not absolute, but that the one may sometimes pass directly into the other. In apogamy, which has been sometimes observed in our type, the Male Fern, and in many other species, the vegetative tissue of the prothallus grows out into the various organs (leaf, stem, and root) of the new Fern- plant, the origin of which cannot be traced to any single cell, or even necessarily to any definite initial group. At the same time vascular bundles appear in the tissue of the prothallus. Archegonia may be absent altogether, or, if present, have nothing to do with the production of the new plant, which arises altogether as a vegetative outgrowth on the prothallus. Every stage of transition between prothallus and plant may be found. In a Fern, nearly related to our type, and in at least one other species, sporangia are sometimes produced on the prothallus itself, among the archegonia and antheridia. Eecent in- vestigations have shown that a fusion of nuclei a kind of false-fertilisation takes place in the prothallus-cells concerned in the apogamous development. In the converse case, that of apospory, which has been observed in several native Ferns, especially garden varieties, either an abortive sporangium grows out into a prothallus, without first forming spores ; or else the sporangia are altogether undeveloped, and the prothallus arises simply as a vegetative growth from the tissues of the leaf itself. In both these cases the sexual generation is formed from the asexual directly, without the intervention of spores. Frequently apospory and apogamy occur together in the same plant, in which case there is no nuclear fusion. We thus see that we must regard the regular alterna- tion of sexual and asexual reproduction as the normal course of life-history in Ferns and their allies, but not as a cast-iron scheme which can never be departed from. 78 STRUCTURAL BOTANY TYPE VI THE FIELD HORSETAIL (Eguisetum arvense, L.) The Vascular Cryptogams at present existing in the world belong to three great stocks or Classes. We have already examined representatives of two of them namely, a Club Moss and a Fern. It remains for us now to make the acquaintance of the third Class, that of the Horsetails. The latter are not now a very important group, for there is only one living genus, containing about twenty species. But small as the family is in these days, it is a very ancient one, and has now no connection whatever with its neighbours among the Ferns and Club Mosses. In early geological times, especially in the far-off period when the coal-beds were being formed, the Horsetail family were in the height of their glory, and were represented by a number of very diverse forms, many of which grew into trees. Hence this good old stock, though now so reduced, is quite as worthy of our study as its more prosperous fellows. Several species of Horsetail are natives of England, and some are very common. In general habit they all bear a strong family likeness to each other, all having stiff, upright, jointed stems, with whorls of little-developed leaves, those of each whorl being united to form a sheath around the stem. If the stem is branched, its branches are also in whorls, the whole plant having a very formal and regular appearance (see Fig. 38). The fructification is in the form of cones, each of which is borne at the end of an upright stem, or of a branch. In some species THE VASCULAR CRYPTOGAMS 79 FIG. 38. Equisetum arvense. 1 and 2, general view of plant, showing underground rhizome, bearing roots, with fertile and sterile aerial stems. 1, Fertile stems ; a, ripe cone. 2, much branched sterile stem ; a (on rhizome), tubers. 3, single peltate scale from cone, showing sporangia. 4, similar scale from below ; sporangia dehiscing. 5, young spore, with elaters not yet expanded. 6, mature spore in damp condition ; elaters curled up. 7, the same in dry condition ; elaters expanded. Figs. 1 and 2 reduced ; Figs. 3 and 4 magnified slightly ; Figs. 5, 6, and 7 very highly magnified. (After "Wossidlo, from Strasburger.) 80 STRUCTURAL BOTANY (as in our type, shown in Fig. 38) there are special fertile stems which only bear the cones, but do not branch, and are not green. In others, the cones are borne on the ordinary green vegetative stems. Underground the plant has a much-branched rhizome, which penetrates to a great depth in the soil, and makes these plants most obstinate weeds. If such a species as E. arvense or E. maximum has once established itself in garden ground, it is almost impossible to get it out again, for its rhizome goes too deep to be easily dug up, and is perpetually giving rise to new shoots. Equisetum, as we shall ficd, resembles other Vascular Cryptogams in having a sharply marked alternation of generations. The plant, as we see it, is the asexual sporophyte, and with this we will begin. I. EXTERNAL CHARACTERS OF THE SPOROPHYTE A. VEGETATIVE ORGANS The general habit of the commonest British species, E. arvcnse, is well shown in Fig. 38, but only some of the upper branches of the rhizome are represented. We must picture to ourselves the main part of the rhizome deep down in the soil, perhaps three feet below the surface, sending up branches which alone are visible in the figure. The characteristic leaf-sheaths are obvious on all the stems whether above or below the ground; on the older parts of the rhizome, however, they often wither away. Each sheath consists of a whorl of coherent leaves, the free parts of which are only repre- sented by the teeth at the top of the sheath. The rhizome bears numerous slender adventitious THE VASCULAR CRYPTOGAMS 81 roots, arising at the nodes, and in this species also produces round tubers, each of which represents a short branch consisting of a single swollen internode. These tubers are capable of giving rise to new plants, and thus form a means of vegetative propagation (see Fig. 38, 2, a). The characters of the stem are best studied in detail on the shoots which rise above the ground. The surface is ribbed lengthwise, each rib lying in the same straight line as one of the leaves of the node next above. Both ribs and leaves alternate regularly in successive inter- nodes. The stems above ground are in this species (E. arvense) of two kinds. First, we have the fertile shoots, which show themselves in spring (March) and have no other function than to bear the cones (Fig. 38, 1). These fertile shoots are unbranched, and are of a pale colour, containing little or no chlorophyll. They die down as soon as the spores are shed. The other shoots are sterile, and their branches constitute the assimilating apparatus of the plant, for the leaves are" of little im- portance in this respect (Fig. 38, 2.) They are of a deep green colour, and are repeatedly branched, the branches breaking out from the stem through the lower part of the leaf-sheaths. In each whorl the branches are equal in number to the leaves, and alternate with them. The ultimate ramifications are very slender, and only have from three to five ribs, while the main stem may have as many as twenty. The surface of the aerial shoots is very hard and somewhat rough, especially at the ridges. We see then that our plant has a very characteristic habit, marked partly by the small development of the leaves, and partly by the great regularity of the whorled branches. Other species differ considerably from this type ; many have only one kind of stem, the cones 6 82 STRUCTURAL BOTANY being borne on ordinary vegetative shoots, while in others the aerial shoots branch little, or not at all. In E. maximum, the largest British species, the barren stems sometimes attain a height of six feet, but some of the tropical kinds, such as E. giganteum, a native of tropical America, are much taller, even, it is said, reaching forty feet. B. BEPEODUCTIVE ORGANS The cone of an Equisetum is unlike the fructification of any other living plant, and cannot be mistaken when once seen, though the male flowers of some Coniferse, such as the Yew, are found to bear a certain resemblance to it when closely examined. The cone is terminal, either on the main fertile shoot (as in E. arvense) or on a branch (as in E. limosum). It consists of a fairly stout axis, giving rise to densely crowded alternating whorls of peltate scales (sporangioplwres) on which the sporangia are borne (see Fig. 38, 1, a). The scales of the cone are usually called sporopliylls, and their mode of development agrees well with their leaf-nature, but some of the fossil forms throw a certain amount of doubt on this interpretation, so we prefer to call them simply sporangium-bearers. In each whorl there are a consider- able number of sporangiophores, about twenty in many cases. Each sporangiophore has a short cylindrical stalk, and expands at the end into a flat disc, to the under-side of which the sporangia are attached, five to ten on each scale. The peltate heads of the sporangiophores are in such close contact that they usually become hexagonal from mutual pressure. The sporangia extend inwards as far as the axis, so as to fill up all the room that is left between the peltate scales. They contain very THE VASCULAK CRYPTOGAMS 83 numerous spores, which are all of one kind. At the bottom of the whole cone is a ring of abortive leaves, called the annulus (see Fig. 38, 1, a); sometimes there are two such rings. These rudimentary structures are of some interest, because in many of the fossil forms there are whorls of barren leaves or bracts between the whorls of sporangiophores. It is possible that we find the last remnants of these bracts in the annulus of living Horsetails. II. INTERNAL STRUCTURE AND DEVELOPMENT OF THE SPOROPHYTE 1. VEGETATIVE ORGANS a. The Stem The general structure of the stem in the genus Eqmsetum is at once simple and characteristic. Among all the Cryptogams now living, these plants approach most nearly, as regards their anatomy, and especially that of the stem, to the simpler Gymnosperms and Dicotyledons, though in other respects they differ widely from them. The stem of Equisetum is invariably traversed by a number of collateral leaf -trace bundles, arranged in a single circle. The course of these bundles is excessively simple ; a single one enters the stem from each leaf, i.e. from each tooth of the coherent sheath. It passes straight down the whole length of the internode, without joining on to any other bundle until it reaches the node below. Here it forks into two, and the forks attach themselves to the two adjoining bundles coming up from below, just where they are beginning to bend out into 84 STRUCTURAL BOTANY the leaves ; consequently every internode contains just as many bundles as there are leaves at the node above, and as the leaves alternate with each other at successive nodes, so also do the bundles in the corresponding inter- nodes. As all the bundles enter the stem to the same vl. sc. .sV, FIG. 39. Equisetum arrense ; transverse section of a branch of sterile stem, x, xylem ; ph, phloem ; ca, carinal cavity ; en, endodermis ; sc, sclerenchyma ; a, assimilating tissue ; rl, vallecular cavities (imperfectly formed) ; st, stomata. Magnified 45 diameters. (R. S. ) depth, and then turn vertically downwards, it follows that, as seen in transverse section, they always form a single ring. It will be seen that the bundle-system is just of the kind typical for Conifers and Dicotyledons, but it is one of the very simplest examples of this type. The stem is always ridged on the surface, as mentioned THE VASCULAR CRYPTOGAMS 85 above. Each of the ridges corresponds in position to one of the vascular bundles (see Fig. 39). The Equiseta are characterised by a great development of intercellular spaces, which have a very definite arrangement. There is usually a ring of large spaces in the cortex, and these cortical cavities are alternate in position with the bundles, and thus lie opposite the depressions or furrows of the external surface. For this reason they bear the name of vallecular cavities. Another ring of intercellular canals accompany the bundles, one on the inner side of each ; these lie opposite the ridges of the stem, and are consequently called the carinal cavities. We shall see presently how they arise. These canals are interrupted at the nodes. Lastly, the whole interior of the pith of the internodes often becomes hollow, leaving only a persistent dia- phragm at each node. This almost always happens in the main aerial stems ; but in the finer aerial branches (see Fig. 39) and in the rhizome the pith often remains solid, as is the case in E. arvense. The intercellular spaces do not all fulfil the same function ; the carinal cavities and the central cavity, if present, usually contain water, while the vallecular cavities are always full of air. In E. arvense and some other species the central cylinder is well defined, a common endodermis sur- rounding the whole ring of vascular bundles on their outer side. In other species, however, there is a separate endodermis round each individual bundle, as in E. limosum (see Fig. 40). In others again there is an intermediate state of things, for a common endodermis is present inside the ring of bundles, as well as outside them (E. variegatum). 86 STRUCTURAL BOTANY These differences, however, do not otherwise affect the anatomy. Although the same general structure is maintained throughout the shoot, yet in the minute ultimate branches the number of bundles becomes much reduced, often down to three, and in these cases the appearance of sf. the transverse section may be very different from that of a main stem or larger branch (see Fig. 40). We will now consider the tissues rather more in detail, and will begin with the vascular bundles. Each bundle is FIG. 40. Equisetum limosum ; transverse normally collateral, section of an ultimate branch of the aerial -,-1 i orr> ^-n stem, v.b, the three vascular bundles, each l - e - W1 | n X J L with its o\vn endodermis ; st, the depressed its inner and stomata. The pith is hollow, but there are -, .. no vallecular cavities. Almost the whole Phloem On ItS Outer cortex is assimilating palisade -tissue. Mag- side. The Cailual nified 100 diameters. (R. S.) .. , cavity marks the position of the protoxylem or first formed tracheides of the bundle (see Fig. 39). Here a few tracheides have become thickened (in an annular or spiral manner) at a very early stage of growth ; consequently they cannot follow the expansion of the surround- ing tissues, and a rupture takes place, forming the cavity. Projecting from the walls of this cavity we see St. THE VASCULAR CRYPTOGAMS 87 the rings or spirals of the disorganised tracheae (see Fig. 39). The later-formed part of the xylem, con- sisting of a few scalariform tracheides (not vessels), is usually separated from the protoxylem by a little parenchyma, and forms two groups to the right and left of the bundle. The whole wood therefore, if continuous and not disturbed by the carinal cavity, would form, as seen in transverse section, a V with the point inwards and the limbs outwards. The phloem lies between the limbs of the V (see Fig. 39, ph). It consists of sieve-tubes (with sieve-plates on their oblique transverse walls) and parenchyma. Beyond this, on the outer side, we come to the pericycle and then to the endodermis (with well-marked cuticularised bands on its radial walls) which marks the beginning of the cortex (Fig. 39). The xylem is often very little developed, especially in the rhizomes and the stems of aquatic species. The pith, or what remains of it, when the stem is fistular, consists of ordinary parenchyma, and presents no features of interest. The cortex, however, at least in the aerial stems and branches, is highly differentiated, as indeed we might expect, considering that it has here to perform the assimilating function usually assigned to the leaves. The inner cortical layers consist of large - celled parenchyma traversed by the air-containing vallecular spaces. The outer cortex is made up of two kinds of tissue, namely, sclerenchyma, fulfilling the mechanical function of strengthening the stem, and chlorophyll- tissue, to which the functions of assimilation and transpiration belong (see Fig. 39). Now both these tissues need to be as near the surface as possible, in order to do their work to the best advantage. The 88 STRUCTURAL BOTANY mechanical tissue offers the greater resistance to bend- ing strain, the further it is removed from the centre- line, or " neutral axis," as it is called in mechanics, of the column, here represented by the stem. For this reason we know that iron columns are always made hollow, for the same amount of material can be used to better advantage if brought as near the exterior as possible, than if distributed all over the transverse section. This mechanical principle is constantly illus- trated in the construction of plants. Again, the assimilating tissue obviously requires to be as near the surface as possible, so as to be fully exposed to light, without which its work cannot go on. Now we will see how in the stem or in a branch of Equisetum a compromise is made between these two competing interests. Each prominent ridge of the stem is occupied by a strand of sclerenchyma, and there are an equal number of additional strands placed at the bottom of the furrows (see Fig. 39, sc). The assimilat- ing tissue occurs in curved bands, each of which lies behind one of the sclerenchymatous ridges, and reaches the surface on either side of it, between the mechanical tissue of the ridge and that of the furrow (see Fig. 39, a). The epidermis has stomata at those places only where the chlorophyll-tissue reaches the surface, so they are placed where they are most needed for transpiration and the passage of gases. We notice also that the bands of chlorophyll - tissue lie directly opposite the vascular bundles, so that they are well situated both for receiving the water and mineral substances from the latter, and also for transferring to them in return the products of assimilation. In the very minute ultimate branches, such as that of which a transverse section is shown in THE VASCULAR CRYPTOGAMS 89 Fig. 40, things are simplified. Here there is little need for mechanical strength, as the weight of the branch is trifling, and so we find the whole cortex utilised for O' assimilation ; the vallecular spaces also are absent. Functionally these little twigs do duty as leaves. The epidermis is chiefly remarkable for its strongly silicified outer cell-walls, which make the surface extremely hard. If all the organic matter be completely burnt away, a perfect skeleton of silex, still showing every marking on the cell-walls, is left behind. The stomata are peculiar, because the guard-cells are. completely covered in on the top by the subsidiary cells, so that a double pair "of guard-cells, one above the other, seems to be present. The description of the structure of the stem, which we have just given, refers more especially to the sterile shoots growing above ground. Both the underground rhizomes and the fertile shoots are somewhat modified in structure. In the former the epidermis is destitute of stomata, and the cortex of chlorophyll-tissue, while mechanical tissues are little needed and little developed ; thus the whole differentiation of the outer tissues is much reduced. In E. arvense the pith of the rhizome is solid, and this is often the case in the smaller aerial branches also, as shown in our Fig. 39. The tubers consist simply of parenchyma crowded with starch, and traversed by a few reduced vascular bundles ; each tuber corresponds to a single internode. The fertile stem being a transitory organ, with no other function than to bear the cone, has a simplified structure, and is destitute at once of stomata, chlorophyll, and sclerenchyma. Throughout all parts of the shoot, however, the vascular system maintains the same struc^ 90 STRUCTURAL BOTANY ture, and this tissue-system is the most constant and characteristic feature in the anatomy. b. The Leaves The leaves of Equisetum are of little importance as organs for gaseous interchange, and probably serve chiefly as a protection to the lateral buds which arise beneath them. However, they no doubt take a certain part in assimilation and transpiration, as is shown by their structure. These functions are of course limited to the leaves of aerial shoots, and in the case of species like E. arvense, to those of the sterile stems. The vascular bundles of the leaf-sheaths are of simple collateral structure, and do not have carinal canals. Each bundle is surrounded by its own endodermis, whether this is the case in the stem or not. As in the stem, the bundles correspond in position to the ridges of the sheath ; outside each bundle lies a strand of sclerenchyma. A narrow band of chlorophyll-containing tissue lies between the sclerenchyma and the vascular bundle, and approaches the surface on either side of the ridge. The stomata are placed where the assimilating cells reach the epidermis, so that there are two longi- tudinal series of stomata corresponding to each vascular bundle. The rest of the leaf -sheath consists of ordinary parenchyma, which thins out between the ridges. The teeth, which alone represent the free part of the leaves, are still further simplified ; a vascular bundle enters each tooth, but gradually dies out. c. The Boots. The roots of Eguisetum are always very slender, and must not be confused with the underground parts of THE VASCULAR CRYPTOGAMS 91 ex. en* the stem, which are much larger (see Fig. 38). All the roots seen on a mature plant are adventitious ; the main root of the embryo only lasts a short time ; its structure is like that of the adventitious roots, and our Fig. 41, which was drawn from the main root, will serve to represent either. The young parts of the root bear numerous root-hairs. They have a wide cortex, enclosing a small and simple central cylinder, the structure of which is usually either triarch ortetrarch. The arrange- ment of the xy lem- and phloem- groups is that usual in roots ; the centre is occupied by a large tracheide. The chief peculiar- ity of the root FIG. 41. Equisetum; transverse section of main root, x, triarch xylem ; ph, phloem (three groups) ; en, double endodermis ; ex, thick - walled exodermis ; ep, epidermis. Magnified about 100 diameters. (After Buchtien. ) is its double endodermis ; the inner layer taking the place of a peri- cycle, which is quite absent. That this layer is really part of the endodermis is proved by the development, and by the fact that its cells fit on accurately to those of the outer sheath, which alone has the usual endo- dermal structure (see Fig. 41, en). This double endodermis is a character quite peculiar to the roots of Equisetum. The origin and mode of growth of the root will be considered in the next section. Apart 92 STKUCTURAL BOTANY from the peculiarity in the endoderrnis, the structure quite agrees with that of a simple root in the higher plants. d. Growing-Points and Branching The growing-points of Equisetum afford perhaps the very best examples of growth by means of a single apical cell, by the divisions of which all the tissues arise. The apex of the stem is acutely conical (see Fig. 42), and the top of the cone is occupied by the large apical cell, which has the form, so common in apical cells, of an inverted three-sided pyramid, of which the curved base is free, while the three sides are adjacent to the surrounding meris- tematic tissue. Divisions take place in the apical cell by walls formed in succession parallel to each of its three sides ; each segment cut off is then divided into two by a wall parallel to the first. The cells thus formed are again divided by approxi- mately radial walls, and then for the first time division takes place in a plane parallel to the external surface of the growing-point. We now have an outer and an inner set of cells. The former, by their further growth and subdivision, give rise to the whole of the vascular tissue, cortex and epidermis, the inner cells only form the pith, which in the main stem soon becomes hollow. There is here no trace of the distinct layers giving rise to epi- dermis, cortex, and stele, such as are sometimes to be recognised in Flowering Plants. The ring of vascular bundles is only marked out at a long distance below the growing-point. About the fifth internode from the apex we find a small-celled zone of tissue, derived from the inner part of the outer layer. This zone gives rise to the vascular bundles, and to the medullary rays between THE VASCULAR CRYPTOGAMS 93 them. The epidermis is also differentiated late, for there is no distinct derinatogen near the apex. The whorls of leaves are at first crowded closely together ; the internodes between them only begin to lengthen some way down the stem. Each whorl arises from the outgrowth of a ring of tissue which extends all FIG 42. Equisctum arvense ; longitudinal median section of the apex of the stem, a, apical cell ; s, segment cut off from it ; ^i> ^2> ^s> youngest leaves, in order of age. L, outline of older leaves. Magnified 180 diameters. (R. S.) round the stem. The circular ridge thus produced, which is at first of equal height all the way round (see Fig. 42, ^ and / 2 ), is the young sheath, and soon grows out at certain places to form the leaf-teeth. We see then that the sheath is formed first, and the free part of the leaves later. The development of the branches in Equiselum is 94 STRUCTURAL BOTANY peculiar. They are apparently of endogenous origin, and for a long time were thought really to arise below the surface, though this is not the case. The branches are arranged in whorls in the axil of each sheath, but alter- nating with the leaf -teeth. The buds arise near the growing-point, each from a single superficial cell, lying FIG. 43.- Equisctum arvcnse ; part of a radial section of stem, just below the apex, to show exogenous origin of branch. a, apical cell of branch ; 1 1} cortex of stem ; 1 2 , base, of leaf below branch ; c, crevice between them, about to close up. Magnified 360 diameters. (R. S.) immediately above the junction between leaf-sheath and stem (see Fig. 43). This cell divides up so as to carve out a pyramidal apical cell like that of the main stem, and the growth of the branch now goes on in the usual way. But while it still consists of a very few cells only, the leaf-sheath grows out above it, and joins on to the tissue of the THE VASCULAR CRYPTOGAMS 95 stem on the upper side of the bud, so as completely to shut it in. Our figure shows the bud just before it is quite enclosed, while there is still a crevice left above it, between the stem and the leaf-sheath. When this passage is once shut, it never opens again ; the bud goes on developing within a closed chamber. It lives to some extent at the expense of the surrounding tissue, and eventually breaks through the base of the leaf-sheath, and at last reaches the light of day. Seen from outside, these branches appear to arise below the node, which of course is not the case really. Endogenous buds are very rare, and we see that those of Equisetum are not among them, but only become enclosed after they have started in the usual way, as superficial outgrowths. Another peculiarity in Equisetum is the arrangement of the adventitious roots, which do not grow on the main stems, but are always in connection with lateral buds. As a rule, one root (occasionally more) is formed at the base of each branch, arising on its lower side, just below its first leaf-sheath. On the aerial branches these roots generally remain undeveloped, while the branch goes on growing. On the rhizome the reverse is the case, for, as a rule, the buds themselves are abortive, while the roots which they bear grow vigorously. The root grows at the apex by means of a single apical cell of the same pyra- midal shape as that of the stem, from which it differs, however, in forming walls parallel to the free base, in addition to those parallel to the sides. The cells thus cut off at the end increase and multiply very rapidly, and form the root-cap. All the rest of the root is formed from the segments cut off from the three sides of the apical cell. The mode of growth is much the same as in the Fern-root, shown in Fig. 27 (p. 54). 96 STRUCTURAL BOTANY The roots of Equisetum branch freely ; the origin of the branches, as in other roots, is deep-seated or endo- genous. In this case it is from the inner layer of the double endodermis that the rootlets are formed, each of them arising from a single cell which lies just on one side of a protoxylem-group. This cell divides up so as to form an apical cell of the usual pyramidal form. The rootlet has to make its way through the whole thickness of the cortex, and in doing so is helped by the presence of a digestive sac (see Part I. p. 171), formed from the outer endodermal layer, which thus constitutes a temporary covering to the young root. 2. KEPRODUCTIVE ORGANS OF THE SPOROPHYTE We have already learnt the main points in the structure of a cone of Equisetum so far as they can be seen with the naked eye or a pocket lens (see p. 82). It remains for us to make ourselves acquainted with the more minute characters. The anatomy of the axis of the cone is in all essentials just the same as that of a vegetative stem, and the development takes place in the same manner, though the growth of the cone is limited. The whorls of sporangiophores are in origin somewhat similar to the whorls of vegetative leaves, but in the fertile cone scarcely any sheath is developed, so the sporangiophores are separate outgrowths almost from the first. The upper part of the sporangiophore soon begins to grow in diameter more rapidly than its base, which thus becomes constricted, so that the mature peltate form is already indicated. At about the same time the spor- angia begin to show themselves as slight outgrowths pro- jecting from the under-side of the expanded portion. Each THE VASCULAE CRYPTOGAMS 97 sporangium, of which there are many, arises from the growth of a little group of cells. The essential part, however, can all be traced to a single superficial cell, which by its repeated divisions gives rise both to the archesporium and to that part of the wall lying over it. u.b. v.b. FIG. 44. Equisetum maximum ; part of transverse section of young cone, showing one complete peltate scale, and parts of two others (P). sp, sporangia ; the shaded part is the archesporium ; v.b, vascular bundles ; the scattered shaded cells are tannin sacs. Magnified 50 diameters. (W. C. W.) At the stage shown in Fig. 44 the archesporium has already grown and divided up, so as to form a good- sized mass of spore-producing tissue. The wall is several cells in thickness. The layer of cells which immediately surrounds the spore-producing tissue on all sides, and may be called the tapetum, is soon used up for nutritive 7 93 STRUCTURAL BOTANY purposes, and the intermediate layers also disappear, so that the wall of the ripe sporangium is only one cell thick. In the mean time the sporogenous tissue goes on increasing, but all its cells do not become mother-cells ; a good many break down and give up their contents, which serve, together with the tapetum, to feed the survivors. The remaining mother-cells, of which there are a large number in the sporangium, then divide each into four, the nucleus dividing twice before the partition- walls are formed. Finally, the four daughter-cells are arranged in a tetrahedron. This rule of the division of o the spore mother-cells into four is wonderfully constant, and holds good throughout the whole of the Mosses and Vascular Cryptogams, as well as in the anthers of Flowering Plants. The young spores of Eguisetum, when first formed, have a thin wall of cellulose only, but as they ripen the structure becomes very complicated and characteristic. The actual membrane of the spore consists of three layers, but outside all these we find a structure quite peculiar to Equisetum, namely, the elaters. They are formed from the fourth or outermost layer of the membrane the epispore, as it is called ; this layer splits along spiral lines into two long bands (with flattened ends), which, until the spore is mature, remain closely wrapped round it (see Fig. 38, 5). When the spores are quite ripe, and getting dry, however, the two elaters stretch themselves out, remaining attached only in the middle of their length, and at one point (Fig. 38, 7). If it is damp they coil themselves up again (Fig. 38, 6). These extraordinary hygroscopic movements may be repeated an indefinite number of times, as we can see by mounting some spores on a dry slide under the microscope, and THE VASCULAR CRYPTOGAMS 99 then breathing on them. The moist air makes the elaters coil up, and as they dry they stretch out again, setting the spores in motion by their contraction and expansion. The use of this curious arrangement has been a good deal discussed. Probably the chief function of the elaters is to help in the dehiscence of the sporangium. As this loses moisture the spores inside begin to stretch out their elaters ; this causes the whole mass of spores to take up more room, and so to press on the wall of the sporangium, which they thus tend to burst. Another use of the elaters may be that they cause the spores to keep entangled together, so that they are obliged to germinate in company. This may be of importance, as the prothalli are usually dioecious. The outermost layer of the sporangial wall, which alone persists till maturity, consists of spirally thickened cells. Dehiscence takes place by a longitudinal slit (Fig. 38,4). The development of the cones, at least in some species of Equisetum, is remarkably slow. Thus the cone from which the section shown in Fig. 44 was made, would not have matured for two years. In this species (E. maximum, the largest in the British Flora) the cones of three successive years are present on the plant at the same time. In March, when the spores are shed, we have not only the ripe cones rising into the air on the fertile steins, but underground, still enclosed in buds, we find the cones of the next year, and the year after that too. In some species, however, as in E. limosum, the development is a great deal quicker. The spores of Eguisetwm contain not only a nucleus, but chlorophyll-granules. As is usually the case with green spores, they must be sown within a few days after ripening, or they will not come up at all. 300 STRUCTURAL BOTANY III. DEVELOPMENT AND STRUCTURE OF THE SEXUAL GENERATION (OOPHYTE) 1. THE PROTHALLUS On the whole, the sexual generation of the Horsetails is much like that of the Ferns, though there are many differences in detail, and the mode of growth and ultimate form of the prothallus are less regular in Equisetum than FIG. 45. Equisetum maximum; large female prothallus, seen from below. I, I, lobes; a, a, arche.fronia ; r.h, root-hairs. Magnified about 30 diameters. (After Buchtien.) in such Ferns as Aspidium. The spore begins by dividing into two unequal cells, and usually the smaller of these grows out into the first root-hair, while the larger gives rise to the green part of the prothallus. The latter THE VASCULAR CRYPTOGAMS 101 usually divides up so as to form at once a flat plate of tissue ; a distinct apical cell is not always to be found. The development is very variable, but generally the prothallus puts out filamentous branches, and forms a kind of cushion with a midrib in the middle, while the sides remain one cell thick. The male prothalli often have no definite growing-point. They remain, on the whole, decidedly smaller than the females, and begin to form antheridia very early. Sometimes antheridia are formed at the ends of branches, while in other cases they arise from the thickened cushion. Fig. 45 represents a very large and complicated female prothallus ; the male individuals are much smaller and less branched. The large female prothalli possess a distinct growing-point, which produces a series of lobes on the lower side of the prothallus. Between these lobes the archegonia are placed. 2. THE SEXUAL ORGANS a. The Antheridia The antheridium of an Equisetum is a very simple structure (see Fig. 46). It arises from a single cell, which divides into two by a wall parallel to the free external surface. The outer of these two cells simply forms the cover ; the inner, after very numerous divisions, gives rise to all the spermatozoid mother-cells. The cover-cell divides up two or three times, by walls at right angles to the surface, usually forming a triangular cell in the middle of the cover, through which dehiscence takes place. In each of the very numerous cells in the interior of the antheridium a single spermatozoid is produced. Its 102 STRUCTURAL BOTANY spirally coiled body is derived almost entirely from the nucleus of the mother-cell, while the cilia themselves, and just that part of the body to which they are attached, are formed from the pro- toplasm. The spermato- zoids are almost exactly like those of Ferns, and go spinning through the FIG. 46. Equisetum maximum; young wa t er ill the same way. antlieridium. w, wall ; sp, tissue from which spermatozoids will be In fact there are some formed. Magnified about 200 dia- TiY> rriQ -in wVn'pTi tVp . / A />, T> 1 j_* \ -L O-L HO -L1J. \Y lllOU LUC meters. (Alter Bucntien.) antheridia as well as the spermatozoids agree in every respect with those of the Horsetails. b. The Archegonia The first archegonia arise on the cushion-like part of the prothallus ; as fresh lobes go on forming, additional archegonia are developed at the base of each lobe (Fig. 45, a). The archegonia at first point downwards as in the Ferns, but subsequently get turned upwards by the growth of the lobes. The development of the individual archegonium is almost exactly like that in some Ferns. O v Usually no basal-cell is formed ; the mother-cell of the archegonium divides at once into central cell and neck. The central cell cuts off two canal-cells at the top, and in the mean time the neck goes on growing. Here also there are four rows of neck-cells, each row consisting of three or four cells. The four cells at the top are very long, and bend far back, leaving a wide opening between them when the organ is ripe (see Fig. 47). THE VASCULAR CRYPTOGAMS 103 a.n. \em. It is remarkable that while the Horsetail plant bears no resemblance whatever to a Fern plant, the prothallus and sexual organs are so much alike in both. It is true that they differ a good deal from those of the Male Fern, but other Ferns come very near the Horsetails as regards their sexual generation. The conditions of life of the sexual prothallus are, as a rule, simple and uniform compared with those to which the asexual plant is exposed, so that the former has less need for varied struc- tural adaptations. Conse- quently we often find that at this stage there is much in common bet ween families which, so far as their sporo- phytes are concerned, have lost all traces of relation- ship. The most striking point about the prothallus of the Horsetails is its being usually (though not without exception) dioecious. We found that in the Ferns very small prothalli often form male organs only, while the better-grown individuals produce archegonia as well. In Horsetails this difference has gone further and become more constant. Even in Horsetails, however, it is not fixed, but depends a great deal upon nutrition. ProthaUi grown on a bad soil (e.g. damp sand) will only produce male organs, while those which are better treated and provided with plenty of food (say in the shape of a food-solution such as that described in Part I. p. 202) Fin. 47. Equisctum maximum; fertilised archegonium. a.n, neck ofarchegonium ; em, young embryo, showing first divisions. Magni- fied about 150 diameters. (After Buchtien.) 104 STRUCTURAL BOTANY will generally become females. This is interesting, for here we see the beginning of sexual differentiation of the prothallus, which has become quite a fixed thing in other families of Cryptogams. It is now a well-established fact that some of the relations of the Horsetails, which lived in the very ancient coal period, were heterosporous.. Evidently our living Equiseta come from some of the less specialised members of the stock. 3. THE EMBRYO Fertilisation, so far as is known, takes place in the Horsetails in the same way as in the Ferns, but the details have not been studied. In fact the whole subject of the sexual reproduction of Equisetum is a difficult one, for the prothalli are by no means easy to cultivate, and only a few observers have succeeded in tracing the whole history. The prothalli will often grow healthily enough up to the time when the first antheridia are formed, but then they generally begin to " damp off." However, the development has been followed throughout by a few botanists, so that we know how the embryo arises from the fertilised ovum. The latter first divides into two by a horizontal wall. The first division in the upper half (that towards the neck of the archegonium) is by a somewhat inclined wall, which separates the first leaf from the unicellular rudiment of the young stem. The latter at once cuts off tw r o segments, which give rise to the second and third leaves. These three leaves form the first whorl of the young Horsetail. Though coherent at the base, they are more distinct from each other than the leaves of later-formed whorls. After these first divisions the apical cell of the stem has THE VASCULAR CRYPTOGAMS 105 a.c. L already assumed the pyramidal form which it keeps all through life. In the mean time similar divisions have taken place in the lower half of the young embryo. Here an inclined wall separates the cell destined to give rise to the main root from one which merely forms the foot, a comparatively unimport- ant structure in Equisetum. The root-cell, which lies ex- actly opposite that from which the stem is derived, divides up so as to form the usual pyra- midal apical cell, from which, by a wall parallel to the free surface, the root-cap is marked off. Thus the young embryo of Equisetum is started, and even at this early stage shows some- thing of the characters of the mature plant, such as the whorled leaves and pyramidal apical cells. Fig. 47 shows a very young embryo enclosed in the venter of the archegonium, when only a few divisions have taken place. In Fig. 48 we see the section of an embryo at a much more advanced stage, when two whorls of leaves are already formed. Up to about this age the embryo remains within the cavity of the enlarged archegonium. So far, the root has not developed much, but now it grows rapidly and breaks through the tissue of the prothallus below it. It is followed by the stem, which bursts the neck r. FIG. 48. Equisetum maxi- mum ; embryo in median longitudinal section, a.c, apical cell of stem ; 1 1} leaf of first whorl ; 1 2 , leaf of second whorl ; r, root. Magnified about 200 dia- meters. (After Buchtien.) 106 STRUCTURAL BOTANY of the archegonium. The young plant is now becoming independent, though for a time it remains connected, by means of the foot, with the prothallus. The main axis formed directly from the embryo has only a limited growth. It remains very slender, and stops growing after it has formed from ten to fifteen whorls of three leaves each. It is interesting to note that in the smallest species of Equisetum, E. scirpoidcs, three-leaved whorls are formed throughout life. At the base of the main stem a stouter lateral branch arises, and this again produces another still more vigorous shoot, and so on. Thus the mature form of the species is gradually built up by the production of successive branches, each more highly developed than the last. After a time one of the lateral shoots turns downwards and penetrates the ground, thus forming the first rhizome of the young plant. The main root is fairly well developed, though only a temporary organ, and shows the same structure as the subsequent adventitious roots. SUMMAEY We have now traced a Horsetail through the com- plete cycle of its life. So far as the general course of development is concerned, we have found an essential agreement with that in the Ferns namely, a sharply marked alternation of generations, spores of one kind, and a well-developed prothallus. Although the latter is usually dioecious, the distinction between male and female prothallus is not a fixed one, but is dependent to a great extent on external circumstances, especially nutrition. The prothallus is not unlike that of some Ferns, but the plant the asexual generation is as THE VASCULAR CRYPTOGAMS 107 different as possible both from Ferns and Lycopods, and this applies both to vegetative structure and spore-bearing organs. Evidently the Horsetails form a perfectly distinct family by themselves. As mentioned above, this family was once, in remote geological ages, an extensive and varied one. Many of its members not only grew into trees, but had the same mode of secondary growth by means of cambium, which is now almost entirely limited to Dicotyledons and Gyninosperms. Their fructifications showed a great variety, some few resembling those of Eguisetum t while most of them were more complicated, and several produced spores of t\vo kinds. In fact we can form a much better idea of the Family Equisetineae from the study of its extinct members, than from that of the small remnant which has survived to our own times. We have now come to the end of our types of Vascular Cryptogams, and may very briefly sum up the characters of this great and ancient Sub-kingdom of plants. They are quite easily characterised as plants with a clear alternation of sexual and asexual generations, each of which leads a more or less independent life, the asexual stage always being much the more highly de- veloped of the two. The fertilisation by means of spermatozoids, which sometimes occurs even among the Gymnosperms, is here a constant character. The hetero- sporous Vascular Cryptogams come nearest to the Flower- ing Plants, as was fully explained in our chapter on Sclaginella, which is the only heterosporous type which we have had space to describe. Heterospory, however, is by no means limited to the Lycopod series ; it occurs also among Ferns (in the widest sense), and, as we have already pointed out, among the fossil Equisetinese. We 108 STRUCTURAL BOTANY cannot say for certain at present which of the hetero- sporous groups really comes nearest to the Phanerogams ; probably none of those now living bear much resem- blance to the real transitional forms (probably belonging to the Fern-series), which existed at an enormously remote period, represented by some of the oldest fossiliferous strata. Selaginella serves as well as any other type, to enable us to form an idea how Cryptogamic and Phanerogamic modes of reproduction are related. In finishing our account of the Vascular Cryptogams, we have also come to the last of our series of vascular plants. So far, the same general system of anatomical structure has prevailed all through ; henceforth we shall leave it behind altogether, and find ourselves among plants with a much simpler, or at least a totally different, internal organisation. CHAPTER II THE BRYOPHYTA THE step which we are about to take, in passing on to our next type, carries us across one of the widest gaps in the Vegetable Kingdom. So far, the plant, in the ordinary sense of the word, has in all cases been represented by the sporophyte generation. We have always found that the stage of the life-cycle, lying between fertilisation and spore-production, is that in which the chief vegetative development is attained. The other stage, namely, that which succeeds spore- production and precedes fertilisation, has up to this point appeared as a comparatively insignificant organism, hardly recognisable as a distinct generation in the Phanerogams or Seldginella, though maintaining a more independent position in the Ferns and Horsetails. Henceforth we shall find the relative proportions of the two generations reversed, the chief vegetative growth taking place in the sexual stage, corresponding to the prothallus of the higher plants, while the sporophyte develops as a fruit rather than as a plant, and serves for little more than the mere production of spores. The sub-kingdom, then, with which we have now to deal, is characterised by the occurrence of a sharply defined alternation of generations, in which the sexual 109 110 STRUCTURAL BOTANY generation is the more important as regards vegetative development, the sporophyte being always dependent upon the oophyte for a great part of its nutrition, and never becoming free. This Sub-kingdom is that of the Bryophyta, or mosslike plants. It includes two great Classes, the true Mosses and the Liver worts. The Mosses, the general appearance of which is familiar to everyone, have a vegetative growth much like that of the higher plants, with well-formed stems and leaves, but all these organs belong to the sexual generation, and so are not directly comparable with the leaves and stems of the higher plants, which belong to the asexual stage. The Liverworts, perhaps less generally known to those who are not botanists, sometimes have distinct leaves and stems not unlike those of the true Mosses, but many of them have a much simpler organisation, the plant showing no distinction of leaf and stem, but consisting of an undifferentiated body performing the functions of both these organs, and called a thallas. We will take one of these simpler Liverworts for our first type of the Bryophyta, because its oophyte generation is much like the prothallus of a Fern, a fact which helps us at once to grasp the true homologies between plants otherwise so different. A. THE LIVERWOBTS TYPE VII. PELLIA EPIPHYLLA 1. THE THALLUS Pellia epipliylla is one of the commonest Liverworts, growing in very various habitats, sometimes by the sides THE BE-YOPHYTA 111 00. of brooks or wells, or in damp woods and hedgerows, sometimes actually living under water ; in other cases, however, it grows on comparatively dry sandy ground. The plant in its vegetative condition is a green, flat, lobed thallus, repeatedly branched, the lobes often overlapping each other (see Fig. 49). The plants grow socially, and may collectively cover a considerable patch of ground. If we cut off a part of the thallus and examine it, we find that it forks repeatedly, all the branches lying nearly in the same plane. The thallus has an upper and under surface, the former darker green than the FlG> 49. General view of a plant of latter ; it is traversed by . a midrib, from which it thins Off 011 either side towards the margins (Fig. 50). On the under-side numerous root-hairs arise, which spring from the midrib and fix the plant to the ground ; for Pellia, like other Bryophyta, possesses no true roots. The whole character of the plant varies greatly according to the conditions under which it grows ; so much so that its different forms would never be supposed to belong to the same species, if the transitional states had not been observed. Under water (where, by the bye, Pellia never fruits) the thallus is long, narrow, and strap-shaped, branched at rather distant intervals, with a very distinct Pdlia epiphyiia. oo, the lobed thallus, constituting the oophyte generation ; sp, the fruit, con- ^tltuting the sporophyte genera- tion. Ihe iruits to the left have opened ; those to the right are 7&* and ^ c /, os f , Hal * natural size. (After Cooke.) 112 STRUCTUKAL BOTANY midrib, and very thin margins. On damp ground, where Pellia attains its greatest luxuriance, the thallus is much broader than in the aquatic form, but still elongated, with the branches spread out nearly flat, and the midrib very strongly marked. On dry sandy soil the plant assumes a very different form ; the thallus remains short and stunted, with densely crowded branches overlapping each other. The whole plant is much thicker and tougher, and consequently the midrib becomes indistinct. In spring, when the plants begin their new growth, they send out a great number of small crowded branches, giving a parsley-like appearance to the growing edge of the thalius. The anatomical structure of the thallus is excessively r.'h. simple. It consists entirely of FIG. 50. Part of the thallus parenchyma, the cells of which of Pellia. seen from above. n , -, i M an, the numerous antheridia ; are elongated in the midrib, r.h, r.k, the root - hairs. an d polyhedral in the rest of the Slightly magnified. (E. S.) , ri , 1 , ., , thallus. Chlorophyll-granules occur chiefly in the more superficial cells. They are most abundant in the cells on the upper surface and in all cells of the thinner marginal portions. The whole tissue is rich in starch-grains. The epidermis scarcely differs from, the rest of the tissue, but has a thin cuticle, at least on the under surface of the thallus. In the interior of the middle part of the thallus there are sometimes groups of cells with much thicker walls than their neighbours. The walls, however, are of cellulose, and there is no further differentiation. The root-hairs, or rliizoids, are THE BRYOPHYTA 113 unicellular ; the cell-walls of the older hairs have a brownish colour, but give cellulose reactions. The growing-point of each branch lies at the base of a depression between the lobes, just as in the prothallus of a Fern. The growth here goes on by means of a single large apical cell, which cuts off segments both at its sides and base. The former build up, by their subsequent growth and divisions, the lateral parts of the thallus, while the basal segments are chiefly concerned in forming the midrib. The tissue derived laterally from the apical cell grows more rapidly than the apex itself, which consequently always lies in a recess of the margin. The branching of the thallus, which as we have seen may take place very freely, is dichotomous, the original growing-point giving rise to two. The way this happens is that a new apical cell is formed from one of the lateral segments, and then both the apical cells go on growing on their own account. The growing-point is surrounded by short glandular hairs, which secrete mucilage and so help to prevent the delicate tissues of this part from drying up. We see then that the thallus of Pcllia is both in external form and internal structure a very simple organism, bearing no resemblance to any of the plants hitherto considered, so far as their asexual generation is concerned. There is, on the other hand, a very marked agreement with the prothallus of a Fern in form, struc- ture, and general mode of growth. In fact, as we shall find, Pcllia and a Fern stand on nearly the same level as regards their sexual generations, though the sporophytes of the two are absolutely different. 8 114 STRUCTURAL BOTANY 2. THE SEXUAL ORGANS a. The Antheridia Pellia is usually monoecious, the thallus producing antheridia at first, and then beginning to form the archegonia. Although our plant bears a general resemblance to the prothallus of a Fern, we must not expect to find an exact agreement. In the position of the reproductive organs there is an important difference ; in the Fern-prothallus they are usually limited to the lower surface, while in Pellia and the Liverworts gener- ally it is always the upper side which bears them. The antheridia are easily seen with the naked eye, dotted over the upper surface on either side of the midrib (see Fig. 50). The antheridia when mature are globular bodies, reaching 0'3 mm. in diameter, attached to the thallus below by a very short multicellular stalk. Each anther- idium is enclosed singly in a flask-like sheath, leaving only a very narrow opening at the top (see Fig. 52). This sheath is formed by the gradual growing up of the thallus-tissue around the young antheridium. The development takes place in the following way : The antheridium arises from a single superficial cell situated on the upper side of the thallus, immediately behind the growing-point. This cell rises above the general level of the thallus, and divides by a transverse wall ; the lower cell thus formed, after undergoing a few further divisions, forms the short stalk. The upper cell divides by a longitudinal wall into two cells, and these rapidly subdivide in such a manner as to form a single superficial layer enclosing a few central cells (see THE BRYOrilYTA 115 an. Fia. 51. Transverse section through the midrib of the thallus of Fellia, showing a young antheridium. an, antheridium ; o, opening of the sheath surrounding the antheridium. Magnified 80. (R. S. ) Fig. 51). The former constitute the wall of the anther- idium, which remains one cell in thickness ; the central cells undergo a great number of divisions, giving rise to a small- celled tissue,which, when mature, is entirely composed of the mother-cells of the spermato- zoids (Fig, 52). During the cell- division rapid growth of the whole organ goes on, and in the mean time a wall of cells grows up around the antheridium, keeping pace with its development, and ultimately closing it in, except for a narrow opening at the top (Figs. 51 and 52). This is the usual course of anther- idial development in the Liverworts ; the sheath, how- ever, is not con- stantly present. FIG. 52. Part of a similar section showing a There is also a nearly ripe antheridium. st, stalk of fairly cloge a g ree _ antheridium ; o, opening of sheath. Mag- nified 80. (R. S.) ment with the 116 STRUCTURAL BOTANY antheridia of Ferns, though there are some differences in the details of development as well as in size. Each of the numerous cells of the central mass of tissue produces a single spermatozoid, just as in Vascular Cryptogams ; the development is also just the same, for the body of the spermatozoid arises almost entirely from the nucleus, while the cilia, which are here two in number, are derived from the protoplasm. The presence of two cilia only is constant through- out the Liverworts and Mosses. Among Vascular Cryptogams we find this number in the spermatozoids of the Club Moss Class, as repre- sented by Selaginetta, while in the Ferns and Horsetails the cilia are FIG 53. -Single sperm- m h more numer0 us. In Pellia atozoid oi Pelha, showing the spirally the body of the spermatozoid is u'X41ma a " while one or both ends grow out into the first root-hairs. The elaters are of use in loosening the mass of spores, so that they are more easily scattered by the wind. They also perform hygrometric movements, which actively disperse the spores. The development of the fruit of Pellia occupies a full year ; when the spores are shed new archegonia are already ripe for fertilisation. el THE BRYOPHYTA 125 SUMMARY We have now completed the simple life-history of this Liverwort. We must not suppose that all Hepaticae are equally simple ; the class is a large one, said to include nearly four thousand species, and embraces a considerable variety of form and structure. In some (e.g. Marchantia) the thalloid form is retained, but a great complexity of anatomical structure exists, while at the same time the thallus bears highly-modified branches for the produc- tion of the sexual organs. Special organs of vegetative propagation gemmae are also very frequently present, which serve to reproduce the thallus directly. In another very numerous series of Liverworts, we find, instead of a thallus, a delicate leafy stem of great beauty; in this group (which includes the majority of the species) we have a high external differentiation of the oophyte, while the anatomical structure remains simple. We will now, however, sum up the essential points in the development of Liverworts, as represented by our type. The thallus of Pellia is obviously comparable to the prothallus of a Fern, while the antheridia and archegonia also are evidently of the same nature as the sexual organs of the Vascular Cryptogams. Fertilisation is accomplished in just the same way, but the product is totally different. In the Ferns the sexually produced embryo grows up into the plant itself, which goes through a long and vigorous course of purely vegetative development, before it proceeds to form the asexual reproductive cells (spores). In Pellia, and Liverworts generally, the sexually pro- duced embryo grows, not into a plant at all, but merely into a fruit, which remains all its life attached to and dependent upon the sexual individual. The capsule, it 126 STRUCTURAL BOTANY is true, contains chlorophyll in its outer layer, and so can do some assimilation on its own account, but for the bulk of its food it must rely on the store produced by the thallus. Spore - production is the one function of the fruit ; all the parts foot, seta, and capsule are means subservient to this end ; there is no vegetative development worth mentioning. This is the great characteristic, not merely of the Liverworts, but of the Bryophyta generally the oophyte is the prominent vegetative generation, while the sporophyte has little more to do than to discharge its purely reproductive functions as a spore-producing organ. The fruit, indeed, is not always so simple as that of Pellia, but still the same rule holds good. We see, then, that in this sub- kingdom we have to do with plants in which the sexual generation is readily comparable to that of the higher Cryptogams, while the product of fertilisation the sporophyte is developed on entirely different lines. The Muscinese, or Bryophyta, are in fact plants with a well-marked alternation of generations, in which the sexual generation is the more conspicuous and indepen- dent. The distinction between Bryophyta and Vascular Cryptogams is so sharp and constant that the gulf between them has been spoken of as the deepest in the vegetable kingdom. R THE MOSSES TYPE VIM. FUNARIA HYGROMETRICA The true Mosses, the general appearance of which will be familiar to everyone, are more highly organised plants than the Liverworts, but at the same time are more THE BRYOPHYTA 127 remote from the Vascular Cryptogams ; this is our reason for taking them after Pellia. Their greater complexity extends to both generations ; the higher development of the oophyte removes all obvious resem- blance to the prothallus of a Fern, while the sporophyte, though a complicated structure, is still only a fruit, and in no way approaches the asexual generation of the higher Cryptogams. The Mosses, unlike Pellia, do not have their vegetative organs in the form of an un- differentiated thallus, but possess a perfectly distinct stem bearing spirally arranged leaves. In fact, the external characters of a Moss plant are essentially similar to those of vascular plants, but in the Mosses leaf and stem belong to the sexual generation, while in the higher plants they form part of the sporophyte. The special Moss (Funaria) on which our description is based, is a very common one, and usually grows on the ground, sometimes occurring on walls also, where, however, it is less abundant than some other kinds. It conies up in great quantities in places where there has been a fire. This Moss grows in close tufts of a bright green colour ; if we separate out a single plant we find that the slender stem is erect, reaching perhaps half an inch in height, and densely clothed with small simple leaves. The lower part, where the plant is kept from the light by the crowding of its fellows, is brown, the leaves here having lost their colour. The upper free part of the stem bears the bright green living leaves, and terminates in a bud. The stem is branched, but not very abundantly, the branches, like the main stem, growing in a vertical direction. At the base of the plant we find a number of absorptive filaments or rhizoids, but there is no real root, an organ which does not exist in any of the Bryophyta. 128 STRUCTURAL BOTANY The leaves are arranged in a spiral, with a divergence of |, that is to say each leaf is separated from the one next above it by three-eighths of the circumference of the stem, so that in following the spiral three times round the stem we pass eight leaves, and find that the ninth lies vertically above that from which we started (see Part I. p. 14). The leaves themselves are inserted on the stem with a fairly broad base ; they are ovate in form, pointed at the tip, and traversed by a distinct midrib, though not otherwise veined (see Fi & 68).. The rhizoids are white a fruiting specimen, g, when quite young, but soon be- K JX'frSt or come brown. The above descrip- sporophyte generation, tion applies especially to the barren consisting of the seta, s. -i -\-\ e *.- and the theca, th ; c, the stems ; we shall refer more parti- calyptra. Slightly mag- cularly later on to those which nined. (After Sachs.) , , bear the reproductive organs. 1. THE LEAFY STEM a. Structure The anatomy of the Moss plant, as represented by Funaria, is simple, but yet shows a fairly well-marked differentiation of tissues. In the mature stem three distinct zones can be distinguished, epidermis, cortex, and central cylinder. The epidermis is one cell thick in most places, though here and there a double row of cells THE BRYOPHYTA 129 may be found. Its cells are small, and in the older part of the stem become very thick walled. The cortex is of relatively great thickness, and made up of parenchyma, the outer cells having thicker walls than the inner. When young, the cortical cells contain chlorophyll. The central cylinder consists of a very sharply defined FIG. 59. A, lower part of a Moss plant, bearing leaves and rhizoids (r), which grow up above the ground and become secondary protonerna (p). At 6 is an underground gemma or bulbil. At k is a bud from which a new leafy stem will grow. Magnified about 20. B, C, and D, successive stages of germination of a spore of Funaria, producing primary protonema. Magnified 200. (After Luerssen. ) cylindrical strand of long, narrow, thin-walled cells, destitute of chlorophyll. There is evidence that this is a water-conducting tissue. In Funaria the central strand is quite uniform throughout ; some of the larger Mosses, however, have a more complicated arrangement, as shown in Fig. 60, which represents the transverse section of the central cylinder of Atriclium. 130 STRUCTURAL BOTANY Here there are elements of various kinds. In the middle, the large, central, rather thick-walled cells, which may be of the nature of tracheides, and serve to con- duct water, are accompanied by parenchyma containing starch; outside this is a zone of smaller cells which have more abundant protoplasm but no starch, and may fulfil the function of a rudimentary phloem. In this zone the sections of leaf - trace bundles are seen. The leaves of Funaria are traversed by a con- spicuous midrib, while the rest of the leaf is only one cell thick. The cells of the thin part FIG. 60. Transverse section of stem are uniform, except at of Atrichum undulatum. showing ,-, -i , , -\ central cylinder and adjacent tissue the Somewhat Serrated t, large water-conducting cells. The edge, where they are finely dotted elements are the sup- -, , .*, posed functional phloem. Among narrower and have rather these are small groups representing thicker walls. The leaf- trace bundles. The more ex- . . ,, ternal cells (containing starch- midrib IS Several Cells granules) belong to the cortex, fo^ an( j con tains a Magnified 190. (After Haberlaudt.) small strand of narrow cells, like those in the central cylinder of the stem. Probably these cells conduct water and assimilated food, while the function of assimilation belongs to the thin part of the leaf, which is very rich in chlorophyll- grains (Fig. 61, d). Moss leaves, by the bye, are very favourable objects for observing the multiplication of the chlorophyll-grains by division. The strand of conducting tissue enters the stem from THE BRYOPHYTA 131 the leaf, but dies out in the cortex without reaching the central cylinder. Consequently there is no complete leaf -trace system in this Moss, though some of the more complicated Mosses (such as Atriclium) have continuous strands connecting the conducting tissue of the stem with that of the leaves. We must remember that the whole mode of life of a Moss plant, especially as regards its water- supply, is very different from that of the higher plants. Many Mosses, and Funaria among them, often grow in places such as the tops of walls, or in sandy soil, where they are liable to be completely dried up in hot weather. Yet they are none the worse, and revive as soon as rain comes again. This rapid recovery is clue to their power of absorbing water by their leaves a power which is either absent or which only exists to an insignificant extent in most of the higher plants. Hence less work falls on the conducting tissues of the stem than in the latter, for only a small part of the water-supply is taken up from below, though this part of the supply is important as it carries with it the necessary mineral food-substance. As we have said, a Moss possesses no true root. The functions of a root are performed by the rliizoids. as they are called, multicellular filaments springing from near the base of the stem (see Fig. 59, r). These rhizoids are very different from ordinary root-hairs, which are generally unicellular, though multicellular rhizoids occur on the prothallus of some few Ferns. The Moss rhizoids consist of a single chain of very long cells separated from one another by oblique walls. They grow entirely by means of the apical cell at the free end of each filament, and branch repeatedly, the diameter diminishing with the successive orders of branching, so that the final ramifications are very 132 STRUCTURAL BOTANY slender indeed in comparison with the main filaments. The whole simulates a regular root system, though totally different in structure. b. Apical Development The stem of all Mosses grows by means of a single apical cell, and the plant is built up in the most regular manner from its segments. The cell is of the same inverted pyramidal form which we found in Equisetum, and divides at first in the same way by walls parallel (or nearly parallel) to the three sides. Each segment first divides into an inner and an outer cell. From the inner cells thus formed the greater part of the tissues of the stem is derived, while the outer cells give rise to leaves, buds, and the outside part of the stem. Each outer cell divides into an upper and a lower half ; from the upper half the leaf is produced, while the lateral buds, where they exist, owe their origin to the lower of the two cells. We see then that every segment produces a leaf, and that each lateral bud stands lelow the leaf to which it belongs, instead of in its axil, a striking difference from the higher plants, though we find something like it among the Ferns. Each leaf grows in length by means of a two-sided apical cell. The chief points then in the development of a Moss stem are the growth from an apical cell, the origin of a leaf from each segment, and the position of the lateral buds beneath the leaves to which they belong. 2. THE SEXUAL ORGANS Funaria is monoecious, 1 though this is not the case 1 As conclusively shown by Mr. L. A. Boodle, "The Monoecism of Funaria hygrometrica," Annals of Botany, vol. xx. 1906, p. 293. THE BRYOPHYTA 133 with all Mosses. The female shoots arise as lateral branches on the male. The male stems are of fair size, reaching a centimetre in height ; in the lower part of the stem the leaves are scattered, but at the top they are crowded together to form a conspicuous rosette. This is not unlike a flower, especially as the middle part of the rosette is of a reddish colour. In some of the larger Mosses (such as Polytrichum, which includes the very large Moss so common on heaths) the resemblance to a flower is still more striking. However, there is of course no direct homology, for these rosettes belong to the oophyte, not to the sporophyte generation, and the organs which they enclose are anthe- ridia, not stamens. On the growing-point, within tht rosette, numerous antheridia arise in long-continued suc- cession without any strict order. Both young and mature antheridia are shown in Fig. 61. As usual, the antheridium owes its origin to a single cell in which one or two transverse walls are formed, after which the growth goes on entirely by means of the apical cell, which cuts off two rows of segments. It is a good general rule in the Mosses that every organ, of whatever kind, grows by means of an apical cell, whereas this mode of growth is nothing like so general among the Liverworts. By further subdivisions of the segments, and finally of the apical cell itself, the antheridium is differentiated into an external wall one cell in thickness, and an internal mass of small-celled tissue, each cell of which becomes the mother-cell of a spermatozoid (Kg. 61,5). The mature antheridium is club-shaped and reaches 0'3 millimetre in length, containing an enormous number of mother-cells. The development of the spermatozoid 134 STRUCTURAL BOTANY within its mother-cell is precisely similar to the same process in Liverworts or Vascular Cryptogams. The antheridia open on access of water ; the cells of the wall swell and press upon the mother-cells within, which are expelled, when the top of the sac ruptures, in a single mass. The mucilaginous cell-walls of the Fio. 61. Longitudinal section through the apical bud of a male shoot of Funaria. d, leaves cut through the midrib ; e, leaves cut through the lamina ; c, paraphyses ; several antheridia are shown ; a, very young ; b, nearly ripe antheridium. Magnified 300. (After Sachs. ) mother-cells disappear and the spermatozoids are set free ; each has a spirally curved body and two cilia, just as in the Liverworts (see Fig. 62, C). The antheridia are accompanied by multicellular hairs with large heads, called the parapliyses. Their function is probably to secrete water, and so to ensure sufficient moisture for the development of the antheridia. THE BRYOPHYTA 135 The female branches are, at first, very small ; usually one such branch is produced, springing from the male stem some distance below the rosette ; sometimes more branches are produced. The leaves converge together at the top, forming a bud within which the archegonia are contained (Fig. 62, A). They arise from the cells of the growing-point, and the apical cell among others is itself used up to form an archegonium, so that no further direct growth of the vegetative axis is possible. The cell from which an arche- gonium is to be formed first divides by a transverse wall. The further growth is by means of the apical cell, which in this case gives rise to four rows of segments, three of which are peripheral and form the wall of the archegonium, while the fourth row is central. From the lowest cell of the central row 3 the ovum and ventral canal- cell (see Fig. 62, B) are pro- duced ; the rest of the series of central cells form the canal of the neck. The external seg- ments undergo further transverse and vertical divisions ; the wall FIG 62. Funaria. A, longi- tudinal section through the apical bud of a female shoot ; a, archegonia ; b, leaves. Magnified 100. B, a single archegonium ; ?', the enlarged venter, with- in which the ovum and ventral canal-cell are seen ; from n to m is the neck, enclosing the neck-canal. Magnified 550. G, unripe spermatozoid in its mother- cell, and mature spermato- zoid with two cilia. Mag- nified 800. (After Sachs). 13G STRUCTURAL BOTANY of the neck ultimately consists of six rows of cells surrounding the canal. The ventral part of the wall, enclosing the ovum, becomes two layers in thickness, and the whole archegonium is seated on a multicellular pedicel. Apart from this last point, the final form of the archegonium is similar to that in the Liverworts, the chief difference consisting in the marked apical growth which goes on in the archegonium, as in other organs, of the true Mosses. When ready for fertilisation the terminal cells of the neck separate widely from each other, leaving an open passage into the canal, which now only contains the mucilage derived from the disorganised canal-cells. Fei tilisation, as in Cryptogams generally, takes place under water. Eaindrops, which have fallen on open male " flowers," and become impregnated with the dis- charged spermatozoids, trickle down on to the lower female plants, and some of the water, carrying the spermatozoids with it, may make its way between the leaves of the archegonial bud, and reach the archegonia themselves. In this way the active male cells are brought into the neighbourhood of the female organs. The rest of the journey they must accomplish by their own movements. Experiments precisely similar to those described in the case of the Ferns have been successfully carried out on Mosses, and here also it appears that the archegonia attract the visits of the swarming sperm-cells by means of a chemical secretion. In Mosses, however, it is not malic acid, but sugar (cane-sugar), which forms the bait. The spermatozoids having been thus lured to the archegonium, penetrate the neck-canal, and one of them ultimately reaches the ovum and effects fertilisa- tion. THE BRYOPHYTA 137 3. THE SPOROGONIUM on FRUIT Funaria fruits very freely, and if we look at a patch of it, at any time of year, we are sure to find plenty of fructifying plants at one stage or another. In the mature state the fruit consists of a long, thin, red-brown stalk, bearing at its end a nodding pear-shaped capsule (see Fig, 58, sp), which at first is green, but finally turns brown. Until almost the last the capsule carries on its top a conical hood (the calyptrd) split along one side ; at an earlier stage this completely envelops the capsule, and is only pushed off as it expands, remaining hanging for a long time. When the calyptra is removed we see the top of the capsule, which is closed by a neat conical lid. The whole of this fruit, including both stalk (seta) and capsule, constitutes the asexual, spore- bearing generation, and is derived from the fertilised ovum. The calyptra, however, is formed from the enlarged wall of the archegonium, which is split off at the base, and borne aloft on the fruit as it grows. The calyptra therefore is, by its origin, a portion of the sexual plant. We will now describe the structure of the fruit when fully formed, and then shortly trace its development from the ovum. Beginning with the capsule, which is the essential part, containing the spores, we find that its base is solid, while the upper portion contains a large hollow space separating the central mass of tissue from the wall (see the longitudinal section shown in Fig. 63, which is from a young capsule in which all the tissues are already marked out). It is the upper part of the capsule which is fertile, while the basal solid portion (apopliysis) performs nutritive 138 STRUCTURAL BOTANY functions. The whole capsule is covered by a well- marked epidermis, which, on the apophysis, contains stomata. In the upper part of the capsule the wall is several cells in thickness ; the hypodermal layers are colourless, w T hile those towards the interior . contain chlorophyll. Connected with these inner cells of the wall are filamentous strands, also con- taining chlorophyll, which extend across the intercellular space, and form a junction with the internal tissues. The central mass is narrow below and ex- panded above, assuming a barrel- like shape. It is from this part that the spores are produced. The archesporium forms at first a single cellular zone, which has a hollow cylindrical form, or, more accurately, is shaped just like a barrel with the ends io 63.-Cai.sule of Funaria knocked out ( see Fig, 63, a). in longitudinal section, ap, ^ 3 \ the apophysis; a, a, the The archesporial layer is separ- avchesporium, forming a , i j ^i p j n fp T . P pll 11 l flr c-nnpp single layer of cells (lightly atecl tr m tn shaded) ; it surrounds the by a zone of sterile tissue called aSatingtiss^isS the outer spore-sac. Within the ly shaded. The operculum archesporium is the large central closes the top of the cap- ,, ., -, -, , sule, above the columella i; HiaSS of Sterile Colourless tlSSlie its individual cells are not ^} ie columella), which is COn- shown. The dark spots , . , in the epidermis of the nected below by a thinner strand apophysis indicate the ^h fae tissue of the apophysis. stomata. Magnified 14. (After Haberlandt.) The layer between archesporium FIG. THE BRYOPHYTA 139 and columella is called the inner spore-sac. The lid (operculum} is at first continuous with the capsule, but eventually becomes detached by the severance of a ring of cells (the annulus) between lid and wall. The apophysis is essentially the assimilating part of the capsule ; beneath the epidermis is a broad zone of chlorophyll-tissue, the cells of which are in many cases of the typical palisade ?/ form (see Figs. 63, 64). The epidermis bears well -developed stomata, which are in essentials similar to those of the higher plants. In Funaria, however, they gener- ally have the pecu- liarity that the wall between the two guard - cells breaks down at the two ends, SO that the part en- FIG. 64. Part of the apophysis of a Moss . . , (Bryuni) in transverse section, p.}), the Closing tne pore IS assimilating palisade parenchyma ; st, left Standing up in stoma. Magnified 130. (After Haber- ,, . , ,, . ,, landt.) the middle 01 the fused guard-cells, like a chimney -shaft passing through a room (Fig. 65). At an earlier stage, however, the stoma is two-celled, just as in vascular plants, and in many Mosses it remains so all through. In other respects these Moss-stomata are quite typical. The guard-cells differ from the ordinary epidermal cells in containing abundant chlorophyll-granules ; the form of the cells, as seen both in surface view and in section, could be exactly matched \^^\f Y ^s^wnrrrivfpiliif 140 STRUCTURAL BOTANY in the stomata of flowering plants. Beneath each stoma is an intercellular space (Fig. 64). It is remarkable to find these organs so perfectly differentiated in plants like the Mosses, which in all other respects are so remote from the higher groups. There is one Liverwort (Antlioceros) which also has well-formed stomata on the. fruit. It is worth noting that typical stomata have in no case so far been found in the sexual generation ; when the ob'phyte bears organs with the same function (as in certain Liverworts), they are constructed on a totally different plan. It is evident from the anatomi- cal structure that the sporophyte is capable of obtaining a great part of its food for itself, and this F,G.65.-Stomaof.F MM ri has been P roved experimentally to be the case. So far as the ocqiTnilofinn n f oxrhn-n i nrm of the fused guard-cells, cerned, a sporogonium such as HabeSt.) 30 ' (After that of Funaria is able to provide for itself, from the time when its assimilating tissue is developed. Water, with the mineral food-substances, is necessarily supplied through the stem of the Moss plant, and passes up to the capsule through the seta, which contains a central conducting cylinder, like that of the stem itself. The cortex of the seta consists of thick-walled tissue, and serves to give the mechanical strength necessary to enable this slender stalk to support the weight of the capsule. The bottom of the seta is fixed in the tissue of the oophyte by a conical foot, but although the in surface view, p, the pore. Note the nuclei and chlorophyll-granules THE BEYOPHYTA 141 contact is a very close one there is never any organic connection between the two generations. We see then that the sporophytic generation of Funaria is in part parasitic on the sexual plant, in part independent. It resembles in this respect a green parasite such as the mistletoe, which, like the Funaria fruit, must obtain all its water and mineral food from the host-plant on which it grows, but can provide its carbonaceous food for itself. In some other Mosses, however, the sporophyte is destitute of chlorophyll, and so has to lead a completely parasitic existence, depending for the whole of its food on the leafy Moss plant. We will now return to the essential part of the capsule, that, namely, in which the spores are formed. The archesporium is at first only a single layer of cells, and occupies but a small part of the capsule (Fig. 63). Eepeated divisions now take place, and the archesporium increases in thickness. Ulti- mately each of the cells formed by it becomes a spore mother-cell, which, as is so usually the case, divides into four spores, arranged tetrahedrally in each mother- cell. As soon as the spores are ripe the capsule begins to dry up. The columella and all the delicate tissues of the fruit collapse, and when the capsule is fully ripe it consists essentially of the wall only, filled with a mass of dark-green spores. The lid becomes detached, but the capsule after this is not left freely open, for in the mean time a double row of teeth (called the peristome) has been formed. These teeth, which project from the edge of the capsule and partly close its mouth, are formed from strips of thickened cell-wall, all other parts of the cells involved having perished. The 142 STRUCTUEAL BOTANY peristome plays a part, as we shall see later, in the dissemination of the spores. The whole of the fruit seta and capsule together constitutes the sporophyte generation, and is derived from the fertilised ovum. The latter first divides by a transverse wall ; further divisions take place, so that . a two-sided apical cell arises at both ends of this embryo, and for a time both apical cells are active, each giving rise to two rows of segments. The upper growing-point, however, is the important one, for it produces the capsule and the greater part of the seta, while the lower apical cell only contributes to the foot, which penetrates downwards into the tissues of the oophyte. The segments derived from the upper apical cell under- go division by walls parallel to the surface. In the part which forms the capsule, the inner cells thua formed constitute what is termed the endothecium, and from this central part the columella and archesporium are ultimately derived. Everything outside the archesporium is the product of the peripheral cells or amphithecium. This origin of the spore-producing layer from the outer part of the endothecium is characteristic of the great majority of Mosses. In the seta the central group of cells, corresponding to the endothecium of the capsule, simply gives rise to the central strand of conducting cells. The differentiation of the capsule from the seta takes place rather late in Funaria, after the whole fruit has grown to a considerable length. 4. GERMINATION OF THE SPORES As the capsule dries, the walls of the cells of the annulus split across and the lid is detached. The THE BRYOPHYTA 143 spores are not all scattered at once ; the dissemination is regulated by the teeth of the peristome, which, when the air is wet, completely close the mouth of the capsule, only allowing the spores to escape in dry weather. The spores themselves contain abundant chlorophyll, and also have a reserve of oil which serves to provide material for germination. When this takes place, the spore does not at once give rise to a Moss plant,but first of all produces a branched filamentous growth of very simple structure, much resembling some of the simpler plants (Algai), as we shall find later on. This filamentous condition of the young Moss, which thus forms the first stage of the oophyte generation, is called the protonema (see Fig. 59). The spore gener- ally sends out filaments in two directions ; one remains green and creeps along the surface of the ground, the other loses its chlorophyll and becomes the first rhizoid. The filaments grow in each case by an apical cell ; they branch freely but remain one cell only in thick- ness ; often the protonema develops to a great extent, forming a tangled green felt, which may cover several square inches of ground. The young Moss plants arise from the protonema as lateral buds (see Fig. 59, k). A cell of the protonema gives rise to a branch ; the branch divides by inclined walls so as to form a tetrahedral apical cell, and as soon as this has taken place regular segmentation begins, and the leafy Moss plant is soon built up. The first leaves are simpler than those of the more mature plant, and may be destitute of a midrib. The protonema which we have just described is formed directly from the spore, and is therefore called primary protonema. It may also arise in a secondary 144 STRUCTURAL BOTANY way from any part of the plant from rhizoids (see Fig. 59) or stem, or detached leaves, or even from the fruit itself. In the latter case we have production of the sexual direct from the asexual generation, affording, in fact, an instance of apospory, such as sometimes occurs in the Ferns (see p. 76). The production of proton ema provides the plant with a most abundant means of vegetative propagation, for every growth of protonema is capable of giving rise to a number of Moss plants. Many Mosses produce special vegetative buds either on their stems or rhizoids (Fig. 59, A, &), or throw off certain of their leaves as organs of propa- gation. In most cases, whatever be the nature of the reproductive body, whether spore or bud, it begins by forming protonema, from which the leafy plants arise at a later stage. This insertion of a filamentous stage of growth in the life-cycle, before the production of the typical form of oophyte, is very characteristic of the true Mosses ; in the Liverworts, the protonema is on the whole much less developed. It may be compared with the early filamentous stage of a Fern-prothallus, with which it is quite homologous. SUMMARY If we now briefly sum up the characteristic points in the life-history of the true Mosses, we see that both generations are decidedly more highly organised than in the Liverworts. The oophyte is here constantly developed as a leafy stem, quite comparable to that of the higher plants, though occupying a different place in the life- history. We find at the same time a considerable degree of anatomical complexity, corresponding to the THE BRYOPHYTA 145 higher external organisation. In like manner, the fruit, or asexual generation, is far more complex than in the Hepaticae. Except in the one point of possessing true stomata, its complexity, however, is on quite different lines from that of the corresponding generation in the higher plants. The Mosses, in fact, constitute a remark- able and very isolated group, highly developed in their own way, but with no near affinities to other Classes of plants The Bryophyta, as a whole, form a perfectly well- defined sub-kingdom, characterised by the occurrence of a well-marked alternation of distinct generations, of which the sexual is the more highly developed, so far as the vegetative organs are concerned. The sexual organs both archegonia and antheridia are constituted on the same general plan as those of the higher Cryptogams, though differing in many details. The Vascular Crypto- gams, together with the Bryophyta, are sometimes classed in one sub-kingdom under the name of Archegoniatce, founded on the general similarity of the female organs all through these groups. The mode of development of the spores, by division of a mother-cell into four, is also common to Bryophyta, together with the higher Crypto- gams, and indeed Phanerogams also, so far as the microspores (pollen-grains) of the latter are concerned. Although, therefore, the Bryophyta are at present uncon- nected by any intermediate forms with the vascular plants, yet they have many points in common with them, and the general lines of homology between all the classes hitherto considered are not difficult to trace. Now we have done with archegoniate plants. The families which remain to be considered are essentially different, both in the organisation of their reproductive organs and in the whole course of their life-history. 10 CHAPTEK III THE THE Sub-kingdom with which we have now to make ourselves acquainted differs profoundly from any of those of which representatives have been already con- sidered. In habit all the plants included under the general heading " Algoe '' are totally different from any hitherto described, and at the same time they differ more among themselves than the lowest Liverwort differs from the most complex Dicotyledon. The members of the group most familiar to ordinary readers are the Seaweeds, for with very few exceptions all plants which grow in the sea belong to the Algge. On the other hand, an immense number of species are inhabitants of fresh water, or can get on, like the American steamboat, " wherever it is a little damp." Generally speaking, the larger and more complex forms are marine ; the fresh-water and terrestrial representatives are both smaller and simpler. Among Seaweeds there are species which rank with the most gigantic members of the Vegetable Kingdom, while there are other Algre which are entirely invisible as individuals to the naked eye. The higher Algae often show a complex external form, with organs analogous to the root, stem, and leaf of the higher plants ; at the same time, their tissues are highly differentiated. On 146 THE AI/LE 147 the other hand, the simplest Algae consist of single isolated cells. Amid this vast range of forms it is evident that only a very few types can be dealt with here. As far as possible, our examples, are selected with a view to illustrate the most striking variations in the life-history and mode of reproduction of Algae. The classification of the Algae into their principal classes roughly follows the colour not that colour is in itself of systematic importance; it happens, however, among these plants that differences in their pigments generally coincide with important morphological distinctions. We will begin with the pure green Algae, those, namely, in which the chlorophyll, like that of most of the higher plants, is not disguised by the presence of any other colouring matter. This class the Chloropliyccce in- cludes the majority of the fresh-water Algae, as well as many Seaweeds. They are, on the whole, among the simpler Algae, and many of the unicellular forms belong here ; but simple as they are in structure, some of them in their mode of development approach nearer to the higher plants than any other Algae. We will take as our first type a fresh-water Alga which, though anatomi- cally simple, shows a very high form of reproduction. A. THE CHLOEOPHYCE^: TYPE IX. (EDOGONIUM 1. STRUCTURE The genus (Edogonium, of which there are a great many species, includes some of the commonest fresh- water Algae, and may be found in almost any pond or 148 STRUCTURAL BOTANY FIG. 66. General view of a very small female plant of (Edogonium ciliatum. b, the attaching disc ; ?, ?, two oogonia, the upper of which has opened by a lid at the top, and contains the fertilised ob'spore ; the lower is still closed, and the ovum unfertilised, cf, $, dwarf males, adhering to the oogonia. The uppermost has opened to discharge a spermato- zoid. Magnified 166. (After Pringsheim. 1 tank, though less common in running water. The CEdogonia are filamentous, the individual threads being only just distin- guishable by the naked eye, and grow attached to stones, piles, larger water- plants, or, in fact, to any submerged object, forming a dark-green downy coating upon it. Fig. 66 shows the whole of a small plant of CEdogon- ium ciliatum, highly magni- fied ; the specimen is much below the usual size. The main outlines of the struc- ture, however, are always the same, the whole plant consisting of a single row of cylindrical cells, attached at one end, which we may call the radical end. The root - cell contains less chlorophyll than the others. It is expanded into a flat- tened disc, which forms a holdfast, but probably does not take any special part in the absorption of food. Fresh -water Algae absorb their food, mineral as well THE ALGM 149 as gaseous, by their whole surface. Both the carbon- dioxide which they require for assimilation, and the oxygen necessary for their breathing, are present in an absorbed state in the water, which at the same time contains salts in solution quite sufficient to supply the needs of these plants. The structure of an ordinary vegetative cell of (Edogonium is as follows : Within the cellulose wall the protoplasm forms a hollow sac the primordial utricle enclosing a large vacuole. The body containing the chlorophyll is very peculiar. In most plants the chloroplasts are small granules, numerous in each cell (though Sclaginella forms an exception to this). In CEdogonium, however, there is only a single, very large chloroplast in each cell. It lies in the primordial utricle, and extends all round the cell, having the form of a hollow cylindrical network. It is so large as to give a green colour to the whole cell, when seen under low powers of the microscope. Within the chloroplast are several proteid granules (the pyrenoids), around which starch-grains are deposited as a result of assimilation in sunlight. Each cell contains a single large nucleus embedded in the protoplasm which lines the wall. (Edogonium has no apical growing-point. In some species (such as that figured) the end cell grows out into a long hair, and takes no further part in the divisions. All the cells of the filament, lying between the radical cell and the terminal hair, divide by transverse walls, as long as growth goes on. The formation of overlapping caps on the cell-wall, at the upper end of some of the cells, is due to the fact that after each division the wall of the mother- cell splits near the top, and a new piece of cell- wall is inserted between the broken edges as the 150 STRUCTURAL BOTANY daughter-cells grow. As the split takes place repeatedly near the same place, a succession of caps is formed, one corresponding to each cell- division. We thus see that the vegetative structure of an (Edogonium is excessively simple far simpler than that of any plant which we have hitherto described. We have now to consider the way in which the Alga reproduces itself. There are two distinct methods, the one asexual, the other sexual. The former serves to propagate the plant rapidly during summer, or as long as the conditions are favourable to its growth ; the latter has for its result the production of resting -spores, which can survive alike the cold of winter and the periods of drought to which (Edogonium, in common with other fresh-water Algre, is often exposed. 2. EEPRODUCTION a. Asexual Any vegetative cell may serve as an organ of asexual reproduction, and many individual plants only show this mode of propagation. The entire contents of a cell are used up to form a single spore. The protoplasm gradually withdraws itself from the cell-wall, the whole mass assuming a rounded form. At the same time a clear, colourless spot appears on one side of the contracted protoplasmic body. From this clear portion of the protoplasm numerous cilia are developed. The cell-wall splits across and the crack opens widely at one side, that, namely, towards which the clear patch of protoplasm is turned. The spore now begins to pass out through the opening, changing its form THE ALG^E 151 as it does so, to adapt itself to the width of the passage (see Fig. 67, A). On first becoming free from the mother- cell, the spore is enclosed within a delicate membrane derived from the ectoplasm of the mother-cell, which soon disappears, so that now the reproductive cell is completely at liberty. In shape it resembles a pear, the more pointed end being colourless ; the chloroplast occupies the wider part, in which also the nucleus is contained. There is no cell-wall, and the whole spore is a purely protoplasmic structure. The cilia form a fringe around the narrow end (see Fig. 67, B). Their oscillations set the spore in motion, and now it swims off through the water, rotating on its axis, and advanc- ing with the pointed end foremost. This is the first instance of an actively-moving spore that we have met with ; among the higher Cryptogams already described it is only the male cells or sperm- atozoids which are capable of locomotion ; in a large proportion of the Algce this power extends also to the spores. On account of its active movements such a spore as that of (Edogonium is called a zoospore, for when first discovered these moving cells were thought to be of animal nature. We now know that spontaneous movement is a power common to all protoplasm, whether belonging to a plant or an animal. The zoospore swims about for some time (an hour or so) ; it is sensitive to light, swimming towards light of moderate intensity, and retreating from it when too bright. As the zoospore becomes older it avoids the FIG. 67. Zoospores of (Edogonium ; A, escaping from the mother-cell ; B, free, with the fringe of cilia. Magnified 350. (After Pringsheim.) 152 STRUCTURAL BOTANY light more than before, and its movements are then directed towards the bottom of the water or solid objects contained in it ; at last it comes to rest, and in doing so attaches itself by its pointed end to some solid body. It loses its cilia, and now for the first time forms a cell- wall of its own. The free end grows out, divides by a transverse wall, and thus starts a new CEdoyonium filament, like that from which it was produced. This mode of reproduction by actively-moving spores, capable of immediate germination, is extremely common among the Algae. It is characteristic of most of the pure-green group, whether inhabitants of fresh water or of the sea, and extends also to certain other families. b. Sexual (Edogonium is propagated very freely by the simple method just described, but it also possesses a mode of sexual reproduction essentially similar to that of the higher Cryptogams, in so far as it consists in the fertilisa- tion of a relatively large and stationary ovum by a small and actively-moving spermatozoid. The distribution of the sexes varies much in the different species of the genus. Some are monoecious, others dioecious, while in a third set (the most numerous) a more complex arrange- ment prevails. In monoecious species, the male organs are formed by successive transverse divisions of one of the thallus-cells, the divisions all taking place near the upper end of the mother-cell, so that a row of rather flat cells is produced. These may divide again further, producing a chain of about a dozen cells in some cases, each of which is an antheridium. In every antheridium the contents divide into two, and each mass becomes a spermatozoid. The spermatozoids resemble the zoospores, THE A.LGM 153 and are ciliated like them (see Fig. 69, B). They are, however, much smaller, and relatively poorer in chlorophyll. The spermatozoid contains a single nucleus, which is placed near the end opposite to the cilia. These spermatozoids have much more the character of complete cells than those of the higher Cryptogams. In the latter, as we have already seen (p. 116), almost the whole body is of nuclear origin, only the cilia and that part of the body to which they are attached being protoplasmic. In (Edogonium, however (and in the lower Cryptogams generally), the greater part of the body is protoplasmic. The resemblance to the zoospores is a point of the greatest importance, as we shall learn later on (p. 165). The female organ, or oogonium, like the antheridium, consists of a single cell (see Figs. 66, 68, and 69), and differs herein from the complex archegonium of the Ferns and Mosses. The oogonium at its first formation is nearly similar to the other cells of the filament. A transverse wall is formed in the usual way ; the upper of the two daughter-cells is the richer in protoplasm, and has the larger nucleus ; this becomes the oogonium ; its lower sister- cell, which is poorer in contents and has a relatively small nucleus, is the supporting-cell, which in some species, however, may feed itself up, undergo further divisions, and give rise to another oogonium. The oogonium swells out, assuming a round or oval outline, and further increases the amount of its pro- toplasm, which thus encroaches considerably upon the central vacuole. The cell - contents meantime with- draw themselves from the wall, and form a free, rounded protoplasmic body- -the ovum (see Fig. 69, A) in the upper part of which the nucleus is placed. The 154 STRUCTURAL BOTANY -Ct'rt* oogonium now opens, either by the formation of a round hole in the membrane, or by the transverse splitting of the cell-wall near the top, in which case the upper part of the membrane acts as a lid (Figs. 66 and 69, A). The gap is at first closed by a new mem- brane formed from the adjacent protoplasm of the oogonium, but this soon disappears again, leaving a free passage to the ovum. Before describing the mode of fertilisation, we will consider the peculiar distribution of the sexes already mentioned, as differing from the ordinary monoecious and dioecious conditions. It is this form which our figures illustrate. The peculiarity consists in the production of dwarf male plants quite different from the ordinary form of the species. By repeated transverse divisions FiG.68. An<]rospore( sillgle cell; - 71 ' imcleus; coccus possesses other means of reproduction (zoospores and sexual motile cells) besides simple cell-division, and that it may grow out into filaments like those of higher green Algae. It thus appears that the life-history is in reality a rather complex one, and that the common unicellular condition is due to reduction from a more advanced type. R THE BEOWN ALG^ (PliMpliycece) The brown Alg< r e, almost all of which are seaweeds, are probably better known to the ordinary observer than even the green group, owing to the large size which many of them attain, and the extraordinary abundance in which they occur on our coasts. In dimensions and structure they present an even wider cli, chloroplastids. B, four cells separating after division. C, group of cells remaining in contact. The two to the left have just divided afresh. D, tetrahedral group. Magnified 540. (After Strasburger.) 184 STRUCTURAL BOTANY range than the Chlorophycese, for though no brown Algae are quite so small or so simple as Pkurococcus, yet many of them very much exceed any of the former division both in size and complexity. The peculiar colour of their thallus results from the fact that in addition to the chlorophyll which they all contain, another pigment of a brown colour is present, which more or less com- pletely disguises the green of the chlorophyll. Unlike the latter, the brown colouring matter is soluble in fresh water, so that we can easily extract it and make the chlorophyll visible. The Phseophycece certainly form a natural group, for from the lowest to the highest there are certain points in their organisation which are common to all. The colour in itself is not a character of much im- portance, but it coincides roughly with structural features, and affords a useful external mark by which the group can in most cases be recognised. This mark must, however, be used with caution, for there are some Algae which resemble the Phaeophycese in colour, but have otherwise nothing in common with them. The majority of the Phseophycese are reproduced by zoospores ; these are called the Phceo-zoosporece. A second order only forms sexually produced resting - spores ; this is the family Fucacece. We will take one example of each, for space will not allow us to do more, though really a large number of types would be necessary if our object were to gain any adequate idea of the diver- sity of the brown seaweeds. THE ALG.E 185 TYPE XIV. ECTOCAEPUS SILICULOSUS 1. STRUCTURE The genus Ectocarpus, various species of which are extremely common on our shores, includes some of the simplest forms of the Phaeophyceae. The thallus is filamentous, and repeatedly branched. It consists of two parts ; a creeping primary portion by which the plant is attached to the substratum (usually one of the larger seaweeds), and a tuft of branched threads, arising from the creeping part and waving freely in the water, often reaching a length of several inches. Throughout the plant the filaments usually remain one cell thick, though in a few cases longitudinal divisions occur. Each cell contains a single nucleus and a varying number of plastids, to the pigment contained in which the brown colour of the whole plant is due. The mode of growth of the free filaments is peculiar. Instead of having an apical growing-point, each branch grows chiefly near its base. In this part short meri- stematic cells are found, which multiply by transverse division, while the more apical part of the branch consists of long, full-grown cells, which have ceased to divide (see Fig. 78). These intercalary growing-points, as they are called, are characteristic of this group of plants, though not by any means constant among them. 2. EEPRODUCTION The reproductive organs are of two kinds, which are distinguished as unilocular and plurilocular sporangia. They are usually borne on distinct plants, but sometimes 186 STRUCTURAL BOTANY on the same individual. Both kinds of sporangia arise as lateral branches, either sessile or stalked. The unilocular sporangia are simply globular or pear- shaped cells, borne at the sides of the branches (Fig. 79). They become densely filled with protoplasm, and their contents divide up into a great number of small zoospores. Each zoospore contains a nucleus and a brown plastid .-sp L and bears two cilia, which are inserted at the side, not at one end of the cell. The sporan- gium dehisces at its apex, and the zoospores escape and swim off into the water. During their movements one cilium points forward in the direction of movement, and the other trails behind. In all Phae- ophycese the motile cells have two cilia each, and they are always inserted laterally, a point which distinguishes them from the corresponding bodies in the green Algae. These zoospores come to FIG. 78. General view of part of . , the thallus of Ectocarpus, show- rest alter a time, and grow ing several branches, sp s P> up i nto new plants. Hence plurilocular sporangia, borne r r laterally on the branches ; the unilocular sporangia are 00, intercalary growing-points, organg O f ase xual reproduc- where cell-division is going on. Magnified 56. (R. S.) tion. THE ALG^E 187 s p: The plurilocular sporangia, unlike those just described, are multicellular structures. In this case the sporangium is divided up by numerous cell-walls, usually longitudinal as well as transverse, into a multitude of small compart- ments (see Figs. 78 and 80). In each of these com- partments one or two zoospores are formed, which do not differ obviously from those arising in the unilocular organ. The zoospores escape from the plurilocular sporangium by a single opening at the end, the walls between the different compartments being absorbed, so that the swarming cells can pass out, one after another, through the same aperture. In a very few cases (one of which is illustrated in Fig. 80), the swarm- cells from plurilocular sporangia have been observed to conjugate. In Ectocarpus siliculosus there seems to be a certain functional difference between the sexes, though in appear- ance the conjugating cells are all alike. Certain of the swarming cells (planogametse) come to rest before the others, and withdraw their cilia. Such a cell, which we may regard as female, exercises a remark- able attraction on the others which are still swim- ming about. They swarm round it in great numbers (as shown in Fig. 80, B), and eventually one of the 8 warmers begins to fuse with the res ting-cell. The two are at first connected by means of one of the cilia of the active cell. This cilium gradually contracts, and the FIG. 79. Unilocular sporangia (sp) borne laterally on a filament of Ectocarpus ovatus. Magnified 300. (After Eeinke.) 188 STRUCTURAL BOTANY A FIG. 80. Ectocarpus siliculosus. A, part of a branch, bearing two plurilocular sporangia (sp\ from one of which the zocnpores (z) are escaping. Magnified 330. B, female cell ($) which has come to rest, with numerous male cells swarmin^ around it. C, two stages of conjugation? B and C magnified 790. (A after Thuret : B and C after Berthold.) protoplasmic bodies are thus brought to- gether, until finally the active and the resting cell com- pletely fuse into one. . We see that this process, by which a zygospore is pro- duced, is a step in advance of the con- jugation of Ulothrix, for in the Ectocarpus there is so far a dif- ference between the two cells, that at the moment of fusion one is at rest and the other active, though previously they had both behaved quite in the same way. We may look upon this as the first slight approach towards the differentiation of sta- tionary ovum and motile spermatozoid. In spite, however, of this distinction, both cells alike are capable of independent ger- mination as asexual THE ALG^E 189 spores, although when germinating alone they are said to produce weaker plants than those formed as the result of conjugation. Otherwise there is no difference between the products of sexual and asexual reproduction, for the zygospore, like the solitary zoospore, gives rise directly to a plant like the parent. 1 In some localities germina- tion without previous conjugation appears to be the rule. It is only in very few species that any form of sexual process has been observed in Phseophycese. In the majority of these Algre, such knowledge as we have goes to show that the motile cells, whether derived from unilocular or plurilocular sporangia, are simply zoospores capable of directly reproducing the plant. There is need for much further observation before we have anything like a satisfactory idea of the propagation and life-history of these plants. The sporangia and zoospores are very uniform throughout the Phreozoosporere, but in the vegetative structure there is the greatest variation. We have chosen one of the simplest examples. In other families of the group, as the oarweeds (Laminarice) and their allies, the thallus attains a vast size, and becomes extremely complex in anatomical structure. TYPE XV. PELVETIA CANALICULATA Among the commonest and most conspicuous seaweeds on the coasts of cold and temperate countries are the members of the order Fucacece. The species chosen for our type is distinguished from all others on our shores by the position in which it grows, which is always close 1 Recent observations have completely confirmed the occurrence of sexual reproduction in Edocarpus, on which some doubt had been cast. 190 STRUCTURAL BOTANY to high-water mark. The plants are thus only under water for a comparatively short time, not more than a quarter of the day, and are able to bear a temporary state of drought without injury. 1. STRUCTURE Pelvetia canaliculata is usually found in abundance on any rocky shore, forming a well-defined band, just below the highest level reached by ordinary tides. The plants are only a few inches high (smaller than most of their relatives), and have a forked, flattened thallus attached to the rocks by a rounded disc (see Fig. 81, A). The thallus shows a conspicuous groove or furrow along one side, to which the species owes its name. In addition to the regular dichotomous branches, adven- titious shoots may arise on any part of the thallus. When the plant is in fructification, which happens chiefly in the late summer and autumn, the ends of some of the branches become enlarged and studded with wart-like projections, each of which has a minute pore at the top (see Fig. 81, A, r). The swollen ends of the branches are called the receptacles ; the wart- like bodies mark the position of the cojiceptaclcs, which are cavities in the tissue, containing the organs of reproduction (see Figs. 81, B, and 82). Pelvctia, though one of the simplest of the Fucacese, is a very highly organised plant compared with the Algae already considered, and shows a rather complex anatomical structure, which we will now very briefly describe. The external layer of tissue consists of small cells with abundant plastids, giving their contents a dull THE ALG.E 191 brown colour. This superficial layer is no doubt the chief assimilating tissue. Within this we come to large parenchymatous cells, less deeply coloured, and as we approach the middle of the thallus the cells become elongated (cf. Fig. 82). In the lower part of the plant the elongation of the central cells is so extreme that they r c FIG. 81. Pelvctia canal iculala. A, small fertile plant; d, attaching disc ; r, r, receptacles, each of which bears a number of wart-like conceptacles. f of natural size. B, transverse section of a receptacle, passing through several conceptacles (c). Magnified about 4. (After Thuret and Bornet.) form a tangled web of branched filaments or hyphce. The elongated cells appear to serve the purpose of conducting food-substances, for they possess regular sieve-plates perforated like those of the vascular plants. Such sieve-plates occur both in the transverse and longitudinal walls of the long cells. The hyphse of the 192 STRUCTURAL BOTANY basal disc and lower parts of the thallus are gener- ally thick-walled. Their function is to strengthen the plant mechanically, and it will be found that the lower portion of the thallus, where these hyphos are most abundant, offers the greatest resistance to tearing. Although it is thus possible to distinguish several. systems of tissue in mature parts of the thallus, it must not be supposed that the different layers remain permanently distinct one from another. As a matter of fact the cells of one system constantly give rise to those of another. For example, the outermost assimi- lating cells divide tangentially, and the inner daughter- cells, thus cut off, contribute to the more internal cortical parenchyma, which appears to discharge the function of storing the assimilated food. Again, the cells of the inner cortex grow in length and may give rise to hyphse, thus adding to the bulk of the central tissue. The elements which correspond to sieve-tubes may subsequently undergo further elongation, thicken their walls, and assume the part of mechanical elements. In the older parts of the thallus the assimilating layer dies away, and is replaced by a secondary tissue answering the same purpose, produced by the repeated divisions of the underlying cortical cells. Thus we see that the various kinds of tissue, which appear so distinct when fully developed, can be derived the one from the other. Owing to the cell-formation in the superficial and other layers, and to the growth of new hyphae which insinuate themselves among the old, a constant increase in the thickness of the thallus goes on. True starch is not formed as the result of assimila- tion ; it is represented by another carbohydrate occurring in granules, but not capable of being stained by iodine. THE ALG.E 193 The growth in length of the thallus takes place with the aid of a definite apical cell, by the division of which all the tissues originate. Such a cell is situated at the apex of each branch, and lies at the bottom of a slit-like depression, which can be detected on examining the tip of the branch with a lens. When branching is to take place, the apical cell simply divides into two by a wall down the middle ; so here we have a true FIG. 82. Conceptacle of Pelvetia, in median section, o, one of the oogonia, each of which contains two ova. Surrounding tissues of thallus also shown. Magnified about 10. (After Thuret and Bornet. ) instance of dichotomy, the two branches being on exactly equal terms from their first origin. Adventitious shoots arise chiefly as the result of accidental injuries. The internal cells lying beneath the wound are stimulated to renewed growth and division, and give rise to a new thallus which may become an independent plant. Thus the Alga ensures itself against permanent loss in consequence of violence, for the damaged parts are replaced by fresh and vigorous shoots. The formation of these new growths, if they become 13 194 STRUCTURAL BOTANY separated from the old plant, affords a simple but effective means of vegetative propagation. The regular repro- duction, however, of Pelvetia, in common with all other Fucacese, is exclusively by the sexual method. 2. EEPEODUCTION As we have seen, each receptacle or enlarged end of a branch contains numerous conceptacles or cavities in which the reproductive organs are placed. When ripe, the conceptacle is an approximately spherical hollow, communicating with the exterior by a narrow pore (see Figs. 81, B, and 82). It arises as a depression on the surface of the thallus, the part which is to form the bottom of the hollow becoming arched over by the greater growth of the surrounding tissue. The formation of the cavity is, however, in some cases at any rate, helped by the breaking down of a central cell, so as to leave a gap in the tissue. In Pelvetia the conceptacle contains organs of three kinds : (1) sterile hairs or paraphyses arising all over the wall of the conceptacle, with their free ends con- verging towards the pore (see Fig. 82); (2) branched filaments, on the lower parts of which the antheridia are borne ; (3) the sessile obgonia, which are placed chiefly in the lower half of the conceptacle (see Fig. 82). This species is therefore hermaphrodite, for the organs of both the sexes occur in the same conceptacle. In most other Fucacese the plants are dioecious, all the conceptacles of each individual containing organs of the same sex. The antheridia arise as single cells, borne laterally in small numbers near the base of the branched filaments THE ALGJE 195 (see Fig. 83, a). At first the antheridium, like other cells in Fucacese, contains a single nucleus. This under- goes repeated division into two, until the total number of sixty-four nuclei is reached. Each of these nuclei now becomes the centre of a distinct cell, the contents of the antheridium dividing up simultaneously into as many protoplasmic bodies as nuclei are present. These bodies become sperm- atozoids, each of which consists of protoplasm, a nucleus, and a plastid; the latter, however, contains but little colouring matter. The spermatozoids are of oval shape, and bear two lateral cilia of unequal length (see Fig. 84, sp). The antheridial wall is double, and when the organ is ripe the outer membrane bursts at the top, and the whole contents, which may be already developed into sperm- atozoids, but are still enclosed within the delicate inner cell-wall, are expelled. The oogonia are single cells, of large size, seated on the tissue at the base of the conceptacle (Figs. 82 and 85) ; they contain a great many plastids, and turn a very dark colour as they become ripe. In each oogonium there is at first a single nucleus, which divides suc- cessively into two, four, and eight. The cell contents, FIG. 83. Antheridia of Pelvciia. p, p, hairs ; a, a, autheridia, some of which are already emptied. Magnified about 260. (After Thuret and Bornet.) 196 STRUCTURAL BOTANY however, divide into two cells only (see Fig. 85) by a transverse wall. Each of the two daughter-cells has one central nucleus ; the remaining six nuclei are expelled from the ova, together with a little protoplasm. The bodies seen close to the transverse septum hi Fig. 86, A, are some of these rejected nuclei. In Fucus itself all the FIG. 84. Spermatozoids of Pelvetia. A, unripe antheridium, already freed from outer membrane ; B, antlieridium open- ing to emit the biciliate spermatozoids (sp) ; c, c, empty mem- branes. Magnified about 450. (After Thuret and Bornet. ) eight nuclei are utilised, for the oogonium there divides into eight cells ; in another genus (Ascophyllum) four cells are formed and four nuclei rejected, while in the majority of the family no division of the cell contents takes place, and of the eight nuclei formed in the oogonium only one serves as the functional nucleus of the ovum. 1 1 The fact that a number of nuclei are always formed suggests that the obgouium of Fucaceoe was originally a structure of the nature of a sporangium. THE ALG.E 197 oo. In Pelvetia, then, the oogonium forms two ova which are surrounded by a thick and very gelatinous cell-wall showing three, distinct layers (see Fig. 86). When ripe, the outer layer of the oogonial wall gives way, and the two ova, surrounded by the thick mucilaginous inner layer of the cell-wall, are set free. The expulsion of the spermatozoids and ova from the conceptacles generally takes place in Pelvetia when the tide has gone down and left the plants exposed to the air, though it may also go on under water. The cavity of the conceptacle is full of mucilage secreted by the hairs which line it. The surround- ing tissue presses on the full conceptacle and forces out the mucilaginous contents through the pore ; mixed with this extruded mucilage are the spermatozoids and ova. If we hang up Pelvetia, or some other Fucaceous seaweed, freshly FlG - 85. Oi>gomum (oo) of , , Pelvetia, already divided to taken from the water, we can form t h e two ova. p. para- see the little slimy drops pluses. To the left of the nnormir appearing at the pores of the conceptacles; and these drops, about no. and Bornet.) if examined under the micro- scope, are found to contain spermatozoids or ova or both, according to the dioecious or hermaphrodite character of the species. In Pelvetia we should find both organs in the same drop. The spermatozoids are expelled while still enclosed in the inner antheridial membrane ; the obgonmm some antheridia are also shown. Magnified about 110. (After Thuret 198 STRUCTURAL BOTANY ova are in pairs, held together by the inner oogonial wall. There are many packets of spermatozoids and many pairs of ova sent out from each conceptacle. As the spray dashes up over the plants with the returning tide, their reproductive cells are washed down from the receptacles, sometimes onto the rocks, sometimes only onto the lower part of the plant itself, where they often come to rest in the groove of the thallus. It is a constant rule among the Fucaceae that fertilisation takes place outside the parent plant, after the sexual cells have been set free. The remaining antheridial membrane bursts after expulsion from the conceptacle, and the spermatozoids are at liberty to swim off by means of their cilia (Fig. 84, B). In this species, however, the ova remain enclosed within the soft mucilaginous membrane derived from the oogonial wall. In most Fucacese this is not the case ; the ova are set free as bare masses of protoplasm ; the peculiar state of things in Pelvetia probably has to do with the long exposure to the air ; the mucilaginous envelope protects the protoplasm within from danger of drought. The spermatozoids during their movements come across the pairs of ova, and swarm around them in large numbers. Some of them make their way into the mucilage, and penetrate to the protoplasm, which it seems is generally approached at the side where the two ova are in contact. It has been shown that ultimately only a single spermatozoid succeeds in entering the protoplasm, and making its way to the nucleus of the ovum. The details of the process have now been thoroughly worked out, and the fusion of the small male nucleus with the large nucleus of the ovum observed, as shown in Fig. 87, which represents the act of fertilisation in another member THE ALG.E 199 of the Fucacese. The proof of sexuality has also been afforded by experiment. If the ova are kept apart from the spermatozoids (as can be easily done in the case of the dioecious species) they soon perish, making perhaps some slight attempt at germination, which comes to nothing. B FIGS 86 A and B. A, oosporcs o, Pelvetia beginning to divide immediately after fertilisation. They are still enclosed m the gelatinous inner wall of the oogonmm. Che small bodies in the gelatinous mass are spermatozoids ; the larger bodies near the septum are the rejected nuclei of the oogonium. B, later stage. The obspores have divided to form many cells, and are sending out rhizoids. Magnified about 120. (After Thuret and Bornet.) If, however, the spermatozoids have access, the result is quite different. The ovum now surrounds itself with a cell-wall of its own, and after a few hours begins to divide. The direction of the first cell-wall formed across the fertilised ovum is said to be determined by light, and to be always at right angles to the incident rays. Other 200 STRUCTURAL BOTANY \ cell-walls follow, and soon the oospore (which here does not pass through a res ting-stage) is converted into a little mass of tissue, but without at first changing its external form. After eight or ten days, several root-hairs begin to grow out at the end away from the light (see Fig. 86, B). They burst through the oogonial wall, which has lasted all this time, and attach the embryo to the _^- -^ rock or whatever else it may be lying upon. The upper part of the embryo now elongates and becomes first 1 -S $ : ''- * \ cylindrical and then flattened at the free end ; a depression soon arises at its apex, in which a definite FIG. 87. Ovum of one of the Fucacea! (^sco- apical cell appears, phyllum nodosum), seen in section at the g^cl now W6 have moment of fertilisation. <$, small male . ,, . , nucleus of a spermatozoid, which has traversed m a ^ essentials a the protoplasm, and is now in contact with new Pelvetia plant the large nucleus of the ovum. The proto- . -, -, plasm of the ovum shows a distinct foam-like I&lTiy structure. Magnified about 650. (After ^@ world. Farmer.) In these Algae the result of fertilisation is a plant just like the parent. There is no kind of asexual reproduction, and therefore no possibility of any alternation of generations. We see, then, that in the order Fucaceae we have the simplest possible life - history combined with a very perfect form of sexual reproduction. The plants are altogether very highly organised, as shown THE ALG.E not only by the elaboration of their reproductive arrangements, but by their whole structure. Some members of the order bear perfectly distinct and well- formed leaves, and rival the flowering plants in the perfection of their external morphology. This is the case notably in the genus Sargassum, of which everybody has heard, from the fact that the plant forms prodigious floating masses, in the mid-Atlantic, giving its name to the well-known Sargasso Sea, which is many thousands of square miles in extent. In anatomical complexity also we have seen that even Pelvetia approaches the level of the vascular plants. It is well to realise at once that Algce may attain a very high organisation. On their own independent lines some of them have reached a degree of differentiation not much inferior to that of the higher land plants, with which, however, they have no direct relation- ship. C. THE RED ALGJE (Floridece) The great majority of the red group of Algae are seaweeds, though some genera are limited to fresh-water streams. The marine Florideae, though they do not reach the great dimensions of some of the brown seaweeds, are well known to every observer, owing to their beauty of colour and form, and are always especially favourite objects with collectors at the seaside. Most of them flourish rather low down on the shore, especially frequenting the sides of deep rock- pools, while many are only found growing beyond low-water mark. In these plants a red pigment, soluble in fresh water, accompanies and usually disguises the green colour of the plastids. The chlorophyll itself is similar to, but not absolutely identical with, that of the higher plants 202 STRUCTURAL BOTANY The shade of colour, produced by the combination of the two pigments, varies much in different species, and in different conditions of the same plant. Sometimes a bright rose colour is the result, sometimes a rich purple, sometimes a reddish brown, while in a few cases so little red colouring matter is developed that the green pigment becomes externally visible. All the plants which have any good claim to rank as Floridese agree closely in their minute organisation, methods of reproduction, and life-history; so that the whole group is a manifestly natural one, though in the degree of complexity of the thallus, and in the elaboration of the reproductive processes, there is a great range of variation. The Florideae stand almost completely isolated in the vegetable kingdom as at present known to us. They form a perfectly well-characterised group, which attains a remarkably high development on its own lines, especially as regards the process of sexual re- production. We shall only be able to describe one representative, and that one of the simpler members of the division. TYPE XVI. CALLITHAMNION CORYMBOSUM 1. STRUCTURE The form of the thallus among red Algae is subject to very great variations ; in some the thallus is finely, in others more coarsely filamentous ; in some, again, it is of stouter build and cylindrical form, while in others the whole plant assumes a flattened leaf-like shape, or consists of a short axis, bearing leaf-like appendages. The type which we have chosen is one of the simplest, THE ALQ^E 203 the whole plant consisting of a repeatedly branched filament, the main axis of which is comparatively thick, the successive branches becoming more and more slender, while the ultimate ramifications terminate at the tips in delicate hairs (see Figs. 88 and 89). The filament is at first only one cell in thickness throughout. In the lower part of the thallus, however, a peculiar kind of secondary cortex is formed, as the plant grows older ; the basal cell of a lateral branch gives rise to delicate filaments, which grow in a downward direction, attach themselves closely to the membrane of the main axis, and eventually form a complete coating over it. This mode of forming a cortex by means of adherent branches is by no means uncommon among filamentous Algse, both of the red and brown divisions. Each cell of the thallus contains, in addition to the colourless protoplasm, a number of plastids (the bearers of the combined red and green pigments) and, at least when young, a single nucleus. The cells communicate with each other by pits in their transverse walls ; the pit- membrane is covered on either side with a pad of callus like that in sieve-tubes (see Part I. p. 60). Fine strands of protoplasm extend through the callus and pit-membrane, thus connecting the contents of the adjacent cells. We see, then, that the protoplasm is continuous in these Algse, as well as in higher plants. The growth of the thallus goes on entirely at the apex of the various branches ; each branch terminates in an apical cell, which divides by transverse walls to form the successive segments composing the filament. When a fresh branch is to be formed, an oblique wall is produced in a segment which has just been cut off from the apical cell. By the oblique wall two unequal cells 204 STRUCTURAL BOTANY are separated, the smaller of which grows out and becomes the apical cell of the new branch. When a branch terminates in a long colourless hair, its growth is at an end ; these branches therefore are of limited length, and in this respect resemble leaves, while other branches retain their apical cell, and are thus capable of indefinite growth. Hence the thallus of a Callithamnion comes to have a regular conformation like that of many higher plants, depending on the relative position of its unlimited and limited branches. 2. KEPRODUCTION a. Asexual The reproduction of Callithamnion and of most Floridece is of two kinds, asexual and sexual. The asexual reproductive cells are called tctrasporcs, because they are almost always produced four together in one sporangium. In this case the tetrasporangia occur on very short lateral branches (see Fig. 88, A), the end cell of which swells up and becomes filled with exceptionally abundant protoplasm and plastids, assuming a very dense red colour. The contents then divide up into four spores, arranged in this particular plant in a tetrahedron (see Fig. 88, A). The membrane of the sporangium ruptures, and the tetraspores are set free ; when they escape they are without any cell-wall, each spore containing a single nucleus. These spores have no cilia, and usually appear to be quite incapable of any spontaneous movements ; no doubt they are disseminated by currents in the water. When a tetraspore comes to rest it forms a cell-wall and THE 205 germinates, sending out a root-hair at one end and dividing up at the other to form a filament, as shown in Fig. 88, B. This mode of propagation is almost FIG. SS.CallitTiamnion corymlosum. A, part of an asexual plant, bearing tetrasporangia ; the branches terminate in long colourless hairs ; t (on left), tetrasporangium containing the tetrahedrally arranged spores ; ts, empty sporangium, from which the tetraspores (t) are just discharged. B, germinat- ing tetraspores. C, part of a female plant, bearing two cystocarps (cp], both of which are the product of a single procarpium. Magnified about 80. (After Thuret and Bornet.) universal among red seaweeds, but the position of the tetrasporangia and the arrangement of the tetraspores in each vary greatly 206 STRUCTURAL BOTANY Most Floridese are dioecious, and as the tetrasporea also are produced on distinct plants, we usually have three forms of each species, asexual, male, and female. Sometimes, however, as has occasionally been observed in our type, all these organs occur on the same individual. b. Sexual If we examine a male plant we find that the antheridia occur in dense clusters on some of the thallus-cells usually just below the point where a branch, is given off (see Fig. 89). Each cluster is really a little system of densely crowded and very short branches, all springing from the same point. Each terminal cell of all these crowded branchlcts becomes an antheridium, and there are so many of these that they form a continuous mass, quite hiding the short branches on which they are borne. It is often easy to recognise the clusters of antheridia in red sea- weeds with the naked eye, for they have no pigment, and so appear as white patches on the red thallus. Every terminal cell of the cluster, then, is an antheridium. Its contents round themselves off, becoming free from the cell -wall, which splits open at the end, often detaching a little lid. Then the cell-contents, which have a single nucleus, escape through the opening; they have only a protoplasmic membrane at first, but no cell- wall. Thus each antheridium produces a single male cell, which in this case is called a spermatium. It has no cilia (in fact cilia are altogether unknown among red seaweeds), but is borne passively along by the movement of the sea. Often, after an antheridium has discharged its contents, the cell next below grows up into the empty THE ALG^E 207 cavity, and thus forms a new antheridium inside the membrane of the old one. The spermatia are excessively minute, not more than 200 millimetre in diameter. FIG. 89, Callithamnion corymlosum ; part of a male plant bearing the clusters of antheridia (a). Magnified about 150. sp, detached group of antheridia, surrounded by free sperm- atia. Magnified about 240. (After Thuret and Bornet. ) 208 STRUCTURAL BOTANY We see, then, that the production of the male cells is a fairly simple process ; the female structures, however, are much more complicated, and unlike any- thing we have met with, so far, in any of our types. The whole apparatus destined to form the fruit con- stitutes a special branch borne laterally on an ordinary filament of the thallus (see Fig. 90). The fertile branch, which is called the procarpium, usually consists in Callithamnion of five cells, of which three form a central group, while the other two are situated laterally, one on each side. The uppermost cell of the middle row (see Fig. 90, a) is prolonged into a slender hair of relatively great length ; this is the trichogyne (Fig. 90, t), or receptive organ; the lower part of the same cell is somewhat enlarged, and bears the name of carpogonium, because the development of the fruit starts from it. The two cells next the carpogonium remain small, and together constitute the trichoplwre ; the two lateral cells are known as the auxiliary cells, because, as we shall see, they contribute in a very important way to the formation of the fruit. The description of Fig. 90 should be carefully studied, to render the com- plicated arrangement intelligible. The carpogonium has a single nucleus ; the trichogyne which forms the upward prolongation of the same cell has, in this case, no nucleus of its own, but contains a strand of protoplasm continu- ous . with that of the carpogonium below ; the outer layers of its membrane are gelatinous. In this condition the procarpium is ready for fertilisation. The long trichogyne is specially adapted for receiving the spermatia. The young fruit is generally placed in a sheltered part of the thallus (as, for example, in this case, among the densely THE ALGJE 209 crowded branches of the bushy stem), where its future development is most secure. In such a position, however, it is not readily accessible to the male cells, and consequently we always find in connection with it the hair-like trichogyne, which projects far out towards the exterior, and thus reaches the exposed part of the thallus to which the spermatia are likely to be conveyed by the chance currents of the surrounding water. When a spermatium happens to reach the trichogyne it adheres to its gelatinous cell-wall, and is thus held fast (see Fig. 90, s). In the mean time the spermatium has formed a membrane round its protoplasm. At the point of contact between spermatium and trichogyne the cell - walls are absorbed, and so the contents of the male cell are enabled to enter the receptive organ. All the parts concerned are very minute, but the details of fertilisation have now been completely followed in certain instances. This was first accomplished in Nemalion, a genus rather simpler in its arrangements than our type. The nucleus of the spermatium travels down the trichogyne and fuses with that of Bornet.) 14 FIG. 90. Calliihamnion corymbosum ; part of a branch of a female plant, bearing a procarpiura. t, apex of the long trichogyne; s, a sperm- atium adhering to it ; a, carpogonium, at base of trichogyne just below this is one cell of the trichophore ; c, the other trichophore cell. The two cells showing to the extreme right and left of the trichophore are the auxiliary cells, which fuse with the carpogon- ium. Magnified 250. (After Thuret and 210 STRUCTUEAL BOTANY the carpogonium. So far, therefore, as the act of fertili- sation is concerned, the Florideae do not differ from other sexual organisms. The result of fertilisation, however, is not the formation of a single ob'spore, but the development of a whole fruit. The fertilised carpogonium is cut off from the trichogyne by a plug of cell-wall, and then sends out short branches, which come into close contact with the auxiliary cells on either side ; there is an actual union of the cell contents of the carpogonium with those of the two auxiliary cells, but the nuclei do not fuse. Each auxiliary cell now divides by a transverse wall (see Fig. 91, c) ; the upper of the two cells in each case becomes a placenta which corymbosum ; part of a gives rise to the spores. Calli- show^^thT^Uest Mamnion is different from most of stage of development of the simpler red seaweeds in so far fertil^S^TheySe as it regularly forms two fruits by the growth and from each procarpium, whereas its division of the two ... ,, , auxiliary cells. Magni- near allies usually torm one only. fied 250. (After Thuret Each placenta buds out into a number of cells, which themselves divide repeatedly, so that eventually two large groups of cells arise, one on each side of the filament (see Fig. 88, c). The groups are really built up of a system of very short and densely crowded unicellular branches, those of each cluster all springing ultimately from the placenta belonging to it. The whole fruit is enclosed in a gelatinous cell - wall, but no cell - walls are formed THE ALG.E 211 between the individual cells, each of which, when all the divisions are complete, becomes a spore, called for distinction a carpospore, as it forms part of the sexually produced fruit, or cystocarp. When ripe, the membrane of the fruit bursts and the spores are set free. They are large, uninucleate, deeply pigmented cells, destitute at the time of their escape of any cell-wall. The main points in the development are : (1) fertilisa- tion by means of a special receptive organ, or trichogyne ; (2) union of the protoplasm of the fertilised cell with that of neighbouring cells ; (3) the production, as the result of this union, of a complex fruit, including a great number of spores. Callitliamnion occupies a middle position among the Floridese as regards the complexity of its spore-forma- tion. The trichogyne is common to all Floridese, but some few members of the order (e.g. Nemalion and the fresh - water genus Batrachospermum) are simpler, in that the carpogonium directly gives rise to the spores, without any preliminary cell - fusion. Many red seaweeds, however, are more complicated, repeated cell-fusion taking place, with the result, in some cases, that a number of fruits may be formed in consequence of a single act of fertilisation ; these fruits often arise at a considerable distance from the directly fertilised cell. In some respects the process of sexual reproduction in the Floridese is more complex than in any other plants. It offers the advantage that a single spermatium, if it once reaches a trichogyne, may ensure the production of a very large number of spores. In many cases a further complication is due to the fact that a multicellular envelope grows up around the spores. To the frequent 212 STRUCTURAL BOTANY presence of such an envelope, the sexually produced fruit owes its special name of cystocarp. ' In so far as the result of fertilisation is the production, not of an obspore, but of an entire fruit, there is a certain analogy between Floridese and Bryophyta ; but in the former the fruit is always in organic connection with the sexual plant, and can therefore scarcely be regarded as a new generation ; whereas in the Mosses the sporo- gonium always remains distinct from the oophyte, though dependent upon it. It is not probable that the Florideae have any direct relation with the Mosses, or with any of the higher plants. Somewhat doubtful affinities with certain green and brown Alga3 have been suggested, but, as at present known to us, the Eed Seaweeds constitute a very isolated and well-defined group. The carpospores germinate in precisely the same way as the asexual tetraspores, but it is only very rarely that the development of the plant has been traced far. Eed seaweeds are difficult to cultivate successfully, and our knowledge of their life-history is still extremely limited ; so far as we know, however, there is nothing to show that any regular alternation of sexual and asexual individuals prevails among them. D. THE CYANOPHYCE.E TYPE XVII. NOSTOC There remains to be considered a group of rather obscure plants of simple structure, which resemble the AlgaB in their habit and mode of life, and are therefore best described in this place, though their real relationships THE 213 are open to doubt. Of the Cyanophyceae some are terrestrial, some aquatic, occurring both in fresh water and in the sea. The representative of the group which we have chosen a species of Nostoc (see Fig. 92) is filamentous. The threads are associated in colonies held together by the soft gelatinous outer walls of the cells. Such colonies often form conspicuous masses of bluish- green jelly on the damp ground, especially in wet weather. Within the mass the filaments wind about in every direction. The cells of which they are made up are rounded, so as to give the whole thread a beaded appearance. The ordinary vegetative cell has a thin inner cell-wall, which is alone visible in Fig. 92, A, the confluent gelatinous layers scarcely showing under the microscope owing to their transparency. The interior of each cell is full of protoplasm, which appears to be coloured throughout its whole mass. No definite plastids have been found to exist in Cyano- phycese, nor has the presence of a nucleus been finally determined, though in certain cases a colourless central body certainly exists, which resembles a nucleus in some of its characters and reactions. In the protoplasm are numerous granules. Many of these plants float on the surface of water, where they sometimes appear rather suddenly in vast quantities in ponds and lakes, covering many acres with a bluish-green scum. It is said that in A FIG. 92. Nostoc Linckii. A, part of a filament ; h, h, heterocysts ; sp, sp, spores. B, isolated spore beginning to germinate. C, young filament formed from a spore, the burst cell- vail of which is shown at the ends. Magnified 470. (After Born et.) these floating 214 STRUCTURAL BOTANY species gas-vacuoles are present in the cells, that is to say, little cavities in the protoplasm containing a gas, the nature of which has not been determined. These gas-vacuoles appear to be important, as they make the plant lighter and enable it to float. The apparent simplicity of the histological structure, due to the want of well-defined nuclei and plastids, is the chief reason why the Cyanophycese are often separated from the Algse. Further observations may, however, at any time abolish these distinctions. The colouring matter appears to be a compound substance consisting of blue-green, yellow, and pure green con- stituents. The tint varies greatly in different forms, but we never find the pure green of chlorophyll. In Nostoc the filament is interrupted at intervals by larger cells with thicker walls ; they may serve for the storage of food-material, becoming emptied as the filament grows. These cells are called the heterocysts (Fig. 92, A, h), and are characteristic of Nostoc and its nearer relations. Sometimes the filaments break across at the heterocysts, and the short rows of living cells between them become isolated. These detached filaments (called the hormo- gonia) are capable of creeping movements, though how they move is quite unknown. They escape from the gelatinous mass, and start new colonies for themselves. This is one mode of propagation. Another is by means of resting-spores, formed directly from some of the vegetative cells, which grow larger than the rest, accumulate more abundant protoplasm, and surround themselves with a thick cell- wall (Fig. 92, A, sp). The spores (Fig. 92, B) can pass through a resting-stage, and endure drought ; when water is supplied they germinate, forming new filaments (Fig. 92, C). THE ALG^E 215 Such is the simple history of Nostoc. Neither in this genus nor in any of the blue-green Algae has any kind of sexual reproduction been observed. The plants of this class must rank, according to our present know- ledge, as among the lowest members of the vegetable kingdom, the only others which are equally simple being the Bacteria (to be subsequently described), some of which appear to be closely allied to the Cyanophyceae. CHAPTER IV THE FUNGI THE Fungi are an immense group by far the largest of all the cryptogamic Classes. Up to the present time about 40,000 species have been described. The whole of this vast mass of most heterogeneous forms is distinguished by one physiological character the absence of chlorophyll. Hence all Fungi alike are incapable of assimilating their carbonaceous food from the carbon-dioxide of the atmosphere ; they must obtain it ready made, as it were, from other sources. So far as carbon-compounds are concerned, Fungi are entirely dependent on organic food. This they obtain either directly from other living creatures, on which they prey, or from dead organic substances produced by living organisms. In the former case we call them parasites, in the latter saprophytes. Parasitic and saprophytic plants wholly or nearly destitute of chlorophyll occur in other classes of the vegetable kingdom, as members of very diverse families. Thus among flowering plants, for example, we have the Dodder (Cuscuta) and the Broomrape (Orolanclie) as parasites; the Bird's Nest (Mbnotropa) and the Bird's Nest Orchid (Neottia nidus-avis) as saprophytes. In all such cases, however, the parasitic or saprophytic forms 216 THE FUNGI 2fl7 are near relations of normal chlorophyll - containing plants, and we attach no great systematic importance to the change in their mode of life. Among Fungi, on the other hand, there seems to be a real bond of relationship throughout the entire class (if we leave a few doubtful cases out of consideration) so here it is probable that the common physiological character coincides with a common origin. We must not, however, suppose that all Thallophyta, which lead the life of parasites or saprophytes, are necessarily Fungi. At the close of this book we shall have to consider two such Families which cannot be classed under this head. Many Fungi are of the greatest practical interest, though chiefly in a disagreeable way. Very many of them are destructive parasites, causing the worst diseases of our field and garden crops and of forest trees. We may mention the rust, smut, and bunt of wheat, the potato disease, the sugar-cane disease, the larch disease, to which innumerable others might be added. Others do harm by injuring timber in buildings, such as the dry-rot fungus, or by destroying articles of food, which are constantly attacked by " mould." Hence, Fungi have been more studied from a practical point of view than any other Cryptogams, and a vast mass of knowledge has now been accumulated as to their physiology and mode of life. Our own point of view is chiefly a morphological one, and we have chosen the few types which we have space to describe, in order to illustrate some of the most striking facts in the com- parative structure and life-history of certain of the more important Families. It must not be supposed that Fungi are altogether to be regarded as injurious to the higher creatures. Not to 218 STRUCTURAL BOTANY mention, what everybody knows, that several of the larger kinds are exceedingly good for food, we may point out that the saprophytes, at any rate, do good service by causing decay, and so ridding the world of the useless remains of dead animals and plants. Masses of dead material would otherwise accumulate to such an extent as to interfere seriously with the life of succeeding generations. Fungi and other saprophytes (notably the Bacteria) bring about the decomposition of dead organic matter, use the products for their own nutrition, and ultimately convert its substance into simple in- organic bodies (such as ammonia and carbon-dioxide), thus rendering it available for the nutrition of green plants, and, through them, for the support of other organisms. We will begin our illustrations of the Fungi with a simple type, representing a .Family which is of special scientific interest, from its evident affinity with certain of the Algae. TYPE XVIII. PYTHIUM BARYANUM We have chosen as our first type of Fungi a genus which stands very near to the Algae, showing an un- mistakable affinity with Vaucheria. The species of Pythium are parasitic on seedlings, and often do great havoc among them, especially if the seed-beds are kept too damp, and not sufficiently exposed to air and light. One of the commonest species, P. Baryanum, can be obtained almost with certainty by growing Cress-seed- lings under a bell-glass, and giving them an excessive amount of water ; but, unfortunately, the parasite THE FUNGI 219 appears often enough when it is not wanted. The disease caused by it is well known to gardeners as the " damping off " of seedlings. The stem of the seedling when attacked by the Fungus soon tumbles over on to the ground. This is because the outer tissues of the o stem, at the part where it gives way, have been so much damaged by the parasite that the stem has not the strength to stand upright. The fallen plants lose their colour and soon completely rot away. 1. STRUCTURE In its vegetative condition, Pythium consists of long, fine, irregularly branched filaments or liyplice ; the latter name is given to the filaments of Fungi in general. These hyphee are for a long time without any trans- verse walls ; they are in fact non-cellular, just like the filaments of a Vaucluria. The inside of the hypha is occupied by vacuolated protoplasm, in which very numerous nuclei are embedded. Unlike Vaucheria, however, Pythium has no chlorophyll and no plastids. Neither is starch formed, either in this genus or in any other Fungus. It is usual to speak of the whole vegetative body or thallus of a Fungus as the mycelium. In Pythium, then, the mycelium of each plant is made up of all the hyphae collectively, which have sprung from a single spore. The mycelium of Pythium penetrates the tissues of its victim or " host," and spends most of its vegetative life within them. A hypha can make its way into the stem either by way of a stoma or by boring directly through the cuticle ; it goes on growing and branching inside the host-plant, where it is not confined to the 220 STRUCTURAL BOTANY intercellular spaces, but can enter the cells themselves. Thus the whole plant comes to be infected, and is traversed throughout by the branched mycelium of the parasite, which lives at its expense. In the First Part of this book (p. 216) we learnt that there are chemical bodies in plants called ferments, (or enzymes), which have the power of changing the con- stitution of other organic substances, converting solids into liquids, and indigestible substances into such as are avail- able for nutrition. The example we specially mentioned was diastase, which converts starch into grape-sugar, but numerous other ferments also occur in plants. Now parasitic Fungi have the power of secreting ferments, which play a very important part in bringing their victims into subjection. The advancing tip of a mycelial hypha secretes a ferment which dissolves the cell-wall lying in its way, and so enables the Fungus to enter living cells, while other bodies of the same class bring the proteids and other organic substances of the host into a condition in which the parasite can assimilate them. In this way, then, the Pythium makes itself thoroughly at home in the body of its victim, infests it in every part, and eventually completely destroys its tissues, converting their materials to its own use. Often the hyphse leave the host, and grow out upon the soil until they reach other victims, which they then infect. In the mean time the Fungus does not neglect to make provision for future generations. The reproduction takes place in two ways, asexual and sexual. We will first describe the former. THE FUNGI 221 2. KEPRODUCTION a. Asexual The hyphse which are to produce the asexual organs of reproduction grow out from the host into the open air. They there form a number of spherical sporangia which are terminal, being seated on the ends of short branches or of the main hyphoe (see Fig. 93, A). The sporangia are beaked at the apex, and, when ripe, the entire protoplasm passes out into the beak, which swells up into a bladder-like sac (see Fig. 93, B). The whole process can only go on when there is water enough to immerse the sporangia. The contents of the sac now divide up into a number of membraneless cells which become zoospores, each bearing two cilia. The zoospores escape and swim away through the water. After some time they come to rest and germinate, producing a hypha, which finds its way into a fresh seedling as soon as opportunity offers. This mode of reproduction, we see, is altogether that of an Alga. Pythium, though a Fungus, is not thoroughly adapted for growth on dry land, for its normal reproduc- tive processes can only go on under water. This is one reason why seedlings attacked by Pythium are said to damp off, for it is when they are kept too damp that their enemy is best able to attack them ; the moisture enables the Pythium to spread. This method of propa- gation by zoospores allows of an enormously rapid multiplication under favourable conditions ; its success, however, is entirely dependent on the presence of water. It is true that only a little water is necessary, but still Pythium is entirely powerless to propagate its 222 STRUCTURAL BOTANY kind in this manner, under such conditions as prevail in nature when the weather is at all dry. The great majority of the Fungi, however, are adapted to the same conditions of life as the ordinary land plants, on which so many of them are parasitic, and this implies that their reproduc- tive bodies are fitted for dissemination through com- paratively dry air. In Pythium and among its near allies we can trace the steps by which this adaptation to an aerial environment has been attained. In some species of Pytliium, as, for example, in the species P. P>aryanum, which is so common on Cress-seedlings, it sometimes happens that FIG. w.pytuum. A, branch the sporangium does not of the mycelium, bearing three zoosporangia (s). Magnified torni ZOOSpores at all, but 145 B, zoosporangium (s) grows out directly into a discharging its contents (6), J which are still enclosed in the hypha, thus Starting a new saffflSas-t. to. 1 *: p lant at . nce > without the zoospores. Magnified 145. c, intervention of the active germinating oospore (osp) form- rp l] q Lb> ing an asexual sporangium (s). Magnified 300. (After De this allows of propagation Bary.) , . , , taking place, even though there should not be water enough to float the zoospores. The same thing happens in the closely-allied genus THE FUNGI 223 Phytophthora, to which the Fungus causing the Potato disease (P. infestans) belongs. Here, and also in a third genus, Peronospora, the sporangia always become detached from the hypha which bears them, before germination (whether by the development of zoospores, or by the direct D FIG. 94. Germination of the sporangia in various species of Peronospora. A, P. nivea ; a, b, c, stages in the formation of zoospores ; d, free biciliate zoospore ; e, zoospores germinating. B, P. dcnsa ; b, commencement of germina- tion ; c, expulsion of undivided contents ; d, empty sporangium ; e, first formation of a hypha from the contents. C, P. Lactucce ; direct germination through the apical papilla. D, P. Radii ; direct germination, hypha formed laterally. Magnified 400. (After De Bary.) formation of a hypha) takes place. The illustrations in Fig. 94 are taken from different species of Peronospora. In P. nivea (parasitic on Umbellifera) the contents of the sporangium divide up at once into a number of biciliate zoospores, which escape by the opening of the terminal papilla (Fig. 94, A). In P. densa (Fig. 94, B), 224 STRUCTURAL BOTANY growing on Scrophulariacese, the protoplasm of the sporangium is expelled entire through the apical opening without dividing into zoospores ; it surrounds itself with a new cell-wall, and germinates directly, to form the mycelium of the next generation. In P. Lactucce (Fig. 94, C) (which infests Lettuces and their allies) a further step is taken ; the contents do not escape at all, but the sporangium simply puts out a hypha which arises at the apex. Lastly, in P. Radii (occurring on flower-heads of Composite), with which the majority of species of Peronospora agree, the apical opening has ceased to be functional, and the hypha grows out laterally. In these latter instances the sporangium has, in fact, become a spore. Such asexual spores of Fungi, germinating directly, bear the name of conidia. We will now return to our type Pytliium. Zoospores and conidia afford a rapid means of propagation so long as a plentiful supply of victims, in the shape of young seedlings, is forthcoming. Pythium is not, however, altogether limited to a parasitic mode of life, for if host- plants are wanting, it can live for some time as a saprophyte on any decaying organic matter which may happen to be at its disposal. Provision, however, has to be made for bad times when food fails altogether, or when there is not enough moisture for active vegetation to go on. Such contingencies are provided against by the formation of resting -spores, which are the result of a sexual process. b. Sexual The organs of sexual reproduction (oogonia and antheridia) may be produced either inside the tissues of the host-plant, or on hypha3 which have grown out THE FUNGI 225 ,-0 a into the air. An oogonium arises as a spherical swelling on a hypha, and may be either terminal, as shown in our Fig. 95, or intercalary, i.e. produced at some in- termediate point in the course of the filament. The young oogonium is cut off from the rest of the hypha by a transverse cell-wall. Its protoplasm now separates into two parts, a central granular portion which becomes the ovum, and a peripheral layer, lining the cell-wall, called the periplasm. The behaviour of the nuclei has now been made out in several Fungi of this group ; in Pythium, the oogonium at first contains a large number of nuclei, nearly all of which pass out into the periplasm, leaving behind, in the central mass, a single nucleus, which is the FIG. 95. Fertilisation of functional female nucleus, and is alone concerned in the act of fertilisation and the subsequent development. In the mean time the anther- idiuin is formed. It is usually a lateral, club-shaped branch, arising either from the same filament which bears the oogonium (see Fig. 95) or from a different one, and separated from the hypha on which it is borne by a transverse wall. The antheridium directs its growth towards the neigh- bouring oogonium, to which it closely applies itself. It may be mentioned here that the mycelium of Pythium and its allies, which is non-cellular during its vegetative growth, generally becomes irregularly parti- tioned up, by a few scattered transverse walls, as the 16 Fythium. A, early stage ; oiigouium (o) and anther- idium (a) still immature. Z>, moment of fertilisation. The contents of the antheridium (a) are pass- ing through the fertilising tube, to unite with the ovum (o). Magnified 800. ( After DeBary.) 226 STRUCTURAL BOTANY period of reproduction approaches. Previous to this the bulk of the protoplasm has travelled into the more terminal portions of the mycelium, where the repro- ductive cells are to be produced ; the transverse walls may serve the purpose of keeping it where it is wanted. The protoplasmic contents of the antheridium, like those of the oogonium, undergo a severance into a central fertile portion and an external layer of periplasrn and here also it is the former alone which plays an active part in the reproductive process. There is no division into spermatozoids, and in fact these bodies are extremely rare among Fungi, another point in which the adaptation to a terrestial habit of life has involved the disappearance of active reproductive cells. The antheridium sends out a short branch, the fertilising tube, which penetrates the wall of the oogonium, and reaches the ovum (Fig. 95). The fertilising tube opens at its end, and now the whole contents of the antheridium (with the exception of the periplasm) pass through the tube, and unite with the protoplasm of the ovum (Fig. 95, E). The whole process can be directly followed with ease, under a high power of the microscope, and, indeed, Pythium is one of the most favourable plants for the immediate observation of the fertilising act. It appears to be now established that only a single nucleus passes over with the male protoplasm, and unites with that of the ovum. As the result of fertilisation, the ovum surrounds itself with a thick cell-wall, the outer layer of which is derived from the periplasm by which it is surrounded. The ovum has now become an oospore ; its contents form a quantity of oil, as a reserve of carbonaceous food, and it next passes into a period of rest. THE FUNGI 227 The germination of the oospore takes place after a long interval, and only when it is brought into contact with water. The process shows very remarkable variations, both among different species and among individuals of the same species, comparable to the variations in the behaviour of the asexual sporangium described above. In some cases the outer thick layers of the oospore membrane are burst, and the contents, surrounded by a delicate cell-wall, grow out into a hypha, thus starting a new mycelium directly. In other cases the process begins in the same way, but the hypha at once forms a sporangium, into which the whole contents pass, dividing up into a number of zoospores (see Fig. 93, C). In a third mode of germination, the formation of the hypha is suppressed, and the zoospores are produced in the interior of the oospore itself. The zoospores swim about like those formed on the vegetative plant, and on coming to rest reproduce the ordinary form of the Fungus. These are differences to which considerable systematic importance would be attached in other groups of plants ; here the different modes of germination are not even constant for the species, but appear to depend on the nutrition, direct germination taking place when food is abundant, while under less favourable conditions zoo- spores are formed at once, thus allowing additional chances of a suitable habitat being reached. We may say, then, that in a Fungus such as Pythium, the ex- ternal circumstances determine whether anything like an alternation of generations takes place or not. Pythium and its allies stand nearer to the Algae than any other Fungi ; in fact they were once themselves placed in the former class, and it is evidently with non-cellular Algae of the type of Vaucheria that they have the closest. 228 STRUCTURAL BOTANY relations. In histological structure the two genera are almost identical, if we leave out of account the chloro- phyll-bodies, which Pythium has given up in adopting a parasitic or saprophytic mode of life. In the reproductive processes there are various deviations from the algal type, the most important being the suppression of the sperrnatozoids, and the gradual replacement (only just beginning in Pythium, but more marked among its allies) of zoosporangia by directly germinating conidia. As we advance towards the higher Fungi we shall find the algal characters dropping more and more out of sight. TYPE XIX. PlLOBOLUS CRYSTALLINUS Among the Algae we found in some of our types (as, for example, in (Edogonium and Vaucheria) a well-marked process of fertilisation, in which the cells taking part in the formation of the oospore showed a distinct difference of sex. In others, however, namely in Ulothrix and Spirogyra, union was found to take place between two essentially similar cells, each having an equal share in the act of conjugation, and the resulting formation of a zygospore. Both these modes of sexual reproduction are represented also among the Fungi. Our last type, Pythium, afforded an example of fertilisation ; the group to which it belongs bears the name of the Oomycetes. We are now about to describe a form in which the sexual act is one of conjugation ; the Fungi in which this process prevails are called the Zygomycetes. A large proportion, though by no means all, of the Oomycetes are parasites on living plants or animals. Among the Zygomycetes parasitism, though it occurs, is THE FUNGI 229 less usual ; the majority are of saprophytic habit, and to this group many of the commonest " moulds " belong. We have chosen as our type a little Fungus which is often found growing on old manure-heaps. In spite of its disagreeable habitat, Pilololus, and especially the species P. crystallinus, is a decidedly pretty object. The part of the plant which alone rises above the surface of the substratum and so meets the eye, consists of the stalks bearing the asexual sporangia. Each of these stalks is about a quarter of an inch high, and swells up near the top into a neat little crystalline globule, surmounted by a kind of black cap, which is the sporangium itself (see Fig. 96, A). These facts will enable us to recognise the plant. We will now consider its structure more in detail. 1. STRUCTURE The mycelium or vegetative thallus of Pilobolus and its allies is made up of repeatedly branched hyphae, which spread in all directions through the substratum. Histologically these hyphae have the same structure as in the Oomycetes ; they are without transverse walls, at least in the vegetative condition, and their protoplasm contains very numerous small nuclei, the whole plant being a non-cellular but multinucleate organism. Thus this order betrays unmistakable affinity to Algse of the Vauclicria type, and therefore Oomycetes and Zygomy- cetes are grouped together in one class, under the name of Phycomycetes or algal Fungi. The group to which Pilololus belongs has, however, departed much further from the algal stock than the Oomycetes have, for the plants are thoroughly adapted to a terrestrial mode of life, and the power of forming zoospores is altogether lost. 230 STRUCTURAL BOTANY 2. EEPRODUCTION a. Asexual The most abundant means of propagation is by asexual spores formed in sporangia. For the purpose of spore-formation certain of the hyphre grow out into the air, and assume a vertical position. The apex of the aerial hypha enlarges to form a nearly spherical sac, into which most of the protoplasm travels from below. This sac is cut off from the stalk by a transverse cell-wall, and becomes the sporangium (see Fig. 96). Its contents divide up simultaneously into a large number of round cells, each of which surrounds itself with a cell-wall and becomes a spore (see Fig. 96, E). Wo see, then, that in this family the sporangium, instead of forming zoospores, as in the last group, gives rise to motionless spores with cell-walls. This is the typical method of asexual spore-formation in these moulds. We will now consider the special adaptations which are characteristic of our type Pilobolus. The upper end of the stalk just below the sporangium swells up into a bladder much larger than the sporangium itself, and a second bladder is often formed in the lower part of the hypha (Fig. 96, B). The lower of these two bladders is separated by a transverse wall from the rest of the mycelium ; the upper is bounded above by the wall which marks off the sporangium. The whole stalk between the two transverse walls constitutes a water- reservoir, in which a high hydrostatic pressure is set up, so that drops of liquid often exude through the membrane (Fig. 96, A, st). Owing to this pressure, the wall bounding the sporangium becomes bulged into its THE FUNGI 231 cavity, forming what is here called the columella (Fig. 96, E, c). The wall of the sporangium itself is not K J H G Fin. 96. Pilololus. A, group of sporangiophores ; st, stalk; r, reservoir ; s, sporangium. Magnified 5. B, hypha and sporangiophore ; r, reservoir ; s, b, sporangium. Magnified 15. C, sporangium in surface-view. Magnified 15. D, sporangium (s) thrown off by bursting of reservoir (r) ; c, columella. Magnified 15. E, reservoir (r) and sporangium (s) seen in section ; b, thin part of wall ; m, mucilaginous layer ; c, columella. F, the same, when the mucilage swells. Magnified about 25. G and H, mycelial branches pre- paring for conjugation. Magnified 120. J, later stage; c, the conjugating cells. Magnified 120. K, after con- jugation : z, the zygospore. Magnified 120. (After Zopf and Van Tieghem.) uniform throughout ; the upper part is thickened and cuticularisedj and bristles with crystals of calcium 232 STRUCTURAL BOTANY oxalate, while the lower part adjoining the columella remains thin. Inside the lower part of the sporangium is a mucilaginous layer derived from its protoplasm ; when the sporangium is wetted, this mucilage takes up water and swells, bursting the delicate cell-wall, and so freeing the upper portion of the sporangium which contains the spores (Fig. 96, F). The pressure on the upper surface of the columella is thus removed, and no longer balances that of the liquid in the reservoir below ; consequently the columella bursts, and a jet of water escapes from the reservoir, driving the sporangium and spores violently before it (Fig. 96, D). The sporangium may thus be hurled to a great distance, amounting it is said to more than a yard in some cases. Hence the name of the plant, which means " a thrower of missiles." The sporangium sticks to any object which it happens to hit, owing to the mucilage which still clings to it. When the spores from the sporangium germinate they reproduce the ordinary form of the Pilobolus plant. Other modes of propagation have occasionally been observed. The normal sporangia are only formed in air ; if, however, the mycelium is forced to grow in a liquid containing plenty of organic food material, another process takes place. The hyphae divide up by trans- verse walls into numerous cells, which may increase in number by budding, each cell putting out a short branch, which becomes separated from the parent. This is called the Oidium condition. In other cases, namely when food is less abundant, some of the cells produced by transverse division of the mycelium may acquire thick walls, and pass into a resting condition. These thick-walled cells are called chlamydospores, and, like the oidiospores, may THE FUNGI 233 either germinate into a new mycelium or give rise at once to sporangia. b. Sexual More important for us is the sexual reproduction, which in Pilobolus and many of its relatives takes place but rarely, though in some other members of the group it is the most frequent means of propagation. Sexual organs are only formed in Pilobolus when some cause hinders the development of the sporangia. It has been found that conjugation can be induced by infecting the aerial hyphae with a parasitic Fungus (which happens to be a relative, for several members of the group prey upon their own family). The parasite hinders the formation of the asexual spores, and the plant is thus led to adopt the other method of propagation, which results in the formation of resting-spores capable of waiting until the bad times are over. When conjugation is about to take place, two neigh- bouring hyphse of the mycelium enlarge, and become club-shaped (Fig. 96, G and H). The swollen portions grow upright, and lay themselves together side by side, accumulating at the same time a large quantity of proto- plasm in their interior. The ends of the conjugating hyphae are next cut off by cell-walls (Fig. 96, J). The terminal parts thus separated, which are the richest in protoplasm, come into close contact, and the cell- walls separating them are absorbed. The protoplasm of the cells now runs together into one mass, and the two cells completely fuse into a single zygospore, which rests upon the two enlarged hyphse, called the suspensors (Fig. 96, K, z). The zygospore grows to a relatively great size, surrounds itself with a very tough and thick cell- 234 STRUCTURAL BOTANY wall, and forms a quantity of oil in its contents. We see that the process is one of perfectly typical conjuga- tion, the two cells concerned taking an exactly equal part in the production of the zygospore. It has recently been shown that in many of the Mucorineoe (the family to which Pilobolus belongs) there is a physiological differ- entiation of sex, definite sexual " strains " existing, which will only conjugate with one another, while hyphse of the same strain are sterile among themselves. After a period of rest, the zygospore, if moistened, germinates. The germination is best known in an allied genus, Mucor. If food enough is to be had, it simply grows out at once into a new mycelium ; if, however, supplies are scanty, it proceeds without delay to form an asexual sporangium, thus increasing the chances of survival. These differences are quite analogous to those which we found in the germination of the oospores of Pythium. The Zygomycetes, so far as their sexual reproduction is concerned, stand on a lower level than the last group. On the other hand, they are more fully adapted to a terres- trial mode of life, and so far are more perfect, as Fungi, than the Oomycetes. We saw in the case of Peronospora how a transition can be traced from the sporangium, to a single conidium, germinating directly. A somewhat similar gradation is to be followed among the immediate relations of Pilobolus. Some of these produce, in addition to the typical large sporangia, very small sporangioles, containing very few spores, or even only one. In other species sporangioles only are known, and these become detached bodily from the supporting hypha, and behave like single conidia. Thus in the Zygomycetes, as in the Oomycetes, a succession of steps leads from the typical sporangium to the simple conidium, the most char- acteristic form of reproductive cell in the Fungi. THE FUNGI 235 TYPE XX. SPH^ROTHECA CASTAGNEI We now come to the higher Fungi, an immense group of plants, which have become completely adapted to every conceivable variety of parasitic and saprophytic life, and no longer show any clear trace of affinity with the Algae. While differing among themselves in every other respect, they agree in possessing a septate, multi- cellular mycelium, with apical growth of the hyphae of which it is built up. Our present type is one of the simplest representatives of the great family of the Ascomycetes, which are characterised by possessing sporangia of very definite size and form (called asci), in the interior of which a definite and usually small number of spores are pro- duced, the number being regularly some multiple of two. In this respect, as in many others, they differ from the Phycomycetes, where, as we have seen, the number of spores formed in a sporangium is quite indefinite, and often very large. It is, in fact, still an open question whether the ascus can be regarded as truly homologous with the sporangium of the lower Fungi. The whole problem of the real relation of the Ascomycetes to the simpler families of Fungi is still unsolved. We will proceed at once to the description of our type, which will serve to give us an elementary idea of the main facts in the structure and development of this important and difficult group of plants. 1. STRUCTURE The species of Sphcerotheca and its nearer allies (form- 236 STRUCTUKAL BOTANY ing the family Erysipluce, so named after its largest genus Erysiphe) are all parasites. One species (S. Castagnei) is exceedingly common on Hops, and produces a very serious disease, the mildew, which causes great loss to the hop-growing industry. Another species (S. pannosa) is equally abundant on the leaves of Roses. All these Fungi are remarkable for being external para- sites, that is to say their much-branched mycelium forms a web on the surface of the leaves and other organs of the host-plant. The presence of the parasite is quite evident to the naked eye, owing to the dirty- white colour of its mycelium, which obscures the natural green of the leaf. The popular name " mildew " (equivalent to mealy dew) refers to this appearance, the leaves looking as if they had been powdered with flour. It must have been to Fungi of this kind that the name mildew was origin- ally applied, though now it is extended in popular usage to other diseases of plants, presenting quite different symptoms. The mycelium which spreads over the leaf, when examined microscopically, is found to be fixed to the host by means of enlarged hyphoe, producing broad, root- like organs of attachment, which anchor themselves by growing out into short plugs wedged in between the epidermal cells of the host. These rhizoids produce other branches, which have a still more important function, for they penetrate into the interior of the cells of the epidermis, forming suckers or haustoria, which absorb the organic substances in the cells attacked, and thus supply the whole Fungus with its food, at the expense of the living tissue of the host. The mycelium, as in all the higher Fungi, is multicellular, consisting of a single chain of cells, each of which in this case has a THE FUNGI 237 single nucleus, though in some allied Fungi the cells are mulfcinucleate. The mycelium, as already mentioned, forms a dense web on the surface of the leaf; its hyphre cross and touch each other at many places. 2. KEPRODUCTION It is at the points where two hyphse cross or come into contact that the fruits originate. Each of the ad- joining hyphse sends out an upright branch; the one enlarges and becomes club-shaped, and is cut off by a transverse wall ; the other remains more slender, comes into close contact with the former, and grows up with it, soon overtopping it and bending over its apex (Fig. 97, A). Two transverse walls are formed in this second branch, one near its base, and the other higher up. Of these two organs the former, i.e. the club-shaped branch, bears the name of the ascogonium, for it is from it that the ascus ultimately arises. The most recent investigations have proved that the second organ is really an antheridium, and that a true act of fertilisation takes place, a point which had previously been much disputed. According to the investigations referred to, fusion takes place between the ascogonium and the terminal cell of the antheridial branch, the cell-walls between them disap- pearing. Then the nucleus of the antheridium passes over through the opening and unites with the nucleus of the ascogonium. In the case of Sphcerotheca and some of its allies these facts are now well established, so it is evident that, in many Ascomycetes at any rate, the development of the ascus-fruit is preceded by a sexual process quite com- parable to that of the Oomycetes (cf. p. 225). In other cases reduced forms of sexual fusion occur, comparable to those observed in apogamous Fern-prothalli (see p. 77). 238 STKUCTUEAL BOTANY As regards the subsequent development of the asco- goniurn, the main facts are clear. After two or three transverse divisions one of the cells of the row (either the last or the last but one) increases in size, and becomes the ascus. It contains two nuclei, which fuse into one and then divide repeatedly, giving rise to eight daughter-nuclei, A P D FIG. 97. A-C, Sphcerotheca Castagnei. A, early stage in forma- tion of fruit ; p, antheridium ; c, ascogonium. 13, more ad- vanced ; p, antheridium ; c, ascogonium ; e, enveloping hyphse. C, ripening fruit in section ; c, ascogonium, from which the young ascus (a) is now developed ; e, e, envelop- ing hyphse, forming perithecium. D and E, S. pannosa ; D, ripe perithecium (e) bursting to set free the ascus (a), in which only six out of the eight ascospores are shown. E, chain of conidia, borne on a vertical branch of the mycelium. Magnified 600. (After De Bary.) around each of which a cell is formed. These eight cells are the ascospores. In the meantime the cell next below has sent out several branches, which grow up around the ascogonium, completely enveloping it in a double layer of densely crowded hyphse (Fig. 97, B and C). From the inner cells of the envelope thus formed short branches filled with very dense protoplasm grow inwards and apply THE FUJNUi 239 themselves closely to the ascus, probably supplying it with food. The outer cells of the envelope become thick- walled, and form a dense protective layer, completely enclosing the ascus. The envelope bears the name of the perithecium ; some of its superficial cells grow out into long hairs (see Fig. 97, D). The ripe perithecia are visible to the naked eye, as little black dots on the surface of the diseased leaf. The dense perithecium serves to protect the ascus during the winter, for the fruits remain inactive on the dead leaves until the following spring. When germina- tion takes place, the ascus absorbs water, swells up, and bursts the perithecium (Fig. 97, D), whereupon its own membrane dehisces, and the ascospores are set free. They at once reproduce the ordinary mycelium of the Fungus. In the case of the Hop, the germinating ascospores infect the young shoot as it first springs up from the soil. In most of the allies of Sphcerotheca each perithecium contains several asci (the product of a single ascogoniuni), instead of one only. Our plant has another means of reproduction, by conidia (see Fig. 97, E). These are produced on vertical hyphee (called the conidiophores) which produce the conidia at the free end. A whole chain of conidia is formed in basipetal order, the oldest thus being at the top. They are detached and scattered by the wind, germinating immediately if they reach a suitable host. They produce mycelium like that from which they sprang, and constitute a ready means of propagation during the summer, while, as we have seen, the ascus -fruits are specially adapted for the winter rest. In some allies of Sphcerotheca, as in the Fungus of the Vine disease, only the conidial fructification is known. 240 STRUCTURAL BOTANY Sphcerotheca represents the course of development of an Ascomycete in almost its simplest form. It is quite exceptional for the ascogonium to produce merely a single ascus ; in the great majority of the Ascomycetes a very large number are formed ; in many cases, how- ever, it has not been found possible to refer the origin of the asci to any definite ascogonium, and there are only very few species in which there is at present any clear evidence for the occurrence of an act of fertilisation. The formation of a completely closed fruit around the ascus or asci is characteristic of the Family (Erysipheae) to which Sphcerotheca immediately belongs. In the majority of the Order Ascomycetes, the fruit is more or less open, either having a small pore at the apex, or taking the form of a widely open cup or disc. Our next type will afford an example of the form of fruit last mentioned. TYPE XXL PHYSCIA PARIETINA A very large group of Ascomycetes have their ascus - fruits in the form of an open cup, or even a flat, shield - like disc. These constitute the Sub- order Discomycetes. The inside of the cup or the free surface of the disc is coated by the hymenium, a name applied in descriptions of the higher Fungi to the layer of spore-producing cells. In the case of Dis- comycetes the hymenium is made up of a large number of vertical asci, with sterile hairs the paraphyses between them. The particular plant we have chosen to represent this THE FUNGI 241 group is a true Discomycetous Fungus as regards its fructification, but it belongs to a set of plants which are so different in habit and mode of life from all other Fungi, that they used until recently to be treated as a distinct class of the vegetable kingdom. These are the Lichens, plants with a definite and often conspicuous thallus, freely exposed to the air and light, very different from the merely filamentous mycelium of ordinary Fungi, which is usually immersed in the substratum. Some Lichens grow on the bark of trees, some on rocks, walls or roofs, and others on the ground. 1. STRUCTURE AND MODE OF LIFE. Our example, Physcia, parietina, is extremely common (especially near the sea) on rocks, old walls and roofs, where it forms a conspicuous and most beautiful object, owing to its brilliant orange colour. We see at once, from the habitat of the plant, that its mode of nutrition must be totally different from that of a typical Fungus. So far from requiring any organic matter, living or dead, on which to feed, Physcia grows on the most barren and unpromising substratum conceivable. Many Lichens, in fact, thrive for years and even centuries under conditions of drought and apparent starvation, which would be absolutely intolerable to any other plants whatsoever. A Lichen, considered as a whole, is neither a parasite nor a saprophyte ; it requires nothing but a little mineral food, and can provide itself with carbon from the Carbon Dioxide of the air, like an ordinary green plant. Lichens therefore can only live in the light, which is not the case with Fungi. We will now proceed to describe the structure of Physcia, and find out the explanation of its remarkable mode of life. 16 242 STRUCTURAL BOTANY Physcia parietina is described as a foliaceous Lichen because its thallus has a flat, somewhat leaf -like form. It is attached to the substratum by its under surface, except at the edges of the thallus, where it is free (see Fio 1 . 98). The upper exposed surface of the thallus is of a deep orange colour, while the lower side is much paler. It is attached to the wall, rock, tree, or whatever else it may be growing on, by fibrous rhizoids, which perform in all respects the functions of roots. The anatomical structure of the thallus shown in transverse IS section in Fig. 99. Towards the upper surface is a dense layer of rather thick-walled tissue, which appears to be parenchymatous. In reality, however, it FIG. 98. Physcia parietina; thallus * s kmli U P> ^6 a ^ seen from above. a, apothecia, of fungal tissues, of fila- which tlie ripest are near the middle. , , ,. , Natural size, h, section of apothecium, mentoUS hyphse, which showing the hymenium. Magnified i n this Case are SO about 5. (After Lauder Lindsay.) .. . , densely packed and closely interwoven that their limits cannot be traced, and the whole structure appears to represent an ordinary cellular tissue. The orange colour is due to crystalline granules of chrysophanic acid deposited outside the cells, both on the free surface of the thallus and between the hyphce of which it is composed. Such acids (which belong chemically to the Benzole series) are common in Lichens, and from some of them Litmus (so much used in chemical testing for acids and alkalis) is prepared. THE FUNGI 243 Underneath the dense upper cortex comes a broad zone of loosely packed hyphae, leaving large air spaces between them. In the upper part of this medullary zone, numerous large green cells are embedded, lying in the interstices between the hyphae (Fig. 99, a). Some of these chlorophyll-containing cells are in course of active division. The green cells of Lichens bear the name of gonidia, and the part of the thallus in which they are contained is distin- guished as the gonidial layer. Underneath the medullary zone is a lower cortical layer resem- bling that on the upper surface. From the lower cortex the strands of hyphae arise, which constitute the rhizoids and are analogous to roots. Now the most important question as to the thallus of a FIG. 99. Physcia parietina -, T i , i vertical section of thallus Lichen concerns the nature of ^ npper cortial Iayer . a> a> the green cells, or gonidia. gonidia ( = Cystococcus) em- rr,, -, * , . T . bedded among medullary They play an essential part m hyphae . w> lo ver cortica j the economy of the plant, for, layer. Magnified 500. ,., ., ,, i 11 (After Sen wendener.) like other chlorophyll -contain- ing cells, they are able to assimilate carbon from the carbon-dioxide of the air, a fact which has been experi- mentally proved, and thus render the Lichen completely independent of organic food. Hence arises the profound physiological difference between Lichens and all other Fungi. For a long time, in fact down to about the year 1868, the gonidia were regarded as forming, like the hyphae, a constituent part of the thallus. So long as that view 244 STRUCTURAL BOTANY remained undisturbed, the Lichens were rightly ranked as a distinct Class, equivalent to Algse and Fungi, and in some respects intermediate between them. Of late years, however, evidence has accumulated, which proves conclusively that the gonidia do not belong to the same plant with the hyphae, but that they are distinct organisms, identical with definite genera and species of the Algae. Hence a Lichen is in reality a compound organism, made up of two totally different plants, an Alga and a Fungus, living in the closest association, and mutually dependent one on the other for certain essential services. The evidence on which this striking conclusion is based is of various kinds. In the first place, the so- called " gonidia " of Lichens are always found to agree exactly with certain species of the lower Algae, which are also well known in a free and independent condition. Thus in our type Physcia parietina, the algal constituent is Cystococcus hnmicola, a unicellular form nearly related to Pleurococcus. Further, the " gonidia " have been isolated from the Lichen-thallus, and are then able to lead a perfectly independent life, growing and multiplying on their own account, just like their fellows which have never been in bondage. More recently it has been found possible to raise a Lichen, that is to say the fungal constituent of a Lichen, by growing its spores in a food solution, which, as Algae are absent, must of course contain organic food substances. In this way a small Lichen- thallus can be produced, but it never contains any gonidia. Thus the fungal as well as the algal constituent can, under suitable conditions, live by itself. Most conclusive of all, however, is the actual synthesis of a Lichen, that is, the building up of a new plant out THE FUNGI 245 of the Fungus and its appropriate Alga. This has been observed in the case of our type, and Fig. 100 represents the process. The ascospores of the Physcia, have been sown among the cells of the Alga, Cystococcus. The spore on germination sends out a hypha, which at once begins FIG. 100. Physcia parietina ; building up of the Lichen out of the Alga and Fungus. A, germinating ascospore (sp) ; the hyphse have seized upon two cells (a, a) of Cystococcus humicola. B, more advanced stage ; sp, sp, ascospores which have produced a web of hyphse, enveloping the Cystococcus cells (a, a} in every direction. Magnified about 400. (After Bonnier.) to branch, and its finer ramifications attach themselves closely to the algal cells (Fig. 100, A). As growth proceeds, more and more of the algal colony becomes involved in the web of hyphae arising from the fungal spore, and one after another the Cystococcus cells are seized upon by the suckers of the Fungus (Fig. 100, B). 246 STRUCTURAL BOTANY Soon the filaments of the Fungus, well-fed at the expense of the Alga, are strong enough to build up a thallus. In the middle of Fig. 100 the hyphse are seen uniting to form a network, which represents the beginning of the cortical layer. Observations such as these have removed all doubt as to the compound nature of the organisms called Lichens. What, then, is the real relation between the Alga and the Fungus of which the Lichen is built up ? It might be supposed that the case is one simply of parasitism, the Alga playing the part of a mere victim to the devouring Fungus, just as a Cress-seedling is preyed upon by Pytliium or a Hop-plant by Splicer otlieca. This does not, however, seem to be the real condition of affairs. The Alga is not, on the whole, injured when the Fungus annexes it. A few of the algal cells may be exhausted and die, but the great majority live and go on multiplying within the Lichen, quite as happily as if living at liberty in the open air. It seems that there are advantages on both sides ; the Alga, by the aid of its chlorophyll-bodies, undertakes the whole duty of the assimilation of carbon, thus providing the Fungus with the organic food which it is unable to manufacture for itself. In many Lichens there are definite pores in the upper cortex, allowing of gaseous interchange between the atmosphere and the gonidial layer. On the other hand, the rhizoids of the fungal partner supply water and mineral food, probably in a more effectual way than the Alga could obtain them for itself. At the same time the tissues of the Fungus shelter the Alga and protect it from the weather, and especially from the effects of drought. It is probable that many unicellular Algae, when enclosed in the thallus of a Lichen, are able to exist, perhaps for centuries, in THE FUNGI 247 places, as, for example, on the surface of exposed rocks, where they could not possibly carry on their life if left to themselves. Such a relation between two organisms which live in common, and perform certain functions each for the good of the other, is kuown by the name of symbiosis, or commensalism, the former word simply calling attention to their living in union, while the latter term means that they share the same table, implying that they mutually help each other to food. 2. KEPRODUCTION. The Lichens being, as we have seen, compound organ- isms, might be described either under the heading " Algre " or " Fungi." It is usual, however, to take them with the latter class, because the organs of fructification, on which classification is chiefly based, belong entirely to the fungal partner. The captive Algre go on increasing by division, but rarely produce any characteristic repro- ductive organs, so long as they form part of the Lichen. Nearly all Lichen - Fungi are Ascomycetes, and the majority belong to the group Discomycetes, in which the hymenium is exposed to the air when mature. So far as the fructification is concerned, there is no essential differ- ence between Lichens and other Fungi of the same group, which lead an ordinary parasitic or saprophytic existence. The ascus-fruits of Physcia parictina are conspicuous to the naked eye as flat, shield-like discs on the upger surface of the thallus, generally of a rather deeper orange colour than the rest of the plant (see Fig. 98). These open fruits of the Discomycetes bear the name of apothecia. In the mature condition there is a rim of 248 STRUCTURAL BOTANY sterile tissue at the edge of the apothecium, the whole disc within this rim being covered by the hymenium or thecium. Below the thecium is a dense mass of closely interwoven hyphae forming the hypothecium. The thecium itself consists of elements of two kinds, the asci and the paraphyses. The asci, of which a great number are present in each fruit, are stout, club- shaped cells set vertically to the surface of the apothecium, each ascus when ripe containing eight ascospores. The paraphyses are sterile hairs rising to a greater height than the asci, both being closely packed together, so that the thecium has a smooth, con- tinuous surface (see Figs. 98 and 101). At an earlier stage of de- velopment the apothecium is closed, and consists of a mass of hyphae surrounded by a cor- tical layer. The paraphyses which arise from the hypo- thecium are the first elements of the thecium to be developed. The asci, which in many cases have been observed to arise FIG. 101. Physcia parieiina ; part of a vertical section through an apothecium. p, paraphyses ; a, asci one immature, the other two containing eight ascospores (sp] each ; c (above), hypo- thecium ; A, A, layers of algal cells ; m, medullary layer ; c (below), the lower cortical layer. Magnified about 250. (After Lauder Lindsay.) from the branches of a distinct hypha, differing from those which produce the paraphyses, are developed re- latively late. They grow up among the paraphyses, insinuating themselves between them until they attain nearly the same height. At the same time the envelope THE FUNGI 249 of the fruit is opened at the apex, and the edges gradually pushed back as the thecium expands. The development of the apothecium may go on for a very long time, even for years in some cases, new asci arising towards the exterior margin. Each ascus at a certain stage of development contains in its protoplasm a single nucleus, which subsequently undergoes repeated division, into two, four, and eight. When the full number is attained, a cell is formed around each nucleus, and these cells become the eight ascospores. The con- tents of the ascus are not, however, completely used up in the process of spore-formation ; a certain part remains over, lying between the spores ; this unused substance becomes gelatinous, and on taking up water tends to swell, and so to burst the aseus. The dissemination of the spores, however, is not entirely due to the pressure from within the asci. The whole thecium, including the paraphyses, endeavours to expand laterally when wetted, and this expansion is resisted by the rim of tissue at the edge. Hence the asci are subjected to very con- siderable pressure, the result of which is that those which are ripe dehisce at the top, expelling all the eight spores with considerable force, so that they are shot up as much as a centimetre into the air. The asci open successively as they become mature, the dehiscence taking place whenever wet weather occurs. The ascospores of Physcia, we have seen, can only complete their germination under natural conditions, and form a new Lichen-thallus, if they come into contact with the cells of Cystococcus, with which they can enter into partnership. The same applies to all Lichens, each having its own particular Alga. There is another form of fructification consisting of 250 STRUCTURAL BOTANY small flask-shaped receptacles called spermogonia (Fig. 102), each of which, in our type and its near allies, is sunk in a wart-like elevation of the thallus. The interior of the spermogonium is occupied by a net-work consisting of sterile cells and fertile lasidia. From the latter, excessively minute cells, FIG? 102. - lp/^ti~a~a- the spermatia, are budded off veruienta. Longitudinal (Fig. 103). The nature of these section of a spermo- ./-IT i_ j- gonium, embedded in an spermatia has been much dis- elevaiion of the cortex. pu ted i there appears to be Magnified. (After Darbi- r . , ., r ^ ,. shire.) weighty evidence tor regarding them, at least in certain cases, as the male cells of the Lichen. FIG. 103. Physciapulvcrulcnta. Portion of the tissue inside a spermogonium, showing the groups of sterile cells (G), and the basidia bearing spermatia. Highly magnified. (After Darbishire.) In most species investigated the apothecium has been found to arise from a special cellular filament, the carpo- THE FUNGI 251 gonium. Its lower end, which usually forms a coil, is embedded in the medullary tissue of the thallus, while the upper portion projects beyond the surface of the cortex, terminating in a long cell, the trichogyne (see Fig. 104). The trichogyne has a gelatinous cell- wall, to which numerous spermatia are found adhering. There is reason to believe that only those carpogonia, of which the trichogynes have been fertilised by spermatia, develop into ascus-fruits. The asci spring from cells of the coil (ascogonia), while the paraphyses arise lower down. It appears, then, that in these Lichens there is a sexual process resembling that in the Floridese and in certain Fungi. The details of fertilisation, however, still re- quire to be worked out. In some cases, on the other hand, the spermatia have been observed to germinate, so their function as male cells seems not to be constant throughout the group. The illustrations (Figs. 102-104) are taken from a Lichen (Physcia puherulenta), closely allied to our type. The ascospores, as we have seen, reproduce only the fungal element of the Lichen ; they must meet with algal cells in order to form a perfect Lichen-thallus. In many members of the group, however, though not in our type, there is a special provision for the re- production of the compound organism as a whole. In these Lichens we find some of the apothecia replaced by patches of a powdery substance, each grain of the powder consisting of a few algal cells invested by fungal hyphse. These little groups become isolated, and are dispersed by the wind or rain, or by the agency of insects. They are called soredia, and serve to reproduce at once the algal and fungal constituents of the Lichen. The group of soredia is here homologous with an 252 STRUCTURAL BOTANY Fio. 104. Pliyscia pulverulenta. Portion of a vertical section of the thallus, showing a complete carpogoniuni. a, projecting trichogyne, with swollen gelatinous cell-wall, in which numerous spermatia are caught ; 6, coiled, ascogonial part of the carpo- gonial filament ; &, dead layers of upper cortex ; c, living cortical hyphse ; d, groups of gonidia ; /, medullary layer ; g, lower cortex; h, rhizoids. Magnified about 1100. (After Darbi- shire.) THE FUNGI 253 apotheciuin ; in other cases similar bodies are formed on other parts of the thallus. Physcia then has served to illustrate, on the one hand, a highly organised Ascomycete, with a com- plex fruit containing a large number of asci; while, on the other hand, it has made us acquainted with the remarkable phenomenon of symbiosis, or the asso- ciated life of two distinct organisms, each performing certain physiological functions for the benefit of the other partner. TYPE XXII. PUCCINIA GEAMINIS. The group of Fungi represented by this type is a comparatively small one, and shows a narrow range of diversity as compared with a great Order like the Ascomycetes. The plants, however, are of much interest, for they afford one of the very best examples of typical parasitic Fungi, which have adapted them- selves exclusively to life at the expense of other plants. Some of them, and especially that species which we have chosen as our type, are extremely injurious to important crops, and so possess a very considerable practical interest. The life - history of these parasites is singularly complicated, at least in their more perfect representatives. Pucdnia graminis is the cause of the rust or mildew of Wheat and other cereals, the two forms of the disease being, as we shall see, stages of one and the same malady. We will begin with the stage known as Rust. 254 STRUCTURAL BOTANY The rust occurs commonly in summer on the leaves and stems of Wheat, Eye, and Oats, as well as on various wild grasses. In this condition the parasite is easily recognised, for it forms conspicuous long, rusty red or orange streaks between the veins of the leaf or along the surface of the stem. When the rust is mature, we see that these streaks are made up of' a fine powder, bursting out through the epidermis of the host-plant, the powder consisting of the conidia or uredospores of the Fungus. Thus it is only the fructification of the parasite which is visible externally. The vegetative part or mycelium is hidden in the tissues, and requires very careful microscopic examina- tion for its detection. It does not spread throughout the whole plant, but is limited to isolated patches of the particular organs attacked. The mycelium consists of a dense web of excessively fine hyphse, growing luxuriantly between the cells of the parts affected, and also sending out haustoria, which penetrate the cavities of the cells themselves. The mycelium is multicellular, the transverse septa, however, only occur- ring at long intervals. The elongated form of the rust- streaks is due to the fact that the Fungus attacks the soft tissues, lying between the longitudinal bands of fibres, which accompany the vascular bundles of the leaf. The fructification characteristic of the rust-stage of the Fungus is produced in great quantities throughout the summer months. Preparatory to its formation, certain of the more superficial hyphse pack themselves closely together, forming a dense layer just below the epidermis of the host-plant. From this layer the conidia are formed. Each conidium is a single cell, borne at the end of a THE FUNGI 25& vertical unicellular stalk (see Fig. 105, E, u). They arise in great numbers close together, forming large groups or sori ; in each sorus the development begins near the middle of the mass and spreads centrifugally. As the sorus develops, the epidermis lying above it is burst, and the ripening conidia are exposed to the air. The single conidium, as it matures, acquires a rather thick cell-wall, consisting of two layers, the exospore and endospore, the outer of which is of a brown colour and is covered with short spines, while the inner ia colourless. There are two germ-pores or thin places in the cell-wall, one on each side of the spore (see Fig. 105, E, u). In the cell-contents a quantity of oily, orange-coloured pigment is present. These conidia are called the uredospores, because they were formerly regarded as belonging to a distinct genus Uredo the species of which are now known to represent merely a particular form of fructification of the Puccinia. These uredospores become detached from their stalks, and are scattered by the wind and possibly also by the agency of insects. They are capable of immediate germination, and give rise to the same form of the Fungus as that which produced them, growing on the same kind of host - plant, or at least on an allied species. The hyphas grow out from the germinating spore through the two germ-pores (see Fig. 105, F). If germination takes place upon a Wheat plant or other host of the Grass family, the hyphse grow along the surface of the epidermis until a stoma is reached, through which an entrance into the inter- cellular spaces of the host is effected. Thenceforth the hyphse at once proceed to develop a new mycelium, from which new crops of uredospores arise. This form of 25G STRUCTURAL BOTANY fructification therefore serves for rapid propagation during the summer, though, as we shall see, it is not of necessity limited to that season. Later in the year another kind of spore, borne on the same mycelium with the uredospores, begins to make its appearance. The external sign of the change of fructification is a change in the colour of the sori, from orange-red to dark brown or nearly black. This is due to the development of the teleutospores, which owe their name (meaning final spores) to the fact that they appear at the end of the season of growth. It was to the teleutospore form of fructification that the name Puccinia was applied in the first instance, before the life-history was completely understood ; for this Fungus was originally put in three distinct genera, which are now known to represent stages in the develop- ment of one and the same plant. The teleutospore condition is popularly known as the Mildew of wheat and other cereals. The teleutospores are produced in just the same way as the uredospores ; in fact both kinds of spore are often found in the same sorus (Fig. 105, E), during the intermediate period while the one fructifica- tion is being gradually replaced by the other. Later in the season we find sori consisting of teleutospores only (Fig. 10 5, A). The teleutospore is borne on a stalk like the uredo- epore, but is quite different from it in structure. The membrane is excessively thick, consisting of a stout outer coat of a dark-brown colour, and an inner colourless layer. The spore is made up of two cells, separated by a comparatively thin transverse septum. We may, if we like, regard the whole structure as a sporangium containing two spores ; but as they never become separated, it is THE FUNGI 257 P- FIG. 105. Puccinia graminis. A, section of the cortex of a wheat-stalk, showing a sorus of teleutospores, with the mycelium below. Magnified 150. B, teleutospore germin- ating ; p, p, germ-pores ; sp, sporidia borne on sterigmata arising from the promycelium. Magnified 230. 0, sporidium germinating directly. D, sporidium (sp) germinating indirectly, forming a secondary sporidium (s). Magnified 370. E, part of a sorus, showing several nredospores (a) and one teleutospore (t) p, p, germ-pores. Magnified 300. F, uredospore germinating. Magnified 300. (After Von Tavel, Tulasne, and De Baiy.) 17 258 STRUCTURAL BOTANY simpler to speak of the whole body as a bicellular spore. Within the protoplasm of each cell are two nuclei, which appear subsequently to fuse into one, and there is also a vacuole containing oil. The surface of the cell-wall is smooth, unlike that of the uredospores. Each cell has a germ-pore, that is to say a deep pit in its membrane ; in the upper cell this pit is situated at the apex, while in the lower it lies on one side, just below the septum (Fig. 105, B). These teleutospores represent the resting-stage of the Fungus, in which it passes through the winter. This completes the history of the parasite, so far as its life on the Wheat or other gramineous 1 host is concerned. The damage which it does to the crop is very serious, though its immediate effect is only local. The chloro- phyll of the part attacked is destroyed, and the tissues thus rendered useless for assimilation, while the cells affected ultimately become exhausted and die. Thus, if the seats of infection be numerous, the plant may gradually lose almost the whole of its effective leaf- surface, and thus become starved and quite incapable of producing good grain. The germination of the teleutospores in this species takes place in the following spring. Each cell sends out a hypha, which starts from the germ- pore, as shown in Fig. 105, B. These hyphse do not develop into a normal mycelium, but are of limited growth, forming what is called a pro-mycelium (see Fig. 105, B), which divides by transverse walls, cutting off a row of about four cells from its terminal portion. 2 Each of these cells sends out a slender lateral outgrowth, which swells up at the end to form a small spore-like cell (Fig. 105, B). The natural order Graminene, or Grasses, includes all cereals. 2 In water, however, the pro -mycelium may grow to a considerable length before producing sporidia. THE FUNGI 259 These cells are the sporidia; the stalks on which they are borne are called the sterigmata. Hence we see that the teleutospore is incapable of directly reproducing the typical form of the Fungus, for it only gives rise to a rudimentary mycelium, which proceeds at once to form yet another kind of spore. The sporidia become detached from their stalks, and are capable of direct germination ; but if they do not happen to be carried by the wind to their proper host-plant, they form only a very short hypha, which at once gives rise to a secondary sporidium (Fig. 105, D), so as to gain another chance of successful dissemination. The sporidia, whether primary or secondary, are quite incapable of infecting any plant of the Grass Family. They are dependent upon a totally different kind of host, namely, the Barberry (Berberis vulgaris) or some of its allies. The sporidium, if it germinates on the leaf of a Barberry bush, sends out a hypha which is able to penetrate the cuticle, and therefore does not need to make use of the stomata, in order to effect an entrance into the tissues of its victim. In this respect the germinating sporidium differs from all the other forms of spore in this Fungus. When the mycelium is once started, it spreads through the tissues of the leaf, just as it did in the Wheat. The fructification produced on the Barberry, however, is of a totally different kind from any of the forms already described. During the spring the Barberry often shows signs of disease, consisting in the appearance of swollen dis- coloured patches on its leaves. When the disease has advanced further we find on the under-side of the leaf, seated upon the swollen place, clusters of exceedingly 260 STRUCTURAL BOTANY pretty little yellow cups containing spores (Fig. 106, A). This is the ^cidium form of the parasite, and, like die C 7. C.- P a, Fro. 106. Pucdnia graminis. A, part of the lower surface of a Barberry leaf ; on the swollen part (P) is a cluster of^Ecidiicm- cups (a). Magnified about 10. B, vertical section of the diseased part of the leaf ; e, e, epidermis ; p, palisade tissue ; s, spongy tissue ; sp, sp, spermogonia of the parasite ; %, ft. 2) a 3, three stages in the development of the ^Scidium- fruit; 1\ peridium ; h, hymenium. Magnified 40. 0, three chains of a.-cidiospores ; h, hymenial cells, i.e. intermediate cells. Magnified about 200. (After Zopf and Kny.) THE FUNGI 261 Urcdo and Puccinia, was long described by botanists as belonging to a distinct genus. When an j&Jcidium - fruit is to be produced, a group of hyphce become densely felted together in an intercellular space of the leaf ; the inner hyphoe of the group enlarge their cells, so as to give rise to a little nest of apparently parenchymatous tissue, surrounded by a web of ordinary mycelium ; at the base of this mass a row of vertically elongated cells- -the liymcnium (Fig. 106, B, Ji) is formed, and it is from these cells that the spores are formed. Each cell of the hymenium divides by transverse walls, and produces in basipetal order a long string of spores, often separated from each other by intermediate sterile cells (Fig. 106, C). In this way the whole interior of the young ^Ecidium becomes filled up by numerous parallel chains of spores, which, as they grow, completely displace the cellular tissue by which the space was at first occupied. The wall or peridium of the cup is built up of vertical rows of sterile cells resembling the chains of spores, but connected together into a permanent tissue. This peridium at first com- pletely encloses the fruit, but as the spores within increase in number, the enveloping layer is burst and thrown open, showing a toothed margin where its edges were torn apart (Fig. 106). The secidiospores, which are of a bright yellow colour, become separated by the breaking down of the sterile cells between them. The spores have a polygonal form, owing to mutual pressure while enclosed in the peridium. Their walls are thick, and each spore possesses six germ-pores or pits, through which, on germinating, the hyphre make their exit. The sccidiospores germinate very readily, within a 2G2 STRUCTURAL BOTANY few hours of their discharge, if sufficient moisture be present. They do not, however, infect the host on which they were produced, but are only able to form a mycelium if conveyed by the wind or rain on to the leaves of some member of the Gramineae, such as the Wheat or Eye. In this case a hypha is sent out through one or more of the germ-pores. The hypha receives the protoplasm from the spore and goes on growing, bending first in one direction and then in another until its tip lights on a stoma. Then the hypha turns in through the pore ,-f the stoma, and so makes its way into the intercellular spaces of the host, where it develops a mycelium from which uredospores are soon produced. Thus the cycle of the parasite's existence is completed. There remains, however, yet another form of repro- ductive structure to be considered before we proceed to sum up the life-history. Accompanying the ^Ecidium on the Barberry, but usually on the upper surface of the leaf, are minute bodies called the spermogonia, which are visible to the naked eye merely as minute black specks. They make their appearance before the cups on the opposite side of the leaf are ripe. Each of these spermogonia, when observed in a section vertical to the surface of the leaf (see Fig. 106, B, sp\ is found to be a little flask-shaped body, consisting of a sheath of slender converging hyphse, leaving a cavity in the middle. The spermogonium arises from the mycelium below the epidermis of the host, but ultimately breaks through it, so that the neck of the flask reaches the surface (Fig. 106, B). The hyphse which project into the cavity form minute cells at their ends, a little row of such cells being formed in each filament. These minute cells the THE FUNGI 263 % spermatia are very much smaller than any other form of spore in the Uredinese, their average diameter being about one two-hundredth of a millimetre. They are pro- duced in great numbers and are accompanied by a gelatinous substance which swells up when wetted, thus pushing out the spermatia through the neck of the flask. The structure of the spermatum, which has a single large nucleus, and little protoplasm, indicates, as Mr. V. H. Blackman has shown, that it is of the nature of a male cell, but in all cases investigated it is certainly func- tionless. A rudimentary form of germination has even been observed, the spermatia, when cultivated in sugar- solution, budding like yeast cells. The true method of sexual reproduction in the Uredineiie has recently been cleared up by Mr. Blackman. In the aecidium of Phragmidium Rubi, for example, a very common parasite on blackberry leaves, a nucleus passes from a vege- tative cell into the fertile cell which gives rise to a chain of ?ecidiospores. The paired nuclei, however, do not unite, but the secidiospores, and the cells of the mycelium produced from them, remain binucleate (as are also the uredospores) until the teleutospore stage is reached. The teleutospores are, at first, themselves binucleate, but while they are ma- turing the two nuclei fuse into one. Thus the life-cycle is divided into two phases, a binucleate phase from the secidiospore to the teleutospore, and a uninucleate phase from the latter to the secidium. Mr. Blackman regards the pairing of nuclei in the aecidium stage, and not their fusion in the teleutospore, as the true sexual act. Various modifi- cations of the process have been met with in other Uredinese. It appears, in all cases, to represent a reduced form of sexu- ality, replacing the fertilisation by spermatia, which was presumably the original process. Analogous phenomena have been observed among some of the Ascomycetes. 2G4 STRUCTURAL BOTAXY We are now acquainted with the full normal life- history of Puccinia graminis, which affords a typical instance of the phenomenon known as hctcrcecism, this term implying that the parasite at different stages of its career necessarily inhabits two distinct hosts. In this case we have seen that on the Wheat or other members of the Grass Family, two forms of fruit the uredospores and the teleutospores are produced. The latter on germination give rise to sporidia which infect the other host, namely, the Barberry. It is only on the Barberry that the jfficidium fructification and the spermogonia are developed. The secidiospores once more infect the Wheat or some allied plant, and the cycle is complete. The fact that the Barberry has something to do with the appearance of rust in Wheat was well known to practical farmers, long before botanists found out the scientific explanation, or even allowed the truth of the observation. Daring the eighteenth century a vast amount of evidence was accumulated showing that Bar- berry bushes acted as centres of infection, from which rust spread over the cornfields. So strong was this con- viction among agriculturists, that in the year 1755 a " Barberry Law ' was enacted in the province of Massachusetts in North America, ordering the rigorous extirpation of Barberry bushes throughout the province. The preamble to the Act runs thus : " Whereas it has been found by experience that the Blasting of Wheat and other English Grain is often occasioned by Barberry Bushes, to the great loss and damage of the inhabitants of this province ..." etc. The true explanation " that the parasitic Fungus of the Barberry and that of Wheat are one and the same species," was first suggested by Sir Joseph Banks in 1805, and fully confirmed a THE FUNGI 265 few years later by the independent experiments of a Danish schoolmaster named Schoeler. Botanists, however, were still unwilling to accept the fact, because the Puccinia of the wheat had quite different characters from the dEcidium of the Barberry. It was not till 1865 that the complete demonstration of all stages of the life-history of the parasite was accomplished by the German botanist De Bary. Now we know of a great many other cases of heteroecism among allied Fungi. We must not suppose, however, that the change of host is absolutely necessary for the perpetuation of a hetercecious parasite such as Puccinia graminis. In Australia, for example, rust is prevalent on Wheat to a serious extent, though there are no Barberry bushes nor any other plant on which the ^Ecidium form has been observed. In parts of England also the disease is well known, though there is no Barberry in the neighbour- hood. In such cases it is evident that the Uredo form must persist through the winter, probably on wild grasses growing as weeds on the cornlands, the uredo- spores infecting the new crop in the following spring. Under such conditions, the teleutospores are useless, for their sporidia can only infect the other host, and not the Gramineae. In the absence of the Barberry the Fungus produces a very large proportion of uredospores in comparison with teleutospores, no doubt because those individuals which are most prolific in the former have had the best chance of perpetuating their race. Some other members of the Uredinece have a very simple life-history compared with Puccinia graminis. In many of them all stages of the Fungus are passed through on the same host-plant, while in others certain of the stages are missing altogether. 2C6 STRUCTURAL BOTANY TYPE XXIII THE MUSHROOM (Agaricus campestris) The Mushroom, which to most people is the best known of all Fungi, represents a group of great extent, including about ten thousand species. The Mushroom and its near allies (most of which are commonly called " Toadstools ") are among the most highly organised of the Fungi. What is known in ordinary language as the Mushroom is simply the fructification ; for the vegetative part of the plant, or mycelium, is very inconspicuous, and remains hidden in the soil. What is called " mushroom spawn," from which the Fungus is raised in cultivation, consists of blocks of richly-manured soil permeated with the mycelium. The vegetative structure is simple enough, the mycelium consisting of long, branched, multicellular hyphse, which traverse the substratum in every direction. The individual hyphse are usually not isolated, but woven together into strands. Fusions of the cells are very common, and take place both between neighbouring cells of the same hyphse and between those of adjacent hyphse. The way in which union takes place is much like the monoecious and dioecious conjugation of Spirogyra, but in the case of the Fungi the process has nothing to do with reproduction, and so far as we know serves no other purpose than to facilitate nutrition. Each cell contains numerous small nuclei in its protoplasm. The matured fructification consists, as everyone knows, of a thick stalk (the stipe) swollen at the base, support- ing a hat-like expansion (the pileus), on the under-side THE FUNGI 267 of which are an immense number of radiating gills or lamellae, pink when young, but of a rich brown colour when mature. If we pull up a Mushroom entire we can see, hanging on to the base of the stalk, remains of the strands of mycelium from which it arose. In Fig. 107, A, is shown a large piece of the mycelium made up of the thick branched bundles of hyphae, and bearing a number of young fructifications. The fruit itself, like every other fungal organ, is entirely built up of hyphse. In the stalk these filaments are closely packed towards the outside, forming an apparently paren- chymatous cortex. Towards the middle they are more loosely arranged, so that the Fr ; ^.-Development of a J Mushroom. A, mycelium individual threads are easily distinguished, and large air- spaces are left between them. The multinucleate cells of which the hyphse are composed communicate with each other by means of pits, one of which (m) giving rise to a number of young fructifications. B, very young mushroom in section ; m, mycelium. C, slightly older ; I, the gills just appearing. D, still older ; Z, gills ; in, mycelium. E, older again ; I, gills ; v, velum ; st, stipe. F, nearly ripe ; h, pileus ; other letters .. . , ,, as before. Reduced. (After is present in the middle or Sachs.) each transverse wall. On the stalk of a ripe Mushroom, rather more than half-way up, is a membranous ring, formed of the remains of the veil, which at an earlier stage covered in 2G8 STRUCTURAL BOTANY the lower surface of the pileus, as shown in Fig. 107, E and F. The tissue of the pileus is like that of the stalk, but rather denser. The gills on the under-sur- face are formed by an extension of the hyphse of the pileus. If we cut a tangential section of the pileus, we see the gills or lamellae in transverse section, and can make out their structure (see Fig. 108). The middle part of each lamella is formed of hyphse coming down from the pileus, and following on the whole a longitudinal course, their lateral branches, however, diverging to- wards the two surfaces. This central tissue of the lamella is called the trama (Fig. 108, B, C, t). Towards the free surfaces the cells of the diverging hyphte are shorter and more closely packed, forming the siib-hy menial layer (sh), and beyond this FIG. 108. Gills of Mushroom. A, part of tangential section of pileus (h), showing gills (I). Slightly magnified. L\ single gill in section ; t, trama ; sh, sub-hymenial layer ; hy, hymenium ; r, lower edge of gill. Magnified about 80. C, part of B enlarged ; t, cells of trama; sh, sub-hymenial layer; q, paraphyses ; s^s^, stages in develop- ment of basidia ; sp, basidiospores. Magnified 370. (After Sachs.) THE FUNGI 269 again we come to the hymenium itself, which is thus composed of the terminal cells of the same hyphag which constitute the trama and sub-hymenial layer (Fig. 108). In this last part of their course the filaments have diverged from their original direction to such an extent o o that they now stand at right angles to the surface of the lamella. The hymenium consists of a palisade-like layer of club-shaped cells rich in protoplasm. Some of these are more slender than the rest, and remain sterile, bearing the name of paraphyses. The others are of stouter build, and are the spore-producing elements, here called lasidia. Each basidium gives rise at its free end to from two to four minute peg-like outgrowths (the sterigmata), each of which enlarges at the tip to form a lasidiospore (see Fig. 108, C, s). The spores when ripe contain oil, and have each two nuclei. These are derived from the basidium, which at an earlier stage possesses a single nucleus formed by the fusion of two or more nuclei which it originally contained. The nucleus of the basidium divides repeatedly, and the daughter- nuclei pass over into the basidiospores, two into each. This mode of fructification, consisting of basidia bear- ing spores on sterigmata, is universal throughout the great order to which Agaricus belongs, hence called the Basidiomycetes. The subdivision of this order, represented by Agaricus, is characterised by the hymenium being ex- posed to the air when ripe, and bears the family name of the Hymenomycetes. The basidium, when it has once produced its two or four spores, is exhausted, and does nothing more ; but for a time new basidia may arise, growing up between the old ones. An immense number of spores are produced from the 270 STRUCTURAL BOTANY gills of a Mushroom. Some idea of their multitude may be obtained by cutting off the pileus of an Agaricus and laying it, gills downward, on a sheet of white paper. If it be removed after a time an exact print of the gills will be found on the paper, in the form of a fine powdery deposit of spores which have fallen from them. Until recently, nothing satisfactory was known as to the germination of the spores of the Mushroom. Of late years, however, Mushrooms have been successfully raised from spores in Paris ; the entire development, up to the formation of ripe fructifications, takes from six to seven months. As a rule they are raised from the mycelium or " spawn." The basidial fructification is quite distinct from that of any other group of Fungi which we have described. There are, however, a number of transitional forms among the lower Basidiomycetes which appear to connect that order with the Uredineaj, and it has been suggested that the basidium of the Mushroom group is homologous with the pro-mycelium produced from the teleutospores of the latter family, the basidiospores corresponding to the sporidia. There are some UredineaB, such as the Piiccinia so common on hollyhocks (P. Malvacearum), in which the teleutospores germinate in situ, i.e. while still in the sorus and attached to the mycelium. In this case the resemblance of the pro-mycelium to the basidia of some of the simpler Basidiomycetes is very striking, but unfortunately we have no space for the description of the supposed intermediate types. In Fig. 107 the development of the Mushroom-fruit is illustrated. The young Mushroom arises from a tangle of hyphse borne on a strand of mycelium. The intertwined hyphae group themselves into a tissue, thus THE FUNGI 271 forming a little oval tubercle. At first the Mushroom is all stalk ; soon, however, the pileus begins to appear at the top. In the earlier stages there is no separation between pileus and stipe (Fig. 107, B, C). The gills are developed endogenously, while enclosed on all sides by continuous tissue (Fig. 107, D). Later on the pileus begins to spread out laterally (Fig. 107, E), but its under-side is still closed in. The tissue which connects the edge of the pileus with the stalk, and thus encloses the gills from below, is called the velum or veil (Fig. 107, F). At last this becomes ruptured as the pileus ex- pands, and its torn remains adhere to the stipe, forming the ring, which we mentioned in describing the ripe fructification. In the case of the Mushroom itself no other form of spore than the basidiospores has so far been discovered. In some nearly allied Fungi, however, additional forms of fructification, such as chains of conidia, are produced on the mycelium. In no case is there any evidence for the occurrence of a sexual process at any stage in the development of Basidiomycetes, unless, indeed, the fusion of nuclei in the basidiuni is an indication of sexuality. The Mushroom itself is a saprophyte, growing in richly manured soil, but some of its near relations are parasitic on trees, to which they do great damage. We have now finished our series of types of Fungi. It has only been possible to consider a very few re- presentatives, and many important groups have been left altogether untouched. We have gained, however, some slight idea of the great range of structure which the class presents, and in our later types we have seen how very far the higher Fungi have diverged from the primi- tive algoid forms with which we started our survey. CHAPTER V THE BA CTE1UA THE Bacteria, which in these days are familiar, by name at any rate, to everyone, are an extensive group of organisms of the most minute size, and, so far as we know them, of the most simple structure. In their mode of life they bear a general resemblance to Fungi, for, with the rarest exceptions, they are destitute of chloro- phyll, and adapted either to a parasitic or saprophytic existence. They are, however, as we shall see, quite different from any known Fungi in structure and development, though it is not impossible that in some cases a real affinity to Fungi may turn out to exist. Both as parasites and as saprophytes, the Bacteria play an enormously important part in the world. Parasitic Bacteria are now known to be the cause of almost all the infectious diseases of man and animals, and in many cases the actual species to which the different diseases are due have been strictly determined. As saprophytes, Bacteria are the great agents of decay of all kinds, owing to the fact that they set up rapid and profound chemical transformations in the organic sub- stances on which they feed. Thus when milk turns sour, or when wine is converted into vinegar, or proteid substances, such as meat, undergo putrefaction, the change 272 THE BACTERIA 273 is in each case due to the action of a definite species of the Bacteria. On the same power of initiating far-reaching decompositions in the bodies which they inhabit, depends the fatal efficiency of the parasitic Bacteria in producing disease. The whole subject of the fermentations set up by these organisms has become in recent years of the greatest possible practical importance in relation both to medicine (as regards the parasitic forms), and to innumer- able branches of industry, as regards the saprophytes. A vast literature has grown up on these subjects, which lie beyond the province of the present Introduction. TYPE XXIV. BACILLUS SUBTILIS This is one of the commonest and best known forms of Bacteria. It occurs constantly in hay, and can be ob- tained with certainty by soaking or boiling hay in water. In the latter case the appearance of the Bacillus depends on the extraordinary resistance to heat shown by its spores, which can stand an hour's boiling with impunity. After a little time the whole of the liquid simply swarms with the cells of the Bacillus, which in its active vegeta- tive condition is a strictly unicellular organism, the isolated cells having the shape of short rods rather more than roVoth of a millimetre in diameter and from 10 5 00 to Tinnr nun. in length. The cells are thus far more minute than those of any plant we have hitherto considered, if we except spermatia or the oidial cells sometimes formed by certain Fungi. The excessive smallness of the cells has placed great difficulties in the way of their investiga- tion, and the histology of Bacteria is still very little understood. So far as we know at present, however, the structure appears to be very simple. There is a definite membrane which, however, does not consist of cellulose, 18 274 STRUCTURAL BOTANY but seems to be chiefly of a proteid nature. The cells move actively, and their movements are now known to be due to cilia which are attached to the protoplasm and penetrate the wall (see Fig. 109, a, d). The whole interior of the cell is usually occupied by FIG. 109. Bacillus suUilis. a, d, ciliated motile cell and filament ; b, non-motile cells and filament ; e, zoogloea, during spore - formation ; c, cells and filament with endo- spores, from the zoogloea. a-d, x!050. e, x 175. (After A. Fischer.) FIG. 110. Bacillus megatherium, a, chain of vegetative rods, each con- sisting of two or more cells, but septa not shown. Magnified 250. p, four-celled rod, after treatment with alcoholic solution of iodine ; 6, vegetative rods ; c-f, rods, show- ing the formation of endo.spores ; r, four-celled rod with ripe spores ; g-^-g.j and h lt Ji 2 , spores swelling before germination ; the mother cell- walls disappear ; k-m, germination of spores. All figures except a magnified 600. (After De Bary.) the protoplasm. It is probable that a nucleus is present, though the minuteness of the cells renders its demonstration extremely difficult. For some time the Bacillus continues in the actively swarming condition, multiplying abundantly by the re- peated transverse division of the cells. After some days the individuals begin to seek the surface of the liquid, THE BACTERIA 275 where they pass into a resting condition. At this stage the cells remain connected together in long filaments, and their outer cell -walls become very gelatinous. This is called the zoogloca condition, and is easily recognised by the gelatinous iridescent film which the colonies of the organism form on the surface. Lastly, when the food in the liquid is getting ex- hausted, the spores begin to form. This takes place after locomotion has ceased and the Bacillus has entered the filamentous condition (see Fig. 109, A). The spores in this species and in a very large group of allied Bacteria, are endospores, one spore being produced in the interior of each cell. A new wall appears round a portion of the contents, in which a nucleus is said to be included. The young spore absorbs the remaining protoplasm, and it becomes elliptical in form, and increases suffi- ciently in bulk for its walls to touch those of the mother- cell. In the mean time it has completely used up the sur- rounding protoplasm, and now lies within a mere empty membrane. The endospore itself acquires a comparatively thick cell-wall, and is extraordinarily tenacious of life. These spores can bear being completely dried up without injury ; they are little affected by poisons, and survive a very high temperature, withstanding even an hour's boiling in the case of the hay Bacillus. Hence spore- forming Bacteria are extremely difficult to extirpate, so that in order to make sure of effectually " sterilising " any substance (i.e. destroying any living things which it con- tains) it is often necessary to expose it to a temperature considerably above the boiling-point of water, or, if that be impracticable, at least to continue boiling for some hours. The spores germinate when brought into a suitable food-solution. The outer-membrane splits across, and 276 STRUCTURAL BOTANY the entire contents escape as an ordinary bacterial cell, which at once begins to move about by means of cilia (see Fig. 109, B). Fig. 110 shows very completely the stages in the forma- tion and germination of the spores in another Bacillus, called B. megatherium, because for one of the Bacteria it is quite a monster, though its cells are only about Twth of a millimetre in diameter. This species was originally found in boiled cabbages, and was afterwards cultivated by its discoverer in solutions of grape-sugar, to which a little extract of meat had been added. The formation of eudospores characterises one great group of Bacteria, and distinguishes them from similar unicellular organisms. O l - J Bacillus suUilis, like most other living things, requires plenty of atmospheric oxygen in order to nourish. Some of the other Bacteria, however, have the remarkable peculiarity that they thrive best in the absence of free oxygen. This is the case, for example, with Bacillus luty- ricus, the organism to which the formation of butyric acid by the fermentation of sugar is due. In this case the oxygen necessary for respiration is not absorbed in the free state, but obtained from the breaking down of the organic substance in which the organism lives. It may be mentioned here that numerous experiments have proved that light has a very unfavourable effect on Bacteria, completely stopping their growth and multi- plication in many cases, and even, when intense enough, killing the cells outright. It is the rays towards the violet end of the spectrum which exercise the greatest retarding effect on the growth of these creatures. The action of light in checking the increase of these agents of decomposition and disease is evidently a fact of great practical importance. THE BACTEKIA 277 It was stated in Part I. (p. 206) that plants of the Pea and Bean kind, unlike ordinary green plants, are able, by the help of certain fungus-like companions, to obtain their nitrogenous food from the free nitrogen of the atmosphere. The plants in question, including most if not all of our native Leguminosse, invariably have swellings or tubercles on their roots. These tubercles are inhabited by a parasitic or symbiotic organism, the entrance of which into the root is the cause of the first formation of the tubercle. It has been proved conclusively that it is only when this organism is present in the soil that the tubercles develop on the roots, and only when the tubercles are formed that free nitrogen can be assimilated. If the plants are grown in sterilised soil, i.e. soil which has been heated sufficiently to kill all living things contained in it, then no tubercles develop, and no nitrogen is absorbed from the air. When the tubercles are present, however, great quantities of nitrogen are assimilated, and the plant can thrive even if nitrogenous compounds be quite absent from the soil. A very important result of this fact is that leguminous crops actually enrich the soil in nitrogen. In Germany one sees whole fields of Yellow Lupine grown for no other purpose than to be ploughed in and so enrich the soil for other crops. The subject is mentioned here because the organism to which this assimilation of gaseous nitrogen is due, has the appearance of a Bacillus, and has been described under the name of B. radicicola. Eecent investigations appear to show that the organism is really one of the Bacteria. In any case the relation of this creature to its leguminous host seems to be one of symbiosis, or mutual service, rather than of one-sided parasitism. 278 STRUCTURAL BOTANY TYPE XXV. CLADOTHEIX DICHOTOMA. We have chosen this plant as an example of another group of Bacteria, which probably, however, are rather remote from the true endosporous Bacilli and their allies. Cladothrix dichotoma is a very common organ- ism in impure water. A very moderate degree of impurity is sufficient to provide it with food, for it sometimes appears in vast quantities in the pipes of an ordinary water-supply, where it forms dirty- white masses, which may even choke up the taps. In its vegetative condition the plant forms long branched threads, attached at one end to some solid substance. The filaments are com- posed of a single series of rod-shaped cells (Fig. Ill), and are enclosed in a gelatinous sheath. The branching is not genuine, for it does not depend on the formation of lateral out- growths, or on a true dichotomy of the growing-point. The gelatinous sheath offers a certain resistance to the growth of the filament within, which consequently breaks across, and the two parts grow past each other at the point of rupture. Exactly the same kind of apparent branching occurs in some of the blue-green Algee, which Cladothrix and its allies closely resemble. The FIG. 111. A, Clado- thrix dichotoma. Branched vegetative filament. Magnified about 450. B, Creno- thrix. Formation of microspores in a filament. Magnified about 700. C, escape of the microspores. Magnified about 700. ( After Zopf.) THE BACTERIA 279 cells of the filament become isolated for reproductive purposes, forming motile cells or zoospores (see Fig. lllA), which pass through a swarming stage, possessing a lateral tuft of cilia. Endospores are not found in any of these forms, but in some of them, as in Crenothrix (Fig. Ill, B and C), the contents of the cells may divide up into numerous small swarm-cells or microzoospores. The re- lations of these organisms to the typical Bacteria, as re- presented by B. subtilis, is quite an open question, though on the other hand their affinity to the Cyano- phycese seems rather closer. Before leaving the Bac- teria, it may be mentioned that while the great ma- jority are either saprophytes or parasites, a few have been found capable of de- riving their food entirely from inorganic substances. In some of these cases carbon- dioxide is decomposed under the influence of light, with the help either of true chlorophyll or of a purple pigment which seems to have similar functions. Another member of the group is able to obtain its carbonaceous food from inorganic carbonates without the aid of light,- a power possessed by no other organisms at present known. The same Bacteria in which this remarkable property resides are of great importance in another respect, as they bring about the oxidation of nitrogen, thus forming the nitrates in the soil. FIG. Ill A. Cladothrix diehotoma. Part of a branched filament, showing formation of ciliated swarm-cells. x 1000. (After A. Fischer.) CHAPTEE VI THE MYXOMYGETES^ OUR last type represents a group of organisms lying on the borderland of the animal and vegetable kingdoms. It may be doubted whether they have any right to a place in a book on botany, but we give them the benefit of the doubt because of their great scientific interest ; for in them we can study living protoplasm and its behaviour on a greater scale than in any other creatures Myxomycetes, unlike Fungi and Bacteria, are of no practical importance, and are probably known to very few people except naturalists ; yet they are common enough, easily visible to the naked eye, and in some conditions extremely conspicuous. In the vegetative state a typical Myxomycete consists of a mass of naked protoplasm, sometimes several inches in extent, which creeps slowly about, on the surface of dead leaves or bark or wood. Such immense aggrega- tions of living matter in so simple a form are quite unknown in any other group of organisms. When reproduction is about to take place, the creature completely changes its character, gradually ceases to be active, and converts itself into a collection of fruits of rather complex structure, in which the microscopic spores 1 Also called Myc to oa. 2SO THE MYXOMYCETES 281 are produced. The spores on germination give rise to swarm-cells, which unite together to build up the great protoplasmic body with which we started. Such are the rough outlines of a Myxomycete's career. We will now proceed to study a particular example more in detail. TYPE XXVI. BADHAMIA UTRICULARIS 1. THE PLASMODIUM This Myxomycete is common in some years, though rare in others: it occurs on the bark or wood of fallen trees, on old garden seats, and in fact in all places where timber is left exposed to damp and decay. In its ordinary state the organism forms irregular flat gelatinous masses of a deep chrome-yellow colour, spreading and creeping over the surface of the rotting wood, especially in damp weather. These creeping masses are called plasmodia. A small plasmodium is shown in Fig. 112. This specimen was only about half an inch across, but much greater dimensions are usually attained, the area covered by one plasmodium sometimes amounting to as much as six square inches. The plas- modium is not uniform in thickness throughout, but is traversed by thicker veins, which unite together to form a kind of network. The thinner protoplasmic layer between the veins is sometimes interrupted so as to leave some of the meshes empty. The whole plasmodium is a mass of living protoplasm which is in constant move- ment. The movement is of two kinds (1) an advancing locomotion of the whole plasmodium, and (2) an internal circulation of the protoplasm, especially in the veins. The locomotion is a slow, creeping movement; the 282 STRUCTURAL BOTANY advancing edge of the mass (a, a in Fig. 112) is con- stantly putting out feelers (pseudopodia, as they are called), which are sometimes withdrawn again, but more often maintain their position, and are increased by the flow of protoplasm from behind. The outer layer of the whole plasmodiura is clear and transparent ; the inner mass is very granular, and the granules are especially abundant in the veins. Many of the granules consist of lime (calcium carbonate), and it is around these lime- Q FIG. \l-2.~Badhamia utricularis ; plasmoclium, from a stained specimen, prepared by Mr. A. Lister, a, a, advancing margin. Magnified about 5. (R. S.) granules that the yellow colouring-matter is chiefly deposited. The clear part of the protoplasm is colour- less. When a pseudopodium is first extended it consists of the clear part (hyaloplasm) only; subsequently the inner granular substance flows into it and increases its mass. The internal movement along the veins is extremely active, and can be followed with ease under the microscope by means of the granules which are swept along with the current. The flow is curiously rhythmical. In each vein the current sets steadily in one direction for THE MYXOMYCETES 283 from one and a half to two minutes, then it slackens and stops altogether for an instant, only to recommence with equal energy in the opposite direction. This internal flow is closely related to the locomotion of the whole plasmodium, for it is found that the current lasts longest in that direction in which the plasmodium is advancing. The movements do not go on at random, but take a definite direction in accordance with the needs of the organism. Thus an active plasmodium, if the wood be wetted on one side of it more than on the other, will move towards the damper side ; if, however, the Myxomycete be about to form spores (for which moisture is not favourable) it will move the opposite way, in the dry direction. Generally speaking, a plasmodium will try to avoid intense light (which no doubt has a bad effect, as in the case of the Bacteria) ; for the purpose of spore-formation, however, it will leave any dark recess of the wood in which it may be hidden, and seek the light. This particular Myxomycete, Badhamia utricularis, feeds on living Fungi, especially on certain members of the Hymenomycetous family, which grow on decaying wood. If a piece of one of these Fungi be placed in its way, the advancing margin of the Badhamia at once begins to flow over it, and the whole plasmodium will turn aside in the direction of the prey. Individual hyphse, or small pieces of the Fungus, become enclosed in vacuoles in the protoplasm of the Badhamia and digested, their use- less remains being afterwards disgorged and left behind on the track. This way of feeding, by taking solid food into the body, and then digesting it, is characteristic of animals, and is not known among true plants. The Myxo- 2S4 STRUCTURAL BOTANY 4.'~vWt. "'Jp J^ r> -. *** p r :MZ&. o'o t-jy&ss mycetes generally have this power, but in most cases they live on dead and decaying substances, such as fragments of bark and wood. Our type is exceptional in preying on living tissues. Its frequency in any particular season depends on the abundance of the Fungi which it feeds on. The plasmodium contains an immense number of nuclei, which are only absent from the clear external portion. The nuclei (see Fig. 113) have the same structure as those of higher organisms, possessing a nucleolus and a framework of delicate threads. They increase in number by division, as the plasmodium grows. We see, then, that the plasmodium of a Myxomycete is a typical example of non- cellular structure, consisting FIG. 113, Badhamiautricularis; p c ^ j.- portion of plasmodium, show- of a perfectly continuous ing a number of spherical protoplasmic body of large nuclei, in each of which the . . . -, -, iibrilkr network and the Slze > containing vacuoles and ?!v eolu L is see T n .' ^ a s nified numerous nuclei, but entirely 1200. (From Lister's Mono- , ... . ,, J graph of the Mycetozoa.) destitute of any cell-wall. Sometimes the plasmodium passes into a resting condition, a change which often, though not always, happens in consequence of drought. The movements cease, and the protoplasm becomes partitioned into a number of irregular cysts or cells, each containing about ten or twenty of the nuclei. The cysts are separated from each other by firm walls, which are hardened portions of the protoplasm. In this resting stage, which is known as the sclcrotium, the plasmodium THE MYXOMYCETES 285 may remain alive for as long as three years. The external appearance of the sclerotium of our type is that of a dry, horny, irregular mass, of a brick-red colour. When moistened, it revives, the walls of the cysts become absorbed, and the contents reunite and recommence the movements characteristic of active life. a 2. THE SPORANGIA AND SPORES When a Myxomycete fructifies it completely changes its appearance. The whole of the active protoplasm is used up to form a sorus of sporangia in which the spores are contained. In Fig. 114 a cluster of sporangia (from an allied genus) is shown. The ripe sporan- gium is a rounded hollow case, borne on a stalk ; it has a firm external wall, and its interior is traversed by a network of threads, among which lime is deposited (see Fig. FlG 114, &). In the meshes of the net- work are contained the numerous 114. Leocarpus vernicosus. , group of sporangia on a frag- ment of dead leaf. Magnified 2|. 6, portion of capillitium with spores. Magni- fied 120. (From Lis- ter's Monograph of the Mycetozoa. ) spores. When fructification is about to take place, the protoplasm accumu- lates at certain points, corresponding to the position of future sporangia. At each of these points the protoplasm heaps itself up to form a projecting mass ; a portion of this hardens and becomes the stalk, while the living part continues to creep upwards, and constitutes the sporangium itself at the top. The outer layer of the terminal mass of protoplasm forms itself into the firm outer wall, while the interior part 283 STRUCTURAL BOTANY builds up a network of hollow branched threads (the capillitium), which traverse the cavity in all directions (see Fig. 114, &) Between these threads masses of living protoplasm remain. After the wall and capillitium are completed, the formation of spores takes place. The protoplasm in the meshes breaks up into distinct masses, the nuclei of which all undergo division. It may be mentioned here that the nuclear division in the sporangia of Myxomycetes, and sometimes that in the plasmodium as well, takes place in just the same complicated way as in the tissues of the higher animals and plants, al- though, so far as the plasmodium is concerned, it is probable that a simpler process of division also goes on. Ultimately the whole of the FIG. H5. Comatricha living protoplasm in the sporangium a, group of f urt h er divides up into spherical sporangia. Natural r r size, b, empty sporan- spores, each of which includes a gium, showing capil- , nnolpnq litium. Magnified 16. LS ' (From Lister's Mono- The cell-wall of the spores, and fozoa.) ( a ^ so the substance of the sporangial wall and the capillitium, resemble the cuticularised membrane of vegetable cells. In a few cases cellulose has been found. It will be noticed that complicated as the structure of the sporangia is, there is no formation of distinct cells until the spores themselves are developed. In Fig. 115 the sporangium of another Myxomycete (Comatricha obtusata) is figured. Here the sporangial wall soon disappears, so that the whole capillitium in connection with the stalk becomes visible. The sporangia of Badhamia open by the breaking down of the membrane, and the spores are exposed. THE MYXOMYCETES 287 They hang for a time on the threads of the capillitium, which acts as a supporting scaffolding, and are gradually scattered. The spores can be kept for an unlimited time dry, and germinate readily when wetted (see Fig. 116, from another Myxomycete). The membrane splits, and the whole contents become free. The protoplasmic mass at first shows " amoeboid " movements, changing its form by putting out and again withdrawing pseu- dopodia. After a few minutes a single cilium is developed at one end, and now the pear-shaped swarm - spore is fully formed (Fig. 116, d). It contains one nucleus, placed near the thin end, and its protoplasm is vacuolated ; one of the FIG. vacuoles is contractile, expanding and contract- ing at regular intervals. The swarm-spore swims . Didymium difforme. a, hatched swarm - cell, containing a nucleus and three vacuoles ; d, ciliated swarm-cell ; e, swarm-cell, with two vacuoles containing bacteria another bacterium is just caught by the pseudopodia ; /, amoeboid swarm -cell. Magnified 720. (From Lister's Monograph of the My cetozoa. ) through the water with a dancing movement, or it creeps along the surface of any solid body like a snail. These swarm-spores, like the plasmodia, can take in their food in the solid state. They catch minute objects in the water by means of pseudopodia put out at the broad posterior end. They are particularly fond of Bacteria, which are often caught in this way, the pseudopodia laying hold of the microbe, and in spite of 288 STRUCTURAL BOTANY its struggles dragging it in until it is enclosed in a vacuole of the protoplasm, and ultimately digested (Fig. 116,0- The swarm - spores multiply repeatedly by division into two, the movement ceasing before the division takes place, and starting again when the daughter-cells are formed. The division of the nucleus precedes that of the cell. The swarm- spore may also become encysted, surrounding itself with a cell -wall, from which it afterwards escapes, resuming its active career. The en- cysted swarm -cells bear the name of microcysts. From the swarm-spores the plasmodium is built iG.H7.Didi/miumdifforme', young r plasmodium with attendant swarm- Up. Before this happens cells. t m microcyst. One microcyst they withdraw their Cilia, is being digested in a vacuole (v). J s, empty spore-membrane. Magni- and henceforth confine fi f e l 47 Si ( F / ora Y ster ' s Monograrh themselves to creeping of the Mycetozoa.) amoeboid movements. When two creeping myxam&bce (as they are called at this stage) meet, their protoplasm flows together into a single mass. Then others join them, and enter into fusion, and so the beginning of a new plasmodium is made. More and more swarm-cells are attracted to the spot, and join on to the fused mass (as shown in Fig. 117) until a large number have united. The cells com- pletely fuse, but their nuclei remain distinct. Thus we see that a plasmodium is a compound structure, built up in the first instance by the union of THE MYXOMYCETES 280 a number of distinct protoplasmic bodies. As there is no fusion of nuclei, the process cannot be regarded as of a sexual nature. When a plasmodium has been started in this way it continues to grow, and the number of its nuclei increases by division to keep pace with its growth, a fact which is now well ascertained. Such are the outlines of the life-history of these extraordinary creatures, which in their whole structure and mode of development differ widely from any other organisms. We can study in them, better perhaps than in anything else, the behaviour of living protoplasm when untrammelled by the bonds of cellular structure. We see how closely movement and growth are connected ; and when the period of fructification comes on, we can observe how the protoplasm by its active exertions literally builds up the new structure out of its own substance. It has been well said that in the plasmodium of a Myxoniycete we have a type of the organisation of all plants, for we see in these organisms, freely exposed to view, the same movements and the same constructive activity of the living matter, on which the growth and development of the highest plants depend. In the latter, however, the living agent is concealed within the framework of the cells, and its successive changes of form are stereotyped by the rigidity of the structures which it has itself built up. iy CHAPTER VII CONCLUSION IT will be useful, at the conclusion of our series of types, to sum up very briefly what we have learnt from them, with reference especially to the relationships of the groups which they represent. Throughout the book we have followed on the whole a descending order, proceed- ing from the more complex to the more simple, though there have been many exceptions to this rule, because it is impossible to arrange any set of plants in a single linear series, whether according to increasing or decreas- ing complexity. In the present summary we will follow the reverse order, starting where we left off, with the lowest forms, following up the various lines of affinity, and concluding with the highest plants, with which we began in Part I. This is the natural succession, and the attempt to follow it will at any rate teach us how complicated the relation- ships are, and how difficult it is to arrange naturally even a few types such as ours. The two groups with which we concluded the series, and with which we therefore begin our summary, have no clear and evident affinities with any of the rest. As regards the Myxomycetes, it is doubtful whether any such relationship exists at all. These organisms have attained a fairly high stage of development (so far at least as 290 CONCLUSION 291 their fruits are concerned) on lines of their own. They are best regarded as forming by themselves a short but distinct line of descent, which may have arisen very far back, among organisms not yet characterised as either animals or plants. In the plasniodial stage the Myxomycetes would most naturally be regarded as animals, especially when we consider their mode of feeding. In the formation of their fruits and spores, however, they rather suggest plants of the nature of Fungi, but probably this is only a case of parallel development, not indicating a real blood-relationship to any undoubted members of the vegetable kingdom. The Bacteria are still more difficult to place, for though in some ways we know so much about them, we still do not know what they are. Possibly several heterogeneous groups are included among them. Such forms as Cladothrix show some affinities with" the Cyauo- phycese, of which we took Nostoc as type, and, if it were not for their ciliated swarm-cells, might be described as Cyanophycese without pigment. The more typical Bac- teria, however (such as Bacillus sultilis and its allies), which are characterised by the production of endospores, are still more unlike any other plants, and might be placed in a neutral group lying at the base of both the animal and vegetable kingdoms. Bacteria, however, offer so few points for morphological comparison with other groups, that nothing definite can be said at present as to their relations. Nostoc, representing the Cyanophycese, is another ex- tremely simple type, so far at least as our present knowledge enables us to judge. It is probable, however, that further research may show the cell-structure to be more like that of the higher plants than it appears at present. It is doubtful whether the Cyanophyceoe should 292 STRUCTURAL BOTANY be classed with the Algae or not ; some botanists group them with the Bacteria in a class by themselves the Schizophyta. As already pointed out, however, this relationship seems remote, except perhaps in the case of Cladotlirix and its immediate allies. Until the histology of the Cyanophycese and Bacteria is better understood, it will remain impossible to assign them to their true position. The absence of sexual reproduction and of ciliated swarm-cells tends to keep the Cyanophyceae in an isolated position, though it now appears probable that they resemble the Algae in possessing nuclei and possibly chromatophores. Leaving these dubious forms, we come to Pleurococcus, a perfectly typical green Algae, the cells of which, though leading a separate existence, possess all the histological characters of the green cells of the higher plants. The life-history of Pleurococcus is no doubt a somewhat com- plex one, but, even apart from that, its structure is a sufficient indication of real affinity with more highly- organised Algae. From unicellular green Algae, or their ancestors among the Flagellata, quite a number of distinct lines of affinity branch out. In one direction we reach the Conjugatae, some of which are themselves unicellular, while in others, such as Spirogyra, the cells are usually united in filaments. This group always remains at a low level anatomically, for no more complex thallus than a simple filament is ever produced. On the other hand, the histological structure shows a great advance, as indicated especially by the highly-differentiated chloro- plastids, which not only assume strange and varied forms, but are much specialised internally, possessing proteid bodies (pyrenoids) around which the formation of starch is localised. In many members of the group, namely CONCLUSION 293 among the Desmids, the external form of the individual cells is also very complex. The sexual process is well marked, but of the simplest kind, consisting in the union of the contents of similar vegetative cells, with, at most, only the slightest indication of any difference between the sexes. The Conjugate, so far as we know, do not lead on to any of the higher groups of plants. In quite a different direction green unicellular organ- isms appear to have given rise to the remarkable group represented among our types by VaucJieria. Here the cell has become niultinucleate, and grows out into a large and in some cases complex thallus, but without dividing, so that we find a highly organised plant with non-cellular structure. VaucJieria itself, though its thallus is simple compared with that of the true Siphonese, has attained a very high level as regards reproduction, for its sexual organs are as sharply differentiated as in the higher multicellular plants. Differences of sex appear quite independently in many diverse lines of affinity, among which that repre- sented by Vaucheria is one of the most distinct. VaucJieria is also of great importance because it leads directly to the Fungi. There can be no doubt of the close relationship of such a Fungus as Pythium to such an Alga as VaucJieria ; in fact Pythium might fairly be described as a member of the Yaucheriaceae which has lost its chlorophyll. The most important difference is the disappearance of the spermatozoids, which are no longer differentiated in the Fungus, the male protoplasm being carried to the ovum by the fertilising tube. This change has been compared to the change from fertilisation by spermatozoids to fertilisation by a pollen-tube, in passing from Cryptogams to Phanerogams. In both cases the disappearance of motile male cells is correlated with the loss of aquatic environment. 1 The gradual extinction 1 See, however, p. 303. 294 STRUCTURAL BOTANY of zoospores among the allies of Pyfhium is due to similar causes, and has been fully traced above (p. 222). The Zygomycetes, as represented by Pilobolus, are more thoroughgoing Fungi than the Pytliium group, though they still show signs of affinity with non-cellular Alga?. Perhaps they may have diverged from the algal series at a somewhat different point. When we come to the higher Fungi, beginning with such forms as Sphcerotheca, we find it impossible, in the present state of our knowledge, to determine their relation to the algoid forms. The ascus has been regarded as corresponding to the sporangium of the Zygomycetes ; this may perhaps be true, but there is evidence, in Sphcerothcca and many other cases, for its being a sexually produced sporangium, comparable perhaps to that which is sometimes formed directly on the germina- tion of the zygospore or oospore of the Phycornycetes (see pp. 227 and 234). In any case the Ascomycetes have diverged very widely from the Phycomycete stock, as shown not only by their fructification but by their septate mycelium. This group reaches a very high development, the ascus-fruit having become a very complex structure in forms like Physcia. The Lichens, most of which are Ascomycetes, are the only Fungi which form a highly-organised aerial thallus. By their association with a green assimilating organism (the captive Alga) they have placed themselves on a level with the higher chlorophyll-containing plants. Among the Ascomycetes the conidial form of fruit, though often important, is subordinate. In the remaining groups the conidia (the origin of which could already be traced in the Phyconiycetes) have more or less completely displaced the sporangia, and assume very various forms, constituting the great means of propagation. The work CONCLUSION 295 of V. H. Blackman has shown that in the Uredineae, the secidium is to be regarded as a sexually produced fruit, comparable to an ascus-fructifi cation, while the spermo- gonia represent male organs which have lost their function. The uredospores, teleutospores, and sporidia, however, are all of the nature of conidia. The Uredineae are adapted to a strictly parasitic mode of life, and in habit differ greatly from the Basidiomy- cetes, especially if we consider a highly organised repre- sentative of the latter, such as the Mushroom. Yet the complex fructifications of these highest of the Fungi are nothing but elaborated conidiophores, and the basidium itself appears to be comparable to a teleutospore germi- nating in situ, a conclusion which is strikingly confirmed by the occurrence of nuclear fusion, alike in the teleuto- spore and the basidium. We see then that the Fungi form by themselves a highly-complex cycle of relationship, touching the lower Algae at one or two points, but otherwise distinct from the rest of the vegetable kingdom. Sexuality has proved to be far more general among this mass of saprophytic and parasitic organisms than was once supposed, though it often assumes a curiously reduced form comparable to the fusion of nuclei in apogamous Fern-prothalli. We must now retrace our steps to the Algae. The Red Seaweeds form a perfectly definite group by them- selves, without clear connections either below or above. Callithamnion is a fair average type ; some forms are simpler, especially in the development of the fruit, but even the simplest of the undoubted Florideae are highly- organised plants, quite unlike any other family. Many are much more complex than our type, but they are complex in their own peculiar way, and do not show any transition towards the higher groups of plants. STRUCTURAL BOTANY The sexual process in Florideas is quite peculiar among Algie, for no definite oospore is ever formed as the result of fertilisation. The whole carpogonium when fertilised remains in complete continuity with the tissues of the thallus, and sends out branches, which ultimately produce numerous spores, usually after various subsidiary cell-fusions have taken place. This continuity of the spore-fruit with the thallus completely severs the Florideae from the Bryophyta, with which there is other- wise a certain analogy, in so far as in both groups the result of fertilisation is a fruit. The Florideie are also remarkable for the entire absence of motile ciliated cells, a point in which they differ from the great majority of the Algai, though certain isolated groups, such as the Conjugate (which certainly have nothing to do with them), have the same peculiarity. We must await the results of further investigation before anything definite can be said as to the affinities of the Bed Seaweeds. The Phaeophyceae are also much isolated from other Algae, but they have more in common with Chlorophycece than is the case with the Red Seaweeds. Ciliated cells are almost universal throughout the group, though in the highest Brown Algte the Fucaceae they only appear as spermatozoids. Fertilisation, so far as known, always takes place outside the oogonium, a point in which these plants differ from the Green Algae. The Fucaceae are on a much higher level than the rest of the order, but transitional forms are not altogether absent. On the whole, we may say that the Brown Algae are a natural group, attaining very great complexity on their own lines, and not clearly related, either to the lower or higher families, though an affinity to some of the Green Algae is not altogether out of the question. CONCLUSION 297 We must now return to the Chlorophycea?. We have in Ulothrix a form scarcely more complex^ than Spirogyra, but evidently on quite a different line of descent. Here the reproductive cells are all ciliated and active. It is between certain of these ciliated zoospores that conjugation takes place, and not between vegetative cells, as in Spirogyra. Evidently the origin of sexuality was quite distinct in these two groups, for in Ulothrix we find its first stages, the sexual cells being still capable of germinating as neutral spores, if unable to conjugate. If we had been able to take a wider survey of the vegetable kingdom, we should have found evidence that this important step from asexual to sexual reproduction was made independently in many groups. Ulothrix is also interesting from the great difference in subsequent development between the asexual and the sexual swarm-cells. The asexual zoospore merely repro- duces the ordinary plant, whereas the zygospore gives rise to a dwarf individual quite distinct from the typical form. In fact we have here, coinciding with the first appearance of sexuality, some slight suggestion of regularly alter- nating generations. For this reason Ulothrix has been regarded as lying more or less in the direct line of descent of the archegoniate plants, in which regular alternation of generations is so strikingly a character. Of course this cannot be taken literally. No form now living can possibly be in the direct line of descent of any other form, any more than a man's cousin can be his ancestor ! One cousin, it is true, may more than an- other inherit the characteristics of some remote ancestor, and this is all we mean in speaking of lines of descent with reference to recent plants. In the case of Ulothrix* 298 STRUCTURAL BOTANY however, there is probably nothing more than a remote analogy with the course of development of Bryophyta. CEdogonium makes in some respects a great step in advance. Here the sexual cells are perfectly differ- entiated ; instead of two similar conjugating zoospores, we find a small moving spermatozoid and a large resting ovum. The casual difference in size sometimes observed in Ulothrix has here become fixed, and other differences are added. Evidently there is a more perfect adjustment of function here ; for while both partners are still on equal terms as regards the union of their nuclei, it is the female cell alone which assumes the whole duty of accumulating food-supplies for the next genera- tion. In order to do this most effectually, it remains at rest, in connection with the vegetative body of the plant. So far as the sexual division of labour is concerned, CEdogonium is as far advanced as any other plant, but its spermatozoids still show external signs of their origin from zoospores. The formation of dwarf male plants in some members of this group is without exact parallel in any other plants, and shows how far the specialisation of the sexes may go, even in simple organisms. This, however, is only a special case, for, as we have seen, there are some species of CEdogonium without dwarf males. The division of the germinating oospore into four swarm-spores is an interesting fact. These spores are just like the zoospores produced by the plant in its ordin- ary condition. The fact that they are always formed by the sexually produced resting - spore immediately on germination, indicates some approach to a regular alter- nation of sexual and asexual reproduction. In this re- spect, however, CEdogonium shows no advance on Ulothrix. CONCLUSION 299 In another genus of green fresh-water Algae, Colco- chccte, the ob'spore, while still enclosed in the oogonium, divides up into a group of cells, in each of which a zoospore is formed. This process shows a certain analogy with the formation of the simplest forms of sporogonium in the Liverworts, among which there is a genus (Eiccia) in which the fruit consists of nothing but a mass of spore mother-cells surrounded by an epidermis. There is, however, no trace of affinity between the Alga and the Hepatic, in which the simplicity of the fruit may be the result of reduction. In the Alga in question the asexual spores produced by the oospore are identical with those formed on the vegetative plant ; whereas in even the lowest Bryophytes the spores are never formed anywhere else than in the sporogonium. The sporophyte of the higher plants, whatever its origin may have been, is specially adapted to the formation of aerial, as distinguished from aquatic, spores. The spores of the Archegoniatae, from the lowest Bryophyta upwards, differ from those of any of the Algre in being almost always suited for dissemination by the air. The sporophyte which bears them is essentially the aerial generation, while the oophyte is dependent on water for the act of fertilisation. The difference is very well shown in Pellia, where the sexual generation is a low- growing thallus, keeping close to the damp ground, or even living under water, while the sporophyte grows up high into the air, exposing its spores as freely as possible, so that they may be dispersed by the wind. The result in other cases is attained in a different way, but the general rule holds good, that the function of the sporophyte the dissemination of spores requires exposure to dry air, while the most important function of the oophyte the act of fertilisation requires the presence of water. 300 STRUCTURAL BOTANY i Possibly the aerial, asexual spores of the Bryophyta may be homologous with the aquatic, asexual spores of the Green Algffi, but, if so, the former have been so completely modified that the homology can no longer be traced with any certainty. The origin of the Arche- goniatse must have taken place in enormously remote geological ages, when plants were first adapting them- selves to terrestrial life, and we cannot be surprised that no transitional forms connecting them with the Algae are known to us. It is commonly assumed that the Bryo- phyta represent an earlier stage of evolution than the Pteridophyta, but palreontological evidence lends no support to this assumption. So far as the vegetative structure of the thallus is concerned, Pellia is a very simple Liverwort ; others are more complex, but in all alike the archegonium and antheridium are totally different from the sexual organs of any Thallophytes. In the true Mosses both the sexual and asexual genera- tions are more highly developed than in the Hepaticse. Not only is the thallus replaced by a leafy stem (a change which is already accomplished in many Liverworts), but the anatomical structure is much more perfect, and a definite system of conducting tissue is differentiated. The sporophyte never develops into anything more than a fruit, yet it is anatomically the more elaborate of the two generations, as shown not only by the arrangements for dispersing the spores, but also by the vegetative tissues of the sporogonium, which have some resemblance to those of vascular plants, especially in the possession of true stomata. The mosses are highly organised plants in their own way, but appear to have no direct affinity with superior groups. If we found a wide gap to cross in passing from Algoe to Bryophyta, this is equally the case when we pass on to CONCLUSION 301 the Vascular Cryptogams. The latter, as at present existing, form three quite distinct stocks, Horsetails, Ferns, and Club Mosses, but in none of the three is there the least evidence for any near relationship to the Bryo- phytes. The sexual generation presents little difficulty ; for instance, the thallus of a Pellia may well be compared with the prothallus of a Fern. It is the sporophyte which is so different in the two classes. Speaking broadly, the asexual generation in the Bryophyta is always a fruit, while in Vascular Cryptogams it is always a plant. Nothing really approaching an intermediate form between the two kinds of sporophyte has ever been discovered, either among recent or fossil plants. Evidently these two great series the Bryophyta and the Pteridophyta diverged very far back indeed. There is no reason to suppose thafc the sporogoniuni of a Moss or a Liverwort ever became modified into the asexual plant of a Fern, Horsetail, or Lycopod. The two forms of sporophytes have probably always been different from the first origin of Archegoniatse onwards. There is direct geological proof of the enormous antiquity of the Vascular Cryptogams, which, together with certain Gynmosperrns, were well developed even in the Devonian period ; their origin is completely lost in the mists of palaeozoic anti- quity, and at present we are entirely without any facts which can throw light on the problem. The Bryophyta may also have been an ancient group, but at present there is little or nothing to show it. The Pteridophyta are much more highly organised than any of the previous groups ; their advance is entirely confined to the asexual generation, for the oophyte remains throughout at a very low level (below that of the simplest Bryophyta), and indeed degenerates as we reach the higher forms, No doubt the aquatic 302 STRUCTURAL BOTANY mode of fertilisation has kept back the oophyte, which throughout the Pteridophyta is always a damp-loving organism, and never completely adapts itself to terres- trial life. The asexual plant, on the other hand, has attained the greatest complexity, rivalling that of the Flowering Plants. In all the three main groups both homosporous and heterosporous forms have occurred, though among the Horsetails heterospory is now extinct. Most Ferns, like our type, are homosporous, though even here rudimentary purely male prothalli are not uncommon. Heterospory in plants of the Fern alliance is now limited to two small families. The origin of heterospory may be compared to the origin of sexual differentiation (see above, p. 298). Just as nutritive functions came to be assigned specially to the ovum, so here they are assigned specially to the female prothallus, which has to feed the embryo during its development. The male prothallus can safely be treated by the plant on strictly economical principles, for it has nothing more to do than to produce a few minute spermatozoids. We see this change beginning both among Ferns and Horsetails ; in the latter the smaller, less vigorous prothalli, are exclusively male. Then the difference extends further back. The insigni- ficant male prothallus only needs a small spore to grow from, while the female must be fed up from the first, and so a large spore, full of reserve food, is set apart for its formation. On the other hand, it is an advantage to have plenty of males in order to ensure fertilisation. Thus we get a large number of microspores forming small male prothalli, and a small number of megaspores forming large female prothalli. CONCLUSION 303 In Selaginella we have an extreme case of heterospory, with an enormous difference between microspores and megaspores. There is, however, another change going on concurrently with that just sketched. Both prothalli have become the mere bearers of sexual organs, and are losing the character of distinct organisms. Hence the female prothallus, though so highly developed relatively to the male, is itself reduced, as compared with the prothallus of a homosporous Cryptogam. The homologies between Selaginella and a gymno- epermous Flowering Plant, such as Picea, are quite clear, and have been fully demonstrated above (p. 31). It is doubtful, however, whether there is any near affinity between Selaginella and the Gyrnnosperms, which, as Paleeontological evidence indicates, probably sprang from the same stock as the Ferns. In any case the homologies hold good, whatever the particular heterosporous family or families may have been from which the Gymnosperms sprang. In the latter Class the female oophyte has lost its independence altogether, and never leaves the megaspore, which itself remains shut up in the sporangium. Ferti- lisation leads to the formation of a seed the character- istic structure of Flowering Plants, consisting of sporan- gium, prothallus, and embryo, united to form one body, and fed entirely at the expense of the parent sporophyte. Until the last few years there appeared to be a sharp distinction between Cryptogams and Phanerogams, in the method of fertilisation, for active spermatozoids were supposed to be peculiar to the former, the generative cell in Phanerogams being carried passively to the ovum by the growth of the pollen-tube. The researches of the Japanese botanists Hirase and Ikeno first broke 304 STRUCTURAL BOTANY down this distinction, for they proved that in some Gymiiosperms true motile spermatozoids are formed. The plants in which this im- portant discovery was first made are the Maidenhair Tree (Ginkgo biloba), a tree of an ancient type, nearly related FIG. ll8.-Ginl-go Ulola. K pair of generative Ponifpr-P cells in the pollen-tube. On the outer side of to tne T6, each cell a spiral coil is seen in connection with an( j (Jvcas TCVO- the nucleus. B, generative cell, showing the spiral spermatozoid in surface view, x 225. iUta (S66 p. 60). From a preparation by Dr. Hirase. (R. S.) j n both these cases (to which others have now been added) a pollen-tube is formed, which enters the nucellus, but does not reach the archegonia, and serves chiefly to anchor the pollen - grain in the right position. Two ^generative cells are formed in the usual way, but each of these gives rise to a large, spirally- coiled spermatozoid, FIG. 119. Cycas revoluta. Pair of genera- w ^h numerous cilia tive cells from a pollen-tube, showing iio r ^ i 1 Q the spirally - coiled spermatozoids, sur- (see rlgS. llo and. Lly, have been rounded by the protoplasm of the cell, The fine striation overlying the spiral coil indicates the cilia. x 190. From sketched tromtheongl- a preparation by Prof. Ikeno. (R. S.) na i preparations, kindly sent by Prof. Ikeno and Dr. Hirase). The spermatozoids break out from the pollen-tube, and by their own active CONCLUSION 305 movements swini to the necks of the archegonia, through a cavity, filled with sap, which is formed in the upper part of the nucellus. These plants thus present a beautiful transition between the Cryptogamic and Phan- erogamic methods of fertilisation. The male cells are conveyed for a short distance by the growth of the pollen-tube, but they have to complete the journey to the ovum by means of their own movements. A condition still more cryptogamic has just been discovered by Caldwell in the Cuban Cycad Microcycas calocoma, in which each pollen-tube produces no less than eight pairs of generative cells, giving rise to sixteen spermatozoids. This multiplication of sperm-cells, un- exampled among Seed-plants, is correlated with the immense number of archegonia (200 or more) produced in the embryo-sac of this strange plant. These remarkable discoveries confirm, in the most striking way, the theoretical conclusions at which Hofmeister arrived forty years ago. From Gryrnnospernis to Angiospernis is another great step, and here the gulf has not yet been bridged. In Angiosperms the female prothallus has almost disap- peared, and even the archegonia are no longer recognis- able. The embryo-sac (the equivalent of the megaspore) proceeds, after only a few preliminary divisions, to the formation of the ovum, and the development of the endosperm is dependent upon fertilisation, obviously an expedient arrangement, for it is not formed at all unless it is wanted. The processes in the pollen-grain are also simplified. The great characteristic of Angio- sperms is the high development of the flower and fruit. Not only does the megaspore remain enclosed in the sporangium or ovule, but the latter is itself enclosed in 20 306 STRUCTURAL BOTANY the ovary, so that fertilisation has to take place through the mediation of the stigma and style. The remarkable development of the floral leaves, characteristic of most Angiosperins, is connected with the occurrence of polli- nation by insects, for which so many Angiosperins are specially adapted. At present we are not in a position to determine either the relation of Angiosperins to Gymnosperms, nor that of Monocotyledons to Dicotyledons. The latter classes are mainly distinguished by vegetative char- acters, the reproductive phenomena being the same in both. On both these questions, however, we may hope for further light, especially from palceontological research, for the first appearance of Angiosperms falls within a geological period from which abundant fossil remains have come down to us. Dr. Wieland's discovery that the Bennettitese, Mesozoic plants allied to Cycads, produced elaborate hermaphrodite flowers, comparable to those of the Angiosperms, has already thrown an entirely new light on these questions. This brief summary has had one main object, to indicate the complicated and difficult nature of all ques- tions as to the affinities of plants. Naturalists in these days are agreed that the different forms of plants and animals arose from one another by descent. If this be so, a natural classification of the vegetable kingdom would take the form of a genealogical tree, just like the pedigree of a family. The genealogical tree of plants must have been complex beyond all power of conception, with boughs, branches, and twigs of every degree starting from each other at every possible point, some long and some short, a few reaching on to our own day, while the CONCLUSION 307 immense majority carne to an end in the long-past geologic ages. If we attempted to construct such a tree, say for our twenty-six types, almost every branch would be marked with a query. If the reader has gained some idea of the difficulty and complexity of the profoundly interesting problems which the comparative study of plants presents to us, the object of this concluding chapter will have been attained. INDEX ADVENTITIOUS SHOOTS OF Pelvetia, 193 ^cidium, 260 ^Ecidiospores, 261, 263 Agaricus campestris. See Mush- room Alga, 146-215 in Lichen, 214 - blue-green. See Cyanophycece - brown. See Pho&ophycece - green. See Chlorophycece red. See Floridece Alternation of generations in Bryo- phyta, 109, 126, 144 in Equisetum, 106 in Male Fern, 43, 73 in (Edogonium, 157 in Selaginella, 74 in Ulotkrix, 167 Amphithecium, 142 Audrospores, 154 Angiosperms, affinities of, 303 Annulus, 56, 83, 139 Antheridium, 24 of Callitkamnion, 206 of Equisetum, 101 of Funaria, 133 - of Male Fern, 43, 64, 65-67 of (Edogonium, 152 ofPellia, 114 ' of Pelvetia, 194 of Pythium, 225 of Selaginella, 24 of Splicer otheca, 237 of V'aucherm, 178 Anthoceros, 140 Apical cell of Callithamnion, 203 of Funaria, 132 of Equisetum (root), 95 of Equisetum (stem), 92 of Male Fern (root), 53 of Male Fern (stem), 52 of Pelvetia, 193 of Selaginella, 14 Apogamy, 76, 77, 263, 295 Apophysis, 137, 139 Apospory, 76, 77, 144 Apothecia, 248 Archegonium, 26 of Equisetum, 102 of Funaria, 135 of Male Fern, 43, 64, 65, 67-69, 77 ofPellia, 116 of Selaginella, 33 SOD Archegoniatse, 145, 299 Archesporium of Equisetum, 97 of Funaria, 138 of Male Fern, 58 of Pell 'ia, 121 of Selaginella, 16 Ascogonium, 237 Ascomycetes, 235 affinities of, 294 Ascophyllum, 200 Ascospores, 238, 248 Ascus, 235, 238, 240, 248 Aspidium Filix-mas, 37-77 Asplenium bulbiferum, 76 viviparum, 76 Atriclium, 129-131 Auxiliary cells, 208 310 INDEX Bacillus butyricut, 276 Bacillus, hay. See Bacillus sub- tilis Bacillus megatherium, 275 radicicola, 277 suUilis, 273-278 Bacteria, 215, 218, 272-279 affinities of, 291 Badhamia utricularis, 281-289 Barberry, 259, 264 Law, 264 Basidiomycetes, -269 affinities of, 295 Basidiospore, 269 Basidium, 269 Batrachospermum, 211 Bennettitese, 306 Berberis vulgaris. See Barberry Broom-rape. See Orolanche Bryophyta, 109-145 affinities of, 299 Bulboclicete, 159 Bud of Equisetum, 94 6aUithamnioncorymbosum,'2Q2-212 affinities of, 295 Calvptra of Funaria, 137 - of Pellia, 123 Capillitium, 286 Capsule of Funaria, 137, 138 Qi Pellia, 120 Carbon, assimilation of, 279 Carinal Cavities, 85 Carpogonium, 208 Carpospore of Callithamnion, 211 Cell-division in S2)irofjyra, 169 Central cylinder of Funaria, 129 Chlamydospores, 232 Chlorophycese, 147-183 Chlorophyll-granules of Vauclicria. 175 Chloroplast of (Edogonium, 149 of Pleurococcus, 182 of Spirogyra, 168 of Ulothrix, 160 Chromatophorc, 292. Sec Chloro- plast Cilia, 21 of Bacteria, 294 - of Cladothrix, 279 - of zoospores in Ectocarpus, 186 Cilia of spermatozoids of Equisetum, 101 of Funaria, 134 - of Male Fern, 65 of zoospores of (Edogonium, 150 of spermatozoids of Pellia, 116 of Pelvetia, 195 - of zoospores of Pythium, 221 - of spermatozoids of Selagin- ella, 21 of zoospores of Ulothrix, 161 of Vaucheria, 176 Circinate vernation, 41 Cladothrix dichotoma, 278-279 Club-moss, 1 Coleoclicete, 299 Collateral bundles in Eguisctum, 83 Columella of Funaria, 138 of Pilobolus, 231 Comatriclia obtusata, 286 Commensalism, 247 Compound leaf, 40 Conceptacles, 190, 194 Cone of Equisetum, 82, 96 of Sclagiiiclla, 5 Conferva, 159 Conidia, 224, 234, 239, 254, 271 Conidiopliores, 239 Conjugatse, 173 affinities of, 292 Conjugation of Ectocarpus, 187 of Pilobolus, 233 of Spirogyra, 170 of Ulothrix, 163, 164 Cortex of Equisetum, 87 of Fnnaria, 129 of Selaginella, 9 Crenothrix, 278, 279 Cryptogams, 2 Cuscuta, 216 Cyanophycere, 212-215 affinities of, 291 Cycadeoe, 35 development of endosperm in, 34 Cycas revoluta, spermatozoids of, 304 Cyst, 284 Cystocarp of Callithamnion, 211 Cystococcus, 244 INDEX 311 DAMPING OFF OF SEEDLINGS, 219 Dehiscence of Sporangium of Equi- setum, 99 of Male Fern, 59 of Sclaginella, 20 Desmids, 169, 172, 293 Dichotomy, 14, 113, 190, 193 Didymium dijfbrme, 287, 288 Dicecious prothallus, of Equisetum, 113 Discomycetes, 240 Diseases caused by Fungi, 217 Dodder. See Cuscuta Dry-rot, 217 Dwarf-males of (Edogonium, 154 Ectocarpus ovatus, 187 siliculosus, 185-189 Elaters of Equisetum, 98 of Pellia, 121-124 Elodea (Part I., 42), 23 Embryo of Equisetum, 104 of Male Fern, 72 of Sclaginella, 29 Embryology, 28 Embryo-sac of Picca, 33 Endodermis, double, of root of Equisetum, 91, 96 of stem of Equisetum, 85 of root of Male Eern, 51 of stem of Male Fern, 47 of Sclaginella, 10 Endosperm, development of, in Angiosperms, 75 ; in Cycadea*, 34 Endospores, 275 Endothecium, 142 Enzymes, 220 Epi basal, 72 Epidermis of Equisetum, 89 - of Funaria, 128, 138 of Male Fern, 49, 50 of Pellia, 112 of Selayinella, 13 Equisetum arvense, 78-108 affinities of, 302 limosum, 85, 99 maximum, 100, 105 scirjioides, 106 variegatum, 85 Enjsiphe, 236 Eye-spot, 162 FERMENTATION, 273 Ferments. See Enzymes Fern, Bracken, 42 Filmy, 38, 42, 44, 76 Male, 37-77 Ferns, affinities of, 302 Tree, 38, 42 Fertilisation, 28, 303 of Calliihamnion, 209 of Equisetum, 104 of. Funaria, 136 - of Male Fern, 69 of (Edogonium, 155 -of Pellia, 119 ofPclvctia, 198 - of Physcia, 251 of Pythium, 226 - of Selaginella, 28 of S'phxrothcca, 237 - of Uredinece, 263 of VaucJieria, 180 Flagcllata, 292 Floridere, 201-212 affinities of, 295 Foot in Equisetum, 105 in Funaria, 142 in Male Fern, 72 - in Pellia, 121 - in Sclaginella, 30 Fossil Equisetaceoe, 107 - Lycopods, 37 Frond of Male Fern, 40 Fruit of Ascomycetes, 240 of Callitliamnion, 210 of Funaria, 137 of Pellia, 120 Fucacese, 184-189 Funaria hygromctrica, 120-145 Fungi, 216-271 affinities of, 293 GAMETOPIIYTE. See OopJnjtc Germination of cecidiospores, 262 - of ascospores of Physcia, 249 of Splicer otheca, 239 of Bacteria, 275 - of carpospores of Callllh- * union, 212 - of conidia, 239 - of Ectocarpus, 186, 188 - ofmegasporesof Selagin- 312 INDEX Germination of microspores of Selaginetta, 20 of oospore of (Edogonium, 156 of oospore of Pelvetia, 199 of Pythium, 227 Vauclieria, 181 of spores of Equisetum, 99, 100 of Male Fern, 43 of Mushroom, 270 of Myxomycetes, 287 of Pildbolus, 232 - of sporidia of Puccinia, 259 of telentospores of Puccinia, of Callith- 258 of tetraspores amnion, 205 of uredospores of Puccinia, 255 of zoospores of (Edogonium, 152 of Pythium, 221 of Ulothrix, 162 Vaucheria, 177 of zygospore of Pilobolus, 234 of Spirogyra, 173 of Ulothrix, 165 Germ-pores, 255, 258, 261 Gills. See Lamella G-inkgo biloba, spermatozoids of, 304 Gengrosira condition of Vauclieria, 181 Gonidia of Lichen, 243 Gro\v ing- point of Equisetum, 92-96 of Funaria, 132 intercalary, 186 - of Male Fern (root), 53 - of Male Fern (stem), 52 of PeH la, 113 of Selaginella, 3, 14 )3 Gymnosperms, affinities of, 30- comparison with Selaginella, 31 spermatozoids in, 29, 32, 75, 107, 303 HAUSTORIA OF Puccinia, 254 of S2)h&rotheca, 236 Hermaphrodite, 35, 306 Heterocysts, 214 Heteroecism, 264 Heterosporous Cryptogams, 302 Equisetaceee, 104, 107 Hofmeister, 36, 305 Homology, 36 Hop, mildew of, 236 Hormogonia, 214 Horsetails. See Equisetum Hymenium, 240, 248, 261 Hymenomycetes, 269 Hyphse of Lichen, 242 of Mushroom. 266 of Pildbolus, 229 of Puccinia, 254 of Pythium, 219 of Sphcerothcca , 236 Hypobasal, 72 Hypocotyl, 30 INDUS i UM, 43 Intercalary growth, 185 Internode, 84 Involucre of Pell La, 117 Jsoetes, 4 KRAKATOA, 60 LAMELLAE, 267 Laminar ia, 189 Leaf, compound, 40 Leaflets of Male Fern, 40-. Leaf-sheaths of Equisetum, 80, 90, 92 Leaf-trace bundles of E'juisctum, 83 of Funaria, 130 of Selaginella, 1 1 Leaves of Equisetum, 80, 90 of Funaria, 128, 130 of Male Fern, 40 of Selaginella, 3, 12 Legummoste, root-tubercles of, 277 Leocarpus rcrnicosus, 285 Lepidodendron, 4, 10 Lichen-Fungi, 247 Lichens, 241 synthesis of, 244 Light, influence of, on Bacteria, 276 on Myxomycetes, 283 on zoospores of CEdo- gonium, 151 - on zoospores of Ulothrix, 161 INDEX 313 Ligule of Selaginella, 4 Linnaeus, 2, 159 Litmus, 242 Liverworts, 110-126 affinities of, 299 Lycopodium, 1 MACROSPOEANGIUM. See Mega- sporangium Macrospore. See Megaspore Male Fern. See Aspidium Filix- mas Malic Acid, 70 Marchantia, 118 Megasporangium of Selaginelld, 7, 19 Megaspore of Selaginella, 7, 19 germination of, 24 Microcycas calocoma, 305 Microcysts, 288 Microsporangiuni of Selaginella, 6, 17 Microspores of Selaginella, 6, 18 germination of, 20 Mildew of hop, 236 of wheat, 251, 256 Monarch roots, 13 Moncecism of Funaria, 132 Monostelic, 11, 44 Monotropa, 216 Mosses, 110, 126-145 affinities of, 300 Mould, 217 Movements of Bacteria, 274 of Myxomycetes, 281, 287 of spermatozoids, 22, 67, 70, 136, 155, 198 of zoospores, 151, 161, 177, 186, 221 Mucor, 234 Muscinese. See Bryophyta Mushroom, 266-271 Mycelium, 219, 229, 236, 254, 259, 266 Myxamoebse, 288 Myxomycetes, 280-289 affinities of, 290 Nemalion, 209 Neottia nidus-avis, 216 Nitrates, 277 Nitrogen, assimilation of, 277 Nostoc, 212-215 - affinities of, 291 Nucleus of Bacteria, 274 of Cyanophycere, 213 - of Mushroom, 267, 269 of Myxomycetes, 284, 286 - of CEdogonium, 149 -- of Pelvetia, 196 - of Pytliium, 219 - of Sirirogyra, 169 of Vaucheria, 175, 180 OAR WEED. See Laminar ia CEdogonium, 147-159 - affinities of, 298 - ciliatum, 148 Oidium condition, 232 Oogonium of CEdogonium, 153 of Pelvetia, 195 - of Pythium, 224 - of Vaucheria, 178, 179 Oomycetes, 228 Oophyte of Equisetum, 100 of Funaria, 128 of Male Fern, 43 - ofPellia, 110 Oospore of CEdogonium, 156 - of Pelvetia, 200 - of Pythium, 226 - of Vaucheria, 181 Operculum of Funaria, 139 Orobanche, 216 Osmunda, 61 Ovum, 32 - oi Equisetum, 104 - of Funaria, 135 - of Male Fern, 69 - of CEdogonium, 153 - ofPellia, 117 - of Pelvetia, 197 - of Pythium, 225 of Selaginella, 26 - of Vaucheria, 180 41, 48 Palisade tissue in Funaria, 139 -- in Male Fern, 49 Palmella condition of Ulothrix, 167 Paraph yses of Funaria, 134 of Lichen, 248 314 INDEX Puraphyses of Mushroom, 269 -otPelvetia, 194 Parasites, 216, 272 Pellia epiphylla, 110-126 affinities of, 299 Peltate scales of Equisctum, 82, 97 Pclvetia canaliculata, 189-201 Pericycle, 47, 51, 87 Peridium, 261 Periplasm, 225 Peristome, 141 Perithecium, 239 Peronospora dcnsa, 223 Lactucce, 224 nivea, 223 - Radii, 224 Phseophycese, 183-201 affinities of, 296 Phseozoosporese, 184 Phloem of Equisettcm, 87 of Male Fern, 47 of Selaginclla, 10 Phycomycetes, 229 Phragmidium Rubi, 263 Physcia parietina, 240-253 pulverulenta, 251 affinities of, 294 Phytophthora infestans, 223 Picea, 26, 33 Pileus, 266 Pilobolus crystallinus, 228-234 affinities of, 294 Pinna, 40 Pinnate, 40 Pith in Equisctum, 85 Pits of Callithammon, 203 Placenta, 55 Planogametae, 166, 187 Plasmodium, 281, 289 Pleurococcus vulgar is, 292 Polystelic, 44 Procarpium, 208 Promycelium, 258 of Equisetum, 100 - of Male Fern, 43, 62-64 - of Sdaginella, 20, 33 Protonema, 143, 144 Protoxylem of Equise/.um, 86 - of Male Fern, 47, 51 - of Selaginella, 10, 11 Psendopodia, 282 Pteridophyta. See Vascular Cryp- togams Pteridosperms, 37 Puccinia graminis, 253-265 Malvacearum, 270 Pyrenoids of GEdogonium, 149 of Spirogyra, 164 of UZothrix, 160 PytJdum Baryanum, 218-228 affinities of, 293 RACHIS, 40 Ramenta, 41, 48 Receptacles, 190, 194 Respiration of Bacteria, 276 Rhizoids of Funaria, 127, 131 - of Lichen, 242 of Pellia, 112 of S/iJicerotheca, 236 of Vaucheria, 175, 178 Rhizophores, 4, 13 Puccia, 299 Root of Equisctum, 80, 90, 95 of Male Fern, 42, 50 - of Selaginella, 13 Root-cap of Equisetum, 95 of Male Fern, 53 of Selaginclla, 15 Root-hairs of Equisctum, 91, 100 - of Male Fern, 52, 61 -of Pellia, 111 of Selaginella, 28 Rootlet of Equisctum, 96 -of Male Fern, 54 Root-tubercles of Leguniinosse. 277 Rust, 253 SAPROPHYTES, 216, 271, 272 Saryassum, 201 Scalariform, 12, 47 Schizophyta, 292 Sclerotium, 284 Seaweeds, 146, 183, 201 Selaginclla, 1-37 - affinities of, 303 Seta of Funaria, 137 of Pellia, 120 Sexual generation. See OopJiytc Sieve-Plates of Pelvetia, 191 Siphoneoe, 174, 177, 181 - affinities of, 293 INDEX 315 Soredium, 251 Sorus, 43, 55 Spawn of Mushroom, 266, 270 Spermatium, 206, 250, 263 Spermatozoids of Equisetum, 101 of Funaria, 134 of Gymnosperms, 29, 32, 75, 107. 303-305 - of Male Fern, 65 - of (Edogonium, 152, 155 -of Pellia, 116 - of Pelvetia, 195 - of Selaginella, 21 - of Vauchcria, 179 Spermogonia, 250, 262 Sphcerotheca, affinities of, 294 Castagnei, 235-240 - pannosa, 236 Spirogyra, 168-174 - affinities of, 292 Sporangium of Ectocarpus, 186 - of Equisetum, 82, 96 of Male Fern, 42 - Myxomycetes, 285 - of Pilobolus, 230 plurilocular, 187 of Pythium, 221, 227 of Selaginella, 6, 16 - unilocular, 186 Sporangioles, 234 Sporangiophores of Equisetum, 82, 96 Spore mother-cell, 17 sac of Funaria, 138 Spores of Bacteria, 274 of Callithamnion, 204, 211 of Ectocarpus, 186 of Equisetum, 98 of Funaria, 141, 142 of Male Fern, 42, 56 of Mushroom, 269 of Myxomycetes, 285 of (Edogonium, 150 of Nostoc, 214 of Pellia, 124 of Pelvetia, 200 - of Physcia, 245, 248 of Pilobolus, 230 ofPuccinia, 254, 256, 260 of Pythium, 224 of Selaginella, 6. 16 Spores of Sphcerotheca, 238 of Spirogyra, 172 swarm, 287 of Ulothrix, 166 - of Vauchcria, 176 Sporidia, 259, 269 Sporogonium of Funaria, 137 - of Pellia, 120 Sporophyll, 8, 16 Sporophyte of Equisetum, 96 of Funaria, 137, 142 of Male Fern, 43 of (Edogonium, 157 -of Pellia, 120, 121 of Ulothrix, 167 Stele, 8 - of Male Fern, 46 - of Selaginella, 8, 11 Stem of Equisetum, 83 of Funaria, 128 of Male Fern, 38, 40, 44 of Selaginella, 8 Sterigmata, "259, 269 Stipe, 266 Stomata of Equisetum, 89 of Funaria, 139, 140 of Male Fern, 50 of Selaginella, 13 Suspensor of Selaginella, 29 Swarm-cell. See Zoospore, and Spores, swarm Symbiosis, 247, 253 TAPETUM of Equisetum, 97 of Male Fern, 58 of Selaginella, 16 Teleutospores, 256, 263 Tetrasporangium, 204 Tetraspores, 204 Thallus, 110, 185, 190, 202, 203, 242 Thecium, 248, 249 Toadstools, 266 Tracheides of Equisetum, 86 - of Selaginella, 12 Trama, 268 Trichogyne, 208, 251 Trichophore, 208 Ulothrix zonata, 159-107, 188 - affinities of, 297 316 INDEX Uredinese, 253-265, 270 affinities of, 295 Uredo, 255, 265 Uredospores, 254, 255, 263 VACUOLES, gas, 214 pulsating, 161 Vallecular cavities, 85 Vascular bundles of Equisetum, 86 Cryptogams, 1-108 affinities of, 293 Vauckeria, 174-182, 218 affinities of, 293 Veil. See Velum Velum, 269 Venation of Male Fern, 41 Vernation, 41 Vessels of Ferns, 47 WATER-FERNS, 34 Wheat, rust and mildew of, 253 Wood of Equisetum, 86 of Male Fern, 46 of Selaginella, 10 XYLEM. See Wood YEAST, 261 ZOOGLCEA, 274 Zoospores of Ectocarpus, 186, 187 of (Edogonium, 151 -ofPythium, 221, 227 of Ulothrix, 160, 165 - of Vaucheria, 176 Zygomycetes, 228 affinities of, 294 Zygospores, 164 of Ectocarpus, 189 of Pilololus, 233 of Spirogyra, 171 of Ulothrix, 164 Printed by MOKRISON & GIBB LIMITED, Eih'nbitryh Large crown Svo, cloth. With about 190 Illustrations. One Volume, price, 10s. 6d. net. Two Volumes : Vol. I price 6s. net. Vol. II. price 5s. net. STUDIES IN FOSSIL BOTANY BY D. H. SCOTT, M.A., LL.D., F.R.S., LATELY HON. KEEPER OF THE JODRELL LABORATORY, ROYAL GARDENS, KEW. SECOND EDITION, REVISED. With many New Illustrations. " It is a great gain to botanists to have in our language so admirable a presentation of the important facts connected with the structure and organisation of the palaeozoic plants." Journal of Botany. "The book before us is a valuable contribution to the literature of the subject. It is one which no student of botany can afford to disregard, and its characteristics may be briefly epitomised as severe accuracy, coupled with clearness of description." Pall Mall Gazette. "Dr. Scott does not give a table of genealogy of the plants with which he deals, but contents himself with attempting to unravel apparent from real points of likeness, and thus clearing the tangled ground to some extent for efforts which will be more effectively made when the rapidly growing knowledge of the past has arrived at greater finality. It is by careful studies such as those which are detailed in the volume before us that such a happy result will be obtained. " Dally Chronicle. " An excellent book. To the botanist it will appeal as a thoroughly sound and scientific piece of exposition, which is a considerable con- tribution to a recent and important branch of the subject." Spectator. Published by ADAM & CHARLES BLACK, SOHO SQUARE, LONDON. Crown 8vo, cloth, price 3s. 6d. With 118 Illustrations. AN INTRODUCTION TO STRUCTURAL BOTANY. BY D. H. SCOTT, M.A., LL.D., PH.D., F.R.S., F.L.S., F.G.S., LATELY HONORARY KEEPER OF TUB JODRELL LABORATORY, ROYAL BOTANIC GARDENS, KEW. PART I. FLOWERING PLANTS (Sixth Edition). Illustrated with 118 Figures. SOME PRESS OPINIONS. " In noticing elementary books in these pages, we have lamented nothing more than the want of a book which should do for structural botany what Prof. Oliver's 'Lessons' has long done for the study of the principal natural orders. It seems hard to realise that this grievance is no more, and that we possess such a book in our own language, and a book that no honest critic will fail to assess at a higher value than any known book in any language that has the same scope and aim. Nothing could well be more plain and simple, or more severely accurate or better judged from beginning to end." Journal of Botany. 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BLACK, 4 SOHO SQUARE, LONDON, W. Crown Svo, cloth, price 3s. 6d. With 120 Illustrations. AN INTRODUCTION TO STRUCTURAL BOTANY. BY D. H. SCOTT, M.A., LL.D., Pn.D., F.R.S., F.L.S.,F.G.S., LATELY HONORARY KEEPER OF THE JODRELL LABORATORY, ROYAL BOTANIC GARDENS, KEW. PART II. FLOWERLBSS PLANTS (Fifth Edition). Illustrated with 120 Figures. SOME PRESS OPINIONS. "The second part of Dr. Scott's admirable manual of Structural Botany is now before us. It consists of a most carefully worked out history of the structure of flowerless plants, which constitute more than half of the vegetable world. . . . Dr. Scott's position in the Royal Gardens at Kew will give a tone of authority for this book, which will carry considerable weight with its readers. It is one which cannot fail to hold its place among the most thoughtful of students of botany." Science Gossip. "We have nothing but praise for this neat little volume. With its companion (Part I. Flowering Plants) it forms as good an introduction as one can imagine, in our present knowledge, to the study of the plant-world of to-day. . . . We only fear lest, amid such a wealth of illustration, the student may deem an examination of the actual specimens to be un- necessary." Guardian. "Students of botany will welcome the second part of Dr. D. H. Scott's 'Introduction to Structural Botany 'which has just appeared. . . . The language is clear and not unnecessarily technical, which is a great advan- tage to a beginner. We believe many are deterred from the fascinating study of botany by the extremely numerous technical terms with which so many manuals abound. . . . We do not remember reading a clearer description of the growth of ferns than that in the chapter on vascular cryptogams." Westminster Preview. " Some time ago we had occasion to notice in favourable terms the first part of this little treatise devoted to the flowering plants. We can speak no less favourably of the present instalment. It is a thoroughly original book, and one well thought out. ... To those who desire to get a clear connected account of the distinctive characteristics and life-history of the great groups of the vegetable kingdom, we most heartily commend Dr. Scott's little volume." Gardeners' Chronicle. Published by A. & C. BLACK, 4 SOHO SQUARE, LONDON, W. SPECIMEN PAGE FROM " FLOWERLESS PLANTS." 40 STRUCTURAL BOTANY gradual strengthening of the growing -point. In this respect, though in no other, Ferns resemble the Mono- cotyledons (see Part I. p. 173). The leaves, often called Fronds, are of very large size, one to three feet long, and much subdivided (see Figs. 17 and 18). This is the first example of a com- pound leaf we have had. A compound leaf is one in which the lamina or blade is completely sub- divided, so that its several parts, called leaflets, re- semble distinct leaves. The leaves of the Male Fern are pinnate, that is, the main stalk r racJiis, bears two rOWS of leaf- i , winner one iets > or P lumL > ( row on each side (see Fig. 18). The pinnae are often subdivided them- selves in the same way, and then the whole leaf is eaid to be bi-pinnate. In the specimen figured, the pinnae are deeply lobed, but not completely subdivided. Each lobe, like that shown singly in Fig 18, B, may be called a segment. The segment is traversed by i * r i -i? j i\ 18. A, leaf of Male Fern (much reduced). B, part of a fertile pinna seen from below ; a, raclris ; s, sorus. Magnified. (After Luerssen.)