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Full text of "The origin of a land flora : a theory based upon the facts of alternation"

LIBRARY 

OF THE 

UNIVERSITY OF CALIFORNIA. 



Class 



THE ORIGIN OF A LAND FLORA 



MACMILLAN AND CO., LIMITED 

LONDON BOMBAY CALCUTTA 
MELBOURNE 

THE MACMILLAN COMPANY 

NEW YORK BOSTON CHICAGO 
ATLANTA SAN FRANCISCO 

THE MACMILLAN CO. OF CANADA, LTD, 

TORONTO 



> 

QS 




LYCOPODI U M, 
S E LA CO. L. 



THE .ORIGIN 

OF A 

LAND FLORA 



A THEORY 
BASED UPON THE FACTS OF ALTERNATION 



BY 

R O. BOWER, Sc.D., RR.S. 

\ V 

REGIUS PROFESSOR OF BOTANY IN THE UNIVERSITY OF GLASGOW 




OF THE 

UNIVERSITY 

OF 



WITH NUMEROUS ILLUSTRATIONS 




MACMILLAN AND CO., LIMITED 

ST. MARTIN'S STREET, LONDON 

1908 



Q\< 



GENERAL 



GLASGOW : PRINTED AT THE UNIVERSITY PRESS 
BY ROBERT MACLEHOSE AND CO. LTD. 






T 

E 

OF 



OF THE 

/ UNIVERSITY 



PREFACE 



IN the year 1874 apogamy was discovered in Ferns by Farlow : and in 
1884 instances of apospory in Ferns were demonstrated before the Linnaean 
Society of London by Druery. These events stimulated a fresh enquiry into 
the nature and origin of Alternation in Archegoniate Plants. My own 
observations on apospory confirmed my interest in this question : it seemed 
to me probable that some biological cause had determined the prevalence 
and constancy of the alternation, to which apogamy and apospory appear as 
occasional exceptions. The theory was entertained that the change of 
conditions involved in the invasion of the Land by organisms originally 
aquatic had played a prominent part 'in the establishment of those 
alternating phases of the life-cycle which are so characteristic of Archegoniate 
Plants. As early as 1889 I had already written several chapters of a 
treatise on this subject : but the necessary facts were found to be then so 
imperfectly known that the work was abandoned, and instead of a full 
discussion of the matter, the Biological Theory of Antithetic Alternation 
was briefly stated in a paper published in the Annals of Botany in 1890 
(vol. iv. p. 347). The main position of Celakovsky in discriminating 
between Homologous and Antithetic Alternation was adopted ; but the 
latter type, as seen in Archegoniate Plants, was recognised as having been 
fixed and perpetuated in^accordance with the adaptation of aquatic organisms 
to a Land- Habit. The Studies in the Morphology of Spore-producing Members 
were then entered upon as preliminary investigations to elucidate the facts 
requisite for a more full statement, and they were published in five parts, 
from 1894 to 1903. Meanwhile, in 1894 Strasburger contributed to the 
Meeting of the British Association in Oxford his paper on the " Periodic 
Reduction of Chromosomes." He brought together a wealth of facts 
establishing the cytological distinction of the alternating generations, and 
his theoretical position was virtually identical with that of my paper of 
four years earlier. 

175G03 



VI 



PREFACE 



Now, after the lapse of seventeen years, it has been possible to state the 
biological argument more fully in the present volume, strengthened by many 
new facts. The First Part (pp. 1-254) deals with the general theory. The 
Second Part (pp. 255-657) is taken up with a detailed statement of the facts, 
together with comparison of the constituents of the several phyla inter se. 
The Third Part (pp. 658-717) is devoted to general comparisons and con- 
clusions. The attempt has been made to work in the results of Palaeonto 
logical research with those of the comparative analysis of living forms. The 
enquiry has related to all the characters, both vegetative and propagative, of 
the sporophyte generation : these include the external form, the embryogeny, 
and anatomical features, and especially the structure and development of 
the Spore-producing members, while the characters of the gametophyte 
have also been taken into account. It is found that the conclusions 
arrived at are supported by general convergence of the lines of evidence 
derived from all of these sources. 

The method adopted in the preparation of this work has been to 
examine not only the mature structure, but also the development of the 
organisms, and of their several parts. While fully utilising the results of 
Palaeontological and anatomical study, considerable weight has throughout 
been given to the facts of the individual development : sometimes the latter 
appear to oppose the former. It is not held that the ontogenetic history 
will always serve as an infallible guide, and opportunity has been taken 
to point out that conclusions based upon it are liable to be overruled by 
the results of wide comparison (pp. 159, 636, and 660, footnote). But it 
is felt that in much of the recent work on Pteridophytes, and especially 
where fossil comparisons come in, the arguments from individual develop- 
ment have been accorded less than their due share of attention. 

I have made no attempt to give comprehensive or complete biblio- 
graphical references : from Campbell's Mosses and Ferns and from other 
sources such references can readily be obtained. But wherever a quotation 
is made, or where a substantial body of information derived from another 
author has been embodied in the text, the reference is fully given. While 
thus acknowledging my indebtedness to those whose work is published, 
I desire also to record the continuous personal help so willingly given by 
three friends and colleagues, who have all allowed me the use of unpub- 
lished drawings and facts. Mr. Kidston's peculiarly exact knowledge has 
greatly strengthened and amplified the Palaeontological statements, while 
Dr. Lang and Mr. Gwynne-Vaughan have given me throughout the 
assistance of friendly criticism, and the support of their special knowledge 
of certain branches of the matter in hand. 

In conclusion, I am well aware that the chief question dealt with 



PREFACE vii 

lies outside the realm of possible proof under present conditions : the 
theory is submitted as a working hypothesis. Naturally it is applicable 
with greater readiness to those organisms which are less advanced, but 
less readily to those which have departed furthest along the lines of 
adaptation to life on exposed Land-Surfaces. Other opinions on the origin 
and nature of Alternation have come into fresh prominence in recent 
years, and especially the view that the present condition of the Arche- 
goniatae has originated by differentiation of phases of a life-cycle originally 
Homologous. This theory has not been disproved any more than the 
theory of Antithetic Alternation has been proved Whatever view be 
ultimately taken of the prime origin of the alternating generations, many 
of the conclusions arrived at here as to the morphological progress and 
phyletic grouping of the Archegoniatae will stand : they have a validity 
of their own quite apart from any question of the ultimate origin of the 
sporophyte, which has finally become the dominant factor in the Flora of 
the Land. 

F. O. BOWER. 



GLASGOW, December, 1907. 



TABLE OF CONTENTS 

PART L 
STATEMENT OF THE WORKING HYPOTHESIS 

CHAPTER HAGR 

INTRODUCTION, - i 

I. THE SCOPE AND LIMITATIONS OF COMPARATIVE MORPHOLOGY, - 5 

II. THE LIFE-HISTORY OF A FERN, 14 

III. ON THE BALANCE OF THE ALTERNATING GENERATIONS OF 

ARCHEGONIATAE, - - 33 

IV. CYTOLOGICAL DISTINCTION OF THE ALTERNATING GENERATIONS 

OF ARCHEGONIATAE, - 46 

V. ALTERNATION IN THALLOPHYTES, - 63 

VI. BIOLOGICAL ASPECT OF ALTERNATION, - 79 

VII. STERILISATION,- 87 

VIII. THE SPORANGIUM DEFINED, - 103 

IX. SOME GENERAL ASPECTS OF THE POLYSPORANGIATE STATE, 113 

X. VARIATIONS IN NUMBER OF SPORANGIA, 119 

XI. THEORY OF THE STROBILUS, - 132 

XII. SPORANGIOPHORES AND SPOROPHYLLS, - 144 

XIII. ON THE RELATIONS BETWEEN THE STERILE AND FERTILE REGIONS 

IN THE SPOROPHYTE, 156 

XIV. EMBRYOLOGY AND THE THEORY OF RECAPITULATION,- 173 
XV. ANATOMICAL EVIDENCE, - 188 

XVI. SYMMETRY OF THE SPOROPHYTE, - - 201 



CONTENTS 

CHAPTER PAGE 

XVII. THE ESTABLISHMENT OF A FREE-LIVING SPOROPHYTE, 218 

XVIII. EVIDENCE FROM PALAEOPHYTOLOGY, - 227 

XIX. AMPLIFICATION AND REDUCTION, - 233 

XX. SUMMARY OF THE WORKING HYPOTHESIS, 244 



PART II. 

DETAILED STATEMENT OF FACTS 

INTRODUCTION, 255 

XXI. BRYOPHYTA, (i.) HEPATICAE, - 257 

XXII. BRYOPHYTA, (n.) Musci, - 272 

XXIII. LVCOPODIALES GENERAL MORPHOLOGY, 288 

XXIV. LYCOPODIALES SPORE-PRODUCING MEMBERS, 311 
XXV. LYCOPODIALES COMPARATIVE ANATOMY, 328 

XXVI. LYCOPODIALES EMBRYOLOGY AND COMPARATIVE SUMMARY, 340 

XXVI I. EQUISETALES, - 366 

XXVIII. SPHENOPHYLLALES, - 39 8 

XXIX. SUMMARY FOR SPORANGIOPHORIC PTERIDOPHYTES, 423 

XXX. OPHIOGLOSSALES, - 43 

XXXI. COMPARATIVE DISCUSSION OF OPHIOGLOSSALES, - 476 

XXXII. FlLICALES BOTRYOPTERIDEAE, 495 

XXXIII. FILICALES MARATTIACEAE, 505 

XXXIV. FILICALES OSMUNDACEAE, 530 
XXXV. FILICALES SCHIZAEACEAE AND MARSILIACEAE, 542 

XXXVI. FILICALES GLEICHENIACEAE AND MATONINEAE, - 553 

XXXVII. FILICALES LOXSOMACEAE AND HYMENOPHYLLACEAE, - 570 

XXXVIII. FILICALES THYRSOPTERIDEAE, DICKSONIEAE, DENNSTAED- 

TIINAE, CYATHEAE AND SALVINIACEAE, 589 

XXXIX. FILICALES MIXTAE, 612 

XL. COMPARISON OF THE FILICALES, - 624 



CONTENTS xi 



PART III. 
CONCLUSION 

CHAPTER PAGE 

XLI. ALGAE AND BRYOPHYTA, 658 

XLII. EMBRYOGENY OF THE PTERIDOPHYTES, - 663 

XLI 1 1. THE VEGETATIVE SYSTEM OF VASCULAR PLANTS ANALYSED, - 678 

XLIV. THE VASCULAR SKELETON,- - - 685 

XLV. THE SPORE-PRODUCING MEMBERS, - - 692 

XLVI. HETEROSPORY AND THE SEED-HABIT, - 703 

XLV 1 1. RESULTS, PHYLETIC AND MORPHOLOGICAL, 709 

INDEX, - 718 



ADDENDUM. 

By a regrettable oversight no mention has been made in the text 
of the interesting new genus Loxsomopsis, described by Dr. Christ as 
having been discovered in Costa Rica (Bull, de VHerb. Boissier, 2me 
sen, tome iv., p. 393, 1904). This rare Fern, still unknown as 
regards stipe and rhizome, shows a habit like that of Loxsoma, but larger 
The outline of the leaf, especially at the base of the pinnae, show 
archaic characters, while the sori correspond in general features to the 
of Loxsoma; but the sporangia have a lateral dehiscence, and a compl 
ring composed of very numerous cells. In these details Loxsomo; 
corresponds to Thyrsopteris, Pending a better knowledge of its characi 
and especially of its anatomy, it may be accorded a place in 
neighbourhood of Loxsoma and Thyrsopteris^ about the base of th 
series of Gradatae (compare p. 655, and Fig. 354). 




INTRODUCTION. 

OF the two branches of the Organic World, the Vegetable Kingdom 

might be expected to present a simpler problem of Descent than the 

Animal Kingdom, on account of the prevalent non-motility of the mature 

individual. That fixity of position which the Higher Plants show, should 

^end to a more obvious record of previous events than the ambulatory 

:.bit of Animals, and especially of their higher types, would seem to 

ow. It is reasonable to expect that organisms of fixed position 

>uld demonstrate in their distribution some traces of their past history; 

se would be specially valuable in the elucidation of the problem of 

Origin of a Land Flora, and of the relation of the Land-growing 

Mants to those of the water. 

But this prima facie probability is largely discounted by the extra- 
ordinary facility shown by Plants for the distribution of their germs. A 
comparison of the Higher Animals with the Higher Plants in respect 
of motility shows that the motile parent in the former is without special 
provision for distribution of its germs, while the Plant with its fixity of 
station shows high elaboration and variety in the methods of their 
dissemination. In consequence of this there will be a natural tendency 
in the vegetable kingdom, as there is also in that of animals, towards 
the obliteration of any such genetic record as the fixity of position of 
the individual plant during its active vegetation might otherwise have 
been expected to have left: Accordingly, on examination of the vegetation 
of any ordinary country-side, its uplands and lower levels, its swamps, 
streams, and pools, plants of the most varied affinity are found to be 
promiscuously shuffled together, and show little sign of ranking in their 
position according to their descent. For instance, the Flora of still 
fresh waters may be found to consist of such plants as various green 
Algae and Characeae ; of Isoetex and Pilularia ; together with Angiosperms, 
such as Littorella, Lobelia, and Subularia. In flowing mountain streams, 
in addition to green Algae may be found Ckantransia and Lemanea^ 
associated with Fontinalis and sundry Angiosperms. Conversely, in various 
positions on land, along with certain Algae in moist spots, representatives 



2 INTRODUCTION 

of the great groups of Bryophytes, Pteridophytes, and Seed-plants may 
be found in close juxtaposition, and sharing the same external conditions. 
On the sea-littoral it is otherwise : there Algae are found associated 
together almost to the exclusion of other plants. Nevertheless, occasional 
Phanerogams do invade the belt between tide-marks, and thus even this 
limit between the Vascular Flora of the land and the Algal Flora of the 
sea-littoral is apt to be blurred. 

It is plain, then, from such simple examples as these, which might 
be indefinitely varied and extended, that the problem of the origin of 
a Land-Flora is not to be solved by any mere reading of the facts of 
distribution into terms of the evolution of the characteristic plants of 
the land. Some other basis than that of distribution at the present day 
must be found for the solution of the problem. It is to be sought for 
in their comparison as regards structure and function, and that not only 
in the most complete condition of full development, but also in the 
successive phases of the individual life-cycle. 

The study of the form and structure of plants, as well as of their 
physiology, directs attention naturally to the water- relation : this more 
than any other single factor dominates the construction of land-living 
plants, while comparison with kindred aquatics shows how profoundly 
land-living plants are influenced by the necessity of adequate water-supply. 
But not only is this dependence of land-plants on water a general 
feature of the whole life-cycle: in certain' large groups of plants it is 
found that leading events in the individual cycle are directly dependent 
upon the presence of external fluid water. The importance of such 
matters in relation to the present problem of the Origin of a Land-Flora 
will be gauged by their prevalence and constancy in large groups of 
organisms. Now in the whole series of Archegoniate Plants (Mosses and 
Ferns), and in some Gymnosperms the act of fertilisation can only be 
carried out in presence of fluid water, outside the actual tissue of the 
organism : their spermatozoids are for a time independently motile in 
external water, and it is a mere detail that in the higher and more 
specialised forms, the distance to be traversed is only short from the 
point of origin of the spermatozoid to the ovum which it is to fertilise. 
The importance of fertilisation need not be insisted on here : everyone 
will admit it to be a crisis, perhaps the most grave crisis, in the life-cycle 
of the plant. When this critical incident in the life is found, in so large a 
series of allied plants as the Archegoniatae, to be absolutely dependent on 
the presence of external fluid water for its realisation, that fact at once 
takes a premier place in any discussion of the relation of plants to water. 

A comparison of the Seed-Plants with the Archegoniatae leads without 
any doubt to the conclusion that their method of fertilisation by means 
of a pollen-tube is a substitution for that by means of the motile 
spermatozoid. The Seed-Plant by adopting this siphonogamic mode of 
fertilisation becomes thereby independent of the presence of external 



INTRODUCTION 3 

fluid water at this critical perioch it may thus be held to have broken 
away from a condition of life inconvenient and embarrassing to organisms 
which live on exposed land-surfaces : and to have established itself in 
this character, as well as in its vegetative development, as a typical 
land-living organism. If this view of the matter be adopted, it follows 
that the Mosses and Ferns occupy a middle position in the relation to 
water: they may almost be described as amphibious, since, though they 
vegetate mostly on land, and show certain advanced structural adaptations 
to such life, they are nevertheless dependent upon external water for the 
important incident of fertilisation in each individual life-cycle. The 
strange feature is that they have retained so persistently this aquatic 
type of fertilisation. 

Looking further down in the scale of vegetation, attention is naturally 
directed towards the Algae, plants resembling, in some superficial 
characters of cell-structure and of colouring, the simpler terms of the 
Archegoniate series, though still more dependent than they upon external 
fluid water for the completion of their life-cycle. It may well, be that 
the affinity which such features suggest is at best only a remote one; 
but at least the existence of such forms would seem to justify the view 
as a probable one, that the great Archegoniate series, which has had 
so large a share in initiating that Land-Flora which we now see occupying 
the exposed land surfaces of the globe, has had its origin in aquatic 
forms : that from these a gradual adaptation to a land-habit has provided 
those forms of vegetation which we group together under the terms, 
Liverworts, Mosses, Club-mosses, Horsetails, and Ferns : and finally, 
with further adaptation to the land-habit, came the Seed-Plants first 
the Gymnosperms and subsequently the higher Flowering Plants. The 
latter culminated in the Gamopetalous Dicotyledons, which are essentially 
of Flowering Plants the most typical elements of a Land-Flora, since 
they include a smaller proportion of aquatic species than either the 
Monocotyledons or the Archichlamydeae. 

This, then, is the general position adopted at the outset : it is in 
accordance with the known facts of Palaeontology, and is the view 
generally entertained by modern morphologists. It will be the object of 
the present work to enquire into the details of such progressions as 
those above mentioned; especially it will be our duty to see how far 
the life-histories of Archegoniate forms will justify the view that the 
present Land-Flora has originated from an aquatic ancestry, and that 
there has been a migration from the water to the land : in that case, 
it will be a further object to ascertain how this has been carried out, 
and to trace those methods of specialisation to a land-habit, which 
have led to the establishment of the higher terms of the series as the 
characteristic representatives of the Flora of exposed land-surfaces. 

It is no new view which is thus to be put forward ; for it has long 
ago been concluded that the origin of life, whether animal or vegetable, 



4 INTRODUCTION 

has been in the water, and that the higher forms of either kingdom 
have assumed such structural and physiological characters as enable them 
to subsist in greater independence of aquatic surroundings than their 
simpler progenitors. The present attempt will be to fill in certain of 
the details into this general scheme, as applied to the vegetable kingdom, 
and to present some connected story of how the transition may have 
come about, as it may be seen reflected in the plants themselves, 
whether of the present day or of the remote past. 



CHAPTER I. 

THE SCOPE AND LIMITATIONS OF COMPARATIVE 
MORPHOLOGY. 

CONFRONTED with the great variety of plant-types which exist living and 
fossil on the earth's crust, the Botanist may regard them in various ways 
with a view to reducing them to some general conception of order. He 
may be satisfied with the mere cataloguing and description of the 
divers J forms which he is able to distinguish, and with the grouping of 
those together which show characters in common : this is the work of 
the Descriptive Botanist, and it naturally took the first place in the 
historical development of the science. Or he may attempt to find in 
such similarities of form as are shown by organisms thus grouped 
together some consecutive account of their probable origin : this is the 
work of the Scientific Systematist, or student of Phylogeny, and it is the 
ultimate aim of all current Morphology. 

In the earlier periods the student of form understood himself to be 
enquiring into the details of the Divine plan, as illustrated in a series 
of isolated creations : and any similarities which species might show 
would demonstrate for him merely the underlying unity of that plan. 
But in these later days he believes that the comparative study of form 
will lead him towards a knowledge of the main lines of descent. 
Contributory to this, which can only result in a balancing of probabilities, 
or often of mere surmises, is the study of the Fossils : Palaeophytology 
gives the only direct and positive clue to the sequence of appearance 
of plant-forms in past time upon the earth. Unfortunately the results 
acquired as yet along this line of observation are so fragmentary that 
they do not suffice to indicate even the general outline of the true 
picture : they must for the present be used rather as a check to phyletic 
theories than as their constant guide. The field is thus left in great 
measure open to other lines of enquiry. 

A second line of evidence which bears upon the evolutionary history 
may be derived from the geographical distribution of plants upon the 



6 SCOPE OF COMPARATIVE MORPHOLOGY 

earth's surface. This is, however, applicable only within certain limits : 
one of those limits is imposed by the wide distribution of germs 
which is so prevalent in plants. Wherever the mechanism for dispersion 
of germs is highly elaborated, and successful, the traces of evolu- 
tionary history, as shown by geographical distribution, are apt to be 
obliterated. The consequence is that in practice such distribution is 
only available as evidence of descent within restricted limits. The 
great geographical barriers, such as the tropics, the greater oceans, and 
the more continuous mountain ranges, it is true, delimit at present certain 
areas of vegetation, within which evidence of value as contributory to 
a knowledge of descent may be gathered; but at best this applies only 
to the later phases of evolution, and geographical distribution of plants 
at the present day gives little clue, or perhaps none at all, to the origin 
of the great groups which constitute the Vegetable kingdom at large. 
The fact that such genera as Equisetum, Lycopodium, Selaginella, Isoetes, 
Marattia, Marsilia, and Pilularia are, within their several limits of 
temperature, virtually cosmopolitan shows how little can be expected from 
geographical distribution of living forms as a key to the evolution of 
early types. Among fossils, Lepidodendron is virtually cosmopolitan. Plants 
of the Glossopteris flora, long thought to be distinctively southern, have 
recently been recognised from Russia. Such examples suggest that neither 
does the geographical distribution of fossils as yet give any certain 
evidence as to descent of the main phyletic lines. 

Another closely related branch of Botanical science is the study of 
organisms from the aspect of function and circumstance, as tested by 
physiological experiment. The intimate connection between form and 
environment is too obvious to need insistence here; but though the 
individual shows a high degree of plasticity under varying conditions, still 
there is a large field, embracing the very fundamentals of plant-form, 
such as the evolutionary origin of leaves, of roots, or of sporangia, which 
lies as yet outside the region of physiological experiment. Thus, however 
interesting the branch of physiological morphology may be, its scope is still 
narrowly limited. The method of experiment, with a view to ascertaining 
the effect of external agencies in determining form, is now nascent, and 
carries with it high possibilities. But it is well in the enthusiasm of the 
moment to keep in view the limitations which must always hedge it 
round. It is to be remembered that the effect of external conditions 
upon form is always subject to hereditary control, and that thus a large 
field is left open still for speculation. This seems to have been forgotten 
by a recent writer, who remarks that "the future lies with experimental 
Morphology, not with speculative Morphology, which is already more than 
full blown." 1 Though we may question the cogency of this antithesis, still 
the assertion contains an important truth, inasmuch as it accords prominence 
to experiment; but the case is overstated. All who follow the development 

1 Flora, 1903, p. 500. 



EXPERIMENTAL MORPHOLOGY 7 

of morphological science will value the results already obtained from the 
application of experiment to the problems of plant-form. But it is 
necessary at the same time to recognise that the two phases of the 
study, the experimental and the speculative, are not antithetic to one 
another, but mutually dependent : the one can never supersede the other. 
The full problem of Morphology is not merely to see how plants behave 
to external circumstances now and this is all that experimental morphology 
can ever tell us but to explain, in the light of their behaviour now, how 
in the past they came to be such as we now see them. To this end the 
experimental morphology of to-day will serve as a most valuable guide, 
and even a check to any more speculative method, by limiting its 
exuberances within the lines of physiological probability. But present-day 
experiment can never do without theory in questions of descent. 
Experiment by itself cannot reconstruct history; for it is impossible to 
rearrange for purposes of experiment all the conditions, such as light, 
moisture, temperature, and seasonal change, on the exact footing of an 
earlier evolutionary period. And even if this were done, are we sure that 
the subjects of experiment themselves are really the same? There remains 
the factor of hereditary character : there is also the question as to the 
circumstances of competition which cannot possibly be put back to the 
exact position in which they once were. Consequently there must always 
be a margin of uncertainty whether a reaction observed under experiment 
to-day would be the exact reaction of a past age. So far, then, from 
experiment competing with, or superseding speculation in Morphology, it 
can only act as a potent stimulus to fresh speculation, whenever the 
attempt is made to elucidate the problem of descent. It will be only 
those who minimise the conservative influences of heredity, or, it may be, 
relegate questions of descent to the background of their minds, who will 
be satisfied by the exercise of the experimental method of morphological 
enquiry, apart from speculation. 

The relations of Morphology and Physiology have been variously 
recognised in the course of development of the science. In the earlier 
periods the two points of view rarely overlapped. Even Sachs, the great 
pioneer of modern experimental physiology, kept the two branches distinct 
in his text-book, recognising the " Difference between Members and 
Organs." But later, in his lectures, he brought them more closely 
together, and habitually regarded morphological facts- in their physiological 
aspect. This is indeed the natural position for any adherent of Evolution: 
and it has been concisely said that morphology deals with the stereotyped 
results of physiology. Such a statement may, however, be criticised as 
assuming too much, in that it accords all initiative in, and determination 
of form, as well as its selection and perpetuation, to the influence of 
circumstance and function. A more apposite summing up of the relations 
of the two branches of Biological science has lately been given by Goebel l 
" Die Grundprobleme der heutigen Pflanzenmorphologie," Biol. CentrbL, Bd. xxv., No. 3. 



8 SCOPE OF COMPARATIVE MORPHOLOGY 

when he said that " Morphology includes such phenomena as are not 
yet physiologically understood." He further indicates that the separation 
of the two points of view has not any foundation in the nature of the 
case, but it is only a preliminary aid to a clear view amid the multiplicity 
of phenomena. The limits between morphology and physiology must 
necessarily fall away as advances are made. But meanwhile Morphology 
must continue to exist, even though it is not and cannot be an exact 
science: it deals comparatively with phenomena imperfectly explained as 
regards their origin in the individual or the race. The history of develop- 
ment of plant-form is an ideal to be approached experimentally, and the 
final object will be not merely a knowledge of the phylogenetic development, 
but of the very essence and cause of the development itself. It will be 
obvious how far present phylogenetic theory falls short of this ideal of 
Causal Morphology, but that is no sufficient reason for discontinuing its 
pursuit as a progressive study. 

For the present the comparative study of plant-form from the point 
of view of descent, as exhibited in the various phases of the individual 
life-cycle, must be pursued as in itself a substantive branch of the science : 
it is clear from what has been said above that it is not co-extensive with 
either Palaepphytology, Plant-Geography, or Plant-Physiology : nevertheless 
it overlaps with all of these, and must be liable to be checked by the 
results of any of these branches. Furthermore, the extension of knowledge 
of any of these branches will inevitably lead to further overlapping, till 
in the end the knowledge derived from the various methods of investigation 
should coincide in conclusions which will be general for them all, and 
constitute a true perception of the evolutionary story. But at the moment 
this consummation is so far from being attained that there is still room 
for the theoretical treatment of the evolution of plants as based on the 
formal comparison of their life-cycles. This must take due cognisance of 
the other branches of study, but will still rest upon its own foojting of 
fact and conclusion. 

There is one assumption involved in such comparative study which 
should be clearly apprehended and considered, rather than tacitly passed 
over. An evolutionary argument based on comparison of life-cycles is 
only valid if the organisms compared have retained the main incidents 
in their individual life unchanged throughout descent. In the main argu- 
ment of this work, the assumption is deliberately made that such constancy 
existed, or, rather, the argument proceeds upon the conclusion derived 
from broad comparison, that the main incidents once initiated have been 
pertinaciously retained. It may be held, and reasonably defended, that 
sexuality may have arisen in many distinct phyletic lines. It is not our 
present purpose to distinguish those different origins, or defend their 
distinctness. But comparison leads us to conclude that, once initiated in 
an evolutionary sequence, sexuality remained throughout descent substantially 
the same process in normal life-cycles. It may be modified in mechanism, 



COMPARISON OF LIFE-CYCLES 



as indeed there is good reason to ^ee that it was ; but it consisted still in 
the fusion of two cells together, bringing, as we believe generally, and see 
proved already in so many cases, a doubling of the chromosome-number 
as a consequence. Seeing sexuality of this nature a constantly recurring 
feature in the life-cycle of various definite phyla leads to the conclusion 
that in. those phyla it was also constant during their descent. Similarly, 
a reduction of chromosome-number has been found to be regularly associated 
with normal spore-production, and spore-production is found to be a 
constantly recurring event in large series of plants. In these it is concluded 
that reduction and spore-production have also been constantly recurring 
incidents throughout the descent of those series. It is hardly right to 
designate this opinion as an assumption : it seems rather to be a natural 
and valid outcome of comparative study. But if, on the other hand, such 
constancy of the leading events of the life-cycle in any phylum during 
descent were to be clearly disproved, then it will follow with equal clearness 
that the comparative argument based upon such facts will have to be 
revised for that phylum. It may seem hardly necessary to put down in 
extenso reasoning which is so obvious; but, on the other hand, it is well 
to see clearly the basis upon which the main argument will proceed. The 
constancy of the events of sexuality and of spore-production in normal 
life-cycles of the several ascending series of green plants is itself the cardinal 
point of the theory to be advanced in relation to the origin of a Land-Flora. 
In so far as inconstancy of either of these events occurs in them it will 
be shown that there is good reason to believe such exceptions to be of 
relatively late origin. 

The further facts which form the basis of Comparative Morphology 
include those relating to the mature external form of the plant, as seen 
in the successive phases of the individual life-cycle : the internal structure, 
as shown by its anatomical study : the form and structure of the parts 
involved in propagation, and the embryology of the individual. Such 
facts relating to living organisms are to be read in the light of comparison 
with the fossils, and the validity of any conclusions tested as far as 
possible according to the results of physiological experiment. 

It has been customary from the earliest times of natural classification 
to group together as akin, according to their degree of similarity, those 
organisms which correspond in form. Such alliances, long ago recognised, 
received a new significance in the light of evolutionary theory : the likeness 
thus comes to be attributed to community of descent, the nearness of the 
kinship being held proportional to the similarity of form, structure, and 
development of the individual. It is essential, however, to bear in mind 
always that this is only an hypothesis, incapable of complete proof under 
present conditions of study, and that the extent of direct evidence as 
yet available is small indeed. It is true that variation in different degrees 
is widespread : that, whatever the causes or methods involved, new races 
may be, and indeed have been established, which come true in more or 



io SCOPE OF COMPARATIVE MORPHOLOGY 

less degree after propagation by seed : and that thus the possibility has 
been demonstrated of origin and establishment of new forms from more 
or less dissimilar parents. This is not the place to discuss the sources of 
variation : whether it arises by a cumulative summation of slight differences, 
or by mutations per saltum, or both : nor whether the characters acquired 
during the individual life are or are not transmitted to the offspring, thus 
giving a positive direction to variation : nor yet to consider the effect of 
sexuality, and of the subsequent reducing-division of the nucleus in dis- 
tributing the qualities inherited from the parents. It suffices for our 
theoretical position that variations do occur, and that they are liable to 
be transmitted to offspring. The struggle for existence in this greatly 
over-populated world necessarily acts as a sieve upon such variants, and 
though the survival of the fittest is in no sense a positively constructive 
factor in itself, it results in the preservation of what is capable of self-support, 
and the elimination of what is physiologically less efficient. But when 
thus much is granted, it amounts only to this : that living organisms 
demonstrate that such an origin as evolutionary theory contemplates is 
at least possible. It does not necessarily follow that all known forms did 
originate in this way. Still, we are justified in accepting this view as a 
"working hypothesis," much more probable than any other explanation 
hitherto given of the existence of various living forms. 

But though we may readily adopt an evolutionary view, as a working 
hypothesis applicable to organic forms at large, it is when we apply it in 
detail that the real difficulties begin. We contemplate, for instance, some 
group of plants which have essentially similar form, structure, and develop- 
ment : we find ' that they differ in certain details and proportions, and 
that it is possible to lay them out in a series extending from one extreme 
form, through minor gradations, to another extreme form. Such a series 
may be strengthened by tracing parallelism of variations of two or more 
characters. Where this can be done the probability of the series representing 
a real evolutionary line is greatly enhanced. But there are at least three 
ways in which such a series may be read: (i) that the simplest form was 
the most primitive, and the whole series one of progression : (2) that the 
most complex was the most primitive, and the whole series one of reduction : 
(3) that the origin was from some central point, and the development 
divergent in two or more directions. Any one of these alternatives would 
be compatible with general evolutionary probability. How are we to 
decide which to adopt in any given case? 

The general principle that progress has been from the simpler to the 
more complex gives to the first alternative a primd facie probability. As 
a matter of fact this consideration weighed largely in the phylogenetic 
decisions of a quarter of a century ago, and the opinions on the descent 
of Ferns serve as a good illustration of it. Those Ferns which have 
the smallest sporangia (Polypodiaceae, Hymenophyllaceae) were held to 
be the most primitive, while those with larger and more complex sporangia 



APPLICATION OF EVOLUTIONARY THEORY n 

were regarded as more advanced (Osmundaceae, Schizaeaceae, Marattiaceae). 
But this, which was little better than an assumption, needed to be tested 
on other grounds, such as comparison with other Pteridophytes, and 
reference to the results of physiological and palaeontological enquiry. It 
is now pointed out, first, on the comparative basis, that the Leptosporangiate 
Ferns are isolated from other plants by the simplicity of their sporangia, 
and that the link in sporangial character with other early types is to be 
found more probably through the Eusporangiate than through the Lepto- 
sporangiate types. Secondly, it can be shown experimentally that reduction 
of complexity of leaf-structure follows the shade-habit; and the "filmy" 
character of the leaf in the Hymenophyllaceae is probably only an extreme 
case of this, while the smaller size of the individual sporangia shows some 
degree of parallelism with this adaptation : certainly it is so in the genus 
Todea. There is also some experimental basis for the conclusion that 
the thin-leaved habit is a derivative condition following on a shade-habit. 
Thirdly, the Palaeontological evidence shows that whereas the Eusporangiate 
Ferns were the characteristic Ferns of the primary rocks, while Lepto- 
sporangiate Ferns were certainly rare, the Leptosporangiates were in the 
ascendant in later strata, and are the dominant Ferns of the present day. 
From such evidence, which will be seen to be convergent along three 
lines of argument, the conclusion is drawn that the general series of 
Ferns has not been one of advancing complexity of sporangial structure, 
but of reduction. This case will serve as an illustration how the primd 
facie probability of advance may be overruled by the cumulative effect 
of other evidence. As a consequence perhaps of such proof of reduction 
in this and other cases, the tendency of the moment among Botanists 
is to look with general mistrust upon ascending series. For my own 
part, I think this tendency has been allowed too free scope : the primd 
fade probability of a series being truly progressive should be kept clearly 
in mind until it is disproved on more exact grounds. 

Wherever a linear series of forms is recognised these two alternatives, 
of the series being an ascending or a descending one, will present them- 
selves. But there is also the third alternative, viz. that the series may 
have been one of divergent development from some central point. It will 
be apparent that this is' in truth merely a combination of the two pre- 
ceding cases, and the lines of argument will be the same, though necessarily 
more complicated. As a consequence such divergent series are less readily 
substantiated than simple series would be. 

But there remains the still more serious question whether a series which 
may have been laid out on ground of form as a presumable evolutionary 
sequence reflects actually any line of evolution at all. It may be composed 
of members of distinct phyletic lines, which have converged in respect of those 
characters which lie at the foundation of our comparison. It has long been 
known that similarity of form may be arrived at along distinct evolutionary 
routes : this is designated parallel, or polyphyletic development, and examples 



12 SCOPE OF COMPARATIVE MORPHOLOGY 

can readily be found in the vegetative and propagative parts of plants both 
high and lower in the scale : it may affect not only the modification of 
parts already present, but also the origin of new parts. As prominent 
examples which will be discussed at length later, the polyphyletic origin 
of leaves, of heterospory, and of the seed-habit may be quoted This 
frequent occurrence of parallel development should serve as a check on 
the too ready acceptance of conclusions based on mere formal comparison, 
and it shows that it is necessary to be sure of the phyletic unity of a 
series before sound conclusions can be arrived at from comparison of 
its components. 

It may be useful to quote a specific case of fallacious reasoning based 
on comparisons which are not within one phyletic unity. It is possible 
to compare the sporangia of Calamostachys with those of Selaginella, of 
Isoetes, and of the Hydropterideae, as examples of heterospory : and general 
conclusions might be drawn from such comparison as to the progressive 
steps of the heterosporous differentiation. But these plants are now 
recognised as representing three (and possibly even four) distinct phyla, 
all of which include homosporous forms. The latter fact indicates that 
heterospory arose after the differentiation of those phyla. It is therefore 
impossible to argue correctly from one phylum to another as to the 
course which a common spore-differentiation has taken, since its course 
must have been distinct in each case from the others. The most that 
can be properly attained is an analogy between the separate progressions 
as seen in those several phyla. 

It is plain then that organic nature is not self-explanatory, and that 
Comparative Morphology is a study beset with pitfalls. There is uncertainty, 
first, in the recognition of true evolutionary sequences : still more in 
their interpretation as ascending, descending, or divergent : and again 
in the connecting 1 of these sequences together so as to construct some 
more or less consecutive story of descent : indeed, this can only be done 
when liberal use is made of the imagination, in bridging over the wide 
gaps in the series, which even the known fossils are so far from filling. 
The details of a story thus constructed depend so largely on comparative 
opinion, or balancing of probabilities, and in so slight a degree upon 
positive demonstration that the history as told by competent experts in 
Comparative Morphology may vary in material features. A little more 
weight allowed for certain observed details, or a little less for others, 
will be sufficient to disturb the balance of the evidence derived from a 
wide field of observation, and consequently to distort the historical 
picture. In the absence of more full "documentary" evidence from the 
fossils there is in truth no finality in discussions on the genesis and 
progress of organic life. But as long as the human mind has the power 
of and inclination towards enquiry, so long will such discussions con 
tinue, together with their kaleidoscopic changes of opinion. Every new 
fact of importance will in some degree affect the weight accorded to 



ITS CONCLUSIONS PLASTIC 13 

others and vary the general result* It will be seen in the discussions 
which follow how largely this is so in the morphology of plants. 

It may be objected that conclusions which are so plastic are little 
better than expressions of personal taste, or even of temperament : that 
the study of Comparative Morphology is therefore calculated to dishearten 
its votaries, while the non-specialist public, which is compelled to take 
its information at second hand, will be bewildered, and will conclude 
that it is useless to pursue a science which shows so little stability. But 
even where problems are apparently insoluble under circumstances of 
present knowledge, it is a satisfaction to most minds to entertain an 
opinion, even if that opinion be of a theoretical nature, and be liable to 
future modification or ultimate disproof. On the other hand, as regards 
the actual progress of morphology, those who follow its history with 
sympathetic care will gain heart when they compare the present position 
with that of a generation ago. And especially for Botanists it is encouraging 
to think that it is little more than half a century since the history of the 
life-cycle of a Fern was first completed by Suminsky. In some sixty years 
a vast array of kindred facts has been acquired, and a theoretic super- 
structure is being raised upon them which, though still protean, is gradually 
acquiring some settled form. Never in its history has the advance of 
morphological thought been so rapid as at present. But in no field of 
morphological research has investigation been more amply rewarded than 
in palaeophytology : the luminous facts derived from fossils are shedding 
a fresh and a direct light upon obscure problems, such as the origin of 
the seed-habit, and helping us to locate such difficult groups as the 
Psilotaceae and Equisetineae. When we regard these rapid advances, and 
truly estimate the influence they bring to bear in strengthening the positions 
already indicated by morphological theory, we shall not only .see that this 
branch of the science is very actively alive, but also that its theorisings 
are not merely unsubstantial figments of the mind. 

Considerations such as these go far to justify the statement in the 
present work of a theoretical view of the origin of a Land-Flora. Some 
may deem the opinions expressed as unduly speculative, but in the first 
place, they are based upon a wide area of fact, and secondly, as above 
remarked, comparative m6rphology must necessarily assume a theoretical 
form under present conditions. We have seen that its conclusions as to 
descent are at best the result of a balancing of probabilities. As long as 
this is clearly understood by the reader, and the author abstains from any 
dogmatic attitude, good should come from any duly reasoned statement, 
even though, like the present, it may be of a theoretical nature. A working 
hypothesis, open like others to refutation, is better than no hypothesis 
at all. This is the position consciously adopted here, for it is believed 
that the full statement of even a speculative view will stimulate enquiry, 
which may lead towards its ultimate proof or disproof. 



CHAPTER II. 

THE LIFE-HISTORY OF A FERN. 

THE middle years of the nineteenth century marked an important epoch 
in the history of Plant Morphology. Before that period this branch of 
botany could hardly be said to exist as a science. What gave distinction 
to that period was the publication of observations which made it possible 
for the first time to give a consecutive account of the various stages in 
the life-history of the Higher Cryptogamia. Up to that time it had been 
the custom to compare Ferns with Flowering Plants, notwithstanding that 
the facts, so far as they were known, gave little support to any view of 
their close similarity ; and to attempt to express the life-story of these 
and others of the lower plants in terms of the higher. But the investigations 
of that period, by following out the actual facts of development, showed 
not only that there was no correlative of the seed in the life-cycle of a 
Fern, but also that there was in the prothallus of Ferns a phase of the 
life-cycle which differed in essential points from anything which was then 
known to exist in the development of Seed-Plants. 

The spores of Ferns were experimentally recognised as reproductive 
organs by Morison (1699), who raised young plants from them. But Kaulfuss 
first observed their germination (1825), and the formation of the prothallus, 
which had already been described by Ehrhart (1788) : it was Bischoff (1842) 
who first recognised the embryo attached to the prothallus. Naegeli (1844) 
discovered the antheridia and spermatozoids, while Suminski (1848) 
ascertained the true nature of the archegonium, and its relation to the 
embryo.. But it remained for Hofmeister to put together, and complete 
the story. In 1849 his description of the germination of Pilularia and 
Salvinia appeared, and two years later, in 1851, he gave to the world his 
Vergleichende Untersuchungen, a work which dealt in the most com- 
prehensive way with the life-story of a number of Liverworts, Mosses, 
Ferns, Fern-Allies, and Gymnosperms. 

It is impossible to exaggerate the importance of the advance in view 
which the publication of Hofmeister's book brought. The middle years 



MATURE SPOROPHYTE 15 

of the nineteenth century were indeed the heroic age of Plant Morphology, 
and the results then attained will always continue to be the basis of com- 
parison, as applied to the ascending series of green plants. It must, 
however, be remarked that those results were achieved on a purely comparative 
footing, and at the moment carried no further interpretation with them. 
For these were the days before evolutionary theory held sway in the 
Biological sciences, and accordingly no underlying phylogenetic meaning 
was as yet seen in the facts observed and compared. But eight years 
later Darwin's Origin of Species was published, and " the Theory of Descent 
had only to accept what genetic morphology had actually brought into 
view." It is also to be noted that at first no past physiological history 
was traced in the facts of the individual life ; this line of interpretation suffered 
much longer delay, and is even now only gradually becoming apparent. 
As we shall see, however, such a meaning and such a history may still 
be found reflected in those successive phases of the individual life which 
Hofmeister and his predecessors were able to detect and to compare. 
It is along lines such as these that we may best seek for the solution of 
the problem presented by the origin of a Land-Flora. 

It will then be essential for our purpose, in the first place, to follow 
through all its phases the life-history of certain typical organisms, and 
we shall best begin with those which occupy a middle position in our 
system, viz. the Pteridophytes. The common Male Shield Fern (Nephrodium 
Filix-mas. Rich.) will serve as a familiar, and also a suitable example. 

This Fern is known to every one as growing in woods and hedgerows, 
and even in more exposed situations, such as the open gills and hill-sides 
of higher-lying districts. It presents a robust appearance, and when fully 
developed it consists of an oblique and massive stock, which is relatively 
short : this is entirely covered over by the bases of the leaves, of which 
the youngest constitute a closely packed terminal bud (Fig. i). Those 
leaves which are situated further from the apex, and immediately below 
the terminal bud, may in summer be found to be of large size and 
compound structure (Fig. 2) ; they are of a rather firm texture : individually 
they are in outline not unlike the ancient Greek shield, and collectively 
they form a crown-like series surrounding the terminal bud. Passing again 
further back from the aprex of the stock, its surface is found to be closely 
invested by the bases of the numerous leaves of former seasons, the 
upper portions of which, having performed their functions, have rotted 
away. If the plant be dug up, and the soil carefully removed from it, 
an ample root-system will be seen, consisting of thin, wiry, and dark- 
looking fibrils, which spring from the basal parts of the leaves, and may 
bear numerous branch-rootlets. 

All these parts consist of tracts of tissue differentiated to subserve 
distinct functions. The Vascular Skeleton, which appears as a cylindrical 
network of strands within the massive axis (Fig. i, E, F), throws off 
continuous and connected branches, on the one hand into the leaves, 



i6 



THE LIFE-HISTORY OF A FERN 



where they ramify and extend upwards to the extreme tips and margins. 
On the other hand, strands of vascular tissue derived from the leaf-bases 
extend towards the tips of the roots, and laterally into their branchlets. 
The Vascular System is thus a connected conducting-system throughout 
the plant. It is embedded in softer parenchymatous tissues, which serve 




FIG. i. 

Nephrodium Filix-mas, Rich. A, stock in longitudinal section ; v, the apex ; st, the 
stem; />, the leaf-stalks; b' , one of the still folded leaves ; g, vascular strands. , leaf- 
stalk bearing at k a bud with root at -w, and several leaves. C, a similar leaf-stalk cut 
longitudinally, bearing bud, /*, with root, w. D, stock from which the leaves have been 
cut away to their bases, leaving only those of the terminal bud. The spaces between the 
leaves are filled with numerous roots, w, w'. E, stock from which the rind has been 
removed to show the vascular network,^-. F, a mesh of the network enlarged, showing 
the strands which pass out into the leaves. (After Sachs.) 

various purposes in the different parts : thus in the young root they may 
be absorbent, or serve to hand on the fluids absorbed to the conducting 
system : in the stem they may serve the purpose of storage of reserve 
materials, while in the leaf the parenchyma carries out the function of 
photosynthesis, together with the passing on of the supply thus acquired 
to the conducting-system. The parts exposed to the air are covered by 



MATURE SPOROPHYTE 17 

an epidermal layer, with a cuticujarised external wall, which prevents 
indiscriminate loss of water by surface-evaporation. But the epidermis 
is perforated by numerous stomata, the motile guard-cells of which can 
control, according to circumstances, the width of the pores leading into 
the intercellular spaces. There is thus a highly organised ventilating 




FIG. 2. 

Xephrodiiun Fili.v-mas, Rich. Fertile leaf about one-sixth natural size, the lower 
part with the under surface exposed. To the left a single fertile segment, enlarged about 
7<imes. (After Luerssen.) 

system. Finally, there are also firm, brown, resistant tissues, disposed 
sometimes near the outer surface, as in the stem and in the leaf-stalk : 
sometimes more deeply seated, as in the root, while in the leaf they 
follow the course of the vascular strands. These give to the several parts 
increased mechanical strength, and power of resistance. 

Thus constituted the Male Shield Fern is an organism which is 
capable of leading an independent life on an exposed land-surface : it 

B 



1 8 THE LIFE-HISTORY OF A FERN 

is in a position to nourish itself by taking up from the soil the water and 
salts which it requires, and to elaborate therefrom, and from the carbon- 
dioxide of the air, fresh supplies of organic food. Further, though for 
reasons to be explained later, it frequently is found growing in situations 
where moisture is abundant and the air moist ; still it can resist considerable 
drought, and is capable of living under as exacting conditions as any 
ordinary terrestrial plant. As proof of this, cases may be quoted of the 
removal of mature Shield Ferns from a more moist habitat to exposed 
situations, where there is no shade, nor any special supply of water : 
they are found to flourish there permanently ; but they show slight 
differences of form from the shade plants : the leaves are more robust, 
of smaller area, and of a paler colour. 

In this power of resisting drought the Shield Fern is by no means 
an isolated exception, nor in any sense an extreme type among Ferns. 
It is a familiar sight in this country to see dry hill-sides covered with the 
common Bracken, and taking no harm from a summer drought. There 
is also a small British Flora of Ferns of dry wall-tops, including such 
species as Polypodium vulgare, Asplenium Ruta-muraria, and Ceterach ; 
these may be found sometimes with their leaves dried to crispness in 
summer. Abroad there are certain genera, such as Nothochlaena, and 
famesonia, and the Niphobolus section of Polypodium, which are typically 
xerophytic : in other cases isolated species may show special adaptation 
to dry surroundings ; for example, Hymenophyllum sericeum which is a 
member of a peculiarly hygrophytic genus. These xerophytic Ferns 
inhabit dry climates, such as the higher Andes : or they are epiphytic 
in habit, and have no access to the water-reservoir of the soil. The forms 
which the xerophytic modification may take are succulence of the smooth 
leaf, with well-developed epidermis, as seen in Polypodium nummulariae- 
folium, and piloselloides, and also in Platy cerium : or a development of 
a thick felt of hairs may cover the surfaces, as in Niphobolus, Nothochlaena, 
and Jamesonia : or of scales, .as in Polypodium (Lepicystis} incanum, or 
Asplenium Ceterach : or there may be a xerotropic folding of the pinnae, 
as in Nothochlaena sinuata and ferruginea, and in Jamesonia. There is 
also a very efficient mode of resisting extreme drought which is not 
shown structurally, viz. the power of retaining vitality after drying up. 
A good example of this is seen in Polypodium (Lepicystis] incanum, which 
grows commonly in Trinidad on tree-trunks, and there shrivels for weeks 
without rain, under a tropical sun ; but when moistened again it swells, 
and continues growth. Such vitality is shared in some degree by the Ferns 
of our wall-tops, and is a common feature leading to the survival of 
many other plants, notably among the Bryophytes. Such cases as these 
quoted will serve to show that a moist habitat is not always a necessity 
for the life of the mature Fern-Plant, and even that Ferns, as a family, 
show a considerable aptitude for resisting extremes of drought. But never- 
theless most Ferns do affect moist situations, while to some, such as 



VEGETATIVE PROPAGATION 



the Filmy Ferns of tropical forests, x an atmosphere approaching complete 
saturation with moisture is a constant necessity. The Male Fern may 
indeed be accepted as a medium type, showing no special adaptation 
nor susceptibility either to moisture or drought, while structurally it 
shows such characters as are usual 
in average Land Vegetation. 

With very few exceptions Ferns 
are perennials, and in the case of 
the Shield Fern there is no theoretical 
limit to the duration of the individual 
life : in point of fact the plant "may 
grow continuously for a long term 
of years, as is shown by the length of 
the stock, and the long succession 
of the bases of leaves! of [former years 
which may be observed persistent 
upon it in the larger specimens. But 
still it is subject to many vicissitudes, 
which are liable to terminate its exist- 
ence. Some provision must be made 
for the maintenance of the race by 
the formation of new individuals. 

The vegetative mode of propaga 
tion in the Shield Fern is by means 
of buds which appear at a late period 
upon the persistent bases of leaves of 
former years. These develop leaves 
similar to those of the parent, with 
roots which form an independent 
system, so that when the progressive 
rotting, which is always advancing 
from the base of the stock onwards, 
isolates the parent leaf from the rest 
of the plant, the bud is in a position 
to subsist as an independent/individual 
(Fig. i, B, c). This is merely one 
example of what is a very common 
phenomenon in Ferns, viz. the vege- 
tative propagation of the individual. 
The details of the process, such as 
the position and the number of buds, may vary greatly in different 
cases (Fig. 3), but the essential point is the same, that by a purely 
vegetative growth, and without any known cytological complications, 
new individuals may be derived, which are similar in all essentials to 
the parent. Such means of increase are styled collectively under the head 




FIG. 3. 

Cystopteris bulbifera (L.) Bernh. A, part of a 
leaf with adventitious buds. Natural size. B, an 
adventitious bud which has fallen off, forming a 
root. C, an adventitious bud further developed. 
B and C somewhat enlarged. (After Matouschek.) 



20 



THE LIFE-HISTORY OF A FERN 



of Sporophytic budding. It is plain that such growths are only methods 
of amplification of the morphological individual ; though ultimately quite 
separate from the parent plant, there is no reorganisation of the protoplasts 
involved in their initiation. 

There is, however, an alternative mode of increase in number of 
individuals, which deals with much larger numbers of potential germs, 
and involves a much greater complexity of the phases of production than 
the mere sporophytic budding : it is by means of spores. Since this 
spores-production is a constant feature in the normal life of all Ferns, and 



J7-. 




FIG. 4 . 
Vertical section of the sorus of Nephrodium Filix-mas. (After Kny.) 

indeed of Archegoniates at large, while sporophytic budding only occurs 
in relatively few, there is good reason to believe that this was a more 
primitive and important form of propagation. It therefore demands more 
serious attention. 

An examination of the leaves of the Male Fern will show in many 
cases, and especially in young plants, merely a smooth, rather pale green 
under surface : these are then the vegetative leaves, or " trophophytts" as 
they are sometimes called, and they always appear first in the develop- 
ment of the individual. But other leaves of older plants, and especially 
those formed later in the season, bear on their lower surface, and chiefly 
near their apical part, numerous roundish patches, which are green or 
brown according to age : these are the sort, and the leaves bearing them 



SPORE-PRODUCTION 



21 



are termed " sporophy/Zs" but they^ do not differ in outline from the 
vegetative leaves (Fig. 2). The sori are disposed in a single linear series 
on either side of the midrib of the pinna, or pinnule, being seated on 
the secondary veins. The kidney-like outline which they present is 
due to a membranous covering called the indusium, which is protective : 
it is attached by a central stalk to a projecting cushion of tissue the 
receptacle which is in close connection with the vein, while there is 




11 



12. 



FIG. 5. 
Young stages of segmentation ;>f the sporangium of Nepkrodium Filix-mas. (After Kny.) 

a vascular extension from the vein into the receptacle. To the latter 
are also attached the numerous sporangia, stalked capsules of lenticular 
form, which are collectively overarched by the umbrella-like indusium. 
Various stages of development of the sporangia may be found together 
in the same sorus : those which are younger are smaller, and of pale 
colour ; those which are mature are larger, and are filled with numerous dark 
brown spores : these showing through the more transparent wall give to the 
ripe sporangium a deep chocolate colour. 

In order properly to understand the arrangement of the parts of the 
sorus it must be cut in vertical section (Fig. 4) : it will then be seen 



22 



THE LIFE-HISTORY OF A FERN 



how the indusium, rising from the receptacle, overarches the sporangia, 
which are also attached to it by long thin stalks. The head of each 
sporangium is shaped like a biconvex lens : its margin is almost com- 
pletely surrounded by a series of indurated cells, which form the mechanically 
effective annulus : this stops short on one side, where several thin-walled 
cells define the stomium, or point where dehiscence shall take place. 
Within are the dark-coloured spores, which, on opening a single sporangium 




FIG. 6. 
Later stages of segmentation of the sporangium of Nephrodiuvi Filix-mas. (After Kny.) 

carefully in' a drop of glycerine, may be counted to the number of 
approximately 48. 

The origin of the sporangium is by outgrowth of a single superficial 
cell of the receptacle, which undergoes successive segmentations as 
illustrated in Figs. 5 : 1-3. A tetrahedral internal cell is thus completely 
segmented off from a single layer of superficial cells constituting the wall. 
The former undergoes further segmentation (Fig. 5. n, 12) to form a 
second layer of transitory nutritive cells, called the tapetum, subsequently 
doubled by tangential fission (Fig. 6. i). The tetrahedral cell which 




SPORE-PRODUCTION 23 

still remains at the centre, having ' rown meanwhile, undergoes successive 
divisions till usually twelve spore-mother-cells are formed (Fig. 6. 2, 6, 7) : 
these become spherical in form, and are suspended in a fluid which, 
together with the now disorganised tapetum, fills the enlarged cavity of 
the sporangium. Each spore-mother-cell then divides twice, so as to form 
a group of four cells, which constitute a spore-tetrad (Fig. 7), the component 
cells showing some differences in their arrangement. Finally, as ripe- 
ness is approached the individual cells of the tetrads separate as the 
spores, each of which has meanwhile developed a protecting wall : owing to 
the absorption of the fluid contents of the sporangium the separate spores 
are dry and dusty, and readily scattered. Since each 
of the 12 spore-mother-cells may form four spores 
their number is 4x12 = 48 in each sporangium. 
Each mature spore consists of a protoplast with 
nucleus, bounded by a colourless inner wall, and 
a brown epispore, which extends outwards into 
irregular projecting folds. 

Meanwhile the wall of the sporangium has 
become differentiated into the thinner lateral walls 
of the lens-shaped head, and the annulus, which 
is a chain of about 16 cells surrounding its margin 

/T ^. , 7X rr.1 ,-, i i Spore-tetrads of Polypodium 

(Fig 6. 40, 40). These constitute a mechanical vu i are. (After Atkinson.) 
spring, which on the rupture of the thin-walled 

stomium becomes slowly everted as the cells dry in the air, and then 
recovering with a sudden jerk, throws out the spores to a considerable 
distance (Fig. 8), each individual spore being separate from its neighbours. 
If a Fern leaf on which the sori are fully matured be laid with its lower 
surface downwards upon a sheet of paper, and left in dry air for some hours, 
or if the drying be accelerated by heat, a fine brown dust, consisting of 
the mature spores, will be deposited on the paper, and they are shed in 
such vast numbers as to give a natural print of the outline of the leaf. 
A rough estimate may be made of the numerical output of spores from 
a large plant of the Shield Fern, as follows. In each sporangium 48 
spores may be formed: a sorus will consist of fully 100 sporangia, usually 
more : 20 is a moderate estimate of the sori on an average pinna : there 
may be fully 50 fertile pinnae on one well-developed leaf, and a strong 
plant would bear 10 fertile leaves. 48x100x20x50x10 = 48,000,000. 
The output of spores of a strong plant in the single season will thus, on 
a moderate estimate, approach the enormous number of fifty millions. 

As we shall see, each of those spores is capable of acting as the starting- 
point of a new individual, and yet Male Ferns are not increasing perceptibly 
in number : the fact is that in open Nature the vast majority of these 
potential germs do not survive the vicissitudes of early life. It is evident, 
however, that the maintenance of the race is very fully provided for, while 
there is an ample margin for the effect of selection of those fittest to survive. 



24 THE LIFE-HISTORY OF A FERN 

In this connection it is well to note further that the spores are produced 
upon the leaves fully exposed to the air, and that dry circumstances 
favour the shedding of the spores : Ferns grown in uniformly moist con- 
ditions show how essential a dry period actually is, for their sporangia 
often do not burst at all. The spores of Todea and of some Hymeno- 
phyllaceae may even be seen germinating within the sporangium. Such 



9 St 




Irrn ( 



125 



FIG 



Dispersion of the spores from sporangium of Aspidium acrostichoides, showing different 
stages of the e version and snapping of the annulus. (After Atkinson.) 

a condition is obviously of no advantage to the plant, and is to be 
looked upon as a failure in the normal action of the annulus. We thus 
see that a relatively dry period, such as the Male Fern is able to undergo 
in summer, is a normal state, and indeed essential for the last phase of spore- 
production, viz. the dissemination of the numerous living germs. 

But the relatively dry conditions which lead up to and are necessary 
for the dissemination of the spores do not suffice for their further 
development : in order that they may germinate moisture is required, 
as it is also throughout the immediately succeeding stages of life. When 



GERMINATION 25 

exposed to suitable conditions of mbisture and temperature each spore may 
germinate : the outer coat bursts, and the inner protrudes and increases 
in size, cell-divisions appearing as the growth proceeds. The body which 
is thus produced is called the prothallus^ and it may vary in its form 
according to the circumstances. In average cases of not too crowded 
culture it usually takes first a short filamentous form, attached by one 
or more rhizoids to the soil (Fig. 9. 4) : it then widens out at the tip 







FIG. 9. 

Germination of the spore in Nephrodivin Filix-iuas^ and early stages of the prothallus. 
(After Kny.) 

to a spathula-like, and finally to a cordate form (Fig. 9. 5 and 6). This 
is the usual type, but when crowded closely together, the filamentous 
form may be longer retained, and prothalli are then of the type shown 
in Fig. ii. i. It is thus seen that the form of the prothallus is plastic, a 
fact which may be brought into further prominence by culture under 
various conditions of lighting, etc. 

The body of the prothallus, exclusive of the downward growing rhizoids, 
consists of cells which are essentially alike, arranged at first in a single- 
layered sheet. This simple structure is maintained permanently by the 



26 THE LIFE-HISTORY OF A FERN 

peripheral parts, but in the central region, below the einarginate apex, 
the cells divide by walls parallel to the flattened surfaces of the prothallus, 
and thus form a somewhat massive central cushion. The mature cells 
are thin-walled, with a peripheral film of cytoplasm surrounding a large 
central vacuole, and embedding the nucleus and numerous chloroplasts. 
The whole body is capable of leading an independent existence, nourishing 
itself by absorption from the soil, and by photosynthesis (Fig. 10). 




FIG. 10. 

Mature prothallus of Ncphrodinm Filix-mas, as seen ffom below, bearing antheridia and 
archegonia. (After Kny./ 

Its structure at once suggests dependence on a continuous and efficient 
water-supply ; for there is a large proportion of surface to bulk, while the 
cell-walls are thin, and the vacuole-contents voluminous. There is no 
arrangement to offer serious resistance to evaporation of water in dry air. 
As a matter of experience prothalli shrivel readily when exposed to dry 
conditions, while in Nature they are regularly found in moist and protected 
positions ; a fact which goes far to determine the habitat also of the 
sporophytes which arise from them, and this cannot fail to act as a 
substantial check upon the distribution of Ferns. But shrivelling under 
drought does not necessarily involve death : in certain cases at least only 
a temporary arrest of activity is the consequence, and prothalli which have 



PROTHALLUS 



27 



been thus dormant for a considerable time have been seen to revive 
when soaked out, and to continue their growth. They share in some 
measure that faculty which is so important to many Bryophyta, of recovery 
after dormancy under drought. Comparing the prothallus with the Fern- 
Plant as regards the water-relation, it is plainly less adapted for life on 
exposed land-surfaces, and more immediately dependent on moisture. 




. FIG. ii. 

i. An attenuated male prothallus of Ncphrodium Filix-mas ; 2-5. stages of development 
of antheridia ; 6, 7. juptured antheridia ; 8. a spermatozoid. (After Kny.) 

The prothallus thus constituted is capable in some cases of vegetative 
propagation, by gemmae, and other forms of " gametophytic budding," 
but this does not occur in the Male Fern. 

Though the close dependence on moisture for functional activity is 
thus seen in the prothallus, it is much more obvious in the behaviour 
of the sexual organs which the prothallus bears. These in the Male Shield 
Fern a are commonly borne, male and female, on the same individual 

1 It is hardly necessary to say that the "Male" Fern is a misnomer, surviving from 
the misconceptions of earlier times. The Fern- Plant is neutral, being neither male nor 
female, while it is on the prothallus that the sexual organs are borne. 



28 THE LIFE-HISTORY OF A FERN 

(Fig. 10) ; but conditions of crowded culture may lead towards a partial, 
or even complete separation of the sexes. The flattened hermaphrodite 
prothallus of the ordinary cordate outline, grown under normal circumstances 
of moisture and moderate lighting, on a horizontal substratum, lies with 
one of its flattened surfaces facing the substratum, and produces upon 
that lower surface antheridia and archegonia, the former in the basal or lateral 
regions, the latter upon the massive cushion : here they develop in acropetal 
succession, the youngest being nearest to the emarginate apex of the 
thallus. This position of the sexual organs is evidently favourable to 
their continued exposure to moist air, or even fluid water: and indeed 
the latter is necessary for the completion of their function. 

The antheridium, which arises by outgrowth and segmentation of a 
single superficial cell, consists when mature of a peripheral wall of tabular 




FIG. 12. 

Archegonia of Polypodium vulgare. A, still closed : o = ovum. K' = canal-cell. K" = ventral- 
canal-cell. B, an archegonium ruptured. X240. (After Strasburger.) 

cells, surrounding a central group of spermatocyr.es (Fig. n. 4, 5). The 
antheridium readily matures in moist air, but does not open except in 
the presence of external fluid water : this causes swelling of the mucilaginous 
walls of the spermatocytes, and increased turgor of the cells of the wall : 
the tension is relieved by rupture of the cell covering the distal end, 
and the spermatocytes are extruded into the water, the cells of the wall 
assisting by their swelling inwards, and consequent shortening (Fig. u. 6). 
The spermatocytes, thus extruded into the water which caused the rupture, 
soon show active movement, and the spermatozoid which had already 
been formed within each of them escapes from its mucilaginous sheath, 
and moves freely in the water by means of active cilia attached near one 
end of its spirally coiled body (Fig. u. 6 and 8). 

The archegonium also originates from a single superficial cell, and 
grows out so as to project from the downward surface of the thallus. It 
consists when mature of a peripheral wall of cells constituting the 
projecting neck, and a central group, arranged serially : the deepest seated 
of these is the large ovum, which is sunk in the tissue of the cushion, 
and above this is a small ventral-canal-cell, and a longer canal-cell 



FERTILISATION 



29 



(Fig. 1 2, A). If prothalli be grswn in moist air, and only watered by 
absorption from below, the archegonia will have no access to fluid water, 







FIG. 13. 

Fertilisation in Onoclca sensibilis : the arrows indicate direction of the growing point of 
the prothallium. A. vertical section through an open archegonium probably within ten 
minute? after the entrance of the first spermatpzoid. X 500. B. vertical section of the 
venter of an archegonium containing spermatozoids, and the collapsed egg with a sperma- 
tozoid within the nucleus. Thirty minutes. X 1200. (After Shaw.) 

and they will then remain closed, and fertilisation will be impossible ; but 
if watered from above, as they would be in the ordinary course of Nature, 
the external fluid water will bathe them, 
and rupture will result. This may be 
observed in living archegonia which 
have been kept relatively dry, and then 
mounted in water under the micro- 
scope : the neck dehisces at the distal 
end owing to internal mucilaginous 
swelling, and its cells diverge widely: 
the canal-cell and ventral- canal-cell are 
extruded, and the ovum remains as a 
deeply seated, spherical protoplast, while 
access to it is gained through the open 
channel of the neck (Fig. 12, B). Thus 
the same conditions lead to the rupture 
both of the male and female organs : 
in Nature a shower of rain would supply 
the necessary external fluid water, and 
would at the same time supply the 



- 




FIG. 13 bis. 



Horizontal section of an egg, showing coiled 
spiral male nucleus within the female. Twelve 
hours. XI200. (After Shaw.) 



30 THE LIFE-HISTORY OF A FERN 

medium of transit of the spermatozoids to the ovum. But the movements 
of the spermatozoids are not subject to blind chance : the chemiotactic 
attraction of the archegonium directs the spermatozoids through the water, 
towards the open neck, which they may be seen to enter, and finally one 
of the spermatozoids coalesces with the ovum : fertilisation is effected 







m 






V 





FIG. 14. 

Young embryo of Adiantum continuum. L = leaf-quadrant ; 5= stem-quadrant ; 
R = root-quadrant ; /''foot-quadrant. (After Atkinson.) 

by the absorption of the male nucleus in that of the egg (Figs. 13 
and i $ bis). Thus the presence of external fluid water is essential for the 
process of fertilisation : without it the prothallus is unable to achieve that 
object of its existence. 

The consequence of fertilisation is the growth and segmentation of 
the ovum, or zygote, as it may now be called, to form a mass of 
embryonic tissue, which at first remains embedded in the tissue of the 



EMBRYO 31 

parent thallus (Fig. 14): as it , grows, leaf, stem, and root become 
differentiated (Fig. 15), which finally emerge; at first the embryo Fern- 
Plant is dependent for its nourishment upon the prothallus which embeds 
it ; but as the first leaf expands it begins to exercise the assimilating 
function, which is taken up also by the later formed leaves. The first 
root also projects into the soil, and soon functionates as an absorbing 
organ : it is followed later by others. Thus the young plant soon 
becomes physiologically independent of the prothallus, which rots away, 







FIG. 15. 

Embryo of Adiantutn concinuntn, older than that in Fig. 14. 
S= stem ; Ffoot. (After Atkinson.) 



leaf; /? = root; 



leaving the young plant established on the soil. It gradually attains the 
mature characters similar to those of the parent from which it originated. 
The above is a bare statement of the salient events in the life-cycle, 
or ontogenetic period of a Fern, as it is seen in its simplest form : and, 
the adjoining diagram may serve to present them graphically to the eye 
(Fig. 1 6). The two most notable points are those where the new 
individual is represented by a single cell, viz. the spore, and the 
fertilised ovum, or zygote. These are two landmarks between which 
intervene two more extensive developments, on the one hand the sexual 
generation, or prothallus, on the other the spore-bearing generation, or 



THE. LIFE-HISTORY OF A FERN 



SPORE 



Fern-Plant. If the events above detailed recur in regular succession 
there will be seen a regular alternation of two phases of life, or generations : 
of these the one, the prothallus, bears the sexual organs, which contain 
the sexual cells or gametes, and it may accordingly be styled the 
gametophyte : the other, the Fern-Plant, is non-sexual or neutral, and bears 
the sporangia, containing the spores : accordingly it may be styled the 
sporophyte. The study of Ferns at large leads to the conclusion that this 

regular alternation of generations 
is typical for the family. These 
two alternating generations differ 
not only in form, but also in 
their relation to external circum- 
stances, and especially in the 
water-relation. The sporophyte 
is structurally a land-growing 
plant, with nutritive, mechanical, 
and conducting tissues, and a 
ventilating system : not only is 
it capable of undergoing free 
exposure to the ordinary atmo- 
spheric conditions, but dryness 
of the air is essential for the 
final end of its existence, viz. 
the distribution of the spores. 
On the other hand, the gameto- 
phyte is structurally a plant 
ill-fitted for exposure, with un- 
differentiated and ill-protected 
tissues, and no ventilating 
system, while the object of its 
existence, viz. the fertilisation 
carried out by the sexual organs, 
can only be achieved in the 
presence of external fluid water. 

There is thus a marked difference between these two phases, and their 
sequence may be said to constitute an antithetic alternation. As regards 
the water-relation, the whole life-cycle of the Fern might not inaptly 
be designated as amphibious, since the one phase is dependent on 
external fluid water for achieving its object of propagation, while the 
other is independent of it. 

It will be seen in the next chapter that this is not a condition for 
Ferns alone, but that such alternation as is seen here has its parallel in 
many other plants, though with great differences in detail, and especially 
in respect of the balance of size of the two generations. 




FIG. 16. 
Scheme of life-cycle of a Fern. 



CHAPTER III. 

ON THE BALANCE OF THE ALTERNATING GENERATIONS 
OF ARCHEGONIATAE. 

HOFMEISTER'S great work on the Higher Cryptogamia, alluded to in the 
previous chapter, was not a mere description of observations, but a com- 
parative treatise. It not only revealed the life-stories of the various types 
of plant-organisation which he examined, but in it he also showed that 
their several stages corresponded in essential features. Notwithstanding 
wide differences of detailed form and of proportion, it was demonstrated 
that, as regards position among the recurrent events of each life-cycle, 
the neutral generation, or sporophyte, and the sexual generation, or 
gametophyte, remained distinct and recognisable in such diverse plants as 
the Bryophytes, Pteridophytes, and Gymnosperms. In arriving at this 
conclusion it was Hofmeister's great merit that he kept his eye securely 
upon those critical points where the individual life is represented by a 
single cell, viz. the zygote, and the spore. However differing in size or 
in complexity, he held as comparable, or, as it is said, ''homologous," the 
phases which intervened respectively between those two events. This great 
generalisation of Hofmeister, stated by him with a brevity and a simplicity 
of language as remarkable as its content was new and far-reaching, has 
formed the essential foundation of all subsequent morphology of Archegoniate 
Plants. A series of examples will now be quoted in illustration of it, and 
these will be selected to show the differences in form and in the relative 
proportions of the two generations ; but it will not be necessary to enter 
into a continuous account of the life of each example, for with certain 
modifications the essentials of sexuality and of spore-production remain 
the same in them all. 

In Riccia, one of the simplest of the Liverworts, the gametophyte, or Ricria- 
plant, as~Tt is called on account of its being more prominent than the 
sporophyte, is a green, dichotomously branched thallus, showing localised 
apical growth, while it is thick in proportion to its area : some species float 
on water, others are attached by rhizoids to the substratum of soil. The 

c 



34 



ALTERNATING GENERATIONS 



only appendages other than rhizoids and occasional hairs are small flat 
scales borne on the lower surface. The gametophyte is thus of very simple 

form (Fig. 17). The sexual organs are borne 
in acropetal succession on its upper surface, 
and are sunk in depressions. The sporophyte, 
which results from fertilisation of the ovum by 
spermatozoids motile through water as in Ferns, 
is a small spherical body, with no distinction 
of apex and base (Fig. 18). It consists of a 
single layer of cells forming a peripheral wall, 
which is, however, disorganised before the 
ripeness of the spores. The latter are pro- 
duced by a tetrad division of the spore-mother- 
cells, which occupy the whole internal space of 
the sporogonium (Fig. i8A); on germination 
the spores yield fresh gametophytes. Thus the 
two generations are here as distinct from one 
another, structurally and in origin, as in the 

Fern, though both are of small size and simple form. It is to be 
noted, however, that the sporophyte is throughout its life dependent 




PIG. 17. 

Riccia minima. A, thallus of 
natural size. B, the same in vertical 
section, showing two sporogonia sunk 
in the tissue of the thallus. Magnified. 
(After Bischoff.) 




FIG. 18. 



Ricciocarpus natans. 
surrounded by 



is natans. Young sporogonia in longitudinal section, 
surrounded by_ the archegonial wall. The younger ( X 666) shows 
the amphithecium (shaded) surrounding the sporogenous cells : in 
the older (xs6o) these are separated, as the free, and rounded 
spore-mother-cells. (After Garber.) 



SA. 

Ricciocarfots natans. The upper figure 
shows the spherical spore-mother-cells 
surrounded by nutritive material. The 
lower shows the tetrads formed from 
them : the sporogpnial wall (shaded) is 
still seen surrounding them, and covered 
externally by the archegonial wall of two 
cell-layers. X666. (After Gather.) 



upon the gametophyte, both mechanically and physiologically, . and that it 
is an almost entirely undifferentiated, spore-producing body. 



IN BRYOPHYTA 



35 



Taking Catharinea undulata (L.), x Web. and Mohr, as an example of the 
condition commonly seen in Mosses, the gametophyte and sporophyte are 
both on a more advanced scale than in Riccia, and both show localised 
apical growth, but their relations remain substantially the same. The " Moss 




FIG. 19. 

Catkarinea (Atrichiim) undnlata (L.), Web. and Mohr. The leafy gametophyte, bearing 
sporogonia. (After Schiniper.) 

Plant," or gametophyte (Fig. 19) appears as an upgrowing, branched, and 
leafy structure, attached to the soil by numerous rhizoids, and nourishing 
itself partly from materials absorbed by them, partly by the activity of 
its chlorophyll-containing shoots: it is thus physiologically an independent 
organism, as is also the simpler thallus of Riccia. In most Mosses the 
plant is ill protected against drought; but they commonly show, as a set 



ALTERNATING GENERATIONS 



off against this, a remarkable power of recovery on the return of moisture 
after being dried up. The sexual organs are usually borne by the Moss- 
Plant at or near to the apex of its upward-growing branches. The result 
of fertilisation here again carried out by spermatozoids motile through 
water is the formation of the Moss-Fruit, or sporophyte, which is 
throughout life a mere appendage on the Moss-Plant. At first it is, 
like that of Riccia^ completely enclosed by the venter of the archegonium 
(Fig. 20) ; but it soon shows apical growth and 
elongation : the venter is then ruptured trans- 
versely, and the sporogonium is exposed. As 
it elongates its base remains embedded in the 
tissue of the Moss-Plant : its apex is still 
covered by the upper part of the archegonial 
wall, the calyptra; but at ripeness this is shed, 
and the enlarged capsule on dehiscence is able 
freely to scatter its spores. After this the 
ephemeral sporophyte dies away. 

Comparing such a Moss with Riccia, the 
phases of the life-history correspond, but their 
elaboration is different : the thalloid gametophyte 
of Riccia is replaced by the upright leafy plant 
of Catharinea. The fertilisation is still dependent 
on fluid water, but its product is more complex : 
there is in Catharinea a distinction of apex and 
base, with localised apical growth ; but the form 
is still relatively simple, the whole construction 
being on the radial type, without appendages. 
The spore-production is restricted to the upper 
region, and takes place in one continuous sac. 
The sporophyte is still borne and nourished 
throughout its life by the parent gametophyte ; 
but it is able by its chlorophyll-containing cells 
to carry on photosynthesis in some degree, as 
an accessory to the supply derived from the 
parent. It has a ventilating system like that characteristic of aerial 
plants, while this is absent from the gametophyte. 

A comparison of such a Moss with a Lycopod shows a different 
balance of the two alternating generations. The gametophyte of Ly co- 
podium cernuum is shown in Fig. 21 as a somewhat massive structure, 
bearing lobes of chlorophyll-containing tissue above, which have sometimes 
been compared with the leaves of a Bryophyte : below it is attached by 
rhizoids to the soil. Like the Moss it is an independent organism capable 
of self-nourishment. It bears its sexual organs about the bases of the 
lobes, and is dependent upon external fluid water for its fertilisation. 
Notwithstanding its massive bulk it is without a ventilating system. The 




FIG. 20. 

Young sporogonium of Physco- 
mitrella patens, Br. and Sch., 
shortly before the rupture of the 
archegonial wall. (After Hy.) 



IN LYCOPODS 37 

sporophyte of L. cernuum, on th other hand, is a large dendroid plant, 
which may attain a height of even 3-4 feet (Fig. 22). In the embryo 
state it is nourished by the gametophyte which bore it, but it soon 
establishes itself independently in the soil as a much-branched plant, 
with relatively massive axes showing localised apical growth and numerous 
small leaves ; while true roots, not mere rhizoids, ramify in the soil. 
The whole plant is traversed by a vascular system, and there is also an 
efficient ventilating system. This ample vegetative development precedes 
the formation of the spores, which is localised in the terminal strobili : 




FIG. 21. 



Young leafy plant of Lycopodium cernMtin, L., with the prothallus, bearing its irregular 
assimilating lobes, attached on its left-hand side. X about 20. (After Treub.) 

these do not differ in general plan from the vegetative shoots, but in 
the axil of each leaf of these fertile branches a single sporangium is 
borne, containing many small spores, which are all alike (Fig. 220, E). 

The gametophyte of Lycopodium is among the most elaborate known 
in Vascular Plants : and yet it falls short of the complexity seen in the 
plant of Catharinea. It is clear that the two correspond from the fact 
that they both arise from spores and bear sexual organs. On the 
other hand, the proportion of the sporophyte. as well as its conformation, 
differs in high degree in the two plants. In place of the dependent 
and ephemeral sporogonium, with limited apical growth, without appen- 
dages, and bearing a single terminal capsule of spores, as in the Moss, 
Lycopodium shows an independent and perennial plant, with apparently 
unlimited apical growth and numerous appendages : it is rooted in 



ALTERNATING GENERATIONS 




Lycopodinm cermium, L., var. Eichleri, Glaz. A, general habit (\ natural size); B, 
end of a branch (natural size) ; C, strobilus ( x 3) ; D, sporopbyll seen from above ; , 
ditto, from the side (xao). (After Pritzel, in Engler and Prantl. Nat. Pflanz.) 



IN LYCOPODS 



39 



the soil, and capable of complete self-nourishment for an extended 
period before the production of spores. Moreover, these are produced, 
not in a single sac, as in the Moss, but in very numerous distinct 
sacs the sporangia. These essential differences of the sporophyte are 
those which clearly define the Bryophytes from the Pteridophytes. In 
the latter the mature sporophyte is always a free-growing organism, and 
a considerable vegetative period usually precedes the formation of the 
spores. 

Referring back to our observations on the Male Fern in the previous 
chapter, it will be seen that these remarks apply there also. The most 
obvious difference between a Lycopod and a Fern is in the size of the 
leaf; but they correspond in all essentials, 
and both show a very marked advance 
of complexity of the sporophyte over the 
Bryophyte sporogonium. On the other 
hand, the prothallus of the Fern is a 
smaller and simpler thing than that of 
Z. cer?mum, and stands thus in still 
stronger antithesis to the leafy plant of 
the Moss. Putting all these points 
together, it is plain that in the Pterido- 
phytes the balance in size of the 
generations is inverted as compared with 
that in the higher Bryophytes. 

In all the Bryophytes, and also in 
many Pteridophytes, the spores are all 
alike, and of small size, as we have seen 
them to be in Nephrodium and Ly co- 
podium : this is described as the 
" homosporous " condition, and it may 
be accepted as a primitive state. But 
in certain other Pteridophytes, and in 

all Seed-Plants, there are two different types of spore : the relatively 
small spore, which is easily transferred when shed, and produces a small 
male prothallus : and the large spore which, though less easily transferred, 
develops within it what is so important to the progeny a bulky female 
prothallus stored with nutriment. This store is derived from the parent 
plant, and is thus ready to supply the young immediately after fertilisation. 
The " heterosporous " condition brings a clear advantage, notwithstanding 
that the separation of the sexes on different prothalli increases the obstacles 
in the way of fertilisation being carried out. In certain cases the two 
types of sporangia and spores start their development alike, and only 
differentiate in the later stages ; for this reason, as well as on grounds 
of general comparison, the heterosporous state may be accepted as the 
later and derivative. From the example of Selaginella it will be seen 




FIG. 23. 

Microsporangium of Selaginella apus in 
median vertical section. Xss- (After Miss 
Lyon.) 



ALTERNATING GENERATIONS 



that heterospory may occur without any essential change in the 
sporophyte; for the plant of Selaginella is of the general Lycopod type, 
with small-leaved, much-branched shoot rooted in the soil, showing 




FIG. 24. 

B, megasporangium of Selaginella. apus in median vertical section, showing three of the 
four megaspores. X2i. A, a single megaspore, with prothallus and an archegonium, 
more highly magnified. (After Miss Lyon.) 

continued apical growth and terminal strobili. These are constructed 
essentially upon the Lycopodinous plan, but instead of the sporangia 
being all alike, some contain numerous small microspores, others contain 





FIG. 25. 

Microspore of Selaginella apus, after 
germination. (After Miss Lyon.) 



FIG. 26. 

Microspore of Selaginella apus, just before extru- 
sion of the spermatozoids. (After Miss Lyon.) 



only four large megaspores. In both cases these result, like other spores, 
from a tetrad division : the chief difference is in their size (Figs. 23, 24 B). 
But though the sporophyte is essentially unaltered, the changes in the 
gametophyte which accompany the heterosporous state are important. The 
prothallus is no longer a free-growing, self-nourishing organism, but it 



IN GYMNOSPERMS 



* -S. 

tends to become, and often actually is, a mere means of working up the 
material stored in the mature spore into gametes and an embryo, and 
does not possess any functional vegetative system. This is exemplified in 
Figs. 25, 26 of Selaginella, which show the contents of the germinated 
microspore developed as little more than an antheridium. In Fig. 26 the 
wall of the spore is ruptured, and the contents are ready to be extruded 
as numerous spermatozoids. Fig. 24 A shows the megaspore with the 
female prothallus within it, bear- 
ing an archegonium. Fertilisation 
takes place as in Ferns through 
the medium of water. The ovum 
after fertilisation forms the embryo 
which remains for a time embedded 
in the prothallus : but later it bursts 
through, and establishes itself as 
the independent sporophyte. 

In many heterosporous plants 
the germination takes place after 
the spores are shed, just as is the 
case in homosporous plants. But 
in others germination of the mega- 
spore may be initiated or even 
carried through within the spor- 
angium. This is the case in 
Selaginella apus (Fig. 24), in 
which it is evident that, even 
when the sporangium has not yet 
opened, the prothallus may be 
well advanced in the megaspores. 
Fertilisation may be carried out 

within rVp> cnnrano-inrn afrp-r itc 
\\ltmn ttie Sporangium after itS 

rnntnrp hv mp^riQ nf cnprmarn 

rupture, oy means ( t spermato- 
zoids derived from spores shed 
from adjoining microsporangia, 

and the embryo may be developed while the megaspore is still within 
the sporangium. It is no great step from this condition to that seen in 
the Seed-Plants, in which the megaspore or embryo-sac as it is called 
in Seed-Plants remains embedded in the tissue of the megasporangium 
or ovule (Fig. 27). The physiological advantage gained by this step is 
an important one : there is no longer any need to hurriedly pass the 
nutritive supplies into the spore before its wall, thickened for protective 
purposes, stops the process of transfer ; for in the Seed-Plants the wall 
of the megaspore, no longer needed for protection, remains thin, and the 
nutrition of the female prothallus can be continued until long after the 
embryo is initiated within it. 




Median longitudinal section of the ovule of Ptce<t 
e ^ cf / sa> at t j me of fertilisation. X 9 . IT, embryo-sac 
filled b y the prothallus; a, the venter; c, the neck of 

an archegonium . ff> ovum . n> its nuc i e us ; nc, 



42 ALTERNATING GENERATIONS 

These points are illustrated in the Gymnosperms, which the positive 
evidence of the geological record shows to have been the primitive Seed- 
Plants. Since the time of Hofmeister comparative morphology has arrived 




FIG. 28. 

. Pinus Laricio, showing a series from the formation of the tetrads to the development of 
the pollen-tube. /, vegetative cells ; st, stalk cell ; b, body cell ; /, tube nucleus. X6oo. 
(After Coulter and Chamberlain.) 

at the same conclusion, though along a distinct line of argument. Taking 
examples from the Pinaceae, the sporophyte is represented by the Tree, 
which is a large, much-branched, independent, and perennial organism, 
with theoretically unlimited apical growth, and a highly differentiated 



IN ANGIOSPERMS 43 

system of root and shoot. A long vegetative period precedes the spore- 
formation. The sporangia are no longer alike as in Selaginella, but differ 
widely in form and position, and are located on distinct male and female 
strobili. The microsporangia, or pollen-sacs, produce after the usual tetrad 
division the microspores, or pollen-grains, which are shed at maturity. 
The male prothallus which they produce is partly formed on the parent 
plant, partly after shedding, and is restricted merely to a few cells (Fig. 28). 
Typically the megasporangia, or ovules, develop each only a single mega- 
spore or embryo-sac as it is called in Seed Plants and within it there is 
at the period of fertilisation a massive female prothallus, bearing archegonia 
(Fig. 27). Since the male and female strobili are distinct, it is necessary 
for fertilisation that the microspores, or pollen-grains, should be shed ; 
but no independent vegetative thallus is produced from them : the pollen- 
grain, -landing on the apex of the megasporangium, forms a pollen tube or 
siphon, which penetrates the sporangial wall, and by its means the non-motile 
male cells are transferred to the ovum. The essential 
point of fertilisation is the same as before, but the 
means are different. The dependence on external 
fluid water, characteristic of all Pteridophytes, is 
dropped, and the siphonogamic method of fertilisation 
may be held to mark the distinctive terrestrial habit. 
But as a lately acquired proof of the justice of 
Hofmeister's comparisons, the fertilisation by a motile 
spermatozoid is still retained, in a somewhat un- FIG 

practical form, in certain primitive Gymnosperms, rradescantia vir&nica. 
Cycadaceae, and Ginkgoaceae. The nursing of the gBfftiS *Sffik8S 
embryo in the female prothallus, or endosperm, ( a Afte a rtfasbur V ger e j 1 ' X54 ' 
follows in the Pine on essentially similar lines as in 

Selaginella, also the final germination to establish again the independent 
sporophyte. 

Lastly, in the higher Seed-Plants, or Angiosperms, which Palaeontology 
indicates as of later origin, the outline of the life-cycle is as in the 
Gymnosperms, but with still further reduction of the prothallial development 
in the pollen-grain (Fig. 29). Fertilisation is of the terrestrial siphonogamic 
type. The embryo-sac remains like that of Gymnosperms embedded in the 
tissue of the parent plant : it contains before fertilisation only an exiguous 
tissue-development, the exact homology of which is- still a question in 
debate (Fig. 30). 

The above sketch illustrates the general trend, though probably not 
the exact course, of evolutionary progress in the Archegoniate series. But 
it is necessary to remark that the examples selected do not form any actual 
phyletic sequence : of them all no two (excepting Lycopodium and Selaginella) 
belong to a single recognised phylum. The general result of their com- 
parison is therefore a history read between the lines. But, with this 
proviso, the following conclusions may be drawn from it, as to the 




44 



ALTERNATING GENERATIONS 



fluctuations of balance of the two generations of the antithetic alternation 
involved in the upward progress of plant-form. 

The gametophyte was at first the predominant feature, and there is 
good reason, as we shall see later, to believe that it was the originally 
pre-existent phase. It was an independent, self-nourishing organism, with 
unlimited apical growth, and is seen in the Bryophytes either in 
the thalloid form, or developed as a more elaborate leafy plant. In 

the Mosses, and in the leafy Liverworts the 
sexual generation reached its morphological 
climax. But nevertheless in the relative 
simplicity of its tissues, and in the absence 
of an internal ventilating system, it remained, 
as its method of sexuality proclaims it to 
be, at best only an imperfect adaptation 
to growth under conditions of subaerial 
exposure. In the homosporous Pterido- 
phytes, though there is in Lycopodium, and 
also in Equisetum, some indication of lateral 
appendages, the gametophyte is thalloid, but 
it still shows its physiological independence, 
while there may be a brief and ill-defined 
apical growth. Nevertheless, in the Pterido- 
phytes the gametophyte as a rule bears the 
stamp of a temporary phase in the cycle 
rather than that of a permanent organism : 
but this becomes much more pronounced 
in the heterosporous forms : in these the 
independent, self-nutritive existence is lost, 
and the prothallus is without localised apical 
growth : the male gametophyte becomes little 
more physiologically than a means of pro- 
ducing spermatozoids : the female is at first 
a producer of ova, and later it is simply a 
means of nourishing the embryo at second 
hand from the plant on which it is depen- 
dent. The morphological reduction which follows the heterosporous state 
is clear enough in Selaginella and in the Pine, and it reaches its climax in 
the Higher Flowering Plants, where the gametophyte is found to have 
dwindled away to an exiguous residuum of a few ill-defined cells, with 
virtually no vegetative characters at all. The whole story indicates the 
eclipse of the generation which appears to have been originally the pre- 
dominant partner in the life-cycle. % 

The sporophyte, on the other hand, has a complementary story. It is 
seen in the simplest Bryophytes as an ephemeral, spherical body, without 
distinct apex or base, and no vegetative system except a temporary 




FIG. 30. 

Ovary of Polygenum convolvulus during 
fertilisation, fs, stalk-like base of ovary ; 
fn, funiculus ; cha, chalaza ; nu, nucellus ; 
mi, micropyle ; zV, inner, ie, outer in- 
tegument ; e, embryo-sac ; ek, nucleus of 
embryo-sac ; ei, egg-apparatus ; an, anti- 
podal cells ; g, style ; n, stigma ; p, pollen- 
rains ; ps, pollen-tubes. X48. (After 
trasburger.) 



INVERSION OF BALANCE 45 

protective wall of cells : it is dependent through life upon the gametophyte, 
and results in a limited number of spores. In more complex Bryophytes 
it is still short-lived and dependent, but larger, with distinction of apex 
and base, a brief apical growth, and a basal vegetative region distinct 
from the terminal capsule : there is entire absence of appendages, but 
a partial differentiation of tissues, with internal ventilating system and 
some assimilatory tissue. The spore-production is on a larger scale, but 
limited usually to the simultaneous development of one continuous 
spore-sac. 

In the Pteridophytes the mature sporophyte is an independent, self- 
supporting organism ; but it is dependent in youth upon the parent 
prothallus : it is commonly perennial. It has theoretically unlimited 
growth of axis and root : the appendages vary greatly in size : there is 
high differentiation of the tissues, with an elaborate ventilating system : 
the plant thus constituted is capable of complete and continued self- 
nutrition. The spores are produced after a more or less prolonged 
vegetative phase, and in perennial forms their production may be 
continued for an unlimited succession of seasons. They are borne 
in separate sporangia, which are commonly seated upon the appendages : 
the sporangia themselves are frequently produced in a continued succession. 
These arrangements are such as to lead to a high and even long-continued 
output of adequately nourished spores. The sporangia are frequently 
restricted to certain shoots, in which the parts are closely aggregated : 
these are termed strobili. 

The heterosporous state seen in all the highest Vascular Plants, 
introduced advantages conducing to certainty in nursing the embryo, and 
led in Seed-Plants to an infinity of special developments which secured 
that transfer of the microspores which is so necessary for fertilisation. 
But the essential plan of the independent, self-nourishing Vascular Plant 
once laid down was not departed from, even in the highest forms. The 
sporophyte, thus sprung from small beginnings, remains the dominant 
generation in all distinctively terrestrial plants. 

The entire inversion of the balance of the two alternating generations 
thus briefly sketched the dwindling away of the one and gradually 
achieved dominance of' the other is a fact which requires some 
physiological explanation. We may be sure that such things do not 
happen without good reason. It will be our object later to enquire into 
this. Meanwhile we recognise the fact itself, and we shall see in the 
comparisons which lead to its recognition an enduring monument to 
the genius of Hofmeister who first pointed them out. 



CHAPTER IV. 

CYTOLOGICAL DISTINCTION OF THE ALTERNATING 
GENERATIONS OF ARCHEGONIATAE. 

ALTERNATION is thus found to be a general 'phenomenon for Archegoniate 
Plants. It was at first recognised chiefly on the basis of the propagative 
organs which the alternating generations respectively bore, and the dis- 
tinction was confirmed on grounds of external form and of anatomical 
structure. The two phases, however, presented no very strict criteria by 
which they could with certainty be told apart. As regards external form, 
a foliar development was found to exist in the sexual generation of the 
Bryophytes, and again in the neutral generation of Vascular Plants : and 
however strongly it might be urged on grounds of detailed comparison 
that these were distinct in origin, and therefore only analogous, still the 
fact that foliar development exists in them both showed that external form 
did not constitute a strict criterion. As regards anatomical structure, the 
presence or absence of vascular tissue, and of intercellular spaces appeared 
at first to give a ready distinction ; but a better knowledge of the anatomy 
of the larger Mosses showed that they also contain conducting tissues closely 
analogous to the vascular strands of Pteridophytes. Again, it is a fact 
that there is an ample ventilating system in the sporophyte, and that 
intercellular spaces are generally absent in the gametophyte; but in the 
leaves of certain Filmy Ferns there may be no intercellular spaces through- 
out considerable tracts, while the statement for the gametophyte is one of 
those negative statements which are at any time open to reversal. Even the 
production of the characteristic organs of propagation, and the transition 
by spore or zygote from one generation to the other, is not so absolute a 
distinction as was once thought ; for first apogamy, and later apospory 
were discovered, and it was thus seen that a vegetative transition might 
take place from either generation to the other, without the critical incident 
of production of spore or zygote intervening as a limit. The climax of 
these difficulties in definition of the two generations was reached when 
Lang described, in 1896, how in certain Ferns sporangia might be borne 
directly upon the prothallus itself. 



CYTOLOGICAL DISTINCTION 47 

This absence of strict criteria Distinguishing between the two alternating 
generations of Archegoniate Plants has given rise to much discussion, 
and the differences of opinion have centred round the question of their 
origin. Were the two generations distinct ab initio, or were they merely 
phases differentiated from a common source? Under the "homologous'' 
theory of alternation the two generations were held to have been similar 
in origin, and the alternation to have originated by a secondary modification 
arising in a pre-existent and independent organisation. The adherents of 
the " antithetic " theory held that the sexual generation was pre-existent, 
and that a new organisation arose, derived by amplification from the 
zygote : the sporophyte was thus originally not a result of change in a 
pre-existent organisation, but it arose as a newly expanded phase, distinct 
in its origin from other phases of the life cycle. The difference of opinion 
entailed in these two theories is essentially one of history, and of method 
of origin. 

In the absence of strict criteria of distinction, such discussions are apt 
to be long and inconsequent. It seemed accordingly to be a welcome 
advance when facts were gradually disclosed, showing that a cytological 
difference exists between the two generations. This appeared to raise 
the whole doctrine of alternation in Archegoniate Plants to a higher plane, 
and to relate the origin of the two alternating phases intimately with the 
existence of a sexual process. In order to understand the nature of 
this new criterion of distinction it is necessary to be acquainted with 
the main features of nuclear division. When a nucleated vegetable cell 
divides, the nucleus takes the initiative, and goes through a series of phases 
as shown in Fig. 31, which is quoted from Strasburger, to whom the 
discovery of the details is chiefly due. Without describing these at 
length it may be stated that the chromatin, that constituent of the nuclear 
body which stains most deeply, distributes itself in the linin : the body 
thus formed changes from a network of fine fibrils in the resting nucleus 
(i) to a thicker convoluted thread, which then divides transversely into- 
segments the chromosomes (3, 4). These segments then divide longitudin- 
ally (6, 7, 8), and the halves of each, separating from one another, pass 
to the opposite poles of the nuclear spindle, which has meanwhile been 
formed (8, 9, 10): they there reconstitute the chromatin-system of the 
two new nuclei (10, u, 12). An essential part of this process is found to be 
that the number of chromosomes is definite, and though in different plants 
and groups of plants it may vary within wide limits, still in the species 
or individual the number is (with some exceptions) strictly maintained. 
But this is so normally only in the cells of the one or of the other 
generation ; for it has been found, in cases which are constantly becoming 
more numerous as observations extend, that there is a numerical difference 
in the chromosomes of dividing nuclei in the two alternating generations 
of the same plant : in the sporophyte the number is twice that in the 
gametophyte : the former has accordingly been styled the " diploid," the 



4 8 



ALTERNATING GENERATIONS 



latter the " haploid " phase. It happens that the facts are more readily 
observed in most Seed-Plants than in Pteridophytes, owing partly to the 
greater size of the spindles, partly to the number of the chromosomes 
being smaller than in most of the Archegoniates. Accordingly we have 
the record of the numbers from over 60 Seed-Plants, showing with remarkable 








FIG. 31. 

Successive stages of nuclear- and cell-division in an embryonic tissue. n = nucleus. 
/=nucleolus. w==nuclear wall. c = cytoplasm. ch = chromosomes. = polar-cap. 
s spindle. kt> = nuclear plate. / = young daughter-nuclei. v connecting threads. 
z = cell-plate. 7w = nevv septum. In i the resting nucleus is shown. In 2 and 3 the 
segregation of the chromosomes. In 4 the chromosomes are seen with transverse discs. 
In 5 the arrangement of the chromosomes to form the nuclear plates and their longitudinal 
fission. In 3-5 the formation of the spindle from the polar-caps. In 6 the longitudinal 
fission of the chromosomes. In 7 their separation towards the poles has begun. In 8 the 
daughter-chromosomes are completely separated. In 9 they are proceeding towards 
the poles. In 10, u, and 12 the daughter nuclei are being formed. In 9-11 the connecting 
threads and the cell-plate are being formed. In 12 the completion of the septum. 
X about 600. (After Strasburger.) 



constancy that the number of chromosomes in the dividing nuclei of the 
sporophyte is double that in those of the gametophyte. Common numbers 
are 32 : 16 24 : 12 16 : 8; while they run as low as 12 : 6 or 6 : 3, and 
as high as 64:32 or 96:48. 

Records from the Gymnosperms bring evidence of the same difference 
between the two generations in various species of Pinus, etc. Among 
Pteridophytes it has been observed in such Ferns as Osmunda, Ahophila, 



TETRAD-DIVISION 



49 




Pollen-mother-cells of a Lily in course of division, somewhat diagrammatic, i. Mother- 
cell with resting nucleus. 2. The segregation of the chromosomes. 3. Synapsis. 
4. Double threads already beginning to coalesce. 5. The spirem derived from the 
coalescent double-thread, apparently a single thread. 6. Subsequent separation of 
the threads. 7. The spirem transversely segmented, double chromosomes. 8. Diakinesis. 
9. Initiation of multi-polar spindle. 10. Spindle of the mother-nucleus, the nuclear plate 
composed of double-chromosomes, n. Reduction-division, the separating chromosomes 
showing a partial separation of their longitudinal halves. 12. Formation of daughter- 
nuclei. 13. The longitudinal halves of the chromosomes (daughter-chromosomes) are 
disposed in the nuclear spindles connected in pairs. 14. Daughter-nuclear-spindles. 
15. Separation of the daughter chromosomes. 16. Formation of the nuclei of the second 
generation, x about 800. (After Strasburger.) 



50 ALTERNATING GENERATIONS 

and various Polypodiaceae. In the Bryophytes, the earliest case described 
was that of Pallavidnia investigated by Farmer, with four chromosomes 
in the gametophyte and eight in the sporophyte : and the same relation 
between the generations has since been found also in other genera of 
Liverworts. In Mosses few observations have yet been made, but Mr. 
M. Wilson has found that in Mnium hornum the numbers are 6 and 12 
respectively in gametophyte and sporophyte. Accordingly the difference 
in chromosome-number may with high probability be held as a general 
diagnostic feature between the two generations in normal representatives of 
the Archegoniate series. 

This being so, the recognised limits between the generations will 
naturally be expected to be the points of transition from the one chromosome- 
number to the other. Now it is found that when the sexual fusion of 
the two nuclei takes place, the subsequent divisions of the fusion-nucleus 
show the doubled number of chromosomes : therefore the zygote will be 
the one limit, and this is in accord with the old distinction of the 
generations dating from the time of Hofmeister. In his practice the 
other limit was the spore, since this is the actual body separated as an 
independent germ. But it is found that the actual reduction of the number 
of chromosomes to one half, that is, to the original pre-sexual number, takes 
place at the tetrad-division of the spore-mother-cell. This cell divides 
twice in rapid succession, and the process is well illustrated in the case 
of the pollen mother-cells of Lilium, in which it has been specially studied 
(Fig. 32). It starts from a cell with a nucleus having the double number 
of chromosomes, as shown by its origin. The nucleus first enters the 
condition of synapsis (Fig. 32. 3, 4), in which a lateral fusion of the 
chromosomes in pairs, respectively of paternal and maternal origin, is 
believed to take place : presently a coiled thread frees itself from the 
tangle of synapsis (Fig. 32. 4, 5), which becomes shorter and thicker, 
still showing, however, indications of its double nature (Fig. 32. 6, 7), 
and divides into segments, which are half as many as those of the parent 
nucleus (Fig. 32. 7, 8) : each individual of the chromosome-pairs then 
moves apart (Fig. 32. 10, n), one of each pair passing to either pole 
of the spindle which has meanwhile been formed : as each half is an 
original chromosome, the number at each pole is one half that of the 
parent nucleus, and the division is styled the heterotype, or reducing 
division (Fig. 32. 12). The second division in each of the two nuclei 
thus formed follows quickly, and is homotypic, that is, each chromosome 
undergoes longitudinal fission into two, as in a vegetative division (Fig. 32. 
13, 14, 15). The four nuclei thus constituted have also half the number 
of chromosomes present in the nucleus of the spore-mother-cell ; but the 
reduction is actually effected, as has been seen, in the first, or reducing 
division. Accordingly, Strasburger has recognised the spore-mother-cell, 
in which the reduction is initiated, as the actual limit between the two 
generations. But it is the spore itself which normally terminates the 




FIG. 33. 

Apogamy in Pteris creticei, L. A and B, development of the first foliar process close 
to the emargination (k) on under surface of prothallus. C, a whole prothallus seen from 
below, showing a young apogamous shoot. /> = prothallus ; 3 l = first leaf; z> = stem-apex ; 
w = root. D, a similar growth more advanced. A and B X 145, C and D highly magnified. 
(From Engler and Prantl, after De Bary.) 



ALTERNATING GENERATIONS 



one generation and forms the starting-point for the next generation : the 
older usage based upon this obvious fact is, therefore, to be preferred, 
and the spore may be still held to be the obvious boundary between 
the two generations. The gametophyte, or haploid phase, will then be 
recognised as extending from the spore to the zygote in each cycle, and 
it shows " n " chromosomes normally in all its nuclear divisions : the 
sporophyte, or diploid phase, is recognised as extending from the zygote 
to the spore, and it shows " 2n " chromosomes in all its normal nuclear 
divisions. However difficult these nuclear details may be to recognise 
in any given case, so far as observation goes within the limits of the 
Archegoniatae they provide a structural basis for the distinction of the 

two generations more exact 
than any other, a distinc- 
tion which runs parallel 
with those less accurate 
criteria on which the 
recognition of the genera- 
tions was first founded. 

The possession of this 
means of diagnosis neces- 
sarily turns attention afresh 
to those cases where the 
transition from one genera- 
tion to the other is bridged 
over by direct vegetative 
growth, viz. the phenomena 
of Apogamy and Apospory, 
which have figured so largely 
in the discussions on alter- 
nation. It has long been 

known that the two alternating generations are not always delimited from 
one another respectively by those unicellular phases of the spore and the 
zygote; but that in certain cases, and among the Archegoniatae most 
commonly in the Ferns, there may be a vegetative transition either from 
the prothallus to the sporophyte without the intervention of a sexual 
process this is termed apogamy; or conversely, from the sporophyte to the 
prothallus without the intervention of spores this is designated apospory. 

In the most frequent examples of apogamy in Ferns the place of an 
embryo is taken by a process which originates from the tissue of the 
cushion as a result of vegetative growth and division of its cells (Fig. 33, A) : 
it soon takes a form corresponding to that of an embryo, with first leaf, 
root, and apex of axis (Fig. 33, B and c), and it finally becomes an established 
plant in the same way as those sexually produced (Fig. 33, D). In some 
cases these developments may take place in entire absence of archegonia 
on the prothallus; in others various conditions of the archegonia may be 




FIG. 34. 

Scolopendrium vulgare. Prothallus from the branched cylindrical 
process of which ten roots arose : eight of these are visible in the 
drawing. X about 6. ('After Lang.) 



APOGAMY 



53 



found, either showing normal struchire or various modifications of it. But 
in other cases, which have been described in detail by Lang, 1 the 
apogamous developments may diverge far from the normal in point of 
the number and position of the parts. Originating by direct vegetative 
growth from the tissues of the thallus, in place of the normal sequence 
and position of the parts the several constituents of the sporophyte, root, 
leaf, sporangium, may appear without order or numerical rule : ten or 
more roots have been found apoga- 
mously produced upon a prothallus 
without other parts of the sporophyte 
(Fig. 34) : sporangia have been ob- 
served without sporophylls, originating 
directly from a formless mass of 
sporophytic tissue apogamously pro- 
duced on the prothallus (Fig. 35), 
or even, in an extreme case, from 
the prothalloid cells of the archegonial 
wall (Fig. 36). The irregularity of 
such growths must be taken into 
consideration in their theoretical 
interpretation, as will be seen later. 
Among the Archegoniatae apogamy 
has hitherto been observed in a score 
or more of species of Ferns, belonging 
to the Osmundaceae, the Hymeno- 
phyllaceae, and chiefly to the Poly- 
podiaceae ; and examples are also 
recorded from the Marsiliaceae. In 
the Bryophytes, the Lycopodiales, 
and the Equisetales no cases are as 
yet recorded. It may be noted, how- 
ever, that similar phenomena have 
been observed in Flowering Plants, 
such as AkhcmiUa, Thalictrum, 
Antennaria, and Taraxacum? 

Turning now to Apospory, 3 that is, the transition by direct vegetative 
growth from the sporophyte to the gametophyte without the intervention 
of spores, instances are recorded from the Liverworts (Anthoceros, Lang), 4 
from the Mosses (Hypnum and Bryum, Pringsheim, 5 Ceratodon, Stahl, G 

1 Phil. Trans., vol. cxc. (1898), p. 187, etc. 

2 For references see Strasburger, Flora, 1907, p. 139. 

y The term " Apospory ; ' was introduced by Vines, in an article on the " Proembryo 
of Chara," Journal of Botany, 1878, p. 355. 

4 Annals of Botany, vol. xv., 1901, p. 503. * Pringsh. Jahrb., xi., 1877. 

*Bot. Zeit., 1876, p. 689. 




FIG. 35. 

Nephrodium dilatatum, Desv., var. cristatum 
gracile. Prothalloid cylindrical process, bearing 
archegonia near its base. It arises by the side of an 
imperfect sporangium (sfi), and bears a similar 
sporangium (sp) on the other side, and on the tip are 
a number of sporangia associated with ramenta. 
X 3 5. (After Lang.) 



54 



ALTERNATING GENERATIONS 



/ ** y 









Funaria, Brizi), 1 and from various species of Ferns belonging to the 
Hymenophyllaceae and Polypodiaceae, 2 but no examples are on record 
from the Lycopodiales or Equisetales. Those cells which would in the 
normal course produce the spores take no part in the formation of the 
gametophytic growths. In Anthoceros the origin of these is commonly 
from sub-epidermal cells: in the Mosses from the cells of the seta, or of 
the sporogonial wall ; while in Ferns the archesporial cell if already denned 
in a sporangium is abortive. Thus the aposporous growths are in no 

sense mere irregularities of 
development from sporogenous 
cells. In Anthoceros each 
growth is apparently referable 
in origin to a single cell, and 
the same is probably the case 
also for Mosses. But in the 
Ferns this is not so : here the 
vegetative development may 
start from a sporangium formed 
in its normal place : a plurality 
of the cells of the stalk, or 
of the sporangial wall surround- 
ing the abortive central cell 
may divide, and assume pro- 
thalloid characters (Fig. 37), 
or the growth may arise from 
the receptacle of the sorus 
(Fig. 37 E): or again, it may 
be initiated at some point 
on the leaf, usually marginal, 
which thus extends directly 
into the prothallial expansion, 
and may bear antheridia and 
archegonia (Fig. 376, c, D). 

The matter may be further 
complicated by the combination 

of apogamy and apospory in the same individual, and this condition has 
been seen in about half the recorded cases of these abnormalities in Ferns. 
The apogamous seedlings of Nephrodium pseudo-mas, var. cristata (Cropper), 
not only sprang themselves in an apogamous manner from the prothalli, 
but proceeded almost at once to an aposporous production of new prothalli 
on the margins of the young leaves. 3 These prothalli bore antheridia, 




FIG. 36. 

Scolopendrium vulgare. Group of sporangia (sf) on a 
projection, the structure of which indicates its relation to an 
archegonium. Occasionally two nuclei are present in a single 
cell. X6oo. (After Lang.) 



1 Ann. Inst. Bot. Rom.) v., p. 54. 

2 For references see Engler and Prantl, Nat. Pfl., I. 4, p. 
1905, p. 239. 

3 Druery, Journ. Linn. Soc., vol. xxix., p. 479. 



I, and Goebel, Flora, 




Apospory. A =Soral-apospory of Athyrium Jilixfoeinina., 
ti. clanssittia. Jones. Part of a pinnule with veins (?>l>) 
id a sorus. In the latter, in place of the sporangia, prothalli 
* formed (f>rtli) with antheridia (antk) and archegonia 
rch). X40. B apical apospory of Polystichuin angulare, ^; 

r. pulcherrimum. Padley. A prothallus at the tip of a 
nnule, as a direct continuation of it. gl marginal glands, c~ = the cushion. 
20. C=the first initiation of a prothallus as in B, at the apex of a pinnule, 
he shading indicates a vein beyond the tip of which the prothallus arises. 
130. D a. similar growth, but borne on an elongated cylindrical process! 
chegonia (arch) are already present. X 10. soral apospory in Polystichuni 
tgulare, var. pnicherrunum. \ prothalloid growth bearing an antheridium 
\ntli) and rhizoids (h) has arisen from the stalk of a sporangium. Xyo. 



E 



ALTERNATING GENERATIONS 



but Dr. Lang's drawing shows how, nevertheless, the prothalli in turn 
hasten to a fresh apogamy (Fig. 38), thus the two transitions may be 
repeated at near intervals of time. The same was seen to be the case 
in Trichomanes alatum, in which apospory and apogamy were found to 
succeed one another, 1 and it has recently been proved to hold also for 
Athyrium filix foemina, var. clarissima^ Jones, and other Ferns. 2 

Lastly, Goebel has shown 3 that when the seedling leaves of certain 
ferns are removed and cultivated on a moist substratum, aposporous 
growths may be induced, which show sometimes the most intimate inter- 
mixture of characters of the sporophyte and 
the gametophyte. These developments appear 
to be similar in kind, though not in detail, to 
those described by Lang and others. It would 
doubtless be possible to erect upon such facts 
a superstructure of theory ; but it is necessary 
to remember that by the abscision of a young 
part it is placed in an anomalous and extreme 
physiological position. It is improbable that 
such circumstances ever arose in the course 
of descent : and accordingly it must remain 
a quite open question what bearing, if any, 
such observations have upon the evolutionary 
story. They demonstrate possibilities : but 
possibilities are not the equivalent of historical 
data. 

The rapid succession of the transitions 
thus actually seen in some Ferns from the 
sporophyte to the gametophyte, and the con- 
verse, give some colour to the suggestion 
made by Goebel, that the sporophytic buds 
he found in the deep-water specimens of 

Isoetes are to be viewed as extreme cases of the telescoping of the 
alternate generations. 4 This state of affairs is very nearly matched by 
certain Adiantums observed by Lang, in which numerous sporophytic buds 
were produced from the sorus. Examination showed that they sprang in 
certain cases from the sporangia themselves, but not from the sporogenous 
tissue. If we imagine the gametophyte stage reduced in such cases, not 
to a very short phase only, as it is in Lang's Nephrodium, but to the 
vanishing point, the result might be as in Goebel's Isoetes. But we may 

1 Ann. of Bot., vol. i., p. 269. 

2 Farmer and Digby, Ann. of Bot., 1907, p. 163-167. 

3 Sitz. d. Math.-phys., Klasse d. K. Bayer. Akad. d. Wiss., xxxvii., 1907. Heft, ii., 
p. 119. This is interesting for comparison with my own negative results on leaves of 
mature plants recorded in Ann. of Bot., iv., p. 168 (1889). 

*Bot. Zeit., 1879, P- I- 




FIG. 38. 






APOGAMY AND APOSPORY 57 

well ask whether such an interpretation does not read into the facts more 
than actually exists? If Isoetes were a plant which habitually showed 
combined apospory and apogamy, and if various steps were present leading 
towards the extreme result, then the conclusions might be accepted. But 
Isoetes is a plant which is structurally stable as a rule, and there is in 
these abnormal growths no prothalloid tissue at all. Thus they appear to 
be merely sporophytic buds formed from sporophyte tissue, and having 
sporophytic character throughout. They will rank with those sporophytic 
buds which are found arising from the sorus in various Ferns, or from 
the nucellus in some Phanerogams : they are, in fact, a mode of vegetative 
continuance of the neutral generation, and nothing more. 

The question necessarily presents itself, what is the cytological state 
of the tissues in the plants which show those vegetative transitions from 
one generation to another, such as have been described for the Mosses 
and Ferns above named? The facts would appear to be inconsistent with 
the structural distinction of the two generations, since the acts of sexuality 
and of spore-formation, by which the cytological changes are normally 
effected, are liable to be omitted. It will be important to know how 
far the distinction between the haploid and the diploid phases will remain 
valid. The facts have lately been elucidated for a number of the 
abnormal Ferns by Prof. Farmer and Miss Digby, 1 and for the very 
peculiar case of the genus Marsilia by Prof. Strasburger.- 

Taking first the case of apogamy : already in 1898 Dr. Lang had observed 
in prothalli of Scolopendrium, in the tissues bordering on the change 
from gametophyte to sporophyte, the frequent presence of two nuclei in a 
single cell (Fig. 36). More detailed observations have since been made on 
other apogamous Ferns, by examination of very young prothalli, before any 
apogamous growths had begun to manifest themselves. 3 Similar cells with 
two nuclei were observed in the case of prothalli of Lastraea pseudo-mas, 
var. polydactyla ; but it was shown that when two nuclei are seen in a single 
cell a neighbouring cell is without one, and cases were found where the 
passage of the nucleus through the cell-wall was actually in progress 
(Fig. 39). This process is regarded as a kind of irregular fertilisation, 
for ultimately the two nuclei fuse. On their division the nuclei of the 
apogamous growth thus pfoduced show, as a consequence of the fusion, 
evidence consistent with a doubling of the chromosomes, just as it 
happens in the normal post-sexual stage. But instead of one cell only 
serving as the starting-point for the new generation, a number of such 
units co-operate loosely to produce it. These results have their interesting 
bearing on the irregularity of number, and the sporadic position of the 
parts in such cases as those observed by Lang. It is thus seen that even 
in these irregular examples the cytological criterion between the two 
generations may hold, and the structural limit will be found in the cells 

1 Ann. of Bot., xxi., p. 161. -Flora, 1907, p. 123. 

3 Farmer, Moore, and Digby, Roy. Soe. Proc., Ixxi., 1903, p. 453. 



58 ALTERNATING GENERATIONS 

in which the doubling of the chromosomes is initiated by the nuclear 
fusion. 

The first case of apospory to be cytologically investigated was that 
of Nephrodium pseudo-mas, var. cristata apospora, where the prothallus 
grows directly out from the margin or surface of the leaf. It was shown 
in this case by Miss Digby * that there is no nuclear change involved, 
but that both sporophyte and gametophyte have a number of chromosomes 
about 50. This result would at first sight appear to show that the 




FIG. 39. 

Neph.rodnt.ni pseudo-mas, var. polydactylum. Tissue of prothallus where an apogamous 
growth is to be formed, showing to the left a cell with two nuclei, while an adjoining cell 
has none. At the centre a nucleus is seen passing through a perforation of the wall, 
and fusing immediately with that of the cell it enters. (After Farmer, and Moore, and 
Miss Digby.) 

chromosome-criterion had hopelessly broken down. But a better under- 
standing of such cases is obtained when the whole nuclear cycle is 
considered, than by contemplation of a single phase of it. It has been 
above noted that there is a frequent relation between apogamy and 
apospory in the life-cycle of the same individual : it is important to 
know the nuclear conditions throughout such cycles. The case of 
Athyrium filix-foemina, var. clarissima, Jones, may be taken as a first 
example where the complete chromosome-cycle is known. 2 In this Fern 

1 Roy. Soc. Proc., Ixxvi., 1905, p. 466. 

2 Farmer and Digby, Ann. of Bot., 1907, pp. 163-7. 



A FOG AMY AND APOSPORY 59 

it has been shown by Farmer- ^and Miss Digby that there is apogamy 
as well as apospory. The cytological investigation shows that in those 
cases where sporophytes were borne on the apogamous prothalli there is 
not any migration of nuclei from one prothallial cell to another, such 
as has been described for some cases of apogamy ; nor is there doubling 
of the chromosomes in any other way. In fact, the chromosome-number 
is the same for the sporophyte as for the prothallus which bears it. 
Investigation of the aposporous transition from the leaf to the prothallus 
showed also that no change of number marks the passage from sporo- 
phyte to gametophyte. There is here a case of cytological uniformity 
throughout the whole cycle, with chromosome-number about 90. This is 
approximately the number found in the diploid stage of a typical A thy Hum 
filix-foemina. The condition of the variety is as though reduction had 
been omitted from the cycle : as a consequence the prothallus being 
itself diploid, fertilisation would be unnecessary to produce a new 
sporophyte : accordingly apogamous budding will suffice, and that is what 
actually occurs. 

A near parallel to this has been worked out with similar exactitude by 
Strasburger in Marsilia Drummondii, A.Br. 1 The typical chromosome - 
numbers are 16 and 32 respectively for gametophyte and sporophyte, 
and normal plants show the usual succession of events. But on ger 
mination of the megaspores borne by certain plants, the gametophyte 
was found to have the diploid character, and this was seen even 
in the division to form the ventral-canal-cell : thus the ovum itself is 
diploid. In such archegonia the neck does not open, so that fertilisation 
by spermatozoids is impossible : the unfertilised diploid egg develops 
apogamously into an embryo, which is naturally diploid also. An 
examination of the sporangia showed further that while in typical Marsilias 
the reduction to 16 chromosomes takes place as usual in the spore- 
mother-cells, in M. Drummondii the megasporangia show two types of 
spore-mother-cells : the one type is normal in number, and shows 
reduction : the other type is produced in smaller numbers in the sporangia, 
for instance only four in place of the usual 16: these on division 
have diploid nuclei, and the interesting fact is that their diploid state 
does not divert them from the usual characters of form and structure. 
Since the apogamous plants produce both diploid and haploid spore- 
mother-cells, it is accordingly not surprising that both apogamous and 
sexual plants should be produced from their sporocarps : and it is apparent 
that among the representatives of the species there will be individual 
cycles completed without any change of chromosome-number : certain 
cycles will accordingly be diploid throughout. In this they correspond to 
what is seen in Athyrium filix-foemina, var. clarissima^ Jones, though 
they differ in the detail that the diploid ovum here forms the embryo, 
while in the Athyrium the embryo arises from the prothallus by apogamous 

1 Flora, 1907, p. 123, etc. 



6o ALTERNATING GENERATIONS 

budding. But an exact parallel is found in Athyriwn filix-foemina, var. 
darissima, Bolton, and in Scolopendrium vulgare, var. crispum Drummondae, 
in which the embryo arises from the unfertilised ovum. 1 It may be 
remarked that the phenomena thus seen in the last-named Ferns and in 
Marsilia correspond essentially to what has been described for certain 
Phanerogams. 2 It thus appears that in a number of cases, systematically 
apart from one another, a diploid condition of the gametophyte is associated 
with apogamous development from a diploid unfertilised ovum : the 
abnormality is initiated by omission of the reduction in the spore-mother- 
cell, and consequently the diploid state is continued in the gametophyte, 
which is normally haploid. It is important to note in such cases that 
a double number of chromosomes may be present without producing 
fundamental change of form or of external character in the gametophyte. 

The further question will then present itself, whether under any 
circumstances the converse is possible, or has been observed, viz. that 
the phase normally diploid, that is, the sporophyte, may be haploid? 
Strasburger states (I.e., p. 166) that no case has come under his 
observation in which the generation normally diploid has only the reduced 
number of chromosomes. No case of a haploid sporophyte has yet been 
proved beyond doubt : but a reasonable probability has been established 
by Farmer and Miss Digby in the case of Lastraea pseudo-mas, var. 
cristata, Druery (I.e., p. 180). The detached leaf of this plant produces 
prothalli from its margin or surface, which bear occasional antheridia, 
but the sporophyte is apogamous. The chromosome-number in the 
prothallus is about 60 : in the embryo the number varies considerably, 
one mean being 60, another mean number being about 78. No migration 
of nuclei was observed, nor is there any reduction in the whole cycle. 
The relatively small number of chromosomes in the nuclei of the sporophyte 
is striking, and suggested to Farmer and Miss Digby that the gametophyte 
character had been impressed on the sporophyte the converse, in fact, of 
what was seen in the varieties of Athyriwn and in Marsilia. A comparison 
of the chromosome-number (60, 78) with that in normal Lastraea 
psetido-mas (144) certainly indicates that this is the probable condition 
of the apogamous sporophyte of Druery's variety : that the sporophyte is 
irregularly haploid, and that the whole cycle is essentially haploid 
throughout. 3 

It still remains to refer briefly to two other modifications of the normal 
cycle of alternation in Archegoniate Plants, so as to complete the tale of 
those which have been observed : I mean sporophytic and gametophytic 
budding. The former has already been mentioned in the case of 

farmer and Digby, I.e., p. 171. 

2 Eii-alcheniilla, Thalictrum, Antennaria, Taraxacum. See Strasburger, I.e., p. 139. 

3 The examples quoted illustrate the more important modifications of the chromosome- 
cycle hitherto described. For further details reference must be made to the papers from 
which these have been derived. 



THEIR CYTOLOGICAL DISTINCTION 61 

Nephrodium Filix Mas, as occurring on the leaf-bases (Fig. i), and in 
Cystopteris bulbifei-a, at points on the upper region of the leaf (Fig. 3). 
It also occurs in various forms in many other Ferns, in Lycopods, and 
Equiseta. The essential feature of it is that a portion of tissue of the 
sporophyte, developed as a bud with axis leaves and roots, on being 
detached from the parent plant, may continue its growth apart from the 
parent. This is plainly a mere vegetative amplification of the sporophyte 
itself, and its tissues are at first continuous into those of the bud : there 
is no reason to think that any nuclear change accompanies the production 
of these growths, and the result is the establishment of physiologically 
independent individuals ; but their origin and detachment do not modify 
our conception of the sporophyte as a whole in any essential point. If 
that conception be based upon nuclear changes accompanying fertilisation 
and reduction, it will include all such results of vegetative amplification : 
they will be held to be intra-sporophytic means of propagation. 

Similarly, in the case of gametophytic budding, which is common in 
Liverworts, Mosses, and in certain Ferns, by means of gemmae : these 
are small bodies, consisting of one or more cells, which are easily 
detached and under suitable conditions develop into new gametophytes. 
Here again the gemmae appear to be mere vegetative growths, and 
they secure increase in number of physiologically independent individuals ; 
but there is no reason to think that there is any nuclear disturbance 
involved : they may be regarded as intra-gametophytic means of propagation. 

How, then, do the irregularities above described affect the general view 
of the cytological distinction between the two alternating generations of the 
Archegoniatae ? It is quite clear that an absolute chromosome-distinction 
cannot be held as universally applicable at the present time to the two 
alternating phases : nor does form depend on the exact number of the 
chromosomes : nor yet is there any obligatory ratio according to which 
the gametophyte is always haploid and the sporophyte always diploid. 
These facts give an opening to the facile conclusion that the chromosome- 
distinction is worthless, and opinions to this effect have already been 
expressed. But the recognition of the present existence of aberrant forms 
does not negative the importance of the relation which is usually seen, 
nor exclude it from taking its due place in the reconstruction of the 
history of the past. It is contrary to all evolutionary theory and 
experience to assume that what has been normal in the past is 
obligatory for the present or the future. Moreover, it seems probable 
that these abnormalities do not represent anything which took a settled 
place in the course of the evolution of the plants in which they appear : 
our opinion might be different if in any of the great phyla it could be 
shown that a definite stock, or line of descent, had been permanently 
established showing aberrant characters ; for instance a permanent Arche- 
goniate phylum showing a cycle without any chromosome-differences. But 
of this there is no evidence at all : every one of the main phyla 



62 ALTERNATING GENERATIONS 

show normally the regular succession of events as described, viz. the 
haploid gametophyte leading through sexuality to the diploid sporophyte, 
which, again, through reduction or meiosis in the spore-mothepcells, leads 
back to the haploid prothallus. The constancy of this is too great 
to allow its recognition as the " normal " to be seriously disturbed by 
the occasional irregularities described irregularities which bear all the 
characters of late, individual, and probably non-permanent aberrations. 
Their existence is suggestive on certain points, but it cannot be held to 
invalidate the view that the cycle as above stated existed in all probability 
throughout the earlier phases of descent of the Archegoniatae. 

Accordingly the cytological distinction of the two generations may be 
upheld as the normal condition for the Archegoniatae. Further, the opinion 
of Farmer may be accepted, that the new facts relating to apogamy and 
apospory leave the question of alternation where it was : they tend neither 
to destroy the one theory of its origin nor to uphold the other (I.e., 
p. 193). Moreover, the facts of the normal chromosome-difference may be 
held to accord with either of the theories of alternation, the homologous 
or the antithetic : they are not finally distinctive for either, and a decision 
must remain still in doubt until the actual history of the genesis of the 
diploid phase in the Archegoniatae can be traced. Towards forming a 
just opinion on this question it is desirable not only to compare the 
Archegoniatae among themselves, but also to take into consideration 
the life-cycles of the Thallophytes ; for these plants often show a simpler 
mode of life, and have always been held to afford suggestions as to the 
probable origin of the more complex land-vegetation. This will be the 
subject of the next chapter. 



CHAPTER V. 

ALTERNATION IN THE THALLOPHYTES. 

THE early recognition by Hofmeister of alternation of generations as a 
general feature in the life-cycle of the Archegoniatae naturally led Botanists 
to enquire whether any similar succession of phases existed in other 
plants : and the question was soon directed towards those lower in the 
scale, which are collectively termed the Thallophytes. Notwithstanding 
that this term covers a most heterogeneous series of organisms, a very 
large number of them show processes of propagation analogous to those 
seen in the Mosses and Ferns. The existence, on the one hand, of the 
phenomenon of sex, and on the other of the means of propagation by 
non-sexual bodies, or spores of various kinds, suggested the comparison 
with corresponding features in the Archegoniatae. Such comparison at 
once raises the further question how far the study of the Thallophytes 
may throw light on the origin of those recurrent and alternating phases 
seen in Archegoniate Plants. 

It will be well at once to realise that the phenomenon of sex, and the 
production of germs, by which the number of individuals may be increased, 
are not necessarily in any way connected in plants at large. It is true 
that in certain plants, and even in large groups of them, experience shows 
us that there is an obligatory succession of such events in the life-history r 
liable, however, as we have seen in the Ferns, to certain exceptional 
modifications. We know from experience that the fertilised zygote of the 
Archegoniatae grows into the sporophyte, which has as its ultimate end 
the production of spores : it has never yet been seen to grow directly into 
a prothallus again. The spore of the Archegoniatae, according to our 
invariable experience, germinates to form a prothallus : it has never 
been seen directly to produce a new sporophyte. There is then an 
obligatory succession of events in the life-history of the Archegoniatae. 
External circumstances may affect the production of fertilised zygotes, or 
of matured spores ; but so far experiment has not altered the product 
which arises respectively from the zygote or the spore, nor has such 
change been observed in Nature. When, however, we turn to organisms 



6 4 ALTERNATION IN THE THALLOPHYTES 

lower in the scale, the case is found to be often different : the first clear 
demonstration for a Thallophyte that the phases do not follow an obligatory 
succession was given by Klebs for Hydrodictyon : subsequently his 
observations were extended to Vaucheria, and later to many other Algae 
and Fungi. 1 Up to 1890, when Klebs' first paper on Hydrodictyon was 
published, the comparison of the various stages of life in the Thallophytes 
and indeed in plants at large had rested on form, and very little was 
known of their behaviour under varying conditions. But Klebs applied 
to them the experimental method, and found in many cases that where 
the organism possesses two or more kinds of propagation, each is directly 
dependent upon quite definite external conditions. There appears in many 
cases to be no cause in the inner nature of the organism for one of these 
to be developed earlier or later : it lies in the hand of the experimenter to 
determine their succession. An exact knowledge of the conditions gives the 
experimenter the secure control over the organism, which can at will be 
forced into any desired mode of propagation within the limits of its species. 
This may be well illustrated by the case of Vaucheria, which happens 
to have had its simple life-history adequately investigated through the 
experiments of Klebs. He found that the formation of the vegetative zoo- 
spores is most active if well-nourished plants are transferred to fresh 
external conditions, and especially to diminished intensity of light; while 
the formation of sexual organs can always be counted on when nutrition 
proceeds slowly, under relatively equable conditions, and under good 
illumination. Similar observations, correlating the phases of life of various 
Algae and Fungi with external conditions, have also been carried out, 
and though the determining circumstances may differ in different plants, 
the fact is now demonstrated for a number of Thallophytes, that there is 
in them no obligatory succession of phases : sexual reproduction or vege- 
tative propagation may be repeated indefinitely, according to the conditions 
of life : neither of these leads of inner necessity to the appearance of the 
other. In fact the relation of the vegetative mode of propagation by 
isolated germs to the life-cycle of such Thallophytes is somewhat similar 
to the production of gemmae on the gametophyte of Archegoniate Plants, 
or to the sporophytic budding of Ferns or Lycopods. In all such cases, 
which may collectively be styled under the general term of somatic 
budding, the increase in number of individuals is secured, but only by an 
incident which takes no part in any rhythmic succession of obligatory 
phases, and involves no cytological change. The result is simply a repetition 
of the same phase from which the bud, gemma, or zoospore was itself 
derived. The experimental method of Klebs, by showing that the order 
of events in such cases is not obligatory, has laid the foundation for a 
more rational comparative study of the life-story in the Thallophytes. It 
may be considered probable that many more of the Fungi and Algae will 
be found to behave like the species which have been tested. As operative 
1 Klebs, Bedingungen der Fortpflanzung bei einigen Algen uud Pilsen, Jena, 1896. 



CYCLE NOT ALWAYS OBLIGATORY 65 

factors, external stimulus, light, temperature, moisture, access to oxygen, 
and the chemical composition of the nutritive medium, have already been 
recognised. These and others in various combination have been found, 
or may in the future be found, to determine the succession of propagative 
methods in many of the Thallophytes. 

But, on the other hand, accurate observation is showing in an increasing 
number of examples that this freedom from obligatory succession of phases 
is not universally the case in the Thallophytes. It is beginning to be 
clear that here, as elsewhere, complications have arisen, associated with the 
phenomenon of sex, which lead frequently to an obligatory succession of 
phases, over which external conditions have little or no control. It has 
been seen in the Archegoniatae and in the Phanerogams that the result 
of sexual coalescence is a doubling of the number of chromosomes in the 
subsequent nuclear divisions, with reduction as the final consequence. The 
similarity in essentials of fertilisation in the Thallophytes to that in the 
Archegoniatae is obvious : it has been found in many cases to result in a 
doubling of the chromosomes in Thallophytes also, and this makes it seem 
probable that there should be post-sexual nuclear complications of somewhat 
the same nature in them also. Strasburger has drawn attention to the 
impossibility of indefinitely continued doubling of chromosomes in fertilisation, 
and the necessity for a reduction-process in plants which show sexuality : 
we must assume that some process of reduction will sooner or later follow 
in each life-cycle where sexual coalescence occurs ; but the mechanism of 
the process, and the period at which it occurs in the life-cycle, may differ 
in different cases. The differentiation of the sexes in the Thallophytes 
has proceeded along many distinct lines. What is then more probable 
than that in different lines of descent the problem of reduction, as a 
necessary consequence of sexual fusion, should have been solved in different 
ways, and at different points in the life-story? 

The facts observed in certain Thallophytes point to the conclusion 
that this has actually happened : reduction has now been shown in some 
of them to follow on sexuality, but its place in the life-cycle varies in 
different cases. The point of interest for present consideration is not so 
much the details of the process of reduction, as the place which it holds 
in the life-cycles of various' Thallophytes, and the influence which it appears 
to have had in determining in them an obligatory succession of phases. 

The question must for the present remain open how the reduction, 
which we may presume to be a necessary consequence on fertilisation, 
is carried out in those Thallophytes which show sexuality but have not 
any fixed succession of phases, such as Vaucheria, etc. Subsequent observ- 
ations will doubtless provide the actual facts, and will probably locate 
the reduction-process either in near proximity to the germination of the 
zygote, or it may be to the production of the gametes. 1 We may even 

1 Oltmanns, Morphol. it. Biol. d. Algen, 1904, p. 324; B. M. Davis, '' Oogenesis in 
Vaucheria, " Bot. Gaz., 1904, vol. xxxviii., p. 81. 

E 



66 ALTERNATION IN THE THALLOPHYTES 

anticipate that one or other of these locations will be found to be a 
general feature of. those plants where there is no obligatory succession of 
phases, and their somatic condition would accordingly? be in the former case 
haploid, in the latter diploid. With this remark they must be left on one 
side for the present. 

In other cases, however, a succession of obligatory phases, denned on 
the one hand by the incident of sexual coalescence, and on the other by 
reduction, has been brought to light ; in fact, examples of alternation 
have been found among the Thallophytes, showing cytological limits closely 
comparable with those which have been accepted in the previous chapter 
for the alternating generations in the Archegoniatae. Among the Algae 
one of the best cases of this, substantiated on both cultural and cytological 
evidence, is that of Dictyota dichotoma, Lamour. It has long been known 
that the tetraspores, antheridia, and oogonia of this plant are distributed 
on different individuals, but it has only recently been shown in what 
relation these plants stand to one another. 1 We now know that the number 
of chromosomes in the somatic divisions of the plants which bear antheridia 
and oogonia is 16 : that there is no change of chromosome-number in 
the formation of the sexual cells, but that the fertilisation results in a 
zygote which on germination gives a plant with 32 chromosomes in its 
somatic divisions. This plant bears tetraspores; but in their production 
the mother-cell, on dividing its nucleus into two and then into four, 
shows a reduction to the original 16, the details of the process being 
closely comparable to those in the tetrad-reduction of Archegoniates and 
Phanerogams. The tetraspores on germination give plants which show 16 
chromosomes on their somatic divisions, and thus correspond to the original 
sexual plants. The only gap which is left in the full demonstration of 
the life-cycle, both by cultures and by cytological observation, is that the 
plants raised by "cultures from tetraspores have not yet been seen to bear 
sexual organs : but still they correspond in their chromosome-number. 
Here, then, is a succession of phases which appears to be obligatory, 
involving two stages which have the same chromosome-relation as the 
alternating generations in the Archegoniatae. But there is this difference : 
that in external form and structure the two alternating generations of Dictyota 
are substantially alike though the one is haploid and the other diploid. 

Somewhat similar phases, which alternate in a less exact and obligatory 
manner, and in which the cytological details have not yet been observed, 
are seen in the life of Cutlerta : they are known as the Cutleria and 
Aglaozonia stages. 2 This case is quoted here as showing that in plants 
probably akin to Dictyota^ the exactitude of the alternation in not maintained. 
But this fact comes out much more strongly in the case of FUCUS, in 

1 J. Lloyd Williams, "Studies in the Dictyotaceae," Annals of Ustany, 1904: D. M". 
Mottier, " Nuclear and Cell Division in Dictyota dichotoma" Annals of Botany, 1900. 

2 See Oltmanns, Morph, u. Biologie der Algen, Jena, 1904, p. 396, etc., where the 
current literature is fullv dealt with. 



CYTOLOGICAL DISTINCTION IN ALGAK 67 

which the thallus itself is diploid, but no alternation is known to exist. 
The cytological observations would indeed seem to exclude it ; for the 
doubling of the chromosomes which follows on fertilisation is maintained 
throughout the somatic divisions, and reduction has been found to take 
place in Fucus in the first divisions respectively of the antheridium and 
the oogonium. 1 Such examples as these, taken from the group of the 
Brown Seaweeds, show that an obligatory alternation, though present in 
some of them in a type comparable cytologically with that of the Archegoniatae, 
is not a constant feature for them all, in the same sense as it is in the 
Mosses and Ferns. 

In the Red Seaweeds the probability has long been contemplated that 
the peculiar developments following on fertilisation consist in the formation 
of a phase of the nature of a sporophyte. This position and a corresponding 
terminology have been accepted and developed for the Florideae generally 
by Oltmanns, in his work on Algae.' 2 Until quite recently the necessary 
cytological details have only been observed in Nemalion, though the 
demonstration is not yet quite convincing. 3 It is stated that on fertilisa- 
tion of the procarp by the spermatium a nuclear fusion takes place : 
this results in a doubling of the chromosome-number from eight, 
which is the number in the somatic divisions of the thallus, to sixteen in 
those post-sexual divisions of the cystocarp which precede the maturing 
of the spores. On the other hand, though no tetrad-division occurs, a 
reduction-division is stated to be immediately associated with the pro- 
duction of the carpospores. If this be so, then the post-sexual stage, 
being diploid, will be cytologically comparable with the sporophyte-stage, 
and the carpospore on germination will initiate again the haploid or 
gametophyte stage. It is, however, to be borne in mind that neither 
Nemalion nor the genera allied with it bear tetraspores, which are so 
marked a feature in most members of the family. Fortunately the cyto- 
logical history of Polysiphonia, a genus which bears tetraspores, is now 
before us, fully worked out by S. Yamanouchi. 4 

He finds in P. violacea that the carpospore on germination shows 
40 chromosomes, and that the same number appear in the vegetative 
mitoses of the tetrasporic plant: so that it may be inferred that the tetrasporic 
plants come from carpospores. The tetraspore on germination shows 
20 chromosomes, and the same number appear in the vegetative mitoses 
of the sexual plant : so it may be inferred that the sexual plants come 
from tetraspores. The nuclei of the gametes contain each 20 chromo- 
somes : the fusion-nucleus in the fertilised carpogonium has 40 chromosomes, 
and gives rise to a series of nuclei in the central cell : some of these 
enter the carpospores, which are consequently a part of the sporophytic 

1 Strasburger, Pringsh. Ja/trb., 1897; Farmer, Phil. Trans., B. 1898. 

2 Morpfwlogie u. Biologic der Algen, 1904-5. 

3 Wolfe, "Cytological studies on Xemalion" Annals of Botany, 1904, p. 607. 
4 Bot. Gazette, 1906, p. 401. 



68 ALTERNATION IN THE THALLOPHYTES 

phase to be continued in the tetrasporic plant. Tetraspore-formation 
terminates the sporophytic phase with typical reduction-phenomena, so 
that the tetraspores are prepared to develop the gametophyte generation. 
There is thus an alternation of a haploid, gametophytic phase with a 
diploid, sporophytic phase in the life-history of Poly sip honia, the cysto- 
carp being included as an early part of the latter. 

It appears, then, from the two types in which alone the cytological 
details are as yet available, that there is a want of uniformity of the cycle 
within the Florideae, not unlike that already noted for the Phaeophyceae. 
The alternation in Nemalion, where there are no tetraspores, is of a more 
restricted type than that in Polysiphonia for in the former reduction 
appears to follow comparatively soon after the fertilisation, but in the 
latter the event is deferred till the diploid plant produces tetraspores. 
Yamanouchi suggests that the tetrasporic plant may -have arisen by a 
suppression of the reduction-phenomena in connection with the carpospore, 
so that it germinates still with the sporophytic number of chromosomes, 
producing a diploid plant, and that the first tetraspore-mother-cells probably 
corresponded to monospores produced on the sexual plant of the simpler 
type, since such reproductive cells would very naturally become the seat 
of the delayed reduction-phenomena. This is a possible, though a some* 
what bold hypothesis. It may be anticipated that as the details become 
more fully known for the Florideae, a comparative basis, illustrated by 
intermediate steps, may provide more certain knowledge of the relation 
of these extreme types of cytological difference. At the moment it is 
interesting to see how great these differences are in the Florideae, as they 
have also been found to be in the , Phaeophyceae : moreover, they are 
marked by no corresponding differences of external form : there is no 
haploid type of plant distinct from the diploid. This fact is probably 
referable to the uniformity of the conditions under which both generations 
live but it also has its own interest in relation with what has been 
seen in the Archegoniatae ; for there it has been shown that a gametophyte 
may be either haploid or diploid without any modification of form. 

In certain Fungi also there has long been a suspicion that there is a 
somewhat similar alternation, and recent observations have tended to 
demonstrate that here also a cytological basis exists in some cases. The 
records of nuclear fusion in Fungi are rapidly increasing : in some cases 
in which such fusion may properly be held to be of a sexual nature, a 
doubling of the chromosomes has been observed in the post-sexual 
divisions ; but it has been found more difficult to locate the necessary 
reduction exactly : among the Fungi there seems indeed to be the same 
want of general uniformity in this as in the Algae. For instance, in the 
Peronosporeae, though the observations on Peronospora and on Albugo 
(Cystopus) are somewhat divergent, 1 there are several records of nuclear 

1 Wager, Annals of Botany, iv., p. 127, x., p. 295, xiv., p. 263 ; Berlese, Jahr. f. wi'ss. 
Bot., xxxi. , p. 159; Stevens, Bot, Gaz., xxviii., p. 149. 






CYTOLOGICAL DISTINCTION IN FUNGI 69 

fusion with consequent doubling x of the chromosome-number taking place 
on fertilisation, but actual chromosome-counting is difficult. There is, on 
the other hand, notwithstanding earlier statements which tended to locate 
reduction at the germination of the oospore, a growing opinion, based in 
part on exact counting, that the reduction in these plants is pre-sexual, 
and takes place at the maturing of the oogonium and antheridium. 
This receives considerable support from Trow's results on Achlya : l he 
finds that doubling occurs as usual on fertilisation, but the necessary 
reduction takes place in gametogenesis in this plant, as in most animals, 
and not in sporogenesis, as in most plants. Such a conclusion from the 
Saprolegniae would thus correspond to what seems probable for the 
Peronosporeae : it has also been seen to be probable according to some 
writers for Vaucheria, and has been conspicuously proved for Fucus. 
In such plants the chromosome-number in the somatic divisions will be 
" 2n," as in animals, and there will be an absence of cytologically distinct 
generations with obligatory alternation. 

/ There are various cases among the higher Fungi in which, on grounds 
of comparison of form combined with nuclear fusion, a sexual process 
is now recognised, for instance in the simpler Ascomycetes. Here the 
carpogonium has long been regarded as a female organ, and the polli- 
nodi-um male ; a position which is now justified by the nuclear fusions 
observed. It naturally follows to regard the ascdgenous hyphae as a 
post-sexual stage analogous to that in the Florideae : they hold the same 
place in the life-cycle as the carpogonial filaments of the latter. The 
condition of this stage as regards chromosome-number is still a matter 
of doubt ; but there is some reason for believing that reduction may take 
place on formation of the ascospores, while their number in each ascus 
is in itself suggestive. Further observations will be required to show 
how far such comparisons have a cytological justification. 

But in the Uredineae the case for an alternation based upon cyto- 
logical detail has been fully made out for Gymnosporangium and Phragmidium, 
the facts being as follows : 2 The mycelium which bears aecidia and 
spermogonia has single nuclei : each is usually in a separate cell, and shows 
two chromatin-masses on division.. This stage is compared with a gameto- 
phyte, capable of bearing sexual organs. The spermatia are held to be 
functionless male cells, and fertilisation is effected by other means. The 
young aecidium is held to be a sorus of female reproductive organs,, 
each of which may be fertilised by the migration into it of the nucleus 
of one of the adjoining undifferentiated mycelial cells. The male and 
female nuclei do not fuse, however, but continue to divide simultaneously, 
and the product of fertilisation is accordingly a growth with paired nuclei : 
this condition is persistent throughout the rest of the life-cycle, includ- 
ing the aecidiospores, the mycelium which germinates from them, the 

l Anna/s of Botany, xviii., p. 541. 

2 V. II. Blackman, Annals of Botany, xviii., p. 323, etc. 



70 ALTERNATION IN THE THALLOPHYTES 

uredospores, and the teleutospores : these collectively are compared with a 
sporophyte-generation, and all show in their cells the paired nuclei, which 
divide in close association together, showing on division four chromatin- 
masses. The final nuclear fusion takes place in the maturing teleutospore, 
while the subsequent division of the fusion-nucleus shows changes which 
correspond to synapsis : at the same time there is a reduction of the 
chromatin-masses from the four characteristic of the paired nuclei 
collectively, to two. It seems thus clear that an alternation of phases, 
the one with "n," the other with " 2n " chromatin-masses exists. It is 
known to be obligatory in those Uredineae which show the full cycle, 
and the limits of the two generations coincide respectively with a process 
of fertilisation (with suspended nuclear fusion), and a process of reduction. 
It is therefore comparable in its broad cytological features with the 
obligatory alternation seen elsewhere. The analogy with the Florideae is 
here again so obvious as to have led to the suggestion of some phyletic 
relation of the Uredineae with that group. As a corollary on these 
observations and conclusions, it has been further suggested that the 
absence of sexuality in the Basidiomycetes may be due to an apogamous 
shortening of the life-cycle, so as to omit the sexual stage altogether. 

There remain for consideration certain of the Algae, which show 
post-sexual complications of an obligatory nature : they have been reserved 
to the last because they have long been singled out as those Thallophytes 
which most naturally suggest the manner in which the alternation in the 
Archegoniatae may have originated. An important feature in them is, 
that in close relation to the sexual fusion, rearrangements of nuclear 
condition occur ; in some, these precede the act of fusion, though commonly 
they follow it; but in either alternative an apparently obligatory phase 
is associated with sexual fusion in the life-cycle, and there is good reason 
to, think that its existence is bound up with the post-sexual reduction. This 
has been specially remarked by Strasburger in connection with the germination 
of zygotes in the Conjugatae 1 and in various Chlorophyceae. The actual 
fact of post-sexual reduction has not yet been established in them by 
chromosome-counting ; but the fact that the post-sexual divisions of the 
nuclei are commonly into four, shows a pregnant analogy with tetrad- 
division, while in some cases the four nuclei are formed notwithstanding 
that only two are eventually required. This would hardly have been the 
case unless there were some importance attached to the division into 
four. Examples will now be given illustrating these points. 

In the unicellular Desmids, where no somatic complications arise, 
conjugation and germination of the zygote have been studied by Klebahn, 
whose drawings of Closterium are here reproduced (Fig. 40). ' 2 The nuclei 
of the conjugating cells remain apart throughout the winter in the resting 
zygote (Fig. 40. i), and only coalesce when germination begins in the 
spring : the contents escape from the thin-walled zygote, and division of 

1 Ueber Rednktions-theilung, etc., 1900, p. 83. - Pringsh. Jahrb. , vol xxii 






NUCLEAR CYCLE IN DESMIDS AND DIATOMS 71 

the fusion-nucleus soon follows v (Fig. 40. 2, 3): this is succeeded by 
further mitosis (Fig. 40. 4), with the result that four nuclei are formed : 
two of these are larger than the other two, and a pair of nuclei of unequal 
size, one small the other large, finds its place in each of the two cells 
into which the zygote divides (Fig. 40. 5, 6). Of these nuclei the 
smaller one in each cell disappears, and it is thought that it becomes 
disorganised, while the other remains as the definitive nucleus of the new 
Closterium cell. 

Certain Diatoms show on conjugation a singular parallelism of behaviour 
to this, but with important differences, and again it is to Klebahn that 
Ave owe the description of the details. 1 In the conjugation of Rhopalodia 









Germination of Clostcrium, after Klebahn. i. Zygote before nuclear fusion. 2. First 
mitosis. 3. Bi-nuclear stage. 4. Second mitosis 5. Bi-cellular stage with large and 
small nuclei. 6. Formation of the two germs. (From Oltmanns.) 

{Fig. 41), the protoplasts of the two cells come into close relation, but 
the nuclei remain distinct (Fig. 41. 2), and undergo each a division into 
two, and again into four x (Fig. 41. 3, 4): of these, two in each cell are 
soon reduced in size, while the others are distinguished by their larger 
size (Fig. 41. 4). Then comes an abstriction of each of the original 
protoplasts into halves, and each half contains two nuclei, one large and 
one small (Fig. 41. 5): these halves coalesce in pairs, and each fusion-pair 
finally develops into an auxospore (Fig. 41. 6, 7): the two larger nuclei 
of each pair meanwhile fuse, while the smaller nuclei become disorganised 
(Fig. 41. 8). 

In these cases there is conjugation, and a tetrad-division of nuclei 
accompanies it. There is no evidence as yet of chromosome-number, 

1 Pringsh. Jahrb., vol. xxix. 



72 ALTERNATION IN THE THALLOPHYTES 

but in both cases half of the nuclei produced are discarded : this would 
seem to point to some special importance attaching to the division into 




FIG. 41. 

Conjugation of Rhopalodia, after Klebahn. k, nucleus ; kk, small nucleus ; gk, large 
nucleus ; fy, pyrenoid ; g, mucilage. (From Oltmanns.) 

four, and by analogy with what is seen elsewhere it would appear probable 
that the importance lies in a process of reduction accompanying it. What- 
ever the actual cytological condition may be, at all events this is clear, 
that, in these strangely similar plants, the tetrad-division of the nuclei 



NUCLEAR CYCLE IN CHLOROPHYCEAE 73 

in the Diatom immediately precedes conjugation, while in the Desmid 
it immediately follows on conjugation. This fact is important for comparison 
with the order of events in other Thallophytes. 

The post-sexual phase in Ulothrix, Oedogoniitm, and Coleochaete has 
been frequently brought into comparison with that of the Bryophytes, 
but the necessary details are still very imperfect as regards their nuclei. 
In Ulothrix -the zygote is formed by the fusion of two motile gametes, 
and after encystment it undergoes a period of rest : parthenospores similar 
to the zygotes are also produced, as shown by Klebs, 1 when the sexual 
cells are exposed to a 0*5% culture solution : these also undergo a period 
of rest. On germination it is found that in either case there is a division 
of the contents into non-motile cells, which grow directly into new filaments. 
But apparently there is this difference, that the parthenospores form only 
two of these, while the zygotes form four. If this be constantly so, the 
point has a special interest as indicating that the tetrad-division is a 
consequence of sexual fusion, and need not happen unless the fusion be 
carried out. Ijjt a similar way the zygote of Oedogonium divides on germina- 
tion into four,^^ number which suggests reduction; it is quite possible 
that the exceptiorfSj^Jto this noted by Pringsheim, 2 may be explained on 
the ground of parthenogenesis, as in Ulothrix. The cell-body formed on 
germination of the zygote of Coleochaete is that which has most frequently 
been compared with the simplest sporogonia of Bryophytes. In this plant 
nuclear fusion has been observed in the ovum (Fig. 42. 6, 7), after which 
the zygote undergoes a period of rest, as a unicellular, uninucleate body, 
covered by a pseudoparenchymatous coat. On germination a transverse 
wall is formed at right angles to the axis of the oogonium : then follow 
longitudinal walls to form octants, and these usually divide further till 8-16 
cells are formed in each hemisphere. The outer coat then bursts about 
the region of the transverse wall, and a zoospore is formed from each 
of the cells, which germinates like any ordinary zoospore. The question 
of homology of this body with a simple sporophyte has recently been 
decided by observations made by C. E. Allen z : he found that reduction 
takes place, with characteristic synapsis, on the first nuclear division in 
the germinating zygote. The cells of the "fruit-body" are then haploid j 
and correspond cytologically with the gametophyte, not with the 
sporophyte as had been commonly assumed. But there is no need 
as an alternative to hold this body as in any sense a derivative of the 
ordinary Coleochaete thallus : it seems more natural to see in it merely 
an extension of the usual tetrad ; for if the second division of a normal 
tetrad were in this case repeated twice or thrice, a cell-group would 
result just as is seen in Coleochaete, and the biological advantage of increase 
in number of the progeny would thus be secured. Morphologically such 

1 Bedinguugen Jer Fortpflanzung, p. 3 21 - 
'-' Gesarymelte Abhandluugen, i., p. 251. 
* Ber. d. D. Bot. Ges., 1905, p. 285. 



74 



ALTERNATION IN THE THALLOPHYTES 



a development stands hitherto alone, 1 but biologically it occupies the 
same place as a simple sporophyte. 




FIG. 42. 



Coleocha.ctepulvina.ta, after Oltmanns. i, Young zoosporangium (?). 2, 3, A 
4, Oogonium shortly before opening. 5, Ditto after 



Antheridia 

(a) and young oogonium (p). 4, Oogonium shortly before opening. 5, Ditto after opening. 
6, Zygote still with two nuclei. 7, Zygote developed to " fruit." 8, Germinating hypno- 
zygote. a, Antheridium. 0, Oogonium. sk, Male nucleus, ek, Female nucleus, ckr, 
Chromatophore. py, Pyrenoid. k, Nucleus. 



Though the cases are thus seen to be still comparatively few in which 
the Thallophytes have had their cytological condition traced throughout 
the whole course of the individual life, there is a growing body of evidence 
to show that an obligatory alternation of cytologically distinct generations 

1 It is possible that the multicellular spores of Ascomycetes supply a parallel. If, 

as is probable, the reduction accompanies the formation of the ascospores, then the 

subsequent divisions in those spores would hold a similar place in the cytological cycle 
to those in the fruit of Coleochaete. 



THREE LEADING TYPES 75 

exists among them. According 16 the criterion of chromosome-difference 
there may be recognised a haploid, pre-sexual phase, characterised by 
having "n" chromosomes this corresponds cytologically to what has 
been termed elsewhere the gametophyte ; and a diploid, post-sexual stage, 
characterised by having " 2n " chromosomes this corresponds in this 
respect at least to the sporophyte. The alternation of these phases 
depends primarily upon sexuality, which doubles the chromosome-number. 
The reduction of the chromosome-number to one half appears to be a 
necessary consequence of it, and the process by which the original number 
is restored is found to be commonly associated, here as elsewhere, with a 
tetrad-division. 

As Professor V. H. Blackman has pointed out (I.e., p. 364), three nuclear 
stages are to be observed in the sexual cycle of animals and plants : 
nuclear association by fusion of the protoplasts which contain them ; nuclear 
reduction^ or fusion, which doubles the chromosome-number; and chromosome- 
reduction, by which their number is halved. Of these three stages the 
second may take place at the same time as the first, or it may be delayed 
for a short time, as in Spirogyra or Cosmarium : or, as in the Uredineae, 
it may be delayed until the stage corresponding to chromosome-reduction. 
According to the relative time of these successive nuclear stages the sexual 
cycle may vary greatly, as we see that it does in the Thallophytes; and 
three leading types of the cycle emerge, though they severally may graduate 
into one another by intermediate steps : they include : 

1. Those in which reduction immediately precedes gametogenesis and 
sexual fusion. The order of events would then be (a) somatic division 
with "2n" chromosomes: (b) chromosome-reduction: (c) gametogenesis 
and sexual fusion. This is the case generally for animals : in plants the 
best demonstration has been in Fucus : it is also seen probably in Rhopalodia ; 
but it probably occurs also in many of those Thallophytes which have 
no obligatory succession of phases, and especially in Achlya, and probably 
in the Peronosporeae. 

2. Those in which reduction immediately follows on sexual fusion. The 
order of events in these would be (a) somatic division with "n" chromo- 
somes : (b) gametogenesis with sexual fusion : (t) chromosome-reduction. 
This is probably the case' in Desmids and other Conjugatae, and in the 
filamentous Chlorophyceae, including Coleochaete. 

3. Those in which a somatic phase of some extent intervenes between sexual 
fusion and reduction, and again between reduction and sexual fusion. This is 

seen in Dictyota, probably in the simpler Ascomycetes, in Uredineae, and 
Florideae : it is comparable with what is seen in the Archegoniate series. 

It is interesting to compare the grouping of types of alternation as 
thus stated with the position adopted by Celakovsky in his paper which 
was published some thirty years ago at Prag. 1 The data, both physiological 
and cytological, are now much more precise, though still very deficient. 

1 Sitz. d. Ges. d. Wiss. in Prag, 1874, p. 30. 



;6 ALTERNATION IN THE THALLOPHYTES 

The criterion of chromosome-number is new : the method of physiological 
experiment is also new. Still, the conclusions are in the main unaltered. 
What was then styled " homologous alternation " now stands on a basis 
of cytological unity as regards the somatic divisions, and denotes such 
recurrent phases in plants as appear to be dependent on external condition, 
not obligatory in their succession, and involve no cytological change : 
this includes the cases grouped under (i) and (2) above. There is hardly 
any need to designate such life-cycles as showing alternation at all, were 
it not that this is the type of life-history for which the term was first 
introduced by the zoologist Steenstrup. The types grouped above under 
the heading (3) were distinguished by Celakovsky as showing "antithetic 
alternation," and it is now found to have its basis in a cytological difference 
of the successive phases, which also show an obligatory succession, not 
determined directly by external conditions. 

The conception of normal antithetic alternation now turns upon the two 
critical points of sexual fusion and reduction : it is necessary to enquire how 
far these events are historically the same in organisms at large. It would 
seem probable that sexual differentiation, and perhaps even sex itself 
originated along several distinct phyletic lines : on this point there is no 
definite information, though the differences of character of the organisms 
which show the simplest types of sexuality distinctly suggest that it had 
not one common source only. In the present state of uncertainty it seems 
.undesirable to depart from the usual convention by which the zygote is 
held to be "homologous"; and, accordingly, it serves as a point for general 
comparison between representatives of distinct phyla. But it must be 
distinctly understood that this is in itself a conventional understanding, and 
that its adoption for convenience of description does not necessarily imply a 
strict "homogeny, " in the sense that sexuality was established once for 
all. Similarly with reduction, which is theoretically a necessary consequence 
of sexual fusion, it is only by a similar conventional understanding that 
in divers organisms the cell where this is initiated is held to be "homologous": 
it is not to be assumed that it is truly "homogenetic" in distinct phyla, 
as though reduction had been initiated once for all in sexual organisms. 
But, on the other hand, in organisms that are akin, such as the members of 
the phyla of the Ferns or the Mosses, it may reasonably be held as probable 
that the zygote and the spore-mother-cell are actually identical things, in fact 
homogenetic for the whole phylum, in the sense that each probably sprang 
from a phyletic source common for the whole phylum. 

A comparison of plants at large as regards the position of the reducing 
process in the life-cycle relative to sexual fusion shows great differences, 
as we have seen. It is not improbable that these may have been due 
in part to initial differences : we have no right to. assume that there was 
uniformity at the outset. Some ground for the view that initial differences 
existed is to be found in such cases as the Desmids and Diatoms ; for in 
Closterium the rejection of the superfluous nuclei, and probably also 



PHYLETIC DELAY IN REDUCTION 77 

reduction, follows on nuclear fusion, while in Rhopalodia these events 
precede nuclear fusion. Such cases seem to point to a probability that 
the problem of reduction was solved independently and in different ways 
in different lines of descent. 

Nevertheless the term sporophyte has been adopted as applicable 
collectively to the non-sexual phase which intervenes between sexual fusion 
and reduction in those plants in which it occurs. But, following the above 
reasoning, it must not be understood to convey necessarily any community 
of descent for all the bodies which it covers. It seems probable that the 
establishment of the sporophyte, whether by a process of intercalation or 
otherwise, has taken place independently in several distinct phyla : thus 
the sporophyte-stage in them, though in some more lax sense it may be 
styled " homologous," is not to be held as " homogenetic " ; nevertheless, 
useful analogies may be drawn between the corresponding phases in 
distinct phyletic lines. 

But, on the other hand, comparison within groups that are held to be 
akin gives strong reason for recognising that there has been a shifting of 
position of the event of chromosome-reduction in certain lines of descent, 
and that the balance of the generations has thus been altered in the 
evolutionary course. For instance, it seems probable that in the Uredineae 
there has been a deferring of the event of reduction after sexuality, with the 
result that the binuclear phase has attained considerable dimensions ; the 
same seems probable also for the Ascomycetous Fungi, though along a quite 
distinct line. A similar intercalation has been suggested in the Florideae. 
Such conclusions can only become cogent when the cytological details are 
known in a large number of related forms. But the most familiar, and at 
the same time the most prominent and permanent example is that of the 
Archegoniatae : in these there is a strong comparative basis for the belief 
that the sporophyte stage has been intercalated, or in any case greatly 
extended, in consequence of the deferring of the event of chromosome- 
reduction. It may be a question whether the post-sexual stage in the 
life-history of certain green Algae represents any phyletic predecessor of the 
sporophyte of the Archegoniatae : it is quite probable that it did not. But 
this much is clear, that it occupies the same position in the life-cycle, and 
it gives at least the suggestion how the Archegoniate sporophyte may 
have originated. According to the antithetic theory as applied to the 
Archegoniatae, the complications of post-sexual nuclear reduction, involv- 
ing, as they are seen to do, at least four nucleated cells, supplied the 
starting-point for a diploid somatic expansion. That is the theory which 
is adopted here as reflecting the probable mode of origin of the alternation 
in the Archegoniate series. But it is only right to acknowledge that it 
is not fully demonstrated either by the cytological facts, or by comparison 
with the alternation in the Thallophytes. The latter can only supply 
suggestive analogies so long as the actual phyletic origin of the Archegoniatae 
remains as obscure as it now is. It becomes, accordingly, an object of all 



78 ALTERNATION IN THE THALLOPHYTES 

the greater interest to trace such internal evidence as there is of the method 
and manner of the intercalation of the diploid phase in the life-history of the 
Archegoniatae : and an important question will be what circumstances they 
probably were which conduced to fixing permanently the alternation that 
resulted, and to making it so constant a feature as it has been in the 
upward development of the green plants of the Land. This will form 
the subject of the next chapter. 



CHAPTER VI. 

THE BIOLOGICAL ASPECT OF ALTERNATION. 

THE phenomenon of Alternation of Generations may be viewed in 
various ways, and the history of the science shows that the different 
aspects of this subject have presented themselves in a natural succession 
according to the progress of biological thought. First it seemed sufficient 
merely to observe the fact that successive, more or less distinct phases 
occur in the life of the individual in certain groups of plants. In the 
case of the Pteridophytes this was the gradually achieved result of the 
labours of various workers antecedent to or contemporary with Hofmeister ; 
but subsequently the discovery of a similar succession of phases in other 
organisms than those in which it was first observed suggested at once their 
comparison : such comparison was placed upon a secure footing for 
Archegoniate plants by Hofmeister himself. Then followed the pursuit 
of such comparisons into the region of the lower and simpler Thallophytes ; 
but this led to some confusion at first, owing to premature attempts to 
reduce all organisms which show sexuality to one rigid scheme as regards 
the successive phases of their lives. On the other hand, in similar com- 
parisons with the higher Flowering plants, the issues were temporarily 
obscured by discussions over " the Alternation of Shoots " found in some 
of them, and by the confounding of this with Alternation of the more 
fundamentally distinct Generations seen in the Archegoniatae. But these 
temporary clouds were in great measure cleared away by Celakovsky, when 
he drew the distinction between "Homologous" and " Antithetic " Alterna- 
tion. His comparisons, however, were still, like those of Hofmeister and of 
Sachs, based essentially on form ; nevertheless, he arrived at conclusions which 
we have seen in a previous chapter to coincide very nearly with the opinions 
current at the present day, and this notwithstanding that those opinions are 
now based on facts which were quite inaccessible to him. The writings of 
Celakovsky may be said to have brought the formal study of Alternation to 
a close. His conclusions were widely accepted at the time, though Botanists 
of the weight and standing of Pringsheim still stood aloof from them. 1 

1 Pringsh. Jahrb., Bd. ix., 1878, also Scott, British Association Report, Liverpool, 1896. 
Address to the Botanical Section. 



So BIOLOGICAL ASPECT OF ALTERNATION 

The succeeding phase of the history has been one involving first 
physiological and subsequently minute cytological considerations. The 
study of the effect of external conditions on the succession of stages of 
the developing organism, initiated by Klebs in 1889, led at once to the 
recognition of the fact that in certain plants that succession is mutable 
according to circumstances, while in others the succession is obligatory. 
The distinction between different types of the life-cycle of organisms thus 
established was found to coincide very nearly with the distinction drawn 
by Celakovsky between "homologous" and "antithetic" alternation, and 
thus his general position came to be greatly strengthened. But another 
effect of the experimental test was to open up more definitely than before 
the problem of origin of this obligatory succession of phases, in those cases 
where it exists : it also accentuated the difference between the antithetic 
or true alternation, and those other appearances which bear a superficial 
resemblance to it. But meanwhile the question of the rise of the neutral 
generation was being approached also from the point of view of adaptation, 
and a theory of its origin in an amphibious mode of life, which it will be 
the object of this chapter to develop, was already being advanced as an 
explanation of the progress and final dominance of the sporophyte in the 
plants of the land. It is clear, however, that adaptation would only account 
for its advance, not for its ultimate origin. This amphibious theory was 
based upon physiological considerations, together with closer observation 
of the origin of the sporogenous cells, their limitations, and their relation to 
the tissues which are merely vegetative. Lastly, more careful observation 
of the details of sexuality and of spore-production led to the generalisation 
on the basis of minute nuclear structure : this put the cytological cachet, as 
well as a structural check upon the conclusions already drawn. But the 
existence of a chromosome-difference between the two generations turns 
attention afresh to the question of the ultimate origin of the obligatory 
succession of phases : it suggests that the origin was in sexuality, and in 
those post-sexual complications which are so frequently the consequence 
of nuclear fusion. Naturally these several phases of the study of alternation 
have overlapped one another, and proceeded in some degree coincidently. 
One of the interesting features in the history is that their results have 
often run so nearly parallel as to yield a high degree of mutual support. 

It has been remarked above that up to the time of Celakovsky the 
study of alternation was on the basis of form ; but it is now clear that the 
merely formal comparison of different organisms, or of their successive 
stages one with another, cannot suffice for the full solution of the question 
as to the real nature of alternation. The case of the structurally similar 
but cybologically distinct generations of Dictyota show this, while the differ- 
ence of the propagative organs which they bear confirms the distinctness 
of the two generations. In the Florideae also, there are no definite structural 
details which serve as formal differentiating characters between the pre-sexual 
and the post-sexual stages. Such examples will probably be multiplied as 



AMPHIBIOUS HABIT 81 

cytological investigation progresses, <but those quoted will suffice to show that, 
for those who accept the cytological distinction of the alternating generations, 
the mere distinction on ground of form is insufficient. 

The existence of certain cases of alternation, demonstrated cytologically, 
where, as in Dictyota^ the form of the two generations is substantially the 
same, and of other cases in which, as in the Archegoniatae, the genera- 
tions differ fundamentally in form as well as in points of structure, raises 
a physiological question which has wide bearings. The external conditions 
to which, in the past as well as now, the two generations have been or are 
exposed must be considered in their relation to such differences. Now, in 
Dictyota, and also in Polysiphonia, since the two forms grow on the same 
coasts, at the same time, and about the same spots, it is obvious that the 
conditions to which they are exposed are substantially the same : knowing 
how closely form is dependent upon circumstance it is not surprising that 
the two generations should be so much alike. 

In the case of the Uredineae, again, the external circumstances of 
the two cytologically distinct stages of the life are alike, both being 
internally parasitic : in form their mycelial stages are also alike, and their 
difference of character only becomes apparent on the formation of the 
propagative organs. Similar remarks will apply, perhaps with less cogency, 
to the post-sexual developments in certain Ascomycetes, the circumstances 
of which do not differ materially from those of the pre-sexual phase, and 
in this may be seen some illumination of the fact that they show for the 
most part mere variants upon the simple filamentous form. Thus, for 
certain Thallophytes in which cytologically distinct generations have been 
recognised, there is a remarkable similarity in form of the two generations : 
this probably finds its true explanation in the fact that the biological 
conditions to which they are respectively exposed are relatively uniform, 
and have been so throughout their descent. 

But with the Archegoniatae the case is quite different : the cyto- 
logically distinct and alternating generations differ widely in their characters 
of form and structure. The question, therefore, presents itself, what is 
the biological and ultimately the phylogenetic bearing of this very obvious, 
and at the same time widespread fact? Far-reaching it is indeed, for 
it affects all the characteristic vegetation of the land. Taking, first, as 
the most prominent example, a typical Fern, the gametophyte, or 
prothallus, shows in its more delicate structure and in its habit, as well 
as in the way in which the sexual process is effected, an adaptation to 
moist conditions, under which it grows best ; while its ultimate function, 
that of sexual reproduction, cannot be carried out without the presence 
of external fluid water : it is, in fact, typically semi-aquatic in its nature, 
sharing many of its main characters with the green Algae from which 
we have some reason to believe that the land-flora originated. 'The 
sporophyte, on the other hand, is fitted by its more robust texture, as 
well as by its differentiation of tissues, for successfully enduring exposure 



82 BIOLOGICAL ASPECT OF ALTERNATION 

to the air under comparatively dry circumstances, while dry weather is 
important for the dispersal of the spores, which it is the final function of 
the sporophyte to produce : thus the Fern, as we normally see it, is an 
organism with, so to speak, one foot in the water, the other on the land. 

Calling in also such evidence from phylogeny as we can command, it 
will be generally accepted that the gametophyte is the older and pre- 
existent generation ; it corresponds to the gametophyte as seen in the 
Liverworts, or in the green Algae : and if we trace the descent of the 
Archegoniate series from some green Algal forms, we may recognise 
that the gametophyte of the Ferns retains the chief Algal characters, as 
regards both its texture and its sexual process. The sporophyte, on the 
other hand, is the younger generation : among the present green Algae 
there is hardly any body strictly comparable to the sporophyte, nor is 
it to be expected that there should be, if, as above stated, the sporophyte 
is typically sub-aerial in its characters, while the green Algae are typically 
aquatic. A comparison of the successive families of the Archegoniate 
series suggests the progress of the sporophyte, from small beginnings, as 
illustrated in the Bryophytes, to larger size and greater complexity of form 
and structure, as seen in the Vascular Cryptogams and Gymnosperms : 
its advance is accompanied by a corresponding reduction of the gameto- 
phyte, and the whole is to be correlated with a progression from the 
aquatic or semi-aquatic habit of the lower forms, to the very distinctly 
sub-aerial habit of the higher. It may accordingly be concluded that the 
alternation which is so prominent in the main Archegoniate series is the 
result of adaptation of originally aquatic organisms to sub-aerial conditions 
of life : it may, in fact, be distinguished physiologically as an amphibious 
alternation, which finds its morphological expression in the difference of 
external form and internal structure between the more ancient gametophyte 
and the more recent sporophyte. 

It is an important fact that in the main Archegoniate series the antithetic 
alternation is normally constant, though the balance of the two generations 
may vary : the constancy of the phenomenon makes us enquire why it 
should be so : the circumstances which have encouraged this constancy 
seem to have been these. The Archegoniate series probably sprang 
from green aquatic forms, inhabiting, as so many of the green Algae 
now do, shallow fresh water, or the higher levels between the marine 
tide-marks : the sexual reproduction was effected through the means 
of external water, and if other conditions were favourable it could be 
carried out at any time through the water which was always present. 
Certain forms, perhaps thereby escaping from competition, spread to the 
land, where access of water was only an occasional occurrence : in these 
the sexual process could only be effected at time of rains or floods 
or copious dews; and even then might not take place unless the sexual 
organs were fully mature : thus less dependence could be placed upon 
sexuality for propagation, and an alternative method of increase of individuals 



AMPHIBIOUS HABIT 83 

had to be substituted. This was x done by the production of the sporo- 
phyte from the zygote : once fertilised a zygote might in these plants 
divide up into a number of portions (carpospores), each of which would 
then serve as a starting-point for a new individual; and dry circumstances, 
under which they would be powdery, would favour their dispersion, as 
in the lower Liverworts. In proportion as these plants spread to higher 
and drier levels (in accordance with the advantage which they gained by 
escape from competition, and more free exposure to light for assimilation) 
the chance of a frequent recurrence of the circumstances necessary for 
M:\uul reproduction would be diminished, and the dependence upon 
carpospores for propagation would increase ; consequently the number of 
spores produced by each sexually formed sporophyte must be larger, if 
the race is to survive, and be in a position to compete. Any increase 
in the number of spores entails greater supply of nourishment during 
their formation : this in the phylum of the Bryophytes is chiefly supplied 
from the gametophyte, which shows distinct adaptation to sub-aerial habit, 
while the means of nutrition on the part of the sporophyte itself are in 
these plants very limited, and the external morphological complexity of 
it very slight. In other phyla, however, such as the Filicales, Lycopodiales, 
and Equisetales, the sporophyte itself assumed the function of nutrition : 
a higher morphological differentiation of the parts followed, and a more 
clear distinction between the organs which were to supply the nutriment 
(stem, leaves, and roots), and the parts devoted to the formation of 
spores (sporangia) : this for the first time stamped the sporophyte with a 
character of independence and permanence, while the number of spores 
produced might now be practically unlimited : in these respects the 
Pteridophytes are immeasurably superior to the Bryophytes. One strange 
point in the whole story is, however, the tenacity with which these plants 
(under the obvious disadvantages which it entails when their habit is 
sub-aerial) retain their aquatic type of fertilisation : it is only when we 
reach the Phanerogams, where the sporophyte attains its climax while 
the gametophyte is almost abortive, that we see the sexual process 
accommodated to that sub-aerial life which had led to the dominant 
position of the sporophyte ; for in them the fertilisation is siphonogamic, 
being carried on by the pollen-tube : these plants are therefore independent 
of external fluid water for their fertilisation, and this fact has doubtless 
contributed largely to their present ascendency. When, as in the preceding 
sketch, we consider what the results of the migration from water to land 
must have been, the permanence and constancy of the antithetic alter- 
nation explains itself. The permanence or morphological fixity of a 
phenomenon in any phylum is in a sense proportional to its importance 
in the well-being of the organisms : given a conservatism in the mode of 
fertilisation (which it is difficult to explain), the rise and progress of 
the sporophyte in the Archegoniate series appears to be a natural 
outcome of the migration from water to land. 



84 BIOLOGICAL ASPECT OF ALTERNATION 

The biological, or adaptation theory of antithetic alternation in the 
Archegoniatae, as embodied in the above paragraphs, was stated in my paper 
on alternation published in iSgo. 1 In 1894 came Strasburger's Address 
on Periodic Reduction, delivered at the British Association meeting at 
Oxford. 2 He there introduced as a structural basis of antithetic alternation 
that cytological distinction of the two generations which had already been 
suggested by Overton : 3 this at once gave a definiteness to alternation 
which it had never possessed before. He adopted a view similar to 
that above stated, as a biological explanation of the rise and final dominance 
of the sporophyte, and pointed out how its gradual development can 
actually be traced, the first indications of it being apparently to be found 
in the Algae : they are to be sought in such post-sexual complications, 
connected probably with reduction, as are seen in Oedogonium. There 
is, however, no direct evidence of the origin of any Archegoniate form 
from any Alga : all that can be said is that, given such a multicellular 
body as the post-sexual stage of certain green Algae, the biological conditions 
of migration from water to land and of an amphibious life will sufficiently 
account for the further advances which are exemplified in land-plants. 

This is, then, the working hypothesis which will form the basis for 
our further enquiry. It will be necessary, however, to analyse the advance 
of the sporophyte, which is thus contemplated, from its simpler beginnings, 
and to consider the several factors which have been involved. Having 
done this, the enquiry will be made, what evidence there is in plants, 
living or fossil, that these factors of advance have actually been operative. 
The initial factor appears to have been " sterilisation" that is, the delay 
of reduction by the conversion of cells which are potentially, and were 
ancestrally, sporogenous, into cells which serve no longer a propagative 
but a vegetative function. It will be readily seen that this is a necessary 
biological consequence of any considerable increase in the number of 
spores ; and it has been pointed out above that such increase is a biological 
advantage, especially in those plants where a land-habit places restrictions 
upon increase in numbers by sexual propagation. The larger the number 
of spores the greater the powers of competition, and the greater the 
probability of survival, and of spread. On a biological theory, the nutrition 
of the increasing number was secured by the conversion of some of the 
potential germs to form a vegetative system, which should provide for 
nutrition and protection. It was naturally important that these tissues 
should be established in the individual before the sporogenous tissue 
which it is their function to nurse : and accordingly the time of spore- 
production was deferred, and a vegetative system, ultimately of great 

1 Annals of Botany, 1890, p. 347. 

^Annals of Botany, viii., 1894; Biol, Centralbl., Dec., 1894. A similar view has also 
been adopted by v. Wettstein, and embodied in his Handbuch aer Syst. Bot., Band ii., 
p. 13, published in 1903. 

3 Annals of Botany, vii. (1893), p. 139. 



FACTORS OF ADVANCE OF SPOROPHYTE 85 

extent, was intercalated. But spore-production follows sooner or later 
in the normal life of every sporophyte, even of the most elaborate ; and 
in the constancy of this process in all normal life-cycles is to be seen 
one of the strongest supports of the antithetic theory. 

A second factor has been the segregation of sporogenous tissue, which 
in the simpler forms is a concrete and continuous tract of cells, into 
numerous more or less distinct pockets, or sporangia. Closely connected 
with this, though not a necessary consequence of it, may follow the 
increase or decrease in number of the sporangia thus distinct from one 
another. There is good reason to believe that the methods of morpho- 
logical advance in former times were not essentially different from those 
to be seen reflected in the plants of the present day. If that be so, 
then a careful study of the modes of variation of number of sporangia 
seen to be effective now, should indicate the methods which have led 
in the past to the condition seen in vascular plants, in which the number 
of sporangia is as a rule large, 

A third factor has been the origin of the appendicular organs of the 
shoot, and the origin of the roots. These changes have more than anything 
else contributed to modification of the form of the plant-body. But though 
these were such important steps, the mode of origin of the appendicular 
organs and roots is still a subject for surmise rather than for definite 
opinion. If, however, the development of the individual be accepted as a 
guide, a reasonable view may be arrived at, which will be stated later. 

Beyond these, which are the three fundamental factors of advance of the 
growing sporophyte, are others which, though later in point of time, have 
had very far-reaching effects upon the organisms in which they appeared ; 
such, for instance, as the abortion of parts : the initiation of the 
heterosporous state, and of its ultimate consequence, the seed-habit : also 
the adoption of the siphonogamic fertilisation, and of the various adaptive 
methods for transfer of the pollen, as exemplified by the higher forms. 
These factors are all closely related to the process of spore-production, 
which we may regard in point of history to have been the primary function 
of the sporophyte. 

On the other hand, the vegetative system, which we may regard as 
being collectively secondary in its origin, has shown many characters 
which may be held as adaptive. The differentiation of tissues has provided 
first the means of construction of fresh organic material within the sporophyte 
itself, so that in place of being a body dependent for its nutrition 
upon the parent gametophyte, it became a physiologically independent, 
self-nourishing organism. Further, the establishment of a conducting 
system secured the necessary facility of transfer of materials from point 
to point : this becomes specially necessary where the formation of appen- 
dicular organs has brought about a large proportion of external surface 
to bulk. Lastly, the appendicular organs themselves are open to 
differentiation, so as to subserve definite functions : such as, absorption, 



86 BIOLOGICAL ASPECT OF ALTERNATION 

carbon assimilation, protection, propagation, and a variety of more special 
duties. 

In the present work it is not proposed to enter into any detailed 
discussion of those later evolutionary advances, which are more especially 
shown by the higher Seed-Plants. The attention will be chiefly directed 
to the more fundamental features involved in the establishment of the 
sporophyte as a factor in terrestrial vegetation, and chapters will be devoted 
to sterilisation, to the methods of increase and decrease in number of 
sporangia, and to the origin of appendicular organs. The attempt will be 
made to gather from living plants (and from fossils also as far as possible) 
such evidence as there is which will bear upon the working hypothesis 
sketched in its broad outlines above. For this is the only satisfactory 
way of testing its validity in the absence of more direct evidence. Definite 
proof of the truth of the biological theory of alternation need not be 
anticipated. What is possible is to show such a degree of reasonable 
probability as will justify its acceptance. For this the evidence available 
appears to be sufficient. 



CHAPTER VII. 

STERILISATION. 

IT has been the practice from early times of Descriptive Botany to designate 
the leaves which produce the sporangia in Ferns and other organisms as 
the fertile leaves, those which carry out a vegetative function only the 
sterile leaves. The similarity of form which these show one to another 
readily established their close relationship : middle forms are frequently 
found between them, partly sterile, partly fertile ; and any conversion of 
the fertile into the sterile would, to preserve uniformity of terms, be 
designated a process of sterilisation. The term thus applied to a leaf, or 
a pinna as a whole, will be properly applied also to its constituent parts, 
and so ultimately to the individual cells composing them; and thus, 
wherever a cell that is normally sporogenous is diverted from that 
function to any vegetative office, the process may be styled one of 
sterilisation of that cell. It seems necessary thus to justify this use of the 
word, since recent investigation has attached a definite structural meaning 
to the change involved in those cells which are diverted from the office of 
spore-production. Its cytological significance lies in the fact that chromo- 
some-reduction, characteristic of fertile cells, does not take place in 
them. Without the historical explanation it might appear strange to 
describe this change of nuclear behaviour as sterilisation ; but on the 
grounds of old custom this term will be retained throughout the present 
discussion. 

In the Archegoniatae and in Seed-Plants the tetrad-division is the 
criterion of the fertile or sporogenous cell. It is true that among the 
highly specialised Seed-Plants this tetrad-division may sometimes be omitted ; 
but putting these exceptions aside, it is the formation of a spore-tetrad 
which is the final distinctive mark of a sporogenous cell as distinct from 
a vegetative cell. But long prior to the appearance of this distinctive 
condition the sporogenous cells may in most cases be recognised with a 
high degree of certainty. They commonly form a clearly defined sporogenous 
group, distinguished by the dense protoplasmic contents of its cells, and 



STERILISATION 




in the later stages by their large nuclei. In Vascular Plants each sporo- 
genous group is as a rule derived by division from a single cell, or a row 

or sheet of cells, and in favour- 
able examples it may present the 
appearance of a compact mass of 
tissue, which may readily be re- 
ferred in origin to a single parent 
cell (Fig. 43). But the details of 
those divisions which result in 
the sporogenous group show great 
diversity in different plants, while 
in a not inconsiderable number of 
cases the limits of the sporogenous 
group are not found to be strictly 
coterminous with the tissue-pro- 
ducts of definite initial cells. It 
is only by a careful study of the 
successive stages of development 
in each individual type of spore- 
producing organ that it is possible 
to assign the limits of origin of 
its sporogenous group. When this 
has been done, and the genetic 
story completely made out, the 
cell or cells which are found to be the ultimate parent-cells of a single 
sporogenous group are designated its archesporium (Fig. 44). The same 
terms are also applied in. the case of the 
Bryophyta, the chief difference being that in 
them the sporogenous tissue of each individual 
forms only one concrete group. 

In not a few cases the whole product of 
the archesporium becomes converted into spore- 
tetrads, and ultimately into spores ; but this is 
not always so. It frequently happens that in 
the course of development certain cells which 
spring from the archesporium are diverted to 
other functions than that of direct spore-pro- 
duction ; a good instance of this is seen in the 
sporangium of Psilotum (Fig. 45). It will be 
well to consider carefully how such a case as 

Botrychinm daiictfoliuin, Wall. 

this is to be regarded from an evolutionary Early stages of development of spor- 

' angia, showing by comparison that the 

point Of View, for it will be Seen later that the sporogenous group originates from a 

single " archesporial" cell. X2oo. 

same reasoning as is used in the present case 

is applicable to a great number of others also. Adequate investigation 

shows that in the sporangium of Psilotum all the cells of the sporogenous. 



FIG. 43. 

Botrychium daucifoliitm, Wall. Sporangium in vertical 
section, with the sporogenous tissue shaded. X2oo. 




FIG. 44. 



CASE OF PSILOTUM 89 

group have a common origin, ahd as sister cells they develop alike at 
first : they may therefore be held to be equipotential cells (Fig. 45 c). 
The conclusion that they are so is supported by comparison with plants 
having some degree of affinity with Psilotum, such as Lycopodium : here 
all the cells of a sporogenous group essentially similar to that of Psilotum 
are equipotential ; all of them normally undergo the tetrad-division 



. 




FIG. 45- 

Psilotiun trigjietnon, Sw. Various stages of development of the synangium and 
sporangium. In C the sporogenous group is shaded. D shows the differentiation of its 
cells, the fertile cells being shaded. E shows the disorganisation of the remaining cells 
without forming spores. X 100. 

and develop spores. But in Psilotum the originally uniform group 
differentiates at a relatively late period into fertile and sterile cells, the 
former having dense protoplasm, and undergoing the tetrad-division ; the 
latter having less dense protoplasm (Fig. 45 D) : these become dis- 
organised without tetrad-division, and their substance goes to nourish 
the young spores (Fig. 45 E). The conclusion to be drawn is that in 
Psilotum all the cells of the sporogenous mass are potentially sporogenous, 
as they are in Lycopodium, and probably were actually fertile in ancestral 
forms : that some of them have been sterilised, that is,' diverted from 



9 o 



STERILISATION 



spore-production before the tetrad-division. It is a secondary matter 
morphologically that in this case their existence is brief; but physiologically 
it is important, for they are sacrificed to furnish better nutrition to the 
others which remain fertile, and produce spores. The structure seen in 
Fig. 45 is thus to be interpreted as indicating the sterilisation of certain of 
the potentially fertile cells in the sporogenous group of Psilotum. 

A second example illustrating this partial sterilisation of a sporogenous 
group may be quoted : it is selected from among the Bryophytes ; but 
the same arguments as in the previous case will equally apply here. 
Fig. 46 illustrates two stages in the development of the sporogonium of 
Aneura : the younger shows the clearly defined, hemispherical internal group 






FIG. 46. 

A, median section of young sporogonium of Aneura ambrosioides. The internal mass 
of cells of the sporogonial head ("archesporium ") is already differentiated so as to 
indicate the sterile elaterophore, and the outer fertile region. B, the same, older : the 
indications of sterilisation have extended outwards, and it is only the peripheral fringe of 
cells (shaded) which will be sporogenous. C, transverse section of the'same. X 150. 

of cells of the sporogonial head, which are equivalent inter se, inasmuch 
as they have a common origin ; but they are already differentiated into two 
distinct regions, a peripheral fertile region, of which the cells are shaded, 
and a central sterile region. The former differentiates at a later stage into 
spore-mother-cells and elaters : the latter forms the sterile elaterophore. 
The whole hemispherical group corresponds in position to the body 
similarly placed in other Hepaticae, which have no elaterophore, and 
in which the whole region develops into spores and elaters. This case 
therefore illustrates an advanced stage of sterilisation of tissue which may 
be held to be ancestrally sporogenous throughout. ' But the final fate of 
the sterile cells here is not merely to serve as evanescent nutritive cells; 
for the elaterophore and elaters are a permanent tissue and permanent 
cells, which remain till the ripeness of the spores, and are functional in 
their dispersal. 



AS SEEN IN BRYOPHYTA 




FIG. 47- 

Fnnaria kygrometrica. A, longitudinal section of a sporogonium showing the first 
differentiation of its parts. X about 96. B, the upper part of the same. x6oo. 

r marks the limits of the theca and operculum. C, basal part of the capsule of the same. 

X 600. The intercellular spaces are beginning to form. ar, archesporium. col> 
columella. 



STERILISATION 



Such examples serve to show that there are good grounds for holding 
that sterilisation of individual cells of the sporophyte, which by their origin 
are to be recognised as potentially fertile, does take place ; and that such 
cells may be diverted to a temporary, or to a more permanent use 
in connection with the production of spores. Thus we are justified 
in holding that sterilisation, which is the initial factor in the 
working hypothesis sketched in the previous chapter, has been actually 
operative. 

It will be impossible here to enumerate all the cases where evidence of 
sterilisation of potentially fertile cells has been brought forward ; but some 

of the more prominent instances of it 
will be quoted. At present it is the 
mere fact of sterilisation which is before 
us, not the biological consequences 
which follow in facilitating the nutrition 
or the dispersal of spores, nor yet the 
morphological advances which may 
result. These aspects of the matter 
will be left over till the several groups 
of the Archegoniatae are specially 
discussed. 

Among the Liverworts the simple 
Ricciaceae have centrally an undiffer- 
entiated sporogenous tissue; but as a 
rule in the Marchantiaceae and Junger- 
manniaceae the almost spherical mass 
of sporogenous tissue becomes differ- 
entiated as development proceeds : 
cells, singly or in groups, instead of 
undergoing the tetrad-division, are 
developed in a vegetative manner, 
either as nutritive cells (Sphaerocarpus\ 
or as elaters of various form and 
arrangement (compare Fig. 46, of 
Aneura]. In the Anthocerotaceae, on the other hand, the archesporium 
is a dome-shaped layer surrounding a central Columella ; but the products 
of this layer do not only form spore-mother-cells, but also numerous 
sterile cells arise from it, which develop as an irregular network enclosing 
the mature spores. In point of fact, in the Liverworts it is the exception 
rather than the rule for the whole of the sporogenous tissue to be fertile, 
though this is the case in the simplest of them. 

In the Musci, on the other hand, the whole of the cells developed 
from the definite, single-layered archesporium normally produce spores ; 
but the archesporium is relatively small compared with the bulk of the 
young sporogonium : it shows an apparently arbitrary limitation at its 




FIG. 48. 

Median longitudinal section of a sporogonium 
of Sphagnum, with bell-shaped archesporium. 
Xijo. (After Waldner.) 



AS SEEN IN PTERIDOPHYTA 



93 



margin. Fig. 47 illustrates howMhe series of cells of the archesporium 
is continuous both upwards and downwards beyond the limit of its fertility : 
this indicates a probability that its marginal limitation has been due to 
sterilisation, a view which is strengthened by comparison with Sphagnum 
(Fig. 48) ; for there the archesporium is in the form of a complete dome : 




FIG. 49. 

Danaea elliptica, Smith. Drawings illustrating partial septations of the sporangium. 
A, tangential section through three sori, showing the loculi in ground plan : the septa are 
often thin, so that pairs of loculi are in close juxtaposition ; the loculi marked (x) 
are large, and show one or more partial septa. X 20. , C, D, E, show such loculi with 
partial septa in greater detail : in D and E it is difficult to decide whether the cells 
marked (?) will develop as tapetum or as spore-mother-cells. X 150. 



a sterile condition of the cells at its apex would give a barrel-shaped 
archesporium, as is seen in the Bryineae. Thus it will appear that any 
evidence of sterilisation in the Musci is less direct than that in the 
Hepaticae. 

Evidence of sterilisation of potential sporogenous cells is common in 
the homosporous Pteridophyta : and, as in the Bryophytes, the function t 



94 



STERILISATION 



of the arrested cells is sometimes simply nutritive, sometimes they form 
permanent tissue-masses. In Lycopodium and Phylloglossum, and in the 
( homosporous Ferns, after the sporogenous tissue is first defined, all its 
cells normally undergo the tetrad-division, and develop spores : occasional 
cells may become disorganised without full development, though, as a 
rule, all the potentialities are realised. But among the Marattiaceae, 
where, as a rule, all the cells of the sporogenous groups are fertile, a number 
of exceptional cases have been noted : the most remarkable is that of 
Danaea, in which it has been shown how certain cells of the larger sporo- 
genous groups remain sterile, and may be develope^d as tapetum, or even 
as component cells of a partial septum (Fig. 49). Somewhat similar 
conditions have been seen in Kaulfussia, Marattia, and Angiopteris* 




FIG. 50. 

A, apex of sporangium of Equisetum litnosum, L., showing the sporogenous cells, 
surrounded by the tapetum (shaded), and sporangial wall. F>, shows part of an older 
sporangium with its tapetum (t) still clearly defined, though the individuality of the cells 
is lost : within this the sporogenous tissue, of which certain cells (a) are abortive. 

X200. 



In other cases it has been found that only a portion of the cells of the 
sporogenous group are fertile, as already described for Psilofum (Fig. 45) : 
this has been shown to be the case also in Tmesipteris, and in Equisetum 
(Fig. 50), and it has been described also as an occasional feature in the 
Ophioglossaceae. In all of these a varying proportion of the sporogenous 
cells are sterile, and become disorganised without forming tetrads. As 
the proportion of the sterile to the fertile cells is not fixed, an elastic 
arrangement exists which leads to the largest number of spores being 
brought to maturity that the plant at the time can support. The limits 
of the sporogenous tissue in early stages are difficult to define in these 
large sporangia, and they show considerable irregularities : this is especially 
so in Psilotum, Tmesipteris, and Ophioglossiim, and it appears to be partly 
due to the ill-defined and broad tapetum which is formed peripherally,. 



AS SEEN IN PTERIDOPHYTA 



95 



partly to the fact that owing to Sterilisation the definitive fertile cells do- 
not form a continuous mass. 

Among heterosporous forms, sterile cells are commonly present in 
the female sporangium (Fig. 51): there is good reason to think that 
arrest of potential sporogenous cells has greatly favoured the advance in 
size of the relatively fe\v remaining megaspores. But apart from this, 
the case of Isoetes is interesting, since there is evidence of sterilisation 
both in the mega- and micro-sporangia, and in both it has resulted in 
permanent tissue-masses. In both types of sporangium an extensive 
potential sporogenous tissue is formed, which is at first uniform in 
structure, as it was also in origin. In the microsporangium considerable 
tracts of this tissue differentiate later as vegetative trabeculae and tapetum, 





FIG. 51. 

Selaginella spinnloa, A. Br. Section of 
megasporangium showing the single fertile 
tetrad still very small, and the rest of the 
sporogenous cells arrested. X TOO. 



FIG. 52. 

Isoetes lacitstris, L. Vertical section of a 
young microsporangium. sp = fertile tissue. 
tr= trabeculae. / = tapetum. X 100. 



while the remainder forms microspores. From the history of development^ 
and from comparison, the conclusion seems justified that the trabeculae and 
tapetum in this case represent sporogenous tissue which has been converted 
into sterile tissue, serving nutritive and mechanical purposes in the very large 
sporangium (Fig. 52). Similarly, in the megasporangium there is sterilisa- 
tion, but it has been carried much further, and it has been possible to 
show that the megaspore-mother-cells are not morphologically predetermined, 
but are physiologically selected from among a large number of potentially 
sporogenous cells : also that each archesporial cell gives rise to several 
megaspore-mother-cells, as well as to trabeculae and tapetum (Fig. 53) 
(Wilson Smith). Thus there has been a differentiation of tissues of uniform 
origin, and a large part has been diverted to functions played by sterile 
vegetative tissue. Very similar sterile tracts of tissue have been seen in 
the large sporangia of Lepidostrobus Broivnii, and their origin by 
sterilisation is highly probable, though naturally this is hardly susceptible 



STERILISATION 



of demonstration in a fossil. The general conclusion may be drawn from 

such cases as those cited, that sterilisation has played a considerable part 

in the sporangia of Pteridophytes. 

In Seed-Plants also there is frequent evidence of sterilisation of cells 

of a potential archesporium, both in megasporangia and in microsporangia. 

In the latter, examples have been seen in which a considerable proportion 

of the cells of the sporogenous group are 
obliterated in much the same way as in Psilotum, 
But in the anthers of not a few Angiosperms 
partial or complete septa of sterile tissue may 
be formed in plants whose near allies have 
their pollen-sacs non-septate. Thus, in the 
Onagraceae the stamens of most of the genera 
are of the ordinary quadrilocular type; but in 
the genera Circaea, Gaura, Clarkia^ and 
Eucharidium the four loculi are each divided 
transversely by one or more sterile septa : these 
septa may consist of only a single layer of cells 
having the character of tapetum, or of two 
layers, or even of four or more, of which the 
middle layers then resemble the tissue of the 
connective. An examination of early states of 
development of these anthers shows that the 
septa result from sterilisation of part of the 
sporogenous tissue, for in sections it is seen 
that the sporogenous cells and those which will 
form the septa originate from a common layer 
corresponding to the archesporium of normal 
anthers of the family (Fig. 54). A similar 
state of things has been described in certain 
of the Mimoseae (Inga, Calliandra, Acacia, 
Albizzia], in many of which there are eight 

. ' c , , , c 

pollen-sacs in place of the normal number of 
four ; while in others (Parkid) the number 
may be much larger. Here, again, the 
developmental history shows that sterilised 
archesporial tissue provides the septa which 
divide the four original pollen-sacs into eight 

or more loculi. With these may also be compared the cases of V^sc^^m 
and Loranthus. Developmental study of the anther of Rhizophora 
has given the same result : in its massive anther the small pollen- 
sacs are very numerous, distributed over a large surface : Warming has 
concluded that the anther became multilocular by the arrest of the 
further development of certain parts of the pollen-forming tissue (see 
Fig. 72, p. 142). Such examples, which by no means exhaust the list, 




FIG. 53. 

Part of a section of a megaspor- 
angium of Isoetes. The cell marked 
(m) is the only fertile spore-mother- 
cell, the rest are undergoing vegeta- 
tive divisions, including the cell (a) 
as shown by other sections of the 
series. Thus sterilisation affects the 
large majority of the cells of the 
sporogenous group. X245- (After 
Wilson Smith.) 



AS SEEN IN SEED-PLANTS 97 

X 

show that sterilisation of sporogenous cells is not uncommon in the anthers 
of Seed-Plants. 

Evidence of sterilisation is also found in the ovules of Seed-Plants. 
Among the Gymnosperms, the Gnetaceae show an archesporium consisting 
of a group of hypodermal cells : in Gnetum Gnemon, which is the best 
known example, these give rise to a considerable mass of sporogenous cells, 
but only one embryo-sac is finally matured. In the Cycads the case is 
similar, inasmuch as there is a considerable tract of sporogenous tissue, 
though only one embryo-sac matures. In the Coniferae also there is 
frequently a multicellular archesporium, and several embryo-sac-mother-cells 
have been seen to enlarge in Taxus and Sequoia^ but in most of them 
only a single one. Among Angiosperms a condition very similar to that 







FIG. 54. 

A, longitudinal section of one loculus of a young stamen of Rucharidium concinuum, 
showing differentiation of the potential archesporium into fertile cells (a) and sterile 
cells (j). , similar section of stamen of Clarkia elegans, more advanced, showing a 
sterile septum dividing the contents of the single loculus into distinct sporogenous groups 



in Gnetum is seen in Casuarina (Figs. 55, 56) : this case is particularly 
interesting, since the potential embryo-sacs are not simply obliterated by 
the growth of the favoured one, but some develop into tracheides with 
thickened walls a proof that permanent sterile tissue may be formed from 
potentially sporogenous cells. In certain Amentiferae also a similar 
formation of tracheides has been seen. A multicellular archesporium is 
common, besides, in other Archichlamydeous Dicotyledons, e.g. in the 
Ranunculaceae and Rosaceae, and some others (Fig. 57); but it is 
apparently less common in the more advanced Dicotyledons and in the 
Monocotyledons. The examples tfius quoted suffice to show that sterilisation 
of potentially sporogenous cells is frequent both in the microsporangia 
and in the megasporangia of the Seed-Plants. 

And thus it is seen that evidence of sterilisation is widespread : it is 
found in all the main groups of the characteristic Flora of the land, both 

G 



9 8 



STERILISATION 



in homosporous and in heterosporous forms ; the sterile cells may be 
functional sometimes only as transitory, nourishing cells ; or they may 
persist as permanent tissue, forming in some cases partial, in others even 
complete septa. 

The converse case, viz. the conversion of cells normally sterile into 
fertile cells, is a much less common phenomenon, though instances of it 
have been observed. This change is not to be confounded with the 
formation of whole organs of propagation, such as sporangia, in places 
where they do not normally exist : what is here meant is the change in 





FIG. 55. 



Ca.sua.rina 

Median section of the nucellus ot an 
ovule, with the group of sporogenous 
cells shaded. X 285. (After Treub.) 



Rum phia.no., Mig. 
dlus of a 



FIG. 56. 

Casuarina glauca, Sieb. Median section of 
nucellus of an ovule showing the cells of the 
sporogenous group differentiated : some are 
becoming elongated in the direction of the 
chalaza : one long cell has divided by six 
swollen walls: another has developed as a 
tracheid. X 285. (After Treub.) 



individual cells, which are normally vegetative, to the sporogenous condition. 
A case of this has been recorded by Lanzius Beninga in a specimen of 
Syntrichia subulata : certain cells of the normally sterile columella were 
found to be undergoing tetrad-division prior to forming spores : a similar 
condition has also been noted by Kienitz Gerloff in a species of Bryuml 
It has also been seen in rare cases in the Pteridophytes, that cells outside 
the limits of the normal sporogenous group, but contiguous with it, may 
show the characters of fertile cells. But the most distinctive case, which 

1 Lanzius Beninga, Beitriige z. Kenntn. d. inn. Banes d. angew. Mooskapsels, 1847,. 
Tab. 58, Figs. 9*, 9**; Kienitz Gerlofif, Bot. Zeit., 1878, p. 47, Taf. 2, Fig. 52. 



STERILE CELLS OCCASIONALLY FERTILE 99 

has been fully made out, is that of Tmesipteris : the normal synangium 
of this plant has when mature two loculi, divided by a septum some five or 
six layers of cells in thickness. Certain synangia of small size are found 
about the limits of the fertile zones : they appear non-septate, and it has 
been shown that the cells of the septum in such cases develop as fertile 
cells, undergoing the tetrad-division (Fig. 58). Such examples show that 
occasionally a reversion may occur from cells normally sterile to the 
function of spore-production. Putting together the two converse series of 




FIG. 57 . 

Longitudinal sections of ovules showing multicellular archesporia. A, B = Astilbf 
japonica. X 550. (After Webb.) C Salix glaucophylla. X6oo. (After Chamberlain.) 
D=Rosa livida. X224- (After Strasburger.) EAlchemilla alpina. X275. (After 
Murbeck.) F^Callipeltis cucitllaria. (After Lloyd.) G = Quercus velutina. Xj2o. 
(After Conrad.) From Coulter and Chamberlain, Morphology of Angiospenns. 

facts of sterilisation which is relatively common, and of reversion to the 
fertile state which is comparatively rare two conclusions may be drawn : 
first, that the facts indicate a preponderance of the former over the latter 
in plants now living : in them sterilisation appears to be a more potent 
factor now than reversion, and it has probably been the same in the past 
also. Secondly, it may be stated generally for Archegoniate and Seed- 
bearing Plants, that spore-production is not always strictly limited to, or 
defined by pre-ordained formative cells or cell-groups. 

Voechting has formulated the proposition that "No living vegetative 
cell of the plant-body, which is capable of growth, has a specific and 



IOO 



STERILISATION 



unalterable function." 1 This thesis should be extended so as to include 
also sporogenous cells : for, as we have seen, many cases can be cited of 
the conversion of cells which are normally sporogenous to a vegetative 
condition, and occasionally the converse. The facts before us show that 
vegetative and sporogenous cells are not things apart or essentially 
different, but that they are on occasions mutually convertible. The 
influences, external or internal, which act upon the embryonic cell, and 
determine whether it shall be vegetative or sporogenous, are still obscure : 
but clearly they act within restricted limits, for in Vascular Plants neither 
superficial cells of the plant-body nor deeply seated cells have ever been 
found to develop as spore-mother-cells. 





FIG. 58. 

Tmesipteris Tannensts, Bernh. A, median section through aynsngnlw*, showing the 
tissue where the septum normally is developing as sporogenous cells (.$). / = tapetum. 
B, part of the contents of a similar synangium, rather older, xx shows the line where 
the septum should normally be, while a chain of fertile cells stretches continuously across 
it. X loo. 

The conversion of potentially fertile cells into vegetative cells was 
recognised by Naegeli, and embodied by him in his fundamental law of 
organic development, as follows : " The phenomenon of reproduction of 
one stage becomes at a higher stage that of vegetation. The cells which 
in the simpler plant are set free as germs, and constitute the initials of 
new individuals, become in the next higher plant part of the individual 
organism, and lengthen the ontogeny to a corresponding extent." 2 The 
sterilisation seen in the sporophyte of the Archegoniatae and Seed-Plants 
is only one special case of that included under Naegeli's general law. 
He points out that the law is realised in three different ways, and the 
case for the sporophyte generation, with which alone we are at present 
concerned, falls under the first head, expressed by him as follows : " The 
propagative cells which arise by division are converted into tissue cells." 
1 Organbildung, p. 241. * Abstammungslehre t p. 352. 



QUESTION OF INNER CAUSES 101 

The general features of the change from a sporogenous to a sterile 
character are associated usually with a less dense protoplasm and a 
smaller and less marked nucleus. If disorganisation be the ultimate fate, 
the wall breaks down, and the identity of the protoplast is lost, with or 
without fragmentation of the nucleus, as in Psilotum : or the proto- 
plasm may shrink and collapse, and the whole protoplast become 
highly refractive before the final absorption, as in angiospermic ovules. 
If the cell is to continue functional in a vegetative capacity, the changes 
are those usual in cells passing from the embryonic to the mature condition. 
It has been stated above that the occurrence or absence of the tetrad- 
division, and of the consequent chromosome-reduction, is the ultimate 
criterion of distinction between a fertile and a sterile cell : in the majority 
of cases the distinction has been drawn on the basis of the results of 
subsequent development, not on that of actual observation of the nuclear 
changes. It is not, however, probable that this has led to any serious 
errors, since the tetrad-formation which follows on chromosome-reduction 
is a sufficiently distinctive feature in all cases except in the ovules of 
Seed-Plants. This being so, it is not surprising that the most exact nuclear 
observations of sporogenous cells, in which the sterile or fertile development 
is a critical question, have been made on the ovules of certain Angiosperms, 
viz. in the case of the apogamous species of the genus Alchemilla. 1 The 
exact questions connected with these plants do not come before us here ; 
but in their elucidation Strasburger had reason to follow carefully through 
the development of certain embryo-sacs, as regards their nuclear condition. 
He found that an archesporial cell having entered the condition of an 
embryo-sac-mother-cell, its nucleus passes through the prophases of the reduc- 
tion-division, up to the stage of synapsis. The embryo-sac-mother-cell then 
alters its trend of development and becomes vegetative, and its nucleus 
passes out of the synapsis condition into that of a typical division, instead 
of continuing the reduction-division. The cell thus remains a part of the 
tissue-system of its parent, not the initial cell of a new generation. Such 
a case is interesting in that it shows how a cell may tremble on the verge 
between the sterile and the fertile state. It leaves, however, still open 
the question as to the influences, external or inner, which determine its 
fate. These probably vary in different cases, and the problem would 
naturally be a simpler one in the Homosporous Archegoniatae than in the 
ovule of an Angiosperm. It seems obvious in the simpler cases to suggest 
nutrition as one potent factor : it is a necessary axiom that an increasing 
spore-output, which is an advantage in increasing the probability of survival 
and dissemination, demands increased nourishment and protection : and 
that a vegetative system increased by sterilisation will tend to provide this. 
But still the advantage gained may be quite independent of the real 
cause : we are not yet in a position to translate the nutritive demand into 
terms of a direct influence upon the individual cell. It seems useless to 
1 Strasburger, "Die Apogamie der Eualchimillen," Pringsh. Jahrb., Band xli., Heft i. 



102 STERILISATION 

speculate upon such questions : for the present it is best to be content 
to recognise as an unsolved problem what those influences are which 
encourage or check reduction in any individual cell of a sporogenous 
tissue at the critical moment. 

In conclusion, the question may be raised how sterilisation is to be 
viewed : is it an advance or a retrogression ? If the antithetic theory of 
alternation be true, then sterilisation must be regarded as an evolutionary 
advance, as far as it influences the whole organism. According to our theory, 
it is by successive stages of sterilisation, following closely upon the heels 
of increase of potential sporogenous tissue, that the vegetative body of the 
sporophyte originated, and enlarged. A new phase of life of increasing 
importance was thus intercalated, the end and result of which was primarily 
an increased spore-output. But its origin was, conversely, in restricted 
propagative development of certain cells. Inasmuch as this has tended 
to a higher state, and greater success of the whole organism, it may be 
held to have been an advance. But as regards the individual cell, 
sterilisation can only be held to be a check to its development, as it 
prevents it from taking direct part in the final end of the sporophyte, 
which is the production of new germs. 

From the examples quoted there is ample proof that sterilisation of 
potentially fertile cells does occur : thus from living plants the evidence 
is supplied of the existence of that factor which is the first essential of 
any theory of origin of the sporophyte by expansion from the zygote. It 
does not necessarily follow that the first vegetative tissues of the sporophyte- 
did originate in this way : all that can be claimed is that plants show 
not uncommonly to-day such a conversion of cells from the propagative 
to the vegetative state as the antithetic theory would demand. 



CHAPTER VIII. 

THE SPORANGIUM DEFINED. 

THERE are two main types of construction of the sporophyte in Archegoniate 
plants which differ in essential features. In the Bryophyte-type it is a 
body dependent on the gametophyte, without appendages of any sort, 
and with the sporogenous tract as one concrete archesporium, while the 
spores are consequently contained in one non-septate sac. In the Pterido- 
phyte-type it is an independent organism, with roots and foliar appendages, 
while the archesporia are discrete and usually numerous : accordingly the 
spores are contained in many distinct pockets : these are the sporangia. 
There is no definite indication how the polysporangiate state of the 
Pteridophytes came into existence ; but with a view to forming an opinion 
it will be necessary to enquire into the characters of the sporangium, 
and to separate the essential features from the non-essential. 

In any ordinary sporangium of a Fern the following parts are to be 
recognised: the stalk, which supports the sporangial head; the latter consists 
of the wall, with an opening-mechanism, the annulus ; within the wall at 
an early stage is the archesporium ; later, the tapetum is differentiated, which 
surrounds the sporogenous group ultimately the spores are matured from 
the latter (compare Figs. 4 to 8). It will be necessary to consider which 
of these parts are constant in sporangia at large, and which are inconstant 
or only occasional parts. It will be possible thus to arrive at some estimate 
as to which of these are essential and which are merely accessory parts 
of the sporangium. 

Comparison shows that though sporangia are often stalked, still there 
are many others which are quite sessile, and indeed immersed in the tissue 
of the part which bears them (Fig. 59) : this indicates clearly that the 
stalk is not an essential part of the sporangium. 

In all the Archegoniatae the spore-mother-cells are covered externally 
by the sporangial wall: this is a protective sheet of cells, which may be 
of variable thickness and structure : it serves the several purposes of protec- 
tion, of nutrition, and in many cases of dehiscence, and of mechanical ejection 



104 



THE SPORANGIUM DEFINED 



of the spores. The extent of the wall as such is closely related to the 
position of the sporogenous cells : where these are deeply sunk, the wall 
is a mere roof over them : where they are carried outwards by the growth 
of the tissues surrounding them, and a projecting sporangium is formed, there 
the wall envelopes them as a tissue of greater extent. But in either case 
it is continuous with the tissue of the sporangium-bearing part, of which it 
is to be regarded as a specially developed region. This view of it accords 
well with the structure of sunken sporangia, as in Ophioglossum (Fig. 59), 
in which the tissues of the wall are continuous with and little differentiated 
from the cognate tissues of the spike. The same is the case with other 
Ophioglossaceae, even in those where the sporangia project; but in these, 
as also in the Lycopods and Equiseta, there is some further specialisation. 

of the wall for dehiscence than 

____ rr ___^ p r - r _ T _^___ in the deeper-seated forms. In the 

Filicineae still more exact specialisa- 
tion is the rule, and the mechanical 
annulus becomes a marked feature 
in the stalked sporangium. But a 
comparison of the Marattiaceae and 
other Ferns leads to the conclusion, 
that the presence of an annulus, 
and its elaborateness, are to be 
correlated with the freedom of the 
sporangium from mutual relations 
with other bodies. The annulus, 
in fact, is still only a specialised 
region of the sporangial wall. This 
conclusion accords with the facts 
FIG. 59 . of its inconstancy, for sporangia 

Ophioglossum reticulatum, L. Vertical section of which Open Under Water may haVC 
the deeply sunk sporangium, with the sporogenous tissue . m, . , 

shaded, xioo. no such mechanism, lhat is the 

case in Isoetes, while the sporangia 

of the Hydropterideae are also entirely without an annulus; nor is there 
any stomium in them, which would localise dehiscence. Loxsoma is a 
specially interesting case, for there the annulus, though indicated by the 
cell-divisions as complete, is only partially indurated : as a matter of fact, 
the part of it which is not indurated could not possibly be mechanically 
effective, owing to the mode of packing of the sporangia in the sorus- 
(Fig. 60). 

The general conclusion to be drawn is, that while the protective wall 
itself is always present, those mechanical arrangements collectively desig- 
nated by the term "annulus" are not essential or constant parts of the 
sporangium, however constant they may seem to be in certain groups :. 
where they cannot be mechanically effective they may be entirely omitted. 

Nor is the tapetum to be looked upon generally as a morphological 




ITS INCONSTANT FEATURES 



105 



constant, notwithstanding that it x shows some constancy of character in 
certain circles of affinity. Sometimes it is not differentiated at all, a condition 
which holds throughout the Bryophyta : in other cases it may appear as 
a more or less definite band of cells, which originates from the tissues 
surrounding the sporogenous cell or group of cells, sometimes from the 
sporogenous group itself. In some cases a large number of cells of the 
sporogenous groups act like a diffused tapetum, becoming disintegrated 
during the development of the spores (Ophioglossum, Psilotum) : in Equisetum 
both a diffused tapetum of 

this nature is found, and also B 

a definite single-layered tape- 
tum, which originates outside 
the sporogenous group. In 
Lycopodivm, and in the Mar- 
attiaceae the tapetum arises 
from cells outside the sporo- 
genous group : in other cases, 
such as Selaginella and the 
Leptosporangiate Ferns, the 
tapetum may consist of cells 
cut off from the sporogenous 
cell or cell-group. There is 
indeed good reason to think 
that there has been a pro- 
gressive change of origin of 
the tapetum within certain 
circles of affinity : speaking 
generally it may be said that 
indefinite and non-specialised 
nutritive arrangements are 
characteristic of larger and 
probably primitive sporangia, 
but more definite tapetal 
layers are found in the smaller 
and probably derivative : and 
further, that while in sporangia 
of relatively large size the 

tapetum usually originates outside the sporogenous group, in smaller 
sporangia of the same affinity it may be cut off from the sporogenous cell 
or cell-group. Such a change appears to be illustrated by comparison of 
Lycopodium with Selaginella, and of the Marattiaceae with the Lepto- 
sporangiate Ferns. The result of such comparisons is that the tapetum, 
however uniform it may be in function, is found to be variable both in 
occurrence and in origin, and accordingly it cannot be regarded as an 
essential or constant feature of the sporangium. 




Loxsoma Cunninghami, Br. E, young sorus. A , rather 
older. &, vertical section of the base of the receptacle, showing 
young sporangia. C, D, mature sporangia, showing the 
1 annulus, and distal point of dehiscence 
5x250. C+Dxso. 



io6 THE SPORANGIUM DEFINED 

It remains to consider the archesporium, and the sporogenous group 
and spores which arise from it. An archesporium, in one form or another, 
is a necessary constant in the development of a fertile sporangium : the 
form, the limits, the mode of origin and of later segmentation which it 
shows, may vary, as indeed is seen to be the case in the different sporangial 
types ; but Avhatever its variations may be, it is in the archesporium, and 
in the sporogenous cells which it produces, that we see the essential feature 
of the sporangium. It will be necessary, then, to examine it carefully, 
and to see how far it is possible to attach a definite meaning to it. 

The term archesporium was introduced by Goebel, 1 and denned as 
follows : " In the Vascular Cryptogams, as in the Phanerogams, the 
spore-producing tissue may be universally referred as regards its origin 
to a cell,- a cell-row, or a cell-sheet : I designate these original parent 
cells of the sporogenous tissue as the archesporium." To this he added 
that "in all Vascular Cryptogams examined an hypodermal archesporium 
exists," thus definitely localising it in a position comparable to that 
in the Spermophyta. He remarks, however, later that he does not lay 
vSpecial stress upon the archesporium being always a cell-row or cell-sheet, 
and contemplates it as possible that sometimes the development may 
proceed otherwise than by the appearance of an archesporium of the 
form described. An examination of all the types of sporangia of living 
Pteridophytes has shown that this is the fact : a considerable number of 
cases have been observed in which the archesporium is not hypodermal, 
in that it is not defined by a single periclinal wall of the parent cells 
involved. The existence of many exceptions among Eusporangiate 
Pteridophytes suggests a reconsideration of the archesporium. We may 
enquire whether a definite meaning is attached to the term, and if so, 
whether that meaning is of general application. 

The effect of Goebel's investigations on the sporangia of Pteridophytes 
was to extend downwards from the Phanerogams the demonstration of a 
formative cell or cells to which the origin of the spores may be ascribed. 
Before 1880 it was held that a 'mass of cells within the young sporangium, 
showing irregular divisions, took upon them the character of a sporogenous 
tissue : Goebel's results led him, as we have seen, to the statement that 
the spore-producing tissue can be referred as regards its origin to a 
cell, a cell-row, or a cell-sheet, which can be distinguished very early 
by the nature of its materials from the rest of the cell-tissue. This 
archesporium was successfully recognised in certain cases, and the 
tendency of the time was to expect similar success in all cases. Thus a 
special significance came to be attached to these cells, quite apart from 
that of the surrounding tissues, as being predestined from the first to the 
important function of spore-production. 

The location of the archesporium in the Phanerogams was found to 
be consistently hypodermal : in a somewhat strained sense the same was 
^Bot. Zeit., 1880, p. 545 etc. 



THE ARCHESPORIUM 107 

found to be the case in many N of the Pteridophytes. But it was not 
sufficiently recognised that between the two lies all the difference between 
stratified and imperfectly stratified meristems. This point was brought 
forward in I896, 1 in the proposition that "the study of the sporangia or 
synangia of a plant should be carried out in the light of a knowledge 
of the segmentation of its apical meristems," and the generalisation was 
laid down that " where the apical meristems are distinctly stratified, the 
structure of the young sporangium is stratified also : in those plants where 
there is a non-stratified structure, with one or more initial cells, and 
frequent periclinal division of superficial cells near the apex of stem, leaf, 
or root, there the structure of the young sporangium is not distinctly 
stratified." Such exceptions to the hypodermal position of the arche- 
sporium as have been found among the Pteridophytes all fall under this 
generalisation, and it may be added as a corollary that in all Vascular 
Cryptogams investigated, the sporogenous tissue is ultimately referable to 
the segmentation of a superficial cell or cells. This has been since 
noted by Wilson Smith, 2 who has accordingly suggested that the term 
" archesporium " should be extended to these also, and he gives the definition 
as follows: "The cell, or group of cells, whether superficial or hypodermal, 
to which in a last analysis all the sporogenous portion of a sporangium 
can be traced, ought to be called the archesporium." 

The use of the term " archesporium " has been discussed afresh by 
Goebel in his Organography (p. 771). He first describes the case for the 
anther of an Angiosperm thus : " In each of the four angles of the anther 
a cell-row or cell layer below the epidermis (hypodermal) divides by 
periclinal walls. Of the cells which thus arise the internal form the 
archesporium, the outer the ' schichtzellen,' which now divide still further 
by periclinal walls." The archesporium is thus defined as the inner product 
of the division of the hypodermal layer ; it gives rise neither to any part of 
the sporangial wall nor to the tapetum, though, since these have a common 
origin with it from the hypodermal layer, it is not clear why the latter 
should not itself be styled the archesporium. Passing on to the Pteridophyta, 
after noting how superficial cells give rise to the essentials of the sporangium, 
and that the details are not uniform, he proceeds (/.<:., p. 774): "We 
may then designate that superficial cell or cell-layer as archesporium which 
sooner or later gives off sterile cells, while in the sporangia of Angiosperms 
the archesporium is a cell-layer lying below the epidermis, which is already 
differentiated : with this the above-noted differences in wall-structure of the 
Pteridophyta and Gymnosperms on the one hand, and of the Angiosperms 
on the other would correspond." This passage calls for the following remarks. 

In the first place, Goebel accepts the conclusion of Wilson Smith, 
that since the sporogenous tissues of Pteridophytes are all referable in 
origin to superficial cells, therefore those cells are to be recognised as 
the archesporium. By accepting this conclusion, it may be presumed that 

1 Studies, ii., p. 8. -Rot. Gazette, vol. xxix., p. 325. 



io8 THE SPORANGIUM DEFINED 

he approves of the method by which it is arrived at : that is, the method 
of recognition of the archesporium by " a last analysis " of cell-origin. 

Secondly, Goebel's method of recognition of the archesporium is not 
consistent : he designates the inner product of the hypodermal layer in 
the Angiospermic stamen the archesporium, notwithstanding that the 
" schichtzellen " and the tapetum are sister-cells with it. But in the 
Pteridophyta, on the ground of common origin by segmentation, not only 
the tapetum but also the sporangial wall itself are described as derived 
from a superficial archesporium. If the recognition of an archesporium 
is to be based upon "a last analysis" of the segmentations, then the 
hypodermal layer of the Angiospermic anther, and not merely the inner 
product of its segmentation, is the archesporium. 

Thirdly, the recognition of the archesporium by the method of "a 
last analysis " brings together under a common head, merely on the ground 
of early segmentations, things which are not really comparable, and ascribes 
a distinct origin to things which are indistinguishable when mature. The 
superficial archesporium of the Pteridophytes gives rise to part of the 
sporangial wall and of the tapetum : the archesporium of the Angiospermic 
anther, on Goebel's definition, gives rise to neither. It is impossible to 
conceive how by any known evolutionary progression the former type of 
" archesporium " could pass into the other, and the superficial cells be covered 
over : therefore the two are to be regarded as not truly comparable. Further, 
the recognition of superficial cells in the Pteridophytes as archesporial 
draws a distinction between part of the sporangial wall which originates 
from them, and the rest which does not : thus in the Leptosporangiate 
Ferns the apical part of the annulus would be archesporial, the lateral 
parts would not. 

With all respect to the opinion of the writer who introduced the term, 
I think that this last change in its application, as suggested by Wilson 
Smith and accepted by Goebel, makes more obscure the meaning of a 
word which never has been clear. The Bryophyta provide a reductio ad 
absurdum of the method of " a last analysis " ; for, following this method, 
in Sphagnum and some others the amphithecium would be reckoned as 
the archesporium, while in the ordinary Bryineae it would be the 
endothecium : or, carrying the analysis in the latter case to its extreme 
limit, the first segments in the upper half of the zygote, or even the 
ovum itself, would be the archesporium. 

The fact is that this sort of analysis of formative tissues has served 
its turn : it has led to much detailed investigation, which has, however, 
shown that the segmentations which lead up to the formation of spore- 
mother-cells are not comparable in all cases. The time has come, in 
presence of many divergent details, to admit frankly that there is no 
general law of segmentation underlying the existence of that cell or cells 
which "a last analysis" may mark out as the "archesporium," and that 
therefore the general application of such a term to those cells which the 



THE ARCHESPORIUM 109 

analysis discloses has no scientific" meaning, beyond the statement of the 
histiogenic fact. At the same time, the value of the details which have 
been acquired by the pursuit of the archesporium must not be under- 
estimated for purposes of comparison. What is dangerous is the attachment 
to them of ulterior ideas : the assumption that because a definite 
" archesporium " is often found, it should by rights be always present : 
and the effort to trace in its appearance homologies which seem based 
on forced rather than on natural comparisons. On the other hand, the 
term has become so established in the literature of the subject that it cannot 
be summarily discarded : it may be retained merely in a descriptive sense, 
in those cases where the cell or cells which give rise to the sporogenous 
.group are obvious, but in a descriptive sense only. 

The discrepancies which become apparent in the course of develop- 
ment between different types of sporangia tend to justify the position 
already adopted by Strasburger on general grounds: he remarks i 
that the centre of gravity of the developmental processes does 
not lie in those cells, cell-rows, or cell-aggregates which have been 
designated " archesporium " by Goebel : the archesporium still belongs to 
the asexual generation, and the presence or absence of a well-defined 
archesporium is not a matter of importance, for it is merely the merismatic 
tissue from which the spore-mother-cells are derived. From the point of 
view of a theory of sterilisation as enunciated above, these spore-mother-cells 
may be held to be, in the simpler cases at least, the residuum which a 
progressive vegetative change has left : in that case there is no reason to 
expect that the demarcation of these islands of fertile tissue should have 
followed any definite system in plants at large, which would be reflected with 
any exact uniformity in the segmentations now involved in their formation. 

The frequency of hypodermal origin of the sporogenous tissue in 
Vascular Plants is readily intelligible biologically. In all except the very 
simplest sporophytes the spores are protected during development by tissues 
which surround them completely : this ensures nutrition and mechanical 
protection. In the Bryophyta the scattering of the spores would be equally 
efficient whether they be produced close to the surface or deeply seated, 
since the dehiscence, whether by a terminal operculum or by longitudinal 
slits, gives free exit to all the spores of the continuous spore-sac, and 
accordingly the spore-mother-cells of the Bryophytes may be central, as 
in many Hepatics, or removed more or less from the centre by the 
occurrence of a columella, as in most Mosses. But in all Vascular Plants, 
where the spores are produced in separate pockets or sporangia, the 
dehiscence leading to dissemination is* referred to the several sporangia 
themselves : this necessitates for them a superficial position on the plant- 
body, or, better, that they shall project beyond the surface. The hypo- 
dermal origin of the sporogenous tissue which is so frequent, may thus be 
recognised as a compromise between the two requirements of effective 

^Annals of Botany, vol. viii., p. 316. 



no THE SPORANGIUM DEFINED 

protection and nutrition on the one hand, and ready dissemination on the 
other. But the compromise may have worked out differently in different 
lines of descent, and, indeed, it appears from the variety of the segmentations 
in the Pteridophytes that this has actually been the case. From this point 
of view no difficulty need be felt to arise from the absence of any general 
law of segmentation, leading up to the formation of spore-mother-cells; 
but, on the other hand, similar and even definite types of segmentation, 
culminating in regularly segmented sporogenous groups resembling one 
another, may have been evolved along more than one line of descent. 

It cannot escape notice that in some cases the individuality of the 
sporangium is not maintained. Certain synangial states are not uncommon, 
which can only be regarded, from the evolutionary point of view, as 
results of either septation or fusion : where the fusion or septation is 
incompletely carried out, and partial septa are present, it becomes a 
question whether the whole or only the part of the complex body is 
correctly to be termed a sporangium. This difficulty is very obvious in 
the sori of Danaea (Fig. 61). The mere application of a term is naturally 
a trivial matter : the question which is really important is, how far the 
conception of the sporangium is to be modified by the existence of such 
cases. The current conception of the sporangium is based upon examples 
where it has a distinct individuality : in the Ferns and Lycopods, and even 
in the pollen-sacs of normal Angiosperms such individualised sporangia are 
seen. But it is a question how far the idea of the individualised sporangium 
so gained is an enlightening one from the point of view of descent. In 
the same way, the old conception of the cell as the structural unit of the 
plant-body was based upon the study of the tissues of the higher plants, 
where the cells are for the most part individualised : it had to give way 
before the accumulated examples of cell-fusions, of polynucleate cells, 
and of non-cellular construction in plants both higher and lower in the 
scale. Just as by comparison of such structures as these the idea of the 
cell has undergone modification, notwithstanding that cells are commonly 
definite bodies in the ordinary tissues of the higher plants, so may the 
existence of evidence pointing to sporangial septations and fusions modify 
the conception of the sporangium. 

The fact that sporangia originally simple have undergone septation has 
only been proved in comparatively few of those cases. The most 
complete demonstrations are those from the anthers of certain Angio- 
sperms, such as the Onagraceae, Mimoseae, Loranthaceae, Rhizophoreae, 
etc. In these the comparative argument is made valid by the existence 
of numerous allied genera, which give ground for close comparison ; for 
while many plants of these orders show the ordinary quadrilocular anthers, 
in others the loculi may be subdivided by further septa, and thus a 
number of sacs take the place of each original one. The development 
shows that the septation results from the conversion of sporogenous tissue 
into sterile septa. Similarly, an argument for fusion of sporangia can also 



THE INDIVIDUALITY OF THE SPORANGIUM in 

be supported on a basis of comparison among Angiosperms, though it is 
a less frequent change : examples are to be found in the coalescent ovules 
of certain Loranthaceae, or in the confluent pollen-sacs of certain Gut- 
tifeae, etc. Among Pteridophytes, on the other hand, the genera are so 
isolated as a rule that the comparative argument is difficult to apply : and 




FIG. 61. 



Danaea ell'ptica, Smith./ Drawings illustrating partial septations of the sporangium. 
A, tangential section through three sori, showing the loculi in ground plan : the septa are 



, , 

often thin, so that pairs of loculi are in close juxtaposition ; the loculi marked 
tial seta. X 20. />' C D show such loculi 

the cells 



, 

are large, and show one or more partial septa. X 20. />', C, D, , show such loculi with 
partial septa in greater detail : in D and E it is difficult to decide whether 
marked (?) will develop as tapetum or as spore-mother-cells. X 150. 



in each case of a synangium it may be a question whether the structure 
results from septation or from fusion. But without entering into these dis- 
cussions at present, it is plain that either way the individuality of the 
sporangium is lost in such cases, just as it is in the Angiospermic anthers, 
and this is particularly clear where, as often happens, partial septa are found. 
In summing up the considerations contained in the above pages, it may 
be asserted for sporangia at large, that the individuality of the sporangium 



H2 THE SPORANGIUM DEFINED 

is not always maintained ; that the elevation of the sporangia relatively to 
the surface of the part which bears them is variable ; that while a sporangial 
wall is always present, the opening mechanisms are inconstant ; that the 
tapetum is inconstant in occurrence and in origin ; and that there is no 
general law underlying the segmentation of the sporogenous cell, or group 
of cells, so as to give it a constant hypodermal or other origin. What then 
remains as the fundamental conception of the sporangium in Vascular Plants? 
Simply the spore-mother-cell or cells, together with the protective wall. The 
definition of a sporangium will then be this : Wherever there is found i?i 
Vascular Plants an isolated spore-mother-cell, or a connected group of them, or 
their products, this, together with its protective tissues, constitutes the essential of 
an individual sporangium. 

This definition is open to criticism, as indeed are all definitions of bodies 
represented in a large series of variable organisms. Still, it brings out the 
point that the essential feature of a sporangium is the presence of one or 
more spore-mother-cells, but without reference to the detail of their 
production, or to the structure of the wall which covers them. It has its 
value in ridding the idea of the sporangium of its accidental accessories, 
and fixing the attention upon what is really essential. 

But it will perhaps be objected that a sporangium may still be a 
sporangium though it may contain no fertile cells ; many imperfect 
structures may be quoted which have the form, position, and other details 
characteristic for the sporangia of the plant on which they are borne : 
they should thus to be ranked as sporangia. That is true ; but as such 
bodies do not as a rule serve any useful purpose, it may be asserted that 
they would never have existed independently of the fully formed sporangia 
of which they are the imperfect representatives. Such vestigial parts, being 
of secondary origin, need not disturb the conception of the sporangium as 
above defined. 

Finally, an important feature of the sporangium is to be seen in the 
fact that in so many cases the archesporium is not strictly circumscribed ; 
the sporogenous group has often ragged edges ; in many of the Eusporangiate 
forms it does not arise from any single archesporial cell, or definite group 
of cells ; moreover, cells which are obviously sister-cells may not unfrequently 
be found to develop the one sterile, the other fertile. This suggests on 
the basis of structure that the fertile tract is a residuum left by advancing 
sterilisation, while the ragged and ill-defined limits point to the conclusion 
that the sterile and fertile tracts are closely related ; in fact, that they had 
a common origin, and that the final condition represents the balance struck 
between sterile and fertile development. From the point of view of the 
hypothesis of progressive sterilisation such sporangia may, at least in the 
simplest cases, be regarded as islands of fertile tissue which have retained 
their spore-producing character. It will be seen later how far this view 
will have to be modified in the more complex and derivative cases, such 
as the Leptosporangiate Ferns. 



CHAPTER IX. 

SOME GENERAL ASPECTS OF THE POLYSPORANGIATE 

STATE. 

IT has been pointed out in Chapter VI., which dealt with the Biological 
Aspect of Alternation, that in the case of plants of aquatic origin migrating 
to the land an increasing production of non-sexual germs, or spores, would 
become important. Since under those circumstances dependence could no 
longer be placed on frequent recurrence of fertilisation, the production of 
numerous spores as a consequence of a single fertilising act will be essential, 
if the race is to survive and be in a position to compete and to extend 
its area. Other things being equal, the larger the spore-output the better. 
This should be constantly before the mind in the comparative study of 
the more primitive types of sporophyte, and the same principle should 
be applied to the more complex forms also, though in them the evidence 
is necessarily less obvious. 

The antithesis between the Bryophyta and the Pteridophyta, as regards 
the method of spore-production, is chiefly marked by the former having 
one concrete sporogenous tissue, the latter numerous discrete sporogenous 
groups which form the centres of more or less distinct sporangia. The 
Bryophyte type is essentially a limited one, for indefinite enlargement 
of the concrete sporogenous tissue introduces mechanical and nutritive 
difficulties : these are mos't urgent at the critical period of separation of 
the spore-mother-cells, when they are floating freely in the fluid contents 
of the spore-sac. In actual life the Bryophyte type is almost always 
annual, and does not extend beyond limited proportions; nor is there 
evidence that it ever attained a larger size in earlier periods. This is 
exactly what biological considerations would have led us to anticipate. 

But in the homosporous Pteridophytes, given an enlarging vegetative 
system, which in them is usually perennial, there seems no limit to the 
number of sporangia which may be borne on the individual plant ; and 
as each sporangium is of moderate size, the mechanical and nutritive 
requirements at the critical period of tetrad-division are suitably met, while 

H 



ii4 THE POLYSPORANGIATE STATE 

the total output of spores from the numerous sporangia may be virtually 
unlimited : moreover, their production may be extended over many years 
on the same individual plant. Since, in the homosporous forms, each 
single spore is small, and therefore conveys with it only a small store 
of nutriment, the risks on germination are great ; a reasonable chance of 
success is, however, secured by their large number. 

But with the heterosporous condition complications arose. Owing to 
the storage arrangements in the enlarged female spore, this more specialised 
state leads to economy in number of the spores necessary to secure survival 
and spread of area ; for each female spore carries with it, in its higher 
store of nourishment, a higher probability of successful establishment of 
an embryo, and a sufficient degree of propagative probability can thus 
be attained with a moderate number of spores. Hence heterosporous 
types may be expected to present examples of reduction of number, not 
only of sporogenous cells, but also of sporangia. That is seen to be actually 
the case, and it might be illustrated by numerous examples. It will then 
be in homosporous types, which are certainly the more primitive, that 
we shall expect to meet with the best evidence as to the origin of the 
polysporangiate state, or with traces of increase in number of sporangia; 
in fact, they will illustrate more faithfully than heterosporous forms the 
upgrade of complexity of their spore-producing parts. 

On grounds of nutrition of the spores, and of stability at the critical 
stage when the spore-mother-cells are floating in fluid, there is a clear 
advantage in the segregation of the spores into separate pockets the 
sporangia as against any method of indefinite enlargement of a single 
sac. It is probably such conditions as these which have also determined 
the limits of size of the individual sporangia of the Pteridophytes, arid led 
to some degree of uniformity in their dimensions. But still considerable 
variations in size of the sporangia are found to occur, even in close 
juxtaposition on the same plant : this is most conspicuous in the 
Eusporangiate forms. Sometimes the difference in size seems to be 
dependent on nutrition ; for instance, it is usual to find about the upper 
and lower limits of the fertile strobilus of Lycopods^ sporangia of smaller 
size than those about the middle of the fertile region : the same is the 
case in the Psilotaceae and in Equisetum. But in other cases this simple 
explanation will not suffice, for smaller sporangia may be found dis- 
tributed between the larger ones : this is especially so in the sori of 
the Marattiaceae, and a general survey shows that in many of the 
Eusporangiate forms the single sporangium is not quantitatively a definite 
unit. 

But though there may thus be wide variation of size of the individual 
sporangia in certain Pteridophytes, still in others their dimensions are 
often very uniform. In the Leptosporangiate Ferns, indeed, the number 
of spores in a single sporangium is often strictly constant. In that case 
change in the output of spores on the plant is effected by change in the 



SIZE AND POSITION OF SPORANGIA 115 

number of the sporangia, not by variation of their dimensions : thus the 
number of the sporangia may come to be an approximate measure of 
the spore-output, as it is in fact in the Leptosporangiate Ferns. 

The several types of Pteridophytes differ greatly in the closeness of 
the relation of their sporangia to the axis of the shoot ; it will be pointed 
out in detail below how the five main series of them the Lycopodiales, 
the Equisetales, the Sphenophyllales, the Ophioglossales, and the Filicales 
exhibit successive degrees of enlargement of the appendicular organs, 
and of the consequent removal of the sporangia from the central axis. 
The strobiloid character, with small appendages, and one sporangium at 
the base of each, or even seated on the axis itself, is characteristic of 
the Lycopods ; but this regularity is not characteristic of the larger-leaved 
types : thus the definiteness in number and in position of the sporangia 
relatively to the other parts, which is as a rule absolutely exact in the 
Lycopods, is less strictly observed in the Equisetales and Sphenophyllales, 
and it is almost entirely absent in the Ophioglossales and Filicales, in 
which the sporangia are borne upon the large leaves, far removed from 
the central axis : their number and their arrangement there tends to be 
indefinite. These facts may be summarised into the statement that in 
the Pteridophytes those forms which bear their sporangia in closest relation 
to the axis show the most strict definiteness in their number and position : 
where the sporangia are removed from the central axis, being borne upon 
larger appendicular organs, they habitually show less definite'ness in number 
and in position. 

The indefiniteness of. number of the sporangia thus seen in the 
Ophioglossales and Filicales is an illustration of the variability of multiple 
structures, alluded to by Darwin as follows : " It seems to be a rule, as 
remarked by Is. Geoffroy St. Hilaire, both with varieties and species, 
that when any part or organ is repeated many times in the same individual 
(as the vertebrae in snakes and the stamens in polyandrous flowers) the 
number is variable : whereas the same part or organ, when it occurs in 
lesser numbers, is constant." That constancy is seen in the Lycopods 
in high degree : it is departed from to some extent in the Sphenophyllales 
and Equisetales, and it becomes unrecognisable in the Ophioglossales 
and Filicales, in which the number of sporangia on each appendage is 
large. 

It has been remarked above that it is still an unsolved problem what 
those intimate influences are which determine the development of any 
specific cell of the plant-body as a spore-mother-cell on the one hand, or 
as a vegetative cell on the other. This determination lies at the root 
not only of the limitation of sporogenous tissues, but also of the initiation 
and consequent number of sporangia. The determining factors are probably 
numerous : suitable nutrition is certainly one. Speaking generally, better 
nutrition is clearly connected with more ample spore-formation ; but it is 
also well known that a plethoric state may lead to sterility in certain 



n6 THE POLYSPORANGIATE STATE 

cases while starvation may conduce to early flowering in many Angiosperms. 
Thus the relation of nutrition to the production of sporangia is not of a 
simple character. Goebel (Organography, p. 498) speaks of other factors, 
such as intensity of light, and internal conditions or correlations, as 
influencing the production of sporophylls. Experimentally it seems easier, 
however, to convert sporophylls into foliage leaves than to make foliage 
leaves fertile. He quotes, nevertheless, the case of a Fern, allied to 
Acrostichum Blumeanum^ in which an arrest of growth of the rhizome, 
after previous good nourishment, led to production of sporangia. This is 
a result similar to that following root-pruning of fruit-trees. It reminds 
us also of the conditions found by Klebs to determine the production of 
reproductive organs in certain Algae and Fungi.' 2 I do not suppose, 
however, that the conditions will be found to be uniform for all sporophytes, 
any more than they have been for Algae or Fungi. In any case, the 
present knowledge of the whole subject for Vascular Plants is indefinite 
and uncertain. 3 

The time of distinctive development of cells as sporogenous cells varies 
in different plants : the following tentative conclusions may be drawn from 
such differences. When in a tissue-tract the distinction between vegetative 
and sporogenous cells takes place relatively late in the individual, the 
presumption is that the distinction has been of late origin in the race. 
On this basis the conclusion has been founded in certain cases that increase 
in number of sporangia by septation has occurred. A large potential 
sporogenous tissue having a common origin is first seen ; but later it 
differentiates, part becoming actually sporogenous, part remaining sterile. 
It is concluded that these late-differentiated sterile tracts were once in 
the race fertile, and that they were subsequently diverted from this previous 
condition ; in fact, that the ontogenetic development reflects the evolutionary 
history. This is exemplified in certain Angiospermic anthers, in the 
synangia of Tmesipteris, and in the partially septate sporangia of Danaea-. 
the same general argument holds also for the sporangium of Isoetes with 
its trabeculae. In other cases where the distinctive characters of the 
sporogenous cells or cell-groups are acquired earlier, the argument for 
septation is less clear, though on grounds of comparison a similar history 
of the structure actually seen appears probable. 

The cases above mentioned involve sporangia which are closely associated 
as synangia, and they are naturally initiated simultaneously. But differences 
of the time of distinctive development of sporogenous cells may become 
more obvious in sporangia which are separate from one another, though 
in close proximity upon the part which bears them. In those types which 
comparison, as well as the Palaeontological record, points out as the 

1 Raciborski, Flora, 1900, p. 25. 2 Die Bedingungen der Fortpjlanzittig, 1896. 

3 The determining conditions have been discussed by various writers. See Uiels, 
Jngendformen und Blutenreife iin Pflanzenreich, Berlin, 1906, where reference is made 
to the literature on the subject. 



SIMULTANEOUS OR SUCCESSIVE 117 

earliest, the sporangia in near juxtaposition show a simultaneous origin ; 
or some degree of succession may be seen from those earlier formed 
near to the base of the shoot or leaf, and leading to the apical region, 
where they appear later. Such simultaneity, or such acropetal succession, 
may be regarded as a primary condition, and it is seen in the Lycopodiales, 
Equisetales, and Sphenophyllales, as well as in those Ferns which are 
designated below the Simplices (see Part II.). But in certain Ferns, which 
the Palaeontological record, as well as comparison, would mark out as 
secondary, the sporangia in near juxtaposition do not arise simultaneously: 
sometimes, as in those which will be styled the Gradatae, there is a regular 
basipetal succession within the sorus, those lowest on the receptacle 
appearing latest. In others, again, there is no such regularity, and sporangia 
of different ages are found promiscuously intermixed : these Ferns are 
styled the Mixtae, and the Palaeontological record indicates that these 
were the latest to appear. Such facts, which will be stated at length 
below (Part II.), may be summed up into the following general statement. 
In the most primitive forms the sporangia in near proximity to one another 
develop simultaneously, though an acropetal succession may often be 
traced on the shoot or strobilus as a whole. Those successions, whether 
in regular order or irregular, which appear in various forms upon the 
leaves, may be held to be later derived, and secondary. 

It will be readily gathered from the contemplation of those plants 
in which sporangia are numerous that accurate comparison of individual 
sporangia as identical bodies in parent and offspring, or in different, less 
closely related specimens, is not possible in plants at large. For the most 
part sporangia are merely examples of "essential correspondence" rather 
than of "individual repetition." The actual sporangia of the offspring are 
not coincident, as a rule, either in exact position or in number with those 
of the parent. This is a consequence of that continued embryology which 
is a leading feature in all vascular sporophytes. As a consequence the 
individual sporangia of any one individual plant or species cannot be 
held to be the exact ontogenetic correlatives of those seen on another 
individual or species. The possibility of such a recognition is most nearly 
approached in the Lycopods, where the sporangia are borne singly in 
definite relation to the axis and leaf. It is departed from furthest in 
the large-leaved Ferns : especially is this so in the Polypodiaceae, where 
the mixed character of the sorus is the rule : but most of all in such a 
case as that of Aspidium anomalum, Hk. and Arn., a Fern found on 
the uplands of Ceylon, and sometimes regarded as a mere variety of 
A. aculeatum, Sw. : its peculiarity consists in the appearance of sori upon 
the upper surface of the leaf, where normally they do not occur. As 
there is no question of mere inversion of the leaf, it can only be assumed 
that there has been a transfer of the stimulus, whatever it be, to soral 
development from the lower to the upper surface of the leaf. Clearly 
the sori which result cannot be the ontogenetic correlatives of any 



n8 THE POLYSPORANGIATE STATE 

previously existent in the race : much less is this possible for the individual 
sporangia of those sori. Such an example shows, in its most extreme 
form, how impossible it may be to compare, as numerically or locally 
identical, the otherwise similar parts, such as sori or sporangia : and 
this is most clearly so in the Ferns, where the leaves are large, and the 
sori and sporangia borne upon them more numerous than in any other 
Vascular Plants. 

We thus see that the homosporous Pteridophytes, which are certainly 
the more primitive, will be the best guide in questions of the origin of 
the sporangial state : and that these may be arranged serially according 
to size of the appendages, the strobiloid types being at one end of the 
series and the large-leaved Ferns at the other. The forms thus arranged 
show more or less clear differences in the sporangial characters : in the 
simpler strobiloid forms the sporangia are less definite units as regards 
spore-output, in the Ferns they have tended to become in the evolutionary 
course more definite units in this respect. In the strobiloid forms the 
relation of the sporangium to the axis is close, and as regards position 
and number it is more definite ; in the larger-leaved forms the sporangia 
are further removed from the axis, and their position and number tends to 
become more and more indefinite. In the strobiloid forms the time of 
origin of the sporangia in near juxtaposition to one another is simultaneous : 
in the larger-leaved forms it tends to become in various ways successive, 
while the palaeontological record shows that the most pronounced succes- 
sions have been of secondary origin. These distinctions will have their 
value in leading to a more precise statement of the problem of origin of 
the sporangial state. To this end it will be found desirable to keep 
distinctly before the mind those vascular types in which the nearest 
approach can be made to a comparison of the sporangia as numerically 
and locally identical. Among the homosporous Pteridophytes this will 
be found to be the case most nearly in the smaller-leaved strobiloid 
forms : and among these especially in the ancient phylum of the 
Lycopodiales. 



CHAPTER X. 

VARIATIONS IN NUMBER OF SPORANGIA. 

NOTWITHSTANDING the familiarity of the fact that the polysporangiate 
state is the constant condition in Vascular Plants, little attempt has 
hitherto been made to analyse the methods of variation in number of 
the sporangia which they bear. But this would appear to be a promising 
line* of enquiry, for it may be held that an adequate knowledge of the 
methods of variation seen to be actually operative now should throw light 
upon the factors which have been operative in the past, and thus some 
suggestion should be obtained how the divers polysporangiate types came 
to be as we now see them. To such an end the facts drawn from those 
organisms which are held to be relatively primitive, such as the homo- 
sporous Pteridophytes, will naturally bear greater weight than those derived 
from more recent and specialised forms, such as the Flowering Plants. 
Nevertheless it will be best to treat the question of change of number of 
sporangia first of all in its relation to the Vascular Plants as a whole, 
so that all the known factors may be disclosed : and it will be a matter 
for subsequent discussion to decide in any individual case which of those 
factors appear to have been operative in bringing that organism to its 
present condition. 

The subject of variation in number of sporangia in the individual or 
the race may be discussed either from the 'physiological or from the 
morphological point of view : it is the morphological question which will 
now be brought forward, though always in the light of physiological con- 
siderations. But I wish at once to meet the objections of those who 
will say from the physiological side that the number of sporangia depends 
on nutrition : this self-evident proposition neither explains nor rules out 
the morphological question how a plurality of sporangia arose, nor how 
the great numerical differences which we see may have been attained : 
nor does it modify the effect which observations of their numerical change 
in the living individual, species, or genus may have on views of Descent 
of the plants observed. By such observation and comparison of living 



120 VARIATIONS IN NUMBER OF SPORANGIA 

plants it is possible to recognise certain methods of numerical change of 
sporangia, now or previously operative : these may either lead to pro- 
gressive increase in number, or to decrease in number. Under these two 
heads the following table shows the several methods of change in number 
of sporangia of which evidence has been found in living plants, but it is 
possible that the table is not exhaustive : 

I. INCREASE IN NUMBER OF SPORANGIA. 

(a) By septation, with or without rounding off of the individual sporangia. 

(b) By formation of new sporangia, or of new spore-bearing organs, which 

may be in addition to, or interpolated between those typically 
present. 

(c) By continued apical or intercalary growth of the parts bearing the 

sporangia. 

(d) By branching of the parts bearing the sporangia. 

(e) Indirectly, by branchings in the non-sporangial region, resulting in 

an increased number of sporangial shoots : this is closely related 
to (c) and (d). 

f 

II. DECREASE IN NUMBER OF SPORANGIA. 

(/) By fusion of sporangia originally separate. 

(g) By abortion, partial or complete, of sporangia. 

(h) By reduction or arrest of apical or intercalary growth in parts bearing 

sporangia. 

(*) By fusion of parts which bear sporangia, or arrest of their branchings. 
(/) Indirectly, by suppression of branchings in the non-sporangial region, 

resulting in decreased number of sporangial shoots : this is closely 

related to (h) and (/). 

Each of these factors of variation will now be discussed, and examples 
of them adduced. At the moment the object is only to recognise that 
such modifications of number of sporangia are or have been operative 
in actual cases, not to estimate the relative prevalence of any one : for 
it is necessary first to distinguish the factors of the problem. 

FACTORS OF INCREASE. 

(a) Increase in number of sporangia by septation, resulting in a plurality 
of loculi, where previously in the race the septa were absent, is shown 
in the septate anthers of various families of Angiosperms (Onagraceae, 
Mimoseae, Rhizophoreae, Myrsinaceae, Loranthaceae, etc.). The details 
have already been sufficiently described in Chapter VIL, p. 97. The 
argument that septation has occurred is in many of these cases quite 
conclusive : thus the plurilocular anthers of. certain genera of the Onagraceae 



FACTORS OF INCREASE 121 

have the numerous small loculi -arranged in four rows, corresponding in 
position to the four pollen-sacs of the normal anthers in other genera of 
the family, the only essential difference from these being the presence of the 
septa which separate the loculi. The .cells which form the septa are similar 
in position to the archesporial cells of the normal anther (Fig. 62), and the 
conclusion is that certain of the archesporial cells are no longer fertile, 
but develop to form the sterile tissue of the septum. A similar argument 
will apply for other families of Angiosperms with varying degrees of 
cogency, according as the septate anthers diverge less or more from the 
usual type, or according to the less or greater isolation of the genera 
which show the multilocular state. In the Pteridophytes also there are 
cases of juxtaposition of loculi similar to those in the septate anthers : 




\ Trrt 




FIG. 62. 

A, longitudinal section of one loculus of a young stamen of Euckaridiuin concinuum, 
showing differentiation of the potential archesporium into fertile cells (a) and sterile 
cells (s). , similar section of stamen of Clarkia elegans, more advanced, showing a 
sterile septum dividing the contents of the single loculus into distinct sporogenous groups 
(i/). * = tapetum. X 365. 

conspicuous instances are found in the spike of Ophioglossum, and in the 
sorus of Danaea. But the development in the Pteridophytes gives less 
conclusive evidence than in the anthers of Angiosperms, owing to their 
meristems not being stratified, and to the fact that the genera are 
more isolated. But though the facts in these plants do not amount to 
a complete demonstration, there seems a reasonable probability that 
septation has played a part among them also. Whatever view be taken 
for these Pteridophytes, it is clear from the examples quoted from the 
Angiosperms that septation of sporangia previously in the race non-septate 
has been a factor of increase in number of sporangia. 

(^) By the formation of new sporangia, or interpolation, is meant the 
initiation of a new sporangial primordium, or of several, at a spot previously 
in the race not so occupied. The results of this mode of increase may 
in some cases be difficult to differentiate from those of septation, where 



122 VARIATIONS IN NUMBER OF SPORANGIA 

the products have been rounded off: they will be best distinguished in 
the early stages of development. A good, clear example is found in the 
genus Gkickenia : the sorus of most species of the genus is of the radiate, 
uniseriate type, in which the sporangia form a series round the receptacle, 
while the centre of it is unoccupied by sporangia : there is good comparative 
reason to believe that this was the primitive type of sorus for the genus. 
But in G. dichotoma additional sporangia, one or more, are often seated 
in the central unoccupied space (Fig. 63). Examination of early stages 
of development shows that these sporangia are not a result of fission of 
sporangia typically present, but develop from new primordia not represented 
in other species (Fig. 64). The interpolation, of which this is a simple 
case, becomes a more prominent feature in the Ferns which palaeontology 




FIG. 63. 

a-h, sori of Gleichenia dichotoina, Willcl. Showing one or more sporangia in the 
entre of the sorus, usually in this genus vacant 
/-, sporangia of Gl. dichotoma, a-/tX about 14. 



teaches us to regard as the more recent. The succession of the sporangia 
characteristic of those Ferns which will be styled on that account the 
Gradatae, may be recognised as a repeated basipetal interpolation of 
sporangia on the receptacle of the sorus (Fig. 65 A). In the Mixtae it is 
still more prominent in the sorus, for new primordia, formed in continued 
succession, appear scattered between those which are more advanced 
(Fig. 66). But interpolation^ may take place not only of sporangia, but 
also of new spore-bearing organs between those typically present. This 
is exemplified in the interpolated stamens in certain flowers, the number 
of which is closely related to their bulk, and to the space available for 
them on the floral receptacle. Such interpolation of sporangium-bearing 
parts will clearly result in an increase in number of sporangia. The 
prevalence of interpolation thus seen in the Leptosporangiate Ferns, has 
given an entirely fictitious importance to this phenomenon in the general 



FACTORS OF INCREASE 



123 




FIG. 64. 

, b, c, j, k Gleichenia ftabellata, Br. d, e, f=Gl. circinata, Sw. g, h, i=GL 
dtchotoma, Willd. a, i>, c, d, show young sori with sporangia marginal, g shows a 
similar condition in Gl. dichotoma, but in h and i the vacant middle space is occupied by 
young sporangia. a-/X2oo. /, xioo. 




FIG. 65. 

A , By Dennstaedtia apiifolia, Hook. A shows the regular basipetal succession of 
sporangia characteristic of the Gradata;. C, D, Dennstaedtia rubiginosa, Kaulf. D 
shows irregularity in the succession. X TOO. 



i2 4 VARIATIONS IN NUMBER OF SPORANGIA 

morphology of Vascular Plants ; while the regularity and constancy in 
number and position of the sporangia in the Lycopods, Equiseta, and 
Eusporangiate Ferns, in which interpolation does not occur, has been 
underestimated. But these, on grounds of comparison, and of fossil history, 
are among the most primitive pf Vascular Plants. Thus interpolation of 
new sporangia is to be recognised as an actual factor of increase in number 
of sporangia, but it is not a general phenomenon, and there is reason to 





FIG. 66. 

Sorus of Davallia. gritffthiana, Hk. Showing sporangia of different ages irregularly 
intermixed. X 100. 



think that it has been initiated as a secondary character, and in certain 
groups only. 

(c) Continued apical growth of the parts bearing the sporangia is a 
marked feature in most Vascular Plants : a concomitant of it, in the case 
of axes, is a continued embryology, with the initiation of an indefinite 
number of successive primordia of spore-producing parts. This is con- 
spicuous in the axes of many Lycopods, and especially so in the Selago 
group of Lycopodium, where it appears to be unlimited : in other species of 
the genus the apical growth of the strobilus also exists, but is of shorter 
duration (Fig. 67). Much the same is the case in other strobiloid types, 
with varying duration of the apical growth. The apical growth of the axis 
is apt to be less prominent where the appendages are large, as in the 



FACTORS OF INCREASE 125 

Ophioglossaceae and Filices, and in these the continued apical growth may 
largely devolve upon the sporophylls. The consequence of such growth 
is increased accommodation for sporangia. Similarly, continued intercalary 
growth may effect the same end : as in the sori of those Ferns designated 




FIG. 67. 

A forked sporangiferous branch of Lycopodium chama>cyparissus in longitudinal section, 
slightly magnified. ff= the axile vascular body. M = leaves. ss = young sporangia. 

the Gradatae. These serve as examples of the way in which, by continued 
growth of the parts which bear the sporangia, a sequence of these may be 
produced, which may be continued so long as the growth itself. It is clear 
-from the examples adduced that such continued growth may occur in parts 
which are not morphologically comparable. In the more primitive Pterido- 
phytes it is answerable for extensive increase in accommodation for primordia 
of sporangia. 



126 VARIATIONS IN NUMBER OF SPORANGIA 




(d] Closely related to (c) is the branching of parts bearing sporangia : 
this may also occur in parts which are in no way morphologically com- 
parable. Branching of strobili is a common feature in Lycopodium and 
Psilotum : branching of the sporophyll is characteristic of most large-leaved 
Pteridophytes : branching of the receptacle of the sorus is common in 

Ferns (Fig. 68) : branching of the fertile spike 
is a marked feature in certain Ophioglossaceae : 
branching of stamens is common in the Angio- 
spermic flower, and is to be distinguished from 
the interpolation above mentioned : a somewhat 
similar branching of the sporangiophores has 
probably occurred in the Sphenophylleae. In 
all such cases, though the parts are not mor- 
phologically comparable, the end is attained 
of an increased accommodation for sporangia, 
which consequently may be produced in 
increased numbers. 

(e) Continued growth and branching, occur- 
ring in the non-sporangial region, may have 
the indirect effect of a further increase in the 
opportunities for production of sporangia; for 

not only is the vegetative system thereby increased, which will have its 
indirect effect in increased powers of nutrition, but also a larger number 
of apices are provided, any one of which may take up the character of a 
sporangial strobilus. The continued apical growth in the vegetative region 
is a general feature of Vascular Plants. Branching is profuse in many of 
the strobiloid Pteridophytes : in the Ferns also it occurs, but the effect in 
increasing the opportunities for spore-production is less obvious here than in 
the strobiloid types. In Flowering Plants also the complicated inflorescences 
and the multiplicity of flowers is dependent upon such apical growth, 
together with repeated branchings. 



FIG. 63. 

a-d, various examples of fission of 
the sorus in Hymenophyllum dila- 
tatum. The sporangia, and one flap 
of the indusium have been removed, 
leaving the receptacle exposed. 
X about 5. 



FACTORS OF DECREASE. 

(/) Decrease in number of sporangia, by fusion of sporangia which 
previously in the race were separate, has been assumed as an explanation 
of synangial states by various writers : but it can only rarely be proved on 
grounds of comparison that fusion of sporangia has actually taken place, 
and the best evidence of it comes from the Angiosperms. Thus the fusion 
of the ovules, leading indeed to the obliteration of their identity, occurs in 
certain species of Loranthus, and comparison leaves little doubt that the 
sunken embryo-sacs represent the individual ovules, the identity of which- 
is lost as regards external form. Fusion of pollen-sacs is more frequent ; 
good examples, showing various states of the fusion, are found in the 
genus Phyllanthus, and in Cyclanthera while the unilocular condition in 






FACTORS OF DECREASE 



127 



Arisantm, and in Najas, Caultma, can hardly be ascribed to any other 
source than the fusion of the pollen-sacs, separate in the ancestry, into a 
single loculus. Thus in the androecium of Angiosperms. and occasionally 












FIG. 69. 

Junipcrus cointmtnis. I., summit of a male flower seen from above. st\, the uppermost 
staminal whorl of these stamens ; st. 2 , the second staminal whorl shows on each stamen 
two pollen-sacs, and the indication of a lamina, /; s/ 3 , the third staminal whorl, of which 
only the tips of the lamina? of two stamens are seen : each of the stamens of this whorl 
had three pollen-sacs, not shown in the figure. II., the same in longitudinal section. 
III., the same in transverse section. There is evidence here of reduction of the lamina, 
and of fusion of the pollen-sacs. (After Goebel.) 

in the gynoecium, a fusion of sporangia is recognised, resulting in a reduc- 
tion in their number. The apex of the male flower in Juniperus communis 
has been quoted by Goebel as a probable example of fusion of pollen-sacs 
(Fig. 69). In certain Pteridophytes the grouping of 
the sporangia is often such as to suggest a previous 
fusion ; but this has not been proved on developmental 
pr comparative grounds for any one case, and the 
question must be left open for discussion on grounds 
of general probability whether the synangial state in 
any individual case has been the result of fusion, or of 
septation with incomplete separation of the sporangia : 
obviously the synangial structure would be compatible 
with either origin. Whatever the final decision for the 
Pteridophyta may be, it is clear that fusion of sporangia 
originally separate has actually occurred in Flowering 
Plants : it is therefore a factor which must be regarded 
as a possible explanation of all synangial states. 

(g) Abortion, partial or complete, of sporangia which 
were fully matured in the type or ancestry is so common 
a feature that special examples need hardly be quoted, 
sacs are commonly found on staminodes, and abortive ovules are frequently 
seen, as in Anemone (Fig. 70), which can only be accepted as the imperfect 
representatives of a plurality of ovules in the ancestry. In the Pteridophytes 
many examples of abortive sporangia have lately been described, and their 
very important morphological bearings will be discussed at length in 
Chapter XIII. But in connection with the circumstance that parts 




FIG. 70. 

Carpel of A nemone 
nemorosa, L. Showing 
one ovule developed, and 
three abortive. (From 
Engler and Prantl.) 



Abortive pollen- 



128 VARIATIONS IN NUMBER OF SPORANGIA 

initiated frequently do not come to full maturity this further fact is to be 
borne in mind : that it is common in the ascending series of plants to 
find a larger number of primordia initiated than the individual can bring 
to full development. Consequently it will be necessary to discriminate 
between those imperfect parts which represent such as were fully matured 
in the ancestry, and those supernumerary primordia which do not represent 
parts ever actually matured in previous generations. In point of fact, 
unperfected primordia may, in some cases, really represent an exuberant 
tendency to progress, and cannot necessarily be assumed to indicate a 
state of reduction. 

(ti) The reduction or arrest of apical or intercalary groivth in parts 
bearing sporangia has doubtless been a potent influence in the past, but 
it is somewhat difficult to point to concrete examples of it. The general 
fact that the floral axis in Phanerogams is abbreviated, while the axis of 
the strobilus in Pteridophytes is frequently endowed with continued apical 
growth, points to the probability of arrest of that apical growth. An 
example of it within a near circle of affinity is seen in the genus Lycopodium; 
for in the section Selago the apical growth of the axis is unlimited, and 
the number of possible sporangia borne on a given axis is unlimited also : 
in other sections of the genus which are held to be more specialised, as 
the strobilus becomes more strictly differentiated from the vegetative region 
the apical growth is arrested early, and the number of sporangia produced 
upon it is limited also. In floral details this is illustrated in some families 
of Angiosperms : thus the Ranunculaceae include such forms as Myosurus, 
with its elongated receptacle and indefinite number of carpels, and Actaea, 
with its abbreviated axis and only a single carpel. It seems probable that 
here also arrest of the apical growth has been associated with reduction 
of the number of sporophylls and of sporangia. 

(/) Fusion of parts which bear sporangia is not so susceptible of illus- 
tration in the Pteridophytes as in the Angiosperms. A most convincing 
series of reduction is laid out in the Araceae by Engler, involving fusion 
of stamens, accompanied by reduction in number of the pollen-sacs (Nat. 
Pflanzenfam. II. i., p. 107). Many examples might also be quoted from 
other Angiosperms, e.g. Cucurbitaceae, Euphorbiaceae, where fusion of parts 
is connected with reduction in number of the sporangia which they 
bear. A suppression of branchings of sporangium-bearing parts may also 
be a factor : it has at times been assumed in theoretical writing, but it is 
difficult to give conclusive examples of it. 

(/) Indirectly the arrest of apical groivth and the partial or complete 
suppression of branchings in the non-sporangial region may affect the 
number of sporangia produced. The abortive buds at the base of 
inflorescences of Aconitum, for instance, or the abortive spikelets in 
Cynosurus are examples of potentially larger numbers of spore-producing 
branches arrested before they bear sporangia. Such arrests of growth 
and of branching may have been more prevalent sources of change than 



DIFFICULTIES OF ANALYSIS 129 

is apparent externally, for in this absence of vestigial parts there would 
be no trace of what had happened. 



The methods of change thus enumerated are, then, the known factors 
which affect the morphological problem of origin and present condition 
of the polysporangiate state. Any one of these, or more, may have be.en 
represented in the history of descent of any polysporangiate sporophyte 
as it is seen to-day. The examples quoted show that the methods 
enumerated are seen to have been actually operative in definite, living 
instances. Possibly these heads do not exhaust the methods of change 
of the present day nor of the past, and the list is open to additions. 
We are justified in assuming that (subject to the possibility of other 
factors having been operative of which we are yet unaware) the condition 
of any polysporangiate sporophyte as we see it is the resultant of modifi- 
cations such as these operative during its descent. The problem will 
therefore be in each case to assign its proper place in the history to any 
or each of these factors. But in each case the physiological probability 
of any modification which the structure would suggest should be con- 
sidered before it is admitted as part of the evolutionary story. Especially 
is this desirable in determining the probable relative prevalence of modifica- 
tions of increase as against those of decrease. It is only in this way that any 
apparent morphological series can be put upon a convincing footing as 
an evolutionary sequence. In complex cases, however, it may be a matter 
of difficulty to analyse a progressive change, and to decide which of the 
factors enumerated have actually been involved. 

It will be obvious that a complete account, in any given case, of the 
steps which have led to its present polysporangiate state involves a full 
knowledge of its evolutionary history a knowledge which js far beyond 
present powers. The advantage which an attempt to analyse the factors 
of sporangial modification brings, however imperfectly it may be carried 
out, is to simplify the problem in certain definite cases. For instance, 
if in a whole phylum of living plants a certain mode of sporangial 
increase be unrepresented, and if the related fossils show a similar absence 
of it, then it seems just' to hold that that mode of increase may be 
dismissed from consideration in the probable evolutionary history of that 
phylum. The case of interpolation already alluded to is one in point : 
in connection with this it is necessary to reconsider and examine certain 
old habits of thought which have too long dominated such discussions 
as the present. About the middle of the nineteenth century it was habitually 
maintained that the Polypodiaceous Ferns were primitive forms, and the 
probable progenitors of all other Pteridophytes. So long as this view 
was held interpolation of new sporangia between older ones, which is so 
prominent in them, was regarded as a general phenomenon which might 
appear anywhere among' the derivative forms. The fundamental idea 



130 VARIATIONS IN NUMBER OF SPORANGIA 

thus came to be that sporangia are bodies essentially indefinite in number 
and variable in place. The habit of interpreting lower forms in terms of 
the higher, which was also prevalent in earlier times, further encouraged 
this view : the common occurrence of indefinite stamens in Angiosperms 
made unduly familiar the idea of the sporangium-bearing parts, as well 
as sporangia themselves, being irregular in number and in position. But 
now there is good reason to believe Jhat both the Angiosperms and the 
Polypodiaceous Ferns were of relatively late origin, and in no sense 
primitive forms. Also that the interpolation of sporangia, or of sporangium- 
bearing parts, which is found in them both, arose secondarily as a con- 
comitant of the increase of the sporangia in either type to high numbers. 
If this be so, then there is no sufficient reason to look upon the sporangia 
of earlier and more definite types as in any way affected by the presence 
of this secondary indefiniteness in number and position. This principle 
finds its application as follows : it may be held that the Lycopods were 
independent of the Ferns in phyletic origin : hence the question of origin 
of their sporangia may be discussed without reference to interpolation at 
all, since it is not a characteristic of that family nor of its relatives. 
Similarly with the Marattiaceous Ferns, interpolation is not seen in the sori 
of any living representative of that family, nor is it demonstrated in their 
fossil prototypes : their sporangia are simultaneous in origin ; the fact that 
the relatively recent Leptosporangiate Ferns show frequent interpolation of 
successive sporangia should not affect our view of those Fern-types which 
certainly preceded them in time. In point of fact, interpolation of spor- 
angia is an occasional and not a general phenomenon : it is restricted to 
certain groups of plants, and is probably of secondary origin : in other 
groups where it has not been observed the sporangia are often seen to 
be stable in number and also in position. Accordingly the problem of 
evolutionary origin becomes more simple in those cases where interpolation 
may be left out of consideration altogether. 

A second case in point, where it is necessary to clear the mind of 
old habits of thought before entering on evolutionary questions, is that of 
synangial sporangia, which are so frequent in the Eusporangiate Pterido- 
phytes. The assumption was formerly common that where this condition 
is seen it is the result of fusion of sporangia previously in the race 
distinct. This assumption is again to be traced to the old habit of 
regarding the Polypodiaceous Ferns as the primitive stock of Pterido- 
phytes : the sporangia in them are all separate ; consequently it was held 
that where in other Ferns they are united, this must be a secondary 
condition. But the synangial condition may just as well have resulted 
from septation as from fusion : the question which is the correct view in 
any individual case is difficult to decide, but evidence of some value may 
be obtained by examination of the structure and development, as well as 
by general comparison of allied forms. In approaching such matters 
opinion must be unbiassed and open. Both fusion and septation have 



EXCLUSION OF CERTAIN FACTORS 131 

occurred in various instances, and in any given case the proper initial 
attitude is to hold that either mode of origin may have been the source 
of the synangial state as it now appears. 

The feature which has probably been most effective of all in distracting 
attention from the methodical analysis of the polysporangiate state in 
Vascular Plants at large has been the swamping effect of continued apical 
growth, and of branching. In the lower Vascular Plants both apical 
growth and branching may be seen in either the sterile or the fertile 
regions. In the higher Flowering Plants the floral region itself is 
characterised by absence of branching, and by restriction of apical growth, 
but both occur freely in the sterile region of the inflorescence. The 
results of this in the Flowering Plants are apt to be so dominating that 
it is often hard to recognise the small terminal and late-produced strobilus 
or flower as the actual residuum which progressive sterilisation and growth 
of the sterile tract have left. 

Among Vascular Plants it is only in the simpler Pteridophytes that this 
aspect of the sporophyte generation clearly emerges : and this is largely due 
to the fact that in them branching of the axis is often less profuse, or may 
even be absent altogether : moreover, the structural similarity between the 
sterile and fertile regions suggests their comparison. As a consequence of 
such comparisons, it follows that the great disproportion of the two regions 
so often seen in the Flowering Plants may be discounted as a secondary 
effect : it has been brought about principally by continued apical growth 
and repeated branching in the vegetative region, together with higher differ- 
entiation of the sterile and fertile shoots. Maintaining consistently this 
point of view, the overpowering effects of continued apical growth and of 
branching will be estimated at their right value, and so the way may be 
prepared for a more exact enquiry into the origin of the polysporangiate 
state, even in the more advanced types. It is by some such analysis as 
that sketched in this chapter that it may be possible to attain to a 
reasonable opinion how the condition seen in the earlier Vascular Plants 
came into existence. The detailed practical application of the method may 
often be difficult, and only partially successful : the present object has been 
to lay the basis for such an analysis, by showing what the recognised 
factors of numerical change of sporangia actually are, and to simplify the 
problem by showing that certain of those factors are of limited application 
only. 



CHAPTER XI. 

THEORY OF THE STROBILUS. 1 

THE term strobilus is commonly applied to those fertile spikes with small 
appendages which are found in a terminal position on the shoots of many 
Pteridophytes. The construction of the vegetative region below the strobilus 
in these plants is on essentially the same plan as the strobili themselves, 
but without the sporangia. The similarity of the two regions, as well as 
the absence of any definite limit between them, is demonstrated with 
peculiar clearness in the case of Lycopodium Selago, as shown in the 
Frontispiece. The absence of the sporangia in the vegetative region may 
be accounted for on the ground of abortion. If this be so, the structure 
of the strobilus will be the prototype for the vegetative shoot, and any 
theory of its origin with its appendages should cover that of the vegetative 
shoot as well. 

The strobiloid condition was common among the earliest Pteridophytes 
of which there is any fossil evidence : there is thus a probability that it 
was a relatively primitive state. It is characteristic of the Equisetales, the 
Lycopodiales, and the Sphenophyllales, which are all relatively micro- 
phyllous ; but the same type of construction is also traceable in radially 
constructed megaphyllous forms, and it will be shown later how the Ophio- 
glossales, and even the large-leaved Filicales, may be referred back to the 
strobiloid type of construction, but with the appendages developed to an 
inordinate size. 

The strobilus, in any of the small-leaved Pteridophytes, consists of a 
relatively bulky axis, endowed with more or less continued apical growth, 
and terminated by an apical cone, upon which the appendages arise 
laterally (compare Fig. 67, p. 125). Those appendages appear in regular 
acropetal order, below the apex of the axis ; and they arise exogenously, 
as more or less massive outgrowths of the tissue of the apical cone (Fig. 71). 
They have these characters in common in all cases, and, according to the 



1 This and the following chapters are largely based upon an address given at the 
International Exposition at St. Louis, U.S.A., 1904. 






POLYPHYLETIC ORIGIN OF LEAVES 



133 



morphological method of the latter half of the nineteenth century, all the 
parts which share these characters would rank as " leaves," and be regarded 
as "homologous." But the progress of the science should be leading 
towards the refinement of the use of the term " homology " : an approach 
must be made, however distant it may yet be, to a classification of parts 
on a basis of Descent. Though this may readily be accepted in theory, 
it is still far from being adopted in the practice of Plant-Morphology. 
None the less, comparison is inevitably leading to the disintegration, on a 
basis of Descent, of the old-accepted categories of parts : and in the case 
of the appendages which are collectively styled " leaves," the question arises 
whether they are all truly comparable in Vascular Plants. This is clearly 
in close relation to the question of their origin, and we shall enquire 




FIG. 71. 

Longitudinal section through the apical cone of the stem of Lycopodiutn Sclago. X 160. 
(After Strasburger. ) 

whether there is not reason to think that the initiation of the foliar 
appendages may have been polyphyletic. 

To those who hold the view that the two alternating generations of 
the Archegoniatae have had a distinct phylogenetic history, it will be clear 
that their parts can not be truly comparable by descent. The leaf of the 
Vascular Plant, accordingly, will not be the correlative of the leaf of a Moss. 
Even those who regard the sporophyte as an unsexed gametophyte will 
still have to show, on a basis of comparison and development, that the 
leaves of the two generations are of common descent. I am not aware 
that this has yet been done by them. 

But the phylogenetic distinctness of origin of the leaves of the sporo- 
phyte and gametophyte is not the only example of parallel foliar develop- 
ment. Goebel has shown with much cogency that the foliar appendages of 
the Bryophytes are not all comparable as regards their origin ; he remarks, 
" It is characteristic that the leaf-formation in the Liverworts has arisen 



134 THEORY OF THE STROBILUS 

independently in quite a number of series," 1 and has shown that they must 
have been produced in different ways. Here then is polyphyleticism in 
high degree, seen in the origin of those parts of the gametophyte which on 
grounds of descent we have already separated from the foliar appendages 
of the sporophyte. 

Such results as these for the gametophyte lead us to enquire how the 
case stands as to the origin of foliar differentiation in Vascular Plants. 
In discussing such questions, it is to be remembered that in different stocks 
the foliar condition of the sporophyte as we see it may have been achieved 
in different ways, just as investigators have found reason to believe that it 
was in the gametophyte. We have no right to assume that the leaf was 
formed once for all in the descent of the sporophyte. But at the moment 
we are unprovided with any definite proof how it occurred. All the evidence 
on the point is necessarily indirect, since no intermediate types are known 
between foliar and non-foliar sporophytes. Physiological experiment has 
as yet nothing to say on the subject. The fossil history of the origin of 
the foliar state in the neutral generation is lost, for the foliar character 
antedated the earliest known fossil-sporophytes. There remain the facts 
of development of the individual, and comparison, while anatomical detail 
may have some bearing also on the question ; but all of these, as indirect 
lines of evidence, fall short of demonstration, and accordingly it is impossible 
to come at present to any decision on the point. For the purposes of 
this discussion, however, we shall proceed on the supposition that all leaves 
of the sporophyte generation originated in essentially the same way, though 
not necessarily along the same phyletic line. 

There are at least three alternatives which may possibly have been 
effective in the origin of a foliar differentiation of the shoot, in any pro- 
gressive line of evolution of vascular sporophytes: (i) That the prototype 
of the leaf was of prior existence, .the axis being a part which gradually 
asserted itself as a basis for the insertion of those appendages ; the leaf in 
such a case would be from the first the predominant part in the con- 
struction of the shoot. (2) That the axis and leaf are the result of 
differentiation of an indifferent branch-system, of which the limbs were 
originally all alike ; in this case neither leaf nor axis would predominate 
from the first. (3) That the axis pre-existed, and the foliar appendages 
arose as outgrowths upon it ; in this case the axis would be from the first 
the predominant part. 

The first of the above alternatives, viz. that the prototype of the leaf 
existed from the first, and was indeed the predominant part in the initial 
composition of the shoot, has been held by certain writers as the basis of 
origin of the leafy shoot in vascular plants. 2 On this view not only is the 

1 Organography, p. 261. 

2 Goethe, "Die Metamorphose der Pflanzen." Gaudichaud, Mem. de V Acad. d. Set., 
1841. Kienitz Gerloff, Bot. Zeit., 1875, p. 55. Celakovsky, " Unters. ueber die Homo- 
logien," Pringsh. Jahrb., xiv., p. 321, 1884; Bot. Zeit., 1901, Heft, v., VI. 



PHYTONIC VIEWS 135 

X. 

whole shoot regarded as being mainly composed of leaves, but some even 
contend that the axis has no real existence as a part distinct from the 
leaf bases. 1 

This view in its general form represented the leafy plant as constructed 
on a plan somewhat similar to that of a complex zoophyte. It has more 
recently culminated in the writings of Celakovsky and Delpino. The former 
in his theory of shoot-segments (" Sprossgliedlehre ") starts from the position 
that the plant is composed of morphological individuals ; the cell, the shoot, 
and the plant-stock are recognised as such. The stock is composed of 
shoots and the shoot of cells. Braun recognised the shoot as the individual 
par excellence; between the cell and the shoot is a great gulf, which has 
not yet been filled ; " between the cell and the bud (shoot) there must 
be intermediate steps, the limitation of which no one has succeeded in 
denning"; the long sought-for individual middle step is the shoot-segment 
(Spross-glied), which is neither leaf only nor stem-segment only, but the leaf 
together with its stem-segment. Now this reasoning appears to involve a 
mistaken method of morphology ; the intermediate step must occur ; we will, 
therefore, discover and define it. The definition of it consists in the draw- 
ing of certain transverse and longitudinal lines partitioning the shoot, lines 
which in the sporophyte have no existence in nature ; the assumed necessity 
of partitioning the shoot into parts of an intermediate category between the 
whole shoot and the cell brings these assumed limits into existence. 

Notwithstanding the ingenuity of the theory as put forward by 
Celakovsky, in the absence of any structural indication of the limits of 
the shoot-segments in the vast majority of cases the theory does not appear 
to be sufficiently upheld by the facts. 

An extreme, and indeed a paradoxical position has been taken on this 
phytonic question by Delpino. As a consequence of his studies on 
phyllotaxis he concluded that the axis is simply composed of the fusion 
of the leaf-bases ; that the leaves are not appendicular organs, but central 
organs; that an axis or stem-system does not exist, and accordingly that 
the higher plants are not cormophytes at all, but phyllophytes. 

The second view, that the axis and leaf are the result of differentiation 
of an indifferent branch-system, of which the limbs were originally all alike, 
has lately been brought into prominence by Potonie. 2 Taking his initiative 
from the branching of the leaves in early fossil Ferns, he recognises the 
frequent occurrence of overtopping (" Uebergipfelung "), that is, the gradual 
process of assertion of certain limbs of a branch-system over others; in 
the branching of Fucoids he finds an analogy for his observations on 
Fern-leaves, and draws the following conclusion, that " the leaves of the 

1 Delpino, " Teoria generale della Filotassi." FY>r ref. see Bot. Jahresbr., viii., 1880, 
p. 118; also vol. xi., 1883, p. 550. 

- Lehrbuch d. Pflanzenpalaeontologie, pp. 156-159. Also Ein Blick in die Geschichte 
a. Bot. Alorph. und d. Pericaulomtheorie, 1903, p. 33, etc. It was, however, suggested 
previously by myself, Phil. Trans., 1884, part ii., p. 605. 



136 THEORY OF THE STROBILUS 

higher plants have been derived in the course of generations from parts 
of an Algal thallus like that of Fucus, or at least from Alga-like plants, 
by means of the overtopping of dichotomous branches, and the develop- 
ment as leaves of the branches, which thus become lateral." Dr. Hallier, 
who adopts Potonie's position, "prefers to draw the comparison with Liver- 
worts, which show a similar sympodial development of a dichotomous 
branch-system. 1 

It seems not improbable that the condition of many branched Fern-leaves 
may have been derived through a process of " overtopping " in an indifferent 
branch-system of the leaf itself. But it lies with Potonie' to show, on a 
basis of comparison of forms more nearly related to them than the Fucoids, 
that the relation of axis to leaf in the Ferns was so derived ; and, further, 
that such an origin is in any way applicable to other foliar developments in 
Vascular Plants, especially in Pteridophytes such as the Lycopods, Equiseta 
and Sphenophylls. I am not aware that this has yet been done. But 
granting that this can be done, the question still remains whether similarity 
of method of branching is any criterion of comparison as to descent. 
For sympodial development of a dichotomous system (and this is all that 
such "overtopping" actually is) has occurred in cases where it cannot 
be held to have resulted in a branching which is foliar ; and of this instances 
can be found without going so far afield as the Fucaceae. If this be so, 
then little value need be attached to the comparison of such branchings 
in plants not nearly allied to one another ; these may be held to be quite 
distinct examples of a general phenomenon of branch-development, without 
the one being in any sense the prototype of the other Such reflections 
as these indicate that the comparison in mode of branching between the 
leaves of Ferns and the thallus of Fucoids, which forms the groundwork of 
the view of Potonie (or between the Ferns and the Thalloid Liverworts, 
as may be preferred by others), are not to be held as more than distant 
analogies ; consequently they are no demonstration of the origin of the leaf 
by a process of "overtopping." 

The view recently advanced by Professor Lignier (" Equisetales et 
Sphenophyllales : leur origine filicineenne commune," Bull Soc. Linn, de 
Normandie, Caen, 1904, p. 93) is analogous to that of Potonie, though 
differing from it in detail. It involves ranking the Lycopod leaf as a 
"phylloid," of the nature of a flattened hair, and comparable to the 
amphigastrium of a Liverwort. The leaf of the Fern, however, is held 
to be a true leaf, or phyllome, derived by differentiation from an indifferent 
system of "cauloids," on which the "phylloids" have become abortive. 
All such hypotheses have critical points in their application ; in the present 
case it lies in the comparison of the Psilotaceae and Sphenophylleae. For 
Lignier regards the leaf-lobes of Tmesipteris as only " phylloids," whereas 
the leaves of the Sphenophylls, and also of the Equisetales, are "phyllomes," 
reduced from the larger-leaved type of the Ferns. The argument is chiefly 
1 Beitriigc z. JHorph. d. Sporophylle n. d. Trophophylls t Hamburg, 1902. 






DICHOTOMOUS THEORY 137 

based on comparisons as to branching and anatomical structure. These 
grounds will not suffice to override the inherent probability that the 
leaves of the Lycopods and Psilotaceae are essentially of the same nature 
as those of the Sphenophylls or Equiseta, and not the consequence of an 
entirely distinct evolutionary history. Moreover, on his own admission the 
" Prohepatic " type, from which Professor Lignier's theory starts, is still wholly 
hypothetical. Further, it may be remarked that the embryology of the 
sporophyte gives no assistance to those who would derive it from a 
dorsiventral thallus. On these as well as other grounds the theory, as 
stated by Professor Lignier, cannot be upheld. 

An essentially similar hypothesis has been enunciated by Tansley (New 
Phytologist, 1907, p. 25, etc.). He contemplates a megaphyllous origin 
of a Fern-like sporophyte from a "hypothetical Archegoniate Alga," which 
showed dichotomous branching : certain branch-systems became specialised 
for assimilatory functions as erect shoots, and assumed radial symmetry, 
while the axis originated by transition through sympodial development of 
the dichotomy to monopodial branching. On this hypothesis the dorsiventral 
symmetry would be the primitive and the radial the derivative state in 
the original sporophyte. The megaphyllous types would be primitive and 
from these the microphyllous would be derived by widespread reduction. 

Putting aside the collateral speculations of Tansley to which exception 
may be taken, such as the homoplastic origin of the archegonia and of the 
spores, as well as of the whole sporophyte in Bryophytes and Pteridophytes, 
and the wholesale resort to reduction in order to explain the origin 
of the ancient microphyllous phyla, there are two points of fact, or of 
absence of fact, which appear specially to oppose his theory : he assumes 
a radial type of construction to be derivative for the sporophyte [and a 
dorsiventral type to be primitive ; but in point of fact, in their individual 
development all sporophytes are originally radial, a condition which has 
probably a very close relation to their production in the archegonium : 
that the dorsiventral state is as a general rule derivative in the sporophyte, 
may be concluded from comparison and shown by experiment (see 
Chapter XVI.). Further, there is no known case of dichotomy in the 
sporophyte, where one branch develops as axis and the other as leaf. 
The known facts derived from living Ferns as well as from the fossils 
point clearly to dichotomous branching of the axis itself and of the leaf 
itself, and to transition from a dichotomous to a monopodial branching 
in the establishment of rachis and pinna. But such evidence is wanting 
in the relations of leaf and axis. It was chiefly the absence of such 
evidence that influenced me in rejecting my own suggestion of origin of 
the shoot from a dichotomous branch-system made in 1884 (Phil. Trans., 
vol. ii., 1884, p. 605): it applies equally to the theory as stated by 
Tansley, which appears thus to break down on the test of fact. 

There remains the third view, which, however, is no new one ; for 
there have not been wanting those who have assigned a more prominent 



138 THEORY OF THE STROBILUS 

place to the axis in the initial differentiation of the shoot. Perhaps the 
most explicit statement on this point is that by Alexander Braun, who 
remarks in his Rejuvenescence in Nature (English edition, p. 107), 
referring to phytonic theories, that "all these attempts to compose the 
plant of leaves are wrecked upon the fact of the existence of the stem as 
an original, independent and connected structure, the more or less distinct 
articulation of which certainly depends upon the leaf-formation, but the 
first formation of which precedes that of the leaves." Unger also, in his 
botanical letters to a friend (No. VIII.), described how "The first endeavour 
is directed towards the building up with cell-elements of an axis" "those 
variously formed supplementary organs which are termed leaves originate 
laterally upon it " and he concludes that " we may [therefore] say with 
perfect justice that the plant ... is, as regards form, essentially a system 
of axes." Naegeli contemplated a somewhat similar origin of the leafy 
shoot as an alternative possibility ; in fact, that the apex of a sporogonium- 
like body elongated directly into that of the leafy stem, in which case 
the axis would be the persistent and prominent part, and the leaves be 
from the first subsidiary, and lateral appendages. In my theory of the 
strobilus in Archegoniate Plants the central idea was somewhat similar 
to this. 1 It may be briefly stated thus : There seems good reason to hold 
that a body of radial construction, having distinction of apex and base, 
and localised apical growth as its leading characters, existed prior to the 
development of lateral appendages in the sporophyte ; the prior existence 
of the axis and lateral origin of the appendages upon it are general for 
normal leafy shoots. The view thus put forward is, indeed, the mere 
reading of the story of the evolution of leaves in terms of their normal 
individual development. 

It is natural to look to the Pteridophytes for guidance as to the origin 
of foliar development in the sporophyte, for they are undoubtedly the most 
primitive plants with leafy shoots. They may be disposed according 
to the prevalent size of their leaves in a series, leading from microphyllous 
to megaphyllous types. I have lately shown that such a seriation is not 
according to one feature only, but that certain other characters which 
have been summarised as " Filicineous " tend to follow with the increasing 
prominence of the leaf. 2 This indicates that such seriation is a natural 
arrangement. Now it is possible to hold either that the large-leaved 
Fern-like plants were the more primitive, and the smaller-leaved, derivatives 
from them by reduction ; or, conversely, that the smaller-leaved were the 
more primitive, and the larger-leaved derivatives from them by leaf- 
enlargement ; other alternative opinions are also possible, such as that 
the leaf-origin has been divergent from some middle type, or that the 
leaves of Vascular Plants may have been of polyphyletic origin. For the 
moment we shall leave these latter alternatives aside. 

Much of the difference of view as to foliar origin centres round the 
^Annals of Botany, vol. viii., p. 343. ^Studies, v., p. 254. 



A SIDELIGHT FROM ANATOMY 139 

question whether originally the leaf was relatively large or small. Those 
who hold that the large-leaved forms were the more primitive will- be 
naturally disposed towards the view of the original preponderance of the 
leaf over the axis, and will favour some phytonic theory ; those who hold 
the smaller-leaved forms to be the more primitive will probably adopt 
a strobiloid theory of origin of the leafy sporophyte. I propose to offer 
some remarks on the relative probability of these alternative views. 

If large-leaved prototypes be assumed generally for Vascular Plants, 
this naturally involves a widespread reduction, since small-leaved forms 
are numerous now, and have been from the earliest times of which we 
have any record. Convincing evidence of reduction of leaf-complexity in 
an evolutionary sequence, supported on grounds of comparison of form 
and structure, and in accordance with the palaeontological facts, has been 
adduced in the progression from Ferns, through Cycado-Filicinean forms 
to the Cycads; and it applies with special force in the case of their 
sporophylls. Ferns, which are essentially shade-loving and typically zoidio- 
gamic, or amphibious, may be understood to have given rise to the 
Cycado-Filices and Cycads, which are more xerophytic, and show that 
essential character of land plants the seed-habit. The case for reduction 
of leaf-complexity seems to be here fully made out, and somewhat similar 
arguments will also apply for other types of Gymnosperms. It must there- 
fore be admitted that extensive reduction of appendages has occurred in 
certain very ancient phyla. 

But while we thus recognise a probability of reduction in certain phyla 
producing relatively smaller-leaved forms, it does not follow that all small- 
leaved Vascular Plants originated thus. On this point the anatomical 
evidence is of importance, as bearing on the origin of the small-leaved 
strobiloid Pteridophytes. Of these (putting aside the Hydropterids as being 
a special reduction problem in themselves) there remain the Lycopodiales, 
the Equisetales, and the Sphenophyllales, which are all cladosiphonic in 
the terminology of Dr. Jeffrey. The question will largely turn upon the 
meaning of this anatomical feature. The cladosiphonic character may be 
held as the anatomical expression of the dominance of the axis in the 
shoot. Here the leaf-trace, is merely an external appendage on the stele, 
which is hardly disturbed by its insertion. This type is seen constantly in 
certain small-leaved Pteridophytes. On the other hand, the condition, 
styled by Dr. Jeffrey the phyllosiphonic, is the anatomical expression of the 
dominance of the leaf over the axis in the shoot. Here the insertion 
of the vascular supply of the leaf profoundly disturbs the vascular arrange- 
ment in the axis, leading to an open communication between the cortex 
and the central medulla at each leaf-insertion. It is characteristic of 
certain large-leaved Pteridophytes, and is seen also generally in Seed-Plants. 
There is thus a probability, supported on anatomical evidence, that the 
seed-bearing plants at large were descended from a large-leaved ancestry, 
and had undergone reduction of leaf-complexity in their descent. 



1 40 THEORY OF THE STROBILUS 

It is a fact of importance that, in the individual life, the one or the other 
anatomical type is usually constant ; but in certain Ferns the progression 
may be traced from the cladosi phonic in the young plant to the phyllosi- 
phonic in the mature, thus suggesting a similar progression in descent, 
viz. that the large-leaved phyllosiphonic Ferns were derived from a smaller- 
leaved cladosiphonic stock. Of the converse, viz. the progression from 
the phyllosiphonic to the cladosiphonic state in the individual life, I know 
of no example among the Pteridophytes, though it is true that there is 
some approach to it in the Marsiliaceae. Thus the anatomical evidence 
indicates a probability that, even in large-leaved Ferns, the cladosiphonic 
was the primitive type ; but that the phyllosiphonic, once initiated, is as 
a rule maintained : this is shown by its persistence in the Seed-Plants, 
even where the leaf has been reduced in size. 

Having thus gained a valuable sidelight from anatomy, indicating 
that small-leaved types were probably primitive, we may now return to 
our central question of the initial relation of leaf to axis. Of the three 
theories already noted, the theory of overtopping as applied to the origin 
of the leaf may, in my opinion, be dismissed, as it is not based upon com- 
parison of nearly related forms, while the facts of embryogeny and of 
leaf-origin do not support it : and further, the sympodial development of 
a dichotomous system, on which it is founded, is a general phenomenon 
of branching, restricted neither to leaves nor to the sporophyte generation. 
As to the other two, the facts, whether of external form or of internal 
structure, seem to me to indicate this conclusion : that the strobiloid 
condition, was primitive for certain types, such as the Equisetales, Lyco- 
podiales, and Sphenophyllales : that in them the leaf was from the first 
a minor appendage upon the dominating axis, and anatomically they have 
never broken away from the cladosiphonic structure which is the internal 
expression of their microphyllous, strobiloid state. That the Filicales and 
"also the Ophioglossales were probably derived from a microphyllous 
strobiloid ancestry, and achieved the phyllosiphonic structure as a conse- 
' quence of leaf-enlargement, this being the derivative rather than the 
primitive condition ; its derivation is even illustrated in the individual 
life of some Ferns. From the Filicales the phyllosiphonic structure was 
probably handed on to the Seed-Plants, and by them retained notwith- 
standing the subsequent leaf-reduction which followed on their adaptation 
to an exposed land-habitat. Thus a strobiloid origin may be attributed 
to all the main types of Vascular Plants. It seems to harmonise more 
readily with the facts than any phytonic theory does. 

A prototype, which was probably a prevalent, though perhaps not a 
general one for the Pteridophytes, may then be sketched as an upright, 
radial, strobiloid structure, consisting of a predominant axis, showing con- 
tinued apical growth, and bearing relatively small and simple appendages. 
On our theory the origin of these appendages in descent would be the 
same as it is to-day in the individual development, viz. by the outgrowth 



ENATION OF LEAF 141 

or enation } of regions of the superficial tissue of the axis to form them, 
and this would occur not simultaneously but successively, the origin of 
the appendages following the continued apical growth of the axis, as it does 
in the developing shoot of the present day. The axis would pre-exist in 
descent, as it actually does in the normal developing shoot. The origin 
of these appendages may have occurred independently along divers lines 
of descent, and the appendages would in that case be not homogenetic 
in the strict sense. Thus there would be no common prototype' of the 
leaf, no morphological abstraction or archetypic form of that part. More 
than one category of appendages might even be produced on the same 
individual shoot, differing in their function on their first appearance. 
Such has perhaps been the case in the Calamarian strobilus, where, as 
will be seen later, the leaf-tooth cannot be readily homologised with the 
sporangiophore. These suggestions will suffice to indicate how elastic a 
strobiloid theory is, and how its application will cover various types of 
construction, even such as are shown by the most complex cones of 
Pteridophytes. 

The objection to a theory of enation will probably be raised that it 
contemplates an origin of new parts rather than a modification of parts 
already present, and that experience indicates the latter as the usual source. 
The reply to this is a double one : first, that the appendages actually 
appear in the ontogeny by enation : a leaf arises as an outgrowth from the 
previously smooth surface of the pre-existent axis : the theory reads the 
descent in terms of the individual life. But secondly, an origin of new 
parts upon a smooth surface of a pre-existent part must necessarily have 
taken place frequently in the formation in isolated genera of emergences 
and prickles, often of large size and with vascular supply. Thus the origin 
of new appendages is not without frequently recurring precedent among 
Vascular Plants. 

An essential feature in the theory of the strobilus is that it involves the 
phyletic pre-existence of the axis. This is a point upon which embryo - 
logical evidence can be adduced, both that of the primary embryology 
and of the continued embryology of the growing shoot (see Chapter XIV.). 

Thus far nothing has been said of the sporangia in relation to this 
theory of the strobilus. It remains to trace the relation of these to the 
appendages. On the above hypothesis the shoot originated from a body 
having a fertile upper region and a sterile base. It is not necessary to fix 
upon any type of sporophyte represented in any living plant as a prototype : 
what is contemplated is an acropetally growing body, with already some 
distinction of a sterile base, and a terminal fertile region endowed with 
apical growth. In two or more types of living Bryophytes the relegation of 
spore-production towards the outer surface is seen, with the formation of a 

1 The term "enation" has long been used in Vegetable Teratology. See Masters, 
"Vegetable Teratology," Roy. Soc,, 1869, p. 443: it connotes the exogenous outgrowth of 
an appendage from a previously vacant surface. 



142 



THEORY OF THE STROBILUS 



sterile central columella : such was probably the case also in the predecessors 
of the strobilus, but the process was more completely carried out, so that 
the spore-formation came to be, as it now is in all Pteridophytes, located 
close to the outer surface. A further step would be the disintegration 
of the sporogenous tissue into- separate pockets or sporangia: of such 
disintegration there is evidence in certain Pteridophytes, but it is exemplified 
in the clearest way in the anthers of various Angiosperms : the condition 
which is actually seen in the anthers of Viscum album, or in the large 
multilocular anthers of Rhizophora illustrates the point (Fig. 72): here 

the numerous, small, isolated loculi 
cover the very considerable surface 
of the enlarged stamen, and develop- 
ment as well as comparison points to 
an origin of these by segregation from 
the normal type of pollen-sacs. The 
outgrowth of appendages by enation, 
from such an apically growing struc- 
ture has been already recognised as a 
probable feature ; if this took place 
either between the segregated loculi 
or below them so as to carry them 
outwards beyond the general surface, 
during its acropetal development, the 
result would then be a strobiloid 
structure with an acropetal succession 
of appendages, such as is seen in 
various Pteridophytes. Sometimes the 
sporangia might be borne in close 
relation to the axis, as in Lycopodium 
or Selaginella, while other Lycopods 
illustrate varying degrees of the carry- 
ing of the sporangia outwards upon 

the appendage. In other cases varying numbers of sporangia are borne 
upon a single appendage, as in the Calamarians and Sphenophylls : and 
according to their form, and their relation to or freedom from sporangia 
divers ranks of these appendages may be distinguished : these matters will 
be discussed in detail later. 

In the hypothesis thus sketched there are several steps which may be 
named as distinct, though actually they may have overlapped : they are 
(i) the differentiation in the primitive sporophyte of a vegetative base,. 
and a fertile upper region having a power of apical growth : (2) the 
relegation of sporogenous cells in the latter to a superficial position : 
(3) the segregation of them into separate pockets or sporangia : (4) the 
enation of the appendages. Every one of these steps has its actual prototype 
among living plants, so that nothing is advanced which is contrary to 




FIG. 72. 

Rhizophora mucronata. Flower in longitudinal 
section. Numerous spherical microsporangia, />, 
in the anther. (After Goebel.) 



ENATION OF LEAF 143 

morphological experience. Therefore the validity of the strobiloid theory 
is not open to a priori objection. The real question is whether those 
processes which are seen to have been in operation elsewhere did actually 
take part in the production of the Pteridophyte strobilus as it is now 
seen? The applicability of the theory to the various known types of 
Pteridophytes will be the true test of its validity. This will be carried 
out in detail in the second part of this work, so far as the very imperfect 
evidence will allow. In questions such as this of the origin of the shoot, 
it is desirable to take the simplest possible reading of the facts as the 
basis of an opinion : on this ground the theory of enation, as accounting 
for the origin of the appendages of the strobilus, seems to be preferable 
to any phytonic theory. It has been remarked that the strobiloid theory 
involves " tremendous morphological assumptions in the way of the origin 
of new organs" (Tansley, New Phytologist, 1907, p. 28, etc.): the only 
assumption, however, which is apparent to the mind of its author, is that 
the order and mode of origin of the appendicular parts in the course of 
Descent has been that which is actually seen in their individual develop- 
ment. They are formed by enation from the axis now, and it is held 
that they originated in the first instance in the same way. 




CHAPTER XII. 



SPORANGIOPHORES AND SPOROPHYLLS. 

THE theoretical position taken up in the last chapter is characterised not only 
by its simplicity but also by its elasticity. It carries with it no obligation 
to . assume that all the appendicular organs should show defmiteness or 
constancy in their disposition upon the axis which bears them, nor even 
that they were all alike in their initial character or function. Now, as a 
matter of fact, an examination of the strobili of such plants as the Psilo- 

taceae, Horsetails, and Calamarians shows that 
irregularities of arrangement of the parts are 
common : it is impossible to reduce the 
arrangement of the appendages in the cone of 
Tmesipteris or of Equisetum maximum to any 
regular scheme (Fig. 73) : the appendages of 
both vary in radial angle and in level of 
insertion. There is also great variability in 
the disposition of the leaves in the genus 
Lycopodium, being sometimes whorled, some- 
times irregularly spiral. It is true that cases 
do exist among the strobiloid Pteridophytes 
which show regularity in the disposition of 
their parts, but in their shoots at large a 
regular disposition of the appendages cannot 

be held to be a general feature. Such irregularities, so far as they ai 
of primary origin, are difficult to explain on the basis of any sympodial 
construction of the strobilus : to a theory of enation, as expounded 
in the previous chapter, they offer no obstacle; for if the appendage 
originated from the surface of the pre-existent axis, as suggested, the] 
might equally well appear in regular positions, or be disposed with greater 
or less irregularity as, indeed, is seen to be the case. 

The comparative classification of those appendages of the strobilus 
which are seen in the different types of Pteridophytes has always presented 




FIG. 73. 

Transverse section through a spor- 
angiferous bud of Tmesipteris. ax = 
axis. f= foliage leaves. I lateral 
lobes. jy = synangia. X2O. 



CLASSIFICATION OF APPENDAGFS 145 

difficulties : of this the Psilotacea~e are a conspicuous example, and the 
analysis of the parts composing their strobili has led to voluminous 
discussions. The difficulties are no less in the Sphenophylls and Cala- 
marians. The presumption upon which morphologists have habitually 
proceeded has been that all the parts are, or at least should be, reducible 
to certain categories, such as axis, leaf, emergence, sporangium these 
being the headings under which the parts of the shoot in the higher 
Vascular Plants are ranged. It is possible by the use of artifices, which 
sometimes appear to be curiously strained, to carry out the classification 
of all the constituent parts of the shoot in Phanerogams into these cate- 
gories. But is the morphologist justified by this measure of success in 
the practice of a somewhat artificial method in assuming that it shall 
always be equally applicable to all Vascular Plants? And further, is it a 
scientific method forcibly to extend the conclusions obtained from the 
study of the higher forms to the lower? The attitude of the believer in 
evolution should be the converse : to examine the lower types with a 
mind untrammelled by conceptions derived from the higher, and a termin- 
ology free to express what is actually seen in the more archaic forms. 
Subsequently his conclusions may be extended to the higher forms. At 
the present day it will seem hardly necessary to put down such simple 
principles as these explicitly ; but doing so finds its justification in the 
habit of thought, still ingrained in the science, of reading the lower Vascular 
Plants in terms of the higher, just as it was done in the pre-Darwinian 
days. From this the mind of the modern morphologist must be entirely 
free. 

The difficulty of reducing the parts of the strobilus in certain Pteri- 
dophytes to the categories above named has already extorted from 
morphologists the adoption of a further term not yet used in reference to 
Flowering Plants. The non-committing word " sporangiophore " is now 
understood to connote a structure which bears sporangia, but is not readily 
referable to the category either of axis or leaf, though it might be included 
under some broad use of the term "emergence." It may contain vascular 
tissue, and be inserted either on the axis or on an appendage. It will 
be the object of this chapter to consider the relations of the sporangia, 
the sporangiophores, and trie sporophylls to one another, and to the axis 
of the whole strobilus, as seen in the various types of Pteridophytes. 

It is a rare thing for sporangia to be borne directly upon the axis 
itself, though there is theoretically no reason against it, but rather the 
reverse. The Lycopodiales include forms which show this position of the 
sporangia, and Selaginella is usually quoted as a case in point (Fig. 74). 
It is true that here the sporangium is inserted on the axis, and springs 
directly from its tissue : it may originate as a swelling quite distinct from 
that which develops into the sporophyll ; but the sporangia are not scattered 
irregularly on the axis, for there is a constant relation of each sporangium 
to the subtending appendage : the sporangium and the sporophyll are in 

K 



146 SPORANGIOPHORES AND SPOROPHYLLS 

the same median plane, and, excepting rare abnormalities, each sporophyll 
subtends only one sporangium: this is seen in all plants belonging to 
the Lycopodiales. But they illustrate various degrees of remoteness of 
the sporangium from the axis, while still retaining the strict numerical 
and subtending relation. Thus Selaginella shows the closest relation of 
the sporangium to the axis ; but the sporangium of Lycopodium originates 
clearly from the tissue of the sporophyll itself (Fig. 75): in some of them 
(Z. Selago) the sporangium may at maturity appear to be thin-stalked 
and axillary, while in others (Z. davatum, Lepidostrobus, and Isoetes) the 
sporangium may extend with a broader base some considerable distance 
along the upper surface of the sporophyll. An extreme condition is that 





FIG. 74. 

Selaginella Martetisii, Spring. Radial 
section of a strobilus, including apex (ap), 
and traversing a young sporophyll (/), and 
sporangium (x). Also another section of 
sporophyll and sporangium, rather older. 



FIG. 75- 

Radial sections through young sporangia of Lycopodium 
Selago. In the youngest the whole sporophyll is shown (I), 
and the axis (st), and it is seen that the sporangium arises 
upon the surface of the sporophyll. X 200. 



of the early fossil Spencerites (Fig. 76), in which the narrow-stalkec 
sporangium is attached some distance from the base of the sporophyll. Il 
is thus seen that while the numerical and radial relations of sporangiui 
and sporophyll are constant, the distance of the sporangium from the axi 
may vary. This arrangement in the Lycopods, which dates back to th< 
earliest fossil records, is certainly the simplest seen in the cones of 
Pteridophytes, and the relation of the sporangium to the axis is habitually 
closer in them than in any other type. 

But other plants, which also have representatives of palaeozoic age, 
bear cones of more complex construction : these present intricate morpho- 
logical problems if the effort is made to classify their parts according to 
the strict categories and the usual successions of axis, leaf, emergence, 
and sporangium ; for instance, the modern Psilotaceae and the ancient 



OF PSILOTACEAE 147 

Sphenophyllaceae raise important questions. Among the former, Tmesipteris 
bears appendages of simple form in the vegetative regio'n ; but the fertile 
appendages are forked at their distal end, and bear on their upper surface, 
just at the point of branching, a bilocular synangium, which has a short 
stalk traversed by a vascular strand (Fig. 77). Various views have been 
propounded in order to read this body in terms of the formal morphology 
of the higher plants : for us, the suggestion would seem to suffice that 




FIG. 76. 
Spencerites insignis. Somewhat diagrammatic radial section of part of the cone, 



Berridge.) From Scott, Progresses rei Botanicae, vol. i. 

the plant is heterophyllous, the vegetative appendages being simple and 
the fertile branched : while to the upper surface of the branched sporophyll 
a sporangiophore is attached with vascular supply and bearing two sporangia. 
In Psilotum the structure is the same, but the number of the sporangia 
is larger. The disposition of the parts in Sphenophyllum majus is again 
very similar to this (Fig. 78) : a synangial group of four to six sporangia 
occupies a position comparable to that of the Psilotaceae on the upper 
surface of a doubly branched appendage ; but these appendages are disposed 



148 SPORANGIOPHORES AND SPOROPHYLLS 

in regular whorls, instead of irregularly as in Tmesipteris. In other Spheno- 
phylls the number of sporangia may be less, and the number of the more 




FIG. 77. 

Tmesipteris tannensis, Bernh. A, habit of a whole plant, pendulous form, showing 
dichotomy. Natural size. -, sporophyll and synangium ; B from the side, C from 
above, D after dehiscence, E from below, all X about 3. .F=rhizome. ^ size, ^trans- 
verse section of stem. X4- (After Engler and Prantl.) 

elongated stalks or sporangiophores, greater : thus in 6". Dawsoni they 
are twice, in S. Rbmeri three times as numerous as the segments of the 



OF EQUISETALES 



149 



verticil : these variations in number of the sporangiophores would be 
difficult to harmonise with any reference to "leaf-segments," as ordinarily 
understood elsewhere; and their disposition suggests the idea of chorisis 
of the sporangiophore similar to that seen frequently in the stamens of 
Angiospermic flowers. 




FIG. 78. 

Forked sporophyll of Spheno- 
phyllum majits, bearing spor- 
angiophore. (After Kidston.) 




FIG. 80. 



of 



Calamostachys. Diagram 
cone in radial section. ax = a.x\s, 
which bears successive verticils of 
bracts (br), and peltate sporangio- 
phores (sp). sm = sporangia borne 
on the sporangiophores. As the 
bracts are alternate with one 
another their upturned tips are 
only shown in every alternate 
verticil. (After Scott.) 




FIG. 79. 

Equisetum maximum, Link. A, the 
upper part of a fertile axis, with the lower 
half of the strobilus. Natural size. b = the 
leaf-sheath. = annulus. jr = stalks of spor- 
angiophores cut off. y = transverse section of 
axis. 2>=sporangioprrores in various posi- 
tions, slightly enlarged. st = stalk. sg-= 
sporangia. s = enlarged distal end. (After 
Sachs.) 



In the Equiseta and Calamarians, spore-bearing bodies of outline not 
unlike those of Psilotaceae are attached directly to the axis itself, and bear 
the pendent sporangia (Fig. 79). They show sometimes almost constant, 
but frequently inconstant, numerical and local relation to the whorls of 



ISO SPORANGIOPHORES AND SPOROPHYLLS 

bracts or leaf-teeth : thus, in the modern Equisetum and in the ancient 
Bornia they occupy the whole strobilus in large numbers, and bracts are 
absent ; in Phyllotheca the fertile spikes are interrupted by occasional whorls 
of vegetative leaves : in Calamostachys the strobilus bears successive whorls 
of bracts, and whorls of sporangiophores alternate with them ; but even here 
they do not show exact numerical correspondence with the bracts, which, 
moreover, alternate independently of them. Further, their longitudinal 




Palaeostachya. Diagram of cone in 
radial section. <z;r = axis, which bears 
verticils of bracts (br) with peltate 
sporangiophores (sfi) in their axils. 
sm = sporangia. (After Renault.) From 
Scott. 





A rchaeocalamites. Part of 
cone showing the axis (<pr) 
in surface view, bearing 
superposed verticils of peltate 
sporangiophores (sp) without 
bracts, sm = sporangia. (After 
Renault.) From Scott. 



FIG. 83. 

Helminthostachys zeylanica.. 
Young spike in oblique profile : 
the primordia of sporangio- 
phores are densely clustered on 
the margin. Magnified. (After 
Goebel.) 



position relatively to the bracts varies, for in Calamostachys they are placed 
midway between the. whorl of bracts, in Palaeostachya in their axils, in 
Cingularia immediately below them. This indefmiteness of relation of 
the spore-bearing bodies to the bract-leaves in number and position, as 
seen among the Equisetales, when taken together with the difference of 
function, points to their being a separate category of members from them 
(Figs. 80, 81, 82). 1 

1 This statement is not in accord with the opinions expressed by Prof. Lignier, which 
will be considered where the Equisetales are specially treated in Part II. 



OF FILICALES 



a. 



In the Ophioglossales the structure of the shoot at large is open to 
various interpretations ; but without entering here into questions which 
will be taken up in detail later it will suffice to mention that in 
Hclminthostachys there are sporangiophores which are broadly similar 
in outline to those of Equisttum, but they are borne in irregularly 
disposed bands on the lateral margins of the fertile spike (Fig. 83). 

It may at first sight seem difficult to bring the very varied disposition 
of the sporangia upon the enlarged sporophyll in modern Ferns into line 
with these examples of spore- 
bearing bodies in smaller-leaved 
types. But it is to be remem- 
bered that in Palaeozoic Ferns 
definite sori were common ; 
they were as a rule of circular 
form, and all their sporangia 
were produced simultaneously. 
The wide-extended sori, such 
as are frequently found among 
the Polypodiaceae, were pro- 
bably of relatively late and 
secondary origin, by extension 
of the sori of the circumscribed 
type. Now, a circular sorus, 
with relatively few sporangia 
formed simultaneously and 
borne upon a more or less pro- 
jecting receptacle, into which, 
as may often be seen, a vascular 
supply extends, differs in no 
essential from such bodies as 
we have been considering. A 
sorus of this simple type is 
seen, for instance, in Kaulfussia, 
which is closely similar to^hat 

rf th<=> Pal-^rwmV ~EWn /Vi>/-A/> Ptychocarpus unitus. Fructification. A, part of a fertile 

trie ralaeOZOlC l<ern, rtycflO- pinnule Q^ surface), showing numerous synangia. B, 

r/irfiuc i/uj/isc /TTirr %A\ Tt- hoc synangia in side view. (A and B X about 6.) (After Grand' 

CarpUl UnttUS ( lg. 84). It has J c> a synangium v in sect ion parallel to the surface of 

Kppri cppn that rrirp> Kp>ariro- tne ' ea f> showing seven confluent sporangia, a, bundle of 

Seen tnat Spore- bearing receptac ', e . ^ its parenchyma ; c, ta^tum; </, spores; e,f, 

bodies mav be borne directly common envelope of synangium. X about 60. (After 

""? Renault.) From Scott's Studies in Fossil Botany. 

on the axis or on the appen- 
dages ; the latter is the case in the Ferns, the chief difference between 
them and the strobiloid forms being that the appendages here are large 
and the sori, or sporangiophores, very numerous. Regarded in this light, 
the Fern-type is not a thing distinct or apart ; the difference from other 
types is mainly one of the degree of development of the sporophyll which 
bears the sori. 





FlG. 84. 



152 SPORANGIOPHORES AND SPOROPHYLLS 

Spore-bearing bodies, or sporangiophores, or sori such as those above 
named, have then the following characters in common : They are out- 
growths of varying length, which bear one or more sporangia: these are, 
when numerous, more or less closely related one to another, and frequently 
synangial : they are usually disposed in a circle round the periphery of a 
disc-like expansion at the distal end of the more or less elongated stalk. 
but other arrangements may be found. A vascular strand usually runs 
through the stalk to its distal end, where it may divide into branches 
which terminate in close relation with the sporangia. The similarity in 
all essentials of structure of the sporangiophores in these early Pteridophytes, 
notwithstanding the diversity of their position, points to the conclusion that 
they are the result of similar response to similar requirements, carried out, 
it may be, in various distinct stocks in the various positions in which 
they are now seen. Their structure suggests that they are simply the 
outcome of placental growth, which has the advantage of securing freedom 
of dehiscence of the sporangia which they bear. In that case there is no 
obligation to hold that they were the result of " metamorphosis " of any 
pre-existent appendage or part. And herein lies the importance of the 
introduction of the term sporangiophore which is now applied to them ; 
for it simply implies the fact that the part so called bears sporangia, but 
does not suggest any view of its further morphological character, nor does 
it impose any limitation upon the position which the sporangiophores 
may hold. 

It remains to consider what relation, if any, subsists between such 
sporangiophores and the " leaves." The difficulty in finding mutual ground 
for discussion of this question lies in the preconceived ideas which the term 
"leaf" carries with it in the minds of many botanists. It is often assumed 
that the vegetative leaf was pre-existent in descent to the appendages of 
the strobilus, the mind naturally translating the successive events of the 
individual life directly into the history of its evolutionary organisation ; in 
fact, the sporophyll has habitually been regarded as a vegetative leaf which 
has become fertile. In the following chapter reasons will be given for 
holding that the converse is nearer the truth, i.e. that foliage leaves are 
often the equivalent of sporophylls which have become in the course of 
descent sterile. If this be so, then signs of the origination of a vegetative 
system are to be sought in the fertile zone rather than the converse. 

A second preconception which is commonly entertained is that " the 
leaf," whether sporophyll or foliage leaf, is a part having a common 
evolutionary origin in all plants in which it appears ; but on grounds 
explained above we shall be prepared to contemplate as possible a 
polyphyletic origin of those parts which are designated collectively 
" leaves." 

A third preconception, which is a common one also, is that those 
bodies which are designated sporangiophores are necessarily of the nature 
of sporophylls, or of segments or parts of sporophylls. Any sporangiophore 



SPORANGIOPHORE A PART SUf GENERIS 153 

attached to an axis would thus be held to be a complete foliar structure, 
and a sporangiophore attached to the upper surface of a bract would be 
recognised as a "ventral lobe" of that bract. But if it be admitted that 
spore-production in the sporophyte was prior to its vegetative development, 
and was a constant phase throughout the evolution of the sporophyte, 
then such a description seems not only unnecessary but highly improbable. 
Moreover, it has been seen that sporangiophores may be present in positions 
which are not those of the normal succession of known vegetative parts ; 
in the endeavour to bring these into line with the customary position and 
succession of vegetative parts, recourse has to be taken to almost fantastic 
explanations. But there is no need for this if the sporangiophore be accepted 
simply to be, what it structurally is, a placental growth. The object of 
the morphologist should be not the forcible reduction of different organisn.s 
to one scheme of construction, but to read in their diverse forms the 
probably diverse story of their origin. This should proceed along the lines 
of the least strained and simplest interpretation. Following these principles, 
the sporangiophore in the Pteridophytes will be held to be a part sift 
generis, itself primitive in its nature, in the sense that it is not the result 
of modification or replacement of any other sort of appendage. 

Certain physiological limitations must necessarily have been operative 
during the transition of the fertile region of any sporophyte from a simpler 
to a more complex condition, such as has been figured to the mind in 
the last chapter. As already pointed out, an increase in spore-production 
is an advantage in homosporous plants, since it increases the chance of 
survival and of distribution. But in any increasing body the formation 
of separate loculi will facilitate the protection and nutrition of the increasing 
mass of spores while young : thus segregation has its biological rationale. 
Projection of the sporangia beyond the surface of the part which produces 
them will facilitate the shedding of the spores, and makes possible those 
mechanical devices which are seen in so many of the Pteridophytes. For 
the protection of the sporangia while young, close juxtaposition of the 
appendages of the strobilus is also important, and illustration of this is 
seen in almost all strobiloid types. But at the same time any projection 
of the spore-producing pa/ts necessitates the conveyance of their nourish- 
ment through a longer distance, and by more restricted channels. Such 
crossing of interests will have tended to keep the appendages which bear 
the spores small, so long as they are themselves not active or essential as 
nutritive organs ; in fact, there would in that case be a tendency to per- 
petuate the strobiloid type. But if the appendages themselves carry on 
efficiently the function of supply of organic material, then there need be 
no limit to their size, provided that the water-supply to them can be 
maintained ; and they may accordingly bear an infinity of sporangia, as is 
seen to be the case in the megaphyllous types. It is in connection with 
these functions of protection and nutrition that the foliar development 
would naturally come into prominence as a feature of the strobilus, and 



154 SPORANGIOPHORES AND SPOROPHYLLS 

this may have taken effect in more than one of the several phyla of the 
early Pteridophytes. 

The relation of the sporangia and sporangiophores to the parts designated 
as bracts, sporophylls, or leaves is habitually, though not always, a close 
one. In the simpler strobiloid - forms the leaf either subtends the spore- 
producing member, or the latter is borne upon its upper surface, commonly 
in a median position. The biological importance of this probably lies in 
the protection which is afforded, and in the ready supply of nourishment 
in cases where the leaf is an effective organ of assimilation. But it is an 
error to assume that there is any obligatory or constant relation for plants 
at large between the spore-producing members and the leaves. This is 
shown, first, by the fact that sporangiophores, even in very early fossils 
s.ich as Bornia, may exist independently of the subtending leaves ; and 
secondly, that when associated with leaves they may vary greatly both in 
numerical and local relation to them, even within near circles of affinity : 
this is seen in the Sphenophylleae with special clearness. Such examples 
taken from early fossils teach that the spore-producing members show a 
high degree of independence from the sporophylls. For the present these 
general remarks must suffice : but later, when the sporangiophoric Pterido- 
phytes have been described in detail, we may attempt some more exact 
recognition of the varying relations which existed between the sporangio- 
phores and the sporophylls in early strobiloid types. 

In this connection the question may be raised whether sporangiophores 
and leaves have always been distinct categories of parts : whether leaves 
or foliar parts have ever developed into forms resembling sporangiophores. 
In the case of the Cycads there is little doubt that the parts usually 
designated female sporophylls or carpels are reduced foliar structures : it 
is shown on the .basis of comparison that their form, so like that of many 
sporangiophores, has been attained by a process of reduction, and thus 
they may be held to be homoplastic with the primitive sporangiophores 
of Pteridophytes. 

Such considerations as these will deter the morphologist from any precise 
definition of the categories of parts borne upon the strobili of early Pterido- 
phytes according to experience derived from study of the Phanerogams. . 
There is indeed no reason to assume that there was any initial uniformity 
of the development such as would lead to their always falling into 
strictly definite categories. Greater uniformity is, however, found among 
the higher forms, and it is this uniformity which has led to the establishment 
of those old morphological categories which are found to fit so ill upon the 
lower Vascular Plants. Each plant-type may be held to have worked out 
its own progressive development, while biological conditions common for 
them all would tend to reduce them to some common scheme. Such 
constancy as appears among the parts of the higher plants would then 
have been achieved by gradual evolution of order from beginnings which 
were less constant : and as a matter of fact the exceptions from that 



THEIR RELATIONS NOT CONSTANT 155 

order of disposition of the parts, or from that structural character which 
has been held as typical, are chiefly found where they would on this view 
be theoretically probable, viz. in the Homosporous Pteridophytes. This 
seems to be the natural way of regarding the various types of strobilus 
seen in early Vascular Plants : it is certainly more satisfactory than any 
attempt forcibly to reduce them to conformity with categories based upon 
the study of those plants which represent a later phase of evolution. 



CHAPTER XIII. 

ON THE RELATIONS BETWEEN THE STERILE AND 
FERTILE REGIONS IN THE SPOROPHYTE. 

FROM the days when Morphology first arose as a branch of the science 
of Botany, the relations between the sterile or vegetative region of Plants, 
and the fertile or reproductive have been the subject of enquiry. Originally 
the question presented itself as one of simple comparison of those regions 
in the Flowering Plants, in which they are clearly differentiated one from 
another : the basis of the comparison was that of their external form, 
with the idea behind it of some degree of unity of plan in the construction 
of the two regions. At the present time the enquiry involves the direct 
question of their physiological relation, but it also extends to the indirect 
historical problem of their genetic relationship. This can best be approached 
by comparison of forms lower in the scale of development, such as the 
Pteridophytes, in which the differentiation is less complete than it is in 
the Flowering Plants. 

From a physiological point of view, the necessity of a due balance 
between the sterile and fertile regions in the case of any fully differentiated, 
self-supporting organism is readily grasped; for the material required to 
build up the strobilus or flower to the point of maturing its spores must 
be derived from an adequate development of the vegetative organs which 
produce it. It is naturally otherwise in sporophytes which are not self- 
supporting, or only partially so, as in the Liverworts and Mosses : also 
in the case of parasites and saprophytes ; but the latter, as derivative or 
secondary conditions, may be put aside when we discuss the adjustment 
of balance between the two regions in its evolutionary aspect. The 
indirect historical question is less readily tangible, but in its solution the 
sources of nutritive supply must be steadily kept in view throughout the 
comparative study of the lower and simpler sporophytes. 

The fact that there is frequently a tendency towards extended production 
of spores in the Homosporous Archegoniatae has been brought forward 
repeatedly in previous chapters, where also the racial advantage which 



GOETHE'S PROGRESSIVE METAMORPHOSIS 157 

follows upon it has been sufficiently recognised. The evidence that this 
tendency actually exists is to be found in the fact, illustrated in so many 
plants, that more numerous spores are habitually initiated than the plant 
is able to bring to maturity. The powers of nutrition impose the actual 
limit of the output of spores in any specific example, and any increase 
of the vegetative system will therefore result in an increased capacity for 
producing mature spores. Where the vegetative region extends so as to 
increase the powers of nutrition, it commonly happens that the initiation 
of potential spores still keeps in advance of such increased supply, and 
so the two seem to advance together. In the present chapter various 
examples from among the Archegoniatae will be examined from this point 
of view : upon these some idea may be based of the general methods of 
progression of the sporophyte, from its less differentiated state towards 
that seen in the Flowering Plants, where the vegetative and reproductive 
regions are clearly distinct, though their construction still shows a funda- 
mental similarity of plan. But before this is entered upon, it will be 
well to clear the ground by consideration of the earlier theoretical views 
on the relation of these two regions of the plant-body. 

Kaspar Friedrich Wolff laid the foundation for a comparative view of 
the appendages of the Higher Plants. In his Teona Generationis, published 
in the latter half of the eighteenth century, he propounded the thesis 
that " in the whole plant, the parts of which differ so extraordinarily from 
one another at first sight, there is nothing to be found on mature con- 
sideration but leaves and stem, for the root belongs to the latter." For 
him all the appendages were of foliar nature. The modifications which 
appear in the parts which compose the flower arose, in his view, from 
the gradual waning of vegetative power, or "vegetatio languescens" as he 
called it ; their development constantly diminishes the longer the vegetation 
is continued, and finally ceases altogether ; consequently the essence of all 
these modifications of the leaf lies in their incomplete development. 

It is but a slight step from ideas such as these to the doctrine of 
Metamorphosis as introduced by Goethe in 1790. He assumed an ideal 
fundamental organ, from which the different leaf-forms in any one of the 
higher plants could be regarded as derived. He designated as " Meta- 
morphosis " that process by which one and the same organ presents itself 
to us in various modifications. This metamorphosis may be of either 
of three kinds : regular, irregular, and occasional. Of these the regular 
or progressive metamorphosis, with which we are specially concerned, is 
that illustrated in any normal Flowering Plant by the progression from 
the cotyledons through the foliage leaves to the flower with its successive 
series of parts. But, as Sachs points out in his History of Botany (Engl. 
Ed., p. 156), Goethe sometimes used the word "Metamorphosis" in its 
literal sense, as meaning an actual change in the organs arising from a 
transmutation of the species ; sometimes his meaning was an ideal one ; 
for, regarding the way in which cotyledons, foliage leaves, bracts, sepals, 



158 STERILE AND FERTILE REGIONS 

petals, etc., originate on the stem, they were all included under the one 
general idea of "leaf." In the words of Sachs, Goethe's doctrine could 
only make its way to logical consistency and clearness of thought by 
deciding for the one or the other meaning of the word : he must either 
assume that the different leaf-forms, which were regarded as alike only 
in the idea, were really produced by change of a previous form a 
conception that at once presupposes a change of the species in time : 
or he must entirely adopt the position of the idealistic philosophy, in 
which the idea and the reality coincide. In this case the assumption 
of a change in time was not necessary : the metamorphosis would remain 
an ideal one, a mere mode of view ; the word leaf would then signify 
only an ideal fundamental form, from which the different forms of leaves 
actually observed may be derived, as De Candolle's constant species, from 
an ideal type. 

Though Goethe did not himself decide finally for either of these 
alternatives, the introduction of a theory of Descent, and a general belief 
in transmutation of species, went far towards clearing away any such 
ambiguity. In its light the facts seemed to point definitely towards a 
conception of a real transformation, and this point of view came into 
prominence pari passu with a better knowledge of the lower Vascular 
Plants, where leaf-differentiation is less fully carried out and gradual transi- 
tions are to be seen between vegetative leaves and sporophylls. Accordingly, 
it seemed to be the plain and simple reading of the facts to accept the 
metamorphosis as a change which had actually been effected in the course 
of descent. The natural progression shown in the life of the individual 
seemed to be that already described as progressive by Goethe : it was 
natural to accept this in terms of the theory of descent as progressive 
in the race also. On this basis the floral appendages would be held to 
be essentially foliage leaves, but altered in character to subserve propa- 
gation ; and the pollen-sacs and ovules which they bear accessories which 
are added to the already existent foliar parts. The experience of zoologists 
had its influence in apparently confirming this position. The analogies 
between the two organic kingdoms are at many points so close that the 
general conclusions of the animal embryologists seemed readily applicable 
to plants also. If the ontogeny of the higher animals is found often to 
recapitulate the history of the race, should not the same conclusion apply 
also to the higher plants? Moreover, such a view presented itself as a 
mere continuation of the theoretical opinion of Goethe : the progressive 
metamorphosis which he recognised would figure, accordingly, as a principal 
feature in the evolution not only of the individual but of the Yace. Thus 
regarded the sporophyll of the individual plant would be an altered 
foliage leaf, and its origin by descent would be the same : the difference 
of their development would then lie in the presence of the sporangia, 
which brings correlative restriction of the foliar development in its train. 

This position may seem satisfactory so long as the Higher Vascular 



GOETHE'S PROGRESSIVE METAMORPHOSIS 159 

Plants alone are considered, or if sporangia are not regarded too scruputously 
from an evolutionary point of view, and if it be assumed that they may 
In- and have been habitually generated at large in the course of descent 
upon pre-existent foliar organs. If these points be granted, then it might 
be possible to retain Goethe's progressive Metamorphosis as the basis of 
an evolutionary story applicable to the Higher Plants. As a matter of 
fact, Botanists continued to analyse and describe the flowers of the Higher 
Plants in this way for a whole generation after the Origin of Species 
had been published. The flower was habitually regarded as the result 
of metamorphosis of a foliage shoot. Though the point was not always 
put into direct terms, the underlying assumption was that a conversion of 
vegetative parts into propagative parts takes place in the individual : that 
sporangia originated sporadically in descent, as they seem to do in certain 
cases now, and that such changes as are seen in the development of 
the individual had their place also in the history of its evolution. But 
increasing knowledge of the life-cycles of the lower forms, and of their 
comparison one with another, was meanwhile leading to sounder views of 
the origin of the higher Vascular Plants. Alternation of generations 
became gradually a more exact factor in the morphology of the last half- 
century. It seems no longer possible to look upon the Vascular Plant as 
a primary entity, as it was held to be in the time of Goethe. The 
sporophyte generally, and consequently the plant-body of all the Higher 
Plants which is a sporophyte, must necessarily be held to be secondary 
by all those who recognise antithetic alternation as a constant feature in 
descent of the Archegoniatae : for them the story of origin of the sporo- 
phyte must affect the interpretation of its parts. 

A fundamental question of method in morphology is involved in this 
discussion, viz. the question of the validity of conclusions based on 
observations of the ontogeny as against the well-founded conclusions of 
phylogeny. It will now be generally agreed that, provided the conclusions 
as to phylogeny be sound, they should have the precedence over those 
based on observation of the individual life. But in the practice of the 
middle part of last century it was customary to act in the opposite sense, 
and to take the successive events in the story of development of the 
individual as the basis of morphological history : such views on descent 
as are based on comparison were often left out of account or given only 
a second place. If this latter principle be adopted, then conclusions 
harmonising with Goethe's progressive metamorphosis will follow, and the 
sporophyll may be accepted as an altered foliage leaf; but if precedence 
be given to the results of a broad comparison, then a converse conclusion 
will necessarily appear the more probable. 

But there is also another question involved in Goethe's view of " pro- 
gressive metamorphosis," that of the origin of the sporangia which appear 
in the strobilus or flower. The assumption that sporangia can be formed 
indiscriminately upon pre-existent vegetative parts was at the back of 



i6o 



STERILE AND FERTILE REGIONS 



Goethe's theory : it must be enquired whether this is a justifiable assump- 
tion. This question has not been too exactly scrutinised by his followers, who 
translated his "progressive metamorphosis" into terms of an evolutionary 
progression. The basis for the assumption was primarily the succession 
of events as seen in the individual life of the higher plants ; but a 
certain laxity of view was further encouraged by the irregularities of 
number and position and of time of appearance of the sporangia in the 
Leptosporangiate Ferns. These plants were accorded an undue prominence 
in the early study of Pteridophytes, and for long the belief was held that 
they were the prototypes of all Vascular Plants. But it is now sufficiently 
clear that the Leptosporangiate Ferns are relatively late derivative forms, 
and that the types of Ferns of the primary rocks were more precise and 




FIG. 85. 

Botrychiuth Lunaria. Sterile laminae, which occasionally produce sporangia (s/>) on 
certain pinnae, and have partly or wholly assumed the form of the fertile spike : /in 
B and C is the fertile spike itself. Natural size. (After Goebel.) 

exact in the arrangement and in the time of origin of their sporangia. 
Such precision is seen in higher degree in the Calamarians and Spheno- 
phylls, and it is specially prominent in the Lycopods. All of these are 
types of quite as early, probably of even earlier, geological history than 
the Leptosporangiate Ferns. Accordingly it may be held that in the 
earliest Vascular Plants which we know the arrangement, time of appear- 
ance, and number of the sporangia showed some degree of definiteness, 
and were in some cases very precise. It cannot be denied that accessory 
sporangia may at the present day appear in some cases where none are 
normally present : conspicuous examples are those described by Lang in 
apogamous Ferns (compare Fig. "35), while a less bizarre case is that of 
the sporangia which appear on the usually sterile leaf of Botrychium 
Lunaria (Fig. 85) : abnormal flowers of Phanerogams also provide 
numerous examples of sporangia not produced in the usual order or 
position. The question is whether the existence of such cases at the 



ORIGIN OF STERILE REGION SECONDARY 161 

present day will justify the assumption that in the evolutionary story 
sporangia originated indiscriminately upon pre-existent vegetative organs. 
My own opinion is that it does not, for a careful examination of such 
cases and comparison of them with the general type to which the plants in 
question belong shows that they are exceptional, if not indeed of the nature of 
monstrosities. It is clear that promiscuous formation of sporangia in 
present-day forms is possible, and that it does at times occur, but it 
does not follow that this was a general mode of their origin in past 
times. 

An essential fact bearing upon the question in point is that spore- 
production is a constantly recurring event in Archegoniate Plants. There 
is good reason to believe that it has found its place in every normally 
completed life-cycle throughout their descent. Cytologically it is now 
seen to be the natural complement of the sexual process. Taking all 
types of Archegoniate Plants into our view, including the more recent 
Flowering Plants as well, there is reason to believe that spore-production 
was the initial function of the sporophyte, and that it has been continued 
and repeated throughout descent. If this be admitted, how can the 
strobilus, or the flower the part bound up with that primary function 
of spore-production be the result of metamorphosis of a vegetative shoot, 
the leading fr ction of which is secondary? The conclusion to be derived 
from broad comparison will be the direct converse : viz. that vegetative 
parts in the sporophyte have originated by change of parts originally 
fertile. 

But in order to carry conviction that this conclusion is correct, it will 
be incumbent on those who hold it to bring forward evidence bearing on the 
origin or increase of the vegetative system, which we see at the present 
day preceding spore-production in the history of the individual life. It 
has already been shown in Chapter VII. that sterilisation of individual 
sporogenous cells, that is, their conversion into cells having a vegetative 
function, is common. It is found in the sporangia of Vascular Plants, 
but it is in the sporogonia of Bryophytes that it has been recognised as 
specially effective in adding to the vegetative system. The sporogonium 
of Aneura (Fig. 86) has already served as an example, while reference to the 
writings of Goebel (Organography, pp. 326-329, Engl. Ed., p. 103), shows 
how fully sterilisation has already been realised, and accepted as a source 
-of increase of the vegetative system in the Bryophyta. Similarly, in 
Vascular Plants it has been shown above, that sterile cells of a sporo- 
genous group may be converted into vegetative tissue of a septum. Such 
examples indicate how sterilisation of individual cells may be effective in 
increasing, and perhaps in the first instance even in originating, the vege- 
tative system. 

A second factor, which has been specially effective in contributing to 
the increase of the vegetative system in the more differentiated types of 
sporophyte, is the abortion of sporangia, or of sporangium-bearing parts. 



162 



STERILE AND FERTILE REGIONS 



As this in my opinion has not yet been accorded its proper place in the 
evolutionary story, I propose to consider it at some length. 

Imperfectly developed parts have played an important role in arguments 
on Evolution. On the Zoological side especially they have been used 
as weighty evidence. Similarly, in Botany they have been the basis of 
discussion : in the morphology' of the flower, abortive stamens, carpels, 
pollen-sacs, and ovules have been cited as foundations for elaborate 
argument. Where present in normal position the existence of an abortive 
stamen or staminode has been habitually held to be sufficient indication 
of the previous existence of a fully developed stamen in the ancestral line ; 
and on such evidence natural affinities have been traced and accepted* 






FK;. 86. 

A, median section of young sporogonium ol Aneura ambrosioides. The internal mass 
of cells of the - sporogonial head ("archesporium ") is already differentiated so as to 
indicate the sterile elaterophore, and the outer fertile region. B, the same, older : the 
indications of sterilisation have extended outwards, and it is only the peripheral fringe of 
cells (shaded) which will be sporogenous. C, transverse section of the same. X 150. 

usually without question. But floral morphology has gone further : com- 
parative study has led to the conclusion that in certain ancestral lines 
of descent parts have existed, which in the individuals of the present day 
are entirely unrepresented by any vestigial growth. This condition of 
complete disappearance of a part or parts has been styled "ablast," as 
distinct from "abortion," where the incompletely developed part has an 
objective existence. Eichler maintained that the conditions distinguished 
as " abort " and " ablast " are not essentially different in kind, but only 
differ in degree. He points out that abortion itself is not susceptible of 
objective proof, and it may be remarked incidentally that it is this fact 
which has prevented the full recognition of the part which it has played 
in the origin of the sporophyte. Speaking of the relations between partial 
abortion, where a vestigial structure is present [" abort "], and complete 
suppression ["ablast"], Eichler remarks (Bluthendiagrammt, p. 52) that in 



ABORTION OF SPORANGIA 163 

cases of abortion " objectively we- see nothing more than that cell-divisions 
occur, that a rudiment appears; thus strictly speaking we observe that 
something develops, not that something is reduced : this may become a 
gland, an emergence, or what not. It is comparison, and usually the 
comparison with other species and genera, etc. that is, the type-method, 
which teaches that it is a reduced organ, and what is its special category. 
Whenever the same comparative method leads even to the assumption 
of a complete suppression, where no rudiment of the organ is seen with 
the bodily eye, in my opinion that is, in point of fact, no more than one 
step further along the same course." This is the position which should 
be the foundation of a correct view as to abortion, or even complete 
suppression of parts : it is currently accepted, and put in practice in the 
morphological treatment of the Angiospermic flower, and it is now high 
time that it should be applied equally to the Pteridophytes, where it has 
probably played a very important part. In the Pteridophytes too little 
attention has hitherto been paid to such subjects, and notably observations 
of arrest of sporangia, or of spore-producing organs, have been neglected. 
It is the isolation of many of the genera, and the paucity of species in 
some of the most important of them, which has stood in the way of 
their detailed comparison in this respect, and consequently arguments from 
arrest have not taken their proper place in the morphology of the 
Pteridophyta. But the argument to be founded on an imperfect sporangium 
at the base of a strobilus of Lycopodium, or on the abortive fertile spike 
of an Ophioglossum seated in the position normal for the fully developed 
part in other individuals, species, or allied genera, is precisely the same 
as that on an imperfect pollen-sac or ovule, or on a stamen or carpel in 
Flowering Plants. Further, a comparison as regards the presence or absence 
of spore-producing parts in species evidently related to one another may 
lead to the conclusion that sporangia entirely unrepresented at the 
present day were probably borne upon ancestral forms : the line of 
reasoning being the same as that in cases of hypothetical complete sup- 
pression of floral parts. It will presently be shown that such hypothetical 
suppression of spore-producing parts may be held accountable for changes in 
balance of the vegetative and propagative regions in the Pteridophytes, and 
be recognised as having Ted to an increasing prominence of the vegetative 
system in the course of their evolution. 

The Lycopodinous type, being represented by numerous species of 
essentially similar construction, lends itself well to such comparative 
treatment, while the comparison is the more pointed owing to the definite 
relation of one sporangium to each subtending leaf, which arrangement, 
with very few exceptions, is the constant rule for the fertile regions of 
these plants. In all known Lycopodinous types a sterile leafy region, 
of greater or less extent, precedes the fertile region in the life of the 
individual plant. In many species of Lycopodium, and especially in those 
which have the vegetative and fertile regions less clearly differentiated, 



10. 1 STI'kli K AND I'KkTILK KKC.IONS 

the initial vegetative stage is soon closed by the appearance of sporangia 
in the axils of the leaves; hut alternate Irrlile ;ind sterile- /ones, merging 
into one another imperceptibly as regards lorm, succeed one another at 
irregular intervals throughout the upper region of the plant. This may 
be styled the Scfago condition, as it is seen conspicuously in /jv/'/W////// 
Selagp (Frontispiece). In others, and especially in those in which Un- 
fertile region appears in the form of a definite terminal strobilus, the 
initial vegetative phase is more extensive, though still essentially similar to 
the strobilus in its construction ; there is, however, a prevalent dilfcrenec 
of form between the sterile and the fertile leaves, but the relation <>i UK- 
sporangia to the latter is the same as in the Sc/w form. It seems 
natural to conclude that the Sclago type is the more primitive, and t In- 
definitely strobiloicl type the derivative. 

The question in cither of these cases is, what genetic relation has 
existed between these sterile and fertile regions which are so similar in 
plan, but differ in the absence or presence of the sporangia. Tin- 
clew is given by examination of the basal limits ot the lertile zones in 
either case; for here, at the point of transition from the sterile to Un- 
fertile, imperfectly developed sporangia are often found, occupying tin- 
place normally taken in the fertile region by those fully developed. 
Applying to these the same argument as in the case of an imperfectly 
developed ovule or pollen-sac in an Angiospeimie flower, they will be 
held to he vestigial representatives of sporangia, noimally present, and 
actually initiated, but not completely developed, Passing from these 
to the vegetative region, where no vestigial sporangia are present, t hough 

UK- arrangement, character, .ind in tin- St'/iig<> type even the loim of 

the leaves is the same as in tin- fertile region, the question arises 
whether these are not essentially sporophylls, in which tin- sporangia 
are completely suppressed? The result of a broad consideration of the 
question will be an answer in the affirmative. The facts indicate that 
in the simple Lycopod type progressive sterilisation has been effective, 
and that it has involved the partial abortion, or even the complete 
<uppression, of whole sporangia; the result is that leaves originally in 
the race fertile have become sterile, and have thus contributed to the 
enlargement of the vegetative region. Tin- fact that the Selago condition 

is seen represented in certain Lycopod fossils of the Coal period is 
important evidence of the validity of [this progression as an early evolu 
nonary factor. 

Such sterilisation as that believed to have occurred in / r.v/W////// in 

the course of descent has been experimentally induced by Goebel in 

:;inel/a> 1 by cutting off young strobili, and treating them as cuttings; 

the- sporangia of the upper legion ahoited, and the sporophylls of the 

newly formed parts of the shoot developed as foliage leaves. Thus the 
result theoietieally contemplated may follow from experiment 

P- (l ^7- 



THK '^SELAGO" CONDITION [65 

With this progressive sterilisation, and the consequent UlCreMC of the 
vegetative region, tlu- apical giowth of tin- axis krcps pace: it secures the 
initiation of additional sporophvlls and sporangia to take- the place ol 
those' transformed or aborted, and as there is no theoretical limit to the 
apical growth ami branching, in such species as L. Selago the halanee 
Can constantly hi- readjusted between tin- sterile and the fertile regions. 
This combination of sterilisation and continued apical growth provides, 
iii a sense, a forward impulse, and ii will be effective up to the limit of 
physiological supply. That it is so is seen in the fact that at the apex 
ot am Lvcopod slrohilus imperfect sporangia are found, which arc to 
he recognised as Supernumeraries, showing the continued exuberance ol 
initiation beyond the power ol the plant to bring to complete maturity. 
\\ e thus acquire, the conception of a zone of reproductive activity or in 
the Selago t\pe it may be several interrupted zones limited below by 
parts which are to he held as vestigial, and above by parts which are 
supernumerary. l',y comparison of living species of AmyW/Ww it is seen 
that the fertile zone is not alwa\s located at the same- level on the plant: 
it is sometimes preceded by a shorter, sometimes by a longer vegetative 
region. There has probably been a phvlogcnctic shifting of the fertile 
/one or /ones: the biological significance of this is obvious, for any 
advance of the fertile /one to a higher point, by abortion of sporangia, 
while the spoioplulls remain in a vegetative capacity as foliage Lea 
provides vegetative region be-low for purposes of nutrition. Such 

a manner of advance has probably been effective in the evolution of the 
Lycopods as we now see them. 

If the Lycopods stood alone in showing such features as those described 
the facts would be of limited inteiest, but they do not; conditions essentially 
similar are si-en in the sporophytes of other Vascular Cryptogams, though 
\arying in detail. The mature plant of Jsoctcs is virtually ol the .SV/r/^v 
type: it bears fertile and sterile leaves intermixed: vestigial representatives 
of sporangia are lomul in the position normal for sporangia upon many of 
the sterile leaves ; further, the probability that the leaves actually sterile 
are so by suppression is as strong here as in the case of Lycopodium 
.SV/r/v". The mature plant is preceded by an embryonic vegetative ph.; 
with leaves bearing no sporangia; but after the first sporangia appear, the 
whole plant m. iv be regarded as a strobilus, imperfectly differentiated, as 
in the Scla^o type, into fertile parts and parts sterile by abortion or by 

complete suppression. 

Similarly, in the 1'silotaceae, the Selago condition, with irregular alter- 
nation of sterile and fertile /ones, is seen in both rsilithnn (Fig. 87) and 
Twt'.\rf>ft-ris. \\hile imperleet lynanglS are found about the limits ot the 
fertile regions. There is, howe\ei, a broad difference in form between the 
simpler sterile appendages and the more elaborate fertile ones; in this 

respect the differentiation of sterile and fertile parts has proceeded further 
than in the Lyeopods. In the allied fossils, the Sphenophvlleae, there 



i66 



STERILE AND FERTILE REGIONS 



is, as a rule, a definite strobilus, which is fertile throughout ; but in .S. majus 
.this is not clearly differentiated by form either at base or apex from the 

vegetative region. Such a condi- 
tion shows an interesting analogy 
with the less differentiated states 
of Lycopodium. 

Among the Equisetales, Equi- 
setum and Bornia have, as a rule, 
a definite strobilus, composed 
entirely of sporangiophores borne 
on an axis, and clearly marked off 
from the vegetative region which 
precedes it in the ontogeny. But 
in the Calamarians, as also in 
the fossil known as Phyllotheca, 
sterile leaf-sheaths are interspersed 
between the sporangiophores, a 
condition occurring also as an 
occasional abnormality in Equi- 
setum (Fig. 88). The morphology 
of these cases will be more fully 
discussed below; meanwhile it may 
be held that while in Equisetum 
and Bornia the differentiation of 
the fertile strobilus from the vege- 
tative region is more distinctively 
marked, Phyllotheca or the Cala- 
marians show some degree of 
analogy with the Selago condition 
seen in Lycopodium. 

The Ophioglossaceae provide 
clear cases where the argument 
of abortion leading to complete 
suppression will apply ; for various 
degrees of development of the 
fertile spike may be found borne 
upon the fully formed leaf, from 
that fully matured to small 
vestigial parts which do little more 
than mark the place where the 
normal spike would be inserted ; 

in other cases again the spike is entirely unrepresented. . The facts here 
correspond to those in Lycopodium SeJago or in Isoetes, except as regards 
the size and number of the parts concerned. Such a condition in an 
Angiospermic flower would certainly be interpreted as abortion, and the 




FIG. 87. 

Shoot of Psilotum. Natural size. Showing " Selago " 
condition in the bifurcate branch-system : the base is 
vegetative : then follows a zone bearing synangia, then 
a vegetative zone, and higher up a second fertile zone. 



THE "SELAGO" CONDITION 



167 



incomplete parts where present as vestigial, and a similar conclusion seems 
justified for the Ophioglossaceae. It may thus be held that in the mature 
plant of the Ophioglossaceae all the leaves are potentially fertile : the 
sterile foliage leaf is merely the part which remains when the spike is 
abortive, and its genetic relation to the fully matured sporophyll is the 
same as that of the sterile to the fertile leaf in Z. Selago or in Isoetes. 

There remains for consideration from this same point of view the large 
series of the Ferns. Notwithstanding the preponderant size of their leaves, 
and the wide distribution of the sori and sporangia over their large surface, 
they should still be studied in the 
same way as other Pteridophytes: their 
difference of conformation should not 
be allowed to affect the recognition of 
such similarity in the relations of the 
vegetative and propagative parts as may 
^xist between them and the smaller- 
leaved forms. Since the relation of 
leaf to axis is essentially the same in 
Ferns as in other Vascular Plants, the 
whole shoot may be held as equivalent 
to the shoot, for instance, of an Isoetes; 
and this aspect of it may be maintained 
equally in those cases where the axis is 
short and the leaves crowded upon it, 
-and also in those where the axis is 
elongated and the leaves isolated at 
long intervals apart. Maintaining this 
point of view of the shoot as a whole, 
there is in the ontogeny of the Ferns 
a preliminary vegetative phase, which 
may be of varying extent ; subsequently 
the fertile phase begins. The broad 
relations of the two phases are thus 
the same as in other Pteridophytes. 

The fertile region in Ferns is imperfectly differentiated, and it is in this 
respect comparable with those imperfectly differentiated forms which show 
what has been called the Selago condition. But the matter is further 
complicated by the fact that in many Ferns the differentiation does not 
involve whole leaves, but only parts of them ; the large Fern-leaf, in fact, 
does not always behave as one unit, but the differentiation of sterile 
and fertile regions may involve only parts of the individual leaf, not the 
whole. 

Taking into consideration first the simpler case, where whole leaves 
are differentiated either as sterile or fertile, examples are seen in such 
cases as the common Hard Fern (Bkchnum boreale) or in the Ostrich 




FIG. 



Phyllotheca. Zigno. A , Ph. equisetiforinis 
from Rovere di Velo, near Verona. Z>, inflores- 
cence from Siberia, placed by Schmalhausen with 
Phyllotheca. (After Solms.) 



i68 



STERILE AND FERTILE REGIONS 




FIG. 89. 

Onoclca Struthiopteris, Hoffm. a, fructifying plant, much reduced, with most of the 
sterile leaves removed ; c, d, lower and upper portions of a sterile leaf; e, lower part of a 
fertile leaf, b, c, two-thirds natural size. (From Rab. Krypt. Flora.) 



THE "SELAGO" CONDITION IN FERNS 169 

Fern (Onoclea Struthiopteris) (Fig. 89). There may be some degree of 
regularity in the succession of sterile and fertile leaves, which may be 
correlated with season ; thus in Blechnum boreale the leaves first expanded 
in the spring are sterile, and they are followed by a series of fertile leaves. 
The condition of the shoot as a whole is, in fact, comparable with that of 
Lycopodium Selago or of Isoetes, with their successive sterile and fertile zones. 

But the commoner case for Ferns is that where leaves are not sterile or 
fertile as a whole, but many or even all the leaves of the mature plant are 
fertile at least in part, and frequently show a correlative reduction of area 
as compared with the rest of the leaf, which is sterile. In the distribution 
of the fertile and sterile parts of the individual leaf there is great diversity, 
and differences may be seen in species of the same genus, or even in 
individuals ; thus in Osmunda regalis the lower parts of the fertile leaf 
are broadly expanded and sterile, the apical region is fertile and correlatively 
exiguous ; but in O. javanica the fertile region extends irregularly over the 
lower pinnae, and the apical region is expanded and sterile (Fig. 90). 

It has been shown by Goebel that the mode of development of such Fern- 
leaves may be experimentally altered : by removing from a plant of Onoclea 
Struthiopteris the foliage leaves which are first expanded in the spring, 
the later expanded leaves, which are normally sporophylls, were induced 
to assume the character of foliage leaves. Similar results were also obtained 
by Atkinson. 

The facts thus briefly summarised for Ferns are evidently comparable 
with those noted for the Lycopods, and the differences in detail which exist 
have their relation to the megaphyllous character. But in Ferns the facts 
are less cogent; for though abortive sporangia and imperfect sori are at 
times found on Fern-leaves, still the evidence that they are vestigial is less 
clear than in Lycopodium, Isoetes, or Ophioglossum, owing to the less 
definite position and number of those parts in Ferns. The conclusion that 
the foliage leaves or parts of leaves in Ferns are phylogenetically sterilised 
sporophylls, or parts of sporophylls, is therefore based rather on broad 
comparisons and on analogies with other Pteridophytes than on the direct 
observation of parts which may be held to be vestigial. That such a 
transmutation may take place in the individual life is fully demonstrated 
by the experiments of Goebel above quoted. It seems therefore reason- 
able to hold for Ferns, as for other Pteridophytes, that sterilisation of 
sporophylls has been effective in the course of their evolution. 

A converse view to that thus stated has been habitual in the past, 
and is maintained by some to the present time. By them the evolutionary 
history is read in direct terms of the ontogeny, and the sterile leaf is thus 
assumed to be the primitive leaf, which has become a sporophyll by the 
superposition upon it of sori and sporangia. Those who take this point 
of view have brought forward in its support the facts that the develop- 
ment and structure of the sterile and fertile leaves is closely alike, and 
that intermediate forms exist frequently between the two, so that the 



170 STERILE AND FERTILE REGIONS 

distinction is very perfectly bridged over. But I submit that the most 
exact demonstration of similarity in detail of development, and the quotation 




FIG. 90. 



Osmnncia, L. A=O. Presliana, I. Small leaf. B and C = O. Regalis, L. 
plant. C = leaf of a mature plant. (After Engler and Prantl.) 



of an infinity of middle-forms drawn from the most varied types of Ferns, 
does not touch the question of phylogenetic priority. Such facts are the 



THEIR GENETIC RELATIONS 171 

N. 

necessary basis either for the older view, that the sporophylls are altered 
foliage leaves, or for the view that the foliage leaves are sterilised sporo 
phylls : but they do not tell distinctively for either. The decision must 
rest primarily upon the presence of vestigial sporangia, together with broad 
comparison rather than upon details of individual development. Still it 
is necessary that any final conclusion should be in accord with the details 
of the individual development, and this is so in the present case, whichever 
of the alternative conclusions be adopted. 

Finally, the interesting demonstration by Goebel, that the sporophyll 
may be experimentally converted into a foliage leaf, does not serve as 
a decisive proof of either view. It demonstrates, however, the close relation 
of the two which either hypothesis will demand. It shows also that 
sterilisation of a sporophyll such as our hypothesis requires can actually 
occur. Such a process of sterilisation, carried out continuously in the 
course of descent, and involving either whole leaves or only parts of them, 
would result in the differentiated character of the leaves of Ferns which 
is actually seen in nature. 

The leading types of Pteridophytes have thus been reviewed as regards 
the relations of their sterile and fertile regions. In the individual life 
of them all, there is at first, as their physiological condition demands, a 
more or less extensive vegetative phase, succeeded sooner or later by 
a fertile phase, though this is often not clearly 'differentiated from it. By 
comparison, it may be concluded that vegetative leaves have been derived 
by sterilisation from sporophylls ; and it is not difficult to realise how a 
vegetative system may thus have been increased, and the production of 
spores have been delayed in the individual life. 

On the other hand, the unlimited apical growth seen in many of the 
Pteridophytes, acts as a set off against the progressive sterilisation, for 
it tends to preserve the balance of the sterile and fertile regions which 
the sterilisation would disturb, and still provides for the initiation of an 
adequate number of spores. In the simpler strobiloid forms, such as 
L. Setago, it is easy to conceive how progressive sterilisation and continued 
apical growth combined would lead to a larger vegetative system and 
an increased final output of y spores. In the more complex Ferns a progression 
of a parallel nature may be traced, though with less exactitude, owing to 
the fact that the large individual leaves do not develop as units. Any 
individual Pteridophyte plant may thus be regarded as being the resultant 
of two progressions : advancing sterilisation below, and apical growth, 
with or without branching, which provides for additional spore-producing 
capacity above ; and it may be pictured to the mind, especially in the 
strobiloid forms, how the fertile zone, which is limited below by the limit of 
sterilisation, may thus have been raised progressively higher on the axis as 
development proceeded, and the time of spore-formation may have been 
correspondingly delayed. But it is essential to remember that however long 
it is delayed, the spore-production which eventually happens is the same 



1 72 STERILE AND FERTILE REGIONS 

process, as regards the whole life-cycle, as that in the simplest sporophytes. 
All the vegetative machinery which precedes and delays it, is, from our 
point of view, a phase intercalated between the two constant and cyto- 
logically complementary events of sexuality and spore-production. 

Once fully differentiated the sterile and fertile regions may vary independ- 
ently of one another. This is already seen in some degree in those species 
of Ly cop odium or Selaginella in which the strobilus is strictly circumscribed ; 
but it becomes a more prominent feature in the higher Flowering Plants, 
where the flower often differs in marked degree from the vegetative system 
of the same plant. Still, even where the sterile and fertile regions are 
the most divergent a comparison of the life-histories as a whole points 
to the conclusion that their genetic relation has ultimately been as it is 
seen in the less advanced Archegoniatae : that the larger part at least, 
if not indeed the whole of the vegetative system is referable in its origin 
to progressive sterilisation of parts originally fertile. The question whether 
the whole is thus referable involves embryological discussion, which must 
be reserved for the next chapter. 



CHAPTER XIV. 

EMBRYOLOGY AND THE THEORY OF RECAPITULATION. 

BEFORE bringing into the discussion any evidence derived from the study 
of comparative embryology, it will be well to enquire briefly into the 
foundations upon which its arguments are based. Here as elsewhere the 
methods and opinions of the present time are founded on the knowledge 
and practice of the past : from time to time, it becomes necessary to re- 
examine the methods currently applied in any special branch of it, and to 
ascertain how far they are in accord with the general position of the science 
as a whole. It will be seen in the matter of embryology that as the point 
of view of the whole science has altered the methods and opinions of 
workers in this field have also undergone modification, and we must 
accordingly be prepared for still further changes so as to keep embryo- 
logical method in accord with the time. A short historical sketch will 
illustrate this, and at the same time it may give some better insight into 
the bases of embryological method as it exists at present. 

Embryology as a branch of the science of Botany can hardly be said 
to have existed before 1840. It is true that there was already some 
knowledge of the form and position of the germ in Flowering Plants 
So early as the seventeenth century both Grew and Malpighi dissected and 
described the embryos of various seeds, while Ray, in his Historia 
Plantarum, founded the distinction of Dicotyledons and Monocotyledons on 
characters of the embryo. ' But up to the early decades of the nineteenth 
century the study of the early stages of development of the individual 
was not used as a systematic means of elucidation of the relations of 
plants. This method was introduced by Schleiden, who saw in the history 
of development the foundation of all insight into morphology. He founded 
the study of development of the flower, which has had such far-reaching 
effects on their comparison and systematic arrangement. He also gave 
special prominence to the initial embryology of the individual plant, and 
to comparison of the higher forms with the Cryptogams. Almost simul- 
taneously the details of cellular construction and of apical segmentation 
in the lower forms were revealed by Naegeli, and as he extended his 



174 THEORY OF RECAPITULATION 

observations, which were thus initiated among the lower Cryptogams, to 
the Archegoniatae and the Phanerogams, he secured that morphological 
ideas, hitherto drawn primarily from the Phanerogams, should be examined 
in the light afforded by the history of development in the Cryptogams. 
And thus the way was prepared for the brilliant embryological work of 
Hofmeister, who, after investigating the embryogeny of the Phanerogams, 
tracing the individual from the egg onwards, proceeded to apply the 
same method to the Bryophytes and Pteridophytes, with the results 
which are now permanently interwoven into the web of the science. It 
may be said that subsequent work in this direction has done little more 
than to fill in the details in the areas of observation left blank upon the 
morphological map thus plotted in broad outline about the middle of the 
last century. It is in the interpretation of the facts, and the recognition 
of the evolutionary history which they convey that there has been room 
for some difference of opinion : and it is this that will now be discussed. 

While the elucidation of the facts by Naegeli, Hofmeister, and others 
was proceeding, the belief in the mutability of species became prevalent : 
the Darwinian theory seemed, as we have already seen, to provide a natural 
explanatory thread running through the facts of genetic morphology and 
connecting them into an evolutionary history. It was held that the 
successive events of the individual life directly illustrated the course of 
descent ; as regards the sporophyte the first stages were accordingly 
regarded as phylogenetically the earliest, and consequently for comparative 
purposes the most important. Embryological detail was thus given a high 
place in comparative morphology. Analogy with the results and arguments 
of zoologists seemed to support this position, and just as some consistent 
reflection of the phylogenetic history was found in the beginnings of the 
individual life of the higher animals, so, it was held, should be the case 
with the plant : the embryology of the sporophyte was accordingly made 
the basis of a consecutive history of its development in the race. For 
instance, the first formed leaves were held to represent the primitive and 
original foliar type, and those formed later on in the individual life were 
regarded as subsequent in the history of the race : or, carrying this lin< 
of thought further into detail, the order and position of the first segmenta- 
tions in the ovum were regarded as of special comparative importance, and 
were used as the basis of elaborate theorising. 

But before such conclusions are accepted, it is well to reflect upon 
the profound differences which exist between the embryology of the higher 
animals and that of the sporophyte in plants. In the first place, the 
embryogeny of the higher animal is carried out once for all after fertilisa- 
tion : the main parts are laid down at a comparatively early stage, and are 
not repeated later. But in the sporophyte of all Vascular Plants the 
initial embryogeny is merely a preliminary phase leading to that continued 
embryogeny which involves the repeated formation of parts : this is main- 
tained throughout the active life of the plant. Hence the initial embryo- 



IN ANIMALS AND IN PLANTS 175 

geny of any higher sporophyte is- a much less essential incident in the 
whole development than that in any higher animal : the embryogeny of 
a higher animal is at best only comparable with the initial embryogeny 
of a plant where the embryo is still enclosed in the tissue of the parent : 
it has' no counterpart corresponding to that continued embryology which is 
so long maintained in the apical region of the plant-body. 

Secondly, the sporophyte is now believed to be itself an intercalated 
phase, which has assumed increasing proportions in the course of descent, 
while the function of spore-formation, which comparison tells us was the 
initial function of the sporophyte, has been proportionally delayed. If 
this be true, so far from the first formed parts being in their present 
form the prototypes, they would be more correctly recognised as derivatives,, 
modified, or it may be transformed, during later evolutionary periods. 

The absence of strict analogy between the embryogeny of the higher 
animals and the higher plants is further illustrated in relation to the theory 
of germinal layers. Following on the experience of animal embryologists 
who found that definite regions of tissue of the mature animal body are 
referable in origin to definite germinal layers of the embryo, Famintzin 
undertook to prove that the same holds for the definite systems of 
epidermis and vascular tissue in the Angiosperms. It is true that the 
origin of the epidermis and of the central stele gives some countenance 
to such a view, though even in these it is not difficult to quote exceptions 
where that regular mode of origin does not exactly apply. But the 
question becomes critical with regard to those parts of the vascular system 
which pass from the stem into the leaves : do these originate from the 
plerome system of the axis, as by the theory of germinal layers they ought 
to do? As De Bary pointed out, 1 this could not be otherwise effected 
than by outgrowths of the plerome pushing between the other layers of 
the young forming leaf. But as a matter of fact, they are derived from 
the primary periblem, and definite bands of this tissue show the corre- 
sponding differentiation, by which means the vascular system of the leaf is 
connected with that of the axis. This almost forgotten discussion is quoted 
here as an example of an attempt, actually made, to impose an embryo- 
logical idea derived from the study of animals upon the embryology of the 
higher plants ; and it shows how, when submitted to the test of detailed 
observation, it has been rejected. It must be clearly understood that such 
comparisons deal only with distant analogies, and that for reasons such as 
those already explained the methods and arguments of animal embryologists 
are not transferable to the embryology of the sporophyte of plants. In 
point of fact, hitherto plant-embryology owes little to animal embryology 
beyond the confusion of thought which follows on fallacious comparisons. 

The success of Naegeli and Leitgeb in recognising and delineating the 
apical cell, and the regular succession of its segmentations in various 
plants, turned the course of accurate observation about the middle of 
1 Conip. Anaf. Engl. Eel., p. 23. 



/ OF 1 



I ;6 



SEGMENTATION 



the last century into this channel. Without their having any clear under- 
standing what the apical segmentation signified, it became an object for 
investigators to define its details in representatives of all the main groups 
of plants, and the attempt was made to correlate the segmentation observed 
with the initiation of definite external parts or internal tracts of tissue. 
With regard to the former, it is undoubtedly the fact that the appendages 
in certain of the lower organisms may be directly correlated with apical 
segmentation : this is seen in many Algae : in Mosses also each segment 
of the apical cell gives rise to a leaf, and Naegeli, who looked upon the 
apex as a dominating influence, held that the same was the case in 
Pteridophytes as well. But a general revision of the question has led 
Schwendener to the conclusion that the origin of the appendages in the 
Pteridophytes is not necessarily connected with or determined by the 




FIG. 91. 

A Apex of Equisetum 'scirpoides : the swelling below shows the highest leaf sheath : this 
extends upwards on the right to the segment-wall, on the left only to the middle of a segment. 

= trans, sec. of t)he same apex : the dotted line indicates the apical cell ; focussing downwards, 
the outline shows 'the youngest sheath, with its three leaf-teeth already indicated by the faintly 
three-lobed outline, //^principal walls. 6'^sextant walls; the position of the leaves is 
independent of these. X 550. (After Schwendener.) 

segmentation at the apex. The genus Equisetum is a good case in 
point ; for though the segments of the apical cell are constantly arranged 
in three longitudinal rows, still the number three does not dominate the 
variable numbers of leaf-teeth in the whorls of the mature plants of the 
genus. Moreover, as the leaves of successive whorls alternate, while the 
successive segments do not, it would be difficult to trace any constant 
connection between them. Even in E. scirpoides, in which the leaves 
are regularly three in a whorl, Schwendener has shown that these are not 
directly related to definite segments (Fig. Qi). 1 The slender apices of 
Salvinia and Azolla have been held to show a constant relation of 
appendages to segments ; but even here, though a numerical correspondence 
may be traced, the successive leaves arise in different parts of the corre- 
sponding segments, being placed alternately in their upper and lower 
halves. In the Ferns there is not any regular numerical relation between 
1 Schwendener, Sitz. d. Akad. zn Berlin, 1885, pp. 927-933, Figs. 7, 8. 



DOES NOT DETERMINE ORGANOGENY 



177 



segments of the apical cell ancl the appearance of leaf-primordia : Schwen- 
dener has even been able to show that where the arrangement of the 
leaves is spiral, the spiral of leaf-arrangement may be antidromous to that 
of the successive segments, and he states that the latter condition is 
almost as common as that where the two spirals are homodromous. It 
thus appears that, in those Pteridophytes in which the apical segmentation 
is most regular, no constant relation exists between the formation of 
segments and the origin of the appendages : Naegeli's conception of the 
apex as a dominating influence in this matter is not supported by the 
facts. And here it may be noted that even in the embryo of the Higher 
Plants there is evidence that the first cleavages in the embryo do not 
define the position of the parts : for it has been found by Westermaier 1 
that the primary median wall of the embryo of Cruciferae has no strict 
relation to the position of the subsequent cotyledons. 




e -j t , r 




FIG. 92. 



Scheme of the succession of cells in the apex of the root of Equisetum hicmale, after 
Naegeli and Leitgeb. A, longitudinal section. B, transverse section at the lower end of 
A. 7z = principal walls. j = sextant walls. <r = the first, = the second, rihe third 
tangential wall. In A the figures I. -XVI. denote the successive segments. = dermatogen. 
k, I, m, n,p successively older portions of the root cap. From Sach's Text-book. 

A somewhat similar Idea to that above discussed was initiated also 
in relation to the internal differentiation of tissues. Naegeli and Leitgeb 
established early the relation of the outer limit of the central vascular 
cylinder to the first periclinal wall in segments at the apex of the root 
in Equisetaceae, Marsiliaceae, and Polypodiaceae (Fig. 92). Subsequently 
Hanstein's study of the meristems in certain well-defined cases of the 
Higher Plants led him to distinguish formative tissues giving rise respec- 
tively to epidermis, cortex, and vascular cylinder: these he designated 
dermatogen, periblem, and plerome. As the study of the tissues became 
more exact, and took form in the stelar theory of Van Tieghem, the 



1 Kef. Bot. Cent., vol. Ixxvii., p. 122, 1899. 
M 



1 78 SEGMENTATION 

generalisation came to be widely accepted that the delimitation of the 
embryonic tissues by Hanstein should coincide exactly with that of the 
mature tissues by Van Tieghem, and that this is generally applicable to 
stems as well as roots of Vascular Plants. 

But this whole subject has recently been submitted to a detailed 
revision by Schoute, 1 and it has been shown that the generalisation of 
Hanstein that the three formative regions exist at the apex was based on 
a very narrow area of observation. As a matter of fact, there is no separate 
origin of them in Pteridophytes, for they all spring from the initial cell or 
cells. In the Phanerogams such separate origin is best seen in roots, but 
even there it is not constant at the extreme tip. In stems it is only seen 
clearly in some few cases ; in most stems hardly at all. The dermatogen 
is the most frequently and the most clearly defined of the three : the 
distinction of periblem and plerome in stems is only rarely carried out. 
Neither is the partitioning of the formative regions in the embryo clearly 
marked at an early stage in vascular plants at large : though there is some 
approach to it in some Dicotyledons, the Monocotyledons and Gymno- 
sperms show little regularity, and it is almost entirely wanting in some 
plants. Thus it cannot be said that the details of development of the 
embryo in any way strengthen the position as regards the formative regions 
of Hanstein. The general conclusion seems a justifiable one, that no 
great morphological importance is to be attached to the formative regions 
of Hanstein since they are so commonly of inconstant occurrence. 

An examination of the further question whether the dermatogen really 
produces epidermis, the periblem cortex, and the plerome the central 
cylinder, also shows inconstant results. In those roots in which periblem 
and plerome are clearly defined at the apex the cortex does originate 
from the periblem and the central cylinder from the plerome, though this 
does not hold exactly for all roots. But in stems the correspondence of 
Van Tieghem's primary tissue-systems with the formative regions does not 
hold : it is not even carried out exactly in the single regular example 
which has been described, viz. Hippuris : for here, according to Schoute, 
the endodermis and part of the cortex may be derived from the plerome. 
It is thus seen that the case is similar to that already discussed of the 
relation of apical segmentation to the origin of the appendages : in certain 
few examples the early segmentation may coincide with the definite mature 
condition, but in the great majority no such regular relation exists in 
either case. Where it does exist it may be held to be casual rather than 
causal, and will bear no constant phylogenetic significance. 

The reasonable degree of success which seemed at first to attend these 
efforts to correlate with early segmentation at the apex not only mature 
external form, but also internal structure, led to a peculiar development 
in the study of the primary origin of the embryo from the egg. However 
clearly we may now see that the position assumed by the investigators of 

1 Die Steldr Theorie^ Groningen, 1902. 



DOES NOT DETERMINE ORGANOGENY 179 

1870-1880 is untenable in face "of present facts, still their reasoning was 
correct : and quite logically (provided the premises were sound) it was 
argued that if the segmentation at the apex of axis or root defines and 
dominates the later development of its tissues or appendages, then a similar 
importance, but enhanced by its earlier position in the individual life, should 
attach to the first segmentations of the zygote. Accordingly the study of 
segmentation was assiduously pursued back to the earliest stages of the 
embryo ; and, as apparently confirming the position, the fact was disclosed 
that a high degree of constancy rules in the first fissions of the ovum of 
the Archegoniatae. Also it was found possible, with some degree of cer- 
tainty, to assign specific developmental functions to the earlier segments : 
thus the first or basal wall was seen to separate a part which habitually 
formed the shoot from a part which habitually formed the foot or root : 
further, the four quarters of the Fern-embryo were shown to correspond to 
the points of origin of stem, leaf, root, and foot : and as the Leptosporangiate 
Ferns were regarded about the time when this work was being done, as a 
fundamental type, the effort was made on the basis of the Fern-embryo, to 
construct what might be called a general embryology founded upon study 
of cell-cleavages. This was extended not only to the various types of 
Pteridophytes, but also, irrespective of the great systematic gulf which lies 
between these classes of Plants, to the Bryophytes. An example of the 
lengths to which this embryology based upon cell-cleavages was driven is 
found in the comparison of the embryo of a Fern and of a Moss, by 
Kienitz-Gerloff. 1 He recognised the basal wall of a Fern-embryo as com- 
parable with that of a Moss : the epibasal half of the embryo in the latter 
divides into quadrants, of which one develops no further, while the other 
forms the whole of the upper part of the sporogonium. Since this quadrant 
corresponds in position, and in some degree in segmentation to that which 
forms the leaf of a Fern, it was suggested that there is a true homology 
between the sporogonial head of a Moss and the leaf of a Fern. Such 
comparisons die hard, and this one still figures in the morphological arena. 
A more reasonable position, and one which is likely to leave still more 
permanent effects on current embryology, was that of allocating certain organs 
of the embryo to certain octants resulting from the primary segmentation 
of the zygote. It is true that the cleavages are relatively constant in certain 
forms : and that the position in which the several parts originate may also 
show a high degree of constancy. The reference of such parts in origin to 
certain octants presupposes that there is some causal connection between 
the two. There are, however, good reasons for not conceding any such 
causal connection. The first is the fact, now demonstrated even in cases. 
where the apical segmentation is regular, that the parts of the mature 
sporophyte are not referable in origin to definite segments. A second is 
that in many cases though the part in question may be referred in origin 
to a definite octant or octants, only a relatively small part of those octants 

1 Bot. Zeit. 1878, p. 55. 



i So EMBRYOLOGY 

may participate in the growth ; while, conversely, the growth may actually 
extend in some cases to other octants than those cited. Further, in certain 
cases, and especially in the Lycopodiales, the relative position of the parts 
of the embryo is not constant. There are thus difficulties in the way of 
according any high importance to the primary segmentations of the embryo. 
From the facts as now known, it would appear more natural to regard the 
embryo as a living whole : to hold that it is liable to be segmented according 
to certain rules at present little understood : that its parts are initiated 
according to principles also as yet only dimly grasped : that there may be, 
and sometimes is, coincidence between the cleavages and the origin of the 
parts, but that the two processes do not stand in any obligatory relation 
the one to the other. 

While the embryology based on cell-cleavages was developing, Sachs was 
engaged in maturing his comprehensive views on the arrangement of cell- 
walls in the youngest parts of plants. His recognition of the prevalence 
of rectangular division of the cells, coupled with the demonstration that 
the same mode of segmentation may occur in such diverse bodies as embryos 
and superficial hairs, went far towards reducing the arrangement of cell 
walls to a general rule : it became apparent that the first cleavages of the 
embryo are not so much the indications of a phylogenetic history, as the 
necessary consequences of rectangular division in a body of approximately 
spherical form. And now that finally the demonstration has come that in 
the continued embryology at the apex of stem and root the segmentation 
has no strict or constant relation either to the formation of the appendages, 
or to the internal differentiation of tissues in plants at large, the logical 
foundation has been swept away from below the feet of the adherents 
of arguments from cleavage. For here as elsewhere we are bound now 
to admit that there is no necessary or constant relation between cell- 
cleavages and differentiation, external or internal. Such relations may 
exist, it is true, and they sometimes do ; but their inconstancy shows that 
they cannot be made the subject of general argument. 

It will thus appear that the methods of embryology hitherto employed 
require considerable revision, so as to bring them into line with the facts 
already observed. Excepting perhaps within narrow circles of affinity, and 
especially in those where definiteness is the rule, arguments from detail 
of segmentation must be discounted: and this will be so in regard to the 
initial embryogeny of the sporophyte, as much as to the continued embry- 
ology close to the growing apex. Concurrently with the gradual acquisition 
of the facts which have led to this general conclusion, there has grown up 
a definite tendency of thought towards a new view of embryological facts. 
The assumption of some unity of plan, or type of construction of the 
embryo in Archegoniate plants, which so long dominated these comparisons, 
has relaxed its hold : in its place has come the desire to study the young 
sporophyte biologically, as a germ to be nursed by the parent plant, in the* 
Bryophytes till full maturity, but in the Vascular Plants for a time only, 



ITS BIOLOGICAL ASPECT 181 

till it is established in the soil. And as the prothallus differs in form and 
position, in size and in duration of life, so the germ itself may differ in the 
place and time of origin of its parts, as well as in their form and structure. 
A few illustrations will show how this point of view gradually asserted itself. 

In 1882 a comparative revision of those parts which serve as haustoria 
in various embryos led me to the conclusion that they are not to be regarded 
as clearly denned morphological members, but rather as swellings of the 
hypocotyl, which arise only where they are required for the first processes 
of development and nutrition of the young embryo. 1 Some years later 
Treub introduced his theory of the " Protocorm " : 2 theoretical considera- 
tions of the biological condition of the young embryo had led him to 
conceive of an organ preceding in descent the leafy shoot, such as is no.w 
seen in Vascular Plants ; and this he recognised as actually present in the 
embryonic tubercle of certain Lycopods : a preliminary stage, in fact, which 
is to them as the protonema is to a Moss. Whatever view we may now 
hold of the protocorm, this theory takes its place as a further step towards 
a biological rather than a purely formal study of embryology. At the hands 
of various other writers such views have been further developed, especially 
in relation to the better knowledge recently acquired of the embryology 
of the Lycopods and Ophioglossaceae : and it was thus open to Goebel to 
formulate the position, as he does in his Organography ? Having shown 
that external forces do not come into consideration in the arrangement in 
space of the parts of the embryo, 4 he points out that we need only consider 
internal factors, and say generally that root, shoot, and haustorium are laid 
down in the positions that are most beneficial for their function. This 
is in fact an extension to the whole embryo of the conclusion which I 
had applied in 1882 to the foot only. Comparative embryology of the 
sporophyte would thus become essentially a study of the circumstances 
and conditions which influenced the embryo during its evolution, and of 
the way in which the germ is formed to meet them. 

But it may be enquired whether the germ itself does not still show 
beneath these adaptive modifications, some characters of a central type? 
Is all trace of the early evolutionary history eliminated by the subsequent 
modifications ? There is at least one leading feature which remains traceable 
with some degree of constancy throughout the series of known embryos 
of the Pteridophyta : it is found in the relation of the parts to that initial 
polarity which is established at a very early stage in them all. This may 
often remain obscured owing to the precocious development of certain 
parts, in response to biological requirements : but nevertheless, it will be 
shown as the several embryos are described, that the apex of the axis has 
constantly a position in close relation to the intercrossing of the octant- 

1 Quart. Jonrn. Micr. Set., xxii., p. 277. 

- Buitciizorg Annals, viii., p. i. The Theory of the Protocorm will be discussed at 
length below, in relation to the embryogeny of the Lycopods. 

3 Organography, ii., p. 246. * Ibid., i., p. 219. 



1 82 EMBRYOLOGY 

walls of the epibasal hemisphere of the embryo. This position of the 
apex of the axis remains the same though the appendages may vary greatly 
in their number, their position, and their relative time of development. 
It is also important to observe that the 'cotyledons show a constant 
orientation to this point, as to a relative axis, whether or not the apex 
of the axis is early developed as an obvious cone. These relations appear 
to be the most constant among the other fluctuating features of the 
various types of Pteridophyte embryos : the theory of the strobilus con- 
templates the phyletic pre-existence of the axis : the early and constant 
definition of the polarity thus seen in the primary embryology is a material 
fact in relation to that theory. 

Goebel further enforces the point that differentiation of the primarily 
similar cells of the embryo takes place gradually, and that the actual 
distinction of the organs begins only late, even though the arrangement 
of the cells may allow of their position being recognised at an earlier 
period. The actual time of their distinctive development may vary in 
different cases, and it is often possible to correlate this with the biological 
requirements ; for instance, the Fern-prothallus is a limited body, with 
small powers of nutritive supply : it is therefore essential that the young 
Fern-plant shall soon establish itself, and accordingly its parts, especially 
the leaf and root, are rushed forward comparatively early. In the case 
of bulky prothalli with large reserves of nutrition, on the other hand, such 
as those of the Lycopodiaceae and Ophioglossaceae, the parts of the 
embryo are differentiated relatively late, not being required for immediate 
action. But even within near circles of affinity there is considerable variety 
in the time of appearance of the organs of the embryo. Jeffrey points 
out how in Marattia and Angiopteris there is a precocious development 
of the cotyledon, while in Danaea it is the root which first shows 
prominently. 1 A somewhat similar state of affairs is seen in the Equisetaceae, 
where . arvense and hiemale have a precocious root, while in E. limosum 
and palustre the root is of later appearance. Among the Ophioglossaceae, 
in O. pedunculosum the cotyledon first emerges, while in O. pendulum, 
and' vufgatum, and in Botrychium Lunaria and virginianum the root takes 
the lead. 

Lastly, there is variability in respect of the suspensor. It is present 
in Lycopodium and Selaginella but absent in Isoetes. It is absent in all 
Equiseta and Ferns, and in all Ophioglossaceae hitherto observed, excepting 
Botrychium obliquum, according to H. L. Lyon. 2 It thus appears that 
within near circles of affinity there is usually constancy of the suspensor, 
but that exceptions may occur even within the single genus. 

There is thus a considerable latitude of detail in the development of 
the embryo in Pteridophytes, and even within near circles of affinity. In 
face of this, the whole conception of embryology should be more plastic 

1 Gainetophyte of Botrychium virginianum^ Toronto, 1898, p. 18. 

2 Bot. Gaz. vol. xl., p. 455. 



INDEPENDENT ORIGIN OF PARTS 



183 



than has often been assumed, and comparative arguments based on 
embryological facts must be used with the greatest caution. 

The independence of origin of the separate parts thus seen in some 
degree in the embryo calls for further consideration, since it is shown also 
elsewhere than in the normal embryo, and it will affect in some degree 
the conception of the nature of the parts of the plant. It is a common 
experience in the plant at large that roots may arise independently of 
other parts: frequently their occurrence is irregular both in number and 
position, and this finds its illustration in almost all the large groups of 
plants. Goebel 1 quotes examples of " free-living " roots, which do not 
spring from a shoot at all, in Pyrola and Monotropa : he regards these 
as derived from the normal in accordance with the saprophytic mode of 
life of these plants. A very peculiar illustration of the detachment of origin 
of roots is shown in the 
abnormal cases of apogamy 
described by Lang (Fig. 
93) ; for here numerous 
roots were formed inde- 
pendently of % any other 
parts of the sporophyte ; 
thus the idea of detach- 
ment of the root is already 
a familiar one. On the 
other hand, the current 
conception of the leaf is 
of a part in close genetic 
connection with the axis : 
but this also has been 
shown by Goebel to be 
open to exceptions. He 





FIG. 93. 



Scolopendrinm vulgare. Prothallus from the branched cylindrical 

., _ . process of which ten roots arose : eight of these are visible in the 

describes Cases Of free- drawing, x about 6. ( After Lang.) 

living leaves. 2 The old 

morphological dogma asserted that a leaf could only arise out of the 
vegetative point of a shoot; but Goebel accepts the facts disclosed in 
Lemna and Utricularia^ as well as the condition of the embryo in many 
Monocotyledons, as overthrowing this dogma. In the latter case the 
cotyledon arises without any vegetative point of an axis being visible. He 
also quotes the case of Adiantum Edgeworthi, a Fern which produces buds 
at each leaf-tip. 3 This case I regard as being important for comparison 
with the condition seen in embryos ; for according to Goebel's description 
and drawings (Fig. 94), the first leaf of the new bud arises not from the 

1 Organography, p. 234. 2 L.c. t p. 235. 

3 L.c., p. 241. See also Kupper, Flora, 1906, p. 337, who found that in Adiantmn- 
species three, and in Aneimia rotundifolia even six leaves originated before the stem-apex 
was defined. 



184 



THEORY OF RECAPITULATION 



leaf-tip which supplies the apex of the new bud, but from a position near 
it upon the convex side of the mother-leaf. As Goebel remarks, this finds 
its parallel in the formation of the embryo and in the apogamous origin of 
a Fern-plant on a prothallus. Through such examples we arrive at a con- 
ception of a leaf also as a part which may be at times of independent 
origin, and not necessarily produced from a pre-existent and obvious axis. 
But the cases above quoted from mature plants are almost certainly 
secondary, and are probably consequent upon peculiar conditions of life. 
The question then presents itself whether the independent origin of a leaf as 
it is seen to occur in certain embryos is not also a secondary condition in 
descent, and a consequence of what might be called anticipatory develop- 
ment of that part of the shoot, to meet such early biological needs as that 
of assimilation or of storage? It is impossible to answer such a question 
with any approach to proof: nevertheless the case of A. Edgworthi is 

very suggestive of such a 
detached and anticipatory 
development of an indivi- 
dual part. Clearly the early 
appearance of a leaf in the 
Fern-embryo would be an 
advantage, while the axis 
is in no way essential for 
the performance of its first 
functions. If such be the 
origin of the first leaf or 
leaves of a sporophyte em- 
bryo, then so far from their 
independent position being, 
as is usually assumed, the 
primitive position, it would 
be secondary, a mere result 
of adaptation to the early 
requirements of the em- 
bryo. This question will 
be specially studied later 
in connection with the 

embryogeny of the Lycopods, a family in which the diversity of character 
of the prothallus has imposed considerable and instructive differences of 
development upon the embryo. Meanwhile I see no sufficient reason, on 
the ground of their position or the mode of their origin, to regard the 
"cotyledon" or "protophyll" as representing a category essentially apart 
from foliage leaves : l nor does the apparently independent existence of 

1 Goebel, Organography, ii., p. 400, remarks specifically for Pteridophytes Unit the 
cotyledons "are without exception arrested forms of foliage leaves": he extends the 
conclusion also to Seed-plants (p. 402). 




; FIG. 94. 

Adiantutn Edgworthi. Origin of leaf borne buds. I. = apex of 
leaf seen from above : the apical cell has divided by a cross- 
wall. X position at which the first leaf of the bud arises, /^posi- 
tion of origin of the lateral leaf-series whence usually in a leaf the 
pinnules develop. II. Apex of leaf seen from the side, lettering the 
same. III. Apex of leaf in optical longitudinal section : j = divided 
apical cell; / = first leaf of the bud. IV. Somewhat older stage. 
V. Apex of leaf in longitudinal section : = apex of bud surrounded 
by scales ; A = first leaf, looking like the continuation of the mother- 
leaf; / = incipient root. I. -I V. highly magnified. V. less highly 
magnified. 



APPLICABLE WITHIN LIMITS 185 

cotyledons or protophylls raise any insuperable obstacle in the way of a 
theory of the strobilus as stated in a previous chapter, so long as they 
are held to be anticipatory growths in the sense above explained. 

From the above pages it will be seen that the foundations of recent or 
current embryology of the sporophyte are open to criticism. The analogies 
with animal embryology are misleading : strict recapitulation is not to be 
assumed where, as in plants, continued embryology holds sway : segmenta- 
tion appears to be a phenomenon connected in no obligatory sense with the 
origin of organs : the relative position of the parts of the embryo, though 
it may be fairly uniform in circles of near affinity, is variable according 
to biological requirements which are readily intelligible in the establish- 
ment of the germ : the relative time of origin of the parts may also be 
variable, even within circles of near affinity. The question will therefore 
be what weight in our comparisons is to be accorded to these somewhat 
fluctuating facts of the primary embryogeny of the sporophyte? They 
have been very highly estimated in the past: while not denying their 
value, I think that they have been given altogether undue precedence over 
the characters of the sporophyte which appear later, and this opinion is 
based both on general considerations and on detailed comparison. Accord- 
ing to the view of alternation advanced above, there does not appear to 
be any sufficient reason for attaching special comparative importance to 
the initial steps of the primary embryology. If it had not been for the 
recapitulation theory of the zoologists, it is improbable that this position 
would ever have been adopted in the case of plants. The more natural 
inference from the facts would probably have been the converse, that is, to 
attach greater weight to the characters of the mature shoot : in fact, the 
position now is that the embryogeny must be interpreted in terms of the 
mature plant rather than the converse which a recapitulation theory would 
demand. For the reasons thus stated the initial embryogeny of the sporo- 
phyte will be accorded only a minor place in our comparisons : when 
once the earlier, and in considerable degree adaptive embryonic phase 
is past, and the form characteristic of the mature plant is by way of 
being established, this would seem to be a more reliable basis for com- 
parison than any minute details of the initial embryogeny. 1 Probably the 
strobilus itself will give the y most trustworthy basis of all. 

hut it is not to be concluded that recapitulation plays no part whatever 
in the development of the sporophyte. Seedlings of many plants with 
highly specialised shoots, such as the phyllodineous Acacias, and spinous 
plants such as Ulex, start with a postcotyledonary shoot of simple and 
not specialised form, characteristic of the group to which they belong: 
they only assume their peculiarly adaptive character later. They thus 
reflect in some degree in their ontogeny the history of their specialisation. 
Such facts are familiar, and the interpretation generally accepted. But 

Thomas (New Phytologist, 1907, p. 77, etc.) has expressed a similar view as 
applied to the embryogeny of Angiosperms. 



1 86 IN THE SHOOT AS A WHOLE 

it does not follow from its acceptance in these cases that a theory of 
recapitulation can be applied consistently, or in detail, to all phases of 
development, or that evidence of it is to be found necessarily in the 
earliest steps of the embryogeny. It remains for the morphologist to draw 
for himself the reasonable limit of its application. If this be done, and 
especially if the variability which exists be duly appreciated, then the early 
stages of the initial embryogeny of the sporophyte will take their right 
place : and recapitulation will be traced as a limited phenomenon only, 
applicable, it is true, to the case of relatively recent adaptations, but not 
with equal certainty to the far-away facts of the past. For reasons such 
as are explained in this chapter, it will not be assumed that plants so 
diverse as are the main groups of Archegoniatae show in their early seg- 
mentation, or in the initial form of their embryos, any detailed reflection of 
ancestral characters. The facts observed should be used with the greatest 
caution, especially where the comparisons are made between representatives 
of phyla which must have diverged early from some primitive stock, if 
indeed they be related at all. 

Certain points touched in the above discussion will help towards an 
understanding of the relations of sporophylls and foliage leaves to the 
first leaves of the embryonic plant. In Chapter XIII. it was concluded 
that in certain cases at least foliage leaves are to be held phylogenetically 
as sterilised sporophylls : and the question remains whether or not all 
non-propagative leaves, including the cotyledons themselves, originated in 
this way. There seems to be a high probability that in the Pteridophytes 
they did. There is no reason to hold that their first leaves differ in any 
essential point from those which are formed later : frequently they resemble 
the later leaves closely in outline ; but they are sometimes characterised by 
peculiarities of form, though these are less marked than in the cotyledons 
of Phanerogams. Sometimes the first leaves in Pteridophytes arise laterally 
on an axis already defined (Equisetum) ; but in other cases, and especially 
in the megaphyllous forms, the first leaf or cotyledon may appear prior 
to any definite outgrowth of the axis itself. This fact may be held to be 
in itself an inherent objection to ranking the cotyledon as the equivalent 
of a foliage leaf which arises from the axis; but this objection is met by 
the fact that free-living leaves, apart from any obvious existent axis, do 
occur elsewhere in certain specialised cases ; these may be interpreted as 
originating by an anticipatory development, though still in relation to an 
axis not yet defined by external growth. And so also the cotyledon in the 
Fern may be held to be essentially an appendage of the axis, the central 
point of which is already defined in close relation to the intersection of 
the octant walls of the epibasal segment, but not characterised as yet by 
external growth : the cotyledon, on the other hand, is hurried forward 
precociously in its development to meet the physiological need for nutrition, 
but maintains nevertheless its orientation relatively to the deferred axis. 



ONLY ONE CATEGORY OF LEAVES 187 

This precocity, however, does not alter its nature as an early foliage leaf, 
bearing essentially the same relation as others do to the sporophylls, and 
to the relative axis. 

In support of this conclusion it may be noted that, according to Prantl, 
even the primordial leaves of Lygodium subalatum are sporophylls, so 
that sterile leaves do not exist at all in that species. 1 No more complete 
demonstration is possible than this of the correctness of the conclusion 
that the cotyledon is the equivalent of the foliage leaf, and of the sporo- 
phyll. The position at which we arrive is then this : that foliage leaves 
are sterilised sporophylls, and the protophylls or cotyledonary leaves are 
further modifications of the same type. In fact, all those parts which 
are commonly styled " leaves " on the Pteridophyte plant belong to the 
same category, but differentiated to meet special needs. 

1 Schizaeaceen, p. 14. 



CHAPTER XV. 

ANATOMICAL EVIDENCE. 

IN the previous chapter it has been shown that early embryological detail 
is an insecure guide for purposes of comparison : that segmentation is not 
related with any general constancy to the origin of the appendages : also 
that the anatomical regions of the mature part are not defined with any 
constancy by early segmentations at the apex. It remains to enquire in 
what way the anatomical characters of the mature parts will affect the 
questions discussed, and especially whether they tend to support or to 
refute the strobiloid theory as put forward in Chapter XI. 

The most pregnant change in anatomical view effected during the last 
half century has been caused by the introduction of the Stelar theory of 
Van Tieghem. Prior to it the individual vascular strand, pursuing its 
course from the appendage into the axis, was regarded generally as the 
structural unit of the vascular system of the whole shoot. This was a 
natural consequence of that detailed investigation of the course of the 
individual vascular strands which was initiated with such success by 
Naegeli, and extended by many other writers'. The position taken up by 
these observers is admirably summarised in the Comparative Anatomy of 
De Bary : from his account it will be seen that the method of anatomical 
study, as well as its result up to 1877, was such as to give prominence 
to the individuality of the leaf: the facts as there stated might almost be 
read as an expression of phytonic theory in terms of internal structure, 
since the chief aim was to follow downwards to its termination each 
individual strand of the leaf-trace. A phytonic view of the facts was 
never explicitly set down by De Bary, though the under-current of 
thought seemed clearly to lead to an analytical rather than an integral 
view of the construction of the shoot. 

But very soon this was corrected, on general and external grounds 
rather than on those of anatomy, by Sachs : for in his Lectures on the 
Physiology of Plants (1882), he strongly insisted on the contemplation of 
the shoot as a whole. It is impossible to say how far this may have 



STELAR THEORY 189 

influenced the thoughts of Van Tieghem, and stimulated 'him in the 
direction of his later generalisation ; but it may be remarked that what 
Sachs did in urging the integral view of the shoot on more general 
grounds, Van Tieghem, by his introduction of the stelar theory in place 
of the mere study of the individual vascular strands, did on a basis of 
anatomical investigation. Both of these reforms tended in the same 
direction, viz. towards the conception of the shoot as a whole, with axis 
and leaf as its constituent parts. It may be said that any step of 
observation or of reasoning which tends to emphasize the primary indi- 
viduality of the leaf, leads towards some phytonic theory of the shoot at 
large : any step which tends to emphasize the primary individuality of 
the axis leads towards some strobiloid view. The effect of the stelar 
theory of Van Tieghem has been in the latter direction. The recognition 
of the vascular column of the axis as a structural unit of the conducting 
system has gone far towards the reinstatement of the axis on the basis 
of structure, as a substantive and essential part of the shoot ; and the 
change of view has been in opposition to those phytonic theories which 
would regard it as a mere congeries of leaf-bases. It will accordingly be 
important to consider this matter carefully in its bearings on the general 
theory of the shoot in the sporophyte. 

Van Tieghem recognised the central cylinder of the axis in the great 
majority of plants as an anatomical region coordinate with the anatomical 
regions of the cortex and the epidermis, which lie outside it : he designated 
it the stele. This cylinder is delimited by certain continuous sheaths : the 
inner, the pericycle, belongs typically to the stele : the outer, the endo- 
dermis, belongs to the cortex : the boundary of the tissue held to be stelar 
is the surface between these contiguous layers. The stele thus denned 
consists of vascular tissue xylem and phloem and of conjunctive tissue 
usually parenchyma. In certain plants throughout, and in certain regions 
of other plants, the structure of the vascular tissue of the axis is relatively 
simple and compact, consisting of a solid central core of xylem, with a 
peripheral band of phloem : this was probably the primitive or original 
type, though it may also result, as in some well-known cases, from reduc- 
tion : it is designated the protostele. But in very many shoots the type 
of stele is more bulky owing to the presence of parenchymatous tissues : 
the vascular tissue may thus be separated into distinct strands, and in 
that case they are usually arranged with a radial symmetry and embedded 
in the conjunctive parenchyma : this tissue occurs partly as the pith, which 
occupies all the central region, partly as a lateral and external packing for 
the several strands. Such a stelar structure, of either the compact or of 
the more bulky type, is found in the axis and in the root of the vast 
majority of sporophytes. 

Exceptional arrangements exist, however, in certain cases : the most 
important of these is the polystelic type of stem-construction, which is 
found in many Pteridophytes and in some few stems of Phanerogams. 



190 ANATOMICAL EVIDENCE 

It is indicated by the presence in the transverse section of two or more 
vascular masses, each being constructed and delimited in the same way 
as the single stele of normal axes. At first it was thought by Van Tieghem 
that to produce this condition the stele, originally simple, underwent a 
branching, notwithstanding that the axis in which this would take place 
remained simple. This suggestion seemed inherently improbable, and it 
has since been shown by direct examination of specific cases that the 
real origin of the polystelic state as it occurs in Ferns is by the formation 
of large leaf-gaps below the bases of insertion of the successive leaves : 
the steps of increasing complexity of stelar structure in Ferns have been 
tentatively outlined by Gwynne-Vaughan as follows : The most primitive 
type of vascular construction was probably the single protostele, with 
uninterrupted central xylem, and this is met with in some of the early 
Fern types in the mature stem, but it is also seen in polystelic types of 
Ferns at the very base of the young plant. Internal parenchyma then 

makes its appearance about the peri- 
phery of the protostele at points just 
above the departure of the leaf-traces : 
this advances gradually inwards from 
these points until the most central 
region of the stele is affected. A struc- 
ture resembling a cylinder or, as it is 
styled, a " solenostele," may then be 

FlG- 95 attained by the gradual differentiation 

onia punctnoba. Diagram ot vascular ' of phloem and endodermis through the 
L in T^ r ^o f ^ leaf-gaps and all round inside of the 
-vIughan^ 06 the bserver ' (After xylem-ring. Below the insertion of 

each leaf a large leaf-gap occurs in the 

solenostele (Fig. 95) : in transverse section at such a point the stele will 
appear as an incomplete ring. If, then, the arrangement of the leaves be 
a close one, two or more of these gaps would occur in a single transverse 
section, and the result would be an appearance as of several steles arranged 
in a ring. These originate, however, not by branching, as Van Tieghem 
thought, but by resolution of the stele, first into a cylinder and then into 
a cylindrical network. The result of this mode of amplification would, 
therefore, be more correctly styled "a dictyostele " rather than a polystelic 
state, and the parts " meristeles " rather than a plurality of distinct steles, 
since the whole is a result of amplification, not of branching, of the 
original monostele. 

But polystely is also found in the genus Selaginella. There is little 
doubt that the monostelic type is the original one for this genus also, 
since it exists in many species. The origin of the more complex state 
is, however, connected rather with the branching of the axis than with 
the insertion of the minute leaves. It has certainly taken place within 
the genus, but the comparative study of the illustrative species from the 




STELAR THEORY 




FIG. 96. 

A, transverse section of the stem of Equisetum palustre (Xa6), and B, part of it 
X 160. C, transverse section of the rhizome of Equis. sylyaticum (X26), and D, 
part of it X 160. E, transverse section of the rhizome of Equis. litorale (X26), and F, 
part of it X 160. cc central cavity. r'=valleculai canals. = carinal canals. j = sheath 
of separate strands, as outer, is = inner general endodermis : in A, C, and E the endo- 
dermis is indicated by a dotted line. (After Pfitzer.) From Rab. Krypt. Flora. 



192 ANATOMICAL EVIDENCE 

developmental point of view is still required before a complete elucidation 
is possible. The case for Medullosa must also remain for the present 
unexplained ; but at least it seems almost certain that it is not a polystelic 
condition arising from overlapping of leaf-gaps, as in Ferns. 

Among Phanerogams the polystely in Auricula has been investigated 
by Gwynne-Vaughan, and the origin of it is again by a resolution of a 
primitive monostele. Perhaps the same may be the case for the more 
complex condition of Gunnera, but that is still uncertain. Of these 
isolated cases of polystely which exist among the vast majority of mono- 
stelic forms the Ferns are the most important : and as their case has been 
shown to be a result of amplification of the monostele, the existence of 
occasional exceptions elsewhere, not yet fully explained, cannot be held as 
a valid objection to the acceptance of the monostele as the fundamental 
type of structure in the sporophyte at large. 

Another mode of amplification of the protostele is exemplified by 
Lycopods, and possibly occurs also elsewhere : it is by the progressive 
conversion of a central tract of the xylem-core into parenchymatous tissue 
of the nature of a pith. This is probably related to another modification 
of the stelar structure found in stems, viz. that designated the schizostelic, 
or by some the astelic, state. Here the several vascular strands are not 
collectively surrounded by an endodermal ring, but are independent, and 
may have a special endodermal sheath surrounding each. This structure 
is found in some species of Equisetum and in certain Phanerogams. It 
seems to be generally admitted that in the stem this condition is derivative 
from the ordinary monostelic state, a conclusion which would naturally 
follow in the case of the genus Equisetum from a comparison of its 
different species (Fig. 96). If this be so, then both the marked excep- 
tions in vascular- structure in the stem are referable in origin to the usual 
monostele, and the conclusion seems justified that in the axis of Vascular 
Plants there is only one fundamental stelar type, and that is the mono- 
stelic type. The morphological importance of any character is held to 
be in accordance with its constancy in a large series of allied organisms : 
the general occurrence of a monostelic structure, or of arrangements 
derivative from it in Vascular Plants at large gives the monostele 
place in the first rank as an internal morphological feature of the axis. 

The prevalent bifacial character of the leaf is apparent in the simplest 
forms of Vascular Plants, where its comparatively small expanse is traversed 
by a single vascular strand. This structure is found in such primitive types 
as the Lycopods, Equiseta, Isoetes, etc. Where the leaf is larger the 
vascular system is expanded in various ways : numerous strands may traverse 
it, diverging from one another towards the margin, but converging towards 
the base, where, with or without fusions, they may form a curved series 
as seen in transverse section (Fig. 97). The orientation with the proto- 
xylem tending adaxially is a constant feature. Each strand is surrounded 
by a definite sheath throughout its individual course, but on fusion two 



FORM OF LEAF-TRACE 193 

or more may pass within a common sheath : a flattened vascular plate, 
or it may be a curved series of strands, is thus produced. In some leaves 
of Ferns and in some Dicotyledons the vascular tis.sue thus disposed in a 
curve as seen in transverse section, with its concave surface upwards, may 
show a closing in of the lateral margins as the leaf-base is approached; it 
seems not improbable that this is connected with mechanical requirements 
consequent on the leverage of. a large leaf on its base. This closing 
in may even be carried so far that the two edges may become contiguous, 
and the result will be a structure not unlike that of a cylindrical stele of the 
axis (Fig. 98). But it would be a mistake, on the mere ground of such 
structure and without the check of comparison, to suggest any close identity 
of character of such " pseudosteles " with the stelar condition of the axis. 
There is reason to believe that these pseudosteles of the leaf are secondary 




FIG. 97. 

Dicksonia Barontetz. Portion of the vascular system of the stem, seen from within, 
and showing the departure of three leaf-traces. (After Gwynne-Vaughan.) 

in their origin, for it is the fact that they are characteristic of leaves of 
relatively large size, while smaller leaves are typically dorsiventral in their 
vascular structure. Moreover, Professor Bertrand and others have been 
able to show by exact comparative analysis that even in very aberrant 
cases of Fern-petioles the pseudostelic structure is referable still to a dorsi- 
ventral origin, and is to be explained as the result of complex foldings 
and fusions of a band-like vascular tract. A somewhat similar explanation 
may be given of the "pseudostelic" petioles of such Dicotyledons as are 
quoted by Schoute (I.e., p 158) and of the "polystelic" petiole of Primula 
Auricula examined by Gwynne-Vaughan ; these may be held to be secondary 
modifications of a structure originally dorsiventral, and the position may 
accordingly be summed up as follows : The construction of the axis is 
essentially cylindrical, and finds its anatomical expression in the cylindrical 
stele ; the construction of the leaf is essentially dorsiventral, and it finds 
its anatomical expression in the isolated vascular strands disposed dorsi- 
ventrally. Both these are liable to modification in special cases, thus by 

N 



194 ANATOMICAL EVIDENCE 

breaking up of the stele in certain axes a schizostelic state with individual 
strands may be attained ; but comparison shows that these are not the 
phylogenetic equivalents of the individual strands of the simple leaf, though 
they may be continued outwards into the leaves : conversely, in the pro- 
gressively developing leaf, a pseudostelic structure may be produced by 
fusion of strands phylogenetically distinct ; but again comparison shows that 
this is not the phylogenetic equivalent of the primitive stele of the axis, 
but a condition secondarily derived. 




FIG. 98. 

Transverse section of base of petiole of Gleichenia dicarpa, showing a pseudo-stelar 
structure resulting from contraction of horse-shoe-like xylem till the margins fuse. Photo- 
graph by R. Kidston from section by Gwynne-Vaughan. 

It is necessary thus to differentiate characters which are primary from 
those which are secondary. It has long been recognised that the distinction 
cannot always be maintained between axis and leaf on the basis of strict 
criteria of form or structure : exceptions can be found to all morphological 
criteria proposed. Still, if on a basis of comparison the primary and 
secondary conditions be clearly kept apart, the divergent anatomical 
characteristics of the two parts become sufficiently obvious. Accordingly, 
in our view the structure of the leaf is recognised as primarily astelic 
throughout, that is, the isolated strands are not to be held as results of 
resolution of a primitive stele ; where an apparently stelar structure appears 



"CAULINE" AND "COMMON" VASCULAR STRANDS 195 

in the leaf it would at best be only a pseudostele, secondary in origin, and 
thus phylogenetically distinct from the stele of the axis. The primary 
structure of the axis is monostelic: where isolated strands occur in the 
axis, each with its sheath is a schizostele, a result of secondary segregation 
of the component tissues of the stele. 

In this connection it is important to recall the old distinction between 
"common" and "cauline " vascular 'bundles. In the former the lower 
part of the course of the individual strand is in the axis, the upper extends 
into the leaf: in the case of the tissues which may be styled cauline, the 
course is within the stem throughout. From a theoretical point of view 
the existence of cauline vascular tracts is important, for it accentuates 
the axis as something more than a mere basis for insertion of leaves. 
The further fact that the axial stele may be followed beyond the youngest 
leaf-traces shows that the vascular system of the axis has an objective 
existence independently of the leaf-traces, however closely it may be 
connected with them in ordinary cases. These cauline extensions are 
prevalent in early Pteridophytes, such as Lycopods, Psilotaceae, and Ferns ; 
this fact must necessarily be of special interest in connection with any 
theory of the origin of foliar developments in Vascular Plants. 

It is evident that the existence of a cauline stele bears directly 
towards a strobiloid theory of the shoot. This suggests the question 
whether any existing group of plants show a nascent condition of the 
vascular system of the shoot such as a strobiloid theory would demand, 
viz. a columnar conducting stele, with no appendages, or with appendages 
anatomically accessory to rather than formative of the central stelar column. 
In a paper on the conducting tissue-system in Bryophyta, Tansley has shown 
that such a structure is found in the more complex Mosses. 1 In discussing 
the points brought forward he very properly disavows at the outset any strict 
homology with Vascular Plants, remarking that it is almost as certain as 
any phylogenetic thesis is likely to be that the conducting tissues of Bryo- 
phytes have nothing directly to do with the origin of the conducting 
tissues of the higher plants. The main seat of the development of these 
tissues in Bryophytes is the gametophyte generation, which is in any case 
excluded from the comparison, since the vascular system in Pteridophytes 
is confined to the sporophyte. And at the least it is extremely unlikely 
that the Pteridophytes have been derived from a Bryophytic ancestor with 
a sporophyte showing anything approaching the specialisation of the rnoss- 
sporogonium, in which conducting tissues also occur. But it must not for 
this reason be supposed that the Bryophytes are of no interest in consider- 
ing the problem of the evolution of the vascular system in Pteridophytes. 

1 Ann. of Botany, xv., 1901, p. 2. For Mr. Tansley's later views on this and kindred 
subjects, especially as affecting the question of origin of the shoot in the Filicales, 
reference should be made to his Lectures (New Phytologist, 1907). This chapter was 
in type before Mr. Tansley's lectures were given. The opinions here expressed may have 
to be modified. 



196 ANATOMICAL EVIDENCE 

We see among the former group plants in the very act, so to speak, of 
developing a conducting system in response to vital needs, and others in 
the most various stages of its evolution in complexity. The conditions 
under which this evolutionary development occurred must have been 
practically identical with those to which the primitive Pteridophytic 
sporophyte was subjected, gradually increasing adaptation of a simple leafy 
form to terrestrial life. And the final result, as seen in the highest Poly- 
trichaceae, is so strikingly like the state of things obtaining in the true 
vascular plant as to furnish probably one of the completest and most 
interesting cases of homoplastic development in the plant-kingdom. It can 
hardly, therefore, be denied that the study of the conducting system in 
Mosses is calculated to throw most valuable side-lights on the question 
of the evolution of the vascular systems of the higher plants. 

As the result of his careful analysis of the tissues, Tansley 1 concluded 
that the highly developed Polytrichaceous stele is in the aerial stem 
essentially double in nature and phylogenetic origin, consisting (i) of a 
central primitive hydrom-cylinder originally developed, and still serving to 
supply the apical bud, sexual organs, and sporogonium with water; and 
(2) of a double peripheral mantle of hydrom and leptom separated by a 
starchy hydrom-sheath, and all three layers composed of the joined bases 
of leaf-traces, and designed between them to conduct water to and formed 
material from the leaves. 

The bearing of these considerations on the problem of the nature and 
origin of the primitive stele among the Pteridophytes, as we find it, for 
instance, among the Sphenophyllales and Lycopodiales, is a very interesting 
question. Two alternative explanations of such a stele are possible 
According to a strobiloid theory, we may suppose the primitive Pterido 
phyte descended* from a form bearing a terminal fruit-body ; this contained 
a primitive hydrom-stele comparable with that of the Mosses, but supplying 
the fruit-body directly, since it is developed in the sporophyte, instead of 
merely leading up to the base of the sporogonium. The lineal descendant 
of such a primitive hydrom-stele would then perhaps be seen in the central 
metaxylem of, for instance, Sphenophyllum, Cheirostrobus, the Lepidodendra 
with solid steles, the monostelic Selagindlas, and (modified in various ways) 
in Psilotuni) Lycopodium, etc. (Fig. 99). Added to this would be the bases 
of the leaf-traces represented by the peripheral protoxylem-strands, and only 
evolved after the primitive sporophyte had thrown out leaves requiring a 
vascular supply connected with the main channel of the stem. The fact 
that they appear before the central xylem in the development of the 
individual stem would be merely in relation to the need for the early 
establishment of conducting channels to the leaves a need which is 
universal in leafy vascular plants. 

On the other hand, under some phytonic theory we might suppose that 
the formation of leaf-structures requiring a vascular supply preceded the 

*L.c. t p. 35- 



RELATION OF LEAF TO AXIS 



197 



formation of a regular stele: in^ which case the leaf-traces, represented in 
the first case by the protoxylem-strands, would be phylogenetically prior : 
the central metaxylem, on the other hand, would be a later formation, 
developed in the larger forms to furnish additional conducting channels to 




FIG. 99 

A forked sporangiferous branch of Lycopodium chamcecyparissus in longitudinal section, 
slightly magnified. ff= the axile vascular body. ^ = leaves. ss = young sporangia. 



supplement the protoxylems in supplying the needs of the higher foliage 
leaves and sporophylls. 

In approaching an opinion on such a question much will turn upon the 
initial relation of the leaf to the axis in the evolution of the leafy sporo- 
phyte, a matter already discussed in Chapter -XI. If the leaf were from 
the first a preponderating influence in the shoot, then the latter explanation 



198 ANATOMICAL EVIDENCE 

would be the more probable : if the leaf were initially a minor appendage, 
then the former of the two explanations suggested by Mr. Tansley will 
naturally follow. The facts are not decisive in indicating either of these 
alternatives ; but a comparative study of the vascular relations of leaf to 
axis in the earlier leafy sporophytes will throw a valuable side-light upon 
the question. Dr. Jeffrey 1 has distinguished two main types of relation 
between the vascular supply of the leaf and the vascular system of the 
axis. The one type he styles " cladosiphonic " : it is characterised by the 
insertion of the leaf-trace on the periphery of the axial stele, which is itself 
hardly disturbed at the point of junction (compare Figs. 71 and 99). This 
is clearly the anatomical expression of the dominance of the axis in the 
shoot, for the leaf-trace is added as a mere appendage on the periphery 
of the otherwise cauline stele. In this respect the structure is like that 
described for some of the larger Mosses. This condition is characteristic 
of the Lycopodiales, Equisetales, and Sphenophyllales ; all of them small- 
leaved forms, and of early occurrence in the scale of vegetation : and 
there is good reason to believe that it is a really primitive condition 
in these early Pteridophytes. 

The other type recognised by Dr. Jeffrey is the "phyllosiphonic," which 
is characterised by the profound disturbance of the vascular tissues of the 
axis at the point where the leaf-supply is inserted : so much so that a 
distinct leaf-gap is produced, and connection may be established at that 
point between the central and peripheral tissues (compare Fig. 95). This 
is the anatomical expression of the dominance of the leaf over the axis in 
the shoot, and it is characteristic of certain large-leaved Pteridophytes, and 
is also general in Seed-Plants. As regards the latter, it has already been 
seen in Chapter XL that certain of the Gymnospermic Seed-Plants have 
probably been derived, with progressive leaf-production, from a Filicinean 
ancestry : their phyllosiphonic character supports this view, which is, how- 
ever, based upon a wide area of comparison on other points besides. It 
maybe held as probable that the seed-bearing plants at large were developed 
from a large-leaved ancestry, having undergone reduction of leaf-complexity 
in the course of their evolution. But while we thus recognise a probability 
of a widespread reduction producing relatively small-leaved forms, it does 
not follow that all smaller-leaved forms originated thus : and the anatomical 
and palaeontological facts together make it probable that such small-leaved 
forms as the Lycopodiales, Equisetales, and Sphenophyllales were primitively 
small-leaved. 

It has been remarked in Chapter XL that in the individual life, one 
or the other anatomical character is usually constant : this is true for the 
mature structure, but the transition from the. cladosiphonic to the phyllo- 
siphonic may frequently be traced as the young plant of the latter type 
passes to the mature state. It has been shown very clearly in the case of 
Alsophila excelsa by Gwynne-Vaughan (I.e., p. 710) (Fig. 100) : here the 

1 Mem. Boston Soc. of Nat. Hist., vol. v., No. 5, 1899. 



SHOWS MICROPHYLLY TO BE PRIMITIVE 199 



axis is protostelic at the base, and' the first leaf-trace may depart without in 
any way altering the structure of this stele, thus showing the cladosiphonic 
character. It is only subsequently, by the overlapping of those intrusive 
pockets of phloem and endodermis which accompany the exit of the leaf- 
trace, that a gradual transition is effected to the 
phyllosiphonic type. Thus the anatomical evidence 
indicates a probability that, even in large-leaved 
Ferns, the cladosiphonic was the primitive type ; 
but that the phyllosiphonic, once initiated, is as a 
rule maintained : this is shown by its persistence 
in the Seed-Plants, even where the leaf has been 
reduced in size. Accordingly the trend of ana- 
tomical evidence is towards the recognition of a 
small-leaved, strobiloid type of construction of the 
primitive sporophyte. 

It may be objected that in certain Pteridophytes 
the condition of the embryo is such as to- militate 
against any strobiloid theory, and that their ana- 
tomical details offer as great an obstacle as their 
external form. The cases which will be cited are 
those of some Lycopods, in which the first leaves 
are isolated, and show no vascular connection with 
the later-formed leaves (Fig. 101): or my own 
observations on Phylloglossum may be held as a 
valid objection, for there also a single vascular 
strand has been observed passing down in one case 
directly from a protophyll into a root, without any 
relation to the other members. 1 Such cases may 
be held to establish the individuality of the leaf 
anatomically before the axis has any existence. In 
the interpretation of these the discussion on embry- 
ology in Chapter XIV. should be recalled, and 
especially the opinion there brought forward that 
the separation of individual leaves in certain cases 
from the rest of the shoot is a secondary condition, 
resulting from what might be called anticipatory 
development, to meet an early need for an assimi- 
latory mechanism. The inconstancy of detail which 
is shown by the embryos of Z. cernuitm, and by the young annual growths 
of Phylloglossum^ is in itself a support of this opinion. The vascular supply 
of the leaves thus isolated is separated it is true from that of the main 
shoot, but its separateness may be held to be secondary, and not a proper 
basis for conclusions as to the primitive construction of the vascular system 
of the normal shoot. 

1 Phil. Trans., part ii., 1885, Plate 73, Fig. 42A. 




FIG. too. 

A Isophila excelsa. Diagram 
of vascular system of a young 
plant in median longitudinal 
section. The xylem is black, 
the phloem lightly shaded, and 
the endodermis is indicated by 
a dotted line, the ground-tissue 
is left white. (After Gwynne- 
Vaughan.) 



200 



ANATOMICAL EVIDENCE 



It appears, then, that the anatomical evidence is consistent with the 
early existence of a small-leaved type of shoot in Vascular Plants. Com- 
parative anatomists are practically unanimous in recognising the non- 
medullated monostele as the primitive stelar type, from which the more 
diffuse vascular types with medulla, and ultimately with separate strands, 
or it may be a dictyostelic state, were derived. Translated into terms of 
general morphology, this opinion indicates a primitive state where the axis 
was structurally dominant in the shoot. The derivation of more complex 
anatomical arrangements from the non-medullated monostele suggests an 





FIG. ioi. 

Embryo of Lycopodium cernuum, after Treub. ,y = suspensor. / = foot. r=root. 
cot cotyledon. The numerous protophylls contain each a vascular strand, which is 
however disconnected from the rest. 

increasing influence of the leaf in the shoot at large, which finds its anatomical 
expression in various types of resolution of the stele into separate strands. 
The general conclusions from anatomy thus appear favourable to a strobiloid 
theory of the shoot, and lead us to contemplate a primitive condition, in 
which the axis was the dominant factor and the appendages of subordinate 
importance. And as this coincides with the story of individual develop- 
ment of the leaf upon the axis in all normal shoots, that coincidence should 
go far in supporting a strobiloid theory of the shoot in the sporophyte 
generation. 






CHAPTER XVI. 

SYMMETRY OF THE SPOROPHYTE. 

AMONG plants at large various types of symmetry are recognised. The 
most simple form is the sphere, a type of symmetry without polarity, that 
is, having no distinction of apex and base : it is the usual initial form of 
the individual, when it is first delimited as the zygote or the spore. Occa- 
sionally this form may be retained to maturity, as in the sporogonium of 
Riccia : but in the vast majority of plants polarity appears early in the 
individual life, usually with growth localised in relation to it. The body 
thus produced may develop variously as regards an imaginary axis of 
construction, which passes between the two poles. Three types of 
symmetry are usually distinguished where polarity exists: (i) the radial 
construction, where the development is equal in all directions round the 
imaginary axis : (2) the bisymmetric or bilateral, in which the construction 
is flattened equally on both sides : and (3) the dorsiventral, where the 
construction is also flattened, but not equally on both sides, the result 
being two faces which differ obviously from one another in form, and 
usually also in inner structure. These types of symmetry may, as a rule,, 
be related to the external conditions under which the parts are developed i 
thus orthotropous, or vertical parts are almost always radial or bilateral - f 
while plagiotropous, that is oblique or horizontal parts, are commonly 
dorsiventral, or occasionally bilateral. Dorsiventrality of the shoot, where 
it exists, has usually some evident relation to the external conditions of 
life, such as the incidence of unequal lighting, or oblique disposition to 
the action of gravity : and it may also be seen in lateral branches to be 
connected with the relation of the part in question to the chief shoot 
which bears it. In some cases it may arise from inner causes, 1 but 
investigation has shown that dorsiventrality of the shoot is usually to be 
referred to some external determining influence. 

1 A striking example of this is brought forward by Willis, in the Podostemaceae, 
where dorsiventrality appears in erect and anemophilous flowers. He suggests that this 
condition has been forced upon them, without reference to any advantage, by the steadily 
increasing dorsiventrality of the vegetative system (Annals of Botany, 1902, p. 593). 



202 SYMMETRY OF THE SPOROPHYTE 

These types of symmetry are not restricted to any of the great groups 
of plants : examples of any one of them may be found in any of the 
great divisions of plant-life. But nevertheless, in certain circles of affinity, 
one or other type of symmetry may be prevalent : thus in the Red and 
Brown Algae the bilateral symmetry is common : among the sporogonia 
of Bryophytes the radial construction prevails : the gametophyte of Liver- 
worts is with very few exceptions dorsiventral. 

The further fact that a single shoot may be at first of one type, and 
subsequently change to another type of symmetry, demonstrates that they 
pass one into another. It can be shown both by comparison and by 
experiment that this occurs within certain limits. The most frequent 
transmutation is that from the radial to the dorsiventral, a change which 
is of special importance in its bearings on the morphology of the 
sporophyte. 

In discussing the subject of symmetry, it has hitherto been usual to 
draw illustrations indifferently, either from the gametophyte or from the 
sporophyte generation. Doubtless, in considering the phenomena of form 
in their general aspects this is right : the wider the net is cast over the 
area of fact, the greater the probability of arriving at a sound conclusion 
as to the qualities and the causes of the several types of symmetry in the 
Plant-body. But it is a different question to enquire into the effect 
which modification of symmetry may have exercised in the evolution of 
the neutral generation. Analogy, with corresponding phenomena in the 
gametophyte, may assist indirectly : but in the elucidation of the actual 
historical record these can only have a theoretical interest. According 
to an antithetic theory the starting-point of the two generations has been 
quite separate and distinct, and this must have its effect on the study 
of their symmetry. 

In the case of the gametophyte various types of symmetry are found 
in the plants of the present day : and since there is no reason to believe 
that there was any common origin of all gametophytes from any one body 
of definite form, there is wide room for speculation as to the source of 
their varying form, and little hope of finality of conclusion. But in the 
case of the sporophyte it is different : the ovum, produced within the venter 
of the archegonium, is normally the starting-point for the sporophyte 
generation in the Archegoniatae : in these plants it is approximately 
spherical in form, and the conclusion follows, on comparative grounds, 
that the initial form of the sporophyte was approximately the sphere a 
body without polarity and of radial construction. The question to be 
discussed in this chapter is, then, what modifications of forms this simple 
body undergoes in the course of its development into the complex sporo- 
phyte, as seen in Archegoniate Plants ; and under what circumstances 
those modifications may have been introduced. 

The development might, in the first instance, consist of simple 
enlargement, together with cell-division, with or without a differentiation 



IX THE BRYOPHYTA 203 

of the products. Such a condition is seen in the spherical sporogonium 
of Riccia, which has habitually been held to be primitive in its simple 
characters of structure and form (compare Fig. 18). Here there is no 
polarity : no distinction of apex and base. This character it shares with 
the earlier .stages of some other embryos of Archegoniatae, which enlarge 
at first as a simple sphere. But a distinction of apex and base soon 
makes its appearance in all the more complex forms, with or without a 
localised apical growth. The two great series of Archegoniatae differ 
widely in the symmetry of their further development. The Bryophyta, with 
very few exceptions, which will require special consideration, show polarity, 
but retain their radial symmetry. Not a few of the Pteridophyta also 
retain their radial symmetry, but under modifications which necessarily 
follow as a consequence of their leafy habit : others, however, depart 
broadly from it, some at an early period of their individual life, others 
at later periods. 

The general view which is implied in the preceding paragraph is that 
the radial type of symmetry is the prior condition for the sporophyte at 
large. This opinion is not based merely on the fact that the ovum from 
which all sporophytes spring is spherical. Much stronger grounds are to 
be found, first, in the high degree of constancy of the radial type of con- 
struction in the sporogonia of Bryophytes : while it is also frequent in 
the Pteridophytes and Seed-Plants, especially in their strobili and flowers. 
Secondly, in the fact that it is possible in many cases to refer the dorsi- 
ventral symmetry, where it exists, to the unequal incidence of external 
conditions, and to see by experiment how such conditions may bring 
about some dorsiventral modification of a structure which is in the first 
instance radial. Examples of this may be quoted occasionally from the 
Bryophytes, and frequently from the Pteridophytes, and from the vegetative 
shoots and flowers of Phanerogams. There is thus not only a compara- 
tive, but also an experimental basis for the opinion that the radial 
symmetry is the primitive, and the dorsiventral the derivative condition in 
the sporophyte. 

Few facts relating to any large group of organisms are more 
impressive than the constancy of the radial symmetry throughout the 
sporogonia of Bryophytes. That body, originally spherical, becomes more 
or less spindle-shaped in its later development, with or without a localised 
apical growth. Zones higher or lower on the spindle-shaped body may 
undergo more strong development than the rest, especially towards the 
distal end, which is to be the fertile capsule. This is commonly seen, 
both in Liverworts and in Mosses, but the Splachnaceae stand out as 
extreme examples, and in Splachnum luteum the apophysis immediately 
below the capsule is expanded into a wide disc (Fig. 102). Nevertheless, 
here also the development is uniform all round in any transverse zone, 
and accordingly the radial construction is accurately maintained. The 
constancy thus usual for the sporogonium in itself directs attention to 



204 



SYMMETRY OF THE SPOROPHYTE 



those relatively few cases among Bryophytes where the radial symmetry is 
departed from. 

Among the Liverworts the only recorded example of departure from 
the radial symmetry is that of Monodea : here the sporogonial head, 
which is borne upon a cylindrical seta, is curved over to one side, and 
it dehisces along the upper surface by a longitudinal slit, the whole 
capsule widening out later into a spoon-like form. Examination of 

developmental stages shows that 

X ^ the young sporogonial head is 

cylindrical in structure. The 
accepted version of this is that 
the capsule is developmentally 
four-valved, but that dehiscence 
is by one slit only, and accord- 
ingly that the four valves remain 
coherent ; but transverse sections 
of the mature capsule show no 
evidence of a structural dorsi- 
ventrality : the transverse section 
is radial up to maturity, and 
there is no apparent structural 
provision for dehiscence. The 
natural conclusion will be that 
Monodea shows only a slight, 
and ontogenetically late and 
unimportant deviation from the 
usual radial type. There are no 
observations connecting this with 
external causes. 

The sporogonia of most 
Mosses are also of the radial 

type throughout ; but in a considerable number a more or less marked 
dorsiventrality is seen in the mature condition. The radial construction is 
as a rule accurately maintained in those sporogonia which grow vertically 
upwards, such as Sphagnum Phascum, or Orthotrichum^ and all sides appear 
equally developed in the longitudinal section (Fig. 103. 3). But in many 
an unequal development is found, which is in relation to the position 
which they assume. As maturity is approached they curve to one side, 
and the capsule becomes oblique, or may even hang over. In these 
cases the earlier stages of the sporogonium are radially constructed, but 
an inequality appears on the two sides, with, it may be, a slightly 
greater development of the assimilating tissue on one side than on the 
other, as in Funaria (Fig. 103. 5) : or this may be carried to such an 
extent that the sporogonium is markedly lopsided, as it is found to be 
in the Buxbaumieae (Fig. 103. 8, 9, 10). 




FIG. 102. 



Splachnum luteum. I. Capsule open. A =apophysis. 
II. Unopened capsule in longitudinal section. j = seta; 
/.jr=leptoxylem ; j/ = stomata on apophysis ; <r^ = colu- 
mella ; / = peristome ; ^j = archesporium ; i intercellular 
space. III. and IV. Diagrams to illustrate the opening 
of the capsule. (From Goebel, after Hedwig, Vaizey, and 
Bryhn.) 



IN THE BRYOPHYTA 



205 



An examination of the origih of this dorsiventrality, thus seen in an 
organ which is as a rule of radial construction, has shown that it appears 
relatively late in the course of the individual development, and is thus 
secondary. The young embryo of Funaria is a radially constructed 
spindle, and the inequality only appears as the capsule approaches 



3. / 






FIG. 103. 

3. Diagrammatic longitudinal section through the green capsule of PhyscoJiiitriwn 
pyriforme. X 14. 5. Median longitudinal section through the mature green capsule 
of Funaria hygrometrica. X 20. 8. Profile view of mature capsule of the dorsiventral 
capsule of Buxbaumia aphylla. 9. Median longitudinal section of the same capsule ; 
I cylindrical air-space; at s the stomata. X 10. 10. Transverse section of the same 
capsule, about the middle. XQ. (After Haberlandt.) 

maturity. In the Buxbaumieae the dorsiventrality is more marked : 
the sporogonium of Diphyscium shows when young a characteristically 
radial cell-net ; but external examination of even young sporogonia shows 
that the dorsiventral form of the capsule as a whole can be recognised 
comparatively early : a condition which accords with its very pronounced 
lopsidedness when mature. An enquiry into the circumstances which 



2O6 



SYMMETRY OF THE SPOROPHYTE 



bving it about has led Goebel to the conclusion that unequal illumination 
is a determining cause ; for he found that in Diphystium the flattening 
of the unilateral sporogonia always takes place on the illuminated side 
(Fig. 104). The primary advantage which is gained by the dorsiventral 
development is the enlargement of the assimilating system. Haberlandt 
has shown how considerable the assimilatory activity is in the capsule 
of Mosses, and has specially pointed out in the case of Buxbaumia how 
much more extensive, as well as better stocked with chloroplasts, the 
enlarged face of the capsule is, than is the side directed downwards. 
A secondary advantage is that the oblique position 
is effective in connection with the scattering of 
the spores. 

Such facts relating to the Bryophyta clearly 
indicate that the radial type of construction is 
the fundamental one for their sporogonia. Not 
only are the departures from that type relatively 
few, and far from being extreme examples as 
compared with dorsiventrality elsewhere, but also 
they may in some cases at least be put in 
definite relation with external causes, and the 
altered form be shown to have a favourable 
biological effect. When to this it is added that 
the dorsiventrality appears comparatively late in 
the individual development, the case seems fully 
made out for the priority of the radial construc- 
tion of the sporogonium of Bryophytes. 

The infinitely greater variety of form among 
the Vascular Plants in some measure confuses 
the question of a fundamental type of symmetry 

for them. Moreover, the issue is further obscured by the diversity of 
their embryogeny : so long as the initial characters of their embryos are 
held accurately to reflect their evolutionary story, this difficulty will 
remain, but in a previous chapter this doctrine has been held open 
to doubt. In the present discussion of the symmetry of the shoot in 
Vascular Plants their embryology will be put temporarily aside, and it 
will be considered towards the close of this chapter. Questions of 
symmetry in Vascular Plants are also complicated by the presence of a 
foliar development. This difficulty will weigh most with those who 
entertain some phytonic theory of the shoot ; but into their difficulties 
we need not enter, since reasons have been given for not sharing their 
view (Chapter XL). Assuming, in accordance with our earlier discussions, 
a strobiloid theory, the shoot will be habitually regarded as an entity, 
and its symmetry as a whole will be held to be determined by the 
equal or unequal development of the appendages, with or without a 
corresponding development of the axis which bears them. 




FIG. 104. 

Diphysciutn foliosum. Longi- 
tudinal section of a stem bearing 
a sporogonium. The arrow indi- 
cates the prevalent incidence of 
light. (After Goebel.) . 



IX VASCULAR PLANTS 207 

In accordance with the general opinions already expressed, it will 
be natural to take first into consideration the fertile region or strobilus : 
or in the higher plants the flower, which is held to be its outcome 
in a more advanced state of development. These may, according to 
a theory of sterilisation, be held to retain the primitive character of 
fertility : it will be seen that they are conservative also in their 
symmetry. 

It has become almost a commonplace of the elementary text-books 
that the radial type of flower in the Angiosperms is the primitive and 
the dorsiventral (or zygomorphic) the derivative condition. The ques- 
tion of symmetry of the flower has been treated so lately and so well 
by Goebel 1 that it is unnecessary here to discuss it in detail. He 
distinguishes two cases : first, that in which the flowers are laid down 
radially, and become dorsiventral in the course of development this 
includes most of the dorsiventral types, and various influences may be 
recognised as conducing to the result, such as the unequal incidence of 
gravity and of light. Secondly, he distinguishes that type in which 
dorsiventrality is brought about before the unfolding of the flower. In 
this case he is of opinion 2 that we have in the position of the flower 
an element of special importance, and the behaviour of the flower in 
becoming dorsiventral only after unfolding must be taken as a starting- 
point in any enquiry into this matter. Lateral flowers are in a different 
position with regard to external forces from terminal flowers. According 
to the sensitiveness of the former to external factors the configuration 
of the flower will be changed more or less early. Such changes may 
become inherited, and flowers so changed will be, of course, favoured 
over others, and many of their parts will be aborted as useless members 
after the introduction of dorsiventral structure. In this connection the 
fact is of importance that in plants normally with zygomorphic flowers, 
when a terminal flower appears, it is frequently of radial type, or is, as 
it is termed, peloric. Goebel remarks, 3 "No doubt these wonderful forms 
of flower exhibit a more primitive type than the dorsiventral flowers, 
which are the normal ones in the plants in which they occur.'' Experi- 
ment has shown in certain cases that peloria is related to intensity of 
insolation, and thus it seems not impossible that the quality of the light- 
ing, as well as position, may have had its influence in leading to 
zygomorphy. But whatever the conclusion drawn from a complete 
analysis of the causes leading to zygomorphy may be, that analysis, as 
far as it has gone, and comparison also, point clearly to the radial type 
of flower in Angiosperms as the primitive, and the dorsiventral as the 
derivative. Similarly, in the case of inflorescences, which are often dorsi- 
ventral in their development, it may be held as probable that the 
dorsiventral inflorescences have proceeded from radial ones. 4 

1 Organography, Engl. edn., vol. i., p. 128. 2 Z.r., p. 132. 

3 L.c., p. 188. 4 Goebel, I.e., p. 138. 



208 SYMMETRY OF THE SPOROPHYTE 

In the Gymnosperms the radial type is constant as a whole for the 
cones, both male and female ; but, as in Welwitschia^ there may be a 
more or less marked zygomorphy in the individual flowers. 

In the strobiloid Pteridophytes also the strobili are for the most 
part of radial construction. In the Equisetineae this is the case with- 
out exception ; also in the Sphenophylleae, so far as they are known. 
The related Psilotaceae are also radial in their fertile region, with the 
exception of the pendent species Ps. complanatum, Sw., which is isobi- 
lateral, probably as a secondary modification in accordance with its pendent 
habit : it bears its leaves and synangia only on the margins of the 
flattened branches. In the genus Lycopodium the strobili are always 
radial, even where the vegetative shoot is strongly dorsiventral. But 
in Selaginella, though the large majority of species show a radial strobilus, 
even where the vegetative shoot is dorsiventral, still the subgenera 
" Homostachys " and " Heterostachys " have dorsiventral strobili. The 
conclusion that these are derivative seems in this case unavoidable. 
Isoetes, and the fossil Lycopodinous genera have all radial strobili. Even 
Ophioglossum and Botrychium may be held to be of the same type, 
their upright axis in the polyphyllous species bearing leaves of equal 
size on all sides ; but in the monophyllous species only one of these 
is as a rule expanded at one time; still, excluding the factor of time, 
the type of arrangement is radial of the shoot as a whole. Helminthostachys, 
however, is dorsiventral as regards the whole shoot; and this may well 
be held, on comparison with the other genera, to be a derivative 
condition. 

The case of the Ferns, including the Hydropterideae, is not so readily 
brought into line with the strobiloid forms, owing to the sori and 
sporangia being distributed widely over their large leaf-surfaces and 
margins, while the sporophylls frequently show no differentiation in form 
or position from the foliage leaves. Their condition will be more fully 
considered later; meanwhile it may be pointed out that, within definite 
circles of affinity, a radial conformation is seen in upright forms, similar 
to that in strobili of other Pteridophytes, while those with oblique or 
horizontal axis show dorsiventrality, such as is seen in the strobili of 
certain Selaginellas and in Helminthostachys. Thus, though no definite 
strobilus is present, the relations of symmetry of the fertile shoot in 
Ferns are still comparable with those in other Pteridophytes. 

Thus a review of the strobili of Pteridophytes as a whole leads to 
the recognition of a very great prevalence of the radial symmetry in 
them. In several large groups, which are certainly primitive in character, 
being represented early in palaeophytological history, the radial type of 
strobilus is never departed from : in others only occasionally ; and this 
radial character of the strobilus may even persist in cases where the 
vegetative system shows dorsiventrality. 

An examination of vegetative shoots of vascular plants at large shows 



IN PTERIDOPHYTES 209 

that they are more highly susceptible of modification of symmetry than 
is the strobilus ; and so they have naturally been the more frequent 
subject of enquiry and of experiment, the observations chiefly relating 
to the post-embryonic shoot. The dorsiventral construction of the 
vegetative shoot is very common in creeping and climbing plants in 
the most different circles of affinity. It also appears in the lateral 
shoots of plants of which the primary shoot is radial. Dorsiventrality 
may make itself apparent either in unequal development of the leaves 
(anisophylly), or in difference of their position ; or it may also affect the 
form of the stem itself. It may be found in one plant that outer 
influences may directly bring about the dorsiventrality, while in others 
it may exist from the beginning, and be hereditary. Goebel 1 has 
pointed out how Vaccinium Myrtillus shows in its lateral shoots a 
transitional state between these two cases ; for in the lateral buds of this 
plant there is an influence exercised, probably by light, which leads 
to a distichous arrangement of the leaves; but it does not take place 
in all buds alike : in some the effect is only a secondary one, acting 
upon the leaves which originate in a spiral succession : in others the 
effect is primary, acting upon the vegetative point itself, on which 
accordingly the leaves arise. The existence of such gradations of effect, 
between dorsiventrality which is the result of immediate impress of outer 
influences and that which is an hereditary condition, is important as 
suggesting how the more fixed dorsiventrality may have come into 
existence. The comparison of such cases, and of the vegetative system 
at large in a number of allied plants, leads to the conviction that in the 
vegetative shoot as well as in the strobilus the radial was the primary 
type, and the dorsiventral the derivative. The causes are probably the 
same in both cases. It is, however, essential to note that the vegetative 
region is more liable to be influenced by them than the fertile; for it 
has been seen in many species of Selaginella and of Lycopodium that 
the vegetative shoot is dorsiventral, while the strobilus is radially con- 
structed. The same is the case with many of the Coniferae. Such 
examples indicate that the strobilus is more conservative of form than 
the vegetative shoot. It is true the converse may be found in some 
of the higher Flowering 'Plants ; for instance, in the Labiatae the vegeta- 
tive shoot is commonly radial, while the flowers are dorsiventral. But 
this condition of the flower is probably one of the relatively late 
specialisation. 

Examining more particularly the vegetative region of the Pteridophytes, 
the radial type of shoot is found with high constancy in the Equisetales, 
both fossil and modern. Also in the ancient Sphenophyllales and the 
modern Psilotaceae : the only exception in the latter being Psilotum 
complanatum, with its bilateral symmetry already mentioned. Of the 
Lycopodiales the early fossil types were characteristically radial in 

1 Orgauography, Eng. ed., vol. i., p. 94. 
O 



210 



SYMMETRY OF THE SPOROPHYTE 



construction, though possibly dorsiventrality may have existed among the 
smaller forms. Of modern Lycopods, the mature shoots of Lycopodium 
show in relation to their position the most gradual transitions from the 
radial to the dorsiventral. The species of the sub-genus Selago, and 
Sub-Selago maintain the radial construction, and are for the most part of 
upright habit. The rest of the genus is very variable : the shoot is some- 
times radial, as in L. i?mndatum : or distinctly distichous forms may occur, 
such as /. Phlegmaria and nummularifolium, which are both pendulous 



A. 



B. 




FIG. 105. 

A, young sporophyte of Danaea simplicifolia still attached to the gametophyte, pr. 
X 3. B, an older sporophyte of the same species. C, gametophyte of A ngiofiteris evecta 
with young sporophyte. (A , B, after Brebner ; C, after Farmer, from Campbell's Mosses 
and Ferns.) 

epiphytes. The change from the radial type may be apparent first in a 
slight inequality of direction of the leaves, otherwise equal, as in the 
creeping shoots of L. annotinum or davatum : or in the marked inequality 
of their size and structure, as in L. complanatum or alpinum. Goebel 
has shown a by experiment on L. complanatum that the dorsiventrality 
is directly induced by light. Finally, the climbing species, /. volubile, 
is specially characterised by a distichous form of the shoot not unlike 
that usual in Selaginella. It has already been concluded on other 
grounds that the Selago type of Lycopodium is the most primitive : it 
is this same type which retains most constantly that radial construction 
which there is reason to believe is a primitive character. 

1 Organography , vol. i., p. 252. 



IN PTERIDOPHYTES 



2 I I 



The genus Selaginella includes eight species which are isophyllous, and 
over three hundred which are anisophyllous in the vegetative region. An 
intermediate condition is found in S. sanguinoletita, which has accordingly 
been investigated by Goebel. 1 He found the dorsiventral structure to 
arise under the influence of external factors, such as moisture and shade. 
In certain strong-growing species, such as 6". caulescens, the upright axes 
may be isophyllous, and the dorsiventrality only become pronounced in 




FIG. 106. 
A small plant of Dp.na.ea alata. x. .r/^stipules. (After Campbell.) 

the plagiotropic upper branches. Others may be dorsiventral from the 
first. Thus the genus illustrates gradual steps of dorsiventral development, 
such as have been noted in Lycopodium and Vacrinium, though the details 
are different. And the same conclusion may be drawn as in those cases, 
viz. that the radial is the primitive form, and the dorsiventral the derivative. 

The Fern-shoot, notwithstanding the preponderance of its leaf-develop- 
ment, may be examined from the same point of view as other Pteridophytes. 
The ancient Marattiaceae illustrate a probable initiation of dorsiventrality 
of the shoot within their own phylum. The young sporophyte seedling 

1 L.f. t p. 105. 



2 I 2 



SYMMETRY OF THE SPOROPHYTE 



I 




FIG. 107. 

Diagrammatic representation of the end of a 
rhizome of Kaulfussia. iv wings of stipule; 
com transverse commissure. (After 
Vaughan. ) 



Gvvynne- 



in all the genera of Marattiaceae investigated hitherto is upright, bursting 
through the prothallus, not recumbent as in other Ferns (Fig. 105). In 

Angiopteris and Marattia this con- 
dition is maintained throughout life, 
and there is no reason to think other- 
wise than that these plants retain 
their primitive position. It is probably 
shared also by Archangiopteris ; a at 
all events there was no dorsiventrality 
in the only specimen with an axis 
hitherto examined. In the genus 
Danaea the same holds for D. 
simplidfolia ; but certain other species 
of Danaea have an oblique rhizome, 
for instance, Danaea alata (Fig. 106). 
Comparison of a number of stocks of 
this Fern shows various degrees of 

inclination and curvature of the axis. It is upright at first, and produces 
leaves and roots uniformly on both sides of the axis; but later the axis 
arches over to one side, and a distichous arrangement of the leaves is 
approached, while the roots originate chiefly from 
the side directed downwards. In Kaulfussia these 
characters are more pronounced ; for there the 
mature rhizome is horizontal, with marked dorsi- 
ventrality, and with internodes of appreciable 
length (Fig. 107). Unfortunately the early 
development of the sporophyte of Kaulfussia is 
still unknown. It seems a reasonable interpre- 
tation of the facts that the upright position, with 
radial symmetry, as seen in Angiopteris and 
Marattia, was the primitive condition here as in 
other Vascular Plants : and that the oblique 
position, already seen in certain Danaeas, became 
more accentuated in the horizontal rhizome of 
Kaulfussia, with its marked dorsiventrality. 

The analogy with what is seen in the Ophio- 
glossaceae greatly strengthens this conclusion. As 
in the Marattiaceae, so also in all the genera of 
the Ophioglossaceae the axis is from the first 
upright (Fig. 108); and that position is maintained 

throughout life in Ophioglossum and Botrychium. But in Helminthostachys, 
notwithstanding its originally vertical position, the axis of the mature 
plant is markedly dorsiventral (Fig. 109), with distichous arrangement 
of the leaves. The conclusion which naturally follows is that in the 

1 Gwynne-Vaughan, Ann. Bof., xix., p. 260. 




FIG. 108. 

Helm in th ostachys zeyla nica. 
Young plant attached to pro- 
thallus. Natural sixe. (After 
Lang.) 



IN PTERIDOPHYTES 



213 



Ophioglossaceae, as in the Marattiaceae, the upright radial shoot is 
primitive, and the dorsiventral character of the shoot derivative : that in 
Helminthostachys, as in Danaea and Kaulfussia, all of them heavily-leaved 
forms in proportion to their relatively elongated stem, the axis has become 
horizontal. 

The Leptosporangiate Ferns present a more varied and difficult problem as 
regards the symmetry of the shoot. They include many upright radial forms, 
such as Cyathea, Alsophila, or Osmunda as well as many which show various 
degrees of obliquity of the axis, accompanied by corresponding degrees 
of dorsiventrality. It is not an uncommon thing to find upright radial 




FIG. 109. 

Naked-eye drawing of rhizome of Helminthostachys zeylanica. / r =stipular flap ; R =root ; 
L leaf; /> = petiole; L S leaf-scar. (After Farmer and Freeman.) 

species, and straggling, dorsiventral species in the same genus : for instance, 
Onoclea germanica is radial and upright, Onodea sensibilis is creeping ; 
Pteris aqiiilina is creeping, while many other species are tufted and radial. 
The same question will arise here as elsewhere, which of the two was 
the primary condition. From analogy with other cases as well as from 
internal evidence, the upright, tufted forms with radial symmetry would 
be held to be the prior type ; but the question is complicated by the fact 
that the embryo itself is not upright in the Leptosporangiate Ferns, and 
it may be thought that its recumbent position defines ab initio the dorsi- 
ventrality of the shoot. 

It is true that the first leaf of the young sporophyte is uniformly on 
the side away from the prothallus: but the position of the second and 



2i 4 SYMMETRY OF THE SPOROPHYTE 

succeeding leaves may vary, as was already noted by Hofmeister. He 
stated specifically 1 that "the similarity in the development of the different 
species of Ferns does not extend beyond the formation of the rudiments 
of the first frond, and of the first root." He then proceeded to show 
that whereas in Pteris aquilina the second frond originates on the side 
of the axis opposite to the first, and distant from it by about half the 
circumference of the stem, in Nephrodium (Aspidium) Filix mas it is 
at a distance of about a third of the circumference 2 : " the third diverges 
from the second, and the fourth again from the third at about 120 
degrees to the right, so that the fourth stands vertically over the first." 
Thus the arrangement of the leaves is radial from the first in the Male 
Fern. The same radial character, with varying divergences, has been 

found in other Fern-seedlings : for 
instance, in Nephrodium dilatatum, 
Asplenium marinum, Pteris tremula, 
Osmunda regalis, and Todea superba. 
The leaf-divergence in such cases is 
variable, but approximates to a half, 
or one-third, and it is spiral from the 
first, without dorsiventral tendency. 
This shows that in plants which are 
radial in the mature state the initial 
condition of the recumbent embryo 
does not impress dorsiventrality' on the 
seedling. In others, however, where the 

Polypodiuni vulgare. X6. Median section i ji i 

through prothaiius, and embryo, partly diagram- mature plant is markedly dorsiventral, 

matic : showing one series only of the distichous .-> i ,-. , i/- i 

leaves A, 4 etc.; *= roots ;/= apex of axis, the dorsiventrality asserts itself early. 
^^^t^JK^tS^SSuL" Among these the case of Polypodium 

vulgare is very instructive, as showing 

that the dorsiventrality of the mature shoot is not a direct consequence 
of the initial procumbent position. The initial embryogeny is as usual : 
the second leaf of the embryo is obliquely on the same side of the 
axis as the first, and as the subsequent leaves are also inserted alternately 
and obliquely on that same side, and the growth of the axis is stronger 
on that same side which faces away from the prothaiius, the whole 
shoot becomes inverted by a strong curvature upwards through the 
apical indentation of the prothaiius ; and thereafter it pursues its horizontal 
course backwards over the top of the prothaiius (Fig. 1 1 o). It is interest- 
ing to note as a consequence of this that the side of the axis initially 
turned downwards in the embryo becomes the upper side of the dorsi- 
ventral rhizome, owing to the inverting curve : thus the creeping posi- 
tion of the permanent shoot is not merely a direct continuance of the 
initial prone position of the embryo. Other examples might be quoted, 
but this will suffice to show that the dorsiventrality of the mature shoot is 
^Higher Cryptogramia^ Eng. ed., p. 208. -/.<;, p. 227. 




IX PTERIDOPHYTES 215 

not a necessary or a direct Consequence of the recumbent position of 
the embryo of Leptosporangiate Ferns. It may also be added that 
the dorsiventrality, in cases where it exists, may be initiated in different 
ways. Such evidence points to its being a secondary condition. 

A further indication that the shoot of Leptosporangiate Ferns is primarily 
radial is to be seen in the internal structure of the axis. It has been 
found in a considerable number of cases that the vascular tissue is 
uniformly developed all round. This is naturally the case in upright 
radial axes : but, apart from the leaf-insertions, it may even be so in 
stems which are horizontal, such as Matonia : though in others, such 



P" 




FIG. in. 

Transverse section of axis of seedling of Lygodiumjaponicunt^ below the first leaf. / one 
of the xylem-parenchyma cells. X^go. (After Boodle.) 

as Pteris aquilina, the vascular system is like the stem itself dorsiventral. 
In young seedlings it has been shown in various cases that the stele 
is cylindrical, and it is found to be so even in Ferns which are markedly 
dorsiventral in the mature state, such as Lygodium japonicum (Fig. in). 
Such examples indicate again a probability that the radial construction 
of the shoot was primitive in the sporophyte of Ferns. 

But it may be urged by those who dissent from this conclusion that 
dorsiventrality is clearly seen in the early embryonic stages of some other 
Pteridophytes, and especially in the case of various species of Lycopodium. 
But here also it seems probable that the condition is adaptive rather 
than primitive : for in the first place the embryo in the genus is singularly 
inconstant in its form : in some species, such as Z. Selago, or Z. davatum, 



2i6 SYMMETRY OF THE SPOROPHYTE 

the embryo settles down at once into an upright radial type of structure : 
in others, and particularly in L. cernuum, which has been made the subject 
of special study and comparison, the embryo may show at first a marked 
dorsiventrality ; but it is at the same time exceedingly variable in form, 
and in some individual cases the embryo of L. cernuum may closely 
resemble the ordinary radial type of other species. This variability will 
in itself discount arguments based upon details of form, and suggests 
that the dorsiventrality where it occurs is the result of relatively direct 
adaptability of a very plastic organism. 1 

The facts and arguments brought forward in this chapter lead up to 
a general view of the symmetry of the sporophyte generation. It would 
appear probable that the original type of its construction has been radial 
throughout, a condition which commonly goes along with a vertically 
upright position. This is the position of the vast majority of Bryophyte 
sporogonia : in them the radial construction is rarely departed from, and 
where this does happen the dorsiventrality is readily referable to a 
modification of a radial type. The greater diversity of habit of the 
Pteridophytes, especially as regards the sporophyte, necessarily brings 
greater difficulties in attaining to any general opinion for them ; but a 
careful review of their various types, and especially a comparison of 
members of the same group of them inter se, leads back constantly to 
the radial type as primitive, even in cases where dorsiventrality is most 
marked. The fact that in the Equisetales and Sphenophyllales the radial 
construction is predominant, while it is also prevalent among the more 
primitive Lycopodiales, and in a less degree in the Filicales, shows the 
strong hold which the radial construction had among very early types. 
In fact the position is fully strong enough to justify the general state- 
ment that the radial mode of construction was primitive for the sporophyte 
at large ; and that where dorsiventrality occurs, it is a secondary 
condition. 

This conclusion is plainly out of harmony with the theoretical posi- 
tion of Lignier, 2 who would refer the sporophyte as well as the gametophyte 
to a hypothetical thalloid origin : this thallus, which was dichotomous, and 
lay flat upon the soil, tended to curve upwards, and consequently to 

1 The more exact comparison of the embryology in the genus Lycopodiuin will be 
taken up in the special part of this work. 

2 " Equisetales et Sphenophyllales. Leur origine filicineenne commune." Bull. Soc. 
Linn, de Nonnandie, 1903, p. 93. A somewhat similar speculation has recently been 
published by Tansley (New Fhytologist, 1907, p. 25) ; he refers the Archegoniatae in 
origin to some " hypothetical Archegoniate Alga." He also passes lightly over the 
transition from a sympodial rhizome to an upright, radially organised type (p. 33). It is 
necessary, however, to remember that, as a matter of observation, all Archegoniate 
sporophytes are initially of radial construction. The same difficulties appear to confront 
both Tansley's and Lignier's hypotheses. To meet them both authors postulate hypo- 
thetical forms which are "of course the purest speculation" (I.e., p. 32). It appears 
preferable to adhere to observed facts. 



IX PTERIDOPHYTES 217 

assume a cylindrical symmetry. On this hypothesis the dorsiventral was 
the prior state for both the sporophyte and the gametophyte, and the 
radial the derivative. The author himself states that this " prohepatic " 
type, from which the two generations were evolved, is still wholly hypo- 
thetical. As regards the sporophyte, since the embryology gives no 
countenance to an originally dorsiventral " prohepatic " state, while 
instances are common of the impress of dorsiventrality upon parts of it 
originally radial, the theory of Lignier cannot be upheld. It may apply 
for the gametophyte, but that has nothing to do with our present dis- 
cussion. It seems the unavoidable conclusion from the facts that the 
primitive symmetry of the sporophyte was radial. 



CHAPTER XVII. 

THE ESTABLISHMENT OF A FREE-LIVING SPOROPHYTE. 

So far the shoot only of the sporophyte has been the subject of discussion : 
it remains to consider the question how the sporophyte, originally dependent 
upon the parent prothallus, became established as a free-living organism 
on the soil. There will be no two opinions which of the principal 
regions of the independent sporophyte, the shoot or the root, was of 
prior existence : it is a necessary outcome of the evolution of the neutral 
generation as sketched above that the shoot was first established, as a 
body dependent on the gametophyte ; it carried out primarily the function 
of spore-production, but ultimately also, as we have seen, that of vegeta- 
tive nutrition. The root is essentially an accessory, which made its 
appearance after those earlier steps were past; it arose from its 
primitive state of dependence to an existence free from the parent 
gametophyte. 

Comparison of living plants indicates, however, a probability that the 
initiation of a root-system followed closely upon the adoption of a free- 
living habit : for roots are present in free-living Pteridophytes with very 
few exceptions, and are, as a rule, formed early in the embryology. It 
seems doubtful, even in the few exceptional cases, whether the rootless 
condition is not due to reduction, rather than representative of a primi- 
tive rootless, but free-living sporophyte. Among the Pteridophytes roots 
are absent in the Psilotaceae, also in certain Hymenophyllaceae, and in 
Salvinia : it seems probable that reduction will correctly account for it in 
such specialised forms as the Hymenophyllaceae ; and also in Salvinia, 
with its peculiar floating habit : the question in the Psilotaceae is more 
problematical, and their rootless condition may perhaps have been really 
primitive, though in the absence of any knowledge of their embryos there 
is no clear indication that it was so : moreover, their habit is so peculiar 
as to make any conclusion difficult. Rootless Phanerogams also exist, 
but there is no reason to regard them as other than results of relatively 
recent reduction. Accordingly, it may be concluded that there is little 






GENETIC RELATION OE AXIS AND ROOT 219 

X 

evidence from plants of the present day of the existence of a primitive, 
permanently free-living, but rootless state of the sporophyte. 

The root in the fully-developed state is broadly different from the 
axis : its endogenous origin, its root-cap, and the radial arrangement of its 
vascular system are its most distinctive features, in addition to the absence 
of appendicular organs, other than root-hairs, or lateral roots. Its full 
character depends upon the collective existence of those features ; for some 
of them are inconstant, and all of them may occasionally be matched by 
axes : 1 thus the two parts are not absolutely distinct in character. 
Sometimes, indeed, it may be found that roots grow on directly into normal 
leafy shoots, as in certain Ferns, Aroids, and Orchids, etc. : ' 2 the con- 
verse, however, has not yet been shown to occur. 

The resolution of the problem what genetic relation, if any, subsisted 
between axis and root will naturally be looked for in such plants as show 
the least degree of differentiation of those parts. As such the living 
Lycopods are pre-eminent, while their fossil relatives also show features 
of importance for comparison. Like axes, the first roots may be exogenous, 
as in certain Lycopod embryos, and in Phylloglossum : in the Lycopods 
the roots show apical dichotomy as do their stems also, while the exarch 
xylem and general disposition of the vascular tissues of the Lycopod stem 
are points of similarity to root- structures which are not equalled in other 
Vascular Plants. Finally, the Selaginellas present features of further 
interest in their so-called " rhizophores," parts which occur in many, but 
not in all species : they are exogenous in order, and capless : they branch 
dichotomously, and upon them the roots with root-cap arise endogenously. 
In structure they are usually like roots, but in some cases the rhizophore 
has a structure resembling that of an axis : for instance in 6". Kraussiana 
the protoxylem is central, and the whole arrangement very like that of the 
stem in S. spinosa? Further, the rhizophores may be readily converted 
in some species into leafy shoots, by suitable cultivation. Thus the 
rhizophores do not show the full characters of roots or of axes, and the 
question has long been debated whether or not they are truly of root- 
nature. Some prefer to distinguish them by a special name, as "rhizo- 
phores " : others describe , them merely as the aerial region of the root. 

1 Exogenous roots are seen in Phylloglossum, and in the embryos of some species of 
Lycopodiitin, as well as in some Phanerogams. Capless roots are known in Aesctilus, and 
in some few others (Goebel, Organography, vol. ii., quote from Engl. ed., p. 267). On 
the other hand, a protective cap has been observed on the apex of the axis in embryos 
of Araucaria, and Cephalotaxus by Strasburger (Angiosp. und Gymnosp., Plates xix. to 
xxi. ) : endogenous shoots are not common, but they occur occasionally, as in the flower- 
buds of Pilostylcs, as well as on the emergence of shoot-buds adventitiously from roots 
(Goebel, I.e., pp. 226, etc.). A radial disposition of the vascular tissue, i.e. with exarch 
xylem, is characteristic of the axes of Lycopods, and of some others of the early types 
of Pteridophytes. 

"Goebel, Organography, vol. ii., p. 226. 

3 Harvey Gibson, Ann. of Bot., 1894, PI. x., Fig. 39. Also 1902, PI. xx., Fig. 17. 



22O 



A FREE-LIVING SPOROPHYTE 



For my own part, I am satisfied to regard them as belonging neither to 
the category of stem nor of root, but as a result of development to meet 
a certain need, and that the growth produced was not of either character 
in phyletic origin. 1 

It is interesting to compare Selaginella with the allied fossils, which 
have as their underground system the enigmatical Stigmarian development 
(Fig. 112). These underground parts of Lepidodendron and Sigillaria 
present morphological questions somewhat similar to those of Selaginella : 
the main Stigmarian trunks are not roots, for their anatomical structure 




Fl(i. 112. 

Ground plan of a Tree-stump with Stigmaria-trunks. One-sixtieth the natural size. 
(After Potonie.) 

is far removed from that of any known roots ; they are not typical rhizomes, 
for the only appendages they are known to bear are the Stigmarian 
rootlets, which are rightly so recognised from their anatomical features. 
They may be best classed with the rhizophores of Selaginella, or more 
especially with the basal knot on the hypocotyl of S. spinulosa (Fig. 113), 
though the correspondence is far from being exact. These, the Stigmarian 
trunks, and the curious processes in Pleuromoia (Fig. H4), 2 may all be 
held to be outgrowths which fall into no recognised category of parts, such 
as stem, leaf, or root ; and they all serve the same purpose, of acting as 
a basis of attachment for the roots themselves. The existence of such 
bodies points to the Lycopodiales as presenting characters of peculiar 



1 Cf. Goebel, Flora, 1905, p. 209. 



Solms, Bot. Zeit., 1899, p. 227. 



ORIGIN OF ROOT 



22 



interest in any discussion of the origin of a subterranean absorptive 
system. 1 

But the presence of such " rhizophores " does not greatly assist the 
solution of the problem of origin of the roots themselves. There is, in 
fact, no sufficient or decisive evidence how the root came into existence 
in Vascular Plants ; but on the facts as they stand two alternative opinions 
are possible. Either that it resulted from the transformation of a leafy 
shoot by loss of the appendages, followed by other special adaptations in 
relation to its life, and to its absorptive function in the soil. Or that it 







Fie. 113. 

Plant of Stlagiitella spinulosa, with root system springing from swollen knot at base of 
the upright hypocotyl. i^ natural size. 

arose as a new type of haustorial outgrowth, not originally of shoot-nature ; 
but nevertheless that in its first and less differentiated condition it 
resembled the shoot from which it arose, in its structure, and in the 
character of its branching. That those features which were helpful in its 
absorptive and conducting functions were permanently maintained, and 
they became distinctive characters of the differentiated root : other charac- 
ters, such as the root-cap and endogenous branching, may have been added 
in accordance with the underground habit. This latter view seems to me 
the more probable alternative. 

Applying it in the case of the Lycopodiales, the root at its inception 
would, like the stem of these plants, be exogenous, with exarch xylem 
1 Compare Goebel, Organography, vol. ii., p. 230. 



222 



A FREE-LIVING SPOROPHYTE 



: 



and dichotomous branching ; and these stem-like characters are actually 
exemplified in the roots of living Lycopods ; but in most cases the 
exogenous origin and dichotomous branching gave way to endogenous 
origin and monopodial branching, both of which are more suitable for 
parts which have to make their way through the soil. The exarch xylem 
was, however, maintained ; and, being biologically convenient in absorptive 
organs, it became a characteristic feature of the root for Vascular Plants 
at large. Many of the primitive types of Vascular Plants had exarch 
xylem in the stem ; and if in the same way their primitive roots resembled 

their stems structurally, they 
also would have exarch xylem. 
On this hypothesis the roots 
would appear to have retained 
a structural character which 
was represented in the early 
structure of stems. In this 
way the origin of roots may 
be presented to the mind 
without their being held to 
have been actually the result 
of transformation of a leafy 
shoot itself, of which there is 
no evidence from abortive 
appendages. But as a matter 
of fact, there is no certain 
knowledge how the root 
originated. 

In most embryo sporo- 
phytes of the present day a 
root-development is initiated 
before the need for it arises, 
that is while the embryo is 
still entirely dependent upon 

the parent gametophyte. But it cannot be assumed that this was always 
the case : indeed, it is thought by some that there is an inherent 
probability that some intermediate condition may have preceded the 
initial formation of the root in descent. Among the embryos of some 
of the early types of Vascular Plants a condition has been found which 
has been held to be primitive, and to illustrate how the transition to 
a free-living condition of the sporophyte may have been effected : it is 
seen in certain species of Lycopodium, L. cernuum, and L. inundatum, 
in which the prothallus is green and subaerial, thereby suggesting a rela- 
tively primitive condition as compared with other types of the genus. In 
these the upper half of the embryo, owing to the rapid extension of the 
massive foot, is soon extruded laterally from the prothallus (Fig. 115). 




FIG. 114. 

Pleuromoia Sternbergii. Swollen base of stem with root- 
scars, and showing part of the aerial stem, with the epidermis 
and leaf-scars on the right, and on the left the sub-epidermal 
sculpture. (After Bischof, from Engler and Prantl.) Two-thirds 
natural size. 



THEORY OF THE PROTOCORM 



223 



The body of the embryo thus exposed bears the cotyledon, and a variable 
number of leaves directed upwards, but it terminates downwards in a tuber- 
like body provided with rhizoids (compare Fig. 21, p. 37). At first there 
is no root, and in extreme examples the appearance of the first root may 
be deferred for a considerable time ; but so soon as the normal aerial 
shoot with leaves is defined, the first root soon penetrates the soil, and 




FIG. 115. 

Young embryo of Lycopodiitm cernuutn, beginning to project from the prothallus. 
ar=archegonium ; s = suspensor ; cot = cotyledon ; ta = embryonic tubercle. X 300. 
(After Treiib.) 

establishes the plant in the usual way. The tuber which thus precedes 
the establishment of the plant by means of a root was called by 
Treub the "protocorm," and he regarded it as a rudimentary structure, 
which was the phyletic forerunner of the leafy shoot as now seen 
generally in Pteridophytes. It is represented, however, only in few cases, 
and is not constant even in the genus Lycopodium : for instance, it 
is absent in L. Se/ago, and also in L. Phlegmaria and L. clavatum. 
In PhylloglossuiHi on the other hand, this type of development is not 
only found in the primary embryology, but is repeated constantly in 



224 A FREE-LIVING SPOROPHYTE 

each season's growth, while the tuber itself is here greatly enlarged for 
purposes of storage. The characteristic "protocorm" is absent from all 
other Pteridophytes. 

The question is, what is the true interpretation of these facts. Does 
the protocorm really represent some condition which existed in the 
phylogeny, intermediate between the fully-rooted sporophyte and that 
more primitive state where it was fully dependent on the prothallus? 
The first point which strikes attention is the way in which the transition 
from dependence to independence of the sporophyte is actually carried 
out in the plants which show this " protocorm " development : assuming 
that there is some difficulty, nutritive or other, in formation of the root 
itself, the case is quite adequately met by the tuberous development with 
rhizoids, as a temporary shift. It seems not improbable that some such 
difficulty should precede in descent the initiation of so important, and so 
characteristic a body as the root. A second point, however, is that a 
protocorm development is exceedingly limited in its distribution among 
living plants : it is not constant even in the genus Lycopodium^ and 
outside the Lycopodiales it is not characteristically developed in any 
other of the early forms : this must be taken fully into consideration 
before assigning to the " protocorm " any general phyletic significance. 
But, on the other hand, it may be urged that the real importance of 
the " protocorm " would exist only in those cases where either the 
root-development has not yet been initiated in the race, or where its 
late development in the individual is a matter of moment, on nutritive 
or other grounds. Immediately any initial difficulty of development of 
a root-system is surmounted in any line of descent, the " protocorm " 
would be liable to be cut out of the ontogeny, as a cumbrous and 
unnecessary stage. This would sufficiently account for the absence of a 
" protocorm " in the great majority of Vascular Plants. But, again, 
Goebel, in arguing against the general phyletic significance of a 
" protocorm," has cited a number of cases of Phanerogamic Plants in 
which, if the formation of the root is suppressed temporarily or entirely 
in the seedling, a protocorm-like body is formed, which is anchored to 
the substratum by hairs. 1 He remarks that this appearance of a 
protocorm in very different circles of affinity seems to him unfavourable 
to the hypothesis of its having a phyletic significance, and he only sees 
in it an organ which corresponds in its development, and especially in 
its formation of roots, to an arrested hypocotylous segment : he suggests 
that a suppression of the formation of the roots may have taken place 
in Lycopodium, as also in the Orchideae, and that this was connected 
with the prolonged development of the germ-plant in them : perhaps 
also the symbiosis with fungi which takes place in these plants, may 
have had its effect. On this view the " protocorm " would be secondary, 
and it would not illustrate an archaic mode of establishment of the 
1 Organography, vol. ii., Engl. ed., p. 232 



THEORY OF THE PROTOCORM 225 

X 

sporophyte on the soil. Such a suggestion certainly accords readily with 
the sporadic occurrence of the " protocorm." 

It is difficult to arrive at a conclusive balance between such conflicting 
facts and arguments as these. So far as any conclusion commends itself 
to my mind it is as follows : A " protocorm " development may have 
been an important phase in the establishment of certain Lycopod embryos, 
in that it serves as a temporary substitute for a root-system delayed in 
its development. But it seems unnecessary to take such cases as proto- 
types for even the genus Lycopodium as a whole : since the Lycopod 
embryo, while showing essential unity in its general plan, seems prone 
to parenchymatous swelling. Two such swellings, somewhat similar in 
structure but differing in place of origin and in function, are known, viz., 
the enlarged " foot " of L. davatum and annotinum, which originates 
from the lower tier of cells of the embryo, and is intra-prothallial ; and 
the " protocorm " of the cernum-type, which originates from the upper 
tier of the embryo, and is extra-prothallial. They are both biologically 
intelligible, for the former acts as an haustorium, the latter may be a 
ready mode of fixation in the soil, and also a specialised place of storage. 
A genus which shows two types of parenchymatous swellings in two 
distinct types of embryo, while both are absent from other species of 
the genus, cannot be expected to have ever had one of these as a 
constant feature in its ancestry. This consideration makes me doubt 
any general application of the theory of the " protocorm " even in the 
genus Lycopodium. These parenchymatous swellings may be looked upon 
as opportunist growths, rather than as persistent relics constant from a 
remote ancestry. This view is greatly strengthened by the occurrence of 
protocorm-like developments in isolated cases among the Angiosperms. 
Phylloglossum with its large storage " protocorm " would then be the 
extreme type of a line of embryological specialisation, not a form pre- 
serving the primitive embryological characters of the whole race. On 
such grounds, while not denying that a " protocorm " may have had a 
certain importance in certain cases, the facts do not appear to justify 
attaching to it any general significance. 

From the above pages it will be plain that the origin of the free- 
living habit of the sporophyte, and of its root-system is quite as obscure 
as that of the leafy shoot itself. The important step from dependent to 
free life was certainly taken at a period before the earliest fossil records of 
Vascular Plants ; for all the best-known types of early fossil Pteridophytes 
have roots assigned to them on secure grounds of observation : so 
naturally the evidence from them does not lead to a solution of the 
difficulty. On the basis of comparison, to which this question must 
necessarily be relegated, no decisive help is forthcoming; the theory of 
the protocorm, which at first sight seemed so full of promise, does not 
give more than a suggestion how the transition from dependence to 
independence may actually be carried out in certain cases, and among 

p 



226 A FREE-LIVING SPOROPHYTE 

the Pteridophytes it is illustrated only within a strictly localised area of 
affinity. The course of transition from the dependent embryo to the 
rooted plant, as it is carried out in the individual life, may be held to 
be the most reasonable guide to the same transition in the past. It is 
seen to be occasionally through the intermediary of a protocorm, but 
oftener without. It may be that this indicates correctly the actual course 
which events took; and suggests that all vascular sporophytes did not 
achieve their independence in the same way. 

It is of course possible to take an entirely different view of the 
relations of the two generations from that here presented, and to consider 
the dependence of the sporophyte as being itself secondary, and the 
haploid and diploid phases as having been originally as independent as 
they are seen to be in Dictyota. In that case the problem would be 
the converse : viz., to trace the origin of the dependent state of the 
sporophyte. There is, however, no serious basis of fact or comparison 
hitherto adduced, which can place this suggestion upon a footing of 
reasonable probability : it will suffice here to have mentioned that the 
suggestion has been made. 



CHAPTER XVIII. 

THE EVIDENCE FROM PALAEOPHYTOLOGY. 

IT has been remarked above (Chapter I.) that the only direct and positive 
clue to the sequence of appearance of Plant-Forms in past time upon the 
earth is to be obtained from the study of fossils. Luminous facts derived 
from them are beginning to shed a fresh and direct light upon problems 
hitherto obscure ; and the last quarter of a century especially has shown 
how greatly a knowledge of the fossil forms may advance the true per- 
ception of affinities of certain groups of plants now living. 

But the success which has already attended Palaeontological investi- 
gation, and has led to such important results, must not be allowed to 
disguise the limits which circumscribe this branch of enquiry : nor should 
it unduly raise the hope that the area of fact available for comparison 
with forms now living will be indefinitely extended. It can hardly be 
anticipated that data derived from fossils will ever take a decisive place 
in discussions of the primary origin of the sporophyte. In the mind 
of the Morphologist there can be no spirit of depreciation of the recent 
advances of Palaeophytology, but rather a very high estimate of their 
value : nevertheless he cannot help recognising how inadequate the 
evidence drawn from fossils is in its bearing on such questions as those 
discussed in the foregoing chapters. Hitherto it has given no clue 
whatever to the origin x of the Bryophyte sporogonium : nor does it 
materially assist in resolving the problem of the origin of the leafy 
sporophyte, or of its adoption of a free-living habit : nor, again, does it 
indicate with any decisiveness the evolutionary relationships of the great 
phyla of the early Pteridophytes. All these questions deal with events 
which we may presume to have preceded the existence of the earliest 
fossils of which any exact record has hitherto been discovered. 1 

1 I am unable to share the very sanguine view of Mr. Arber (Annals of Botany, 1906, 
p. 216), who remarks that " the imperfection of the Record, largely exaggerated in the 
past, can be wholly neglected where we are considering the larger divisions of the vege- 
table kingdom, such as phyla, classes, or groups of Plants." 



228 EVIDENCE FROM PALAEOPHYTOLOGY 

There are three palpable deficiencies in the Palaeontological evidence : 
one, as has been said, is its incompleteness as regards the prime origins 
of the leading types which are lower in the scale of vegetation ; another 
is the usual, and almost necessary absence of developmental detail ; the 
third arises from the frequency with which fossils are known by impressions 
only, without the material sufficing for study of the internal structure. 
This is especially so for some of the earliest, and from an evolutionary 
point of view the most important forms. The first is by far the most 
serious shortcoming. . 

The earliest fossil-bearing strata contain plant-remains which are more 
in the nature of independent problems than an assistance, on any basis 
of comparison, to the understanding of the known types of the vegetable 
kingdom. Such plants as Nematophycus and Pachytheca suggest the 
existence of Algae in the Silurian age, but are not readily ranked with 
more modern forms. Similarly, the plant-remains from the Lower Old 
Red Sandstone are highly problematical, though they indicate a probability 
of terrestrial life. This seems more clear in the Middle Devonian, where 
among other remains of plants apparently of the land, Palaeopitys Milleri 
has been found : this is a stem with structure, showing tracheides arranged 
evidently as having been produced from a cambium, while pits are seen 
in the longitudinal sections : the whole structure is reminiscent of some 
Cordaitean structure. But it is only in the upper Devonian that the 
'remains of a Land-Flora are such as to be referable with any degree of 
confidence to known types : thus Bothrodendron Kiltorchensc seems plainly 
to be a large Lycopod ; Archaeopteris hibernica has usually been referred 
to the Filicales, though it has recently been suggested that it may not 
improbably be in reality the male fructification of a Pteridosperm ; 
Pseudobornia ursina lately described from Bear Island by Nathorst, is a 
Calamarian type with relatively large fimbriated leaves; characteristic 
Cordaitean remains are also to be found. These may all be referred to 
well-known groups of Land-growing Plants, and though they may differ 
in certain important respects from related forms of later date, they show 
in complexity of character, and often also in size, features which are 
definitely those of the highly organised phyla to which they are referred. 
Thus the early representatives give little clear information beyond the fact 
of the early existence of those phyla to which they belong : they do not 
provide an explanation of their origin, and help only slightly to form 
opinions as to their mutual relations. Few facts are more striking than 
this apparently sudden presentment of certain vegetable types, already 
showing in a high degree the characteristics of their class. An extreme 
case of this is pointed out by Zeiller. 1 He remarks that evidence of the 
existence of the Gymnosperms, " dates from the base of the strata of Gaspe 
in Canada; that is to say, from the most ancient epoch which has left to 
us the remains of terrestrial plants : they are there represented by the 
1 Elements de Palaeobotanique^ p. 369. 



ITS LIMITATIONS 229 

Cordaiteae, a type already very, perfect and specialised. We cannot then 
draw from the data of Palaeobotany which we possess any indication of 
the origin of these first Gymnosperms." This illustrates how hopeless it 
must be, so long as earlier strata yield only indefinite remains or none at 
all, to base upon stratigraphical evidence any consecutive story of the 
rise of a Land-Flora : for on a comparative basis these Gymnosperms which 
thus appear so early stand high in the scale of Vascular Plants. Other 
examples might be quoted, but this will suffice to illustrate the deficiency 
of .the record as regards prime origins. It has already been noted that 
developmental detail is usually absent from fossils, and that many are 
known only as impressions, without the possibility of minute structural 
examination of their tissues under the microscope. These considerations 
only show still further how scanty is the positive information from study 
of the fossils which is available for elucidating the . early origin of the 
sporophyte. 

There is also a converse line of information, which involves negative 
evidence, based on the absence of certain types from strata where others 
are present. It may be held that organisms which first appear in the 
earlier strata are more primitive branches of the evolutionary tree than 
those which appear only in the more recent strata. But the fact that the 
record is, as we have seen, so very incomplete as regards the prime origins 
of the leading phyla will at once strike the note of caution in use of 
such negative arguments. Moreover, the probability of preservation of the 
representatives of any group may depend greatly on the character of the 
organisms in question : thus it need be no surprise that the small and 
delicate Bryophytes are conspicuous by their absence from the earlier 
records, while Algae are but rarely preserved. Again, the non-representation 
of any group may depend in some measure on the position in which the 
plants grew : thus the flora of uplands will be less likely to be preserved 
than that of low-lying lands or swamps ; this argument has sometimes 
been applied in explanation of the absence of Angiosperms till a relatively 
late period. It is often possible to make out a plausible case from such 
negative evidence : but its insecurity is obvious. To use it with effect 
it must be supported by other considerations, such as argument from 
comparison. Thus the' absence of evidence that Polypodiaceous Ferns 
existed in Palaeozoic times, must be taken with the position which is 
assigned to them on grounds of comparison among other Filicales; it 
then becomes a very convincing argument as showing their later derivative 
character, and the more so that Fern-types are among the best-preserved 
of early fossils. On the other hand, the entire absence of well-authenticated 
Ophioglossaceous remains from all the earlier formations only intensifies the 
difficulty of the problem which surrounds these curious plants, and cannot in 
itself be accepted as demonstrating that they are of relatively recent origin. 

These remarks are intended to indicate the limitations to which the use 
of palaeophytological evidence must necessarily be subject. It is when these 



230 EVIDENCE FROM PALAEOPHYTOLOGY 

are clearly apprehended that the true value of that evidence will begin to 
emerge. Though, as we see, it cannot yet be held to throw any direct 
light on the prime origin of terrestrial plants, still it has valuable bearings 
on the mutual relations of the earlier known types. It is especially valid 
in supplying . a knowledge of " synthetic types," that is, plants now extinct, 
which include among their characteristics some of the peculiarities of two 
or more distinct lines of descent. The most important of these hitherto 
disclosed are the Sphenophylls, which constitute a series separate from the 
three great phyla of living Pteridophytes, though some affinity is to_be 
recognised between them and the modern Psilotaceae. Their leaves agree 
with those of the Equisetales in being whorled, and being superposed 
they are most nearly like the oldest known Calamite Archaeocalamites. 
Their whorled arrangement also corresponds with that of one of the 
earliest Lycopods, Lycopodites Stockii, from the calciferous sandstone. 
The anatomy of the stem of Sphenophyllum is Lycopodial rather than 
Equisetal, but the strobili are nearer to those of the Equisetales than to those 
of any other known family. The interest in the group which showed such 
mixed characters was further intensified by the discovery of Cheirostr obits. x 
" This strobilus presents the same combination of Lycopodial with Equi- 
setal characters which we find in Sphenophyllum itself, but in both directions 
the agreement is more striking. . . . We may express its probable natural 
position by placing it in the main division Sphenophyllales, but in a 
family by itself, distinct from the Sphenophylleae in the narrower sense. 
The threefold affinities of Cheirostrobus, firstly with the Sphenophylleae, 
secondly with the Equisetales, and thirdly with the Lycopodiales, appear 
indisputable, and indicate that this genus, and the Sphenophyllales gene- 
rally, represent a phylum intermediate between the other two, which we 
must suppose to have originated with them, from a common ancestral 
group. In this way, the study of the extinct Sphenophyllales has thrown 
quite a new light on the obscure affinities of the Equisetal stock, for it 
indicates clearly that this phylum had a common origin with that of the 
Lycopodiales, a conclusion which the exclusive investigation of their recent 
representatives could never have suggested." Another important synthetic 
group of plants, of early occurrence, is 'that of the Cycadofilices, which 
link together the Pteridophytes and the Gymnosperms. Such examples 
illustrate what may be held to be the most important results obtained 
hitherto from Palaeophytology, as aiding the study of descent in Plants. 

Another line of argument from Palaeontological data is now beginning 
to ,be used, though only sparingly, since it is rare as yet to find that the 
facts suffice for its application. It consists in the comparison of plants of 
near affinity from different strata, and deducing from their stratigraphical 
sequence a progression as regards some single character. This method 
has been carried out successfully by Mr. Kidston, in respect of the structure 
of the stele of Lycopods: he has concluded that "it is probable that the 
* See Scott, Studies in Fossil Botany, pp. 494-497. 



STRATIGRAPHICAL SEQUENCES 231 

continuous ring of primary xyl&m is the older type of Sigillarian stem 
structure, and that the circle of isolated strands which form the primary 
xylem of the Clathrarian Sigillariae of the higher geological horizons has 
originated by a splitting up of the continuous-ring type of bundle ; and 
as already mentioned, even in the few Clathrarian Sigillariae from the 
higher horizon of which the structure is known, the actual transition from 
the one type to the other can be observed." 

" The Lepidodendra form, however, an older genus than Sigillaria, and 
extend to the base of the Carboniferous Formation. In beds not far above 
the base, and low down in the Calciferous Sandstone Series, specimens of 
Lepidodendron showing structure have been found ; and of two of these 
occurring in the same bed, one species shows the continuous ring of 
primary wood, while the other possesses a solid cylinder of primary wood 
without any trace of pith ; and although there occur here the two types 
of primary wood, side by side, still the solid cylinder type seems to be 
more common in the lower than in the upper horizons of Carboniferous 
Rocks, and the sequence of changes in the development of the primary 
xylem of the palaeozoic Arborescent Lycopods seems to point to the solid 
vascular cylinder as the oldest type, from which has been derived the 
medullate cylinder with a continuous ring of primary wood, and this con- 
tinuous ring of primary wood has, in turn, broken up to form the isolated 
strands of primary wood found in the Clathrarian Sigillariae." J This is 
a good example of an evolutionary story, shown among plants of near 
affinity in respect of a single character, and based upon stratigraphical as 
well as structural comparison. Similar conclusions are emerging at various 
other points. 

Another result of importance derived from Palaeontological study is 
less direct in its bearings on the story of descent : it is that by comparison 
of fossils with modern plants certain stereotyped views, based primarily 
on the study of modern plants, are liable to be revised, and relaxed. 
This may be illustrated by reference to secondary thickening in stems. 
It was formerly held that stems which showed well-developed secondary 
wood were necessarily referable to Seed-bearing Plants. Difficulties followed 
from the acceptance of this doctrine, and they culminated in the case of 
the Calamarieae. Here the better knowledge of their anatomy, and of 
their fructifications showed clearly that a true Pteridophyte might attain 
large dimensions, and show a secondary thickening of its stem. Similar 
results are now familiar for other phyla of the Pteridophytes, and these 
facts, together with a better knowledge of recent plants, has shown that 
secondary thickening is a feature restricted to no single group of plants. 
Similarly, fossils have led to a relaxing of ideas respecting heterospory, 
and the seed-habit, and have helped quite as much as any study of recent 
forms, to the acceptance of a doctrine of parallel origin of marked char- 
acters independently in more than one line of descent. 

1 Trans. Roy. Soc.^ Edin., 1905, vol. xvi., p. 548. 



232 EVIDENCE FROM PALAEOPHYTOLOGY 

But however valuable such results may be in leading towards a better 
knowledge, and more rational views, still they deal with relatively minor 
matters, and do not directly touch questions of prime origin. As to the 
early stages of evolution, of Bryophytes as well as Pteridophytes, the study 
of Fossils is still silent, and it seems not improbable that it will remain 
so. In order to frame some view of the prime origin of Land-Plants 
recourse must accordingly be taken to the only other method available 
for resolution of these problems, viz., the comparison of living forms. 
Experiment, another possible line of enquiry, but still in its infancy, is 
left out of account at present, for reasons explained above (p. 7). Those 
who deal habitually with the stronger weapon of direct historical fact 
involved in the study of the fossils are apt to feel some distrust of the 
more delicate weapon of comparison : it is liable to be weak and indecisive, 
and its results are much more in the nature of expressions of opinion 
than of actual demonstration. Still, so long as comparison is the only 
means available, it is necessary to use it, notwithstanding its weakness 
and uncertainty : while its conclusions will be checked, wherever possible, 
by reference to the more direct results of Palaeophytology. Such con- 
clusions may ultimately come also under revision, on the grounds of 
their probability in the past, at the hands of the experimental morphologist 
But as his experiments can never apply directly to any organisms except 
those now existing on the earth, the conclusions which he arrives at 
can never have the direct cogency which is inherent in Palaeo-phytological 
fact. 

For reasons thus explained, it is upon comparative study that we must 
chiefly depend at present, when we attempt to trace the origin of the 
sporophyte generation, whether as exhibited in forms now living, or in 
those which the palaeophytologists are disclosing with such amazing 
rapidity. 



CHAPTER XIX. 

AMPLIFICATION AND REDUCTION. 

WHEREVER the attempt has been made by studying plants as they are 
seen living or fossil, to link them together into some coherent evolutionary 
story, theories of phyletic amplification and reduction have been freely 
employed. Sometimes greater prominence has been given to the one, 
sometimes to the other. 

The term amplification is used to embrace all changes leading to 
increased formal or structural complexity of the plant. It is necessary 
to distinguish between those changes of amplification which are indivi- 
dual and those which are phyletic. The former are the result of development 
traceable in some degree to the direct effect of external circumstances upon 
the individual organism : phyletic changes of amplification are those trace- 
able as inherited from generation to generation in an advancing stock. 
But in actual practice it is difficult to discriminate between them, for the 
two are not different in kind : in point of fact it is only on a basis of 
comparison that phyletic amplification can be recognised : it may indeed 
be held to be a perpetuation of such individual amplifications as are 
transmitted in descent. 

In the simplest cases amplification may be a consequence of mere 
non-localised distension of the plant-body; but in all more complex 
organisms growth is localised and continued at certain initial points, 
which thus take the character of apical cones, and define the polarity 
of the resulting structure. Or, furthermore, a secondary activity may 
appear in some intermediate zone, and new tissue be there intercalated : 
the common and obvious type of this is where increase in length or 
in width of the whole organ is the result, and that is what is usually 
understood as intercalary growth. But it would also include those develop- 
ments of vascular tissue designated as secondary thickening. Closely 
associated with apical growth, but less commonly with intercalary growth, 
is the initiation of new apical points, which lead to the various modes 
of branching of parts. This has also played an important role in the 
origin of complex plants as we see them. 



234 AMPLIFICATION AND REDUCTION 

Reduction is the term used to connote the converse of amplification, 
and it also may be either individual or phyletic, where the develop- 
ment of the mature organism, either in whole or in part, in external 
form or in internal structure, falls short of that of the ancestry, the 
condition would be described as reduced : such a state may be held 
to result from a check in the development before maturity, as shown 
in the ancestry, had been attained. If such a condition become a 
character of an evolutionary sequence, then it would rank as a phyletic 
reduction. 

Progressive amplification and progressive reduction are phenomena 
which may be illustrated in any phyletic sequence, and the question 
whether or not, and how far either has been operative in the history 
of descent in any specific case is virtually the equivalent of enquiry 
into its evolutionary history. The character of the progression may have 
varied at different times : in any stock a period of evolutionary advance 
may have been succeeded by a period of retrogression or the converse. 
Further, it is to be noted that amplification or reduction may affect the 
organism as a whole, or only special parts of it. Moreover, different 
parts of the same organism may show evidence of having behaved in 
exactly converse ways in the course of descent. Examples of this are 
seen in every case of correlation, the amplification of one part habitually 
entailing the reduction of another. 

To produce any organism as it is seen to-day, the two factors of 
amplification and reduction have been constantly possible throughout 
descent. The organism itself may be held to represent the sum of all 
such progressions and retrogressions, phyletic and -individual. It is obvious 
that while reduction may have been active in the later phases, the balance 
taken over the whole evolutionary history must have been on the side 
of amplification, otherwise the organism would be non-existent. This 
may seem a mere platitude ; but it is essential to state it, in view of the 
overestimate of the factor of reduction, as shown in most morphological 
discussions. This has resulted from the greater readiness with which 
evidence of reduction comes to hand, together with the method of our 
comparisons, which habitually start from pronounced "types." 

The common criterion is that of mere size, but this carries with it 
differences of complexity, either of external form, or of internal structure, 
or usually of both. As a rule it is impossible to. tell from a single 
specimen, or even from a number of representatives of a constant species 
whether the organism has been reduced or amplified in the course of 
its antecedent phyletic history : it does not bear any certain index of 
these points in its individual characters, unless in cases where reduction 
has led to change of the original function of a part. It is primarily upon 
the comparison of organisms related to any given species that an opinion 
may be based how far amplification or reduction respectively have been 
operative in its evolution. 



THEIR COMPARATIVE BASIS 235 

In cases where there is good reason to believe that the phyletic origin 
is correctly recognised, and where the type is represented by numerous 
well-known species, a very strong presumption may be accepted, amounting 
almost to a demonstration, of what has taken place in the more recent 
steps of descent. This is more easily illustrated in respect of a given 
part, than of the whole organism. For instance, in the phyllodineous 
Acacias the progressive amplification of the phyllode and the progressive 
reduction of the lamina are practically demonstrated by comparison of 
the various species included in the single genus : the conclusion is 
further supported by the facts of development of the individual seedling ; 
for the young plants frequently show in their ontogeny the steps which 
comparison among distinct species had already suggested. It is unnecessary 
to multiply examples of such phenomena, for they are familiar to every 
student. 

It is, however, the familiarity with such ideas, in cases where sufficient 
evidence is available (a condition frequently seen among the Higher 
Plants), which has led to their misuse in cases where the evidence is 
less complete. Where ordinal or generic types are isolated, and the 
genera represented, it may be, by few species, or even by a single one, 
as is so often the case in the Pteridophytes, the weapon of comparison 
is apt to lose its temper and its edge. Still, it has been used, but 
in these isolated cases the comparative argument is less cogent, its 
application being more violent and less exact. The cogency of all 
morphological comparisons varies inversely with the distinctness of the 
organisms compared : this is especially to be borne in mind in dealing 
with questions of progressive amplification or reduction among the 
Archegoniatae.* 

Looking back upon the theories of amplification or of reduction which 
have been suggested in the past, it becomes evident that they have often 
been applied at random. That one or the other has been advanced 
according to the taste, or, one might almost say, according to the tempera- 
ment of the writer : frequently they have been invoked under the pressure 
of doubt, or in support of an insecure hypothesis. More especially was 
this so in the days when monophyletic views ruled more than they at 
present do. A full recognition of the probability of polyphyletic origins 
has obviated the necessity which was once felt to refer all related 
organisms to one scheme : there is no present obligation to explain their 
form as derivative from one type, either by amplification, or by the more 
common deus ex machina reduction. 

Goebel has drawn attention to the prevalence in phyletic speculation 
of theories of arrest of development over theories involving amplification. 
He remarks that most of our phylogenetic series- are reduction-series, 1 
and traces this to the fact that a definite type is habitually recognised as 
a starting point for comparison. Naturally such a type must already be 

1 Organography ', Part I., p. 60. 



236 AMPLIFICATION AND REDUCTION 

a thing with pronounced characters, otherwise it would not be held as 
typical : there will then be an inherent probability that allied forms would 
range themselves as reductions from such a type. On the other hand, 
in series which have really been ascending series, the original forms would 
not be prominent as types, and so would not be likely to command 
attention. 

Commonly it has been on a basis of simple comparison that phyletic 
series have been traced ; but it is plain that apparent sequences should 
be checked according to other considerations than those of mere formal 
comparison. The most important of such checks is that of physiological 
probability, or even in some cases possibility. In those phyla where the 
organisms are relatively isolated, and the wide gaps in the series make 
comparisons less certain, such checks are specially necessary, and in none 
more so than in the Pteridophyta. 

There is overwhelming evidence that the homosporous state was the 
original condition of all the known phyla of Pteridophytes, as it is the 
uniform condition of all the Bryophytes. It may be assumed that it was 
while still in this condition that the leading characters of their several 
sporophytes were established, though in many of them the heterosporous 
state supervened at a later date. This brought with it complications of 
the factors which originally determined the form of the sporophyte. It 
is desirable to avoid any confusion of these later factors with those which 
determined the character of the sporophyte in its more primitive homo- 
sporous state. It will be best to put them on one side for the moment, 
and to confine the attention at first to the simpler problem of the evolution 
of the homosporous types : for this will be found to give a better insight 
into the principles relating to amplification and reduction, and the part 
which they respectively played in the evolution of the primitive 
sporophyte. 

According to the adaptive theory of alternation, as stated in Chapter VI., 
the extended development of the sporophyte acted as an offset to those 
obstacles to fertilisation which faced aquatic organisms as they extended 
to a land habit. Where all germs are alike (homosporous), the larger the 
number of them produced the greater the probability of survival : thus 
selection would favour those with the highest spore-output. But to secure 
a high output of spores there must be an adequate supply of nutritive 
material : thus a condition of any extension of spore-output will be a due 
nutritive supply ; and, conversely, any diminution of nutritive supply will 
reduce the output The two systems, that of nutrition and that of 
propagation, will thus tend to vary together as regards amplification or 
reduction. And since in homosporous forms the highest chance of survival 
and of spread lies with those organisms capable of the highest numerical 
propagation, we should naturally anticipate that in them, other things 
being equal, a general progressive amplification would have the upper 
hand. 



IX VEGETATIVE AND PROPAGATIVE REGIONS 237 

But it is to be remembered that in the plant-body the two functional 
systems, the vegetative and propagative, are not equally free of one 
another. In any independent organism the vegetative system may increase 
without any corresponding amplification of the propagative; but the latter 
cannot do so without the former, since it is dependent on the vegetative 
system for its nutritive supply. In the Archegoniatae this statement will 
hold for the organism as a whole, taking -gametophyte and sporophyte 
as one. But if, as in the present work, attention be centred on the 
sporophyte, qualifications will require to be made : for a considerable 
proportion of the nutritive supply of the sporophyte may originate from 
the parent gametophyte. In the embryos of all the Archegoniatae this 
is the initial condition, and some of the simplest have never broken 
away from it; but in all the more advanced types the vegetation inde- 
pendence of the sporophyte is fully attained, while others hover in varying 
degree between self-nutrition and dependence. It thus becomes a question 
of the source of the nutritive supply in each separate case before it is 
possible to decide how the balance of the nutritive to the propagative 
system in the sporophyte has been adjusted in descent ; and this is a 
necessary preliminary to any view as to the probable amplification or 
reduction of either. 

It will be well to consider a few examples illustrative of the various 
degrees of embryonic dependence in Archegoniate Plants. In the sporo- 
gonium of Riccia there is no self-nutritive tissue : the supply comes entirely 
from the gametophyte : it may be a question for discussion whether the 
absence of a nutritive system is due here to reduction, or is itself the 
actual primitive state; but the latter is the view usually accepted. In most 
other Liverworts there is little or no functional nutritive system in the 
sporophyte. But the Anthoceroteae form an exception, and in them it 
is represented in varying degrees : in Dendroceros and Notothylas, and 
part of the genus Anthoceros there is chlorophyll-parenchyma in the sporo- 
gonial wall, but no stomata ; but in the two sections of the genus Anthoceros 
with non-spiral elaters, the presence of stomata is a structural indication 
of the efficiency of the sporophyte in self-nutrition. It may, however, be 
a question whether the simpler Anthoceroteae are on the up-grade or the 
down-grade of development. That a down-grade of development may 
occur even among simple Liverworts has been placed upon a reasonable 
footing of probability by Lang, in the case of the genus Cyathodium J 
(Fig. 1 1 6), where it appears to be a consequence of growth in a moist, 
shaded habitat. Not only is the reduction effective in size, but also in 
complexity of the whole sporogonium ; but the spores themselves, though 
numerically fewer, fully maintain their individual bulk. The foot is also 
reduced, and it is suggested as possible that the absence of a foot in the 
Riaia cell may be the consequence of still further reduction in them of 
a similar nature to that seen in Cyathodium. 

1 Annals of Botany, xix., p. 241. 



2 3 8 



AMPLIFICATION AND REDUCTION 



Among the Mosses the small Cleistocarpic forms are virtually dependent 
for all their nutritive supply upon the Moss-Plant. In larger forms, such as 
Mnium, Splachnum, and Buxbaumia, there is a well-developed assimilatory 
system with functional stomata, and there is no doubt that it con- 
tributes materially to the nutrition of the sporophyte. But in some cases, 
such as Sphagnum, Ephemerum, and Nanomitrium, stomata, though present, 
are non-functional, a fact which indicates a probability that these sporo- 
gonia are now more dependent for nutrition upon the Moss-Plant than 
their ancestors were. There seems some probability also that there has 
been, in the genera last named, a reduction in the numerical spore-output. 
These examples from the Bryophyta illustrate how the sporophyte is 
variously dependent upon the gametophyte for nutrition ; and that while 




FIG. 116. 

Longitudinal sections through sporogonia of Cyathodium cavernarum (A), and Cyatho- 
diumfoetidissimum (), to show their position on the thallus and their relative size. In 
both cases the sporogonia contained spores and elaters with their walls thickened, but had 
not quite attained their full size. ^74. (After Lang.) 

in some cases provision has been made for some degree of self-nutrition, 
in others the dependence may have increased in the course of descent, 
as shown by reduction of the assimilatory system of the sporophyte; and 
there is also some indication that the spore-output has suffered by the 
change. Thus, notwithstanding their homoporous state, it would seem 
probable that phyletic reduction both of the vegetative system and of the 
spore-output has been operative among them in some cases in their 
neutral generation. 

Among the Pteridophytes the embryonic dependence is usually brief: 
the young plant hastens to elaborate its own assimilatory system, and to 
become physiologically independent, as in any mature Fern, or Horsetail. 
But under some circumstances the period of dependence is liable to be 
extended, a condition which brings with it evidences of a corresponding 
reduction of the first-formed appendages. This is seen in certain embryos 
borne on underground, mycorhizal prothalli, and examples of it are seen 
in the Lycopods, and in the Ophioglossaceae. For instance, while Lyco- 



REDUCTION FOLLOWS DEPENDENCE 



239 



pcdium Selago expands its first leaves as green assimilating leaves, those 
of L. clavatum are developed underground, and appear as minute colour- 
less scales, succeeded later by green foliage leaves (Fig. 117). Again, in 
Rotrychium virginianum the cotyledon is a green, expanded foliage leaf: 
in B. Lunaria the first leaves are minute colourless scales. These cases 
from among the Pteridophytes illustrate in two distinct series how, where 
physiological dependence of the sporophyte is extended, owing to peculiar 
circumstances, a local reduction of its vegetative system may follow. 
They also have their interest for comparison with those Bryophytes which 
have non-functional stomata, for in both the gametophyte appears to 
have assumed increased responsibilities. Nevertheless, in these cases from 
the Pteridophytes, the plant when ultimately 
free shows no general reduction : the effect 
is local, and does not extend to the mature 
organism ; moreover, there is no reason to see 
in such effects any reducing influence upon 
the ultimate spore-output. 

Passing on to the independent sporophyte 
as seen in the Pteridophyta after the embryonic 
period is past, two cases require consideration : 
the autotrophic types, on the one hand, and 
on the other those sporophytes which show 
indirect nutrition, such as is seen in the 
mycorhizic types. In independent autotrophic, 
homosporous Pteridophytes, the presumption, 
as has been seen above, would be that they 
would show evidences of amplification rather 
than of reduction. So strong does this pre- 

Seedling of Lycopodinm clavatum, 

sumption appear that, wherever a line of (After Bmchmann.) xio. /-foot; 

i , r , a; = root ; bl= leaves here represented 

reduction IS Suggested for a homOSpOrOUS type, as minute underground scales. 

it should be incumbent upon its author to 

show physiological reasons why it should have occurred. Mere mor- 
phological comparison without physiological support should be held as 
an insufficient basis for theories of general reduction in homosporous 
forms. 

But examples of special reduction, affecting parts or details of 
homosporous Pteridophytes, are not uncommon. It seems not improbable 
that the leaves of modern species of Equisetum are reduced as com- 
pared with those of early Calamarian forms, and this may be held 
as correlative to the development of the cortex in Equisetu?n as an 
effective assimilating tissue. Certain of the leaves of Osmunda have an 
arrested lamina, while the leaf-base remains as part of the protective 
armour which covers the axis : potentially these are complete leaves, 
and their arrest before maturity may be held as a case of reduction. 
Such examples as these are in the nature of correlative adjustment of 




FIG. 117. 



240 AMPLIFICATION AND REDUCTION 

parts of the shoot, inter se, and cannot be held to be examples of general 
reduction. 

There remain to be considered those sporophytes which show some 
form of indirect nutrition, the commonest of which is the mycorhizic 
symbiosis. The occurrence of a symbiotic state is often loosely held 
to be equivalent in itself to a demonstration that the organism in which 
it occurs has been the subject of general reduction ; and reference is apt 
to be made in support of this to extreme cases, where it has in fact led 
to complete saprophytism. But it is necessary to be clear what effects 
they are which necessarily follow upon this habit, as apart from those 
which are occasional and extreme : for it is only the former which can 
properly be counted on for argument. Stahl has indicated that the usual 
structural concomitants of mycorhiza in green plants are such as lead 
to economy of the water-interchange : l viz., a restricted root-development, 
with thick unbranched roots, and absence of root-hairs : little structural 
provision for water- transfer and an absence of organs of water-secretion ; 
while a leathery texture of the leaf, a feature of other plants which 
economise water, is not uncommon. But these characters are by no 
means uniformly or exclusively found in mycorhizic plants : for instance 
Cyathea is mycorhizic, but it shows such characters as the leathery leaf 
less obviously than Aspletiium nidus and Osmunda regalis, which are not. 
When present the features above named may be held to be indicative of 
a probable reduction in respect of the parts immediately affected ; but 
that is a very different thing from the general reduction which is some- 
times assumed to follow mycorhiza as a necessary consequence. General 
reduction implies an effect on both the nutritive system and the propagative 
system. But it is to be clearly understood that so far as the mycorhizic 
habit affects nutrition, by yielding as it does in some cases an efficient 
saprophytic supply,- the reduction will appear in the vegetative system 
only, and not in the propagative. This is amply illustrated in Phanero- 
gamic plants such as Neottia and Sarcodes? where the flowers and fruits 
remain of the usual types though the vegetative system is reduced. 
Similarly, among Pteridophytes, if mycorhiza were really effective in them 
as a considerable means of saprophytic nourishment, we should expect the 
consequent reduction to appear in the vegetative system, with a loss of 
chlorophyll in extreme cases ; but that the spore-producing parts should 
remain of the usual dimensions and character of the family : that is, 
supposing the saprophytic supply to be as efficient as the normal chloro- 
phyll nutrition. Now, putting aside certain exceptions to be noted below, 
such a condition is unknown among Pteridophytes, and its absence goes far 
to show that the mycorhizic symbiosis seen in them is not a fully effective 
source of organic nutritive supply. The facts do not bear out the 
general assumption that mycorhizic symbiosis, as seen in certain of 

1 Pringsh. Jahrb., xxxiv. , p. 539. 

2 See F. W. Oliver, on Sarcodes, Ann. of Dot., iv. 



MYCORHIZA AND REDUCTION 241 

the Pteridophytes, leads to a general reduction of the infected sporophyte 
as a whole. 

The particular family in which the argument relating to mycorhiza has 
been specially applied is the Ophioglossaceae. A more detailed account 
of it will be given where the family is specially described below; but 
meanwhile it is to be noted that there is throughout the Adder's Tongues 
a close parallelism of proportion of the sterile lamina to the fertile spike : 
this is indeed one of the most remarkable features in the morphology of 
the family, the parallelism extending not only to the size of the respective 
parts, but also to the character and extent of the branching of each. If 
saprophytic nourishment by the. mycorhiza were in this case a real 
substitute for chlorophyll-assimilation the sterile lamina would fall behind 
the spike in its dimensions ; but in the normal representatives of the 
family it does not. The conclusion follows that in the mature sporophyte 
of the Ophioglossaceae the mycorhiza is not functionally an effective 
substitute for nutrition by chlorophyll-assimilation. 

There is, however, one series of species in that family in which the 
proportion of the two parts is not maintained, viz., the section Ophioderma 
of the genus Ophioglossum. Here the epiphytic O. pendulum shows 
approximately the usual balance ; but in O. intermedium, a land-growing 
species, the sterile lamina is relatively small, while in O. simplex, also a 
ground-growing species in which mycorhiza is present, the lamina appears 
to be altogether unrepresented. I regard this section, Ophioderma, as a 
series in which mycorhiza has become effective as a substitute for chlorophyll- 
nutrition, and that reduction of the vegetative system has actually followed 
as a consequence : nevertheless the spike, being effectively nourished, 
retains its dimensions. But disturbance of the balance of the vegetative 
and reproductive systems such as this is a very different thing from any 
general reduction of both, such as is sometimes assumed to follow in 
consequence of a symbiotic habit. 1 

Another family which provides an interesting parallel in this respect to 
these Ophioglossaceae is that of the Psilotaceae. In Tmesipteris there is a 
reasonable balance of size between the forked sporophyll and the bilocular 
synangium. In Psilotum this balance is not maintained, for the small 
sporophylls are ineffective as assimilating organs while the trilocular 
synangium is still of large size. It is true the green axis is an effective 
organ of assimilation, but it would appear probable that the mycorhizic 
state also assists. 

The. discussion of the parts played respectively by amplification and 
reduction in the genesis of the homosporous sporophyte may now be 
summed up. The end of its development is the production of the 
largest number of effective germs. To increase their number involves 
amplification of the propagative system. This involves also in many 
cases amplification of the nutritive system. However, this is not an 

'See Scott, Studies, p. 511. 
Q 



242 AMPLIFICATION AND REDUCTION 

end in itself, but only a means to the end, viz., the suitable nutrition 
of the nascent germs. There are several ways in which this nutrition 
may be effected ; they are these : 

(1) Nutrition by the gametophyte, which was the most primitive 

method. 

(2) Self-nutrition of the sporophyte by its own assimilatory system. 

(3) Indirect nutrition of the sporophyte, e.g. by mycorhiza. 

Provided the spore-production be maintained, it matters not which of 
these is effective, or dominant in any individual case ; and in point of 
fact they have varied in the phyletic history. In the original state of the 
sporophyte there was undoubtedly nutrition by the first method. Subse- 
quently the second supervened ; and there is reason to think that during 
the phyletic history there has been a varying balance of the effectiveness 
of these two factors. Generally speaking (i) has waned in importance 
proportionately to the whole requirement ; but in such cases as the 
Moss-sporogonia with non-functional stomata, and in the large under- 
ground prothalli of Lycopods and Ophioglossaceae (i) appears again to 
have increased in proportional importance, encroaching upon the effective- 
ness of (2), with the result that local reduction of the mechanism of 
self-nutrition in the sporophyte followed ; but still that may have pro- 
duced no ill-effect upon the spore-output. Passing to the independent 
sporophyte, its primitive nutrition was autotrophic (2), and there was a 
suitable balance of the nutritive and propagative systems, the method of 
which differed in the different phyla. Lastly, in those cases where 
indirect nutrition (3) by mycorhiza contributes effectively, a reduction of 
the normal nutritive system of the sporophyte may take place ; but so 
long as the sum of nutrition is maintained the propagative system would 
not be reduced. If, however, for any reason the sum of nutrition fall, 
then general reduction would ensue. 

It is not then enough to suggest reduction on mere grounds of com- 
parative convenience : to make the suggestion convincing in any group 
where general reduction is believed to have occurred, it will be necessary 
to prove that the sum of nutrition, from whatever source, has diminished 
in the course of descent, and that reduced spore-output has been the 
result. Until this has shown to have occurred in any case, there seems 
no sufficient reason to accept as more than a quite open hypothesis any 
suggestion of general reduction of its sporophyte. The biological probability 
is against extensive, or general, reduction in homosporous forms, .and in 
any case the positive balance during the whole phyletic history must 
have been on the side of amplification. 

But where there is heterospory, and especially in plants showing the 
seed-habit, where a high certainty of a germ becoming effectively established 
is attained by storage in the enlarged spore, reduction in the number of 
spores followed, and the cognate reduction of other parts assumed many 



THE BIOLOGICAL CRITERION 243 

different forms. These need nob be detailed here : it will suffice to quote 
as one example of a case fully made out the reduction of the sporophyll 
in the Cycadales. 

It is thus seen that hypotheses of relative primitiveness, or of reduction 
as applied to living organisms, do not stand on an equal footing. The 
former has the logically prior claim, and should be accepted as a 
working theory until good grounds can be given for preferring the latter ; 
and the mere exigencies of comparison will not be sufficient : a proper 
foundation can only be sought in the biological circumstances of the 
organism in question. Such evidence is specially necessary when dealing 
with homosporous forms, in which the problem is more directly one of 
size, nutritive capacity, and consequent spore-number, than in the case 
of those which are heterosporous. 1 

1 Compare Bower, Science Progress, vol. iv., p. 358, etc. Also Tansley, Neiv Phytologist, 
vol. i., p. 131. 



CHAPTER XX. 

SUMMARY OF THE WORKING HYPOTHESIS. 


IT will be useful to collect the substance of the preceding chapters into a 

more concise form, hypothetical and uncertain as in their very nature 
any conclusions must necessarily be. 

The general problem of the origin of a Land-Flora is not to be solved 
by mere observation of the present-day distribution of the organisms 
composing it; some other basis for an opinion must be sought. The 
problem has been approached primarily from the point of view of the 
individual life; and special regard has been given to the relation which 
subsists between the environment and fertilisation, the most critical incident 
in the life of any organism (Introduction). 

It seems probable that certain Algae represent in their general characters 
the original source from which the Land-Flora sprang. Their prevalent 
method of fertilisation by motile gametes is by many held to show a 
reminiscence of their ultimate origin from the free-living Flagellates : however 
this may be, the gamete motile in water is a character which many Algae 
share with the Archegoniatae ; it is a feature essentially typical of aquatic 
vegetation. 

In respect of their whole life-cycle the Archegoniatae may be said to 
show an amphibial existence, the aquatic and the terrestrial characters 
being reflected in its two alternating phases (Chapters II. and III.). The 
gametophyte is as a rule delicate in texture, without intercellular spaces 
in its tissues, or a fully developed water-conducting system, while its sexual 
organs only become functional on their rupture in water outside the 
plant-body : the gametophyte thus proclaims its ultimate dependence on 
external fluid water as thoroughly as an Alga. The sporophyte, on the 
other hand, is a characteristically subaerial body : this is shown by its 
more robust habit, its effective ventilating system, and its vascular strands 
for the conducting function seen in all the higher forms : its final result, 
the maturing and dissemination of spores, is normally carried out under 
circumstances of dryness. All these features mark it as an essentially 
terrestrial phase. 



ORIGIN OF THE SPOROPHYTE 245 

The Archegoniatae themselves retain with remarkable pertinacity the 
awkward and embarrassing mode of fertilisation through the medium of 
external fluid water. But with the advent of the Seed- Habit this became 
modified : finally the sperm was no longer set free as a cell motile in 
external water, but fertilisation came to be effected by means of a closed 
pollen-tube. Thus the higher Seed-Plants at last became typically terrestrial 
organisms, breaking away from the last vestige of the amphibious habit 
of their progenitors, the Archegoniatae. 

But all this was not achieved suddenly. From living organisms, and 
in some degree from fossils, indications may be gathered of the various 
steps which led to the establishment of the sporophyte as the essential 
feature of a Land-Flora. Tracing these steps backwards it is possible to 
obtain a clue from the simpler aquatic organisms : these plants give the 
best indication available how the initial start was probably made. There 
is reason to believe, on grounds of comparison, that the sexual generation 
or gametophyte was the prior existent, and that the neutral generation or 
sporophyte arose as a phase intercalated in the course of descent between 
successive gametophytes : that the initial step which led to this was the 
existence of those complications of cell-division which appear in so many 
of the lower plants as a consequence of sexuality, and are connected 
with the reduction of chromosomes already doubled in the sexual fusion 
of nuclei. It is certainly the fact that in some Algae such post-sexual 
divisions do result in the production of a plurality of germs : biological 
circumstances which would encourage the multiplication of those germs 
might be expected to lead towards the establishment of a neutral generation. 
Ir) plants exposed to changing conditions of moisture and of drought, 
such circumstances would be specially effective, and this must naturally 
be the position of any which spread to a land-habit. Here access to 
external fluid water would be an occasional rather than a constant 
occurrence : consequently sexuality could only be carried out occasionally, 
when water was available, while it would be precluded under dry conditions. 
Less dependence could then be placed on sexuality for increase in 
number of individuals, and a premium would be put on an alternative 
mode of propagation, suitable for dryer circumstances. The post-sexual 
divisions accompany ing 'reduction would supply the initial state upon which 
variation and selection could work towards this end, and by an increase 
of these divisions the number of post-sexual germs would be increased. 
It is thus seen that the biological conditions involved in the transition 
from water to land would naturally encourage some form of amphibious 
alternation (Chapters V. and VI.). 

The establishment of a Land-Flora thus involves the origin of a body 
adapted to terrestrial life ; and as such the sporophyte is to be recog- 
nised. Its first function, as it is also its final office even in its most 
elaborate forms, is to produce spores. The spores of the simpler 
Archegoniatae are all similar and equivalent germs : the larger their 



246 SUMMARY OF THE WORKING HYPOTHESIS 

number the better the chance of survival ; in this may be found the 
rationale of the enormous numbers of spores habitually produced by 
the homosporous Archegoniatae. To protect them while young, [and to 
nourish them during their development presupposes some vegetative 
system, which will require to be more elaborate the larger the number 
of spores. The protection is in part supplied by the parent gametophyte, 
though in all but the simplest it falls on the sporophyte. The nutrition 
may also in some cases be supplied by the gametophyte, as it is in the 
simpler Liverworts and Mosses ; but in the more advanced forms, after 
the first embryonic stages are passed this duty falls on the sporophyte 
itself, as in the Vascular Plants. The comparative study of the sporophyte 
in its various living forms suggests certain factors of advance, which led 
to its becoming efficient for carrying out these functions of protection 
and self-nutrition, and thus conduced to its final independence ; the most 
important of these are : (i) sterilisation of cells potentially sporogenous, so 
as to supply a vegetative system (Chapter VIII.) ; (ii) the segregation of 
the sporogenous tissue into distinct pockets, or sporangia, thereby facilitating 
nutrition and dispersal (Chapters VIII. and IX.); and (iii) the origin of 
appendicular organs, which serve a variety of purposes beyond the usual 
direct ones of supporting the sporangia, and of nutrition (Chapter XI.). 

Sterilisation of cells potentially sporogenous is a feature which is very 
widespread among living sporophytes : evidence of its occurrence may be 
drawn from all the main groups composing the characteristic Flora of 
the Land (Chapter VII.). The argument to be based on this fact is as 
follows : it is seen in plants of the present day that in definite cell-groups 
of the sporophyte, which may be recognised as sporogenous, sometimes 
the whole body of the cells undergo the tetrad-division, and form spores ; 
in other cases, while certain cells of such groups are fertile, other cells 
of like origin with' them remain sterile : these may, however, subserve 
various purposes in less direct relation to the production of the spores : 
in certain cases the sterile cells may even develop as permanent tissue. 
The conclusion from this is first ontogenetic : viz., that the sterile cells, 
being sister cells with those which are fertile, are potentially sporogenous 
cells which have been diverted from their original purpose, and that their 
potential spore-producing capacity has been sacrificed to ensure the 
success of those which remain fertile. The second conclusion is phylo- 
genetic, and it follows from the fact that examples of such sterilisation 
may be drawn from all the main groups of Plants which form the 
characteristic Flora of the Land : it is that such transformation of cells 
from the fertile to the sterile condition as is seen so commonly at the 
present day, was also of common occurrence in the course of evolution 
of the sporophyte. It would be going too far to say that there is in 
this any demonstration of the source from which all vegetative tissues 
of the sporophyte have been traced ; but at least this is a justifiable 
working hypothesis. 



AMPLIFICATION OF THE SPOROPHYTE 247 

It is possible to conceive of an indefinite increase of the sporophyte, 
by continued cell-division and progressive sterilisation, in a body main- 
taining a simple form ; but mechanical and physiological checks impose 
a moderate limit. The Bryophytes illustrate in some of their forms such 
progress in the sporophyte successfully carried out Ito a relatively high 
degree of complexity. But in all their more advanced types there is a 
distinction of apex and base, the basal region being sterile and the apical 
region fertile. Their sporogonia, however, always retain a simple form, 
and with few exceptions the radial type of construction : they are all 
alike also in having a single continuous spore-sac. This is plainly a type 
of construction which has its limits imposed by mechanical and physio- 
logical conditions. Reasons such as these have tended to prevent the 
Bryophytes from developing their sporogonia beyond a very moderate 
size. They show, however, very clearly on comparison the successive 
steps by which progressive sterilisation may advance the complexity of 
a simple type of sporophyte (Chapters III. and IX.). 

But the Vascular Plants, while showing the same plan of life-cycle, 
have been able to continue development without those mechanical and 
physiological checks operating upon their spore-output. The outstanding 
features in which they are more free than the Bryophytes follow from 
the segregation of sporogenous tissue in distinct sporangia, and the 
formation of appendicular organs. The biological advantages- thus attained 
are obvious : a plurality of sporangia makes possible the separate, and 
more efficient nutrition of each : thereby also the mechanical difficulties, 
which act in limiting the Bryophyte sporogonium, are effectively avoided. 
On the other hand, the development of appendicular organs makes 
independent self-nutrition of the sporophyte really effective, while the 
position of the sporangia on the appendages facilitates the dispersal of 
the spores. The palaeontological record shows conclusively that both of 
these features were of very early date, and their consequences are 
illustrated in the earliest fossils of which there is any detailed knowledge 
(Chapter XVIII.). The advantages secured by an unrestricted type of 
development were doubtless such as to lead to a rapid advance. It 
can therefore be no matter for surprise that connecting links between 
the two states are absent, even supposing the two phyla, in which they 
are characteristically shown, to have had some degree of community of 
origin. 

The Pteridophytes show diversity of type, according to the size of 
their appendages : those which are smaller-leaved, as in the Lycopods, 
Equiseta, and Sphenophylls, have as a rule a terminal strobiloid fructifica- 
tion, though this is not always clearly differentiated from the vegetative 
region. In the Fern-like types the fructification is disposed more 
generally over the enlarged leaves. As in the Bryophyta so in the 
strobiloid Vascular Plants, a sterile basal region precedes the terminal 
fertile strobilus. This vegetative region may be held to be a phase 



248 SUMMARY OF THE WORKING HYPOTHESIS 

intercalated as a consequence of sterilisation, and will therefore take a 
secondary place. An important question will then be how this more 
elaborate condition of the strobilus of Vascular Plants came into exist- 
ence. Any theory of the origin of the strobilus should be based upon 
detailed knowledge of its structure and development, in forms living 
and fossil, and of its parts : these are the axis, which is the central 
part in any strobilus ; the appendages ; and the sporangia, which are 
usually produced in relation to the latter. These parts will require 
separate consideration. 

A detailed study of the sporangia of Vascular Plants has led to the 
following definition of the sporangium (Chapter VIII.), which discards 
non-essential and fluctuating characters, and retains only what is essential 
and constant. " Wherever there is found in Vascular Plants a single spore- 
mother-cell, or connected group of them, or their products, this, together 
with its protective tissues, constitutes the essential of an individual 
sporangium." In many cases the sporogenous group is not strictly cir- 
cumscribed, but has ragged edges : cells which are sister-cells may not 
unfrequently be found to develop the one sterile, the other fertile. On 
the basis of structure this is consistent with the view that each fertile 
tract is a residuum left by advancing sterilisation. In the simpler stro- 
biloid types the sporangia are associated, singly or in small numbers, 
with appendages of various form and nature, which arise laterally, and 
in acropetal succession, as superficial outgrowths from the pre-existent 
axis : these are designated in various cases sporophylls or sporangiophores. 
The theory of the strobilus, stated in Chapter XL, uses the structural 
and developmental facts thus briefly summarised in the following way. 
It assumes, first, a sporophyte-body, already showing a distinction of a 
basal vegetative and an apical fertile region. This was endowed with 
apical growth, and an acropetal succession of its spore-development. The 
latter was relegated towards the surface, a change clearly indicated by the 
analogy of the Liverworts and Mosses. That by advancing sterilisation 
the fertile tissue underwent segregation into separate pockets, or sporangia, 
and that, by enation from the surface, appendages were formed in acro- 
petal succession, of the nature of sporangiophores, or sporophylls : upon 
these the fertile loculi would be borne outwards, as they are seen to be 
in the individual development of sporangiophores to-day. The apically 
growing axis would thus have been the pre-existent portion of the shoot, 
and the successively formed appendages secondary, as they are in the 
actual development. It has been shown that every one of these steps 
has its prototype among living plants : moreover the theory is in accord- 
ance with the ontogeny at every step (Chapter XL). 

In the strobiloid type of the Lycopods the sporangia are definite in 
position and in number : while the relation of them to the bulky axis is 
very close. This is held to be a primitive condition, and palaeophy- 
tology shows that it was existent among the earliest fossils. In others, 



THE POLY-SPORANGIATE STATE 249 

also probably primitive and certainly early, the appendages are larger, and 
the sporangia more removed from the axis ; and in proportion as this is 
so their number is less precise. But even where the appendages are 
largest, as in Ferns, or Ophioglossaceae, the relation of leaf to axis remains 
essentially the same. 

The variations of number of sporangia actually seen as effective, in 
Vascular Plants have been discussed in Chapter X., in which methods of 
increase are separated from those of decrease. Both of these are liable 
to be disguised by the swamping effect of continued apical growth, and of 
branching of axes and appendages, which are so prevalent in Vascular 
Plants. But, putting these more obvious sources of numerical change of 
sporangia aside, there are others which have also been effective, and have 
probably played an important part in evolution. As factors of increase in 
number of sporangia septation and interpolation are to be recognised. 
The former of these has probably been underestimated hitherto in its 
evolutionary effect : numerous synangial bodies in Pteridophytes are com- 
patible with it, and each must be considered on its merits; moreover, 
septation is demonstrated to have actually occurred in the anthers of a 
number of Angiosperms. Interpolation of new sporangia among those 
previously present, on the other hand, has hitherto been overestimated : 
in certain of the simplest forms, and particularly in the Lycopods, it is 
non-existent : it is more prominent in larger-leaved forms, where sporangia 
are indefinite in number, such as the Ferns, and it has played an 
important part among the later Polypodiaceae ; but no clear case of it 
is known among Palaeozoic Plants. It is held as a relatively late mode 
of increase, initiated as a secondary phenomenon, and it cannot be 
assumed to have been of general occurrence in the course of descent. 

Among the factors of decrease in number of sporangia the arrest of 
apical growth in axes, or in appendages, has probably been one of the 
most effective, and especially so in the later types of development ; but 
as this, where operative, would leave no trace behind of what had actually 
occurred, it is liable to be underrated in its effect. The chief remaining 
factors of decrease are fusion of sporangia originally distinct, and abortion. 
The former of these has probably been overestimated hitherto in its 
evolutionary effect : tne assumption has been too generally made that 
synangia are the result of fusion of sporangia originally separate. Each 
such case must be considered on its merits, but with the full conscious- 
ness that septation will produce results structurally similar to those of 
fusion. Abortion has been altogether underestimated in dealing with 
early Vascular Plants. In the Lycopods there is ample evidence of its 
effect ; and it is to be remembered that where abortion is complete, 
no vestige remains to show what has taken place (Chapter XIII.). 

The condition of any poly-sporangiate sporophyte, regarded from an 
evolutionary aspect, may be held to be the resultant of such conflicting 
factors of increase or decrease as those mentioned, which were operative 



250 SUMMARY OF THE WORKING HYPOTHESIS 

during its descent. The problem will therefore be to assign its proper 
place in the evolutionary history to any or each of these factors. But 
to do this presumes a knowledge of that history more complete than is 
at present accessible : still it is well thus to formulate the problem, with 
a view to clearing the points at issue. 

. The sporangia are rarely inserted directly on the axis, but usually on 
appendicular organs of various form and size : these have been designated 
in some cases sporophylls, in others sporangiophores. Reasons have 
been assigned in Chapter XII. for the opinion J;hat all these appendages 
are not to be held as referable to any single original category of parts, 
such as the formal morphology of the higher plants would recognise. 
According to a strobiloid theory there is no need to assume that all 
appendicular organs were alike in their initial character, though circum- 
stances may have led to their ultimately settling down to a more or less 
uniform type among plants of advanced development. 

The term sporangiophore is applied to certain appendages which bear 
one or more sporangia, and are traversed as a rule by a vascular strand 
for their supply. Their position may be directly upon the axis, as in 
the Equisetales ; or upon some lateral appendage, as in Helminthostachys ; 
or on the surface or margin of a leaf, as in Ferns, where they are commonly 
called sori. The sporangiophore, wherever found in primitive forms, may 
be held to be itself a primitive structure, and is not to be assumed to 
be a result of modification of any other sort of appendage (Chapter XII.). 
The position which " foliar " parts hold relatively to sporangia or sporangio- 
phores is frequently that of subtending them, as though determined by some 
function of protection, or, in some cases, of nutrition. It is illustrated in 
the Lycopods, the Sphenophylls, and the Ophioglossaceae ; and with less 
regularity in the Calamarians. These relations are probably due to some 
common causal circumstances. 

Such discussions naturally open up the question of the nature and 
origin of those parts which are comprehended under the term " leaf." 
So long as the fossil record remains as imperfect as at present, there 
can be no certain knowledge on these points, since the foliar development 
was present in the earliest vascular fossils of which there is certain or 
detailed evidence : accordingly the question can only be approached on 
grounds of comparison. There is reason to believe that the Bryophytes 
acquired their leaves polyphyletically, and this consideration would suggest 
that the foliar appendages of Vascular Plants may also have been poly- 
phyletic ; this position, which accords with their differences of character, 
is quite compatible with the strobiloid theory (Chapter XII.). One point 
which follows naturally from the observation of the earliest stages of 
development of foliar organs, whether in the sterile or the fertile shoot, 
is their lateral origin below the apex of the axis which bears them. In 
the ontogeny the axis pre-exists the youngest leaves : this is believed 
to have been the case also throughout descent (Chapter XL). 



ORIGIN OF THE FOLIAGE LEAF 251 

Though the evolutionary origin of the leaf must be still a matter of 
doubt, there is less uncertainty as to the relation of the sporophyll to 
the foliage leaf (Chapter XIII.). The idea of "progressive metamorphosis" 
from the foliage leaf to the sporophyll, as propounded by Goethe, is 
incompatible with the strobiloid theory as above stated : the converse of 
Goethe's progressive metamorphosis will appear to have occurred, 
viz., that at least in some cases, and perhaps in all, the foliage 
leaf is a sterilised sporophyll : thus the vegetative region, though 
ontogenetically the first, is held to be phylogenetically the derivative 
not the primitive condition of the shoot. The evidence that this is 
so is primarily based upon broad comparison ; but secondarily upon 
the existence of abortive sporangia in certain Pteridophytes, about 
the limits of the vegetative region. It is further pointed out that in 
cases of complete suppression, where no vestige remains of the undeveloped 
part, there is no structural evidence that the abortive part ever existed : 
this will account for the apparent deficiency of more direct evidence 
bearing on the origin of the vegetative system. The result is a basal 
vegetative region, more or less clearly defined from a terminal strobiloid 
region, the latter retaining the primitive characters and the spore-producing 
function. A vegetative region thus established in any phylum as distinct 
from the fertile, may undergo a distinct progressive evolution of its own, 
according to its special nutritive or other function ; and the result may 
be as wide a divergence in character of the two parts. But in many 
Pteridophytes the differentiation is not effectively carried out; as is seen 
in the " Selago" condition of Lycopodium; or in many Ferns, in which any 
of the vegetative leaves may bear sori. 

The anatomical characters of the shoot accord readily with the theory 
of the strobilus (Chapter XV.). The non-medullated monostele is generally 
accepted as the primitive type, and the more diffuse vascular conditions 
with medulla and ultimately with separate strands as derivative types ; 
and this holds even in the megaphyllous forms, for their individual life 
habitually opens with a protostelic condition of the axis, which may sub- 
sequently pass into some more diffuse structure. This is held to indicate 
a prior state of the shoot where the axis was structurally dominant, and 
the appendages small : ' the more complex vascular arrangements go along 
with an increasing influence of the leaf in the shoot, and are the internal 
expression of it. On the theory of the strobilus this is a secondary con- 
dition, as in the anatomical history of the individual it is seen to be. 

The embryology of the sporophyte generation has figured largely in 
comparative argument. It is pointed out in Chapter XIV. that the im- 
portance of the earliest stages has been greatly overestimated. It has 
been shown that neither the initial segmentation of the embryo, nor the 
continued segmentation at the growing point bears any constant relation 
to the genesis of appendages, or of specific tissues. It thus becomes 
apparent that the early details of segmentation themselves are not 



252 SUMMARY OF THE WORKING HYPOTHESIS 

sufficiently trustworthy guides in the study of the origin of members, 
except perhaps between closely allied organisms. The present tendency 
is to study the embryo biologically, rather than as an embodiment of 
early historical fact : and to recognise that the various appendages of the 
embryo originate in such positions, and at such times as are most suitable 
for the performance of their functions. The demonstration that "free- 
living " leaves or roots may occasionally exist, suggests that some such 
degree of freedom may rule also in the first stages of the embryo. 

There is, however, one relatively constant and fixed point in the 
embryology of Pteridophytes ; it is the position of the apex of the axis 
in close proximity to the intersection of the octant walls in the epi-basal 
hemisphere. This at once defines the polarity of the embryo, whether 
or not the axis may assert itself early by active growth. But when once 
the more plastic stage of the embryo is past, and the characteristic form 
of the plant established, this would seem to be a more reliable basis 
for comparison than the first phases of the embryo (Chapter XIV.). 

A general comparison of the shoot in the sporophyte generation as 
regards symmetry leads to the conclusion that it was originally radial 
(Chapter XVL). In the Bryophytes the sporogonium is a body which 
shows polarity, but retains with very few exceptions the radial symmetry. 
In the Pteridophytes many retain the radial symmetry also ; but others 
depart broadly from it, some at an early period of the individual life, 
others at later periods. These changes may be referred to the unequal 
incidence of external conditions, and it has been shown experimentally 
that a radial structure may be influenced towards dorsiventrality by such 
external causes as unequal incidence of light, or of gravity. This has 
been the probable origin of the dorsiventrality as seen in the sporophyte. 
A comparison of the representatives of the same phylum among them- 
selves frequently indicates that those genera or species which are radial 
are less specialised in other respects than those which are dorsiventral : 
this is particularly clear in the Lycopodiales, as also in the large-leaved 
Ophioglossaceae and Marattiaceae. A careful review of the various phyla 
of Pteridophytes leads back constantly to the radial type as primitive. 
The fact that the radial construction is predominant in the Equisetales, 
Sphenophyllales, and Lycopodiales, while it is prevalent also in the 
Palaeozoic Filicales, shows how strong a hold it had among the earliest 
types of Vascular Plants. 

There is little evidence from plants of the present day of the existence 
of a primitive, permanently free-living, but rootless state of the sporophyte 
(Chapter XVII. ). There is no certain knowledge how the root originated : 
it is clear, however, that in the Lycopodiales the structure of the root 
is more like that of their axis than in other plants, while the comparison 
may also be strengthened by the fact of its occasional exogenous origin 
in those plants, and its dichotomous branching. Further in the same 
phylum there exist in the ^Stigmarian trunks, and the rhizophores of 



ORIGIN OF THE FREE-LIVING STATE 253 

Selagmel/a, parts which are neither true roots nor axes, but serve as bases 
of attachment for roots. Though these bodies do not as yet greatly help 
to solve the question of origin of the root, they draw attention to the 
Lycopodiales in connection with any discussion how a subterranean 
absorptive system originated. In the vast majority of Vascular Plants the 
root is formed early, and is ready as soon as the embryo projects from 
the prothallus, to take up its physiological duty. There seems in them 
no need to assume that they achieved their independence through an 
intermediate " protocorm " stage. It is quite as probable that the transition 
was in descent, as it is to-day, directly to the rooted state. 

The general conception of the rise of the sporophyte embodied in the 
preceding chapters differs in its tone and tendency from some of the writings 
which have preceded it. The attempt has here been made to treat the 
sporophyte consistently throughout : to apply the same point of view to 
the Vascular Plants as to the Bryophytes. One important difference 
between the morphological method adopted here and that of some other 
writers is that it gives a less prominent place to hypotheses involving 
reduction from a more complex ancestry. The general principle here 
has been to assume that morphological characters are in the up-grade of 
development, unless there is good reason for holding a contrary opinion ; 
and only to admit that an organ has been reduced from some more 
elaborate body when there is some assignable reason (comparative or 
physiological, but preferably both) for that conclusion (Chapter XIX.). 
A theory of reduction has never been resorted to merely as a means 
of resolving a difficulty of comparison. The position adopted has accord- 
ingly been to regard it as probable that the smaller-leaved types were 
themselves primitive as a rule, though in some there is evidence of 
probable reduction ; and to contemplate it as probable that microphyllous 
have given rise to megaphyllous types. It is highly probable, on the 
other hand, that reduction of size and complexity has been highly effective 
in certain phyletic lines : for instance, the recognised evolutionary story 
of the Cycads involves extensive reduction of the sporophylls. But for 
the primitive strobiloid forms, it would seem to harmonise better with 
their early occurrence, and their morphological characters, to hold that 
they represent a relatively primitive condition, rather than a down-grade 
of morphological complexity. 

The recent changes of view have been in great measure due to the 
extension of the knowledge of the fossils, both stratigraphically and 
morphologically. It is true that such data are seldom decisive on ques- 
tions of comparison (Chapter XVIII.), but the case here is a strong one. 
It is found that strobiloid forms are at least as early in occurrence as 
large-leaved Fern-like types. Further, they appear not only to have been 
present in the earliest fossil-bearing strata, but also well represented in 
numbers and in variety of type. This has brought with it the conviction 
that the strobiloid type has quite an equal right with any other to be 



254 SUMMARY OF THE WORKING HYPOTHESIS 

held as itself of primitive character. Whether the facts justify the con- 
clusion that the megaphyllous types are derivations of a microphyllous 
strobiloid ancestry, the fossil record does not disclose. The only avenue 
to an opinion is then a detailed comparison of the known representatives. 
It will' be the object of the Second Part of this work to supply such 
comparisons, relating not only to this important question, but also to the 
whole theory of the strobilus, as stated in the First Part. 



PART II. 
INTRODUCTION. 

WE proceed now to deal with the detailed statement of facts bearing on 
the theoretical position expounded in the First Part of this work. The 
arrangement to be adopted must not be understood as indicating any definite 
opinion as to kinship of the several phyla described : it is often dictated 
by convenience of exposition, as much as by the estimate of degrees of 
affinity. Moreover, to any who entertain a belief in polyphyletic origins, 
it will be clear that any simple serial sequence must be misleading. The 
primary end here pursued is not to assign degrees of affinity to the 
relatively isolated relics of a former World-Flora : such relations must 
always remain highly problematical, so long as the data remain as incom- 
plete as they at present are. The object is rather to frame some general 
idea of the methods of advance of the sporophyte ; and to trace the effects 
of those methods from its simpler beginnings to its final condition as an 
independent plant, forming the essential feature of the Flora of the Land. 
Such a study must depend largely on details. Those details will now be 
put together in systematic sequence. 

It may be objected that the scheme of this book is a reversal of the 
ordinary logical procedure of using the facts as a basis for the conclusions. 
But in point of fact, it is not so : for in writing the preceding chapters 
which have dealt with the general theory, all the data now to be 
described were before the mind of the author, and formed the natural 
foundation of his thoughts. It is for the convenience of readers that the 
working hypothesis has been stated first, so as to convey the point of 
view from which the facts may be examined and appraised. The detailed 
statement will thus be more intelligible in its bearing on the question of 
the origin of a Land-Flora, than would otherwise have been possible. 
It will hardly be necessary to repeat again that the general [theory of the 
foundation of a Land-Flora as a concomitant of antithetic alternation 
has been stated only as a "working hypothesis^': it is now to be tested 
by its applicability to the details which are to follow. The course adopted 



256 INTRODUCTION 

will be to start with the simpler types, and to proceed to the more 
complex. The presumable course of progressive evolution will thus be 
followed, but only in the broadest lines. Paragraphs will be inserted from 
time to time, pointing comparisons from one phylum to another ; and thus 
some general conclusion may be arrived at as to the stability of the 
" working hypothesis." 

It may be anticipated that the first place in the detailed description 
will accordingly be given to those Algae which show post-sexual develop- 
ments of the nature of a sporophyte, inasmuch as their nuclei have a 
double chromosome-number. But it seems unnecessary to give any more 
detailed account of these than that already embodied in Chapter V. : 
for at best these Algae only show that such post-sexual complications do 
exist among them, while none of them can be accepted as direct lineal 
progenitors of even the simplest of the Archegoniatae. It is therefore 
sufficient for our present purpose to recognise again the fact that they 
suggest how the antithetic alternation seen in the Archegoniatae may have 
originated. 

With these remarks the Thallophytes may be left on one side : it is 
reasonable to expect, however, that in the future a better knowledge of them 
may result in their being drawn more directly into discussions of the origin 
of alternation ; but at present they have only a remote, and chiefly a 
theoretical connection with the question of the origin of a Land-Flora. 
Such materials as are available for the elucidation of this question are to 
be sought for in the study of the Archegoniatae, organisms which show 
themselves already fitted in greater or less degree for life on exposed 
land-surfaces. 

In treating the Archegoniatae there will be no need to give any detailed 
description of the gametophyte : this is already adequately done in the 
Mosses and ferns of Campbell, and in the Organography of Goebel. It 
may be necessary to refer in some special cases to the gametophyte in 
order properly to understand the sporophyte which it bears ; but excepting 
in such cases the gametophyte will be omitted from our descriptions : and 
thus the subject resolves itself into a comparative examination of the 
sporophyte in the Archegoniatae, from the general point of view laid down 
in the foregoing chapters. 



CHAPTER XXI. 

BRYOPHYTA. 

I. HEPATICAE. 

THE Liverworts include three main series, which differ, not only in the 
structure of the gametophyte but also in the details of the sporophyte ; 
the differences are sufficient to require their separate treatment : the 
three series are the Marchantiales, the Jungermanniales, and the Antho- 
cerotales. The results acquired from any one of these series may be 
brought into comparison with those from any other, and suggestive 
side-lights may thus be gained on the methods of advance of the 
sporophyte which they illustrate ; but the extent of their differences 
shows that they cannot readily be held to constitute one consecutive 
evolutionary sequence. 

A. THE MARCHANTIALES. 

These include those Hepatics which show the simplest of all Archegoniate 
sporophytes ; and the fruit-body of Riccia is the extreme example of 
simplicity of construction. As in all other Archegoniatae the sporophyte 
of Riccia originates from the ovum, contained in the venter of the 
archegonium (Fig. 118), which is here deeply sunk in the tissue of 
the thallus. The naked ovum at fertilisation is contracted away from the 
wall of the archegonium, but after fertilisation it forms a cell-wall, and 
expands till it completely fills the cavity. It then segments, the first 
plane of segmentation being inclined to the axis of the archegonium : this 
is followed by other cell-divisions resulting in cleavage of the sphere into 
octants, after which the segmentation becomes less regular. It is only at 
a comparatively late period that walls parallel to the outer surface separate 
a superficial series of cells forming the wall of the fruit-body, from the 
mass of cells which lie within (Fig. 119). All these latter cells are fertile, 
while the superficial cells composing the wall are sterile and subsequently 
they become disorganised, their substance being absorbed by the developing 



2 5 8 



BRYOPHYTA 



spores so that at maturity they are not to be recognised. A similar fate 
is described for the cells of the inner layer into which the archegonial 
wall has meanwhile divided; and thus there is a supply of nutritive 
material to the developing spores, comparable to that from the tapetum in 
the higher Archegoniatae. The cells that lie within become rounded off, 
and thus separate from one another in the enlarging fruit (Fig. 120): all 
of them undergo the tetrad-division, and develop into spores, which have 



V. 





FIG. 

A archegonium of Riccia trichocarpa showing ventral canal cell (v) and ovum. X 525. 
Z> = ripe archegonium of Riccia glauca. X26o. (After Campbell.) 

a thick, darkly coloured outer wall. |These are set free by the dis- 
organisation of the archegonial wall, or calyptra, the outer layer of which 
persists till the spores are ripe. Under suitable conditions the spores 
germinate, and each forms a new gametophyte. 

Hitherto no observations have been described as to the reduction of 
chromosomes in the tetrad-division of Riccia but it may be assumed from 
analogy with other Liverworts that it takes place here also. It is, however, 
recorded by Garber l that the number of chromosomes in the gametophyte 
of Ricciocarpus natans is four, while in the sporophyte it is eight ; but 
the actual fact of reduction in the tetrad-division was not observed. 

The sporophyte of Riccia thus described is the simplest sporophyte 
known among the Archegoniatae. It has been habitually regarded as a 

1 Bot. Gaz., 1904, p. 171. 



MARCHANTIALES 



259 



primitive type, though the suggestion has also been made that it may 
really be reduced; but in view of the fact that the gametophyte in the 
Ricciaceae is a well-developed structure, amply capable of nourishing not 
only one but many such sporogonia, there appears no immediate reason to 
hold that this sporophyte is other than primitive in its simplicity. The 
points of special interest in it for the purposes of comparison with the 
more complex Archegoniatae are these : that it shows no distinction of 
apex and base : . that the whole central mass of cells is fertile, each cell 
producing spores, while none are diverted to purposes of nutrition or of 




FIG. 120. 



Ricciocarpus natans. The upper figure 

FIG. IIQ. shows the spherical spore-mother-cells 

surrounded by nutritive material. The 

Ricciocarpus natans. Young sporogonia in longitudinal section, lower shows the tetrads formed from 
surrounded by the archegonial wall. The younger (x666) shows them: the sporogonial wall (shaded) is 
the amphithecium (shaded) surrounding the sporogenous cells : in still seen surrounding them, and covered 
the older (Xs6o) these are separated, as the free, and rounded externally by the archegonial wall of two 
spore-mother-cells. (After Garber.) cell-layers. X666. (After Gar ber.) 

dispersal : and that the superficial cells forming the wall are segmented off 
by periclinal walls of ' relatively late origin, indicating some relatively 
recent differentiation of them from the cells which lie within. 

A reasonable theory of the phyletic origin of a simple sporogonium, 
such as that of Riccia founded on these facts, would then be, that it 
sprang from the simple zygote, as in point of fact all normal sporophytes 
do. The simplest possible case of a sporophyte would be that the 
chromosome-reduction which follows on fertilisation should take place on 
the first segmentation of the zygote, and in certain Algae this appears 
actually to occur (Chapter V.). But in the sporogonium of Riccia the 
reduction which accompanies tetrad-division is held over till a limited 
number of segmentations of the zygote have been completed : this suggests 



26o BRYOPHYTA 

that the event of reduction was deferred in the course of its descent. The 
cell-mass thus produced in Riccia is at first homogeneous, as was probably 
the case definitively in certain of its ancestry. Differentiation comes 
later in the sporogonium of Riccia, as it probably did also in the race : 
in place of every cell being equally liable to the tetrad-division, this is 
carried out only by those which lie internally : those forming the 
superficial wall are sterile, and form only somatic tissue. There is ample 
evidence of such sterilisation of fertile cells occurring elsewhere, both in 
plants related to Riccia, and in other phyla (Chapter VII.), so that no 
a priori objection can be taken to its place in the theory : there is, 
however, no direct proof that this was actually the case. The remaining 
cells which lie centrally then all undergo the tetrad-division, which on 
the above theory was the primitive condition for all the cells of the 
sporophyte. 

Till recently it was thought that the fruit-body of Coleochaete supplied 
a prototype of an undifferentiated mass of cells, all fertile, such as this 
theory contemplates; but it has now been shown that in Coleochaete 
reduction occurs at the first segmentation of the zygote, and accordingly 
the old comparison is no longer permissible. There is, however, a growing 
body of evidence, from several distinct phyla of Thallophytes, that the 
event of chromosome-reduction consequent on sexuality may be deferred 
in the individual life : that a sterile, or vegetative phase of the nature 
of a sporophyte, varying in structure and in mode of origin, but similar 
in being partly somatic, partly fertile, may be thus intercalated between 
the two events. The Florideae, the Ascomycetous Fungi, and the Uredineae 
provide examples of such intercalation of a sporophytic phase : these point 
an analogy in this respect with the simplest Archegoniatae, though along 
phyletic lines almost certainly apart from the latter (Chapter V.). Thus 
the view now stated of the phyletic origin of the simple sporogonium of 
Riccia by continued segmentation of the zygote, and deferred tetrad-division, 
with sterilisation of the superficial cells, is in the main hypothetical, it 
is true \ but it has a reasonable basis, partly on the facts of the individual 
development, partly on analogy. In the absence of still simpler sporo- 
phytes affording comparisons within the series of the Archegoniatae 
themselves, this analogy, together with the facts of the individual develop- 
ment in Riccia itself, make the view thus stated appear more probable 
than any alternative hitherto proposed. 

Riccia being the simplest type of sporogonium in the Archegoniatae, 
the basis of the antithetic theory has been fully stated, as applied to the 
facts of its development. The same theory may be extended from it to 
other forms also, in which the sporophyte, though more complicated, 
arises from the zygote by similar though more extended segmentation. 
Steps in advance are illustrated in other Marchantiales, which will now 
be described. 

The sporogonium of the Marchantiaceae, of which Fegatella (Conocephalus) 



MARCHANTIALES 



261 



may be taken as a fair example (Fig. T2i), is more complex than that 
of the Ricciaceae in having polarity of structure, and differentiation of 
the internal mass of sporogenous cells. Both these characteristics suggest 
further steps in sterilisation of potentially fertile cells. The polarity is 
marked in the more or less oblong external form, but more definitely 
by the fact that the basal tissue is sterile, and develops as the foot 



I. 




W. 



FIG. 121. 



Sporogonium of Fegatella (Conocephalus). I., venter of fertilised archegonium with 
eight-celled embryo. II., transverse section of a similar embryo. III., IV., older em- 
bryos in longitudinal section. V., part of a longitudinal section of a developing capsule, 
showing differentiation of archesponal tissue into elater-forming cells (el), and sporogenous 
cells (sfi). VI., part of longitudinal section of receptacle with embryo. VII., similar 
section showing two manure sporogonia in outline, ac = air-chambers ; az/=calyptra; 
crips = capsule ;_/==foot ; = neck of archegonium ; /) = pore ; rec. st.= stalk of receptacle ; 
rhiz. =rhizoids; j = setaof sporogonium. I.-V.X3<5o. VI.X76. VII. X2O. (After Cavers.) 

which supports the fertile capsule (Fig. 121, vn.). It is stated that the 
whole hypobasal half of the zygote is thus sterile, though this limitation 
may not apply for all cases. This state as compared with that of Riccia 
might be expressed as a consequence of sterilisation of the whole product 
of the hypobasal half of the zygote; but it is still a matter for debate 
whether the Ricciaceae really represent the progenitors of the Mar- 
chantiaceae. The presence of this polarity may be held as a biological 
concomitant of the position of these larger sporogonia relatively to the 
thallus which bears them : the spherical form of Riccia is suitable to its 



262 



BRYOPHYTA 



habit, surrounded as it is up to full maturity by the tissues of the parent 
thallus. But the larger sporogonia of the Marchantiaceae project at 
maturity from their envelopes, and even during development their relation 
to the parent thallus is not uniform all round, their nutrition emanating 
mainly from base of the archegonium : a certain degree of polarity, 
expressed in the formation of a sterile foot for nutritive and mechanical 
purposes, is thus intelligible. 




FIG. 122 

Monoclea. Forsteri, Hook. 47 = part of a longitudinal section of a capsule showing 
elaters and rows of spore-mother-cells. XSSQ. 48 = longitudinal section of the tip of a 
nearly mature capsule, showing the lobed spore-mother-cells. X 160. 49 = elaters, tetrads 
of spores, and a cell from the wall of a still more mature capsule. X35o. (After Johnson.) 



But much more interest attaches to the internal differentiation of the 
capsule. The wall is initiated at a relatively early stage, and remains a 
single layer, excepting at the extreme apex : the mass of tissue which lies 
within, corresponding as it does in position to the sporogenous cells of 
Riccia, is composed of cells all alike in origin, and it is often designated 
the archesporium (Fig. 121 iv.). But they do not all develop as spore- 
mother-cells: some become elongated, and form the well-known sterile 
elaters (Fig. 121 v. el) ; others, undergoing more numerous divisions, 
remain fertile, and divide into spore-tetrads : a later stage of this differ- 
entiation is well illustrated in Fig. 122 for Monoclea, a genus of doubtful 



MARCHANTIALES 



263 



affinity with the Marchantiaceae, though showing a similar relation of 

spore-mother-cells and elaters. Since the whole mass is uniform in origin, 

and since the similar mass in Riccia is wholly fertile, it is the natural 

conclusion that certain of the potentially fertile cells have been sterilised 

to form the elaters : or, in other words, remain as somatic cells without 

undergoing chromosome-reduction. The final function of the elaters is to 

assist mechanically in the dispersal of the mature spores ; but it is possible 

that in such a plant as Fegatella (Conocephalus] they may in some 

degree assist in the early nutrition of 

the cells which remain fertile. This 

seems almost certainly to be the case 

in Corsima, and also in those genera 

of more doubtful affinity, viz. Sphaero- 

carpus, Gcothallus, and Riella, where 

the sterile cells are not mechanically 

strengthened by spiral or annular 

thickenings of their walls : they are 

here recognised as " nutritive cells," 

and they undoubtedly aid in the supply 

of nourishment, and perhaps also in 

dispersal of the spores by swelling 

of their mucilaginous remains. The 

obvious importance of these nutritive 

cells, as well as of the elaters, is 

further evidence of the probability that 

a progressive sterilisation, or conversion 

of reproductive into somatic cells, has 

occurred. 

In the Marchantiaceae there is 
regularly present at the distal end of 




FIG. 123. 



Cynthodium cavernarum, longitudinal section 
... of an almost mature sporogonium showing apical 

the CapSUle a Small maSS OI tiSSUe disc. X 200. Above, the apical disc of the same 
. , . 1*1 sporogonium in median section. X 200. (After 

within the one-layered wall, which Lang.) 

remains sterile, and comes away at 

dehiscence as a cap, or lid. This also originates from the archesporium : 

its development has been clearly shown by Lang in Cyathodium: Fig. 123 

illustrates the apex of a sporogonium, and from comparison of the young 

state it is plain that certain cells of the archesporium are told off as 

sterile from the first. 

From these notes it appears that in the Marchantiaceae, as compared 
with the Ricciaceae, the evidence is strong for the conclusion that the 
sporogenous tissue is liable to be reduced at various points by diverting 
cells, or groups of cells, from their original function as fertile cells : the 
somatic functions which they then perform have obvious uses, and this 
gives biological probability to the conclusion. 



264 



BRYOPHYTA 



B. THE JUNGERMANNIALES. 

The same principle is illustrated also in the Jungermanniales, but with 
differences of detail. In these the first segmentation of the zygote 
separates a hypobasal cell, which in some cases develops as a multicellular 




FIG. 124. 

Frullania dilatata, development of the embryo. X 300. (After Leitgeb). x, x^ the 
archesporial cells. The numbers indicate the primary transverse divisions. (From 
Campbell, Mosses and Ferns.) 

haustorium (Frullanid), but more frequently remains as a minute appendage 
at the base of the fruit, while the epibasal cell gives rise to the capsule, 
stalk, and foot. So far as a comparison on the basis of the segmentation 
of the zygote is valid, this would indicate in the Jungermanniales a still 

further delegation of function from the 
hypobasal to the epibasal half of the 
embryo. 

The epibasal half of the zygote under- 
goes segmentation, so that a number of 
transverse discs are formed, each composed 
of four cells, while the terminal tier is 
composed of four octants of a hemisphere 
(Fig. 124). The segmentation is often 
very regular, but exceptions exist. 1 The 
FIG. 125. uppermost tier of cells appears to be cut 

Embryos of Radula compianata. X2 3 o. off by the first transverse wall in the 

(After Leitgeb.) 7, 8, show the basal ap- ., . , , r r , , . 

pendage cut off b y the first segmentation of epibasal half of the zygote, and in many 

the zygote : a shows the upper region of a c i i T 

more advanced embryo. of the Anakrogynous Jungermanniaceae 

the whole of the capsule is derived from 

these cells : subsequent intercalary divisions in the lower half of the 
epibasal cell gave rise to the sterile seta. But, on the other hand, in many 
of the Akrogynous Jungermanniaceae for instance, in Radula it is shown 
by Leitgeb 2 that some of the lower tiers of cells also take part in the 
formation of the capsule ; in Radula the number of these appears to be 
three (Fig. 125). Thus there is in such cases no general distinction, on 




1 See Campbell's Figs, of Porella ; Mosses and Ferns, Fig. 55. 

2 Lebennoose, ii., p. 55. 



JUNGERMANNIALES 



265 



the basis of the first segmentations, of the capsule from the seta. So far 
as segmentations offer a basis for argument, the general conclusion may 
be drawn that the seta and capsule are not always distinct ab ijntio. The 
developmental facts suggest that the fertile region may be held to be a 
residuum left by sterilisation, which has been basifugally progressive : the 
result of such sterilisation is the region of the seta as it is seen in these 
plants. We may regard as the most primi- 
tive case that in which all the tiers of cells 
of the embryo form the archesporium : 
those cases in which the differentiation 
of the archesporium is deferred in the 
individual life may be held to be relatively 
less primitive. 

The four cells of the uppermost tier, 
which thus as a rule form the capsule 
in the Jungermanniales, divide first 
transversely to form four terminal cover- 
cells : the four larger cells below again 
segment to form four inner cells and 
eight to twelve peripheral cells. This 
is the same segmentation as occurs also 
in the lower tiers : the similarity is in 
accordance with the view above expressed 
as to the origin of the seta by basifugal 
sterilisation, and supports the conclusion 
that seta and capsule had a common 
origin. 

However interesting such questions 
may be, they are more or less speculative. 
A much greater interest, proportional to 
the greater cogency of the facts, attaches 
to the various modes of development of 
the capsule itself in the Jungermanniales. 
Jt has been seen that the inner cells 
above described constitute the arche- 
sporium. In many of the Akrogynous 

Jungermanniaceae the cells, after repeated divisions, undergo a differen- 
tiation as in the Marchantiaceae, into spore-mother-cells and sterile elaters : 
these are associated in various ways, and the case of Porella will serve 
as an average example (Fig. 126). The argument from differentiation of 
sterile and fertile cells during development applies equally here as in the 
Marchantiaceae. The same is the case with many of the Anakrogynae; 
but in some of the latter there is a more specialised tissue-differentiation 
leading to the formation of a coherent mass of sterile tissue, with a more 
definitely localised residuum of fertile tissue : this sterile mass has been 




FIG. 126. 

Porella Bolanderi. Longitudinal section of 
a sporogonium after the final division of the 
archesporial cells. ><35- (After Campbell.) 



266 



BRYOPHYTA 



styled an elaterophore. It is illustrated in the cases of Metzgeria and 
Aneura (Fig. 127), and also in Pellia (Fig. 128). In the former the 
elaterophore is attached internally at the distal end of the capsule, in the 
latter at its base. In Aneura the capsule- wall consists of two layers of 
cells, while the archesporium lies within. The differentiation of this body 
is initiated early: in so young a sporogonium as that of Fig. 129 A 
there is a distinction already marked by the protoplasmic contents between 
a central group of more transparent cells and a peripheral band with 




FIG. 127. 

Aneura pinguis. Ripe capsule 
in longitudinal section. From the 
summit the elaterophore hangs into 
the spore-cavity, in which are many 
spores and elaters. Magnified. 
(After Goebel.) 




FIG. 128. 

Pellia calycina. Sporo- 
gonium opened, and emptied, 
showing the valves of the 
wall recurved, and an elatero- 
phore of many threads. (After 
Goebel.) 



denser protoplasm. The former constitutes only a central part of the 
elaterophore : it is clearly shown at a later stage that the differentiation 
has extended (Fig. 129 B), and that some of the inner products of the 
darker band shown in Fig. A are also developing as sterile cells, while 
it is only the fertile outer fringe which is the final residuum after these 
progressive steps of sterilisation. This point comes out even more clearly 
in a transverse section (Fig. 129 c), where the central group of cells 
first differentiated are readily distinguished from those differentiated later. 
There can be little doubt, after comparison with other Jungermanniaceae, 
that the history of the individual development in this case gives a correct 



jrNGERMANNIALES 



267 



clue to the steps of evolution of the elaterophore : that it arose by partial 
sterilisation of the archesporium, with the consequence that the fertile 
zone is relegated to the exterior. The development in Pellia is essentially 
the same, but the attachment of the elaterophore is to the base of the 
capsular cavity. It is worthy of note that these modifications occur in 
the Aqakrogynous Jungermanniaceae, which, as we have already seen, are 
more advanced than the Akrogynous as regards the differentiation of the 
seta and capsule. 





FIG. 129. 

A, median section of young sporogonium of Aneura ambrosioides. The internal mass 
of cells of the sporogonial head ("archesporium") is already differentiated so as to 
indicate the sterile elaterophore, and the outer fertile region. , the same, older : the 
indications of sterilisation have extended outwards, and it is only the peripheral fringe of 
cells (shaded) which will be sporogenous. C, transverse section of the same. X 150. 

It thus appears that in the Jungermanniales partial sterilisation of the 
sporogenous tissue has occurred analogous to that in the Marchantiales ; 
but in addition there is reason to believe that the elongated seta has 
here originated also by relegation of the spore-producing function from 
the lower segments to the upper, or even to that tier of them which is 
apical. 

'C. THE ANTHOCEROTALES. 

The Anthocerotales stand clearly apart in the mature characters of the 
sporogonium. Its large size and early freedom from the calyptra, the 
continued intercalary growth, and the specialisation for self-nourishment 
are external characters which dissociate the family from other Liverworts : 
while internally, the presence of a columella, at least in the larger forms, 
and the origin of the archesporium from the outer cells of the young 
capsule, also point in the same direction. There are, however, differences 
of detail within the family, which indicate with some degree of probability 
a line of derivation from a Jungermanniaceous type, the nearest approach 
being in the genus Notothylas. 



268 



BRYOPHYTA 



The development of the sporogonium has been carefully studied in 
Anthoceros by Campbell. 1 The early segmentations result in three tiers 
composed of four cells each (Fig. 130 A): of these the lowest is derived 
from the hypobasal half of the zygote, and it forms the main part, if 
not indeed the whole of the foot. The highest tier gives rise to the 
primary capsule, while from the intermediate zone derived from the 
second tier the meristematic part of the older sporogonium is formed 
(Fig. 130 ?:). The highest tier of four cells segments further to form 







FIG. 130. 

Anthoceros Pearsoni. Development of the embryo. X 300. A, C, E, median longi- 
tudinal sections. B and D, successive cross-sections of embryos of about the age of A 
and C respectively. In E the archesporium is differentiated. (After Campbell.) 

a central group of four, and a peripheral series (Fig. 130 c, D) : from 
the former the sterile columella originates, although in all other Liverworts 
the corresponding cells give rise to the archesporium. This, however, 
is primarily formed in Anthoceros from the inner cells resulting from 
periclinal division of the peripheral cells of the highest tier, and its 
form is that of a dome completely covering the rounded apex of the 
columella (Fig. 130 E). The columella thus initiated in Anthoceros 
develops entirely as sterile tissue. The layer of cells immediately outside 
it, recognised as the archesporium by their denser protoplasm, differentiates 

1 Mosses and Ferns, p. 134. 



AXTHOCEROTALES 



269 



later, in the well-known way, iqto elaters and spore-mother-cells, showing 
thus a partial sterilisation (Fig. 131 A). Subsequently the intercalary 
activity begins in the middle zone, and it adds by basipetal increments 
respectively to the columella, the archesporial layer, and the capsular wall. 
Such intercalary development may be held to have been of secondary 
origin from the region corresponding to the seta of other forms, and the 



\v-\ 




FIG. 131. 

Xotothylas Breutclii. A , longitudinal section of the basal region of an almost mature 
capsule, showing a short sterile columella derived from the endothecium. X 130. , a 
similar section, but without sterile columella. In the upper part the wall of the capsule 
can be distinguished from the sporogenous tissue, the cells of which are differentiated into 
spore-mother-cells (shaded), and elaters. Traced downwards, the central portion of the 
sporogenous tissue is continuous with the endothecium, while the peripheral portion comes 
from a layer of cells which have a common origin with the wall. X 170. C, transverse 
section of a sporogoniuni like B, at the base of the fertile region ; the uniform sporogenous 
tissue (shaded) can be seen to be composed of four central groups of cells (endothecium) 
and a surrounding layer derived from the amphithecium. X 170. D, , f, G, H , succes- 
sive transverse sections of a sporogonium, in which one of the four rows of cells of the 
endothecium is forming a sterile columella, left clear in G and H. The origin of the 
amphithecial archesporium can be followed. X 170. (After Lang.) 

primary condition of the sporogonium will thus be a more satisfactory 
basis for comparison with other Liverworts than its mature state. 

It might be difficult, in the absence of intermediate forms, to bring 
this peculiar sporogonium of Anthoceros into relation at all with those of 
other Liverworts. But within the Anthocerotales there is the genus 
Notothylas, which bears sporogonia of small size, and of limited inter- 
calary growth, whereas in the other genera the large sporogonia appear 



270 BRYOPHYTA 

to have the intercalary activity unlimited. Moreover, in these small 
sporogonia, though a sterile columella is often present, sometimes its 
place is taken by/ fertile tissue ; and the difference may be seen in sporo- 
gonia of the same species. The details of this were long ago described 
by Leitgeb/ but doubts have since been raised regarding his conclusions by 
investigators who, working chiefly with other species, did not obtain the 
same results. 2 Recently, however, Lang has made observations which go 
far to explain the discrepancies ; and though they do not exactly coincide 
with Leitgeb's account as regards the development of Notothylas, they show 
that, as regards the fertility of the columella, he was substantially correct. 3 
It appears that the embryonic structure of the sporogonium of 
Notothylas is essentially like that of Anthoceros, in respect of the relations 
at its base of columella, archesporium, and capsule-wall. In those cases 
where the columella is present in the mature state, the spore-mother-cells 
originate only from the archesporium. But in other cases where a definite 
columella is not present in the mature state, any cell of the tract laid 
down structurally as columella may become a spore-mother-cell. Many 
do so, and thus, as Leitgeb described, the place of the sterile columella 
may be taken by a spongy mass of sterile tissue, in the meshes of which 
the spores are included. In addition to this, however, fertile cells and 
elaters are also produced from the archesporium, which lies, as in 
Anthoceros, outside the columella (Figs. 131 A-F). Two interpretations 
of this state are possible : either that the columella-less forms are primitive, 
and their partly fertile condition intermediate towards the establishment 
of a completely sterile columella : or that the forms with a columella are 
primitive, and the columella-less forms a reversion, some of its cells 
resuming fertility which had previously been lost. Dr. Lang is inclined 
to consider the columella-less forms as reduced : but whether reduced or 
not the facts throw considerable light upon the relation of the columella- 
less to the columelloid forms: they increase the justification for considering 
the central group of cells, which in all other Anthocerotaceae is wholly 
devoted to the formation of a sterile columella, as the original sporogenous 
tissue, and the amphithecial archesporium as of secondary origin. The 
duty of producing spores would seem to have been transferred from the 
central to the superficial set of cells. It is thus possible to bring the 
apparently divergent sporogonium of the Anthocerotales into relation to 
that of the simpler and probably more primitive Jungermanniales. The 
causes of the change of the products of the endothecium from the fertile 
to the sterile condition must be looked for in influences acting on the 
primary meristematic tissue of the embryo, or on the intercalary zone of 
secondary meristem. Dr. Lang holds 4 that the idea of grouping of elaters 
in a central position to form the columella is not in this case in accordance 

1 Lebermoose, v. , p. 39. 

Mottier, Bot. Gas., 1894; Campbell, Mosses and Ferns, Edn. ii., pp. 151-155. 
3 Lang, Ann. of Bot., vol. xxi., p. 201, etc. 4 Z.r., p. 208. 



ANTHOCEROTALES 271 

with the facts. These suggest^ rather the influence of nutritive factors 
acting on the young embryo while still enclosed in the tissue of the 
gametophyte. 

The characters of progress achieved by the more complex Antho- 
cerotales, in advance of the Jungermanniales, appear accordingly to be 
these: (i) a continued intercalary growth at the base, originating from 
the seta, and giving an unlimited sequence of spore-production; (2) 
provision for the nourishment and ultimate dispersal of the spores by 
means of the columella ; (3) relegation of spore-development to a more 
superficial source, as the sterilisation at the centre becomes established ; 
and (4) development of an assimilatory apparatus for self-nourishment 
from the tissues of the capsular wall. All these advances are readily 
intelligible on biological grounds, and are due either directly to steri- 
lisation of fertile cells, or to secondary modifications in tissues already 
sterile in the simpler types. The theory of progressive sterilisation has 
already been traced in its application to the sporogonia of other 
Liverworts, as elucidating the origin of the protective capsular wall, 
the seta, the elaters, and elaterophores. It is now seen that the origin 
of the sporogonium of the Anthocerotales, though the most advanced of 
all the Hepaticae, falls naturally within the lines of a theory of progressive 
sterilisation, which starts from relatively simple post-sexual cell-divisions. 



CHAPTER XXII. 

II. MUSCI. 

THE Mosses for the most part show greater uniformity of plan in their 
sporogonia, and give less indication of the steps of their evolution than 
do the Liverworts. It is a question open for discussion what, if any, 
are the genetic relations of these two classes. Whatever view may be 
held on this point, there are certainly strong features of similarity between 
their sporogonia. Without necessarily accepting these resemblances as 
indications of near genetic affinity, they must at least be held to point 
a strong analogy between the two series : so strong indeed that it will 
go far to justify an application of a theory of sterilisation in the Musci, 
even where the gradual steps of the process are less clearly indicated 
than they are in the Liverworts. 

The Musci include the Sphagnales, the Andreaeales, and the Bryales : 
these are sufficiently distinct in their sporophyte-structure to require 
separate description; and the Sphagnales will be taken first, as showing 
the clearest analogies with the Hepatics. 

A. SPHAGNALES. 

Notwithstanding the strong divergence of their gametophytes, both in 
form and in structure, the sporogonia of the Sphagnales and Anthocerotales 
show marked similarity, both in form and in development. Alone among 
the Mosses the embryo of Sphagnum segments by successive transverse 
walls, like a Liverwort : there is no continued apical growth, the further 
enlargement after the first segmentations being intercalary in the segments 
already laid down (Fig. 132 A): of these only the upper three or four 
go to form the capsule : the rest form the short seta, and the foot. In 
the upper region each segment divides into quarters, which again divide 
so as to form in each transverse section a central group of four (endothecium), 
and a peripheral series (amphithecium) (Fig. 132 c y D, E). The former 
give rise to the columella alone, which is in Sphagnum a bulky mass 



SPHAGNALES 



273 



of tissue with rounded apex (Fig. 133 F, H). The peripheral series of 
cells, or amphithecium, divides periclinally to give off internally the single 




FIG. 132. 

Development of sporogonium of SpJtagnum acutifplium, Ehrh. A = embryo with four 
tiers; J = apical cell; =basal cell with oblique division. j6? = embryo with five tiers. 
C = optical section of the same embryo; one quadrant is still undivided; rt = anticflnal; 
/ = periclinal walls; h = principal walls. D transverse section of the lower part of an 
embryo. E = & rather older stage; spr = spore-forming layer. ,F=median longitudinal 
section of a sporogonium showing the bell-shaped sporogenous layer, and the wall covering 
it externally. G = transverse section of a sporogonium of similar age ; lettering as in E, 
// = median longitudinal section, though a half-ripe sporogonium; ca/=calypira ; spr 
spore-cavity, in which the spore-mother-cells are isolated ; sps = spore-sac ; epi = epidermis ; 
7 = furrow in wall where the operculum will separate ; / > j = pseudopodium ; ^ = vaginula ; 
^W=perichaetial leaves ; co/=coluinella. (After Waldner, from Engler and Prantl.) 

layer of the archespo'rium ; this appears as a continuous dome closely 
investing the columella. The external product of the amphithecium forms 

s 



274 BRYOPHYTA 

the rnany-layered capsular wall (Fig. 132 F, G, H). The dome-shaped 
archesporial layer divides later into four layers, and every cell undergoes 
the tetrad-division to form spores. At maturity dehiscence takes place 
by a transverse rupture, setting free a circular operculum. The foot is 
considerably enlarged as an haustorium, which is marked off at maturity 
by the narrow neck of the short seta : and the whole is borne upwards 
on a more or less elongated pseudopodium developed by the parent 
gametophyte (Fig. 132 H). 

In the facts thus briefly sketched there is no obvious evidence of 
sterilisation : it is only when the peculiarly close analogies with the 
Anthoceroteae are traced that any relation to the theory emerges. The 
points of similarity with the typical Anthocerotales are seen in the form 
of the sporogonium, in its absence of apical growth, and in the manner of 
its primary segmentation : also in the origin of the columella from the 
whole of the central group of cells, and of the completely dome-shaped 
archesporium from the primary capsular wall outside it. It differs, how- 
ever, in the mode of dehiscence, and in the fact that elaters are absent, 
while the columella is not mechanically functional : it serves no purpose 
beyond the nutrition of the considerable mass of spores. But as methods 
of dehiscence, and of distribution -of spores not unfrequently vary within 
near circles of affinity, this discrepancy does not seem of prime import- 
ance. Lastly, however, there is, as a point of difference from Anthoceros^ 
the absence of any functional assimilatory system in the sporogonium of 
Sphagnum, though it is so well developed in Anthoceros. But, as Haberlandt 
has shown, 1 functionless stomata, without pores and without intercellular 
spaces below them, are present in large numbers on the capsules of 
Sphagnum : from this he concludes " that it is certain not only that the 
ancestors of the present Bog-Mosses had normal functional stomata on 
their capsules, but also that the capsules of these ancestors possessed a 
relatively well developed assimilatory system as well." All these con- 
siderations taken together point to a close analogy (if nothing more) 
between the two types. 

On the other hand, Sphagnum has always been ranked as a Moss on 
such grounds as habit, absence of elaters, and structure of the archegonium : 
but it differs from all other Mosses in the transverse segmentation of 
the embryo, and in the absence of an apical cell : also (excepting Andreaea) 
in the complete dome-shape of the archesporium, and in its origin from 
the amphithecmm ; these all being features of correspondence with 
Anthoceros. Such equivocal comparisons, with Liverworts on the one 
hand, and with Mosses on the other, give Sphagnum itself a special 
interest : at the same time they serve to link together the two large groups 
of Bryophytes, and point to the propriety of regarding their sporogonia 
equally from the point of view of a theory of sterilisation. The columella 
would thus be held in both cases to be a consequence of sterilisation 
1 Pringsh. Jahrb,, xvii., p. 474. 



ANDREAEALES 



275 



progressive from within, which has 
extended in Sphagnum^ as in Anthoceros, 
to the whole product of the endothecium, 
while in both the archesporium takes its 
origin wholly from the amphithecium. A 
key to this difference of Sphagnum from 
all other Mosses may be found in Noto- 
thylas, in which the sporogenous cells 
may arise from both sources, the centri- 
fugal progress of sterilisation being less 
completely carried out there than in 
Anthoceros. In both cases the difference 
appears referable to the degree of centri- 
fugal sterilisation in a body in which the 
spore-production was originally central. 
Thus the condition of Sphagnum is in 
this respect the most advanced in the 
Mosses, as that of Anthoceros is among 
the Liverworts. As regards decentrali- 
sation of the fertile tissue the rest of the 
Mosses will be seen to correspond to 
the less extreme types of the Hepatics, 
while the columella-less sporogonia of 
Notothylas link together the two degrees 
of decentralisation. 



B. ANDREAEALES. 

The mature sporogonium of Andreaea 
offers analogies with that of Sphagnum 
in its form, with its short seta and large 
foot, the whole being borne up on an 
elongated pseudopodium : it also corre- 
sponds in the fact that the columella is 
interrupted at the apex^ and covered by 
the archesporium which forms a complete 
dome (Fig. 133); but it differs in the 
dehiscence when mature by longitudinal 
slits. 

The segmentation of the zygote corre- 
sponds to that of other Mosses rather 
than to that of the Hepatics, for after 
the appearance of the transverse basal 
wall, the cleavages are oblique, a two-sided initial cell being present (Fig. 
134 A-D) : but the number of such segmentations is limited to about a dozen. 




FIG. 133. 

Median longitudinal section of sporogonium 
of Andreaea rupestris, at the time of division 
of the archesporium. f> = pseudopodium ; _/"= 
foot; z; = vaginula; h = neck; c = columella; 
w = wall of sporogonium j e = epidermis; s = 
spore sac; *=archesporial cells dividing; 
r=calyptra; s = neckof archegonium. (After 
-Kuhn.) X8o. 



BRYOPHYTA 



These undergo further sub-divisions to constitute an endothecium of four cells, 
and a many-celled amphithecium (Fig. 134 G). The hypobasal half, which 
has meanwhile undergone irregular divisions, together with the two lowest 
segments of the epibasal region remains sterile, and constitutes the short 
seta and enlarged foot. Three or four only of the upper segments are 
fertile, while the rest go to form the sterile apex of the capsule (Fig. 134 E). 




FIG. 134. 

Development of sporogonium of Andreaea. A= young embryo of A. crassinervia, 
Brch. B = A. petrophila, Ehrh. C = of A . crassinervia, older. D = of A . sp. , older still. 
E= optical longitudinal section of A. petrophila ; ,y/r= archesporium. J F=optical trans- 
verse section of a young embryo ; jj = segment- walls ; rr = radial walls ; aa = first divisions 
of the quadrants. G = older stage ; ^="grund-quadrat." // = older stage with division 
to form archesporium. / = archesporium differentiated. /if = archesporium divided into 
two layers; as/> = outer spore-sac ; col. =columella. (After Waldner.) D after C. Miiller- 
Berol. (From Engler and Prantl.) 

The archesporium is here, as in all other Mosses, derived from the endo- 
thecium : the peripheral cells which result from the segmentation of the 
endothecium, become densely granular, and give rise to spore-mother-cells, 
the internal cells form the columella (Fig. 134 E, j, K). It is not stated 
by Waldner how the archesporial dome is completed at its apex: probably 
it is by certain cells of the inner product of the endothecium, forming 
spore-mother-cells, in place of sterile cells of the columella. In this there 
would be no theoretical difficulty, for on the present theory all cells derived 
from the endothecium were at first potentially fertile cells ; moreover, as 
bearing indirectly on this point, the internal limit between the archesporium 



BRYALES 



277 



\J 



and the columella is in Andreaea a very 
irregular one (Fig. 135). Further, in Archidtum, 
which some writers put in close relation to 
Andreaea, any cell derived from the endo- 
thecium may apparently become a spore- 
mother-cell. 

It is thus seen that though Sphagnum and 
Andreaea have certain apparent characters of 
the sporogonium in common, their segmen- 
tation is essentially different, and the dome- 
shaped archesporium is produced in different 
ways in the two. It remains doubtful there- 
fore how far the similarities indicate a real 
affinity. In any case the relation of the 
Andreaeales to the simpler Bryales is much 
nearer than to the Sphagnales, and it is in 
this direction that comparison of the sporo- 
gonia will bring the more interesting con- 
clusions concerning them. 

C. BRYALES. 

These include the vast majority of Mosses. 
In their sporophyte generation there is uni- 
formity of the general scheme, though 
considerable fluctuation in size, as well as 
in minor detail. Some of the smallest forms, 
which show irregular opening of the capsule 
on maturity, are classed as the Cleistocarpae ; 
the more elaborate forms, which dehisce 
transversely, setting free an operculum, are 
designated the Stegocarpae. The latter, as 
they represent the prevalent type in Mosses, 
will be taken first, while the Cleistocarpic 
forms being taken later, will then be better 
appreciated in their value for purposes of 
comparison. 

(a) Stegocarpae. 

The embryo of all these Mosses takes at 
an early stage the form of a more or less 
slender, but always simple spindle: any later 
deviations from this are of secondary origin. 
Its development from the zygote is first by 
the appearance of a transverse basal wall, which is succeeded by oblique 
segmentations in the epibasal half: these appear alternately on opposite 




Ceratodon purpureus. A, B, young 
embryo seen from points of view at 
right angles to one another. C = an 
older embryo ; gg outer limit of endo- 
thecium ; sps = outer spore-sac. (After 
Kienitz-Gerloff.) 



278 BRYOPHYTA 

sides, so as to cut off two rows of segments successively from the terminal, 
two-sided initial cell (Fig. 135 A and B). The apical growth is, however, 
not very long continued, and gives place later to intercalary activity. The 
hypobasal cell, as also sometimes the lowermost segments, undergo less 
regular sub-divisions, but the upper segments sub-divide with greater 
regularity though still with some differences of detail in such a way 
that a definite result is arrived at, viz., the formation of a central tract 
of tissue (endothecium), consisting of four rows of cells, and a peripheral 
series (amphithecium), consisting of more numerous cells, which soon 
divide both radially and periclinally to form a thick wall (Fig. 135 c). 
It is important, from a theoretical point of view, to note that the endothe- 
cium thus established, though less definite in the lowest of the epibasal 
segments, extends upwards throughout the length of the capsule to its 
apex : it is not merely a local development in that part which is ultimately 
to be the fertile region, but it is a continuous and definite column of 
tissue, occupying the centre of the spindle-shaped sporogonium. It may 
be a question what is the morphological importance of a tract thus defined 
by embryonic segmentation. In Chapter XIV. the relation of the leading 
anatomical regions of axis and root to the apical segmentation has been 
discussed, and it was seen that there is no obligatory correspondence 
between early segmentation and the definition of mature tissue-tracts : for 
it has been found that, in parts of such complicated outline as the leaf- 
bearing shoot, the correspondence between early segmentation and mature 
structure is not strictly maintained. But it is the fact that in parts of 
such simple outline as roots there is a definite correspondence of that 
nature, and this is particularly clear in certain Pteridophytes. The case 
of the simple spindle-shaped sporogonium of a Moss is comparable, in its 
form as well as in the early segmentation of its central tract, with such 
roots ; and there seems good reason to regard the endothecium accordingly 
as being in fact a morphologically definite region throughout its length. 
The most important function of the endothecium is that it is the exclusive 
source of spore-formation ; but as a matter of fact, it is only a relatively 
small extent of it which carries this into effect, the rest remaining sterile, 
performs other duties. 

The archesporium originates from a restricted region of the endothecium 
some distance back from the apex of the sporogonium, and a very con- 
siderable distance from its base : the sterile region of the capsule at the 
distal end forms the calyptra and peristome : the much longer sterile 
region at the base forms the apophysis and the seta. These regions may 
vary in their proportion to the fertile region in different types of Mosses : 
a fair average is that seen in Funaria (Fig. 136 A). The origin of the 
archesporium is by periclinal division cutting off a single layer of cells 
from the periphery of the endothecium : this ultimately divides up into 
several layers of minute cubical cells, all of which undergo the tetrad- 
division in the usual way, and produce spores. 



BRYALES 



279 



JB. 




FIG. 136. 

Fnnaria hygronietrica. A longitudinal section of a sporogonium showing the first 
differentiation of its parts. X about 96. .5 = the upper part of the same. X6oo. 
r marks the limits of theca and operculum. C basal part of capsule of the same. x6oo. 
r=archesporium ; r0/=columella. (After Campbell.) 



280 



BRYOPHYTA 



Meanwhile, however, other changes supervene, and they are especially 
marked in the amphithecium ; but all such changes are nothing more than 
secondary modifications of the amphithecial tissue : there is no develop- 
ment of new parts, however greatly the external appearance of the sporo- 
gonium may be affected by their presence. They may be considered in 





FIG. 137. 

3. Diagrammatic longitudinal section through the green capsule of Physcomitrium 
pyriforme. X 14. 5. Median longitudinal section through the mature green capsule 
of Funnria hygrometrica. X2o. 8. Profile view of mature capsule of the dorsiventral 
capsule of Buxbaumia aphylla. g. Median longitudinal section of the same capsule ; 
/=cylindrical air-space; at j the stomata. X 10. 10. Transverse section of the same 
capsule, about the middle. Xg. (After Haberlandt.) 

succession from below upwards. The lower part of the sporogonium which 
forms the seta elongates more or less in different types : where well 
developed, as for instance in Funarta, its structure shows a peripheral 
sclerotic band, which merges gradually into thinner-walled parenchymatous 
tissue : centrally lies a strand of thin-walled water-conducting tissue, 
without cell-contents when mature, surrounded by a thicker-walled sheath. 



BRYALES 



281 



No accurate statements are to >hand showing the genetic relation of the 
endothecium to this central tract, but it certainly gives rise to the greater 
part, if not exclusively to the whole, of the conducting strand. 

In the upper region assimilating tissue is usually developed in more 
or less close relation to the capsule itself, together with numerous and 
often large intercellular air-spaces. These developments are derived chiefly 
from the amphithecium, while at the same time the sterile cells of the 
columella usually expand, as a tissue for water-storage : this may also 
contain some chlorophyll, and occasionally forms air-spaces. The swollen 
shape of the capsule is chiefly 
due to these changes, which are 
obviously secondary. Different 
types may be distinguished 
according as the assimilatory 
system is developed from the 
wall of the capsule itself, or 
partly here and partly in the 
apophysis below, or entirely in 
the region of the apophysis. 
For instance, in Bartramia the 
assimilatory system is chiefly in 
the wall of the capsule, where 
it is equally developed all round. 
In Buxbaumia (Fig. 137, 8, 9) the 
same is the case, except that 
the development is dorsiventral : 
the capsule early takes an oblique 
position, and the assimilatory 
tissue is developed more strongly 
on the better-lighted side. In 
other cases, however, the assimi- 
latory system extends some dis- 
tance below the actual capsule, 

constituting the swollen region of the apophysis : this is of small size in 
Polytrichum, and the assimilatory system is chiefly here also in the wall 
of the capsule, but it extends downwards to the small apophysis, while the 
numerous stomata lie in the narrow neck between these parts. In many 
Mosses, again, the apophysis itself becomes the chief seat of assimilation, 
as, for instance, in Funaria (Fig. 137, 5), and this leads to its enlargement: 
so much so that it becomes the most prominent feature in the whole 
sporogonium : thus in the Splachnaceae it is commonly larger than the 
capsule which it is to nourish, and in S. luteum (Fig. 138) it appears as 
a wide umbrella-like expansion, which shows a structure not unlike a 
leaf-lamina, with well-marked epidermis, spongy mesophyll, and stomata 
upon the upper surface. Still, with all these variants at or near to the 




FIG. 138. 

SplacJinum luteum. I. Capsule open. A apophysis. 
II. Unopened capsule in longitudinal section. ,r=seta ; 
-jr = leptoxylem ; jr/ = stomata on apophysis ; /=colu- 
mella ; / = peristome ; ^4j = archesporium ; i intercellular 
space. III. and IV. Diagrams to illustrate the opening 
of the capsule. (From Goebel, after Hedwig, Vaizey, and 
Bryhn.) 



282 BRYOPHYTA 

fertile region of the capsule, the original formation is the same : it is 
based upon an amphithecium, capable as we see of variously extended 
development, and an endothecium which is less variable, though it may 
expand also to form a more bulky tissue for water-storage : its most 
distinctive function, however, is to give rise to spores, while below it 
serves for conducting purposes. 

Above the fertile region the endothecium as a rule develops only cells 
similar to those of the columella below : the amphithecium, however, 
undergoes changes of induration of the walls, variously distributed, which 
result in the formation of the operculum, the annulus by which it is 
detached, and the peristome which is laid bare when the operculum falls 
away. The details of the peristome may vary considerably in different 
Mosses, but in all cases it appears to take its origin from the innermost 
layer of the amphithecium. 1 The columella may in some cases co-operate 
with it mechanically in the function of spore-distribution, but neither the 
endothecium nor any of its products take any share in the development of 
the peristome. 

It thus appears that the sporogonium of the stegocarpic Bryales is 
composed of two tissue-tracts, distinguished early from one another in 
segmentation, and divergent in their later development. The outer is 
always sterile, while the other is fertile only in part. The question arises 
as to the initial condition, and the origin of these regions. It may be in 
some degree elucidated by comparison of some of those smaller forms in 
which the sporophyte is of simpler construction : they have in common 
the feature that the mechanism of dehiscence is absent, or imperfect, and 
on this account they have been grouped together as the Cleistocarpae. 
It is clear at the outset that this condition may either have been 
primitive or the result of reduction : these alternatives must be kept in 
mind in any discussion of such forms, even though no definite conclusion 
be arrived at. 

(b) Cleistocarpae. 

Of the various genera grouped as the Cleistocarpae, Phascum has been 
examined developmentally by Kienitz-Gerloff : the primary segmentation is 
according to the type of the Bryales, with well-marked endothecium and 
amphithecium (Fig. 139). From the periphery of the former the arche- 
sporium is derived in the usual way, while the formation of an air-space 
and spore-sac, and the enlargement of the cells of the columella, are all 
according to the usual type : stomata may also be present, but there is 
neither operculum nor peristome. Developmentally there is a near similarity 
to Andreaea, though on the ground of its peculiar dehiscence and domed 
archesporium this genus is usually kept apart. It has, however, been 
pointed out above that a very slight modification of the ordinary type of 
Andreaea would produce the condition of the archesporium seen in 

1 Goebel, Organography, p. 383. 



BRYALES 



283 



Phascum : the sterilisation of the whole of the upper products of the 
endothecium at the narrow distal end of the fertile tract would interrupt 
the dome, and complete the columella, just as it is seen in Phascum. 
There is no inherent im- 
probability in this, but 
rather the reverse : for it 
would be only introducing 
one further step in sterili- 
sation. On such grounds 
the relation of the Phasca- 
ceae to the Andreaeaceae 
would appear to be a near 
one. 

Another simple Cleisto- 
carpic form which has not 
only been observed exter- 
nally, but also worked out 
developmentally, is Nano 
mitrium tenerum. The 
small capsule has here a 
provision for dehiscence 
by the formation of a 
rudimentary annulus. The 
segmentation of the embryo 
begins on the plan of the 
Bryales (Fig. 140, i.), and 
there is as usual a differen- 

r , j ..u through a sporogonium after formation of the air-space ; s/ = arche- 

tiatlOn OI the endOtneClUm sporium; .y/.r = spore-sac; = limits between amphithecium and 

j uvu ' /T?' endothecium. J3 = transverse section of the same. (After Kienitz- 

and amphithecium (rig. Gerioff.) 

140, ii.). The cells of the 

latter, after further division, become differentiated into an exiguous central 

columella, surrounded by relatively numerous and somewhat irregularly 

arranged spore-mother-cells (Fig. 140, iv.) : but as maturity approaches the 

columella disappears, its materials having served for nourishing the spores 

which fill the cavity of the capsule. 

The genus Ephemerum is closely related with that of Nanomitrium, 
from which it differs in the absence of any definite operculum : the 
condition of the columella is the same, but while it disappears at 
maturity in some species (E. papillosum\ in others it may still be seen 
in the mature capsule (E. crassineruiutri). The fact that stomata occur 
on the capsule-wall, though when ripe this is only a single layer in 
thickness, has its bearing on the question whether these simple Mosses 
are primitive or reduced forms. A systematic position is now assigned 
to them by many writers apart from other Cleistocarpic forms, in close 
relation to the Funariaceae. 




FIG. 139. 
Phascum cuspidatum, Schreb. Schut. A longitudinal section 



28 4 



BRYOPHYTA 



Another Cleistocarpic type, but again one of doubtful affinity, is 
Archidium, in which the small sporogonium has been examined develop- 
mentally by Leitgeb. 1 The first stages agree with those of the Phascaceae ; 
but the tissue of the endothecium shows no differentiation into archesporium 
and columella : certain few cells of it, definite neither in number nor in 
position, become spore-mother-cells, while the sterile cells in which they 
are embedded are absorbed as the spores become matured. This con- 
dition in Archidium suggested to Leitgeb a comparison with that in certain 
Liverworts, for instance, Riella ; but in view of the facts ascertained by 





FIG. 140. 

Nanoinitriiun tenerum. Archegonium after fertilisation' and voung sporogonium at 
different stages of development, in longitudinal section. In II. the endothecium is 
shaded. F foot ; S = stalk. IV. Sporogonium showing the sporocytes in greater part 
separate round the columella. All magnified, I. the most highly. (After Goebel.) 

Lang, the comparison with Notothylas would seem more pertinent. 
Without suggesting even a remote relationship, these two forms both 
illustrate how individual cells, distributed without order in an otherwise 
sterile columella, are partially fertile ; and they suggest that the whole of 
the columella was originally fertile. Of this in the Liverworts there is 
substantial comparative evidence, and this adds probability to the similar 
conclusion for the phylum of the Mosses. 

While it is thus seen that in normal Cleistocarpic forms, which may 
be held to be either primitive or reduced, internal cells of the endothecium 
may develop as spore-mother-cells, a similar condition is also seen 
occasionally in Stegocarpic forms as an abnormality : cases have been 
described of the appearance of fertile cells among the normally sterile 

^Sitz. Akad. Wiss.> Wien, 1879. 



COMPARISON OF SPOROGONIA 285 

cells of the columella. 1 Suctk facts again indicate a probability that 
the whole product of the endothecium was fertile in more primitive 
forms. 

A general comparison of the sporogonia of Mosses (excluding the 
Sphagnaceae) thus leads to the conclusion that two distinct tissue-tracts 
are consistently produced in them by early segmentation, the endothecium 
and the amphithecium. As these are differentiated early, and with great 
constancy, while they differ also in their products, they are to be accepted 
as morphologically distinct. The amphithecium is always sterile, and to its 
modifications the chief mechanical and assimilatory tissues owe their 
origin ; the modifications may involve expansion of tissues, but no initia- 
tion of new parts. The endothecium, theoretically fertile in the first 
instance throughout its length and breadth, underwent progressive sterili- 
sation, parts of it being diverted to other uses : a central tract became 
the sterile columella, while the fertile region became abbreviated both 
at its upper and lower limits ; and thus the actual archesporium in 
typical Bryales is a mere truncated residuum, with its barrel-like form 
open at both ends : the structural indication that its origin was as thus 
suggested is seen in its apparently arbitrary limitations at either end 
(compare Figs. 135, 136, 139). 

This is well illustrated in Funaria and Phascum, where there is a 
continued growth with an initial cell at the apex of the sporogonium ; 
the archesporium appears in longitudinal sections of young sporogonia as 
a definite row of cells on either side of the columella ; but it is impossible 
at first to tell in those rows of cells where the exact limit of spore- 
development will be. Below the lower limit the cells of the row develop 
sterile, above it fertile ; but in either case the segmentations which define 
the cell-row are the same. Passing to the apex, the archesporial row is 
continued beyond the limit of fertility : passing downwards, the cell 
row may also be traced into the seta : structurally the possibility of 
further spore-production seems to be there, but arrested. In different 
types of Mosses the fertile zone thus limited is not always located at 
the same level in the sporogonium as a whole : it is sometimes preceded 
by a shorter, sometimes by a longer, seta. By comparison of these 
different types, an idea is acquired of a residual and limited fertile zone, which 
has been liable to be shifted in the course of descent ; and such shifting 
is made possible by the continued apical growth seen in the developing 
sporogonium. It is important to have a clear conception of the fertile 
zone as a residuum thus movable in the course of descent; the vari- 
able balance thus established between sterile and fertile tissues is not 
only interesting in its bearing on the study of sporogonia, but it will 
come into comparison later with similar features seen in certain strobiloid 

1 Lantzius Beninga, Beitr. z. Kenntn. d. Mooskapsels, 1847, Tab. 58, Figs. 9', 9". 
Also Kienitz-Gerloff, Bot. Zeit., 1878, p. 47, Taf. 2, Fig. 52. 



286 BRYOPHYTA 

Pteridophytes, in which the apical growth is longer continued, and the 
shifting of the fertile zone consequently more obvious. 

The results of progressive sterilisation should not only be studied in 
their longitudinal aspect, but also in the transverse, as leading towards 
decentralisation of the fertile residuum, and establishment of a central 
sterile tract. There is reason to believe that the original type, both of 
Liverworts and Mosses, had a solid core of sporogenous cells. In the 
Liverworts a step toward decentralisation is seen in the partial elatero- 
phores of the Jungermanniaceae, but it attains a greater completeness in 
the Anthoceroteae, where, excepting in Notothylas, the spore-production 
is relegated even to the amphithecium. In Sphagnum the same is the 
case, though there is no guide as to the evolutionary steps which led to 
it. In the Bryales also decentralisation has been effective, but has not 
attained the length of relegating spore-production to the amphithecium. 
The biological significance of decentralisation is plain, as the presence 
of a central conducting column provides a means of better nutrition 
for the increasing mass of spores than where these constitute a solid 
core. In respect of the degree of decentralisation it may be said that 
the Anthoceroteae and Sphagnaceae are the most advanced of the 
Bryophytes ; but all Bryophytes stand far behind the Pteridophytes in 
this respect, for as we shall see, in all the Pteridophytes the spore- 
production is referable in origin, not to deeply seated, but to superficial 
cells of the plant-body. 

The biological circumstances of dispersal of the ripe spores, as well 
as those of nutrition, have doubtless affected the position of the 
archesporium in Archegoniate Plants. Where, as in the Bryophytes 
(excepting the Anthoceroteae), the spores are all produced simultaneously 
in one capsule, which collapses at their maturity, a superficial position of 
the archesporium is immaterial : indeed a relatively central position will be 
advantageous as simplifying the problem of nutrition. Dispersal of the 
ripe spores is then carried out by some drastic method of decay or of 
dehiscence of the protective wall, and the whole sporogonium ceases its 
functional activity with the liberation of the mature spores. The central 
tissues can be sacrificed with impunity where, as in the Bryophytes, the 
spore-production is simultaneous. But in Vascular Plants the spore- 
production is in one way or another successive, and the succession, 
acropetal as a rule, brings with- it the great biological advantage of 
spreading the physiological drain for nutrition over a longer period. In 
this case the central tissue cannot be sacrificed, but must be maintained 
as a nutritive core, in the interest of the later-formed spores of the 
acropetal succession. A more superficial position of the archesporium 
thus becomes necessary, while the projection of the separate sporangia 
beyond the surface which bears them will increase the facility for 
scattering the spores when mature. Thus the difference between the 
deep-seated position of the archesporium of the Bryophytes, and its 



COMPARISON OF SPOROGONIA 287 

superficial position in the Ptecjdophyt.es is intelligible on biological 
grounds : it is closely related to the simultaneous development of the 
spores in Bryophytes, as against the successive spore-production in the 
Pteridophytes. Still in the former some degree of decentralisation, as we 
have seen, brings advantages of nutrition, and its structural expression 
is the sterile columella; but decentralisation does not become a peremptory 
condition of success of the Bryophyte-type, as it appears to have been in 
the Pteridophytes. There is thus a biological reason for the nearer 
relation which all Bryophytes show to that condition which comparison 
indicates as primitive, where the fertile tissue is deeply seated, or even 
occupies a central position in the simpler types. Such considerations lend 
a biological probability to the theory of progressive sterilisation applied in 
the above pages to the sporogonia of the Bryophyta. 

Reviewing the Musci as a whole, the evidence of progressive sterilisation 
in them is less cogent than it is in the Hepaticae. They probably represent 
a more or less distinct phyletic sequence from the latter; but still analogies 
may be drawn between the two ; such analogies strengthen the weaker 
evidence in the Musci ; and, as there appear to be no facts which preclude 
such a view, while many give a reasonable measure of support, it may be 
held that progressive sterilisation has been effective here in essentially the 
same way as it is more clearly demonstrated in the Hepaticae. 



CHAPTER XXIII. 

INTRODUCTORY REMARKS ON PTERIDOPHYTA. 

IN the comparative sketch of the sporophyte in the Bryophyta which has 
been given in the preceding chapters, it has been seen that for these plants 
a theory of sterilisation of potentially fertile cells accords well with the 
developmental facts. Numerous cases have been seen of cells, similar in 
origin to the sporogenous cells, being diverted to other uses than that 
of propagation : these form somatic tissue : there is indeed good reason to 
think that most, if not even the whole, of the somatic tissue of the 
sporogonium originated in this way. This is no new conception : it is a very 
natural corollary on the fundamental conclusions of Hofmeister : it was 
first clearly stated in the writings of Leitgeb on Liverworts, and was 
extended by him also to the Mosses : it was adopted by Goebel in his 
work on the Muscineae in Schenk's Handbuch, and it is now more 
definitely formulated in his Organography, Eng. edn., vol. ii., pp. 93-167. 
It may be held as the generally accepted hypothesis underlying any 
comparative study of the -sporogonia of the Bryophytes at the present 
time. 

But the hypothesis of sterilisation has not been extended with the 
same readiness to other Archegoniate forms. In treating the Pterido- 
phytes, notwithstanding that they have an essentially similar life-cycle, 
there is rarely any reference in the current literature to the effect which 
progressive sterilisation may have had in their evolution. A certain 
excuse for this want of consistency may be found in the fact that in the 
Pteridophytes the proportion of somatic to propagative tissue is very large : 
any hypothesis of sterilisation must therefore recognise the process as 
having extended much further in them than in the Bryophytes. The form 
of the sporophyte also is much more complicated than in the Bryophytes : 
consequently the difficulties of application of a theory of sterilisation to 
the Pteridophytes are much greater, and the results less secure. This is 
certainly true, but it does not appear to be a sufficient reason for a plain 
departure from a theoretical position which has illuminated the comparative 



INTRODUCTORY ON PTERIDOPHYTA 289 

study of the Bryophytes. Norton the other hand, does it justify the 
initial assumption that the origin of the sporophyte in Vascular Plants 
differed essentially from that in the Bryophytes. Accordingly, the theory 
of progressive sterilisation will here be applied to the study of the 
Pteridophytes also, along lines parallel to those observed for the Bryo- 
phytes. It is not to be expected that the facts will amount to a complete 
demonstration : the present object will be to see how far they accord with 
a theory which has its more obvious application in the simpler series of 
Archegoniate Plants. 

The most important evidence will naturally be obtained from the study 
of the spore-producing members themselves ; and these will be described 
in detail in the several types of Pteridophytes. But facts of value bearing 
indirectly on the general hypothesis, are also to be derived from the form 
and structure of the vegetative parts, as well as from their origin and 
early development. In fact, the whole sporophyte is to be studied in 
relation to the question of its origin, just as much in the more complex 
as in the simpler Archegoniate forms. One guiding line must constantly 
be maintained, and it is this : that however late in the individual life the 
production of spores may appear, still spore-production was on our general 
hypothesis the first office of the sporophyte. By various means the vege- 
tative phase may have attained a large size, and great complexity of 
structure : but however preponderant it may appear, still we should be 
prepared to regard it theoretically as secondary, that is, as a phase 
intercalated between the events of nuclear fusion in the zygote and 
reduction in the spore-mother-cell. 

It will be well to observe some regular order in the discussion of 
the large area of fact involved. The several groups of the Pteridophytes 
will accordingly be taken in succession, starting from those with relatively 
small appendages and strobiloid habit, and proceeding to those with 
appendages of larger size. The fossil representatives will be included in 
the discussion, together with the living forms. In each group a pre- 
liminary section will deal with the external characters of the mature 
organism, 'with special reference to the balance of the vegetative and 
reproductive regions. It will be followed by a detailed examination of 
the spore-producing members, and lastly, certain facts of anatomy and 
of embryology will be considered in their bearings on the general ques- 
tion. The characters of the gametophyte will only be referred to 
incidentally, so far as they affect the biological circumstances of the 
young sporophyte. 

If then the Pteridophyta be arranged according to the complexity of 
the appendages, and especially of their spore-producing parts, the Lycopodiales 
will come first, since in them each isolated sporangium is attached in the 
median plane to its subtending sporophyll. 

A second series is characterised by having one or more sporangia 
borne on a vascular pedicel : when the number is more than one they 

T 



290 LYCOPODIALES 

are disposed in radiate fashion around its distal end, which is usually 
enlarged. The whole structure, which is called a " sporangiophore," may 
be inserted directly on the axis, as in the Equisetales, or upon the appen- 
dages of the axis, as in the Sphenophy Hales (including the Psilotaceae). In 
the latter case the position is, as a rule, in the median plane of the 
subtending leaf; but in cases where the sporangiophore is more elaborate 
and shows indications of branching the position may be less obvious. 
Extreme elaboration of the sporangiophore, sometimes including its branch- 
ing, is seen in the series of the Ophioglossale-s, which appear as the most 
advanced examples of this pedicellate or Sporangiophoric Type. 

In a third series, the Filicales, the sporangia are usually grouped in 
" sori," which have features in common with the sporangiophores, but they 
differ from the sporangiophoric types in that the sori are distributed over 
the margins or surfaces of the leaf itself, which is here of relatively large 
size and complex construction. 

The order of description will follow the sequence thus laid down, and 
it will become apparent that the elaboration of the leaves themselves 
follows roughly parallel with that of the sporangial arrangement : in fact the 
whole series may be regarded as progressing from simpler to more complex 
types of the whole shoot. The arrangement thus adopted is convenient 
for description. The question will be reserved for later discussion how far 
it indicates a true evolutionary progression. 



LYCOPODIALES. 

/. General Morphology. 

These plants are taken first because in them the spore-producing members 
are more simple and regular in their disposition on the shoot than in any 
other Vascular Plants. Throughout this phylum (as now limited by the 
exclusion of the Psilotaceae), each single sporangium is subtended by a 
sporophyll (Frontispiece), the median planes of the sporophyll and of the 
sporangium coincide, and typically no more than one sporangium is 
associated with each sporophyll. 1 These appendages are borne laterally 
upon the axis, which is endowed with apical growth. The arrangement 
of the appendages, either sterile or fertile, is sometimes in regular 
whorls, but frequently it is according to some more or less interrupted 
spiral scheme (Fig. 141). The axis may undergo frequent branching, 
typically in a dichotomous manner, though intermediate steps are seen 
in certain species to the monopodial type : in some of the Lycopodiales, 
however, branching is rare, or absent. It is thus evident that the whole 
shoot is of a simple strobiloid type. It bears roots at its base, and in the 

1 Occasional exceptions have been noted, where two small sporangia, side by side, are 
subtended by a single sporophyll. These are rare, and appear to originate in some form 
of fission of the normal sporangium (Annals of Bot., xvii., p. 278). 



GENERAL MORPHOLOGY 



291 



straggling or creeping forms these may arise adventitiously at points far 
up along the axis. It is by comparison as regards the differences which 
occur in this otherwise uniform family that some knowledge of the course 
of development of the Lycopod-type may be derived. 

The Lycopodiales are divided, according to the presence or absence 
of a ligule, into two divisions ; the Eligulatae, which include the Lycopo 
diaceae, that is the living genera Lycopodium and Phylloglossum, with which 
are also to be associated certain early fossils designated Lycopodites\ and 




FIG. 141, 



Shoots of several species ot Lycopodium to show the form and arrangement of the 
leaves. A=L. rufescens, Hook. X 2. B = L. mandioccanum, Raddi. Natural size. 
C L,. reflexum, Lam. Xa. /? = /,. casuarinoides, Spring. : part of a terminal branch of 
an old plant. X4- E = L. cernwttn, L. X 2. F~L. volubiie, Forst, seen from above. 
X2. (From Engler and Prantl.) 



the Ligulatae, which include the Selaginellaceae and Isoetaceae of living 
forms, together with the fossil Lepidodendraceae and Sigillariaceae. These 
will be severally considered as illustrating variants on the simple strobiloid 
type of the whole phylum. 



A. ELIGULATAE. 

The genus Lycopodium, which includes about' a hundred living species, 
was arranged by Spring according to the degree of differentiation of the 
several species. 1 He distinguished two main sections of the genus, the 
first including those with sporangia scattered over the length of the shoot : 
the second including those with the sporangia associated in definite cones. 
The former section was again sub-divided according as the leaves were 
all alike, or as a distinction appeared between sterile and fertile leaves : 
the latter section according as the shoot was developed radially or 
dorsiventrally. The details of Spring's scheme have since been modified, 
but the principle remains the same in the classifications of the present 
day : it is to arrange the genus along lines which clearly indicate a 
progressive differentiation and specialisation of sterile and fertile tracts. Such 
an arrangement naturally harmonises with evolutionary theory. The species 

aphic des Lycopodiac^es^ 1841. 



292 LYCOPODIALES 

which Spring placed first of all was L. Selago ; and though this may not 
be actually the most elementary living species in the genus, still it is the 
best known of those which show a low degree of differentiation. 

The plant of L. Selago is shrubby, with dichotomously branched axes, 
bearing numerous leaves of approximately equal size and simple form 
(Frontispiece). There is usually a sterile region at the base of the plant: 
this is followed by the well-known alternating sterile and fertile zones, 
the length of which corresponds with a high degree of exactitude on the 
several branches. They are stated to be determined by successive seasons, 
the middle region of each year's increment of growth being fertile. These 
zones are not definitely marked by any distinction of the leaves themselves, 
but by the presence or absence of sporangia : nor are they strictly delimited 
in this respect, for occasionally a single sporangium may be found in an 
otherwise sterile region. About the limits of these zones sporangia of 
smaller size may be found, which sometimes remain closed when all those 
near them have dehisced. These are those incomplete sporangia which 
have already been referred to in Chapter XIII., in connection with the 
argument for sterilisation as affecting the balance of the sterile and fertile 
regions. The condition thus seen in L. Selago is shared in more or less 
complete degree by about 40 living species, which constitute the section 
Selago : they are mostly ground-growing plants. An examination of them 
shows that while most of them, .have, like Z. Selago, a sterile basal region 
of considerable length, still in certain species (L. compactum, Hook., and 
L. Trendlla Sodiro) sporangia have been found in the leaf-axils down to 
the base of the mature plant : this has been noted also, but less completely, 
in L. firmum, Mett., and L. rigidum, Gmel. Unfortunately these species 
are unknown in the embryonic state, so that it is impossible to tell how 
early in the individual life the formation of sporangia actually begins ; 
but practically the whole of the mature plant is a fertile strobilus. The 
incomplete differentiation of the sterile and fertile zones is seen in all the 
40 species : isolated sporangia are frequently found in an otherwise sterile 
zone, and occasionally sterile leaves occur in a fertile zone : these facts, 
together with the occurrence of incompletely developed sporangia at the 
limits of the zones, and the very uniform character of the leaves whether 
sterile or fertile, have their direct bearing on the theory of sterilisation 
enunciated in Chapter XIII. 

Some ten other species were grouped by Baker under the heading 
Sub-Selago, and are characterised by having the sterile leaves a little 
different from the fertile, but passing into them gradually, while the 
sporangia are aggregated into indistinct terminal spikes. All the species 
thus grouped have a sterile basal region : above this follows a recurrence 
of sterile and fertile zones, as in Selago (L. Dalhousiatanum) \ reversion 
from the fertile strobilus to a permanently sterile state is more common 
(L. carinatum, gnidioides). A progressive diminution of size of the fertile 
leaves upwards is seen : it is sometimes gradual (L. squarrosum\ but 



GENERAL MORPHOLOGY 



293 



sometimes more sudden (L. E>alhousiaea?ium). Isolated sporangia in the 
sterile region are more rare than in ^Selago, but they do occur (L. carinatum, 
gnidioides, si/uarrosum} : also partially abortive sporangia have been seen 
at the base of the strobilus (L. carinatum). All these characters together 




FIG. 142. 

Lycopodium Phlegtnaria> L. A figure showing habit of the whole plant. One-third. 
Z? = end of a branch. Natural size. C a sterile leaf, somewhat enlarged. /} = sporo- 
phyll seen from below, enlarged. = a sporophyll seen from above, enlarged. (From 
Engler and Prantl.) 

show a very close similarity to what is seen in the Selago group, but with 
gradually increasing definition of the strobilus from the lower vegetative 
region. 

In the Phkgmaria group, which includes about eighteen species of 
epiphytic character, the spikes are slender, and dichotomously forked, 
with sporophylls as a rule very different in size from the foliage leaves 
(Fig. 142). Occasionally sporangia may be found in the vegetative region 



294 LYCOPODIALES 

subtended by leaves of the foliage type (L. varium)\ while, on the other 
hand, leaves of the sporophyll type may develop no sporangia (L. subulatum> 
Phlegmaria). There may occasionally be alternating sterile and fertile 
zones (L. nummularifolium). Transitions from the fertile strobilus to the 
larger-leaved foliage shoot are frequent (L. nummularifolium, subulatttm, 
ophioglossoides, pinifolium, Phlegmaria). Thus the differentiation of the 
strobilus is one of external form rather than a rigid difference of intimate 
character. The converse conditions of L. varium and L. subulatum show 
that the difference of size of sporophylls and foliage leaves is not due 
directly to correlation in the individual parts, but rather to the general 
condition of the shoot as a whole. 

The above groups, including fully one half of the living species of 
Lycopodium are now associated together under the sub-genus Urostachya x ; 
the characters assigned are not only those of the distribution of the sterile 
and fertile zones, but extend also to other features. The branching of 
the axis is almost, or entirely, absent in the simplest of the upright forms : 
in the trailing or pendulous forms it is more frequent. It is of the 
dichotomous type, and usually in planes successively at right angles. The 
roots arise primarily from the basal region of the axis ; in no case is 
there a creeping monopodial axis, with adventitious roots arising along 
its whole length. In about half of the species there is no formal dis- 
tinction of sporophylls from the foliage leaves : where such a distinction 
exists the sporophylls still have a green colour, and as a rule an entire 
margin. The spores have a pitted surface, without external processes. 
These general characters indicate a natural grouping of species which are 
certainly the simpler living representatives of the Lycopod type. 

The second sub-genus, designated Rhopalostachya, includes the remain- 
ing species which are more differentiated than the first in many of their 
characters. The branching of the axis is only dichotomous in the younger 
parts, and becomes monopodial later, often with a well-marked main axis. 
All the species are ground-growing; a few are climbers. The upright 
species are freely branched (Fig. 143); in the creeping species the 
recumbent axis gives off upright branches, and is attached to the soil by 
successive adventitious roots ; the fertile strobili are for the most part 
definitely marked off from the sterile region, and are often carried upon 
elongated stalks, which bear minute scale-leaves (Fig. 144). The differ- 
entiation of the sterile and fertile leaves is constant ; the sporophylls are 
pale, often chaffy scales, with toothed, ciliate margin (Fig. 143 D, E), 
while the spores bear reticulate flanges or prickles on their outer wall. 
These characters collectively mark off Rhopalostachya as more differentiated 
than Urostachya. But it includes some species which approach the latter ; 
thus the three species associated as the inundatum group show only slight 
differentiation of the strobilus from the vegetative shoot, while abortive 
sporangia are found at the base in L. inundatum. L. Drummondii even 

1 See Pritzel, Engler u. Prantl, Nat. Pflanzenfam. , i. iv. , p. 591. 



GENERAL MORPHOLOGY 



295 




FIG. 143. 

Lycopodium cernuutn^ L., var. Eichleri, Glaz. A, general habit (J natural size); , 
end of a branch (natural size) ; C, strobilus ( X 3) ; Z>, sporophyll seen from above ; , 
ditto, from the side ( X 20). (After Pritzel, in Engler and Prantl. Nat. PJtanz.) 



296 



LYCOPODIALES 



approaches L, Selago in its alternation of successive sterile and fertile 
zones. On such grounds the innndafam group has been associated with 
Phlegmaria by Baker in the sub-genus Lepidotis, though this association 

is not now upheld. In the rest, however, the 
definition is more exact, and is strictly main- 
tained ; for instance, in L. cernuum in a very 
large number of specimens a transition from 
the strobilus back to the vegetative shoot was 
never observed. The same is the case in the 
species associated as the group of L. clavatum, 
which are terrestrial trailing species, with well- 
defined strobili. In this series the intercalation 
of a peduncle, with small distant scales, between 
the larger-leaved foliage shoot and the definite 
strobilus is indicated (Fig. 144). The question 
whether the peduncle is directly derived from the 
basal part of the strobilus, or from a specialised 
part of the already sterile foliage region, may be 
left open ; but as sporangia are not found on it, 
nor even any vestiges of arrested sporangia, the 
latter seems the more probable source of the 
peduncle : the biological importance of it in 
ground-growing forms is readily understood. 

Finally, the dorsiventral species, previously 
grouped as the sub-genus Diphasium, are now 
distributed according to their obvious affinities; 
the dorsiventral character of their vegetative 
shoot being held as a secondary adaptation : 
the strobilus, however, remains as clearly defined 
as in the more advanced representatives of the 
sub-genus Rhopalostachya, and does not share in 
the dorsiventral development. 

A comparison of the living species of Lyco- 
podium thus appears to demonstrate a progression 
from a less differentiated to a more differentiated 
state. In the simplest forms the whole of the 
unbranched or sparsely-branched shoot is practi- 
cally a fertile strobilus, which serves the double 
purpose of assimilation and of spore-production. 

By gradual steps the living species suggest how the two functions became 
separated : a purely vegetative region was established by abortion of the 
sporangia, and it was naturally located in the first-developed or lower part 
of the plant, since the function of nutrition must necessarily precede that 
of spore-production. The fertile upper region also became more specialised, 
and in the species where it is most clearly defined from the vegetative region 




FIG. 144. 

Lycopodium carolinianutn, L. 
.<4=figure showing habit. About 
two-thirds natural size. B stro- 
bilus. Natural size. C = sporophyll 
seen from above. x6. Z> = the 
same from the side. x6. (After 
Engler and Prantl.) 



GENERAL MORPHOLOGY 



297 




the sporophylls no longer servers assimilating leaves, but appear as chaffy 

scales, performing a protective function. It would be difficult to read from 

the comparative study of the mature 

sporophyte in the genus Lycopodium 

any other evolutionary story than this. 
The only other living genus of 

eligulate Lycopods is the monotypic 

Phylloglossutit) long recognised as the 

simplest of them all. The mature plant 

as seen above ground consists of a tuft 

of almost cylindrical assimilating leaves, 

from the midst of which rises the 

simple axis terminated by the short 

strobilus ; below ground are found two 

ovate storage tubers, one dating from 

the preceding year and in course of 

exhaustion, the other in course of 

development as a store for the succeed- 
ing year. There are also one or more 

roots (Fig. 145). The lower parts of 

this curious little plant cannot be 

properly understood till it is compared 

with the embryos of certain species of 

Lycopodium, for it repeats in its annual 

growths their embryonic characters : the 

discussion of them will therefore be 

postponed (p. 351). The very short strobilus shows a similarity to the 

Urostachya rather than to the Rhopalostachya section of the genus : this is 
seen in the smooth margin of the rather fleshy 
sporophylls, as well as in the incomplete protection 
of the sporangia. It is interesting to note that tran- 
sitions are occasionally found between the foliage 
leaves (protophylls) and the sporophylls : Fig. 

\S I/ .^** 146 A shows a case where a single sporophyll of 

larger size than the normal, with a sporangium 
in its axil, stands isolated some way below the 
strobilus : thus it is intermediate both in position 
and in character between the two types. A small 
protophyll without any sporangium may also 
sometimes be found at the base of the pedicel. 
Dichotomous branching of the strobilus is some- 
times seen, but it is rare : an example is shown 
in Fig. 146 B. Such features are important for 
comparison with Lycopodium, and indicate that 
there is a close relation between the two genera. 



+ 15 



FIG. 145. 

Phylloglossum Drummondir, Kunze. A = 
apex of a germinated tuber ; b-, 6-2, b%, leaves ; sp 
= the young strobilus. X 15. j5=the whole 
plant. X one^-half. /j = the old tuber; t% = 
the young tuber; r=root. C sporophyll with 
sporangium seen from above. X 12. (After 
Engler and Prantl.) 




FIG. 146. 

Phylloglossum Drummondii 
Kunze. A = a plant showing pro 



tophylls and strobilus : one sporo- 
phyll of the latter is at a distance 
below the rest, intercalary growth 



having taken place in the axis 
above it. X3. B = a. plant with 
the strobilus branched into two 
unequal parts. X 3. 



298 



LYCOPODIALES 



Of the fossils which have been referred to a near affinity with 
Lycopodium under the name Lycopodites, many have been shown to find 





FIG. 147. 

Lycopodites Stockii, Kidst. i=specimen, natural size; a-f, sporangia; g, sporophyll. 
2 = sporophyll enlarged. 4 = small portion of stem, enlarged, showing verticillate leaf- 
bases. (After Kidston.) 

their true place elsewhere. 1 But some at least of them show distinct 
Lycopodinous characters : for instance, Lycopodites Stockii, Kidston, from 
the calciferous sandstone of Dumfriesshire. In habit it is like Lycopodium 

1 Kidston, Trans. Nat. Hist. Soc., Glasgow, vol. vi., p. 32. 



GENERAL MORPHOLOGY 299 

Phlegmaria, showing a terminal strebilus, with sporangia, and the sporophylls 
smaller than those borne by the more lax region of the shoot below (Fig. 
147). The leaves are arranged in whorls a condition not unknown among 
species of the Phlegmaria group. The sporangia do not appear to have been 
restricted to the terminal strobilus, but to have occurred also in relation 
to the larger foliage leaves : this is a condition which has been seen 
to occur in Z. varium, as well as in the living species of the group 
sub-Selago, from which the Phlegmaria group appear to be a specialised 
offset. So far from this distribution of the sporangia raising a difficulty, 
it seems to point to the existence in very early strata of a Lycopodinous 
type showing characters which exist in living species, and which com- 
parison indicates as primitive. These fossils are unfortunately rare, and 
in the particular case of L. Stockii the essential facts are based upon a 
single specimen. 

B. LIGULATAE. 

The ligulate Lycopodiales resemble the Eligulatae in general habit, 
but they differ from them in the presence of a small process the ligule 
borne on the upper surface of the leaf, near its base : also whereas the 
living Eligulatae are all homosporous, all the living Ligulatae are hetero- 
sporous. Selaginella is the preponderant genus of the living Ligulatae : 
its vegetative development is characterised by frequently repeated branch- 
ing of the axis, which bears numerous small leaves : but whereas in 
Lycopodium the dorsiventral development of the shoot is the exception, 
and the radial the rule, in Selaginella only a few species show the radial 
construction as a permanent character : the latter, as Goebel remarks, 1 
usually grow on dryer and brighter spots than the dorsiventral. As 
the result of experiments on species such as S. sanguinolenta^ in which 
anisophylly is not constant, but appears under the influence of external 
factors, Goebel concludes that the dorsiventrality is a phenomenon of 
adaptation brought about by light : thus the radial type will naturally 
be the more primitive. In the great majority of the living species, 
however, the strobilus is isophyllous, even where the vegetative shoot 
is anisophyllous : thus indicating that it is the more conservative part 
of the plants. But in ' some ten per cent, of the living species the 
strobilus itself is also anisophyllous. The definition of the strobilus from 
the vegetative shoot in Selaginella is more marked than in Lycopodium : 
a condition corresponding to that of the Se/ag0-group of Lycopodium is 
unknown, nor have isolated sporangia ever been observed in the vegeta- 
tive region : the differentiation of the sporophyte of the genus as a whole 
corresponds to that of the more specialised types of Lycopodium. But 
imperfect sporangia have been observed at the base of the strobilus of 
.S. spinulosa, and Martensii, and would doubtless be found in many 
other species : this condition is open to the same interpretation as 

1 Organography ', vol. i., p. 105. 



300 LYCOPODIALES 

in Lycopodium. As is well known, the megasporangia and microsporangia 
are alike in their early stages of development, though differing later in 
the spores which they produce : this additional degree of differentiation 
in the genus falls in with the higher differentiation noted in the vegetative 
organs, as compared with Lycopodium. 

Of the species with radial construction, the best known is . spinulosa, 
specially investigated by Bruchmann : x this will be briefly described for 
purposes of comparison on the one hand with Lycopodium, and on the 
other with the related fossils, while the dorsiventral Selaginellas may be 
regarded as specialised offsets from some such radial type as this. The 
seedling of 6". spinulosa is like other Selaginellas in having an upright 
elongated hypocotyl (Fig. 148), which is continued directly into the primary 
root : the hypocotyl bears two cotyledons, after which a variable number 

of pairs of epicotylar leaves are formed 
before the first branching, which is a 
true dichotomy. The limbs thus formed 
branch repeatedly, at first dichotomously, 
but later monopodially, all the branch- 
ings being in one plane, at right angles 
to that of the first dichotomy : thus two 
fan-like branch-systems are produced, of 
which certain stronger branches are 

Selaginella spinulosa. G=young seedling . ., , ., /T -,. x ,. 

with megaspore attached, showing elongated fertile, the TCSt Sterile (Fig. 149). The 
hypocotyl (H) and cotyledons K. ff= seedling f ,1 i r .1 

more advanced showing dichotomy. ^=base arrangement of the leaves of the primary 

of hypocotyl with swollen knot. W= roots. j 

^hypocotyl. .ff^suspensor (after Bruch- aXIS IS deCUSSate, but On the later 





branches there are transitions to spiral, 

while in the thicker strobili the arrangement is on a complex spiral 
plan. The main axis terminating below in the hypocotyl remains 
permanent, and its' base swells at the level of the suspensor to form a 
knot, from which alone the later roots originate ; they are formed 
endogenously in swellings of tissue of the knot, and burst their way 
outwards through the superficial tissue. The whole plant of S. spinulosa 
is thus dependent upon a central source of water-supply from the base 
of the main axis. In most species of Selaginella^ however, the well- 
known rhizophores are formed, at the branchings of the axes of higher 
order, and thus their rooting may be efficiently carried out at a distance 
from the primary axis : this is probably a derivative condition, just as 
the dorsiventral development, of which it is the usual concomitant, is 
also derivative. Both in the form of the shoot, and in the central root- 
ing, the type of S. spinulosa may be held to be more primitive than 
the common dorsiventral type of the genus : in these respects it will be 
seen to correspond more nearly with the large fossils than do the more 
specialised species of the genus. 2 

1 Unters. iiber S. spinulosa, A. Br., Gotha, 1897. 

2 See Goebel, Organography, vol. ii., p. 230. 



GENERAL MORPHOLOGY 



301 



But the Fossil Ligulates wer< not all large. There is evidence that 
small organisms, corresponding in habit to the heterophyllous Selagindlas, 
existed also in early geological times. The fossil from the Upper Coal 
Measures, described as Lycopodites Gutbieri, Gopp, can hardly have been 
anything else. Lycopodites primaevus, Schr., from the Westphalian Middle 
Coal Measures, though it shows no distinctly Selaginelloid shoot, has 
heterosporous sporangia, with megaspores more numerous than four in 
each sporangium, as shown me by Mr. Kidston, in specimens belonging to 
the Brussels Museum. A similar condition has been described by Zeiller 1 





FIG. 149. 

Plant of Sclaginella spinulosa, with root-system springing from swollen knot at base of 
the upright hypocotyl. Natural size. 

in a plant from Blanzy, named by him Lycopodites Suissei, where the 
number of megaspores ^vas found to be 16 to 24. In these cases the 
reduction in number of the spores as a consequence of heterospory 
appears to have proceeded less far than in the modern Selagtnella. 
But, on the other hand, the carboniferous plant described by Bertrand 
as Miadesmia corresponds in structure, as well as in the heterophyllous 
arrangement of the leaves and in the presence of a ligule, to Selaginella, 
while it appears to have progressed towards a seed-like fructification. 
The minute new species Miadesmia membranacea, Bertrand, has been 
directly compared with Selaginella spinulosa ( = S. selaginoides, Link) by 
Miss Benson,- in respect of characters other than the seed-like structure 
1 Comptes Rcndus, April, 17, 1900. - Proc. Roy. Soc., Series B, vol. Ixxix, p. 473 



302 LYCOPODIALES 

borne by the megasporophyll. So far as it goes, then, the evidence from 
the fossils favours the conclusion that plants resembling Selaginella existed 
in the primary rocks, and that even the more specialised heterophyllous 
type of Selaginella dates at least from the Carboniferous period, while 
it seems possible that a seed-like habit had already been established. 

The dendroid Lycopodiales are among the earliest known fossils, 
dating from the Lower Devonian period to the Trias. They include the 
families of the Lepidodendraceae, Bothrodendraceae, Sigillariaceae, and 




FIG. 150. 

Ground plan of a Tree-stump with Stigmaria-trunks. One-sixtieth the natural size. 
(After Potonie.) 

Pleuromoiaceae. Underlying the differences of detail according to which 
these families are distinguished, there is a general unity of morphological 
plan : the essential features of it are as follows. The main axis was 
upright, rising in some cases to a height of 100 feet. It was bulky 
relatively to the numerous simple leaves which it bore : it branched 
upwards in a dichotomous manner, in most cases profusely : in some of 
the Sigillariaceae, however, and in Pleuromoia branching may be entirely 
absent. The development of the branches of the dichotomy were in 
various cases either equal or unequal, a fact which leads to differences of 
habit, as is seen to be the case in Lycopodium or Selaginella. The axis 
was fixed in the soil by a shallow and broadly spreading system of 
Stigmarian trunks (Fig. 150). In Lepidodendron the main Stigmarian 
trunks usually numbered four, which bifurcated repeatedly, thus forming a 



GENERAL MORPHOLOGY 



303 




widely spreading system : from these the rootlets radiated in all directions, 
developing to a length of a foot or more, and showing dichotomous 
branching. The underground system was thus proportional to that above 
ground. In the Sigillariaceae similar trunks are found, but it seems 
doubtful whether they show the same constancy of initial type as in 
Lepidodendron. In Pleuromoia the base of the upright stock swells into 
a tuberous body, which is very Stigmaria-like in the fact that it is 
covered by root-scars, while it extends into four blunt processes corre- 
sponding in position and character to Stigmarian trunks, though much 
shorter (Fig. 151). It would 
seem probable that in this 
relatively late Triassic fossil 
(which is unfortunately known 
only in the form of casts, 
not structurally), a simple 
representative of the Lepido- 
dendroid basal region is cor- 
rectly recognised. In all of 
the dendroid forms the Stig- 
marian trunk appears to have 
been present, as a basis for 
the roots : but the latter were 
not restricted to that position : 
Potonie shows how the scars 
of their insertion may be 
sometimes found on the leaf- 
bearing axes also, associated 
with some degree of regularity 
with the leaf-scars. 1 

The leaves of the fossil 
Lycopodiales were sometimes 
of considerable size, but un- 
branched and of simple form. 
They expanded at the base into the well-known cushions, which in many 
forms occupy the whole, external surface of the axis : this corresponds to 
what is seen among the living Lycopods. On the upper surface of the 
leaf, near its base, the ligule is seated : it appears to have been a constant 
feature in the dendroid Lycopodiales, and the occurrence of it links them 
rather with Selaginella than with Lycopodium. It was often seated in a 
deep pit as it is in some living Selaginellas and this pit persists as a 
marked feature in the neighbourhood of the leaf-scars, whenever the cast 
of a stem-surface is well preserved (Fig. 152). 

The vegetative region appears to have been, as a rule, purely 
vegetative : the sporangia are restricted to well-defined cones or strobili, 

1 Lehrbnch der Pflanzenpalaeontologie, p. 212, Fig. 215. 




FIG. 151. 

Pleuromoia Sternbergii. Swollen base of stem with root- 
scars, and showing part of the aerial stem, with the epidermis 
and leaf-scars on the right, and on the left the sub-epidermal 
sculpture. (After Bischof, from Engler and Prantl.) Two-thirds 
natural size. 



304 



LYCOPODIALES 



similar in their general characters, as also in their clear definition, to 
those of the more differentiated types of Lycopodium or to Selaginella. 
The general structure of the strobilus is, as in other Lycopods, essentially 
the same as that of the vegetative shoot, excepting in the presence of the 
sporangia. These are of very large size, and are commonly extended radially 
outwards from the axis, being interposed between the axis and the 
ligule : the latter then appears on the upper surface of the sporophyll, 
beyond the distal limit of the sporangium (Fig. 153). In Lepidodendron 
the cones thus constructed were borne on the ends of the ordinary 




B 




FIG. 152. 

Lepidophloios, sp. A = tangential section from the outside of a stem, passing through 
the leaf-bases, and showing their characteristic form, slightly enlarged. = a single 
leaf-base, to show details; = collateral vascular bundle; /a = the two parichnos- 
strands; lg= ligule in its pit. x 10. Will. Coll., 1974 A. (After Scott.) 

branches; but in Sigillaria they appear to have arisen laterally upon the 
main axis, from which after maturity they were deciduous, and each was 
borne upon an elongated pedicel covered with acicular bracts, while the 
cone itself showed a construction essentially similar to that of a small 
Lepidodendron . 

Though the type with a definite cone marked off from the sterile 
region was usual for the fossil Lycopodiales, it was not universal. In the 
imperfectly known plant, Pleuromoia from the Trias, the whole main axis 
seems to have been a strobilus (Fig. 154), borne upon a Stigmarian base 
(compare Fig. 151). But a much more satisfactory example, from the 
Westphalian series (Middle Coal Measures) is that of Ptnakodendron 
musivitm, Weiss, specimens of which, discovered by M. Hector Delteure 



GENERAL MORPHOLOGY 305 

at Mariemont in Belgium, are -about to be described by Mr. Kidston : to 
him I am indebted for the information that this large Lycopod bore its 
sporangia associated with the leaves of certain portions of the stem, without 
any cone-formation, or alteration of the form or disposition of the leaves 
which bear them : the fertile and sterile portions are distinguished only 
by the presence or absence of sporangia. It is, in fact, a typical repre- 
sentative of the " Selago " type, but of dendroid dimensions. In this 
connection it is interesting to note that Solms Laubach mentions certain 
" remains of great size, remarkable for the unusual thickness of the axis 
classed by Lesquereux with Lepidophloios. Weiss also has described 
a similarly colossal cone as Lomatophloios macrolepidotus, but, unfortunately, 




FIG 153. 

Lepidostrobus. Diagram showing axis and sporophylls in radial section. # = axis of 
strobilus ; = sporophylls and sporangia; .r^stele; i, c = inner cortex; ni, c = middle 
cortex ; o, c outer cortex ; / = pedicel ; la lamina of sporophyll ; /z'=ligule ; /, t= leaf- 
trace ; sp, / = wall of sporangium. (Enlarged after Maslen, from Scott, Studies in 
Fossil Botany.) 

there is no detailed account of it. The enormous size of the axis in 
these specimens gives rise to the suspicion that the fructification was not 
confined to special fertile shoots, but might occasionally appear on the 
leaves even of the main stem, which then increased in thickness, much 
as we see in the present day in the female flower of Cycas, and mutatis 
mutandis in Lycopodium Selago. We naturally ask, on what sort of scars 
could such cones be seated as lateral organs?" 1 Kidston's description of 
Pinakodcndron shows that the "Selago" condition did actually exist in 
dendroid types, and thus resolves the difficulty. A similar condition is 
shown by the small Lycopodites ciliatus^ Kidst, from the Middle Coal 
Measures, 2 while the still earlier Lycopodites Stockii (compare Fig. 147 
above) also has its sporangia associated with leaves of the foliage type. 
Finally, the imperfectly known Lycopodites Reidii, from the Devonian of 

^Fossil Botany, Engl. edn., p. 235. 
2 Trans. Nat. Hist. Sac., Glasgow, vol. vi., p. 37. 
U 



306 



LYCOPODIALES 



Scotland, has been compared by Penhallow 
with Lycopodium Selago as regards the un- 
differeritiated shoot. 1 Without attaching too 
much importance to the last example, it 
appears certain that Lycopods, even of large 
size, existed in very early times, in which 
there was no clear differentiation of vege- 
tative and fertile regions : in fact, the 
" Selago " condition dates back to the 
Primary Rocks. 

There can be no question of the Lyco- 
podinous affinity of the fossils thus described 
briefly in their general morphology : it 
remains then* to indicate where the nearest 
correspondence is to be found between 
them and living forms. They are plainly 
related to the Ligulate Lycopodiales, and, 
being of a radial type of shoot, and usually, 
if not always heterosporous, the correspon- 
dence is nearer to the radial species of 
Selaginella \ this suggests a comparison with 
.5. spinulosa, from which some interesting 
points will emerge. In the first place, the 
difference of size is to be discounted : how- 
ever diverse the gigantic Lepidodendron may 
seem from the minute S. spinulosa, the com- 
parison really relates to the relative position 
and character of the parts composing the 
plant-body. The parts which form the shoot 
axis, foliage-leaf and sporophyll, the ligule, 
and the sporangium are identical in both 
as regards their relative positions, though 
differing greatly in their number and dimen- 
sions : in the dichotomous branching, and 
in the relation of the resulting shoots to the 
upright main axis they are alike : also in 
the dependence of the whole plant for its 
water-supply upon the base of the primary 
axis. In fact, Selaginella spinulosa is like a 
Lepidodendron in miniature, as regards the 
scheme of its construction. The comparison 
extends also to that curious knot which is 
found at the base of the main axis in S. spinulosa : here the origin of the 
roots is strictly localised : they appear endogenotisly on indeterminate 

1 Canadian Record of Science, 1892, p. 8. 




I 



FIG. 



154. 



Pleuromoia Sternbergi. Axis, with 
the lower part of the terminal strpbilus. 
Two-thirds natural size. After Bischof. 
(From Engler and Prantl.) 



GENERAL MORPHOLOGY 307 

outgrowths from the axis itself, ^vhich have been regarded as rudimentary 
rhizophores. It does not seem an undue strain of comparison to suggest 
that in this basal knot is still to be seen, on a minimal scale, a living 
representative of those larger growths known as the Stigmarian trunks. 
These would thus be in their nature indeterminate outgrowths of the 
hypocotyl, as are these rudimentary rhizophores ; but like them, strictly 
localised in origin, instead of being dispersed over the branch-system, as 
are the rhizophores in most modern Selaginellas. It is thus possible to 
bring the general morphology of Lepidodendron into relation to that of 
the modern Selaginella, a type which there is reason to believe itself 
dated from the Carboniferous period. 

On the other hand, there are obvious relations between the dendroid 
Lycopodiales and the living genus Isoetes : this type has been found 
fossil in the Tertiaries, and back as far as the Lower Chalk, while in the 
Trias the curious fossil Pleuromoia is represented : but there is no suffi- 
cient evidence of the genus Isoetes having itself figured among the earliest 
fossils. 

The plant of Isoetes consists of a short upright axis covered by relatively 
large leaves (Fig. 155) : the axis is usually unbranched, though bifurcation 
occasionally occurs, a fact that has its interest for comparison with the 
Lycopods. 1 The leaves are essentially of one type, with broad base and 
acicular upper part, while seated in a pit on the upper surface, at some 
little distance from the base, is the ligule. The leaves may be either 
sterile or fertile, and in some species there is a difference in size, the 
sterile leaves being the smaller. The plant is heterosporous. Where the 
leaf is fertile the large cake-like sporangium lies in a depression of the 
leaf-surface, between the ligule and the leaf-base, that region being 
elongated to accommodate it : in the sterile leaves it is shorter. An 
examination of the sterile leaves of /. lacustris (and Wilson Smith made 
similar observations in /. echinospora) shows that sporangia in various 
degrees of abortion may be found upon them : in some of these spores 
are developed, but in smaller numbers than the normal : other sporangia 
may remain quite small, and produce no spores. Dissections show that, 
in the majority of leaves that are apparently sterile, a rudimentary sporan- 
gium is really present hi a normal position. It is stated that a regular 
seasonal sequence is followed in the distribution of the megasporophylls, 
the microsporophylls, and the foliage leaves : that the megasporangia are 
borne on the first or outermost leaves of each annual increment, then 
follow leaves with microsporangia, while the sterile leaves form the transi- 
tion from one year's increment to the next. It is thus seen that in the 
distribution of its sporangia Isoetes shows a condition similar to that of 
Lycopodium Se/ago, but that the various degrees of their abortion are 
better represented. *It follows from the facts that after the embryonic 
stages are past in which no sporangia are produced the whole plant, is 

1 Solms Laubach, Bot. Zeit., 1902, p. 179. 



308 



LYCOPODIALES 




FIG. 155. 

Isoetes lacustris, L. A= plant of natural size. B = base of a fertile leaf with 
ligule (/), and sporangium (sp) seen from above. C = longitudinal and D transverse 
sections of the leaf-base; sp = sporangium ; /=ligule. (From Rabenhorst's, Krypt. 
Flora.') 






GENERAL MORPHOLOGY 309 

potentially a fertile strobilus, in which the vegetative and reproductive 
systems are not differentiated from one another. This, together with its 
prevalent absence of terminal branching, points out Isoetes as a near 
approach in its general construction to the strobiloid type theoretically 
primitive for the Lycopodiales : this it shares with the simplest Selago- 
forms of Lycopodium. But it is with the dendroid Lycopodiales that 
Isoetes shows common characters of the sporangia themselves : there is 
also some similarity to them in the structure of its abbreviated but bulky 
stock : on this also the very similar bifurcating roots are inserted, but in 
Isoetes their origin is localised in the depressed grooves which traverse 
the stock longitudinally, instead of their being borne on Stigmarian out- 
growths, as in the fossils. The Isoetes plant is then like a partially 
differentiated Lepidostrobus seated upon a Lepidodendroid base : in fact, 
like a stunted Lepidodendron, with its preliminary vegetative phase very 
short. Its mature shoot still carries on both vegetative and propagative 
functions, and in this lack of differentiation a primitive character is to be 
recognised. 

The account thus given of the general morphology of the mature 
sporophyte in the Lycopodiales, living and fossil, shows the essential 
identity of their plan of construction throughout the phylum, and how in 
the two series, the ligulate and the eligulate, parallel conditions of differ- 
entiation are represented. In both the structure of the shoot is essentially 
strobiloid, with a constant numerical relation of the sporangium to the 
subtending sporophyll. In both series the branching of the axis is primarily 
by dichotomy, with a deviation in the more specialised types, and especially 
in the higher ramifications to the monopodial branching : but in certain 
simple -types branching is rare, or even absent. The shoot is fixed in the 
soil by roots, formed chiefly, or even exclusively, at the base of the axis 
in the simpler types; but in the more specialised they may be formed at 
various other points on the shoot-system, or on outgrowths from it of an 
indeterminate character. In both series there is evidence of abortion of 
sporangia, leading to a segregation of definite tracts of the shoot-system 
devoted to nutrition and to propagation respectively : in the higher types 
the strobilus becomes a definite cone of limited growth, clearly marked 
off from the lower vegetative region : the production of spores is thus 
deferred in the individual life, and a more lengthy vegetative phase 
intercalated before that event. This progressive differentiation is best 
illustrated in the eligulate series, which is also the more primitive in 
respect of its homosporous condition. We are thus led by comparison 
of the Lycopodiales, living and fossil, to contemplate as a fundamental 
type of their shoot a simple unbranched strobilus with unlimited 
apical growth, bearing un differentiated leaves, and having one sporangium 
associated with each leaf. This may not improbably have been the 
primitive type from which, by branching, by formation of a root-system, 
by differentiation of the sterile from the fertile region, and, finally, by 



3io LYCOPODIALES 

cessation of apical growth in the fertile branches, the whole series of 
forms included in the Lycopodiales arose. It will remain to be seen 
how far the detailed study of the sporangia, and especially of the 
anatomy and embryogeny of the Lycopodiales, will support this hypo- 
thetical origin. 






CHAPTER XXIV. 



SPORE-PRODUCING MEMBERS OF THE LYCOPODIALES. 

THE normal sporangia in the Lycopodiales are always non-septate sacs, 

excepting that in the megasporangia of Isoetes there may be an isolation of 

the megaspore-mother-cells (see p. 320). The form is that of a kidney, of 

which the curvature and proportions are liable to considerable variation. 

The position is, as we have 

seen, essentially constant, each 

sporangium being subtended by, 

or inserted in a median position 

relatively to its sporophyll; the 

curvature is in the tangential plane, 

showing in tangential section a 

more or less pronounced fan-like 

outline. A series of examples 

of sporangia will be selected as 

illustrating the structure and 

mode of development, and the 

degree of variation in form and 

proportion which exists within 

the phylum. 

The genus Lycopodium will 
be taken first, and the spor- 
angia compared in a number 
of species. It will become 

apparent from this comparison that the differences which they show are 
not at haphazard, but that they follow with some degree of accuracy those 
lines of external differentiation, upon which the systematic arrangement of 
the genus has been based. In order to make this clear the description will 
follow the accepted systematic order,, beginning with the least differentiated 
types. In Z. Selago J the sporangium originates at the base of the sporophyll, 

^Studies, i., p. 511. 




FIG. 156. 

Radial sections through young sporangia of Lycopodium 
Selago. In the youngest the whole sporophyll is shown (/), 
and the axis (sf), and it is seen that the sporangium arises 
upon the surface of the sporophyll. The older stages show 
the segmentation of the sporangium. X 200. 



312 



LYCOPODIALES 



but clearly upon its upper surface as a transversely extended cushion 
(Fig. 157 A). In median radial section it appears as a convex growth, in 
which a central row of three cells, the result of periclinal division of one 
parent cell, is dominant (Fig. 156): of these the middle cell is of arche- 
sporial character. A tangential section of a sporangium of similar age 
(Fig. 157 B) shows that there are a number of these archesporial cells: 
in the example shown there are seven : but the number is not constant, 
as is shown by comparison of various tangential sections, and supported 
by sections cut transversely (Fig. 157 c). The young sporangium consists 
thus of a single tangential row of archesporial cells, covered in completely 



Yr ~*S2 




. 



FIG. 157. 



Lycopodium Selago. A= young sporangium seen in superficial view: s< = stem; 
/=sporophyll. .Z> = tangential section of a similar sporangium, the cells numbered i, 
ii, iii correspond to those similarly marked in Fig. 156. C = a sporangium of like age in 
transverse section, as along a line s, s, in Fig. B. The archesporial cells are shaded. 
Z> = an older sporangium, in radial section, showing the spore-mother-cells separated, 
before tetrad-division. A , B, C X 200. D X 100. 



by a single layer of cells forming the sporangial wall, and supported 
below by cells which grow more actively in the middle region of the 
sporangium, thus leading to the curved form which it assumes later. It 
is clear also that all the essential parts of the sporangium originate from 
several superficial cells of the sporophyll, and that it is impossible to refer 
them in origin to any single parent cell. 

At first the parts thus laid down often grow uniformly, so that their 
mode of origin may still be traced in a more advanced state : but later the 
more numerous divisions are less regular. Superficially they result in the 
formation of a sporangial wall, composed of three layers, or of more 
towards the base of the sporangium (Fig. 157 D): of these the innermost is 
the transitory tapetum : the tapetal investment of the sporogenous tissue is 
completed by development of the adjacent cells of the sub-archesporial 



SPORE-PRODUCING MEMBERS 



313 




FIG. 158. 

d, e,f= sections radial, tangential, 
and transverse of matu 



tissue also as tapetum. The dehiscence is along a transverse line, and the 

preparation for this is already indicated at the distal end in Fig. 157 D, d. 

Meanwhile, the sporogenous group within, in 

the formation of which the whole products of 

the archesporium are involved, has been subject 

to repeated cell-division: its cells finally separate, 

round themselves off, and all of them, as a rule, 

undergo the tetrad-division. In the mature 

sporangium the form is less strongly curved 

in L. Selago than in many other species, while 

the stalk is a relatively narrow one. The 

general proportions, as well as the imperfect 

protection of the sporangium afforded by the 

rather narrow sporophylls, are shown in Fig. 

158 d, e, f. 

The type of sporangium thus described for 
L. Selago, with its single row of archesporial 

Cells, relatively narrOW Stalk, and imperfect ana transverse ot mature sporangia 

of Lycopodium Selago. g, n, i 

protection while VOUnSJ, appears tO be Character- similar sections of mature sporangia 
* . . . f Lycopodmm phlegmai'ia. X 12. 

istic, with relatively slight modifications, of the 

sub-genus Urostachya : other species of the sub-genus which have been 
examined, viz., L. dichotomum, Jacq., carinatum, Desv., nummularifoliitm, 

Blume, and Phlegmaria, L., are all alike in 
showing an archesporium consisting of a single 
tangential row of cells, though the number of 
these in the row may vary ; the simplest case 
observed was that of L. Phlegmaria, where the 
single series consisted of certainly not more than 
five cells, and perhaps of less. The further 
development in these species was also the same 
as in L. Selago, though the proportions were 
different. In L. dichotomum, however, there is 
the peculiarity that the sporangial wall is found 
to be more massive, consisting of 4-7 layers. 
Putting such differences aside there seems 
reason to regard the single tangential series of 
archesporial cells as a common feature of the 
sub-genus Urostachya : further, the sporangia 
are inefficiently protected by the sporophylls 





FIG. 159. 



Lycopodium innndatuiti, 
Radial sections of sporangia. 



In 



the upper, younger figure periclinal /Fi" 1^8 " Jl l\ 
divisions are shown in two cells, and X O" 06' 

the archesporial ceils are shaded. j n p r itzel's arrangement of the genus the 

In the lower, older figure the pro- 
duct of division of these ceils is section Inundata is separated from Phlegmaria, 

shown. X 200. 

and placed in the second sub-genus Rhopalo- 

stachya. We shall see that the sporangial character upholds this change. The 
sporangia are from the first more bulky than in Urostachya (Fig. 160 k, /, m\ 



314 



LYCOPODIALES 




_ 



and this has been found in L. inundatum to go along with a more bulky 
origin. The sporangium as seen in radial section arises as a broad swell- 
ing, while two cells have been seen to divide periclinally, indicating at 
least two tangential rows of archesporial cells in place of the single row 
in L. Selago. This origin of the sporogenous tissue may still be traced in 
the older stages (Fig. 159). It may be that this condition is not 
actually constant in all cases, but it has certainly been observed to exist 
in L. inundatum. 

Of the rest of the sub-genus Rhopalostachya, Z. clavatum and L. alpinum 
have been examined, and they both show a still more massive type of 

sporangium. This is seen 
in the mature state (Fig. 
1 60 q, r, s: t, u, v, w), 
where the stalk appears 
to be short and thick : 
moreover, it is seen that 
the strobilus is constructed 
in these species so as to 
afford more complete pro- 
tection to the sporangium 
while young, than is the 
case in the simpler type 
of Z. Selago. This is 
effected by special de- 

, r , , 

VCIOpment OI the lower 
c i , -,-, 

parts of the sporophylls 
(Fig. 22 D, E): in some 

cases, as in L. cernuum, the sporophyll takes a peltate form. Radial sections 
of the young sporangium show, both in L. clavatum and in L. alpinum, 
that from the first the form is broader still than in the types previously 
described. At least three cells in each radial section are involved in the 
origin of the archesporium, sometimes even more than three (Fig. 161 A, B). 
Occasionally periclinal divisions appear in the superficial cells, by which 
subsequent additions may be made to the archesporial tissue (Fig. 161 B, 
cells marked x). The . tangential sections also show an advance on the 
Selago type : for twelve is not an uncommon number of the archesporial 
cells in one tangential row as against seven in Z. Selago, or five in 
Z. Phlegmaria. Countings of the sporogenous cells laid bare in sections 
of sporangia at an age approaching the tetrad-condition show that their 
number is far in advance of those of the Selago type : this is the condition 
to be anticipated from the bulky character of the sporangia (Fig. 161 c, D) : 
moreover, their thicker and shorter stalks would be well fitted to transfer 
the necessary nourishment for the larger spore-output. It may be noted 
that in these large sporangia occasional irregular processes project 
upwards from the base of the sporangium into its cavity, which would 



FIG. 160. 



Drawings to illustrate the form and manner of protection of the 
sporangia in the sub-genus A' hopalostachya, of Lycopodium. k,Z,m, 
sections of L. inundatum ', ff, r. s. of L. alpimnn ; t, u. z>, TV, of L. 

ciavatum. x 12. 



SPORE-PRODUCING MEMBERS 



315 



assist in conveyance of nourishment to the large mass of developing 
spores. Thus in the main features of form and dehiscence the sporangia 
of Rhopalostachya conform to the type of Z. Selago, but are larger and 
more productive; while the sporophylls have a more elaborate form for 
purposes of protection. This goes along with the differentiation of the 
vegetative from the propagative regions, the steps of which have been 
traced above in the genus Lycopodinm. The conclusion seems justly to 
follow that with this differentiation, which has apparently involved a 
diminution in the actual number of sporangia by abortion, there has come 




FIG. 161. 

Lycoppdium alpinutn, L. A Radial section through a sporophyll and young 
sporangium. .5 = the same older; in both the sporogenous tissue is shaded. D radial 
section of an older sporangium ; s/ = stem. C= tangential section of a sporangium of Lye. 
clavatum, of similar age to D ; in both these figures the sporogenous tissue is referable 
in origin to three rows of cells. A, fi, Cx 200. D X 100. 

into existence a more massive type of sporangium, together with a more 
extensive spore-output from each of them, and a more specialised protec- 
tion of them while young. 

It has been seen that the strobilus of Phylloglossum resembles that of 
the sub-genus Urostachya^ rather than that of Rhopalostachya. An exami- 
nation of the developing sporangium supports this comparison, for only a 
single row of about six archesporial cells is found; but, on the other 
hand, the outline of the sporangium, and the relative thickness of the 
stalk, show some similarity to Z. immdatum. 

The sporangium of Selaginella corresponds in general type to that of 
Lycopodium. It is usually described as arising from the surface of the 
axis: in some species it does so (S. Martensii) (Fig. 162), but in others 
it is seated more nearly upon the surface of the leaf; in fact its position 



LYCOPODIALES 




may vary in different species though the numerical relation of one to each 
subtending leaf is strictly maintained. There is considerable divergence 
of opinion as to the details of its early development, which not 
improbably arises in part from want of exact uniformity in different 
species, partly from difficulty of observation, owing to the small size of 
the cells in young stages. 1 It has been seen above that S. spinulosa is 

among the least differentiated species, as 
regards external form, and on that account 
it deserves special attention. The de- 
scription here given will be based on that 
species. According to Goebel the whole 
sporogenous tissue, as seen in the radial 
section in 6". spinulosa, is referable in 
origin to a single archesporial cell, which 
is, however, one only of several forming 
a tangential series. I do not deny that 
this may sometimes be the case ; but in 
my sections two primary archesporial 
cells were usually present (Fig. 163 A, B), 
somewhat as in Lycopodium inundatum. 
Tangential sections show that these 
represent two rows of archesporial cells, 
with about four cells in each (Fig. 163 D). 
Thus the correspondence in sporangial 

type with that of Lycopodium is very striking, as regards early development : 
the chief difference is in the origin of the tapetum, for this in Selaginella 
is cut oft' by tangential divisions from the sporogenous tissue (Fig. 163 c, E), 
of which it is thus a sterilised part. There is reason, however, to think 
that the first periclinal divisions in the young sporangium do not always 

1 Observations have been made on various species of the genus: Goebel (Bot. Zeit., 
1881, p. 697) investigated S. spinulosa, helvetica, and Wallichii, and his results are 
restated in his Organography, vol. ii., p. 600 ; allowance is, however, made by him for some 
degree of variation in details. My own observations on S. spinulosa, and Martensii are 
described in my Studies, i., p. 522. Campbell, in his Mosses and Ferns, 2nd edition, 
p. 530, describes the development for S. Kratissiana, but his figures are by no means 
convincing that his reference of the whole sporogenous tissue to a single parent cell in 
the radial section is correct. Miss Lyon (Bot. Gaz., xxxii., p. 124) has made a 
careful study of the development in S. apus, and rupestris, and traces the sporangium 
frequently if not always to a single superficial cell, which she designates the archesporium ; 
but as the results from radial sections were not accurately checked by comparison of 
tangential or transverse sections, the point of ultimate origin of the whole sporangium 
from a single superficial parent cell cannot be regarded as demonstrated for this species. 
Before the details for the genus as a whole can be properly understood, the development 
will have to be studied in tangential as well as in radial sections, in a number of different 
species selected from different sections of the genus ; meanwhile the substantial agreement 
of the sporangial type between the less differentiated S. spinulosa and the genus 
Lycopodium is the main point of interest for the present discussion. 



FIG. 162. 

Selaginella Martensii, Spring. Sporangia 
in radial section. A traverses the stem apex 
(/), the sporophyll (/), and sporangium (x) ; 
in the latter two archesporial cells are seen, 
shaded. B shows an older stage. X 350. 



SPORE-PRODUCING MEMBERS 



317 



define the future sporogenous tksue from the sporangial wall, but that by 

further periclinal divisions of the superficial cells additions may be made 

to it. If this be so, then 

the distinction between A 

the two sources of origin 

of the tapetum does not 

appear so marked as at 

first sight it might appear 

to be. 

In the microsporangia 
all the cells of the sporo- 
genous group may under- 
go the tetrad-division, 
and form microspores ; 
but Miss Lyon found 
that in S. apus not more 
than five-sixths of them 
were fertile, the rest dis- 
appear. In the mega- 
sporangia, as a rule, a 
single cell is early differ- 
entiated by its denser 
protoplasm from the rest : 




FIG. 163. 

Selaginella spinnlosa. A, - = radial sections through young 
sporangia of successive ages. C=a transverse section of one more 
advanced. D a tangential section. E=a radial section of an older 
sporangium showing all its essential parts, together with the ligule and 
part of the sporophyll. A,,C, 0x350. .fix 200. 



this alone undergoes the 

tetrad-division, and forms megaspores (Fig. 164). But in S. rupestris a 
smaller number of megaspores, or even only one, may come to maturity : 

in S. apus, however, two mother-cells 
may become matured, and eight mega- 
spores be thus formed in one sporangium. 
These fluctuations have their interesting 
bearing upon the origin of the hetero- 
sporous differentiation, showing that there 
is some margin of variation in the num- 
ber of spore-mother-cells which are fertile 
even in forms now living. 

The facts relating to the sporangium 
in Selaginella^ though imperfectly known 
for the genus at large, show that in 
position and in general plan the spor- 
angium is of the usual Lycopod type ; 
but that its dimensions are smaller than 
is usually the case in Lycopodium : the 
difference in origin of the tapetum is probably related to the smaller size 
of the whole sporangium. The heterosporous condition appears to have 
brought with it only minor modifications of the original sporangial type. 




FIG. 164. 

Selaginella spinulosa, A. Br. Section of 
megasporangium showing the single fertile 
tetrad still very small, and the rest of the 
sporogenous cells arrested. X 100. 



318 LYCOPODIALES 

In Isoetes the position of the large sporangium, between the ligule 
and the axis, corresponds to that in Selagznella, though it is here more 
definitely inserted on the leaf-base, and is sunk in a deep depression of 
its upper surface (Fig. 155 B, c, D) ; but these differences of detail do 
not obscure the essential unity of the plan in the two genera. Instead 
of being a body more or less flattened between the sporophyll and the 
axis, as in Lycopodium and Selaginella, the sporangium is here extended 
radially outwards from the axis into a broad cake-like body. It may 
best be regarded as a result of such variation of dimensions as has been 




FIG. 165. 

Isoetes lacustrh; L. A radial section through base of sporophyll with ligule (/), 
velum (f), and sporangium, in which the archesporium is shaded. B &. similar section 
of an older sporangium. C = part of an older microsporangium, showing the potential 
archesporium differentiated into trabeculae (t>-\ and sporogenous tissue (sfi), while the 
tapetum (t) is clearly defined /-> = an older stage with spore-mother-cells separated, and 
tapetum shaded covering the trabeculae. A, Z>'X 200. C, D x 100. 

seen in minor degree within the genus Lycopodium, but here carried to 
greater lengths. The developmental details harmonise readily with this 
view. The microsporangium is naturally a better basis for comparison 
with the homosporous Lycopods than the megasporangium, and it will 
therefore be taken first The mature structure of a microsporangium is 
shown in Fig. 155 D, which indicates how the very large internal space 
is traversed by the sterile trabeculae : these extend, with many irregularities 
of branching and wing-like expansions, which are not shown in the figure, 
from the sub-archesporial tissue to the covering wall. The type of the 
megasporangium is the same, though the trabeculae are here fewer in 
number but more massive, so that the proportion of sterile tissue to the 
fertile is much larger in the megasporangium. As the development shows, 



SPORE-PRODUCING MEMBERS 319 

the trabeculae have a common^ origin with the fertile sporogenous cells: 
there has in fact been a sterilisation of potentially fertile tissue, which 
proceeds to a greater length in the megasporangium than in the micro- 
sporangium. The early development of both types of sporangia is alike 
up to a fairly advanced condition, as is the case also in Selaginella ; this 
fact has its bearing on the origin of their differentiated state. 

The sporangium of /. laacstris originates from superficial cells of the 
leaf-base of small number, lying below the ligule (Fig. 165 A.) The cell seen 
immediately below the ligule in the longitudinal section of the young 
leaf forms the velum : the rest show some evidence of common origin 
by earlier anticlinal segmentation : this may very well have been so, but 
the comparative interest begins with their periclinal divisions, and it is 
then that a basis appears for comparison with what has been seen in 
Lycopodium. The periclinal division appears first in the central part of 
the young sporangium, and thence it extends in either direction : in the 
longitudinal section some four or five cells are involved in /. lacustris, 
though apparently the number may be smaller in 7. echinospora. 1 Com- 
paring this with the condition as seen in Lycopodium, it appears to be 
an advance on even the most complex type, such as 7. alpinum ; and 
this is completely in accordance with the radially extended form of the 
mature sporangium of Isoetes. Moreover, the differences beween Wilson 
Smith's description for 7. echinospora and my own for 7 lacustris suggest 
that differences of radial extension of the sporangium exist in different 
species of Isoetes similar to those which have been shown to occur within 
the genus Lycopodium. But there does not appear to be any such cor- 
relation of them with the morphological differentiation of the plant at 
large as that which was traced in Lycopodium, and gave a special interest 
to the sporangial differences in that genus. 

The internal cells thus cut off by the first periclinal divisions are 
destined to be sporogenous ; but the first periclinal divisons thus initiated 
do not absolutely define the future sporogenous tissue : it has been 
repeatedly seen that additions to it may be made by subsequent periclinal 
division of the superficial cells, especially in the middle region of the 

1 Wilson Smith found in /. echinospora that he was able to trace the origin of the 
sporangium back in longitudinal sections of the leaf to a single cell lying between the 
ligule and the leaf-base : this corresponded to a transverse row of three to five cells, which 
formed the rudiment of the sporangium ; but the cell thus recognised in the longitudinal 
section also formed the velum, which on that account he accepts as a sterilised part of 
the sporangium. Doubtless this is a logical outcome of a last analysis of cell-origins, 
provided it be assumed that all things are homologous which have a common origin from 
ultimate parent cells (see Chapter VIII.). But is there any other line of evidence than 
that of cell-origin to show that the velum was ever a part of a sporangium, or anything 
but sterile ? Without such evidence the mere fact of common origin from a very early 
segmentation seems a somewhat shadowy ground for the conclusion which Wilson Smith 
proposes. If this criterion of homology be accepted, then all parts of the plant are 
ultimately homologous, for they all originate from the ovum. (See Wilson Smith, Hot. 
Gaz., 1900, p. 225). 



320 



LYCOPODIALES 



sporangium (Fig. 165 B). The potential sporogenous tissue thus produced, 
after successive sub-divisions, forms a very considerable sheet of tissue, 
several cells in thickness. Of this, however, only a portion develops into 
spores : in the case of a microsporangium certain tracts of cells of this 
tissue assume dense protoplasm, and the cells, ultimately separating from one 

another, undergo the tetrad-division, producing 
[microspores (Fig. 165 c, D); but other tracts of 
cells, neither showing any regular outline or 
arrangement, nor referable in origin to pre- 
determined cells of the genetic tissue, become 
less densely protoplasmic, and form the sterile 
trabeculae : a tapetal tissue invests the fertile 
tracts : it is derived partly from the innermost 
layer of the sporangial wall, as in Lycopodium, 
partly from the superficial cells of the trabeculae. 
A similar differentiation of the potentially sporo- 
genous -tissue is found also in the megasporangia, 
the early stages of which are quite indistinguish- 
able from those of the microsporangia ; but in 
the former a relatively smaller number of cells, 
usually lying isolated in the potential sporo- 
genous tissue, and distributed with no constant 
relation to their ultimate parent cells, enlarge 
and divide to form the megaspores (Fig. 166). 
As there is no opening mechanism in the 
submerged sporangia of Isoetes, no basis for 
comparison is yielded from that source. The 
study of the development in Isoetes thus leads 
clearly to the conclusion that there has been 
a differentiation, within the sporangia, of tissues 
at first of uniform character : that part of the 
potential sporogenous tissue remains fertile, but 
a large proportion in the microsporangium, and 
a still larger proportion in the megasporangium, 
is diverted to other uses, and remains sterile. 
As regards the origin of the potential sporo- 
genous tissue, and the form and position of the 
sporangium, there is clear correspondence to the Lycopod-type, and 
especially to those forms with the more bulky sporangia : in fact if we 
imagine a heterosporous Lycopod, with its sporangium widened out radially 
along the leaf-surface and its enlarged sporogenous tissue partly sterilised 
so as to form trabeculae, the result would be practically what is seen in 
Isoetes. 

A study of the sporangia of the fossil Lycopods is a necessary adjunct 
to that of the modern forms, though the usual absence of developmental 




FIG. 166. 

Part of a section of a megaspor- 
angium of Isoetes. The cell marked 
(m) is the only fertile spore-mother- 
cell, the rest are undergoing vegeta- 
tive divisions, including the cell (a) 
as shown by other sections of the 
series. Thus sterilisation affects the 
large majority of the cells of the 
sporogenous group. X245. (After 
Wilson Smith.) 



SPORE-PRODUCING MEMBERS 



321 



details in them restricts the comparison to the basis of mature structure. 
On this footing it appears that the type of sporangium characteristic of 
the sub-genus Urastachya, and showing special resemblance to that of 
Lycopodium Phlegmaria, dates back at least to the calciferous sandstone, 
for it is seen in Lycopodites Stockii (compare Fig. 147). Sporangia apparently 
of the same type have been recognised also in other early fossils referred 
to Lycopodites, but their small size and the state of preservation do not 




Spoiceritcs insignis. Somewhat diagrammatic radial section of part of the cone, 
showing two sporophylls in connection with the axis. On the lower sporophyll the 
sporangium is shown attached at its distal end to the ventral outgrowth of the sporophyll : 
within the sporangium some of the characteristic winged spores are shown. (After Miss 
Berridge.) From Scott, Progressus rei Botanicae, vol. i. 

allow of any exact comparison. Of other apparently non-ligulate types one 
of the best known as regards the details of the strobilus, though its vegetative 
region is still unknown, is Spencerites (Fig. 167), which has been described 
by Scott and others from specimens showing microscopic structure. Here 
the verticillate or spiral sporophylls consist of a narrow pedicel bearing an 
upturned lamina ; at the base of the lamina is a massive ventral outgrowth, 
to which the distal end of the sporangium is attached by a narrow neck. 
The presence of the ventral sporangiferous lobe has suggested to Dr. Scott 
a comparison with the Sphenophyllales, though the absence of any vascular 
supply to the " ventral lobe " renders the analogy somewhat remote. It 

x 



322 LYCOPODIALES 

is doubtful what is the evolutionary relation between the distal and the 
basal insertion of the sporangium upon the sporophyll ; whether the one 
or the other is the more primitive in the Lycopodiales must be left for the 
present open, but it is evident that such differences as these are of degree 
only, in a type which is constant as regards the numerical relation of the 
sporangia to the sporophylls, and in the coincidence of the median planes 
of both of those parts. There seems little reason to hold that these 
peculiarities of Spencerites are archaic relatively to those of the ordinary 
Lycopodinous type. Comparison does not make it necessary, nor even 
probable, while stratigraphically the ordinary Lycopod type is quite as 
early as Spencerites. 

The same relation of sporangium to sporophyll as is seen in the 
living Lycopbds is maintained in the Lepidodendroid cones, which are 




FIG. 168. 

Lepidpstrobus Brownii. A radial section traversing the axis, a sporophyll, and a 
sporangium. In the latter numerous spores are seen partially filling it, while sterile 
processes project upwards into the cavity. (From Sowerby's drawing.) 

known in many cases to bear ligules, and to be heterosporous, thus 
corresponding more especially to the ligulate series of the Lycopodiales; 
but this may possibly not be the case for all of them. An examination 
of the details of the sporangium will naturally be best carried out in the 
best preserved specimens, though these may not be generally typical of 
all others. Lepidostrobus Brownii, Schpr., is probably the best preserved 
of Lepidodendroid cones, and it will therefore be taken first. The large 
silicified specimen in the British Museum was first described by Robert 
Brown, with drawings by Sowerby. 1 The original specimen was about two 
inches in length, and of about the same diameter : it was evidently only 
the upper half of a strobilus, as the internal structure, which is preserved 
with singularly little distortion, shows to be the case. It has been cut 
into transverse, radial and tangential sections, and consequently a very 
adequate knowledge of the details can be obtained. The central axis 

1 Linn. Trans., vol. xx. See also Misc. Bot. Works of Robert Brown, vol. i., p. 583. 



SPORE-PRODUCING MEMBERS 



323 



shows a structure similar to thatsof Lepidodendron Harcourtii^ and though 
there is no direct indication of the source of the cone, comparison of the 
structure with that of Lepidodendron stems, and with other Lepidodendroid 
cones, leaves no doubt of its being the strobilus of a Lepidodendron. The 
axis bears numerous sporophylls, of which thirteen are usually represented 
in each transverse section : the basal region of each extends horizontally 
from the axis, and supports the sporangium, which may extend for fully 
half an inch along its surface (Fig. 168). The distal end of the sporo- 
phyll turns upwards, without any peltate expansion. 




FIG. 169. 

Lepidostrobus Brotvnii. A= wall of sporangium, showing outer sclerotic cells (scl), 
with several thin-walled layers within. X 100. B = three sporangia in transverse section 
of the cone ; r= median ridge. X 3. C = cone in tangential section. D = sporangium in 
tangential section of cone, slightly diagrammatic; .$/ = sporophyll ; r=sub-archesporial 
ridge ; v, b = vascular bundle ; / = processes rising from the ridge. X6. E= small part of 
the base of a sporangium in radial section, showing the processes /$, /, which rise from it. 

X20. 

Comparison of transverse and tangential sections of the cone (Fig. 
169 B, c) gives a clear idea of the form of the very large sporangium, 
which is a radially extended body, broader and deeper at the distal end 
than at the proximal ; it is attached throughout its length by a relatively 
narrow median, flange-like insertion to the upper surface of the sporophyll, 
and immediately above the course of its vascular bundle. Comparison of 
its outline with that of the sporangium of Isoetes shows a very striking 
similarity ; but this is not limited to the form only : in L. Brownii above 

1 Or to that of L. Williavisoni, according to Solms Laubach, Fossil Botany, Engl. 
ed., p. 226. 



324 LYCOPODIALES 

the flange of insertion an internal ridge of sterile tissue extends upwards 
into the sporangium, just as in Isoetes (r. Fig. 169 B, D), while from it 
sterile processes project further upwards, extending far into the cavity, 
and traversing the mass of the spores (Fig. 169 E). In the mature 
sporangium they stop short of the upper sporangial wall, but in the young 
state as seen in the arrested sporangia towards the apex of the. cone 
they may extend completely across the cavity : in position and in number 
they are irregular, as are the trabeculae of Isoetes, to which they show a 
striking similarity. It seems probable that they are truly comparable to 
the trabeculae of Isoetes; but, on the other hand, it is possible that they 
may correspond rather to those irregular upgrowths from the sub-arche- 
sporial tissue mentioned as occurring in some of the larger sporangia of 
Lycopodium. The large cavities of the sporangia are filled with small 
spores, arranged in tetrads, and it is probable that the trabeculae were 
of importance in the nourishment of the large sporogenous mass, as 
also mechanically. The wall of the sporangium in L. Brownii consists 
of an outer layer of indurated prismatic cells, supported by four or more 
layers of thin-walled cells (Fig. 169 A). It is impossible to miss the 
general similarity of this large sporangium to the microsporangium of 
Isoetes : the size, the position, the outline, and the presence of trabeculae 
all point to the close correspondence : a ligule has, it is true, not been 
noted in the fossil ; but as only a few sections have been available, and 
as the ligule in other Lepidodendrons is only small, it would be rash to 
lay any great stress upon this negative observation. The points of 
similarity of this remarkable fossil to the fertile plant of Isoetes are such 
as can hardly have been the result of parallel development : they strongly 
support the view expressed above, that the plant of Isoetes is like a 
stunted Lepidodendron. 

On the other hand, Brown's cone shows only microsporangia, while 
Isoetes^ like certain other Lepidostrobi, is heterosporous. But the specimen 
itself was incomplete : only the upper part of the .cone is represented, and 
it is now known that in other species the apical region bore microspor- 
angia, while the lower bore megasporangia, as in Z. Veltheimianus (Fig. 
170) : it is quite possible that the lower portion, which is missing from 
Brown's cone, bore megasporangia; but on this point there is no positive 
evidence. 

In other Lepidostrobi the general form of the sporangium is the same 
as that above described : there is great radial extension, while in a number 
of cases a ligule has been found at the distal end, thus corresponding in 
position to that in Isoetes. The sporophylls are liable to peltate expansion 
at the apex : they are then so disposed that the downward-turned lips of 
the upper sporophylls are enveloped by the upturned lips of the lower, 
thus giving very complete protection to the sporangia. This may be held 
to be a secondary adaptation of their form, comparable to that seen in 
some of the more specialised cones of Lycopodium belonging to the 



SPORE-PRODUCING MEMBERS 



325 



sub-genus Rhopalostachya, while hi the simpler Sctago-forms the sporophylls 
are as in L. Brownii, or, better still, in Pinakodendro?i. The wall of the 
mature sporangium is frequently represented by the single prismatic outer 
layer alone, the inner thin-walled layers seen in Z. Brownii being absent : 
this difference is comparable to that seen in Lycopodium, where the mature 
wall usually consists of a single layer, but in L. dichotomum of several 
layers. There is also some 
divergence in detail of the 
internal upgrowths from the 
basal ridge : in most Lepi- 
dostrobi these take the form 
of longitudinally disposed 
plates, of which one or more 
project upwards into the 
sporangial cavity. Lastly, 
there is the fact of hetero- 



spory, 



which has now been 




established in a number of 
examples, though it must 
not be assumed for them all 
without actual demonstra- 
tion. Such differences as 
those mentioned are, how- 
ever, of secondary impor- 
tance, and in the general 
morphological character of 
the Lepidodendroid cones 
there is substantial unifor- 
mity as regards the relation of 
sporophyll and sporangium, 
as well as in their form. 

The fructifications of 
Sigillaria appear as cones 
sessile, but more 
borne on long 
lateral branches, which are 
covered below with acicular 
bracts : such strobili are thus more strongly differentiated from the 
vegetative axes than is the case in Lepidodendron. The plan of construction 
of the cone itself appears to have been the same, and though its preservation 
is commonly imperfect, it seems that the sporangia of Sigillaria resembled 
those of Lepidodendron in their form and mode of insertion, as also in the 
existence in them of a heterosporous condition. They were sometimes of 
large dimensions : frequently, however, of smaller size. Among them a 
small cone, described by Zeiller as Sigillariostrobus Crepini, differs from 



cue 

FIG. 170. 

Lepidostrobus Veltheimianus. Longitudinal section of cone, 
showing microsporangia above and megasporangia below. ax= 
axis of cone, snowing stele, /, and leaf-traces, passing out to 
sporophylls, br', nii= microsporangia ; trta = megasporangia con- 
taining a few spinose megaspores. X about 4. (From Scott's 
Studies in Fossil Botany.) 



326 LYCOPODIALES 

the rest in the distal insertion of the sporangium upon the sporophyll, 
corresponding in this respect to Spencerites. 

Taking a general view of the fructifications of the Lycopodiales, the 
most salient feature is the constancy of the numerical relation of sporan- 
gium to sporophyll. In the whole phylum of the Lycopodiales each 
sporangium is subtended by its sporophyll, while the median planes of 
both those parts coincide. In most cases the sporangium is in close 
proximity to the axis, or it may even be inserted upon it : occasionally 
its position is further removed from the axis and inserted towards the 
distal end of the sporophyll : these differences are of secondary importance 
so long as the median position is regularly preserved. It is to be noted 
that such extreme conservatism in number and in place of the sporangia 
is peculiar to this phylum of Vascular Plants, in which also the closest 
relation exists between the sporangia and the axis : in all other types 
the sporangia show not only a less close relation to the axis, but also 
less definiteness in number and in position : there is often, indeed, some 
rough proportion between the size of the appendages and the number of 
the sporangia which they bean 

The type of the sporangium itself is constant, though liable to differences 
in proportion : it is always more or less fan-shaped in tangential section, 
but the angle of spread of the fan is liable to considerable variation. It 
is, however, in the extension radially outwards from the axis that the 
greatest differences of proportion are seen, and it has been shown above 
that in the living species of Lycopodium the differences may be correlated 
with the degrees of differentiation of the strobilus from the vegetative 
region; the narrow compressed form of sporangium with relatively thin 
stalk is found in the less differentiated, the sporangium more radially 
extended with short thick stalk in those with more clearly differentiated 
strobili. The extremes of radial extension are seen in the dendroid fossils, 
as well as in Isoetes. It would seem probable, as suggested by the com- 
parative study of the living species of Lycopodium, that the larger sporangia 
are derivative types, and that the enlargement was consequent upon 
increased facilities of nutrition : such increased facilities are afforded by 
the large size of the assimilating leaves in Isoetes; but in the more 
differentiated species of Lycopodium, and in still higher degree in the 
dendroid fossils, by the extensive vegetative system which precedes the 
production of cones. The abortion of sporangia, and consequent reduc- 
tion of their number in proportion to the foliage leaves, would tend in 
the same direction. Such circumstances would encourage enlargement 
of the spore-output, which is most readily and directly secured by increase 
in size of the individual sporangium in so hide-bound a type as that of 
the Lycopodiales. The extreme enlargement led to mechanical and 
nutritive difficulties, which were met, perhaps independently, in Isoetes 
and in some Lepidodendrons by the formation of trabeculae : these origi- 
nated in Isoetes by partial sterilisation of sporogenous tissue. But though 



SPORE-PRODUCING MEMBERS 327 

there is thus evidence of great "fluctuation in size of the sporangia, and 
though the presence of the sterile trabeculae indicates that the limits of 
convenience as regards nourishment and mechanical support are approached, 
still there is no evidence that within the Lycopodinous phylum (as now 
limited by the exclusion of the Psilotaceae) any actual septation has 
occurred. The relation of one sporangium to each sporophyll, and no 
more, is maintained throughout with some rare exceptions, which as they 
never became characters of a race may be held as abnormalities. There 
is, moreover, no evidence of interpolation of sporangia, those which exist 
are all found to arise in strictly acropetal order. 

Finally, it would seem probable that the heterosporous condition, where 
it occurs, supervened after the individual sporangia had already acquired 
approximately the dimensions and characteristics seen in the different types 
in which it appears. 



CHAPTER XXV. 

COMPARATIVE ANATOMY OF THE LYCOPODIALES. 

IT has been already noted that the Lycopods are marked off from other 
Vascular Plants by the simple and regular arrangement of their sporangia 
in relation to the other parts of the shoot : also that the characters of 
the shoot themselves suggest in their simple form and arrangement a 
primitive state. The Lycopods are no less notable for their anatomical 
characters, and especially those of the Vascular System. They stand apart 
from almost all other Vascular Plants in the presence in their mature axes 
of a stele having peripheral protoxylem, and often showing the solid 
xylem-core characteristic of the protostele. The leaf-traces insert them- 
selves with the minimum of local disturbance upon the periphery of the 
columnar stele, which is further shown by its development to be cauline 
(compare Fig. 67, p. 125). Exceptions from this simple vascular construc- 
tion occur within the phylum : but a comparative examination of the various 
forms will show that the non-medullated monostele may be accepted as 
a central type of construction for them all, upon which certain modi- 
fications and variants have arisen : some of these are exemplified in the 
fossils, some in plants now living. The comparisons will be primarily 
based upon the structure of the mature shoot. The same order will be 
maintained as in the description of the external morphology, and it will 
be found that the anatomical complexity follows, with some degree of 
exactness, that of the external form. 

Taking, therefore, first the less differentiated Selago section of the genus 
Lycopodium, as seen in L. Selago, serratum, or lucidulum, the cylindrical 
stele is there found to consist of a connected central mass of xylem of 
irregularly star-like form : the rays of the star vary in number in different 
species, as well as in different regions of the same plant, and are specially 
characterised by the form of the periphery of the rays : these expand 
outwards into a wide-spread, almost fan-like outline, as seen in the trans- 
verse section (Fig. 171 c). Small tracheides forming the protoxylem lie at 
the extreme periphery, while the centrally-disposed metaxylem is composed 



COMPARATIVE ANATOMY 



329 



of larger elements without any x parenchyma interspersed between them. 
The spaces between the xylem-rays are occupied by the sieve-tubes, with 
the protophloem lying at the periphery, while conjunctive parenchyma 
forms a complete sheath intervening between the phloem and the xylem. 
The whole is invested by a parenchymatous sheath resembling a pericycle, 
but derived, according to Strasburger, 1 from the cortex : outside this is 
the endodermis, recognisable while young as a single layer, but later 
obscured by extension of the corky development. A very similar structure 
to the above is seen also in the thinner branches of L. inundatum, a 
species, which as we have seen above, stands in its external morphology in 
near relation to the section Selago. These species may be taken as repre- 
senting the structure usually found in the simpler upright, ground-growing 
members of the genus. 




FIG. 171. 

Diagrammatic transverse sections of the stele of various species of Lycopodium ; the 
phloem is dotted, the xylem drawn as tracheides. C = Lyc. serraturn, Thbg., with 
stellate arrangement. D = upright stem of L. annotiuum, L., with somewhat stellate 
arrangement. =L. cernunm, L., with uniform distribution of the small groups. F= 
L. volubile, Forst, with strongly bilateral structure. X8o. (From Engler and Prantl.) 

But a more elaborate construction of the stele is found to accompany 
the greater differentiation of external form. In creeping and climbing stems 
there is apt to be an increase in the number of the protoxylems, accom- 
panied by a development of alternating bands of xylem and phloem : the 
xylem becomes isolated into distinct masses as seen in the transverse section, 
and these are roughly disposed parallel to the surface of the substratum 
(Fig. 171 F). In other cases, and especially in the epiphytes, the xylem 
and phloem are more uniformly distributed, the former as patches em- 
bedded in the latter, as seen in tranverse section (Fig. 171 E). Both these 
conditions may be connected by intermediate steps with the simpler type 
seen in L. Selago, and as they occur in plants with more specialised form 
and habit, it may be concluded with some degree of certainty that the type 
with a connected xylem-tract shows the more primitive state. 

It would seem hardly necessary to insist on this rather obvious outcome 
of comparison within the genus Lycopodium, were it not that a certain 
misconception, which dates back to the Text-book of Sachs, still survives 

1 Leitunosbahnen, p. 460. 



330 LYCOPODIALES 

as to the nature and origin of the more complicated steles of Lycopodium. 
Comparison of these with polystelic stems of Selaginella long ago suggested 
that the former structure was derived phylogenetically from the latter, by 
the lateral fusion of several distinct steles; and thus that the stele of 
Lycopodium is in reality a compound one. 1 But the polystelic condition 
seen in some Selaginellas is not uniform for that genus, as we shall see 
below : moreover it seems improbable that the simpler, homosporous 
Lycopodium should show structural derivation from the heterosporous Sela- 
ginella, while it is only in the more complicated Selaginellas that the 
polystelic condition appears : again, the species of Lycopodium which show 
distinct xylem-plates are in our view morphologically more advanced than 
those with the xylem more closely connected. Such considerations go 
far to negative any idea of the more complex steles of Lycopodium, being 
compound in their origin. Comparison within the genus is usually a safer 
guide in such questions than more far-fetched references; and in the 
present case it suggests a different explanation, which is as follows : that, 
in a primitively protostelic Lycopod-stock with cylindrical solid xylem, the 
phloem became progressively intrusive as the morphological differentiation 
of the plant increased : at first it appeared in the transverse section as 
occupying a few narrow involutions of the margin of the still connected 
xylem, this then showing the stellate outline, with fan-like peripheral arms, 
as seen in the Selago type. But in others the number of the involutions 
and their depth became greater, till the coherence of the xylem-tract as 
seen in the single transverse section became interrupted, and the appearance 
of more or less isolated plates with narrow peripheral edges was attained, 
as in Z. clavatum. The origin of the xylem-islands as seen in L. 
squarrosum or Z. cernuum was substantially the same, the difference being 
that they are not merely intrusive from the margin, but the xylem-tracts 
are actually for some distance occluded in the phloem. In point of fact 
these two types of more complicated derivative structure are not strongly 
differentiated from one another. Thus, from comparison within the genus, 
it may be figured how from the condition of a primitive protostele with 
phloem about its periphery the Lycopod-stele became in the more advanced 
cases a sort of xylem-sponge, with phloem and conjunctive parenchyma 
occupying the interstices. It will be seen later that the simple protostelic 
state without intrusive phloem is represented among the fossil Lycopods. 

The relation of the leaves to the central stele in Lycopodium is interest- 
ing, both in respect to the young and to the mature condition. If the 
apex of the shoot be investigated, the plerome-cylinder is seen to extend 
beyond the youngest leaves, to a point immediately below the apical group 
of cells ; and thus the central region of the stele is cauline in its origin 
(Fig. 172). The leaves originate from the three or four outer layers of 
cells of the growing point, quite apart from the plerome, while procambium- 

1 This is specifically stated in Strasburger's Leitungsbahnen, p. 458, and the view has 
been retained in his Text-book, in the German edition of 1906. 



COMPARATIVE ANATOMY 331 

strands become differentiated in the intervening tissue, which form a connec- 
tion with the central cylinder : upon this they are inserted laterally. It is 
thus clear that in the ontogeny of the shoot the leaf is an accessory which 
arises after the stele is already in existence. Its relative unimportance is 
not only apparent from this late origin, but also from the fact that the 
arrangement of the leaves upon the shoot does not dominate the number 
or position of the protoxylem-groups of the stele. It has long been known 
that the number of the xylem-rays is independent of the position of 
the leaves. In L. davatum Jones has found that though in shoots 
with simple leaf-arrangement it is usual for the protoxylems to correspond 
to the leaf-insertions, still, where the number of protoxylems is beyond 
six, there is no apparent relation between them and the leaf-insertions. 1 




FIG. 172. 

Longitudinal section through the apical cone of the stem of Lycopodium Selago. X 160. 
(After Strasburger.) 

When the above facts are taken together,- it is apparent that the leaf 
in Lycopodium is but an accessory appendage, and that the axis is the 
dominant feature of the shoot. This conclusion probably applies for 
Lycopods at large, and it has its important bearing on the relation of 
leaf to axis, discussed in Chapter XL 

Hitherto no definite knowledge of the anatomy of the smaller fossil 
eligulate Lycopods included under the name Lycopodites has come to hand : 
whenever such facts are available they will provide interesting material for 
comparison with the modern species of Lycopodium. The ligulate and hetero- 
sporous forms would be equally important for comparison with Selaginella. 

The discussion of the external morphology of the latter genus has led 
to the recognition of the radial type as relatively primitive, while those 
species with dorsiventral shoots are held to be more specialised and 

1 Linn. Trans., 2nd series, vol. vii., p. 19. 



332 



LYCOPODIALES 



derivative. Of the former S. spmnlosa, A. Br., 'is the best known, and it 
will be seen that its vascular anatomy, which differs from that of all 
other Selagincllas, shows points of interesting comparison on the one 
hand with Lycopodium, and on the other with the dendroid Club-Mosses. 
The hypocotyl, and the lower parts of the axis, with its branches, are 
traversed by a cylindrical stele, which is peculiar in having a central 

A 



H.G. 



F.E. 




FIG. 



173. 



Selaginella spinulosa. A. Transverse section of the trailing stem showing central 
protoxylem. # = pericycle ; <5 = protophloem ; c = phloem parenchyma ; rf=metaxylem. 

X275- B = transverse section of upper part of axis, showing seven protoxylems. .X35o. 

C-G scheme of arrangement of the protoxylems in sections taken successively from 
below upwards. (After Harvey-Gibson.) 

strand of protoxylem surrounded by metaxylem : this is further invested 
by a narrow band of phloem surrounded peripherally by a sheath 
resembling a pericycle, and by the trabecular endodermis so characteristic 
of Selaginella (Fig. 173): according to Strasburger both of the latter 
layers are derived from the cortex, as they are also in Lycopodium. 1 
In passing upwards in the strobilus the stele loses its peculiarity of 
having a central protoxylem : for the strand divides, and the branches 
diverge outwards to the periphery of the wood, where they appear in 

1 Leitungsbahnen, p. 458. 



COMPARATIVE ANATOMY 



333 



number from three to eight, as ^lightly projecting groups of small spiral 
tracheides. The condition thus attained is very similar to that seen in 
the simpler types ot Lycopodium : or a better comparison may perhaps 




B 



FIG. 174. 

Lepidodendron Harcourtii. A= transverse section of stem; st= stele; o.c = outer 
cortex ; both here and in the outer cortex the leaf-traces are shown about natural size. 
B stele of same ; / = pith, hollow in the middle ; .r = xylem-ring ; />-r = protoxylem-points. 
The leaf-traces join the stele between them ; l.t = leaf-trace bundles, of which the outer, 
/.*', show xylem and phloem ; i.c = inner cortex. Xj. (From Scott's Studies in Fossil 
Botany.) 

be drawn with certain stems of Lepidodendron. The presence of central 
and peripheral protoxylem in different parts of the Jsame shoot should be 
a -warning against too great reliance upon such characters ; it may, perhaps, 
indicate that the central position was the more primitive, as is believed 
by some on general comparative grounds. At the same time it is of 



334 LYCOPODIALES 

interest for comparison with the Lepidodendroid fossils, in which a 
peripheral protoxylem is found in the shoot, while a central protoxylem, 
adjoining the medulla, is found in the Stigmarian trunks. 

Other species of Selaginella show further elaboration along distinct 
lines. The simpler dorsiventral species, and even such radial species as 
S. rupestris and oregana, show ribbon-like steles with marginal protoxylems, 
upon which the leaf-traces are inserted. In the more complex cases the 
axis becomes polystelic (S. inaequalifolia and Willdonovii}, or in some cases 
solenostelic (rhizome of S. laevigatd), thus resembling similar vascular 
complications seen in the stems of Ferns. These may be held to be 
relatively late, and special developments from the non-medullated, mono- 
stelic type : their origin shows parallelism of development rather than any 
nearer relation with the similar structure seen in the Ferns. 

The near correspondence of the ancient Lepidodendron-type to that of 
the modern Lycopodiales appears not only in their external form, but also 
in their internal structure, though special modifications of type, different 
from those of the modern forms, appear in accordance with the larger 
dimensions so prevalent in the fossils. The similarity consists in the 
presence of a single cylindrical stele, with a centripetal wood, and peripheral 
protoxylem, in relation to which the leaf-traces are inserted with the 
minimum of local disturbance. 

The general structure of one of the more simple types may be gathered 
from Scott's figure of Lepidodendron Harcourtii (Fig. 174), which shows 
(A) the relatively small proportion of the stele to the whole axis : (B) the 
peripheral protoxylem, with its relation to the incoming leaf-traces, and the 
uninterrupted metaxylem, not separated into strands : while centrally a large 
pith is seen hollow in the middle. The steles of Lepidodendron varied in 
structure towards the centre : in some cases such as the very ancient 
Lepidodendron rhodumnense, Renault, and Lepidodendron saalfeldense, Solms, 
from the Culm, there was a solid stele, without secondary thickening; 
or, as in Lepidodendron Petticurensis, Kidston (Roy. Soc. Edin. Proc., 
1906-7, p. 207), the solid xylem-core was surrounded by secondary wood. 
But often, and especially in more recent forms, the xylem was medullated, 
and in this they differ from modern Lycopods. It is obvious in some 
cases that the pith originated by incomplete development of tissue originally 
tracheidal: this is clearly indicated in Fig. 175. This drawing also shows 
that outside the xylem came a narrow band, probably of phloem, which 
is usually ill preserved, while in some cases there is evidence of an 
endodermis, as in the present case. Thus, putting aside the larger size, 
and the medullation which is its frequent concomitant, there is substantial 
similarity in the structure of the stele to that of a simple Lycopodium, 
or of Selaginella spinulosa at its distal region. 

A more striking concomitant of the larger growth was, however, the 
secondary thickening represented in the majority of the known species of 
Lepidodendron, though absent from some of the earliest. It was carried 



COMPARATIVE ANATOMY 335 

out by two distinct zones of cajnbial activity, the one immediately sur- 
rounding the primary xylem, and resulting in a band of radially seriated 
secondary wood, contiguous usually with the protoxylem of the primary 
development. Externally an exiguous secondary phloem appears (Fig. 176). 
Outside the thickening ring of the stele a second zone of cambial activity 
arises in the cortex, below the persistent bases of the leaves : this results 
in the formation of a broad band of secondary cortical tissue, or periderm. 




FIG. 17;. 

Lepidostrobus Rrovunii. Part of a transverse section showing the central parenchy- 
matous pith (/*), the wood (xy), the innermost band of cortex (<:), the endodermis (?) (sh). 

X200. 

Such secondary activity ^extended from the main trunk into the branches, 
and in some cases into those of quite moderate dimensions. Comparison 
of the various known types of Lepidodendron suggest unmistakably that 
even the most elaborate are the result of expansion of a non-medullated 
monostelic construction, to serve dendroid purposes. A first step, following 
on the increasing size of the stele, would be the formation of a paren- 
chymatous pith : this probably originated directly, by the incomplete 
development of a primitively solid tracheidal core, as is suggested in the 
case of L. Brownii: and in support of this it is found that tracheides 
and parenchymatous cells may be intermixed in the central region, a 
condition held to represent an imperfectly, formed pith : it is seen in 

31TY 



336 



LYCOPODIALES 



L. selaginoides. It would seem probable that the non-medullated condition, 
so persistently maintained in the smaller living Lycopods, was the primitive 
state also for the larger dendroid fossils. The other factor of expansion, by 
cambial activity, appears to have originated independently of medullation, 
since it occurs both in medullated and in non-medullated axes. Physio- 
logically it counterbalanced medullation where both occur together, for it 




FIG. 176. 

Transverse section of an axis of Lepidodcndron selaginoides. Cy = centre of the 
vascular system ; tr = tracheae ; V vessels of the primary cylinder \fp primitive fibres 
of the primary wood; .Z?2~trachei(les f tne secondary wood ; r=ray of the secondary 
wood ;A = secondary parenchyma; zc cambial zone; L = liber ; s = foliar traces detached 
from the primary cylinder. (After Hovelacque.) 

substituted an enlarging peripheral vascular supply for the reduction in 
efficiency in the limited central system. This was indeed a necessary 
condition for dendroid development. 

However large the proportion of pith to the primary wood became in 
Lepidodendron, the continuity of the ring was as a rule unbroken, and the 
leaf-traces were simply inserted upon the primary xylem with the minimum 
of local disturbance. But in Sigillaria, in which the leaves sometimes 
attained a very large size, the case is different : though they show in all 



COMPARATIVE ANATOMY 337 

essentials the same construction x of the stele as in Lepidodendron, they 
illustrate steps towards the breaking up of the primary wood of the 
medullated stele into separate bundles. The details derived from various 
Sigillarian fossils have lately been put together in stratigraphical sequence 
by Kidston, 1 and his conclusion has already been quoted above (Chapter 
XVIII., p. 230): he has shown a strong support for the view that the 
condition with primary xylem forming a closed ring surrounding the large 
medulla was the most primitive for Sigillaria : such a structure is found 
in the more ancient specimens from the Lower Coal Measures (S. elongata, 
Brongn., and 6". elegans, Brongn.) : those from the lower Permian, however, 
(S. menardi, Brongn., and S. spinulosa, Rost. sp.) show the primary xylem 
as a circle of separate bundles, though some of them may cohere laterally 
in the last-named species. This indicates an evolutionary progression from 
a concrete primary xylem to a condition where it is separated into strands. 
In such forms the pith, being of relatively very large size, the primary 
wood is reduced to a comparatively narrow investment round it, liable 
as we have seen to be broken up into distinct strands. The secondary 
tissues make their appearance, however, as in Lepidodendron ; there being 
in Sigillaria a broad zone of secondary xylem, and a highly organised 
periderm. It is thus seen that the later Sigillarias have departed further 
in their structure from the simple protostele than other dendroid Lycopods, 
for they show not only medullation, and a secondary thickening, but 
breaking up of the primary xylem as well. 

It has been concluded above, on the basis of external comparison, 
that the plant of Isoetes is like a partially differentiated Lepidostrobus 
seated upon a Lepidodendroid base. The question will now be how far 
its anatomy will countenance such an opinion. There has been some 
confusion in the descriptions given by various investigators, owing doubtless 
to the difficulty in decyphering a complex mass of tissues affected by 
the reduction which follows on an aquatic habit. But this has been in 
great measure cleared by Scott and Hill in their Memoir on Isoetes 
hystrix, one of the few land-growing species. 2 Nevertheless the terrestrial 
habit of this plant does not greatly affect its structure as compared with 
other species, a circumstance which is held to point to the conclusion 
that Isoetes is a genus which has long hovered about the limits of terrestrial 
and aquatic life. The statement here given is based upon the Memoir 
of Scott and Hill. 

The stele of the mature plant is not composed merely of the united 
leaf-traces, but is best interpreted as a cauline structure, comparable to 
that of the simpler monostelic Lycopods, but much shorter than is usual 
in them. The crowded leaf-traces are inserted upon it, the stelar wood 
serving to join up the xylem of the leaf-traces, but it does not belong to 
one trace more than another, and in structure it differs from them. The 

1 Trans. Roy. Soc., Edin., vol. xvi., Part iii., No. 23. 
-Ann. of Bot., vol. xiv., 1900, p. 413. 
Y 



338 



LYCOPODIALES 



differentiation of' the primary wood is nearly simultaneous over its whole 
area, but with indications of centripetal succession. The cambial activity 
starts early, being continuous from that of the primary meristem. As a 
rule the same cambium is active throughout, producing secondary ground 
tissue, wood, and phloem on its inner side, and cortical parenchyma 
only towards its exterior; but other arrangements are found, while in 
some cases a second cambial activity may arise inside or without the 
first. The adjoining diagram, quoted from Scott and Hill (Fig. 177), 
shows the relation of the primary and secondary tissues usual in /. hystrix^ 
and it will be noted that the secondary phloem is internal to the secondary 
xylem ; the cambium lies outside the latter in direct contiguity with the 

secondary cortex, which arises externally 
from it. The stele which is cylindrical 
above becomes in /. hystrix triquetrous 
below, in /. lacustris it is usually flattened 
bilaterally : the change of form is a 
secondary consequence of the abutment 
of the numerous, successively formed 
root-bases upon it, and does not affect 
the general comparisons. Scott and Hill 
conclude that the anatomy of the stem 
with its solid stele, from which the 
densely crowded small and simple leaf- 
traces pass off, is just what might be 
expected in a stunted Lycopod, while the 
anomalous character of the secondary 
thickening in Isoetes agrees in some 
measure with that in certain fossil Lyco- 
pods. Scott 1 has remarked on the stem 
of Lepidodendron fuliginosiim as having 
an anomalous cambium producing a 
good deal of secondary parenchyma, among which there are scattered 
groups of wood ; and he regards this species as exhibiting a primitive 
and rudimentary form of secondary growth. It seems to offer a distinct 
analogy with Isoetes. On the other hand, the slight cambial increase 
discovered in Selaginella spinulosa by Bruchmann affords some link as 
regards secondary thickening, though a feeble one, with a living Lycopod. 
The general result of this anatomical examination and comparison of 
Isoetes is accordingly to strengthen its position among the Lycopodiales, 
and to show that its primary vascular arrangement corresponds in 
essentials to the type as exemplified in living species of Lycopodium, 
but much abbreviated, and with the xylem reduced in accordance 
with the aquatic or amphibious habit prevalent in the genus. It also 
appears that the secondary development, though showing fluctuating 

1 Studies, p. 143. 




FIG T 

Diagrammatic transverse section of the upper 



dary phloem has developed ; ,r 2 , that in which 
secondary xylem has developed ; r< = cambium ; 
c<i secondary cortex; It leaf-traces. X42. 
(After Scott and Hill.) 



COMPARATIVE ANATOMY 339 

anomalies, finds its nearest paraNel in certain fossils belonging to the 
Lycopodiales. 

It may thus be concluded from comparative examination of all the 
leading types of the Lycopodiales that the vascular structure of the mature 
shoot is referable in origin in all cases to the non-medullated monostele. 
This is actually seen existent in the stems of Selaginella spmulosa, though 
in its lower portion the protoxylem is central ; but it is also shown more 
amply developed, and with the protoxylem in the accustomed position at 
the periphery in the upper region of that plant, as well as in certain 
stems of Lepidodendron. Comparative study of the Lycopodiales shows 
that all the variants of vascular structure known in them may be referred 
in origin to this simple type. In Lycopodium the modification has been 
by intrusion of the phloem more or less deeply into the xylem-core, till 
this may at last be divided into distinct plates, or riddled like a sponge. 
In Selaginella there is amplification in various ways, the most obvious 
being by the adoption of a solenostelic structure, or more commonly by 
segregation of the enlarging stele to form a varying number of meri- 
steles. Among the dendroid fossils, where the demands on the conducting, 
system were large in consequence of the large size of the plants, the 
extended stele became first medullated, as seen in most stems of Lepido- 
dendron : and then in the later Sigillarias the residual ring of xylem 
became broken up into more or less distinct strands. In these types 
additional vascular tissue was supplied by the potentially unlimited 
developments from an external cambium. Finally, in Isoetes a complicated 
structure, partly primary, partly secondary, is found, which would be 
hardly intelligible except when studied in the light of the dendroid fossils ; 
but even this, in common with the rest, is referable in origin to the 
non-medullated monostelic type, together with the results of secondary 
thickening. The bearing which this constant reference to a primitive 
monostele has upon a strobiloid theory is plain : as is also the fact that 
throughout the Lycopodiales the foliar traces are inserted peripherally, and 
with only slight local disturbance upon the periphery of the cauline 
xylem-core : for this indicates structurally that the leaf is in them all the 
minor, while the axis is the dominant feature of the shoot. 

Scott and Hill rightly' point out that the view of the central cylinder 
as cauline applies only to the adult stem of Isoetes: in embryonic stages 
the construction of the vascular system is from the union of definite 
leaf-traces : this is the case also in the embryonic stages of certain other 
Lycopods. The question of the relation of these facts to a theory of 
the strobilus will be taken up in connection with the embryology of the 
Lycopods, which forms the subject of the next chapter. 



CHAPTER XXVI. 

EMBRYOLOGY OF THE LYCOPODIALES. 

(A) ELIGULATE LYCOPODIALES. 

IN Chapter XIV. the modern aspect of comparative embryology of the 
sporophyte has been discussed. For reasons there stated it was concluded 
that only a minor place in comparisons is to be conceded to the details 
of the initial embryology of the sporophyte : the characteristic form of 
the mature plant, established after the earlier and in considerable degree 
adaptive phase of development is past, is held to give a more reliable 
basis for argument than does the embryonic state. Especially is this the 
case among the Pteridophyta, and it happens that the Lycopods supply 
examples of peculiar interest in relation to such questions ; they will serve 
at once as an illustration, and as a test of the principle thus briefly 
stated. For in the general conformation of their mature sporophyte there 
is - a remarkable uniformity throughout the whole phylum : the differences 
are those of secondary detail : the main facts of plan and proportion of 
their srioot- and root-systems, of their branching, and of the relation of 
the sporangia to the other parts, leave no doubt of a natural affinity as 
based on the character of the mature sporophyte. But in the embryology 
there are points of marked divergence, which may be more or less clearly 
correlated with differences of character of the parent prothallus. There is 
reason to think that within the genus Lycopodium the prothallus and embryo 
have undergone a cognate divergent development from a central type, 
though the mature sporophyte has still retained a substantial uniformity. 

The differences in character of the prothallus within the genus Lycopodium 
are found to be those of habit and of mode of nutrition rather than of 
fundamental structure. According to their mode of life three main types 
may be distinguished, which, however, graduate into one another in such 
a way as to suggest their intimate connection by descent from some 
common source. The type shown by L. cernuum t and shared also by 
L. inundatum and sa/akense, consists of a massive cylindrical thallus, of 
which the conical lower part is sunk in the soil, while ^the upper part is 



EMBRYOLOGY 



341 



exposed freely above ground, ano> is of a green colour : in L. cernuum 
and inundatum it bears numerous irregular leaf-like lobes, though in 
Z. salakense the lobes are rudimentary or absent (Fig. 178). The pro- 
thallus is evidently in the main a self-nourishing body, though an endo- 
phytic fungus is almost constantly present, indicating a second but 
subsidiary line of saprophytic nutrition. As the prothallus grows a 
merismatic zone is localised surrounding the upper part of the cylindrical 
body, but below its apex : this contributes to increase both the upper and 




. r 



FIG. 178. 

Young leafy plant of Lycopodium cernuum, L., with the prothallus, bearing its irregular 
assimilating lobes, attached on its left-hand side. X about 20. ('After Treub.) 

>wer regions, while above it the green expanded lobes are formed. The 
jxual organs appear between the latter, the youngest being nearest to the 
icrismatic zone. 

A second type shows in the ascendant that method of nutrition which 
was subsidiary in the first : it is exemplified by the large subterranean 
)thalli of L. complanatum, davatum, and annotinum\ being shut off from 
light these prothalli are colourless, and the leaf-like lobes are absent. The 
ssive prothallus is composed of a lower region which takes a conical 
>rm, the angle of the cone being greater in Z. davatum and annotinum 
than in Z. complanatum: it is in this region,, as in Z. cernuum, that the 
idophytic fungus is present. The merismatic zone is active as before at 
its upper limit, and above it is the part which bears the sexual organs, 
>ut without any vegetative lobes as in Z. cermium (Fig. 179 B). It is clear 



342 



LYCOPODIALES 



that the general plan of construction of the prothallus is the same as in 
the L. cernuum-type, but modified in accordance with the saprophytic 
method of nutrition. 




FIG. 179. 

A =old prothallus of Lycopodium annotinuui, L. , with young plant projecting beyond 
the earth-surface (o). Natural size. B= median section through a young prothallus of 
Lycopodium clavatum, L. X about 30. C = part of this from the middle region of the 
upper surface, with antheridia in different stages. X 52. Z> = part of the margin of the 
median section, with meristem and archegonia. X 52. = the epidermis devoid of 
fungus, with rhizoids ; r = cortical layers, with their cells filled by hyphal coils ; / = the 
palisade layer, also filled with hypha? ; s^> the storage tissue; ; = the meristem; 
an antheridia; arch archegonia ; em = an embryo ; f= its foot; zw = its root. (After 
Bruchmann, from Engler and Prantl.) 

In the third type, exemplified by Z. phlegmaria and other epiphytic 
species, the prothallus is more attenuated, and repeatedly branched. The 
delicate colourless branches extend widely through the dead bark on 



EMBRYOLOGY 



343 







which the prothalli grow, and they are attached by hairs which project 
in all directions. Here again a fungus plays an important part in the 
nutrition, which is exclusively saprophytic. The prothalli reproduce readily 
by gemmae, as also by progressive decay, which separates the ultimate 
branches as distinct individuals. The sexual organs are borne upon the 
upper surface of enlarged branches of the thallus, and are always accom- 
panied by paraphyses. 

Such different types of prothallus, when studied separately, appear 
widely divergent : and at first the underlying unity of their construction 
was less appreciated than the differences which they show ; so little indeed 
that Bruchmann, to whose labours so many of the important facts are 
due, was disposed to make those differences the basis of a division of 
the genus Lycopodium into distinct groups, or even genera. 1 But Lang,, who 
had simultaneously with him been 
at work on the prothallus of Z. 
clavatum? pointed out clearly the 
relation of the divergent types 
to one general plan, recognising 
especially how the prothallus of 
L. Selago, one of the species 
described by Bruchmann, gives 
the clue to their connection. For 

itS prothallus appears tO be Variable 

j c j , /T -,. 

in ItS mode Of development (Fig. 

1 80). It is usually a pale under- 
ground body; but at other times 

it grows above ground, and is coloured a full green. The spores appear to 
germinate either at the surface or below it. The form of the prothallus is 
determined largely by the soil in which it develops : thus, the elongated 
cylindrical form is usually found in firm ground, though less deeply 
buried than in the annotinum-\.y$Q : the thallus seems, in fact, to stretch 
upward as though to bring as near to the surface as possible the seedling 
unsuited for subterranean growth. The subterranean prothalli may be 
simple, or be branched so as to take a coral-like form. In more open 
soil, however, and especially near to the surface, the prothalli are more 
compressed and flattened. Each prothallus tapers off as in the other 
types at its lower end into a conical point, which indicates where it 
issued from the spore, while towards its upper end the sexual organs are 
formed. In the half-saprophytic prothalli, grown to the surface of the soil, 
the conical form similar to that of other types is clearly seen (Fig. 181) : the 
saprophytic lower region, the meristem, and the crown bearing the sexual 
organs and paraphyses holding the usual positions. 

1 Ueber die Prothallien und die Keinipflanzen mehrerer europaischen Lycopodien, 
Gotha, 1898, p. 108. 

-Annals of Botany ', xiii., p. 279. 



FIG. 180. 

Prothalli of Lycopodium Selago, bearing seedlings. 
o, o shows the level of the soil, and the seedlings in their 
development show varying proportions so that the first. 

a 



344 



LYCOPODIALES 



In considering these various prothalli it is then clear that they are 
all modifications of the same conical form : that the several parts, though 
differing in proportion, have the same positions relative to one another 
and to the sexual organs which they bear, while the differences are closely 
related to the differences of circumstance and of nutrition. There is 
reason to believe that the full chlorophyll-nutrition was the primitive state 
for them all, and the saprophytic nutrition, seen in the subterranean or 

the epiphytic types, a deriva- 
tive state. On this basis the 
cernuum-type would be recog- 
nised as relatively primitive, 
while Z. Selago, being less 
specialised than the annotinum 
or phlegmaria-typzs, would 
approach it more nearly than 
they do. But it does not 
follow necessarily that a species 
which is recognised as primi- 
tive in respect of one prominent 
feature, is to be held as primi- 
tive in all its features. This 
applies to L. cernuum : it is 
true that its prothallus is green 
and assimilating, and in this 
respect probably primitive; but 
its sporophyte is a fairly 
advanced one, with definite 
strobili, and with peltate chaffy 
sporophylls strongly differen- 
tiated from the assimilating 
leaves : its axis, too, shows 
an advanced condition of the 
stele. Thus in its general 
characters Z. cernuum cannot 
be held as a consistent proto- 
type of the genus. But, on 

the other hand, Z. Selago has a prothallus little removed from the condition 
seen in Z. cernuum, while in addition the sporophyte of that species has 
been seen to represent the least differentiated type in the whole genus. 
On the general sum of its characters it would accordingly take a place as 
a relatively primitive form. But its prothallus shows distinct plasticity in 
the directions along which specialisation has extended to produce the 
more extreme types : on the one hand, its subterranean specimens, with 
elongated cylindrical form, prefigure the more specialised developments of 
Z. complanatum and annotinum : the compressed and flattened form 





FIG. 181. 

Median longitudinal section through a young prothallus of 
Lycopodiujii Selago. /> = basal cell; n/t, root hairs; ^ = epi- 
dermis ; r=the investing tissue, stored with reserve materials, 
and harbouring an endophyte ; t=the central ; ^ = the genera- 
tive tissue ; r=archegonium ; k = young embryo ; # = anthe- 
ridia beginning to develop: gh sexual hairs, x 35. (After 
Bruchmann.) 



EMBRYOLOGY 



345 




developed in more open soil suggests the origin of the phlegmaria-type : 
while its green sub-aerial forms are reminiscent of the cernuum-type. A 
plant which shows such plasticity is clearly not far removed from the self- 
nourishing condition of the prothallus, which was probably the primitive 
condition for them all. 

These remarks upon the curiously divergent development of the pro- 
thallus in the genus Lycopodium are a necessary preliminary to the study of 
the embryogeny in the genus ; for it is impossible to understand the 
comparisons of the different forms of embryo without some knowledge of 
the prothalli which produce them. In all the species of Lycopodium in which 
the embryogeny is accurately known, an early stage of the embryo is 
found in which it consists of a suspensor, and of two tiers, each composed 
of four cells (Fig. 182). The first cleavages are variable in their succession,, 
as is found to be the case also in other embryos ; 
but their position shows considerable constancy. 
It is stated that from the lower tier of cells, 
i.e. that adjoining the suspensor, the structure 
designated the foot arises, while the upper tier 
gives origin to all the other parts of the 
embryo, and the correctness of the statement 
is borne out by numerous drawings. But after 
the first stages are past there is usually no 
sharp limit between the tissue composing the 
foot and that of the other parts : in the simplest 
cases it appears as though the foot were merely 
a region of tissue lying between the suspensor 
and the upper tier, rather than a definite organ 
or part. Functionally, the foot does not appear 

to be differentiated from the suspensor in the genus Lycopodium, and it 
shares with it the office of maintaining connection with the prothallus. Not- 
withstanding the initial similarity which thus rules in the embryos of the 
genus, the further steps of the embryogeny differ according to the different 
forms of prothallus above described ; and it becomes a question which of 
the divergent types is to be held as the most nearly reflecting the original 
condition, and which as' later and derivative. 

The type of L. Selago may be taken first, since it does not show any 
high degree of specialisation in its variable gametophyte, while it has 
been seen above that its mature sporophyte is one of the least differen- 
tiated in the genus. Its early embryogeny, so far as is known, conforms 
to the usual type, as above stated. The foot originates from the lower 
tier, and the various parts of the embryo from the upper. 1 But the foot 
is only slightly developed. The upper tier of cells soon assumes a green 
colour and unsymmetrical form, owing to the lateral upgrowth of the first 
leaf or cotyledon, while the apex of the axis also originates early, near 

1 Bruchmann, I.e., pp. 97-103. 



FIG. 182. 

Diagram illustrating the primary 
segmentation of the zygote in Lyco- 
podium. /, I first segmentation 
wall which separates the suspensor, 
here cross-hatched, b, b separates 
a lower tier (foot-tier) here dotted, 
from an upper tier (stem-tier) left 
clear ; each tier consists at first of 
four cells. The wall b, b corresponds 
to the wall IV. -IV. in Figs. 183, 186, 
and to wall II.-II. in Fig. 185. 



346 



LYCOPODIALES 



Bl 



to its base : it is clear that the relation of the apex to the intersection of 
the first walls has been a close one (Fig. 183 A). The axis soon proceeds 
to form successive leaves spirally arranged. The cotyledon and subsequent 
leaves have the ordinary characters of the foliage leaves of the species. 
The tissue below soon becomes elongated as the hypocotyl, the length of 
which is determined by the level at which the prothallus lies in the soil : 
where it is at or near to the surface the hypocotyl may be quite short: 
where deeply seated it lengthens, so that the first leaves are exposed 

above ground (Fig. 184). It is 
traversed by a vascular strand, 
which is monarch below, but 
near to the first leaf, or later, it 
becomes diarch, and shows two 
lateral tracheidal strands. The 
first root originates exogenously 
from the upper tier, just above 
the foot, and is succeeded by 
other roots of endogenous origin 
at higher points (Fig. 183 B). 

Here, then, is an embryogeny 
characterised by its great direct- 
ness and simplicity. The only 
complication is the varying elon- 
gation of the hypocotyl according 
to the level of the prothallus in 
the soil ; and there is good reason 
to think that this is an immediate 
adaptation to meet the varying 
levels of development of the game- 
tophyte in the soil, in an embryo 
which is pertinaciously subaerial. 
The nursing of the embryo by the 
prothallus is not long continued, 
nor is it structurally provided for, there being no development of an 
elaborate "calyptra," as in some other species: the embryo soon escapes 
from the prothallus, and fends for itself. The whole condition of the 
embryo is such as bespeaks a simple and primitive state. Probably this 
view would never have been in doubt had it not been for the existence of 
different arrangements seen in other species of the genus, which happened 
to have been described some years earlier. 

Of these the type which corresponds most nearly to L. Selago is that 
of L. Phlegmaria, so accurately described by Treub. 1 Here the segmen- 
tation of the embryo, as well as the origin of all the parts and their 
proportions while young, have been fully made out, and appear to be 

1 Ann.Jard. Bot. de Buitenzorg^ vol. v., p. 87, etc. 




FIG. 183. 

Lycopodium Selago. A young embryo. X 150. 
^=foot ; IV.-IV. =wall separating the foot from the 
stem-tier ; K= cotyledon ; a = apex ; .EV=suspensor. 
B = embryo more advanced, with prothallus still attached ; 
Z?/=cotyledon ; f^, W^ = young roots ; V=suspensor 
attached to the foot, which is clearly defined from the 
base of the axis. X 20. (After Bruchmann.) 



EMBRYOLOGY 



347 






substantially the same : the hypocotyl becomes elongated as the seedling 
develops, and the whole appearance of the seedling resembles that of 
Z. Selago. The primary segmentation in L. Phlegmaria is according to 
the scheme (Fig. 182), and the lower tier, as in Z. Selago, forms only the 
foot, which attains no great size (Fig. 185). The upper tier develops 
unsymmetrically from the first, the side which will form the first leaf 
growing more strongly; close to the base of the cotyledon, and apparently 
lateral owing to the stronger growth of the latter, but in reality terminal, 
arises the apex of the axis (T, Figs. 185 A, B) ; it is, in fact, initiated in 
close proximity to the organic centre of the upper tier. The root 
(R, Fig. 185 B) also originates from 
the upper tier. A comparison of 
Treub's drawings of Z. Phlegmaria 
with Bruchmann's less complete 
series for Z. Selago shows clearly the 
substantial similarity of the embryo- 
geny in the two species. It will 
be remembered that the flattened 
prothalli of the latter species, formed 
near the level of the soil, have been 
held to prefigure the strap-shaped 
sexual branches of the Phlegmaria 
prothallus, though the latter shows 
its higher specialisation for a sa- 
prophytic habit in its filamentous 
development and in its frequent 
branching. On the other hand, as 
regards the sporophyte, it has been 
seen that the Phlegmaria type is not 
one of the highest developed, but is 
associated with Selago in the sub- 
genus Urostachya. This being so, it is natural to find their embryos so 
similar, notwithstanding the difference in specialisation of the prothalli 
themselves. Lastly, both embryos are from the first subaerial : their first 
leaves are green assimilating organs, and differ in no essential degree from 
the normal foliage leaves. This may probably be held to be a primitive 
condition. 

But in the davatum-annotinum-ty^e the case is different. It has been 
seen that there the prothallus is developed underground, often at a considerable 
depth, and this brings with it modifications of the embryogeny. The first 
steps in the development are the same as in the types described above 
(Fig. 1 86 A); but very soon there is a conspicuous enlargement of the 
tissue of the foot, derived from the lower tier, adjoining the suspensor 
(Fig- i86B): a large spherical swelling is thus formed, which remains as 
an intra-prothallial haustorium (compare Fig. 1796 with Fig. i86c). The 




FIG. 184. 

Prothalli of Lycopodiunt Selago with seedling plants 
X 3. (After Bruchmann.) 



34$ 



LYCOPODIALES 



upper tier meanwhile progresses only slowly : two opposite leaves, one on 
either side of the stem-apex, appear late as compared with other species, 



B. 



A -m 




FIG. 185. 

Embryos of Lycopodium Phlegmaria, in longitudinal section. p = foot ; c = cotyledon 
T = apex of axis. The wall marked II. in A and B corresponds to the wall marked IV. in 
Bruchmann's drawings (Figs. 183, 186) and to the wall b, b in Fig. 182. C and D represent 
older stages : in D, the cotyledon (c) has been followed by a plumular leaf f\ and the apex 
of the axis lies between them. A and B X 200. CX3J. Z>X2oo. (After Treub.) 

their position relatively to the foot and to the suspensor not being constant 
(Fig. i86c). This is ascribed by Bruchmann 1 to inconstancy of the foot: 

l L.c., p. 46. 



EMBRYOLOGY 



349 



its greatest development is nor> always in the plane of the median wall, 
but on that side from which the greatest quantity of nutriment flows from 
the prothallus, and this brings about a torsion which the suspensor does 
not prevent. In fact, the "foot" is here an opportunist growth, inconstant 
in position itself, and distorting in a variable manner the rest of the embryo. 
Soon after the origin of the first two leaves follows the origin of the first 







FIG. 1 86. 

A young embryo ot Lycopodium annotinum. I.-I. =the basal wall; Il.-II. =the 
transverse wall; IV. -IV. =the wall separating the foot-tier from the stem-tier. Z? = an 
older embryo of L. clavatum, showing more advanced development of the two tiers, and 
especially of the foot-tier. L =an older embryo detached, with cotyledons (BJ i), a further 
leaf (Biz), and the firsj; root (W), and foot (F). D young underground, colourless 
seedling ; ^=foot ; JV=root ; /F 2 = origin of a second root ; Bl leaf-scales, of which the 
first pair are the cotyledons. A and 2>X 150. 0x52. DX.IO. (After Bruchmann.) 

root, in a position variable relatively to them (Fig. i86c). The embryo 
then bursts the tissue of the prothallus, as a consequence of active inter- 
calary growth of the hypocotyl, which emerges upwards, while the root 
enters the soil downwards (Fig. 1860). The axis while growing through 
the soil is pale, and bears only colourless scale-leaves, but on emerging 
ultimately at the surface these pass into green leaves of the ordinary foliage 
type (Fig. 179 A). The embryology thus described is more complex than 
that of the Selago type : its details are plainly in accordance with the 
saprophytic specialisation of the prothallus, and with its position deeply 



350 



LYCOPODIALES 



sunken in the soil. The embryo is long dependent for nourishment entirely 
upon the large prothallus ; hence its swollen haustorial foot, which is de- 
veloped most strongly in the direction of the largest nutritive supply, reacting 
meanwhile upon the disposition of the other parts of the embryo : in point of 
origin this is the consequence of unequal turgid distension and division 
of cells of the foot-tier, which in the Se/ago-type remain small. The first 




FIG. 187. 

Lycopodium cernuum. Young embryo emerging from the prothallus. a?- = neck of 
archegonium ; s = suspensor ; I. -I. basal wall, corresponding to b, b in Fig. 182, to II. -II. 
in Fig. 185, and to IV.-IV. in Figs. 183 and 186 ; cot- cotyledon ; tub - tubercle of 
protocorm. X 300. (After Treub.) 



leaves here an opposite pair, though in other species there is a single 
cotyledon are only scale-leaves, which may serve for protection of the 
apex in forcing its way upwards through the soil; but this is only a 
derivative function, and it can hardly be doubted, after comparison with 
the embryo of L. Selago, that the foliage character of the first leaves was 
the prototype, and that the early formation of colourless scale-leaves in the 
davatum-annotinum-\y\& is a concomitant of the subterranean habit adopted 
by their prothalli. 



EMBRYOLOGY 351 

There remains the type of elnbryogeny of Z. cernnum? shared in all 
essentials by L. inundatum.- Here the initial steps appear to be like 
those of other species, but the lower tier of cells which elsewhere forms the 
foot remains small, and as a body consisting of but few cells it serves to 
maintain a connection with the parent prothallus (Fig. 187). The upper tier 
as usual originates the several parts of the embryo : breaking through the 
prothallial tissue it emerges early as a free-growing structure; but it swells 
early into an undifferentiated tuberous body, the "protocorm," which is 
roughly spherical in form, composed exclusively of parenchyma, and attached 
to the soil by root-hairs. It is occupied by a symbiotic fungus. However 
similar to the swollen foot of the clavatum-^^e this " protocorm " may 
be, it is essentially a body of different origin : the foot springs from the 
lower tier of the embryo, and remains intra-prothallial : the protocorm 
originates from the upper tier, and is extra-prothallial. It was at first 
regarded as a foot which had quitted the prothallus; but developmentally 
it is distinct, while there is no evidence that an escape of the foot from 
the prothallus ever took place. The protocorm must therefore be held 
to be a body different in origin and nature from the foot in the davatum- 
type. The part of the " protocorm " directed upwards bears a conical 
papilla of tissue, which develops into a cylindrical cotyledon : this is a 
green assimilating organ, with or without vascular tissue : it is succeeded 
by other leaves of similar type, which are, however, indefinite both in 
number and in position (Fig. 188). Relatively late the apex of the axis 
is recognised : its position is described as being near to the latest formed 
leaf, and the subsequent leaves arise from it in the usual acropetal succession, 
thus constituting the normal shoot. Close to its base the first root is also 
formed, and thus the normal plant is at length established. 

The existence of a tuberous stage, prior to the establishment of the 
normal sporophyte in these species, has given rise to Treub's well-known 
Theory of the Protocorm, while the very similar structure which is found 
perpetuated, and annually repeated in the life of Phylloglossum, added 
interest to the question of the real nature of the tuber in Z. cernuum ; 
but before its nature is discussed, it will be well to describe the leading 
facts in Phylloglossum. The prothallus of Phylloglossum appears, from the 
description of Thomas, 3 to be of the cernuum-type, but it resembles most 
nearly that of Z. inundatum : it has, however, no leaf-like assimilating 
lobes on the green crown, which projects above the soil. The archegonia 
appear upon the assimilating crown, and produce an embryo which is 
sim ilar to that of Z. cernuum : it projects early from the prothallus, the 
cot yledon being the first part to emerge : this develops as a green assimilating 
leaf similar to those of subsequent years. A "protocorm" is formed at 
ic e below the first leaf, and apparently in the same manner as the 
idult plant forms its tuber. No root has been observed during the 



Buit. Ann., viii., p. i. 2 Goebel, Bot. Zeit., 1887, p. 183. 

3 Proc. Roy. Soc., vol. Ixix., p. 285. 



352 



LYCOPODIALES 



first year's growth. From the description of Thomas it thus appears that 
the embryology is just what would be expected of a plant which had 
already been recognised as repeating in its annual cycle a development 
similar to that of L. cernuum. 

The yearly growth of Phylloglossum resembles in many features that 
of the embryo : it originates at the apex of the storage-tuber formed 
during the preceding year, and its punctum vegetationis retains its identity 
as the centre of the new growth. Sometimes only a single leaf is formed, 






FIG. 1 88. 

A and B embryos of Lycopodium cernuum, showing protocorm. j^suspensor; 
/ = foot; cot = cotyledon ; _/', _/' 2 , etc. = successive leaves; ? root; vtpunctinn 
vcgetationis. X 35. (After Treub.) 

but usually several more in strong plants : they arise in succession laterally 
around the apex, but are definite neither in number nor in position. In 
those cases where the plant does not form a strobilus, the apex, which 
lies centrally among the leaves, becomes depressed, while the tissue 
surrounding it, continuing to grow actively but unequally, a process is 
formed which develops into the new tuber (Fig. 189 A, B). Where a 
strobilus is formed it arises directly from the apex (Fig. 189 c, D, E), and 
a new provision has to be made for the formation of the new tuber. 
This appears adventitiously at the base of the peduncle, as a depression 
which is carried outwards on an elongating process due to active and 



EMBRYOLOGY 



353 






unequal growth, as in the previous case (Fig. 189 F, G). Comparing 
the tuber of PhyUoglossum with the protocorm of L. cernuum, it is clear 
that the relations of both to the protophylls and to the definitive axis 
are the same : further, the relation of the foot in the embryo to the 
protocorm is as that of the stalk to the tuber in the perennating 
PhyUoglossum. It follows that the tuber in PhyUoglossum may be held to 
be a " protocorm " repeated 
annually in the life-cycle. 

In Treub's description for 
Z. cernuum, the origin of the 
definitive apex of the axis is 
not brought into relation to 
the primary segmentation of 
the embryo. His account of 
it is that " at the end of 
the second phase the tubercle 
ceases to grow, and its point 
of vegetation gives rise to 
the vegetative cone of a leafy 
Lycopod-shoot," etc. 1 Nor is 
the origin of the axis clearly 
made out by Goebel for Lye. 
inundatum, though its close 
relation to the cotyledon is 
again recognised.' 2 But the 
continuity of existence of the 
apex, which may be traced 
throughout the development 
in PhyUoglossum, suggests a 

i v T plants, showing arrangement of the protophylls. E, a plant 

Similar Continuity in L. Cer- forming a strobilus. F, G, similar plants, older, showing mode 

a,-,r] /*f/Wxr/s4 T of origin of new tuber; /= protophylls ; = apex ; / = tuber; 

nnum ana initnaatum. 1 r =root. A-EXI*. FX^. GXZ. 
venture to think that a 

renewed investigation of the embryology of these species, especially in 
their simpler types, would bring them into line with other Lycopods, and 
show that the apex originates as in them from the central point of the 
upper tier of the embryo ; but that the assertion of its characters is 
correlatively deferred, and its identity disguised by the early prevalence of 
the tuberous swellings and consequent irregularity of the first leaves. 

Treub's theory of the " protocorm " has already been considered in 
Chapter XVII. Reasons were there given for not sharing the opinion 
that the tuberous developments seen in the embryogeny of L. cernuum 
and inundatum represent a primitive condition once wide-spread. The posi- 
tion was not accepted that the " protocorm " embodies an early evolutionary 
step towards the establishment of a free sporophyte prior to the formation 




FIG. 189. 

Drummondii. A, B, frontal, and side views 
of young plant which will not form a strobilus. C, D, similar 



/,.('., p. II. 



-L.C., p. 184. 



354 LYCOPOD1ALES 

of a root. An alternative view was there propounded that the Lycopod 
embryo is a body prone to parenchymatous swelling, and that the 
"protocorm" is a consequence of secondary specialisation. It remains to 
group the facts of embryogeny in the eligulate Lycopods in accordance 
with that alternative view. 

The simplest type of embryogeny in the genus is that of Z. Selago, 
a species already recognised as primitive in the characters of the sporophyte. 
The embryo accommodates its growth in length to the level of its parent 
prothallus ; excepting for this the embryo is of a constant type, without 
any complications of parenchymatous swelling. I regard this as a primitive 
condition for the genus, and the main features are these : a suspensor 
and foot of moderate size, passing directly into the primitive shoot, which 
escapes early from the prothallus, and expands its first leaves as green 
assimilating leaves. The apex of the axis, which provides a definite leaf- 
succession, is established early at the centre of the upper tier ; the first 
root is formed early and exogenously, and it is followed soon by others of 
endogenous origin. Thus the young plant is simply and directly set up 
as an independent unit (Figs. 183, 184). The type most nearly corre- 
sponding to Z. Selago is that of Z. Phlegmaria. Notwithstanding the pro- 
nounced saprophytism of the prothallus, the embryogeny is practically 
identical in all essentials with that of Z. Selago, though more exactly 
worked out (Fig. 185). But it is different with the davatum-annotinum-\y^e. 
Here the primary embryogeny is the same as in Z. Phlegwaria, but the 
deeply underground position of the saprophytic prothallus necessitates longer 
and more efficient nursing of the embryo before it can establish its 
physiological independence. The absorptive surface and storage capacity 
of the embryo are accordingly enlarged by parenchymatous swelling of 
the foot. The directness of the adaptation is here indicated' by the fact 
that the enlargement is on whatever side is nearest to the greatest source 
of supply. The late differentiation of the several parts, and the tardy 
emergence of the embryo from the prothallus, are all in accord with the 
necessarily longer nursing period : while the colourless scale-character of 
the earliest leaves is also a natural and secondary consequence of the 
subterranean embryogeny. It is not difficult to see in the clavatiim-\ypz 
an embryogeny essentially like that of Selago, but secondarily modified in 
relation to the subterranean habit of the prothallus. This accords well 
with the fact that the species included are more highly specialised than 
Z. Selago as regards the characters of the sporophyte (Fig. 186). 

The ctrnuum-inuhdatum-\ypz on general characters of its prothallus and 
sporophyte takes a middle position. The embryogeny opens as in other 
Lycopods : but the foot-tier, which is enlarged in the davatum-\y^z, here 
remains small. The origin of the cotyledon is as in Z. Selago, but the 
swelling in the upper tier, which begins early on the side directed down- 
wards, profoundly disturbs the subsequent arrangements, so that detailed 
comparisons become difficult, and, as a consequence, the origin of the axis 



EMBRYOLOGY 355 

is still obscure. The type of teaf seen in the cotyledon is repeated in 
the "protophylls," but without definiteness of position or number upon the 
enlarging tuber : their sequence is closed at last by the activity of the 
stem-apex, close to which in time and in position the first root appears. 
It is as though a rootless phase of morphological anomaly, initiated by the 
parenchymatous swelling in the upper tier, were intercalated in the regular 
embryogeny of the Selago type, immediately after the origin of the cotyledon : 
and after a period of digression the normal embryogeny were then resumed. 
The swelling is associated in L. cernuum and inundatum with the entry of 
a mycorrhizal fungus, which occupies the tuber : it must at present remain 
uncertain whether or not this symbiotic state is the cause or a mere 
concomitant of the tuberous condition : and what the relation of it to 
the late appearance of the root ; but given the tuberous state, the other 
anomalous foliar conditions readily follow. The proneness of the Lycopod- 
embryo to such secondary swelling as contemplated is seen also in the 
embryos of the clavatiim-\y\)e : it is also shown by the repetition of such 
swelling upon the roots in Z. cernuum itself, as have been fully described 
by Treub. 

The ferwtum-type of embryo is shared by L. inundatum, but not in 
its extreme form. It is this species rather than L. cernuum itself which 
gives the link to Phylloglossum. The strobilus of the latter is like a very 
simple strobilus of L. inundatum : this species, as is well known, perishes 
in winter, excepting the tip of the trailing stem, which perennates. If 
such a condition were still further prepared for, and condensed by the 
formation of an adventitious protocorm in cases where the plant has been 
fertile, or of a similar body as the product of direct apical growth where 
the plant of the previous year was sterile, the condition of Phylloglossum 
would be attained. It is interesting to note in this connection that Goebel 
has found that adventitious protocorms are formed in L. inundatum, a fact 
which strengthens the suggestion here made. 1 It would thus appear that 
Phylloglossum, so far from being a prototype of Lycopodinous development, 
is more probably a specialised offset from it. I still adhere to my thesis as 
stated in 1885, that "it is a permanently embryonic form of Lycopod." But 
it may now be added that the characters which it repeats each year appear 
to be those of a secondary rather than of a primitive embryonic type. 

And thus the embryogeny of the Eligulate Lycopods, so far as at 
present known, conforms to a single central scheme with variations upon 
it. The type of L. Selago, the only species of the "Selago" section of 
the genus in which the embryo has hitherto been observed, is held to 
be the most primitive, as it is certainly the simplest. The rest may be 
held to be secondary variants on that type, due to changes for the 
most part biologically intelligible. 

^ Bot. Zeit., 1887, Plate II., Fig. 32. 



356 LYCOPODIALES 

(B) LIGULATE LYCOPODIALES. 

It is an unfortunate circumstance that the embryogeny of fossils is 
usually inaccessible, for that of the dendroid Lycopods would greatly 
strengthen views as to their relation to modern forms. As it is, Sela- 
ginella and Isoefes provide the only facts of the embryogeny in the 
Ligulate Lycopods : it will be seen that they are strangely divergent in 
the form of the embryo. 

It has been pointed out that Selaginella spinulosa may be held to 
be more primitive as regards the morphology of the mature plant than 
the dorsiventral species of the genus \ and further, it has been seen that 
it differs from them anatomically, showing a vascular structure which is 
probably more primitive also. This gives a special interest to its em- 
bryology, which has been fully worked out by Bruchmann. 1 The early 
stages are essentially as in Lycopodium, resulting in a suspensor, and two 
tiers of four cells each, forming the embryo : the whole structure is at 
first straight, with the apex flattened (Fig. 190 A, B). The susp'ensor 
remains as in Lycopodium, and is a means of thrusting the embryo down- 
wards into the tissue of the prothallus. The lower tier of cells of the 
embryo (i.e. that between walls i., i. and iv., iv. in Fig. 190 A and c) itself 
forms the hypocotyl, which may here be greatly elongated and curved. 
and becomes thus a prominent feature of the embryo ; at its base, in 
close relation to the suspensor, the first root arises in a lateral position. 
The products of the upper tier at first remain small (i.e. above wall 
iv., iv., Fig. 190 A, c) : the formation of the first cotyledon may in some 
cases be long delayed, sometimes it may still be wanting even when the 
axis has already curved obliquely to the suspensor. The second cotyledon 
may be even longer delayed : in some cases it only appears after the shoot 
issues from the spore. But sooner or later two opposite but unequal 
cotyledons successively make their appearance. Their orientation relatively 
to the suspensor is liable to vary. The apex of the axis, 'which has no 
single initial cell, lies between them, originating from the centre of the 
flattened apex of the embryo (Fig. 190 A, c, D). As the hypocotyl 
elongates the embryo curves so that the axis takes a vertical position, while 
the suspensor is pushed to one side by the growing root. Finally the 
shoot emerges above ground, and the two cotyledons, developing at last 
to equal size, appear as green assimilating leaves (Fig. 190 G, H). 
According to Bruchmann, no enlarged " foot " is formed in this species, 
and the same appears to be the case in Selagintlla apus. 

Comparing this embryogeny with that so well known in S. Martensii, 
there is essential similarity in the disposition of the parts. The chief 
difference lies in the presence of an haustorial swelling of the hypocotyl 
in S. Martensii, which has been called a "foot," and in the fact that 
single initials are found at the apices of stem and root : this accords 

^ U liters, ueber ''''Selaginella spinulosa," Gotha, 1897. 



EMBRYOLOGY 



357 



with the mode of development^ of these parts in mature plants. For 
the variability in haustorial development within the genus the study of 
the embryology of Lycopodium has already prepared the way. Both genera 
demonstrate the inconstancy of the haustorial organs of the embryo, and 
justify my conclusion of more than twenty years ago, that these swellings 




FIG. 190. 

Embryos of Selaginelfyi spinulosa. A-D illustrate the segmentation. I. I., first wall, 
separating the suspensor ; IV. IV., corresponds to wall similarly marked in Figs. 183, 186, 
and to wall 6, b in Fig. 182; /5 = wall marking off the vascular strand of the axis. 
A'j A' 2 cotyledons ; = ligule; W=root. E = section of germinated spore with embryo 
in situ. G, H seedlings. H, natural sixe ; G, enlarged. F the basal knot enlarged ; 
Et - suspensor ; W\ W. 2 -roo\.s. (After Bruchmann.) 

of the hypocotyl arise when and where they are required, and are not 
to be held to be clearly denned or constant morphological members. 1 In 
both of the points named it would seem probable that 6". spi?mlosa 
represents a more primitive type than S. Martensii. 

Comparing this embryogeny with that of Lycopodium, it seems remark- 
able that the similarity of detail should be so great when the difference 

1 Quart, fourn, Micr. Sci., xxii., p. 292, etc. 



358 LYCOPODIALES 

of the parent prothalli is so marked ; the difference being between a 
free-growing, self-nourishing bisexual prothallus in Lycopodium, and an 
endosporic, unisexual, storage prothallus in Selaginella. The early form 
and structure of the embryo, consisting of a pluricellular suspensor and 
two tiers of cells of the embryo, is virtually the same in both cases. In 
the later development the nearest similarity is between L. Se/ago and 
S. spinulosa : there is in both a marked elongation of the hypocotyl, with 
the first root originating laterally near its base : there is the same absence 
of any determinate foot : and as a rule the same origin of a first cotyledon 
laterally, with the apex of the axis between it and the next-formed leaf. 
The axis thus originates in both in close relation to the intersection of 
the primary segmentation-walls of the upper tier of the embryo. 1 One 
point of difference is in the part played by the lower of the two tiers 
of cells of the embryo : in Lycopodium it remains in close relation with 
the suspensor, and may be more or less swollen into a foot in some 
species ; but it does not elongate, or form any permanent part of the 
embryo, the whole of which originates from the upper tier. But in Sela- 
ginella the lower tier elongates to form the greater part of the hypocotyl, 
while the first root originates from its base. The position of the root 
relatively to the other parts is otherwise alike in L. Selago and in S. spinulosa. 
The similarity of the embryogeny of the genus Selaginella to that of 
Lycopodium is thus established by comparison of species both of which 
are held to be primitive in their respective genera, on the basis of com- 
parison of their mature sporophytes. 

At first sight the embryogeny of Isoetes seems to differ radically from 
that of Selaginella. notwithstanding that the endosporic prothallus is so 
similar in both cases. The key to the difference is first the inversion 
of the embryo, as compared with Selaginella, and secondly, the entire 
absence of any representative of the suspensor : what remains in Isoetes 
may be held to correspond to the product of the two upper embryonic 
tiers only. 2 

The first division of the zygote is by a wall more or less inclined to the 
axis of the archegonium, but occasionally almost including it (Fig. 191 B) : 
this indeterminate position of the " basal wall " is theoretically important, 
as bearing on the inversion of the embryo in the archegonium as com- 
pared with that in Lycopodium or Selaginella : the variations seen in 
Isoetes suggest how that inversion may have come about. The two tiers 
thus initiated are usually called the hypobasal and the epibasal ; but the 
octant divisions commonly seen in other embryos are not always clearly 
defined in Isoetes. The hypobasal tier here forms the foot only : all the 

1 On this point the facts have been better made out in /_. phlegmaria and L. clavatuni 
than in L. Selago ; but the facts for the latter, so far as they go, are consistent with the 
same conclusion. 

2 See Campbell, Mosses and Ferns, p. 545, etc. Also Kienitz-Gerloff, Bot. Zeit., 
1881, p. 761. 



EMBRYOLOGY 



359 



other parts originate from the epibasal tier : the cotyledon with its ligule 
is the first part to be organised : it is followed soon by the first root, 
which arises at the opposite side of the epibasal tier to the cotyledon 
(Fig. 191 c, D, E, F). Between these parts a slight depression is formed, 
and it is surrounded by a semicircular ridge : within this the apex of the 
axis is at last organised, and it soon gives rise to the second leaf, which 




St.,' 



FIG. 191. 

Isoetcs echinospora, var. Braunti, Dur. X 365. /4=an archegonium. B = a. two- 
celled embryo within the archegonium. D, E, F= three successive horizontal sections 
of a somewhat advanced embryo; /? = root; cot cotyledon ; st-^t&m ; /=ligule. x 175. 
C = median longitudinal section of a young embryo; /=ligule. X 200. G = median 
section of a young sporophyte with second leaf, h, already formed; r 2 = second root; 
*^ = stem apex, x 150. (After Campbell.) 

faces the cotyledon : the very exiguous apical cone lies between them 
(Fig. 191 G). And so the shoot is established, bearing successive leaves 
with spiral arrangement, and successive roots, of which the second is below 
the base of the second leaf. 

Comparing this apparently divergent embryogeny with that of the 
Lycopods above described, if the whole embryo be imagined inverted 
in its orientation relative to the archegonial neck, and their suspensor 
be imagined entirely away, then the two embryonic tiers may be com- 



360 LYCOPODIALES 

pared with those seen in Isoetes. Here, as in L. Selago and L. Phlegmaria, 
the hypobasal tier forms the suctorial organ only, and takes no direct 
part in the establishment of the plant. The epibasal tier is like that of 
L. Selago as regards the parts which it initiates and in the positions 
which they severally hold, but differs in its growth in length being 
stunted, and in the early ascendancy of the cotyledon, which condition it 
shares, however, in some measure with L. Phlegmaria : it differs also in 
the late definition of the apex. But the position of the latter relatively 
to the whole embryo is the same, for the stem originates in close 
relation to the centre of the upper tier of the embryo, as it does in 
all the Lycopods where the embryogeny has been exactly followed. The 
apical cone is small in bulk and late in appearance, these being probably 
correlative consequences of the early advance of the cotyledon. It is 
thus possible to see even in the embryo of Isoetes some clear relation 
to the plan which, with such curious modifications, underlies the embryo- 
geny of the Lycopods. 

i 

We are now in a position to enunciate a comparative view of the 
embryogeny as known in the Lycopodiales, and to state it so as to place 
the several curiously divergent types in what is believed to be their 
natural relation to a probable primitive embryogeny. In so doing pro- 
minence is given to the more constant features, while only a subsidiary 
place is given to those characters which are less stable. 

In those Lycopods in which the embryogeny has been exactly followed, 
the embryo consists of a suspensor and two tiers of four cells each 
composing the embryonic body : the two tiers are separated by a wall 
which may be called the "basal" wall (Fig. 182 b, /;). This seems to be 
a general condition, subject only to minor modifications : in Isoetes, 
however, the suspensor is entirely wanting. In the very various develop- 
ments which follow, the most constant feature is undoubtedly the close 
relation of the stem-apex to the point of intersection of the octant-walls 
in the epibasal tier. In the simplest cases the axis of the embryo is 
thus defined at once as lying between that point and the base of the 
suspensor. The whole embryo is thus primarily a spindle-like body, and 
this may be held to have been the primitive condition for them all. 

But this simple form is subject to early modifications, which disguise 
the position of the axis by delaying its apical growth, and by distorting 
the form : so much so that the position and identity of the apex is liable 
to be lost. The least distorted types are those of L. Selago (Figs. 183, 184) 
and Phlegmaria (Fig. 185), and of S. spitiulosa (Fig. 190), all plants which 
are relatively primitive in their genera as recognised by the characters 
of the mature sporophyte. In L. Selago and Phlegmaria no haustorial 
swellings exist. The early development of the single cotyledon at first 
throws the apex of the axis to one side, but this is rectified later when 
the second leaf appears on the side opposite to the first. The apex thus 






EMBRYOLOGY 361 

" righted " is then carried up together with the two leaves by the elong- 
ating hypocotyl, while the first root appears laterally at its base. The 
whole arrangement is relatively simple, but illustrates a slight degree of 
distortion of the apex, which is, however, temporary only. 

In the c/avatum-type (Figs. 170, 186) the hypobasal tier is enlarged, 
and curved to one side, with correlative late differentiation of the epibasal 
region, and absence at first of localised intercalary growth. The orien- 
tation of the foot is not constant, but it is directed towards the chief 
source of nutriment, a fact which indicates its opportunist character. In 
the epibasal region the apex of the axis is clearly of central origin, 
between the two small but equal cotyledons, which appear relatively late. 
The root originates in a position corresponding to that of the former 
type. The whole embryo may be held to be a biologically intelligible 
modification of the Se/ago-type consequent on the underground habit of 
the large mycorrhizal prothallus. The enlargement of the haustorial foot 
leads correlatively to slow development of the epibasal region, while the 
first leaves, having no nutritive function, are not hurried on in their 
development so as either to distort or to produce correlative reduction of 
the apical cone. 

In the cernuum-type (Figs. 187, 188) the tuberous swelling is not in the 
hypobasal but in the epibasal region, and it profoundly disturbs its develop- 
ment. The biological cause of the swelling, which is extra-prothallial and 
liable to repetition, may be the intrusion of the symbiotic fungus which is 
present, or there may be some other reason for the tuberous development, 
associated as it is with the late origin of the root. But whatever the 
cause, the form is such as might be expected in a secondary tuberous 
modification of a green leafy shoot of a young Lycopod. The bulky 
development below, and the rapid enlargement of the assimilating leaves 
act correlatively in keeping the stem-apex inconspicuous. Its identity 
throughout the embryogeny of L. cernuum itself is not yet demonstrated ; 
but in Phylloglossum the definitive apex of the shoot has been seen to 
coincide with the apex of the tuber (Fig. 189) ; it is therefore probable 
that in the embryogeny of L. cernuum the apex of the axis is present 
in the very young embryo in the usual position, but has escaped recog- 
nition owing to its correlative diminution. The protophylls would then 
be leaves of the normal type, altered in relation to the gouty habit of the 
axis which bears them, and disposed in an apparently irregular and isolated 
fashion upon the swollen axis. The root is long deferred, perhaps in 
relation to the mycorrhizal habit; but when it does appear, its relation to 
the leafy shoot is like that which it has in the other types of the genus. 
The normal leafy shoot and the root-system, thus delayed by the gouty 
interlude called the " protocorm," ultimately continue their development 
as in other Lycopods. Thus the different types of the genus appear to 
start alike, and when established in the soil continue alike, but show 
divergent intermediate phases. The simple development of Z. Selago and 



362 LYCOPODIALES 

Phlegmaria is believed to be primitive, the turgid developments of Z. 
clavatum, cernuum and inundatnm, and also of Phylloglossum, are believed 
to be secondary. 

The embryogeny of Selaginella (Fig. 190) corresponds in all essentials to 
that of Lycopodium, and shows only minor distortion or swelling. In the 
simple case of S. spinulosa the apex of the axis originates as before from 
the centre of the upper tier : the active growth of the first leaf throws 
the apex of the axis to one side ; but it is " righted " again on the 
appearance of the second, and the identity of the apex is clearly 
maintained throughout. The whole epibasal tier is then carried upwards 
by intercalary growth of the hypobasal region, but the hypocotyl thus 
formed is without lateral swelling, and the first root originates laterally 
at its base. As regards distortions, this case is quite similar to that of 
Z. Selago or Phlegmaria. But in other Selaginellas, as exemplified by 
6". Martensii, the cotyledons arise equally, as indeed they sometimes 
do in *$". spinulosa, and the temporary distortion of the apex does not 
appear ; but a lateral swelling, absent in S. spinulosa, constitutes the 
" foot." Thus Selaginella shows only minor and inconstant deviations 
from the simple type. 

The embryogeny of Isoetes is less easily compared, but the following 
tentative suggestion is given (Fig. 191). The suspensor is entirely absent, 
and the embryo, composed only of the two tiers corresponding to those of 
other Lycopods, is usually orientated so that its apex is from the first 
directed towards the neck of the archegonium. That the rotation necessary 
to bring this about may occur is indicated by the differences of position 
of the basal wall noted by Campbell. The product of the hypobasal tier is 
the haustorial foot only : the upper tier hastens at once to form the large 
cotyledon, with the effect that the stem-apex is delayed, and remains 
minute : it only becomes clearly recognisable after the appearance of the 
second leaf opposite the first. In relative position, however, these parts of 
the shoot correspond to those of S. spinulosa or Z. Phlegmaria. The 
first root originates from the epibasal tier as in Lycopodium rather than 
Selaginella, and unlike Z. Phlegmaria and S. spinulosa on the side opposite 
to the cotyledon ; but the orientation of the root relatively to the cotyle- 
don has been seen to vary within the Lycopodiales, so no great importance 
need attach to this discrepancy. The primary embryogeny of Isoetes may 
thus be held as related to that of the other Lycopodiales, but without a 
suspensor, and greatly abbreviated, and with the apex of the axis correlatively 
reduced and delayed in its development, owing to the early production of 
the cotyledon and the root Nevertheless, its position at the centre of the 
epibasal tier is maintained. 

It is thus seen that the embryos of all the Lycopodiales may be held 
as variants on a single type, and fundamentally of spindle-like form. 



SUMMARY 363 

SUMMARY OK THE COMPARATIVE EXAMINATION OK THE 
LYCOPODIALES. 

The sporophyte of the Lycopodiales has now been studied compara- 
tively as regards its external form, its spore-producing members, its 
anatomy, and embryology. The conclusion arrived at from all these 
quarters is favourable to a strobiloid origin, with subsequent specialisation 
along lines variously divergent. By the comparison of known representa- 
tives of the Lycopodiales, living and fossil, certain characters have been 
recognised as relatively primitive, others as derivative : and thus a general 
idea has been obtained of a primitive type of Lycopod-sporophyte, which 
forms the basis of a theory how such a sporophyte came into being. In 
form this primitive sporophyte was probably a simple, unbranched, radially 
constructed shoot, endowed with unlimited apical growth, while local 
intercalary growth might also occur. The axis bore undifferen dated leaves, 
each of which had one sporangium associated with it in a median position. 
It was rooted at its base, but the origin of the root may be held 
to have been accessory in evolution, as it is seen to be late and variable 
in the individual development. The internal construction of the shoot 
showed a non-medullated monostele, continuous as a cauline column to 
the apex of the axis, while the foliar strands were inserted with but slight 
local disturbance upon its periphery. Its sporangia were kidney-shaped, 
and not greatly extended radially. The primitive body thus sketched in 
its broad outlines was derived from a spindle-shaped embryo, without 
any haustorial swelling, or tuberous protocorm. The theory of the 
strobilus, as enunciated in Chapter XL, would adequately account for 
the origin of so simple a sporophyte as this, from a still more primitive 
body, with sterile base and fertile apical region, by segregation of the 
fertile tissue into separate sporangia, and by enation of sporophylls. 

The nearest living representative of such a sporophyte which has been 
adequately investigated is Lycopodium Selago ; but it is to be remembered 
that this is the only one of 39 species of the && < &-section of the genus 
so examined, and there are indications, derived as yet from external 
characters only, that other and more primitive types than L. Selago exist 
among them : these await, further investigation. The first leaves formed 
on the embryo of L. Selago are lateral in origin, and become aerial and 
green, but are sterile : sporangia were noted by Bruchmann, 1 as first 
appearing after the second branching of the axis, which, however, is 
early as compared with the other European species, though not as 
compared with the large Andean forms. Their early appearance, as 
well as the similarity of the sterile and fertile leaves, coupled with 
the evidence of abortion of sporangia in the upper region, all point 
to the conclusion that originally all the leaves were sporophylls, while 
all arose laterally upon the axis. 

'/..f., p. 100. 



364 LYCOPODIALES 

From such a starting-point various lines of elaboration may be traced, 
open often to ready biological explanations : and these appear to have 
run in some degree parallel in the ligulate and non-ligulate series. The 
steps which may be traced on a basis of comparison are as follows : 
First, the progressive sterilisation by abortion of sporangia increased the 
vegetative region: this led to more definite specialisation of the strobilus: 
in the more advanced forms the sporophylls are no longer nutritive, but 
only protective in function, so that the differentiation of the nutritive 
from the vegetative tract has become clearly marked. The vegetative shoot 
once distinct from the propagative strobilus was susceptible of various 
specialisation. In the dendroid fossils it attained large size, with secondary 
increase of its tissues, both stelar and extra-stelar, but still it maintained 
its radial symmetry. In the smaller forms, the straggling or climbing 
habit led not uncommonly to dorsiventral development, which occasionally 
extended to the more conservative strobilus itself. Such advances were 
accompanied by various elaboration of the vascular tissues, such as 
medullation, disintegration into separate strands, or even into meristeles. 
But these are all referable back in origin to the primitive monostele, just 
as the variations of external character are referable by comparison to the 
primitive strobilus. 

The sporangia all conform to one general fan-shaped type, with 
singular constancy of number and position relatively to the leaves. But 
the dimensions vary, and at least in Lycopodium there is a relation 
between the size of the sporangium and the definition of the strobilus : 
where the shoot is undifferentiated, as in L. Se/ago, the sporangium is 
radially compressed : where the strobilus is clearly defined, and the 
vegetative region more specialised, as in L. clavatum or alpinum, it is 
radially elongated. The most extreme cases of this are found among the 
ligulate forms, as in the dendroid fossils with their ample vegetative 
system. But, on the other hand, this relation is not constant, for 
the sporangia of Isoetes are radially elongated, though there is no 
differentiation of the strobilus, while the sporangia of Selaginella are 
compressed, though the strobili are clearly defined. One of the most 
interesting points in these large sporangia is the partial sterilisation of 
their sporogenous tissues, probably to meet mechanical and nutritive 
requirements : sterile trabeculae are thus formed in the sporangia of 
Isoetes, and in certain Lepidostrobi. This leads towards a condition 
of septation, but in the Lycopods the step is never taken to complete 
partition of the sporangium. Finally, the heterosporous differentiation is 
probably a condition assumed after the character of the sporangium was 
already defined, and it has not greatly affected the general morphology 
of the shoots where it has occurred. 

In the eligulate series the embryo is simple and spindle-shaped. In 
L. Selago. which on other grounds is regarded as a primitive type, it 
grows directly and without complications into the seedling, with its green 



SUMMARY 365 

assimilating leaves. In other cases it shows various modifications. Where 
the thallus is buried deeply underground, as in L. clavatum, the lower tier 
of the embryo enlarges as an haustorial foot, while the first leaves are 
modified into colourless protective scales, evidently a secondary condition. 
In the iw;//////-type and in Phylloglossum a distinct extra-prothallial 
swelling appears in the upper tier of the embryo, disturbing the position 
and even the arrangement of its parts. Since the first stages of this 
embryo resemble those of other Lycopods, and since the normal shoot, when 
ultimately established, is also of the usual Lycopod type, it is concluded 
that the swollen stage, styled the "protocorm" by Treub, is a gouty 
interlude, introduced secondarily into the normal development, and not a 
stage of general significance. In the ligulate series, Selaginella spinulosa, 
which is held as a relatively primitive type on comparison of its mature 
sporophyte, the seedling is very similar to that of Z. Selago, notwithstanding 
the striking difference of their prothalli. But the simple spindle-form 
which it shows is departed from in other species, by the lateral formation 
of a swollen haustorium : this "foot" is again held to be a secondary 
development. The apparently divergent ernbryogeny of Isoetes is carried 
out without a suspensor, but the position of the parts in relation to the 
greatly abbreviated axis is essentially similar to that in Selaginella. It thus 
appears that in both series the most primitive type has an embryo in 
the form of a simple spindle : it forms its first leaves as normal green 
foliage leaves, and those species in which this is departed from are held 
as the result of secondary modification. The first foliage leaves in these 
simple forms differ in no essential respect from the subsequently formed 
sporophylls, except in the absence of the sporangium. Hence the observed 
facts support the view that all the leaves were originally sporophylls, and 
the whole plant originally a simple strobilus. 

It has thus been seen that a strobiloid theory is applicable to all 
known types of the Lycopodiales. This matter has been dealt with at 
:onsiderable length because, in the first place, this phylum of Vascular 
Plants dates back fully as far as any other in the Palaeontological record. 
Secondly, because it is represented by many living species susceptible ot 
minute investigation throughout their life-cycle : and, thirdly, because these 
and the fossils together how gradual, and at the same time considerable 
divergence of detail in the one uniform scheme. They thus provide a 
better basis for comparison than any other series of Pteridophytes of equal 
age. The conclusions arrived at will be susceptible of comparison with 
those relating to other phyla of Vascular Plants. But though the applica- 
tion of the theory of the strobilus may be extended to other phyla, it 
must be remembered that the arguments and conclusions relative to the 
Lycopodiales stand by themselves, and would still be equally cogent if no 
other Vascular Plants existed on the earth's surface. 



CHAPTER XXVII. 

SPORANGIOPHORIC PTERIDOPHYTES. 

I. EQUISETALES. 

UNDER the common designation of the " Sporangiophoric Pteridophytes " 
may be grouped together those forms whose sporangia are disposed, either 
singly or in larger numbers, upon more or less elongated vascular stalks, 
which are enlarged as a rule at their distal ends. The existence of the 
sporangiophore clearly distinguishes these plants from the Lycopodiales, 
though it may for the present remain an open question whether any 
genetic connection existed between the latter and the sporangiophoric types. 
Under this designation are included the Equisetales and the Sphenophyllales 
(incl. Psilotaceae), while, according to the view which will be developed 
below, the Ophioglossales will also appear as an outlying group sharing 
the same character, though in a more elaborated form. It will be a 
matter for later discussion how far the existence of the sporangiophore 
as the immediate sporangium-bearing member will supply a valid basis 
on which to trace affinity : the decision must rest on the degree of 
correspondence of the sporangiophoric types in other characters, such as 
the external morphology and anatomy of the vegetative organs, and the 
details of the gametophyte. Unfortunately these are often so imperfectly 
known that we are thrown back in great measure upon the spore-producing 
members : but on grounds previously explained these are held to be the 
most important of all. 

The Equisetales, which are taken first of the sporangiophoric types, 
are distinguished from the rest by the fact that their sporangiophores are 
inserted directly upon the axis, not on appendicular parts : in some 
cases they show a definite relation to the bracts which subtend them : in 
others no such relation exists. Other less distinctive characters of the 
vegetative organs are the constantly radial construction of the shoot : 
the elongation of the internodes which are longitudinally striated, the 
verticillate arrangement of the leaves, a high degree of branching, and a 
structure of the stele with a ring of isolated vascular strands ; these 



EQUISETALES 



367 







FIG. 192. 

Equisctiun pratcnse, Ehrh. Rhizome with unbranched fertile shoots (a), a fertile 
shoot which has begun to form branches (b), and a young sterile shoot (c). Natural size. 
(After Duval-Jouve, from Rabenhorst's Krypt. Flora.) 



368 



EOUISETALES 



collectively characterise the group as a definite one. As regards its past 
history, the evidences of the existence of the Equisetales extend back to 
the Devonian period, where they already showed a high degree of 
elaboration. But these plants formed a more conspicuous feature in the 
Carboniferous Flora, where they attained their maximum development in 

point of numbers as well as in 
size. Subsequently the type 
became less prevalent, till at 
the present day it is repre- 
sented only by the cosmo- 
politan genus Equisetum, with 
its twenty-four species, showing 
remarkable uniformity of type. 
The essential characteristics of 
the living genus will be taken 
first, as it is susceptible of more 
complete study than the fossils : 
these will be worked in on a 
basis of comparison with what 
is seen in Equisetitm itself. 

EXTERNAL CHARACTERS. 

It will be unnecessary to 
describe the characters of the 
shoot in Equisetum in full 
detail, or the comparatively 
slight modifications of it upon 
which the species are distin- 
guished : a brief account will 
suffice to indicate the essential 
features, for beneath them all 
lies a general unity of plan 
which is closely followed, 

whptVipr trip srinnr HP nnHpr 
WHCtHCr 

g round or exposed to the air 
(Fig. 192). The axis is plainly 

the dominant feature of the shoot, and it is always of radial construction : 
it is terminated by a conical apex with well-marked initial cell. Upon the 
vegetative axis the leaf-sheaths arise laterally, in close acropetal succession : 
they are webbed from a very early stage, and when mature consist of 
clearly marked leaf-teeth projecting upwards from the webbed sheath below 
(Fig. 193). As the developing internodes lengthen by intercalary growth 
of the bud thus constructed the leaf-sheaths separate, while the internodes 
themselves are then seen to be marked by flutings corresponding to the 




FIG. 193. 

Equisetum maximum, Link. Left half of a radial longi- 
tudinal section below the apex of an underground bud (in 
September). vK, lower part of the apical cone ; b 1 , b" ', b'" = 
leaves; z = pith; v, z/ = meristematic ring; g; g= cell-layer 
from which the bundles of the leaf-teeth arise ; /, z"=the first 

ind 



- , 

Sachs> fr m Engkr 






GENERAL MORPHOLOGY 369 

markings of the leaf-sheath next above : at the nodes it is clearly seen that 
the teeth of the successive leaf-sheaths alternate. The leaves themselves 
are mostly dry and chaffy, while the tissues of the stem contain chloro- 
phyll, and constitute the chief assimilating tissue of the plant. The 
number of teeth in the sheath, their proportions, and their permanence 
or deciduous character may vary : the internodes may be swollen for 
storage purposes in underground stems, while on the aerial stems the 
extent of the chlorophyll-parenchyma, and the number and disposition 
of the stomata may fluctuate; but, putting aside such differences, which 
are only of secondary importance, the plan of the shoot is the same 
in all living Horsetails. It is a notable fact that in none of them is there 
any departure from the radial symmetry of construction of the shoot, or 
from the verticillate disposition of the leaves. 

The normal branching of the shoot is exclusively monopodial, 1 and 
originates from cells lying immediately above the leaf sheaths, and in a 
position alternating with its teeth (cells marked /, /, in Fig. 193); the 
branches are therefore not axillary. The shoots thus initiated burst 
through the subtending sheath, giving the appearance of an endogenous 
origin, and on further development they repeat, though usually on a 
simplified scale, the characters of the original shoot. Such branches are 
not initiated at every available point intervening between the leaf-teeth : 
moreover, where they are initiated, they are frequently not developed 
beyond the earliest stages, in which case there may be no external sign 
of their presence. The branches thus formed are plainly accessory to the 
parent shoot, while they repeat its characters : they are not to be held 
as any necessary constituent part of the parent shoot, but as parts added 
to those of the simple shSot itself. 

The roots, excepting the primary root of the embryo, are formed in 
regular relation to the accessory buds above described : one root is 
initiated at the base of each bud, and thus the roots, though formed 
like the buds in definite positions relative to the other parts, are 
held none the less to be accessory also. Their further branching is 
monopodial. 

Both roots and shoots are susceptible of different degrees of development 
according to circumstances, with results which lead to striking external 
differences ; and upon these the specific distinctions are partly based. 
Either shoots or roots may remain dormant though initiated : this is 
especially seen in the case of the roots in aerial parts, and of the lateral 
shoots in the parts that are underground. This circumstance provides 
specific characters : thus, in some species many or all of the branches 
may remain dormant, even on the aerial stems (e.g. E. limosum and 
hiemale). It also contributes largely to the general aspect of the individual 

1 Occasional terminal branchings have been described, especially in the region of the 
strobilus, which would be comparable with the terminal branchings in the Lycopodiales, 
but they are sufficiently uncommon to be held as abnormalities. 

2 A 



370 EQUISETALES 

organism, as is clearly seen in the case of such species as E. pratense 
(Fig. 192). In other species again the development or non-development 
of the branches differentiates the vegetative axes from those which are 
fertile, as in E. aruense and maximum : in others the lateral branches 
on fertile axes are only delayed in their development, as in E. palustre 
and sylvaticum : in others again there is little difference as regards branching 
between the fertile and sterile shoots. But it has been shown experimentally 
by Goebel 1 that even in so pronounced a case of the absence of lateral 
branches as the fertile axis of E. arvense the development of green lateral 
branches could be induced : this was done by culture of the lower 
internodes in a moist chamber, when green assimilating branches were 
put out from the nodes, as in the vegetative shoot. The apparently 
branchless fertile shoot was thus brought into line with the ordinary 
branched type prevalent in the genus. Such facts indicate that the 
branched condition was probably common for the genus, but in certain 
cases a late differentiation has arisen between the colourless fertile shoots 
where the branches are dormant, and the branched green assimilating 
shoots. 2 

The fertile strobilus of Equisetum is normally terminal on the axis, 
and is usually borne on the relative main axis only. Many cases exist, 
however, of the development of the strobili on lateral branches : this may 
be normal for certain species, such as E. myriochaetum, Cham, of the 
sub-section Pleiostachya, Milde, well shown in Engler and Prantl, Pflanzen- 
familien, i., 4, Fig. 343, p. 547 ; but it also occurs occasionally in others, 
where a single terminal strobilus is normally present ("forma polystachya"}? 
In the case of Equisetum sylvaticum polystachyum (Fig. 194), where 
numerous lateral branches normally sterile bear small strobili, Luerssen 
has been able to correlate the change with external conditions : 4 this is 
the next step to bringing its determination within the limits of experiment. 
On the other hand, numerous cases have been recorded of the continued 
growth of the strobilus, at its apex, with a return to the ordinary vegetative 
characters. Such facts show that the lateral branches are not essentially 
different from the relative main axis, as regards the final end of spore- 
production : also, that there is no absolute barrier between the vegetative 
and the fertile regions in Equisetum. Speaking generally, the fertile strobilus 
is not restricted to axes of any definite order. Thus it requires no great 
effort of imagination to see in the shoot-system of Equisetum the result 
of amplification of a simple unit, the shoot, composed of axis and successive 

^ Ber. d. Deutsch. Bot. GeselL, 1886, p. 184. 

2 For an interesting discussion of the biological relations of the sterile and fertile shoots 
in living species of Equisetum, see Goebel, Organography , vol. ii., p. 501. 

3 For records of such developments in European species, see Rab. Krypt. Flora, iii., 
p. 622, etc.; and especially Luerssen, " Beitr. z. Kenntn. d. Flora, W. and Ostpreussens," 
Bibl. Bot., 1894, Heft 28. 

*L.c., p. 13. 






(iKXKRAL MORPHOLOGY 



leaf-sheaths, and capable of spore -production 
by a terminal strobilus. The branching, 
however complex, may be held as accessory, 
as also the formation of roots so closely 
associated with the branches. The funda- 
mental idea of the plant is thus carried 
back to the first shoot which originates 
with the embryogeny. It may be held that 
from this, by successive accessory branch- 
ings, the complex shoot-system arose, while 
the spore-production was deferred to the 
later branchings : it is on these that the 
fructification ultimately appears in the living 
species, while the primary axis and earlier 
branchings are normally sterile. 

The strobilus itself consists of a con- 
tinuation of the axis which bears it, and 
upon this the sporangiophores are disposed, 
but often with less regularity than rules in 
the case of the leaf-sheaths. The whole 
strobilus is normally occupied by the spor- 
angiophores, without any intervening bracts 
(Fig. 195 A). The sporangiophore itself 
consists of a central stalk supporting a 
polygonal distal end : from the margin of 
this the sporangia hang in variable number, 
forming a series surrounding the stalk (Fig. 
195 B). The spores are all of one type 
(Isosporous). At the base of the strobilus 
a ring-like structure is found the annulus 
which is like a reduced leaf-sheath, and it 
has usually been held to show a transitional 
stage between the vegetative leaf-sheaths 
and the first whorl of the sporangiophores, 
these being recognised as" equivalent parts. 
Reasons will be advanced below for not 
accepting this apparently simple view. The 
strobilus of Equisetum is liable to variations 
of development, which have their importance 
in relation to certain fossil forms. The most 
notable of these is proliferation, the apex of 
the strobilus being continued as a vegetative 
shoot : the effect is thus gained of a fertile 
zone bearing sporangiophores, threaded upon 
an axis, or of a succession of such zones, 



371 



FIG. 194. 

Equisetnm syfoaticum, L. , forma poly- 
stachya, Milde. Plant with 18 secondary 
strobili, in three whorls of branches which 
are normally sterile. Natural size. (After 
Luerssen.) 



372 



EOUISETALES 



separated by leaf-sheaths (Fig. 196). It is thus seen that the strobilus of 
Equisetum is not always that circumscribed terminal body which is typical 
for the living species. 

The Equisetum-^^ has been recognised, though with some uncertainty, 
and only in few specimens, as far ^back as the Middle Coal Measures ; l 

but it is seen represented more commonly, 
and by large forms, in the Mesozoic rocks. 
Related to it are two other fossil forms : 
the genus Phyllotheca of Permian age 
resembles Equisetum in the general features 
of the shoot, with its cup-like leaf-sheaths 
webbed at the base, but differing in the 
form of the leaves and in the fertile 
region : this is constructed on the general 
plan of Equisetum, but with the strobilus 
interrupted at intervals by sheaths of 
sterile leaves, as in some abnormal con- 
ditions of Equisetum (Fig. 197). Some 
specimens of Phyllotheca have, however, 
been described by M. Zeiller as having 
strobili like those of Annularia, that is, 
of the Calamostachys-type? The other 
genus is Schizoneura, of Triassic age, 
characterised by the whorled leaves being 
associated in webbed sheaths, which may, 
however, be slit longitudinally to the base. 
They thus form leaf-like lobes which stand 
off at a considerable angle from the axis 
(Fig. 198). The axis is marked by longi- 
tudinal grooves, which are continuous 
longitudinally from internode to internode, 
thus showing that the leaves of successive 
whorls did not alternate. The fructifi- 
cation is unknown. 

Most of the older Equisetal fossils, 
however, belong to the Calamarian type. 
These plants were often of dendroid habit, 
with secondary thickening of the stem, but with a similar primary 
construction of the shoot to that seen in Equisetum. The leaf-whorls 
are frequently webbed at the base, though often only slightly, as in 
Annularia ; but in Asterophyllites, which is traced back to the Devonian 
period, the leaves appear quite separate, in widely divergent whorls. 

1 Kidston, "On the occurrence of the genus Equisetum, etc.," Annals Mag. Nat. 
Hist., ix., p. 138, 1892. 

2 Zeiller, Palaeobotanique, p. 164. 




FIG. 195. 

Equisetum maximum, Link. A, the 
upper part of a fertile axis, with the lower 
half of the strobilus. Natural size, b the 
leaf-sheath. n; = annulus. x stalks of spor- 
angiophores cut off. y = transverse section of 
axis. j9 = sporangiophores in various posi- 
tions, slightly enlarged. s/ = stalk. sg 
sporangia. 5 = enlarged distal end. (After 
Sachs.) 



GENERAL MORPHOLOGY 



373 



The leaves themselves were usually simple, as in Ec/uisetum, though of 
greater dimensions, and accordingly more effective as assimilating organs; 
but among the earliest forms, such as Asterocalamites (Schimper), from 
the Culm, the leaves were branched in repeated dichotomies (Fig. 199). 
In the very early Pseudobornia, from the upper Devonian of Bear Island, 1 
the foliage was forked in a fan-like fashion, and of considerable dimensions. 
Another feature, in which certain of the earliest forms differed from the 
later, was in the fact that the members of successive whorls were super- 
posed, and did not alternate (e.g.. Asterocalamites). Such forms have been 



FIG. 196. 



Equisetum pratense, Ehrh. Shoots showing recurrent whorls 
of sporangiophores and of bracts. (After Milde.) 




B 



FIG. 197. 



Pltyllotheca. Zigno. A, Ph. eqnisetiforntis 
from Rovere di Velo, near Verona. B, inflores- 
cence from Siberia, placed by Schmalhausen with 
Phyllotheca. (After Solms.) 



associated by Potonie as" a family of " Protocalamariaceae." The facts 
would seem to indicate then a primitive construction of the Equisetoid 
shoot as having relatively large \vhorled and superposed leaves, effective 
as assimilating foliage : these were also separate from one another, and 
liable to bifurcation. The condition, as seen in the present Equisetum, 
might be understood as attained by reduction of the coalescent and simple 
leaves, which became also alternate instead of superposed, while the 
assimilatory function was relegated almost entirely to the axis. But there 
is no certain proof that the actual evolution of Equisetum itself was along 
such a line as this. 

' Nathorst, Z. Foss. Flora d. Folarliinder. i., Lief. 3, Taf. 7, 8. 



374 



EQUISETALES 



The Calamarian strobili were terminal on the axes, but they had a more 
elongated form than is usual in Equisetum (Fig. 200) : sometimes they 




FIG. 198. 

Schizoneura Godwanensis. Two-thirds the natural size. (After O. Feistmantel, from 
Engler and Prantl, Nat. Pflanzenfain.} 

extended to a length of 30 cm. (Potonie). They differed also in their 
construction : the nearest to the Equisetum-typQ is the ancient Archaeo- 

calamites (Bornia)?- characteristic of the oldest 
Carboniferous strata, and of the upper Devonian 
(Fig. 201). Its strobilus is essentially like that 
of Equisetum, having no sterile bracts intervening 
between the whorls of eight to ten sporangio- 
phores. These whorls did not alternate, but 
neither did the whorls of branched leaves in this 
early type. Here it would appear that there is a 
more complete differentiation of the reproductive 
from the vegetative region than is the case where, 
as in other Calamarians, sterile bracts are dis- 
tributed throughout the strobilus. 
Asterocaiamites scrobicuiatus. ^ } atter was t he more prevalent type 

bchlotneim (sf) from the culm. 

Fragment of a leafy shoot, re- amO ng the early Equisetales : in them the sterile 

duced to hall its natural size. 

(After Stur, from Zeiiier, Paieo- leaf-whorls and the whorls of sporangiophores 

botanique.) 

regularly succeeded one another, as it is seen 

in Calamostachys, and is well shown in C. Binneyana, which is the 
best known type (Fig. 202). The sterile whorls are commonly composed 

1 Renault, Bassin Hoitiller et Per mien d? Autir.i et tfEpinac, vol. ii., p. 80, Plate 42. 




FIG. 






GKNKRAI, MORPHOLOGY 



375 



of twelve coherent Jeaves, but thkteen have been counted : the sporangio- 

phores are usually six, that is, half the usual number of the leaves of the 

sterile whorls; but seven and eight have been seen in a single whorl of 

them, while no whorl of sixteen bracts has been seen. Hence it is clear 

that the sporangiophores bear no strict 

numerical relation to the sterile bracts. 

The position of the bracts in successive 

whorls of them alternates : the successive 

whorls of sporangiophores, on the other 

hand, do not alternate, " but are placed one 

above the other in vertical rows. Hence it 

is evident that their position can bear no 

constant relation to that of the bracts." 1 

This absence of a strict relation of the 
sporangiophores to the bracts comes out 
also in C. Ludwigi, described in detail by 
Weiss.' 2 He remarks of this species that 
the number of leaves in the sterile whorl is 
evidently variable : he made several count- 
ings, and concludes, " accordingly it may 
be accepted that there were sixteen leaves 
in the whorl, but that they might be re- 
duced to twelve (or thirteen ? ) by abortion 
of some of them." The leaves of the 
neighbouring whorls certainly alternated. 
Of the sporangiophores he says, the number 
in each whorl is six, and the successive 
whorls of sporangiophores stand vertically 
above one another ; but he notes slight 
deviations from this, perhaps due to torsion. 
A still further step is depicted by Weiss, 3 
in the case of Calamostachys germanica, 
where apparently the narrow bracts are 
approximately three times the number of 
the sporangiophores ; but this is not speci- 
fically stated to be the case in the text. 

On the other hand, it has been shown 
clearly in the case Palaeostachya vera that 
the number of bracts approximately corresponded directly to the number 
of sporangiophores, though possibly in some cases they somewhat exceeded 

1 Williamson and Scott, " Further Observations on the Organisation of Fossil Plants, etc.,'' 
part i., Phil. Trans., 1894, B, pp. 902-3. See also Scott, Studies, p. 47, etc. 
~ Abhandl. z. Geol. Spezialkarte, vol. ii., part i. , p. 38. 
A L.c. , vol ii., part i., Taf. xvi., Fig. 3 K. 




FIG. 200. 

Palaeostachyapedunculata. Specimen 
from the coal-shales, showing a fertile 
shoot bearing about a dozen cones, and a 
few leaves. >t = stem. About two-thirds 
natural size. (After Williamson, Phil. 
Trans. Will. Coll., 1060.) From Scott's 
Sttuiics in Fossil Botany. 



376 



EQUISETALES 



it. In fact it is to be recognised that, speaking of the bracts, " a tendency 
to multiply the number of appendages in each whorl seems to have been 




Archaeocctlajiiites. Part of 
cone showing the axis (ax) 
in surface view, bearing 
superposed verticils of peltate 
sporangiophores (sp) without 
bracts, sm sporangia. (After 
Renault.) From Scott. 



FIG. 202. 



Calamostachys. Diagram- of 
cone in radial section. a.r = axis, 
which bears successive vercicils of 
bracts (br), and peltate sporangio- 
phores (sp). sm sporangia borne 
on the sporangiophores. As the 
bracts are alternate with one 
another their upturned tips are 
only shown in every alternate 
verticil. (After Scott.) 



FIG. 207. 



Palaeostacliya. Diagram of cone 
in radial section. ax axis, which 
bears verticils of bracts (l>r) with 
peltate sporangiophores (s/>) in 
their axils, sm = sporangia. (After 
Renault.) From Scott. 



a characteristic Calamarian feature." 1 There appears, consequently, to 
have been no constant relation either of number or of radial position 

between the bracts and the 
sporangiophores. 

The relation of these 
two parts as regards vertical 
position is also variable 
within the fossil Equise 
tales ; for, as is well known, 
the sporangiophores occupy 
in Palaeostachya a position 
at the base of the internode 
(Fig. 203), in Calamostachys 
p. 882. 




Cingularia, typica, Weiss. From the Westphalian. Diagrammatic 
drawing of part of a shoot. X about 2. After Weiss. 

1 Hickling, Ann. of Bot. , 1907, 



SPORE-PRODUCING MEMBERS 



377 



in the middle of the internode^(Fig. 202), and in Stachannularia or 
Cingularia at the top of the internode (Fig. 204). Such facts as these, 
here only briefly sketched, must be taken into account in discussing the 
morphology of the strobilus of the Equisetales, and in deciding the true 
chararacter of the sporangiophores. But before this is entered upon their 
detailed structure and development must be examined. 



SPORE-PRODUCING MEMBERS. 

Naturally the development of the spore-producing members can only 
be followed in the living genus, though from the similarity of their mature 
features to those seen in the fossils it 
is probable that there was substantial 
similarity in these also. In Equisetum 
the axis, which is about to produce 
a strobilus, ceases active growth in 
length, retaining a conical form : the 
sporangiophores arise upon it in aero- 
petal order, as convex swellings (Fig. 
205). The details show some varia- 
tion in different species : they are here 
described for Equisetum arvense and 
Itmosum* In the first stages the spor- 
angiophores are not unlike the sterile 
leaf-sheaths, involving, as seen in 
longitudinal section, some six cells, 
which grow out with a fan-like tracery 
and repeated anticlinal walls (Fig. 
206 A). This similarity has been used 
as an argument favouring the view that 
the sporangiophore and the bract-leaf 
are results of " metamorphosis " of 
essentially the same part, a point 

which will be taken up later. Single superficial cells near the margins 
of the convex outgrowths' are early recognisable as the parent cells which 
give rise to all the essential parts of the sporangia, though adjoining 
cells also grow out together with these* to form the sporangial body : the 
origin of the sporangium is thus of the eusporangiate type (Fig. 206 A, B). 
At an early stage there is active growth in the middle region of the 
sporangiophore, which results in an inversion of the young sporangia, so 
that they come to point with their apices towards the axis. Each parent 
cell first divides periclinally (Fig. 206 A) : the inner cell gives rise only 
to a portion of the sporogenous tissue, the outer undergoes further division, 
first by anticlinal, later by periclinal walls (Fig. 206 B, c, D). The inner 

1 Studies, i. , p. 496, etc. 




FIG. 205. 

Half-developed strobilus of Equisetum arvense, 
in longitudinal section, taken at end of October. 
X 50. (After Hofmeister.) 



78 



EOUISETALES 



products thus formed share with the product of the inner cell already 
described in constituting the large sporogenous tissue, which, though entirely 
derived from the single parent cell, is not defined by its first periclinal 
wall : it is indicated by shading in the figures, while the products of the 
subsequent periclinal divisions are marked with a cross. Transverse sections 
at the stage represented in Fig. 207 A show the sporogenous tissue in a 
central position surrounded by several rather irregular layers forming the 
sporangial wall (Fig. 207 B). The size and construction of the sporangia, 
even of those in near juxtaposition, may vary greatly : this has been 
especially seen in the case of E. limosum. As the sporogenous group 
enlarges a layer of cells immediately adjoining it externally becomes glandular 




FIG. 206. 

Equisetum arvense, L. A, radial longitudinal section of part of young strobilus, 
showing two sporangiophores in a very young state. B, C, >, individual sporangia, in 
older states, cut in median section. X 200. 



in appearance, and develops as the tapetum (Fig. 208 A). Later the cells 
of the sporogenous tissue itself separate, and round themselves off as spore- 
mother-cells ; but it is only about two-thirds of these cells which undergo 
the tetrad-division, about one-third of them shrivel, and become disorganised, 
their substance mingling with that of the tapetum, which becomes intrusive 
as a multinucleate plasma into the interstices between the spore-mother- 
cells (Fig. 208 B) : the fertile cells which remain are nourished by this 
as they develop into the mature spores. Finally the superficial cells of 
the wall become indurated and spirally thickened, while those within it, 
excepting at the base of the sporangium, are disorganised. The mature 
sporangium, consisting thus of a single layer of cells of the wall, and 
containing the ripe spores which are all alike, dehisces along a longitudinal 
line facing inwards towards the stalk, which line had previously been defined 
by the cell-structure. 



SPORE-PRODUCIXG MEMBERS 



379 



A. 



Each of the sporangiophores, ' from which the sporangia thus depend, 
is traversed from the stalk upwards by a vascular strand, which branches 
in the enlarged head, and each branch terminates immediately below the 
base of one sporangium. The sporangiophores are in close juxtaposition 
while young, and thus the sporangia are effectively protected. At the base 
of- the strobilus lies the annulus, which completes the investment of the 
lowermost series of sporangiophores : it has as a rule no vascular supply 
(Fig. 209). Goebel has pointed out the 
protective biological use of the annulus ; l 
also that at the apex the highest spor- 
angiophores may be imperfectly developed 
and concrescent, thus forming a terminal 
cap : the protection of the young sporangia 
is thus very complete. 

The number of sporangiophores in the 
Eguisetum-strobi\us is not strictly defined, 
while the number of sporangia on each 
sporangiophore is also variable : it is usually 
larger in Equisetum than in the Cala- 
marians: this raises the question of evidence 
of variability of number of sporangia. There 
is in Equisetum no structural evidence of 
the septation of sporangia such as might 
lead to their increase in number, nor is 
there any interpolation of later sporangia 
between those first formed. In some of 
the larger cones, such as E. maximum, 
branched sporangiophores are commonly 
found, which appear to indicate a possible 
increase in their number by fission : the 
irregularity of their number and arrange- ?1tisftum *, L . A, section ua- 
ment in these large cones would seem to ^f^^^^ii^h^b! 
support this (compare Fig. 195.) Excepting f^^^^gZZ^^ 
for such indications there is no evidence first . P ericlinal division. />, a similar sp or- 

angmm cut transversely, x 200. 

among living species of methods of increase 

in number of sporangia. Even the apical growth of the strobilus itself is, 
as a rule, strictly limited. Of reduction in number of sporangia there is 
as little direct evidence, but it is to be remembered that complete abortion 
leaves no trace of what has occurred (see Chapter X.). On grounds to be 
mentioned below it would seem probable that such complete abortion of 
sporangiophores has figured in the evolution of the Equisetales, contributing 
to the origin of the initial vegetative system of the individual plant. 

The structure of the mature sporangiophore and of the sporangia in 
the Calamarians is so similar to that of Equisetum that, taken together 

1 Organography, ii., p. 500. 




FIG. 207. 



3 8o 



EOUISETALES 



with their insertion directly on the axis, there can be no doubt of their 
true homology. 1 This is illustrated by Scott's figure of the sporangiophore 
of Calamostachys Casheana (Fig. 210), which shows the position and 
structure of the sporangia; but the number of the sporangia on each was, 
as a rule, only four. In some species there was heterospory, megasporangia 
and microsporangia being found even upon the same sporangiophore : 
this is illustrated by Scott in Calamostachys Casheana^ 1 He has also noted 
in C. Binneyana the abortion of certain spores of the tetrad : 3 this, taken 
with the condition as seen in C. Casheana, indicates that in the palaeozoic 
genus "we are able to trace how heterospory originated. The facts suggest 
that in the first instance a certain number of spores became abortive, and 




FIG. 208. 

A, apex of sporangium of Eqtiisetum limosum, L., showing the sporogenous cells, 
surrounded by the tapetum (/). and sporangial wall. B, shows part of an older spor- 
angium with its tapetum (/) still clearly defined, though the individuality of the cells is 
lost : within this the sporogenous tissue, of which certain cells (a) are abortive. X 200. 

so allowed of better nutrition for the remainder : this process, going on 
more freely in some sporangia than in others, may ultimately have rendered 
possible the excessive development of those spores that survived at the 
expense of the others, and may thus have led to the development of 
specialised megaspores." 4 In this respect Calamostachys was in advance 
of Equisetum. 

It has been shown above how completely the young sporangia are 
protected in the strobilus of Equisetum by the close aggregation of the 
sporangiophores, together with the covering afforded by the basal annulus 
and terminal cap. In the more lax strobili of the Calamarians the pro- 
tection must have been chiefly carried out by the intermediate whorls of 
bracts, which overtopped the sporangiophores, a condition more nearly 
comparable with what is seen in other strobiloid types. 

1 The relation of the strobili of the type of Calamostachys as regards their anatomy to 
the Calamitean stem has been pointed out by Scott ; it will be unnecessary here to enter 
into the evidence on such questions ; it suffices to refer to Scott, Studies, pp. 45, etc. 

2 Scott, Studies, Fig. 22. * L.c., p. 51. 4 /,.r., p. 53. 



SPORE-PRODUCING MEMBERS 



It remains now to consider the > morphological character of the sporangio- 
phore in the Equisetales. The current view of the strobilus of Equisdum 
is that it is a product of meta- 
morphosis of the sterile shoot, 
and that the sporangiophore 
is an altered sterile leaf. This 
has been re-stated lately by 
Goebel, 1 on the basis of de- 
velopment of the individual, 
but without bringing the fossil 
Calamarian strobili into the 
comparison. It may, however, 
be safely asserted that if Equise- 
tum and Ec/uisetites had never 
existed, a comparison of the 
Calamarian strobili with those 
of other Pteridophytes would 
have led to a different view ; 
it will be necessary therefore 
to examine this natural group 
of the Equisetales as a whole, 
and not only one isolated genus, 
even though that type be the 
well-known one now living. 

Taking first the developmental evidence derived from Equisetum, as 




FIG. 209. 

Equisetum limosum, L. Median longitudinal section of a 
sporangium at the base of the strobilus, together with the 
annulus (a), x 200. 




FIG. 210. 

Calatnostackys Caskeana. Tangential section, showing four sporangia grouped around 
their sporangiophore (s/>). Three contain megaspores and one microspores. X30. Phil. 
Trans. II'. and S. Will. Coll., 1587. (From Scott, Studies in Fossil Botany:) 

;iven by Goebel, 2 it is found that, notwithstanding the difference in mature 
form (which Goebel notes, and from which he concludes that the distinction 

1 Organography, vol. ii., pp. 499-503. 2 Z..c., p. 500. 



382 EQUISETALES 

arose at an early date), the origin of the two bodies is alike; but the 
sporangiophore, which is the more bulky, soon adopts a mode of growth 
which leads to a shield-like form. He concludes that the simpler develop- 
ment of the sterile leaf was the more primitive type, and that the stronger 
growth of the lower surface of the sporangiophore, so as to give it the 
hypo-peltate form, is a new development. He also alludes to the transi- 
tional forms between the two types, such as have been described by Gliick 
and others. 1 

Before the homology of the leaf-teeth with the sporangiophores is 
accepted, the grounds upon which it is based are to be examined ; they 
appear to be these : 

(1) Similarity of the cell-structure on first origin. 

(2) Similarity of position relatively to the axis. 

(3) Transitions, through the annulus and its malformations, from the 

one type to the other. 

The similarity of structure of the two as shown in vertical sections 
was pointed out by Gliick, though, as he himself remarks (p. 362), it 
holds only for the very earliest stages. But similarity of segmentation 
has long ago been shown to be no proof of morphological identity in the 
case of embryos and hairs ; without going so far afield as this, a com- 
parison of a vertical section through the leaf-margin of Angiopteris? with 
a vertical section through its sorus, 3 shows a near similarity of the 
cell-net : yet this does not suggest any homology of the leaf-margin with 
the lip of the sorus ; and no more can the similarity of segmentation at 
the outset of that of the bract be held to prove the foliar nature of the 
sporangiophore. 

Both sporangiophores and sterile leaves are lateral appendages of the 
axis, but this does not of itself prove the point ; for instance, in plants 
which bear prickles, the prickles and the leaves occur together on the 
shoot ; and the former arise not much later than the latter, while similar 
tissues take part in the formation of both. If both arose simultaneously 
close to the apex, the early distinction of them would be a matter of 
difficulty, though they are parts of different morphological character. It is 
possible thus to contemplate the origin of parts of similar cellular structure, 
but not morphologically comparable with one another, laterally upon the 
same axis. 

The occurrence of middle forms between the teeth of the normal 
annulus and sporangiophores appears at first sight important evidence ; but, 
as is well known, intermediate forms occur between ovules and "foliage 
leaves, and, nevertheless, the opinion is widely, and in my view rightly, 

1 Gliick, "Die Sporophyll-Metamorphose," Flora, vol. Ixxx., 1895, p. 364, and Plate 5. 
References are there given also to Milde and other writers. 

2 Annals of Botany, vol. iii., Plate 23, Fig. 71. 

3 Phil. Tram., B, 1897, Plate 10, Fig. 66. 






SPORE-PRODUCING MEMBERS 383 

accepted that the ovule, like otheK sporangia, is an organ si/i generis, and 
not the result of modification of a leaf or leaf-segment. The occasional 
existence of sporangia, or even of imperfect sporangiophores upon the 
annulus, is not necessarily a proof of evolutionary transition from the one 
structure to the other, but is rather to be held as indicating that the 
primofdium in its ontogenetic origin was not defined in its character. 

. The strength of the view stated by Goebel lies in the fact that it is 
supported by all three lines of argument above noted, and if it were 
not for the fossils, which he does not introduce into his discussion of 
the matter, it would probably not be called in question. But comparison 
with them suggests an alternative view. viz. that the sporangiophores are 
not of the nature of phyllomes, but are comparable rather with the 
sporangiophores of the Psilotaceae or Sphenophylleae; these they certainly 
resemble in form and function, though they differ from most of them in 
maintaining no strict relation of position to the true leaves. This sug- 
gestion must now be examined. 

It is based primarily upon those Calamarian strobili in which each 
leaf-whorl is regularly succeeded by a whorl of sporangiophores. In the 
strobili the leaves of 'successive whorls show a radial alternation, as in 
the vegetative shoot, and it seems natural to suppose that they accordingly 
correspond to the ordinary succession of them in the vegetative region. 
But in addition to the sterile leaves the sporangiophores are present, and 
their presence does not disturb the alternate succession of the leaves. If 
the sporangiophores were rightly regarded as leaves, it might be anticipated 
that the alternate succession of the sterile leaves would be disturbed where 
the sporangiophores intervene between their whorls, but it is not. Again, 
though the number of the sporangiophores is frequently half that of the 
jrile leaves, that numerical relation is not strictly maintained, while their 
lisposition in vertical, non-alternating series is on a plan apart from that 
)f the alternating whorls of sterile leaves. Their position on the internode 
also, sometimes at the base, sometimes at the upper limit, often in the 
mkldle, again shows their independence of the sterile leaves. These facts 
together point to their being structures of a different nature from the 
leaves of the strobilus. 

It may be asked how this non-phyllome theory of the sporangiophores 
is compatible with the facts in Equisetum, in which the annulus has 
usually been accepted as a transition from the foliage-whorls to the 
sporangiophores. It is true the annulus lies at the boundary between the 
sterile and fertile regions, and that in Equisetum no vestiges of leaf-whorls 
are found higher up among the sporangiophores. Goebel has pointed out 
an obvious protective use for the annulus, which would sufficiently account 
for its constancy and limited size in the genus. 1 A comparison of other 
types of Equisetineous strobili affords the following explanation of the 
Equisetum strobilus in terms of the fossils. In the genus Archaeocalamites 

1 Organography, p. 68 1. 



384 EQUISETALES 

(Bornid) Renault describes 1 for B. radiata, Brongn., how the fructifications 
are simple, or interrupted in their length by verticils of leaves, which 
render the spike itself, so to speak, articulated and of very variable 
length. The condition of these spikes is then different in proportion, 
rather than in essential points from that described for Phyllotheca (Fig. 
197), and so curiously reproduced in the abnormal Equiseta described 
above (Fig. 196). This again differs from Calamostachys mainly in the 
number of the sporangiophores which intervene between the successive 
leaf-whorls. The tracts which bear the sporangia being thus variable, it 
would appear that the Equisetum-ty^te is merely an extreme case, in 
which the whole series of sporangiophores which form the terminal 
strobilus are collectively above the last leaf-sheath, and that last leaf sheath 
is of a reduced type, and appears as the annulus. 

It is obvious that in the present state of our knowledge the case is 
not proved either for the phyllome-theory of the sporangiophore in the 
Equisetales, which is out of harmony with the known facts in the fossils, 
or for the non-phyllome theory, which is certainly a less obvious explanation 
of the simple strobilus of Equisetum. But the balance of evidence is 
strongly in favour of the latter, as without undue pressure it covers the 
whole area of facts, including those relating to the fossil Equisetales. - 

1 Bassin Houiller d'Autun et d'Epinac, p. 8 1. 

2 It is necessary briefly to mention another view, advanced by Jeffrey (Mem. Boston 
Soc. of Nat. Hist., vol. v., pp. 184-5), as applicable to those Calamitean cones where 
the bracts in each whorl are stated to be double the number of the sporangiophores. 
He suggests that the pairs of the sterile leaves were really dichotomously divided dorsal 
segments of sporophylls, of which the sporangiophores were the ventral segments. It is 
necessary to remember, however, that in the best known cones of Calamostachys the 
bracts of successive whorls alternate, while the successive whorls of the sporangiophores, 
considered by themselves, are strictly superposed (Scott, Progressus, p. 158) : this fact 
appears to be fatal' to Jeffrey's suggestion, as will be obvious if the arrangement be 
plotted out diagrammatically in one plane. It will then appear that the proposed scheme 
would only apply to each alternate whorl of bracts, not to them all. There is also 
against it the fact that in the Equisetales at large the arrangement of the cone with the 
bracts approximately doubling the number of the sporangiophores is only one among 
several different arrangements : the proposed scheme is quite inapplicable for Archaeo- 
calamites or for Equisetum ^ and equally so for Palaeostachya (cf. Hickling, I.e.). 

Akin to Jeffrey's theory, though not coincident with it, is that of Lignier (Bull, de la 
Soc. Linn, de Normandie, Caen, 1903, p. 162, etc.), which also is based primarily on the 
data for the cone of Calamostachys, and upon comparisons with the Sphenophylls. His 
view is that the sporangiophores in Calamostachys are the result of concrescence in pairs 
of fertile lateral lobes of the leaves forming the verticil. The anatomical facts are 
derived from Renault (Bassin Houiller et Perm. d^Autun et d^Epinac, iv. , 2, p. 130, and 
PI. Ix.) ; the details shown in his figure, 6, of the single transverse section partially 
depicted would accord with the theory ; but the evidence seems insufficient, and there are 
the following positive objections to it. First, there is no structural evidence in the 
sporangiophores themselves of Calamostachys, or in any other of the Equisetales, of the 
presumed fusion. Secondly, in the single drawing of a complete transverse section of 
the cone of C. Zeilleri by Renault (I.e. PI. ix., Fig. 5) there are 14 sporangiophores, but 
only 27 sterile bracts : so that the numerical relation does not hold in the one case on 



ANATOMY 385 

ANATOMY. 

For the purposes of the present discussion the chief points of 
importance in the anatomy of the Equisetales relate to the structure of the 
axis : the leaves and roots carry only a minor interest. It will suffice to 
say of the former that their structure in Equisetum points to a probability 
of reduction from a condition more effective in assimilation, which was 
their state in some at least of the Calamites. The roots of Equisetum are 
essentially of the Fern-type, though with some peculiarities of detail of 
their own : the roots of the Calamites show in their primary structure 
striking similarity to those of Equisetum, including the peculiar double 
endodermis ; but they show in addition a cambial thickening, which is 
quite in keeping with the secondary growth of the axis which they 
support. 

In discussing the structure of the axis the same order may be observed 
as in the external morphology, and the living genus Equisetum will be 
taken first. Transverse sections of the internode show the well-known 
disposition of the chief tissue-tracts, though with varying proportion and 
structure of the several tissues according to the species and the grade of 
the axis cut : viz. a peripheral epidermis, a broad cortex, and a central 
stelar region. The chief interest naturally centres in the tissues of the 
stele, and indeed it is unnecessary to discuss here the special characters 
of the superficial tracts. It may be noted first that the outer limit of the 
stele is not defined by the first apical segmentations : the inner cell cut 
off by the first periclinal wall in each segment of the apical cell forms 
only the pith, while the vascular tissues originate together with the 
cortex from the outer products of each segment. 1 But it has been 
seen that early segmentation is not a constant index of morphological 
character, and, accordingly, the stelar condition of Equisetum may 
properly be compared with that of other Vascular Plants, irrespective 
of its origin in the primary segmentation. The stele consists of a 

\\hich the whole theory is based. Thirdly, the same difficulty will arise from the alternation 
of the whorls of bracts, and the superposition of the sporangiophores as opposed to Jeffrey's 
suggestion. Fourthly, the theory is quite inapplicable to the Equisetales at large, as is 
admitted by Lignier (I.e., p. 131). He himself suggests a different origin of the sporangio- 
phore for Equisetum and Archaeocalamites, where they are held to represent whole 
leaves. These two hypotheses of origin of the sporangiophore put forward by Lignier seem 
too divergent to explain satisfactorily the nature of substantially the same part within 
the same natural phylum. Such difficulties are sure to arise where the attempt is made 
to reduce variable forms to a strict morphological scheme. This Lignier has done with 
some ingenuity for the individual case ; but the more elastic view of the sporangiophore as 
a part sui generis appears to accord better with the natural facts. The sporangiophore 
may have a more or less definite relation to the sterile bracts, and it often has ; but the 
facts for the Equisetal phylum do not indicate this as an obligatory relation. The 
nature of that relation will be best considered when corresponding facts from other 
sporangiophoric types are available (see part iii.). 

1 Campbell, Mosses and Ferns, p. 460. 

2 B 



386 EQUISETALES 

large pith with a central cavity interrupted by diaphragms at the 
nodes : around it is disposed a ring of vascular strands of number varying 
according to the species, or according to the rank of the axis in question. 
They are separated laterally by broad parenchymatous rays, while the 
whole is surrounded in most species by a continuous endodermis 
(Fig. 2ii A, B). There is, however, a good deal of difference in the 
disposition of the endodermis in various species, and these differences are 
of such a nature as to raise questions as to the validity of the simple 
character of the stele itself. The simplest case is that above described, 
and it may be seen in the aerial shoots of E. arvense and palustre, where 
there is a simple endodermal sheath of sinuous outline, formed from the 
innermost layer of the cortex ; in fact, the arrangement is that most usual 
in Vascular Plants. In this case the term " stele " will naturally connote 
all that lies within that sheath. A second type is that seen in the 
rhizomes, but not in the aerial shoots of E. sylvaticum (Fig. 211 c, D), 
in which a second endodermis is present as a sinuous layer, forming an 
inner barrier of demarcation from the inner-lying pith. A third type is 
seen in the rhizomes of E. hiemale and some others, but not in the aerial 
stems of those species : it is characterised by each single strand being 
individually surrounded by a closed endodermal sheath (Fig. 211 E, F), 
while there is no general endodermis delimiting the whole stele. Such 
individual endodermal sheaths also surround the strands in the tubers of 
E. arvense, sylvaticum, and palustre, species in which, however, a general 
endodermis is found in the ordinary axes. The inconstancy of the 
arrangements thus seen, even in the different regions of the same plant, 
indicates them as special and secondary peculiarities, which need not 
seriously affect the conception of the stem as essentially monostelic. The 
fact that the differences of the endodermis do not otherwise affect the 
anatomy confirms this conclusion. It may then be held that the stem 
of Equisetum is monostelic throughout, but subject to disintegration of 
the stele. 

The structure of the individual vascular strands, as seen in the transverse 
section of the internode, is fairly uniform in the different species. Each 
strand shows towards its central limit a canal designated "carinal," 
because it is on the same radius as one of the keel-flanges which mark 
the fluted internode externally (Fig. 211 c). These canals indicate the 
position of the protoxylem-strands, which become obliterated as the sur- 
rounding tissues expand in development; for the primary tracheides are unable 
to keep pace in their own growth with the expansion of the surrounding 
tissues, and accordingly break down. Close on either side of the margin 
of each carinal canal the annular thickenings of one or two or more 
tracheides remain to maturity, and permanently record the position of the 
protoxylem. As we shall see later, these are directly continuous with 
the protoxylem of the leaf-trace. Further out from the centre than the 
canal, and right and left of it, two other groups of xylem arise later : 



ANATOMY 



387 




FlCi. 211. 

A, transverse section of the stem of Equisetum palustre (X26), and B, part of it 
X 160. C, transverse section of the rhizome of Eqnis. sylvaticum ( X 26), and D, 
part of it X 160. E, transverse section of the rhizome of Equis. litorale ( X 26), and f, 
part of it x 160. cc central cavity. r = valleculai canals. t- = carinal canals. i = sheath 
of separate strands, as outer, is = inner general endodermis : in A, C, and E the endo- 
dermis is indicated by a dotted line. (After Pfitzer.) From Rab. Krypt. Flora. 



EQUISETALES 



the number of the tracheides in these varies in different species 
(Fig. 211 B, D, F). It will be shown that these are not directly con- 
tinuous with the xylem of the leaf-trace. The phloem lies between them, 
and consists of sieve-tubes and parenchyma. 

If the vascular tissues be followed onwards into the nodes, the structure 
there displayed will give ground for a proper understanding of the inter- 
nodal strands. Hitherto it has been customary to treat these as integral 
" vascular bundles " of the collateral type, comparable with the leaf-trace 
bundles of Phanerogams : they have been assumed to enter the axis from 
the leaves as integral bundles, and to pursue their course down one 

internode, maintaining their identity as 
integral bundles to its base : there 
each was held to bifurcate, and the 
shanks to affix themselves right and 
left on the nearest lateral bundles 
which pass in at the lower node. This 
was the scheme contemplated by De 
Bary ; l but it is a scheme characteristi- 
cally Phanerogamic, and it has always 
presented difficulties of comparison with 
other Pteridophyte-types. An advance 
to a more intelligible view, based upon 
more exact analysis of the nodal 
structure, has been the result of the 
investigations of Gwynne-Vaughan.' 2 to 
whom I owe the use of hitherto un- 
published drawings, as well as the 
description which follows. He found 
that in E. Telmateja, of the three 
strands of xylem present in each bundle 
of the internode the carinal strand alone 

passes out at the node as a leaf-trace. The two lateral strands join on 
to the xylem of the nodal ring, where the xylem is much more amply 
developed than in the internode, and even shows some slight degree of 
secondary increase. 3 In certain species (E. hiemale, and better still in 
E. giganteum] the lateral strands of the internodal bundles may be traced 
as externally projecting ridges over the nodal xylem into the internode 
above. In passing through the node they diverge from one another, so 
that in the internode above they are found on the adjacent sides of two 
different bundles. At the node above they approach each other, and in 
the next internode they both occur in the same bundle once again. The 

1 Comp.-Anat., pp. 279 and 327. 

2 Gwynne-Vaughan, Report Brit. Ass., Glasgow, 1901, p. 850; also Ann. of Bot. 
1901, p. 774. 

3 Cor mack, Annals of Botany, vii., p. 63. 




FIG. 212. 

Diagram constructed by Mr. Gwynne-Vaughan 
to represent a tangential view of the vascular 
system of Equisetum. The dotted lines indicate 
the course of the true leaf-trace strands : the 
continuous lines indicate the cauline strands. 




ANATOMY 389 

leaf-trace protoxylem, having entered the bundle, runs downwards for one 
internode between but internally to the two lateral strands : at the node 
below it divides into two branches, which curve to the right and the 
left in order to fuse with the neighbouring leaf-traces that enter at this 
node (Fig. 212). So the xylem of the so-called vascular bundle of 
E</idsetum consists of three strands, two of which are lateral and cauline, 
while the median or carinal strand is common to both stem and leaf. The 
fact that only a small portion passes out as a leaf-trace, and not the 
bundle as a whole, constitutes an essential point of difference between 
it and the bundle of a Phanerogam. The general conformation of the 
vascular tissue at the node, according to the 
above description, is shown in the diagram 
(Fig. 213). 

The tracheides in each strand are very 
few, and consequently it is difficult to deter- 
mine the direction of their development. 
However, as regards the leaf-trace and the 
carinal strand it appears clear that they are 
not exarch but endarch, or perhaps slightly 
mesarch on the adaxial side. The lateral 
strands, as a whole, are 4 differentiated later { 

than the carinal strand, but they do not 
seem to be a continuation of its centrifugal 
development. On the contrary, in E. gigan- 
teum, where as many as ten to fifteen elements 
are present in each lateral strand, the smallest 
of them are invariably at the outer extremity, 
and they gradually increase in size inwards. 

Longitudinal Sections Show that the largest form the external parts of the composite 

vascular bundle. 

tracheides are coarsely reticulate, with large 

pits and very broad bands of thickening between them : in the smaller 
elements the reticulation becomes finer and more regular, and in the 
smallest it closely resembles true spiral thickening. To state definitely 
whether the lateral strands are exarch or not was not possible m this 
species, because no incornpletely differentiated portions of the stem were 
available : so the question must remain at present undecided, although 
the mature structure certainly gives a strong impression of centripetal 
development. 

It is suggested by Gwynne-Vaughan that the lateral xylem-strands in the 
vascular bundles of the existing species of Equisetum may perhaps be 
taken to represent the last remnants of a primitive central mass : this 
would be in entire agreement with their apparently centripetal develop- 
ment, and in particular with their cauline course. The probability of 
this suggestion can best be gauged by comparison with the fossil Equisetales, 
and with other Pteridophytes. For Calamites the case has been succinctly 



390 EQUISETALES 

stated by Scott : 1 he remarks that " the Calamite, so far as anatomy goes, 
is simply an Equisetum with secondary thickening." The secondary increase 
commences at the nodes, and extends thence through the internodes. 
This again adds point to the similarity with Equisetum, since the trace of 
secondary increase present in Equisetum is seen at the nodes, though it 
does not extend into the internodes. The result of the secondary growth 
in Catamites may be a woody mass of great bulk, and varying in the 
details of its structure : into these matters it is unnecessary to enter here : 
it will suffice to quote further from Scott 2 that "we may therefore express 
the general characteristics of the Calamarian vascular system by the state- 
ment that the whole arrangement is of the type of Equisetum but more 
varied, and sometimes more complex"; and, further, that 3 "the position 
of the branches with reference to the nodes and leaf-traces was precisely 
the same in Calamites as in the recent Equisetum" Thus, as regards 
stelar problems the two stand together, and the hypothesis put forward 
by Gwynne-Vaughan for the elucidation of the stelar structure in Equisetum 
should find its application in Calamites also. It will now be shown that 
certain facts derived from these fossils strongly support it. 

In his Pflanzen-palaeontologie (p. 205) Potonie established a comparison 
between the secondary vascular tissues of the Calamarieae and the Spheno- 
phyllaceae by mentally doing away with the Central mass of primary 
xylem that exists in the latter. Gwynne-Vaughan suggested that by 
inverting this procedure, and considering it possible that the ancestors 
of Equisetum may have possessed a xylem that extended to the centre of 
the stem, one is led to derive their structure, as it exists at present, 
from the modification of a stele with a solid central mass of centripetal 
xylem such as that of Sphenophyllum or of certain Lepidodendreae. To 
illustrate the nature of the modifications that such a stele would have to 
undergo, a series' of parallel developments was pointed out by Gwynne- 
Vaughan within the latter group, viz. Lepidodendron Rhoduinnense, Selaginoides, 
Harcourtii, Si^iharia spinosa, and Menardi: here parenchyma appears in 
the xylem, and gradually increases in quantity until only an attenuated 
peripheral ring of xylem remains, which then becomes more or less broken 
up into separate strands. This suggestion raises the question whether any 
Calamarian stem is known in which the hypothetical primary xylem is 
better represented, and is shown to be centripetal in its development? . 

At the very same meeting at which Gwynne-Vaughan developed his 
theory Scott described a new species, Calamites petty curensis, which gave 
the requisite answer. It comes from the Calciferous sandstone of Burnt- 
island. 4 The interest depends on the fact that each vascular bundle 
possesses a distinct arc of centripetal wood on the side next the pith. 
The carinal canals are present as in an ordinary Calamite, and contain, as 
usual, the remains of the disorganised protoxylem. They do not, however, 

1 Studies, p. 23. 2 Z.r., p. 25. 

*L.c., p. 31. 4 Scott, Brit. Ass. Report, 1901, p. 849. 



ANATOMY 391 

as in other Kquisetales, form the^ inner limit of the wood; but xylem of 
a considerable thickness, and consisting of typical tracheides, extends into 
the pith on the inner side of the canal, which is thus completely enclosed 
by the wood. Hence, starting from the spiral tracheides of the protoxylem, 
there was here a considerable development of the xylem in a centripetal 
as well as a centrifugal direction. This appears to be the first case of 
centripetal wood observed in a Calamarian stem ; it serves to furnish a 
new link between the Palaeozoic Equisetales and the Sphenophyllales, and 
through them also with the Lycopods. 

The question remains whether the young plant of Equisetum shows in 
its axis a structure indicative of a protostelic origin. Jeffrey l has traced 
the details for E. hiemale, and finds that the central cylinder of the first 
shoot makes its appearance as an unbroken tube of reticulated tracheides. 
There are no protoxylem elements, although the internal tracheides are 
formed first. The primitive axis, in fact, starts out with a similar organisa- 
tion to that which is subsequently found to recur in the nodes. These 
facts, though not in themselves conclusive, would tally well enough with 
an origin of the shoot from a protostelic ancestry. 

The facts and arguments contained in the preceding pages clearly 
indicate the line of comparison of the stelar state of the Equisetales 
with that of the other Pteridophytes. The axis is monostelic, as in 
other primitive forms. It presents the appearance of a mere attenuated 
remnant of the probable archaic state of the protostele. Comparison 
makes it probable that in place of the solid xylem-core, which is seen 
in other phyla to be the primitive condition, the central part has become 
parenchymatous : in the early fossil, Calamites pettycurensis, the change 
had advanced so far as to reduce the volume of the xylem, though a 
centripetal remnant still persisted, and serves to indicate the probability 
of a protostelic origin, comparable to that condition seen in some 
Lycopodiales and in the Sphenophyllales. In the ordinary Calamites, 
as well as in Equisetnm, the change has advanced so far that only minute 
remnants of the centripetal wood are to be recognised, and that recogni- 
tion would itself be uncertain were it not for the confirmation brought 
by the fossil from the Calciferous sandstone. But together the evidence 
appears conclusive, and trie result is to place the Equisetales, which have 
so long been a structural problem, in line with other strobiloid forms : 
they, like the rest, have probably sprung from a protostelic ancestry. 
Physiologically the changes involved appear as a natural result of life in 
a semi-aquatic and muddy habitat, while the reduction of the leaves 
from effective assimilatory organs as they appear to have been in the 
early Calamites, to the protective sheaths of Equisetum, would also 
harmonise with ,the anatomical change contemplated. 

The leaves and the sterile bracts of the strobilus in the Equisetales 
are supplied with simple strands, which call for no special remark. But 

1 /,.*-., p. 171. 



392 EQUISETALES 

some curious features have lately been disclosed for the strands entering 
the sporangiophores. In the case of Palaeostachya vera, where the 
sporangiophores in each whorl approximately equal the bracts, and are 
apparently axillary, the strand for each originates immediately above 
the bract-bundle ; it does not, however, pass out, but ascends with 
the main bundle of the axis through half the internode : it is then 
sharply reflexed, and drops again to the upper limit of the nodal 
disc, whence it passes outwards to the sporangiophore. 1 In 
Calamostachys the course seems to be the same, but with the points of 
difference that the sporangiophore-trace drops less than in Palaeostachya, 
in accordance with the position of the sporangiophore, and that Calamo- 
stachys has commonly two bracts to each sporangiophore, the latter 
being inserted in a plane between them. The anatomical condition in 
Stachannularia and Cingularia is unfortunately unknown : so far as the 
facts are available they indicate that the vascular supply of the sporangio- 
phore is regularly derived from the bracteal node next below. This 
suggests a certain anatomical relation of the sporangiophores to the bracts 
in Calamarians at large ; but the details of that relation are variable, 
and they cannot be held to support any general theory of lateral fusion 
of leaf-segments to form the sporangiophores, such as that suggested 
by Lignier in the case of C. Zeilleri. As regards the position of the 
sporangiophore on the internode, the anatomy, so far as known, appears 
to indicate the condition of Calamostachys, with its sporangiophore 
halfway up the internode, as a central type : and that while Cingularia 
probably shows an exaggeration of this displacement, so that the spor- 
angiophores appear immediately below the bracts of the next upper 
whorl, Palaeostachya is a modification of the Calamostachys type in the 
opposite direction, so that the sporangiophores are axillary in position. 2 

EMBRYOLOGY. 

The archegonium of Equisetum lies with the neck directed upwards. 
The basal wall, which first segments the zygote, appears approximately 
horizontal : the embryo is thereby divided into epibasal and hypobasal 
halves : the shoot arises from the former, the foot from the latter. There 
is some conflict of evidence as to the place of origin of the first root : it 
is referred by Sadebeck to the hypobasal half in E. arvense and palustre 
(Fig. 2i4); 3 but Jeffrey traces the origin of the root to the epibasal half 
in E. hiemale, though with some uncertainty; but in any case it arises 
high up on the side of the embryo in that species, and in close relation 
to the primitive shoot. 4 The absence of a suspensor simplifies the 
embryogeny. As in the Lycopodiales, so here also it will be found 

1 Hickling, I.e., p. 375. 2 Compare Scott, Progresses, i., pp. 160-161. 

3 See Engler and Prantl, Nat. Pfanzenfam . , i. 4, p. 520, where the literature is cited. 

4 Mem. Bost. Soc. of Nat. Hist., vol. v., No. 5, p. 168. 



EMBRYOLOGY 



393 




FIG. 214. 

Embryo of Equisetum. I. -IV., Equisetum arvense, L. I. and II., the same embryo 
in different positions : in I. the median wall is visible, in II. the transverse wall. X3oo. 
1 1 1. -IV., a more advanced embryo showing development of the stem and leaf-sheath. 
X250. V., an embryo still further developed, but not dissected free from the prothallus, 
and showing the orientation relatively to the archegonium. st = the stem apex ; z> = the 
first leaf-sheath; w = the root. Xg8. VI. and VII., Equisetum palustre, L. VI. = 
young embryo still in the archegonium, stem and foot are visible. X^oo, VII. =an 
embryo further advanced, and dissected free, and orientated 90 as compared with VI. 
The root (/) and stem (st) are visible. X 300. b basal wall ; T= transverse wall ; m = 
median wall; t' = epibasal and //=hypobasal region; e/ = the first leaf-sheath. (After 
Sadebeck, in Engler and Prantl. Nat. PJlanzenfam.} 



394 EOUISETALES 

essential to a proper understanding of the embryogeny to fix the attention 
primarily upon the origin of the apex of the axis, which is defined at a 
very early stage in Equisetum. The epibasal half of the embryo is 
described as dividing into octants by walls at right angles to one another : 
one of the octants then takes the lead over the others, and it is this one 
which gives rise to the axis, with its tetrahedral apical cell like that in the 
mature plant : the product of this octant soon constitutes the greater part 
of the epibasal region (Fig. 214 iv.). A little consideration of the facts 
thus stated will show, first, that the octant-walls are the natural preliminary 
steps to the definition of a tetrahedral initial cell centrally in the epibasal 
hemisphere : the octant-walls might even be held to be themselves the 
first segmentations in the definition of that cell ; secondly, that, con- 
sistently with the initiation of a conical initial cell, the origin of the axis is 
in the closest possible relation to the point of intersection of the octant- 
walls, just as it is found to be in the Lycopodiales. There is, however, 
this difference, that the apex asserts itself very early in Equisetum, which 
is in accord with the early dominance of the axis over the appendages. 
These arise as three (or sometimes only two) leaf-teeth, borne upon a 
coalescent sheath, which is described as originating partly from the 
remaining three octants of the epibasal half, but partly also from the 
lower portion of that which gives rise to the apex of the axis itself. It 
seems quite unnecessary in such a case as this to attempt to allocate the 
several parts to definite octants : clearly if the leaf-sheath be partly derived 
from the stem-octant, this is not rightly so named. Probably the allocation 
of parts to definite octants would not have been attempted in Equisetum 
had it not been found to apply with apparent success elsewhere, and 
especially in the Ferns. In the present case it seems more natural to 
regard the whole epibasal hemisphere as formative of the shoot : from this 
the stem-tip originates at the central point by the simplest course of 
segmentation, which happens to involve octant-walls, while the peripheral 
region of the epibasal hemisphere gives rise to the first leaf-sheath with its 
three teeth. The shoot thus established continues its apical growth 
directly upwards, forming successive three-leaved sheaths, followed soon by 
the appearance of accessory branches. The hypobasal half of the embryo 
meanwhile becomes slightly distended, as the "foot," which remains in 
contact with the prothallus after the young plant emerges. The root 
originates laterally in the hypobasal hemisphere in E. arvense and palustre 
(Fig. 214), but in E. hiemale it appears to be formed laterally at some 
distance from the base, and even from the epibasal hemisphere. 

This embryogeny accords readily with a strobiloid theory. The apex 
of the axis arises early at the usual point in close proximity to the 
intersection of the first octants, and it is dominant from true first. The 
leaves, which are minor appendages in the mature shoot, arise relatively 
late, and are not prominent features in the embryogeny. The branching 
is clearly accessory, as it is also relatively late in the time of its appearance. 






SUMMARY 395 

It has been seen in the LycopodsMhat the root is constant neither in the 
time nor in the place of its appearance: it has been also seen that it 
originates in the epibasal region in Lycopodinm and Isoetes, but in Sclagintlla 
in the hypobasal. It need therefore be no cause for surprise, but rather of 
increased interest that the point of origin of the first root should fluctuate 
within the genus Equisetum. Its indefinite position in different cases 
stamps upon it with special clearness the character of an accessory to 
the shoot itself, which its late appearance in certain Lycopods seems 
further to confirm. The whole embryo thus consists of a spindle-like axis 
with continued apical growth ; its base is like that of Isoetes without any 
suspensor. The leaves and roots appear as appendages upon this spindle- 
like axis. 

Naturally, the erabryogeny of the fossil Equisetales is not accessible 
for comparison. 

From the account of the Equisetales given in the above pages, it is 
possible to form some idea of a primitive general type for the phylum. 
They were probably, from the first, organisms with a prominent axis, 
while the leaves, of moderate size, were arranged in whorls, with 
elongated internodes between them. The root was an accessory addition 
to the shoot. Spore-production, which is so important an event in the 
antithetic alternation, does not figure in the early stages of life in 
any known Equisetal type, but appears only late in the individual life. 
There is little direct evidence among the Equisetales of any deferring 
of spore-production, by abortion of sporangia or of sporangiophores, com- 
parable with that which is so clearly indicated in the Lycopodiales. But 
comparative evidence shows that in the Equisetales spore-production is not 
restricted to branches of any definite rank, and transfers of the reproduc- 
tive function from branches of one rank to those of a higher rank may 
occur in nature, and are illustrated in various living species of Equisetum. 
This, coupled with the fact that there is essential structural similarity 
between axes of all ranks in these plants, makes it seem probable that 
axes of lower rank, and finally even the primary axis itself, may have 
been fertile in a primitive Equisetoid type : that a deferring of spore- 
production by transfer from axes of lower to those of higher order 
occurred, and that thus the initial vegetative system was greatly extended. 
In the Calamarians a secondary development of tissues in the axis accom- 
panies the enlargement of the vegetative system, which thus attained 
dendroid characters, now only faintly reflected in the smaller living forms. 

It would appear from the elongated form of the lax cone in such types 
as Calamostachys, and especially from the usual intermixture of bract-leaves 
and sporangiophores in them, that among early Equisetal types a condition 
existed not unlike that of the undifferentiated Lycopod shoot of the Selago 
type : that is, a general-purposes shoot, in which the office of spore- 
production was not strictly differentiated from the function of nutrition, 



396 EQUISETALES 

in point of fact bract-leaves and sporangiophores are associated together 
in Calamarian strobili: these may typify the primitive shoot as the strobilus 
of Lye. selago does that of the Lycopods. A separation of these appendages 
might be effected in ways which are here suggested by analogy with other 
phyla, rather than by direct observation in this. In the Lycopodiales it 
has been seen that abortion of sporangia occurred in certain regions, 
which thus became more effectively vegetative : such abortion of sporangio- 
phores would produce a vegetative region in place of a Calamarian strobilus. 
On the other hand, abortion of the bracts in the strobilus would produce 
the condition seen in Archaeocalamites or in Equisetum : moreover, in the 
Equisetales, where the sporangia are borne upon sporangiophores with 
enlarged distal ends, such protective structures are not required in cases 
where the sporangiophores are crowded; in fact the abortion of the whole 
bract-leaf in the specialised strobilus would bring with it no biological diffi- 
culty. It seems probable that both of these factors may have been effective 
in producing the conditions shown among the Equisetales. In the Calamites 
the chief distinction between the strobilus and the vegetative shoot is in 
the absence of the sporangiophores in the latter. It is true that no 
observations of vestigial sporangiophores have been recorded, but it is to 
be remembered that where abortion is complete no record remains of 
what has happened, and that this is the case in many Locopods where 
there is good reason to hold that abortion of sporangia has occurred. 
It seems probable, then, from comparison of strobili and vegetative shoots, 
as well as from analogy with the Lycopods, that abortion of sporangio- 
phores will account for the distinction of the strobili from the vegetative 
region in C alamo st achy s. 

But in other cases the segregation of leaves and sporangiophores was 
more fully carried out. In Phyllotheca successive fertile zones appear, inter- 
rupted by whorls of sterile bracts. On the other hand, the strobilus of 
Equisetum is without sterile bracts at all : this condition, which may be 
held as the more advanced, is shared by Archaeocalamites. It is, 
however, uncertain how the Equisetoid type of strobilus arose : possibly 
it was without bracts from the first : but more probably it originated 
by the complete disappearance, from the fertile head, of bracts origi- 
nally present : in this case the annulus, which survives as having a 
biological value for protective purposes, may be held to represent the last 
remnant of the series of abortive bract-whorls. The evidence for such 
progressive separation of the vegetative and reproductive functions is not 
so conclusive in the Equisetales as it is in the Lycopodiales ; but the facts, 
so far as they go, are at least in accord with a theory of such a process 
acting on a shoot in which the two functions were originally combined in 
a manner similar to that seen in other primitive Pteridophytes. 

As in many other phyla, terminal bifurcation of the axis is seen, 
but here it appears only as a rare abnormality. The normal branchings 
are accessory in their origin, and are effective as reduplications of the 



SUMMARY 397 

original shoot. The appendages ^spring laterally below the apex of the 
axis, but even in extreme types they never attain to very great dimensions. 
An interesting point is the dichotomous branching of the leaf in early 
forms : this is important for comparison with other phyla, and will have 
its place especially in the comparative morphology of the strobili. 

It is possible to account for even the most complex types of the 
Equisetales as resulting from advances along the lines of ramification and 
of progressive sterilisation above indicated, but starting from the simple 
shoot with its appendages. With this view of the general Equisetal 
morphology the development of the embryo of Eqiiisetum coincides, the 
axis taking the lead from the first, while the variability of position of the 
first root is a further indication of its accessory character. Finally, 
the vascular anatomy, so long held to be Phanerogamic in its character 
rather than Pteridophytic, is now shown to be referable in origin to a 
primitive monostele : the structure in the known forms is far removed, it 
is true, from the condition of a solid xylem-core ; but it has been shown 
that the structure of the xylem that remains is clearly indicative of origin 
from a primitive type of monostele. These characters taken collectively 
point in no uncertain way to a strobiloid origin of the Equisetal 
sporophyte. 



CHAPTER XXVIII. 

II. SPHENOPHYLLALES. 

THIS second phylum of the sporangiophoric Pteridophytes includes the 
title-family of extinct fossil plants, the Sphenophylleae, and associated with 
them, though perhaps somewhat aloof, as differing in certain important 
features, is the living family of the Psilotaceae : this contains the genera 
Psilotum and Tmesipteris. Certain other imperfectly known fossils may 
also find their best place in this relationship. The Sphenophyllales are 
characterised by having a dominant axis, with protostelic structure, which 
bears leaves of moderate size, with more or less furcate branching, and 
arranged either in whorls (Sphenophylluni) or alternate (Psilotaceae). An 
important distinctive character is the insertion of the sporangiophores not 
directly on the axis, but upon the appendages : they are thus marked 
off clearly from the Equisetales, notwithstanding that they have many 
points of resemblance to them : these points are more marked in the 
Sphenophylleae, while the relation of the Psilotaceae is rather towards 
the Lycopodiales. The whole phylum thus occupies an intermediate, or 
perhaps a central position, which gives its study a very special interest. 

A. SPHENOPHYLLEAE. 

This ancient and long extinct family is represented according to present 
knowledge by the undivided genus Sphenophyllum : but associated more 
or less distinctly with it is the complex strobilus known as Ckeirostrobus. 
The Sphenophylleae as at present known dated from the Calciferous 
Sandstone series of the Lower Carboniferous formation, and extended 
upwards to the Permian. They were plants of straggling habit, with the 
usual vegetative region preceding the spore-producing parts : these were 
commonly borne upon definite terminal strobili, but at least one case is 
known where the definition of the vegetative and reproductive regions was 
less clearly marked. In the case of Cheirostrobus the vegetative region 
is still unknown 



(iKXKRAL MORPHOLOGY 



399 



The vegetative system of Sphtnophyllum consisted of a slender axis 
(Fig. 215), with elongated and fluted internodes intervening between 
successive superposed whorls of leaves, which in the cone, and sometimes 
in the vegetative region, were more or less webbed below. The branching 
of the shoot was irregular and monopodial : the branches were isolated 
and apparently axillary, 1 though it seems uncertain whether they were 
not actually, as in Equisetum^ inserted 
between two of the whorled leaves rather 
than in the axil of one. 

The leaves in each whorl numbered, 
as a rule, some multiple of three, six being 
a frequent number, though as many as 
twelve, or even eighteen, may be found 
in some species. They were commonly 
wedge-shaped, and more or less forked 
in the venation, with very various cutting 
extending more or less deeply between 
the forks. In some of them, and especi- 
ally in the early forms, the leaves were 
divided into linear or even filamentous 
segments (Fig. 216, A, B.). Potonie points 
out 2 that the earliest forms had narrowly 
linear, branched leaves, those of later 
occurrence had larger, more broadly wedge- 
shaped, and unbranched leaves : thus the 
size of the leaf increased in the rising 
geological scale, while the branching of it 
fell off. But, on the other hand, a striking 
feature illustrated in the well-known S. cunei- 
folium was the heterophyllous character. 
Here on the same plant finely cut leaves 
may be found below and broader, wedge- 
shaped leaves above, while in the strobilus 
the leaves are again finely cut (compare 
Fig. 215). Commonly the members of one 
whorl were equally developed, but in the 
forms from the Glossopteris Flora, named 
Trizygia, they were unequal. Examples of the leafage of different types of 
Sphenophylls are shown in Fig. 216, A, B, c, D. The plants were fixed in 
the soil by diarch roots, which appear to have been borne on the nodes ; 
but the details regarding them are imperfectly known." The whole plant 
seems to have been of a weak, straggling character. 

The internal structure possessed greater distinctiveness than the external 
form, and showed a marked secondary thickening : this originated very 
1 Scott, Studies, p. 82. - Engler and Prantl, i., 4, p. 516. a Scott, Studies, p. 92. 




Fu;. 215. 

Sphenophyllitm s/>., branched stem, bearing 
linear and cuneate whorled leaves on different 
parts. The branch (a) terminates in a long 
and slender cone. Half natural size. (After 
Stur, from Scott's Stjidies in Fossil Botany.) 



400 SPHENOPHYLLALES. A. SPHENOPHYLLEAE 

early, so that the unaltered primary state is seen only in small twigs. 
Here a protostelic structure is seen, without any pith or conjunctive 
parenchyma. The primary xylem is of triangular form, the groups of 
protoxylem, either single or double, being at the projecting angles; or 
the angles may be duplicated, and a hexarch form be attained. The 
vascular system is strictly cauline : it passes through the nodes without any 
appreciable change of structure, a point of interest for comparison with 
the Equisetal structure as interpreted by Gwynne-Vaughan. 1 A peculiarity 
of some importance for further comparison is shown in the primary wood 







D 



FIG. 216. 



A = a leaf-whorl of Sphenophylhun ctmeifolium, and one leaf of it somewhat enlarged. 
B a. leaf- whorl of Sphenophllynm tenerrimum. C Sphenophyllum verticillatum. 
(From Potonie's Lehrbuch der Pflanzenpalciontologie.) D = " Trizygia " speciosa. 
Royle, from the Glossopteris-facies, (after O. Feistmantel.) 

of the ancient species, S. insigne, from the calciferous sandstone : here a 
canal is formed at each of the three angles of the primary wood, pre- 
sumably by disorganisation of the protoxylem, as in the Equisetales 2 
(Fig. 217). The cambial activity commences immediately outside the 
primary wood, and results in a broad zone of secondary wood, which 
completely surrounds the primary : it is traversed by continuous medullary 
rays in S. insigne, but in the later species these are represented only by 
little groups of thin-walled cells, which form, nevertheless, a continuous 
system. Outside the wood lie the phloem and the cortex, the latter 
showing a formation of periderm, which may be repeated, resulting in 
a scaly bark. 



Compare Williamson and Scott, Phil. Trans., vol. clxxxv., part, ii., p. 922. 
2 Scott, Studies, p. 88. 



ANATOMY 



401 



The leaves appear both from, their size and from their structure 
to have been the assimilating organs, while the axis took little part in 
that function. Their parenchymatous tissue was, however, mechanically 
strengthened by bands of sclerenchyma. The vascular strands given off 
at the nodes usually branched within the cortex of the stem into strands 
which passed out as the veins of the leaf, though in some cases a single 
strand entered the leaf. 




FIG. 217. 

Splienophyllutn iusigne. Transverse section of rather young stem, showing triangular 
primary wood with a canal at each angle, marking the protoxylem, then secondary 
wood, remains of phloem, and the primary cortex showing two of the furrows. X about 
30. From a photograph, Phil. Trans. W. and S. Will. Coll., 919. (Block from Scott's 
Studies in Fossil otany.) 

The strobilus of Sphettophyllum was constructed on a plan similar to 
that of the vegetative shoot, with slight structural differences, and with 
the additional fact that the spore-producing parts are present. These took 
the form of sporangiophores, resembling in their main features those of 
other sporangiophoric Pteridophytes. The most obvious differences between 
the strobilus and the vegetative shoot are that the internodes are shorter, 
and the leaves, which are elongated as before, frequently show a distinct 
webbing below. The result is that the whole cone appears externally as 
a compact body, with the sporangiophores very adequately protected till 
mature (compare Fig. 215). The various fossils described under the 
generic name of Sphenophyllum show differences of detail in the number 

2 c 



402 SPHENOFHYLLALES. A. SPHENOPHYLLEAE 

and position of the sporangiophores, as well as in the number of the 
sporangia borne by each of them. These differences offer curious analogies 
to those of floral construction in Angiosperms : but the latitude of 
variation here shown is such as would in Angiospermic flowers form the 
basis of much wider distinctions than those of species, or even of genera. 
It is not improbable that upon this basis the genus will ultimately be 
broken up, as detailed knowledge of it increases : meanwhile the following 
types of disposition of the sporangia have been described. 

The simplest is that seen in S. trichomatosum, Stur, from the Middle 
Coal Measures, where the sporangia appear solitary near to the axils of 

the subtending bracts, which were here of very 
narrow form. It is an open question whether 
y the single sporangium was here really sessile, 

'7 v j or was borne upon a vascular stalk, as in other 

BU* species, but in this case exceptionally short. The 

/ \ evidence derived from impressions does not suffice 

FlG 2l8 to decide this point (Fig. 218). From the guarded 

.statements of Zeiller, 1 it appears probable that a 

$pfienof>nyllum trichomatosum, ^ r 

Stur. Diagrammatic figure of the similar disposition of the sporangia is found also 

arrangement of the sporangia. 

(After Kidston.) J n , angustifolium and tcncrrimum, and it may 

be noted that these are all small species with 

narrow leaves. In the well-known S. cuneifolium, Stern (S. Dawsoni, 
Will, and Scott), each sporangium, single as in the foregoing species, is 
borne upon an elongated pedicel the sporangiophore. The sporangio- 
phores in this case are, as a rule, twice as many as the bracts of the 
subtending whorl: each is traversed by a vascular strand which terminates 
at the base of the sporangium. The sporangiophores are inserted close to 
the base of the leaf-verticil, which is here webbed into a wide cup : and 
to this the pedicels may be adherent for varying distances upwards (Fig. 
219). The vascular supply of the sporangiophores is derived by branching 
from that of the subtending bract, of which they thus seem to be 
appendages. In the regular cases the foliar strand on entering the verticil 
divides into three, the single lower branch supplies the bract, while the 
other two enter the two sporangiophores.' 2 A further complication is seen 
in S. Romeri, Solms Laubach, for in this cone two sporangia are borne 
on each sporangiophore, hanging down from its peltate distal end. The 
sporangiophores are disposed in three concentric verticils on each whorl of 
bracts, and are attached by short stalks traversed by a vascular strand, 
which branches to supply the two sporangia (Fig. 220). The analogy with 
the sporangiophore of the Equisetales is more obvious here than in the 
previous cases, where only a single sporangium is borne on each. But it 
appears still more plainly in S. majus, Brongn., from the Middle Coal 
Measures, but as yet known only from impressions. This species is 

1 1} Appareil Fruct. d. Sphenophyllitm, pp. 31, 32. 
2 For details, see Scott, Studies, p. 93, etc. 



SPORE-PRODUCING MEMBERS 



403 



interesting from the fact that its ^strobilus is not a strictly definite one 
(Fig. 221). The number of the leaves in the whorl of the vegetative 
region is not constant : six to eight have been observed by Mr. Kidston. 




FIG. 219. 

Spkcnophyllum Daivsoni. / Obliquely transverse section of a cone, showing parts of 
three whorls of bracts, a" = hollow axis (stele missing); l>, d= cortex of axis; e, e? = bracts 
cut at different levels ; /"= sporangiophores, the innermost just springing from a whorl of 
bracts, which are here coherent ; f sporangiophores in connection with their sporangia ; 
") g', S" sporangia of the three whorls. Xj. After Williamson, Phil. Trans. Will. 
Coll., 1049 B. (From Scott's Studies in Fossil Botany. ,) 

The branching of the leaves is variable, and even the two halves of one 
leaf may be unequal : the sporophylls are especially narrow as compared 
with the foliage leaves. The strobilus is characterised by the shorter 
length of the internodes, though this is variable also in the vegetative 
region : a gradual transition occurs at the limits of the fertile tract, but 
without any sudden alteration of the size or form of the leaf: the sporo- 
phylls stood out from the axis just like the ordinary foliage leaves, but 



404 SPHENOPHYLLALES. A. SPHENOPHYLLEAE 

were united at the base into a narrow sheath or collar surrounding the 
axis. The transition to the strobilus is plainly seen at the lower limit of 
the large specimen from the Brussels Museum, described and figured by 
Kidston : towards the upper limit of the specimen, where the sporangia 
cease, the axis is continued in the vegetative manner, with longer internodes. 
These facts plainly point to the absence of a highly differentiated strobilus, 
and the existence in this species of a " Se/ago" condition, where the fertile 

region is a mere zone on a continued axis. Not 
j only does S. majus stand as yet alone in the genus 

by the indefiniteness of its cone, but also in the 
character of its sporangiophores. One of these is 
borne near to the base of each forked sporophyll 
(Fig. 222) : the sporangia, which are 4-6 in 
number, but usually four, are grouped round a 
central attachment ; and though no elongated 
pedicel can be seen, still the fact that when they 
are removed from the bracts they still remain 

FIG. 220. . , . _ - .-._. 

united in groups of four to six indicates that 

Sphenophylluin Roemeri. Dia- 111 i T c 

grammatic sketch of the arrange- they had a common base. In favourable cases 
the SP rangia - (Aftei Kidston has been able to demonstrate that a 




radial line of dehiscence is clearly marked, 

corresponding in position to that of the synangium of Psilotum^ to which 
the whole structure shows a remarkable resemblance. As a last type, and 
not the least remarkable of this variable genus, may be mentioned the 
fructification of S. fertile, recently described by Scott. 1 It is characterised 
by the fact that both the "dorsal and ventral lobes are fertile," by which 
is meant that the bract bears sporangia as well as the sporangiophore, 
which it subtends. Dr. Scott remarks that this is "more probably due to 
special modification than to retention of a primitive condition." With 
this opinion I readily concur, adding the further comparison of this 
condition with the common variation of Botrychium Lunaria, where the 
sterile leaf is often partially, or even completely fertile (compare Fig. 85). 
Lastly, there remains to be described that remarkable cone from the 
Calciferous Sandstone of Burntisland, named by Scott Cheirostrobus, and 
placed by him in relation to the Sphenophyllales, while recognising also 
its affinities with the Equisetales and Lycopodiales. 2 The vegetative system 
of the plant of which this is the fructification is still unknown. The cone 
itself is of large size, and shows greater complexity than any of the known 
sporangiophoric types. The robust axis shows structural characters sug- 
gestive of a Lycopodinous rather than of a Sphenophylloid affinity : the 
central stele in transverse section has a solid star-shaped xylem-core, with 
twelve projecting protoxylem-groups, corresponding to the series of sporo- 

1 Proc. Roy- Soi\, Dec., 1904, and Ann. of />'<?/., xix., p. 168, also Progresses Rei 
Botanicce, i. p. 151. 

2 Scott, Phil. Trans., vol. 1896, 1897, "On Cheirostrobus." 



SPORE-PRODUCING MEMBERS 



405 





FIG. 221 



Slab showing fertile shoots of Sphcnofihyllum majus, Bronn. sp. After a photograph 
from the specimen in the Musee roy. d'hist. nat. de Belgique, Brussels, and here inserted 
by permission of the director, M. E. Dupont. The curved specimen running up the 
middle of the slab shows a vegetative region with long internodes above and below, and 
a fertile region showing short internodes between them. 




4 o6 SPHENOPHYLLALES. A. SPHENOPHYLLEAE 

phylls. ] These were arranged in whorls of twelve, and were superposed : 
each consisted of three sterile lobes palmately divided, and it bore upon 
its upper surface, and inserted close to its base three sporangiophores ; 
each of these was provided with four long pendent sporangia attached to 
its peltate distal end (Fig. 223). So far as the vascular connections are 
a guide, it may be concluded that the sporangiophores are appendages 
of the branched sporophyll, and especially of its middle 
segment, since a vascular strand supplying them originates 
from the bundle which runs into the middle segment of 
the sporophyll. This strand divides then into three, 
and one branch enters each of the sporangiophores 
(Fig. 224). Thus, as Scott himself points out, 2 the 
course of the vascular bundles supplying the sporangio- 
phores and bracts is essentially the same in Spheno- 
phyiium and Cheirostrabus, though necessarily more 
complex in the latter. 

FlG. 222. 

There can be little doubt of the fundamental 

Forked sporophyll of 

sphenophyiium majus, correspondence of the various types above described : 

bearing sporangiophore. ..,'.. , 

(After Kidston.) they all coincide in the presence of spore-producing 

parts subtended by sterile bracts arranged in whorls : 
and notwithstanding their differences in number, and in the number of 
sporangia which they individually bear, it is safe to conclude that the 
sporangiophores are homologous throughout the series. Their similarity of 
general structure to the sporangiophores of the Equisetales is most clearly 
seen in Sphenophyiium majus ^ or in greatly elongated form in Cheirostrobus : 
considering this in conjunction with their correspondence in function, there 
is reason also to recognise a distinct relation to the sporangiophores of 
the Equisetales. In point of position there is the difference of their being 
leaf-borne, as against the axial insertion of the Equisetales ; in fact the 
relation to the leaf is similar to that often seen in them, but closer. 
The conclusion seems inevitable that the sporangiophore in these two 
phyla is a member of similar morphological rank, though it may in both 
phyla show some variety in its exact position. 

And here it will not be inapposite to point out in support of this view 
some features of structural similarity which exist between the Equisetales and 
the Sphenophyllales. They will be best illustrated in brief by the juxta- 
position of Scott's two figures (compare Fig. 217 with Fig. 225). The 
former shows the transverse section of the ancient S. insigne from 
Burntisland, which differs from the later Sphenophylls in having a canal 
marking the position of the protoxylem at each angle of the primary wood ; 
also in having continuous medullary rays in the secondary wood, and 
scalariform tracheides in place of those with numerous bordered pits. 

1 A second specimen, belonging also to Mr. Kidston, to whom the original discovery 
was due, shows only eleven protoxylems. 
2 Z.r., p. 113. 



SPORE-PRODUCING MEMBERS 



407 



Comparing this with Fig. 225, Which is from the axis of Calamostachys 
Binneyana, .there is a similarity in outline of the primary stele ; but as 
this is not constant in the species it cannot bear weight in the comparison. 
The points of importance are, the similar canals, of like position to those 
of 6". insigne, and like them showing the position of the protoxylem ; the 
continuous medullary rays, and the similarity of the tracheides. These 




Fit;. 223. 

Chcirostrobns Pettycurensis. Diagram. The upper part in transverse, the lower in 
radial section ; the position of the organs corresponds in the two sections, i. Transverse 
section. Six complete sporophylls, each with three segments, are shown ; Sp. a = section 
passing through sterile segments; S^.6 ditto through fertile segments, or sporangio- 
phores ; j/=lamina of sterile segment ; ^j = downward outgrowths of sterile laminae cut 
transversely ; $/ = their apices, transverse ; _/== peltate sporangiophores ; ^. in sporangia. 
Note that in Sp.a each peltate lamina,/", is seen in two distinct lobes, with the sterile lamina 
between ; v.b\, -v.ba vascular bundles of two whorls. 2. Radial section. The sporophylls 
are separated from one airbther for clearness' sake, in nature they are in close contact. 
,*/.r = axis of cone; fj^its stele; ^/* = base of sporophyll. Other lettering as in trans- 
verse section. The diagram is true to nature as regards proportions of parts, as well as 
their relative position. X about 2. (From Scott's Studies in Fossil Botany.) 

features appear to indicate a real structural resemblance, and it is important 
to note that the nearest approach is between the oldest of the Sphenophylls 
and the strobilar structure of a Calamite : for according to the views here 
advanced, it is in the strobilus that the more primitive structure might be 
anticipated. 

A special interest in relation to the strobiloid theory attaches to 
Sphenophyllum majus, with its ill-defined cone. It is important to note 
that this state, so prominent in Lycopodium, is found in that species 



408 



SPHENOPHYLLALES. B. PSILOTACEAE 



of Sphenophyllum in which the arrangement of the sporangia is in a group 
disposed radially around a central attachment a condition comparable 
with that of the Calamarians, and which was 
probably a relatively primitive state. Finding these 
two features combined in the same plant gives to 
both additional weight. But they are also combined 
in that other series which, following the suggestion of 
Thomas, are here included with the Sphenophyllales, 
viz. the Psilotaceae. These will now be described, 
and the general discussion of the morphology of the 
sporangiophoric Pteridophytes will be reserved till it 
can be illuminated by the facts which these living 
genera supply. 




FIG. 224. 



B. PSILOTACEAE. 



Diagram cf the vascular 
supply to the sterile lobes 
(st), and to the sporangio- 
phores (f) in Cheirostrobus. 



The genera Tmesipte?-is and Psilotum are the only 
living representatives of this peculiar and somewhat 
isolated family, while there is nothing known among 
Fossils which can with any certainty be ascribed to 

it. They have commonly been classed with the Lycopodiales, and, as 
we shall see, there are many undoubted points of resemblance in that 




FIG. 225. 

Calamostachys Binneyana. Transverse section of axis of cone, showing stele and part 
of cortex. Surrounding the pith there are six bundles, in groups of two, with secondary 
wood. /jr = protoxylem groups. X about 60. Phil. Trans. W. andS. Will. Coll., 1016. 
(From Scott's Studies in Fossil Botany.') 

direction. But increasing knowledge of the Sphenophylleae, as well as 
of the Psilotaceae themselves, has indicated a more natural position of 
both together in the phylum of the Sphenophyllales. The two genera of 
the Psilotaceae are so similar in their general characters that there is no 
doubt of their close affinity : on the other hand the differences of detail 



CiKNKRAL MORPHOLOGY 



409 



between them, as well as the variations in the individuals in either genus, 
afford an important basis for 
comparison with other forms, 
throwing light upon fluctuations 
of structure which would other- 
wise be more puzzling than 
they now appear to be. 

Both genera are rootless. 
The green, more or less shrubby 
shoot, is established in the 
substratum, which is usually of 
humus character, by means of 
a plexus of leafless rhizomes 
invested with rhizoids, and 
penetrated by a mycorhizic 
fungus. The nutrition of these 
plants is thus of a mixed char- 
acter, partly saprophytic, partly 
by photosynthesis. The aerial 
shoots bear appendages of two 
sorts, described as foliage leaves, 
which are simple, and sporo- 
phylls, which are forked. These 
may be associated together 
irregularly on the same shoot 
which thus takes the character 
of a lax, undifferentiated stro- 
bilus. 

In Tmcsipteris, of which the 
single species T. tannensis is 
native in Australasia, though 
extending northwards to the 
Philippines, the structure is 
more simple than in Psilotum. 
Its habit is peculiar, the plant 
being established on the trunks 
of tree-ferns, though occasion- 
ally it has also been found 
growing upon the ground. 
The rhizome, which fixes it in 
the substratum, is repeatedly 
branched in a dichotomous 
manner and is without appen- 
dages other than rhizoids. Branches of this system turn upwards to the 
light, and develop as the aerial shoots : these are usually themselves 




D 



FIG. 226. 



Tinesipteris tanncnsis, Bernh. A = Habit-figure of a whole 
plant (pendent form), snowing a dichotomy. Natural size. 
B-E, sporophylls, with synangia ; J5, seen from the side ; C, 
from above ; D, after dehiscence ; E, from the under (dorsal) 
side, all X about 3. F rhizome V z natural size, 
old ster 



verse section of old stem, X4- 
Prantl, Nat. Pflanzenfam.) 



(After Prit/el in Engler and 



4io 



SPHENOPHYLLALES. B. PSILOTACEAE 



unbranched, though occasionally a dichotomy may be observed (Fig. 226 A). 
The aerial shoots differ from the rhizomes in bearing appendages : first, 
small scale-like bodies are produced, but higher up they enlarge gradually, 
till the condition of the fully-formed foliage leaf is attained : this is about 
half an inch, long, and flattened in a vertical plane. The basal vegetative 
region is continued directly into the fertile region : here the distinctive 
feature is the forked sporophyll, 1 which bears the sporangiophore seated 
at the fork, and on its adaxial surface : each sporangiophore supports 
two large and confluent sporangia (Fig. 226 B, c). The disposition of 
the leaves upon the mature axis is irregularly alternate, and this appears 
in transverse section of the apical bud (Fig. 227) : here the axis shows 





FIG. 227. 

Transverse section through a spor- 
angiferous bud of Tmesipteris. ax = 
axis. , y=foliage leaves. I lateral 
lobes, .y^synangia. X2o. 



FIG. 228. 

Tmesipteris tannensis, various unusual forms 
of sporophyll and sporangiophore ; in i. thesyn- 
angium is abortive ; in ii. and iii. one loculus is 
abortive ; others show a larger number of loculi 
than two ; others again, right and left on the 
lower row, show a single loculus, the septum 
being imperfect, or absent. 






a very irregular outline owing to the decurrent bases of the appendages : 
it is also apparent that these are alternate : it may also be noted that 
in the case figured three foliage leaves (/) are inserted above the three 
sporophylls (/, sy, /). 

The fertile region forms a very lax strobilus, in which the following 
features may be noticed. It does not differ markedly from the vegetative 
region in the size of the parts which it bears : it is not composed exclusively 
of sporophylls, but foliage leaves of the usual type may be interspersed 

1 The terminology here used is that of Scott (Stttdies, p. 479). I regret having in 
1893 (Sttidies, part i.) used the term sporangiophore in a wider sense than here, so 
as to include the bifid sporophyll itself. Such an extension of the term obscures the 
natural comparisons not only with the Sphenophylleae, but also with other sporangio- 
phoric types. It is best to restrict the use of the word in the Psilotaceae to the 
body borne by the bifid sporophyll, often designated also the synangium. The various 
opinions previously held as to the morphology of these appendages need not be discussed 
again here. It will suffice to refer to my Studies, i., p. 539, where they have been 
considered at some length, with references to the literature relating to them. See 
also Lignier, Bull. Soc. Linn, de Normandie, 1904, p. 95, and footnote. 



GENERAL MORPHOLOGY 411 

among the bifid sporophylls : ndt v uncommonly there is a reversion from 
the strobilus back to the ordinary vegetative state. In fact, as regards 
relation of foliage leaves and sporophylls, the condition is the same as 
that in the " Setago " section of Lvcopodium, with its successive, but little 
differentiated, sterile and fertile zones. But not uncommonly the fertile 
zones of Tmesipteris show differences from the normal as regards the details 
of the spore-bearing members at the limits, or about the middle of the 
fertile zones : l about the upper and lower limits, but especially at the upper, 
variations of reduction from the normal, both of sporophylls and of 
synangia may be found : these may appear in the abortion of either 
loculus, or of both of them (Fig. 228 i. ii. iii.) : or the two loculi may be 
imperfectly formed, the septum being incomplete between them, and the 
synangium is then replaced by a single loculus (Fig. 228 lower row). It 
would appear that these reductions are to be correlated with deficient 
nutrition at the limits of the fertile zone. Conversely, about the middle of 
a fertile zone, where presumably the nutrition at initiation of the parts is 
most efficient, certain sporophylls may be developed beyond the normal 
limits : in the simpler cases an additional loculus may appear in the 
synangium (Fig. 228); but in well-developed plants Thomas has found 
that not infrequently the sporophylls may show a repeated dichotomy, and 
two or even three normally shaped synangia, or sporangiophores, may be 
produced, one at each fork of the sporophyll. He has also described how 
the sporangiophore is not always sessile as it is normally, but may be 
raised up on a longer or shorter stalk ; also that it may at times be replaced 
by a leaf-lobe of outline like those which are normal. The theoretical 
bearings of these several variations, which do not appear to be uncommon 
where the plant flourishes well, will be discussed later. 

In Psilotum the main features resemble those in Tmesipteris, but with 
differences of detail. The genus consists of two well-marked species, 
P. triquetrum, which is upright and shrubby, with a radially constructed 
shoot, and P. flaccidum, which is weak and pendulous, with a bilaterally 
flattened shoot, bearing the appendages on its margins. The underground 
rhizomes are rootless and leafless, as in Tmesipteris, but are more profusely 
bifurcate : they are covered with rhizoids, and show mycorrhiza. They 
produce gemmae, which' freely propagate the plant vegetatively. The 
aerial shoots also bifurcate much more freely than in Tmesipteris, in 
planes successively at right angles (Fig. 229). On these the minute vege- 
tative leaves are disposed, but with no constant or definite arrangement : 
they appear as small subulate processes, arising from the projecting angles 
of the green axis, and are commonly without vascular tissue. In the upper 
regions of strong shoots they are replaced by sporophylls which are bifurcate 
as in Tmesipteris, though very small : each bears a short-stalked sporangio- 
phore, which supports three synangial sporangia. Here as in Tmesipteris 

1 Many of the details here embodied are taken from Thomas, Proc. Roy. Soc., vol. Ixix., 
P- 343- 



412 



SPHENOPHYLLALES. B. PSILOTACEAE 



the fertile shoot is very lax, and does not form a definite strobilus, while 
foliage leaves are interspersed irregularly between the sporophylls. There 

is thus a " Selago " condition in 
Psilotum also, and it is even more 
obvious than it is in Tmesipteris. 
Psilotum is also open to deviations'" 
of structure of the spore-producing 
parts : reduction of the sporangia 
from the normal three to two is 
not uncommon, though it appears 
frequently to be the result of arrest 
of one of the loculi. In other 
cases the number of the loculi 
may be increased to four or five. 1 
Thus in the Psilotaceae sporangio- 
phores may be found bearing 
sporangia variable in number from 
one to five. The observations of 
Thomas also show 2 that there are 
fairly numerous instances in Psilo- 
tum of a second dichotomy of one, 
or even of both branches of the 
forked sporophyll : in the former 
case two sporangiophores are 
present and three minute leaf-lobes, 
in the latter case three sporangio- 
phores and four leaf-lobes. The 
synangia are in these instances 
closely crowded together, and in 
some cases at least irregular quin- 
quilocular synangia are due to the 
fusion of two original primordia in 
close proximity. There are no 
statements as to the position in 
the fertile region which these 
abnormalities hold. From the 
above facts in Psilotum, as well 

aS th S6 in TmesiptertS, Thomas 

Hrnwc trip> rnnrlncinn " f haf rpnparpr? 
araWS tne ' 

dichotomy of the Sporophylls of 

' 

the family Psilotaceae is an ancient 
feature." Without accepting this position, this much at least is clear, 
that the present Psilotaceae possess morphological possibilities of further 

1 Solms Laubach, Ann. fard. Bot., Buitenzorg, iv., p. 174. 
'* Proc. Roy. Soc., vol. Ixix., p. 349. 




FIG. 229. 

Psilotum triquetn<m, Sw. Shoot showing repeated 
dichotomy, bearing minute tooth-like sterile leaves, and 
turgid synangia attached to the minute forked sporo- 
phylls. It is to be noted that a sterile region intervenes 
between two fertile regions. Natural size. 



SPORE-flRODUCING MEMBERS 413 

amplification beyond what is heleK to be the normal for them. It is a 
different question whether these were ever effectively realised in the past, 
and thus figured as normal features in any ancestral race. Nevertheless it 
is hardly possible to avoid the comparison of the forked sporophylls of 
the Psilotaceae, and these extra branchings, also with forked leaves, which 
'are so prominent a characteristic of the Sphenophylleae. 

From the study of the external characters of the living Psilotaceae it 
appears that the sporophyte is readily referable to a strobiloid origin. The 
rootless condition and the leafless rhizomes present no difficulty, but 
rather the reverse. It may, however, be a question whether this condition 
was primitive in them, or the result of reduction in accordance with their 
peculiar habit. As regards the lax shoots, the dichotomous branching is 
reminiscent of the Lycopodiales rather than the Sphenophyllales. The 
vegetative development of the lower parts of the aerial shoots, as well as 
the " Selago" condition so clearly seen in their upper regions, corresponds 
to that of the simpler Lycopods, while it finds its correlative also in 
Sphenophyllum majus. The chief points of divergence as regards external 
form are the shape of the leaves and sporophylls, and their alternate 
arrangement, though they share the latter with most of the Lycopods. 
The reduction or abortion of sporangiophores about the limits of the 
fertile zones compares with the imperfect development of abortion of 
sporangia in a similar position in Lycopodium Selago and others ; while the 
amplifications noted by Thomas about the middle of the fertile zones in 
Tmesipteris only accentuate the recognition of those zones as distinct from 
the sterile parts. Accordingly the general reference to a strobiloid origin 
will apply to the Psilotaceae with equal force to that in the case of 
Lycopodium, and this will be so upon the facts themselves, whatever the 
genetic relations may have been between the Psilotaceae, the Lycopods, 
and the Sphenophylls. 



DEVELOPMENT OF THE SPORE-PRODUCING PARTS. 

The apical cone of Tmesipteris is very variable in bulk : in strong 
young shoots it may be a broad dome, while in weaker specimens, or 
those in which the apical growth is beginning to fail, it may be com- 
paratively narrow. Passing from the actual apex the sides of the cone are 
covered externally by deep prismatic cells, which are of somewhat irregular 
origin, depth, and arrangement. When a leaf or sporophyll is about to be 
formed, certain of these increase in size, and undergo both periclinal and 
anticlinal divisions so as to form a massive outgrowth, the summit of 
which is occupied, as seen in radial section, by a single cell of a wedge- 
like or prismatic form : it is not improbable that the latter passes over to 
the wedge-like form as the part develops. In these early states it is 
impossible to say whether the part in question will be a vegetative leaf or 
a sporophyll, and even when older it is still a matter of uncertainty, so 



414 



SPHENOPHYLLALES. B. PSILOTACEAE 



similar are they in their initial stages, though so different when mature. 
Those, however, which are to develop as sporophylls soon show an 
increase in thickness, while they grow less in length ; an excrescence of 
the adaxial surface soon becomes apparent (Fig. 230 A), in which the 
superficial cells are chiefly involved : this constitutes the sporangiophore. 
The superficial cells at first form a rather regular series: they undergo more 







FIG. 



230. 



Tmesipteris tannensis. /4=very young synangium arising from the adaxial surface 
of the sporophyll. X 100. .5 = a sporophyll bearing a much older synangium ; the sporo- 
genous masses are shaded. X 100. C = part of a radial section of a mature synangium 
showing the insertion of the septum. X 100. 

or less regular divisions: a band of tissue some four or more layers in depth 
is thus produced. At about this period certain masses of cells assume 
the characters of a sporogenous tissue (Fig. 230 B) ; but though they can 
be recognised as such by the character of the cells, it is exceedingly 
difficult to define the actual limits of these sporogenous masses. The 
more superficial tissues, as well as the band intervening between the two 
sporogenous masses remain sterile, the latter developing into the septum, 
while the former develop into the walls of the synangium: it is specially 



SPOkK-PRODUCIXG MKMBKRS ' 415 

to be noted that the origin of .tjie tissue of the sterile septum, which 
separates the sporangia, seems to be similar to that of the sporogenous 
masses themselves. 

A- the development proceeds, it is still difficult to recognise with 
certainty the exact limits of the sporogenous masses : this is probably due 
to the fact that there is no very clearly defined tapetum, nor is the whole 
of the sporogenous mass used up in the actual formation of spores ; but a 
considerable proportion of the cells composing it, acting as a diffused 
tapetum, become broken down, and disappear in a manner similar to that 
to be described more in detail in Psilotum (Fig. 231). 

Finally, a strand of vascular tissue, of which the origin may be traced 
in Figs. 236 A, B, is formed, extending upwards into the sporophyll ; 
on entering the sporangiophore, it passes up to the base of the septum, 
and there branches right and left, the two branch-bundles traversing the 
margins of the septum. 

When mature, the wall of the synangium consists of a superficial layer 
of deep cells, with thick cell-walls, a band of thinner-walled compressed 
cells, three to four layers thick, supporting the superficial layer internally 
(Fig. 230 c). These cells have pitted walls, and are not definitely 
limited internally, but irregular tatters of cell-wall project into the cavities 
of the sporangia, showing thus that there is no clear limit between the 
wall of the sporangium and the tapetum. 

The septum shows in the main a structure similar to this inner band 
of the wall, with which it is continuous ; it consists of a firm plate of 
narrow tabular cells, four to six layers in thickness, with profusely pitted, 
woody walls. The septum is also coated by the remains of thinner-walled 
disorganised cells. As already noted, the branches of the vascular bundle 
which enters the sporangiophore pass right and left up the margin of the 
septum ; these bundles are seen as bands of tracheides in transverse sections 
through the lower part of the septum (Fig. 230 c.); the bundles are not 
sharply differentiated from the surrounding tissues, and they appear to consist 
only of xylem. A number of tracheides, continuous with the bundle, 
extend along the central part of the septum ; and from the position of the 
bundle, it appears to belong to the septum, rather than the external wall 
of the synangium. The results obtained thus from radial sections have 
been verified by comparison also of sections in other directions. 1 

An examination of the imperfect synangial sporangiophores which 
occur at the limits of the fertile region shows them to be frequently 
uriilocular : this may be in some cases due to the abortion of one or 
other of the two loculi, but in other cases it is clearly by imperfect 
development of the septum between them, as vestigial remains of it may 
often be found. It has been demonstrated that tissue which would nor- 
mally develop as septum may in these cases develop as tapetum, or 
even as sporogenous tissue (Fig. 231); and thus the unilocular condition 

1 See Studies, i. , p. 543. 



416 SPHENOPHYLLALES. B. PSILOTACEAE 






is attained. But the tissue of the normal septum is of common origin 
with the sporogenous tissue itself, and in the earliest stages does not differ 
from it in position or structure. The normal development, in fact, would 
be compatible with the view that the septum is formed from potential 
sporogenous tissue sterilised : the unilocular condition would then follow 
from reversion of that sterile tissue to its original fertile state again. 
This question is intimately connected with that of the origin of the 
sporangiophore, and opportunity will be taken later to discuss it. 

The detailed study of the sporangiophore or synangium of Psilotum by 
means of sections is more difficult than that of Tmesipteris on account of 
its trilocular character. In radial sections through the terminal bud the 








FIG. 231. 

Tmesipteris tanncnsis, Bernh. A, median section through synangium, showing the 
tissue where the septum normally is developing as sporogenous cells (s). / = tapetum. 
, part of the contents of a similar synangium, rather older, xx shows the line where 
the septum should normally be, while a chain of fertile cells stretches continuously across 
it. This drawing also indicates how cells distributed through the sporogenous tissue 
become disorganised. X 100. 

young sporophylls are found to present a general outline and structure 
similar to those of Tmesipteris. Fig. 232 A shows one such : the cell ( x ) 
is believed to represent the organic apex of the sporophyll, though it is 
doubtful whether it be this initial which gives rise to the whole mass of 
the tissue. The sporangiophore appears as an outgrowth of the upper 
surface of the sporophyll, while the tissue on the abaxial side of it is 
already growing out into a bulky projection. But it has not been possible 
to trace the development of the essential parts of the loculi of the 
synangium from the superficial cells of the adaxial side of the sporophyll 
in this case with the same certainty as in Tmesipteris : this is chiefly 
owing to the stalk being here narrower, and to the fact that only one 
loculus can be cut in a median direction in any one section ; supposing 
this to be the median plane of the whole sporophyll, then it will be the 



SPORE-PRODUCING MEMBERS 



417 



abaxial loculus which will be thus v traversed. A truly radial section of a 
young synangium is shown in Fig. 232 B, the arrow indicating the direc- 
tion of the main axis : the cell ( x ) is a conical cell, which is commonly 
though perhaps not constantly found occupying the centre of the apical 
surface of the synangium. The cell shaded is believed to be the arche- 
sporial cell for one of the loculi, but after comparison of a large number 




FIG. 232. 

Psilotuin triquctruin, Sw. Various stages of development of the synangium and 
sporangium. In C the sporogenous group is shaded. D shows the differentiation of its 
cells, the fertile cells being shaded. E shows the disorganisation of the remaining cells 
without forming spores. X 100. 

of sections I am still uncertain whether the whole of the sporogenous tissue 
in each loculus is really referable to a single parent cell, for just the same 
difficulty arises here as in Tmesipteris in recognising the exact limits of 
the sporogenous masses. 

The subsequent stages of development are illustrated by Figs. 232 c, D, E, 
and it will be seen from these how the sporogenous masses assume large 
dimensions, and are at first composed of uniform cells. The wall of the 
synangium meanwhile becomes multiseriate, and the cells of the outermost 
layer assume a deep and prismatic form, while the inner layers are narrow. 

2D 



4i 8 SPHENOPHYLLALES. B. PSILOTACEAE 

The same is the character of the more superficial cells of the sporogenous 
mass (Fig. 232 D), so that it is almost impossible to recognise the limit 
between the tissue of the wall and of the sporogenous mass : the superficial 
portions of the latter become disorganised without the formation of spores, 
but there is no clearly defined tapetum. Such is also the fate of a 
considerable proportion of the more central cells : for as the synangium 
develops, irregular groups of cells of the sporogenous masses assume 
dense granular contents, and subdivide, while the others remain paler, 
with more watery contents, and do not divide : the former undergo the 
final tetrad-division and form spores, while the latter become disorganised. 
The actual state of partial disorganisation is shown in Fig. 232 E: thus a 
partial sterilisation of cells of the sporogenous tissue, essentially similar to 
that in Equisetum^ is seen also in Psilotum, and, as above stated, it occurs 
also in Tmesipteris. 

In both genera the lines of dehiscence at maturity are defined 
structurally. In Psilotum the lines radiate from the centre, and it has 
been seen that a similar condition may be recognised in the sporangio- 
phores of Sphenophyllum majus. Thus, in the broad outlines of 
structure, in function, and in position the sporangiophore of the Psilotaceae 
is the correlative of that in the Sphenophylleae. It remains to see how 
far there is a correspondence also in the anatomical structure of these 
plants. 

ANATOMY. 

The internal structure of the Psilotaceae is relatively simple, in 
accordance with their outward form. The axis is traversed by a stele, 
which is fundamentally of the protostelic type, and limited by an 
endodermis which is more definite in Psilotum than in Tmesipteris. The 
broad cortex which surrounds the stele is variously differentiated in the 
rhizome and in the aerial shoot : in the former it consists of starchy 
parenchyma, with endophytic mycorhiza in the outer layers, while the 
superficial cells grow out into rhizoids : in the aerial shoot the stele is 
surrounded successively by thin-walled parenchyma, sclerenchyma, and 
assimilating tissue, while peripherally is an epidermis with stomata. Such 
characters, however, present little that is of comparative value ; it is in 
the vascular tissue that a better basis for comparison is found. 

The structure of the stele in Psilotum^ varies according to the position 
and size of the part : in the rhizome there is often no protoxylem, but 
the xylem consists of a somewhat irregular, and exiguous group of 
scalariform tracheides, surrounded by an ill-developed phloem, while peri- 
pherally there is a definite endodermis. At the base of an aerial shoot 
the xylem increases in bulk, with interspersed parenchyma cells, and 

1 These statements are based partly on personal observations, but also on the writings 
of Bertrand (Arch. hot. du Nord., i.), of Boodle (Ann. of Bot., xviii., p. 505), and of 
Miss Ford (Ann. of Bot., xviii., p. 589). 



ANATOMY 419 

protoxylem makes its appearance : x where the protoxylem is clear it is 
evident that the differentiation of the primary xylem is centripetal, as it 
is, with local exceptions, throughout the aerial shoot. Passing upwards 
along the aerial shoot, the peripherally projecting protoxylems increase 
in number, the whole stele enlarging : finally, in transverse section the 
xylem appears as a hollow, many-rayed star, while the centre is occupied 
by sclerotic tissue. Peripherally as before lie the phloem and the endo- 
dermis (Fig. 233). An examination of the lower part of the aerial shoot, 
and of some adjoining parts of the rhizome, shows a feeble secondary 
formation of xylem : there is no definite cambium, but the additional 
tracheides which arise from the tissue outside the primary wood often 
show signs of a radial arrangement. The secondary development fades 
off as the stellate structure of the upper region is attained. Thus 




FIG. 233. 
Psilotutn triqitetrutn. Part of a transverse section of the central stele. X 100. 

structurally the base of the stem of Psilotum recalls the stem of Spheno- 
phylhtm, though with very feeble secondary growth, while the upper part 
of the stem resembles the axis of the cone of Cheirostrobus, though on a 
simple scale, and with fewer xylem-rays. 

In the stem of Tmesipteris no secondary development has hitherto 
been found. The rhizome exhibits much the same structure as that of 
Psilotum, with a solid group of tracheides at the centre, or in weaker 
branches an irregular xylem-ring, surrounded by phloem, and a very ill- 
defined endodermis. Often there is no distinct protoxylem ; but, as the 
passage is made to the aerial stem, protoxylem may appear : its position 
in all the upper region is mesarch. In weaker shoots, and especially in 
the upper regions, the cauline tissue of the stele fades out : the leaf- 
traces become the main feature of the vascular system, which in trans- 
verse sections is then represented by a, ring of separate strands : each 
of these has its mesarch protoxylem corresponding to that seen in the 
leaf-trace (Fig. 234) : in point of fact this becomes a phyllosiphonic type 
of structure. In strong axes, however, the xylern may still form a 



420 SPHENOPHYLLALES. B. PSILOTACEAE 

coherent ring surrounding a central pith, but with the protoxylem 
mesarch. 

The chief anatomical difference between the two genera appears thus 
to be in the position of the protoxylem. But Boodle points out that 
locally a mesarch position may be found in Psilotum also, and he con- 
cludes that both genera might be referred to a common parent form, 
in which the aerial stem had a rayed mesarch xylem-mass, the suppression 
of leaf-traces having caused the loss of centrifugal wood in the one genus, 
and the influence of the leaf-traces in the other genus having broken up 




FIG. 234. 

Tinesipteris tannensis. Transverse section of the sterile region, high up. The proto- 
xylem {pr. -xy>) is mesarch. The xylem of the stele is fading out, and being replaced 
by parenchyma ; three of the tracheides (/. tr.~) show incomplete development ; there 
is no longer a complete ring, and the leaf-trace bundles (/. t.~) enter the gaps which result, 
in much the same way as in a phyllosiphonic type. There is no definite endodermis. 
Xi 50. 

the xylem into distinct bundles. 1 He further recalls the fact that in 
Cheirostrobus there are indications of a mesarch structure, while paren- 
chyma is present among the tracheides towards the centre of its stele : 
such cells in response to mechanical requirements might readily be 
converted into mechanical tissue, as in Psilotum. It thus appears that 
the Sphenophylleae and Psilotaceae show uniformity in the general type 
of their vascular construction, though the details are subject to consider- 
able fluctuation. This result adds point to the comparisons already 
based upon the external characters and the spore-producing parts. At 
the same time, it is to be remembered that a structure resembling that 
of Psilotum and Cheirostrobus is seen in certain of the Lycopodiales ; in 

1 L.c., p. 515. 



ANATOMY 421 

particular it is found in LepidostKolnis Brownii, where also the leaf-trace 
bundles are of the mesarch type. The comparison has also been made 
by Miss Ford with Bothrodendron rnundum : in this case the corre- 
spondence seems to be rather with the rhizome of Psilotum than with 
its aerial shoot. 

EMBRYOLOGY. 

Of the embryology of the Psilotaceae nothing is at present known. 
Even the prothallus has not been recognised with certainty, though 
Dr. Lang 1 has described the structure of one which may with a 
reasonable degree of probability be referred to Psilotum. It was 
closely associated with a plant of Psilotum, in a locality where no 
species of Lycopodium (with which a mistake of identity might occur) 
were observed growing in the same situation. This, as well as 
certain comparative reasons, made Lang regard it as probable that his 
prothallus is really that of Psilotum. It was a prothallus of the wholly 
saprophytic, subterranean type, corresponding to that of L. downturn or 
complanatum : it bore antheridia, but no archegonia or embryos. 

The initial embryology of the Psilotaceae is thus a complete blank. 
It is to be hoped that ultimately this blank may be filled : meanwhile the 
following remarks may be made as indicating the nature of the problem 
which the further data may be expected to solve. The relationship of 
the Psilotaceae to the Lycopods, long recognised on characters of the 
mature sporophyte, has lately been in a measure discounted by a better 
knowledge of the Sphenophylleae, though the prothallus provisionally 
attributed by Lang to Psilotum would appear to point to a strengthening 
of the former relationship. A connection also with the Equisetales is now 
more clearly recognised than formerly ; and it will be remembered that in 
these the axis asserts itself early, while the first leaf-sheath appears 
relatively late, as a subsidiary appendage. In the sporophyte of the 
Psilotaceae we see a rootless plant, with branched, leafless rhizome, while 
the appendages appear first on the aerial shoot. It may be expected that 
the embryology should show some evidence bearing on the question 
whether the leafless and rootless condition of the lower parts is primitive 
or the result of reduction.' If the embryo showed, like that of Lycopodium, 
cotyledons and a primary root, that would be positive evidence that the 
rootless and leafless condition seen in more advanced stages of the plant 
was a result of reduction. If, on the other hand, the embryo developed 
without appendages directly into the rootless and leafless rhizome, then 
either of two interpretations would be possible : either that reduction 
had been effective back to the earliest phases of the individual : or 
that the sporophyte at first represented that primitive state of an axis 
without any appendages, which a strobiloid theory contemplates in the far 
back ancestry : it is significant that some remote approach to this is seen 

1 Ann. of Bot., xviii., 1904, p. 571. 



422 SPHENOPHYLLALES. B. PSILOTACEAE 

in the embryo of Equisetum, with its direct and early assertion of the 
axis, and the relatively late and subsidiary character of its first leaves. 
It is naturally impossible to express any opinion on such points at 
present; but it is to be remarked that the facts relating to the mature 
plants of the Psilotaceae as they stand would bear either of those inter- 
pretations. So far as expressed, current opinion appears to favour the 
probability of reduction in accordance with habit, and especially so in 
the case of Psilotum, where the leaves lend themselves readily to an 
interpretation as reduced structures. But whichever view be ultimately 
taken, a strobiloid theory would meet the facts more readily than any 
phytonic theory of the shoot. 



CHAPTER XXIX. 

SUMMARY FOR SPHENOPHYLLALES AND 
FOR SPORANGIOPHORIC PTERIDOPHYTES GENERALLY. 

IT has now been seen that the two living genera of Psilotaceae, though 
differing in the number of sporangia on each sporangiophore, as well as 
in the size of the appendages, correspond nevertheless in the essential 
characters of form ; they are alike in the rootless and leafless rhizome, 
in the irregular alternate arrangement of the appendages, and in the 
relation of the sporangiophore to the forked sporophyll. The anatomical 
characters also correspond, though with differences open to biological 
explanation. No one will therefore doubt the natural affinity of these two 
genera. 

The relation of the Psilotaceae to the Sphenophylleae has been only 
lately recognised. Previously they were placed with the Lycopodiaceae, 
and in the above pages points of similarity to these plants have been 
repeatedly noted ; such as the dichotomous branching of the primitive 
monostelic axis, the imperfect differentiation of the vegetative and fertile 
regions, and the relation of the sporangiophore in the one and of the 
sporangium in the other to the sporophyll. It was Dr. Scott who first 
indicated the closer relation between the Psilotaceae and the extinct 
Sphenophylleae, on the ground of anatomical resemblance, as well as the 
similarity of the spore-producing parts ; 1 this view was further developed 
by Thomas, on the basis of observation of many specimens in their 
nat