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THE ORIGIN OF A LAND FLORA

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LYCOPODI U M,
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THE .ORIGIN

OF A

LAND FLORA

A THEORY
BASED UPON THE FACTS OF ALTERNATION

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

\ V

REGIUS PROFESSOR OF BOTANY IN THE UNIVERSITY OF GLASGOW

OF THE

UNIVERSITY

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.

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

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,

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

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.)

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

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

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

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

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

(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

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

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

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

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

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

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

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.)

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.

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.

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.

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

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

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

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

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

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

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.

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).

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

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

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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.

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.

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.

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

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

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

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.

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

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,

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.

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.)

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.)

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

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

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

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.

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
native habitat- The chief difference seems to lie in the fact that the
appendages of the former are irregularly alternate and distinct, while in
the latter they are in regular whorls, and webbed at the base. But the
genus Lycopodium, which includes species with whorled and others with
irregularly alternate leaves, shows that too much weight must not be
attached to such a distinction relating to kindred forms. 3 There is also
the difference of branching, which is terminal and dichotomous in the

1 Studies, p. 499. 2 Proc. Roy. Soc., vol. Ixix., p. 343.

3 Compare Scott, Progressus, i., p. 166.

4 2 4 SPORANGIOPHORIC PTERIDOPHYTES

Psilotaceae, and apparently axillary in the Sphenophylleae. But the analogy
of Equisetum is interesting in this relation, for there, though the normal
branching is monopodial, a terminal branching of the strobilus sometimes
occurs. Thus the points of resemblance appear greatly to outweigh the
differences, and the Psilotaceae and Sphenophylleae may well be grouped
together as representing one phylum the Sphenophyllales.

Examining the plants thus designated from the point of view of a
strobiloid theory, the Psilotaceae show in the most pronounced way the
" Selago" condition, while about the limits of the fertile zone in Tmesipteris
imperfectly formed sporangiophores are often seen, which bear a similar
interpretation to the abortive sporangia in Lycopodium. The matter is
complicated here, it is true, by the marked difference between the simple
sterile leaf and the forked sporophyll : it has been shown, however, that
in the first stages of the individual development these parts are indis-
tinguishable. Though in Sphenophyllum the strobilus is definitely marked
as a rule from the vegetative region, it is important to note that
Sphenophyllum majus, which most nearly resembles the Psilotaceae in the
form of its appendages, has also an indefinite strobilus, with continuation
again upwards into a vegetative state. Thus in both families the shoot
shows examples of imperfect differentiation. This goes along with a
development of the sporophyll, both in Tmesipteris and in Sph. majus,
as an effective organ of assimilation, which is a further mark of a low
differentiation. These facts may be held as justifying for the Sphenophyllales
a line of argument similar to that for the Lycopodiales : that a definite
strobilus has been the result of differentiation in a shoot in which the
vegetative and reproductive functions were not originally separate. It is
true that the case is not so clear for the Sphenophyllales as for the
Lycopodiales : this is consequent on the number of the known species
and of individuals available for comparison being much less, and the
knowledge of the fossils more limited. It will perhaps be objected that
the earliest known cone of this series, Cheirostrobus, was perfectly definite
and highly specialised, while the earliest indication of a less specialised
type is in Sph. majus. But the fact that so often the earliest known
fossils of any phylum are very complex does not prove that such types
were earliest in evolution. Thus Cheirostrobus among the Sphenophyllales
and Pseudobornia among the Equisetales, though respectively the oldest
representatives known, are both extreme forms, as compared with the other
members of the phyla to which they respectively belong. In estimating such
facts we should reflect that at the present day primitive and recent forms
grow side by side, and both or either might be preserved as fossils ; also
that the chances of this happening depend upon many varied factors, of
opportunity, texture, habitat, etc. The chances of discovery at the present
day are equally varied. When these points are fully considered it will
be clear that stratigraphical position of those isolated fossils which happen
to have been discovered and described should not suffice to prove an

SUMMARY 425

evolutionary priority in face of strong comparativ