FLOWER LESS PLANTS
R2.S753
AN INTRODUCTION
TO
STRUCTURAL B OTAR Y
PART II
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AN INTRODUCTION
TO
STRUCTURAL BOTANY
PART II
FLOWERLESS PLANTS
By DUKINFIELD HENRY SCOTT
M. A. , PH.D. , F. II. S., F.L.S., F.G.S.
HONORARY KEEPER OF THE JODRELL LABORATORY, ROYAL GARDENS, KEW
FORMERLY ASSISTANT PROFESSOR IN BIOLOGY (BOTANY) AT THE ROYAL COLLEGE OF SCIENCE, LONDON
LONDON
ADAM AND CHARLES BLACK 1896
All Rvjhts Reserved
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
PBEFACE
viii
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. 110—114, from Mr. Arthur Lister’s Monograph of the Mycetozoa. As in Part I., the figures signed K. 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 Fucacese.
D. H. SCOTT.
October 12, 1896.
CONTENTS OF PART II
4
PAGE
Preface ........ vii
CHAPTER I
THE VASCULAR CRYPTOGAMS . . 1
Type IV
Selaginella Keaussiana ..... 1
I. External Characters —
A. Vegetative Organs ..... 2
B. Reproductive Organs ..... 6
II. Internal Characters of the Vegetative Organs , . 7
а. The Stem ...... 8
б. The Leaves . . . . .12
c. The Rliizophores and Roots . . .13
d. Growing Points and Mode of Branching —
a. The Stem . . . . .14
(3. Rliizophores and Roots . . .14
III. Reproduction and Life-History —
a. The Sporangia and Spores . . .16
b. Germination of the Microspores . . .20
c. Germination of the Megaspores . . .24
d. Fertilisation and Embryology . . .28
Comparison between Selaginella and the Gymnosperms . 31
ix
X
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
/3. Other Tissues of the Stem . . .48
2. The Leaf ...... 48
3. The Root ...... 50
4. The Growing Points —
a. The Stem ..... 52
/?. 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
F. Comparison between the Life-History of Ferns and
that of the Higher Plants . . .73
Type VI
The Field Horsetail (Equisetuvi arvense) . . .78
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 — - PAGE
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 Arcliegonia . . . . .102
3. The Embryo . . . . . .104
Summary ....... 106
CHAPTER II
THE BRYOPHYTA . . .109
A. THE LIVERWORTS
Type VII
Pellia epiphylla —
1. The Tliallus ...... 110
2. The Sexual Organs —
a. The Antheridia ..... 114
b. The Arcliegonia . . . . .116
c. Fertilisation ..... 119
3. The Sporophyte or Fruit —
a. External Characters .... 120
b. Development . . . . .121
Summary . . . . . . .125
xn
CONTENTS
B. THE MOSSES
Type VIII
PAGE
Funaria hygrometrica . . . . . .126
1. The Leafy Stem —
a. Structure . . . . . .128
b. Apical Development . . ... 1 32
2. The Sexual Organs . . . . .132
3. The Sporophyte Generation (Fruit) . . . 137
4. Germination of the Spores .... 142
Summary ....... 144
CHAPTER III
THE ALGTE . . .146
A. THE CHLOROPHYCEA3
Type IX
(Edogonium —
1. Structure . . . . .147
2. Reproduction —
a. Asexual ...... 150
b. Sexual ...... 152
Type X
Ulothrix zonata—
1. Structure . . . . . .159
2. Reproduction ...... 160
Type XI
Spirogyra —
1. Structure ...... 168
2. Reproduction . . . . . .170
CONTENTS
xm
Type XII
PAGE
Vaucheria . . . . . . . .174
1. Structure ...... 175
2. Reproduction . . . . . .176
Type XIII
Pleurococcus vulgaris . . . . . .182
B. THE PHAiOPHY CEAS
Type XIV
Eotocarpus siliculosus —
1. Structure ...... 185
2. Reproduction . . . . . .185
Type XV
Pelvetia canaliculata ...... 189
1. Structure . . . . . .190
2. Reproduction . . . . . .194
O. THE FLORIDEAE . . .201
Type XVI
Callitiiamnion corymbosum —
1. Structure ...... 202
2. Reproduction —
a. Asexual ...... 204
b. Sexual ...... 206
D. THE CYANOPHYCEAE
Nostoc
Type XVII
212
XIV
CONTENTS
CHAPTER IV
PAGE
THE FUNGI . . .216
Type XVIII
Pythtum Baryanum ...... 218
1. Structure . . .. . . .219
2. Reproduction —
a. Asexual . . . . . .221
b. Sexual ...... 224
Type XIX
PlLOBOLUS CRYSTALL1NUS ...... 228
1. Structure ...... 229
2. Reproduction —
a. Asexual ...... 230
b. Sexual ...... 233
Type XX
Sphxerotheca Castagnei —
1. Structure . . . . . 235
2. Reproduction ...... 237
Type XXI
Physcia parietina ...... 240
1. Structure and Mode of Life .... 241
2. Reproduction . . . . . .247
Type XXII
Puccinia GRAMINIS ...... 251
Type XXIII
The Mushroom ( Agnricus campestris) .... 264
CONTENTS
xv
CHAPTER V
PAGE
THE BACTERIA . . .270
Type XXIV
Bacillus subtilis . . . . . .271
Type XXV
Cladothrix dichotoma ...... 276
CHAPTER VI
THE MYXOMYCETES . . .278
Type XXVI
Badhamia utricularis —
1. The Plasmodium ..... 279
2. The Sporangia and Spores .... 283
CHAPTER VII
CONCLUSION . . .288
Index
305
STRUCTURAL BOTANY
part II
FLOWERLESS PLANTS
♦
CHAPTER I
THE VASCULAR CRYPTOGAMS
Type IV
Selaginella Kraussiana
Selaginella 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 hillsides, in mountainous districts. Some of the tropical species are universally grown in hothouses, and are often popularly called Lycopodium, though the real Lycopodium, or Club-moss, is quite a different 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
it*
1
2
STRUCTURAL BOTANY
mosses, however, which we shall describe later on, they have nothing whatever to do.
Selaginclla is taken 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 1 now living. In other respects, such as its vegetative ana- tomy, the structure of Selaginclla 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 Organs
Selaginclla 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 Linneus, who lived in the eighteenth century. It implies that in 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.
- 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
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
4
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 stem. The leaves on the lower side are much larger than those on the upper (see Fig. 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, Iso'etes, and, unimportant as it seems, is a very ancient character, for it is found in certain fossil plants of the coal period ( Lepidodcndron ).
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,
Fig. 2. — Sclaginella Kraussiana ; young plant, m, megaspore still in connection with plant ; c, two cotyledons ; r, main root ; ?■', first lateral root. Note the two kinds oflraves. Magnified 6 diameters. (R. S.)
THE VASCULAR CRYPTOGAMS
5
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.
These 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 unbranched stems (S. simplex) ; others have climbing stems, which ascend tall trees, and may attain a length of 6 0 feet (S. cxaltata ) ;
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.
Fig. 3. — Selaginella hclvctica, showing procumbent stem and two fertile spikes, r, rliizopliore ; sp, sporangia. Slightly magnified. (After Dodel-Port.)
6
STRUCTURAL BOTANY
Kraussiana and S. hclvetica (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.
B. Reproductive 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 (25 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 (microspores), compar- able in size to pollen- grains. The other
1 The word spore is applied to any single cell which becomes isolated from the parent plant for reproductive purposes ; of. Part I. p. 113.
Fig. 4. — Selaginella hclvetica ; part 01 longitudinal section through spike, showing two sporophylls. via, mega- sporangium dehiscing ; three out of four megaspores visible ; note abortive mother - cells ; mi, microsporangium with numerous mierospores. Magnified about 15 diameters. (After Dodel-Port.)
THE VASCULAR CRYPTOGAMS
7
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.
The development and structure of the sporangia will be further described below.
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 so it was with our other types of flowering plants. With the Cryptogams the case is quite different. Not only is the anatomy of Selaginclla peculiar to that one genus among plants now living, but the variation of
Fig. 5. — Selaginclla spinosa ; fertile spike. Magnified diameters. (W. C. W.)
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 phloem 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.
Fig. 6. — Selaginella spinosa ; transverse section Oi stem, cp epidermis ; en, trabeculae 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 seen 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 Fig. 5, Part I.
1 In the trailing part of the stem of S. spinosa it ajipears that the protoxylem is central.
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.
As we have already mentioned, the structure of this species is exceptional in the genus ; its interest lies chiefly in the resem- 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 monostdic ; 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- Fig. 7. — Selarjinclla Kraussiana; dia- i ,■«, , grammatie longitudinal section of stem,
ly dlffeient anailge- si, the two steles, anastomosing at base ment. In this species, of branches; 1. 1, leaf- trace bundles, only . , , . , shown in upper part. (After Harvey
the stem is traversed 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
Fig. 8. — Sclac/inella Kraussiana ; part of transverse section of stem showing one stele, x, the wood ; px, protoxylem ; ph, phloem ; pe, pericycle ; cn, endodermal cells forming the whole or part of trabecula; ; c, inner layers of cortex'. Magnified about 100 diameters. (After Harvey Gibson.)
of the stem of the genus Selaginella may be summed up as follows : —
(1) The stele con- tains no pith.
(2) The xylemand phloem of the stele are not arranged in separate bundles.
(3) The xylem is usually developed centripetally.
(4) In many species there is more than one stele.
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 scalarifonn (see Fig. 23), 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 Selaginella true vessels, arising by cell-fusion, occur in the wood.
b. The Leaves
The leaves of Selaginella 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 phloem. Around the whole is a bundle-sheath.
The mesophyll 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 are usually found on the under-side of the leaf only, and especially 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 Selaginella and its allies. It is a very ancient character, for the rootlets of the fossil relations of Selaginella, which lived in the Carboniferous epoch, had an almost identical structure. The stele of the root, like that of the stem, is surrounded by an intercellular space.
14
STRUCTURAL BOTANY
d. Growing Points and Mode of Branching
a. The Stem
The growing-point of the stem in Selayinella differs from that in flowering plants in the fact that the meristem at the apex shows no trace of differentiation into distinct layers giving rise to definite tissues. In many species, among which is S. Kraussiana, we find at the apex a single cell, larger than its neighbours, from the divisions of which all the new tissues and organs are ultimately produced. The presence of this single apical cell, as it is called, is very general, though not universal, among Cryptogams, and contrasts sharply with the small-celled meristem characteristic of the flowering plants. We shall, however, have better opportunities of studying growth by an apical cell when we come to other groups of Cryptogams, so we will defer the further consideration of this subject.
The tranches arise laterally by the growth of a group of cells just below the apex ; the main axis and its lateral branch, however, often develop so equally that the stem appears to be forked. The branching of Selayinella may be described as an apparent bifurcation or dichotomy. True dichotomy only exists when the growing-point itself is equally divided. The branches of Selayinella, unlike those of flowering plants, are not axillary.
The development of the leaves proceeds in the same way as in the Phanerogams (see Part I. pp. 84, 169, 259).
/3. Ehisophores and Boots
In those species which have special rhizophores, the latter may either grow by means of an apical cell, or
THE VASCULAR CRYPTOGAMS
15
may have a small-celled meristem more like that of the higher plants. As soon as the rhizophore reaches the ground, roots begin to grow out at its end. The growing-points of these roots arise endogenously , i.e. from the interior of the tissue at the end of the rhizophore. The roots themselves, which grow by means of an apical cell, differ from most other roots in their manner of branching. The rootlets arise quite close to the growing-point, where they are only covered by the root- cap of the parent root, so that they are not really endogenous in origin like other roots, which arise beneath the cortex. This is a very rare case, and does not hold good for most other vascular Cryptogams.
We have found great differences between Selaginella and the flowering plants, even as regards the vegetative organs. The structure of both stem and root, their apical growth, and their mode of branching are all quite distinct from anything we met with in our Phanerogamic types. These differences, however, are unimportant compared with those which we shall find in the re- productive organs and life-history. We will now go on to describe the reproductive processes. We shall find that they at first seem quite distinct from those in flowering plants, but a careful comparison will enable us to see that there is, after all, a close relation between the two methods of reproduction. Among the flowerless plants Selaginella is one of those which in its course of development approaches most nearly to the Phanerogams. It is this fact which makes it of special interest to the student of botany.
16
STRUCTURAL BOTANY
III. REPRODUCTION AND LIFE-HISTORY
a. The Sporangia and Spores
We have already learnt that Selaginella is repro- duced by spores, which are of two kinds. The organ in which the spores are immediately produced is the sporangium. In Selaginella a single sporangium is borne in the axil of each fertile leaf or sporophyll of the cone. The cone is a vertical shoot differing but little from the ordinary vegetative shoots of the plant, and bearing many sporophylls (see Figs. 3, 4, and 5).
We will now trace the development of a single sporangium. It does not matter whether we take a microsporangium or a megasporangium, for up to a certain point they develop in the same way.
Each sporangium arises just below the growing-point of the cone, from the outgrowth of a little group of meristematic cells, situated either exactly in the axil between leaf and stem or rather higher up, on the stem itself. A little ridge of tissue is thus produced, which at first consists of uniform cells. Very soon, however, a few cells in the middle of the young sporangium, lying immediately below its epidermis, begin to be distinguished by their more abundant protoplasm (see Fig. 9, B). Only one or two such cells are visible in a radial section such as that shown in the figure. This little group of cells is called the archesporium (see Part I. pp. 113, 264), for ultimately, after much growth and numerous cell divisions, it produces the spores. The archesporium soon becomes surrounded by a well-marked layer of cells, the tapetum, formed partly from the surrounding tissue, and partly from the archesporium itself (see Fig. 9, A and B, t).
THE VASCULAR CRYPTOGAMS
17
The whole sporangium continues to grow and its cells to
divide. At the stage shown a short stalk, and the arche- sporium has given rise to a many-celled tissue.
Up to this point both kinds of sporangia behave exactly alike. The reader will at once see that, thus far, the development of the young sporangium is in all respects similar to that of a pollen-sac (compare Fig. 9, A, with one of the four pollen-sacs shown in Fig. 39, A, Part I. p. 110). It is also evident that in the youngest stages there is a considerable resemblance to an ovule at its first origin (cf. Fig. 9, B , with Fig. 44, 1 or 2, in Part I. p. 121).
Henceforth it will be necessary to distinguish between a microsporangium and a megasporangium. We will first describe the former.
in Fig. 9, A, it already has
Fig. 9. — Sclaginella spinosa. A, young microsporangium in longitudinal section ; t, tapetum ; a (darkly shaded), mass of sporogenous cells derived from archesporium. B, very young megasporangium ; t, tapetum ; a, archesporium ; l, ligule of sporophyll. Magnified about 300 diameters. (After Goebel.)
The divisions of the archesporial cells of a microspor- angium give rise to a mass of spore-producing tissue, each cell of which now rounds itself off and becomes a spore mother-cell. Each of the spore mother-cells, lying free within the cavity of the sporangium, next divides into four, the division taking place exactly in the same 2
18
STRUCTURAL BOTANY
way as in the pollen mother-cells of Dicotyledons (see Part I. Fig. 39, B, p. 110). The fonr danghter-cells become the microstores. Each group of four is tetra- hedrally arranged. The microspore acquires a cell-wall
of its own, the outer layer of which is thickened and cuti- cularised.
The microsporangium is now ripe, and the space within the sporangium-wall is filled with thousands of microspores (see Figs. 4 and 10).
We see that in every respect the microsporangium, through- out its whole development, closely resembles a pollen-sac, while the microspores, in their structure and mode of origin, precisely correspond to the pollen-grains.
We will consider the further destiny of the microspores later on, and will now pass on to the megasporangium.
Up to the time when the spore mother - cells become isolated from one another, the development of the mega- sporangium goes on in just the same way as that of the microsporangium. Now, however, a striking differ- ence manifests itself. In the megasporangium, out of all the numerous mother-cells only a single one undergoes division ; all the rest remain undivided and are abortive.
Fig. 10. — Sdagincllcc spinosa ; microsporangium in radial section. L, sporopliyll ; lig, ligule of sporopliyll ; mi, niicrospores still grouped in tetrads inside the sporangium ; t, persistent tapetum ; d, place of dehiscence. Magni- fied about 40 diameters. (R. S.)
THE VASCULAR CRYPTOGAMS
19
The one favoured mother-cell divides into four tetra- hedrally arranged daughter - cells ; each daughter - cell becomes a megaspore. The four megaspores develop enormously, and gradually displace and absorb all the remaining mother-cells, which, however, can be seen for a long time lying inert in the sporangial cavity. The four megaspores, as they grow, gradually take possession of the whole interior of the sporangium, which itself grows to a greater size than the micro- sporangium (see Figs. 4 and 11).
The megaspores acquire enor- mously thickened cell - walls, the outer layers of which are cuti- cularised, and often have a rough and warty surface. '
We see then that a megasporangium differs from a microsporangium in the fact that only one mother-cell divides, and that its daughter-cells occupy the whole sporangium, which thus contains four spores only.
The megaspores, the diameter of which is about twenty times that of the microspores, attain their
Fig. 11. — Sclaginella spinosa ; megasporangium from same cone in tangential section, mg, megaspores, three out of the four are visible ; t, persistent tapetum ; d, line of dehiscence ; lig, ligule ; L, sporophyll. Magnified about 40 diameters. (R. S.)
20
STRUCTURAL BOTANY
great dimensions at the expense of the abortive mother- cells.
We cannot understand the relation of the microspores and megaspores to each other, or to the reproductive cells of flowering plants, until we are made acquainted with their further history. We will therefore now go on to describe the changes which take place in the spores, on their germination.
b. Germination of the Microspores
The microsporangium, when ripe, opens by transverse dehiscence, the walls splitting along a line parallel to the surface of the adjoining leaf (see Fig. 10). The microspores are thus set free, and if they fall on to damp earth germination takes place.
The first change that happens is that the spore divides into two cells of very unequal size. A very small cell is cut off on one side of the spore (see Fig. 12, A, p). This little cell takes no further part in the development, and may be called the prothallus-cell. The large cell now divides into two equal parts (see Fig. 12, A); each half undergoes several further divisions. The final result is that we find the spore divided up into about a dozen cells. One of these is the little prothallus - cell first formed, which remains unaltered. Of the remainder, eight form an external layer, within which the central cells are enclosed, their number being either two or four according to the species. In S. Kraus- siayia there are four (Fig. 12, B, c). While these divisions are in progress, the spore grows a little, becomes more spherical, and bursts its outer hard membrane, so that its contents are now enclosed only by its inner cellulose wall. The central cells next undergo numerous successive
THE VASCULAR CRYPTOGAMS
21
divisions ; the small cells thus formed become rounded and lie freely in the space enclosed by the external layer (Fig. 12, C). Subsequently the external cells become disorganised, their contents contributing to the nutrition of the central group. The prothallus - cell, however, persists all through. The round central cells are alone concerned in the further development.
When the external layer becomes dis- organised, its cell-walls disappear and the contents flow together
into a structureless mass, in which the round cells are em- bedded. In the mean- time importan t changes go on in the contents of the latter. The large nucleus, which each cell contains, be- comes converted into a long rather club-
Fio. 12. — Selaginclla ; germinating micro- spores. A, young stage ; p, prothallus- cell ; first wall formed in antheridium. B, more advanced stage ; c, central cells which will form spermatozoids. C, mature stage ; sp. to, spermatozoid mother-cells, surrounded by cells of wall of anther- idium. D, free spermatozoids, each with two cilia. A, magnified 290 diameters ; B, magnified 290 diameters ; C, magni- fied 640 diameters ; D, magnified 780 diameters. (After Relayed'.)
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 soon the sperm- atozoid 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 2000 of an inch 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 267). 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 is conveyed passively 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 Selnginella ; 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. 267) of the germination of the pollen-grain in the Spruce Fir, we shall recall the fact that several cell- visions 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. 108, cells marked 2 and 3, p. 267). 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 Gymnosperms the antheridium is only represented by the stalk-cell and the two genera- tive cells (see Part I. Fig. 108, 4 and 5, p. 267). The whole is enclosed in the undifferentiated 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
Fjg. 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
26
STRUCTURAL BOTANY
in other cases the lower part of the tissue is added after the spores have fallen on to the ground.
The tissue which fdls 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 the neck. This outgrowth is cut off as a distinct cell, called the neck canal cell. Another smaller cell, the ventral canal cell, is cut off below it ; the remaining lower portion of the original ventral cell is the ovum. We see, then, that the development 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 neck canal cell breaks down, and an open passage, con- taining 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. After the dehiscence of the
m.
archegonium ready for fertilisation. 0, ovum ; n, cells of neck ; m, mucilage in canal. Mag- nified about 500 dia- meters. (After Pfeifer. )
THE VASCULAR CRYPTOGAMS
27
megasporangium, which takes place as in the micro- sporangium by a transverse slit, the megaspores are shed on to the ground. The growth of the prothallus con- tinues, the coats of the megaspore are ruptured at its
Fig. 15. — Selaginclla Martensii; longitudinal section through old prothallus showing two embryos, yj, and p.,, prothallus ; a, unfertilised areliegonium ; r.h, root-hairs; s, suspensors of embryos, larger embryo shaded ; no cells shown ; r, root ; /, foot ; st, stem ; l, l, cotyledons ; Ex, wall of megaspore. Magnified 165 diameters. (After Pfeffer.)
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, m). 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 a 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 the 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. — Selaginella ; advanced embryo in longitudinal section.
S, suspensor ; H, root ; F, foot ; G, cotyledons ; L, ligules ;
St. apex of stem ; H, liypoeotyl. Magnified 165 diameters.
(After Pfeffer.)
portions of their protoplasm also, is the same in both sub-kingdoms.
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. 15 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. 13 gives a general idea of the position of these organs, and their relation to the prothallus. In Fig. 14 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 inclosed 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, Selaginella 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. Ill, F, and 112, pp. 277, 278). 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 between Selaginella and the Gymnospcrms
At first sight the differences between even the highest flowerless plants, such as Selaginella, and the flowering plants, seems 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 with which we have become familiar in Phanerogams. If, however, we carefully compare the development of Selaginella 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 quite exactly with those of a pollen-sac in the flowering plants. The microspores formed in the former are developed precisely in the same way, and have just the same structure as the pollen-grains produced 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. The generative cells are essentially spermatozoids which have lost the power of active movement, a power which they no longer need, as they are conveyed to their destination by means of the pollen-tube. The antheridium, i.e. the organ in which the male cells are formed, is more complex in Selaginella than in the Gymnosperms, 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 rnicrospore are used up to form the prothallus-cell and antheridium. The comparison of the development gives us then the following chief results : —
Selaginella.
1. Spermatozoids
2. Protliallus and antheridium
3. Microspore
4. Microsporangium
Gymnosperm.
= Generative cells.
= Cell group in pollen-grain. = Pollen-grain.
= 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 ovide.
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 there- fore 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 : 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 megasporangium, so this difference is not so great after all. The thick, cuti- cularised wall of the megaspore is obviously a necessity for its protection when it becomes freely exposed. It is interesting, however, to know that in some Gymno= 3
STRUCTURAL BOTANY
34
sperms the membrane of the embryo-sac likewise becomes cuticularised. It has also been found that in some plants of this class (Cycadete) the development of the endosperm is 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 Selaginella.
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. The most probable explanation is that in most Gymnosperms the embryo-sac represents a mother-cell, which directly becomes a megaspore, without going through the fourfold division which is usual in Cryptogams.1
The organ in which the megaspores are produced is a megasporangium ; 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 megasporangium corresponds to the ovule, or more strictly to the nucellus of the ovule, for the megasporangium has no integument.
We have found, however, that the megasporangium and microsporangium 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
1 In Sequoia, however (the genus to which the famous “ Big Trees” of California belong), the embryo-sacs are formed in fours, just like megaspores though only one comes to maturity. Very likely this also takes place in some of the other Gymnosperms.
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.
1. Ovum
2. Archegonium
3. Protliallus
4. Megaspore
5. Megasporangium
Gymnospcrm.
1. Ovum.
2. Arcliegonium.
3. Endosperm.
4. Embryo-sac.
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 oi carpels, according to whether the adjoining sporangium be a micro- or megasporangium. There is, however, no differentiation between carpels and stamens in Selaginella or in any Cryptogam. We may compare the whole spike of Selaginella to a hermaphrodite 1 flower with no perianth, and with stamens and carpels resembling each other. In some Gymnosperms 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 Lily, the stamens and carpels are both contained in the same flower.
2 As the Cyeadese have been referred to more than once, it may he worth while to mention that they are a family of Gymnosperms 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 Cycadeaj in the Palm- house at Kew.
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 Gymnosperm 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 possible modifica- tions 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.
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 set free from the megasporangium before fertilisation takes place, and therefore long previous to the develop- ment of an embryo. The megaspore, when it is filled with prothallus and contains an embryo, bears a certain resemblance to a seed, but there is no true homology ; for, as we have already seen, the development shows that the megaspore is homologous with the embryo-sac only.
The seed, in fact, is a new development, peculiar to flowering plants ; it corresponds to a persistent megasporangium containing a single megaspore, which has produced a prothallus and, after fertilisation, an embryo, while still in situ ; the shedding of the seed would correspond to the detachment of the whole megasporangium. Something of this kind takes place in certain Cryptogams allied to the Ferns, though not in Selaginella.
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 genera and about forty species are represented. The limits of both genera and species are, however, very
38
STRUCTURAL BOTANY
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 complete idea of the whole order.
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
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 others, 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 com- pletely subdivided, so that its branches, 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, or pinnae, one row on each side (see Fig. 18). The pinnse. are often subdivided them- selves in the same way, and then the whole leaf is said to be bi-pinnate. In the specimen figured, the pinnre 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
Fir. 18. — A, leaf of Male 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 'jpalcce, 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 in their mode of 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. Reproductive 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 the 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 VASCULAR CRYPTOGAMS
43
the clusters of sporangia, or sori, as they are called, are very conspicuous on the under-surface. They are usually limited to the apical 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 prothallus, which is a much larger structure here than in S claginclla, 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 antlieridia 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 a perfectly sharp alternation of generations. The Fern plant is the asexual generation, or sporophyte, producing the sporangia, and vdtimately the spores. The prothallus is the sexual generation, or oophyte, producing the antheridia and archegonia, in which the sexual cells are developed.
44
STRUCTURAL BOTANY
IT. INTERNAL STRUCTURE OF THE SPOROPHYTE
A. The Vegetative Organs 1. The Stem
a. The Vascular System
In all the plants with which we have had to do so far (except some of the Selaginellas), the stem contains only one vascular cylinder, or stele. In the Male Fern and most other Ferns this is only the case while the stem is still quite young. As the growing point strengthens, a more complex structure is gradually
built up, and the stem soon becomes polystelic, he., all the later -formed parts of the stem contain a number of distinct steles, or cylinders ; in fact the simple monostelic structure is only found at the very base or oldest portion of the stem ; this part soon dies away alto- gether, so the whole stem of a Male Fern, as soon as it is once fairly established in the ground, is polystelic. This is the case with the majority of Ferns ; but a few, such as the Filmy Ferns, retain monostelic structure through- out life. We will now confine our attention to the mature stem of the Male Fern.
If we examine with the naked eye, or with the aid of a lens, the transverse section of the stem, we at once
Fig. 19. — Longitudinal section of small stem of Male Fern, a, apex ; l, l, bases of leaves ; st, stem ; g, leaf gap. Natural size. (After Sachs.)
THE VASCULAR CRYPTOGAMS
45
see the cut ends of the steles, arranged in a ring and embedded in ground tissue (see Fig. 21).
In order to understand the arrangement it is necessary
st
Fir. 20. — Bundle system of the Male Fern dissected out. st, principal steles of the stem ; l.g, leaf gap corresponding to the insertion of a leaf ; t, steles passing out into the leaf. Slightly magnified. (After Reinke. )
Fig. 21. — Transverse section of stem of Male Fern, show- ing the bases of leaves, st, principal steles of the stem. (After De Bary.)
to make a dissection, carefully removing the parenchyma and leaving behind the vascular skeleton only. Such a pre- paration is shown in Fig. 20. We now see that the steles form a hollow network, with large diamond-shaped meshes. Each of these meshes corre- sponds to the base of a leaf ; the steles bordering the mesh give oft' branches, which enter
the petiole (see Figs. 19 and 20). As the leaf-bases of the Male Fern completely cover the surface of the stem, and no internodes are developed, every transverse section must necessarily be surrounded by the bases of leaves, cut across at various levels, and
46
STRUCTURAL BOTANY
showing the steles entering them obliquely from the stem (see Fig. 21).
Every stele consists essentially of a central mass of wood surrounded by a ring of phloem (see Fig. 22). This arrangement is usual in Ferns, and vascular strands
Fig. 22. — Transverse section of small stele from the petiole of the Male Fern, c, c, cortical cells ; e, endodermis ; st, starch sheath, made up of inner endodermis and pericyele ; ph, phloem ; x, x, xylem ; px, protoxylem. Magnified 200 diameters. (R. S.)
of this kind are often called concentric bundles. As, however, in many cases each of them almost exactly repeats the structure of the whole vascular cylinder of the embryonic stem, it is better to regard them as steles or cylinders.
THE VASCULAR CRYPTOGAMS
47
The wood consists of tracheides and parenchyma ; the tracheides of Ferns are generally of the kind called scalariform, or ladder-like, from the peculiar structure of their walls, shown in Fig. depends on the form of 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. The tracheides are of great length, with pointed ends. True vessels, arising from the fusion of distinct cells, are rare in Ferns.
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 endodermis. 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
23. This structure
Fig. 23. — Portions of scala- riform tracheae. A, part of wall in surface view. Magnified 187 diameters. £, part of wall in section, showing bordered pits. t, torus on closing mem- brane. Magnified 375 diameters. (After De Bary. )
48
STRUCTURAL BOTANY
to those of the outer enclodermal layer. This outer layer alone has the usual structure of an endodermis (see p. 75, Part I.) and becomes thick-walled.
/3. 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 palece , 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 stem (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 scales, 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.
Keturning 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 stomata, 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. (R. S.)
The cells contain chlorophyll, as is generally the case in 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
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- formed tracheides are spirally thickened, the larger elements, devel- oped later, are scalari- 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
Fig. 25. — Transverse section of central part of root of a Fern, showing origin of rootlet, xy, diarch xylem ; ph, phloem ;pe, pericycle ; en, endodermis ; c', cortical cells ; a, apical cell of root- let ; r.c, root-cap ; p, cortex of rootlet ; pi, stele of rootlet. Magnified about 150 diameters. (After Van Tieghem.)
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STRUCTURAL 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 cortex. In a polystelic stem, such as that of the Male Fern, the steles are not marked out 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.
/3. The Boot
The root, like the stem, carries on its apical growth by means of a single cell, which here also has the form of a 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. The segments cut off from the base of the apical cell (see Fig. 27) go to form the root- cap ; those formed at its sides build up the tissues of the root itself, in much the same way as in the case of a monostelic stem.
The mode of branching of the root in Ferns differs
Fig. 26. — Apex of stem of a Fern (stolon of Nc.phrolcpis ) in longitudinal section, a, apical cell ; Sj, s2, segments ; c, cortex ; p, stele ; r, cell from which a root will arise. Magnified 80 diameters. (After Van Tieghem.)
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STRUCTURAL BOTANY
in one important respect from that in the higher plants. In Ferns each rootlet arises, not from the pericycle, bnt from the enclodermis, 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
Fig. 27. — Apex of root of a Fern in longitudinal section, showing triangular apical cell, pi, stelej^c, pericycle; en, endodermis; p, cortex ; r.c, root-cap. The dark lines mark out the groups of cells 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. Reproductive Organs of the Sporophyte
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 immedE 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 and 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. 2 8 , B, 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-walls 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 Fern passing through a sorus ; i, indusium. Magnified 80 diameters. B, 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 100 dia- meters. B, spore mother-cell divided (only three spores visible). Magnified 350 diameters. F, 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.)
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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 of them in each sporangium, but in the Male Fern there are 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 hack 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.
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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 towards the light, and divides at first transversely. A few more transverse wyalls are formed, the end cell being always the one to divide, so that
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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 undergo some transverse divisions.
Soon, however, the transverse divisions cease, an oblique wall appears in the 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 base of a depression between two lobes. This is due to 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
Fig. 30. — Apex of young prothallus of Aspidium, older than in Fig. 29 seen in surface view, a, apical cell. Magnified 210. (R. S.)
Fig. 29. — Very young prothallus of an Aspidium. sp, membrane of spore; r.h, first root-hair; a, apical cell. Magnified 210. (R. S.)
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
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
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STRUCTURAL BOTANY
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 herma- phrodite, 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 only, is shown in Fig. 32, but still smaller ones occur.
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 Sclaginella and other hetero- sporous forms, appears occasionally as a more or less
Fig. 32. — Young male prothallus of Male Fern. a.n, antherida ; sp, spermatozoids escaping ; r.li, root- hairs. Magnified about 70 diameters. (After Kny. )
THE VASCULAR CRYPTOGAMS
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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 qre 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.
B. 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
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zoid is formed from the nucleus of the mother-cell ; the cilia, however, and the narrow end of the body are derived from the protoplasm. In Fig. 33, B the young spermatozoids are shown enclosed in their mother-cells.
A
Fig. 33. — Antheridia of Male Fern. A, 1, 2, and 3. antlieridia at three successive stages seated on prothailus ; c, central cell ; iv, wall. B, older antheridium ; sp, mass of spermatozoid mother-cells ; w, wall. 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 antheridia, 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
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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- times the little bladders are Fig. 34.— Ripe antheridium,
left behind, sometimes they escaping. Magnified 350
hang on all the time, until diameters. (After Luers-
an archegonium is reached. seu')
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 only rises 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,
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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-
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 (see Fig. 35, A and B), which separate a little to
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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).
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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 which fills it, and so at last reaches the ovum below. Quite a number of spermatozoids maybe 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
Fio. 36. — Archegonium ready for fer- tilisation. o, ovum ; n, neck ; to, mucilage extruded from canal ; p, cells of prothallus. Magnified 350. (After Strasburger. )
THE VASCULAR CRYPTOGAMS
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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.
I). Embryology
The first change after fertilisation is the formation of a cell-wall around the protoplasm of the fertilised ovum.
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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 whole of the Fern-group there is considerable
uniformity in the 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 ejpibasal and a hypobasal half ; the former faces the apex 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
Fig. 37. — Embryo of a Fern ( Pteris ) in median section, s, apex of stem ; e, first leaf ; r, root ; /, foot by which embryo is attached to pro- thallus ; p, prothallus. Magnified 150. (After Hofmeister. )
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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, which before long is used up, so that the embryo now becomes an independent plant, which continues its growth, producing fresh leaves and roots. The leaves which are first formed are always of a very 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
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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, as is the case, for example, in the Wallflower. We have just seen that in Ferns, when the prothallus happens to be dioecious, the male specimens often remain rudimentary. In Selaginella, 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 Gyrnnosperms — we find the male prothallus at an equally low level with that of Selaginella — it could
THE VASCULAR CRYPTOGAMS
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scarcely be lower ; but as the mode of fertilisation has changed, all outward resemblance to a cryptogamic pro- thallus has vanished. Only in the facts that a pollen- grain is set free, and germinates, do we find any evident suggestion that it marks the beginning of a separate genera- tion. The female prothallus, though not less bulky than that of Sdacjinella, 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
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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 lengtlieriing of the life-history. In other, rarer cases, it is cut short, that is to say, the one generation passes 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 (ajpogamy), 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
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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.
In a Fern, nearly related to our type, sporangia are sometimes produced on the prothallus itself, among the archegonia and antheridia. This is the most extra- ordinary departure from the normal course of develop- ment yet observed.
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 inter- vention of spores.
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.
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Type VI
The Field Horsetail ( Equisetum arvense, L.)
The Vascular Cryptogams at present existing in the world belong to three great stocks or families. 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 family, 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 exclusive one, and has no connection whatever with its neighbours among the Ferns and Club Mosses. In early geological days, 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, 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
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Fig. 38. — Equisctum 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. )
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(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 dug up, and is perpetually giving rise to new shoots.
Equisetum, as we shall see, 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. arvense, 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
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roots, which arise 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
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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. gigantcum, a native of tropical America, are much taller, even, it is said, reaching forty feet.
B. Reproductive 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 Coniferae, 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 ( sporangiophores ) on which the sporangia are borne (see Fig. 38, 1, a). The scales of the cone are usually called sporophylls, 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 con- siderable number of sporangiophores, — about twenty in many cases. Each of them 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
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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, l,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 Equisetum 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
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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
Fig. 39. — Equisetum arvensc ; transverse section of a branch of sterile stem, a1, xylem ;ph, phloem ; ca, carinal cavity ; en, endodermis ; sc, sclerenchyma ; a, assimilating tissue ; vl, 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).
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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 normally collateral, i.e. with xylem on its inner and phloem on its outer side. The carinal cavity marks the first formed tracheides Here a few tracheides annular or spiral
Fig. 40. — Eqwisetum limosum; transverse section of an ultimate branch of the aerial stem, v.b, the three vascular bundles, each with its own endodermis ; st, the depressed stomata. The pith is hollow, but there are no vallecular cavities. Almost the whole cortex is assimilating tissue. Magnified 100 diameters. (R. S.)
an
position of the protoxylem or of the bundle (see Fig. 39). have become thickened (in 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
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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 earinal 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, pli). 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, the one sclerenchyma, fulfilling the mechanical function of strengthening the stem ; the other chlorophyll-tissue, to which the functions of assimila- tion 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
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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 he as near the surface as possible, so as to be fully exposed to light, without which its work cannot go on at all.
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
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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 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 flint, 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-
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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 case of species like E. cirvense, 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-sheatli consists of more or less thick-walled 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 Roots
The roots of Equisetum are always very slender, and must not be confused with the underground parts of
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much larger
(see
Fig.
38). All
the stem, which are 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
The young parts of the root bear They have a wide cortex, enclosing
to represent either, numerous root-hairs, a small and simple central cylinder, the structure of which is usually either triarch ortetrarch.
The arrange- ment of the xylem- and phloem- groups is that usual in roots ; the centre is occupied by a large tracheide.
The chief peculiar- ity of the root 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 Equisefoim. The origin and mode of growth of the root will be considered in the next section. Apart
Fig. 41. — Uquisetum; 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. )
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STRUCTURAL BOTANY
from the peculiarity in the endoclermis, 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 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
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93
them. The epidermis is also differentiated late, for there is no distinct dermatogen 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
a.
Fig. 42. — Equisctum arvcnse ; longitudinal median section of the apex of the stem, a, apical cell ; s, segment cut off from it ; llt lo, l3, 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 Equisetuvi is
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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. — Equisetum arvense ; part of a radial section of stem, just below the apex, to show exogenous origin of branch. a, apical cell of branch ; lx and 1.2, bases of leaves above and 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 Equiseturn are not among them, but only become enclosed after they have started in the usual way, as superficial outgrowths.
The way in which the adventitious roots of Equisetum arise is remarkable, for they 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).
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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. Reproductive Organs of the Sporophyte
We have already learnt the main points in the structure of a cone of Equisehim 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. About the same time the sporangia begin to show themselves as slight outgrowths projecting from the under-side of the expanded portions. Each
THE VASCULAR 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.
Fig. 44. — Equisctum 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
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STRUCTURAL BOTANY
purposes. 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 great many in the sporangium, then divide each into four. The nucleus divides twice before the partition- walls are formed. Finally, the four daughter-cells are arranged in a tetrahedron. This rule of the division of 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 all flowering plants.
The young spores of Equisetum, when first formed, have a thin wall of cellulose only, but as they ripen the structure becomes very complicated and peculiar. 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 ejnspore, 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 them 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 stems, 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 Equisetum 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.
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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, l, lobes ; a, a, archegonia ; r.h, root-hairs. Magnified about 30 diameters. (After Buclitien.)
in such Ferns as Asjoidium. 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
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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
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STRUCTURAL BOTANY
spirally coiled body is derived almost entirely from the nucleus of the mother-cell, while the cilia themselves,
and perhaps the point 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 water in the same way. In fact there are some Ferns in which the 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 (see 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. 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 meantime the neck goes on growing. Here also there are four rows of neck-cells, each row consisting of three or four cells. The top four cells are very long, and bend far back, leaving a wide opening between them when the organ is ripe (see Fig. 47).
Fig. 46. — Equisetum maximum ; young antheridiuni. w, wall ; sp, tissue from which spermatozoids will be formed. Magnified about 200 dia- meters. (After Bucli tien.)
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103
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.
Whatever view we take of the origin of the two generations in the Vascular Cryptogams, there is no doubt that the sexual generation is much the more primitive, i.e. the less altered, of the two. Con- sequently we find that at this stage there is much in common between 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. Prothalli 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. 200)
a. 7i.
Fig. 47. — Equisetum maximum ; fertilised arcliegonium. a.n, neck ofarcliegonium ; cm, young embryo, showing first divisions. Magni- fied about 150 diameters. (After Buchtien.)
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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 Embkyo
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 JEquisetum 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 two 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
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105
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 Equis'etum.
The root-cell, which lies ex- actly opposite that from which the stem is formed, 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
a.c.
mum ; embryo in median longitudinal section, a.c, apical cell of stem ; lu leaf of first whorl ; l2, leaf of second whorl ; r, root. Magnified about 200 dia- meters. (After Buchtien.)
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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 Eguisetvm , 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.
Summary
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
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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 Gymnosperms. Their fructification showed a great variety, some few resembling those of Equisetum, while most were very complicated, and several produced spores of two 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. To these characters we may add, as a sharp distinction from the flowering plants, the constant fertilisation by means of spermatozoids. The heterospor- ous Vascular Cryptogams come nearest to the flowering plants, as was fully explained in our chapter on Selaginella, 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 Equisetineae. We
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cannot say for certain at present which of the hetero- sporous forms really comes nearest to the Phanerogams ; probably none of those now living bear much resemblance to the real transitional forms, which we suppose must have 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 the widest gap 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 Selaginella, though maintaining a more independent position in the Perns 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 is on the whole less highly developed, 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
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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 Liverworts. 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 thallus. 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 LIVERWORTS
Type VII. Pellia epiphylla 1. The Thallus
Pellia epiphylla is one of the commonest Liverworts growing in very various habitats, sometimes by the sides
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of brooks or wells, or in damp woods and hedgerows, sometimes actually living under water ; sometimes, on the other hand, 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 latter ; it is traversed by a midrib, from which it thins off on 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
Pig. 49. — General view of a plant of Pellia epiphylla. oo, tlie lobed thallus, constituting the oophyte generation ; sp, the fruit, con- stituting the sporophyte genera- tion. The fruits to the left have opened ; those to the right are younger and still closed. Half natural size. (After Cooke.)
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midrib, and very thin margins. In damp positions, 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, with 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 thallus.
The anatomical structure of the thallus is excessively simple. It consists entirely of parenchyma, the cells of which are elongated in the midrib, and polyhedral in the rest of the 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, ai'e of cellulose, and there is no further differentiation. The root-hairs are
Tig. 50.- — Part of the thallus of Pellia, seen from above. an, the numerous antheridia ; r.h, r.h, the root - hairs. Slightly magnified. (R. S.)
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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 Pellia 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, Pellia and a Fern stand on nearly the same level as regards their sexual generations, though the sporophytes of the two are absolutely different.
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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 pro thallus 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
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Tig. 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. There is also a fairly close agree- ment with the
O.
Fig. 52. — Part of a similar section showing a nearly ripe antheridium. st, stalk of antheridium ; o, opening of sheath. Mag- nified 80. (R. S.)
o. an.
Fig. 51. — Transverse section through the midrib of the thallus of Pellia, showing a young antheridium. an, antheridium ; o, opening of the sheath surrounding the antheridium. Magnified 80. (R. S.)
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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,
1 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 Selaginella, while in the Ferns and Horsetails the cilia are
Fig. 53. Single sperm- muc]1 more numerous. In Pellia atozoid oi Pellia,
showing the spirally the body of the sperrnatozoid is
zoid is swimming (Fig. 53). Here also a little bladder, formed from the remains of the protoplasm and nucleus, hangs on to the spermatozoid when it is first set free.
The female organs, which here, as in the Vascular Cryptogams, bear the name of archegonia, arise in large numbers just behind the growing-points of the older thalli on the upper side. The thallus always thickens where they are formed. The thickened part comes to a sudden end towards the margin of the prothallus, and
for the body of the spermatozoid arises almost entirely from the nucleus, while the cilia, which are
b. The Archegonia
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the archegonia thus come to be seated on a steep slope, facing towards the growing-point. In the mean time the thallus goes on growing below the thickened part, forming a thin membrane, while simultaneously a membranous outgrowth arises above, behind the arche- gonia, and completely overlaps the whole group, which thus appear to be enclosed in a kind of pocket on the upper surface of the thallus. This pocket is called the involucre. The development of the involucre varies much according to the position in which the plant grows ; in dry habitats it reaches a great length, while in wet places it remains short.
We will now follow the development of the arche- gonium itself. Like the antheridium, it arises from a single superficial cell. This grows out and cuts off a basal cell by a transverse wall. From the upper cell the archegonium itself is developed. Three vertical walls are first formed, separating a central cell from three peripheral cells. A transverse wall cuts off a cap-cell from the top of the central cell. Then the peripheral cells, or some of them, divide vertically, so that we have a ring of about five cells surrounding the central one. Next, all the cells divide across by a transverse wall, cutting the whole archegonium into two halves, the lower being the venter and the upper the neck (Fig. 54). The principal parts are now marked out. The external cells of the lower ventral part grow and divide, giving rise to a wall two cells thick, while the central cell undergoes a single transverse division into two very unequal halves ; the small upper part is the ventral canal-cell, the lower and larger cell be- comes the ovum itself (Fig. 54). In the mean time the neck elongates greatly, and all its cells divide
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repeatedly by transverse walls, so that the ripe archegonium consists of a chimney-like neck, enclosing a row of canal-cells leading down to the ovum at the bottom (see Fig. 54). The cap-cell at the top of the neck divides into four by vertical walls crossing each other at right angles. We see that the archegonium of
a Liverwort differs
from the corre- sponding organ of a Fern or other Vascular Crypto- gam, not only in the much greater length of the neck, but also in the origin of the neck- canal. In the Liverworts this is derived from the upper part of the archegonium , while
Fig. 54. — Arcliegonia of a Liverwort {March- antia). The youngest stages are shown on the left. In the more mature archegonia, the venter, neck, and canal are clearly shown. 0, ovum ; V.C.C, ventral canal- cells. Magnified about 200.
in the Vascular Cryptogams it is formed from an outgrowth of the central cell. The
final result, however, is much the same in both cases, and on the whole there is more reason to lay stress on the essential similarity of the sexual organs in plants so remote from each other, than to dwell on their somewhat minute differences.
When ready for fertilisation, the archegonium opens. This is due to the pressure of the mucilaginous substance
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in the canal, arising from the disorganised neck canal- cells. This substance takes up water, swells, and so forces the four cap-cells apart, causing the neck to open, while at the same time a portion of the mucilage pro- trudes through the opening.
c. Fertilisation
In Pellia, as in the Cryptogams generally, fertilisation must take place under water ; after rain or dew the surface of the thallus is wet enough for the spermato- zoids to accomplish their journey. The cells of the antheridial wall take up water, swell, and press upon the mass of spermatozoid mother-cells. The antheridium bursts, and its contents are set free. As soon as the spermatozoids are released from their mother-cells, they swim through the water, rotating as they go, in much the same way as those of a Fern. They are also drawn towards the archegonia as soon as they come within their “ sphere of influence,” but in this case the chemical nature of the attractive substance has not yet been determined. They are caught in the mucilage, wriggle down the neck of the archegonium, and one of them effects fertilisation by union with the nucleus of the ovum. In all this process, so far as the details have been worked out, there is exact agreement with the Vascular Cryptogams.
So much the more surprising is the remarkable difference in the ultimate product of fertilisation. The ovum when fertilised surrounds itself as usual with a cell - wall, and begins to divide. The result of
this development will be considered in the next section.
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3. The Sporophyte or Fruit
a. External Characters
If we examine a fertile specimen of Pellia about February, we can easily recognise the young fruits on the upper surface of the thallus. At this stage each fruit appears as a little dark green ball, about one- sixteenth of an inch in diameter, projecting from the involucre (see Fig. 49, lower part). It is attached by a short thick stalk of a lighter green colour, the bottom of which is tightly fixed in the body of the thallus. If we look at the fruit with a lens we see that the upper spherical part — the capsule — is partly enclosed in a light coloured membrane which it is just begin- ning to burst ; the capsule, where its surface is exposed, is smooth and glossy. The stalk is called the seta.
Later in the season, about May, a great change happens. The seta grows with astonishing rapidity, and in three or four days attains a length of perhaps as much as three inches (see Fig. 49). The seta in its elongated condition is of a pure white colour, rather transparent, and bears the dark green, or now almost black, capsule aloft on its top, the whole looking like a thick pin with a round head (Fig. 49, on right). Shortly afterwards the dehiscence of the capsule takes place, by four longitudinal fissures, splitting the walls into four valves, which straighten themselves out, forming a horizontal cross on the top of the seta (see Fig. 49, on left). The spores are then set free. It is not long before the seta collapses, and the whole structure, when once the spores are shed, soon perishes. We will now
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go back to the fertilised ovum, and see how the fruit, which here represents the asexual generation, is developed.
b. Development
After fertilisation, the ovum first divides by a trans- verse wall, but of the two cells thus produced the lower takes no part in the further development. Capsule and seta are both entirely derived from the upper cell, which next divides by a vertical and then by a transverse wall, the latter marking the boundary between capsule and seta. Cell- walls parallel to the external surface now appear in both parts. In the young capsule these divisions separate the external layer from the archesporium, which constitutes the whole of the internal group. In- numerable cell-divisions take place, keeping pace with the growth both in seta and capsule, until a stage is reached like that shown in longitudinal section in Fig. 55. The seta at its lower end develops a conical foot, with a rim projecting upwards, firmly fixing the seta in the thallus, with which, however, there is never any real continuity of tissue.
The seta itself consists throughout of very uniform short-celled parenchyma. The capsule has a wall three or more cells in thickness ; from the internal mass of cells the sporogenous tissue is produced, but the whole of this is not used up to form the mother-cells of spores. A certain number of the cells grow in length, while remaining narrow, and ultimately form long tubular structures with a double spiral thickening. These curious elements — very characteristic of the Liverworts — are called the elaters. When young, the elaters no doubt serve to convey food-substance from the seta to the
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developing spores. At a later stage they play a part in the dissemination of the spores, as we shall see further on. In Pellia the elaters radiate from the base of the
capsule, where they are attached to a mass of shorter
cells likewise spirally thickened. They extend from
this part, upwards and outwards, passing between the mother-cells, which 'SP- chiefly occupy the outer and upper part of the capsule (Fig. 55). Young elaters, with the spiral bands just beginning to
form, are shown in Fig. 56, and nearly mature ones are represented in Fig. 57 among the spores.
The mother-cells of the spores, which are very numerous, are of a peculiar shape. At an early stage they become very deeply
Fig. 55.— Young fruit of Pellia, m f0ur-lobed, the lobes being longitudinal section. It is enclosed 0
within the calyptra. n, neck of tetrahedrally arranged, so archegonium, in which the fruit was thafc only three are seen formed ; ar , abortive arcliegoma ; J
f, foot of sporogonium ; sp, spore in one plane (Fig. 56, sjo').
SreS.;sJ elaters- Magni‘ The lobes are connected
in the middle by a quite narrow neck, in which the nucleus, which remains for a long time undivided, is situated. Eventually the nucleus of the mother - cell divides into four, each daughter-nucleus travelling out into one of the lobes, which now become separated from one another by cell-
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walls, so that the division is complete. We must not, however, suppose that this peculiar form of mother-cell is general in all Liverworts, though it is very common among them.
The venter of the archegonium enlarges very rapidly to keep pace with the growth of the fruit inside, which it completely envelops for a long time ; in Pellia, how- ever, this envelope (called the calyptra) is not entirely formed from the venter of the archegonium, but the neighbouring thallus-tissue also takes part in the growth,
Fig. 56. — Elaters and spore mother-cells of Pellia. sp and sp.2, spore mother-cells about to divide ; el, el, young elaters with indications of spiral thickenings. Magnified 360. (R. S. )
so that we see abortive, unfertilised archegonia carried up on the sides of the calyptra (see Fig. 55, ar), while at the top, the neck of the fertilised archegonium itself can still be recognised (Fig. 55, n). The cells of the thallus surrounding the foot become especially crowded with starch, and thus provide food for the developing fruit. The whole structure of the fruit is already com- plete while the seta still remains quite short. Its elongation, as we have seen, is a comparatively sudden process, and is due to the great stretching of cells which
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are already formed. The calyptra, which was already ruptured, is now left behind as a torn membrane at the base of the stalk.
The spores of Pellia present a peculiarity which is quite exceptional among Liverworts, for they become multicellular while still enclosed in the capsule. In Fig. 57 the oval spores are shown at various stages of cell-
division ; when ripe they consist of several tiers of cells. They con- tain chlorophyll, and continue their germination, under favourable con- ditions, as soon as shed. A wedge- shaped cell, such as is shown in Fig. 57 in the lower spore on the left, becomes the growing-point of the new plant, while one or both The elaters are so that they are
Fig. 57. —Spores and elaters from an almost ripe capsule of Pellia. The spores ( sp . ) are dividing into numerous cells. At sp.j a stage of division is shown, el, el, elaters. Magnified 360. (R. S.)
ends grow out into the first root-hairs, of use in loosening the mass of spores 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.
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Summary
We have now completed the simple life-history of this Liverwort. We must not suppose that all Hepatic* 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 ( c.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 — gemmse — 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 sexual generation is obviously comparable to that of a Tern, 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
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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 Pcllia, 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 Muscineae, 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.
B. THE MOSSES
TYPE VIII. 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
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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 comes 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 root-hairs, but there is no real root, — an organ which does not exist in any of the Bryophyta.
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]Sp.
The leaves are arranged in a spiral, with a divergence of f , 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 eighth 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 Fig. 58). The root-hairs are white when quite young, but soon be- come brown. The above descrip- tion applies especially to the barren stems ; we shall refer more parti- cularly later on to those which bear the reproductive organs.
Fig. 58. — Funaria liygro- metrica ; general view of a fruiting specimen, g, the leafy Moss plant or oophyte ; sp, the fruit or sporophyte generation, consisting of the seta, s, and the theca, tli ; c, the calyptra. Slightly mag- nified. (After Sachs.)
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
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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 protonema ( 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 Atrichum.
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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 these 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 are uniform, except at the somewhat serrated edge, where they are narrower and have rather thicker walls. The midrib is several cells thick, and contains a 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
Fig. 60. — Transverse section of stem of Atrichum undulatum, showing central cylinder and adjacent tissue. t, large water-conducting cells. The finely dotted elements are the sup- posed functional phloem. Among these are small groups representing leaf-trace bundles. The more ex- ternal cells (containing starch- granules) belong to the cortex. Magnified 190. (After Haberlandt.)
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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 Atrichum ) have continuous strands connecting the copducting 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 due 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 rhizoids, as they are called, multicellular root-hairs 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
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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 below 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 dioecious, though this is not the case
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with all Mosses. The two sexes are quite distinguish- able by the habit of the plants. The male plants 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 they enclose are antheridia, not stamens. On the growing-point within the rosette numerous antheridia arise in long-continued succession 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 (Fig. 61, b).
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
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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
Fig. til . — Longitudinal section through the apical bud of a male plant 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 paraphyses. Their function is probably to secrete water, and so to ensure sufficient moisture for the development of the antheridia.
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The female plants are very small and not in any way con- spicuous. The leaves converge together at the top, forming a bud within which the archegonia are contained (Fig. 62, At). 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. Here, however, as in the male plant, the growth may be continued later by means of a lateral bud.
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, the ovum and ventral canal-cell (see Fig. 62, B) are produced ; 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
m
bud of a small female plant ; a, archegonia ; b, leaves. Magnified 100. B, a single archegonium ; v, the en- larged venter, within which the ovum and ventral canal- cell are seen ; from n to m is the neck, enclosing the neck - canal. Magnified 550. C, unripe spermato- zoid in its mother-cell, and mature spermatozoid with two cilia. Magnified 800. (After Sachs.)
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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.
Fertilisation, as in Cryptogams generally, takes place under water. Raindrops, 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. Here, 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.
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3. The Sporophyte Generation (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 calyptra ) 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 ( apophysis ) performs nutritive
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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
F ig. 63. — Capsule of Funaria inlougitudinalsection. ap, the apophysis ; a, a, the archesporium, forming a single layer of cells (lightly shaded) ; it surrounds the columella, c. The green assimilating tissue is dark- ly shaded. The operculum closes the top of the cap- sule, above the columella ; its individual cells are not shown. The dark spots in the epidermis of the apophysis indicate the stomata. Magnified 14. (After Haherlandt.)
several cells in thickness ; the hypodermal layers are colourless, while 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 knocked out (see Fig, 63, a). The archesporial layer is separ- ated from the intercellular space by a zone of sterile tissue called the outer spore-sac. Within the archesporium is the large central mass of sterile colourless tissue (the columella ), which is con- nected below by a thinner strand with the tissue of the apophysis. The layer between archesporium
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139
and columella is called the inner spore-sac. The lid (i 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 - d eveloped 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- closing the pore is left standing up in the middle of the fused guard-cells, like a double screen set in the middle of 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
st.
Fig. 64. — Part of the apophysis of a Moss ( Bryum ) in transverse section, p.p, the assimilating palisade parenchyma ; st, stoma. Magnified 130. (After Haber- landt.)
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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 (. Anthoceros ) 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 oophyte 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 has been proved experimentally in surface view, p, the to be the case. So far as the uiNot\ viie nuc?ei assimilation of carbon is con- of the fused guard-cells, cerned, a sporogonium such as Haberkndt )3°’ 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
Fig. 65. — Stoma of Funaria
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141
contact is a very close one there is never any organic connection between the two generations.
We see then that the sporophyte 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 foodJ 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). Repeated 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 tissue of the fruit collapses, 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 (forming 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
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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, however, is the really important growing-point, 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 thus 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 amjohithecium. 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
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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 (Algce), 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
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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 ease 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, b), or throw off certain of their leaves as organs of propa- gation. In every case, 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
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higher external organisation. On the other hand, 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 Archegoniatcc , 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
CHAPTER III
THE ALGAE
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 “ Algae ” 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 Algae. 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 Algae 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
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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. So 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 Ohlorophycece — in- cludes the majority of the fresh-water Algae, as well as many Seaweeds. They are, on the whole, among the simpler Alga?, 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 Algae which, though anatomi- cally simple, shows a very high form of reproduction.
A. THE CHLOROPH Y CEiE
Type IX. (Edogonium 1. Structure
The genus (Eclocjonium , 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
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STRUCTURAL BOTANY
Fig. 66. — General view of a very small female plant of (Eclogonium ciliatum. b, the attaching disc ; 5,9, two oogonia, the upper of which has opened by a lid at the top, and contains the fertilised oospore ; the lower is still closed, and the ovum unfertilised, cf, cf, dwarf males, adhering to the oiigonia. The uppermost has opened to discharge a spermato- zoid. Magnified 166. (After Pringsheim.)
tank, though less common in running water. The (Edogonia 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. 6 6 shows the whole of a small plant of CEdogon- ium ciliatum ; the speci- men is much below the usual size. The main outlines of the structure, however, are always the same, the whole plant con- sisting 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
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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 CEdogonium 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 Selaginella 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,
CEdogonium 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, so 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
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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 so 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 Algse, is often exposed.
2. Reproduction
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
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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 Algae 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 GEdogonium ; A, escaping from the mother-cell ; B, free, with the fringe of cilia. Magnified 350. (After Pringslieim. )
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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 (Edogonium 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,
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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 a very small portion at one end of the body being protoplasmic. In CEdogonium, 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, so that it encroaches considerably upon the central vacuole. The whole 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
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STRUCTURAL BOTANY
now
formed
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 membrane from the neighbouring 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 in parts of the filament a chain of small cells is produced much shorter than the ordinary vegetative cells of the plant. Each of these short cells becomes the mother- cell of a single zoospore of the usual structure, but of a size intermediate between a normal vegetative zoospore and a spermatozoid (Fig. 68, an). These small spores (called androspores) are most commonly produced from the same filaments which bear the oogonia ; more rarely they occur on distinct filaments. Each androspore swims about for a time, and then comes to rest, attaching itself to the
Fig. 68. — Androspore {an) of (Edogonium ciliatum escaping. At 5 is an oogonium. Magnified 350. (After Pring- slieini.)
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female plant either near or actually upon the oogonium (Figs. 66 and 69, A). The androspore surrounds itself with a cell-wall and germinates. The plant which it produces is always of very small size ; it may consist of a basal vegetative cell with one or more antheridial cells, or the vegetative part may be absent, and the whole dwarf male be reduced to an antheridium only. In the antheridial cell, or in each of them if more than one are present, two sperm- atozoids are produced.
They escape by a lid-like opening of the anther- idium,and make theirway to the oogonial aperture (Figs. 66 and 69, A).
Like the ordinary sperm- atozoids, they move by a crown of cilia ; they are also able to help themselves along by the contractions of their whole body. The same power of contraction is of service when the oogonial opening is reached, for the entrance may be much narrower than the body of the spermatozoid, which can only pass through by accommodating its form to the size of the passage.
The act of fertilisation now takes place ; the spermatozoid comes into contact with the protoplasm of the ovum, and the twTo cells unite. The details of fertilisation have been exactly followed in CEdogonium ;
plant from which a spermatozoid lias escaped ; sp, the spermatozoid in contact with the ovum, §.■ Mag- nified 350. B, a single spermatozoid showing cilia. Magnified 350. C, germination of the oospore in Bul- bochcvte, showing the contents divided up to form four zoospores. Magnified 250. (After Pringsheim.)
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after the protoplasm of the spermatozoid has united with that of the female cell, the nucleus of the former can still be distinguished. It is much smaller than that of the ovum. The male nucleus passes through the protoplasm until it reaches the female nucleus, and then the two unite to form a single nucleus. Thus even in so simple an Alga as this we see that fertilisation is really precisely the same process as in the highest Phanerogams, such as the Lily.
The fertilised ovum (which is now called the oospore) contracts further, surrounds itself with a cell-wall, and gradually passes into a resting state. The contents undergo great changes ; the chlorophyll disappears and is replaced by a brown or red colouring matter, while large quantities of oil appear in the protoplasm, and at the same time its wall becomes much thickened. The resting stage may only last for a few weeks. It appears that in some at least of the species germination takes place before winter comes on. In the mean time the oospore has remained enclosed within the oogonial wall.
When germination begins, the inner layers of the cell- wall of the oospore swell, and burst the hard outer coat ; the entire contents surrounded only by a delicate membrane now become free, leaving behind both the outer oospore-wall and that of the oogonium. In the normal course of development the oospore does not immediately give rise to a new plant ; its contents divide into four cells, each of which rounds itself off and becomes a ciliated zoospore, exactly resembling the zoospores formed in the vegetative cells, except that the contents are wholly or partly of a red colour (see Fig. 69, C). These zoospores free themselves from the
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enclosing membrane, swarm actively for a time, and then come to rest, each giving rise to an ordinary (Edogonium plant. It appears that in many cases the plants remain through the winter in the state of young filaments of three or four cells. Their contents gradually assume the usual green colour. In a few exceptional cases the oospore grows out directly into a filament, and the contents of an unfertilised oogonium sometimes behave in the same way. In other cases again the contents divide as usual, but each of the cells germinates directly within the oospore without passing through the swarming stage. All these cases, however, are exceptions ; the regular process is the production from the oospore of the four zoospores, each of which gives rise to a new plant.
We have now completed the life -history of this remarkable Alga. In what way can we compare it with that of the higher Cryptogams ? One thing is evident ; we have here no such clear and regular alternation of generations as we have hitherto found. We cannot draw any sharp distinction between the asexual and sexual individuals, for in most cases the male and female plants themselves give rise also to the asexual zoospores. On the other hand, we may regard the germinating oospore, producing four asexual repro- ductive cells, as representing an extremely simple sporophyte generation. The oospore, however, is in some cases capable of direct germination into a new plant, which is as if the fertilised ovum of a Moss were to grow out directly into a Moss-plant without form- ing a spore-fruit. It is evident that the well-marked alternation of generations by which all the higher plants are characterised has not yet been established in Algre
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such as (Edogonium, though we can perhaps trace some indication of the process in a rudimentary form.
It has been well pointed out that the great develop- ment of the sporophyte generation in the higher Cryptogams may be viewed as an adaptation to life on land. The sexual plant, owing to its mode of fertilisa- tion, is always dependent on the presence of water ; the formation of spores suitable for dissemination through the air is advantageously handed over to a distinct generation. In the purely aquatic Algae there is no need for this double adaptation, and so we do not find among them any evident distinction between a sexual and a spore-bearing generation. The asexual reproduction by locomotive spores is evidently adapted to aquatic life, and is extremely common among Algae and some of their nearer allies.
As regards the mode of fertilisation, we still find in Edogonium (which in this respect is one of the highest Algae) a sharp differentiation of the sexual cells — on the one hand the larger stationary ovum, on the other the small active spermatozoid. The spermatozoid, however, is here much more like an ordinary cell, i.e. less specially adapted for its function, than in the higher Cryptogams. At the same time the unmistakable resemblance between a spermatozoid and an asexual zoospore is very striking ; we shall see the significance of this resemblance when we come to our next type.
It is particularly interesting that in (Edogonium we have in the androspores of some species a form of cell exactly intermediate between the ordinary zoospores and the spermatozoids. This formation of androspores giving- rise to dwarf-males is quite a peculiar case, and only occurs in some species of (Edogonium and in the allied
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genus Bulbochccte, which chiefly differs from CEclogonium in having branched instead of simple filaments. Its significance is not perfectly understood ; possibly by dividing the period of locomotion into two stages (first, the swarming androspore, and secondly, the swarming spermatozoid) the chance of the male cell reaching an oogonium at the right time may be increased.
Type X. Ulothrix zonata 1. Structure
We will next consider another fresh-water Alga, not at all unlike (Edogonium in general habit, but representing a very different and much lower type of reproduction. Ulothrix zonata is an extremely common Alga, of the filamentous or confervoid kind,1 growing by preference in running streams, or in water which is constantly renewed, as in brooks, open watercourses, or in open-air tanks with a constant supply.
The thread which constitutes the Alga is very slender, not usually exceeding about '025 millimetre (one-thousandth of an inch) in diameter. The filament consists of a single row of cells, often very numerous, amounting to a thousand or more in a single thread. The threads are usually attached at one end to the stones or other things in the water, but they can live equally well detached, floating in tangled masses on the surface.
1 Simple filamentous Algae are often spoken of as confervoid, because all such forms were in the days of Linneus included in the old genus Conferva, which since that time has been subdivided again and again, as the important distinctions among the species originally referred to it came to be better understood.
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The cells of Ulotlirix are somewhat similar in structure to those of our last type. Within the cell-wall is the protoplasmic layer, or primordial utricle, in which the nucleus is embedded. The chloroplast has the form of a broad transverse band, in which pyrenoids are contained. When, as is often the case, the cells remain short, the chloroplast may occupy almost the whole length of the cell, but when the latter grows longer the green band forms a girdle around its middle part. In the interior of the cell is a large vacuole containing only cell-sap. When the filament is attached by one end, the root-cell is nearly or quite colourless ; sometimes this organ of attachment is branched, though the rest of the thread always remains simple. The thallus grows throughout its length, and has no special growing-point. The cells divide by transverse walls to keep pace with the growth.
2. Reproduction
The reproductive cells of XJlothrix are active zoospores. They are of two kinds, the most constant distinction con- sisting in the number of the cilia, of which there are four in the one kind and two in the other. Both vary much in size, but on the average the quadriciliate is decidedly larger than the biciliate form. We will first consider the former. The larger zoospores arise from ordinary cells of the filament ; each mother-cell may produce one, two, four, or eight of these bodies from its contents. In the first case the whole protoplasm (with the exception of a thin external layer) is used up to form the single zoospore ; if two are to be produced a transverse cell- division first takes place, and each half of the contents con- stitutes a zoospore ; if the number be four, a longitudinal
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division succeeds the transverse one ; and if eight are to be formed, another division again, at right angles to the other two, completes the partition of the contents.
In any case the resulting zoospores are pear-shaped, contain each one nucleus, and have the chloroplast in the broader part of the cell, while the pointed end is made up of clear protoplasm. In each zoospore is a “ pulsating vacuole ” which alternately contracts and expands about every ten or fifteen seconds ; in this respect these vegetable cells precisely resemble some of the unicellular animals. At the point are the four long cilia by which the locomotion is brought about. The mother cell-wall breaks down at one side, and the zoospores escape. Their movements are very active, and they have formed the subject of some of the best observations on the locomotion of vegetable cells. Eelatively to their own length their speed is great. A zoospore will travel through twice or three times its own length in a second, while the fastest ship requires from ten to fifteen seconds to traverse its own length ; on the other hand, zoospores are so small that their absolute speed is slow (about a metre in an hour).1 Considering the distance they are required to travel, we may say that their locomotion is very active. Under the microscope their size and speed are magnified together, so that we are enabled to realise the true relation between their movements and their dimensions.
We saw some time back (pp. 70 and 136) that the spermatozoids of the higher Cryptogams are sensitive to certain chemical substances when dissolved in the water. Zoospores of Algse are also sensitive, though in different ways ; in this case they react especially to the influence
1 Noll, in Strasburger, Noll, Schenck, and Schimper, Lehrbuch der Botcinik.
11
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STRUCTURAL BOTANY
of light. In darkness these zoospores wander aimlessly about in all directions, and do not come to rest until they are exhausted and die. In darkness therefore they cannot reproduce the plant, nor would it be any good if they could, for without light assimilation cannot go on, and therefore life is impossible for a green plant. Light, how- ever, shining from one side, at once exercises a directive effect on the movements of the zoospores. If the light be of moderate intensity, they swim towards its source ; if the brightness be excessive, they hasten with equal decision in the opposite direction. As the swarming period nears its end, the zoospores tend more and more to avoid the light, and so are brought into contact with dark solid objects in the water, on which they can come to rest and germinate. Hence the reaction to light is of service to the plant, for the zoospores are thus induced to disperse themselves in the open water where their own assimilation can go on, while at the same time seeking shelter from too intense sunlight, which is dangerous to unprotected protoplasm. Finally, their changing sensitiveness ultimately brings them to rest in a position where germination can take place.
The zoospores of Ulothrix and of many other Algae are provided with a red pigment - spot at one side of the clear protoplasm ; this has been called the eye-spot, a fanciful name, though it is possible that the red pigment may really have something to do with the action of light on the zoospore. These comparatively large, four-ciliated zoospores of Ulothrix swim about for an hour or two, and when they come to rest attach themselves by the colourless end, and grow out at once into a new Ulothrix filament, like the parent.
In other cells, it may be of the same or of different
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filaments, zoospores of a somewhat different kind are formed. These are characterised by having two cilia instead of four, but in other respects are like the former kind, though smaller. They are produced to the number of 8, 46, 32, or even more, in a mother-cell, by successive bipartition, just as in the former case. Except for the number of cilia there is no visible difference between the two kinds. It is these small zoospores which are the sexual cells of Ulotlirix. When they escape from their mother-cell they swim off through the water just like the larger kind of spore. If, however, they meet, as often happens, with another swarm of their own kind, just set free from another cell of the same or of a different thread, an extraordinary process takes place. When two zoospores from different mother-cells happen to touch one another they at first become entangled by their cilia, and the pair go on spinning through the water together. They gradually become more closely united ; their bodies come into contact laterally, and soon begin to fuse. The fusion starts at the pointed colourless ends, and after these parts are quite joined up the opposite ends remain for a time separate. Soon however, in fact within a very few minutes, fusion is complete ; the two cells have become one, and now constitute a single four-ciliated spore (see Fig. 70). Its origin from two cells can long be recognised by the two eye-spots and the two chlorophyll-bodies.
The movements do not last long after fusion is complete. The spore resulting from this union with- draws its four cilia and comes to rest, attaching itself like an ordinary zoospore by the colourless end. In most cases the two cells which unite are of about the same size ; sometimes it happens that a smaller cell unites with
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a larger (Fig. 7 0, C), for of course the size of the swarmers varies according to the number produced in a mother-cell, and the dimensions of the latter.
The important point is that we have here a union of perfectly similar cells, whereas in all the plants described above there was a sharp distinction between the male
and female cells. We have in fact reached in Ulothrix the lowest and simplest stage of sexual reproduction in plants. The pro- cess in this rudi- mentary form requires a special name ; fer- tilisation is the union of unlike cells, e.g. that of an ovum with a spermatozoid, or with a generative cell from the pollen-tube ;
zygospores, immediately after conjuga- the sexual fusion of tion ; E, ripe zygospore ; F, unicellular •
plant grown from zygospore ; G, similar Similar cells IS termed plant producing zoospores, which are conjugation, and the about to escape. Magnified 482. (After , „ .
Dodel-Port.) result oi such a union
is called a zygospore, to distinguish it from the oospore, which is the product of fertilisation. Every possible intermediate stage between the two processes is, however, known in various Algte, so we need not hesitate to regard conjugation as really a primitive form of sexual reproduction. One point of importance must be noticed : only swarm-cells from different mother-cells will conjugate, not those
Fig. 70. — Ulothrix zonata. A, part of a filament ; most cells are already empty ; from one the biciliate zoospores are escaping. B, zoospores ; C, two in the act of conjugating ; D, two young
THE alga:
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from the same. Hence we see that in this very simple plant self-conjugation is avoided, just as self -fertilisation is so often avoided among the higher plants.
We will now see what becomes of the zygospore. It soon acquires a cell-wall, and puts out a colourless root-hair (of variable length) by which it is attached to the sub- stratum. The germinating zygospore goes on growing slowly for some weeks, but never attains any great length, and always remains unicellular (see Fig. 70, F). Its contents become denser and of a dark -green colour, while the cell-wall is much thickened, and now the little plant enters on a stage of rest.
It is a curious point about Ulotlirix that its dead season is not the winter, but the height of summer. Growth and asexual reproduction go on actively all through the winter and spring, only stopping while the plants are actually frozen hard. When the hot weather comes, however, growth is checked, and then the zygospores are produced. After a few weeks of slow growth they remain dormant, until late in autumn or even into the winter. Then, when cold weather returns, the plantlets produced from the zygospores wake up, but they never grow into Ulothrix filaments. Their contents divide up into a number of cells (from three up to about fourteen), and in this case the division is simultaneous (Fig. 70, G). These cells are zoospores, and when they become free they no doubt reproduce the ordinary form of the Ulothrix plant, though all the stages have not yet been observed.
We have already called attention to the close similarity between the conjugating cells and the asexual zoospores ; it will be remembered that in CEdogonium we already noticed how much the spermatozoids resemble
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STRUCTURAL BOTANY
the zoospores of that plant. Now in Ulothrix it is not a mere matter of similar appearance ; the sexual cells actually are zoospores, for if conjugation be not effected they can germinate on their own account, behaving exactly like the purely asexual quadriciliate zoospores. This may happen either within the mother-cell, if they fail to free themselves, or in the open, if no opportunity for conjugation has arisen. The little zoospores then come to rest, and grow out at once into Ulothrix plants smaller than those formed from the big zoospores, but otherwise quite like them. It is true that the very smallest swarm- cells (produced to the number of thirty-two or more in a mother-cell) sometimes only give rise to a very feeble and rudimentary thread, which may never grow up into a mature plant. We see, however, this important fact : the conjugating cells have not yet become exclusively adapted to a sexual function ; they are still spores, capable of more or less successful independent germina- tion, though fitted under favourable conditions for con- jugation. This makes Ulothrix and other Algae like it of quite special interest, for in them we can trace the very first rise of the sexual process among cells which are still, to all intents and purposes, spores.
Conjugating motile cells are often called planogametce (which implies that they are wandering sexual cells), but we have not used that word so far, because it is important to bring home to our minds the really fundamental fact, that the cells which conjugate are themselves spores. We see, however, that conjugation, when it does take place, makes a great difference ; with- out it, the isolated micro-zoospores germinate directly into new plants, but if union has taken place, the develop- ment is totally different, resulting in a resting unicellular
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plantlet, which is destined to produce asexual spores, but which never itself becomes a normal plant. In fact, simple as Ulotlirix is, it recalls to our minds, even more than CEdogonium, the life-history of the higher Cryptogams. The result of fertilisation is a dwarf form of plant, limited almost entirely to reproductive functions, which may be regarded as an extremely rudimentary sporophyte generation.
Occasionally Ulothrix reproduces itself in another way again, for the filaments may divide up in all directions into colonies of rounded - cells, which go on multiplying on their own account for an indefinite time before giving rise to the ordinary form of the plant. This has been called the Palmella condition, because the plant when in this state has been mistaken for a distinct genus, called Palmella. In other cases the cells of the filament round themselves off, acquire thick walls, and may pass into a resting condition. Ultimately they reproduce the normal filaments, either by direct germination or by forming zoospores in their interior, which escape and ultimately grow into new plants. The Palmella state of Ulothrix is chiefly found when the plant is left by the receding water on the damp sides of the pool or stream in which it grows. The thick-walled resting- cells are a means of protection against death by drought.
The special scientific interest of Ulothrix depends on the fact that in it we see sexual reproduction in its very earliest beginnings, and at the same time find at least a slight indication of a definite alternation of generations, suggesting that of the higher Cryptogams.
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STRUCTURAL BOTANY
Type XI. Spirogyra 1. Structure
Our next type is another of the filamentous fresh-water Algae, but very distinct from those already dealt with. The genus Spirogyra contains about seventy species, differing much among themselves both in size and in details of structure. We will give a general account of the genus, noting when necessary the points in which the specific differences are of interest.
Spirogyra occurs chiefly in ponds and lakes, that is to say in still, rather than in running water. It is often present in immense quantities, the filaments forming floating masses, sometimes several acres in extent, buoyed up by the bubbles of oxygen which their assimilation has produced. It is characteristic of Spirogyra to float, for its filaments are perfectly free, without attachment to any kind of substratum. There is no distinction of apex and base, and all the cells are alike throughout the thread.
In the larger species of Spirogyra the cells are, microscopically speaking, of great diameter (reaching •25 mm. = one-hundredth of an inch, in extreme cases). They are therefore very favourable for study. Within the stratified cell-wall is the primordial utricle, in which protoplasmic circulation can be well observed in vigorous plants. Imbedded in this are the very con- spicuous chloroplasts, which here take the form of green spiral bands with toothed edges. They form the most striking feature of the plant under the microscope, and from them the genus derives its name. The chloroplasts vary in number from one to ten in each cell (see Figs.
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71 and 72) according to the species, but the number is not always constant even in the cells of the same filament. Each chloroplast is studded at intervals with large pyrcnoids, which can be well seen here, and have a crystalline form. It is around these proteid bodies that the starch is formed when assimilation is taking place. Each pyrenoid then becomes surrounded by a coating of minute starch-granules. In every cell there is one large nucleus, either imbedded in the primordial utricle, or suspended in the middle of the cell by protoplasmic fibrils. These fibrils are. attached, at the ends remote from the nucleus, to the chloroplast, abutting on it at the points where the pyrenoids are situated. The nucleus contains a large nucleolus.
All the cells are similar, and all take an equal part by growth and transverse division in the development of the plant. Spirogyrct may therefore be spoken of as physiologically a unicellular organism, for all its cells are equivalent, and each appears to be capable of carrying on all the necessary functions for itself. In the ordinary state the plant is morphologically multicellular, but some- times the thread actually breaks up into its separate cells, and we then find that each of these on its own account is capable of independent life, and can start a new plant. In a large number of species allied to our type, the cells normally lead an isolated existence, separ- ating after each division.
In Spirogyra the division of the cell coincides with that of the nucleus, which takes place by a complicated process (much like that in the higher plants) resulting in the formation of two exactly similar daughter-nuclei. While this is going on, a transverse septum gradually grows in from the outside of the cell, first appearing as
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STRUCTURAL BOTANY
a mere ring, and gradually advancing towards the middle, until a complete disc of cellulose is formed across the cell, dividing its contents into two parts and separating the newly-formed daughter-nuclei (see Fig. 71). The growth of the cell-wall is entirely due to additions from the protoplasm. This mode of cell-division by ingrowth from the outer wall is common among the lower plants, while in the rest of the vegetable kingdom the new wall is as a rule formed simultaneously over its whole area.
W
Fig. 71. — Cell of Spirogyra nitida during division. There are several spiral ehloroplasts, with numerous pyrenoids. The two daughter-nuclei have already separated ; at W — • IV the new cell- wall is growing in between them. Protoplasm con- centrated at the growing edge of the cell- wall. Magnified about 200. (R. S.)
2. Reproduction
Except for the occasional breaking up of a filament into its constituent cells, Spirogyra possesses no asexual means of propagation. Its normal reproduction is always sexual, and is a form of conjugation, consisting in the union of like cells. As we shall see, however, it is quite different from the mode of conjugation in UlotJirix, for in Spirogyra and its allies it is the ordinary vegetative cells which unite. Conjugation, in most cases, takes place between two filaments lying side
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by side. The cells of each filament send out lateral outgrowths, which exactly correspond to each other in position. These outgrowths come into contact, and adhere together ; the walls separating them are then absorbed, so as to leave an open passage between the opposite cells of the two threads (see Fig. 72).
The contents of the cells in one of the two filaments now contract, receding from the cell-wall, and round them- selves off ; for a time the (jells of the other filament remain unchanged. The contracted cell-contents next begin to insinuate themselves into the connecting passage, and gradu- ally work through it, passing into the opposite cell (Fig.
73). They then unite with the contents of the latter, which in the mean time have themselves somewhat con- tracted.
In this case the fusion of the nuclei of the two conjuga- ting cells has been observed.
The united protoplasmic mass assumes a. rounded or oval
form, and surrounds itself with a cell-wall, which becomes thickened and cuticularised on its external surface. It appears that in some species at any rate the chloroplast or chloroplasts of the active cell disintegrate in the zygospore, so that those of the new generation belong
Fig. 72. — Sinrogyo-a longata conjugating. The two parallel filaments are putting out the conjugating outgrowths (a), which at b are already in con- tact ; ch, spiral chloroplastid ; n, nucleus, with radiating protoplasmic fibrils. Magni- fied about 350. (After Sachs. )
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STRUCTURAL BOTANY
only to the receptive partner. If this be so, it points to a certain degree of sexual differentiation.
That there is actually a certain distinction of sex in Spirogyra is evident from the fact that all the cells of each conjugating filament usually behave in the same way, either giving up their own protoplasm or receiving that of the fellow-filament. That the difference, how- ever, is not fixed is shown by the fact that conjugation sometimes takes place monceciously, i.e. between the cells of the same filament. This mode of union is called
“ chain-like,” in opposition to the more usual dioecious or “ ladder-like ” method, and both processes may occur in the same species. In monoecious conjugation two adjoining cells put out very short lateral processes, which arise close to- gether, on either side of the transverse wall separating the two cells. The processes unite, and the contents of the one cell pass through the side passage and fuse with those of its neighbour. In this case there may be a certain sexual difference among the individual cells, but the filament is obviously bisexual.
In most of the allies of Spirogyra there is no trace of any difference of sex, for the cell-contents meet and fuse midway between the parent cells, each of which thus takes an equal part in the process.
The contents of the zygospore assume a darker colour, and the starch disappears, giving place to oil, which constitutes the reserve carbonaceous material during the
Fig. 73. — Conjugation more advanced. At a tlie proto- plasm is in the act of passing over ; at b the union is com- plete, one cell having trans- ferred all its contents to the other. Magnified about 350. (After Sachs.)
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interval of rest. The zygospore can now survive either a period of drought or the cold of winter, as the case may be. On germination the zygospore, unlike that of Ulotlirix and its allies, gives rise directly to a new plant. The outer wall is burst, the protoplasm grows out, clothed only by the inner cellulose wall, the bright green colour of the chlorophyll reappears, and starch is once more formed. On first germinating, the young plant shows a distinction between apex and base, for it remains for a time attached by one end, which is pointed and colourless. 'This distinction is soon lost, and the filament floats freely in the water. In a few rare cases, where conjugation fails to take place, a single cell has been observed to form a resting-spore by itself.
The life-history of Spirogyra, which represents a con- siderable order of fresh-water Algae, is, as we see, extremely simple ; there is not the slightest indication of any alternation of generations. All individuals alike are capable of conjugation, and are therefore sexual. The name Conjugatse is applied to this order, for it was among them that the union of similar cells to form a zygospore was first observed. In this group it is always ordinary vegetative stationary cells which unite, so the process is essentially different from the union of actively moving spores such as takes place in Ulothrix. The Conjugatce appear to be an isolated group, showing no near relation to any of the higher forms, whereas from the motile conjugation of Ulotlirix we can trace a series of steps leading on to the well-marked fertilisation of Algae like OEdogonium. In Spirogyra, however, we have really the type of all sexual reproduction, which consists essentially in the fusion of two distinct cells, accompanied by the union of their nuclei. The structural differences
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between male and female, so generally found in the higher plants, are of only secondary importance, and must be regarded as special adaptations to secure this union with the least expenditure of material and energy. In one word, we may regard conjugation as the primitive form of fertilisation.
Type XII. Vaucheria
We now come to a group of plants, which in their general structure differ more profoundly from all our previous types than any of the latter differ from one another. So far, all the plants we have considered have been cellular : in Vauclieria the protoplasm is not partitioned by cell-walls at all, but is perfectly con- tinuous throughout the whole plant. A large family of green Algie — the Siphonese — are distinguished by this non-cellular structure. Peculiar as these plants are in their internal organisation, they are none the less capable of attaining on their own lines a high develop- ment. Many of them are of large size, and some possess organs analogous with the stem, leaf, and root of higher plants.
Vaucheria is one of the simplest of the Siphoneas in its vegetative organs, but as regards its reproduction is more highly differentiated than any other member of the family as at present known. Some few species of Vaucheria are marine, but most are either fresh-water or terrestrial plants. Some of them are among the very commonest of Algae, occurring everywhere in ponds and ditches, or on damp earth, as, for instance, on the soil of neglected flowerpots, where they form a tangled green
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web, and are often troublesome weeds, especially to Fern cultivators. Their threads are rather coarse, darkish- green, and branched. When submerged, they often form dense masses of large size, becoming conspicuous objects in the water.
1. Structure
In the vegetative condition a Vaucheria consists of a cylindrical green filament, repeatedly branched at irregular intervals, and attached by a colourless branched root or rhizoid (see Fig. 75, 1>). The contents are uniform throughout the green part of the plant. The filament is bounded externally by a cellulose membrane, but internally there is nowhere the least trace of cell-walls, so long as the plant remains in its normal vegetative condition ; the cell-contents are uninterrupted throughout its entire length (which may amount to many inches), and extend without a break into the branches.
The protoplasm forms a thick layer lining the external wall, and contains an immense number of roundish chlorophyll-granules, the abundance of which accounts for the deep-green colour of the threads. Imbedded in the protoplasm, just inside the layer of chlorophyll-granules, there are innumerable minute nuclei, which multiply by division as the plant grows. This feature is very character- istic of the Siphoneae and some allied Algae, which always have very numerous nuclei scattered in their protoplasm. We see, then, that Vaucheria possesses the essentials of histological structure — protoplasm and nuclei — though not partitioned into distinct cells. The growth of the stem and its branches is apical ; at the extreme growing end of each filament the protoplasm is colourless and transparent. Vaucheria does not form starch ; the pro-
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duct of its assimilation is deposited in the form of a fatty oil.
2. Reproduction
Vauclieria produces reproductive cells of two kinds — asexual and sexual. The asexual reproduction affords a rapid means of propagation, and goes on chiefly when the plant is growing in abundance of water, and generally under conditions that suit it.
The protoplasm accumu- lates and becomes denser at the end of a branch, which assumes a club-like form. The enlarged end is then separated from the rest of the filament by a transverse septum, for Vauclieria forms cell-walls in connection with its repro- ductive organs, though not elsewhere (see Tigs. 74 and 75, A). The cell thus formed may be called the zoosporangium. The entire contents of the zoosporangium constitute a single zoospore of relatively large size, clothed over its whole surface with numerous short cilia (see rig. 74).
The sporangium opens at the apex ; the expulsion of the zoospore is helped by the expansion of mucilage contained in the cell, but depends to a great extent on its own movements. The opening is much narrower than the zoospore, which has to push its way through, and to change its form in the process (see Tig. 75, A).
Fig. 74. — Zoospore of Vaucheria scssilis. To the left is shown the end of a filament, just cut off, by the wall (vi), to form a zoospor- angium. Magnified 25. To the right is a zoospore covered with the numerous cilia (c). Magnified 95. (After Strasburger.)
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Sometimes the front part, which tries to rotate as it becomes free, gets twisted off from the rest of the cell, and then two zoospores are formed instead of one.
The zoospore contains a great number of chloroplasts and of nuclei. In this case the nuclei lie near the outside in a clear zone of protoplasm. The numerous cilia are in pairs, each pair corresponding to a nucleus. Evidently the whole of this great zoospore corresponds to a multitude of small zoospores, not separated from
Fig. 75. — Zoospore and its germination in Vaucheria scssilis. A, zoospore ( sp ) in the act of escaping from the sporangium. Magnified about 30. B, germinating zoospore (sp), which has formed two green filaments and a rhizoid (r). Magnified about 20. (After Sachs. )
each other. In most other Siphonete we find that numerous zoospores, usually with two cilia each, are formed in the sporangium.
The escape of the zoospores of Vaucheria generally takes place in the morning. They swim about
rather lazily for a quarter of an hour or so, and are so big that their movements can be followed with the naked eye. Then they come to rest, and immedi- ately withdraw their cilia. The zoospore germinates at once, sending out a filament which attaches itself to 12
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the soil by a colourless rhizoid, and soon begins to branch (see Fig. 75, B).
This mode of reproduction may go on indefinitely, from generation to generation, so long as weather and water remain favourable. But under other circum- stances, and especially if the level of the water is sinking, and a danger of drought threatens, Vaucheria proceeds to form reproductive bodies of another kind. This plant, so simple in its vegetative structure, possesses more sharply differentiated antheridia and oogonia than any of the Algae already described. Both organs arise as lateral outgrowths. In some species they are seated directly on the main filament ; in others a short special branch forms a pedicel on which the sexual organs are borne, as shown in Fig. 76, where the oogonium is terminal on the pedicel, while the anther- idium is seated laterally below it.
The antheridium at its first origin resembles a young vegetative branch. It contains abundant protoplasm with chloroplasts and very numerous nuclei. As it approaches maturity, it usually becomes curved like a horn (Fig. 76). The terminal part is cut off from the rest by a transverse wall, and forms the true antheridium. The nuclei assemble in the interior of the cell, leaving the chloroplasts and part of the protoplasm in contact with the wall. The more internal portion of the contents now breaks up into the excessively minute spermatozoids, each of which consists of one of the nuclei, together with an ex- tremely small quantity of protoplasm, and is provided with two cilia. They begin to swarm within the antheridium, which now opens at the apex, and expels the spermatozoids, together with a portion of the
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unused protoplasm, some of which, however, remains behind.
In the mean time the development of the oogonium has been going on. Arising, like the antheridium, as a
Fig. 76. — Fertilisation of Vaucheria terrestris. A, part of filament, bearing two reproductive branches ; a, antheridium ; o, oogonium. On the right an early stage is shown. On the left the antheridium is already empty, and the oogonium fertilised. B, right hand group of Fig. A later on, showing fertilisation. The minute spermatozoids (sp) are caught in the drop of mucilage expelled from the oogonium. Anther- idium of same branch not yet open. C, some time after fertilisation. The antheridium and filament are empty, and the oospore is ripening. D, ripe oospore, containing oil- drops. Magnified 110. (E. S. )
lateral outgrowth (or as in the species figured, at the end of the pedicel), it soon assumes a globular form. The protoplasm of the oogonium contains a large quantity of oil, as well as chloroplasts, and at first a
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very large number of nuclei. After a time a sort of papilla or beak is formed (Fig. 76, A) on one side. A remarkable change now takes place in the contents. A portion of the protoplasm wanders back into the filament, carrying with it all the nuclei except one, so that the oogonium becomes a uninucleate cell. It is now cut off by a transverse wall from the filament. The contents rearrange themselves so that the portion towards the papilla becomes clear and free from chloroplasts, forming the receptive spot. The now solitary nucleus (which has grown rather larger than before) lies towards the middle of the cell, connected by a strand of protoplasm with the clear receptive spot. The wall of the oogonium opens at the papilla, and at the same moment a portion of the colourless protoplasm is expelled. It is probably due to the pressure of the protoplasmic mass that the wall is opened ; sometimes the extruded part of the contents remains in connection with the protoplasm within the oogonium. From this point onwards we may speak of the contents of the oogonium as the ovum.
Fertilisation now takes place. The minute sperm- atozoids swarm in at the oogonial aperture, but do not necessarily select the particular oogonium near which they were formed. In Fig. 76, B, for example, an antheridium is shown which has not yet discharged its spermatozoids, although the adjoining oogonium is actually being fertilised. Numerous spermatozoids may enter an oogonium, but only one fuses with the ovum, which it penetrates at the receptive spot where the protoplasm is clear. The nucleus of the sperm- atozoid has been traced through the protoplasm of the ovum, until it reaches the sole remaining nucleus, with
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which it unites. It appears then that in Vauclieria, in spite of the remarkable peculiarities of its organisa- tion, fertilisation is precisely the same process as in the higher plants.
The fertilised ovum surrounds itself with a cell-wall of some thickness, and passes into a resting-stage (see Fig. 76, D ) during which a period of drought can be endured with safety. Then germination takes place ; the oospore gives rise directly to a new Vauclieria plant, which soon begins forming the asexual zoospores.
We see that there is in this plant no indication of a regular alternation of generations. Sexual and asexual reproduction may both occur in the same individual, and it depends on the external conditions whether the one or the other takes place.
The sexually produced resting-spore is itself a pro- vision against drought, but the plant can also protect itself against this danger in a more rough and ready fashion. If the water sinks and the filaments are left stranded on the mud, it sometimes happens that they divide up into a number of cells, each of which surrounds itself with a thick wall. This is called the Gongrosira state, because specimens of Vauclieria in this condition used to be placed in a different genus under that name. We see then that Siphonaceous filaments can separate into distinct cells when necessary, though this does not happen in the normal course of their vegetative life.
Vauclieria stands quite alone among its relatives in the perfection of its reproductive arrangements. In other Siphonese, in which any sexual process has been observed, it takes the form of the conjugation of motile
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cells. Vaucheria therefore, though one of the simplest of its order in vegetative respects, appears to rank highest as regards reproduction.
TYPE XIII. Pleurococcus vulgaris
Before leaving the green Algse we will take one more example, the very simplest we can find, as an illustra- tion of a unicellular plant. Pleurococcus vulgaris is in our climate perhaps the most abundant of all plants. Everyone must have noticed how commonly the trunks of trees, palings, and wet walls are covered by a bright green powdery layer, especially on the side away from the sun. In damp winter weather the green coating is most developed. This substance, though it may include many different organisms, is chiefly made up of Pleurococcus vulgaris.
This Alga, which occurs in prodigious numbers, consists of small rounded cells, sometimes quite separate, some- times grouped together in little packets of two, four, or eight. When adhering together, the sides in contact are rather flattened.
Each cell has a thin cellulose wall, and is densely filled with protoplasm, which at first sight appears to be coloured uniformly green. This, however, is not the case, for the chlorophyll is really limited to definite chloroplasts, of which there are usually several occupying the outer part of the cell-contents. The rest of the protoplasm is colourless. About the middle of the cell is a nucleus, containing a nucleolus (Fig. 77). The cells divide freely into two ; successive divisions take place in all three directions, and are at right angles
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to each other. The cells may either round themselves off and separate immediately after each division, or may remain grouped together for a few generations. Sooner or later, however, they fall apart. The plant forms small starch-grains in
the chloroplasts when exposed to light.
We have in Pleurococcus an example of a typically unicel- lular plant, in which the cells lead a perfectly independent life ; each individual cell, how- ever, has the same structure as in higher plants. Evidence has recently been brought forward to show that Pleuro- coccus possesses other means of reproduction besides simple cell-division, and that it passes through other stages of exist- ence than that described. It
Fig. 77 .—Pleurococcus vulgaris. A, single cell ; n, nucleus ; ch, 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.)
is probable that the life-history is in reality a rather complex one, but at present the whole of the facts have not been made clear.
B. THE BROWN ALGAE {Phceophycece)
The brown Algte, 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
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range than the Chlorophyceae, for though no brown Algae are quite so small or so simple as Pleurococcus, 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 Phaeophyceae 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 Phaeophyceae in colour, but have otherwise nothing in common with them.
The majority of the Phaeophyceae are reproduced by zoospores ; these are called the Phceo-zoosporece. A second order only forms sexually produced resting - spores ; this is the family Fucacecc. 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.
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Type XIV. Ectocarpus SILICULOSUS 1. Structure
The genus Ectocarpus, various species of which are extremely common on our shores, includes some of the simplest forms of the Phseophyceae. 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. Reproduction
The reproductive organs are of two kinds, which are distinguished as unilocular and plurilocular sporangia. They are usually borne on distinct plants, but sometimes
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on the same individual. Both kinds of sporangia arise as lateral branches, either sessile or stalked.
The unilocular sporangia are simply globular or pear-
Fm. 78. — General view of part of tlie thallus of Edocarpus, show- ing several branches, sp, sp, plurilocular sporangia, borne laterally on the branches ; g, g, intercalary growing-points, where cell-division is going on. Magnified 56. (R. S.)
shaped cells, borne at the sides of the free branches. They become densely filled with protoplasm, and their con- tents divide up into a great number of small zoospores. Each zoospore contains a nucleus and a brown plastid, 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 Phfe- ophycefe 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 Algse. These zoospores come to rest after a time, and grow up into new plants. Hence the unilocular sporangia are organs of asexual reproduc- tion.
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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 pnd, 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 zoospores from plurilocular sporangia have been observed to conjugate.
In Ectocarpus silicidosus there is even a certain functional difference be- tween 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 swarmers begins to fuse with the resting-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 Reinke.)
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Fig. 80. — Ectocarpus siliculosus. A, part of a branch, bearing two plurilocular sporangia ( sp ), from one of which the zoospores (z) are escaping. Magnified 330. B, female cell which has come to rest, with numerous male cells swarming 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 is a step in advance of the con- jugation of Ulothrix, for in the Ectocarpus there is so far a difference between the two cells, that at the moment of fusion one is at rest and the other active, though previously they had both be- haved quite in the same way. We may look upon this as the first slight approach towards the differ- entiation of station- ary ovum and motile spermatozoid. In spite, however, of this distinction, both cells alike are capable of independent asexual germination, although when germinating
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alone they 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.
In this species, then, as in Ulotlirix, we have an extremely low form of sexual reproduction, in which the conjugating cells retain the power of germinating on their own account. It is only in very few species that any- thing like a sexual process has been observed in Phseophycem. In the Vast majority of the forms, 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 spor- angia and zoospores are very uniform throughout the Phceozoosporese, 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 which we have chosen for our type is distinguished from all others by the position in which it grows, which is always close
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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 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 conceptacles, which are cavities in the tissue containing the organs of reproduction (see Figs. 8 1, B, and 82).
Pelvdia, 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
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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
Fig. 81. — Pehetia canaliculata. A, small fertile plant; d, attaching disc ; r, r, receptacles, each of which bears a number of wart-like conceptacles. 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 liyplice. 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 hyphee of the
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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 hyphae 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 hyphae, 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 growth 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.
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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 tliallus 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
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STRUCTURAL BOTANY
separated from the old plant, affords a simple but effective means of vegetative propagation. The regular repro- duction, however, of Pelvdia, in common with all other Fucacese, is exclusively by the sexual method.
2. Reproduction
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 parapliyses 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 odgonia, 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
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(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 Pelvetia. p, p, hairs ; a, a, antheridia, some of which are already emptied. Magnified about 260. (After Thuret and Bornet. )
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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 in Fig. 86, A, are some of these rejected nuclei. In Fucus itself all the
Fig. 84. — Spermatozoids of Pclvetia. A, unripe antheridium, already freed from outer membrane ; B, antheridium 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 oogonium of Fucaceae was originally a structure of the nature of a sporangium.
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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 Fig- 85. — Oogonium (oo) of
Pelvetia , already divided to taken from the water, we can forai the two ova. p. para-
see the little slimy drops physes. To the left of the
J oogonium some anthendia
appearing at the pores of the are also shown. Magnified
conceptacles; and these drops, and Bornet ) (Aftei rhmet
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
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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 on to the rocks, sometimes only on to 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 Fucacete 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 Fucacem this is not the case ; the ova are set free as bare masses of protoplasm ; the peculiar state of things in Pelvctici 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
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of the Fucacere. 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.
Figs. 86, A and B. — A, oospores ot Pelvetia beginning to divide immediately after fertilisation. They are still enclosed in the gelatinous inner wall of the oogonium. The 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 oospores 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
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STRUCTURAL BOTANY
cell-walls follow, and soon the oospore (which here does not pass through a resting-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
cylindrical and
then flattened at the free end ; a depression soon arises at its apex, in which a definite apical cell appears, and now we have in all essentials a new Pelvetia plant fairly started in the world.
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
Fig. 87. — Ovum of one of the Fucaceae ( Asco - phyllum nodosum ), seen in section at the moment of fertilisation. cf, small male nucleus of a spermatozoid, which has traversed the protoplasm, and is now in contact with the large nucleus of the ovum. The proto- plasm of the ovum shows a distinct foam-like structure. Magnified about 650. (After Farmer. )
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201
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 Algae 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 ALG/E ( Floridecc )
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.
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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 Floride?e 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 corymbosujvi 1. Structure
The form of the thallus among red Algse 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 ALGJ5
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 Algte, 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 Algae, 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
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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 Callitkamnion comes to have a regular conformation like that of many higher plants, depending on the relative position of its unlimited and limited branches.
2. Reproduction
a. Asexual
The reproduction of Callithamnion and of most Floride;e is of two kinds, asexual and sexual. The asexual reproductive cells are called tetrcispores, 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
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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. 88. — Callithamnion corymbosum. 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.
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STRUCTURAL BOTANY
Most Floridese are dioecious, and as the tetraspores 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 branchlets 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 ALGiE
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 corymbosum ; part of a male plant bearing the clusters of antheridia (a). Magnified about 150. sp, detached group of antheridia, surrounded by free sperni- atia. Magnified about 240. (After Thuret and Bornet.)
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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 it is concerned in the formation of the fruit. The two cells next the carpogonium remain small, and together constitute the trichophore ; 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 complicated 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 continuous 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, and the details of fertilisation have only been completely followed in one instance ( Ncmalion , a genus rather simpler in its ari’angements than our type). In that case the nucleus of the spermatium was observed to travel down the trichogyne and fuse with that of 14
Fiu. 90. — Callithamnion corymbosum ; part of a branch of a female plant, bearing a procarpium. 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 tricliophore ; c, the other trichopliore cell. The two cells showing to the extreme right and left of the tricliophore are the auxiliary cells, which fuse with the carpogon- ium. Magnified 250. (After Thuret and Bornet. )
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STRUCTURAL BOTANY
the carpogonium. Probably it is so in Callithamnion also, but more observations on the fertilisation of these plants are much wanted.
The result of fertilisation is not the formation of a single oospore, 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. It is probable that there is an actual union of the cell contents of the carpogonium with those of the two auxiliary cells. Each of the latter now divides by a transverse wall (see Fig. 91, c) ; the upper of the two cells in' each case becomes a placenta which gives rise to the spores. Callithamnion is different from most of the simpler red seaweeds in so far as it regularly forms two fruits from each pro- carpium, whereas its near allies usually form one only. '
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
Fjc:. 91. — Callithamnion corymbosum ; part of a branch of a female plant, showing the earliest stage of development of the cystocarps (c, c) after fertilisation. They arise by the growth and division of the two auxiliary cells. Magni- fied 250. (After Thuret and Bornet.)
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211
between the individual cells, each of which, when all the divisions are complete, becomes a spore, called for distinction a cctrpospore, 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.
Callithamnion occupies a middle position among the Floridere 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 Florideae 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
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STRUCTURAL BOTANY
presence of such an envelope, the sexually produced fruit owes its special name of eystocarp.
In so far as the result of fertilisation is the production, not of an oospore, 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 Floridese have any direct relation with the Mosses, or with any of the higher plants. They show somewhat doubtful affinities with certain green and brown Algae, but, as at present known to us, they 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. Bed 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 generation prevails among them.
I). THE cyanophycea:
Type XVII. Nostoc
There remains to be considered a group of rather obscure plants of simple structure, which resemble the Algae in their habit and mode of life, and are therefore best described in this place, though their real relationships
THE ALGtE
213
are open to doubt. Of the Cyanophycese 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 Tig. 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 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 these floating
Fir;. 92. — Nostoc Linckii. A, part of a filament ; h, h, lieterocysts ; sp, sp, spores. B, isolated spore beginning to germinate. C, young filament formed from a spore, the burst cell- wall of which is shown at the ends. Magnified 470. (After Bornet.)
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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 Algae. 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, apparently destitute of pro- toplasm. These cells are called the heterocysts (Fig. 92, A, li), and are characteristic of Nostoc and its nearer relations. Sometimes the filaments break across at the heterocysts, and the short row of living cells between them become isolated. These detached filaments (called the Jiormogonia ) 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).
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215
Such is the simple history of Nustoc. Neither in this genus nor in any of the blue-green Algte 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 ( Orobanelie ) as parasites ; the Bird’s Nest ( Monotropa ) and the Bird’s Nest Orchid ( Neottia nidus-avis) as saprophytes. In all such cases, however, the parasitic or saprophytic forms
21C
THE FUNGI
217
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 thallophytes, 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 heath
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
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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
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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 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 hyphce ; the latter name is given to the filaments of Fungi in general. These hyphse are for a long time without any trans- verse walls ; they are in fact non-cellular, just like the filaments of a Vaucheria. The inside of the hypha is occupied by vacuolated protoplasm, in which very numerous nuclei are embedded. Unlike Vauclieria, however, Pytliium 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 hyplne 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
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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. 214) 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 Pytliium 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.
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2. Reproduction
a. Asexual
The hyphte 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 hyphte (see Fig. 93, A). The sporangia are beaked at the apex. 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. Pytliium, 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 Pytliium are said to clamp off, for it is when they are kept too damp that their enemy is best able to attack them ; the moisture enables the Pytliium 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 Pytliium is entirely powerless to propagate its
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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 Pythium, as, for example, in the species P. Baryanum, which is so common on cress-seedlings, it sometimes happens that
Big. 93. — Pythium. A, branch the sporangium does not of the mycelium, bearing three „ ,
zoosporangia (s). Magnified form ZOOSpOl’eS at all, but
145. B, ^oosporangium (s) grows out directly into a discharging its contents (&), f
Which are still enclosed in the hypha, thus starting a new
enlarged papilla, but have pjant at once, without the already divided to form the f.
intervention of the active
zoospores. Magnified 145. C, germinating oospore (osp) form- ing an asexual sporangium (s). Magnified 300. (After De Bary. )
aquatic cells. Evidently this allows of propagation taking place, even though there should not be water enough to boat the zoospores. The same thing happens in the closely-allied genus
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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
Fig. 94. — Germination of the sporangia in various species of Peronospora. A, P. nivca ; a, b, c, stages in the formation of zoospores ; cl, free bieiliate zoospore ; e, zoospores germinating. B, P. densa ; 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 Umbelliferoe) the contents of the sporangium divide up at once into a number of bieiliate zoospores, which escape by the opening of the terminal papilla (Fig. 94, A). In P. densa (Fig. 94, B),
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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. Laducce (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. Raclii (occurring on flower-heads of Composite), with which the majority of species of Pcronospora 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, hear 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. Pytliium 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 hyphte which have grown out
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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 not yet been made quite clear in Pythium, but there is no doubt that the oogonium at first contains a large number of nuclei ; judging from the analogy of allied genera, it is probable that nearly all of them pass into the periplasm, leaving behind, in the central mass, a single nucleus, which is alone concerned in the act of fertilisa- tion.
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 15
Pythium. yf, early stage ; oogonium (o) and anther- idium (a) still immature. B, 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 De Bary.)
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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 periplasm, 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 almost unknown among Fungi, — another point in which the adaptation to a terrestrial 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, B). 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 is probable, from analogy with allied Fungi, 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 now passes into a period of rest.
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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, G). 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 Pytliium, the ex- ternal circumstances determine whether anything like an alternation of generations takes place or not.
Pytliium and its allies stand nearer to the Algae than any other Fungi; in fact they were formerly themselves placed among Algae, and it is evidently with Siphonese of the type of Vaucheria that they have the closest
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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 spermatozoids, 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. Pilobolus crystallinus
Among the Algee we found in some of our types (as, for example, in GEdogonium and Vauclieria) 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
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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. Though its habitat is so unattractive, Pilobolus itself, and especially j 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 np 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 the Sipho- naceous Algae, and therefore Oomycetes and Zygomycetes are grouped together in one class, under the name of Phycomycetes or algal Fungi. The group to which Pilobolus 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.
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2. Reproduction
a. Asexual
The most abundant means of propagation is by asexual spores formed in sporangia. For the purpose of spore-formation certain of the hyphae grow out into the air, and assume a vertical position. The apex of the aerial hypha enlarges into 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). We 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 this group. 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 hypha ; 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
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cavity, forming what is here called the columella (Fig. 96, E, c). The wall of the sporangium itself is not
Fig. 96. — Pilobolus. A, group of sporangiophores ; st, stalk ; r, reservoir ; s, sporangium. Magnified 5. B, hypha and sporangiopliore ; 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 1 5. E, reservoir (?■) 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 Tiegliem.)
uniform throughout ; the upper part is thickened and cuticularised, and bristles with crystals of calcium
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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 hursts, 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
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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 me'ans 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 hyphse 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 branches 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 hyphse 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-
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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.
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 terrestrial 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 characteristic form of reproductive cell in the Fungi.
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TYPE XX. Sph^erotheca 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 Algte. While differing among themselves in every other respect, they agree in possessing a septate, multi- cellular mycelium, with apical growth of the hyphse 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 Sjohceroiheca and its nearer allies (form-
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STRUCTURAL BOTANY
ing the family Erysiphece, 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 hyphse, 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
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single nucleus, though in some allied Fungi the cells are multinucleate. 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. Reproduction
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 appear to prove 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 disappearing. Then the nucleus of the antheridium passes over through the opening and unites with the nucleus of the ascogonium. If these statements are confirmed (and there seems no reason to doubt their accuracy), it is evident that, in this Ascomycete at any rate, the development of the ascus-fruit is preceded by a sexual process quite comparable to that of the Oomycetes.
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As regards the subsequent development of the ascogonium, 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 be- comes the ascus. Its nucleus divides repeatedly, and gives rise to eight daughter-nuclei, around each of which
Fig. 97. — A-C, Sphcerotheca Castagnei. A, early stage in forma- tion of fruit ; p, antheridium ; c, ascogonium. B, 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 hyphae, forming perithecium. D and E, S. pannosa ; D, ripe perithecium ( c ) 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.)
a cell is formed. These eight cells are the ascospores. Iu the mean time the cell next below has sent out several branches, which grow up around the ascogonium, com- pletely enveloping it in a double layer of densely crowded hyphte (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 themselves closely to
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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 ascogonium), instead of one only.
Our plant has another means of reproduction, by conidia (see Fig. 97, E). These are produced on vertical hyplue (called the conidiopliores ) 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.
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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 evidence at all for the occurrence of an act of fertilisation.
The formation of a completely closed fruit around the ascus or asci is characteristic of the immediate family (Erysipliese) to which Sphcerotheca 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 XXI. 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 liymenium , 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
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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, some on walls or roofs, and others on the ground.
1. Structure and Mode of Life
Our example, Physcia parietina, is extremely common on 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 con- ceivable. 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 C02 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.
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Pliyscia 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 Fig. 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 is shown in transverse 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 is built up, like all fungal tissues, of fila- mentous hyphfe, which in this case are so 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 hyphse 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.
which the ripest are near the middle. Natural size, h, section of apothecium, showing the hynienium. Magnified about 5. (After Lauder Lindsay.)
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Underneath the dense upper cortex conies a broad zone of loosely packed hyphse, 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 hyphse (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 hyphse arise, which constitute the rhizoids and are analogous to roots.
Now the most important question as to the thallus of a Lichen concerns the nature of the green cells, or gonidia.
They play an essential part in the economy of the plant, for, 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 hyphse, a constituent part of the thallus. So long as that view
Wmm-
Fig. 99. — Physcia parietina ; vertical section of thallus t, upper cortial layer ; a, a, gonidia ( = Cystococcus ) em- bedded among medullary hyphse ; u, lower cortical layer. Magnified 500. (After Schwendener. )
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remained undisturbed, the Lichens were rightly ranked as a distinct class, equivalent to Algte and Fungi, and in some respects intermediate between them. Of late years, however, evidence has accumulated, which proves con- clusively that the gonidia do not belong to the same plant with the hyphse, but that they are independent 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 Algse, 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 Algfe 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
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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
Fio. 100. — Physcia parietina ; building up of the Lichen out of the Alga and Fungus. A, germinating ascospore (sp) ; the hyphffi have seized upon two cells (a, a) of Cystococcus humicola. B, more advanced stage ; sp, sp, ascospores which have produced a web of hypha?, enveloping the Cystococms 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 hyphte arising from the fungal spore, and one after another the Cystococcus cells are seized upon by the suckers of the Fungus (Fig. 100, B).
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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 hypliae 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 Pythium or a hop-plant by Splicer otheca. 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. On the other hand, the rhizoids of its fungal companion 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 places, as, for example, on the surface of exposed rocks, where they could not possibly carry on their life if left to themselves.
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Such a relation between two organisms which live in common, and perform certain functions each for the good of the other, is known 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. Reproduction
TheLichens being, as we have seen, compound organisms, might be described either under the heading “ Algae ” 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 Algae 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.
Most Lichens, however, have also a means of vegetative propagation of their own, which is directly connected with their symbiotic mode of life. It often happens that the tissue of the thallus in Physcia and many other Lichens appears to break up into a powdery mass. This is due to a change in the gonidial layer. The algal cells become enveloped, either singly or in little groups, by a luxuriant growth of the fungal hyphen among which they lie. The hyphse surrounding each
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such cell or group isolate themselves from the rest of the medullary tissue. Ultimately the cortex is broken through, and the mass of algal cells, with their envelop- ing hyphce, is exposed as a powdery layer, which is easily dispersed by the wind, or carried away by trickling raindrops. Each grain of the powder is called a soredium, and consists of one or more algal cells enveloped in a little web of fungal filaments. The soredia may grow and break up into new groups, or may develop directly into a Lichen-thallus, if they come to rest in a suitable place. The great point is that each soredium, however small, contains both the Alga and the Fungus, and so provides the two constituent elements of which a Lichen is built up.
The ascus-fruits of Physcia parietina are conspicuous to the naked eye as flat, shield-like discs on the upper 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 sterile tissue at the edge of the apothecium, the whole disc within this rim being covered by the hymenium. Below the hymenium is a dense mass of closely interwoven hyphae forming the hypothecium. The hymenium 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 the same height as the asci, both being closely packed together, so that the hymenium has a smooth, continuous surface (see Figs. 98 and 101).
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At an earlier stage of development the apothecium is closed, and consists of a mass of hyphte surrounded by a cortical layer. The paraphyses which arise from the hypothecium are the first elements of the hymenium to be developed. The asci, which in many cases have been observed to arise from the branches of a distinct hypha, differing from those which produce the paraphyses, are developed relatively late. They grow up among the paraphyses, insinuating themselves between them until they attain an equal height. At the same time the envelope of the fruit is opened at the apex, and the edges gradually pushed back as the hymenium 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 contents of the ascus are not, however, completely used up in the process of spore - formation ; a certain part remains over, lying
Fig. 101.- — Physcia parietina ; 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.)
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between the spores ; this unused substance becomes gelatinous, and on taking up water tends to swell, and so to burst the ascus. The dissemination of the spores, however, is not entirely due to the pressure from within the asci. The whole hymenium, 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 considerable pres- sure, 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 small flask-shaped receptacles called spermogonia. From the walls of these organs filaments project into the cavity, and from the ends of these filaments excessively minute cells (the spermatia) are cut off. The whole structure is not unlike the spermogonium of JEcidium figured below (see Fig. 103). There is no certainty as to the nature of these spermatia. They have been regarded as male cells, and in a few Lichens evidence has been adduced, to show that, by their means, the hypha from which the asci are produced is fertilised. In a much larger number of cases, however, the so-called spermatia have been observed to germinate, and even to produce,
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in the presence of appropriate Algre, a fertile thallus. At present the weight of evidence appears to be against the occurrence of a sexual process in the development of the ascus-fruit of Lichens, though in a few members of the group such a possibility is not excluded.
Physcia then has served to illustrate, on the one hand, a highly organised Ascomycete, with a complex fruit containing a large number of asci ; while, on the other hand, it has made us acquainted with the remark- able phenomenon of symbiosis, or the associated life of two distinct organisms, each performing certain physio- logical functions for the benefit of the other partner.
Type XXII. PUCCINIA GRAMINIS
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.
Puccinia 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 Bust.
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The rust occurs commonly in summer on the leaves and stems of wheat, rye, 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
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vertical unicellular stalk (see Fig. 102). 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 is colourless. There are two germ-pores or thin places in the cell-wall, one on each side of the spore (see Fig. 102). 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 hyphae grow out from the germinating spore through the two germ-pores (see Fig. 102). If germination takes place upon a wheat plant or other host of the Grass family, the hyphae 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 hyphae at once proceed to develop a new mycelium, from which new crops of uredospores arise. This form of
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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. 102), 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. 102).
The teleutospore is borne on a stalk like the uredo- spore, 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
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Fig. 102. — Puccinia graminis. A, section of tlie 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. C, sporidium germinating directly. D, sporidium (sp) germinating indirectly, forming a secondary sporidium (s). Magnified 370. E, part of a sorus, showing several uredospores (a) and one teleutospore ( t ) ; p, p, germ-pores. Magnified 300. F, uredospore germinating. Magnified 300. (After Von Tavel, Tulasne, and De Bary. )
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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. 102). 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. 102. These hyphse never develop into a normal mycelium, but are of strictly limited growth, forming what is called a pro-mycelium (see Fig. 102), which divides by transverse walls, cutting off a row of about four cells from its terminal portion. Each of these cells sends out a slender lateral outgrowth, which swells up at the end to form a small spore-like cell (Fig. 102).
1 The natural order Graminese, or Grasses, includes all cereals.
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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. 102, 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 17
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pretty little yellow cups containing spores (Fig. 103, A). This is the JEcidium form of the parasite, and, like the
Fig. 103. — Puccinia graminis. A, part of the lower surface of a Barberry leaf ; on the swollen part (P) is a cluster of Aicidium- 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 ; alt «2, «3, three stages in the development of the JEcidium-ixmt ; P, peridium ; h, hymenium. Magnified 40. C, three chains of secidiospores ; li, hy menial cells, i.c. intermediate cells. Magnified about 200. (After Zopf and Ivny.)
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Uredo and Puccinia, was long described by botanists as belonging to a distinct genus.
When an JEcidium - fruit is to be produced, a group of hyphse become densely felted together in an intercellular space of the leaf ; the inner hyphse 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 hymenium (Fig. 103, B, h) — 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. 103, C). In this way the whole interior of the young JEcidmm 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. 103).
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 hyphse make their exit.
The secidiospores germinate very readily, within a
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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 Graminese, such as the wheat or rye. 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 of 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 JEcidium 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. 103, B, sp), is found to be a little flask-shaped body, consisting of a sheath of slender converging hyphae, 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. 103). The hyphae 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
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spermatia— are very much smaller than any other form of spore in the Uredinese, their average diameter being about five-thousandths 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.
At present we are quite unable to say what these spermatia are. At one time they were supposed to be male cells, perhaps serving for the fertilisation of the young JEcidium- fruit, but for this view there is no evidence at present. Against it we have to set the fact that the spermatia are capable of a sort of germination, though they have never been known to produce a mycelium. When cultivated in sugar and water they produce new cells by “ budding,” each spermatium forming a little branch which becomes detached from the mother-cell, and repeats the process indefinitely. This is the same mode of growth which is found in Yeast, the Fungus causing the alcoholic fermentation of sugar. However, the spermatia are certainly not identical with true Yeast, and indeed the same mode of growth is common enough among all sorts of Fungi.
It is remarkable that the spermatia are accompanied by a sugary secretion, which is sometimes so abundant that a sweet flavour is perceptible if the surface of the infected leaf be tasted. Further, these bodies often have a pleasant, flower-like smell, and are observed to be highly attractive to flies and other insects, which get smeared with the sticky masses of spermatia and help to scatter them about. But until we have some idea of the function of the spermatia, it is quite impossible to say whether the insect-visitors are of any use to the Fungus.
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We are now acquainted with the full normal life- history of Puccinia graminis, which affords a typical instance of the phenomenon known as hetercecism, 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 JEcidium fructification and the spermogonia are developed. The fecidiospores 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. During the last century a vast amount of evidence was accumulated showing that Barberry bushes acted as centres of infection, from which rust spread over the cornfields. So strong was this convic- tion among agriculturalists, 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
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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 JEcidium 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 chauge 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 iEcidium 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 Gramineee. 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 the best chance of perpetuating their race.
Some other members of the Uredinecc 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.
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Type XXIII
The Mushroom ( Agaricus cavipestris)
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 hyph;e, 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 hyphce and between those of adjacent hyphae. 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
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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. 104,
A, is shown a large piece of the mycelium made up of the thick branched bundles of hyphse, and bearing a number of young fructifications.
The fruit itself, like every other fungal organ, is entirely built up of hyphae. In the stalk these filaments are closety packed towards the outside, forming an apparently paren- chymatous cortex. Towards the middle they are more loosely arranged, so that the individual threads are easily distinguished, and large air- spaces are left between them.
The multinucleate cells of which the hyphae are composed communicate with each other by means of pits, one of which is present in the middle of 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
Fig. 104. — Development of a Mushroom. A, mycelium (m) giving rise to a number of young fructifications.
B , very young mushroom in section ; m, mycelium.
C, slightly older ; l, the gills just appearing. D, still older; l, gills ; m, mycelium. E, older again ; l , gills ; v, velum ; st, stipe. F, nearly ripe ; h, pileus ; other letters as before. Reduced. (After Sachs.)
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the lower surface of the pileus, as shown in Fig. 104, E and F.
The tissue of the pileus is like that of the stalk, but
rather denser. The
Fig. 105. — Gills of Mushroom. A, part of tangential section of pileus (h), showing gills ( l ). Slightly magnified. B, single gill in section ; t, trama ; sh, sub-hymenial layer ; hy, liymenium ; r, lower edge of gill. Magnified about 80. C, part of B enlarged ; t, cells of trama ; sh, sub-hymenial layer ; q, paraphyses ; stages in develop-
ment of basidia ; sp, basidiospores. Magnified 370. (After Sachs.)
gills on the under-sur- face are formed by an extension of the hyphae 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. 1 05). The middle part of each lamella is formed of hyphae 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. 105, B, C, t). Towards the free surfaces the cells of the diverging hyphae are shorter and more closely packed, forming the sub-hymenial layer (sh), and beyond this
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again we come to the hymenium itself, which is thus composed of the terminal cells of the same hyphte which constitute the trama and sub-hymenial layer (Fig. 105). In this last part of their course the filaments have diverged from their original direction to such an extent 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 basidia. 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 basidiospore (see Fig. 105, 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
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gills of a Mushroom. Some idea of their multitude may be obtained by cutting off the pileus of an Agaricits 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.
Curiously enough, nothing is known as to the germina- tion of the spores of the Mushroom, although a few related Fungi have been raised from spores. In the case of the Mushroom the right conditions for spore- cultivation have never been determined, and the Fungus is exclusively propagated from the mycelium or “ spawn.”
Th.e 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 Uredinese, and it appears probable 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 Uredinese, such as the Puccinia 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 these intermediate types.
In Fig. 104 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 hyphse group themselves into a tissue, thus
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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. 104, B, C). The gills are developed endogenously, while enclosed on all sides by continuous tissue (Fig. 104, D). Later on the pileus begins to spread out laterally (Fig. 104, 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. 104, 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. 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 BACTERIA
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
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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 fermenta- tions set up by these organisms has become in recent years of the greatest possible practical importance, both in relation to medicine (as regards the parasitic forms), and to innumerable 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 iVooth of a millimetre in diameter and from jq5oo to 7ooo mm. 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
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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, 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 membrane, — a proof in itself that the latter must be of a very different
character from the cell -wall of ordinary plants (see Fig. 106, B).
The whole interior of the cell is usually occupied by the pro- toplasm. It has not been proved as yet that a nucleus is present, though at certain stages rod-like bodies have been detected which may turn out to be of that nature.
For some time the Bacillus continues in the actively swarm- ing condition, multiplying abund- antly by the repeated transverse division of the cells. After some days the individuals begin to seek the surface of the liquid, 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 zoogloea 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. 106, A). The spores
Fig. 106. — Bacillus subtilis. A, cells cohering in fila- ments ; the dark bodies are spores. B, swarming ciliated cells produced on germination of the spores. Magnified 800. (After Strasburger. )
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in this species and in a very large group of allied Bacteria, are endospores, one spore being produced in the interior of each cell. At its first appearance the young spore is of small size, appearing like a mere granule in the protoplasm of the parent cell. As its growth pro- ceeds 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 surrounding 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 com- pletely dried up without injury ; they are little affected by poisons, and survive a very high temperature, with- standing 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 contains) 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 the entire contents escape as an ordinary bacterial cell, which at once begins to move about by means of cilia (see Fig. 106, B).
Fig. 107 shows very completely the stages in the formation 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 jJoth of a millimetre in diameter. This species was originally found in boiled cabbages, and was after- 18
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STRUCTURAL BOTANY
wards cultivated by its discoverer in solutions of grape- sugar, to which a little extract of meat had been added. The formation of endospores characterises one great group of Bacteria, and distinguishes them from similar unicellular organisms.
Bacillus subtilis, like most other living things, requires
plenty of atmospheric oxygen in order to flour- ish. Some of the other Bacteria, however, have the remarkable peculiar- ity that they thrive best in the absence of free oxygen. This is the case, for example, with Bacillus butyricus, the organism to which the formation of butyric acid by the fer- mentation 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 ex- periments have proved that light has a very unfavourable effect on Bacteria, completely stopping their growth and multiplication 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
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 ; b, vegetative rods ; c-f, successive stages in formation of endospores ; r, four-celled rod with ripe spores ; (h-9a and h1, h2, spores swelling before germination ; the mother cell- walls disappear ; k-m, germination of spores. All figures except « magnified 600. (After De Bary.)
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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.
It was stated in Part I. (p. 204) 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. It is very doubtful,
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however, whether this is its true nature ; neither ciliated swarm-cells nor endospores have been observed, and it is not unlikely that the organism may be an oidial form of some Fungus rather than a true member 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.
Type XXV. Cladothrix dichotoma
We have chosen this plant as an example of another group of so-called Bacteria, which probably, however, have little to do with the true Bacilli and their allies. Cladothrix dichotoma is a very common organism 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 composed of a single series of rod-shaped cells (Fig. 108), and are enclosed in a gelatinous sheath. The branching is not genuine, for it does not depend on the formation of lateral outgrowths, or on a true dichotomy of the growing-point. The gelatinous sheath offers a certain resistance to the growth of the filaments 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 Algse, which Cladothrix and its allies closely resemble. The contents of the
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cells divide up for reproductive purposes into small round bodies (cocci) (see Fig. 108, B and C), which are said to pass through a swarming stage. Endospores are not found in any of these forms, but in some of them it appears that cells of the filament may thicken their walls and serve as resting-spores (arthro- spores) exactly as happens in Nostoc.
The relations of these organisms to the typical Bacteria, as represented by B. suhtilis, is quite an open question, though on the other hand their affinity to the Cyanophycese seems clear.
Before leaving the Bacteria, it may be mentioned that while the great majority are either sapro- phytes or parasites, a few have been found capable of deriving 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 func- tions. Another member of the group is able to obtain its carbon- aceous 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. 108. — Cladothrix dichotoma. A,
branched vegetative filament. Magnified about 450. B, forma- tion of cocci in a filament. Magnified about 700. C, escape of the cocci. Magni- fied about 700. (After Zopf.)
CHAPTER YI
THE MYXOMYCETES1
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 Mycctozoa.
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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. Tiie Plasmodium
This is a very common Myxomycete, occurring 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 creep- ing masses are called plasmodia. A small plasmodium is shown in Fig. 109. 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 — (l)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
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advancing edge of the mass (a, a in Fig. 109) is con- stantly putting out feelers {psmdopodia, 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 plasmodium 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-
f Q
Fig. 109. — Badliamia utricularis ; plasmodium 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
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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, fpr 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, Badliamia utricular is, 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 Badliamia 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 Badliamia and digested, their useless remains being afterwards disgorged and left behind on the track. The way of feeding, by taking solid food into the body, and then digesting it, is characteristic of animals, and is not known among
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true plants. The Myxomycetes 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.
The plasmodium contains an immense number of nuclei, which are only absent from the clear external
portion. The nuclei (see Fig. 110) 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 of a perfectly continuous protoplasmic body of large size, containing vacuoles and numerous nuclei, but entirely 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 sclerotium, the plasmodium
Fig. 1 1 0. — Baclhamia utricular is ; portion of plasmodium, show- ing a number of spherical nuclei, in each of which the fibrillar network and the nucleolus is seen. Magnified 1200. (From Lister’s Mono- graph of the Mycetozoa.)
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may remain alive for as long as three years. The external appearance of the sclerotium 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.
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. Ill 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.
Ill, &). In the meshes of the net- work are contained the numerous 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
Fig. 111. — Leocarpus vernicosus. a, group of sporangia on a frag- ment of dead leaf. Magnified 2|. b, portion of capillitium with spores. Magni- fied 120. (From Lis- ter’s Monograph of the Mycetozoa. )
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STRUCTURAL BOTANY
builds up a network of hollow branched threads (the capillitium), which traverse the cavity in all directions (see Fig. Ill, b). 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 living protoplasm in the sporangium further divides up into spherical spores, each of which includes a single nucleus.
The cell- wall of the spores, and also 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. 112 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.
Fig. 112. — Comatricha obtusata. a, group of sporangia. Natural size, b, empty sporan- gium, showing capil- litium. Magnified 16. (From Lister’s Mono- graph of the Myce- tozoa.)
THE MYXOMYCETES
285
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. 113, 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. 113, d). It contains one nucleus, placed near the thin end, and its protoplasm is
vacuolated; one of the Fig. 113. — Didymium difforme. a,
spore ; b, swarm-cell escaping from the spore - membrane ; c, newly - 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 Mycetozoa. )
vacuoles is contractile, expanding and contract- ing at regular intervals.
The swarm-spore swims 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
286
STRUCTURAL BOTANY
its struggles dragging it in until it is enclosed in a vacuole of the protoplasm, and ultimately digested (Fig. 113, e).
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 up. Before this happens they withdraw their cilia,, and henceforth confine themselves to creeping amoeboid movements. When two creeping mymmceboe, (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. 114) 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
Fig. 114. — Bidymium difforme ; young plasmodium with attendant swarm- cells. to, microcyst. One microcyst is being digested in a vacuole (v). s, empty spore-membrane. Magni- fied 470. (From Lister’s Monograph of the Mycetozoa. )
THE MYXOMYCETES
287
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 Myxomycete 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.
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
288
CONCLUSION
289
a fairly high stage of development (so far at least as 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 plasmodial 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 Cladotlirix show pretty clear affinities with Cyanophyceae, of which we took Nostoc as type, and may be described as Cyanophyceae without pigment. The more typical Bacteria, however (such as Bacillus subtil is and its allies), which are characterised by their cilia and endospores, are 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, are so little known morphologically that nothing definite can be said at present as to their relations.
Nostoc, representing the Cyanophyceae, is another ex- tremely simple type, so far at least as our present knowledge enables us to judge. It is possible, however, that further research may show the cell-structure to be more like that of the higher plants than it appears at 19
290
STRUCTURAL BOTANY
present. It is doubtful whether the Cyanophyceoe should be classed with the Algse or not ; some botanists group them with the Bacteria in a class by themselves — the Schizophyta. As already pointed out, however, this relationship only seems to hold good for a portion of the Bacteria, such as Cladothrix and its immediate allies. Until the histology of the Cyanophycese is better under- stood, it will remain impossible to assign them to their true position. Their present isolation depends on negative characters (absence of sexual reproduction and of nuclei and chloroplastids), which may cease to hold good when our knowledge is more complete.
Leaving these dubious forms, we come to Pleurococcus , a perfectly typical green Alga, 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 probably a somewhat com- plex one, but, however that may be, its structure is a sufficient indication of real affinity with more highly- organised Algse.
From unicellular green Algae 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 com- plex 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 chloroplastids, 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
CONCLUSION
291
members of the group also (though not in our type) the external form of the individual cells is 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 Conjugatse, so far as we know, do not lead on to any of the higher groups of plants.
In quite a different direction green unicellular Algte appear to have given rise to the remarkable family represented among our types by Vaucheria. Here the cell has become multinucleate, 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. Vaucheria itself, though its thallus is simple compared with that of other 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.
Vaucheria 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 Vaucheria ; in fact Pythium might fairly be described as a member of the Yaucheriacem 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
292
STRUCTURAL BOTANY
from Cryptogams to Phanerogams. In both cases the disappearance of motile male cells is correlated with the loss of aquatic environment. The gradual extinction of zoospores among the allies of Pythium 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 Pythium group, though they still show signs of affinity with Siphoneous Algse. 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 cases like Sphcerotheca, 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 Phycomycetes (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
CONCLUSION
293
groups the conidia (the origin of which could already be traced in the Phycomycetes) have displaced the sporangia, and assume very various forms, constituting the great means of propagation. This high differentiation of the conidia is very well marked among the Uredinete, where we find four distinct kinds of conidial fructification : the cecidiospores, the uredospores, the tedcutospores, and the sporidia, not to mention the spermatia, which are of doubtful nature.
The Uredinese are adapted to a strictly parasitic mode of life, and in habit differ greatly from the Basidiomycetes, especially if we consider a highly-organised representative 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 germinating in situ.
We see then that the Fungi form by themselves a highly-complex cycle of relationship, touching the lower Algse at one or two points, but otherwise distinct from the rest of the vegetable kingdom. It is remarkable in how small a proportion of this mass of saprophytic and parasitic forms there is any evidence for the occurrence of sexual reproduction.
We must now retrace our steps to the Algae. The Bed 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,
294
STRUCTURAL BOTANY
The sexual process in Florideae is quite peculiar among Algae, 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 a fruit is the result of fertilisation. The Florideae are also remarkable for the entire absence of motile ciliated cells, in which they differ from the great majority of the Algae, though certain isolated groups, such as the Conjugatae (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 Chlorophyceae than is the case with the Bed Seaweeds. Ciliated cells are almost universal throughout the group, though in the highest Brown Algae — 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
295
We must now return to the Chlorophyceae. We have in Ulotlirix 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 like ordinary zoospores, 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 peculiarly interesting, because when con- jugation is accomplished it has a marked effect on the future development. The solitary 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, the first indication of regularly alternating generations. For this reason we regard Ulothrix 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, however, may more than another inherit the characteristics of some remote ancestor, and this is all we n ean in speaking of lines of descent among plants. Ulothrix enables us to form an idea
296
STRUCTURAL BOTANY
of what the simple algal predecessor of the lowest Bryophyte may possibly have been like.
Edogonium 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, (Edogonium 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 (Edogonium 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, is an important step towards the regular alternation of sexual and asexual reproduction. In this respect, however, Edogonium shows no advance on Ulothrix.
CONCLUSION
297
In another genus of green fresh-water algfe, Coleo- chcete, the oospore, while still enclosed in the oogonium, divides up into a cellular tissue, in each cell 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 ( Riccia ) in which the fruit consists of nothing but a mass of spore mother-cells surrounded by an epidermis. There is, however, a very wide gap between the Alga and the Hepatic. 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 Archegoniatse, from the lowest Bryophyta upwards, differ from those of any of the Algte in being almost always suited for dissemination by the air. The sporophyte which bears them is essentially the aerial generation, while the odphyte 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.
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STRUCTURAL BOTANY
Possibly the aerial, asexual spores of the Bryophyta may be homologous with the aquatic, asexual spores of the Green Algae, 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 Archegoniatae must have taken place in enormously remote geological ages, when plants were first adapting themselves to terrestrial life, and we cannot be surprised that no transitional forms connecting them with the Algae are known to us.
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 generations are more highly developed than in the Hepaticae. 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 Algae to Bryophyta, still more is this the case when we advance from Bryophyta to the Vascular Cryptogams.
CONCLUSION
299
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 Bryophytes. 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 that the sporogonium 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 an- tiquity of the Vascular Cryptogams, which, together with certain Gymnosperms, were well developed even in the Devonian period ; their origin is completely lost in the mists of palaeozoic antiquity, and at present we are entirely without any facts which can throw light on the mystery.
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
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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 homospoi’ous and heterosporous forms occur, 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. 296). 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 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
301
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 plants. 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- spermous flowering plant, such as Picea, are quite clear, and have been fully demonstrated above (p. 31). It is doubtful, however, whether there is any rear affinity between Selaginella and the Gymnosperms, which may have had quite a different origin. In any case the homologies hold good, whatever the particular hetero- sporous 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. The male prothallus no longer gives rise to spermatozoids, but the generative nucleus is carried passively to the ovum by the growth of the pollen-tube. Fertilisation leads to the formation of a seed — the characteristic structure of flowering plants, consisting of sporangium, prothallus, and embryo, united to form one body, and fed entirely at the expense of the parent sporophyte.
From Gymnosperms to Angiosperms is another great step, and here again the gulf has not yet been bridged. In Angiosperms the female prothallus has almost dis- appeared, and even the archegonia are no longer recognis- able, The embryo-sac (the equivalent of the megaspore)
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STRUCTURAL BOTANY
proceeds, after only a few preliminary divisions, to the formation of the ovum, and the development of the endo- sperm is postponed till after 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 Angiosperms 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 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 Angiosperms, is connected with the occurrence of pollination by insects, which is quite a new departure, peculiar to Angiosperms.
At present we are not in a position to determine either the relation of Angiosperms to Gymnosperms, nor that of Monocotyledons to Dicotyledons. The latter classes are mainly distinguished by vegetative characters, the reproductive phenomena being the same in both. On both these questions, however, we may hope for further light, especially from palaeontological research, for the first appearance of both Monocotyledons and Dicotyledons falls within a geological period from which abundant fossil remains have come down to us.
This brief summary has had one main object,- — to indicate how complicated and difficult, questions as to the affinities of plants really are. Most 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
CONCLUSION
303
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 immense majority came to an end in the long past geologic ages.
If we attempted to construct such a tree, even 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 Pclvetia, 193
iEcidium, 258 iEcidiospores, 259 Agaricus eampestris. — See Mush- room
Algse, 146-215
in Liclien, 244
blue-green. — See Cyanophycece
brown. — See Phceophycece
green. — See Chlorophycece
red. — See Floridecc
Alternation of generations in Bryo- phyta, 109, 126, 144
in Equisetum, 106
— in Male Fern, 43, 73
in (Edogonium , 157
in Selaginella, 74
in Ulotlirix, 167
Amphithecium, 142 Androspores, 154 Angiosperms, affinities of, 301 Annulus, 56, 83, 139 Antheridium, 24
of Callithamnion, 206
of Equisetum , 101
— — of Funaria, 133
of Male Fern, 43, 64, 65-67
of CEdogonium,, 152
of Pellia, 114
of Pelvetia, 194
of Pythium, 225
of Selaginella, 24
of Sphcerotheca, 237
of Vaucheria, 178
Anthoceros, 140
20
Apical cell of Callithamnion, 203
of Funaria, 1 32
of Equisetum (root), 95
of Equisetum (stem), 92
of Male Fern (root), 53
of Male Fern (stein), 52
of Pclvetia, 193
of Selaginella, 14
Apogamy, 76 Apophysis, 137 Apospory, 76, 144 Apothecia, 248 Archegonium, 26
of Equisetum, 102
of Funaria, 135
of Male Fern, 43, 64, 65, 67-69
of Pellia, 116
of Selaginella, 33
Archegoniatae, 145, 297 Archesporium of Equisetum, 97
of Funaria, 138
of Male Fern, 58
of Pellia, 121
of Selaginella, 16
Arthrospores, 277 Ascogonium, 237 Ascomycetes, 235
affinities of, 292
Ascophyllum, 200 Ascospores, 238, 248 Ascus, 235, 238, 240, 248 Aspidium Filix-mas, 37-77 Asplenium buTbiferum, 76
viviparum, 7 6
Atriehum, 129, 131 Auxiliary cells, 208
306
INDEX
Bacillus butyricus, 274 Bacillus, hay. — See Bacillus sub- tilis
Bacillus megatherium, 273
radicicola, 27 5
subtilis, 271-276
Bacteria, 215, 218, 270-277
affinities of, 289
Badliamia utricularis, 279-287 Barberry, 257, 262
Law, 262
Basidiomycetes, 267
affinities of, 293
Basidiospore, 267 Basidium, 267 Batrachospermum, 211 Berberis vulgaris.— See Barberry Broom-rape. — See Orobanche Bryopliyta, 109-145
affinities of, 297
Bulbochcete, 159 Bud of Bquisetum, 94
Callithamnion corymbosum, 202- 212
affinities of, 293
Calyptra of Funaria, 137
of Pellia, 123
Capillitium, 284 Capsule of Funaria , 137
of Pellia, 1 20
Carbon, assimilation of, 277 Carinal cavities, 85 Carpogonium, 208 Carpospore of Callithamnion, 211 Cell-division in Spirogyra, 169 Central cylinder of Funaria, 129 Clilamydospores, 232 Chlorophycese, 147-183 Chlorophyll-granules of Vaucheria, 175
Cliloroplast of QUdogonium, 149
of Pleurococcus, 182
of Sptirogyra, 168
of Ulothrix, 160
Cilia, 21
of Bacteria, 292
of zoospores in Ectocarpus, 186
of spermatozoids of Equisetum,
101
of Funaria, 134
Cilia of spermatozoids of Male Fern, 65
of zoospores of (Edogonium,\bti
of spermatozoids of Pellia, 116
of Pelvetia, 195
of zoospores of Pythium, 221
of spermatozoids of Selagin-
ella, 21
of zoospores of TJlothrix, 161
of Vauclicria, 176
Circinate vernation, 41 Cladothrix dichotoma, 276-277 Club-moss, 1 Cocci, 277 Coleochcde, 297
Collateral bundles in Equisetum, 83 Columella of Funaria, 138
of Pilobolus, 231
Comatricha obtusata, 284 Commensalism, 247 Compound leaf, 40 Conceptacles, 190, 194 Cone of Equisetum, 82, 96
of Selaginella, 5
Conferva, 159
Conidia, 224, 234, 239, 252, 269 Conidiophores, 239 Conjugate, 173
affinities of, 290
Conjugation of Ectocarpus, 187
of Pilobolus, 233
of Spirogyra, 170
of Ulothrix, 163, 164
of Equisetum, 87
Cortex of Funaria, 129
of Selaginella, 9
Cryptogams, 2 Cuscuta, 216 Cyanophycere, 212-215
affinities of, 289
Cycadese, 35
development of endosperm in,
34
Cyst, 282
Cystocarp of Callithamnion, 211 Cystococcus, 244
Damping off of seedlings, 219 Dehiscence of Sporangium of Equis- etum, 99
of Male Fern, 59
INDEX
307
Dehiscence of Sporangium of Sela- ginella, 20 Dichotomous, 3 Dichotomy, 14, 113, 190, 193 Didymium difforme, 285 Dioecious prothallus of Equisetum, 113
Discomycetes, 240 Diseases caused by Fungi, 217 Dodder. — See Cuscuta Dry-rot, 217
Dwarf-males of (Edogonium, 154
Ectocarpus siliculosus, 185-189 Elaters of Equisetum, 98
of Pellict, 121
Elodea (Part I., 42), 23 Embryo of Equisetum, 104
of Male Fern, 72
of Selaginella, 29
Embryologjq 28 Embryo-sac of Picea, 33
formation of, in Sequoia, 34
Endodermis, double, in Equisetum, 91
of root of Equisetum, 91, 96
of stem of Equisetum, 85
of root of Male Fern, 51
of stem of Male Fern, 47
of Selaginella, 10
Endosperm, development of, in Cycadeae, 34 Endospores, 273 Endothecium, 142 Enzymes, 220 Epibasal, 72
Epidermis of Equisetum, 89
of Funaria, 128, 138
of Male Fern, 49, 50
of Pallia, 112
of Selaginella, 13
Equisetum arvensc, 78-108
affinities of, 300
limosum, 85, 99
maximum, 99
variegatum, 85
Enjsiphc, 236 Eye-spot, 162
Fermentation, 271 Ferments. — See Enzymes
Fern, Bracken, 42 — — - Filmy, 35, 42, 44, 76
Male, 37-77
Ferns, affinities of, 300
Tree, 38, 42
Fertilisation, 28
of Callithamnion, 20.9
of Equisetum, 104
of Funaria, 136
of Male Fern, 69
of (Edogonium , 155
-of Pellia, 119
of Pelvetia, 198
of Pythium, 226
of Selaginella, 28
of Sphcerotheca, 237
of Vaucheria, 180
Florideae, 201-212
affinities of, 293
Foot in Equisetum, 105
in Funaria, 1 42
in Male Fern, 72
in Pellia, 121
in Selaginella, 30
Fossil Equisctaceae, 107 Frond of Male Fern, 40 Fruit of Ascomycetes, 240
of Callithamnion, 210
of Funaria, 137
of Pellia, 120
Fucaceae, 184-189 Funaria hygrometrica, 126-145 Fungi, 216-269 affinities of, 291
Germination of aicidiospores, 260
of aseospores of Physcia, 250
— — of Sphcerotheca, 239
of Bacteria, 273
of carpospores of Callith- amnion, 212
of conidia, 239
of Ectocarpus, 186, 188
of megaspores of Selaginella, 24
of microspores of Selaginella, 20
of oospore of (Edogonium , 156
of Pelvetia, i 99
of oospore of Pythium, 227
of Vaucheria, 181
of spores of Equisetmn, 99, 100
of Male Fern, 43
308
INDEX
Germination of spores of Myxomy- cetes, 285
of Pilobolus, 232
of sporidia of Puccinia, 257
of telentospores of Puccinia,
256
— — of tetraspores of Callith- aninion, 205
— of nredospores of Puccinia,
of zoospores of Edogonium, 152
of Pythium, 221
of Ulothrix, 162
of Vaucheria, 177
of zygospore of Pilobolus, 234
■ of Spirogyra, 173
of Ulothrix, 165
Germ-pores, 253, 256, 259 Gills. — See Lamellae Gongrosira condition of Vaucheria,
- 181
Gonidia of Liclien, 243 Growing point of Equisctum, 92-96
of Funaria, 1 32
intercalary, 186
of Male Fern (root), 53
of Male Fern (stem), 52
of P cilia, 113
of Selaginella, 3, 1 4
Gymnosperms, affinities of, 301
■ comparison with Selaginella, 31
Haustoiua of Puccinia, 252
of Splicer otheca, 236
Hermaphrodite, 35 Heterocysts, 214 Heterceeism, 262 Heterosporous Cryptogams, 300
Equiseta, 104
Hofmeister, 35 Homology, 36 Hop, mildew of, 236 Hormogonia, 214 Horsetails. — See Equisctum Hymenium, 240, 248, 259 Hymenomycetes, 267 Hyphse of Lichen, 242
of Mushroom, 264
of Pilobolus, 229
— — - of Puccinia, 252 — — of Pythium, 219 — - of Sphcerotheca, 236
Hypobasal, 72 Hypocotyl, 30
Lsdusium, 43 Internode, 84 Involucre of Pallia, 117 Isoetes, 4
Krakatoa, 60
Lamellae, 265
Laminaria, 189
Leaf, compound, 40
Leaflets of Male Fern, 40
Leaf-sheaths of Equisctum, 80, 90,92
Leaf-trace bundles of Equisetum, 83
of Funaria, 130
of Selaginella, 11
Leives of Equisetum, 80, 90
of Funaria, 128, 130
of Male Fern, 40
of Selaginella, 3, 12
Leguminosfe, root-tubercles of, 275 Leocarpus vcrnicosus, 283 Lepidodendron, 4, 10 Lichen-Fungi, 247 Lichens, 241
synthesis of, 244
Light, influence of, on Bacteria, 274
on Myxomycetes, 281
— on zoospores of Edo -
gonium, 151
-= on zoospores of Ulothrix,
161
Ligule of Selaginella, 4 Litmus, 242 Liverworts, 110-126
affinities of, 297
Lycopodium, 1
Macrosporangium. — See Mcgcis- porangium
Macrospore. — See Megaspore Male Fern. — See Aspidium Filix- Mas
Malic acid, 70 Marchantia, 118
Megasporangium of Selaginella, 7, 1 9 Megaspore of Selaginella, 7, 19
germination of, 24
Microcysts, 286
INDEX
309
Microsporangium of Selaginella, 6, 17
Microspores of Selaginella, 6, 18
germination of, 20
Mildew of hop, 236
of wheat, 251, 254
Monarch, 13 Monostelic, 11, 44 Monotropa, 216 Mosses, 110, 126-145
affinities of, 298
Mould, 217
Movements of Bacteria, 272
of Myxomycetes, 279, 285
of spermatozoids, 22, 67, 70,
136, 155, 198
of zoospores, 151, 161, 177,
186, 221 Mucor, 234
Muscine®. — See Bryophyta Mushroom, 264-269 Mycelium, 219, 229, 236, 252, 257, 264
Myxamceb®, 286 Myxomycetes, 278-287 affinities of, 288
Ncmalion, 209 Neottia nidus-avis, 216 Nitrates, 277
Nitrogen, assimilation of, 275 JYostoc, 212-215
affinities of, 289
Nucleus of Mushroom, 265, 267
of Myxomycetes, 252, 254
of CEdogonium, 149
of Pelvctia, 196
of Pythium, 219
of Spirogyra, 169
of Vaucheria, 175, 180
O a uweed. — See Laminaria, (Jiclogonium, 1 4 7-159
affinities of, 296
ciliatum, 148
Oidium condition, 232 Ougonium of CEdogonium , 153
of Pelvetia, 195
of Pythium, 224
of Vaucheria, 178, 179
Odmyeetes, 228
Odphyte of Equisetum, 100
of Funaria, 128
of Male Fern, 43
of Pcllia, 110
Oospore of CEdogonium, 156
of Pelvctia, 200
of Pythium, 226
of Vaucheria, 181
Operculum of Funaria, 139 Orobanche, 216 Osmunda, 61 Ovum, 32
of Equisetum, 104
of Funaria, 135
of Male Fern, 69
of CEdogonium, 153
of Pcllia, 117
of Pelvctia, 197
of Pythium, 225
of Selaginella, 26
of Vaucheria, 180
Pale.®, 41, 48
Palisade tissue in Funaria, 139
in Male Fern, 49
Palmella condition of Vlothrix, 167 Parapliyses of Funaria, 134
of Lichen, 248
of Mushroom, 267
of Pelvetia, 194
Parasites, 216, 270 Pcllia cpiphylla, 110-126
affinities of, 297
Peltate scales of Equisetum, 82, 97 Pelvetia canaliculata, 189-201 Pericycle, 47, 51, 87 Peridium, 259 Periplasm, 225 Peristome, 141 Perithecium, 239 Peronospora densa, 223
Lactucoc, 224
nivea, 223
Radii, 224
Ph®ophyce®, 183-201
affinities of, 294
Phreozoospore®, 184 Phloem of Equisetum, 87
of Male Fern, 47
of Selaginella, 10
Phycomycetes, 229
310
INDEX
Physcia parietina, 240-251
affinities of, 292
Phytophthora infestans, 223 Picea, 26, 33 Pileus, 264
Pilobolus crystallinus, 228-234
affinities of, 292
Pinna, 40 Pinnate, 40 Pitli in Equisetum, 85 Pits of Callithamnion, 203 Placenta, 55 Planogametfe, 166, 187 Plasmodium, 279, 287 Pleurococcus vulgaris, 290 Polystelic, 44 Procarpinm, 208 Promycelinm, 256
of Equisetum, 100
of Male Fern, 43, 62-64
of Selaginella., 20, 33
l’rotonema, 143
Proto xylem of Equisetum, 86
of Male Fern, 47
of Selaginella, 10, 11
Psendopodia, 280
Pteridophyta. — See Vascular' Cryp- togams
Puccinia graminis, 251-263
Malvacearum, 268
Pyrenoids of CEdogonium, 149
of Spirogyra, 164
of Ulothrix, 1 60
Pythium Baryanum, 218-228 affinities of, 291
Rachis, 40 Receptacles, 190, 194 Respiration of Bacteria, 274 Rhizoid of Funaria, 131
of Lichen, 242
of Sphicrotheca, 236
of Vaucheria, 175, 178
Rhizophores, 4, 13 R i ccia, 297
Root of Equisetum, 80, 90, 95 - — — of Male Fern, 42, 50
of Selaginella, 13
Root-cap of Equisetum, 95
of Male Fern, 53
of Selaginella, 15
Root-liairs of Equisetum, 91, 100
of Funaria, 127, 131
of Male Fern, 52, 61
of Pellia, 111
of Selaginella, 28
Rootlet of Equisetum, 96
of Male Fern, 54
Root-tnbercles of Leguminosre, 275 Rust, 251
Saprophytes, 216, 269, 270 Sargassum, 201 Scalariform, 12, 47 Schizophyta, 290 Sclerotic m, 282 Seaweeds, 146, 183, 201 Selaginella, 1-37
affinities of, 301
Sequoia, formation of embryo-sac.
34
Seta of Funaria, 137
of Pellia, 120
Sexual generation. — See Oophyte Sieve-plates of Pelvetia, 191 Siphonere, 174, 177, 181
affinities of, 291
Soredium, 248 Sorus, 43, 55
Spawn of Mushroom, 264, 268 Spermagonia, 250, 260 Spermatium, 206, 250, 261 Spermatozoids of Equisetum, 101
of Funaria, 134
of Male Fern, 65
of CEdogonium, 152, 155
of Pellia, 116
of Pelvetia, 195
of Selaginella, 21
of Vaucheria, 179
Sphcerotlieca, affinities of, 292
Castagnei, 235-240
pannosa, 236
Spirogyra, 168-174
affinities of, 290
Sporangium of Edocarpus, 186
of Equisetum, 82, 96
of Male Fern, 42
of Myxomycetes, 283
of Pilobolus, 230
plurilocular, 187
of Pythium, 221, 227
INDEX
311
Sporangium of Selaginella, 6, 16
unilocular, 186
Sporangioles, 234
Sporangiophores of Equisetum , 82, 96
Spore mother-cell, 17
sac of Funaria, 138
Spores of Bacteria, 272
of Callithamnion, 204, 211
of Ectocarpus, 186
of Equisetum, 98
of Funaria, 141, 142
of Male Fern, 42, 56
of Mushroom, 267
of Myxomycetes, 283
of lEdogonium, 150
of Nostoc, 214
of Pellia, 124
of Pelvetia, 200
of Physcia, 245, 248-
of Pilobolus, 230
of Puccinia, 252, 254, 258
of Pythium, 224
of Selaginella, 6, 16
of Sphcerotheca, 238
of Spirogyra, 172
swarm, 285
of Ulothrix, 166
of Vaucheria, 176
Sporidia, 257, 267 Sporogonium of Funaria, 137
of Pellia, 120
Sporophyll, 8, 16 Sporophyte of Equisetum, 96
of Funaria, 137, 142
of Male Fern, 43
of Qfdogonium, 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, 257, 267 Stipe, 264
Stomata of Equisetum, 89
of Funaria, 139 .
of Male Fern, 50
Stomata of Selaginella, 13 Suspensor of Selaginella, 29 Swarm-cell. — See Zoospore, and Spores, swarm Symbiosis, 247, 251
Tapetum of Equisetum, 97
of Male Fern, 58
of Selaginella, 16
Teleutospores, 254 Tetrasporangium, 204 Tetraspores, 204
Thallus, 110, 185, 190,202, 203, 242 Toadstools, 264 Tracheides of Equisetum, 86
of Male Fern, 47
of Selaginella, 12
Trama, 266 Trieliogyne, 208 Trichopliore, 208
Ulothrix sonata, 159-167, 188
affinities of, 295
Uredine®, 251-263, 268
affinities of, 293
Uredo, 253, 263 Uredospores, 252, 253
Vacuoles, gas, 214
pulsating, 161
Vallecular cavities, 85 Vascular bundles of Equisetum, 86
Cryptogams, 1-108
affinities of, 299
Vaucheria, 174-182, 218
affinities of, 291
Veil. — See Velum Velum, 269
Venation of Male Fern, 41 Vernation, 41
Wheat, rust and mildew of, 251 Wood of Equisetum, 86
of Male Fern, 46
of Selaginella, 10
Xylem. — See TVood
Yeast, 261
Zooglcea, 272
312
INDEX
Zoospores of Eclocarpus, 186, 187
of (Edogonium , 151
of Pytliium, 221, 227
of Ulotlirix, 160, 165
of Vaucheria, 176
Zygomycetes, 228
Zygomycetes, affinities of, 292 Zygospores, 164
o i Ectocarp us, 189
— — of Pilobolus, 233
of Spirogyra, 171
of Ulotlirix, 164
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