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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
BY
R O. BOWER, Sc.D., RR.S.
\ V
REGIUS PROFESSOR OF BOTANY IN THE UNIVERSITY OF GLASGOW
OF THE
UNIVERSITY
OF
WITH NUMEROUS ILLUSTRATIONS
MACMILLAN AND CO., LIMITED
ST. MARTIN'S STREET, LONDON
1908
Q\<
GENERAL
GLASGOW : PRINTED AT THE UNIVERSITY PRESS
BY ROBERT MACLEHOSE AND CO. LTD.
T
E
OF
OF THE
/ UNIVERSITY
PREFACE
IN the year 1874 apogamy was discovered in Ferns by Farlow : and in
1884 instances of apospory in Ferns were demonstrated before the Linnaean
Society of London by Druery. These events stimulated a fresh enquiry into
the nature and origin of Alternation in Archegoniate Plants. My own
observations on apospory confirmed my interest in this question : it seemed
to me probable that some biological cause had determined the prevalence
and constancy of the alternation, to which apogamy and apospory appear as
occasional exceptions. The theory was entertained that the change of
conditions involved in the invasion of the Land by organisms originally
aquatic had played a prominent part 'in the establishment of those
alternating phases of the life-cycle which are so characteristic of Archegoniate
Plants. As early as 1889 I had already written several chapters of a
treatise on this subject : but the necessary facts were found to be then so
imperfectly known that the work was abandoned, and instead of a full
discussion of the matter, the Biological Theory of Antithetic Alternation
was briefly stated in a paper published in the Annals of Botany in 1890
(vol. iv. p. 347). The main position of Celakovsky in discriminating
between Homologous and Antithetic Alternation was adopted ; but the
latter type, as seen in Archegoniate Plants, was recognised as having been
fixed and perpetuated in^accordance with the adaptation of aquatic organisms
to a Land- Habit. The Studies in the Morphology of Spore-producing Members
were then entered upon as preliminary investigations to elucidate the facts
requisite for a more full statement, and they were published in five parts,
from 1894 to 1903. Meanwhile, in 1894 Strasburger contributed to the
Meeting of the British Association in Oxford his paper on the " Periodic
Reduction of Chromosomes." He brought together a wealth of facts
establishing the cytological distinction of the alternating generations, and
his theoretical position was virtually identical with that of my paper of
four years earlier.
175G03
VI
PREFACE
Now, after the lapse of seventeen years, it has been possible to state the
biological argument more fully in the present volume, strengthened by many
new facts. The First Part (pp. 1-254) deals with the general theory. The
Second Part (pp. 255-657) is taken up with a detailed statement of the facts,
together with comparison of the constituents of the several phyla inter se.
The Third Part (pp. 658-717) is devoted to general comparisons and con-
clusions. The attempt has been made to work in the results of Palaeonto
logical research with those of the comparative analysis of living forms. The
enquiry has related to all the characters, both vegetative and propagative, of
the sporophyte generation : these include the external form, the embryogeny,
and anatomical features, and especially the structure and development of
the Spore-producing members, while the characters of the gametophyte
have also been taken into account. It is found that the conclusions
arrived at are supported by general convergence of the lines of evidence
derived from all of these sources.
The method adopted in the preparation of this work has been to
examine not only the mature structure, but also the development of the
organisms, and of their several parts. While fully utilising the results of
Palaeontological and anatomical study, considerable weight has throughout
been given to the facts of the individual development : sometimes the latter
appear to oppose the former. It is not held that the ontogenetic history
will always serve as an infallible guide, and opportunity has been taken
to point out that conclusions based upon it are liable to be overruled by
the results of wide comparison (pp. 159, 636, and 660, footnote). But it
is felt that in much of the recent work on Pteridophytes, and especially
where fossil comparisons come in, the arguments from individual develop-
ment have been accorded less than their due share of attention.
I have made no attempt to give comprehensive or complete biblio-
graphical references : from Campbell's Mosses and Ferns and from other
sources such references can readily be obtained. But wherever a quotation
is made, or where a substantial body of information derived from another
author has been embodied in the text, the reference is fully given. While
thus acknowledging my indebtedness to those whose work is published,
I desire also to record the continuous personal help so willingly given by
three friends and colleagues, who have all allowed me the use of unpub-
lished drawings and facts. Mr. Kidston's peculiarly exact knowledge has
greatly strengthened and amplified the Palaeontological statements, while
Dr. Lang and Mr. Gwynne-Vaughan have given me throughout the
assistance of friendly criticism, and the support of their special knowledge
of certain branches of the matter in hand.
In conclusion, I am well aware that the chief question dealt with
PREFACE vii
lies outside the realm of possible proof under present conditions : the
theory is submitted as a working hypothesis. Naturally it is applicable
with greater readiness to those organisms which are less advanced, but
less readily to those which have departed furthest along the lines of
adaptation to life on exposed Land-Surfaces. Other opinions on the origin
and nature of Alternation have come into fresh prominence in recent
years, and especially the view that the present condition of the Arche-
goniatae has originated by differentiation of phases of a life-cycle originally
Homologous. This theory has not been disproved any more than the
theory of Antithetic Alternation has been proved Whatever view be
ultimately taken of the prime origin of the alternating generations, many
of the conclusions arrived at here as to the morphological progress and
phyletic grouping of the Archegoniatae will stand : they have a validity
of their own quite apart from any question of the ultimate origin of the
sporophyte, which has finally become the dominant factor in the Flora of
the Land.
F. O. BOWER.
GLASGOW, December, 1907.
TABLE OF CONTENTS
PART L
STATEMENT OF THE WORKING HYPOTHESIS
CHAPTER HAGR
INTRODUCTION, - i
I. THE SCOPE AND LIMITATIONS OF COMPARATIVE MORPHOLOGY, - 5
II. THE LIFE-HISTORY OF A FERN, 14
III. ON THE BALANCE OF THE ALTERNATING GENERATIONS OF
ARCHEGONIATAE, - - 33
IV. CYTOLOGICAL DISTINCTION OF THE ALTERNATING GENERATIONS
OF ARCHEGONIATAE, - 46
V. ALTERNATION IN THALLOPHYTES, - 63
VI. BIOLOGICAL ASPECT OF ALTERNATION, - 79
VII. STERILISATION,- 87
VIII. THE SPORANGIUM DEFINED, - 103
IX. SOME GENERAL ASPECTS OF THE POLYSPORANGIATE STATE, 113
X. VARIATIONS IN NUMBER OF SPORANGIA, 119
XI. THEORY OF THE STROBILUS, - 132
XII. SPORANGIOPHORES AND SPOROPHYLLS, - 144
XIII. ON THE RELATIONS BETWEEN THE STERILE AND FERTILE REGIONS
IN THE SPOROPHYTE, 156
XIV. EMBRYOLOGY AND THE THEORY OF RECAPITULATION,- 173
XV. ANATOMICAL EVIDENCE, - 188
XVI. SYMMETRY OF THE SPOROPHYTE, • - - 201
CONTENTS
CHAPTER PAGE
XVII. THE ESTABLISHMENT OF A FREE-LIVING SPOROPHYTE, 218
XVIII. EVIDENCE FROM PALAEOPHYTOLOGY, - 227
XIX. AMPLIFICATION AND REDUCTION, - 233
XX. SUMMARY OF THE WORKING HYPOTHESIS, 244
PART II.
DETAILED STATEMENT OF FACTS
INTRODUCTION, 255
XXI. BRYOPHYTA, (i.) HEPATICAE, - 257
XXII. BRYOPHYTA, (n.) Musci, - 272
XXIII. LVCOPODIALES— GENERAL MORPHOLOGY, 288
XXIV. LYCOPODIALES— SPORE-PRODUCING MEMBERS, 311
XXV. LYCOPODIALES— COMPARATIVE ANATOMY, 328
XXVI. LYCOPODIALES — EMBRYOLOGY AND COMPARATIVE SUMMARY, 340
XXVI I. EQUISETALES, - 366
XXVIII. SPHENOPHYLLALES, - 398
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 connecting1 of these sequences together so as to construct some
more or less consecutive story of descent : indeed, this can only be done
when liberal use is made of the imagination, in bridging over the wide
gaps in the series, which even the known fossils are so far from filling.
The details of a story thus constructed depend so largely on comparative
opinion, or balancing of probabilities, and in so slight a degree upon
positive demonstration that the history as told by competent experts in
Comparative Morphology may vary in material features. A little more
weight allowed for certain observed details, or a little less for others,
will be sufficient to disturb the balance of the evidence derived from a
wide field of observation, and consequently to distort the historical
picture. In the absence of more full "documentary" evidence from the
fossils there is in truth no finality in discussions on the genesis and
progress of organic life. But as long as the human mind has the power
of and inclination towards enquiry, so long will such discussions con
tinue, together with their kaleidoscopic changes of opinion. Every new
fact of importance will in some degree affect the weight accorded to
ITS CONCLUSIONS PLASTIC 13
others and vary the general result* It will be seen in the discussions
which follow how largely this is so in the morphology of plants.
It may be objected that conclusions which are so plastic are little
better than expressions of personal taste, or even of temperament : that
the study of Comparative Morphology is therefore calculated to dishearten
its votaries, while the non-specialist public, which is compelled to take
its information at second hand, will be bewildered, and will conclude
that it is useless to pursue a science which shows so little stability. But
even where problems are apparently insoluble under circumstances of
present knowledge, it is a satisfaction to most minds to entertain an
opinion, even if that opinion be of a theoretical nature, and be liable to
future modification or ultimate disproof. On the other hand, as regards
the actual progress of morphology, those who follow its history with
sympathetic care will gain heart when they compare the present position
with that of a generation ago. And especially for Botanists it is encouraging
to think that it is little more than half a century since the history of the
life-cycle of a Fern was first completed by Suminsky. In some sixty years
a vast array of kindred facts has been acquired, and a theoretic super-
structure is being raised upon them which, though still protean, is gradually
acquiring some settled form. Never in its history has the advance of
morphological thought been so rapid as at present. But in no field of
morphological research has investigation been more amply rewarded than
in palaeophytology : the luminous facts derived from fossils are shedding
a fresh and a direct light upon obscure problems, such as the origin of
the seed-habit, and helping us to locate such difficult groups as the
Psilotaceae and Equisetineae. When we regard these rapid advances, and
truly estimate the influence they bring to bear in strengthening the positions
already indicated by morphological theory, we shall not only .see that this
branch of the science is very actively alive, but also that its theorisings
are not merely unsubstantial figments of the mind.
Considerations such as these go far to justify the statement in the
present work of a theoretical view of the origin of a Land-Flora. Some
may deem the opinions expressed as unduly speculative, but in the first
place, they are based upon a wide area of fact, and secondly, as above
remarked, comparative m6rphology must necessarily assume a theoretical
form under present conditions. We have seen that its conclusions as to
descent are at best the result of a balancing of probabilities. As long as
this is clearly understood by the reader, and the author abstains from any
dogmatic attitude, good should come from any duly reasoned statement,
even though, like the present, it may be of a theoretical nature. A working
hypothesis, open like others to refutation, is better than no hypothesis
at all. This is the position consciously adopted here, for it is believed
that the full statement of even a speculative view will stimulate enquiry,
which may lead towards its ultimate proof or disproof.
CHAPTER II.
THE LIFE-HISTORY OF A FERN.
THE middle years of the nineteenth century marked an important epoch
in the history of Plant Morphology. Before that period this branch of
botany could hardly be said to exist as a science. What gave distinction
to that period was the publication of observations which made it possible
for the first time to give a consecutive account of the various stages in
the life-history of the Higher Cryptogamia. Up to that time it had been
the custom to compare Ferns with Flowering Plants, notwithstanding that
the facts, so far as they were known, gave little support to any view of
their close similarity ; and to attempt to express the life-story of these
and others of the lower plants in terms of the higher. But the investigations
of that period, by following out the actual facts of development, showed
not only that there was no correlative of the seed in the life-cycle of a
Fern, but also that there was in the prothallus of Ferns a phase of the
life-cycle which differed in essential points from anything which was then
known to exist in the development of Seed-Plants.
The spores of Ferns were experimentally recognised as reproductive
organs by Morison (1699), who raised young plants from them. But Kaulfuss
first observed their germination (1825), and the formation of the prothallus,
which had already been described by Ehrhart (1788) : it was Bischoff (1842)
who first recognised the embryo attached to the prothallus. Naegeli (1844)
discovered the antheridia and spermatozoids, while Suminski (1848)
ascertained the true nature of the archegonium, and its relation to the
embryo.. But it remained for Hofmeister to put together, and complete
the story. In 1849 his description of the germination of Pilularia and
Salvinia appeared, and two years later, in 1851, he gave to the world his
Vergleichende Untersuchungen, a work which dealt in the most com-
prehensive way with the life-story of a number of Liverworts, Mosses,
Ferns, Fern-Allies, and Gymnosperms.
It is impossible to exaggerate the importance of the advance in view
which the publication of Hofmeister's book brought. The middle years
MATURE SPOROPHYTE 15
of the nineteenth century were indeed the heroic age of Plant Morphology,
and the results then attained will always continue to be the basis of com-
parison, as applied to the ascending series of green plants. It must,
however, be remarked that those results were achieved on a purely comparative
footing, and at the moment carried no further interpretation with them.
For these were the days before evolutionary theory held sway in the
Biological sciences, and accordingly no underlying phylogenetic meaning
was as yet seen in the facts observed and compared. But eight years
later Darwin's Origin of Species was published, and " the Theory of Descent
had only to accept what genetic morphology had actually brought into
view." It is also to be noted that at first no past physiological history
was traced in the facts of the individual life ; this line of interpretation suffered
much longer delay, and is even now only gradually becoming apparent.
As we shall see, however, such a meaning and such a history may still
be found reflected in those successive phases of the individual life which
Hofmeister and his predecessors were able to detect and to compare.
It is along lines such as these that we may best seek for the solution of
the problem presented by the origin of a Land-Flora.
It will then be essential for our purpose, in the first place, to follow
through all its phases the life-history of certain typical organisms, and
we shall best begin with those which occupy a middle position in our
system, viz. the Pteridophytes. The common Male Shield Fern (Nephrodium
Filix-mas. Rich.) will serve as a familiar, and also a suitable example.
This Fern is known to every one as growing in woods and hedgerows,
and even in more exposed situations, such as the open gills and hill-sides
of higher-lying districts. It presents a robust appearance, and when fully
developed it consists of an oblique and massive stock, which is relatively
short : this is entirely covered over by the bases of the leaves, of which
the youngest constitute a closely packed terminal bud (Fig. i). Those
leaves which are situated further from the apex, and immediately below
the terminal bud, may in summer be found to be of large size and
compound structure (Fig. 2) ; they are of a rather firm texture : individually
they are in outline not unlike the ancient Greek shield, and collectively
they form a crown-like series surrounding the terminal bud. Passing again
further back from the aprex of the stock, its surface is found to be closely
invested by the bases of the numerous leaves of former seasons, the
upper portions of which, having performed their functions, have rotted
away. If the plant be dug up, and the soil carefully removed from it,
an ample root-system will be seen, consisting of thin, wiry, and dark-
looking fibrils, which spring from the basal parts of the leaves, and may
bear numerous branch-rootlets.
All these parts consist of tracts of tissue differentiated to subserve
distinct functions. The Vascular Skeleton, which appears as a cylindrical
network of strands within the massive axis (Fig. i, E, F), throws off
continuous and connected branches, on the one hand into the leaves,
i6
THE LIFE-HISTORY OF A FERN
where they ramify and extend upwards to the extreme tips and margins.
On the other hand, strands of vascular tissue derived from the leaf-bases
extend towards the tips of the roots, and laterally into their branchlets.
The Vascular System is thus a connected conducting-system throughout
the plant. It is embedded in softer parenchymatous tissues, which serve
FIG. i.
Nephrodium Filix-mas, Rich. A, stock in longitudinal section ; v, the apex ; st, the
stem; />, the leaf-stalks; b' , one of the still folded leaves ; g, vascular strands. £, leaf-
stalk bearing at k a bud with root at -w, and several leaves. C, a similar leaf-stalk cut
longitudinally, bearing bud, /*, with root, w. D, stock from which the leaves have been
cut away to their bases, leaving only those of the terminal bud. The spaces between the
leaves are filled with numerous roots, w, w'. E, stock from which the rind has been
removed to show the vascular network,^-. F, a mesh of the network enlarged, showing
the strands which pass out into the leaves. (After Sachs.)
various purposes in the different parts : thus in the young root they may
be absorbent, or serve to hand on the fluids absorbed to the conducting
system : in the stem they may serve the purpose of storage of reserve
materials, while in the leaf the parenchyma carries out the function of
photosynthesis, together with the passing on of the supply thus acquired
to the conducting-system. The parts exposed to the air are covered by
MATURE SPOROPHYTE 17
an epidermal layer, with a cuticujarised external wall, which prevents
indiscriminate loss of water by surface-evaporation. But the epidermis
is perforated by numerous stomata, the motile guard-cells of which can
control, according to circumstances, the width of the pores leading into
the intercellular spaces. There is thus a highly organised ventilating
FIG. 2.
Xephrodiiun Fili.v-mas, Rich. Fertile leaf about one-sixth natural size, the lower
part with the under surface exposed. To the left a single fertile segment, enlarged about
7<imes. (After Luerssen.)
system. Finally, there are also firm, brown, resistant tissues, disposed
sometimes near the outer surface, as in the stem and in the leaf-stalk :
sometimes more deeply seated, as in the root, while in the leaf they
follow the course of the vascular strands. These give to the several parts
increased mechanical strength, and power of resistance.
Thus constituted the Male Shield Fern is an organism which is
capable of leading an independent life on an exposed land-surface : it
B
1 8 THE LIFE-HISTORY OF A FERN
is in a position to nourish itself by taking up from the soil the water and
salts which it requires, and to elaborate therefrom, and from the carbon-
dioxide of the air, fresh supplies of organic food. Further, though for
reasons to be explained later, it frequently is found growing in situations
where moisture is abundant and the air moist ; still it can resist considerable
drought, and is capable of living under as exacting conditions as any
ordinary terrestrial plant. As proof of this, cases may be quoted of the
removal of mature Shield Ferns from a more moist habitat to exposed
situations, where there is no shade, nor any special supply of water :
they are found to flourish there permanently ; but they show slight
differences of form from the shade plants : the leaves are more robust,
of smaller area, and of a paler colour.
In this power of resisting drought the Shield Fern is by no means
an isolated exception, nor in any sense an extreme type among Ferns.
It is a familiar sight in this country to see dry hill-sides covered with the
common Bracken, and taking no harm from a summer drought. There
is also a small British Flora of Ferns of dry wall-tops, including such
species as Polypodium vulgare, Asplenium Ruta-muraria, and Ceterach ;
these may be found sometimes with their leaves dried to crispness in
summer. Abroad there are certain genera, such as Nothochlaena, and
famesonia, and the Niphobolus section of Polypodium, which are typically
xerophytic : in other cases isolated species may show special adaptation
to dry surroundings ; for example, Hymenophyllum sericeum which is a
member of a peculiarly hygrophytic genus. These xerophytic Ferns
inhabit dry climates, such as the higher Andes : or they are epiphytic
in habit, and have no access to the water-reservoir of the soil. The forms
which the xerophytic modification may take are succulence of the smooth
leaf, with well-developed epidermis, as seen in Polypodium nummulariae-
folium, and piloselloides, and also in Platy cerium : or a development of
a thick felt of hairs may cover the surfaces, as in Niphobolus, Nothochlaena,
and Jamesonia : or of scales, .as in Polypodium (Lepicystis} incanum, or
Asplenium Ceterach : or there may be a xerotropic folding of the pinnae,
as in Nothochlaena sinuata and ferruginea, and in Jamesonia. There is
also a very efficient mode of resisting extreme drought which is not
shown structurally, viz. the power of retaining vitality after drying up.
A good example of this is seen in Polypodium (Lepicystis] incanum, which
grows commonly in Trinidad on tree-trunks, and there shrivels for weeks
without rain, under a tropical sun ; but when moistened again it swells,
and continues growth. Such vitality is shared in some degree by the Ferns
of our wall-tops, and is a common feature leading to the survival of
many other plants, notably among the Bryophytes. Such cases as these
quoted will serve to show that a moist habitat is not always a necessity
for the life of the mature Fern-Plant, and even that Ferns, as a family,
show a considerable aptitude for resisting extremes of drought. But never-
theless most Ferns do affect moist situations, while to some, such as
VEGETATIVE PROPAGATION
the Filmy Ferns of tropical forests, xan atmosphere approaching complete
saturation with moisture is a constant necessity. The Male Fern may
indeed be accepted as a medium type, showing no special adaptation
nor susceptibility either to moisture or drought, while structurally it
shows such characters as are usual
in average Land Vegetation.
With very few exceptions Ferns
are perennials, and in the case of
the Shield Fern there is no theoretical
limit to the duration of the individual
life : in point of fact the plant "may
grow continuously for a long term
of years, as is shown by the length of
the stock, and the long succession
of the bases of leaves! of [former years
which may be observed persistent
upon it in the larger specimens. But
still it is subject to many vicissitudes,
which are liable to terminate its exist-
ence. Some provision must be made
for the maintenance of the race by
the formation of new individuals.
The vegetative mode of propaga
tion in the Shield Fern is by means
of buds which appear at a late period
upon the persistent bases of leaves of
former years. These develop leaves
similar to those of the parent, with
roots which form an independent
system, so that when the progressive
rotting, which is always advancing
from the base of the stock onwards,
isolates the parent leaf from the rest
of the plant, the bud is in a position
to subsist as an independent/individual
(Fig. i, B, c). This is merely one
example of what is a very common
phenomenon in Ferns, viz. the vege-
tative propagation of the individual.
The details of the process, such as
the position and the number of buds, may vary greatly in different
cases (Fig. 3), but the essential point is the same, that by a purely
vegetative growth, and without any known cytological complications,
new individuals may be derived, which are similar in all essentials to
the parent. Such means of increase are styled collectively under the head
FIG. 3.
Cystopteris bulbifera (L.) Bernh. A, part of a
leaf with adventitious buds. Natural size. B, an
adventitious bud which has fallen off, forming a
root. C, an adventitious bud further developed.
B and C somewhat enlarged. (After Matouschek.)
20
THE LIFE-HISTORY OF A FERN
of Sporophytic budding. It is plain that such growths are only methods
of amplification of the morphological individual ; though ultimately quite
separate from the parent plant, there is no reorganisation of the protoplasts
involved in their initiation.
There is, however, an alternative mode of increase in number of
individuals, which deals with much larger numbers of potential germs,
and involves a much greater complexity of the phases of production than
the mere sporophytic budding : it is by means of spores. Since this
spores-production is a constant feature in the normal life of all Ferns, and
J7-.
FIG. 4.
Vertical section of the sorus of Nephrodium Filix-mas. (After Kny.)
indeed of Archegoniates at large, while sporophytic budding only occurs
in relatively few, there is good reason to believe that this was a more
primitive and important form of propagation. It therefore demands more
serious attention.
An examination of the leaves of the Male Fern will show in many
cases, and especially in young plants, merely a smooth, rather pale green
under surface : these are then the vegetative leaves, or " trophophytts" as
they are sometimes called, and they always appear first in the develop-
ment of the individual. But other leaves of older plants, and especially
those formed later in the season, bear on their lower surface, and chiefly
near their apical part, numerous roundish patches, which are green or
brown according to age : these are the sort, and the leaves bearing them
SPORE-PRODUCTION
21
are termed " sporophy/Zs" but they^ do not differ in outline from the
vegetative leaves (Fig. 2). The sori are disposed in a single linear series
on either side of the midrib of the pinna, or pinnule, being seated on
the secondary veins. The kidney-like outline which they present is
due to a membranous covering called the indusium, which is protective :
it is attached by a central stalk to a projecting cushion of tissue — the
receptacle— which is in close connection with the vein, while there is
11
12.
FIG. 5.
Young stages of segmentation ;>f the sporangium of Nepkrodium Filix-mas. (After Kny.)
a vascular extension from the vein into the receptacle. To the latter
are also attached the numerous sporangia, stalked capsules of lenticular
form, which are collectively overarched by the umbrella-like indusium.
Various stages of development of the sporangia may be found together
in the same sorus : those which are younger are smaller, and of pale
colour ; those which are mature are larger, and are filled with numerous dark
brown spores : these showing through the more transparent wall give to the
ripe sporangium a deep chocolate colour.
In order properly to understand the arrangement of the parts of the
sorus it must be cut in vertical section (Fig. 4) : it will then be seen
22
THE LIFE-HISTORY OF A FERN
how the indusium, rising from the receptacle, overarches the sporangia,
which are also attached to it by long thin stalks. The head of each
sporangium is shaped like a biconvex lens : its margin is almost com-
pletely surrounded by a series of indurated cells, which form the mechanically
effective annulus : this stops short on one side, where several thin-walled
cells define the stomium, or point where dehiscence shall take place.
Within are the dark-coloured spores, which, on opening a single sporangium
FIG. 6.
Later stages of segmentation of the sporangium of Nephrodiuvi Filix-mas. (After Kny.)
carefully in' a drop of glycerine, may be counted to the number of
approximately 48.
The origin of the sporangium is by outgrowth of a single superficial
cell of the receptacle, which undergoes successive segmentations as
illustrated in Figs. 5 : 1-3. A tetrahedral internal cell is thus completely
segmented off from a single layer of superficial cells constituting the wall.
The former undergoes further segmentation (Fig. 5. n, 12) to form a
second layer of transitory nutritive cells, called the tapetum, subsequently
doubled by tangential fission (Fig. 6. i). The tetrahedral cell which
SPORE-PRODUCTION 23
still remains at the centre, having ' £rown meanwhile, undergoes successive
divisions till usually twelve spore-mother-cells are formed (Fig. 6. 2, 6, 7) :
these become spherical in form, and are suspended in a fluid which,
together with the now disorganised tapetum, fills the enlarged cavity of
the sporangium. Each spore-mother-cell then divides twice, so as to form
a group of four cells, which constitute a spore-tetrad (Fig. 7), the component
cells showing some differences in their arrangement. Finally, as ripe-
ness is approached the individual cells of the tetrads separate as the
spores, each of which has meanwhile developed a protecting wall : owing to
the absorption of the fluid contents of the sporangium the separate spores
are dry and dusty, and readily scattered. Since each
of the 12 spore-mother-cells may form four spores
their number is 4x12 = 48 in each sporangium.
Each mature spore consists of a protoplast with
nucleus, bounded by a colourless inner wall, and
a brown epispore, which extends outwards into
irregular projecting folds.
Meanwhile the wall of the sporangium has
become differentiated into the thinner lateral walls
of the lens-shaped head, and the annulus, which
is a chain of about 16 cells surrounding its margin
/T^. , 7X rr.1 ,-, i • i Spore-tetrads of Polypodium
(Fig 6. 40, 40). These constitute a mechanical vui£are. (After Atkinson.)
spring, which on the rupture of the thin-walled
stomium becomes slowly everted as the cells dry in the air, and then
recovering with a sudden jerk, throws out the spores to a considerable
distance (Fig. 8), each individual spore being separate from its neighbours.
If a Fern leaf on which the sori are fully matured be laid with its lower
surface downwards upon a sheet of paper, and left in dry air for some hours,
or if the drying be accelerated by heat, a fine brown dust, consisting of
the mature spores, will be deposited on the paper, and they are shed in
such vast numbers as to give a natural print of the outline of the leaf.
A rough estimate may be made of the numerical output of spores from
a large plant of the Shield Fern, as follows. In each sporangium 48
spores may be formed: a sorus will consist of fully 100 sporangia, usually
more : 20 is a moderate estimate of the sori on an average pinna : there
may be fully 50 fertile pinnae on one well-developed leaf, and a strong
plant would bear 10 fertile leaves. 48x100x20x50x10 = 48,000,000.
The output of spores of a strong plant in the single season will thus, on
a moderate estimate, approach the enormous number of fifty millions.
As we shall see, each of those spores is capable of acting as the starting-
point of a new individual, and yet Male Ferns are not increasing perceptibly
in number : the fact is that in open Nature the vast majority of these
potential germs do not survive the vicissitudes of early life. It is evident,
however, that the maintenance of the race is very fully provided for, while
there is an ample margin for the effect of selection of those fittest to survive.
24 THE LIFE-HISTORY OF A FERN
In this connection it is well to note further that the spores are produced
upon the leaves fully exposed to the air, and that dry circumstances
favour the shedding of the spores : Ferns grown in uniformly moist con-
ditions show how essential a dry period actually is, for their sporangia
often do not burst at all. The spores of Todea and of some Hymeno-
phyllaceae may even be seen germinating within the sporangium. Such
9 St
Irrn (
125
FIG
Dispersion of the spores from sporangium of Aspidium acrostichoides, showing different
stages of the e version and snapping of the annulus. (After Atkinson.)
a condition is obviously of no advantage to the plant, and is to be
looked upon as a failure in the normal action of the annulus. We thus
see that a relatively dry period, such as the Male Fern is able to undergo
in summer, is a normal state, and indeed essential for the last phase of spore-
production, viz. the dissemination of the numerous living germs.
But the relatively dry conditions which lead up to and are necessary
for the dissemination of the spores do not suffice for their further
development : in order that they may germinate moisture is required,
as it is also throughout the immediately succeeding stages of life. When
GERMINATION 25
exposed to suitable conditions of mbisture and temperature each spore may
germinate : the outer coat bursts, and the inner protrudes and increases
in size, cell-divisions appearing as the growth proceeds. The body which
is thus produced is called the prothallus^ and it may vary in its form
according to the circumstances. In average cases of not too crowded
culture it usually takes first a short filamentous form, attached by one
or more rhizoids to the soil (Fig. 9. 4) : it then widens out at the tip
FIG. 9.
Germination of the spore in Nephrodivin Filix-iuas^ and early stages of the prothallus.
(After Kny.)
to a spathula-like, and finally to a cordate form (Fig. 9. 5 and 6). This
is the usual type, but when crowded closely together, the filamentous
form may be longer retained, and prothalli are then of the type shown
in Fig. ii. i. It is thus seen that the form of the prothallus is plastic, a
fact which may be brought into further prominence by culture under
various conditions of lighting, etc.
The body of the prothallus, exclusive of the downward growing rhizoids,
consists of cells which are essentially alike, arranged at first in a single-
layered sheet. This simple structure is maintained permanently by the
26 THE LIFE-HISTORY OF A FERN
peripheral parts, but in the central region, below the einarginate apex,
the cells divide by walls parallel to the flattened surfaces of the prothallus,
and thus form a somewhat massive central cushion. The mature cells
are thin-walled, with a peripheral film of cytoplasm surrounding a large
central vacuole, and embedding the nucleus and numerous chloroplasts.
The whole body is capable of leading an independent existence, nourishing
itself by absorption from the soil, and by photosynthesis (Fig. 10).
FIG. 10.
Mature prothallus of Ncphrodinm Filix-mas, as seen ffom below, bearing antheridia and
archegonia. (After Kny./
Its structure at once suggests dependence on a continuous and efficient
water-supply ; for there is a large proportion of surface to bulk, while the
cell-walls are thin, and the vacuole-contents voluminous. There is no
arrangement to offer serious resistance to evaporation of water in dry air.
As a matter of experience prothalli shrivel readily when exposed to dry
conditions, while in Nature they are regularly found in moist and protected
positions ; a fact which goes far to determine the habitat also of the
sporophytes which arise from them, and this cannot fail to act as a
substantial check upon the distribution of Ferns. But shrivelling under
drought does not necessarily involve death : in certain cases at least only
a temporary arrest of activity is the consequence, and prothalli which have
PROTHALLUS
27
been thus dormant for a considerable time have been seen to revive
when soaked out, and to continue their growth. They share in some
measure that faculty which is so important to many Bryophyta, of recovery
after dormancy under drought. Comparing the prothallus with the Fern-
Plant as regards the water-relation, it is plainly less adapted for life on
exposed land-surfaces, and more immediately dependent on moisture.
. FIG. ii.
i. An attenuated male prothallus of Ncphrodium Filix-mas ; 2-5. stages of development
of antheridia ; 6, 7. juptured antheridia ; 8. a spermatozoid. (After Kny.)
The prothallus thus constituted is capable in some cases of vegetative
propagation, by gemmae, and other forms of " gametophytic budding,"
but this does not occur in the Male Fern.
Though the close dependence on moisture for functional activity is
thus seen in the prothallus, it is much more obvious in the behaviour
of the sexual organs which the prothallus bears. These in the Male Shield
Fern a are commonly borne, male and female, on the same individual
1 It is hardly necessary to say that the "Male" Fern is a misnomer, surviving from
the misconceptions of earlier times. The Fern- Plant is neutral, being neither male nor
female, while it is on the prothallus that the sexual organs are borne.
28 THE LIFE-HISTORY OF A FERN
(Fig. 10) ; but conditions of crowded culture may lead towards a partial,
or even complete separation of the sexes. The flattened hermaphrodite
prothallus of the ordinary cordate outline, grown under normal circumstances
of moisture and moderate lighting, on a horizontal substratum, lies with
one of its flattened surfaces facing the substratum, and produces upon
that lower surface antheridia and archegonia, the former in the basal or lateral
regions, the latter upon the massive cushion : here they develop in acropetal
succession, the youngest being nearest to the emarginate apex of the
thallus. This position of the sexual organs is evidently favourable to
their continued exposure to moist air, or even fluid water: and indeed
the latter is necessary for the completion of their function.
The antheridium, which arises by outgrowth and segmentation of a
single superficial cell, consists when mature of a peripheral wall of tabular
FIG. 12.
Archegonia of Polypodium vulgare. A, still closed : o = ovum. K' = canal-cell. K" = ventral-
canal-cell. B, an archegonium ruptured. X240. (After Strasburger.)
cells, surrounding a central group of spermatocyr.es (Fig. n. 4, 5). The
antheridium readily matures in moist air, but does not open except in
the presence of external fluid water : this causes swelling of the mucilaginous
walls of the spermatocytes, and increased turgor of the cells of the wall :
the tension is relieved by rupture of the cell covering the distal end,
and the spermatocytes are extruded into the water, the cells of the wall
assisting by their swelling inwards, and consequent shortening (Fig. u. 6).
The spermatocytes, thus extruded into the water which caused the rupture,
soon show active movement, and the spermatozoid which had already
been formed within each of them escapes from its mucilaginous sheath,
and moves freely in the water by means of active cilia attached near one
end of its spirally coiled body (Fig. u. 6 and 8).
The archegonium also originates from a single superficial cell, and
grows out so as to project from the downward surface of the thallus. It
consists when mature of a peripheral wall of cells constituting the
projecting neck, and a central group, arranged serially : the deepest seated
of these is the large ovum, which is sunk in the tissue of the cushion,
and above this is a small ventral-canal-cell, and a longer canal-cell
FERTILISATION
29
(Fig. 1 2, A). If prothalli be grswn in moist air, and only watered by
absorption from below, the archegonia will have no access to fluid water,
FIG. 13.
Fertilisation in Onoclca sensibilis : the arrows indicate direction of the growing point of
the prothallium. A. vertical section through an open archegonium probably within ten
minute? after the entrance of the first spermatpzoid. X 500. B. vertical section of the
venter of an archegonium containing spermatozoids, and the collapsed egg with a sperma-
tozoid within the nucleus. Thirty minutes. X 1200. (After Shaw.)
and they will then remain closed, and fertilisation will be impossible ; but
if watered from above, as they would be in the ordinary course of Nature,
the external fluid water will bathe them,
and rupture will result. This may be
observed in living archegonia which
have been kept relatively dry, and then
mounted in water under the micro-
scope : the neck dehisces at the distal
end owing to internal mucilaginous
swelling, and its cells diverge widely:
the canal-cell and ventral- canal-cell are
extruded, and the ovum remains as a
deeply seated, spherical protoplast, while
access to it is gained through the open
channel of the neck (Fig. 12, B). Thus
the same conditions lead to the rupture
both of the male and female organs :
in Nature a shower of rain would supply
the necessary external fluid water, and
would at the same time supply the
-
FIG. 13 bis.
Horizontal section of an egg, showing coiled
spiral male nucleus within the female. Twelve
hours. XI200. (After Shaw.)
30 THE LIFE-HISTORY OF A FERN
medium of transit of the spermatozoids to the ovum. But the movements
of the spermatozoids are not subject to blind chance : the chemiotactic
attraction of the archegonium directs the spermatozoids through the water,
towards the open neck, which they may be seen to enter, and finally one
of the spermatozoids coalesces with the ovum : fertilisation is effected
m
V
FIG. 14.
Young embryo of Adiantum continuum. L = leaf-quadrant ; 5= stem-quadrant ;
R = root-quadrant ; /''—foot-quadrant. (After Atkinson.)
by the absorption of the male nucleus in that of the egg (Figs. 13
and i $ bis). Thus the presence of external fluid water is essential for the
process of fertilisation : without it the prothallus is unable to achieve that
object of its existence.
The consequence of fertilisation is the growth and segmentation of
the ovum, or zygote, as it may now be called, to form a mass of
embryonic tissue, which at first remains embedded in the tissue of the
EMBRYO 31
parent thallus (Fig. 14): as it , grows, leaf, stem, and root become
differentiated (Fig. 15), which finally emerge; at first the embryo Fern-
Plant is dependent for its nourishment upon the prothallus which embeds
it ; but as the first leaf expands it begins to exercise the assimilating
function, which is taken up also by the later formed leaves. The first
root also projects into the soil, and soon functionates as an absorbing
organ : it is followed later by others. Thus the young plant soon
becomes physiologically independent of the prothallus, which rots away,
FIG. 15.
Embryo of Adiantutn concinuntn, older than that in Fig. 14.
S= stem ; F—foot. (After Atkinson.)
leaf; /? = root;
leaving the young plant established on the soil. It gradually attains the
mature characters similar to those of the parent from which it originated.
The above is a bare statement of the salient events in the life-cycle,
or ontogenetic period of a Fern, as it is seen in its simplest form : and,
the adjoining diagram may serve to present them graphically to the eye
(Fig. 1 6). The two most notable points are those where the new
individual is represented by a single cell, viz. the spore, and the
fertilised ovum, or zygote. These are two landmarks between which
intervene two more extensive developments, on the one hand the sexual
generation, or prothallus, on the other the spore-bearing generation, or
THE. LIFE-HISTORY OF A FERN
SPORE
Fern-Plant. If the events above detailed recur in regular succession
there will be seen a regular alternation of two phases of life, or generations :
of these the one, the prothallus, bears the sexual organs, which contain
the sexual cells or gametes, and it may accordingly be styled the
gametophyte : the other, the Fern-Plant, is non-sexual or neutral, and bears
the sporangia, containing the spores : accordingly it may be styled the
sporophyte. The study of Ferns at large leads to the conclusion that this
regular alternation of generations
is typical for the family. These
two alternating generations differ
not only in form, but also in
their relation to external circum-
stances, and especially in the
water-relation. The sporophyte
is structurally a land-growing
plant, with nutritive, mechanical,
and conducting tissues, and a
ventilating system : not only is
it capable of undergoing free
exposure to the ordinary atmo-
spheric conditions, but dryness
of the air is essential for the
final end of its existence, viz.
the distribution of the spores.
On the other hand, the gameto-
phyte is structurally a plant
ill-fitted for exposure, with un-
differentiated and ill-protected
tissues, and no ventilating
system, while the object of its
existence, viz. the fertilisation
carried out by the sexual organs,
can only be achieved in the
presence of external fluid water.
There is thus a marked difference between these two phases, and their
sequence may be said to constitute an antithetic alternation. As regards
the water-relation, the whole life-cycle of the Fern might not inaptly
be designated as amphibious, since the one phase is dependent on
external fluid water for achieving its object of propagation, while the
other is independent of it.
It will be seen in the next chapter that this is not a condition for
Ferns alone, but that such alternation as is seen here has its parallel in
many other plants, though with great differences in detail, and especially
in respect of the balance of size of the two generations.
FIG. 16.
Scheme of life-cycle of a Fern.
CHAPTER III.
ON THE BALANCE OF THE ALTERNATING GENERATIONS
OF ARCHEGONIATAE.
HOFMEISTER'S great work on the Higher Cryptogamia, alluded to in the
previous chapter, was not a mere description of observations, but a com-
parative treatise. It not only revealed the life-stories of the various types
of plant-organisation which he examined, but in it he also showed that
their several stages corresponded in essential features. Notwithstanding
wide differences of detailed form and of proportion, it was demonstrated
that, as regards position among the recurrent events of each life-cycle,
the neutral generation, or sporophyte, and the sexual generation, or
gametophyte, remained distinct and recognisable in such diverse plants as
the Bryophytes, Pteridophytes, and Gymnosperms. In arriving at this
conclusion it was Hofmeister's great merit that he kept his eye securely
upon those critical points where the individual life is represented by a
single cell, viz. the zygote, and the spore. However differing in size or
in complexity, he held as comparable, or, as it is said, ''homologous," the
phases which intervened respectively between those two events. This great
generalisation of Hofmeister, stated by him with a brevity and a simplicity
of language as remarkable as its content was new and far-reaching, has
formed the essential foundation of all subsequent morphology of Archegoniate
Plants. A series of examples will now be quoted in illustration of it, and
these will be selected to show the differences in form and in the relative
proportions of the two generations ; but it will not be necessary to enter
into a continuous account of the life of each example, for with certain
modifications the essentials of sexuality and of spore-production remain
the same in them all.
In Riccia, one of the simplest of the Liverworts, the gametophyte, or Ricria-
plant, as~Tt is called on account of its being more prominent than the
sporophyte, is a green, dichotomously branched thallus, showing localised
apical growth, while it is thick in proportion to its area : some species float
on water, others are attached by rhizoids to the substratum of soil. The
c
34
ALTERNATING GENERATIONS
only appendages other than rhizoids and occasional hairs are small flat
scales borne on the lower surface. The gametophyte is thus of very simple
form (Fig. 17). The sexual organs are borne
in acropetal succession on its upper surface,
and are sunk in depressions. The sporophyte,
which results from fertilisation of the ovum by
spermatozoids motile through water as in Ferns,
is a small spherical body, with no distinction
of apex and base (Fig. 18). It consists of a
single layer of cells forming a peripheral wall,
which is, however, disorganised before the
ripeness of the spores. The latter are pro-
duced by a tetrad division of the spore-mother-
cells, which occupy the whole internal space of
the sporogonium (Fig. i8A); on germination
the spores yield fresh gametophytes. Thus the
two generations are here as distinct from one
another, structurally and in origin, as in the
Fern, though both are of small size and simple form. It is to be
noted, however, that the sporophyte is throughout its life dependent
PIG. 17.
Riccia minima. A, thallus of
natural size. B, the same in vertical
section, showing two sporogonia sunk
in the tissue of the thallus. Magnified.
(After Bischoff.)
FIG. 18.
Ricciocarpus natans.
surrounded by
•is natans. Young sporogonia in longitudinal section,
surrounded by_ the archegonial wall. The younger ( X 666) shows
the amphithecium (shaded) surrounding the sporogenous cells : in
the older (xs6o) these are separated, as the free, and rounded
spore-mother-cells. (After Garber.)
SA.
Ricciocarfots natans. The upper figure
shows the spherical spore-mother-cells
surrounded by nutritive material. The
lower shows the tetrads formed from
them : the sporogpnial wall (shaded) is
still seen surrounding them, and covered
externally by the archegonial wall of two
cell-layers. X666. (After Gather.)
upon the gametophyte, both mechanically and physiologically, . and that it
is an almost entirely undifferentiated, spore-producing body.
IN BRYOPHYTA
35
Taking Catharinea undulata (L.),xWeb. and Mohr, as an example of the
condition commonly seen in Mosses, the gametophyte and sporophyte are
both on a more advanced scale than in Riccia, and both show localised
apical growth, but their relations remain substantially the same. The " Moss
FIG. 19.
Catkarinea (Atrichiim) undnlata (L.), Web. and Mohr. The leafy gametophyte, bearing
sporogonia. (After Schiniper.)
Plant," or gametophyte (Fig. 19) appears as an upgrowing, branched, and
leafy structure, attached to the soil by numerous rhizoids, and nourishing
itself partly from materials absorbed by them, partly by the activity of
its chlorophyll-containing shoots: it is thus physiologically an independent
organism, as is also the simpler thallus of Riccia. In most Mosses the
plant is ill protected against drought; but they commonly show, as a set
ALTERNATING GENERATIONS
off against this, a remarkable power of recovery on the return of moisture
after being dried up. The sexual organs are usually borne by the Moss-
Plant at or near to the apex of its upward-growing branches. The result
of fertilisation — here again carried out by spermatozoids motile through
water — is the formation of the Moss-Fruit, or sporophyte, which is
throughout life a mere appendage on the Moss-Plant. At first it is,
like that of Riccia^ completely enclosed by the venter of the archegonium
(Fig. 20) ; but it soon shows apical growth and
elongation : the venter is then ruptured trans-
versely, and the sporogonium is exposed. As
it elongates its base remains embedded in the
tissue of the Moss-Plant : its apex is still
covered by the upper part of the archegonial
wall, the calyptra; but at ripeness this is shed,
and the enlarged capsule on dehiscence is able
freely to scatter its spores. After this the
ephemeral sporophyte dies away.
Comparing such a Moss with Riccia, the
phases of the life-history correspond, but their
elaboration is different : the thalloid gametophyte
of Riccia is replaced by the upright leafy plant
of Catharinea. The fertilisation is still dependent
on fluid water, but its product is more complex :
there is in Catharinea a distinction of apex and
base, with localised apical growth ; but the form
is still relatively simple, the whole construction
being on the radial type, without appendages.
The spore-production is restricted to the upper
region, and takes place in one continuous sac.
The sporophyte is still borne and nourished
throughout its life by the parent gametophyte ;
but it is able by its chlorophyll-containing cells
to carry on photosynthesis in some degree, as
an accessory to the supply derived from the
parent. It has a ventilating system like that characteristic of aerial
plants, while this is absent from the gametophyte.
A comparison of such a Moss with a Lycopod shows a different
balance of the two alternating generations. The gametophyte of Ly co-
podium cernuum is shown in Fig. 21 as a somewhat massive structure,
bearing lobes of chlorophyll-containing tissue above, which have sometimes
been compared with the leaves of a Bryophyte : below it is attached by
rhizoids to the soil. Like the Moss it is an independent organism capable
of self-nourishment. It bears its sexual organs about the bases of the
lobes, and is dependent upon external fluid water for its fertilisation.
Notwithstanding its massive bulk it is without a ventilating system. The
FIG. 20.
Young sporogonium of Physco-
mitrella patens, Br. and Sch.,
shortly before the rupture of the
archegonial wall. (After Hy.)
IN LYCOPODS 37
sporophyte of L. cernuum, on th£ other hand, is a large dendroid plant,
which may attain a height of even 3-4 feet (Fig. 22). In the embryo
state it is nourished by the gametophyte which bore it, but it soon
establishes itself independently in the soil as a much-branched plant,
with relatively massive axes showing localised apical growth and numerous
small leaves ; while true roots, not mere rhizoids, ramify in the soil.
The whole plant is traversed by a vascular system, and there is also an
efficient ventilating system. This ample vegetative development precedes
the formation of the spores, which is localised in the terminal strobili :
FIG. 21.
Young leafy plant of Lycopodium cernMtin, L., with the prothallus, bearing its irregular
assimilating lobes, attached on its left-hand side. X about 20. (After Treub.)
these do not differ in general plan from the vegetative shoots, but in
the axil of each leaf of these fertile branches a single sporangium is
borne, containing many small spores, which are all alike (Fig. 220, E).
The gametophyte of Lycopodium is among the most elaborate known
in Vascular Plants : and yet it falls short of the complexity seen in the
plant of Catharinea. It is clear that the two correspond from the fact
that they both arise from spores and bear sexual organs. On the
other hand, the proportion of the sporophyte. as well as its conformation,
differs in high degree in the two plants. In place of the dependent
and ephemeral sporogonium, with limited apical growth, without appen-
dages, and bearing a single terminal capsule of spores, as in the Moss,
Lycopodium shows an independent and perennial plant, with apparently
unlimited apical growth and numerous appendages : it is rooted in
ALTERNATING GENERATIONS
Lycopodinm cermium, L., var. Eichleri, Glaz. A, general habit (\ natural size); B,
end of a branch (natural size) ; C, strobilus ( x 3) ; D, sporopbyll seen from above ; £,
ditto, from the side (xao). (After Pritzel, in Engler and Prantl. Nat. Pflanz.)
IN LYCOPODS
39
the soil, and capable of complete self-nourishment for an extended
period before the production of spores. Moreover, these are produced,
not in a single sac, as in the Moss, but in very numerous distinct
sacs— the sporangia. These essential differences of the sporophyte are
those which clearly define the Bryophytes from the Pteridophytes. In
the latter the mature sporophyte is always a free-growing organism, and
a considerable vegetative period usually precedes the formation of the
spores.
Referring back to our observations on the Male Fern in the previous
chapter, it will be seen that these remarks apply there also. The most
obvious difference between a Lycopod and a Fern is in the size of the
leaf; but they correspond in all essentials,
and both show a very marked advance
of complexity of the sporophyte over the
Bryophyte sporogonium. On the other
hand, the prothallus of the Fern is a
smaller and simpler thing than that of
Z. cer?mum, and stands thus in still
stronger antithesis to the leafy plant of
the Moss. Putting all these points
together, it is plain that in the Pterido-
phytes the balance in size of the
generations is inverted as compared with
that in the higher Bryophytes.
In all the Bryophytes, and also in
many Pteridophytes, the spores are all
alike, and of small size, as we have seen
them to be in Nephrodium and Ly co-
podium : this is described as the
" homosporous " condition, and it may
be accepted as a primitive state. But
in certain other Pteridophytes, and in
all Seed-Plants, there are two different types of spore : — the relatively
small spore, which is easily transferred when shed, and produces a small
male prothallus : and the large spore which, though less easily transferred,
develops within it what is so important to the progeny — a bulky female
prothallus stored with nutriment. This store is derived from the parent
plant, and is thus ready to supply the young immediately after fertilisation.
The " heterosporous " condition brings a clear advantage, notwithstanding
that the separation of the sexes on different prothalli increases the obstacles
in the way of fertilisation being carried out. In certain cases the two
types of sporangia and spores start their development alike, and only
differentiate in the later stages ; for this reason, as well as on grounds
of general comparison, the heterosporous state may be accepted as the
later and derivative. From the example of Selaginella it will be seen
FIG. 23.
Microsporangium of Selaginella apus in
median vertical section. Xss- (After Miss
Lyon.)
ALTERNATING GENERATIONS
that heterospory may occur without any essential change in the
sporophyte; for the plant of Selaginella is of the general Lycopod type,
with small-leaved, much-branched shoot rooted in the soil, showing
FIG. 24.
B, megasporangium of Selaginella. apus in median vertical section, showing three of the
four megaspores. X2i. A, a single megaspore, with prothallus and an archegonium,
more highly magnified. (After Miss Lyon.)
continued apical growth and terminal strobili. These are constructed
essentially upon the Lycopodinous plan, but instead of the sporangia
being all alike, some contain numerous small microspores, others contain
FIG. 25.
Microspore of Selaginella apus, after
germination. (After Miss Lyon.)
FIG. 26.
Microspore of Selaginella apus, just before extru-
sion of the spermatozoids. (After Miss Lyon.)
only four large megaspores. In both cases these result, like other spores,
from a tetrad division : the chief difference is in their size (Figs. 23, 24 B).
But though the sporophyte is essentially unaltered, the changes in the
gametophyte which accompany the heterosporous state are important. The
prothallus is no longer a free-growing, self-nourishing organism, but it
IN GYMNOSPERMS
* -S.
tends to become, and often actually is, a mere means of working up the
material stored in the mature spore into gametes and an embryo, and
does not possess any functional vegetative system. This is exemplified in
Figs. 25, 26 of Selaginella, which show the contents of the germinated
microspore developed as little more than an antheridium. In Fig. 26 the
wall of the spore is ruptured, and the contents are ready to be extruded
as numerous spermatozoids. Fig. 24 A shows the megaspore with the
female prothallus within it, bear-
ing an archegonium. Fertilisation
takes place as in Ferns through
the medium of water. The ovum
after fertilisation forms the embryo
which remains for a time embedded
in the prothallus : but later it bursts
through, and establishes itself as
the independent sporophyte.
In many heterosporous plants
the germination takes place after
the spores are shed, just as is the
case in homosporous plants. But
in others germination of the mega-
spore may be initiated or even
carried through within the spor-
angium. This is the case in
Selaginella apus (Fig. 24), in
which it is evident that, even
when the sporangium has not yet
opened, the prothallus may be
well advanced in the megaspores.
Fertilisation may be carried out
within rV«p> 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 tjme of fertilisation. X9. IT, embryo-sac
filled by the prothallus; a, the venter; c, the neck of
an archegonium . ff> ovum . n> its nucieus ; 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, (aAftear¥tfasburVgerej1' X54°'
follows in the Pine on essentially similar lines as in
Selaginella, also the final germination to establish again the independent
sporophyte.
Lastly, in the higher Seed-Plants, or Angiosperms, which Palaeontology
indicates as of later origin, the outline of the life-cycle is as in the
Gymnosperms, but with still further reduction of the prothallial development
in the pollen-grain (Fig. 29). Fertilisation is of the terrestrial siphonogamic
type. The embryo-sac remains like that of Gymnosperms embedded in the
tissue of the parent plant : it contains before fertilisation only an exiguous
tissue-development, the exact homology of which is- still a question in
debate (Fig. 30).
The above sketch illustrates the general trend, though probably not
the exact course, of evolutionary progress in the Archegoniate series. But
it is necessary to remark that the examples selected do not form any actual
phyletic sequence : of them all no two (excepting Lycopodium and Selaginella)
belong to a single recognised phylum. The general result of their com-
parison is therefore a history read between the lines. But, with this
proviso, the following conclusions may be drawn from it, as to the
44
ALTERNATING GENERATIONS
fluctuations of balance of the two generations of the antithetic alternation
involved in the upward progress of plant-form.
The gametophyte was at first the predominant feature, and there is
good reason, as we shall see later, to believe that it was the originally
pre-existent phase. It was an independent, self-nourishing organism, with
unlimited apical growth, and is seen in the Bryophytes either in
the thalloid form, or developed as a more elaborate leafy plant. In
the Mosses, and in the leafy Liverworts the
sexual generation reached its morphological
climax. But nevertheless in the relative
simplicity of its tissues, and in the absence
of an internal ventilating system, it remained,
as its method of sexuality proclaims it to
be, at best only an imperfect adaptation
to growth under conditions of subaerial
exposure. In the homosporous Pterido-
phytes, though there is in Lycopodium, and
also in Equisetum, some indication of lateral
appendages, the gametophyte is thalloid, but
it still shows its physiological independence,
while there may be a brief and ill-defined
apical growth. Nevertheless, in the Pterido-
phytes the gametophyte as a rule bears the
stamp of a temporary phase in the cycle
rather than that of a permanent organism :
but this becomes much more pronounced
in the heterosporous forms : in these the
independent, self-nutritive existence is lost,
and the prothallus is without localised apical
growth : the male gametophyte becomes little
more physiologically than a means of pro-
ducing spermatozoids : the female is at first
a producer of ova, and later it is simply a
means of nourishing the embryo at second
hand from the plant on which it is depen-
dent. The morphological reduction which follows the heterosporous state
is clear enough in Selaginella and in the Pine, and it reaches its climax in
the Higher Flowering Plants, where the gametophyte is found to have
dwindled away to an exiguous residuum of a few ill-defined cells, with
virtually no vegetative characters at all. The whole story indicates the
eclipse of the generation which appears to have been originally the pre-
dominant partner in the life-cycle. %
The sporophyte, on the other hand, has a complementary story. It is
seen in the simplest Bryophytes as an ephemeral, spherical body, without
distinct apex or base, and no vegetative system except a temporary
FIG. 30.
Ovary of Polygenum convolvulus during
fertilisation, fs, stalk-like base of ovary ;
fn, funiculus ; cha, chalaza ; nu, nucellus ;
mi, micropyle ; zV, inner, ie, outer in-
tegument ; e, embryo-sac ; ek, nucleus of
embryo-sac ; ei, egg-apparatus ; an, anti-
podal cells ; g, style ; n, stigma ; p, pollen-
§ rains ; ps, pollen-tubes. X48. (After
trasburger.)
INVERSION OF BALANCE 45
protective wall of cells : it is dependent through life upon the gametophyte,
and results in a limited number of spores. In more complex Bryophytes
it is still short-lived and dependent, but larger, with distinction of apex
and base, a brief apical growth, and a basal vegetative region distinct
from the terminal capsule : there is entire absence of appendages, but
a partial differentiation of tissues, with internal ventilating system and
some assimilatory tissue. The spore-production is on a larger scale, but
limited usually to the simultaneous development of one continuous
spore-sac.
In the Pteridophytes the mature sporophyte is an independent, self-
supporting organism ; but it is dependent in youth upon the parent
prothallus : it is commonly perennial. It has theoretically unlimited
growth of axis and root : the appendages vary greatly in size : there is
high differentiation of the tissues, with an elaborate ventilating system :
the plant thus constituted is capable of complete and continued self-
nutrition. The spores are produced after a more or less prolonged
vegetative phase, and in perennial forms their production may be
continued for an unlimited succession of seasons. They are borne
in separate sporangia, which are commonly seated upon the appendages :
the sporangia themselves are frequently produced in a continued succession.
These arrangements are such as to lead to a high and even long-continued
output of adequately nourished spores. The sporangia are frequently
restricted to certain shoots, in which the parts are closely aggregated :
these are termed strobili.
The heterosporous state seen in all the highest Vascular Plants,
introduced advantages conducing to certainty in nursing the embryo, and
led in Seed-Plants to an infinity of special developments which secured
that transfer of the microspores which is so necessary for fertilisation.
But the essential plan of the independent, self-nourishing Vascular Plant
once laid down was not departed from, even in the highest forms. The
sporophyte, thus sprung from small beginnings, remains the dominant
generation in all distinctively terrestrial plants.
The entire inversion of the balance of the two alternating generations
thus briefly sketched — the dwindling away of the one and gradually
achieved dominance of' the other— is a fact which requires some
physiological explanation. We may be sure that such things do not
happen without good reason. It will be our object later to enquire into
this. Meanwhile we recognise the fact itself, and we shall see in the
comparisons which lead to its recognition an enduring monument to
the genius of Hofmeister who first pointed them out.
CHAPTER IV.
CYTOLOGICAL DISTINCTION OF THE ALTERNATING
GENERATIONS OF ARCHEGONIATAE.
ALTERNATION is thus found to be a general 'phenomenon for Archegoniate
Plants. It was at first recognised chiefly on the basis of the propagative
organs which the alternating generations respectively bore, and the dis-
tinction was confirmed on grounds of external form and of anatomical
structure. The two phases, however, presented no very strict criteria by
which they could with certainty be told apart. As regards external form,
a foliar development was found to exist in the sexual generation of the
Bryophytes, and again in the neutral generation of Vascular Plants : and
however strongly it might be urged on grounds of detailed comparison
that these were distinct in origin, and therefore only analogous, still the
fact that foliar development exists in them both showed that external form
did not constitute a strict criterion. As regards anatomical structure, the
presence or absence of vascular tissue, and of intercellular spaces appeared
at first to give a ready distinction ; but a better knowledge of the anatomy
of the larger Mosses showed that they also contain conducting tissues closely
analogous to the vascular strands of Pteridophytes. Again, it is a fact
that there is an ample ventilating system in the sporophyte, and that
intercellular spaces are generally absent in the gametophyte; but in the
leaves of certain Filmy Ferns there may be no intercellular spaces through-
out considerable tracts, while the statement for the gametophyte is one of
those negative statements which are at any time open to reversal. Even the
production of the characteristic organs of propagation, and the transition
by spore or zygote from one generation to the other, is not so absolute a
distinction as was once thought ; for first apogamy, and later apospory
were discovered, and it was thus seen that a vegetative transition might
take place from either generation to the other, without the critical incident
of production of spore or zygote intervening as a limit. The climax of
these difficulties in definition of the two generations was reached when
Lang described, in 1896, how in certain Ferns sporangia might be borne
directly upon the prothallus itself.
CYTOLOGICAL DISTINCTION 47
This absence of strict criteria Distinguishing between the two alternating
generations of Archegoniate Plants has given rise to much discussion,
and the differences of opinion have centred round the question of their
origin. Were the two generations distinct ab initio, or were they merely
phases differentiated from a common source? Under the "homologous''
theory of alternation the two generations were held to have been similar
in origin, and the alternation to have originated by a secondary modification
arising in a pre-existent and independent organisation. The adherents of
the " antithetic " theory held that the sexual generation was pre-existent,
and that a new organisation arose, derived by amplification from the
zygote : the sporophyte was thus originally not a result of change in a
pre-existent organisation, but it arose as a newly expanded phase, distinct
in its origin from other phases of the life cycle. The difference of opinion
entailed in these two theories is essentially one of history, and of method
of origin.
In the absence of strict criteria of distinction, such discussions are apt
to be long and inconsequent. It seemed accordingly to be a welcome
advance when facts were gradually disclosed, showing that a cytological
difference exists between the two generations. This appeared to raise
the whole doctrine of alternation in Archegoniate Plants to a higher plane,
and to relate the origin of the two alternating phases intimately with the
existence of a sexual process. In order to understand the nature of
this new criterion of distinction it is necessary to be acquainted with
the main features of nuclear division. When a nucleated vegetable cell
divides, the nucleus takes the initiative, and goes through a series of phases
as shown in Fig. 31, which is quoted from Strasburger, to whom the
discovery of the details is chiefly due. Without describing these at
length it may be stated that the chromatin, that constituent of the nuclear
body which stains most deeply, distributes itself in the linin : the body
thus formed changes from a network of fine fibrils in the resting nucleus
(i) to a thicker convoluted thread, which then divides transversely into-
segments — the chromosomes (3, 4). These segments then divide longitudin-
ally (6, 7, 8), and the halves of each, separating from one another, pass
to the opposite poles of the nuclear spindle, which has meanwhile been
formed (8, 9, 10): they there reconstitute the chromatin-system of the
two new nuclei (10, u, 12). An essential part of this process is found to be
that the number of chromosomes is definite, and though in different plants
and groups of plants it may vary within wide limits, still in the species
or individual the number is (with some exceptions) strictly maintained.
But this is so normally only in the cells of the one or of the other
generation ; for it has been found, in cases which are constantly becoming
more numerous as observations extend, that there is a numerical difference
in the chromosomes of dividing nuclei in the two alternating generations
of the same plant : in the sporophyte the number is twice that in the
gametophyte : the former has accordingly been styled the " diploid," the
48
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
±0
FIG. 31.
Successive stages of nuclear- and cell-division in an embryonic tissue. n = nucleus.
«/=nucleolus. w==nuclear wall. c = cytoplasm. ch = chromosomes. £ = polar-cap.
s — spindle. kt> = nuclear plate. / = young daughter-nuclei. v — connecting threads.
z = cell-plate. 7w = nevv septum. In i the resting nucleus is shown. In 2 and 3 the
segregation of the chromosomes. In 4 the chromosomes are seen with transverse discs.
In 5 the arrangement of the chromosomes to form the nuclear plates and their longitudinal
fission. In 3-5 the formation of the spindle from the polar-caps. In 6 the longitudinal
fission of the chromosomes. In 7 their separation towards the poles has begun. In 8 the
daughter-chromosomes are completely separated. In 9 they are proceeding towards
the poles. In 10, u, and 12 the daughter nuclei are being formed. In 9-11 the connecting
threads and the cell-plate are being formed. In 12 the completion of the septum.
X about 600. (After Strasburger.)
constancy that the number of chromosomes in the dividing nuclei of the
sporophyte is double that in those of the gametophyte. Common numbers
are 32 : 16 — 24 : 12 — 16 : 8; while they run as low as 12 : 6 or 6 : 3, and
as high as 64:32 or 96:48.
Records from the Gymnosperms bring evidence of the same difference
between the two generations in various species of Pinus, etc. Among
Pteridophytes it has been observed in such Ferns as Osmunda, Ahophila,
TETRAD-DIVISION
49
Pollen-mother-cells of a Lily in course of division, somewhat diagrammatic, i. Mother-
cell with resting nucleus. 2. The segregation of the chromosomes. 3. Synapsis.
4. Double threads already beginning to coalesce. 5. The spirem derived from the
coalescent double-thread, apparently a single thread. 6. Subsequent separation of
the threads. 7. The spirem transversely segmented, double chromosomes. 8. Diakinesis.
9. Initiation of multi-polar spindle. 10. Spindle of the mother-nucleus, the nuclear plate
composed of double-chromosomes, n. Reduction-division, the separating chromosomes
showing a partial separation of their longitudinal halves. 12. Formation of daughter-
nuclei. 13. The longitudinal halves of the chromosomes (daughter-chromosomes) are
disposed in the nuclear spindles connected in pairs. 14. Daughter-nuclear-spindles.
15. Separation of the daughter chromosomes. 16. Formation of the nuclei of the second
generation, x about 800. (After Strasburger.)
50 ALTERNATING GENERATIONS
and various Polypodiaceae. In the Bryophytes, the earliest case described
was that of Pallavidnia investigated by Farmer, with four chromosomes
in the gametophyte and eight in the sporophyte : and the same relation
between the generations has since been found also in other genera of
Liverworts. In Mosses few observations have yet been made, but Mr.
M. Wilson has found that in Mnium hornum the numbers are 6 and 12
respectively in gametophyte and sporophyte. Accordingly the difference
in chromosome-number may with high probability be held as a general
diagnostic feature between the two generations in normal representatives of
the Archegoniate series.
This being so, the recognised limits between the generations will
naturally be expected to be the points of transition from the one chromosome-
number to the other. Now it is found that when the sexual fusion of
the two nuclei takes place, the subsequent divisions of the fusion-nucleus
show the doubled number of chromosomes : therefore the zygote will be
the one limit, and this is in accord with the old distinction of the
generations dating from the time of Hofmeister. In his practice the
other limit was the spore, since this is the actual body separated as an
independent germ. But it is found that the actual reduction of the number
of chromosomes to one half, that is, to the original pre-sexual number, takes
place at the tetrad-division of the spore-mother-cell. This cell divides
twice in rapid succession, and the process is well illustrated in the case
of the pollen mother-cells of Lilium, in which it has been specially studied
(Fig. 32). It starts from a cell with a nucleus having the double number
of chromosomes, as shown by its origin. The nucleus first enters the
condition of synapsis (Fig. 32. 3, 4), in which a lateral fusion of the
chromosomes in pairs, respectively of paternal and maternal origin, is
believed to take place : presently a coiled thread frees itself from the
tangle of synapsis (Fig. 32. 4, 5), which becomes shorter and thicker,
still showing, however, indications of its double nature (Fig. 32. 6, 7),
and divides into segments, which are half as many as those of the parent
nucleus (Fig. 32. 7, 8) : each individual of the chromosome-pairs then
moves apart (Fig. 32. 10, n), one of each pair passing to either pole
of the spindle which has meanwhile been formed : as each half is an
original chromosome, the number at each pole is one half that of the
parent nucleus, and the division is styled the heterotype, or reducing
division (Fig. 32. 12). The second division in each of the two nuclei
thus formed follows quickly, and is homotypic, that is, each chromosome
undergoes longitudinal fission into two, as in a vegetative division (Fig. 32.
13, 14, 15). The four nuclei thus constituted have also half the number
of chromosomes present in the nucleus of the spore-mother-cell ; but the
reduction is actually effected, as has been seen, in the first, or reducing
division. Accordingly, Strasburger has recognised the spore-mother-cell,
in which the reduction is initiated, as the actual limit between the two
generations. But it is the spore itself which normally terminates the
FIG. 33.
Apogamy in Pteris creticei, L. A and B, development of the first foliar process close
to the emargination (k) on under surface of prothallus. C, a whole prothallus seen from
below, showing a young apogamous shoot. /> = prothallus ; 3l = first leaf; z> = stem-apex ;
w = root. D, a similar growth more advanced. A and B X 145, C and D highly magnified.
(From Engler and Prantl, after De Bary.)
ALTERNATING GENERATIONS
one generation and forms the starting-point for the next generation : the
older usage based upon this obvious fact is, therefore, to be preferred,
and the spore may be still held to be the obvious boundary between
the two generations. The gametophyte, or haploid phase, will then be
recognised as extending from the spore to the zygote in each cycle, and
it shows " n " chromosomes normally in all its nuclear divisions : the
sporophyte, or diploid phase, is recognised as extending from the zygote
to the spore, and it shows " 2n " chromosomes in all its normal nuclear
divisions. However difficult these nuclear details may be to recognise
in any given case, so far as observation goes within the limits of the
Archegoniatae they provide a structural basis for the distinction of the
two generations more exact
than any other, a distinc-
tion which runs parallel
with those less accurate
criteria on which the
recognition of the genera-
tions was first founded.
The possession of this
means of diagnosis neces-
sarily turns attention afresh
to those cases where the
transition from one genera-
tion to the other is bridged
over by direct vegetative
growth, viz. the phenomena
of Apogamy and Apospory,
which have figured so largely
in the discussions on alter-
nation. It has long been
known that the two alternating generations are not always delimited from
one another respectively by those unicellular phases of the spore and the
zygote; but that in certain cases, and among the Archegoniatae most
commonly in the Ferns, there may be a vegetative transition either from
the prothallus to the sporophyte without the intervention of a sexual
process— this is termed apogamy; or conversely, from the sporophyte to the
prothallus without the intervention of spores— this is designated apospory.
In the most frequent examples of apogamy in Ferns the place of an
embryo is taken by a process which originates from the tissue of the
cushion as a result of vegetative growth and division of its cells (Fig. 33, A) :
it soon takes a form corresponding to that of an embryo, with first leaf,
root, and apex of axis (Fig. 33, B and c), and it finally becomes an established
plant in the same way as those sexually produced (Fig. 33, D). In some
cases these developments may take place in entire absence of archegonia
on the prothallus; in others various conditions of the archegonia may be
FIG. 34.
Scolopendrium vulgare. Prothallus from the branched cylindrical
process of which ten roots arose : eight of these are visible in the
drawing. X about 6. ('After Lang.)
APOGAMY
53
found, either showing normal struchire or various modifications of it. But
in other cases, which have been described in detail by Lang,1 the
apogamous developments may diverge far from the normal in point of
the number and position of the parts. Originating by direct vegetative
growth from the tissues of the thallus, in place of the normal sequence
and position of the parts the several constituents of the sporophyte, root,
leaf, sporangium, may appear without order or numerical rule : ten or
more roots have been found apoga-
mously produced upon a prothallus
without other parts of the sporophyte
(Fig. 34) : sporangia have been ob-
served without sporophylls, originating
directly from a formless mass of
sporophytic tissue apogamously pro-
duced on the prothallus (Fig. 35),
or even, in an extreme case, from
the prothalloid cells of the archegonial
wall (Fig. 36). The irregularity of
such growths must be taken into
consideration in their theoretical
interpretation, as will be seen later.
Among the Archegoniatae apogamy
has hitherto been observed in a score
or more of species of Ferns, belonging
to the Osmundaceae, the Hymeno-
phyllaceae, and chiefly to the Poly-
podiaceae ; and examples are also
recorded from the Marsiliaceae. In
the Bryophytes, the Lycopodiales,
and the Equisetales no cases are as
yet recorded. It may be noted, how-
ever, that similar phenomena have
been observed in Flowering Plants,
such as AkhcmiUa, Thalictrum,
Antennaria, and Taraxacum?
Turning now to Apospory,3 that is, the transition by direct vegetative
growth from the sporophyte to the gametophyte without the intervention
of spores, instances are recorded from the Liverworts (Anthoceros, Lang),4
from the Mosses (Hypnum and Bryum, Pringsheim,5 Ceratodon, Stahl,G
1 Phil. Trans., vol. cxc. (1898), p. 187, etc.
2 For references see Strasburger, Flora, 1907, p. 139.
y The term " Apospory ;' was introduced by Vines, in an article on the " Proembryo
of Chara," Journal of Botany, 1878, p. 355.
4 Annals of Botany, vol. xv., 1901, p. 503. * Pringsh. Jahrb., xi., 1877.
*Bot. Zeit., 1876, p. 689.
FIG. 35.
Nephrodium dilatatum, Desv., var. cristatum
gracile. Prothalloid cylindrical process, bearing
archegonia near its base. It arises by the side of an
imperfect sporangium (sfi), and bears a similar
sporangium (sp) on the other side, and on the tip are
a number of sporangia associated with ramenta.
X35. (After Lang.)
54
ALTERNATING GENERATIONS
/ ** y
Funaria, Brizi),1 and from various species of Ferns belonging to the
Hymenophyllaceae and Polypodiaceae,2 but no examples are on record
from the Lycopodiales or Equisetales. Those cells which would in the
normal course produce the spores take no part in the formation of the
gametophytic growths. In Anthoceros the origin of these is commonly
from sub-epidermal cells: in the Mosses from the cells of the seta, or of
the sporogonial wall ; while in Ferns the archesporial cell if already denned
in a sporangium is abortive. Thus the aposporous growths are in no
sense mere irregularities of
development from sporogenous
cells. In Anthoceros each
growth is apparently referable
in origin to a single cell, and
the same is probably the case
also for Mosses. But in the
Ferns this is not so : here the
vegetative development may
start from a sporangium formed
in its normal place : a plurality
of the cells of the stalk, or
of the sporangial wall surround-
ing the abortive central cell
may divide, and assume pro-
thalloid characters (Fig. 37),
or the growth may arise from
the receptacle of the sorus
(Fig. 37 E): or again, it may
be initiated at some point
on the leaf, usually marginal,
which thus extends directly
into the prothallial expansion,
and may bear antheridia and
archegonia (Fig. 376, c, D).
The matter may be further
complicated by the combination
of apogamy and apospory in the same individual, and this condition has
been seen in about half the recorded cases of these abnormalities in Ferns.
The apogamous seedlings of Nephrodium pseudo-mas, var. cristata (Cropper),
not only sprang themselves in an apogamous manner from the prothalli,
but proceeded almost at once to an aposporous production of new prothalli
on the margins of the young leaves.3 These prothalli bore antheridia,
FIG. 36.
Scolopendrium vulgare. Group of sporangia (sf) on a
projection, the structure of which indicates its relation to an
archegonium. Occasionally two nuclei are present in a single
cell. X6oo. (After Lang.)
1 Ann. Inst. Bot. Rom.) v., p. 54.
2 For references see Engler and Prantl, Nat. Pfl., I. 4, p.
1905, p. 239.
3 Druery, Journ. Linn. Soc., vol. xxix., p. 479.
I, and Goebel, Flora,
Apospory. A =Soral-apospory of Athyrium Jilixfoeinina.,
ti. clanssittia. Jones. Part of a pinnule with veins (?>l>)
id a sorus. In the latter, in place of the sporangia, prothalli
* formed (f>rtli) with antheridia (antk) and archegonia
rch). X40. B — apical apospory of Polystichuin angulare, ^;
r. pulcherrimum. Padley. A prothallus at the tip of a
nnule, as a direct continuation of it. gl— marginal glands, c~ = the cushion.
20. C=the first initiation of a prothallus as in B, at the apex of a pinnule,
he shading indicates a vein beyond the tip of which the prothallus arises.
130. D — a. similar growth, but borne on an elongated cylindrical process!
chegonia (arch) are already present. X 10. £ — soral apospory in Polystichuni
tgulare, var. pnicherrunum. \ prothalloid growth bearing an antheridium
\ntli) and rhizoids (h) has arisen from the stalk of a sporangium. Xyo.
E
ALTERNATING GENERATIONS
but Dr. Lang's drawing shows how, nevertheless, the prothalli in turn
hasten to a fresh apogamy (Fig. 38), thus the two transitions may be
repeated at near intervals of time. The same was seen to be the case
in Trichomanes alatum, in which apospory and apogamy were found to
succeed one another,1 and it has recently been proved to hold also for
Athyrium filix foemina, var. clarissima^ Jones, and other Ferns.2
Lastly, Goebel has shown 3 that when the seedling leaves of certain
ferns are removed and cultivated on a moist substratum, aposporous
growths may be induced, which show sometimes the most intimate inter-
mixture of characters of the sporophyte and
the gametophyte. These developments appear
to be similar in kind, though not in detail, to
those described by Lang and others. It would
doubtless be possible to erect upon such facts
a superstructure of theory ; but it is necessary
to remember that by the abscision of a young
part it is placed in an anomalous and extreme
physiological position. It is improbable that
such circumstances ever arose in the course
of descent : and accordingly it must remain
a quite open question what bearing, if any,
such observations have upon the evolutionary
story. They demonstrate possibilities : but
possibilities are not the equivalent of historical
data.
The rapid succession of the transitions
thus actually seen in some Ferns from the
sporophyte to the gametophyte, and the con-
verse, give some colour to the suggestion
made by Goebel, that the sporophytic buds
he found in the deep-water specimens of
Isoetes are to be viewed as extreme cases of the telescoping of the
alternate generations.4 This state of affairs is very nearly matched by
certain Adiantums observed by Lang, in which numerous sporophytic buds
were produced from the sorus. Examination showed that they sprang in
certain cases from the sporangia themselves, but not from the sporogenous
tissue. If we imagine the gametophyte stage reduced in such cases, not
to a very short phase only, as it is in Lang's Nephrodium, but to the
vanishing point, the result might be as in Goebel's Isoetes. But we may
1 Ann. of Bot., vol. i., p. 269.
2 Farmer and Digby, Ann. of Bot., 1907, p. 163-167.
3 Sitz. d. Math.-phys., Klasse d. K. Bayer. Akad. d. Wiss., xxxvii., 1907. Heft, ii.,
p. 119. This is interesting for comparison with my own negative results on leaves of
mature plants recorded in Ann. of Bot., iv., p. 168 (1889).
*Bot. Zeit., 1879, P- I-
FIG. 38.
APOGAMY AND APOSPORY 57
well ask whether such an interpretation does not read into the facts more
than actually exists? If Isoetes were a plant which habitually showed
combined apospory and apogamy, and if various steps were present leading
towards the extreme result, then the conclusions might be accepted. But
Isoetes is a plant which is structurally stable as a rule, and there is in
these abnormal growths no prothalloid tissue at all. Thus they appear to
be merely sporophytic buds formed from sporophyte tissue, and having
sporophytic character throughout. They will rank with those sporophytic
buds which are found arising from the sorus in various Ferns, or from
the nucellus in some Phanerogams : they are, in fact, a mode of vegetative
continuance of the neutral generation, and nothing more.
The question necessarily presents itself, what is the cytological state
of the tissues in the plants which show those vegetative transitions from
one generation to another, such as have been described for the Mosses
and Ferns above named? The facts would appear to be inconsistent with
the structural distinction of the two generations, since the acts of sexuality
and of spore-formation, by which the cytological changes are normally
effected, are liable to be omitted. It will be important to know how
far the distinction between the haploid and the diploid phases will remain
valid. The facts have lately been elucidated for a number of the
abnormal Ferns by Prof. Farmer and Miss Digby,1 and for the very
peculiar case of the genus Marsilia by Prof. Strasburger.-
Taking first the case of apogamy : already in 1898 Dr. Lang had observed
in prothalli of Scolopendrium, in the tissues bordering on the change
from gametophyte to sporophyte, the frequent presence of two nuclei in a
single cell (Fig. 36). More detailed observations have since been made on
other apogamous Ferns, by examination of very young prothalli, before any
apogamous growths had begun to manifest themselves.3 Similar cells with
two nuclei were observed in the case of prothalli of Lastraea pseudo-mas,
var. polydactyla ; but it was shown that when two nuclei are seen in a single
cell a neighbouring cell is without one, and cases were found where the
passage of the nucleus through the cell-wall was actually in progress
(Fig. 39). This process is regarded as a kind of irregular fertilisation,
for ultimately the two nuclei fuse. On their division the nuclei of the
apogamous growth thus pfoduced show, as a consequence of the fusion,
evidence consistent with a doubling of the chromosomes, just as it
happens in the normal post-sexual stage. But instead of one cell only
serving as the starting-point for the new generation, a number of such
units co-operate loosely to produce it. These results have their interesting
bearing on the irregularity of number, and the sporadic position of the
parts in such cases as those observed by Lang. It is thus seen that even
in these irregular examples the cytological criterion between the two
generations may hold, and the structural limit will be found in the cells
1 Ann. of Bot., xxi., p. 161. -Flora, 1907, p. 123.
3 Farmer, Moore, and Digby, Roy. Soe. Proc., Ixxi., 1903, p. 453.
58 ALTERNATING GENERATIONS
in which the doubling of the chromosomes is initiated by the nuclear
fusion.
The first case of apospory to be cytologically investigated was that
of Nephrodium pseudo-mas, var. cristata apospora, where the prothallus
grows directly out from the margin or surface of the leaf. It was shown
in this case by Miss Digby * that there is no nuclear change involved,
but that both sporophyte and gametophyte have a number of chromosomes
about 50. This result would at first sight appear to show that the
FIG. 39.
Neph.rodnt.ni pseudo-mas, var. polydactylum. Tissue of prothallus where an apogamous
growth is to be formed, showing to the left a cell with two nuclei, while an adjoining cell
has none. At the centre a nucleus is seen passing through a perforation of the wall,
and fusing immediately with that of the cell it enters. (After Farmer, and Moore, and
Miss Digby.)
chromosome-criterion had hopelessly broken down. But a better under-
standing of such cases is obtained when the whole nuclear cycle is
considered, than by contemplation of a single phase of it. It has been
above noted that there is a frequent relation between apogamy and
apospory in the life-cycle of the same individual : it is important to
know the nuclear conditions throughout such cycles. The case of
Athyrium filix-foemina, var. clarissima, Jones, may be taken as a first
example where the complete chromosome-cycle is known.2 In this Fern
1 Roy. Soc. Proc., Ixxvi., 1905, p. 466.
2 Farmer and Digby, Ann. of Bot., 1907, pp. 163-7.
A FOG AMY AND APOSPORY 59
it has been shown by Farmer- ^and Miss Digby that there is apogamy
as well as apospory. The cytological investigation shows that in those
cases where sporophytes were borne on the apogamous prothalli there is
not any migration of nuclei from one prothallial cell to another, such
as has been described for some cases of apogamy ; nor is there doubling
of the chromosomes in any other way. In fact, the chromosome-number
is the same for the sporophyte as for the prothallus which bears it.
Investigation of the aposporous transition from the leaf to the prothallus
showed also that no change of number marks the passage from sporo-
phyte to gametophyte. There is here a case of cytological uniformity
throughout the whole cycle, with chromosome-number about 90. This is
approximately the number found in the diploid stage of a typical A thy Hum
filix-foemina. The condition of the variety is as though reduction had
been omitted from the cycle : as a consequence the prothallus being
itself diploid, fertilisation would be unnecessary to produce a new
sporophyte : accordingly apogamous budding will suffice, and that is what
actually occurs.
A near parallel to this has been worked out with similar exactitude by
Strasburger in Marsilia Drummondii, A.Br.1 The typical chromosome -
numbers are 16 and 32 respectively for gametophyte and sporophyte,
and normal plants show the usual succession of events. But on ger
mination of the megaspores borne by certain plants, the gametophyte
was found to have the diploid character, and this was seen even
in the division to form the ventral-canal-cell : thus the ovum itself is
diploid. In such archegonia the neck does not open, so that fertilisation
by spermatozoids is impossible : the unfertilised diploid egg develops
apogamously into an embryo, which is naturally diploid also. An
examination of the sporangia showed further that while in typical Marsilias
the reduction to 16 chromosomes takes place as usual in the spore-
mother-cells, in M. Drummondii the megasporangia show two types of
spore-mother-cells : the one type is normal in number, and shows
reduction : the other type is produced in smaller numbers in the sporangia,
for instance only four in place of the usual 16: these on division
have diploid nuclei, and the interesting fact is that their diploid state
does not divert them from the usual characters of form and structure.
Since the apogamous plants produce both diploid and haploid spore-
mother-cells, it is accordingly not surprising that both apogamous and
sexual plants should be produced from their sporocarps : and it is apparent
that among the representatives of the species there will be individual
cycles completed without any change of chromosome-number : certain
cycles will accordingly be diploid throughout. In this they correspond to
what is seen in Athyrium filix-foemina, var. clarissima^ Jones, though
they differ in the detail that the diploid ovum here forms the embryo,
while in the Athyrium the embryo arises from the prothallus by apogamous
1 Flora, 1907, p. 123, etc.
6o ALTERNATING GENERATIONS
budding. But an exact parallel is found in Athyriwn filix-foemina, var.
darissima, Bolton, and in Scolopendrium vulgare, var. crispum Drummondae,
in which the embryo arises from the unfertilised ovum.1 It may be
remarked that the phenomena thus seen in the last-named Ferns and in
Marsilia correspond essentially to what has been described for certain
Phanerogams.2 It thus appears that in a number of cases, systematically
apart from one another, a diploid condition of the gametophyte is associated
with apogamous development from a diploid unfertilised ovum : the
abnormality is initiated by omission of the reduction in the spore-mother-
cell, and consequently the diploid state is continued in the gametophyte,
which is normally haploid. It is important to note in such cases that
a double number of chromosomes may be present without producing
fundamental change of form or of external character in the gametophyte.
The further question will then present itself, whether under any
circumstances the converse is possible, or has been observed, viz. that
the phase normally diploid, that is, the sporophyte, may be haploid?
Strasburger states (I.e., p. 166) that no case has come under his
observation in which the generation normally diploid has only the reduced
number of chromosomes. No case of a haploid sporophyte has yet been
proved beyond doubt : but a reasonable probability has been established
by Farmer and Miss Digby in the case of Lastraea pseudo-mas, var.
cristata, Druery (I.e., p. 180). The detached leaf of this plant produces
prothalli from its margin or surface, which bear occasional antheridia,
but the sporophyte is apogamous. The chromosome-number in the
prothallus is about 60 : in the embryo the number varies considerably,
one mean being 60, another mean number being about 78. No migration
of nuclei was observed, nor is there any reduction in the whole cycle.
The relatively small number of chromosomes in the nuclei of the sporophyte
is striking, and suggested to Farmer and Miss Digby that the gametophyte
character had been impressed on the sporophyte — the converse, in fact, of
what was seen in the varieties of Athyriwn and in Marsilia. A comparison
of the chromosome-number (60, 78) with that in normal Lastraea
psetido-mas (144) certainly indicates that this is the probable condition
of the apogamous sporophyte of Druery's variety : that the sporophyte is
irregularly haploid, and that the whole cycle is essentially haploid
throughout.3
It still remains to refer briefly to two other modifications of the normal
cycle of alternation in Archegoniate Plants, so as to complete the tale of
those which have been observed : I mean sporophytic and gametophytic
budding. The former has already been mentioned in the case of
farmer and Digby, I.e., p. 171.
2 Eii-alcheniilla, Thalictrum, Antennaria, Taraxacum. See Strasburger, I.e., p. 139.
3 The examples quoted illustrate the more important modifications of the chromosome-
cycle hitherto described. For further details reference must be made to the papers from
which these have been derived.
THEIR CYTOLOGICAL DISTINCTION 61
Nephrodium Filix Mas, as occurring on the leaf-bases (Fig. i), and in
Cystopteris bulbifei-a, at points on the upper region of the leaf (Fig. 3).
It also occurs in various forms in many other Ferns, in Lycopods, and
Equiseta. The essential feature of it is that a portion of tissue of the
sporophyte, developed as a bud with axis leaves and roots, on being
detached from the parent plant, may continue its growth apart from the
parent. This is plainly a mere vegetative amplification of the sporophyte
itself, and its tissues are at first continuous into those of the bud : there
is no reason to think that any nuclear change accompanies the production
of these growths, and the result is the establishment of physiologically
independent individuals ; but their origin and detachment do not modify
our conception of the sporophyte as a whole in any essential point. If
that conception be based upon nuclear changes accompanying fertilisation
and reduction, it will include all such results of vegetative amplification :
they will be held to be intra-sporophytic means of propagation.
Similarly, in the case of gametophytic budding, which is common in
Liverworts, Mosses, and in certain Ferns, by means of gemmae : these
are small bodies, consisting of one or more cells, which are easily
detached and under suitable conditions develop into new gametophytes.
Here again the gemmae appear to be mere vegetative growths, and
they secure increase in number of physiologically independent individuals ;
but there is no reason to think that there is any nuclear disturbance
involved : they may be regarded as intra-gametophytic means of propagation.
How, then, do the irregularities above described affect the general view
of the cytological distinction between the two alternating generations of the
Archegoniatae ? It is quite clear that an absolute chromosome-distinction
cannot be held as universally applicable at the present time to the two
alternating phases : nor does form depend on the exact number of the
chromosomes : nor yet is there any obligatory ratio according to which
the gametophyte is always haploid and the sporophyte always diploid.
These facts give an opening to the facile conclusion that the chromosome-
distinction is worthless, and opinions to this effect have already been
expressed. But the recognition of the present existence of aberrant forms
does not negative the importance of the relation which is usually seen,
nor exclude it from taking its due place in the reconstruction of the
history of the past. It is contrary to all evolutionary theory and
experience to assume that what has been normal in the past is
obligatory for the present or the future. Moreover, it seems probable
that these abnormalities do not represent anything which took a settled
place in the course of the evolution of the plants in which they appear :
our opinion might be different if in any of the great phyla it could be
shown that a definite stock, or line of descent, had been permanently
established showing aberrant characters ; for instance a permanent Arche-
goniate phylum showing a cycle without any chromosome-differences. But
of this there is no evidence at all : every one of the main phyla
62 ALTERNATING GENERATIONS
show normally the regular succession of events as described, viz. the
haploid gametophyte leading through sexuality to the diploid sporophyte,
which, again, through reduction or meiosis in the spore-mothepcells, leads
back to the haploid prothallus. The constancy of this is too great
to allow its recognition as the " normal " to be seriously disturbed by
the occasional irregularities described — irregularities which bear all the
characters of late, individual, and probably non-permanent aberrations.
Their existence is suggestive on certain points, but it cannot be held to
invalidate the view that the cycle as above stated existed in all probability
throughout the earlier phases of descent of the Archegoniatae.
Accordingly the cytological distinction of the two generations may be
upheld as the normal condition for the Archegoniatae. Further, the opinion
of Farmer may be accepted, that the new facts relating to apogamy and
apospory leave the question of alternation where it was : they tend neither
to destroy the one theory of its origin nor to uphold the other (I.e.,
p. 193). Moreover, the facts of the normal chromosome-difference may be
held to accord with either of the theories of alternation, the homologous
or the antithetic : they are not finally distinctive for either, and a decision
must remain still in doubt until the actual history of the genesis of the
diploid phase in the Archegoniatae can be traced. Towards forming a
just opinion on this question it is desirable not only to compare the
Archegoniatae among themselves, but also to take into consideration
the life-cycles of the Thallophytes ; for these plants often show a simpler
mode of life, and have always been held to afford suggestions as to the
probable origin of the more complex land-vegetation. This will be the
subject of the next chapter.
CHAPTER V.
ALTERNATION IN THE THALLOPHYTES.
THE early recognition by Hofmeister of alternation of generations as a
general feature in the life-cycle of the Archegoniatae naturally led Botanists
to enquire whether any similar succession of phases existed in other
plants : and the question was soon directed towards those lower in the
scale, which are collectively termed the Thallophytes. Notwithstanding
that this term covers a most heterogeneous series of organisms, a very
large number of them show processes of propagation analogous to those
seen in the Mosses and Ferns. The existence, on the one hand, of the
phenomenon of sex, and on the other of the means of propagation by
non-sexual bodies, or spores of various kinds, suggested the comparison
with corresponding features in the Archegoniatae. Such comparison at
once raises the further question how far the study of the Thallophytes
may throw light on the origin of those recurrent and alternating phases
seen in Archegoniate Plants.
It will be well at once to realise that the phenomenon of sex, and the
production of germs, by which the number of individuals may be increased,
are not necessarily in any way connected in plants at large. It is true
that in certain plants, and even in large groups of them, experience shows
us that there is an obligatory succession of such events in the life-historyr
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
64 ALTERNATION IN THE THALLOPHYTES
lower in the scale, the case is found to be often different : the first clear
demonstration for a Thallophyte that the phases do not follow an obligatory
succession was given by Klebs for Hydrodictyon : subsequently his
observations were extended to Vaucheria, and later to many other Algae
and Fungi.1 Up to 1890, when Klebs' first paper on Hydrodictyon was
published, the comparison of the various stages of life in the Thallophytes —
and indeed in plants at large — had rested on form, and very little was
known of their behaviour under varying conditions. But Klebs applied
to them the experimental method, and found in many cases that where
the organism possesses two or more kinds of propagation, each is directly
dependent upon quite definite external conditions. There appears in many
cases to be no cause in the inner nature of the organism for one of these
to be developed earlier or later : it lies in the hand of the experimenter to
determine their succession. An exact knowledge of the conditions gives the
experimenter the secure control over the organism, which can at will be
forced into any desired mode of propagation within the limits of its species.
This may be well illustrated by the case of Vaucheria, which happens
to have had its simple life-history adequately investigated through the
experiments of Klebs. He found that the formation of the vegetative zoo-
spores is most active if well-nourished plants are transferred to fresh
external conditions, and especially to diminished intensity of light; while
the formation of sexual organs can always be counted on when nutrition
proceeds slowly, under relatively equable conditions, and under good
illumination. Similar observations, correlating the phases of life of various
Algae and Fungi with external conditions, have also been carried out,
and though the determining circumstances may differ in different plants,
the fact is now demonstrated for a number of Thallophytes, that there is
in them no obligatory succession of phases : sexual reproduction or vege-
tative propagation may be repeated indefinitely, according to the conditions
of life : neither of these leads of inner necessity to the appearance of the
other. In fact the relation of the vegetative mode of propagation by
isolated germs to the life-cycle of such Thallophytes is somewhat similar
to the production of gemmae on the gametophyte of Archegoniate Plants,
or to the sporophytic budding of Ferns or Lycopods. In all such cases,
which may collectively be styled under the general term of somatic
budding, the increase in number of individuals is secured, but only by an
incident which takes no part in any rhythmic succession of obligatory
phases, and involves no cytological change. The result is simply a repetition
of the same phase from which the bud, gemma, or zoospore was itself
derived. The experimental method of Klebs, by showing that the order
of events in such cases is not obligatory, has laid the foundation for a
more rational comparative study of the life-story in the Thallophytes. It
may be considered probable that many more of the Fungi and Algae will
be found to behave like the species which have been tested. As operative
1 Klebs, Bedingungen der Fortpflanzung bei einigen Algen uud Pilsen, Jena, 1896.
CYCLE NOT ALWAYS OBLIGATORY 65
factors, external stimulus, light, temperature, moisture, access to oxygen,
and the chemical composition of the nutritive medium, have already been
recognised. These and others in various combination have been found,
or may in the future be found, to determine the succession of propagative
methods in many of the Thallophytes.
But, on the other hand, accurate observation is showing in an increasing
number of examples that this freedom from obligatory succession of phases
is not universally the case in the Thallophytes. It is beginning to be
clear that here, as elsewhere, complications have arisen, associated with the
phenomenon of sex, which lead frequently to an obligatory succession of
phases, over which external conditions have little or no control. It has
been seen in the Archegoniatae and in the Phanerogams that the result
•of sexual coalescence is a doubling of the number of chromosomes in the
subsequent nuclear divisions, with reduction as the final consequence. The
similarity in essentials of fertilisation in the Thallophytes to that in the
Archegoniatae is obvious : it has been found in many cases to result in a
•doubling of the chromosomes in Thallophytes also, and this makes it seem
probable that there should be post-sexual nuclear complications of somewhat
the same nature in them also. Strasburger has drawn attention to the
impossibility of indefinitely continued doubling of chromosomes in fertilisation,
and the necessity for a reduction-process in plants which show sexuality :
we must assume that some process of reduction will sooner or later follow
in each life-cycle where sexual coalescence occurs ; but the mechanism of
the process, and the period at which it occurs in the life-cycle, may differ
in different cases. The differentiation of the sexes in the Thallophytes
has proceeded along many distinct lines. What is then more probable
than that in different lines of descent the problem of reduction, as a
necessary consequence of sexual fusion, should have been solved in different
ways, and at different points in the life-story?
The facts observed in certain Thallophytes point to the conclusion
that this has actually happened : reduction has now been shown in some
of them to follow on sexuality, but its place in the life-cycle varies in
different cases. The point of interest for present consideration is not so
much the details of the process of reduction, as the place which it holds
in the life-cycles of various' Thallophytes, and the influence which it appears
to have had in determining in them an obligatory succession of phases.
The question must for the present remain open how the reduction,
which we may presume to be a necessary consequence on fertilisation,
is carried out in those Thallophytes which show sexuality but have not
any fixed succession of phases, such as Vaucheria, etc. Subsequent observ-
ations will doubtless provide the actual facts, and will probably locate
the reduction-process either in near proximity to the germination of the
zygote, or it may be to the production of the gametes.1 We may even
1 Oltmanns, Morphol. it. Biol. d. Algen, 1904, p. 324; B. M. Davis, '' Oogenesis in
Vaucheria, " Bot. Gaz., 1904, vol. xxxviii., p. 81.
E
66 ALTERNATION IN THE THALLOPHYTES
anticipate that one or other of these locations will be found to be a
general feature of. those plants where there is no obligatory succession of
phases, and their somatic condition would accordingly? be in the former case
haploid, in the latter diploid. With this remark they must be left on one
side for the present.
In other cases, however, a succession of obligatory phases, denned on
the one hand by the incident of sexual coalescence, and on the other by
reduction, has been brought to light ; in fact, examples of alternation
have been found among the Thallophytes, showing cytological limits closely
comparable with those which have been accepted in the previous chapter
for the alternating generations in the Archegoniatae. Among the Algae
one of the best cases of this, substantiated on both cultural and cytological
evidence, is that of Dictyota dichotoma, Lamour. It has long been known
that the tetraspores, antheridia, and oogonia of this plant are distributed
on different individuals, but it has only recently been shown in what
relation these plants stand to one another.1 We now know that the number
of chromosomes in the somatic divisions of the plants which bear antheridia
and oogonia is 16 : that there is no change of chromosome-number in
the formation of the sexual cells, but that the fertilisation results in a
zygote which on germination gives a plant with 32 chromosomes in its
somatic divisions. This plant bears tetraspores; but in their production
the mother-cell, on dividing its nucleus into two and then into four,
shows a reduction to the original 16, the details of the process being
closely comparable to those in the tetrad-reduction of Archegoniates and
Phanerogams. The tetraspores on germination give plants which show 16
chromosomes on their somatic divisions, and thus correspond to the original
sexual plants. The only gap which is left in the full demonstration of
the life-cycle, both by cultures and by cytological observation, is that the
plants raised by "cultures from tetraspores have not yet been seen to bear
sexual organs : but still they correspond in their chromosome-number.
Here, then, is a succession of phases which appears to be obligatory,
involving two stages which have the same chromosome-relation as the
alternating generations in the Archegoniatae. But there is this difference :
that in external form and structure the two alternating generations of Dictyota
are substantially alike though the one is haploid and the other diploid.
Somewhat similar phases, which alternate in a less exact and obligatory
manner, and in which the cytological details have not yet been observed,
are seen in the life of Cutlerta : they are known as the Cutleria and
Aglaozonia stages.2 This case is quoted here as showing that in plants
probably akin to Dictyota^ the exactitude of the alternation in not maintained.
But this fact comes out much more strongly in the case of FUCUS, in
1J. 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 xof 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
lAnna/s of Botany, xviii., p. 541.
2V. 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 Conjugatae1 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 haploidj
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. 321-
'-' Gesarymelte Abhandluugen, i., p. 251.
* Ber. d. D. Bot. Ges., 1905, p. 285.
74
ALTERNATION IN THE THALLOPHYTES
a development stands hitherto alone,1 but biologically it occupies the
same place as a simple sporophyte.
FIG. 42.
Coleocha.ctepulvina.ta, after Oltmanns. i, Young zoosporangium (?). 2, 3, A
4, Oogonium shortly before opening. 5, Ditto after
Antheridia
(a) and young oogonium (p). 4, Oogonium shortly before opening. 5, Ditto after opening.
6, Zygote still with two nuclei. 7, Zygote developed to " fruit." 8, Germinating hypno-
zygote. a, Antheridium. 0, Oogonium. sk, Male nucleus, ek, Female nucleus, ckr,
Chromatophore. py, Pyrenoid. k, Nucleus.
Though the cases are thus seen to be still comparatively few in which
the Thallophytes have had their cytological condition traced throughout
the whole course of the individual life, there is a growing body of evidence
to show that an obligatory alternation of cytologically distinct generations
1 It is possible that the multicellular spores of Ascomycetes supply a parallel. If,
as is probable, the reduction accompanies the formation of the ascospores, then the
subsequent divisions in those spores would hold a similar place in the cytological cycle
to those in the fruit of Coleochaete.
THREE LEADING TYPES 75
exists among them. According 16 the criterion of chromosome-difference
there may be recognised a haploid, pre-sexual phase, characterised by
having "n" chromosomes — this corresponds cytologically to what has
been termed elsewhere the gametophyte ; and a diploid, post-sexual stage,
characterised by having " 2n " chromosomes — this corresponds in this
respect at least to the sporophyte. The alternation of these phases
depends primarily upon sexuality, which doubles the chromosome-number.
The reduction of the chromosome-number to one half appears to be a
necessary consequence of it, and the process by which the original number
is restored is found to be commonly associated, here as elsewhere, with a
tetrad-division.
As Professor V. H. Blackman has pointed out (I.e., p. 364), three nuclear
stages are to be observed in the sexual cycle of animals and plants :
nuclear association by fusion of the protoplasts which contain them ; nuclear
reduction^ or fusion, which doubles the chromosome-number; and chromosome-
reduction, by which their number is halved. Of these three stages the
second may take place at the same time as the first, or it may be delayed
for a short time, as in Spirogyra or Cosmarium : or, as in the Uredineae,
it may be delayed until the stage corresponding to chromosome-reduction.
According to the relative time of these successive nuclear stages the sexual
cycle may vary greatly, as we see that it does in the Thallophytes; and
three leading types of the cycle emerge, though they severally may graduate
into one another by intermediate steps : they include :
1. Those in which reduction immediately precedes gametogenesis and
sexual fusion. The order of events would then be (a) somatic division
with "2n" chromosomes: (b) chromosome-reduction: (c) gametogenesis
and sexual fusion. This is the case generally for animals : in plants the
best demonstration has been in Fucus : it is also seen probably in Rhopalodia ;
but it probably occurs also in many of those Thallophytes which have
no obligatory succession of phases, and especially in Achlya, and probably
in the Peronosporeae.
2. Those in which reduction immediately follows on sexual fusion. The
order of events in these would be (a) somatic division with "n" chromo-
somes : (b) gametogenesis with sexual fusion : (t) chromosome-reduction.
This is probably the case' in Desmids and other Conjugatae, and in the
filamentous Chlorophyceae, including Coleochaete.
3. Those in which a somatic phase of some extent intervenes between sexual
fusion and reduction, and again between reduction and sexual fusion. This is
seen in Dictyota, probably in the simpler Ascomycetes, in Uredineae, and
Florideae : it is comparable with what is seen in the Archegoniate series.
It is interesting to compare the grouping of types of alternation as
thus stated with the position adopted by Celakovsky in his paper which
was published some thirty years ago at Prag.1 The data, both physiological
and cytological, are now much more precise, though still very deficient.
1 Sitz. d. Ges. d. Wiss. in Prag, 1874, p. 30.
;6 ALTERNATION IN THE THALLOPHYTES
The criterion of chromosome-number is new : the method of physiological
experiment is also new. Still, the conclusions are in the main unaltered.
What was then styled " homologous alternation " now stands on a basis
of cytological unity as regards the somatic divisions, and denotes such
recurrent phases in plants as appear to be dependent on external condition,
not obligatory in their succession, and involve no cytological change :
this includes the cases grouped under (i) and (2) above. There is hardly
any need to designate such life-cycles as showing alternation at all, were
it not that this is the type of life-history for which the term was first
introduced by the zoologist Steenstrup. The types grouped above under
the heading (3) were distinguished by Celakovsky as showing "antithetic
alternation," and it is now found to have its basis in a cytological difference
of the successive phases, which also show an obligatory succession, not
determined directly by external conditions.
The conception of normal antithetic alternation now turns upon the two
critical points of sexual fusion and reduction : it is necessary to enquire how
far these events are historically the same in organisms at large. It would
seem probable that sexual differentiation, and perhaps even sex itself
originated along several distinct phyletic lines : on this point there is no
definite information, though the differences of character of the organisms
which show the simplest types of sexuality distinctly suggest that it had
not one common source only. In the present state of uncertainty it seems
.undesirable to depart from the usual convention by which the zygote is
held to be "homologous"; and, accordingly, it serves as a point for general
comparison between representatives of distinct phyla. But it must be
distinctly understood that this is in itself a conventional understanding, and
that its adoption for convenience of description does not necessarily imply a
strict "homogeny, " in the sense that sexuality was established once for
all. Similarly with reduction, which is theoretically a necessary consequence
of sexual fusion, it is only by a similar conventional understanding that
in divers organisms the cell where this is initiated is held to be "homologous":
it is not to be assumed that it is truly "homogenetic" in distinct phyla,
as though reduction had been initiated once for all in sexual organisms.
But, on the other hand, in organisms that are akin, such as the members of
the phyla of the Ferns or the Mosses, it may reasonably be held as probable
that the zygote and the spore-mother-cell are actually identical things, in fact
homogenetic for the whole phylum, in the sense that each probably sprang
from a phyletic source common for the whole phylum.
A comparison of plants at large as regards the position of the reducing
process in the life-cycle relative to sexual fusion shows great differences,
as we have seen. It is not improbable that these may have been due
in part to initial differences : we have no right to. assume that there was
uniformity at the outset. Some ground for the view that initial differences
existed is to be found in such cases as the Desmids and Diatoms ; for in
Closterium the rejection of the superfluous nuclei, and probably also
PHYLETIC DELAY IN REDUCTION 77
reduction, follows on nuclear fusion, while in Rhopalodia these events
precede nuclear fusion. Such cases seem to point to a probability that
the problem of reduction was solved independently and in different ways
in different lines of descent.
Nevertheless the term sporophyte has been adopted as applicable
collectively to the non-sexual phase which intervenes between sexual fusion
and reduction in those plants in which it occurs. But, following the above
reasoning, it must not be understood to convey necessarily any community
of descent for all the bodies which it covers. It seems probable that the
establishment of the sporophyte, whether by a process of intercalation or
otherwise, has taken place independently in several distinct phyla : thus
the sporophyte-stage in them, though in some more lax sense it may be
styled " homologous," is not to be held as " homogenetic " ; nevertheless,
useful analogies may be drawn between the corresponding phases in
distinct phyletic lines.
But, on the other hand, comparison within groups that are held to be
akin gives strong reason for recognising that there has been a shifting of
position of the event of chromosome-reduction in certain lines of descent,
and that the balance of the generations has thus been altered in the
evolutionary course. For instance, it seems probable that in the Uredineae
there has been a deferring of the event of reduction after sexuality, with the
result that the binuclear phase has attained considerable dimensions ; the
same seems probable also for the Ascomycetous Fungi, though along a quite
distinct line. A similar intercalation has been suggested in the Florideae.
Such conclusions can only become cogent when the cytological details are
known in a large number of related forms. But the most familiar, and at
the same time the most prominent and permanent example is that of the
Archegoniatae : in these there is a strong comparative basis for the belief
that the sporophyte stage has been intercalated, or in any case greatly
extended, in consequence of the deferring of the event of chromosome-
reduction. It may be a question whether the post-sexual stage in the
life-history of certain green Algae represents any phyletic predecessor of the
sporophyte of the Archegoniatae : it is quite probable that it did not. But
this much is clear, that it occupies the same position in the life-cycle, and
it gives at least the suggestion how the Archegoniate sporophyte may
have originated. According to the antithetic theory as applied to the
Archegoniatae, the complications of post-sexual nuclear reduction, involv-
ing, as they are seen to do, at least four nucleated cells, supplied the
starting-point for a diploid somatic expansion. That is the theory which
is adopted here as reflecting the probable mode of origin of the alternation
in the Archegoniate series. But it is only right to acknowledge that it
is not fully demonstrated either by the cytological facts, or by comparison
with the alternation in the Thallophytes. The latter can only supply
suggestive analogies so long as the actual phyletic origin of the Archegoniatae
remains as obscure as it now is. It becomes, accordingly, an object of all
78 ALTERNATION IN THE THALLOPHYTES
the greater interest to trace such internal evidence as there is of the method
and manner of the intercalation of the diploid phase in the life-history of the
Archegoniatae : and an important question will be what circumstances they
probably were which conduced to fixing permanently the alternation that
resulted, and to making it so constant a feature as it has been in the
upward development of the green plants of the Land. This will form
the subject of the next chapter.
CHAPTER VI.
THE BIOLOGICAL ASPECT OF ALTERNATION.
THE phenomenon of Alternation of Generations may be viewed in
various ways, and the history of the science shows that the different
aspects of this subject have presented themselves in a natural succession
according to the progress of biological thought. First it seemed sufficient
merely to observe the fact that successive, more or less distinct phases
occur in the life of the individual in certain groups of plants. In the
case of the Pteridophytes this was the gradually achieved result of the
labours of various workers antecedent to or contemporary with Hofmeister ;
but subsequently the discovery of a similar succession of phases in other
organisms than those in which it was first observed suggested at once their
comparison : such comparison was placed upon a secure footing for
Archegoniate plants by Hofmeister himself. Then followed the pursuit
of such comparisons into the region of the lower and simpler Thallophytes ;
but this led to some confusion at first, owing to premature attempts to
reduce all organisms which show sexuality to one rigid scheme as regards
the successive phases of their lives. On the other hand, in similar com-
parisons with the higher Flowering plants, the issues were temporarily
obscured by discussions over " the Alternation of Shoots " found in some
of them, and by the confounding of this with Alternation of the more
fundamentally distinct Generations seen in the Archegoniatae. But these
temporary clouds were in great measure cleared away by Celakovsky, when
he drew the distinction between "Homologous" and " Antithetic " Alterna-
tion. His comparisons, however, were still, like those of Hofmeister and of
Sachs, based essentially on form ; nevertheless, he arrived at conclusions which
we have seen in a previous chapter to coincide very nearly with the opinions
current at the present day, and this notwithstanding that those opinions are
now based on facts which were quite inaccessible to him. The writings of
Celakovsky may be said to have brought the formal study of Alternation to
a close. His conclusions were widely accepted at the time, though Botanists
of the weight and standing of Pringsheim still stood aloof from them.1
1 Pringsh. Jahrb., Bd. ix., 1878, also Scott, British Association Report, Liverpool, 1896.
Address to the Botanical Section.
So BIOLOGICAL ASPECT OF ALTERNATION
The succeeding phase of the history has been one involving first
physiological and subsequently minute cytological considerations. The
study of the effect of external conditions on the succession of stages of
the developing organism, initiated by Klebs in 1889, led at once to the
recognition of the fact that in certain plants that succession is mutable
according to circumstances, while in others the succession is obligatory.
The distinction between different types of the life-cycle of organisms thus
established was found to coincide very nearly with the distinction drawn
by Celakovsky between "homologous" and "antithetic" alternation, and
thus his general position came to be greatly strengthened. But another
effect of the experimental test was to open up more definitely than before
the problem of origin of this obligatory succession of phases, in those cases
where it exists : it also accentuated the difference between the antithetic
or true alternation, and those other appearances which bear a superficial
resemblance to it. But meanwhile the question of the rise of the neutral
generation was being approached also from the point of view of adaptation,
and a theory of its origin in an amphibious mode of life, which it will be
the object of this chapter to develop, was already being advanced as an
explanation of the progress and final dominance of the sporophyte in the
plants of the land. It is clear, however, that adaptation would only account
for its advance, not for its ultimate origin. This amphibious theory was
based upon physiological considerations, together with closer observation
of the origin of the sporogenous cells, their limitations, and their relation to
the tissues which are merely vegetative. Lastly, more careful observation
of the details of sexuality and of spore-production led to the generalisation
on the basis of minute nuclear structure : this put the cytological cachet, as
well as a structural check upon the conclusions already drawn. But the
existence of a chromosome-difference between the two generations turns
attention afresh to the question of the ultimate origin of the obligatory
succession of phases : it suggests that the origin was in sexuality, and in
those post-sexual complications which are so frequently the consequence
of nuclear fusion. Naturally these several phases of the study of alternation
have overlapped one another, and proceeded in some degree coincidently.
One of the interesting features in the history is that their results have
often run so nearly parallel as to yield a high degree of mutual support.
It has been remarked above that up to the time of Celakovsky the
study of alternation was on the basis of form ; but it is now clear that the
merely formal comparison of different organisms, or of their successive
stages one with another, cannot suffice for the full solution of the question
as to the real nature of alternation. The case of the structurally similar
but cybologically distinct generations of Dictyota show this, while the differ-
ence of the propagative organs which they bear confirms the distinctness
of the two generations. In the Florideae also, there are no definite structural
details which serve as formal differentiating characters between the pre-sexual
and the post-sexual stages. Such examples will probably be multiplied as
AMPHIBIOUS HABIT 81
cytological investigation progresses, <but those quoted will suffice to show that,
for those who accept the cytological distinction of the alternating generations,
the mere distinction on ground of form is insufficient.
The existence of certain cases of alternation, demonstrated cytologically,
where, as in Dictyota^ the form of the two generations is substantially the
same, and of other cases in which, as in the Archegoniatae, the genera-
tions differ fundamentally in form as well as in points of structure, raises
a physiological question which has wide bearings. The external conditions
to which, in the past as well as now, the two generations have been or are
exposed must be considered in their relation to such differences. Now, in
Dictyota, and also in Polysiphonia, since the two forms grow on the same
coasts, at the same time, and about the same spots, it is obvious that the
conditions to which they are exposed are substantially the same : knowing
how closely form is dependent upon circumstance it is not surprising that
the two generations should be so much alike.
In the case of the Uredineae, again, the external circumstances of
the two cytologically distinct stages of the life are alike, both being
internally parasitic : in form their mycelial stages are also alike, and their
difference of character only becomes apparent on the formation of the
propagative organs. Similar remarks will apply, perhaps with less cogency,
to the post-sexual developments in certain Ascomycetes, the circumstances
of which do not differ materially from those of the pre-sexual phase, and
in this may be seen some illumination of the fact that they show for the
most part mere variants upon the simple filamentous form. Thus, for
•certain Thallophytes in which cytologically distinct generations have been
recognised, there is a remarkable similarity in form of the two generations :
this probably finds its true explanation in the fact that the biological
conditions to which they are respectively exposed are relatively uniform,
and have been so throughout their descent.
But with the Archegoniatae the case is quite different : the cyto-
logically distinct and alternating generations differ widely in their characters
of form and structure. The question, therefore, presents itself, what is
the biological and ultimately the phylogenetic bearing of this very obvious,
and at the same time widespread fact? Far-reaching it is indeed, for
it affects all the characteristic vegetation of the land. Taking, first, as
the most prominent example, a typical Fern, the gametophyte, or
prothallus, shows in its more delicate structure and in its habit, as well
as in the way in which the sexual process is effected, an adaptation to
moist conditions, under which it grows best ; while its ultimate function,
that of sexual reproduction, cannot be carried out without the presence
of external fluid water : it is, in fact, typically semi-aquatic in its nature,
sharing many of its main characters with the green Algae from which
we have some reason to believe that the land-flora originated. 'The
sporophyte, on the other hand, is fitted by its more robust texture, as
well as by its differentiation of tissues, for successfully enduring exposure
82 BIOLOGICAL ASPECT OF ALTERNATION
to the air under comparatively dry circumstances, while dry weather is
important for the dispersal of the spores, which it is the final function of
the sporophyte to produce : thus the Fern, as we normally see it, is an
organism with, so to speak, one foot in the water, the other on the land.
Calling in also such evidence from phylogeny as we can command, it
will be generally accepted that the gametophyte is the older and pre-
existent generation ; it corresponds to the gametophyte as seen in the
Liverworts, or in the green Algae : and if we trace the descent of the
Archegoniate series from some green Algal forms, we may recognise
that the gametophyte of the Ferns retains the chief Algal characters, as
regards both its texture and its sexual process. The sporophyte, on the
other hand, is the younger generation : among the present green Algae
there is hardly any body strictly comparable to the sporophyte, nor is
it to be expected that there should be, if, as above stated, the sporophyte
is typically sub-aerial in its characters, while the green Algae are typically
aquatic. A comparison of the successive families of the Archegoniate
series suggests the progress of the sporophyte, from small beginnings, as
illustrated in the Bryophytes, to larger size and greater complexity of form
and structure, as seen in the Vascular Cryptogams and Gymnosperms :
its advance is accompanied by a corresponding reduction of the gameto-
phyte, and the whole is to be correlated with a progression from the
aquatic or semi-aquatic habit of the lower forms, to the very distinctly
sub-aerial habit of the higher. It may accordingly be concluded that the
alternation which is so prominent in the main Archegoniate series is the
result of adaptation of originally aquatic organisms to sub-aerial conditions
of life : it may, in fact, be distinguished physiologically as an amphibious
alternation, which finds its morphological expression in the difference of
external form and internal structure between the more ancient gametophyte
and the more recent sporophyte.
It is an important fact that in the main Archegoniate series the antithetic
alternation is normally constant, though the balance of the two generations
may vary : the constancy of the phenomenon makes us enquire why it
should be so : the circumstances which have encouraged this constancy
seem to have been these. The Archegoniate series probably sprang
from green aquatic forms, inhabiting, as so many of the green Algae
now do, shallow fresh water, or the higher levels between the marine
tide-marks : the sexual reproduction was effected through the means
of external water, and if other conditions were favourable it could be
carried out at any time through the water which was always present.
Certain forms, perhaps thereby escaping from competition, spread to the
land, where access of water was only an occasional occurrence : in these
the sexual process could only be effected at time of rains or floods
or copious dews; and even then might not take place unless the sexual
organs were fully mature : thus less dependence could be placed upon
sexuality for propagation, and an alternative method of increase of individuals
AMPHIBIOUS HABIT 83
had to be substituted. This wasxdone 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-
0 ' 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
9o
STERILISATION
spore-production before the tetrad-division. It is a secondary matter
morphologically that in this case their existence is brief; but physiologically
it is important, for they are sacrificed to furnish better nutrition to the
others which remain fertile, and produce spores. The structure seen in
Fig. 45 is thus to be interpreted as indicating the sterilisation of certain of
the potentially fertile cells in the sporogenous group of Psilotum.
A second example illustrating this partial sterilisation of a sporogenous
group may be quoted : it is selected from among the Bryophytes ; but
the same arguments as in the previous case will equally apply here.
Fig. 46 illustrates two stages in the development of the sporogonium of
Aneura : the younger shows the clearly defined, hemispherical internal group
FIG. 46.
A, median section of young sporogonium of Aneura ambrosioides. The internal mass
of cells of the sporogonial head ("archesporium ") is already differentiated so as to
indicate the sterile elaterophore, and the outer fertile region. B, the same, older : the
indications of sterilisation have extended outwards, and it is only the peripheral fringe of
cells (shaded) which will be sporogenous. C, transverse section of the'same. X 150.
of cells of the sporogonial head, which are equivalent inter se, inasmuch
as they have a common origin ; but they are already differentiated into two
distinct regions, a peripheral fertile region, of which the cells are shaded,
and a central sterile region. The former differentiates at a later stage into
spore-mother-cells and elaters : the latter forms the sterile elaterophore.
The whole hemispherical group corresponds in position to the body
similarly placed in other Hepaticae, which have no elaterophore, and
in which the whole region develops into spores and elaters. This case
therefore illustrates an advanced stage of sterilisation of tissue which may
be held to be ancestrally sporogenous throughout. ' But the final fate of
the sterile cells here is not merely to serve as evanescent nutritive cells;
for the elaterophore and elaters are a permanent tissue and permanent
cells, which remain till the ripeness of the spores, and are functional in
their dispersal.
AS SEEN IN BRYOPHYTA
FIG. 47-
Fnnaria kygrometrica. A, longitudinal section of a sporogonium showing the first
differentiation of its parts. X about 96. B, the upper part of the same. x6oo.
r marks the limits of the theca and operculum. C, basal part of the capsule of the same.
X 600. The intercellular spaces are beginning to form. ar, archesporium. col>
columella.
STERILISATION
Such examples serve to show that there are good grounds for holding
that sterilisation of individual cells of the sporophyte, which by their origin
are to be recognised as potentially fertile, does take place ; and that such
cells may be diverted to a temporary, or to a more permanent use
in connection with the production of spores. Thus we are justified
in holding that sterilisation, which is the initial factor in the
working hypothesis sketched in the previous chapter, has been actually
operative.
It will be impossible here to enumerate all the cases where evidence of
sterilisation of potentially fertile cells has been brought forward ; but some
of the more prominent instances of it
will be quoted. At present it is the
mere fact of sterilisation which is before
us, not the biological consequences
which follow in facilitating the nutrition
or the dispersal of spores, nor yet the
morphological advances which may
result. These aspects of the matter
will be left over till the several groups
of the Archegoniatae are specially
discussed.
Among the Liverworts the simple
Ricciaceae have centrally an undiffer-
entiated sporogenous tissue; but as a
rule in the Marchantiaceae and Junger-
manniaceae the almost spherical mass
of sporogenous tissue becomes differ-
entiated as development proceeds :
cells, singly or in groups, instead of
undergoing the tetrad-division, are
developed in a vegetative manner,
either as nutritive cells (Sphaerocarpus\
or as elaters of various form and
arrangement (compare Fig. 46, of
Aneura]. In the Anthocerotaceae, on the other hand, the archesporium
is a dome-shaped layer surrounding a central Columella ; but the products
of this layer do not only form spore-mother-cells, but also numerous
sterile cells arise from it, which develop as an irregular network enclosing
the mature spores. In point of fact, in the Liverworts it is the exception
rather than the rule for the whole of the sporogenous tissue to be fertile,
though this is the case in the simplest of them.
In the Musci, on the other hand, the whole of the cells developed
from the definite, single-layered archesporium normally produce spores ;
but the archesporium is relatively small compared with the bulk of the
young sporogonium : it shows an apparently arbitrary limitation at its
FIG. 48.
Median longitudinal section of a sporogonium
of Sphagnum, with bell-shaped archesporium.
Xijo. (After Waldner.)
AS SEEN IN PTERIDOPHYTA
93
margin. Fig. 47 illustrates howMhe series of cells of the archesporium
is continuous both upwards and downwards beyond the limit of its fertility :
this indicates a probability that its marginal limitation has been due to
sterilisation, a view which is strengthened by comparison with Sphagnum
(Fig. 48) ; for there the archesporium is in the form of a complete dome :
FIG. 49.
Danaea elliptica, Smith. Drawings illustrating partial septations of the sporangium.
A, tangential section through three sori, showing the loculi in ground plan : the septa are
often thin, so that pairs of loculi are in close juxtaposition ; the loculi marked (x)
are large, and show one or more partial septa. X 20. £, C, D, E, show such loculi with
partial septa in greater detail : in D and E it is difficult to decide whether the cells
marked (?) will develop as tapetum or as spore-mother-cells. X 150.
a sterile condition of the cells at its apex would give a barrel-shaped
archesporium, as is seen in the Bryineae. Thus it will appear that any
evidence of sterilisation in the Musci is less direct than that in the
Hepaticae.
Evidence of sterilisation of potential sporogenous cells is common in
the homosporous Pteridophyta : and, as in the Bryophytes, the function t
94
STERILISATION
of the arrested cells is sometimes simply nutritive, sometimes they form
permanent tissue-masses. In Lycopodium and Phylloglossum, and in the
( homosporous Ferns, after the sporogenous tissue is first defined, all its
cells normally undergo the tetrad-division, and develop spores : occasional
cells may become disorganised without full development, though, as a
rule, all the potentialities are realised. But among the Marattiaceae,
where, as a rule, all the cells of the sporogenous groups are fertile, a number
of exceptional cases have been noted : the most remarkable is that of
Danaea, in which it has been shown how certain cells of the larger sporo-
genous groups remain sterile, and may be develope^d as tapetum, or even
as component cells of a partial septum (Fig. 49). Somewhat similar
conditions have been seen in Kaulfussia, Marattia, and Angiopteris*
FIG. 50.
A, apex of sporangium of Equisetum litnosum, L., showing the sporogenous cells,
surrounded by the tapetum (shaded), and sporangial wall. F>, shows part of an older
sporangium with its tapetum (t) still clearly defined, though the individuality of the cells
is lost : within this the sporogenous tissue, of which certain cells (a) are abortive.
X200.
In other cases it has been found that only a portion of the cells of the
sporogenous group are fertile, as already described for Psilofum (Fig. 45) :
this has been shown to be the case also in Tmesipteris, and in Equisetum
(Fig. 50), and it has been described also as an occasional feature in the
Ophioglossaceae. In all of these a varying proportion of the sporogenous
cells are sterile, and become disorganised without forming tetrads. As
the proportion of the sterile to the fertile cells is not fixed, an elastic
arrangement exists which leads to the largest number of spores being
brought to maturity that the plant at the time can support. The limits
of the sporogenous tissue in early stages are difficult to define in these
large sporangia, and they show considerable irregularities : this is especially
so in Psilotum, Tmesipteris, and Ophioglossiim, and it appears to be partly
due to the ill-defined and broad tapetum which is formed peripherally,.
AS SEEN IN PTERIDOPHYTA
95
partly to the fact that owing to Sterilisation the definitive fertile cells do-
not form a continuous mass.
Among heterosporous forms, sterile cells are commonly present in
the female sporangium (Fig. 51): there is good reason to think that
arrest of potential sporogenous cells has greatly favoured the advance in
size of the relatively fe\v remaining megaspores. But apart from this,
the case of Isoetes is interesting, since there is evidence of sterilisation
both in the mega- and micro-sporangia, and in both it has resulted in
permanent tissue-masses. In both types of sporangium an extensive
potential sporogenous tissue is formed, which is at first uniform in
structure, as it was also in origin. In the microsporangium considerable
tracts of this tissue differentiate later as vegetative trabeculae and tapetum,
FIG. 51.
Selaginella spinnlo»a, A. Br. Section of
megasporangium showing the single fertile
tetrad still very small, and the rest of the
sporogenous cells arrested. X TOO.
FIG. 52.
Isoetes lacitstris, L. Vertical section of a
young microsporangium. sp = fertile tissue.
tr= trabeculae. / = tapetum. X 100.
while the remainder forms microspores. From the history of development^
and from comparison, the conclusion seems justified that the trabeculae and
tapetum in this case represent sporogenous tissue which has been converted
into sterile tissue, serving nutritive and mechanical purposes in the very large
sporangium (Fig. 52). Similarly, in the megasporangium there is sterilisa-
tion, but it has been carried much further, and it has been possible to
show that the megaspore-mother-cells are not morphologically predetermined,
but are physiologically selected from among a large number of potentially
sporogenous cells : also that each archesporial cell gives rise to several
megaspore-mother-cells, as well as to trabeculae and tapetum (Fig. 53)
(Wilson Smith). Thus there has been a differentiation of tissues of uniform
origin, and a large part has been diverted to functions played by sterile
vegetative tissue. Very similar sterile tracts of tissue have been seen in
the large sporangia of Lepidostrobus Broivnii, and their origin by
sterilisation is highly probable, though naturally this is hardly susceptible
STERILISATION
of demonstration in a fossil. The general conclusion may be drawn from
such cases as those cited, that sterilisation has played a considerable part
in the sporangia of Pteridophytes.
In Seed-Plants also there is frequent evidence of sterilisation of cells
of a potential archesporium, both in megasporangia and in microsporangia.
In the latter, examples have been seen in which a considerable proportion
of the cells of the sporogenous group are
obliterated in much the same way as in Psilotum,
But in the anthers of not a few Angiosperms
partial or complete septa of sterile tissue may
be formed in plants whose near allies have
their pollen-sacs non-septate. Thus, in the
Onagraceae the stamens of most of the genera
are of the ordinary quadrilocular type; but in
the genera Circaea, Gaura, Clarkia^ and
Eucharidium the four loculi are each divided
transversely by one or more sterile septa : these
septa may consist of only a single layer of cells
having the character of tapetum, or of two
layers, or even of four or more, of which the
middle layers then resemble the tissue of the
connective. An examination of early states of
development of these anthers shows that the
septa result from sterilisation of part of the
sporogenous tissue, for in sections it is seen
that the sporogenous cells and those which will
form the septa originate from a common layer
corresponding to the archesporium of normal
anthers of the family (Fig. 54). A similar
state of things has been described in certain
of the Mimoseae (Inga, Calliandra, Acacia,
Albizzia], in many of which there are eight
. ' c , , , c
pollen-sacs in place of the normal number of
four ; while in others (Parkid) the number
may be much larger. Here, again, the
developmental history shows that sterilised
archesporial tissue provides the septa which
divide the four original pollen-sacs into eight
or more loculi. With these may also be compared the cases of V^sc^^m
and Loranthus. Developmental study of the anther of Rhizophora
has given the same result : in its massive anther the small pollen-
sacs are very numerous, distributed over a large surface : Warming has
concluded that the anther became multilocular by the arrest of the
further development of certain parts of the pollen-forming tissue (see
Fig. 72, p. 142). Such examples, which by no means exhaust the list,
FIG. 53.
Part of a section of a megaspor-
angium of Isoetes. The cell marked
(m) is the only fertile spore-mother-
cell, the rest are undergoing vegeta-
tive divisions, including the cell (a)
as shown by other sections of the
series. Thus sterilisation affects the
large majority of the cells of the
sporogenous group. X245- (After
Wilson Smith.)
AS SEEN IN SEED-PLANTS 97
• X
show that sterilisation of sporogenous cells is not uncommon in the anthers
of Seed-Plants.
Evidence of sterilisation is also found in the ovules of Seed-Plants.
Among the Gymnosperms, the Gnetaceae show an archesporium consisting
of a group of hypodermal cells : in Gnetum Gnemon, which is the best
known example, these give rise to a considerable mass of sporogenous cells,
but only one embryo-sac is finally matured. In the Cycads the case is
similar, inasmuch as there is a considerable tract of sporogenous tissue,
though only one embryo-sac matures. In the Coniferae also there is
frequently a multicellular archesporium, and several embryo-sac-mother-cells
have been seen to enlarge in Taxus and Sequoia^ but in most of them
only a single one. Among Angiosperms a condition very similar to that
FIG. 54.
A, longitudinal section of one loculus of a young stamen of Rucharidium concinuum,
showing differentiation of the potential archesporium into fertile cells (a) and sterile
cells (j). £, similar section of stamen of Clarkia elegans, more advanced, showing a
sterile septum dividing the contents of the single loculus into distinct sporogenous groups
in Gnetum is seen in Casuarina (Figs. 55, 56) : this case is particularly
interesting, since the potential embryo-sacs are not simply obliterated by
the growth of the favoured one, but some develop into tracheides with
thickened walls — a proof that permanent sterile tissue may be formed from
potentially sporogenous cells. In certain Amentiferae also a similar
formation of tracheides has been seen. A multicellular archesporium is
common, besides, in other Archichlamydeous Dicotyledons, e.g. in the
Ranunculaceae and Rosaceae, and some others (Fig. 57); but it is
apparently less common in the more advanced Dicotyledons and in the
Monocotyledons. The examples tfius quoted suffice to show that sterilisation
of potentially sporogenous cells is frequent both in the microsporangia
and in the megasporangia of the Seed-Plants.
And thus it is seen that evidence of sterilisation is widespread : it is
found in all the main groups of the characteristic Flora of the land, both
G
98
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.) E—Alchemilla 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. * Abstammungslehret 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___^ — pr-r_T_^___ in the deeper-seated forms. In the
Filicineae still more exact specialisa-
tion is the rule, and the mechanical
annulus becomes a marked feature
in the stalked sporangium. But a
comparison of the Marattiaceae and
other Ferns leads to the conclusion,
that the presence of an annulus,
and its elaborateness, are to be
correlated with the freedom of the
sporangium from mutual relations
with other bodies. The annulus,
in fact, is still only a specialised
region of the sporangial wall. This
conclusion accords with the facts
FIG. 59. of its inconstancy, for sporangia
Ophioglossum reticulatum, L. Vertical section of which Open Under Water may haVC
the deeply sunk sporangium, with the sporogenous tissue . m, . ,
shaded, xioo. no such mechanism, lhat is the
case in Isoetes, while the sporangia
of the Hydropterideae are also entirely without an annulus; nor is there
any stomium in them, which would localise dehiscence. Loxsoma is a
specially interesting case, for there the annulus, though indicated by the
cell-divisions as complete, is only partially indurated : as a matter of fact,
the part of it which is not indurated could not possibly be mechanically
effective, owing to the mode of packing of the sporangia in the sorus-
(Fig. 60).
The general conclusion to be drawn is, that while the protective wall
itself is always present, those mechanical arrangements collectively desig-
nated by the term "annulus" are not essential or constant parts of the
sporangium, however constant they may seem to be in certain groups :.
where they cannot be mechanically effective they may be entirely omitted.
Nor is the tapetum to be looked upon generally as a morphological
ITS INCONSTANT FEATURES
105
constant, notwithstanding that it x shows some constancy of character in
certain circles of affinity. Sometimes it is not differentiated at all, a condition
which holds throughout the Bryophyta : in other cases it may appear as
a more or less definite band of cells, which originates from the tissues
surrounding the sporogenous cell or group of cells, sometimes from the
sporogenous group itself. In some cases a large number of cells of the
sporogenous groups act like a diffused tapetum, becoming disintegrated
during the development of the spores (Ophioglossum, Psilotum) : in Equisetum
both a diffused tapetum of
this nature is found, and also B
a definite single-layered tape-
tum, which originates outside
the sporogenous group. In
Lycopodivm, and in the Mar-
attiaceae the tapetum arises
from cells outside the sporo-
genous group : in other cases,
such as Selaginella and the
Leptosporangiate Ferns, the
tapetum may consist of cells
cut off from the sporogenous
cell or cell-group. There is
indeed good reason to think
that there has been a pro-
gressive change of origin of
the tapetum within certain
circles of affinity : speaking
generally it may be said that
indefinite and non-specialised
nutritive arrangements are
characteristic of larger and
probably primitive sporangia,
but more definite tapetal
layers are found in the smaller
and probably derivative : and
further, that while in sporangia
of relatively large size the
tapetum usually originates outside the sporogenous group, in smaller
sporangia of the same affinity it may be cut off from the sporogenous cell
or cell-group. Such a change appears to be illustrated by comparison of
Lycopodium with Selaginella, and of the Marattiaceae with the Lepto-
sporangiate Ferns. The result of such comparisons is that the tapetum,
however uniform it may be in function, is found to be variable both in
occurrence and in origin, and accordingly it cannot be regarded as an
essential or constant feature of the sporangium.
Loxsoma Cunninghami, Br. E, young sorus. A , rather
older. &, vertical section of the base of the receptacle, showing
young sporangia. C, D, mature sporangia, showing the
1 annulus, and distal point of dehiscence
5x250. C+Dxso.
io6 THE SPORANGIUM DEFINED
It remains to consider the archesporium, and the sporogenous group
•and spores which arise from it. An archesporium, in one form or another,
is a necessary constant in the development of a fertile sporangium : the
form, the limits, the mode of origin and of later segmentation which it
shows, may vary, as indeed is seen to be the case in the different sporangial
types ; but Avhatever its variations may be, it is in the archesporium, and
in the sporogenous cells which it produces, that we see the essential feature
of the sporangium. It will be necessary, then, to examine it carefully,
and to see how far it is possible to attach a definite meaning to it.
The term archesporium was introduced by Goebel,1 and denned as
follows : " In the Vascular Cryptogams, as in the Phanerogams, the
spore-producing tissue may be universally referred as regards its origin
to a cell,- a cell-row, or a cell-sheet : — I designate these original parent
cells of the sporogenous tissue as the archesporium." To this he added
that "in all Vascular Cryptogams examined an hypodermal archesporium
exists," thus definitely localising it in a position comparable to that
in the Spermophyta. He remarks, however, later that he does not lay
vSpecial stress upon the archesporium being always a cell-row or cell-sheet,
and contemplates it as possible that sometimes the development may
proceed otherwise than by the appearance of an archesporium of the
form described. An examination of all the types of sporangia of living
Pteridophytes has shown that this is the fact : a considerable number of
cases have been observed in which the archesporium is not hypodermal,
in that it is not defined by a single periclinal wall of the parent cells
involved. The existence of many exceptions among Eusporangiate
Pteridophytes suggests a reconsideration of the archesporium. We may
enquire whether a definite meaning is attached to the term, and if so,
whether that meaning is of general application.
The effect of Goebel's investigations on the sporangia of Pteridophytes
was to extend downwards from the Phanerogams the demonstration of a
formative cell or cells to which the origin of the spores may be ascribed.
Before 1880 it was held that a 'mass of cells within the young sporangium,
showing irregular divisions, took upon them the character of a sporogenous
tissue : Goebel's results led him, as we have seen, to the statement that
the spore-producing tissue can be referred as regards its origin to a
cell, a cell-row, or a cell-sheet, which can be distinguished very early
by the nature of its materials from the rest of the cell-tissue. This
archesporium was successfully recognised in certain cases, and the
tendency of the time was to expect similar success in all cases. Thus a
special significance came to be attached to these cells, quite apart from
that of the surrounding tissues, as being predestined from the first to the
important function of spore-production.
The location of the archesporium in the Phanerogams was found to
be consistently hypodermal : in a somewhat strained sense the same was
^Bot. Zeit., 1880, p. 545 etc.
THE ARCHESPORIUM 107
found to be the case in many N of the Pteridophytes. But it was not
sufficiently recognised that between the two lies all the difference between
stratified and imperfectly stratified meristems. This point was brought
forward in I896,1 in the proposition that "the study of the sporangia or
synangia of a plant should be carried out in the light of a knowledge
of the segmentation of its apical meristems," and the generalisation was
laid down that " where the apical meristems are distinctly stratified, the
structure of the young sporangium is stratified also : in those plants where
there is a non-stratified structure, with one or more initial cells, and
frequent periclinal division of superficial cells near the apex of stem, leaf,
or root, there the structure of the young sporangium is not distinctly
stratified." Such exceptions to the hypodermal position of the arche-
sporium as have been found among the Pteridophytes all fall under this
generalisation, and it may be added as a corollary that in all Vascular
Cryptogams investigated, the sporogenous tissue is ultimately referable to
the segmentation of a superficial cell or cells. This has been since
noted by Wilson Smith,2 who has accordingly suggested that the term
" archesporium " should be extended to these also, and he gives the definition
as follows: "The cell, or group of cells, whether superficial or hypodermal,
to which in a last analysis all the sporogenous portion of a sporangium
can be traced, ought to be called the archesporium."
The use of the term " archesporium " has been discussed afresh by
Goebel in his Organography (p. 771). He first describes the case for the
anther of an Angiosperm thus : " In each of the four angles of the anther
a cell-row or cell layer below the epidermis (hypodermal) divides by
periclinal walls. Of the cells which thus arise the internal form the
archesporium, the outer the ' schichtzellen,' which now divide still further
by periclinal walls." The archesporium is thus defined as the inner product
of the division of the hypodermal layer ; it gives rise neither to any part of
the sporangial wall nor to the tapetum, though, since these have a common
origin with it from the hypodermal layer, it is not clear why the latter
should not itself be styled the archesporium. Passing on to the Pteridophyta,
after noting how superficial cells give rise to the essentials of the sporangium,
and that the details are not uniform, he proceeds (/.<:., p. 774): "We
may then designate that superficial cell or cell-layer as archesporium which
sooner or later gives off sterile cells, while in the sporangia of Angiosperms
the archesporium is a cell-layer lying below the epidermis, which is already
differentiated : with this the above-noted differences in wall-structure of the
Pteridophyta and Gymnosperms on the one hand, and of the Angiosperms
on the other would correspond." This passage calls for the following remarks.
In the first place, Goebel accepts the conclusion of Wilson Smith,
that since the sporogenous tissues of Pteridophytes are all referable in
origin to superficial cells, therefore those cells are to be recognised as
the archesporium. By accepting this conclusion, it may be presumed that
1 Studies, ii., p. 8. -Rot. Gazette, vol. xxix., p. 325.
io8 THE SPORANGIUM DEFINED
he approves of the method by which it is arrived at : that is, the method
of recognition of the archesporium by " a last analysis " of cell-origin.
Secondly, Goebel's method of recognition of the archesporium is not
consistent : he designates the inner product of the hypodermal layer in
the Angiospermic stamen the archesporium, notwithstanding that the
" schichtzellen " and the tapetum are sister-cells with it. But in the
Pteridophyta, on the ground of common origin by segmentation, not only
the tapetum but also the sporangial wall itself are described as derived
from a superficial archesporium. If the recognition of an archesporium
is to be based upon "a last analysis" of the segmentations, then the
hypodermal layer of the Angiospermic anther, and not merely the inner
product of its segmentation, is the archesporium.
Thirdly, the recognition of the archesporium by the method of "a
last analysis " brings together under a common head, merely on the ground
of early segmentations, things which are not really comparable, and ascribes
a distinct origin to things which are indistinguishable when mature. The
superficial archesporium of the Pteridophytes gives rise to part of the
sporangial wall and of the tapetum : the archesporium of the Angiospermic
anther, on Goebel's definition, gives rise to neither. It is impossible to
conceive how by any known evolutionary progression the former type of
" archesporium " could pass into the other, and the superficial cells be covered
over : therefore the two are to be regarded as not truly comparable. Further,
the recognition of superficial cells in the Pteridophytes as archesporial
draws a distinction between part of the sporangial wall which originates
from them, and the rest which does not : thus in the Leptosporangiate
Ferns the apical part of the annulus would be archesporial, the lateral
parts would not.
With all respect to the opinion of the writer who introduced the term,
I think that this last change in its application, as suggested by Wilson
Smith and accepted by Goebel, makes more obscure the meaning of a
word which never has been clear. The Bryophyta provide a reductio ad
absurdum of the method of " a last analysis " ; for, following this method,
in Sphagnum and some others the amphithecium would be reckoned as
the archesporium, while in the ordinary Bryineae it would be the
endothecium : or, carrying the analysis in the latter case to its extreme
limit, the first segments in the upper half of the zygote, or even the
ovum itself, would be the archesporium.
The fact is that this sort of analysis of formative tissues has served
its turn : it has led to much detailed investigation, which has, however,
shown that the segmentations which lead up to the formation of spore-
mother-cells are not comparable in all cases. The time has come, in
presence of many divergent details, to admit frankly that there is no
general law of segmentation underlying the existence of that cell or cells
which "a last analysis" may mark out as the "archesporium," and that
therefore the general application of such a term to those cells which the
THE ARCHESPORIUM 109
analysis discloses has no scientific" meaning, beyond the statement of the
histiogenic fact. At the same time, the value of the details which have
been acquired by the pursuit of the archesporium must not be under-
estimated for purposes of comparison. What is dangerous is the attachment
to them of ulterior ideas : the assumption that because a definite
" archesporium " is often found, it should by rights be always present :
and the effort to trace in its appearance homologies which seem based
on forced rather than on natural comparisons. On the other hand, the
term has become so established in the literature of the subject that it cannot
be summarily discarded : it may be retained merely in a descriptive sense,
in those cases where the cell or cells which give rise to the sporogenous
.group are obvious, but in a descriptive sense only.
The discrepancies which become apparent in the course of develop-
ment between different types of sporangia tend to justify the position
already adopted by Strasburger on general grounds: he remarks i
that the centre of gravity of the developmental processes does
not lie in those cells, cell-rows, or cell-aggregates which have been
designated " archesporium " by Goebel : the archesporium still belongs to
the asexual generation, and the presence or absence of a well-defined
archesporium is not a matter of importance, for it is merely the merismatic
tissue from which the spore-mother-cells are derived. From the point of
view of a theory of sterilisation as enunciated above, these spore-mother-cells
may be held to be, in the simpler cases at least, the residuum which a
progressive vegetative change has left : in that case there is no reason to
expect that the demarcation of these islands of fertile tissue should have
followed any definite system in plants at large, which would be reflected with
any exact uniformity in the segmentations now involved in their formation.
The frequency of hypodermal origin of the sporogenous tissue in
Vascular Plants is readily intelligible biologically. In all except the very
simplest sporophytes the spores are protected during development by tissues
which surround them completely : this ensures nutrition and mechanical
protection. In the Bryophyta the scattering of the spores would be equally
efficient whether they be produced close to the surface or deeply seated,
since the dehiscence, whether by a terminal operculum or by longitudinal
slits, gives free exit to all the spores of the continuous spore-sac, and
accordingly the spore-mother-cells of the Bryophytes may be central, as
in many Hepatics, or removed more or less from the centre by the
occurrence of a columella, as in most Mosses. But in all Vascular Plants,
where the spores are produced in separate pockets or sporangia, the
dehiscence leading to dissemination is* referred to the several sporangia
themselves : this necessitates for them a superficial position on the plant-
body, or, better, that they shall project beyond the surface. The hypo-
dermal origin of the sporogenous tissue which is so frequent, may thus be
recognised as a compromise between the two requirements of effective
^Annals of Botany, vol. viii., p. 316.
no THE SPORANGIUM DEFINED
protection and nutrition on the one hand, and ready dissemination on the
other. But the compromise may have worked out differently in different
lines of descent, and, indeed, it appears from the variety of the segmentations
in the Pteridophytes that this has actually been the case. From this point
of view no difficulty need be felt to arise from the absence of any general
law of segmentation, leading up to the formation of spore-mother-cells;
but, on the other hand, similar and even definite types of segmentation,
culminating in regularly segmented sporogenous groups resembling one
another, may have been evolved along more than one line of descent.
It cannot escape notice that in some cases the individuality of the
sporangium is not maintained. Certain synangial states are not uncommon,
which can only be regarded, from the evolutionary point of view, as
results of either septation or fusion : where the fusion or septation is
incompletely carried out, and partial septa are present, it becomes a
question whether the whole or only the part of the complex body is
correctly to be termed a sporangium. This difficulty is very obvious in
the sori of Danaea (Fig. 61). The mere application of a term is naturally
a trivial matter : the question which is really important is, how far the
conception of the sporangium is to be modified by the existence of such
cases. The current conception of the sporangium is based upon examples
where it has a distinct individuality : in the Ferns and Lycopods, and even
in the pollen-sacs of normal Angiosperms such individualised sporangia are
seen. But it is a question how far the idea of the individualised sporangium
so gained is an enlightening one from the point of view of descent. In
the same way, the old conception of the cell as the structural unit of the
plant-body was based upon the study of the tissues of the higher plants,
where the cells are for the most part individualised : it had to give way
before the accumulated examples of cell-fusions, of polynucleate cells,
and of non-cellular construction in plants both higher and lower in the
scale. Just as by comparison of such structures as these the idea of the
cell has undergone modification, notwithstanding that cells are commonly
definite bodies in the ordinary tissues of the higher plants, so may the
existence of evidence pointing to sporangial septations and fusions modify
the conception of the sporangium.
The fact that sporangia originally simple have undergone septation has
only been proved in comparatively few of those cases. The most
complete demonstrations are those from the anthers of certain Angio-
sperms, such as the Onagraceae, Mimoseae, Loranthaceae, Rhizophoreae,
etc. In these the comparative argument is made valid by the existence
of numerous allied genera, which give ground for close comparison ; for
while many plants of these orders show the ordinary quadrilocular anthers,
in others the loculi may be subdivided by further septa, and thus a
number of sacs take the place of each original one. The development
shows that the septation results from the conversion of sporogenous tissue
into sterile septa. Similarly, an argument for fusion of sporangia can also
THE INDIVIDUALITY OF THE SPORANGIUM in
be supported on a basis of comparison among Angiosperms, though it is
a less frequent change : examples are to be found in the coalescent ovules
of certain Loranthaceae, or in the confluent pollen-sacs of certain Gut-
tifeae, etc. Among Pteridophytes, on the other hand, the genera are so
isolated as a rule that the comparative argument is difficult to apply : and
FIG. 61.
Danaea ell'ptica, Smith./ Drawings illustrating partial septations of the sporangium.
A, tangential section through three sori, showing the loculi in ground plan : the septa are
, ,
often thin, so that pairs of loculi are in close juxtaposition ; the loculi marked
tial seta. X 20. />' C D £ show such loculi
the cells
,
are large, and show one or more partial septa. X 20. />', C, D, £, show such loculi with
partial septa in greater detail : in D and E it is difficult to decide whether
marked (?) will develop as tapetum or as spore-mother-cells. X 150.
in each case of a synangium it may be a question whether the structure
results from septation or from fusion. But without entering into these dis-
cussions at present, it is plain that either way the individuality of the
sporangium is lost in such cases, just as it is in the Angiospermic anthers,
and this is particularly clear where, as often happens, partial septa are found.
In summing up the considerations contained in the above pages, it may
be asserted for sporangia at large, that the individuality of the sporangium
H2 THE SPORANGIUM DEFINED
is not always maintained ; that the elevation of the sporangia relatively to
the surface of the part which bears them is variable ; that while a sporangial
wall is always present, the opening mechanisms are inconstant ; that the
tapetum is inconstant in occurrence and in origin ; and that there is no
general law underlying the segmentation of the sporogenous cell, or group
of cells, so as to give it a constant hypodermal or other origin. What then
remains as the fundamental conception of the sporangium in Vascular Plants?
Simply the spore-mother-cell or cells, together with the protective wall. The
definition of a sporangium will then be this : Wherever there is found i?i
Vascular Plants an isolated spore-mother-cell, or a connected group of them, or
their products, this, together with its protective tissues, constitutes the essential of
an individual sporangium.
This definition is open to criticism, as indeed are all definitions of bodies
represented in a large series of variable organisms. Still, it brings out the
point that the essential feature of a sporangium is the presence of one or
more spore-mother-cells, but without reference to the detail of their
production, or to the structure of the wall which covers them. It has its
value in ridding the idea of the sporangium of its accidental accessories,
and fixing the attention upon what is really essential.
But it will perhaps be objected that a sporangium may still be a
sporangium though it may contain no fertile cells ; many imperfect
structures may be quoted which have the form, position, and other details
characteristic for the sporangia of the plant on which they are borne :
they should thus to be ranked as sporangia. That is true ; but as such
bodies do not as a rule serve any useful purpose, it may be asserted that
they would never have existed independently of the fully formed sporangia
of which they are the imperfect representatives. Such vestigial parts, being
of secondary origin, need not disturb the conception of the sporangium as
above defined.
Finally, an important feature of the sporangium is to be seen in the
fact that in so many cases the archesporium is not strictly circumscribed ;
the sporogenous group has often ragged edges ; in many of the Eusporangiate
forms it does not arise from any single archesporial cell, or definite group
of cells ; moreover, cells which are obviously sister-cells may not unfrequently
be found to develop the one sterile, the other fertile. This suggests on
the basis of structure that the fertile tract is a residuum left by advancing
sterilisation, while the ragged and ill-defined limits point to the conclusion
that the sterile and fertile tracts are closely related ; in fact, that they had
a common origin, and that the final condition represents the balance struck
between sterile and fertile development. From the point of view of the
hypothesis of progressive sterilisation such sporangia may, at least in the
simplest cases, be regarded as islands of fertile tissue which have retained
their spore-producing character. It will be seen later how far this view
will have to be modified in the more complex and derivative cases, such
as the Leptosporangiate Ferns.
CHAPTER IX.
SOME GENERAL ASPECTS OF THE POLYSPORANGIATE
STATE.
IT has been pointed out in Chapter VI., which dealt with the Biological
Aspect of Alternation, that in the case of plants of aquatic origin migrating
to the land an increasing production of non-sexual germs, or spores, would
become important. Since under those circumstances dependence could no
longer be placed on frequent recurrence of fertilisation, the production of
numerous spores as a consequence of a single fertilising act will be essential,
if the race is to survive and be in a position to compete and to extend
its area. Other things being equal, the larger the spore-output the better.
This should be constantly before the mind in the comparative study of
the more primitive types of sporophyte, and the same principle should
be applied to the more complex forms also, though in them the evidence
is necessarily less obvious.
The antithesis between the Bryophyta and the Pteridophyta, as regards
the method of spore-production, is chiefly marked by the former having
one concrete sporogenous tissue, the latter numerous discrete sporogenous
groups which form the centres of more or less distinct sporangia. The
Bryophyte type is essentially a limited one, for indefinite enlargement
of the concrete sporogenous tissue introduces mechanical and nutritive
difficulties : these are mos't urgent at the critical period of separation of
the spore-mother-cells, when they are floating freely in the fluid contents
of the spore-sac. In actual life the Bryophyte type is almost always
annual, and does not extend beyond limited proportions; nor is there
evidence that it ever attained a larger size in earlier periods. This is
exactly what biological considerations would have led us to anticipate.
But in the homosporous Pteridophytes, given an enlarging vegetative
system, which in them is usually perennial, there seems no limit to the
number of sporangia which may be borne on the individual plant ; and
as each sporangium is of moderate size, the mechanical and nutritive
requirements at the critical period of tetrad-division are suitably met, while
H
ii4 THE POLYSPORANGIATE STATE
the total output of spores from the numerous sporangia may be virtually
unlimited : moreover, their production may be extended over many years
on the same individual plant. Since, in the homosporous forms, each
single spore is small, and therefore conveys with it only a small store
of nutriment, the risks on germination are great ; a reasonable chance of
success is, however, secured by their large number.
But with the heterosporous condition complications arose. Owing to
the storage arrangements in the enlarged female spore, this more specialised
state leads to economy in number of the spores necessary to secure survival
and spread of area ; for each female spore carries with it, in its higher
store of nourishment, a higher probability of successful establishment of
an embryo, and a sufficient degree of propagative probability can thus
be attained with a moderate number of spores. Hence heterosporous
types may be expected to present examples of reduction of number, not
only of sporogenous cells, but also of sporangia. That is seen to be actually
the case, and it might be illustrated by numerous examples. It will then
be in homosporous types, which are certainly the more primitive, that
we shall expect to meet with the best evidence as to the origin of the
polysporangiate state, or with traces of increase in number of sporangia;
in fact, they will illustrate more faithfully than heterosporous forms the
upgrade of complexity of their spore-producing parts.
On grounds of nutrition of the spores, and of stability at the critical
stage when the spore-mother-cells are floating in fluid, there is a clear
advantage in the segregation of the spores into separate pockets — the
sporangia— as against any method of indefinite enlargement of a single
sac. It is probably such conditions as these which have also determined
the limits of size of the individual sporangia of the Pteridophytes, arid led
to some degree of uniformity in their dimensions. But still considerable
variations in size of the sporangia are found to occur, even in close
juxtaposition on the same plant : this is most conspicuous in the
Eusporangiate forms. Sometimes the difference in size seems to be
dependent on nutrition ; for instance, it is usual to find about the upper
and lower limits of the fertile strobilus of Lycopods^ sporangia of smaller
size than those about the middle of the fertile region : the same is the
case in the Psilotaceae and in Equisetum. But in other cases this simple
explanation will not suffice, for smaller sporangia may be found dis-
tributed between the larger ones : this is especially so in the sori of
the Marattiaceae, and a general survey shows that in many of the
Eusporangiate forms the single sporangium is not quantitatively a definite
unit.
But though there may thus be wide variation of size of the individual
sporangia in certain Pteridophytes, still in others their dimensions are
often very uniform. In the Leptosporangiate Ferns, indeed, the number
of spores in a single sporangium is often strictly constant. In that case
change in the output of spores on the plant is effected by change in the
SIZE AND POSITION OF SPORANGIA 115
number of the sporangia, not by variation of their dimensions : thus the
number of the sporangia may come to be an approximate measure of
the spore-output, as it is in fact in the Leptosporangiate Ferns.
The several types of Pteridophytes differ greatly in the closeness of
the relation of their sporangia to the axis of the shoot ; it will be pointed
out in detail below how the five main series of them — the Lycopodiales,
the Equisetales, the Sphenophyllales, the Ophioglossales, and the Filicales
— exhibit successive degrees of enlargement of the appendicular organs,
and of the consequent removal of the sporangia from the central axis.
The strobiloid character, with small appendages, and one sporangium at
the base of each, or even seated on the axis itself, is characteristic of
the Lycopods ; but this regularity is not characteristic of the larger-leaved
types : thus the definiteness in number and in position of the sporangia
relatively to the other parts, which is as a rule absolutely exact in the
Lycopods, is less strictly observed in the Equisetales and Sphenophyllales,
and it is almost entirely absent in the Ophioglossales and Filicales, in
which the sporangia are borne upon the large leaves, far removed from
the central axis : their number and their arrangement there tends to be
indefinite. These facts may be summarised into the statement that in
the Pteridophytes those forms which bear their sporangia in closest relation
to the axis show the most strict definiteness in their number and position :
where the sporangia are removed from the central axis, being borne upon
larger appendicular organs, they habitually show less definite'ness in number
and in position.
The indefiniteness of. number of the sporangia thus seen in the
Ophioglossales and Filicales is an illustration of the variability of multiple
structures, alluded to by Darwin as follows : " It seems to be a rule, as
remarked by Is. Geoffroy St. Hilaire, both with varieties and species,
that when any part or organ is repeated many times in the same individual
(as the vertebrae in snakes and the stamens in polyandrous flowers) the
number is variable : whereas the same part or organ, when it occurs in
lesser numbers, is constant." That constancy is seen in the Lycopods
in high degree : it is departed from to some extent in the Sphenophyllales
and Equisetales, and it becomes unrecognisable in the Ophioglossales
and Filicales, in which the number of sporangia on each appendage is
large.
It has been remarked above that it is still an unsolved problem what
those intimate influences are which determine the development of any
specific cell of the plant-body as a spore-mother-cell on the one hand, or
as a vegetative cell on the other. This determination lies at the root
not only of the limitation of sporogenous tissues, but also of the initiation
and consequent number of sporangia. The determining factors are probably
numerous : suitable nutrition is certainly one. Speaking generally, better
nutrition is clearly connected with more ample spore-formation ; but it is
also well known that a plethoric state may lead to sterility in certain
n6 THE POLYSPORANGIATE STATE
cases while starvation may conduce to early flowering in many Angiosperms.
Thus the relation of nutrition to the production of sporangia is not of a
simple character. Goebel (Organography, p. 498) speaks of other factors,
such as intensity of light, and internal conditions or correlations, as
influencing the production of sporophylls. Experimentally it seems easier,
however, to convert sporophylls into foliage leaves than to make foliage
leaves fertile. He quotes, nevertheless, the case of a Fern, allied to
Acrostichum Blumeanum^ in which an arrest of growth of the rhizome,
after previous good nourishment, led to production of sporangia. This is
a result similar to that following root-pruning of fruit-trees. It reminds
us also of the conditions found by Klebs to determine the production of
reproductive organs in certain Algae and Fungi.'2 I do not suppose,
however, that the conditions will be found to be uniform for all sporophytes,
any more than they have been for Algae or Fungi. In any case, the
present knowledge of the whole subject for Vascular Plants is indefinite
and uncertain.3
The time of distinctive development of cells as sporogenous cells varies
in different plants : the following tentative conclusions may be drawn from
such differences. When in a tissue-tract the distinction between vegetative
and sporogenous cells takes place relatively late in the individual, the
presumption is that the distinction has been of late origin in the race.
On this basis the conclusion has been founded in certain cases that increase
in number of sporangia by septation has occurred. A large potential
sporogenous tissue having a common origin is first seen ; but later it
differentiates, part becoming actually sporogenous, part remaining sterile.
It is concluded that these late-differentiated sterile tracts were once in
the race fertile, and that they were subsequently diverted from this previous
condition ; in fact, that the ontogenetic development reflects the evolutionary
history. This is exemplified in certain Angiospermic anthers, in the
synangia of Tmesipteris, and in the partially septate sporangia of Danaea-.
the same general argument holds also for the sporangium of Isoetes with
its trabeculae. In other cases where the distinctive characters of the
sporogenous cells or cell-groups are acquired earlier, the argument for
septation is less clear, though on grounds of comparison a similar history
of the structure actually seen appears probable.
The cases above mentioned involve sporangia which are closely associated
as synangia, and they are naturally initiated simultaneously. But differences
of the time of distinctive development of sporogenous cells may become
more obvious in sporangia which are separate from one another, though
in close proximity upon the part which bears them. In those types which
comparison, as well as the Palaeontological record, points out as the
1 Raciborski, Flora, 1900, p. 25. 2 Die Bedingungen der Fortpjlanzittig, 1896.
3 The determining conditions have been discussed by various writers. See Uiels,
Jngendformen und Blutenreife iin Pflanzenreich, Berlin, 1906, where reference is made
to the literature on the subject.
SIMULTANEOUS OR SUCCESSIVE 117
earliest, the sporangia in near juxtaposition show a simultaneous origin ;
or some degree of succession may be seen from those earlier formed
near to the base of the shoot or leaf, and leading to the apical region,
where they appear later. Such simultaneity, or such acropetal succession,
may be regarded as a primary condition, and it is seen in the Lycopodiales,
Equisetales, and Sphenophyllales, as well as in those Ferns which are
designated below the Simplices (see Part II.). But in certain Ferns, which
the Palaeontological record, as well as comparison, would mark out as
secondary, the sporangia in near juxtaposition do not arise simultaneously:
sometimes, as in those which will be styled the Gradatae, there is a regular
basipetal succession within the sorus, those lowest on the receptacle
appearing latest. In others, again, there is no such regularity, and sporangia
of different ages are found promiscuously intermixed : these Ferns are
styled the Mixtae, and the Palaeontological record indicates that these
were the latest to appear. Such facts, which will be stated at length
below (Part II.), may be summed up into the following general statement.
In the most primitive forms the sporangia in near proximity to one another
develop simultaneously, though an acropetal succession may often be
traced on the shoot or strobilus as a whole. Those successions, whether
in regular order or irregular, which appear in various forms upon the
leaves, may be held to be later derived, and secondary.
It will be readily gathered from the contemplation of those plants
in which sporangia are numerous that accurate comparison of individual
sporangia as identical bodies in parent and offspring, or in different, less
closely related specimens, is not possible in plants at large. For the most
part sporangia are merely examples of "essential correspondence" rather
than of "individual repetition." The actual sporangia of the offspring are
not coincident, as a rule, either in exact position or in number with those
of the parent. This is a consequence of that continued embryology which
is a leading feature in all vascular sporophytes. As a consequence the
individual sporangia of any one individual plant or species cannot be
held to be the exact ontogenetic correlatives of those seen on another
individual or species. The possibility of such a recognition is most nearly
approached in the Lycopods, where the sporangia are borne singly in
definite relation to the axis and leaf. It is departed from furthest in
the large-leaved Ferns : especially is this so in the Polypodiaceae, where
the mixed character of the sorus is the rule : but most of all in such a
case as that of Aspidium anomalum, Hk. and Arn., a Fern found on
the uplands of Ceylon, and sometimes regarded as a mere variety of
A. aculeatum, Sw. : its peculiarity consists in the appearance of sori upon
the upper surface of the leaf, where normally they do not occur. As
there is no question of mere inversion of the leaf, it can only be assumed
that there has been a transfer of the stimulus, whatever it be, to soral
development from the lower to the upper surface of the leaf. Clearly
the sori which result cannot be the ontogenetic correlatives of any
n8 THE POLYSPORANGIATE STATE
previously existent in the race : much less is this possible for the individual
sporangia of those sori. Such an example shows, in its most extreme
form, how impossible it may be to compare, as numerically or locally
identical, the otherwise similar parts, such as sori or sporangia : and
this is most clearly so in the Ferns, where the leaves are large, and the
sori and sporangia borne upon them more numerous than in any other
Vascular Plants.
We thus see that the homosporous Pteridophytes, which are certainly
the more primitive, will be the best guide in questions of the origin of
the sporangial state : and that these may be arranged serially according
to size of the appendages, the strobiloid types being at one end of the
series and the large-leaved Ferns at the other. The forms thus arranged
show more or less clear differences in the sporangial characters : in the
simpler strobiloid forms the sporangia are less definite units as regards
spore-output, in the Ferns they have tended to become in the evolutionary
course more definite units in this respect. In the strobiloid forms the
relation of the sporangium to the axis is close, and as regards position
and number it is more definite ; in the larger-leaved forms the sporangia
are further removed from the axis, and their position and number tends to
become more and more indefinite. In the strobiloid forms the time of
origin of the sporangia in near juxtaposition to one another is simultaneous :
in the larger-leaved forms it tends to become in various ways successive,
while the palaeontological record shows that the most pronounced succes-
sions have been of secondary origin. These distinctions will have their
value in leading to a more precise statement of the problem of origin of
the sporangial state. To this end it will be found desirable to keep
distinctly before the mind those vascular types in which the nearest
approach can be made to a comparison of the sporangia as numerically
and locally identical. Among the homosporous Pteridophytes this will
be found to be the case most nearly in the smaller-leaved strobiloid
forms : and among these especially in the ancient phylum of the
Lycopodiales.
CHAPTER X.
VARIATIONS IN NUMBER OF SPORANGIA.
NOTWITHSTANDING the familiarity of the fact that the polysporangiate
state is the constant condition in Vascular Plants, little attempt has
hitherto been made to analyse the methods of variation in number of
the sporangia which they bear. But this would appear to be a promising
line* of enquiry, for it may be held that an adequate knowledge of the
methods of variation seen to be actually operative now should throw light
upon the factors which have been operative in the past, and thus some
suggestion should be obtained how the divers polysporangiate types came
to be as we now see them. To such an end the facts drawn from those
organisms which are held to be relatively primitive, such as the homo-
sporous Pteridophytes, will naturally bear greater weight than those derived
from more recent and specialised forms, such as the Flowering Plants.
Nevertheless it will be best to treat the question of change of number of
sporangia first of all in its relation to the Vascular Plants as a whole,
so that all the known factors may be disclosed : and it will be a matter
for subsequent discussion to decide in any individual case which of those
factors appear to have been operative in bringing that organism to its
present condition.
The subject of variation in number of sporangia in the individual or
the race may be discussed either from the 'physiological or from the
morphological point of view : it is the morphological question which will
now be brought forward, though always in the light of physiological con-
siderations. But I wish at once to meet the objections of those who
will say from the physiological side that the number of sporangia depends
on nutrition : this self-evident proposition neither explains nor rules out
the morphological question how a plurality of sporangia arose, nor how
the great numerical differences which we see may have been attained :
nor does it modify the effect which observations of their numerical change
in the living individual, species, or genus may have on views of Descent
of the plants observed. By such observation and comparison of living
120 VARIATIONS IN NUMBER OF SPORANGIA
plants it is possible to recognise certain methods of numerical change of
sporangia, now or previously operative : these may either lead to pro-
gressive increase in number, or to decrease in number. Under these two
heads the following table shows the several methods of change in number
of sporangia of which evidence has been found in living plants, but it is
possible that the table is not exhaustive :
I. INCREASE IN NUMBER OF SPORANGIA.
(a) By septation, with or without rounding off of the individual sporangia.
(b) By formation of new sporangia, or of new spore-bearing organs, which
may be in addition to, or interpolated between those typically
present.
(c) By continued apical or intercalary growth of the parts bearing the
sporangia.
(d) By branching of the parts bearing the sporangia.
(e) Indirectly, by branchings in the non-sporangial region, resulting in
an increased number of sporangial shoots : this is closely related
to (c) and (d).
f
II. DECREASE IN NUMBER OF SPORANGIA.
(/) By fusion of sporangia originally separate.
(g) By abortion, partial or complete, of sporangia.
(h) By reduction or arrest of apical or intercalary growth in parts bearing
sporangia.
(*) By fusion of parts which bear sporangia, or arrest of their branchings.
(/) Indirectly, by suppression of branchings in the non-sporangial region,
resulting in decreased number of sporangial shoots : this is closely
related to (h) and (/).
Each of these factors of variation will now be discussed, and examples
of them adduced. At the moment the object is only to recognise that
such modifications of number of sporangia are or have been operative
in actual cases, not to estimate the relative prevalence of any one : for
it is necessary first to distinguish the factors of the problem.
FACTORS OF INCREASE.
(a) Increase in number of sporangia by septation, resulting in a plurality
of loculi, where previously in the race the septa were absent, is shown
in the septate anthers of various families of Angiosperms (Onagraceae,
Mimoseae, Rhizophoreae, Myrsinaceae, Loranthaceae, etc.). The details
have already been sufficiently described in Chapter VIL, p. 97. The
argument that septation has occurred is in many of these cases quite
conclusive : thus the plurilocular anthers of. certain genera of the Onagraceae
FACTORS OF INCREASE 121
have the numerous small loculi -arranged in four rows, corresponding in
position to the four pollen-sacs of the normal anthers in other genera of
the family, the only essential difference from these being the presence of the
septa which separate the loculi. The .cells which form the septa are similar
in position to the archesporial cells of the normal anther (Fig. 62), and the
conclusion is that certain of the archesporial cells are no longer fertile,
but develop to form the sterile tissue of the septum. A similar argument
will apply for other families of Angiosperms with varying degrees of
cogency, according as the septate anthers diverge less or more from the
usual type, or according to the less or greater isolation of the genera
which show the multilocular state. In the Pteridophytes also there are
cases of juxtaposition of loculi similar to those in the septate anthers :
\ Trrt
FIG. 62.
A, longitudinal section of one loculus of a young stamen of Euckaridiuin concinuum,
showing differentiation of the potential archesporium into fertile cells (a) and sterile
cells (s). £, similar section of stamen of Clarkia elegans, more advanced, showing a
sterile septum dividing the contents of the single loculus into distinct sporogenous groups
(i/). * = tapetum. X 365.
conspicuous instances are found in the spike of Ophioglossum, and in the
sorus of Danaea. But the development in the Pteridophytes gives less
conclusive evidence than in the anthers of Angiosperms, owing to their
meristems not being stratified, and to the fact that the genera are
more isolated. But though the facts in these plants do not amount to
a complete demonstration, there seems a reasonable probability that
septation has played a part among them also. Whatever view be taken
for these Pteridophytes, it is clear from the examples quoted from the
Angiosperms that septation of sporangia previously in the race non-septate
has been a factor of increase in number of sporangia.
(^) By the formation of new sporangia, or interpolation, is meant the
initiation of a new sporangial primordium, or of several, at a spot previously
in the race not so occupied. The results of this mode of increase may
in some cases be difficult to differentiate from those of septation, where
122 VARIATIONS IN NUMBER OF SPORANGIA
the products have been rounded off: they will be best distinguished in
the early stages of development. A good, clear example is found in the
genus Gkickenia : the sorus of most species of the genus is of the radiate,
uniseriate type, in which the sporangia form a series round the receptacle,
while the centre of it is unoccupied by sporangia : there is good comparative
reason to believe that this was the primitive type of sorus for the genus.
But in G. dichotoma additional sporangia, one or more, are often seated
in the central unoccupied space (Fig. 63). Examination of early stages
of development shows that these sporangia are not a result of fission of
sporangia typically present, but develop from new primordia not represented
in other species (Fig. 64). The interpolation, of which this is a simple
case, becomes a more prominent feature in the Ferns which palaeontology
FIG. 63.
a-h, sori of Gleichenia dichotoina, Willcl. Showing one or more sporangia in the
entre of the sorus, usually in this genus vacant
/-«, sporangia of Gl. dichotoma, a-/tX about 14.
teaches us to regard as the more recent. The succession of the sporangia
characteristic of those Ferns which will be styled on that account the
Gradatae, may be recognised as a repeated basipetal interpolation of
sporangia on the receptacle of the sorus (Fig. 65 A). In the Mixtae it is
still more prominent in the sorus, for new primordia, formed in continued
succession, appear scattered between those which are more advanced
(Fig. 66). But interpolation^ may take place not only of sporangia, but
also of new spore-bearing organs between those typically present. This
is exemplified in the interpolated stamens in certain flowers, the number
of which is closely related to their bulk, and to the space available for
them on the floral receptacle. Such interpolation of sporangium-bearing
parts will clearly result in an increase in number of sporangia. The
prevalence of interpolation thus seen in the Leptosporangiate Ferns, has
given an entirely fictitious importance to this phenomenon in the general
FACTORS OF INCREASE
123
FIG. 64.
«, b, c, j, k — Gleichenia ftabellata, Br. d, e, f=Gl. circinata, Sw. g, h, i=GL
dtchotoma, Willd. a, i>, c, d, show young sori with sporangia marginal, g shows a
similar condition in Gl. dichotoma, but in h and i the vacant middle space is occupied by
young sporangia. a-/X2oo. /, £xioo.
FIG. 65.
A , By Dennstaedtia apiifolia, Hook. A shows the regular basipetal succession of
sporangia characteristic of the Gradata;. C, D, Dennstaedtia rubiginosa, Kaulf. D
shows irregularity in the succession. X TOO.
i24 VARIATIONS IN NUMBER OF SPORANGIA
morphology of Vascular Plants ; while the regularity and constancy in
number and position of the sporangia in the Lycopods, Equiseta, and
Eusporangiate Ferns, in which interpolation does not occur, has been
underestimated. But these, on grounds of comparison, and of fossil history,
are among the most primitive pf Vascular Plants. Thus interpolation of
new sporangia is to be recognised as an actual factor of increase in number
of sporangia, but it is not a general phenomenon, and there is reason to
FIG. 66.
Sorus of Davallia. gritffthiana, Hk. Showing sporangia of different ages irregularly
intermixed. X 100.
think that it has been initiated as a secondary character, and in certain
groups only.
(c) Continued apical growth of the parts bearing the sporangia is a
marked feature in most Vascular Plants : a concomitant of it, in the case
of axes, is a continued embryology, with the initiation of an indefinite
number of successive primordia of spore-producing parts. This is con-
spicuous in the axes of many Lycopods, and especially so in the Selago
group of Lycopodium, where it appears to be unlimited : in other species of
the genus the apical growth of the strobilus also exists, but is of shorter
duration (Fig. 67). Much the same is the case in other strobiloid types,
with varying duration of the apical growth. The apical growth of the axis
is apt to be less prominent where the appendages are large, as in the
FACTORS OF INCREASE 125
Ophioglossaceae and Filices, and in these the continued apical growth may
largely devolve upon the sporophylls. The consequence of such growth
is increased accommodation for sporangia. Similarly, continued intercalary
growth may effect the same end : as in the sori of those Ferns designated
FIG. 67.
A forked sporangiferous branch of Lycopodium chama>cyparissus in longitudinal section,
slightly magnified. ff= the axile vascular body. M = leaves. ss = young sporangia.
the Gradatae. These serve as examples of the way in which, by continued
growth of the parts which bear the sporangia, a sequence of these may be
produced, which may be continued so long as the growth itself. It is clear
-from the examples adduced that such continued growth may occur in parts
which are not morphologically comparable. In the more primitive Pterido-
phytes it is answerable for extensive increase in accommodation for primordia
of sporangia.
126 VARIATIONS IN NUMBER OF SPORANGIA
(d] Closely related to (c) is the branching of parts bearing sporangia :
this may also occur in parts which are in no way morphologically com-
parable. Branching of strobili is a common feature in Lycopodium and
Psilotum : branching of the sporophyll is characteristic of most large-leaved
Pteridophytes : branching of the receptacle of the sorus is common in
Ferns (Fig. 68) : branching of the fertile spike
is a marked feature in certain Ophioglossaceae :
branching of stamens is common in the Angio-
spermic flower, and is to be distinguished from
the interpolation above mentioned : a somewhat
similar branching of the sporangiophores has
probably occurred in the Sphenophylleae. In
all such cases, though the parts are not mor-
phologically comparable, the end is attained
of an increased accommodation for sporangia,
which consequently may be produced in
increased numbers.
(e) Continued growth and branching, occur-
ring in the non-sporangial region, may have
the indirect effect of a further increase in the
opportunities for production of sporangia; for
not only is the vegetative system thereby increased, which will have its
indirect effect in increased powers of nutrition, but also a larger number
of apices are provided, any one of which may take up the character of a
sporangial strobilus. The continued apical growth in the vegetative region
is a general feature of Vascular Plants. Branching is profuse in many of
the strobiloid Pteridophytes : in the Ferns also it occurs, but the effect in
increasing the opportunities for spore-production is less obvious here than in
the strobiloid types. In Flowering Plants also the complicated inflorescences
and the multiplicity of flowers is dependent upon such apical growth,
together with repeated branchings.
FIG. 63.
a-d, various examples of fission of
the sorus in Hymenophyllum dila-
tatum. The sporangia, and one flap
of the indusium have been removed,
leaving the receptacle exposed.
X about 5.
FACTORS OF DECREASE.
(/) Decrease in number of sporangia, by fusion of sporangia which
previously in the race were separate, has been assumed as an explanation
of synangial states by various writers : but it can only rarely be proved on
grounds of comparison that fusion of sporangia has actually taken place,
and the best evidence of it comes from the Angiosperms. Thus the fusion
of the ovules, leading indeed to the obliteration of their identity, occurs in
certain species of Loranthus, and comparison leaves little doubt that the
sunken embryo-sacs represent the individual ovules, the identity of which-
is lost as regards external form. Fusion of pollen-sacs is more frequent ;
good examples, showing various states of the fusion, are found in the
genus Phyllanthus, and in Cyclanthera • while the unilocular condition in
FACTORS OF DECREASE
127
Arisantm, and in Najas, §Caultma, can hardly be ascribed to any other
source than the fusion of the pollen-sacs, separate in the ancestry, into a
single loculus. Thus in the androecium of Angiosperms. and occasionally
„
FIG. 69.
Junipcrus cointmtnis. I., summit of a male flower seen from above. st\, the uppermost
staminal whorl of these stamens ; st.2, the second staminal whorl shows on each stamen
two pollen-sacs, and the indication of a lamina, /; s/3, the third staminal whorl, of which
only the tips of the lamina? of two stamens are seen : each of the stamens of this whorl
had three pollen-sacs, not shown in the figure. II., the same in longitudinal section.
III., the same in transverse section. There is evidence here of reduction of the lamina,
and of fusion of the pollen-sacs. (After Goebel.)
in the gynoecium, a fusion of sporangia is recognised, resulting in a reduc-
tion in their number. The apex of the male flower in Juniperus communis
has been quoted by Goebel as a probable example of fusion of pollen-sacs
(Fig. 69). In certain Pteridophytes the grouping of
the sporangia is often such as to suggest a previous
fusion ; but this has not been proved on developmental
pr comparative grounds for any one case, and the
question must be left open for discussion on grounds
of general probability whether the synangial state in
any individual case has been the result of fusion, or of
septation with incomplete separation of the sporangia :
obviously the synangial structure would be compatible
with either origin. Whatever the final decision for the
Pteridophyta may be, it is clear that fusion of sporangia
originally separate has actually occurred in Flowering
Plants : it is therefore a factor which must be regarded
as a possible explanation of all synangial states.
(g) Abortion, partial or complete, of sporangia which
were fully matured in the type or ancestry is so common
a feature that special examples need hardly be quoted,
sacs are commonly found on staminodes, and abortive ovules are frequently
seen, as in Anemone (Fig. 70), which can only be accepted as the imperfect
representatives of a plurality of ovules in the ancestry. In the Pteridophytes
many examples of abortive sporangia have lately been described, and their
very important morphological bearings will be discussed at length in
Chapter XIII. But in connection with the circumstance that parts
FIG. 70.
Carpel of A nemone
nemorosa, L. Showing
one ovule developed, and
three abortive. (From
Engler and Prantl.)
Abortive pollen-
128 VARIATIONS IN NUMBER OF SPORANGIA
initiated frequently do not come to full maturity this further fact is to be
borne in mind : that it is common in the ascending series of plants to
find a larger number of primordia initiated than the individual can bring
to full development. Consequently it will be necessary to discriminate
between those imperfect parts which represent such as were fully matured
in the ancestry, and those supernumerary primordia which do not represent
parts ever actually matured in previous generations. In point of fact,
unperfected primordia may, in some cases, really represent an exuberant
tendency to progress, and cannot necessarily be assumed to indicate a
state of reduction.
(ti) The reduction or arrest of apical or intercalary groivth in parts
bearing sporangia has doubtless been a potent influence in the past, but
it is somewhat difficult to point to concrete examples of it. The general
fact that the floral axis in Phanerogams is abbreviated, while the axis of
the strobilus in Pteridophytes is frequently endowed with continued apical
growth, points to the probability of arrest of that apical growth. An
example of it within a near circle of affinity is seen in the genus Lycopodium;
for in the section Selago the apical growth of the axis is unlimited, and
the number of possible sporangia borne on a given axis is unlimited also :
in other sections of the genus which are held to be more specialised, as
the strobilus becomes more strictly differentiated from the vegetative region
the apical growth is arrested early, and the number of sporangia produced
upon it is limited also. In floral details this is illustrated in some families
of Angiosperms : thus the Ranunculaceae include such forms as Myosurus,
with its elongated receptacle and indefinite number of carpels, and Actaea,
with its abbreviated axis and only a single carpel. It seems probable that
here also arrest of the apical growth has been associated with reduction
of the number of sporophylls and of sporangia.
(/) Fusion of parts which bear sporangia is not so susceptible of illus-
tration in the Pteridophytes as in the Angiosperms. A most convincing
series of reduction is laid out in the Araceae by Engler, involving fusion
of stamens, accompanied by reduction in number of the pollen-sacs (Nat.
Pflanzenfam. II. i., p. 107). Many examples might also be quoted from
other Angiosperms, e.g. Cucurbitaceae, Euphorbiaceae, where fusion of parts
is connected with reduction in number of the sporangia which they
bear. A suppression of branchings of sporangium-bearing parts may also
be a factor : it has at times been assumed in theoretical writing, but it is
difficult to give conclusive examples of it.
(/) Indirectly the arrest of apical groivth and the partial or complete
suppression of branchings in the non-sporangial region may affect the
number of sporangia produced. The abortive buds at the base of
inflorescences of Aconitum, for instance, or the abortive spikelets in
Cynosurus are examples of potentially larger numbers of spore-producing
branches arrested before they bear sporangia. Such arrests of growth
and of branching may have been more prevalent sources of change than
DIFFICULTIES OF ANALYSIS 129
is apparent externally, for in this absence of vestigial parts there would
be no trace of what had happened.
The methods of change thus enumerated are, then, the known factors
which affect the morphological problem of origin and present condition
of the polysporangiate state. Any one of these, or more, may have be.en
represented in the history of descent of any polysporangiate sporophyte
as it is seen to-day. The examples quoted show that the methods
enumerated are seen to have been actually operative in definite, living
instances. Possibly these heads do not exhaust the methods of change
of the present day nor of the past, and the list is open to additions.
We are justified in assuming that (subject to the possibility of other
factors having been operative of which we are yet unaware) the condition
of any polysporangiate sporophyte as we see it is the resultant of modifi-
cations such as these operative during its descent. The problem will
therefore be in each case to assign its proper place in the history to any
or each of these factors. But in each case the physiological probability
of any modification which the structure would suggest should be con-
sidered before it is admitted as part of the evolutionary story. Especially
is this desirable in determining the probable relative prevalence of modifica-
tions of increase as against those of decrease. It is only in this way that any
apparent morphological series can be put upon a convincing footing as
an evolutionary sequence. In complex cases, however, it may be a matter
of difficulty to analyse a progressive change, and to decide which of the
factors enumerated have actually been involved.
It will be obvious that a complete account, in any given case, of the
steps which have led to its present polysporangiate state involves a full
knowledge of its evolutionary history — a knowledge which js far beyond
present powers. The advantage which an attempt to analyse the factors
of sporangial modification brings, however imperfectly it may be carried
out, is to simplify the problem in certain definite cases. For instance,
if in a whole phylum of living plants a certain mode of sporangial
increase be unrepresented, and if the related fossils show a similar absence
of it, then it seems just' to hold that that mode of increase may be
dismissed from consideration in the probable evolutionary history of that
phylum. The case of interpolation already alluded to is one in point :
in connection with this it is necessary to reconsider and examine certain
old habits of thought which have too long dominated such discussions
as the present. About the middle of the nineteenth century it was habitually
maintained that the Polypodiaceous Ferns were primitive forms, and the
probable progenitors of all other Pteridophytes. So long as this view
was held interpolation of new sporangia between older ones, which is so
prominent in them, was regarded as a general phenomenon which might
appear anywhere among' the derivative forms. The fundamental idea
130 VARIATIONS IN NUMBER OF SPORANGIA
thus came to be that sporangia are bodies essentially indefinite in number
and variable in place. The habit of interpreting lower forms in terms of
the higher, which was also prevalent in earlier times, further encouraged
this view : the common occurrence of indefinite stamens in Angiosperms
made unduly familiar the idea of the sporangium-bearing parts, as well
as sporangia themselves, being irregular in number and in position. But
now there is good reason to believe Jhat both the Angiosperms and the
Polypodiaceous Ferns were of relatively late origin, and in no sense
primitive forms. Also that the interpolation of sporangia, or of sporangium-
bearing parts, which is found in them both, arose secondarily as a con-
comitant of the increase of the sporangia in either type to high numbers.
If this be so, then there is no sufficient reason to look upon the sporangia
of earlier and more definite types as in any way affected by the presence
of this secondary indefiniteness in number and position. This principle
finds its application as follows : it may be held that the Lycopods were
independent of the Ferns in phyletic origin : hence the question of origin
of their sporangia may be discussed without reference to interpolation at
all, since it is not a characteristic of that family nor of its relatives.
Similarly with the Marattiaceous Ferns, interpolation is not seen in the sori
of any living representative of that family, nor is it demonstrated in their
fossil prototypes : their sporangia are simultaneous in origin ; the fact that
the relatively recent Leptosporangiate Ferns show frequent interpolation of
successive sporangia should not affect our view of those Fern-types which
certainly preceded them in time. In point of fact, interpolation of spor-
angia is an occasional and not a general phenomenon : it is restricted to
certain groups of plants, and is probably of secondary origin : in other
groups where it has not been observed the sporangia are often seen to
be stable in number and also in position. Accordingly the problem of
evolutionary origin becomes more simple in those cases where interpolation
may be left out of consideration altogether.
A second case in point, where it is necessary to clear the mind of
old habits of thought before entering on evolutionary questions, is that of
synangial sporangia, which are so frequent in the Eusporangiate Pterido-
phytes. The assumption was formerly common that where this condition
is seen it is the result of fusion of sporangia previously in the race
distinct. This assumption is again to be traced to the old habit of
regarding the Polypodiaceous Ferns as the primitive stock of Pterido-
phytes : the sporangia in them are all separate ; consequently it was held
that where in other Ferns they are united, this must be a secondary
condition. But the synangial condition may just as well have resulted
from septation as from fusion : the question which is the correct view in
any individual case is difficult to decide, but evidence of some value may
be obtained by examination of the structure and development, as well as
by general comparison of allied forms. In approaching such matters
opinion must be unbiassed and open. Both fusion and septation have
EXCLUSION OF CERTAIN FACTORS 131
occurred in various instances, and in any given case the proper initial
attitude is to hold that either mode of origin may have been the source
of the synangial state as it now appears.
The feature which has probably been most effective of all in distracting
attention from the methodical analysis of the polysporangiate state in
Vascular Plants at large has been the swamping effect of continued apical
growth, and of branching. In the lower Vascular Plants both apical
growth and branching may be seen in either the sterile or the fertile
regions. In the higher Flowering Plants the floral region itself is
characterised by absence of branching, and by restriction of apical growth,
but both occur freely in the sterile region of the inflorescence. The
results of this in the Flowering Plants are apt to be so dominating that
it is often hard to recognise the small terminal and late-produced strobilus
or flower as the actual residuum which progressive sterilisation and growth
of the sterile tract have left.
Among Vascular Plants it is only in the simpler Pteridophytes that this
aspect of the sporophyte generation clearly emerges : and this is largely due
to the fact that in them branching of the axis is often less profuse, or may
even be absent altogether : moreover, the structural similarity between the
sterile and fertile regions suggests their comparison. As a consequence of
such comparisons, it follows that the great disproportion of the two regions
so often seen in the Flowering Plants may be discounted as a secondary
effect : it has been brought about principally by continued apical growth
and repeated branching in the vegetative region, together with higher differ-
entiation of the sterile and fertile shoots. Maintaining consistently this
point of view, the overpowering effects of continued apical growth and of
branching will be estimated at their right value, and so the way may be
prepared for a more exact enquiry into the origin of the polysporangiate
state, even in the more advanced types. It is by some such analysis as
that sketched in this chapter that it may be possible to attain to a
reasonable opinion how the condition seen in the earlier Vascular Plants
came into existence. The detailed practical application of the method may
often be difficult, and only partially successful : the present object has been
to lay the basis for such an analysis, by showing what the recognised
factors of numerical change of sporangia actually are, and to simplify the
problem by showing that certain of those factors are of limited application
only.
CHAPTER XI.
THEORY OF THE STROBILUS.1
THE term strobilus is commonly applied to those fertile spikes with small
appendages which are found in a terminal position on the shoots of many
Pteridophytes. The construction of the vegetative region below the strobilus
in these plants is on essentially the same plan as the strobili themselves,
but without the sporangia. The similarity of the two regions, as well as
the absence of any definite limit between them, is demonstrated with
peculiar clearness in the case of Lycopodium Selago, as shown in the
Frontispiece. The absence of the sporangia in the vegetative region may
be accounted for on the ground of abortion. If this be so, the structure
of the strobilus will be the prototype for the vegetative shoot, and any
theory of its origin with its appendages should cover that of the vegetative
shoot as well.
The strobiloid condition was common among the earliest Pteridophytes
of which there is any fossil evidence : there is thus a probability that it
was a relatively primitive state. It is characteristic of the Equisetales, the
Lycopodiales, and the Sphenophyllales, which are all relatively micro-
phyllous ; but the same type of construction is also traceable in radially
constructed megaphyllous forms, and it will be shown later how the Ophio-
glossales, and even the large-leaved Filicales, may be referred back to the
strobiloid type of construction, but with the appendages developed to an
inordinate size.
The strobilus, in any of the small-leaved Pteridophytes, consists of a
relatively bulky axis, endowed with more or less continued apical growth,
and terminated by an apical cone, upon which the appendages arise
laterally (compare Fig. 67, p. 125). Those appendages appear in regular
acropetal order, below the apex of the axis ; and they arise exogenously,
as more or less massive outgrowths of the tissue of the apical cone (Fig. 71).
They have these characters in common in all cases, and, according to the
1 This and the following chapters are largely based upon an address given at the
International Exposition at St. Louis, U.S.A., 1904.
POLYPHYLETIC ORIGIN OF LEAVES
133
morphological method of the latter half of the nineteenth century, all the
parts which share these characters would rank as " leaves," and be regarded
as "homologous." But the progress of the science should be leading
towards the refinement of the use of the term " homology " : an approach
must be made, however distant it may yet be, to a classification of parts
on a basis of Descent. Though this may readily be accepted in theory,
it is still far from being adopted in the practice of Plant-Morphology.
None the less, comparison is inevitably leading to the disintegration, on a
basis of Descent, of the old-accepted categories of parts : and in the case
of the appendages which are collectively styled " leaves," the question arises
whether they are all truly comparable in Vascular Plants. This is clearly
in close relation to the question of their origin, and we shall enquire
FIG. 71.
Longitudinal section through the apical cone of the stem of Lycopodiutn Sclago. X 160.
(After Strasburger. )
whether there is not reason to think that the initiation of the foliar
appendages may have been polyphyletic.
To those who hold the view that the two alternating generations of
the Archegoniatae have had a distinct phylogenetic history, it will be clear
that their parts can not be truly comparable by descent. The leaf of the
Vascular Plant, accordingly, will not be the correlative of the leaf of a Moss.
Even those who regard the sporophyte as an unsexed gametophyte will
still have to show, on a basis of comparison and development, that the
leaves of the two generations are of common descent. I am not aware
that this has yet been done by them.
But the phylogenetic distinctness of origin of the leaves of the sporo-
phyte and gametophyte is not the only example of parallel foliar develop-
ment. Goebel has shown with much cogency that the foliar appendages of
the Bryophytes are not all comparable as regards their origin ; he remarks,
" It is characteristic that the leaf-formation in the Liverworts has arisen
134 THEORY OF THE STROBILUS
independently in quite a number of series,"1 and has shown that they must
have been produced in different ways. Here then is polyphyleticism in
high degree, seen in the origin of those parts of the gametophyte which on
grounds of descent we have already separated from the foliar appendages
of the sporophyte.
Such results as these for the gametophyte lead us to enquire how the
case stands as to the origin of foliar differentiation in Vascular Plants.
In discussing such questions, it is to be remembered that in different stocks
the foliar condition of the sporophyte as we see it may have been achieved
in different ways, just as investigators have found reason to believe that it
was in the gametophyte. We have no right to assume that the leaf was
formed once for all in the descent of the sporophyte. But at the moment
we are unprovided with any definite proof how it occurred. All the evidence
on the point is necessarily indirect, since no intermediate types are known
between foliar and non-foliar sporophytes. Physiological experiment has
as yet nothing to say on the subject. The fossil history of the origin of
the foliar state in the neutral generation is lost, for the foliar character
antedated the earliest known fossil-sporophytes. There remain the facts
of development of the individual, and comparison, while anatomical detail
may have some bearing also on the question ; but all of these, as indirect
lines of evidence, fall short of demonstration, and accordingly it is impossible
to come at present to any decision on the point. For the purposes of
this discussion, however, we shall proceed on the supposition that all leaves
of the sporophyte generation originated in essentially the same way, though
not necessarily along the same phyletic line.
There are at least three alternatives which may possibly have been
effective in the origin of a foliar differentiation of the shoot, in any pro-
gressive line of evolution of vascular sporophytes: (i) That the prototype
of the leaf was of prior existence, .the axis being a part which gradually
asserted itself as a basis for the insertion of those appendages ; the leaf in
such a case would be from the first the predominant part in the con-
struction of the shoot. (2) That the axis and leaf are the result of
differentiation of an indifferent branch-system, of which the limbs were
originally all alike ; in this case neither leaf nor axis would predominate
from the first. (3) That the axis pre-existed, and the foliar appendages
arose as outgrowths upon it ; in this case the axis would be from the first
the predominant part.
The first of the above alternatives, viz. that the prototype of the leaf
existed from the first, and was indeed the predominant part in the initial
composition of the shoot, has been held by certain writers as the basis of
origin of the leafy shoot in vascular plants.2 On this view not only is the
1 Organography, p. 261.
2 Goethe, "Die Metamorphose der Pflanzen." Gaudichaud, Mem. de V Acad. d. Set.,
1841. Kienitz Gerloff, Bot. Zeit., 1875, p. 55. Celakovsky, " Unters. ueber die Homo-
logien," Pringsh. Jahrb., xiv., p. 321, 1884; Bot. Zeit., 1901, Heft, v., VI.
PHYTONIC VIEWS 135
• X.
whole shoot regarded as being mainly composed of leaves, but some even
•contend that the axis has no real existence as a part distinct from the
leaf bases.1
This view in its general form represented the leafy plant as constructed
on a plan somewhat similar to that of a complex zoophyte. It has more
recently culminated in the writings of Celakovsky and Delpino. The former
in his theory of shoot-segments (" Sprossgliedlehre ") starts from the position
that the plant is composed of morphological individuals ; the cell, the shoot,
and the plant-stock are recognised as such. The stock is composed of
shoots and the shoot of cells. Braun recognised the shoot as the individual
par excellence; between the cell and the shoot is a great gulf, which has
not yet been filled ; " between the cell and the bud (shoot) there must
be intermediate steps, the limitation of which no one has succeeded in
•denning"; the long sought-for individual middle step is the shoot-segment
(Spross-glied), which is neither leaf only nor stem-segment only, but the leaf
together with its stem-segment. Now this reasoning appears to involve a
mistaken method of morphology ; the intermediate step must occur ; we will,
therefore, discover and define it. The definition of it consists in the draw-
ing of certain transverse and longitudinal lines partitioning the shoot, lines
which in the sporophyte have no existence in nature ; the assumed necessity
of partitioning the shoot into parts of an intermediate category between the
whole shoot and the cell brings these assumed limits into existence.
Notwithstanding the ingenuity of the theory as put forward by
Celakovsky, in the absence of any structural indication of the limits of
the shoot-segments in the vast majority of cases the theory does not appear
to be sufficiently upheld by the facts.
An extreme, and indeed a paradoxical position has been taken on this
phytonic question by Delpino. As a consequence of his studies on
phyllotaxis he concluded that the axis is simply composed of the fusion
of the leaf-bases ; that the leaves are not appendicular organs, but central
organs; that an axis or stem-system does not exist, and accordingly that
the higher plants are not cormophytes at all, but phyllophytes.
The second view, that the axis and leaf are the result of differentiation
of an indifferent branch-system, of which the limbs were originally all alike,
has lately been brought into prominence by Potonie.2 Taking his initiative
from the branching of the leaves in early fossil Ferns, he recognises the
frequent occurrence of overtopping (" Uebergipfelung "), that is, the gradual
process of assertion of certain limbs of a branch-system over others; in
the branching of Fucoids he finds an analogy for his observations on
Fern-leaves, and draws the following conclusion, that " the leaves of the
1 Delpino, " Teoria generale della Filotassi." FY>r ref. see Bot. Jahresbr., viii., 1880,
p. 118; also vol. xi., 1883, p. 550.
- Lehrbuch d. Pflanzenpalaeontologie, pp. 156-159. Also Ein Blick in die Geschichte
a. Bot. Alorph. und d. Pericaulomtheorie, 1903, p. 33, etc. It was, however, suggested
previously by myself, Phil. Trans., 1884, part ii., p. 605.
136 THEORY OF THE STROBILUS
higher plants have been derived in the course of generations from parts
of an Algal thallus like that of Fucus, or at least from Alga-like plants,
by means of the overtopping of dichotomous branches, and the develop-
ment as leaves of the branches, which thus become lateral." Dr. Hallier,
who adopts Potonie's position, "prefers to draw the comparison with Liver-
worts, which show a similar sympodial development of a dichotomous
branch-system.1
It seems not improbable that the condition of many branched Fern-leaves
may have been derived through a process of " overtopping " in an indifferent
branch-system of the leaf itself. But it lies with Potonie' to show, on a
basis of comparison of forms more nearly related to them than the Fucoids,
that the relation of axis to leaf in the Ferns was so derived ; and, further,
that such an origin is in any way applicable to other foliar developments in
Vascular Plants, especially in Pteridophytes such as the Lycopods, Equiseta
and Sphenophylls. I am not aware that this has yet been done. But
granting that this can be done, the question still remains whether similarity
of method of branching is any criterion of comparison as to descent.
For sympodial development of a dichotomous system (and this is all that
such "overtopping" actually is) has occurred in cases where it cannot
be held to have resulted in a branching which is foliar ; and of this instances
can be found without going so far afield as the Fucaceae. If this be so,
then little value need be attached to the comparison of such branchings
in plants not nearly allied to one another ; these may be held to be quite
distinct examples of a general phenomenon of branch-development, without
the one being in any sense the prototype of the other Such reflections
as these indicate that the comparison in mode of branching between the
leaves of Ferns and the thallus of Fucoids, which forms the groundwork of
the view of Potonie (or between the Ferns and the Thalloid Liverworts,
as may be preferred by others), are not to be held as more than distant
analogies ; consequently they are no demonstration of the origin of the leaf
by a process of "overtopping."
The view recently advanced by Professor Lignier (" Equisetales et
Sphenophyllales : leur origine filicineenne commune," Bull Soc. Linn, de
Normandie, Caen, 1904, p. 93) is analogous to that of Potonie, though
differing from it in detail. It involves ranking the Lycopod leaf as a
"phylloid," of the nature of a flattened hair, and comparable to the
amphigastrium of a Liverwort. The leaf of the Fern, however, is held
to be a true leaf, or phyllome, derived by differentiation from an indifferent
system of "cauloids," on which the "phylloids" have become abortive.
All such hypotheses have critical points in their application ; in the present
case it lies in the comparison of the Psilotaceae and Sphenophylleae. For
Lignier regards the leaf-lobes of Tmesipteris as only " phylloids," whereas
the leaves of the Sphenophylls, and also of the Equisetales, are "phyllomes,"
reduced from the larger-leaved type of the Ferns. The argument is chiefly
1 Beitriigc z. JHorph. d. Sporophylle n. d. Trophophyllst Hamburg, 1902.
DICHOTOMOUS THEORY 137
based on comparisons as to branching and anatomical structure. These
grounds will not suffice to override the inherent probability that the
leaves of the Lycopods and Psilotaceae are essentially of the same nature
as those of the Sphenophylls or Equiseta, and not the consequence of an
entirely distinct evolutionary history. Moreover, on his own admission the
" Prohepatic " type, from which Professor Lignier's theory starts, is still wholly
hypothetical. Further, it may be remarked that the embryology of the
sporophyte gives no assistance to those who would derive it from a
dorsiventral thallus. On these as well as other grounds the theory, as
stated by Professor Lignier, cannot be upheld.
An essentially similar hypothesis has been enunciated by Tansley (New
Phytologist, 1907, p. 25, etc.). He contemplates a megaphyllous origin
of a Fern-like sporophyte from a "hypothetical Archegoniate Alga," which
showed dichotomous branching : certain branch-systems became specialised
for assimilatory functions as erect shoots, and assumed radial symmetry,
while the axis originated by transition through sympodial development of
the dichotomy to monopodial branching. On this hypothesis the dorsiventral
symmetry would be the primitive and the radial the derivative state in
the original sporophyte. The megaphyllous types would be primitive and
from these the microphyllous would be derived by widespread reduction.
Putting aside the collateral speculations of Tansley to which exception
may be taken, such as the homoplastic origin of the archegonia and of the
spores, as well as of the whole sporophyte in Bryophytes and Pteridophytes,
and the wholesale resort to reduction in order to explain the origin
of the ancient microphyllous phyla, there are two points of fact, or of
absence of fact, which appear specially to oppose his theory : he assumes
a radial type of construction to be derivative for the sporophyte [and a
dorsiventral type to be primitive ; but in point of fact, in their individual
development all sporophytes are originally radial, a condition which has
probably a very close relation to their production in the archegonium :
that the dorsiventral state is as a general rule derivative in the sporophyte,
may be concluded from comparison and shown by experiment (see
Chapter XVI.). Further, there is no known case of dichotomy in the
sporophyte, where one branch develops as axis and the other as leaf.
The known facts derived from living Ferns as well as from the fossils
point clearly to dichotomous branching of the axis itself and of the leaf
itself, and to transition from a dichotomous to a monopodial branching
in the establishment of rachis and pinna. But such evidence is wanting
in the relations of leaf and axis. It was chiefly the absence of such
evidence that influenced me in rejecting my own suggestion of origin of
the shoot from a dichotomous branch-system made in 1884 (Phil. Trans.,
vol. ii., 1884, p. 605): it applies equally to the theory as stated by
Tansley, which appears thus to break down on the test of fact.
There remains the third view, which, however, is no new one ; for
there have not been wanting those who have assigned a more prominent
138 THEORY OF THE STROBILUS
place to the axis in the initial differentiation of the shoot. Perhaps the
most explicit statement on this point is that by Alexander Braun, who
remarks in his Rejuvenescence in Nature (English edition, p. 107),
referring to phytonic theories, that "all these attempts to compose the
plant of leaves are wrecked upon the fact of the existence of the stem as
an original, independent and connected structure, the more or less distinct
articulation of which certainly depends upon the leaf-formation, but the
first formation of which precedes that of the leaves." Unger also, in his
botanical letters to a friend (No. VIII.), described how "The first endeavour
is directed towards the building up with cell-elements of an axis" — "those
variously formed supplementary organs which are termed leaves originate
laterally upon it " and he concludes that " we may [therefore] say with
perfect justice that the plant ... is, as regards form, essentially a system
of axes." Naegeli contemplated a somewhat similar origin of the leafy
shoot as an alternative possibility ; in fact, that the apex of a sporogonium-
like body elongated directly into that of the leafy stem, in which case
the axis would be the persistent and prominent part, and the leaves be
from the first subsidiary, and lateral appendages. In my theory of the
strobilus in Archegoniate Plants the central idea was somewhat similar
to this.1 It may be briefly stated thus : There seems good reason to hold
that a body of radial construction, having distinction of apex and base,
and localised apical growth as its leading characters, existed prior to the
development of lateral appendages in the sporophyte ; the prior existence
of the axis and lateral origin of the appendages upon it are general for
normal leafy shoots. The view thus put forward is, indeed, the mere
reading of the story of the evolution of leaves in terms of their normal
individual development.
It is natural to look to the Pteridophytes for guidance as to the origin
of foliar development in the sporophyte, for they are undoubtedly the most
primitive plants with leafy shoots. They may be disposed according
to the prevalent size of their leaves in a series, leading from microphyllous
to megaphyllous types. I have lately shown that such a seriation is not
according to one feature only, but that certain other characters which
have been summarised as " Filicineous " tend to follow with the increasing
prominence of the leaf.2 This indicates that such seriation is a natural
arrangement. Now it is possible to hold either that the large-leaved
Fern-like plants were the more primitive, and the smaller-leaved, derivatives
from them by reduction ; or, conversely, that the smaller-leaved were the
more primitive, and the larger-leaved derivatives from them by leaf-
enlargement ; other alternative opinions are also possible, such as that
the leaf-origin has been divergent from some middle type, or that the
leaves of Vascular Plants may have been of polyphyletic origin. For the
moment we shall leave these latter alternatives aside.
Much of the difference of view as to foliar origin centres round the
^Annals of Botany, vol. viii., p. 343. ^Studies, v., p. 254.
A SIDELIGHT FROM ANATOMY 139
question whether originally the leaf was relatively large or small. Those
who hold that the large-leaved forms were the more primitive will- be
naturally disposed towards the view of the original preponderance of the
leaf over the axis, and will favour some phytonic theory ; those who hold
the smaller-leaved forms to be the more primitive will probably adopt
a strobiloid theory of origin of the leafy sporophyte. I propose to offer
some remarks on the relative probability of these alternative views.
If large-leaved prototypes be assumed generally for Vascular Plants,
this naturally involves a widespread reduction, since small-leaved forms
are numerous now, and have been from the earliest times of which we
have any record. Convincing evidence of reduction of leaf-complexity in
an evolutionary sequence, supported on grounds of comparison of form
and structure, and in accordance with the palaeontological facts, has been
adduced in the progression from Ferns, through Cycado-Filicinean forms
to the Cycads; and it applies with special force in the case of their
sporophylls. Ferns, which are essentially shade-loving and typically zoidio-
gamic, or amphibious, may be understood to have given rise to the
Cycado-Filices and Cycads, which are more xerophytic, and show that
essential character of land plants — the seed-habit. The case for reduction
of leaf-complexity seems to be here fully made out, and somewhat similar
arguments will also apply for other types of Gymnosperms. It must there-
fore be admitted that extensive reduction of appendages has occurred in
certain very ancient phyla.
But while we thus recognise a probability of reduction in certain phyla
producing relatively smaller-leaved forms, it does not follow that all small-
leaved Vascular Plants originated thus. On this point the anatomical
evidence is of importance, as bearing on the origin of the small-leaved
strobiloid Pteridophytes. Of these (putting aside the Hydropterids as being
a special reduction problem in themselves) there remain the Lycopodiales,
the Equisetales, and the Sphenophyllales, which are all cladosiphonic in
the terminology of Dr. Jeffrey. The question will largely turn upon the
meaning of this anatomical feature. The cladosiphonic character may be
held as the anatomical expression of the dominance of the axis in the
shoot. Here the leaf-trace, is merely an external appendage on the stele,
which is hardly disturbed by its insertion. This type is seen constantly in
certain small-leaved Pteridophytes. On the other hand, the condition,
styled by Dr. Jeffrey the phyllosiphonic, is the anatomical expression of the
dominance of the leaf over the axis in the shoot. Here the insertion
of the vascular supply of the leaf profoundly disturbs the vascular arrange-
ment in the axis, leading to an open communication between the cortex
and the central medulla at each leaf-insertion. It is characteristic of
certain large-leaved Pteridophytes, and is seen also generally in Seed-Plants.
There is thus a probability, supported on anatomical evidence, that the
seed-bearing plants at large were descended from a large-leaved ancestry,
and had undergone reduction of leaf-complexity in their descent.
1 40 THEORY OF THE STROBILUS
It is a fact of importance that, in the individual life, the one or the other
anatomical type is usually constant ; but in certain Ferns the progression
may be traced from the cladosi phonic in the young plant to the phyllosi-
phonic in the mature, thus suggesting a similar progression in descent,
viz. that the large-leaved phyllosiphonic Ferns were derived from a smaller-
leaved cladosiphonic stock. Of the converse, viz. the progression from
the phyllosiphonic to the cladosiphonic state in the individual life, I know
of no example among the Pteridophytes, though it is true that there is
some approach to it in the Marsiliaceae. Thus the anatomical evidence
indicates a probability that, even in large-leaved Ferns, the cladosiphonic
was the primitive type ; but that the phyllosiphonic, once initiated, is as
a rule maintained : this is shown by its persistence in the Seed-Plants,
even where the leaf has been reduced in size.
Having thus gained a valuable sidelight from anatomy, indicating
that small-leaved types were probably primitive, we may now return to
our central question of the initial relation of leaf to axis. Of the three
theories already noted, the theory of overtopping as applied to the origin
of the leaf may, in my opinion, be dismissed, as it is not based upon com-
parison of nearly related forms, while the facts of embryogeny and of
leaf-origin do not support it : and further, the sympodial development of
a dichotomous system, on which it is founded, is a general phenomenon
of branching, restricted neither to leaves nor to the sporophyte generation.
As to the other two, the facts, whether of external form or of internal
structure, seem to me to indicate this conclusion : that the strobiloid
condition, was primitive for certain types, such as the Equisetales, Lyco-
podiales, and Sphenophyllales : that in them the leaf was from the first
a minor appendage upon the dominating axis, and anatomically they have
never broken away from the cladosiphonic structure which is the internal
expression of their microphyllous, strobiloid state. That the Filicales and
"also the Ophioglossales were probably derived from a microphyllous
strobiloid ancestry, and achieved the phyllosiphonic structure as a conse-
' quence of leaf-enlargement, this being the derivative rather than the
primitive condition ; its derivation is even illustrated in the individual
life of some Ferns. From the Filicales the phyllosiphonic structure was
probably handed on to the Seed-Plants, and by them retained notwith-
standing the subsequent leaf-reduction which followed on their adaptation
to an exposed land-habitat. Thus a strobiloid origin may be attributed
to all the main types of Vascular Plants. It seems to harmonise more
readily with the facts than any phytonic theory does.
A prototype, which was probably a prevalent, though perhaps not a
general one for the Pteridophytes, may then be sketched as an upright,
radial, strobiloid structure, consisting of a predominant axis, showing con-
tinued apical growth, and bearing relatively small and simple appendages.
On our theory the origin of these appendages in descent would be the
same as it is to-day in the individual development, viz. by the outgrowth
ENATION OF LEAF 141
or enation } of regions of the superficial tissue of the axis to form them,
and this would occur not simultaneously but successively, the origin of
the appendages following the continued apical growth of the axis, as it does
in the developing shoot of the present day. The axis would pre-exist in
descent, as it actually does in the normal developing shoot. The origin
of these appendages may have occurred independently along divers lines
of descent, and the appendages would in that case be not homogenetic
in the strict sense. Thus there would be no common prototype' of the
leaf, no morphological abstraction or archetypic form of that part. More
than one category of appendages might even be produced on the same
individual shoot, differing in their function on their first appearance.
Such has perhaps been the case in the Calamarian strobilus, where, as
will be seen later, the leaf-tooth cannot be readily homologised with the
sporangiophore. These suggestions will suffice to indicate how elastic a
strobiloid theory is, and how its application will cover various types of
construction, even such as are shown by the most complex cones of
Pteridophytes.
The objection to a theory of enation will probably be raised that it
contemplates an origin of new parts rather than a modification of parts
already present, and that experience indicates the latter as the usual source.
The reply to this is a double one : first, that the appendages actually
appear in the ontogeny by enation : a leaf arises as an outgrowth from the
previously smooth surface of the pre-existent axis : the theory reads the
descent in terms of the individual life. But secondly, an origin of new
parts upon a smooth surface of a pre-existent part must necessarily have
taken place frequently in the formation in isolated genera of emergences
and prickles, often of large size and with vascular supply. Thus the origin
of new appendages is not without frequently recurring precedent among
Vascular Plants.
An essential feature in the theory of the strobilus is that it involves the
phyletic pre-existence of the axis. This is a point upon which embryo -
logical evidence can be adduced, both that of the primary embryology
and of the continued embryology of the growing shoot (see Chapter XIV.).
Thus far nothing has been said of the sporangia in relation to this
theory of the strobilus. It remains to trace the relation of these to the
appendages. On the above hypothesis the shoot originated from a body
having a fertile upper region and a sterile base. It is not necessary to fix
upon any type of sporophyte represented in any living plant as a prototype :
what is contemplated is an acropetally growing body, with already some
distinction of a sterile base, and a terminal fertile region endowed with
apical growth. In two or more types of living Bryophytes the relegation of
spore-production towards the outer surface is seen, with the formation of a
1 The term "enation" has long been used in Vegetable Teratology. See Masters,
"Vegetable Teratology," Roy. Soc,, 1869, p. 443: it connotes the exogenous outgrowth of
an appendage from a previously vacant surface.
142
THEORY OF THE STROBILUS
sterile central columella : such was probably the case also in the predecessors
of the strobilus, but the process was more completely carried out, so that
the spore-formation came to be, as it now is in all Pteridophytes, located
close to the outer surface. A further step would be the disintegration
of the sporogenous tissue into- separate pockets or sporangia: of such
disintegration there is evidence in certain Pteridophytes, but it is exemplified
in the clearest way in the anthers of various Angiosperms : the condition
which is actually seen in the anthers of Viscum album, or in the large
multilocular anthers of Rhizophora illustrates the point (Fig. 72): here
the numerous, small, isolated loculi
cover the very considerable surface
of the enlarged stamen, and develop-
ment as well as comparison points to
an origin of these by segregation from
the normal type of pollen-sacs. The
outgrowth of appendages by enation,
from such an apically growing struc-
ture has been already recognised as a
probable feature ; if this took place
either between the segregated loculi
or below them so as to carry them
outwards beyond the general surface,
during its acropetal development, the
result would then be a strobiloid
structure with an acropetal succession
of appendages, such as is seen in
various Pteridophytes. Sometimes the
sporangia might be borne in close
relation to the axis, as in Lycopodium
or Selaginella, while other Lycopods
illustrate varying degrees of the carry-
ing of the sporangia outwards upon
the appendage. In other cases varying numbers of sporangia are borne
upon a single appendage, as in the Calamarians and Sphenophylls : and
according to their form, and their relation to or freedom from sporangia
divers ranks of these appendages may be distinguished : these matters will
be discussed in detail later.
In the hypothesis thus sketched there are several steps which may be
named as distinct, though actually they may have overlapped : they are
(i) the differentiation in the primitive sporophyte of a vegetative base,.
and a fertile upper region having a power of apical growth : (2) the
relegation of sporogenous cells in the latter to a superficial position :
(3) the segregation of them into separate pockets or sporangia : (4) the
enation of the appendages. Every one of these steps has its actual prototype
among living plants, so that nothing is advanced which is contrary to
FIG. 72.
Rhizophora mucronata. Flower in longitudinal
section. Numerous spherical microsporangia, />,
in the anther. (After Goebel.)
ENATION OF LEAF 143
morphological experience. Therefore the validity of the strobiloid theory
is not open to a priori objection. The real question is whether those
processes which are seen to have been in operation elsewhere did actually
take part in the production of the Pteridophyte strobilus as it is now
seen? The applicability of the theory to the various known types of
Pteridophytes will be the true test of its validity. This will be carried
out in detail in the second part of this work, so far as the very imperfect
evidence will allow. In questions such as this of the origin of the shoot,
it is desirable to take the simplest possible reading of the facts as the
basis of an opinion : on this ground the theory of enation, as accounting
for the origin of the appendages of the strobilus, seems to be preferable
to any phytonic theory. It has been remarked that the strobiloid theory
involves " tremendous morphological assumptions in the way of the origin
of new organs" (Tansley, New Phytologist, 1907, p. 28, etc.): the only
assumption, however, which is apparent to the mind of its author, is that
the order and mode of origin of the appendicular parts in the course of
Descent has been that which is actually seen in their individual develop-
ment. They are formed by enation from the axis now, and it is held
that they originated in the first instance in the same way.
CHAPTER XII.
SPORANGIOPHORES AND SPOROPHYLLS.
THE theoretical position taken up in the last chapter is characterised not only
by its simplicity but also by its elasticity. It carries with it no obligation
to . assume that all the appendicular organs should show defmiteness or
constancy in their disposition upon the axis which bears them, nor even
that they were all alike in their initial character or function. Now, as a
matter of fact, an examination of the strobili of such plants as the Psilo-
taceae, Horsetails, and Calamarians shows that
irregularities of arrangement of the parts are
common : it is impossible to reduce the
arrangement of the appendages in the cone of
Tmesipteris or of Equisetum maximum to any
regular scheme (Fig. 73) : the appendages of
both vary in radial angle and in level of
insertion. There is also great variability in
the disposition of the leaves in the genus
Lycopodium, being sometimes whorled, some-
times irregularly spiral. It is true that cases
do exist among the strobiloid Pteridophytes
which show regularity in the disposition of
their parts, but in their shoots at large a
regular disposition of the appendages cannot
be held to be a general feature. Such irregularities, so far as they ai
of primary origin, are difficult to explain on the basis of any sympodial
construction of the strobilus : to a theory of enation, as expounded
in the previous chapter, they offer no obstacle; for if the appendage
originated from the surface of the pre-existent axis, as suggested, the]
might equally well appear in regular positions, or be disposed with greater
or less irregularity — as, indeed, is seen to be the case.
The comparative classification of those appendages of the strobilus
which are seen in the different types of Pteridophytes has always presented
FIG. 73.
Transverse section through a spor-
angiferous bud of Tmesipteris. ax =
axis. f= foliage leaves. I— lateral
lobes. jy = synangia. X2O.
CLASSIFICATION OF APPENDAGFS 145
difficulties : of this the Psilotacea~e are a conspicuous example, and the
analysis of the parts composing their strobili has led to voluminous
discussions. The difficulties are no less in the Sphenophylls and Cala-
marians. The presumption upon which morphologists have habitually
proceeded has been that all the parts are, or at least should be, reducible
to certain categories, such as axis, leaf, emergence, sporangium — these
being the headings under which the parts of the shoot in the higher
Vascular Plants are ranged. It is possible by the use of artifices, which
sometimes appear to be curiously strained, to carry out the classification
of all the constituent parts of the shoot in Phanerogams into these cate-
gories. But is the morphologist justified by this measure of success in
the practice of a somewhat artificial method in assuming that it shall
always be equally applicable to all Vascular Plants? And further, is it a
scientific method forcibly to extend the conclusions obtained from the
study of the higher forms to the lower? The attitude of the believer in
evolution should be the converse : to examine the lower types with a
mind untrammelled by conceptions derived from the higher, and a termin-
ology free to express what is actually seen in the more archaic forms.
Subsequently his conclusions may be extended to the higher forms. At
the present day it will seem hardly necessary to put down such simple
principles as these explicitly ; but doing so finds its justification in the
habit of thought, still ingrained in the science, of reading the lower Vascular
Plants in terms of the higher, just as it was done in the pre-Darwinian
days. From this the mind of the modern morphologist must be entirely
free.
The difficulty of reducing the parts of the strobilus in certain Pteri-
dophytes to the categories above named has already extorted from
morphologists the adoption of a further term not yet used in reference to
Flowering Plants. The non-committing word " sporangiophore " is now
understood to connote a structure which bears sporangia, but is not readily
referable to the category either of axis or leaf, though it might be included
under some broad use of the term "emergence." It may contain vascular
tissue, and be inserted either on the axis or on an appendage. It will
be the object of this chapter to consider the relations of the sporangia,
the sporangiophores, and trie sporophylls to one another, and to the axis
of the whole strobilus, as seen in the various types of Pteridophytes.
It is a rare thing for sporangia to be borne directly upon the axis
itself, though there is theoretically no reason against it, but rather the
reverse. The Lycopodiales include forms which show this position of the
sporangia, and Selaginella is usually quoted as a case in point (Fig. 74).
It is true that here the sporangium is inserted on the axis, and springs
directly from its tissue : it may originate as a swelling quite distinct from
that which develops into the sporophyll ; but the sporangia are not scattered
irregularly on the axis, for there is a constant relation of each sporangium
to the subtending appendage : the sporangium and the sporophyll are in
K
146 SPORANGIOPHORES AND SPOROPHYLLS
the same median plane, and, excepting rare abnormalities, each sporophyll
subtends only one sporangium: this is seen in all plants belonging to
the Lycopodiales. But they illustrate various degrees of remoteness of
the sporangium from the axis, while still retaining the strict numerical
and subtending relation. Thus Selaginella shows the closest relation of
the sporangium to the axis ; but the sporangium of Lycopodium originates
clearly from the tissue of the sporophyll itself (Fig. 75): in some of them
(Z. Selago) the sporangium may at maturity appear to be thin-stalked
and axillary, while in others (Z. davatum, Lepidostrobus, and Isoetes) the
sporangium may extend with a broader base some considerable distance
along the upper surface of the sporophyll. An extreme condition is that
FIG. 74.
Selaginella Martetisii, Spring. Radial
section of a strobilus, including apex (ap),
and traversing a young sporophyll (/), and
sporangium (x). Also another section of
sporophyll and sporangium, rather older.
FIG. 75-
Radial sections through young sporangia of Lycopodium
Selago. In the youngest the whole sporophyll is shown (I),
and the axis (st), and it is seen that the sporangium arises
upon the surface of the sporophyll. X 200.
of the early fossil Spencerites (Fig. 76), in which the narrow-stalkec
sporangium is attached some distance from the base of the sporophyll. Il
is thus seen that while the numerical and radial relations of sporangiui
and sporophyll are constant, the distance of the sporangium from the axi
may vary. This arrangement in the Lycopods, which dates back to th<
earliest fossil records, is certainly the simplest seen in the cones of
Pteridophytes, and the relation of the sporangium to the axis is habitually
closer in them than in any other type.
But other plants, which also have representatives of palaeozoic age,
bear cones of more complex construction : these present intricate morpho-
logical problems if the effort is made to classify their parts according to
the strict categories and the usual successions of axis, leaf, emergence,
and sporangium ; for instance, the modern Psilotaceae and the ancient
OF PSILOTACEAE 147
Sphenophyllaceae raise important questions. Among the former, Tmesipteris
bears appendages of simple form in the vegetative regio'n ; but the fertile
appendages are forked at their distal end, and bear on their upper surface,
just at the point of branching, a bilocular synangium, which has a short
stalk traversed by a vascular strand (Fig. 77). Various views have been
propounded in order to read this body in terms of the formal morphology
of the higher plants : for us, the suggestion would seem to suffice that
FIG. 76.
Spencerites insignis. Somewhat diagrammatic radial section of part of the cone,
Berridge.) From Scott, Progresses rei Botanicae, vol. i.
the plant is heterophyllous, the vegetative appendages being simple and
the fertile branched : while to the upper surface of the branched sporophyll
a sporangiophore is attached with vascular supply and bearing two sporangia.
In Psilotum the structure is the same, but the number of the sporangia
is larger. The disposition of the parts in Sphenophyllum majus is again
very similar to this (Fig. 78) : a synangial group of four to six sporangia
occupies a position comparable to that of the Psilotaceae on the upper
surface of a doubly branched appendage ; but these appendages are disposed
148 SPORANGIOPHORES AND SPOROPHYLLS
in regular whorls, instead of irregularly as in Tmesipteris. In other Spheno-
phylls the number of sporangia may be less, and the number of the more
FIG. 77.
Tmesipteris tannensis, Bernh. A, habit of a whole plant, pendulous form, showing
dichotomy. Natural size. £-£, sporophyll and synangium ; B from the side, C from
above, D after dehiscence, E from below, all X about 3. .F=rhizome. ^ size, ^trans-
verse section of stem. X4- (After Engler and Prantl.)
elongated stalks or sporangiophores, greater : thus in 6". Dawsoni they
are twice, in S. Rbmeri three times as numerous as the segments of the
OF EQUISETALES
149
verticil : these variations in number of the sporangiophores would be
difficult to harmonise with any reference to "leaf-segments," as ordinarily
understood elsewhere; and their disposition suggests the idea of chorisis
of the sporangiophore similar to that seen frequently in the stamens of
Angiospermic flowers.
FIG. 78.
Forked sporophyll of Spheno-
phyllum majits, bearing spor-
angiophore. (After Kidston.)
FIG. 80.
of
Calamostachys. Diagram
cone in radial section. ax = a.x\s,
which bears successive verticils of
bracts (br), and peltate sporangio-
phores (sp). sm = sporangia borne
on the sporangiophores. As the
bracts are alternate with one
another their upturned tips are
only shown in every alternate
verticil. (After Scott.)
FIG. 79.
Equisetum maximum, Link. A, the
upper part of a fertile axis, with the lower
half of the strobilus. Natural size. b = the
leaf-sheath. « = annulus. jr = stalks of spor-
angiophores cut off. y = transverse section of
axis. 2>=sporangioprrores in various posi-
tions, slightly enlarged. st = stalk. sg-=
sporangia. s = enlarged distal end. (After
Sachs.)
In the Equiseta and Calamarians, spore-bearing bodies of outline not
unlike those of Psilotaceae are attached directly to the axis itself, and bear
the pendent sporangia (Fig. 79). They show sometimes almost constant,
but frequently inconstant, numerical and local relation to the whorls of
ISO SPORANGIOPHORES AND SPOROPHYLLS
bracts or leaf-teeth : thus, in the modern Equisetum and in the ancient
Bornia they occupy the whole strobilus in large numbers, and bracts are
absent ; in Phyllotheca the fertile spikes are interrupted by occasional whorls
of vegetative leaves : in Calamostachys the strobilus bears successive whorls
of bracts, and whorls of sporangiophores alternate with them ; but even here
they do not show exact numerical correspondence with the bracts, which,
moreover, alternate independently of them. Further, their longitudinal
Palaeostachya. Diagram of cone in
radial section. <z;r = axis, which bears
verticils of bracts (br) with peltate
sporangiophores (sfi) in their axils.
sm = sporangia. (After Renault.) From
Scott.
A rchaeocalamites. Part of
cone showing the axis (<pr)
in surface view, bearing
superposed verticils of peltate
sporangiophores (sp) without
bracts, sm = sporangia. (After
Renault.) From Scott.
FIG. 83.
Helminthostachys zeylanica..
Young spike in oblique profile :
the primordia of sporangio-
phores are densely clustered on
the margin. Magnified. (After
Goebel.)
position relatively to the bracts varies, for in Calamostachys they are placed
midway between the. whorl of bracts, in Palaeostachya in their axils, in
Cingularia immediately below them. This indefmiteness of relation of
the spore-bearing bodies to the bract-leaves in number and position, as
seen among the Equisetales, when taken together with the difference of
function, points to their being a separate category of members from them
(Figs. 80, 81, 82).1
1 This statement is not in accord with the opinions expressed by Prof. Lignier, which
will be considered where the Equisetales are specially treated in Part II.
OF FILICALES
a.
In the Ophioglossales the structure of the shoot at large is open to
various interpretations ; but without entering here into questions which
will be taken up in detail later it will suffice to mention that in
Hclminthostachys there are sporangiophores which are broadly similar
in outline to those of Equisttum, but they are borne in irregularly
disposed bands on the lateral margins of the fertile spike (Fig. 83).
It may at first sight seem difficult to bring the very varied disposition
of the sporangia upon the enlarged sporophyll in modern Ferns into line
with these examples of spore-
bearing bodies in smaller-leaved
types. But it is to be remem-
bered that in Palaeozoic Ferns
definite sori were common ;
they were as a rule of circular
form, and all their sporangia
were produced simultaneously.
The wide-extended sori, such
as are frequently found among
the Polypodiaceae, were pro-
bably of relatively late and
secondary origin, by extension
of the sori of the circumscribed
type. Now, a circular sorus,
with relatively few sporangia
formed simultaneously and
borne upon a more or less pro-
jecting receptacle, into which,
as may often be seen, a vascular
supply extends, differs in no
essential from such bodies as
we have been considering. A
sorus of this simple type is
seen, for instance, in Kaulfussia,
which is closely similar to^hat
r»f 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 synangiumv in section parallel to the surface of
Kppri cppn that «r»rirp> Kp>arir»o- tne 'eaf> showing seven confluent sporangia, a, bundle of
Seen tnat Spore- bearing receptac',e. ^ its parenchyma ; c, ta^tum; </, spores; e,f,
bodies mav be borne directly common envelope of synangium. X about 60. (After
""? Renault.) From Scott's Studies in Fossil Botany.
on the axis or on the appen-
dages; the latter is the case in the Ferns, the chief difference between
them and the strobiloid forms being that the appendages here are large
and the sori, or sporangiophores, very numerous. Regarded in this light,
the Fern-type is not a thing distinct or apart ; the difference from other
types is mainly one of the degree of development of the sporophyll which
bears the sori.
FlG. 84.
152 SPORANGIOPHORES AND SPOROPHYLLS
Spore-bearing bodies, or sporangiophores, or sori such as those above
named, have then the following characters in common : They are out-
growths of varying length, which bear one or more sporangia: these are,
when numerous, more or less closely related one to another, and frequently
synangial : they are usually disposed in a circle round the periphery of a
disc-like expansion at the distal end of the more or less elongated stalk.
but other arrangements may be found. A vascular strand usually runs
through the stalk to its distal end, where it may divide into branches
which terminate in close relation with the sporangia. The similarity in
all essentials of structure of the sporangiophores in these early Pteridophytes,
notwithstanding the diversity of their position, points to the conclusion that
they are the result of similar response to similar requirements, carried out,
it may be, in various distinct stocks in the various positions in which
they are now seen. Their structure suggests that they are simply the
outcome of placental growth, which has the advantage of securing freedom
of dehiscence of the sporangia which they bear. In that case there is no
obligation to hold that they were the result of " metamorphosis " of any
pre-existent appendage or part. And herein lies the importance of the
introduction of the term sporangiophore which is now applied to them ;
for it simply implies the fact that the part so called bears sporangia, but
does not suggest any view of its further morphological character, nor does
it impose any limitation upon the position which the sporangiophores
may hold.
It remains to consider what relation, if any, subsists between such
sporangiophores and the " leaves." The difficulty in finding mutual ground
for discussion of this question lies in the preconceived ideas which the term
"leaf" carries with it in the minds of many botanists. It is often assumed
that the vegetative leaf was pre-existent in descent to the appendages of
the strobilus, the mind naturally translating the successive events of the
individual life directly into the history of its evolutionary organisation ; in
fact, the sporophyll has habitually been regarded as a vegetative leaf which
has become fertile. In the following chapter reasons will be given for
holding that the converse is nearer the truth, i.e. that foliage leaves are
often the equivalent of sporophylls which have become in the course of
descent sterile. If this be so, then signs of the origination of a vegetative
system are to be sought in the fertile zone rather than the converse.
A second preconception which is commonly entertained is that " the
leaf," whether sporophyll or foliage leaf, is a part having a common
evolutionary origin in all plants in which it appears ; but on grounds
explained above we shall be prepared to contemplate as possible a
polyphyletic origin of those parts which are designated collectively
" leaves."
A third preconception, which is a common one also, is that those
bodies which are designated sporangiophores are necessarily of the nature
of sporophylls, or of segments or parts of sporophylls. Any sporangiophore
SPORANGIOPHORE A PART SUf GENERIS 153
attached to an axis would thus be held to be a complete foliar structure,
and a sporangiophore attached to the upper surface of a bract would be
recognised as a "ventral lobe" of that bract. But if it be admitted that
spore-production in the sporophyte was prior to its vegetative development,
and was a constant phase throughout the evolution of the sporophyte,
then such a description seems not only unnecessary but highly improbable.
Moreover, it has been seen that sporangiophores may be present in positions
which are not those of the normal succession of known vegetative parts ;
in the endeavour to bring these into line with the customary position and
succession of vegetative parts, recourse has to be taken to almost fantastic
explanations. But there is no need for this if the sporangiophore be accepted
simply to be, what it structurally is, a placental growth. The object of
the morphologist should be not the forcible reduction of different organisn.s
to one scheme of construction, but to read in their diverse forms the
probably diverse story of their origin. This should proceed along the lines
of the least strained and simplest interpretation. Following these principles,
the sporangiophore in the Pteridophytes will be held to be a part sift
generis, itself primitive in its nature, in the sense that it is not the result
of modification or replacement of any other sort of appendage.
Certain physiological limitations must necessarily have been operative
during the transition of the fertile region of any sporophyte from a simpler
to a more complex condition, such as has been figured to the mind in
the last chapter. As already pointed out, an increase in spore-production
is an advantage in homosporous plants, since it increases the chance of
survival and of distribution. But in any increasing body the formation
of separate loculi will facilitate the protection and nutrition of the increasing
mass of spores while young : thus segregation has its biological rationale.
Projection of the sporangia beyond the surface of the part which produces
them will facilitate the shedding of the spores, and makes possible those
mechanical devices which are seen in so many of the Pteridophytes. For
the protection of the sporangia while young, close juxtaposition of the
appendages of the strobilus is also important, and illustration of this is
seen in almost all strobiloid types. But at the same time any projection
of the spore-producing pa/ts necessitates the conveyance of their nourish-
ment through a longer distance, and by more restricted channels. Such
crossing of interests will have tended to keep the appendages which bear
the spores small, so long as they are themselves not active or essential as
nutritive organs ; in fact, there would in that case be a tendency to per-
petuate the strobiloid type. But if the appendages themselves carry on
efficiently the function of supply of organic material, then there need be
no limit to their size, provided that the water-supply to them can be
maintained ; and they may accordingly bear an infinity of sporangia, as is
seen to be the case in the megaphyllous types. It is in connection with
these functions of protection and nutrition that the foliar development
would naturally come into prominence as a feature of the strobilus, and
154 SPORANGIOPHORES AND SPOROPHYLLS
this may have taken effect in more than one of the several phyla of the
early Pteridophytes.
The relation of the sporangia and sporangiophores to the parts designated
as bracts, sporophylls, or leaves is habitually, though not always, a close
one. In the simpler strobiloid - forms the leaf either subtends the spore-
producing member, or the latter is borne upon its upper surface, commonly
in a median position. The biological importance of this probably lies in
the protection which is afforded, and in the ready supply of nourishment
in cases where the leaf is an effective organ of assimilation. But it is an
error to assume that there is any obligatory or constant relation for plants
at large between the spore-producing members and the leaves. This is
shown, first, by the fact that sporangiophores, even in very early fossils
s.ich as Bornia, may exist independently of the subtending leaves ; and
secondly, that when associated with leaves they may vary greatly both in
numerical and local relation to them, even within near circles of affinity :
this is seen in the Sphenophylleae with special clearness. Such examples
taken from early fossils teach that the spore-producing members show a
high degree of independence from the sporophylls. For the present these
general remarks must suffice : but later, when the sporangiophoric Pterido-
phytes have been described in detail, we may attempt some more exact
recognition of the varying relations which existed between the sporangio-
phores and the sporophylls in early strobiloid types.
In this connection the question may be raised whether sporangiophores
and leaves have always been distinct categories of parts : whether leaves
or foliar parts have ever developed into forms resembling sporangiophores.
In the case of the Cycads there is little doubt that the parts usually
designated female sporophylls or carpels are reduced foliar structures : it
is shown on the .basis of comparison that their form, so like that of many
sporangiophores, has been attained by a process of reduction, and thus
they may be held to be homoplastic with the primitive sporangiophores
of Pteridophytes.
Such considerations as these will deter the morphologist from any precise
definition of the categories of parts borne upon the strobili of early Pterido-
phytes according to experience derived from study of the Phanerogams. .
There is indeed no reason to assume that there was any initial uniformity
of the development such as would lead to their always falling into
strictly definite categories. Greater uniformity is, however, found among
the higher forms, and it is this uniformity which has led to the establishment
of those old morphological categories which are found to fit so ill upon the
lower Vascular Plants. Each plant-type may be held to have worked out
its own progressive development, while biological conditions common for
them all would tend to reduce them to some common scheme. Such
constancy as appears among the parts of the higher plants would then
have been achieved by gradual evolution of order from beginnings which
were less constant : and as a matter of fact the exceptions from that
THEIR RELATIONS NOT CONSTANT 155
order of disposition of the parts, or from that structural character which
has been held as typical, are chiefly found where they would on this view
be theoretically probable, viz. in the Homosporous Pteridophytes. This
seems to be the natural way of regarding the various types of strobilus
seen in early Vascular Plants : it is certainly more satisfactory than any
attempt forcibly to reduce them to conformity with categories based upon
the study of those plants which represent a later phase of evolution.
CHAPTER XIII.
ON THE RELATIONS BETWEEN THE STERILE AND
FERTILE REGIONS IN THE SPOROPHYTE.
FROM the days when Morphology first arose as a branch of the science
of Botany, the relations between the sterile or vegetative region of Plants,
and the fertile or reproductive have been the subject of enquiry. Originally
the question presented itself as one of simple comparison of those regions
in the Flowering Plants, in which they are clearly differentiated one from
another : the basis of the comparison was that of their external form,
with the idea behind it of some degree of unity of plan in the construction
of the two regions. At the present time the enquiry involves the direct
question of their physiological relation, but it also extends to the indirect
historical problem of their genetic relationship. This can best be approached
by comparison of forms lower in the scale of development, such as the
Pteridophytes, in which the differentiation is less complete than it is in
the Flowering Plants.
From a physiological point of view, the necessity of a due balance
between the sterile and fertile regions in the case of any fully differentiated,
self-supporting organism is readily grasped; for the material required to
build up the strobilus or flower to the point of maturing its spores must
be derived from an adequate development of the vegetative organs which
produce it. It is naturally otherwise in sporophytes which are not self-
supporting, or only partially so, as in the Liverworts and Mosses : also
in the case of parasites and saprophytes ; but the latter, as derivative or
secondary conditions, may be put aside when we discuss the adjustment
of balance between the two regions in its evolutionary aspect. The
indirect historical question is less readily tangible, but in its solution the
sources of nutritive supply must be steadily kept in view throughout the
comparative study of the lower and simpler sporophytes.
The fact that there is frequently a tendency towards extended production
of spores in the Homosporous Archegoniatae has been brought forward
repeatedly in previous chapters, where also the racial advantage which
GOETHE'S PROGRESSIVE METAMORPHOSIS 157
follows upon it has been sufficiently recognised. The evidence that this
tendency actually exists is to be found in the fact, illustrated in so many
plants, that more numerous spores are habitually initiated than the plant
is able to bring to maturity. The powers of nutrition impose the actual
limit of the output of spores in any specific example, and any increase
of the vegetative system will therefore result in an increased capacity for
producing mature spores. Where the vegetative region extends so as to
increase the powers of nutrition, it commonly happens that the initiation
of potential spores still keeps in advance of such increased supply, and
so the two seem to advance together. In the present chapter various
examples from among the Archegoniatae will be examined from this point
of view : upon these some idea may be based of the general methods of
progression of the sporophyte, from its less differentiated state towards
that seen in the Flowering Plants, where the vegetative and reproductive
regions are clearly distinct, though their construction still shows a funda-
mental similarity of plan. But before this is entered upon, it will be
well to clear the ground by consideration of the earlier theoretical views
on the relation of these two regions of the plant-body.
Kaspar Friedrich Wolff laid the foundation for a comparative view of
the appendages of the Higher Plants. In his Teona Generationis, published
in the latter half of the eighteenth century, he propounded the thesis
that " in the whole plant, the parts of which differ so extraordinarily from
one another at first sight, there is nothing to be found on mature con-
sideration but leaves and stem, for the root belongs to the latter." For
him all the appendages were of foliar nature. The modifications which
appear in the parts which compose the flower arose, in his view, from
the gradual waning of vegetative power, or "vegetatio languescens" as he
called it ; their development constantly diminishes the longer the vegetation
is continued, and finally ceases altogether ; consequently the essence of all
these modifications of the leaf lies in their incomplete development.
It is but a slight step from ideas such as these to the doctrine of
Metamorphosis as introduced by Goethe in 1790. He assumed an ideal
fundamental organ, from which the different leaf-forms in any one of the
higher plants could be regarded as derived. He designated as " Meta-
morphosis " that process by which one and the same organ presents itself
to us in various modifications. This metamorphosis may be of either
of three kinds : regular, irregular, and occasional. Of these the regular
or progressive metamorphosis, with which we are specially concerned, is
that illustrated in any normal Flowering Plant by the progression from
the cotyledons through the foliage leaves to the flower with its successive
series of parts. But, as Sachs points out in his History of Botany (Engl.
Ed., p. 156), Goethe sometimes used the word "Metamorphosis" in its
literal sense, as meaning an actual change in the organs arising from a
transmutation of the species ; sometimes his meaning was an ideal one ;
for, regarding the way in which cotyledons, foliage leaves, bracts, sepals,
158 STERILE AND FERTILE REGIONS
petals, etc., originate on the stem, they were all included under the one
general idea of "leaf." In the words of Sachs, Goethe's doctrine could
only make its way to logical consistency and clearness of thought by
deciding for the one or the other meaning of the word : he must either
assume that the different leaf-forms, which were regarded as alike only
in the idea, were really produced by change of a previous form — a
conception that at once presupposes a change of the species in time :—
or he must entirely adopt the position of the idealistic philosophy, in
which the idea and the reality coincide. In this case the assumption
of a change in time was not necessary : the metamorphosis would remain
an ideal one, a mere mode of view ; the word leaf would then signify
only an ideal fundamental form, from which the different forms of leaves
actually observed may be derived, as De Candolle's constant species, from
an ideal type.
Though Goethe did not himself decide finally for either of these
alternatives, the introduction of a theory of Descent, and a general belief
in transmutation of species, went far towards clearing away any such
ambiguity. In its light the facts seemed to point definitely towards a
conception of a real transformation, and this point of view came into
prominence pari passu with a better knowledge of the lower Vascular
Plants, where leaf-differentiation is less fully carried out and gradual transi-
tions are to be seen between vegetative leaves and sporophylls. Accordingly,
it seemed to be the plain and simple reading of the facts to accept the
metamorphosis as a change which had actually been effected in the course
of descent. The natural progression shown in the life of the individual
seemed to be that already described as progressive by Goethe : it was
natural to accept this in terms of the theory of descent as progressive
in the race also. On this basis the floral appendages would be held to
be essentially foliage leaves, but altered in character to subserve propa-
gation ; and the pollen-sacs and ovules which they bear accessories which
are added to the already existent foliar parts. The experience of zoologists
had its influence in apparently confirming this position. The analogies
between the two organic kingdoms are at many points so close that the
general conclusions of the animal embryologists seemed readily applicable
to plants also. If the ontogeny of the higher animals is found often to
recapitulate the history of the race, should not the same conclusion apply
also to the higher plants? Moreover, such a view presented itself as a
mere continuation of the theoretical opinion of Goethe : the progressive
metamorphosis which he recognised would figure, accordingly, as a principal
feature in the evolution not only of the individual but of the Yace. Thus
regarded the sporophyll of the individual plant would be an altered
foliage leaf, and its origin by descent would be the same : the difference
of their development would then lie in the presence of the sporangia,
which brings correlative restriction of the foliar development in its train.
This position may seem satisfactory so long as the Higher Vascular
GOETHE'S PROGRESSIVE METAMORPHOSIS 159
Plants alone are considered, or if sporangia are not regarded too scruputously
from an evolutionary point of view, and if it be assumed that they may
In- and have been habitually generated at large in the course of descent
upon pre-existent foliar organs. If these points be granted, then it might
be possible to retain Goethe's progressive Metamorphosis as the basis of
an evolutionary story applicable to the Higher Plants. As a matter of
fact, Botanists continued to analyse and describe the flowers of the Higher
Plants in this way for a whole generation after the Origin of Species
had been published. The flower was habitually regarded as the result
of metamorphosis of a foliage shoot. Though the point was not always
put into direct terms, the underlying assumption was that a conversion of
vegetative parts into propagative parts takes place in the individual : that
sporangia originated sporadically in descent, as they seem to do in certain
cases now, and that such changes as are seen in the development of
the individual had their place also in the history of its evolution. But
increasing knowledge of the life-cycles of the lower forms, and of their
comparison one with another, was meanwhile leading to sounder views of
the origin of the higher Vascular Plants. Alternation of generations
became gradually a more exact factor in the morphology of the last half-
century. It seems no longer possible to look upon the Vascular Plant as
a primary entity, as it was held to be in the time of Goethe. The
sporophyte generally, and consequently the plant-body of all the Higher
Plants which is a sporophyte, must necessarily be held to be secondary
by all those who recognise antithetic alternation as a constant feature in
descent of the Archegoniatae : for them the story of origin of the sporo-
phyte must affect the interpretation of its parts.
A fundamental question of method in morphology is involved in this
discussion, viz. the question of the validity of conclusions based on
observations of the ontogeny as against the well-founded conclusions of
phylogeny. It will now be generally agreed that, provided the conclusions
as to phylogeny be sound, they should have the precedence over those
based on observation of the individual life. But in the practice of the
middle part of last century it was customary to act in the opposite sense,
and to take the successive events in the story of development of the
individual as the basis of morphological history : such views on descent
as are based on comparison were often left out of account or given only
a second place. If this latter principle be adopted, then conclusions
harmonising with Goethe's progressive metamorphosis will follow, and the
sporophyll may be accepted as an altered foliage leaf; but if precedence
be given to the results of a broad comparison, then a converse conclusion
will necessarily appear the more probable.
But there is also another question involved in Goethe's view of " pro-
gressive metamorphosis," that of the origin of the sporangia which appear
in the strobilus or flower. The assumption that sporangia can be formed
indiscriminately upon pre-existent vegetative parts was at the back of
i6o
STERILE AND FERTILE REGIONS
Goethe's theory : it must be enquired whether this is a justifiable assump-
tion. This question has not been too exactly scrutinised by his followers, who
translated his "progressive metamorphosis" into terms of an evolutionary
progression. The basis for the assumption was primarily the succession
of events as seen in the individual life of the higher plants ; but a
certain laxity of view was further encouraged by the irregularities of
number and position and of time of appearance of the sporangia in the
Leptosporangiate Ferns. These plants were accorded an undue prominence
in the early study of Pteridophytes, and for long the belief was held that
they were the prototypes of all Vascular Plants. But it is now sufficiently
clear that the Leptosporangiate Ferns are relatively late derivative forms,
and that the types of Ferns of the primary rocks were more precise and
FIG. 85.
Botrychiuth Lunaria. Sterile laminae, which occasionally produce sporangia (s/>) on
certain pinnae, and have partly or wholly assumed the form of the fertile spike : /in
B and C is the fertile spike itself. Natural size. (After Goebel.)
exact in the arrangement and in the time of origin of their sporangia.
Such precision is seen in higher degree in the Calamarians and Spheno-
phylls, and it is specially prominent in the Lycopods. All of these are
types of quite as early, probably of even earlier, geological history than
the Leptosporangiate Ferns. Accordingly it may be held that in the
earliest Vascular Plants which we know the arrangement, time of appear-
ance, and number of the sporangia showed some degree of definiteness,
and were in some cases very precise. It cannot be denied that accessory
sporangia may at the present day appear in some cases where none are
normally present : conspicuous examples are those described by Lang in
apogamous Ferns (compare Fig. "35), while a less bizarre case is that of
the sporangia which appear on the usually sterile leaf of Botrychium
Lunaria (Fig. 85) : abnormal flowers of Phanerogams also provide
numerous examples of sporangia not produced in the usual order or
position. The question is whether the existence of such cases at the
ORIGIN OF STERILE REGION— SECONDARY 161
present day will justify the assumption that in the evolutionary story
sporangia originated indiscriminately upon pre-existent vegetative organs.
My own opinion is that it does not, for a careful examination of such
cases and comparison of them with the general type to which the plants in
question belong shows that they are exceptional, if not indeed of the nature of
monstrosities. It is clear that promiscuous formation of sporangia in
present-day forms is possible, and that it does at times occur, but it
•does not follow that this was a general mode of their origin in past
times.
An essential fact bearing upon the question in point is that spore-
production is a constantly recurring event in Archegoniate Plants. There
is good reason to believe that it has found its place in every normally
completed life-cycle throughout their descent. Cytologically it is now
seen to be the natural complement of the sexual process. Taking all
types of Archegoniate Plants into our view, including the more recent
Flowering Plants as well, there is reason to believe that spore-production
was the initial function of the sporophyte, and that it has been continued
and repeated throughout descent. If this be admitted, how can the
strobilus, or the flower — the part bound up with that primary function
of spore-production — be the result of metamorphosis of a vegetative shoot,
the leading fr ction of which is secondary? The conclusion to be derived
from broad comparison will be the direct converse : viz. that vegetative
parts in the sporophyte have originated by change of parts originally
fertile.
But in order to carry conviction that this conclusion is correct, it will
be incumbent on those who hold it to bring forward evidence bearing on the
origin or increase of the vegetative system, which we see at the present
day preceding spore-production in the history of the individual life. It
has already been shown in Chapter VII. that sterilisation of individual
sporogenous cells, that is, their conversion into cells having a vegetative
function, is common. It is found in the sporangia of Vascular Plants,
but it is in the sporogonia of Bryophytes that it has been recognised as
specially effective in adding to the vegetative system. The sporogonium
of Aneura (Fig. 86) has already served as an example, while reference to the
writings of Goebel (Organography, pp. 326-329, Engl. Ed., p. 103), shows
how fully sterilisation has already been realised, and accepted as a source
-of increase of the vegetative system in the Bryophyta. Similarly, in
Vascular Plants it has been shown above, that sterile cells of a sporo-
genous group may be converted into vegetative tissue of a septum. Such
examples indicate how sterilisation of individual cells may be effective in
increasing, and perhaps in the first instance even in originating, the vege-
tative system.
A second factor, which has been specially effective in contributing to
the increase of the vegetative system in the more differentiated types of
sporophyte, is the abortion of sporangia, or of sporangium-bearing parts.
162
STERILE AND FERTILE REGIONS
As this in my opinion has not yet been accorded its proper place in the
evolutionary story, I propose to consider it at some length.
Imperfectly developed parts have played an important role in arguments
on Evolution. On the Zoological side especially they have been used
as weighty evidence. Similarly, in Botany they have been the basis of
discussion : in the morphology' of the flower, abortive stamens, carpels,
pollen-sacs, and ovules have been cited as foundations for elaborate
argument. Where present in normal position the existence of an abortive
stamen or staminode has been habitually held to be sufficient indication
of the previous existence of a fully developed stamen in the ancestral line ;
and on such evidence natural affinities have been traced and accepted*
FK;. 86.
A, median section of young sporogonium ol Aneura ambrosioides. The internal mass
of cells of the - sporogonial head ("archesporium ") is already differentiated so as to
indicate the sterile elaterophore, and the outer fertile region. B, the same, older : the
indications of sterilisation have extended outwards, and it is only the peripheral fringe of
cells (shaded) which will be sporogenous. C, transverse section of the same. X 150.
usually without question. But floral morphology has gone further : com-
parative study has led to the conclusion that in certain ancestral lines
of descent parts have existed, which in the individuals of the present day
are entirely unrepresented by any vestigial growth. This condition of
complete disappearance of a part or parts has been styled "ablast," as
distinct from "abortion," where the incompletely developed part has an
objective existence. Eichler maintained that the conditions distinguished
as " abort " and " ablast " are not essentially different in kind, but only
differ in degree. He points out that abortion itself is not susceptible of
objective proof, and it may be remarked incidentally that it is this fact
which has prevented the full recognition of the part which it has played
in the origin of the sporophyte. Speaking of the relations between partial
abortion, where a vestigial structure is present [" abort "], and complete
suppression ["ablast"], Eichler remarks (Bluthendiagrammt, p. 52) that in
ABORTION OF SPORANGIA 163
cases of abortion " objectively we- see nothing more than that cell-divisions
occur, that a rudiment appears; thus strictly speaking we observe that
something develops, not that something is reduced : this may become a
gland, an emergence, or what not. It is comparison, and usually the
comparison with other species and genera, etc. — that is, the type-method,—
which teaches that it is a reduced organ, and what is its special category.
Whenever the same comparative method leads even to the assumption
of a complete suppression, where no rudiment of the organ is seen with
the bodily eye, in my opinion that is, in point of fact, no more than one
step further along the same course." This is the position which should
be the foundation of a correct view as to abortion, or even complete
suppression of parts : it is currently accepted, and put in practice in the
morphological treatment of the Angiospermic flower, and it is now high
time that it should be applied equally to the Pteridophytes, where it has
probably played a very important part. In the Pteridophytes too little
attention has hitherto been paid to such subjects, and notably observations
of arrest of sporangia, or of spore-producing organs, have been neglected.
It is the isolation of many of the genera, and the paucity of species in
some of the most important of them, which has stood in the way of
their detailed comparison in this respect, and consequently arguments from
arrest have not taken their proper place in the morphology of the
Pteridophyta. But the argument to be founded on an imperfect sporangium
at the base of a strobilus of Lycopodium, or on the abortive fertile spike
of an Ophioglossum seated in the position normal for the fully developed
part in other individuals, species, or allied genera, is precisely the same
as that on an imperfect pollen-sac or ovule, or on a stamen or carpel in
Flowering Plants. Further, a comparison as regards the presence or absence
of spore-producing parts in species evidently related to one another may
lead to the conclusion that sporangia entirely unrepresented at the
present day were probably borne upon ancestral forms : the line of
reasoning being the same as that in cases of hypothetical complete sup-
pression of floral parts. It will presently be shown that such hypothetical
suppression of spore-producing parts may be held accountable for changes in
balance of the vegetative and propagative regions in the Pteridophytes, and
be recognised as having Ted to an increasing prominence of the vegetative
system in the course of their evolution.
The Lycopodinous type, being represented by numerous species of
essentially similar construction, lends itself well to such comparative
treatment, while the comparison is the more pointed owing to the definite
relation of one sporangium to each subtending leaf, which arrangement,
with very few exceptions, is the constant rule for the fertile regions of
these plants. In all known Lycopodinous types a sterile leafy region,
of greater or less extent, precedes the fertile region in the life of the
individual plant. In many species of Lycopodium, and especially in those
which have the vegetative and fertile regions less clearly differentiated,
10. 1 STI'kli K AND I'KkTILK KKC.IONS
the initial vegetative stage is soon closed by the appearance of sporangia
in the axils of the leaves; hut alternate Irrlile ;ind sterile- /ones, merging
into one another imperceptibly as regards lorm, succeed one another at
irregular intervals throughout the upper region of the plant. This may
be styled the Scfago condition, as it is seen conspicuously in /jv/'/W//////
Selagp (Frontispiece). In others, and especially in those in which Un-
fertile region appears in the form of a definite terminal strobilus, the
initial vegetative phase is more extensive, though still essentially similar to
the strobilus in its construction ; there is, however, a prevalent dilfcrenec
of form between the sterile and the fertile leaves, but the relation <>i UK-
sporangia to the latter is the same as in the Sc/w form. It seems
natural to conclude that the Sclago type is the more primitive, and t In-
definitely strobiloicl type the derivative.
The question in cither of these cases is, what genetic relation has
existed between these sterile and fertile regions which are so similar in
plan, but differ in the absence or presence of the sporangia. Tin-
clew is given by examination of the basal limits ot the lertile zones in
either case; for here, at the point of transition from the sterile to Un-
fertile, imperfectly developed sporangia are often found, occupying tin-
place normally taken in the fertile region by those fully developed.
Applying to these the same argument as in the case of an imperfectly
developed ovule or pollen-sac in an Angiospeimie flower, they will be
held to he vestigial representatives of sporangia, noimally present, and
actually initiated, but not completely developed, Passing from these
to the vegetative region, where no vestigial sporangia are present, t hough
UK- arrangement, character, .ind in tin- St'/iig<> type even the loim of
the leaves is the same as in tin- fertile region, the question arises
whether these are not essentially sporophylls, in which tin- sporangia
are completely suppressed? The result of a broad consideration of the
question will be an answer in the affirmative. The facts indicate that
in the simple Lycopod type progressive sterilisation has been effective,
and that it has involved the partial abortion, or even the complete
•<uppression, of whole sporangia; the result is that leaves originally in
the race fertile have become sterile, and have thus contributed to the
enlargement of the vegetative region. Tin- fact that the Selago condition
is seen represented in certain Lycopod fossils of the Coal period is
important evidence of the validity of [this progression as an early evolu
nonary factor.
Such sterilisation as that believed to have occurred in / r.v/W////// in
the course of descent has been experimentally induced by Goebel in
:;inel/a>1 by cutting off young strobili, and treating them as cuttings;
the- sporangia of the upper legion ahoited, and the sporophylls of the
newly formed parts of the shoot developed as foliage leaves. Thus the
result theoietieally contemplated may follow from experiment
• P- (l^7-
THK '^SELAGO" CONDITION [65
With this progressive sterilisation, and the consequent UlCreMC of the
vegetative region, tlu- apical giowth of tin- axis krcps pace: it secures the
initiation of additional sporophvlls and sporangia to take- the place ol
those' transformed or aborted, and as there is no theoretical limit to the
apical growth ami branching, in such species as L. Selago the halanee
Can constantly hi- readjusted between tin- sterile and the fertile regions.
This combination of sterilisation and continued apical growth provides,
iii a sense, a forward impulse, and ii will be effective up to the limit of
physiological supply. That it is so is seen in the fact that at the apex
ot am Lvcopod slrohilus imperfect sporangia are found, which arc to
he recognised as Supernumeraries, showing the continued exuberance ol
initiation beyond the power ol the plant to bring to complete maturity.
\\ e thus acquire, the conception of a zone of reproductive activity — or in
the Selago t\pe it may be several interrupted zones — limited below by
parts which are to he held as vestigial, and above by parts which are
supernumerary. l',y comparison of living species of AmyW/Ww it is seen
that the fertile zone is not alwa\s located at the same- level on the plant:
it is sometimes preceded by a shorter, sometimes by a longer vegetative
region. There has probably been a phvlogcnctic shifting of the fertile
/one or /ones: the biological significance of this is obvious, for any
advance of the fertile /one to a higher point, by abortion of sporangia,
while the spoioplulls remain in a vegetative capacity as foliage Lea
provides vegetative region be-low for purposes of nutrition. Such
a manner of advance has probably been effective in the evolution of the
Lycopods as we now see them.
If the Lycopods stood alone in showing such features as those described
the facts would be of limited inteiest, but they do not; conditions essentially
similar are si-en in the sporophytes of other Vascular Cryptogams, though
\arying in detail. The mature plant of Jsoctcs is virtually ol the .SV/r/^v
type: it bears fertile and sterile leaves intermixed: vestigial representatives
of sporangia are lomul in the position normal for sporangia upon many of
the sterile leaves ; further, the probability that the leaves actually sterile
are so by suppression is as strong here as in the case of Lycopodium
.SV/r/v". The mature plant is preceded by an embryonic vegetative ph.;
with leaves bearing no sporangia; but after the first sporangia appear, the
whole plant m. iv be regarded as a strobilus, imperfectly differentiated, as
in the Scla^o type, into fertile parts and parts sterile by abortion or by
complete suppression.
Similarly, in the 1'silotaceae, the Selago condition, with irregular alter-
nation of sterile and fertile /ones, is seen in both rsilithnn (Fig. 87) and
Twt'.\rf>ft-ris. \\hile imperleet lynanglS are found about the limits ot the
fertile regions. There is, howe\ei, a broad difference in form between the
simpler sterile appendages and the more elaborate fertile ones; in this
respect the differentiation of sterile and fertile parts has proceeded further
than in the Lyeopods. In the allied fossils, the Sphenophvlleae, there
i66
STERILE AND FERTILE REGIONS
is, as a rule, a definite strobilus, which is fertile throughout ; but in .S. majus
.this is not clearly differentiated by form either at base or apex from the
vegetative region. Such a condi-
tion shows an interesting analogy
with the less differentiated states
of Lycopodium.
Among the Equisetales, Equi-
setum and Bornia have, as a rule,
a definite strobilus, composed
entirely of sporangiophores borne
on an axis, and clearly marked off
from the vegetative region which
precedes it in the ontogeny. But
in the Calamarians, as also in
the fossil known as Phyllotheca,
sterile leaf-sheaths are interspersed
between the sporangiophores, a
condition occurring also as an
occasional abnormality in Equi-
setum (Fig. 88). The morphology
of these cases will be more fully
discussed below; meanwhile it may
be held that while in Equisetum
and Bornia the differentiation of
the fertile strobilus from the vege-
tative region is more distinctively
marked, Phyllotheca or the Cala-
marians show some degree of
analogy with the Selago condition
seen in Lycopodium.
The Ophioglossaceae provide
clear cases where the argument
of abortion leading to complete
suppression will apply ; for various
degrees of development of the
fertile spike may be found borne
upon the fully formed leaf, from
that fully matured to small
vestigial parts which do little more
than mark the place where the
normal spike would be inserted ;
in other cases again the spike is entirely unrepresented. . The facts here
correspond to those in Lycopodium SeJago or in Isoetes, except as regards
the size and number of the parts concerned. Such a condition in an
Angiospermic flower would certainly be interpreted as abortion, and the
FIG. 87.
Shoot of Psilotum. Natural size. Showing " Selago "
condition in the bifurcate branch-system : the base is
vegetative : then follows a zone bearing synangia, then
a vegetative zone, and higher up a second fertile zone.
THE "SELAGO" CONDITION
167
incomplete parts where present as vestigial, and a similar conclusion seems
justified for the Ophioglossaceae. It may thus be held that in the mature
plant of the Ophioglossaceae all the leaves are potentially fertile : the
sterile foliage leaf is merely the part which remains when the spike is
abortive, and its genetic relation to the fully matured sporophyll is the
same as that of the sterile to the fertile leaf in Z. Selago or in Isoetes.
There remains for consideration from this same point of view the large
series of the Ferns. Notwithstanding the preponderant size of their leaves,
and the wide distribution of the sori and sporangia over their large surface,
they should still be studied in the
same way as other Pteridophytes: their
difference of conformation should not
be allowed to affect the recognition of
such similarity in the relations of the
vegetative and propagative parts as may
^xist between them and the smaller-
leaved forms. Since the relation of
leaf to axis is essentially the same in
Ferns as in other Vascular Plants, the
whole shoot may be held as equivalent
to the shoot, for instance, of an Isoetes;
and this aspect of it may be maintained
equally in those cases where the axis is
short and the leaves crowded upon it,
-and also in those where the axis is
elongated and the leaves isolated at
long intervals apart. Maintaining this
point of view of the shoot as a whole,
there is in the ontogeny of the Ferns
a preliminary vegetative phase, which
may be of varying extent ; subsequently
the fertile phase begins. The broad
relations of the two phases are thus
the same as in other Pteridophytes.
The fertile region in Ferns is imperfectly differentiated, and it is in this
respect comparable with those imperfectly differentiated forms which show
what has been called the Selago condition. But the matter is further
complicated by the fact that in many Ferns the differentiation does not
involve whole leaves, but only parts of them ; the large Fern-leaf, in fact,
does not always behave as one unit, but the differentiation of sterile
and fertile regions may involve only parts of the individual leaf, not the
whole.
Taking into consideration first the simpler case, where whole leaves
are differentiated either as sterile or fertile, examples are seen in such
cases as the common Hard Fern (Bkchnum boreale) or in the Ostrich
FIG.
Phyllotheca. Zigno. A , Ph. equisetiforinis
from Rovere di Velo, near Verona. Z>, inflores-
cence from Siberia, placed by Schmalhausen with
Phyllotheca. (After Solms.)
i68
STERILE AND FERTILE REGIONS
FIG. 89.
Onoclca Struthiopteris, Hoffm. a, fructifying plant, much reduced, with most of the
sterile leaves removed ; c, d, lower and upper portions of a sterile leaf; e, lower part of a
fertile leaf, b, c, two-thirds natural size. (From Rab. Krypt. Flora.)
THE "SELAGO" CONDITION IN FERNS 169
Fern (Onoclea Struthiopteris) (Fig. 89). There may be some degree of
regularity in the succession of sterile and fertile leaves, which may be
correlated with season ; thus in Blechnum boreale the leaves first expanded
in the spring are sterile, and they are followed by a series of fertile leaves.
The condition of the shoot as a whole is, in fact, comparable with that of
Lycopodium Selago or of Isoetes, with their successive sterile and fertile zones.
But the commoner case for Ferns is that where leaves are not sterile or
fertile as a whole, but many or even all the leaves of the mature plant are
fertile at least in part, and frequently show a correlative reduction of area
as compared with the rest of the leaf, which is sterile. In the distribution
of the fertile and sterile parts of the individual leaf there is great diversity,
and differences may be seen in species of the same genus, or even in
individuals ; thus in Osmunda regalis the lower parts of the fertile leaf
are broadly expanded and sterile, the apical region is fertile and correlatively
exiguous ; but in O. javanica the fertile region extends irregularly over the
lower pinnae, and the apical region is expanded and sterile (Fig. 90).
It has been shown by Goebel that the mode of development of such Fern-
leaves may be experimentally altered : by removing from a plant of Onoclea
Struthiopteris the foliage leaves which are first expanded in the spring,
the later expanded leaves, which are normally sporophylls, were induced
to assume the character of foliage leaves. Similar results were also obtained
by Atkinson.
The facts thus briefly summarised for Ferns are evidently comparable
with those noted for the Lycopods, and the differences in detail which exist
have their relation to the megaphyllous character. But in Ferns the facts
are less cogent; for though abortive sporangia and imperfect sori are at
times found on Fern-leaves, still the evidence that they are vestigial is less
clear than in Lycopodium, Isoetes, or Ophioglossum, owing to the less
definite position and number of those parts in Ferns. The conclusion that
the foliage leaves or parts of leaves in Ferns are phylogenetically sterilised
sporophylls, or parts of sporophylls, is therefore based rather on broad
comparisons and on analogies with other Pteridophytes than on the direct
observation of parts which may be held to be vestigial. That such a
transmutation may take place in the individual life is fully demonstrated
by the experiments of Goebel above quoted. It seems therefore reason-
able to hold for Ferns, as for other Pteridophytes, that sterilisation of
sporophylls has been effective in the course of their evolution.
A converse view to that thus stated has been habitual in the past,
and is maintained by some to the present time. By them the evolutionary
history is read in direct terms of the ontogeny, and the sterile leaf is thus
assumed to be the primitive leaf, which has become a sporophyll by the
superposition upon it of sori and sporangia. Those who take this point
of view have brought forward in its support the facts that the develop-
ment and structure of the sterile and fertile leaves is closely alike, and
that intermediate forms exist frequently between the two, so that the
170 STERILE AND FERTILE REGIONS
distinction is very perfectly bridged over. But I submit that the most
exact demonstration of similarity in detail of development, and the quotation
FIG. 90.
Osmnncia, L. A=O. Presliana, I. Small leaf. B and C = O. Regalis, L.
plant. C = leaf of a mature plant. (After Engler and Prantl.)
of an infinity of middle-forms drawn from the most varied types of Ferns,
does not touch the question of phylogenetic priority. Such facts are the
THEIR GENETIC RELATIONS 171
• N.
necessary basis either for the older view, that the sporophylls are altered
foliage leaves, or for the view that the foliage leaves are sterilised sporo
phylls : but they do not tell distinctively for either. The decision must
rest primarily upon the presence of vestigial sporangia, together with broad
comparison rather than upon details of individual development. Still it
is necessary that any final conclusion should be in accord with the details
of the individual development, and this is so in the present case, whichever
of the alternative conclusions be adopted.
Finally, the interesting demonstration by Goebel, that the sporophyll
may be experimentally converted into a foliage leaf, does not serve as
a decisive proof of either view. It demonstrates, however, the close relation
of the two which either hypothesis will demand. It shows also that
sterilisation of a sporophyll such as our hypothesis requires can actually
occur. Such a process of sterilisation, carried out continuously in the
course of descent, and involving either whole leaves or only parts of them,
would result in the differentiated character of the leaves of Ferns which
is actually seen in nature.
The leading types of Pteridophytes have thus been reviewed as regards
the relations of their sterile and fertile regions. In the individual life
of them all, there is at first, as their physiological condition demands, a
more or less extensive vegetative phase, succeeded sooner or later by
a fertile phase, though this is often not clearly 'differentiated from it. By
comparison, it may be concluded that vegetative leaves have been derived
by sterilisation from sporophylls ; and it is not difficult to realise how a
vegetative system may thus have been increased, and the production of
spores have been delayed in the individual life.
On the other hand, the unlimited apical growth seen in many of the
Pteridophytes, acts as a set off against the progressive sterilisation, for
it tends to preserve the balance of the sterile and fertile regions which
the sterilisation would disturb, and still provides for the initiation of an
adequate number of spores. In the simpler strobiloid forms, such as
L. Setago, it is easy to conceive how progressive sterilisation and continued
apical growth combined would lead to a larger vegetative system and
an increased final output ofyspores. 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 -jt , 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, r—ihe third
tangential wall. In A the figures I. -XVI. denote the successive segments. 0 = 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. 2L.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 they 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
™LinT^r^of^ leaf-gaps and all round inside of the
-vIughan^06 the °bserver' (After xylem-ring. Below the insertion of
each leaf a large leaf-gap occurs in the
solenostele (Fig. 95) : in transverse section at such a point the stele will
appear as an incomplete ring. If, then, the arrangement of the leaves be
a close one, two or more of these gaps would occur in a single transverse
section, and the result would be an appearance as of several steles arranged
in a ring. These originate, however, not by branching, as Van Tieghem
thought, but by resolution of the stele, first into a cylinder and then into
a cylindrical network. The result of this mode of amplification would,
therefore, be more correctly styled "a dictyostele " rather than a polystelic
state, and the parts " meristeles " rather than a plurality of distinct steles,
since the whole is a result of amplification, not of branching, of the
original monostele.
But polystely is also found in the genus Selaginella. There is little
doubt that the monostelic type is the original one for this genus also,
since it exists in many species. The origin of the more complex state
is, however, connected rather with the branching of the axis than with
the insertion of the minute leaves. It has certainly taken place within
the genus, but the comparative study of the illustrative species from the
STELAR THEORY
FIG. 96.
A, transverse section of the stem of Equisetum palustre (Xa6), and B, part of it
X 160. C, transverse section of the rhizome of Equis. sylyaticum (X26), and D,
part of it X 160. E, transverse section of the rhizome of Equis. litorale (X26), and F,
part of it X 160. cc — central cavity. r'=valleculai canals. £ = carinal canals. j = sheath
of separate strands, as — outer, is = inner general endodermis : in A, C, and E the endo-
dermis is indicated by a dotted line. (After Pfitzer.) From Rab. Krypt. Flora.
192 ANATOMICAL EVIDENCE
developmental point of view is still required before a complete elucidation
is possible. The case for Medullosa must also remain for the present
unexplained ; but at least it seems almost certain that it is not a polystelic
condition arising from overlapping of leaf-gaps, as in Ferns.
Among Phanerogams the polystely in Auricula has been investigated
by Gwynne-Vaughan, and the origin of it is again by a resolution of a
primitive monostele. Perhaps the same may be the case for the more
complex condition of Gunnera, but that is still uncertain. Of these
isolated cases of polystely which exist among the vast majority of mono-
stelic forms the Ferns are the most important : and as their case has been
shown to be a result of amplification of the monostele, the existence of
occasional exceptions elsewhere, not yet» fully explained, cannot be held as
a valid objection to the acceptance of the monostele as the fundamental
type of structure in the sporophyte at large.
Another mode of amplification of the protostele is exemplified by
Lycopods, and possibly occurs also elsewhere : it is by the progressive
conversion of a central tract of the xylem-core into parenchymatous tissue
of the nature of a pith. This is probably related to another modification
of the stelar structure found in stems, viz. that designated the schizostelic,
or by some the astelic, state. Here the several vascular strands are not
collectively surrounded by an endodermal ring, but are independent, and
may have a special endodermal sheath surrounding each. This structure
is found in some species of Equisetum and in certain Phanerogams. It
seems to be generally admitted that in the stem this condition is derivative
from the ordinary monostelic state, a conclusion which would naturally
follow in the case of the genus Equisetum from a comparison of its
different species (Fig. 96). If this be so, then both the marked excep-
tions in vascular- structure in the stem are referable in origin to the usual
monostele, and the conclusion seems justified that in the axis of Vascular
Plants there is only one fundamental stelar type, and that is the mono-
stelic type. The morphological importance of any character is held to
be in accordance with its constancy in a large series of allied organisms :
the general occurrence of a monostelic structure, or of arrangements
derivative from it in Vascular Plants at large gives the monostele
place in the first rank as an internal morphological feature of the axis.
The prevalent bifacial character of the leaf is apparent in the simplest
forms of Vascular Plants, where its comparatively small expanse is traversed
by a single vascular strand. This structure is found in such primitive types
as the Lycopods, Equiseta, Isoetes, etc. Where the leaf is larger the
vascular system is expanded in various ways : numerous strands may traverse
it, diverging from one another towards the margin, but converging towards
the base, where, with or without fusions, they may form a curved series
as seen in transverse section (Fig. 97). The orientation with the proto-
xylem tending adaxially is a constant feature. Each strand is surrounded
by a definite sheath throughout its individual course, but on fusion two
FORM OF LEAF-TRACE 193
or more may pass within a common sheath : a flattened vascular plate,
or it may be a curved series of strands, is thus produced. In some leaves
of Ferns and in some Dicotyledons the vascular tis.sue thus disposed in a
curve as seen in transverse section, with its concave surface upwards, may
show a closing in of the lateral margins as the leaf-base is approached; it
seems not improbable that this is connected with mechanical requirements
consequent on the leverage of. a large leaf on its base. This closing
in may even be carried so far that the two edges may become contiguous,
and the result will be a structure not unlike that of a cylindrical stele of the
axis (Fig. 98). But it would be a mistake, on the mere ground of such
structure and without the check of comparison, to suggest any close identity
of character of such " pseudosteles " with the stelar condition of the axis.
There is reason to believe that these pseudosteles of the leaf are secondary
FIG. 97.
Dicksonia Barontetz. Portion of the vascular system of the stem, seen from within,
and showing the departure of three leaf-traces. (After Gwynne-Vaughan.)
in their origin, for it is the fact that they are characteristic of leaves of
relatively large size, while smaller leaves are typically dorsiventral in their
vascular structure. Moreover, Professor Bertrand and others have been
able to show by exact comparative analysis that even in very aberrant
cases of Fern-petioles the pseudostelic structure is referable still to a dorsi-
ventral origin, and is to be explained as the result of complex foldings
and fusions of a band-like vascular tract. A somewhat similar explanation
may be given of the "pseudostelic" petioles of such Dicotyledons as are
quoted by Schoute (I.e., p 158) and of the "polystelic" petiole of Primula
Auricula examined by Gwynne-Vaughan ; these may be held to be secondary
modifications of a structure originally dorsiventral, and the position may
accordingly be summed up as follows : The construction of the axis is
essentially cylindrical, and finds its anatomical expression in the cylindrical
stele ; the construction of the leaf is essentially dorsiventral, and it finds
its anatomical expression in the isolated vascular strands disposed dorsi-
ventrally. Both these are liable to modification in special cases, thus by
N
194 ANATOMICAL EVIDENCE
breaking up of the stele in certain axes a schizostelic state with individual
strands may be attained ; but comparison shows that these are not the
phylogenetic equivalents of the individual strands of the simple leaf, though
they may be continued outwards into the leaves : conversely, in the pro-
gressively developing leaf, a pseudostelic structure may be produced by
fusion of strands phylogenetically distinct ; but again comparison shows that
this is not the phylogenetic equivalent of the primitive stele of the axis,
but a condition secondarily derived.
FIG. 98.
Transverse section of base of petiole of Gleichenia dicarpa, showing a pseudo-stelar
structure resulting from contraction of horse-shoe-like xylem till the margins fuse. Photo-
graph by R. Kidston from section by Gwynne-Vaughan.
It is necessary thus to differentiate characters which are primary from
those which are secondary. It has long been recognised that the distinction
cannot always be maintained between axis and leaf on the basis of strict
criteria of form or structure : exceptions can be found to all morphological
criteria proposed. Still, if on a basis of comparison the primary and
secondary conditions be clearly kept apart, the divergent anatomical
characteristics of the two parts become sufficiently obvious. Accordingly,
in our view the structure of the leaf is recognised as primarily astelic
throughout, that is, the isolated strands are not to be held as results of
resolution of a primitive stele ; where an apparently stelar structure appears
"CAULINE" AND "COMMON" VASCULAR STRANDS 195
in the leaf it would at best be only a pseudostele, secondary in origin, and
thus phylogenetically distinct from the stele of the axis. The primary
structure of the axis is monostelic: where isolated strands occur in the
axis, each with its sheath is a schizostele, a result of secondary segregation
of the component tissues of the stele.
In this connection it is important to recall the old distinction between
"common" and "cauline " vascular 'bundles. In the former the lower
part of the course of the individual strand is in the axis, the upper extends
into the leaf: in the case of the tissues which may be styled cauline, the
course is within the stem throughout. From a theoretical point of view
the existence of cauline vascular tracts is important, for it accentuates
the axis as something more than a mere basis for insertion of leaves.
The further fact that the axial stele may be followed beyond the youngest
leaf-traces shows that the vascular system of the axis has an objective
existence independently of the leaf-traces, however closely it may be
connected with them in ordinary cases. These cauline extensions are
prevalent in early Pteridophytes, such as Lycopods, Psilotaceae, and Ferns ;
this fact must necessarily be of special interest in connection with any
theory of the origin of foliar developments in Vascular Plants.
It is evident that the existence of a cauline stele bears directly
towards a strobiloid theory of the shoot. This suggests the question
whether any existing group of plants show a nascent condition of the
vascular system of the shoot such as a strobiloid theory would demand,
viz. a columnar conducting stele, with no appendages, or with appendages
anatomically accessory to rather than formative of the central stelar column.
In a paper on the conducting tissue-system in Bryophyta, Tansley has shown
that such a structure is found in the more complex Mosses.1 In discussing
the points brought forward he very properly disavows at the outset any strict
homology with Vascular Plants, remarking that it is almost as certain as
any phylogenetic thesis is likely to be that the conducting tissues of Bryo-
phytes have nothing directly to do with the origin of the conducting
tissues of the higher plants. The main seat of the development of these
tissues in Bryophytes is the gametophyte generation, which is in any case
excluded from the comparison, since the vascular system in Pteridophytes
is confined to the sporophyte. And at the least it is extremely unlikely
that the Pteridophytes have been derived from a Bryophytic ancestor with
a sporophyte showing anything approaching the specialisation of the rnoss-
sporogonium, in which conducting tissues also occur. But it must not for
this reason be supposed that the Bryophytes are of no interest in consider-
ing the problem of the evolution of the vascular system in Pteridophytes.
1 Ann. of Botany, xv., 1901, p. 2. For Mr. Tansley's later views on this and kindred
subjects, especially as affecting the question of origin of the shoot in the Filicales,
reference should be made to his Lectures (New Phytologist, 1907). This chapter was
in type before Mr. Tansley's lectures were given. The opinions here expressed may have
to be modified.
196 ANATOMICAL EVIDENCE
We see among the former group plants in the very act, so to speak, of
developing a conducting system in response to vital needs, and others in
the most various stages of its evolution in complexity. The conditions
under which this evolutionary development occurred must have been
practically identical with those to which the primitive Pteridophytic
sporophyte was subjected, — gradually increasing adaptation of a simple leafy
form to terrestrial life. And the final result, as seen in the highest Poly-
trichaceae, is so strikingly like the state of things obtaining in the true
vascular plant as to furnish probably one of the completest and most
interesting cases of homoplastic development in the plant-kingdom. It can
hardly, therefore, be denied that the study of the conducting system in
Mosses is calculated to throw most valuable side-lights on the question
of the evolution of the vascular systems of the higher plants.
As the result of his careful analysis of the tissues, Tansley1 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. Jeffrey1 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 Goebel1 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 opinion2 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. 2Z.r., p. 132.
3L.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. Goebel1 has
pointed out how Vaccinium Myrtillus shows in its lateral shoots a
transitional state between these two cases ; for in the lateral buds of this
plant there is an influence exercised, probably by light, which leads
to a distichous arrangement of the leaves; but it does not take place
in all buds alike : in some the effect is only a secondary one, acting
upon the leaves which originate in a spiral succession : in others the
effect is primary, acting upon the vegetative point itself, on which
accordingly the leaves arise. The existence of such gradations of effect,
between dorsiventrality which is the result of immediate impress of outer
influences and that which is an hereditary condition, is important as
suggesting how the more fixed dorsiventrality may have come into
existence. The comparison of such cases, and of the vegetative system
at large in a number of allied plants, leads to the conviction that in the
vegetative shoot as well as in the strobilus the radial was the primary
type, and the dorsiventral the derivative. The causes are probably the
same in both cases. It is, however, essential to note that the vegetative
region is more liable to be influenced by them than the fertile; for it
has been seen in many species of Selaginella and of Lycopodium that
the vegetative shoot is dorsiventral, while the strobilus is radially con-
structed. The same is the case with many of the Coniferae. Such
examples indicate that the strobilus is more conservative of form than
the vegetative shoot. It is true the converse may be found in some
of the higher Flowering 'Plants ; for instance, in the Labiatae the vegeta-
tive shoot is commonly radial, while the flowers are dorsiventral. But
this condition of the flower is probably one of the relatively late
specialisation.
Examining more particularly the vegetative region of the Pteridophytes,
the radial type of shoot is found with high constancy in the Equisetales,
both fossil and modern. Also in the ancient Sphenophyllales and the
modern Psilotaceae : the only exception in the latter being Psilotum
complanatum, with its bilateral symmetry already mentioned. Of the
Lycopodiales the early fossil types were characteristically radial in
1 Orgauography, Eng. ed., vol. i., p. 94.
O
210
SYMMETRY OF THE SPOROPHYTE
construction, though possibly dorsiventrality may have existed among the
smaller forms. Of modern Lycopods, the mature shoots of Lycopodium
show in relation to their position the most gradual transitions from the
radial to the dorsiventral. The species of the sub-genus Selago, and
Sub-Selago maintain the radial construction, and are for the most part of
upright habit. The rest of the genus is very variable : the shoot is some-
times radial, as in L. i?mndatum : or distinctly distichous forms may occur,
such as /. Phlegmaria and nummularifolium, which are both pendulous
A.
B.
FIG. 105.
A, young sporophyte of Danaea simplicifolia still attached to the gametophyte, pr.
X 3. B, an older sporophyte of the same species. C, gametophyte of A ngiofiteris evecta
with young sporophyte. (A , B, after Brebner ; C, after Farmer, from Campbell's Mosses
and Ferns.)
epiphytes. The change from the radial type may be apparent first in a
slight inequality of direction of the leaves, otherwise equal, as in the
creeping shoots of L. annotinum or davatum : or in the marked inequality
of their size and structure, as in L. complanatum or alpinum. Goebel
has shown a by experiment on L. complanatum that the dorsiventrality
is directly induced by light. Finally, the climbing species, /. volubile,
is specially characterised by a distichous form of the shoot not unlike
that usual in Selaginella. It has already been concluded on other
grounds that the Selago type of Lycopodium is the most primitive : it
is this same type which retains most constantly that radial construction
which there is reason to believe is a primitive character.
1 Organography , vol. i., p. 252.
IN PTERIDOPHYTES
2 I I
The genus Selaginella includes eight species which are isophyllous, and
over three hundred which are anisophyllous in the vegetative region. An
intermediate condition is found in S. sanguinoletita, which has accordingly
been investigated by Goebel.1 He found the dorsiventral structure to
arise under the influence of external factors, such as moisture and shade.
In certain strong-growing species, such as 6". caulescens, the upright axes
may be isophyllous, and the dorsiventrality only become pronounced in
FIG. 106.
A small plant of Dp.na.ea alata. x£. .r/^stipules. (After Campbell.)
the plagiotropic upper branches. Others may be dorsiventral from the
first. Thus the genus illustrates gradual steps of dorsiventral development,
such as have been noted in Lycopodium and Vacrinium, though the details
are different. And the same conclusion may be drawn as in those cases,
viz. that the radial is the primitive form, and the dorsiventral the derivative.
The Fern-shoot, notwithstanding the preponderance of its leaf-develop-
ment, may be examined from the same point of view as other Pteridophytes.
The ancient Marattiaceae illustrate a probable initiation of dorsiventrality
of the shoot within their own phylum. The young sporophyte seedling
1L.f.t p. 105.
2 I 2
SYMMETRY OF THE SPOROPHYTE
I
FIG. 107.
Diagrammatic representation of the end of a
rhizome of Kaulfussia. iv — wings of stipule;
com — transverse commissure. (After
Vaughan. )
Gvvynne-
in all the genera of Marattiaceae investigated hitherto is upright, bursting
through the prothallus, not recumbent as in other Ferns (Fig. 105). In
Angiopteris and Marattia this con-
dition is maintained throughout life,
and there is no reason to think other-
wise than that these plants retain
their primitive position. It is probably
shared also by Archangiopteris ; a at
all events there was no dorsiventrality
in the only specimen with an axis
hitherto examined. In the genus
Danaea the same holds for D.
simplidfolia ; but certain other species
of Danaea have an oblique rhizome,
for instance, Danaea alata (Fig. 106).
Comparison of a number of stocks of
this Fern shows various degrees of
inclination and curvature of the axis. It is upright at first, and produces
leaves and roots uniformly on both sides of the axis; but later the axis
arches over to one side, and a distichous arrangement of the leaves is
approached, while the roots originate chiefly from
the side directed downwards. In Kaulfussia these
characters are more pronounced ; for there the
mature rhizome is horizontal, with marked dorsi-
ventrality, and with internodes of appreciable
length (Fig. 107). Unfortunately the early
development of the sporophyte of Kaulfussia is
still unknown. It seems a reasonable interpre-
tation of the facts that the upright position, with
radial symmetry, as seen in Angiopteris and
Marattia, was the primitive condition here as in
other Vascular Plants : and that the oblique
position, already seen in certain Danaeas, became
more accentuated in the horizontal rhizome of
Kaulfussia, with its marked dorsiventrality.
The analogy with what is seen in the Ophio-
glossaceae greatly strengthens this conclusion. As
in the Marattiaceae, so also in all the genera of
the Ophioglossaceae the axis is from the first
upright (Fig. 108); and that position is maintained
throughout life in Ophioglossum and Botrychium. But in Helminthostachys,
notwithstanding its originally vertical position, the axis of the mature
plant is markedly dorsiventral (Fig. 109), with distichous arrangement
of the leaves. The conclusion which naturally follows is that in the
1 Gwynne-Vaughan, Ann. Bof., xix., p. 260.
FIG. 108.
Helm in th ostachys zeyla nica.
Young plant attached to pro-
thallus. Natural sixe. (After
Lang.)
IN PTERIDOPHYTES
213
Ophioglossaceae, as in the Marattiaceae, the upright radial shoot is
primitive, and the dorsiventral character of the shoot derivative : that in
Helminthostachys, as in Danaea and Kaulfussia, all of them heavily-leaved
forms in proportion to their relatively elongated stem, the axis has become
horizontal.
The Leptosporangiate Ferns present a more varied and difficult problem as
regards the symmetry of the shoot. They include many upright radial forms,
such as Cyathea, Alsophila, or Osmunda • as well as many which show various
degrees of obliquity of the axis, accompanied by corresponding degrees
of dorsiventrality. It is not an uncommon thing to find upright radial
FIG. 109.
Naked-eye drawing of rhizome of Helminthostachys zeylanica. /r=stipular flap ; R =root ;
L — leaf; /> = petiole; L S— leaf-scar. (After Farmer and Freeman.)
species, and straggling, dorsiventral species in the same genus : for instance,
Onoclea germanica is radial and upright, Onodea sensibilis is creeping ;
Pteris aqiiilina is creeping, while many other species are tufted and radial.
The same question will arise here as elsewhere, which of the two was
the primary condition. From analogy with other cases as well as from
internal evidence, the upright, tufted forms with radial symmetry would
be held to be the prior type ; but the question is complicated by the fact
that the embryo itself is not upright in the Leptosporangiate Ferns, and
it may be thought that its recumbent position defines ab initio the dorsi-
ventrality of the shoot.
It is true that the first leaf of the young sporophyte is uniformly on
the side away from the prothallus: but the position of the second and
2i4 SYMMETRY OF THE SPOROPHYTE
succeeding leaves may vary, as was already noted by Hofmeister. He
stated specifically1 that "the similarity in the development of the different
species of Ferns does not extend beyond the formation of the rudiments
of the first frond, and of the first root." He then proceeded to show
that whereas in Pteris aquilina the second frond originates on the side
of the axis opposite to the first, and distant from it by about half the
circumference of the stem, in Nephrodium (Aspidium) Filix mas it is
at a distance of about a third of the circumference 2 : " the third diverges
from the second, and the fourth again from the third at about 120
degrees to the right, so that the fourth stands vertically over the first."
Thus the arrangement of the leaves is radial from the first in the Male
Fern. The same radial character, with varying divergences, has been
found in other Fern-seedlings : for
instance, in Nephrodium dilatatum,
Asplenium marinum, Pteris tremula,
Osmunda regalis, and Todea superba.
The leaf-divergence in such cases is
variable, but approximates to a half,
or one-third, and it is spiral from the
first, without dorsiventral tendency.
This shows that in plants which are
radial in the mature state the initial
condition of the recumbent embryo
does not impress dorsiventrality' on the
seedling. In others, however, where the
Polypodiuni vulgare. X6. Median section • i ji i
through prothaiius, and embryo, partly diagram- mature plant is markedly dorsiventral,
matic : showing one series only of the distichous .-> i • •,-. • , i/- i
leaves A, 4 etc.; *= roots ;«/= apex of axis, the dorsiventrality asserts itself early.
^^^t^JK^tS^SSuL" Among these the case of Polypodium
vulgare is very instructive, as showing
that the dorsiventrality of the mature shoot is not a direct consequence
of the initial procumbent position. The initial embryogeny is as usual :
the second leaf of the embryo is obliquely on the same side of the
axis as the first, and as the subsequent leaves are also inserted alternately
and obliquely on that same side, and the growth of the axis is stronger
on that same side which faces away from the prothaiius, the whole
shoot becomes inverted by a strong curvature upwards through the
apical indentation of the prothaiius ; and thereafter it pursues its horizontal
course backwards over the top of the prothaiius (Fig. 1 1 o). It is interest-
ing to note as a consequence of this that the side of the axis initially
turned downwards in the embryo becomes the upper side of the dorsi-
ventral rhizome, owing to the inverting curve : thus the creeping posi-
tion of the permanent shoot is not merely a direct continuance of the
initial prone position of the embryo. Other examples might be quoted,
but this will suffice to show that the dorsiventrality of the mature shoot is
^Higher Cryptogramia^ Eng. ed., p. 208. -/.<;, p. 227.
IX PTERIDOPHYTES 215
not a necessary or a direct Consequence of the recumbent position of
the embryo of Leptosporangiate Ferns. It may also be added that
the dorsiventrality, in cases where it exists, may be initiated in different
ways. Such evidence points to its being a secondary condition.
A further indication that the shoot of Leptosporangiate Ferns is primarily
radial is to be seen in the internal structure of the axis. It has been
found in a considerable number of cases that the vascular tissue is
uniformly developed all round. This is naturally the case in upright
radial axes : but, apart from the leaf-insertions, it may even be so in
stems which are horizontal, such as Matonia : though in others, such
P"
FIG. in.
Transverse section of axis of seedling of Lygodiumjaponicunt^ below the first leaf. / — one
of the xylem-parenchyma cells. X^go. (After Boodle.)
as Pteris aquilina, the vascular system is like the stem itself dorsiventral.
In young seedlings it has been shown in various cases that the stele
is cylindrical, and it is found to be so even in Ferns which are markedly
dorsiventral in the mature state, such as Lygodium japonicum (Fig. in).
Such examples indicate again a probability that the radial construction
of the shoot was primitive in the sporophyte of Ferns.
But it may be urged by those who dissent from this conclusion that
dorsiventrality is clearly seen in the early embryonic stages of some other
Pteridophytes, and especially in the case of various species of Lycopodium.
But here also it seems probable that the condition is adaptive rather
than primitive : for in the first place the embryo in the genus is singularly
inconstant in its form : in some species, such as Z. Selago, or Z. davatum,
2i6 SYMMETRY OF THE SPOROPHYTE
the embryo settles down at once into an upright radial type of structure :
in others, and particularly in L. cernuum, which has been made the subject
of special study and comparison, the embryo may show at first a marked
dorsiventrality ; but it is at the same time exceedingly variable in form,
and in some individual cases the embryo of L. cernuum may closely
resemble the ordinary radial type of other species. This variability will
in itself discount arguments based upon details of form, and suggests
that the dorsiventrality where it occurs is the result of relatively direct
adaptability of a very plastic organism.1
The facts and arguments brought forward in this chapter lead up to
a general view of the symmetry of the sporophyte generation. It would
appear probable that the original type of its construction has been radial
throughout, a condition which commonly goes along with a vertically
upright position. This is the position of the vast majority of Bryophyte
sporogonia : in them the radial construction is rarely departed from, and
where this does happen the dorsiventrality is readily referable to a
modification of a radial type. The greater diversity of habit of the
Pteridophytes, especially as regards the sporophyte, necessarily brings
greater difficulties in attaining to any general opinion for them ; but a
careful review of their various types, and especially a comparison of
members of the same group of them inter se, leads back constantly to
the radial type as primitive, even in cases where dorsiventrality is most
marked. The fact that in the Equisetales and Sphenophyllales the radial
construction is predominant, while it is also prevalent among the more
primitive Lycopodiales, and in a less degree in the Filicales, shows the
strong hold which the radial construction had among very early types.
In fact the position is fully strong enough to justify the general state-
ment that the radial mode of construction was primitive for the sporophyte
at large ; and that where dorsiventrality occurs, it is a secondary
condition.
This conclusion is plainly out of harmony with the theoretical posi-
tion of Lignier,2 who would refer the sporophyte as well as the gametophyte
to a hypothetical thalloid origin : this thallus, which was dichotomous, and
lay flat upon the soil, tended to curve upwards, and consequently to
1 The more exact comparison of the embryology in the genus Lycopodiuin will be
taken up in the special part of this work.
2 " Equisetales et Sphenophyllales. Leur origine filicineenne commune." Bull. Soc.
Linn, de Nonnandie, 1903, p. 93. A somewhat similar speculation has recently been
published by Tansley (New Fhytologist, 1907, p. 25) ; he refers the Archegoniatae in
origin to some " hypothetical Archegoniate Alga." He also passes lightly over the
transition from a sympodial rhizome to an upright, radially organised type (p. 33). It is
necessary, however, to remember that, as a matter of observation, all Archegoniate
sporophytes are initially of radial construction. The same difficulties appear to confront
both Tansley's and Lignier's hypotheses. To meet them both authors postulate hypo-
thetical forms which are "of course the purest speculation" (I.e., p. 32). It appears
preferable to adhere to observed facts.
IX PTERIDOPHYTES 217
assume a cylindrical symmetry. On this hypothesis the dorsiventral was
the prior state for both the sporophyte and the gametophyte, and the
radial the derivative. The author himself states that this " prohepatic "
type, from which the two generations were evolved, is still wholly hypo-
thetical. As regards the sporophyte, since the embryology gives no
countenance to an originally dorsiventral " prohepatic " state, while
instances are common of the impress of dorsiventrality upon parts of it
originally radial, the theory of Lignier cannot be upheld. It may apply
for the gametophyte, but that has nothing to do with our present dis-
cussion. It seems the unavoidable conclusion from the facts that the
primitive symmetry of the sporophyte was radial.
CHAPTER XVII.
THE ESTABLISHMENT OF A FREE-LIVING SPOROPHYTE.
So far the shoot only of the sporophyte has been the subject of discussion :
it remains to consider the question how the sporophyte, originally dependent
upon the parent prothallus, became established as a free-living organism
on the soil. There will be no two opinions which of the principal
regions of the independent sporophyte, the shoot or the root, was of
prior existence : it is a necessary outcome of the evolution of the neutral
generation as sketched above that the shoot was first established, as a
body dependent on the gametophyte ; it carried out primarily the function
of spore-production, but ultimately also, as we have seen, that of vegeta-
tive nutrition. The root is essentially an accessory, which made its
appearance after those earlier steps were past; it arose from its
primitive state of dependence to an existence free from the parent
gametophyte.
Comparison of living plants indicates, however, a probability that the
initiation of a root-system followed closely upon the adoption of a free-
living habit : for roots are present in free-living Pteridophytes with very
few exceptions, and are, as a rule, formed early in the embryology. It
seems doubtful, even in the few exceptional cases, whether the rootless
condition is not due to reduction, rather than representative of a primi-
tive rootless, but free-living sporophyte. Among the Pteridophytes roots
are absent in the Psilotaceae, also in certain Hymenophyllaceae, and in
Salvinia : it seems probable that reduction will correctly account for it in
such specialised forms as the Hymenophyllaceae ; and also in Salvinia,
with its peculiar floating habit : the question in the Psilotaceae is more
problematical, and their rootless condition may perhaps have been really
primitive, though in the absence of any knowledge of their embryos there
is no clear indication that it was so : moreover, their habit is so peculiar
as to make any conclusion difficult. Rootless Phanerogams also exist,
but there is no reason to regard them as other than results of relatively
recent reduction. Accordingly, it may be concluded that there is little
GENETIC RELATION OE AXIS AND ROOT 219
• X
evidence from plants of the present day of the existence of a primitive,
permanently free-living, but rootless state of the sporophyte.
The root in the fully-developed state is broadly different from the
axis : its endogenous origin, its root-cap, and the radial arrangement of its
vascular system are its most distinctive features, in addition to the absence
of appendicular organs, other than root-hairs, or lateral roots. Its full
character depends upon the collective existence of those features ; for some
of them are inconstant, and all of them may occasionally be matched by
axes : 1 thus the two parts are not absolutely distinct in character.
Sometimes, indeed, it may be found that roots grow on directly into normal
leafy shoots, as in certain Ferns, Aroids, and Orchids, etc. : '2 the con-
verse, however, has not yet been shown to occur.
The resolution of the problem what genetic relation, if any, subsisted
between axis and root will naturally be looked for in such plants as show
the least degree of differentiation of those parts. As such the living
Lycopods are pre-eminent, while their fossil relatives also show features
of importance for comparison. Like axes, the first roots may be exogenous,
as in certain Lycopod embryos, and in Phylloglossum : in the Lycopods
the roots show apical dichotomy as do their stems also, while the exarch
xylem and general disposition of the vascular tissues of the Lycopod stem
are points of similarity to root- structures which are not equalled in other
Vascular Plants. Finally, the Selaginellas present features of further
interest in their so-called " rhizophores," parts which occur in many, but
not in all species : they are exogenous in order, and capless : they branch
dichotomously, and upon them the roots with root-cap arise endogenously.
In structure they are usually like roots, but in some cases the rhizophore
has a structure resembling that of an axis : for instance in 6". Kraussiana
the protoxylem is central, and the whole arrangement very like that of the
stem in S. spinosa? Further, the rhizophores may be readily converted
in some species into leafy shoots, by suitable cultivation. Thus the
rhizophores do not show the full characters of roots or of axes, and the
question has long been debated whether or not they are truly of root-
nature. Some prefer to distinguish them by a special name, as "rhizo-
phores " : others describe , them merely as the aerial region of the root.
1 Exogenous roots are seen in Phylloglossum, and in the embryos of some species of
Lycopodiitin, as well as in some Phanerogams. Capless roots are known in Aesctilus, and
in some few others (Goebel, Organography, vol. ii., quote from Engl. ed., p. 267). On
the other hand, a protective cap has been observed on the apex of the axis in embryos
of Araucaria, and Cephalotaxus by Strasburger (Angiosp. und Gymnosp., Plates xix. to
xxi. ) : endogenous shoots are not common, but they occur occasionally, as in the flower-
buds of Pilostylcs, as well as on the emergence of shoot-buds adventitiously from roots
(Goebel, I.e., pp. 226, etc.). A radial disposition of the vascular tissue, i.e. with exarch
xylem, is characteristic of the axes of Lycopods, and of some others of the early types
of Pteridophytes.
"Goebel, Organography, vol. ii., p. 226.
3 Harvey Gibson, Ann. of Bot., 1894, PI. x., Fig. 39. Also 1902, PI. xx., Fig. 17.
22O
A FREE-LIVING SPOROPHYTE
For my own part, I am satisfied to regard them as belonging neither to
the category of stem nor of root, but as a result of development to meet
a certain need, and that the growth produced was not of either character
in phyletic origin.1
It is interesting to compare Selaginella • with the allied fossils, which
have as their underground system the enigmatical Stigmarian development
(Fig. 112). These underground parts of Lepidodendron and Sigillaria
present morphological questions somewhat similar to those of Selaginella :
the main Stigmarian trunks are not roots, for their anatomical structure
Fl(i. 112.
Ground plan of a Tree-stump with Stigmaria-trunks. One-sixtieth the natural size.
(After Potonie.)
is far removed from that of any known roots ; they are not typical rhizomes,
for the only appendages they are known to bear are the Stigmarian
rootlets, which are rightly so recognised from their anatomical features.
They may be best classed with the rhizophores of Selaginella, or more
especially with the basal knot on the hypocotyl of S. spinulosa (Fig. 113),
though the correspondence is far from being exact. These, the Stigmarian
trunks, and the curious processes in Pleuromoia (Fig. H4),2 may all be
held to be outgrowths which fall into no recognised category of parts, such
as stem, leaf, or root ; and they all serve the same purpose, of acting as
a basis of attachment for the roots themselves. The existence of such
bodies points to the Lycopodiales as presenting characters of peculiar
1 Cf. Goebel, Flora, 1905, p. 209.
Solms, Bot. Zeit., 1899, p. 227.
ORIGIN OF ROOT
22
interest in any discussion of the origin of a subterranean absorptive
system.1
But the presence of such " rhizophores " does not greatly assist the
solution of the problem of origin of the roots themselves. There is, in
fact, no sufficient or decisive evidence how the root came into existence
in Vascular Plants ; but on the facts as they stand two alternative opinions
are possible. Either that it resulted from the transformation of a leafy
shoot by loss of the appendages, followed by other special adaptations in
relation to its life, and to its absorptive function in the soil. Or that it
Fie. 113.
Plant of Stlagiitella spinulosa, with root system springing from swollen knot at base of
the upright hypocotyl. i^ natural size.
arose as a new type of haustorial outgrowth, not originally of shoot-nature ;
but nevertheless that in its first and less differentiated condition it
resembled the shoot from which it arose, in its structure, and in the
character of its branching. That those features which were helpful in its
absorptive and conducting functions were permanently maintained, and
they became distinctive characters of the differentiated root : other charac-
ters, such as the root-cap and endogenous branching, may have been added
in accordance with the underground habit. This latter view seems to me
the more probable alternative.
Applying it in the case of the Lycopodiales, the root at its inception
would, like the stem of these plants, be exogenous, with exarch xylem
1 Compare Goebel, Organography, vol. ii., p. 230.
222
A FREE-LIVING SPOROPHYTE
:
and dichotomous branching ; and these stem-like characters are actually
exemplified in the roots of living Lycopods ; but in most cases the
exogenous origin and dichotomous branching gave way to endogenous
origin and monopodial branching, both of which are more suitable for
parts which have to make their way through the soil. The exarch xylem
was, however, maintained ; and, being biologically convenient in absorptive
organs, it became a characteristic feature of the root for Vascular Plants
at large. Many of the primitive types of Vascular Plants had exarch
xylem in the stem ; and if in the same way their primitive roots resembled
their stems structurally, they
also would have exarch xylem.
On this hypothesis the roots
would appear to have retained
a structural character which
was represented in the early
structure of stems. In this
way the origin of roots may
be presented to the mind
without their being held to
have been actually the result
of transformation of a leafy
shoot itself, of which there is
no evidence from abortive
appendages. But as a matter
of fact, there is no certain
knowledge how the root
originated.
In most embryo sporo-
phytes of the present day a
root-development is initiated
before the need for it arises,
that is while the embryo is
still entirely dependent upon
the parent gametophyte. But it cannot be assumed that this was always
the case : indeed, it is thought by some that there is an inherent
probability that some intermediate condition may have preceded the
initial formation of the root in descent. Among the embryos of some
of the early types of Vascular Plants a condition has been found which
has been held to be primitive, and to illustrate how the transition to
a free-living condition of the sporophyte may have been effected : it is
seen in certain species of Lycopodium, L. cernuum, and L. inundatum,
in which the prothallus is green and subaerial, thereby suggesting a rela-
tively primitive condition as compared with other types of the genus. In
these the upper half of the embryo, owing to the rapid extension of the
massive foot, is soon extruded laterally from the prothallus (Fig. 115).
FIG. 114.
Pleuromoia Sternbergii. Swollen base of stem with root-
scars, and showing part of the aerial stem, with the epidermis
and leaf-scars on the right, and on the left the sub-epidermal
sculpture. (After Bischof, from Engler and Prantl.) Two-thirds
natural size.
THEORY OF THE PROTOCORM
223
The body of the embryo thus exposed bears the cotyledon, and a variable
number of leaves directed upwards, but it terminates downwards in a tuber-
like body provided with rhizoids (compare Fig. 21, p. 37). At first there
is no root, and in extreme examples the appearance of the first root may
be deferred for a considerable time ; but so soon as the normal aerial
shoot with leaves is defined, the first root soon penetrates the soil, and
FIG. 115.
Young embryo of Lycopodiitm cernuutn, beginning to project from the prothallus.
ar=archegonium ; s = suspensor ; cot = cotyledon ; ta£ = embryonic tubercle. X 300.
(After Treiib.)
establishes the plant in the usual way. The tuber which thus precedes
the establishment of the plant by means of a root was called by
Treub the "protocorm," and he regarded it as a rudimentary structure,
which was the phyletic forerunner of the leafy shoot as now seen
generally in Pteridophytes. It is represented, however, only in few cases,
and is not constant even in the genus Lycopodium : for instance, it
is absent in L. Se/ago, and also in L. Phlegmaria and L. clavatum.
In PhylloglossuiHi on the other hand, this type of development is not
only found in the primary embryology, but is repeated constantly in
224 A FREE-LIVING SPOROPHYTE
each season's growth, while the tuber itself is here greatly enlarged for
purposes of storage. The characteristic "protocorm" is absent from all
other Pteridophytes.
The question is, what is the true interpretation of these facts. Does
the protocorm really represent some condition which existed in the
phylogeny, intermediate between the fully-rooted sporophyte and that
more primitive state where it was fully dependent on the prothallus?
The first point which strikes attention is the way in which the transition
from dependence to independence of the sporophyte is actually carried
out in the plants which show this " protocorm " development : assuming
that there is some difficulty, nutritive or other, in formation of the root
itself, the case is quite adequately met by the tuberous development with
rhizoids, as a temporary shift. It seems not improbable that some such
difficulty should precede in descent the initiation of so important, and so
characteristic a body as the root. A second point, however, is that a
protocorm development is exceedingly limited in its distribution among
living plants : it is not constant even in the genus Lycopodium^ and
outside the Lycopodiales it is not characteristically developed in any
other of the early forms : this must be taken fully into consideration
before assigning to the " protocorm " any general phyletic significance.
But, on the other hand, it may be urged that the real importance of
the " protocorm " would exist only in those cases where either the
root-development has not yet been initiated in the race, or where its
late development in the individual is a matter of moment, on nutritive
or other grounds. Immediately any initial difficulty of development of
a root-system is surmounted in any line of descent, the " protocorm "
would be liable to be cut out of the ontogeny, as a cumbrous and
unnecessary stage. This would sufficiently account for the absence of a
" protocorm " in the great majority of Vascular Plants. But, again,
Goebel, in arguing against the general phyletic significance of a
" protocorm," has cited a number of cases of Phanerogamic Plants in
which, if the formation of the root is suppressed temporarily or entirely
in the seedling, a protocorm-like body is formed, which is anchored to
the substratum by hairs.1 He remarks that this appearance of a
protocorm in very different circles of affinity seems to him unfavourable
to the hypothesis of its having a phyletic significance, and he only sees
in it an organ which corresponds in its development, and especially in
its formation of roots, to an arrested hypocotylous segment : he suggests
that a suppression of the formation of the roots may have taken place
in Lycopodium, as also in the Orchideae, and that this was connected
with the prolonged development of the germ-plant in them : perhaps
also the symbiosis with fungi which takes place in these plants, may
have had its effect. On this view the " protocorm " would be secondary,
and it would not illustrate an archaic mode of establishment of the
1 Organography, vol. ii., Engl. ed., p. 232
THEORY OF THE PROTOCORM 225
X
sporophyte on the soil. Such a suggestion certainly accords readily with
the sporadic occurrence of the " protocorm."
It is difficult to arrive at a conclusive balance between such conflicting
facts and arguments as these. So far as any conclusion commends itself
to my mind it is as follows : A " protocorm " development may have
been an important phase in the establishment of certain Lycopod embryos,
in that it serves as a temporary substitute for a root-system delayed in
its development. But it seems unnecessary to take such cases as proto-
types for even the genus Lycopodium as a whole : since the Lycopod
embryo, while showing essential unity in its general plan, seems prone
to parenchymatous swelling. Two such swellings, somewhat similar in
structure but differing in place of origin and in function, are known, viz.,
the enlarged " foot " of L. davatum and annotinum, which originates
from the lower tier of cells of the embryo, and is intra-prothallial ; and
the " protocorm " of the cernum-type, which originates from the upper
tier of the embryo, and is extra-prothallial. They are both biologically
intelligible, for the former acts as an haustorium, the latter may be a
ready mode of fixation in the soil, and also a specialised place of storage.
A genus which shows two types of parenchymatous swellings in two
distinct types of embryo, while both are absent from other species of
the genus, cannot be expected to have ever had one of these as a
constant feature in its ancestry. This consideration makes me doubt
any general application of the theory of the " protocorm " even in the
genus Lycopodium. These parenchymatous swellings may be looked upon
as opportunist growths, rather than as persistent relics constant from a
remote ancestry. This view is greatly strengthened by the occurrence of
protocorm-like developments in isolated cases among the Angiosperms.
Phylloglossum with its large storage " protocorm " would then be the
extreme type of a line of embryological specialisation, not a form pre-
serving the primitive embryological characters of the whole race. On
such grounds, while not denying that a " protocorm " may have had a
certain importance in certain cases, the facts do not appear to justify
attaching to it any general significance.
From the above pages it will be plain that the origin of the free-
living habit of the sporophyte, and of its root-system is quite as obscure
as that of the leafy shoot itself. The important step from dependent to
free life was certainly taken at a period before the earliest fossil records of
Vascular Plants ; for all the best-known types of early fossil Pteridophytes
have roots assigned to them on secure grounds of observation : so
naturally the evidence from them does not lead to a solution of the
difficulty. On the basis of comparison, to which this question must
necessarily be relegated, no decisive help is forthcoming; the theory of
the protocorm, which at first sight seemed so full of promise, does not
give more than a suggestion how the transition from dependence to
independence may actually be carried out in certain cases, and among
p
226 A FREE-LIVING SPOROPHYTE
the Pteridophytes it is illustrated only within a strictly localised area of
affinity. The course of transition from the dependent embryo to the
rooted plant, as it is carried out in the individual life, may be held to
be the most reasonable guide to the same transition in the past. It is
seen to be occasionally through the intermediary of a protocorm, but
oftener without. It may be that this indicates correctly the actual course
which events took; and suggests that all vascular sporophytes did not
achieve their independence in the same way.
It is of course possible to take an entirely different view of the
relations of the two generations from that here presented, and to consider
the dependence of the sporophyte as being itself secondary, and the
haploid and diploid phases as having been originally as independent as
they are seen to be in Dictyota. In that case the problem would be
the converse : viz., to trace the origin of the dependent state of the
sporophyte. There is, however, no serious basis of fact or comparison
hitherto adduced, which can place this suggestion upon a footing of
reasonable probability : it will suffice here to have mentioned that the
suggestion has been made.
CHAPTER XVIII.
THE EVIDENCE FROM PALAEOPHYTOLOGY.
IT has been remarked above (Chapter I.) that the only direct and positive
clue to the sequence of appearance of Plant-Forms in past time upon the
earth is to be obtained from the study of fossils. Luminous facts derived
from them are beginning to shed a fresh and direct light upon problems
hitherto obscure ; and the last quarter of a century especially has shown
how greatly a knowledge of the fossil forms may advance the true per-
ception of affinities of certain groups of plants now living.
But the success which has already attended Palaeontological investi-
gation, and has led to such important results, must not be allowed to
disguise the limits which circumscribe this branch of enquiry : nor should
it unduly raise the hope that the area of fact available for comparison
with forms now living will be indefinitely extended. It can hardly be
anticipated that data derived from fossils will ever take a decisive place
in discussions of the primary origin of the sporophyte. In the mind
of the Morphologist there can be no spirit of depreciation of the recent
advances of Palaeophytology, but rather a very high estimate of their
value : nevertheless he cannot help recognising how inadequate the
evidence drawn from fossils is in its bearing on such questions as those
discussed in the foregoing chapters. Hitherto it has given no clue
whatever to the originx 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.
238
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
258
BRYOPHYTA
spores so that at maturity they are not to be recognised. A similar fate
is described for the cells of the inner layer into which the archegonial
wall has meanwhile divided; and thus there is a supply of nutritive
material to the developing spores, comparable to that from the tapetum in
the higher Archegoniatae. The cells that lie within become rounded off,
and thus separate from one another in the enlarging fruit (Fig. 120): all
of them undergo the tetrad-division, and develop into spores, which have
V.
FIG.
A — archegonium of Riccia trichocarpa showing ventral canal cell (v) and ovum. X 525.
Z> = ripe archegonium of Riccia glauca. X26o. (After Campbell.)
a thick, darkly coloured outer wall. |These are set free by the dis-
organisation of the archegonial wall, or calyptra, the outer layer of which
persists till the spores are ripe. Under suitable conditions the spores
germinate, and each forms a new gametophyte.
Hitherto no observations have been described as to the reduction of
chromosomes in the tetrad-division of Riccia • but it may be assumed from
analogy with other Liverworts that it takes place here also. It is, however,
recorded by Garber l that the number of chromosomes in the gametophyte
of Ricciocarpus natans is four, while in the sporophyte it is eight ; but
the actual fact of reduction in the tetrad-division was not observed.
The sporophyte of Riccia thus described is the simplest sporophyte
known among the Archegoniatae. It has been habitually regarded as a
1 Bot. Gaz., 1904, p. 171.
MARCHANTIALES
259
primitive type, though the suggestion has also been made that it may
really be reduced; but in view of the fact that the gametophyte in the
Ricciaceae is a well-developed structure, amply capable of nourishing not
only one but many such sporogonia, there appears no immediate reason to
hold that this sporophyte is other than primitive in its simplicity. The
points of special interest in it for the purposes of comparison with the
more complex Archegoniatae are these : that it shows no distinction of
apex and base : . that the whole central mass of cells is fertile, each cell
producing spores, while none are diverted to purposes of nutrition or of
FIG. 120.
Ricciocarpus natans. The upper figure
FIG. IIQ. shows the spherical spore-mother-cells
surrounded by nutritive material. The
Ricciocarpus natans. Young sporogonia in longitudinal section, lower shows the tetrads formed from
surrounded by the archegonial wall. The younger (x666) shows them: the sporogonial wall (shaded) is
the amphithecium (shaded) surrounding the sporogenous cells : in still seen surrounding them, and covered
the older (Xs6o) these are separated, as the free, and rounded externally by the archegonial wall of two
spore-mother-cells. (After Garber.) cell-layers. X666. (After Gar ber.)
dispersal : and that the superficial cells forming the wall are segmented off
by periclinal walls of ' relatively late origin, indicating some relatively
recent differentiation of them from the cells which lie within.
A reasonable theory of the phyletic origin of a simple sporogonium,
such as that of Riccia founded on these facts, would then be, that it
sprang from the simple zygote, as in point of fact all normal sporophytes
do. The simplest possible case of a sporophyte would be that the
chromosome-reduction which follows on fertilisation should take place on
the first segmentation of the zygote, and in certain Algae this appears
actually to occur (Chapter V.). But in the sporogonium of Riccia the
reduction which accompanies tetrad-division is held over till a limited
number of segmentations of the zygote have been completed : this suggests
26o BRYOPHYTA
that the event of reduction was deferred in the course of its descent. The
cell-mass thus produced in Riccia is at first homogeneous, as was probably
the case definitively in certain of its ancestry. Differentiation comes
later in the sporogonium of Riccia, as it probably did also in the race :
in place of every cell being equally liable to the tetrad-division, this is
carried out only by those which lie internally : those forming the
superficial wall are sterile, and form only somatic tissue. There is ample
evidence of such sterilisation of fertile cells occurring elsewhere, both in
plants related to Riccia, and in other phyla (Chapter VII.), so that no
a priori objection can be taken to its place in the theory : there is,
however, no direct proof that this was actually the case. The remaining
cells which lie centrally then all undergo the tetrad-division, which on
the above theory was the primitive condition for all the cells of the
sporophyte.
Till recently it was thought that the fruit-body of Coleochaete supplied
a prototype of an undifferentiated mass of cells, all fertile, such as this
theory contemplates; but it has now been shown that in Coleochaete
reduction occurs at the first segmentation of the zygote, and accordingly
the old comparison is no longer permissible. There is, however, a growing
body of evidence, from several distinct phyla of Thallophytes, that the
event of chromosome-reduction consequent on sexuality may be deferred
in the individual life : that a sterile, or vegetative phase of the nature
of a sporophyte, varying in structure and in mode of origin, but similar
in being partly somatic, partly fertile, may be thus intercalated between
the two events. The Florideae, the Ascomycetous Fungi, and the Uredineae
provide examples of such intercalation of a sporophytic phase : these point
an analogy in this respect with the simplest Archegoniatae, though along
phyletic lines almost certainly apart from the latter (Chapter V.). Thus
the view now stated of the phyletic origin of the simple sporogonium of
Riccia by continued segmentation of the zygote, and deferred tetrad-division,
with sterilisation of the superficial cells, is in the main hypothetical, it
is true \ but it has a reasonable basis, partly on the facts of the individual
development, partly on analogy. In the absence of still simpler sporo-
phytes affording comparisons within the series of the Archegoniatae
themselves, this analogy, together with the facts of the individual develop-
ment in Riccia itself, make the view thus stated appear more probable
than any alternative hitherto proposed.
Riccia being the simplest type of sporogonium in the Archegoniatae,
the basis of the antithetic theory has been fully stated, as applied to the
facts of its development. The same theory may be extended from it to
other forms also, in which the sporophyte, though more complicated,
arises from the zygote by similar though more extended segmentation.
Steps in advance are illustrated in other Marchantiales, which will now
be described.
The sporogonium of the Marchantiaceae, of which Fegatella (Conocephalus)
MARCHANTIALES
261
may be taken as a fair example (Fig. T2i), is more complex than that
of the Ricciaceae in having polarity of structure, and differentiation of
the internal mass of sporogenous cells. Both these characteristics suggest
further steps in sterilisation of potentially fertile cells. The polarity is
marked in the more or less oblong external form, but more definitely
by the fact that the basal tissue is sterile, and develops as the foot
I.
W.
FIG. 121.
Sporogonium of Fegatella (Conocephalus). I., venter of fertilised archegonium with
eight-celled embryo. II., transverse section of a similar embryo. III., IV., older em-
bryos in longitudinal section. V., part of a longitudinal section of a developing capsule,
showing differentiation of archesponal tissue into elater-forming cells (el), and sporogenous
cells (sfi). VI., part of longitudinal section of receptacle with embryo. VII., similar
section showing two manure sporogonia in outline, ac = air-chambers ; az/=calyptra;
crips = capsule ;_/==foot ; « = neck of archegonium ; /) = pore ; rec. st.= stalk of receptacle ;
rhiz. =rhizoids; j = setaof sporogonium. I.-V.X3<5o. VI.X76. VII. X2O. (After Cavers.)
which supports the fertile capsule (Fig. 121, vn.). It is stated that the
whole hypobasal half of the zygote is thus sterile, though this limitation
may not apply for all cases. This state as compared with that of Riccia
might be expressed as a consequence of sterilisation of the whole product
of the hypobasal half of the zygote; but it is still a matter for debate
whether the Ricciaceae really represent the progenitors of the Mar-
chantiaceae. The presence of this polarity may be held as a biological
concomitant of the position of these larger sporogonia relatively to the
thallus which bears them : the spherical form of Riccia is suitable to its
262
BRYOPHYTA
habit, surrounded as it is up to full maturity by the tissues of the parent
thallus. But the larger sporogonia of the Marchantiaceae project at
maturity from their envelopes, and even during development their relation
to the parent thallus is not uniform all round, their nutrition emanating
mainly from base of the archegonium : a certain degree of polarity,
expressed in the formation of a sterile foot for nutritive and mechanical
purposes, is thus intelligible.
FIG. 122
Monoclea. Forsteri, Hook. 47 = part of a longitudinal section of a capsule showing
elaters and rows of spore-mother-cells. XSSQ. 48 = longitudinal section of the tip of a
nearly mature capsule, showing the lobed spore-mother-cells. X 160. 49 = elaters, tetrads
of spores, and a cell from the wall of a still more mature capsule. X35o. (After Johnson.)
But much more interest attaches to the internal differentiation of the
capsule. The wall is initiated at a relatively early stage, and remains a
single layer, excepting at the extreme apex : the mass of tissue which lies
within, corresponding as it does in position to the sporogenous cells of
Riccia, is composed of cells all alike in origin, and it is often designated
the archesporium (Fig. 121 iv.). But they do not all develop as spore-
mother-cells: some become elongated, and form the well-known sterile
elaters (Fig. 121 v. el) ; others, undergoing more numerous divisions,
remain fertile, and divide into spore-tetrads : a later stage of this differ-
entiation is well illustrated in Fig. 122 for Monoclea, a genus of doubtful
MARCHANTIALES
263
affinity with the Marchantiaceae, though showing a similar relation of
spore-mother-cells and elaters. Since the whole mass is uniform in origin,
and since the similar mass in Riccia is wholly fertile, it is the natural
conclusion that certain of the potentially fertile cells have been sterilised
to form the elaters : or, in other words, remain as somatic cells without
undergoing chromosome-reduction. The final function of the elaters is to
assist mechanically in the dispersal of the mature spores ; but it is possible
that in such a plant as Fegatella (Conocephalus] they may in some
degree assist in the early nutrition of
the cells which remain fertile. This
seems almost certainly to be the case
in Corsima, and also in those genera
of more doubtful affinity, viz. Sphaero-
carpus, Gcothallus, and Riella, where
the sterile cells are not mechanically
strengthened by spiral or annular
thickenings of their walls : they are
here recognised as " nutritive cells,"
and they undoubtedly aid in the supply
of nourishment, and perhaps also in
dispersal of the spores by swelling
of their mucilaginous remains. The
obvious importance of these nutritive
cells, as well as of the elaters, is
further evidence of the probability that
a progressive sterilisation, or conversion
of reproductive into somatic cells, has
occurred.
In the Marchantiaceae there is
regularly present at the distal end of
FIG. 123.
Cynthodium cavernarum, longitudinal section
... of an almost mature sporogonium showing apical
the CapSUle a Small maSS OI tiSSUe disc. X 200. Above, the apical disc of the same
. , . 1*1 sporogonium in median section. X 200. (After
within the one-layered wall, which Lang.)
remains sterile, and comes away at
dehiscence as a cap, or lid. This also originates from the archesporium :
its development has been clearly shown by Lang in Cyathodium: Fig. 123
illustrates the apex of a sporogonium, and from comparison of the young
state it is plain that certain cells of the archesporium are told off as
sterile from the first.
From these notes it appears that in the Marchantiaceae, as compared
with the Ricciaceae, the evidence is strong for the conclusion that the
sporogenous tissue is liable to be reduced at various points by diverting
cells, or groups of cells, from their original function as fertile cells : the
somatic functions which they then perform have obvious uses, and this
gives biological probability to the conclusion.
264
BRYOPHYTA
B. THE JUNGERMANNIALES.
The same principle is illustrated also in the Jungermanniales, but with
differences of detail. In these the first segmentation of the zygote
separates a hypobasal cell, which in some cases develops as a multicellular
FIG. 124.
Frullania dilatata, development of the embryo. X 300. (After Leitgeb). x, x^ the
archesporial cells. The numbers indicate the primary transverse divisions. (From
Campbell, Mosses and Ferns.)
haustorium (Frullanid), but more frequently remains as a minute appendage
at the base of the fruit, while the epibasal cell gives rise to the capsule,
stalk, and foot. So far as a comparison on the basis of the segmentation
of the zygote is valid, this would indicate in the Jungermanniales a still
further delegation of function from the
hypobasal to the epibasal half of the
embryo.
The epibasal half of the zygote under-
goes segmentation, so that a number of
transverse discs are formed, each composed
of four cells, while the terminal tier is
composed of four octants of a hemisphere
(Fig. 124). The segmentation is often
very regular, but exceptions exist.1 The
FIG. 125. uppermost tier of cells appears to be cut
Embryos of Radula compianata. X23o. off by the first transverse wall in the
(After Leitgeb.) 7, 8, show the basal ap- ., . , ,r r , , .
pendage cut off by 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 holds4 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. 4Z.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 cy D, E). The former
give rise to the columella alone, which is in Sphagnum a bulky mass
SPHAGNALES
273
of tissue with rounded apex (Fig. 133 F, H). The peripheral series of
cells, or amphithecium, divides periclinally to give off internally the single
FIG. 132.
Development of sporogonium of SpJtagnum acutifplium, Ehrh. A = embryo with four
tiers; J = apical cell; £=basal cell with oblique division. j6? = embryo with five tiers.
C = optical section of the same embryo; one quadrant is still undivided; rt = anticflnal;
/ = periclinal walls; h = principal walls. D — transverse section of the lower part of an
embryo. E = & rather older stage; spr = spore-forming layer. ,F=median longitudinal
section of a sporogonium showing the bell-shaped sporogenous layer, and the wall covering
it externally. G = transverse section of a sporogonium of similar age ; lettering as in E,
// = median longitudinal section, though a half-ripe sporogonium; ca/=calypira ; spr—
spore-cavity, in which the spore-mother-cells are isolated ; sps = spore-sac ; epi = epidermis ;
«7 = furrow in wall where the operculum will separate ; />j = pseudopodium ; ^ = vaginula ;
^W=perichaetial leaves ; co/=coluinella. (After Waldner, from Engler and Prantl.)
layer of the archespo'rium ; this appears as a continuous dome closely
investing the columella. The external product of the amphithecium forms
s
274 BRYOPHYTA
the rnany-layered capsular wall (Fig. 132 F, G, H). The dome-shaped
archesporial layer divides later into four layers, and every cell undergoes
the tetrad-division to form spores. At maturity dehiscence takes place
by a transverse rupture, setting free a circular operculum. The foot is
considerably enlarged as an haustorium, which is marked off at maturity
by the narrow neck of the short seta : and the whole is borne upwards
on a more or less elongated pseudopodium developed by the parent
gametophyte (Fig. 132 H).
In the facts thus briefly sketched there is no obvious evidence of
sterilisation : it is only when the peculiarly close analogies with the
Anthoceroteae are traced that any relation to the theory emerges. The
points of similarity with the typical Anthocerotales are seen in the form
of the sporogonium, in its absence of apical growth, and in the manner of
its primary segmentation : also in the origin of the columella from the
whole of the central group of cells, and of the completely dome-shaped
archesporium from the primary capsular wall outside it. It differs, how-
ever, in the mode of dehiscence, and in the fact that elaters are absent,
while the columella is not mechanically functional : it serves no purpose
beyond the nutrition of the considerable mass of spores. But as methods
of dehiscence, and of distribution -of spores not unfrequently vary within
near circles of affinity, this discrepancy does not seem of prime import-
ance. Lastly, however, there is, as a point of difference from Anthoceros^
the absence of any functional assimilatory system in the sporogonium of
Sphagnum, though it is so well developed in Anthoceros. But, as Haberlandt
has shown,1 functionless stomata, without pores and without intercellular
spaces below them, are present in large numbers on the capsules of
Sphagnum : from this he concludes " that it is certain not only that the
ancestors of the present Bog-Mosses had normal functional stomata on
their capsules, but also that the capsules of these ancestors possessed a
relatively well developed assimilatory system as well." All these con-
siderations taken together point to a close analogy (if nothing more)
between the two types.
On the other hand, Sphagnum has always been ranked as a Moss on
such grounds as habit, absence of elaters, and structure of the archegonium :
but it differs from all other Mosses in the transverse segmentation of
the embryo, and in the absence of an apical cell : also (excepting Andreaea)
in the complete dome-shape of the archesporium, and in its origin from
the amphithecmm ; these all being features of correspondence with
Anthoceros. Such equivocal comparisons, with Liverworts on the one
hand, and with Mosses on the other, give Sphagnum itself a special
interest : at the same time they serve to link together the two large groups
of Bryophytes, and point to the propriety of regarding their sporogonia
equally from the point of view of a theory of sterilisation. The columella
would thus be held in both cases to be a consequence of sterilisation
1 Pringsh. Jahrb,, xvii., p. 474.
ANDREAEALES
275
progressive from within, which has
extended in Sphagnum^ as in Anthoceros,
to the whole product of the endothecium,
while in both the archesporium takes its
origin wholly from the amphithecium. A
key to this difference of Sphagnum from
all other Mosses may be found in Noto-
thylas, in which the sporogenous cells
may arise from both sources, the centri-
fugal progress of sterilisation being less
completely carried out there than in
Anthoceros. In both cases the difference
appears referable to the degree of centri-
fugal sterilisation in a body in which the
spore-production was originally central.
Thus the condition of Sphagnum is in
this respect the most advanced in the
Mosses, as that of Anthoceros is among
the Liverworts. As regards decentrali-
sation of the fertile tissue the rest of the
Mosses will be seen to correspond to
the less extreme types of the Hepatics,
while the columella-less sporogonia of
Notothylas link together the two degrees
of decentralisation.
B. ANDREAEALES.
The mature sporogonium of Andreaea
offers analogies with that of Sphagnum
in its form, with its short seta and large
foot, the whole being borne up on an
elongated pseudopodium : it also corre-
sponds in the fact that the columella is
interrupted at the apex^ and covered by
the archesporium which forms a complete
dome (Fig. 133); but it differs in the
dehiscence when mature by longitudinal
slits.
The segmentation of the zygote corre-
sponds to that of other Mosses rather
than to that of the Hepatics, for after
the appearance of the transverse basal
wall, the cleavages are oblique, a two-sided initial cell being present (Fig.
134 A-D) : but the number of such segmentations is limited to about a dozen.
FIG. 133.
Median longitudinal section of sporogonium
of Andreaea rupestris, at the time of division
of the archesporium. f> = pseudopodium ; _/"=
foot; z; = vaginula; h = neck; c = columella;
w = wall of sporogonium j e = epidermis; s =
spore sac; *=archesporial cells dividing;
r=calyptra; s = neckof archegonium. (After
-Kuhn.) X8o.
BRYOPHYTA
These undergo further sub-divisions to constitute an endothecium of four cells,
and a many-celled amphithecium (Fig. 134 G). The hypobasal half, which
has meanwhile undergone irregular divisions, together with the two lowest
segments of the epibasal region remains sterile, and constitutes the short
seta and enlarged foot. Three or four only of the upper segments are
fertile, while the rest go to form the sterile apex of the capsule (Fig. 134 E).
FIG. 134.
Development of sporogonium of Andreaea. A= young embryo of A. crassinervia,
Brch. B = A. petrophila, Ehrh. C = of A . crassinervia, older. D = of A . sp. , older still.
E= optical longitudinal section of A. petrophila ; ,y/r= archesporium. JF=optical trans-
verse section of a young embryo ; jj = segment- walls ; rr = radial walls ; aa = first divisions
of the quadrants. G = older stage ; ^="grund-quadrat." // = older stage with division
to form archesporium. / = archesporium differentiated. /if = archesporium divided into
two layers; as/> = outer spore-sac ; col. =columella. (After Waldner.) D after C. Miiller-
Berol. (From Engler and Prantl.)
The archesporium is here, as in all other Mosses, derived from the endo-
thecium : the peripheral cells which result from the segmentation of the
endothecium, become densely granular, and give rise to spore-mother-cells,
the internal cells form the columella (Fig. 134 E, j, K). It is not stated
by Waldner how the archesporial dome is completed at its apex: probably
it is by certain cells of the inner product of the endothecium, forming
spore-mother-cells, in place of sterile cells of the columella. In this there
would be no theoretical difficulty, for on the present theory all cells derived
from the endothecium were at first potentially fertile cells ; moreover, as
bearing indirectly on this point, the internal limit between the archesporium
BRYALES
277
\J
and the columella is in Andreaea a very
irregular one (Fig. 135). Further, in Archidtum,
which some writers put in close relation to
Andreaea, any cell derived from the endo-
thecium may apparently become a spore-
mother-cell.
It is thus seen that though Sphagnum and
Andreaea have certain apparent characters of
the sporogonium in common, their segmen-
tation is essentially different, and the dome-
shaped archesporium is produced in different
ways in the two. It remains doubtful there-
fore how far the similarities indicate a real
affinity. In» any case the relation of the
Andreaeales to the simpler Bryales is much
nearer than to the Sphagnales, and it is in
this direction that comparison of the sporo-
gonia will bring the more interesting con-
clusions concerning them.
C. BRYALES.
These include the vast majority of Mosses.
In their sporophyte generation there is uni-
formity of the general scheme, though
considerable fluctuation in size, as well as
in minor detail. Some of the smallest forms,
which show irregular opening of the capsule
on maturity, are classed as the Cleistocarpae ;
the more elaborate forms, which dehisce
transversely, setting free an operculum, are
designated the Stegocarpae. The latter, as
they represent the prevalent type in Mosses,
will be taken first, while the Cleistocarpic
forms being taken later, will then be better
appreciated in their value for purposes of
comparison.
(a) Stegocarpae.
The embryo of all these Mosses takes at
an early stage the form of a more or less
slender, but always simple spindle: any later
deviations from this are of secondary origin.
Its development from the zygote is first by
the appearance of a transverse basal wall, which is succeeded by oblique
segmentations in the epibasal half: these appear alternately on opposite
Ceratodon purpureus. A, B, young
embryo seen from points of view at
right angles to one another. C = an
older embryo ; gg — outer limit of endo-
thecium ; sps = outer spore-sac. (After
Kienitz-Gerloff.)
278 BRYOPHYTA
sides, so as to cut off two rows of segments successively from the terminal,
two-sided initial cell (Fig. 135 A and B). The apical growth is, however,
not very long continued, and gives place later to intercalary activity. The
hypobasal cell, as also sometimes the lowermost segments, undergo less
regular sub-divisions, but the upper segments sub-divide with greater
regularity — though still with some differences of detail — in such a way
that a definite result is arrived at, viz., the formation of a central tract
of tissue (endothecium), consisting of four rows of cells, and a peripheral
series (amphithecium), consisting of more numerous cells, which soon
divide both radially and periclinally to form a thick wall (Fig. 135 c).
It is important, from a theoretical point of view, to note that the endothe-
cium thus established, though less definite in the lowest of the epibasal
segments, extends upwards throughout the length of the capsule to its
apex : it is not merely a local development in that part which is ultimately
to be the fertile region, but it is a continuous and definite column of
tissue, occupying the centre of the spindle-shaped sporogonium. It may
be a question what is the morphological importance of a tract thus defined
by embryonic segmentation. In Chapter XIV. the relation of the leading
anatomical regions of axis and root to the apical segmentation has been
discussed, and it was seen that there is no obligatory correspondence
between early segmentation and the definition of mature tissue-tracts : for
it has been found that, in parts of such complicated outline as the leaf-
bearing shoot, the correspondence between early segmentation and mature
structure is not strictly maintained. But it is the fact that in parts of
such simple outline as roots there is a definite correspondence of that
nature, and this is particularly clear in certain Pteridophytes. The case
of the simple spindle-shaped sporogonium of a Moss is comparable, in its
form as well as in the early segmentation of its central tract, with such
roots ; and there seems good reason to regard the endothecium accordingly
as being in fact a morphologically definite region throughout its length.
The most important function of the endothecium is that it is the exclusive
source of spore-formation ; but as a matter of fact, it is only a relatively
small extent of it which carries this into effect, the rest remaining sterile,
performs other duties.
The archesporium originates from a restricted region of the endothecium
some distance back from the apex of the sporogonium, and a very con-
siderable distance from its base : the sterile region of the capsule at the
distal end forms the calyptra and peristome : the much longer sterile
region at the base forms the apophysis and the seta. These regions may
vary in their proportion to the fertile region in different types of Mosses :
a fair average is that seen in Funaria (Fig. 136 A). The origin of the
archesporium is by periclinal division cutting off a single layer of cells
from the periphery of the endothecium : this ultimately divides up into
several layers of minute cubical cells, all of which undergo the tetrad-
division in the usual way, and produce spores.
BRYALES
279
JB.
FIG. 136.
Fnnaria hygronietrica. A — longitudinal section of a sporogonium showing the first
•differentiation of its parts. X about 96. .5 = the upper part of the same. X6oo.
r marks the limits of theca and operculum. C — basal part of capsule of the same. x6oo.
«r=archesporium ; r0/=columella. (After Campbell.)
280
BRYOPHYTA
Meanwhile, however, other changes supervene, and they are especially
marked in the amphithecium ; but all such changes are nothing more than
secondary modifications of the amphithecial tissue : there is no develop-
ment of new parts, however greatly the external appearance of the sporo-
gonium may be affected by their presence. They may be considered in
FIG. 137.
3. Diagrammatic longitudinal section through the green capsule of Physcomitrium
pyriforme. X 14. 5. Median longitudinal section through the mature green capsule
of Funnria hygrometrica. X2o. 8. Profile view of mature capsule of the dorsiventral
capsule of Buxbaumia aphylla. g. Median longitudinal section of the same capsule ;
/=cylindrical air-space; at j the stomata. X 10. 10. Transverse section of the same
capsule, about the middle. Xg. (After Haberlandt.)
succession from below upwards. The lower part of the sporogonium which
forms the seta elongates more or less in different types : where well
developed, as for instance in Funarta, its structure shows a peripheral
sclerotic band, which merges gradually into thinner-walled parenchymatous
tissue : centrally lies a strand of thin-walled water-conducting tissue,
without cell-contents when mature, surrounded by a thicker-walled sheath.
BRYALES
281
No accurate statements are to >hand showing the genetic relation of the
endothecium to this central tract, but it certainly gives rise to the greater
part, if not exclusively to the whole, of the conducting strand.
In the upper region assimilating tissue is usually developed in more
or less close relation to the capsule itself, together with numerous and
often large intercellular air-spaces. These developments are derived chiefly
from the amphithecium, while at the same time the sterile cells of the
columella usually expand, as a tissue for water-storage : this may also
contain some chlorophyll, and occasionally forms air-spaces. The swollen
shape of the capsule is chiefly
due to these changes, which are
obviously secondary. Different
types may be distinguished
according as the assimilatory
system is developed from the
wall of the capsule itself, or
partly here and partly in the
apophysis below, or entirely in
the region of the apophysis.
For instance, in Bartramia the
assimilatory system is chiefly in
the wall of the capsule, where
it is equally developed all round.
In Buxbaumia (Fig. 137, 8, 9) the
same is the case, except that
the development is dorsiventral :
the capsule early takes an oblique
position, and the assimilatory
tissue is developed more strongly
on the better-lighted side. In
other cases, however, the assimi-
latory system extends some dis-
tance below the actual capsule,
constituting the swollen region of the apophysis : this is of small size in
Polytrichum, and the assimilatory system is chiefly here also in the wall
of the capsule, but it extends downwards to the small apophysis, while the
numerous stomata lie in the narrow neck between these parts. In many
Mosses, again, the apophysis itself becomes the chief seat of assimilation,
as, for instance, in Funaria (Fig. 137, 5), and this leads to its enlargement:
so much so that it becomes the most prominent feature in the whole
sporogonium : thus in the Splachnaceae it is commonly larger than the
capsule which it is to nourish, and in S. luteum (Fig. 138) it appears as
a wide umbrella-like expansion, which shows a structure not unlike a
leaf-lamina, with well-marked epidermis, spongy mesophyll, and stomata
upon the upper surface. Still, with all these variants at or near to the
FIG. 138.
SplacJinum luteum. I. Capsule open. A— apophysis.
II. Unopened capsule in longitudinal section. ,r=seta ;
-£jr = leptoxylem ; jr/ = stomata on apophysis ; £/=colu-
mella ; / = peristome ; ^4j = archesporium ; i— intercellular
space. III. and IV. Diagrams to illustrate the opening
of the capsule. (From Goebel, after Hedwig, Vaizey, and
Bryhn.)
282 BRYOPHYTA
fertile region of the capsule, the original formation is the same : it is
based upon an amphithecium, capable as we see of variously extended
development, and an endothecium which is less variable, though it may
expand also to form a more bulky tissue for water-storage : its most
distinctive function, however, is to give rise to spores, while below it
serves for conducting purposes.
Above the fertile region the endothecium as a rule develops only cells
similar to those of the columella below : the amphithecium, however,
undergoes changes of induration of the walls, variously distributed, which
result in the formation of the operculum, the annulus by which it is
detached, and the peristome which is laid bare when the operculum falls
away. The details of the peristome may vary considerably in different
Mosses, but in all cases it appears to take its origin from the innermost
layer of the amphithecium.1 The columella may in some cases co-operate
with it mechanically in the function of spore-distribution, but neither the
endothecium nor any of its products take any share in the development of
the peristome.
It thus appears that the sporogonium of the stegocarpic Bryales is
composed of two tissue-tracts, distinguished early from one another in
segmentation, and divergent in their later development. The outer is
always sterile, while the other is fertile only in part. The question arises
as to the initial condition, and the origin of these regions. It may be in
some degree elucidated by comparison of some of those smaller forms in
which the sporophyte is of simpler construction : they have in common
the feature that the mechanism of dehiscence is absent, or imperfect, and
on this account they have been grouped together as the Cleistocarpae.
It is clear at the outset that this condition may either have been
primitive or the result of reduction : these alternatives must be kept in
mind in any discussion of such forms, even though no definite conclusion
be arrived at.
(b) Cleistocarpae.
Of the various genera grouped as the Cleistocarpae, Phascum has been
examined developmentally by Kienitz-Gerloff : the primary segmentation is
according to the type of the Bryales, with well-marked endothecium and
amphithecium (Fig. 139). From the periphery of the former the arche-
sporium is derived in the usual way, while the formation of an air-space
and spore-sac, and the enlargement of the cells of the columella, are all
according to the usual type : stomata may also be present, but there is
neither operculum nor peristome. Developmentally there is a near similarity
to Andreaea, though on the ground of its peculiar dehiscence and domed
archesporium this genus is usually kept apart. It has, however, been
pointed out above that a very slight modification of the ordinary type of
Andreaea would produce the condition of the archesporium seen in
1 Goebel, Organography, p. 383.
BRYALES
283
Phascum : the sterilisation of the whole of the upper products of the
endothecium at the narrow distal end of the fertile tract would interrupt
the dome, and complete the columella, just as it is seen in Phascum.
There is no inherent im-
probability in this, but
rather the reverse : for it
would be only introducing
one further step in sterili-
sation. On such grounds
the relation of the Phasca-
ceae to the Andreaeaceae
would appear to be a near
one.
Another simple Cleisto-
carpic form which has not
only been observed exter-
nally, but also worked out
developmentally, is Nano
mitrium tenerum. The
small capsule has here a
provision for dehiscence
by the formation of a
rudimentary annulus. The
segmentation of the embryo
begins on the plan of the
Bryales (Fig. 140, i.), and
there is as usual a differen-
r , j ..u through a sporogonium after formation of the air-space ; s/ = arche-
tiatlOn OI the endOtneClUm sporium; .y/.r = spore-sac; « = limits between amphithecium and
j uvu ' /T?' endothecium. J3 = transverse section of the same. (After Kienitz-
and amphithecium (rig. Gerioff.)
140, ii.). The cells of the
latter, after further division, become differentiated into an exiguous central
columella, surrounded by relatively numerous and somewhat irregularly
arranged spore-mother-cells (Fig. 140, iv.) : but as maturity approaches the
columella disappears, its materials having served for nourishing the spores
which fill the cavity of the capsule.
The genus Ephemerum is closely related with that of Nanomitrium,
from which it differs in the absence of any definite operculum : the
condition of the columella is the same, but while it disappears at
maturity in some species (E. papillosum\ in others it may still be seen
in the mature capsule (E. crassineruiutri). The fact that stomata occur
on the capsule-wall, though when ripe this is only a single layer in
thickness, has its bearing on the question whether these simple Mosses
are primitive or reduced forms. A systematic position is now assigned
to them by many writers apart from other Cleistocarpic forms, in close
relation to the Funariaceae.
FIG. 139.
Phascum cuspidatum, Schreb. Schut. A — longitudinal section
284
BRYOPHYTA
Another Cleistocarpic type, but again one of doubtful affinity, is
Archidium, in which the small sporogonium has been examined develop-
mentally by Leitgeb.1 The first stages agree with those of the Phascaceae ;
but the tissue of the endothecium shows no differentiation into archesporium
and columella : certain few cells of it, definite neither in number nor in
position, become spore-mother-cells, while the sterile cells in which they
are embedded are absorbed as the spores become matured. This con-
dition in Archidium suggested to Leitgeb a comparison with that in certain
Liverworts, for instance, Riella ; but in view of the facts ascertained by
FIG. 140.
Nanoinitriiun tenerum. Archegonium after fertilisation' and voung sporogonium at
different stages of development, in longitudinal section. In II. the endothecium is
shaded. F— foot ; S = stalk. IV. Sporogonium showing the sporocytes in greater part
separate round the columella. All magnified, I. the most highly. (After Goebel.)
Lang, the comparison with Notothylas would seem more pertinent.
Without suggesting even a remote relationship, these two forms both
illustrate how individual cells, distributed without order in an otherwise
sterile columella, are partially fertile ; and they suggest that the whole of
the columella was originally fertile. Of this in the Liverworts there is
substantial comparative evidence, and this adds probability to the similar
conclusion for the phylum of the Mosses.
While it is thus seen that in normal Cleistocarpic forms, which may
be held to be either primitive or reduced, internal cells of the endothecium
may develop as spore-mother-cells, a similar condition is also seen
occasionally in Stegocarpic forms as an abnormality : cases have been
described of the appearance of fertile cells among the normally sterile
^Sitz. Akad. Wiss.> Wien, 1879.
COMPARISON OF SPOROGONIA 285
cells of the columella.1 Suctk facts again indicate a probability that
the whole product of the endothecium was fertile in more primitive
forms.
A general comparison of the sporogonia of Mosses (excluding the
Sphagnaceae) thus leads to the conclusion that two distinct tissue-tracts
are consistently produced in them by early segmentation, the endothecium
and the amphithecium. As these are differentiated early, and with great
constancy, while they differ also in their products, they are to be accepted
as morphologically distinct. The amphithecium is always sterile, and to its
modifications the chief mechanical and assimilatory tissues owe their
origin ; the modifications may involve expansion of tissues, but no initia-
tion of new parts. The endothecium, theoretically fertile in the first
instance throughout its length and breadth, underwent progressive sterili-
sation, parts of it being diverted to other uses : a central tract became
the sterile columella, while the fertile region became abbreviated both
at its upper and lower limits ; and thus the actual archesporium in
typical Bryales is a mere truncated residuum, with its barrel-like form
open at both ends : the structural indication that its origin was as thus
suggested is seen in its apparently arbitrary limitations at either end
(compare Figs. 135, 136, 139).
This is well illustrated in Funaria and Phascum, where there is a
continued growth with an initial cell at the apex of the sporogonium ;
the archesporium appears in longitudinal sections of young sporogonia as
a definite row of cells on either side of the columella ; but it is impossible
at first to tell in those rows of cells where the exact limit of spore-
development will be. Below the lower limit the cells of the row develop
sterile, above it fertile ; but in either case the segmentations which define
the cell-row are the same. Passing to the apex, the archesporial row is
continued beyond the limit of fertility : passing downwards, the cell
row may also be traced into the seta : structurally the possibility of
further spore-production seems to be there, but arrested. In different
types of Mosses the fertile zone thus limited is not always located at
the same level in the sporogonium as a whole : it is sometimes preceded
by a shorter, sometimes by a longer, seta. By comparison of these
different types, an idea is acquired of a residual and limited fertile zone, which
has been liable to be shifted in the course of descent ; and such shifting
is made possible by the continued apical growth seen in the developing
sporogonium. It is important to have a clear conception of the fertile
zone as a residuum thus movable in the course of descent; the vari-
able balance thus established between sterile and fertile tissues is not
only interesting in its bearing on the study of sporogonia, but it will
come into comparison later with similar features seen in certain strobiloid
1 Lantzius Beninga, Beitr. z. Kenntn. d. Mooskapsels, 1847, Tab. 58, Figs. 9', 9".
Also Kienitz-Gerloff, Bot. Zeit., 1878, p. 47, Taf. 2, Fig. 52.
286 BRYOPHYTA
Pteridophytes, in which the apical growth is longer continued, and the
shifting of the fertile zone consequently more obvious.
The results of progressive sterilisation should not only be studied in
their longitudinal aspect, but also in the transverse, as leading towards
decentralisation of the fertile residuum, and establishment of a central
sterile tract. There is reason to believe that the original type, both of
Liverworts and Mosses, had a solid core of sporogenous cells. In the
Liverworts a step toward decentralisation is seen in the partial elatero-
phores of the Jungermanniaceae, but it attains a greater completeness in
the Anthoceroteae, where, excepting in Notothylas, the spore-production
is relegated even to the amphithecium. In Sphagnum the same is the
case, though there is no guide as to the evolutionary steps which led to
it. In the Bryales also decentralisation has been effective, but has not
attained the length of relegating spore-production to the amphithecium.
The biological significance of decentralisation is plain, as the presence
of a central conducting column provides a means of better nutrition
for the increasing mass of spores than where these constitute a solid
core. In respect of the degree of decentralisation it may be said that
the Anthoceroteae and Sphagnaceae are the most advanced of the
Bryophytes ; but all Bryophytes stand far behind the Pteridophytes in
this respect, for as we shall see, in all the Pteridophytes the spore-
production is referable in origin, not to deeply seated, but to superficial
cells of the plant-body.
The biological circumstances of dispersal of the ripe spores, as well
as those of nutrition, have doubtless affected the position of the
archesporium in Archegoniate Plants. Where, as in the Bryophytes
(excepting the Anthoceroteae), the spores are all produced simultaneously
in one capsule, which collapses at their maturity, a superficial position of
the archesporium is immaterial : indeed a relatively central position will be
advantageous as simplifying the problem of nutrition. Dispersal of the
ripe spores is then carried out by some drastic method of decay or of
dehiscence of the protective wall, and the whole sporogonium ceases its
functional activity with the liberation of the mature spores. The central
tissues can be sacrificed with impunity where, as in the Bryophytes, the
spore-production is simultaneous. But in Vascular Plants the spore-
production is in one way or another successive, and the succession,
acropetal as a rule, brings with- it the great biological advantage of
spreading the physiological drain for nutrition over a longer period. In
this case the central tissue cannot be sacrificed, but must be maintained
as a nutritive core, in the interest of the later-formed spores of the
acropetal succession. A more superficial position of the archesporium
thus becomes necessary, while the projection of the separate sporangia
beyond the surface which bears them will increase the facility for
scattering the spores when mature. Thus the difference between the
deep-seated position of the archesporium of the Bryophytes, and its
COMPARISON OF SPOROGONIA 287
superficial position in the Ptecjdophyt.es is intelligible on biological
grounds : it is closely related to the simultaneous development of the
spores in Bryophytes, as against the successive spore-production in the
Pteridophytes. Still in the former some degree of decentralisation, as we
have seen, brings advantages of nutrition, and its structural expression
is the sterile columella; but decentralisation does not become a peremptory
condition of success of the Bryophyte-type, as it appears to have been in
the Pteridophytes. There is thus a biological reason for the nearer
relation which all Bryophytes show to that condition which comparison
indicates as primitive, where the fertile tissue is deeply seated, or even
occupies a central position in the simpler types. Such considerations lend
a biological probability to the theory of progressive sterilisation applied in
the above pages to the sporogonia of the Bryophyta.
Reviewing the Musci as a whole, the evidence of progressive sterilisation
in them is less cogent than it is in the Hepaticae. They probably represent
a more or less distinct phyletic sequence from the latter; but still analogies
may be drawn between the two ; such analogies strengthen the weaker
evidence in the Musci ; and, as there appear to be no facts which preclude
such a view, while many give a reasonable measure of support, it may be
held that progressive sterilisation has been effective here in essentially the
same way as it is more clearly demonstrated in the Hepaticae.
CHAPTER XXIII.
INTRODUCTORY REMARKS ON PTERIDOPHYTA.
IN the comparative sketch of the sporophyte in the Bryophyta which has
been given in the preceding chapters, it has been seen that for these plants
a theory of sterilisation of potentially fertile cells accords well with the
developmental facts. Numerous cases have been seen of cells, similar in
origin to the sporogenous cells, being diverted to other uses than that
of propagation : these form somatic tissue : there is indeed good reason to
think that most, if not even the whole, of the somatic tissue of the
sporogonium originated in this way. This is no new conception : it is a very
natural corollary on the fundamental conclusions of Hofmeister : it was
first clearly stated in the writings of Leitgeb on Liverworts, and was
extended by him also to the Mosses : it was adopted by Goebel in his
work on the Muscineae in Schenk's Handbuch, and it is now more
definitely formulated in his Organography, Eng. edn., vol. ii., pp. 93-167.
It may be held as the generally accepted hypothesis underlying any
comparative study of the -sporogonia of the Bryophytes at the present
time.
But the hypothesis of sterilisation has not been extended with the
same readiness to other Archegoniate forms. In treating the Pterido-
phytes, notwithstanding that they have an essentially similar life-cycle,
there is rarely any reference in the current literature to the effect which
progressive sterilisation may have had in their evolution. A certain
excuse for this want of consistency may be found in the fact that in the
Pteridophytes the proportion of somatic to propagative tissue is very large :
any hypothesis of sterilisation must therefore recognise the process as
having extended much further in them than in the Bryophytes. The form
of the sporophyte also is much more complicated than in the Bryophytes :
consequently the difficulties of application of a theory of sterilisation to
the Pteridophytes are much greater, and the results less secure. This is
certainly true, but it does not appear to be a sufficient reason for a plain
departure from a theoretical position which has illuminated the comparative
INTRODUCTORY ON PTERIDOPHYTA 289
study of the Bryophytes. Norton the other hand, does it justify the
initial assumption that the origin of the sporophyte in Vascular Plants
differed essentially from that in the Bryophytes. Accordingly, the theory
of progressive sterilisation will here be applied to the study of the
Pteridophytes also, along lines parallel to those observed for the Bryo-
phytes. It is not to be expected that the facts will amount to a complete
demonstration : the present object will be to see how far they accord with
a theory which has its more obvious application in the simpler series of
Archegoniate Plants.
The most important evidence will naturally be obtained from the study
of the spore-producing members themselves ; and these will be described
in detail in the several types of Pteridophytes. But facts of value bearing
indirectly on the general hypothesis, are also to be derived from the form
and structure of the vegetative parts, as well as from their origin and
early development. In fact, the whole sporophyte is to be studied in
relation to the question of its origin, just as much in the more complex
as in the simpler Archegoniate forms. One guiding line must constantly
be maintained, and it is this : that however late in the individual life the
production of spores may appear, still spore-production was on our general
hypothesis the first office of the sporophyte. By various means the vege-
tative phase may have attained a large size, and great complexity of
structure : but however preponderant it may appear, still we should be
prepared to regard it theoretically as secondary, that is, as a phase
intercalated between the events of nuclear fusion in the zygote and
reduction in the spore-mother-cell.
It will be well to observe some regular order in the discussion of
the large area of fact involved. The several groups of the Pteridophytes
will accordingly be taken in succession, starting from those with relatively
small appendages and strobiloid habit, and proceeding to those with
appendages of larger size. The fossil representatives will be included in
the discussion, together with the living forms. In each group a pre-
liminary section will deal with the external characters of the mature
organism, 'with special reference to the balance of the vegetative and
reproductive regions. It will be followed by a detailed examination of
the spore-producing members, and lastly, certain facts of anatomy and
of embryology will be considered in their bearings on the general ques-
tion. The characters of the gametophyte will only be referred to
incidentally, so far as they affect the biological circumstances of the
young sporophyte.
If then the Pteridophyta be arranged according to the complexity of
the appendages, and especially of their spore-producing parts, the Lycopodiales
will come first, since in them each isolated sporangium is attached in the
median plane to its subtending sporophyll.
A second series is characterised by having one or more sporangia
borne on a vascular pedicel : when the number is more than one they
T
290 LYCOPODIALES
are disposed in radiate fashion around its distal end, which is usually
enlarged. The whole structure, which is called a " sporangiophore," may
be inserted directly on the axis, as in the Equisetales, or upon the appen-
dages of the axis, as in the Sphenophy Hales (including the Psilotaceae). In
the latter case the position is, as a rule, in the median plane of the
subtending leaf; but in cases where the sporangiophore is more elaborate
and shows indications of branching the position may be less obvious.
Extreme elaboration of the sporangiophore, sometimes including its branch-
ing, is seen in the series of the Ophioglossale-s, which appear as the most
advanced examples of this pedicellate or Sporangiophoric Type.
In a third series, the Filicales, the sporangia are usually grouped in
" sori," which have features in common with the sporangiophores, but they
differ from the sporangiophoric types in that the sori are distributed over
the margins or surfaces of the leaf itself, which is here of relatively large
size and complex construction.
The order of description will follow the sequence thus laid down, and
it will become apparent that the elaboration of the leaves themselves
follows roughly parallel with that of the sporangial arrangement : in fact the
whole series may be regarded as progressing from simpler to more complex
types of the whole shoot. The arrangement thus adopted is convenient
for description. The question will be reserved for later discussion how far
it indicates a true evolutionary progression.
LYCOPODIALES.
/. General Morphology.
These plants are taken first because in them the spore-producing members
are more simple and regular in their disposition on the shoot than in any
other Vascular Plants. Throughout this phylum (as now limited by the
exclusion of the Psilotaceae), each single sporangium is subtended by a
sporophyll (Frontispiece), the median planes of the sporophyll and of the
sporangium coincide, and typically no more than one sporangium is
associated with each sporophyll.1 These appendages are borne laterally
upon the axis, which is endowed with apical growth. The arrangement
of the appendages, either sterile or fertile, is sometimes in regular
whorls, but frequently it is according to some more or less interrupted
spiral scheme (Fig. 141). The axis may undergo frequent branching,
typically in a dichotomous manner, though intermediate steps are seen
in certain species to the monopodial type : in some of the Lycopodiales,
however, branching is rare, or absent. It is thus evident that the whole
shoot is of a simple strobiloid type. It bears roots at its base, and in the
1 Occasional exceptions have been noted, where two small sporangia, side by side, are
subtended by a single sporophyll. These are rare, and appear to originate in some form
of fission of the normal sporangium (Annals of Bot., xvii., p. 278).
GENERAL MORPHOLOGY
291
straggling or creeping forms these may arise adventitiously at points far
up along the axis. It is by comparison as regards the differences which
occur in this otherwise uniform family that some knowledge of the course
of development of the Lycopod-type may be derived.
The Lycopodiales are divided, according to the presence or absence
of a ligule, into two divisions ; the Eligulatae, which include the Lycopo
diaceae, that is the living genera Lycopodium and Phylloglossum, with which
are also to be associated certain early fossils designated Lycopodites\ and
FIG. 141,
Shoots of several species ot Lycopodium to show the form and arrangement of the
leaves. A=L. rufescens, Hook. X 2. B = L. mandioccanum, Raddi. Natural size.
C — L,. reflexum, Lam. Xa. /? = /,. casuarinoides, Spring. : part of a terminal branch of
an old plant. X4- E = L. cernwttn, L. X 2. F~L. volubiie, Forst, seen from above.
X2. (From Engler and Prantl.)
the Ligulatae, which include the Selaginellaceae and Isoetaceae of living
forms, together with the fossil Lepidodendraceae and Sigillariaceae. These
will be severally considered as illustrating variants on the simple strobiloid
type of the whole phylum.
A. ELIGULATAE.
The genus Lycopodium, which includes about' a hundred living species,
was arranged by Spring according to the degree of differentiation of the
several species.1 He distinguished two main sections of the genus, the
first including those with sporangia scattered over the length of the shoot :
the second including those with the sporangia associated in definite cones.
The former section was again sub-divided according as the leaves were
all alike, or as a distinction appeared between sterile and fertile leaves :
the latter section according as the shoot was developed radially or
dorsiventrally. The details of Spring's scheme have since been modified,
but the principle remains the same in the classifications of the present
day : it is to arrange the genus along lines which clearly indicate a
progressive differentiation and specialisation of sterile and fertile tracts. Such
an arrangement naturally harmonises with evolutionary theory. The species
aphic des Lycopodiac^es^ 1841.
292 LYCOPODIALES
which Spring placed first of all was L. Selago ; and though this may not
be actually the most elementary living species in the genus, still it is the
best known of those which show a low degree of differentiation.
The plant of L. Selago is shrubby, with dichotomously branched axes,
bearing numerous leaves of approximately equal size and simple form
(Frontispiece). There is usually a sterile region at the base of the plant:
this is followed by the well-known alternating sterile and fertile zones,
the length of which corresponds with a high degree of exactitude on the
several branches. They are stated to be determined by successive seasons,
the middle region of each year's increment of growth being fertile. These
zones are not definitely marked by any distinction of the leaves themselves,
but by the presence or absence of sporangia : nor are they strictly delimited
in this respect, for occasionally a single sporangium may be found in an
otherwise sterile region. About the limits of these zones sporangia of
smaller size may be found, which sometimes remain closed when all those
near them have dehisced. These are those incomplete sporangia which
have already been referred to in Chapter XIII., in connection with the
argument for sterilisation as affecting the balance of the sterile and fertile
regions. The condition thus seen in L. Selago is shared in more or less
complete degree by about 40 living species, which constitute the section
Selago : they are mostly ground-growing plants. An examination of them
shows that while most of them, .have, like Z. Selago, a sterile basal region
of considerable length, still in certain species (L. compactum, Hook., and
L. Trendlla Sodiro) sporangia have been found in the leaf-axils down to
the base of the mature plant : this has been noted also, but less completely,
in L. firmum, Mett., and L. rigidum, Gmel. Unfortunately these species
are unknown in the embryonic state, so that it is impossible to tell how
early in the individual life the formation of sporangia actually begins ;
but practically the whole of the mature plant is a fertile strobilus. The
incomplete differentiation of the sterile and fertile zones is seen in all the
40 species : isolated sporangia are frequently found in an otherwise sterile
zone, and occasionally sterile leaves occur in a fertile zone : these facts,
together with the occurrence of incompletely developed sporangia at the
limits of the zones, and the very uniform character of the leaves whether
sterile or fertile, have their direct bearing on the theory of sterilisation
enunciated in Chapter XIII.
Some ten other species were grouped by Baker under the heading
Sub-Selago, and are characterised by having the sterile leaves a little
different from the fertile, but passing into them gradually, while the
sporangia are aggregated into indistinct terminal spikes. All the species
thus grouped have a sterile basal region : above this follows a recurrence
of sterile and fertile zones, as in § Selago (L. Dalhousiatanum) \ reversion
from the fertile strobilus to a permanently sterile state is more common
(L. carinatum, gnidioides). A progressive diminution of size of the fertile
leaves upwards is seen : it is sometimes gradual (L. squarrosum\ but
GENERAL MORPHOLOGY
293
sometimes more sudden (L. E>alhousiaea?ium). Isolated sporangia in the
sterile region are more rare than in ^Selago, but they do occur (L. carinatum,
gnidioides, si/uarrosum} : also partially abortive sporangia have been seen
at the base of the strobilus (L. carinatum). All these characters together
FIG. 142.
Lycopodium Phlegtnaria> L. A — figure showing habit of the whole plant. One-third.
Z? = end of a branch. Natural size. C — a sterile leaf, somewhat enlarged. /} = sporo-
phyll seen from below, enlarged. £ = a sporophyll seen from above, enlarged. (From
Engler and Prantl.)
show a very close similarity to what is seen in the Selago group, but with
gradually increasing definition of the strobilus from the lower vegetative
region.
In the Phkgmaria group, which includes about eighteen species of
epiphytic character, the spikes are slender, and dichotomously forked,
with sporophylls as a rule very different in size from the foliage leaves
(Fig. 142). Occasionally sporangia may be found in the vegetative region
294 LYCOPODIALES
subtended by leaves of the foliage type (L. varium)\ while, on the other
hand, leaves of the sporophyll type may develop no sporangia (L. subulatum>
Phlegmaria). There may occasionally be alternating sterile and fertile
zones (L. nummularifolium). Transitions from the fertile strobilus to the
larger-leaved foliage shoot are frequent (L. nummularifolium, subulatttm,
ophioglossoides, pinifolium, Phlegmaria). Thus the differentiation of the
strobilus is one of external form rather than a rigid difference of intimate
character. The converse conditions of L. varium and L. subulatum show
that the difference of size of sporophylls and foliage leaves is not due
directly to correlation in the individual parts, but rather to the general
condition of the shoot as a whole.
The above groups, including fully one half of the living species of
Lycopodium are now associated together under the sub-genus Urostachya x ;
the characters assigned are not only those of the distribution of the sterile
and fertile zones, but extend also to other features. The branching of
the axis is almost, or entirely, absent in the simplest of the upright forms :
in the trailing or pendulous forms it is more frequent. It is of the
dichotomous type, and usually in planes successively at right angles. The
roots arise primarily from the basal region of the axis ; in no case is
there a creeping monopodial axis, with adventitious roots arising along
its whole length. In about half of the species there is no formal dis-
tinction of sporophylls from the foliage leaves : where such a distinction
exists the sporophylls still have a green colour, and as a rule an entire
margin. The spores have a pitted surface, without external processes.
These general characters indicate a natural grouping of species which are
certainly the simpler living representatives of the Lycopod type.
The second sub-genus, designated Rhopalostachya, includes the remain-
ing species which are more differentiated than the first in many of their
characters. The branching of the axis is only dichotomous in the younger
parts, and becomes monopodial later, often with a well-marked main axis.
All the species are ground-growing; a few are climbers. The upright
species are freely branched (Fig. 143); in the creeping species the
recumbent axis gives off upright branches, and is attached to the soil by
successive adventitious roots ; the fertile strobili are for the most part
definitely marked off from the sterile region, and are often carried upon
elongated stalks, which bear minute scale-leaves (Fig. 144). The differ-
entiation of the sterile and fertile leaves is constant ; the sporophylls are
pale, often chaffy scales, with toothed, ciliate margin (Fig. 143 D, E),
while the spores bear reticulate flanges or prickles on their outer wall.
These characters collectively mark off Rhopalostachya as more differentiated
than Urostachya. But it includes some species which approach the latter ;
thus the three species associated as the inundatum group show only slight
differentiation of the strobilus from the vegetative shoot, while abortive
sporangia are found at the base in L. inundatum. L. Drummondii even
1 See Pritzel, Engler u. Prantl, Nat. Pflanzenfam. , i. iv. , p. 591.
GENERAL MORPHOLOGY
295
FIG. 143.
Lycopodium cernuutn^ L., var. Eichleri, Glaz. A, general habit (J natural size); £,
end of a branch (natural size) ; C, strobilus ( X 3) ; Z>, sporophyll seen from above ; £,
ditto, from the side ( X 20). (After Pritzel, in Engler and Prantl. Nat. PJtanz.)
296
LYCOPODIALES
approaches L, Selago in its alternation of successive sterile and fertile
zones. On such grounds the innndafam group has been associated with
Phlegmaria by Baker in the sub-genus Lepidotis, though this association
is not now upheld. In the rest, however, the
definition is more exact, and is strictly main-
tained ; for instance, in L. cernuum in a very
large number of specimens a transition from
the strobilus back to the vegetative shoot was
never observed. The same is the case in the
species associated as the group of L. clavatum,
which are terrestrial trailing species, with well-
defined strobili. In this series the intercalation
of a peduncle, with small distant scales, between
the larger-leaved foliage shoot and the definite
strobilus is indicated (Fig. 144). The question
whether the peduncle is directly derived from the
basal part of the strobilus, or from a specialised
part of the already sterile foliage region, may be
left open ; but as sporangia are not found on it,
nor even any vestiges of arrested sporangia, the
latter seems the more probable source of the
peduncle : the biological importance of it in
ground-growing forms is readily understood.
Finally, the dorsiventral species, previously
grouped as the sub-genus Diphasium, are now
distributed according to their obvious affinities;
the dorsiventral character of their vegetative
shoot being held as a secondary adaptation :
the strobilus, however, remains as clearly defined
as in the more advanced representatives of the
sub-genus Rhopalostachya, and does not share in
the dorsiventral development.
A comparison of the living species of Lyco-
podium thus appears to demonstrate a progression
from a less differentiated to a more differentiated
state. In the simplest forms the whole of the
unbranched or sparsely-branched shoot is practi-
cally a fertile strobilus, which serves the double
purpose of assimilation and of spore-production.
By gradual steps the living species suggest how the two functions became
separated : a purely vegetative region was established by abortion of the
sporangia, and it was naturally located in the first-developed or lower part
of the plant, since the function of nutrition must necessarily precede that
of spore-production. The fertile upper region also became more specialised,
and in the species where it is most clearly defined from the vegetative region
FIG. 144.
Lycopodium carolinianutn, L.
.<4=figure showing habit. About
two-thirds natural size. B — stro-
bilus. Natural size. C = sporophyll
seen from above. x6. Z> = the
same from the side. x6. (After
Engler and Prantl.)
GENERAL MORPHOLOGY
297
the sporophylls no longer servers assimilating leaves, but appear as chaffy
scales, performing a protective function. It would be difficult to read from
the comparative study of the mature
sporophyte in the genus Lycopodium
any other evolutionary story than this.
The only other living genus of
eligulate Lycopods is the monotypic
Phylloglossutit) long recognised as the
simplest of them all. The mature plant
as seen above ground consists of a tuft
of almost cylindrical assimilating leaves,
from the midst of which rises the
simple axis terminated by the short
strobilus ; below ground are found two
ovate storage tubers, one dating from
the preceding year and in course of
exhaustion, the other in course of
development as a store for the succeed-
ing year. There are also one or more
roots (Fig. 145). The lower parts of
this curious little plant cannot be
properly understood till it is compared
with the embryos of certain species of
Lycopodium, for it repeats in its annual
growths their embryonic characters : the
discussion of them will therefore be
postponed (p. 351). The very short strobilus shows a similarity to the
Urostachya rather than to the Rhopalostachya section of the genus : this is
seen in the smooth margin of the rather fleshy
sporophylls, as well as in the incomplete protection
of the sporangia. It is interesting to note that tran-
sitions are occasionally found between the foliage
leaves (protophylls) and the sporophylls : Fig.
\S I/ .^** 146 A shows a case where a single sporophyll of
larger size than the normal, with a sporangium
in its axil, stands isolated some way below the
strobilus : thus it is intermediate both in position
and in character between the two types. A small
protophyll without any sporangium may also
sometimes be found at the base of the pedicel.
Dichotomous branching of the strobilus is some-
times seen, but it is rare : an example is shown
in Fig. 146 B. Such features are important for
comparison with Lycopodium, and indicate that
there is a close relation between the two genera.
+ 15
FIG. 145.
Phylloglossum Drummondir, Kunze. A =
apex of a germinated tuber ; b-±, 6-2, b%, leaves ; sp
= the young strobilus. X 15. j5=the whole
plant. X one^-half. /j = the old tuber; t% =
the young tuber; r=root. C — sporophyll with
sporangium seen from above. X 12. (After
Engler and Prantl.)
FIG. 146.
Phylloglossum Drummondii
Kunze. A = a plant showing pro
tophylls and strobilus : one sporo-
phyll of the latter is at a distance
below the rest, intercalary growth
having taken place in the axis
above it. X3. B = a. plant with
the strobilus branched into two
unequal parts. X 3.
298
LYCOPODIALES
Of the fossils which have been referred to a near affinity with
Lycopodium under the name Lycopodites, many have been shown to find
FIG. 147.
Lycopodites Stockii, Kidst. i=specimen, natural size; a-f, sporangia; g, sporophyll.
2 = sporophyll enlarged. 4 = small portion of stem, enlarged, showing verticillate leaf-
bases. (After Kidston.)
their true place elsewhere.1 But some at least of them show distinct
Lycopodinous characters : for instance, Lycopodites Stockii, Kidston, from
the calciferous sandstone of Dumfriesshire. In habit it is like Lycopodium
1 Kidston, Trans. Nat. Hist. Soc., Glasgow, vol. vi., p. 32.
GENERAL MORPHOLOGY 299
Phlegmaria, showing a terminal strebilus, with sporangia, and the sporophylls
smaller than those borne by the more lax region of the shoot below (Fig.
147). The leaves are arranged in whorls — a condition not unknown among
species of the Phlegmaria group. The sporangia do not appear to have been
restricted to the terminal strobilus, but to have occurred also in relation
to the larger foliage leaves : this is a condition which has been seen
to occur in Z. varium, as well as in the living species of the group
sub-Selago, from which the Phlegmaria group appear to be a specialised
offset. So far from this distribution of the sporangia raising a difficulty,
it seems to point to the existence in very early strata of a Lycopodinous
type showing characters which exist in living species, and which com-
parison indicates as primitive. These fossils are unfortunately rare, and
in the particular case of L. Stockii the essential facts are based upon a
single specimen.
B. LIGULATAE.
The ligulate Lycopodiales resemble the Eligulatae in general habit,
but they differ from them in the presence of a small process — the ligule —
borne on the upper surface of the leaf, near its base : also whereas the
living Eligulatae are all homosporous, all the living Ligulatae are hetero-
sporous. Selaginella is the preponderant genus of the living Ligulatae :
its vegetative development is characterised by frequently repeated branch-
ing of the axis, which bears numerous small leaves : but whereas in
Lycopodium the dorsiventral development of the shoot is the exception,
and the radial the rule, in Selaginella only a few species show the radial
construction as a permanent character : the latter, as Goebel remarks,1
usually grow on dryer and brighter spots than the dorsiventral. As
the result of experiments on species such as S. sanguinolenta^ in which
anisophylly is not constant, but appears under the influence of external
factors, Goebel concludes that the dorsiventrality is a phenomenon of
adaptation brought about by light : thus the radial type will naturally
be the more primitive. In the great majority of the living species,
however, the strobilus is isophyllous, even where the vegetative shoot
is anisophyllous : thus indicating that it is the more conservative part
of the plants. But in ' some ten per cent, of the living species the
strobilus itself is also anisophyllous. The definition of the strobilus from
the vegetative shoot in Selaginella is more marked than in Lycopodium :
a condition corresponding to that of the Se/ag0-group of Lycopodium is
unknown, nor have isolated sporangia ever been observed in the vegeta-
tive region : the differentiation of the sporophyte of the genus as a whole
corresponds to that of the more specialised types of Lycopodium. But
imperfect sporangia have been observed at the base of the strobilus of
.S. spinulosa, and Martensii, and would doubtless be found in many
other species : this condition is open to the same interpretation as
1 Organography ', vol. i., p. 105.
300 LYCOPODIALES
in Lycopodium. As is well known, the megasporangia and microsporangia
are alike in their early stages of development, though differing later in
the spores which they produce : this additional degree of differentiation
in the genus falls in with the higher differentiation noted in the vegetative
organs, as compared with Lycopodium.
Of the species with radial construction, the best known is £. spinulosa,
specially investigated by Bruchmann : x this will be briefly described for
purposes of comparison on the one hand with Lycopodium, and on the
other with the related fossils, while the dorsiventral Selaginellas may be
regarded as specialised offsets from some such radial type as this. The
seedling of 6". spinulosa is like other Selaginellas in having an upright
elongated hypocotyl (Fig. 148), which is continued directly into the primary
root : the hypocotyl bears two cotyledons, after which a variable number
of pairs of epicotylar leaves are formed
before the first branching, which is a
true dichotomy. The limbs thus formed
branch repeatedly, at first dichotomously,
but later monopodially, all the branch-
ings being in one plane, at right angles
to that of the first dichotomy : thus two
fan-like branch-systems are produced, of
which certain stronger branches are
Selaginella spinulosa. G=young seedling . ., , ., /T-,. x — ,.
with megaspore attached, showing elongated fertile, the TCSt Sterile (Fig. 149). The
hypocotyl (H) and cotyledons K. ff= seedling f ,1 i r .1
more advanced showing dichotomy. ^=base arrangement of the leaves of the primary
of hypocotyl with swollen knot. W= roots. • • j
^hypocotyl. .ff^suspensor (after Bruch- aXIS IS deCUSSate, but On the later
branches there are transitions to spiral,
while in the thicker strobili the arrangement is on a complex spiral
plan. The main axis terminating below in the hypocotyl remains
permanent, and its' base swells at the level of the suspensor to form a
knot, from which alone the later roots originate ; they are formed
endogenously in swellings of tissue of the knot, and burst their way
outwards through the superficial tissue. The whole plant of S. spinulosa
is thus dependent upon a central source of water-supply from the base
of the main axis. In most species of Selaginella^ however, the well-
known rhizophores are formed, at the branchings of the axes of higher
order, and thus their rooting may be efficiently carried out at a distance
from the primary axis : this is probably a derivative condition, just as
the dorsiventral development, of which it is the usual concomitant, is
also derivative. Both in the form of the shoot, and in the central root-
ing, the type of S. spinulosa may be held to be more primitive than
the common dorsiventral type of the genus : in these respects it will be
seen to correspond more nearly with the large fossils than do the more
specialised species of the genus.2
1 Unters. iiber S. spinulosa, A. Br., Gotha, 1897.
2 See Goebel, Organography, vol. ii., p. 230.
GENERAL MORPHOLOGY
301
But the Fossil Ligulates wer< not all large. There is evidence that
small organisms, corresponding in habit to the heterophyllous Selagindlas,
existed also in early geological times. The fossil from the Upper Coal
Measures, described as Lycopodites Gutbieri, Gopp, can hardly have been
anything else. Lycopodites primaevus, Schr., from the Westphalian Middle
Coal Measures, though it shows no distinctly Selaginelloid shoot, has
heterosporous sporangia, with megaspores more numerous than four in
each sporangium, as shown me by Mr. Kidston, in specimens belonging to
the Brussels Museum. A similar condition has been described by Zeiller1
FIG. 149.
Plant of Sclaginella spinulosa, with root-system springing from swollen knot at base of
the upright hypocotyl. Natural size.
in a plant from Blanzy, named by him Lycopodites Suissei, where the
number of megaspores ^vas found to be 16 to 24. In these cases the
reduction in number of the spores as a consequence of heterospory
appears to have proceeded less far than in the modern Selagtnella.
But, on the other hand, the carboniferous plant described by Bertrand
as Miadesmia corresponds in structure, as well as in the heterophyllous
arrangement of the leaves and in the presence of a ligule, to Selaginella,
while it appears to have progressed towards a seed-like fructification.
The minute new species Miadesmia membranacea, Bertrand, has been
directly compared with Selaginella spinulosa ( = S. selaginoides, Link) by
Miss Benson,- in respect of characters other than the seed-like structure
1 Comptes Rcndus, April, 17, 1900. - Proc. Roy. Soc., Series B, vol. Ixxix, p. 473
302 LYCOPODIALES
borne by the megasporophyll. So far as it goes, then, the evidence from
the fossils favours the conclusion that plants resembling Selaginella existed
in the primary rocks, and that even the more specialised heterophyllous
type of Selaginella dates at least from the Carboniferous period, while
it seems possible that a seed-like habit had already been established.
The dendroid Lycopodiales are among the earliest known fossils,
dating from the Lower Devonian period to the Trias. They include the
families of the Lepidodendraceae, Bothrodendraceae, Sigillariaceae, and
FIG. 150.
Ground plan of a Tree-stump with Stigmaria-trunks. One-sixtieth the natural size.
(After Potonie.)
Pleuromoiaceae. Underlying the differences of detail according to which
these families are distinguished, there is a general unity of morphological
plan : the essential features of it are as follows. The main axis was
upright, rising in some cases to a height of 100 feet. It was bulky
relatively to the numerous simple leaves which it bore : it branched
upwards in a dichotomous manner, in most cases profusely : in some of
the Sigillariaceae, however, and in Pleuromoia branching may be entirely
absent. The development of the branches of the dichotomy were in
various cases either equal or unequal, a fact which leads to differences of
habit, as is seen to be the case in Lycopodium or Selaginella. The axis
was fixed in the soil by a shallow and broadly spreading system of
Stigmarian trunks (Fig. 150). In Lepidodendron the main Stigmarian
trunks usually numbered four, which bifurcated repeatedly, thus forming a
GENERAL MORPHOLOGY
303
widely spreading system : from these the rootlets radiated in all directions,
developing to a length of a foot or more, and showing dichotomous
branching. The underground system was thus proportional to that above
ground. In the Sigillariaceae similar trunks are found, but it seems
doubtful whether they show the same constancy of initial type as in
Lepidodendron. In Pleuromoia the base of the upright stock swells into
a tuberous body, which is very Stigmaria-like in the fact that it is
covered by root-scars, while it extends into four blunt processes corre-
sponding in position and character to Stigmarian trunks, though much
shorter (Fig. 151). It would
seem probable that in this
relatively late Triassic fossil
(which is unfortunately known
only in the form of casts,
not structurally), a simple
representative of the Lepido-
dendroid basal region is cor-
rectly recognised. In all of
the dendroid forms the Stig-
marian trunk appears to have
been present, as a basis for
the roots : but the latter were
not restricted to that position :
Potonie shows how the scars
of their insertion may be
sometimes found on the leaf-
bearing axes also, associated
with some degree of regularity
with the leaf-scars.1
The leaves of the fossil
Lycopodiales were sometimes
of considerable size, but un-
branched and of simple form.
They expanded at the base into the well-known cushions, which in many
forms occupy the whole, external surface of the axis : this corresponds to
what is seen among the living Lycopods. On the upper surface of the
leaf, near its base, the ligule is seated : it appears to have been a constant
feature in the dendroid Lycopodiales, and the occurrence of it links them
rather with Selaginella than with Lycopodium. It was often seated in a
deep pit — as it is in some living Selaginellas— and this pit persists as a
marked feature in the neighbourhood of the leaf-scars, whenever the cast
of a stem-surface is well preserved (Fig. 152).
The vegetative region appears to have been, as a rule, purely
vegetative : the sporangia are restricted to well-defined cones or strobili,
1 Lehrbnch der Pflanzenpalaeontologie, p. 212, Fig. 215.
FIG. 151.
Pleuromoia Sternbergii. Swollen base of stem with root-
scars, and showing part of the aerial stem, with the epidermis
and leaf-scars on the right, and on the left the sub-epidermal
sculpture. (After Bischof, from Engler and Prantl.) Two-thirds
natural size.
304
LYCOPODIALES
similar in their general characters, as also in their clear definition, to
those of the more differentiated types of Lycopodium or to Selaginella.
The general structure of the strobilus is, as in other Lycopods, essentially
the same as that of the vegetative shoot, excepting in the presence of the
sporangia. These are of very large size, and are commonly extended radially
outwards from the axis, being interposed between the axis and the
ligule : the latter then appears on the upper surface of the sporophyll,
beyond the distal limit of the sporangium (Fig. 153). In Lepidodendron
the cones thus constructed were borne on the ends of the ordinary
B
FIG. 152.
Lepidophloios, sp. A = tangential section from the outside of a stem, passing through
the leaf-bases, and showing their characteristic form, slightly enlarged. £ = a single
leaf-base, to show details; £•£ = collateral vascular bundle; /a = the two parichnos-
strands; lg= ligule in its pit. x 10. Will. Coll., 1974 A. (After Scott.)
branches; but in Sigillaria they appear to have arisen laterally upon the
main axis, from which after maturity they were deciduous, and each was
borne upon an elongated pedicel covered with acicular bracts, while the
cone itself showed a construction essentially similar to that of a small
Lepidodendron .
Though the type with a definite cone marked off from the sterile
region was usual for the fossil Lycopodiales, it was not universal. In the
imperfectly known plant, Pleuromoia from the Trias, the whole main axis
seems to have been a strobilus (Fig. 154), borne upon a Stigmarian base
(compare Fig. 151). But a much more satisfactory example, from the
Westphalian series (Middle Coal Measures) is that of Ptnakodendron
musivitm, Weiss, specimens of which, discovered by M. Hector Delteure
GENERAL MORPHOLOGY 305
at Mariemont in Belgium, are -about to be described by Mr. Kidston : to
him I am indebted for the information that this large Lycopod bore its
sporangia associated with the leaves of certain portions of the stem, without
any cone-formation, or alteration of the form or disposition of the leaves
which bear them : the fertile and sterile portions are distinguished only
by the presence or absence of sporangia. It is, in fact, a typical repre-
sentative of the " Selago " type, but of dendroid dimensions. In this
connection it is interesting to note that Solms Laubach mentions certain
" remains of great size, remarkable for the unusual thickness of the axis
— classed by Lesquereux with Lepidophloios. Weiss also has described
a similarly colossal cone as Lomatophloios macrolepidotus, but, unfortunately,
FIG 153.
Lepidostrobus. Diagram showing axis and sporophylls in radial section. # = axis of
strobilus ; £ = sporophylls and sporangia; .r^stele; i, c = inner cortex; ni, c = middle
cortex ; o, c — outer cortex ; / = pedicel ; la — lamina of sporophyll ; /z'=ligule ; /, t= leaf-
trace ; sp, «/ = wall of sporangium. (Enlarged after Maslen, from Scott, Studies in
Fossil Botany.)
there is no detailed account of it. The enormous size of the axis in
these specimens gives rise to the suspicion that the fructification was not
confined to special fertile shoots, but might occasionally appear on the
leaves even of the main stem, which then increased in thickness, much
as we see in the present day in the female flower of Cycas, and mutatis
mutandis in Lycopodium Selago. We naturally ask, on what sort of scars
could such cones be seated as lateral organs?"1 Kidston's description of
Pinakodcndron shows that the "Selago" condition did actually exist in
dendroid types, and thus resolves the difficulty. A similar condition is
shown by the small Lycopodites ciliatus^ Kidst, from the Middle Coal
Measures,2 while the still earlier Lycopodites Stockii (compare Fig. 147
above) also has its sporangia associated with leaves of the foliage type.
Finally, the imperfectly known Lycopodites Reidii, from the Devonian of
^Fossil Botany, Engl. edn., p. 235.
2 Trans. Nat. Hist. Sac., Glasgow, vol. vi., p. 37.
U
306
LYCOPODIALES
Scotland, has been compared by Penhallow
with Lycopodium Selago as regards the un-
differeritiated shoot.1 Without attaching too
much importance to the last example, it
appears certain that Lycopods, even of large
size, existed in very early times, in which
there was no clear differentiation of vege-
tative and fertile regions : in fact, the
" Selago " condition dates back to the
Primary Rocks.
There can be no question of the Lyco-
podinous affinity of the fossils thus described
briefly in their general morphology : it
remains then* to indicate where the nearest
correspondence is to be found between
them and living forms. They are plainly
related to the Ligulate Lycopodiales, and,
being of a radial type of shoot, and usually,
if not always heterosporous, the correspon-
dence is nearer to the radial species of
Selaginella \ this suggests a comparison with
.5. spinulosa, from which some interesting
points will emerge. In the first place, the
difference of size is to be discounted : how-
ever diverse the gigantic Lepidodendron may
seem from the minute S. spinulosa, the com-
parison really relates to the relative position
and character of the parts composing the
plant-body. The parts which form the shoot
— axis, foliage-leaf and sporophyll, the ligule,
and the sporangium — are identical in both
as regards their relative positions, though
differing greatly in their number and dimen-
sions : in the dichotomous branching, and
in the relation of the resulting shoots to the
upright main axis they are alike : also in
the dependence of the whole plant for its
water-supply upon the base of the primary
axis. In fact, Selaginella spinulosa is like a
Lepidodendron in miniature, as regards the
scheme of its construction. The comparison
extends also to that curious knot which is
found at the base of the main axis in S. spinulosa : here the origin of the
roots is strictly localised : they appear endogenotisly on indeterminate
1 Canadian Record of Science, 1892, p. 8.
I
FIG.
154.
Pleuromoia Sternbergi. Axis, with
the lower part of the terminal strpbilus.
Two-thirds natural size. After Bischof.
(From Engler and Prantl.)
GENERAL MORPHOLOGY 307
outgrowths from the axis itself, ^vhich have been regarded as rudimentary
rhizophores. It does not seem an undue strain of comparison to suggest
that in this basal knot is still to be seen, on a minimal scale, a living
representative of those larger growths known as the Stigmarian trunks.
These would thus be in their nature indeterminate outgrowths of the
hypocotyl, as are these rudimentary rhizophores ; but like them, strictly
localised in origin, instead of being dispersed over the branch-system, as
are the rhizophores in most modern Selaginellas. It is thus possible to
bring the general morphology of Lepidodendron into relation to that of
the modern Selaginella, a type which there is reason to believe itself
dated from the Carboniferous period.
On the other hand, there are obvious relations between the dendroid
Lycopodiales and the living genus Isoetes : this type has been found
fossil in the Tertiaries, and back as far as the Lower Chalk, while in the
Trias the curious fossil Pleuromoia is represented : but there is no suffi-
cient evidence of the genus Isoetes having itself figured among the earliest
fossils.
The plant of Isoetes consists of a short upright axis covered by relatively
large leaves (Fig. 155) : the axis is usually unbranched, though bifurcation
occasionally occurs, a fact that has its interest for comparison with the
Lycopods.1 The leaves are essentially of one type, with broad base and
acicular upper part, while seated in a pit on the upper surface, at some
little distance from the base, is the ligule. The leaves may be either
sterile or fertile, and in some species there is a difference in size, the
sterile leaves being the smaller. The plant is heterosporous. Where the
leaf is fertile the large cake-like sporangium lies in a depression of the
leaf-surface, between the ligule and the leaf-base, that region being
elongated to accommodate it : in the sterile leaves it is shorter. An
examination of the sterile leaves of /. lacustris (and Wilson Smith made
similar observations in /. echinospora) shows that sporangia in various
degrees of abortion may be found upon them : in some of these spores
are developed, but in smaller numbers than the normal : other sporangia
may remain quite small, and produce no spores. Dissections show that,
in the majority of leaves that are apparently sterile, a rudimentary sporan-
gium is really present hi a normal position. It is stated that a regular
seasonal sequence is followed in the distribution of the megasporophylls,
the microsporophylls, and the foliage leaves : that the megasporangia are
borne on the first or outermost leaves of each annual increment, then
follow leaves with microsporangia, while the sterile leaves form the transi-
tion from one year's increment to the next. It is thus seen that in the
distribution of its sporangia Isoetes shows a condition similar to that of
Lycopodium Se/ago, but that the various degrees of their abortion are
better represented. *It follows from the facts that after the embryonic
stages are past — in which no sporangia are produced — the whole plant, is
1 Solms Laubach, Bot. Zeit., 1902, p. 179.
308
LYCOPODIALES
FIG. 155.
Isoetes lacustris, L. A= plant of natural size. B = base of a fertile leaf with
ligule (/), and sporangium (sp) seen from above. C = longitudinal and D transverse
sections of the leaf-base; sp = sporangium ; /=ligule. (From Rabenhorst's, Krypt.
Flora.')
GENERAL MORPHOLOGY 309
potentially a fertile strobilus, in which the vegetative and reproductive
systems are not differentiated from one another. This, together with its
prevalent absence of terminal branching, points out Isoetes as a near
approach in its general construction to the strobiloid type theoretically
primitive for the Lycopodiales : this it shares with the simplest Selago-
forms of Lycopodium. But it is with the dendroid Lycopodiales that
Isoetes shows common characters of the sporangia themselves : there is
also some similarity to them in the structure of its abbreviated but bulky
stock : on this also the very similar bifurcating roots are inserted, but in
Isoetes their origin is localised in the depressed grooves which traverse
the stock longitudinally, instead of their being borne on Stigmarian out-
growths, as in the fossils. The Isoetes plant is then like a partially
differentiated Lepidostrobus seated upon a Lepidodendroid base : in fact,
like a stunted Lepidodendron, with its preliminary vegetative phase very
short. Its mature shoot still carries on both vegetative and propagative
functions, and in this lack of differentiation a primitive character is to be
recognised.
The account thus given of the general morphology of the mature
sporophyte in the Lycopodiales, living and fossil, shows the essential
identity of their plan of construction throughout the phylum, and how in
the two series, the ligulate and the eligulate, parallel conditions of differ-
entiation are represented. In both the structure of the shoot is essentially
strobiloid, with a constant numerical relation of the sporangium to the
subtending sporophyll. In both series the branching of the axis is primarily
by dichotomy, with a deviation in the more specialised types, and especially
in the higher ramifications to the monopodial branching : but in certain
simple -types branching is rare, or even absent. The shoot is fixed in the
soil by roots, formed chiefly, or even exclusively, at the base of the axis
in the simpler types; but in the more specialised they may be formed at
various other points on the shoot-system, or on outgrowths from it of an
indeterminate character. In both series there is evidence of abortion of
sporangia, leading to a segregation of definite tracts of the shoot-system
devoted to nutrition and to propagation respectively : in the higher types
the strobilus becomes a definite cone of limited growth, clearly marked
off from the lower vegetative region : the production of spores is thus
deferred in the individual life, and a more lengthy vegetative phase
intercalated before that event. This progressive differentiation is best
illustrated in the eligulate series, which is also the more primitive in
respect of its homosporous condition. We are thus led by comparison
of the Lycopodiales, living and fossil, to contemplate as a fundamental
type of their shoot a simple unbranched strobilus with unlimited
apical growth, bearing un differentiated leaves, and having one sporangium
associated with each leaf. This may not improbably have been the
primitive type from which, by branching, by formation of a root-system,
by differentiation of the sterile from the fertile region, and, finally, by
3io LYCOPODIALES
cessation of apical growth in the fertile branches, the whole series of
forms included in the Lycopodiales arose. It will remain to be seen
how far the detailed study of the sporangia, and especially of the
anatomy and embryogeny of the Lycopodiales, will support this hypo-
thetical origin.
CHAPTER XXIV.
SPORE-PRODUCING MEMBERS OF THE LYCOPODIALES.
THE normal sporangia in the Lycopodiales are always non-septate sacs,
excepting that in the megasporangia of Isoetes there may be an isolation of
the megaspore-mother-cells (see p. 320). The form is that of a kidney, of
which the curvature and proportions are liable to considerable variation.
The position is, as we have
seen, essentially constant, each
sporangium being subtended by,
or inserted in a median position
relatively to its sporophyll; the
curvature is in the tangential plane,
showing in tangential section a
more or less pronounced fan-like
outline. A series of examples
of sporangia will be selected as
illustrating the structure and
mode of development, and the
degree of variation in form and
proportion which exists within
the phylum.
The genus Lycopodium will
be taken first, and the spor-
angia compared in a number
of species. It will become
apparent from this comparison that the differences which they show are
not at haphazard, but that they follow with some degree of accuracy those
lines of external differentiation, upon which the systematic arrangement of
the genus has been based. In order to make this clear the description will
follow the accepted systematic order,, beginning with the least differentiated
types. In Z. Selago J the sporangium originates at the base of the sporophyll,
^Studies, i., p. 511.
FIG. 156.
Radial sections through young sporangia of Lycopodium
Selago. In the youngest the whole sporophyll is shown (/),
and the axis (sf), and it is seen that the sporangium arises
upon the surface of the sporophyll. The older stages show
the segmentation of the sporangium. X 200.
312
LYCOPODIALES
but clearly upon its upper surface as a transversely extended cushion
(Fig. 157 A). In median radial section it appears as a convex growth, in
which a central row of three cells, the result of periclinal division of one
parent cell, is dominant (Fig. 156): of these the middle cell is of arche-
sporial character. A tangential section of a sporangium of similar age
(Fig. 157 B) shows that there are a number of these archesporial cells:
in the example shown there are seven : but the number is not constant,
as is shown by comparison of various tangential sections, and supported
by sections cut transversely (Fig. 157 c). The young sporangium consists
thus of a single tangential row of archesporial cells, covered in completely
Yr ~*S2
.
FIG. 157.
Lycopodium Selago. A= young sporangium seen in superficial view: s< = stem;
/=sporophyll. .Z> = tangential section of a similar sporangium, the cells numbered i,
ii, iii correspond to those similarly marked in Fig. 156. C = a sporangium of like age in
transverse section, as along a line s, s, in Fig. B. The archesporial cells are shaded.
Z> = an older sporangium, in radial section, showing the spore-mother-cells separated,
before tetrad-division. A , B, C X 200. D X 100.
by a single layer of cells forming the sporangial wall, and supported
below by cells which grow more actively in the middle region of the
sporangium, thus leading to the curved form which it assumes later. It
is clear also that all the essential parts of the sporangium originate from
several superficial cells of the sporophyll, and that it is impossible to refer
them in origin to any single parent cell.
At first the parts thus laid down often grow uniformly, so that their
mode of origin may still be traced in a more advanced state : but later the
more numerous divisions are less regular. Superficially they result in the
formation of a sporangial wall, composed of three layers, or of more
towards the base of the sporangium (Fig. 157 D): of these the innermost is
the transitory tapetum : the tapetal investment of the sporogenous tissue is
completed by development of the adjacent cells of the sub-archesporial
SPORE-PRODUCING MEMBERS
313
FIG. 158.
d, e,f= sections radial, tangential,
and transverse of matu
tissue also as tapetum. The dehiscence is along a transverse line, and the
preparation for this is already indicated at the distal end in Fig. 157 D, d.
Meanwhile, the sporogenous group within, in
the formation of which the whole products of
the archesporium are involved, has been subject
to repeated cell-division: its cells finally separate,
round themselves off, and all of them, as a rule,
undergo the tetrad-division. In the mature
sporangium the form is less strongly curved
in L. Selago than in many other species, while
the stalk is a relatively narrow one. The
general proportions, as well as the imperfect
protection of the sporangium afforded by the
rather narrow sporophylls, are shown in Fig.
158 d, e, f.
The type of sporangium thus described for
L. Selago, with its single row of archesporial
Cells, relatively narrOW Stalk, and imperfect ana transverse ot mature sporangia
of Lycopodium Selago. g, n, i —
protection while VOUnSJ, appears tO be Character- similar sections of mature sporangia
* . . . °f Lycopodmm phlegmai'ia. X 12.
istic, with relatively slight modifications, of the
sub-genus Urostachya : other species of the sub-genus which have been
examined, viz., L. dichotomum, Jacq., carinatum, Desv., nummularifoliitm,
Blume, and Phlegmaria, L., are all alike in
showing an archesporium consisting of a single
tangential row of cells, though the number of
these in the row may vary ; the simplest case
observed was that of L. Phlegmaria, where the
single series consisted of certainly not more than
five cells, and perhaps of less. The further
development in these species was also the same
as in L. Selago, though the proportions were
different. In L. dichotomum, however, there is
the peculiarity that the sporangial wall is found
to be more massive, consisting of 4-7 layers.
Putting such differences aside there seems
reason to regard the single tangential series of
archesporial cells as a common feature of the
sub-genus Urostachya : further, the sporangia
are inefficiently protected by the sporophylls
FIG. 159.
Lycopodium innndatuiti,
Radial sections of sporangia.
In
the upper, younger figure periclinal /Fi°" 1^8 °" Jl l\
divisions are shown in two cells, and X O" 06'
the archesporial ceils are shaded. jn pritzel's arrangement of the genus the
In the lower, older figure the pro-
duct of division of these ceils is section Inundata is separated from Phlegmaria,
shown. X 200.
and placed in the second sub-genus Rhopalo-
stachya. We shall see that the sporangial character upholds this change. The
sporangia are from the first more bulky than in Urostachya (Fig. 160 k, /, m\
314
LYCOPODIALES
_
and this has been found in L. inundatum to go along with a more bulky
origin. The sporangium as seen in radial section arises as a broad swell-
ing, while two cells have been seen to divide periclinally, indicating at
least two tangential rows of archesporial cells in place of the single row
in L. Selago. This origin of the sporogenous tissue may still be traced in
the older stages (Fig. 159). It may be that this condition is not
actually constant in all cases, but it has certainly been observed to exist
in L. inundatum.
Of the rest of the sub-genus Rhopalostachya, Z. clavatum and L. alpinum
have been examined, and they both show a still more massive type of
sporangium. This is seen
in the mature state (Fig.
1 60 q, r, s: t, u, v, w),
where the stalk appears
to be short and thick :
moreover, it is seen that
the strobilus is constructed
in these species so as to
afford more complete pro-
tection to the sporangium
while young, than is the
case in the simpler type
of Z. Selago. This is
effected by special de-
, r , ,
VCIOpment OI the lower
c i •, -,-,
parts of the sporophylls
(Fig. 22 D, E): in some
cases, as in L. cernuum, the sporophyll takes a peltate form. Radial sections
of the young sporangium show, both in L. clavatum and in L. alpinum,
that from the first the form is broader still than in the types previously
described. At least three cells in each radial section are involved in the
origin of the archesporium, sometimes even more than three (Fig. 161 A, B).
Occasionally periclinal divisions appear in the superficial cells, by which
subsequent additions may be made to the archesporial tissue (Fig. 161 B,
cells marked x). The . tangential sections also show an advance on the
Selago type : for twelve is not an uncommon number of the archesporial
cells in one tangential row as against seven in Z. Selago, or five in
Z. Phlegmaria. Countings of the sporogenous cells laid bare in sections
of sporangia at an age approaching the tetrad-condition show that their
number is far in advance of those of the Selago type : this is the condition
to be anticipated from the bulky character of the sporangia (Fig. 161 c, D) :
moreover, their thicker and shorter stalks would be well fitted to transfer
the necessary nourishment for the larger spore-output. It may be noted
that in these large sporangia occasional irregular processes project
upwards from the base of the sporangium into its cavity, which would
FIG. 160.
Drawings to illustrate the form and manner of protection of the
sporangia in the sub-genus A' hopalostachya, of Lycopodium. k,Z,m,
sections of L. inundatum ', ff, r. s. of L. alpimnn ; t, u. z>, TV, of L.
ciavatum. x 12.
SPORE-PRODUCING MEMBERS
315
assist in conveyance of nourishment to the large mass of developing
spores. Thus in the main features of form and dehiscence the sporangia
of Rhopalostachya conform to the type of Z. Selago, but are larger and
more productive; while the sporophylls have a more elaborate form for
purposes of protection. This goes along with the differentiation of the
vegetative from the propagative regions, the steps of which have been
traced above in the genus Lycopodinm. The conclusion seems justly to
follow that with this differentiation, which has apparently involved a
diminution in the actual number of sporangia by abortion, there has come
FIG. 161.
Lycoppdium alpinutn, L. A — Radial section through a sporophyll and young
sporangium. .5 = the same older; in both the sporogenous tissue is shaded. D — radial
section of an older sporangium ; s/ = stem. C= tangential section of a sporangium of Lye.
clavatum, of similar age to D ; in both these figures the sporogenous tissue is referable
in origin to three rows of cells. A, fi, Cx 200. D X 100.
into existence a more massive type of sporangium, together with a more
extensive spore-output from each of them, and a more specialised protec-
tion of them while young.
It has been seen that the strobilus of Phylloglossum resembles that of
the sub-genus Urostachya^ rather than that of Rhopalostachya. An exami-
nation of the developing sporangium supports this comparison, for only a
single row of about six archesporial cells is found; but, on the other
hand, the outline of the sporangium, and the relative thickness of the
stalk, show some similarity to Z. immdatum.
The sporangium of Selaginella corresponds in general type to that of
Lycopodium. It is usually described as arising from the surface of the
axis: in some species it does so (S. Martensii) (Fig. 162), but in others
it is seated more nearly upon the surface of the leaf; in fact its position
LYCOPODIALES
may vary in different species though the numerical relation of one to each
subtending leaf is strictly maintained. There is considerable divergence
of opinion as to the details of its early development, which not
improbably arises in part from want of exact uniformity in different
species, partly from difficulty of observation, owing to the small size of
the cells in young stages.1 It has been seen above that S. spinulosa is
among the least differentiated species, as
regards external form, and on that account
it deserves special attention. The de-
scription here given will be based on that
species. According to Goebel the whole
sporogenous tissue, as seen in the radial
section in 6". spinulosa, is referable in
origin to a single archesporial cell, which
is, however, one only of several forming
a tangential series. I do not deny that
this may sometimes be the case ; but in
my sections two primary archesporial
cells were usually present (Fig. 163 A, B),
somewhat as in Lycopodium inundatum.
Tangential sections show that these
represent two rows of archesporial cells,
with about four cells in each (Fig. 163 D).
Thus the correspondence in sporangial
type with that of Lycopodium is very striking, as regards early development :
the chief difference is in the origin of the tapetum, for this in Selaginella
is cut oft' by tangential divisions from the sporogenous tissue (Fig. 163 c, E),
of which it is thus a sterilised part. There is reason, however, to think
that the first periclinal divisions in the young sporangium do not always
1 Observations have been made on various species of the genus: Goebel (Bot. Zeit.,
1881, p. 697) investigated S. spinulosa, helvetica, and Wallichii, and his results are
restated in his Organography, vol. ii., p. 600 ; allowance is, however, made by him for some
degree of variation in details. My own observations on S. spinulosa, and Martensii are
described in my Studies, i., p. 522. Campbell, in his Mosses and Ferns, 2nd edition,
p. 530, describes the development for S. Kratissiana, but his figures are by no means
convincing that his reference of the whole sporogenous tissue to a single parent cell in
the radial section is correct. Miss Lyon (Bot. Gaz., xxxii., p. 124) has made a
careful study of the development in S. apus, and rupestris, and traces the sporangium
frequently if not always to a single superficial cell, which she designates the archesporium ;
but as the results from radial sections were not accurately checked by comparison of
tangential or transverse sections, the point of ultimate origin of the whole sporangium
from a single superficial parent cell cannot be regarded as demonstrated for this species.
Before the details for the genus as a whole can be properly understood, the development
will have to be studied in tangential as well as in radial sections, in a number of different
species selected from different sections of the genus ; meanwhile the substantial agreement
of the sporangial type between the less differentiated S. spinulosa and the genus
Lycopodium is the main point of interest for the present discussion.
FIG. 162.
Selaginella Martensii, Spring. Sporangia
in radial section. A traverses the stem apex
(«/), the sporophyll (/), and sporangium (x) ;
in the latter two archesporial cells are seen,
shaded. B shows an older stage. X 350.
SPORE-PRODUCING MEMBERS
317
define the future sporogenous tksue from the sporangial wall, but that by
further periclinal divisions of the superficial cells additions may be made
to it. If this be so, then
the distinction between A
the two sources of origin
of the tapetum does not
appear so marked as at
first sight it might appear
to be.
In the microsporangia
all the cells of the sporo-
genous group may under-
go the tetrad-division,
and form microspores ;
but Miss Lyon found
that in S. apus not more
than five-sixths of them
were fertile, the rest dis-
appear. In the mega-
sporangia, as a rule, a
single cell is early differ-
entiated by its denser
protoplasm from the rest :
FIG. 163.
Selaginella spinnlosa. A, -£ = radial sections through young
sporangia of successive ages. C=a transverse section of one more
advanced. D — a tangential section. E=a radial section of an older
sporangium showing all its essential parts, together with the ligule and
part of the sporophyll. A,£,C, 0x350. .fix 200.
this alone undergoes the
tetrad-division, and forms megaspores (Fig. 164). But in S. rupestris a
smaller number of megaspores, or even only one, may come to maturity :
in S. apus, however, two mother-cells
may become matured, and eight mega-
spores be thus formed in one sporangium.
These fluctuations have their interesting
bearing upon the origin of the hetero-
sporous differentiation, showing that there
is some margin of variation in the num-
ber of spore-mother-cells which are fertile
even in forms now living.
The facts relating to the sporangium
in Selaginella^ though imperfectly known
for the genus at large, show that in
position and in general plan the spor-
angium is of the usual Lycopod type ;
but that its dimensions are smaller than
is usually the case in Lycopodium : the
difference in origin of the tapetum is probably related to the smaller size
of the whole sporangium. The heterosporous condition appears to have
brought with it only minor modifications of the original sporangial type.
FIG. 164.
Selaginella spinulosa, A. Br. Section of
megasporangium showing the single fertile
tetrad still very small, and the rest of the
sporogenous cells arrested. X 100.
318 LYCOPODIALES
In Isoetes the position of the large sporangium, between the ligule
and the axis, corresponds to that in Selagznella, though it is here more
definitely inserted on the leaf-base, and is sunk in a deep depression of
its upper surface (Fig. 155 B, c, D) ; but these differences of detail do
not obscure the essential unity of the plan in the two genera. Instead
of being a body more or less flattened between the sporophyll and the
axis, as in Lycopodium and Selaginella, the sporangium is here extended
radially outwards from the axis into a broad cake-like body. It may
best be regarded as a result of such variation of dimensions as has been
FIG. 165.
Isoetes lacustrh; L. A— radial section through base of sporophyll with ligule (/),
velum (f), and sporangium, in which the archesporium is shaded. B — &. similar section
of an older sporangium. C = part of an older microsporangium, showing the potential
archesporium differentiated into trabeculae (t>-\ and sporogenous tissue (sfi), while the
tapetum (t) is clearly defined /-> = an older stage with spore-mother-cells separated, and
tapetum shaded covering the trabeculae. A, Z>'X 200. C, D x 100.
seen in minor degree within the genus Lycopodium, but here carried to
greater lengths. The developmental details harmonise readily with this
view. The microsporangium is naturally a better basis for comparison
with the homosporous Lycopods than the megasporangium, and it will
therefore be taken first The mature structure of a microsporangium is
shown in Fig. 155 D, which indicates how the very large internal space
is traversed by the sterile trabeculae : these extend, with many irregularities
of branching and wing-like expansions, which are not shown in the figure,
from the sub-archesporial tissue to the covering wall. The type of the
megasporangium is the same, though the trabeculae are here fewer in
number but more massive, so that the proportion of sterile tissue to the
fertile is much larger in the megasporangium. As the development shows,
SPORE-PRODUCING MEMBERS 319
the trabeculae have a common^ origin with the fertile sporogenous cells:
there has in fact been a sterilisation of potentially fertile tissue, which
proceeds to a greater length in the megasporangium than in the micro-
sporangium. The early development of both types of sporangia is alike
up to a fairly advanced condition, as is the case also in Selaginella ; this
fact has its bearing on the origin of their differentiated state.
The sporangium of /. laacstris originates from superficial cells of the
leaf-base of small number, lying below the ligule (Fig. 165 A.) The cell seen
immediately below the ligule in the longitudinal section of the young
leaf forms the velum : the rest show some evidence of common origin
by earlier anticlinal segmentation : this may very well have been so, but
the comparative interest begins with their periclinal divisions, and it is
then that a basis appears for comparison with what has been seen in
Lycopodium. The periclinal division appears first in the central part of
the young sporangium, and thence it extends in either direction : in the
longitudinal section some four or five cells are involved in /. lacustris,
though apparently the number may be smaller in 7. echinospora.1 Com-
paring this with the condition as seen in Lycopodium, it appears to be
an advance on even the most complex type, such as 7. alpinum ; and
this is completely in accordance with the radially extended form of the
mature sporangium of Isoetes. Moreover, the differences beween Wilson
Smith's description for 7. echinospora and my own for 7 lacustris suggest
that differences of radial extension of the sporangium exist in different
species of Isoetes similar to those which have been shown to occur within
the genus Lycopodium. But there does not appear to be any such cor-
relation of them with the morphological differentiation of the plant at
large as that which was traced in Lycopodium, and gave a special interest
to the sporangial differences in that genus.
The internal cells thus cut off by the first periclinal divisions are
destined to be sporogenous ; but the first periclinal divisons thus initiated
do not absolutely define the future sporogenous tissue : it has been
repeatedly seen that additions to it may be made by subsequent periclinal
division of the superficial cells, especially in the middle region of the
1 Wilson Smith found in /. echinospora that he was able to trace the origin of the
sporangium back in longitudinal sections of the leaf to a single cell lying between the
ligule and the leaf-base : this corresponded to a transverse row of three to five cells, which
formed the rudiment of the sporangium ; but the cell thus recognised in the longitudinal
section also formed the velum, which on that account he accepts as a sterilised part of
the sporangium. Doubtless this is a logical outcome of a last analysis of cell-origins,
provided it be assumed that all things are homologous which have a common origin from
ultimate parent cells (see Chapter VIII.). But is there any other line of evidence than
that of cell-origin to show that the velum was ever a part of a sporangium, or anything
but sterile ? Without such evidence the mere fact of common origin from a very early
segmentation seems a somewhat shadowy ground for the conclusion which Wilson Smith
proposes. If this criterion of homology be accepted, then all parts of the plant are
ultimately homologous, for they all originate from the ovum. (See Wilson Smith, Hot.
Gaz., 1900, p. 225).
320
LYCOPODIALES
sporangium (Fig. 165 B). The potential sporogenous tissue thus produced,
after successive sub-divisions, forms a very considerable sheet of tissue,
several cells in thickness. Of this, however, only a portion develops into
spores : in the case of a microsporangium certain tracts of cells of this
tissue assume dense protoplasm, and the cells, ultimately separating from one
another, undergo the tetrad-division, producing
[microspores (Fig. 165 c, D); but other tracts of
cells, neither showing any regular outline or
arrangement, nor referable in origin to pre-
determined cells of the genetic tissue, become
less densely protoplasmic, and form the sterile
trabeculae : a tapetal tissue invests the fertile
tracts : it is derived partly from the innermost
layer of the sporangial wall, as in Lycopodium,
partly from the superficial cells of the trabeculae.
A similar differentiation of the potentially sporo-
genous-tissue is found also in the megasporangia,
the early stages of which are quite indistinguish-
able from those of the microsporangia ; but in
the former a relatively smaller number of cells,
usually lying isolated in the potential sporo-
genous tissue, and distributed with no constant
relation to their ultimate parent cells, enlarge
and divide to form the megaspores (Fig. 166).
As there is no opening mechanism in the
submerged sporangia of Isoetes, no basis for
comparison is yielded from that source. The
study of the development in Isoetes thus leads
clearly to the conclusion that there has been
a differentiation, within the sporangia, of tissues
at first of uniform character : that part of the
potential sporogenous tissue remains fertile, but
a large proportion in the microsporangium, and
a still larger proportion in the megasporangium,
is diverted to other uses, and remains sterile.
As regards the origin of the potential sporo-
genous tissue, and the form and position of the
sporangium, there is clear correspondence to the Lycopod-type, and
especially to those forms with the more bulky sporangia : in fact if we
imagine a heterosporous Lycopod, with its sporangium widened out radially
along the leaf-surface and its enlarged sporogenous tissue partly sterilised
so as to form trabeculae, the result would be practically what is seen in
Isoetes.
A study of the sporangia of the fossil Lycopods is a necessary adjunct
to that of the modern forms, though the usual absence of developmental
FIG. 166.
Part of a section of a megaspor-
angium of Isoetes. The cell marked
(m) is the only fertile spore-mother-
cell, the rest are undergoing vegeta-
tive divisions, including the cell (a)
as shown by other sections of the
series. Thus sterilisation affects the
large majority of the cells of the
sporogenous group. X245. (After
Wilson Smith.)
SPORE-PRODUCING MEMBERS
321
details in them restricts the comparison to the basis of mature structure.
On this footing it appears that the type of sporangium characteristic of
the sub-genus Urastachya, and showing special resemblance to that of
Lycopodium Phlegmaria, dates back at least to the calciferous sandstone,
for it is seen in Lycopodites Stockii (compare Fig. 147). Sporangia apparently
of the same type have been recognised also in other early fossils referred
to Lycopodites, but their small size and the state of preservation do not
Spoiceritcs insignis. Somewhat diagrammatic radial section of part of the cone,
showing two sporophylls in connection with the axis. On the lower sporophyll the
sporangium is shown attached at its distal end to the ventral outgrowth of the sporophyll :
within the sporangium some of the characteristic winged spores are shown. (After Miss
Berridge.) From Scott, Progressus rei Botanicae, vol. i.
allow of any exact comparison. Of other apparently non-ligulate types one
of the best known as regards the details of the strobilus, though its vegetative
region is still unknown, is Spencerites (Fig. 167), which has been described
by Scott and others from specimens showing microscopic structure. Here
the verticillate or spiral sporophylls consist of a narrow pedicel bearing an
upturned lamina ; at the base of the lamina is a massive ventral outgrowth,
to which the distal end of the sporangium is attached by a narrow neck.
The presence of the ventral sporangiferous lobe has suggested to Dr. Scott
a comparison with the Sphenophyllales, though the absence of any vascular
supply to the " ventral lobe " renders the analogy somewhat remote. It
x
322 LYCOPODIALES
is doubtful what is the evolutionary relation between the distal and the
basal insertion of the sporangium upon the sporophyll ; whether the one
or the other is the more primitive in the Lycopodiales must be left for the
present open, but it is evident that such differences as these are of degree
only, in a type which is constant as regards the numerical relation of the
sporangia to the sporophylls, and in the coincidence of the median planes
of both of those parts. There seems little reason to hold that these
peculiarities of Spencerites are archaic relatively to those of the ordinary
Lycopodinous type. Comparison does not make it necessary, nor even
probable, while stratigraphically the ordinary Lycopod type is quite as
early as Spencerites.
The same relation of sporangium to sporophyll as is seen in the
living Lycopbds is maintained in the Lepidodendroid cones, which are
FIG. 168.
Lepidpstrobus Brownii. A radial section traversing the axis, a sporophyll, and a
sporangium. In the latter numerous spores are seen partially filling it, while sterile
processes project upwards into the cavity. (From Sowerby's drawing.)
known in many cases to bear ligules, and to be heterosporous, thus
corresponding more especially to the ligulate series of the Lycopodiales;
but this may possibly not be the case for all of them. An examination
of the details of the sporangium will naturally be best carried out in the
best preserved specimens, though these may not be generally typical of
all others. Lepidostrobus Brownii, Schpr., is probably the best preserved
of Lepidodendroid cones, and it will therefore be taken first. The large
silicified specimen in the British Museum was first described by Robert
Brown, with drawings by Sowerby.1 The original specimen was about two
inches in length, and of about the same diameter : it was evidently only
the upper half of a strobilus, as the internal structure, which is preserved
with singularly little distortion, shows to be the case. It has been cut
into transverse, radial and tangential sections, and consequently a very
adequate knowledge of the details can be obtained. The central axis
1 Linn. Trans., vol. xx. See also Misc. Bot. Works of Robert Brown, vol. i., p. 583.
SPORE-PRODUCING MEMBERS
323
shows a structure similar to thatsof Lepidodendron Harcourtii^ and though
there is no direct indication of the source of the cone, comparison of the
structure with that of Lepidodendron stems, and with other Lepidodendroid
cones, leaves no doubt of its being the strobilus of a Lepidodendron. The
axis bears numerous sporophylls, of which thirteen are usually represented
in each transverse section : the basal region of each extends horizontally
from the axis, and supports the sporangium, which may extend for fully
half an inch along its surface (Fig. 168). The distal end of the sporo-
phyll turns upwards, without any peltate expansion.
FIG. 169.
Lepidostrobus Brotvnii. A= wall of sporangium, showing outer sclerotic cells (scl),
with several thin-walled layers within. X 100. B = three sporangia in transverse section
of the cone ; r= median ridge. X 3. C = cone in tangential section. D = sporangium in
tangential section of cone, slightly diagrammatic; .$•/ = sporophyll ; r=sub-archesporial
ridge ; v, b = vascular bundle ; / = processes rising from the ridge. X6. E= small part of
the base of a sporangium in radial section, showing the processes /$, /, which rise from it.
X20.
Comparison of transverse and tangential sections of the cone (Fig.
169 B, c) gives a clear idea of the form of the very large sporangium,
which is a radially extended body, broader and deeper at the distal end
than at the proximal ; it is attached throughout its length by a relatively
narrow median, flange-like insertion to the upper surface of the sporophyll,
and immediately above the course of its vascular bundle. Comparison of
its outline with that of the sporangium of Isoetes shows a very striking
similarity ; but this is not limited to the form only : in L. Brownii above
1Or 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 anyx parenchyma interspersed between them.
The spaces between the xylem-rays are occupied by the sieve-tubes, with
the protophloem lying at the periphery, while conjunctive parenchyma
forms a complete sheath intervening between the phloem and the xylem.
The whole is invested by a parenchymatous sheath resembling a pericycle,
but derived, according to Strasburger,1 from the cortex : outside this is
the endodermis, recognisable while young as a single layer, but later
obscured by extension of the corky development. A very similar structure
to the above is seen also in the thinner branches of L. inundatum, a
species, which as we have seen above, stands in its external morphology in
near relation to the section Selago. These species may be taken as repre-
senting the structure usually found in the simpler upright, ground-growing
members of the genus.
FIG. 171.
Diagrammatic transverse sections of the stele of various species of Lycopodium ; the
phloem is dotted, the xylem drawn as tracheides. C = Lyc. serraturn, Thbg., with
stellate arrangement. D = upright stem of L. annotiuum, L., with somewhat stellate
arrangement. £=L. cernunm, L., with uniform distribution of the small groups. F=
L. volubile, Forst, with strongly bilateral structure. X8o. (From Engler and Prantl.)
But a more elaborate construction of the stele is found to accompany
the greater differentiation of external form. In creeping and climbing stems
there is apt to be an increase in the number of the protoxylems, accom-
panied by a development of alternating bands of xylem and phloem : the
xylem becomes isolated into distinct masses as seen in the transverse section,
and these are roughly disposed parallel to the surface of the substratum
(Fig. 171 F). In other cases, and especially in the epiphytes, the xylem
and phloem are more uniformly distributed, the former as patches em-
bedded in the latter, as seen in tranverse section (Fig. 171 E). Both these
conditions may be connected by intermediate steps with the simpler type
seen in L. Selago, and as they occur in plants with more specialised form
and habit, it may be concluded with some degree of certainty that the type
with a connected xylem-tract shows the more primitive state.
It would seem hardly necessary to insist on this rather obvious outcome
of comparison within the genus Lycopodium, were it not that a certain
misconception, which dates back to the Text-book of Sachs, still survives
1 Leitunosbahnen, p. 460.
330 LYCOPODIALES
as to the nature and origin of the more complicated steles of Lycopodium.
Comparison of these with polystelic stems of Selaginella long ago suggested
that the former structure was derived phylogenetically from the latter, by
the lateral fusion of several distinct steles; and thus that the stele of
Lycopodium is in reality a compound one.1 But the polystelic condition
seen in some Selaginellas is not uniform for that genus, as we shall see
below : moreover it seems improbable that the simpler, homosporous
Lycopodium should show structural derivation from the heterosporous Sela-
ginella, while it is only in the more complicated Selaginellas that the
polystelic condition appears : again, the species of Lycopodium which show
distinct xylem-plates are in our view morphologically more advanced than
those with the xylem more closely connected. Such considerations go
far to negative any idea of the more complex steles of Lycopodium, being
compound in their origin. Comparison within the genus is usually a safer
guide in such questions than more far-fetched references; and in the
present case it suggests a different explanation, which is as follows : that,
in a primitively protostelic Lycopod-stock with cylindrical solid xylem, the
phloem became progressively intrusive as the morphological differentiation
of the plant increased : at first it appeared in the transverse section as
occupying a few narrow involutions of the margin of the still connected
xylem, this then showing the stellate outline, with fan-like peripheral arms,
as seen in the Selago type. But in others the number of the involutions
and their depth became greater, till the coherence of the xylem-tract as
seen in the single transverse section became interrupted, and the appearance
of more or less isolated plates with narrow peripheral edges was attained,
as in Z. clavatum. The origin of the xylem-islands as seen in L.
squarrosum or Z. cernuum was substantially the same, the difference being
that they are not merely intrusive from the margin, but the xylem-tracts
are actually for some distance occluded in the phloem. In point of fact
these two types of more complicated derivative structure are not strongly
differentiated from one another. Thus, from comparison within the genus,
it may be figured how from the condition of a primitive protostele with
phloem about its periphery the Lycopod-stele became in the more advanced
cases a sort of xylem-sponge, with phloem and conjunctive parenchyma
occupying the interstices. It will be seen later that the simple protostelic
state without intrusive phloem is represented among the fossil Lycopods.
The relation of the leaves to the central stele in Lycopodium is interest-
ing, both in respect to the young and to the mature condition. If the
apex of the shoot be investigated, the plerome-cylinder is seen to extend
beyond the youngest leaves, to a point immediately below the apical group
of cells ; and thus the central region of the stele is cauline in its origin
(Fig. 172). The leaves originate from the three or four outer layers of
cells of the growing point, quite apart from the plerome, while procambium-
1 This is specifically stated in Strasburger's Leitungsbahnen, p. 458, and the view has
been retained in his Text-book, in the German edition of 1906.
COMPARATIVE ANATOMY 331
strands become differentiated in the intervening tissue, which form a connec-
tion with the central cylinder : upon this they are inserted laterally. It is
thus clear that in the ontogeny of the shoot the leaf is an accessory which
arises after the stele is already in existence. Its relative unimportance is
not only apparent from this late origin, but also from the fact that the
arrangement of the leaves upon the shoot does not dominate the number
or position of the protoxylem-groups of the stele. It has long been known
that the number of the xylem-rays is independent of the position of
the leaves. In L. davatum Jones has found that though in shoots
with simple leaf-arrangement it is usual for the protoxylems to correspond
to the leaf-insertions, still, where the number of protoxylems is beyond
six, there is no apparent relation between them and the leaf-insertions.1
FIG. 172.
Longitudinal section through the apical cone of the stem of Lycopodium Selago. X 160.
(After Strasburger.)
When the above facts are taken together,- it is apparent that the leaf
in Lycopodium is but an accessory appendage, and that the axis is the
dominant feature of the shoot. This conclusion probably applies for
Lycopods at large, and it has its important bearing on the relation of
leaf to axis, discussed in Chapter XL
Hitherto no definite knowledge of the anatomy of the smaller fossil
eligulate Lycopods included under the name Lycopodites has come to hand :
whenever such facts are available they will provide interesting material for
comparison with the modern species of Lycopodium. The ligulate and hetero-
sporous forms would be equally important for comparison with Selaginella.
The discussion of the external morphology of the latter genus has led
to the recognition of the radial type as relatively primitive, while those
species with dorsiventral shoots are held to be more specialised and
1 Linn. Trans., 2nd series, vol. vii., p. 19.
332
LYCOPODIALES
derivative. Of the former S. spmnlosa, A. Br., 'is the best known, and it
will be seen that its vascular anatomy, which differs from that of all
other Selagincllas, shows points of interesting comparison on the one
hand with Lycopodium, and on the other with the dendroid Club-Mosses.
The hypocotyl, and the lower parts of the axis, with its branches, are
traversed by a cylindrical stele, which is peculiar in having a central
A
H.G.
F.E.
FIG.
173.
Selaginella spinulosa. A. Transverse section of the trailing stem showing central
protoxylem. # = pericycle ; <5 = protophloem ; c = phloem parenchyma ; rf=metaxylem.
X275- B = transverse section of upper part of axis, showing seven protoxylems. .X35o.
C-G — scheme of arrangement of the protoxylems in sections taken successively from
below upwards. (After Harvey-Gibson.)
strand of protoxylem surrounded by metaxylem : this is further invested
by a narrow band of phloem surrounded peripherally by a sheath
resembling a pericycle, and by the trabecular endodermis so characteristic
of Selaginella (Fig. 173): according to Strasburger both of the latter
layers are derived from the cortex, as they are also in Lycopodium.1
In passing upwards in the strobilus the stele loses its peculiarity of
having a central protoxylem : for the strand divides, and the branches
diverge outwards to the periphery of the wood, where they appear in
1 Leitungsbahnen, p. 458.
COMPARATIVE ANATOMY
333
number from three to eight, as ^lightly projecting groups of small spiral
tracheides. The condition thus attained is very similar to that seen in
the simpler types ot Lycopodium : or a better comparison may perhaps
B
FIG. 174.
Lepidodendron Harcourtii. A= transverse section of stem; st= stele; o.c = outer
cortex ; both here and in the outer cortex the leaf-traces are shown about natural size.
B — stele of same ; / = pith, hollow in the middle ; .r = xylem-ring ; />-r = protoxylem-points.
The leaf-traces join the stele between them ; l.t = leaf-trace bundles, of which the outer,
/.*', show xylem and phloem ; i.c = inner cortex. Xj. (From Scott's Studies in Fossil
Botany.)
be drawn with certain stems of Lepidodendron. The presence of central
and peripheral protoxylem in different parts of the Jsame shoot should be
a -warning against too great reliance upon such characters ; it may, perhaps,
indicate that the central position was the more primitive, as is believed
by some on general comparative grounds. At the same time it is of
334 LYCOPODIALES
interest for comparison with the Lepidodendroid fossils, in which a
peripheral protoxylem is found in the shoot, while a central protoxylem,
adjoining the medulla, is found in the Stigmarian trunks.
Other species of Selaginella show further elaboration along distinct
lines. The simpler dorsiventral species, and even such radial species as
S. rupestris and oregana, show ribbon-like steles with marginal protoxylems,
upon which the leaf-traces are inserted. In the more complex cases the
axis becomes polystelic (S. inaequalifolia and Willdonovii}, or in some cases
solenostelic (rhizome of S. laevigatd), thus resembling similar vascular
complications seen in the stems of Ferns. These may be held to be
relatively late, and special developments from the non-medullated, mono-
stelic type : their origin shows parallelism of development rather than any
nearer relation with the similar structure seen in the Ferns.
The near correspondence of the ancient Lepidodendron-type to that of
the modern Lycopodiales appears not only in their external form, but also
in their internal structure, though special modifications of type, different
from those of the modern forms, appear in accordance with the larger
dimensions so prevalent in the fossils. The similarity consists in the
presence of a single cylindrical stele, with a centripetal wood, and peripheral
protoxylem, in relation to which the leaf-traces are inserted with the
minimum of local disturbance.
The general structure of one of the more simple types may be gathered
from Scott's figure of Lepidodendron Harcourtii (Fig. 174), which shows
(A) the relatively small proportion of the stele to the whole axis : (B) the
peripheral protoxylem, with its relation to the incoming leaf-traces, and the
uninterrupted metaxylem, not separated into strands : while centrally a large
pith is seen hollow in the middle. The steles of Lepidodendron varied in
structure towards the centre : in some cases such as the very ancient
Lepidodendron rhodumnense, Renault, and Lepidodendron saalfeldense, Solms,
from the Culm, there was a solid stele, without secondary thickening;
or, as in Lepidodendron Petticurensis, Kidston (Roy. Soc. Edin. Proc.,
1906-7, p. 207), the solid xylem-core was surrounded by secondary wood.
But often, and especially in more recent forms, the xylem was medullated,
and in this they differ from modern Lycopods. It is obvious in some
cases that the pith originated by incomplete development of tissue originally
tracheidal: this is clearly indicated in Fig. 175. This drawing also shows
that outside the xylem came a narrow band, probably of phloem, which
is usually ill preserved, while in some cases there is evidence of an
endodermis, as in the present case. Thus, putting aside the larger size,
and the medullation which is its frequent concomitant, there is substantial
similarity in the structure of the stele to that of a simple Lycopodium,
or of Selaginella spinulosa at its distal region.
A more striking concomitant of the larger growth was, however, the
secondary thickening represented in the majority of the known species of
Lepidodendron, though absent from some of the earliest. It was carried
COMPARATIVE ANATOMY 335
out by two distinct zones of cajnbial activity, the one immediately sur-
rounding the primary xylem, and resulting in a band of radially seriated
secondary wood, contiguous usually with the protoxylem of the primary
development. Externally an exiguous secondary phloem appears (Fig. 176).
Outside the thickening ring of the stele a second zone of cambial activity
arises in the cortex, below the persistent bases of the leaves : this results
in the formation of a broad band of secondary cortical tissue, or periderm.
FIG. 17;.
Lepidostrobus Rrovunii. Part of a transverse section showing the central parenchy-
matous pith (/*), the wood (xy), the innermost band of cortex (<:), the endodermis (?) (sh).
X200.
Such secondary activity ^extended from the main trunk into the branches,
and in some cases into those of quite moderate dimensions. Comparison
of the various known types of Lepidodendron suggest unmistakably that
even the most elaborate are the result of expansion of a non-medullated
monostelic construction, to serve dendroid purposes. A first step, following
on the increasing size of the stele, would be the formation of a paren-
chymatous pith : this probably originated directly, by the incomplete
development of a primitively solid tracheidal core, as is suggested in the
case of L. Brownii: and in support of this it is found that tracheides
and parenchymatous cells may be intermixed in the central region, a
condition held to represent an imperfectly, formed pith : it is seen in
31TY
336
LYCOPODIALES
L. selaginoides. It would seem probable that the non-medullated condition,
so persistently maintained in the smaller living Lycopods, was the primitive
state also for the larger dendroid fossils. The other factor of expansion, by
cambial activity, appears to have originated independently of medullation,
since it occurs both in medullated and in non-medullated axes. Physio-
logically it counterbalanced medullation where both occur together, for it
FIG. 176.
Transverse section of an axis of Lepidodcndron selaginoides. Cy = centre of the
vascular system ; tr = tracheae ; V— vessels of the primary cylinder \fp — primitive fibres
of the primary wood; .Z?2~trachei(les °f tne secondary wood ; r=ray of the secondary
wood ;A = secondary parenchyma; zc — cambial zone; L = liber ; s = foliar traces detached
from the primary cylinder. (After Hovelacque.)
substituted an enlarging peripheral vascular supply for the reduction in
efficiency in the limited central system. This was indeed a necessary
condition for dendroid development.
However large the proportion of pith to the primary wood became in
Lepidodendron, the continuity of the ring was as a rule unbroken, and the
leaf-traces were simply inserted upon the primary xylem with the minimum
of local disturbance. But in Sigillaria, in which the leaves sometimes
attained a very large size, the case is different : though they show in all
COMPARATIVE ANATOMY 337
essentials the same construction x of the stele as in Lepidodendron, they
illustrate steps towards the breaking up of the primary wood of the
medullated stele into separate bundles. The details derived from various
Sigillarian fossils have lately been put together in stratigraphical sequence
by Kidston,1 and his conclusion has already been quoted above (Chapter
XVIII., p. 230): he has shown a strong support for the view that the
condition with primary xylem forming a closed ring surrounding the large
medulla was the most primitive for Sigillaria : such a structure is found
in the more ancient specimens from the Lower Coal Measures (S. elongata,
Brongn., and 6". elegans, Brongn.) : those from the lower Permian, however,
(S. menardi, Brongn., and S. spinulosa, Rost. sp.) show the primary xylem
as a circle of separate bundles, though some of them may cohere laterally
in the last-named species. This indicates an evolutionary progression from
a concrete primary xylem to a condition where it is separated into strands.
In such forms the pith, being of relatively very large size, the primary
wood is reduced to a comparatively narrow investment round it, liable
as we have seen to be broken up into distinct strands. The secondary
tissues make their appearance, however, as in Lepidodendron ; there being
in Sigillaria a broad zone of secondary xylem, and a highly organised
periderm. It is thus seen that the later Sigillarias have departed further
in their structure from the simple protostele than other dendroid Lycopods,
for they show not only medullation, and a secondary thickening, but
breaking up of the primary xylem as well.
It has been concluded above, on the basis of external comparison,
that the plant of Isoetes is like a partially differentiated Lepidostrobus
seated upon a Lepidodendroid base. The question will now be how far
its anatomy will countenance such an opinion. There has been some
confusion in the descriptions given by various investigators, owing doubtless
to the difficulty in decyphering a complex mass of tissues affected by
the reduction which follows on an aquatic habit. But this has been in
great measure cleared by Scott and Hill in their Memoir on Isoetes
hystrix, one of the few land-growing species.2 Nevertheless the terrestrial
habit of this plant does not greatly affect its structure as compared with
other species, a circumstance which is held to point to the conclusion
that Isoetes is a genus which has long hovered about the limits of terrestrial
and aquatic life. The statement here given is based upon the Memoir
of Scott and Hill.
The stele of the mature plant is not composed merely of the united
leaf-traces, but is best interpreted as a cauline structure, comparable to
that of the simpler monostelic Lycopods, but much shorter than is usual
in them. The crowded leaf-traces are inserted upon it, the stelar wood
serving to join up the xylem of the leaf-traces, but it does not belong to
one trace more than another, and in structure it differs from them. The
1 Trans. Roy. Soc., Edin., vol. xvi., Part iii., No. 23.
-Ann. of Bot., vol. xiv., 1900, p. 413.
Y
338
LYCOPODIALES
differentiation of' the primary wood is nearly simultaneous over its whole
area, but with indications of centripetal succession. The cambial activity
starts early, being continuous from that of the primary meristem. As a
rule the same cambium is active throughout, producing secondary ground
tissue, wood, and phloem on its inner side, and cortical parenchyma
only towards its exterior; but other arrangements are found, while in
some cases a second cambial activity may arise inside or without the
first. The adjoining diagram, quoted from Scott and Hill (Fig. 177),
shows the relation of the primary and secondary tissues usual in /. hystrix^
and it will be noted that the secondary phloem is internal to the secondary
xylem ; the cambium lies outside the latter in direct contiguity with the
secondary cortex, which arises externally
from it. The stele which is cylindrical
above becomes in /. hystrix triquetrous
below, in /. lacustris it is usually flattened
bilaterally : the change of form is a
secondary consequence of the abutment
of the numerous, successively formed
root-bases upon it, and does not affect
the general comparisons. Scott and Hill
conclude that the anatomy of the stem
with its solid stele, from which the
densely crowded small and simple leaf-
traces pass off, is just what might be
expected in a stunted Lycopod, while the
anomalous character of the secondary
thickening in Isoetes agrees in some
measure with that in certain fossil Lyco-
pods. Scott1 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 ; ,r2, 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. cernuumt and shared also by
L. inundatum and sa/akense, consists of a massive cylindrical thallus, of
which the conical lower part is sunk in the soil, while ^the upper part is
EMBRYOLOGY
341
exposed freely above ground, ano> is of a green colour : in L. cernuum
and inundatum it bears numerous irregular leaf-like lobes, though in
Z. salakense the lobes are rudimentary or absent (Fig. 178). The pro-
thallus is evidently in the main a self-nourishing body, though an endo-
phytic fungus is almost constantly present, indicating a second but
subsidiary line of saprophytic nutrition. As the prothallus grows a
merismatic zone is localised surrounding the upper part of the cylindrical
body, but below its apex : this contributes to increase both the upper and
. •• r
FIG. 178.
Young leafy plant of Lycopodium cernuum, L., with the prothallus, bearing its irregular
assimilating lobes, attached on its left-hand side. X about 20. ('After Treub.)
>wer regions, while above it the green expanded lobes are formed. The
jxual organs appear between the latter, the youngest being nearest to the
icrismatic zone.
A second type shows in the ascendant that method of nutrition which
was subsidiary in the first : it is exemplified by the large subterranean
)thalli of L. complanatum, davatum, and annotinum\ being shut off from
light these prothalli are colourless, and the leaf-like lobes are absent. The
ssive prothallus is composed of a lower region which takes a conical
>rm, the angle of the cone being greater in Z. davatum and annotinum
than in Z. complanatum: it is in this region,, as in Z. cernuum, that the
idophytic fungus is present. The merismatic zone is active as before at
its upper limit, and above it is the part which bears the sexual organs,
>ut without any vegetative lobes as in Z. cermium (Fig. 179 B). It is clear
342
LYCOPODIALES
that the general plan of construction of the prothallus is the same as in
the L. cernuum-type, but modified in accordance with the saprophytic
method of nutrition.
FIG. 179.
A =old prothallus of Lycopodium annotinuui, L. , with young plant projecting beyond
the earth-surface (o). Natural size. B= median section through a young prothallus of
Lycopodium clavatum, L. X about 30. C = part of this from the middle region of the
upper surface, with antheridia in different stages. X 52. Z> = part of the margin of the
median section, with meristem and archegonia. X 52. £ = the epidermis devoid of
fungus, with rhizoids ; r = cortical layers, with their cells filled by hyphal coils ; / = the
palisade layer, also filled with hypha? ; s^> — the storage tissue; ;« = the meristem;
an — antheridia; arch — archegonia ; em = an embryo ; f= its foot; zw = its root. (After
Bruchmann, from Engler and Prantl.)
In the third type, exemplified by Z. phlegmaria and other epiphytic
species, the prothallus is more attenuated, and repeatedly branched. The
delicate colourless branches extend widely through the dead bark on
EMBRYOLOGY
343
which the prothalli grow, and they are attached by hairs which project
in all directions. Here again a fungus plays an important part in the
nutrition, which is exclusively saprophytic. The prothalli reproduce readily
by gemmae, as also by progressive decay, which separates the ultimate
branches as distinct individuals. The sexual organs are borne upon the
upper surface of enlarged branches of the thallus, and are always accom-
panied by paraphyses.
Such different types of prothallus, when studied separately, appear
widely divergent : and at first the underlying unity of their construction
was less appreciated than the differences which they show ; so little indeed
that Bruchmann, to whose labours so many of the important facts are
due, was disposed to make those differences the basis of a division of
the genus Lycopodium into distinct groups, or even genera.1 But Lang,, who
had simultaneously with him been
at work on the prothallus of Z.
clavatum? pointed out clearly the
relation of the divergent types
to one general plan, recognising
especially how the prothallus of
L. Selago, one of the species
described by Bruchmann, gives
the clue to their connection. For
itS prothallus appears tO be Variable
j c j , /T-,.
in ItS mode Of development (Fig.
1 80). It is usually a pale under-
ground body; but at other times
it grows above ground, and is coloured a full green. The spores appear to
germinate either at the surface or below it. The form of the prothallus is
determined largely by the soil in which it develops : thus, the elongated
cylindrical form is usually found in firm ground, though less deeply
buried than in the annotinum-\.y$Q : the thallus seems, in fact, to stretch
upward as though to bring as near to the surface as possible the seedling
unsuited for subterranean growth. The subterranean prothalli may be
simple, or be branched so as to take a coral-like form. In more open
soil, however, and especially near to the surface, the prothalli are more
compressed and flattened. Each prothallus tapers off as in the other
types at its lower end into a conical point, which indicates where it
issued from the spore, while towards its upper end the sexual organs are
formed. In the half-saprophytic prothalli, grown to the surface of the soil,
the conical form similar to that of other types is clearly seen (Fig. 181) : the
saprophytic lower region, the meristem, and the crown bearing the sexual
organs and paraphyses holding the usual positions.
1 Ueber die Prothallien und die Keinipflanzen mehrerer europaischen Lycopodien,
Gotha, 1898, p. 108.
-Annals of Botany ', xiii., p. 279.
FIG. 180.
Prothalli of Lycopodium Selago, bearing seedlings.
o, o shows the level of the soil, and the seedlings in their
development show varying proportions so that the first.
a
344
LYCOPODIALES
In considering these various prothalli it is then clear that they are
all modifications of the same conical form : that the several parts, though
differing in proportion, have the same positions relative to one another
and to the sexual organs which they bear, while the differences are closely
related to the differences of circumstance and of nutrition. There is
reason to believe that the full chlorophyll-nutrition was the primitive state
for them all, and the saprophytic nutrition, seen in the subterranean or
the epiphytic types, a deriva-
tive state. On this basis the
cernuum-type would be recog-
nised as relatively primitive,
while Z. Selago, being less
specialised than the annotinum
or phlegmaria-typzs, would
approach it more nearly than
they do. But it does not
follow necessarily that a species
which is recognised as primi-
tive in respect of one prominent
feature, is to be held as primi-
tive in all its features. This
applies to L. cernuum : it is
true that its prothallus is green
and assimilating, and in this
respect probably primitive; but
its sporophyte is a fairly
advanced one, with definite
strobili, and with peltate chaffy
sporophylls strongly differen-
tiated from the assimilating
leaves : its axis, too, shows
an advanced condition of the
stele. Thus in its general
characters Z. cernuum cannot
be held as a consistent proto-
type of the genus. But, on
the other hand, Z. Selago has a prothallus little removed from the condition
seen in Z. cernuum, while in addition the sporophyte of that species has
been seen to represent the least differentiated type in the whole genus.
On the general sum of its characters it would accordingly take a place as
a relatively primitive form. But its prothallus shows distinct plasticity in
the directions along which specialisation has extended to produce the
more extreme types : on the one hand, its subterranean specimens, with
elongated cylindrical form, prefigure the more specialised developments of
Z. complanatum and annotinum : the compressed and flattened form
FIG. 181.
Median longitudinal section through a young prothallus of
Lycopodiujii Selago. /> = basal cell; n/t, root hairs; ^ = epi-
dermis ; r=the investing tissue, stored with reserve materials,
and harbouring an endophyte ; t=the central ; ^ = the genera-
tive tissue ; «r=archegonium ; k = young embryo ; #« = anthe-
ridia beginning to develop: gh — sexual hairs, x 35. (After
Bruchmann.)
EMBRYOLOGY
345
developed in more open soil suggests the origin of the phlegmaria-type :
while its green sub-aerial forms are reminiscent of the cernuum-type. A
plant which shows such plasticity is clearly not far removed from the self-
nourishing condition of the prothallus, which was probably the primitive
condition for them all.
These remarks upon the curiously divergent development of the pro-
thallus in the genus Lycopodium are a necessary preliminary to the study of
the embryogeny in the genus ; for it is impossible to understand the
comparisons of the different forms of embryo without some knowledge of
the prothalli which produce them. In all the species of Lycopodium in which
the embryogeny is accurately known, an early stage of the embryo is
found in which it consists of a suspensor, and of two tiers, each composed
of four cells (Fig. 182). The first cleavages are variable in their succession,,
as is found to be the case also in other embryos ;
but their position shows considerable constancy.
It is stated that from the lower tier of cells,
i.e. that adjoining the suspensor, the structure
designated the foot arises, while the upper tier
gives origin to all the other parts of the
embryo, and the correctness of the statement
is borne out by numerous drawings. But after
the first stages are past there is usually no
sharp limit between the tissue composing the
foot and that of the other parts : in the simplest
cases it appears as though the foot were merely
a region of tissue lying between the suspensor
and the upper tier, rather than a definite organ
or part. Functionally, the foot does not appear
to be differentiated from the suspensor in the genus Lycopodium, and it
shares with it the office of maintaining connection with the prothallus. Not-
withstanding the initial similarity which thus rules in the embryos of the
genus, the further steps of the embryogeny differ according to the different
forms of prothallus above described ; and it becomes a question which of
the divergent types is to be held as the most nearly reflecting the original
condition, and which as' later and derivative.
The type of L. Selago may be taken first, since it does not show any
high degree of specialisation in its variable gametophyte, while it has
been seen above that its mature sporophyte is one of the least differen-
tiated in the genus. Its early embryogeny, so far as is known, conforms
to the usual type, as above stated. The foot originates from the lower
tier, and the various parts of the embryo from the upper.1 But the foot
is only slightly developed. The upper tier of cells soon assumes a green
colour and unsymmetrical form, owing to the lateral upgrowth of the first
leaf or cotyledon, while the apex of the axis also originates early, near
1 Bruchmann, I.e., pp. 97-103.
FIG. 182.
Diagram illustrating the primary
segmentation of the zygote in Lyco-
podium. /, I— first segmentation
wall which separates the suspensor,
here cross-hatched, b, b separates
a lower tier (foot-tier) here dotted,
from an upper tier (stem-tier) left
clear ; each tier consists at first of
four cells. The wall b, b corresponds
to the wall IV. -IV. in Figs. 183, 186,
and to wall II.-II. in Fig. 185.
346
LYCOPODIALES
Bl
to its base : it is clear that the relation of the apex to the intersection of
the first walls has been a close one (Fig. 183 A). The axis soon proceeds
to form successive leaves spirally arranged. The cotyledon and subsequent
leaves have the ordinary characters of the foliage leaves of the species.
The tissue below soon becomes elongated as the hypocotyl, the length of
which is determined by the level at which the prothallus lies in the soil :
where it is at or near to the surface the hypocotyl may be quite short:
where deeply seated it lengthens, so that the first leaves are exposed
above ground (Fig. 184). It is
traversed by a vascular strand,
which is monarch below, but
near to the first leaf, or later, it
becomes diarch, and shows two
lateral tracheidal strands. The
first root originates exogenously
from the upper tier, just above
the foot, and is succeeded by
other roots of endogenous origin
at higher points (Fig. 183 B).
Here, then, is an embryogeny
characterised by its great direct-
ness and simplicity. The only
complication is the varying elon-
gation of the hypocotyl according
to the level of the prothallus in
the soil ; and there is good reason
to think that this is an immediate
adaptation to meet the varying
levels of development of the game-
tophyte in the soil, in an embryo
which is pertinaciously subaerial.
The nursing of the embryo by the
prothallus is not long continued,
nor is it structurally provided for, there being no development of an
elaborate "calyptra," as in some other species: the embryo soon escapes
from the prothallus, and fends for itself. The whole condition of the
embryo is such as bespeaks a simple and primitive state. Probably this
view would never have been in doubt had it not been for the existence of
different arrangements seen in other species of the genus, which happened
to have been described some years earlier.
Of these the type which corresponds most nearly to L. Selago is that
of L. Phlegmaria, so accurately described by Treub.1 Here the segmen-
tation of the embryo, as well as the origin of all the parts and their
proportions while young, have been fully made out, and appear to be
1 Ann.Jard. Bot. de Buitenzorg^ vol. v., p. 87, etc.
FIG. 183.
Lycopodium Selago. A— young embryo. X 150.
^=foot ; IV.-IV. =wall separating the foot from the
stem-tier ; K= cotyledon ; a = apex ; .EV=suspensor.
B = embryo more advanced, with prothallus still attached ;
Z?/=cotyledon ; f^, W^ = young roots ; £V=suspensor
attached to the foot, which is clearly defined from the
base of the axis. X 20. (After Bruchmann.)
EMBRYOLOGY
347
substantially the same : the hypocotyl becomes elongated as the seedling
develops, and the whole appearance of the seedling resembles that of
Z. Selago. The primary segmentation in L. Phlegmaria is according to
the scheme (Fig. 182), and the lower tier, as in Z. Selago, forms only the
foot, which attains no great size (Fig. 185). The upper tier develops
unsymmetrically from the first, the side which will form the first leaf
growing more strongly; close to the base of the cotyledon, and apparently
lateral owing to the stronger growth of the latter, but in reality terminal,
arises the apex of the axis (T, Figs. 185 A, B) ; it is, in fact, initiated in
close proximity to the organic centre of the upper tier. The root
(R, Fig. 185 B) also originates from
the upper tier. A comparison of
Treub's drawings of Z. Phlegmaria
with Bruchmann's less complete
series for Z. Selago shows clearly the
substantial similarity of the embryo-
geny in the two species. It will
be remembered that the flattened
prothalli of the latter species, formed
near the level of the soil, have been
held to prefigure the strap-shaped
sexual branches of the Phlegmaria
prothallus, though the latter shows
its higher specialisation for a sa-
prophytic habit in its filamentous
development and in its frequent
branching. On the other hand, as
regards the sporophyte, it has been
seen that the Phlegmaria type is not
one of the highest developed, but is
associated with Selago in the sub-
genus Urostachya. This being so, it is natural to find their embryos so
similar, notwithstanding the difference in specialisation of the prothalli
themselves. Lastly, both embryos are from the first subaerial : their first
leaves are green assimilating organs, and differ in no essential degree from
the normal foliage leaves. This may probably be held to be a primitive
condition.
But in the davatum-annotinum-ty^e the case is different. It has been
seen that there the prothallus is developed underground, often at a considerable
depth, and this brings with it modifications of the embryogeny. The first
steps in the development are the same as in the types described above
(Fig. 1 86 A); but very soon there is a conspicuous enlargement of the
tissue of the foot, derived from the lower tier, adjoining the suspensor
(Fig- i86B): a large spherical swelling is thus formed, which remains as
an intra-prothallial haustorium (compare Fig. 1796 with Fig. i86c). The
FIG. 184.
Prothalli of Lycopodiunt Selago with seedling plants
X 3. (After Bruchmann.)
34$
LYCOPODIALES
upper tier meanwhile progresses only slowly : two opposite leaves, one on
either side of the stem-apex, appear late as compared with other species,
B.
A -m
FIG. 185.
Embryos of Lycopodium Phlegmaria, in longitudinal section. p = foot ; c = cotyledon
T = apex of axis. The wall marked II. in A and B corresponds to the wall marked IV. in
Bruchmann's drawings (Figs. 183, 186) and to the wall b, b in Fig. 182. C and D represent
older stages : in D, the cotyledon (c) has been followed by a plumular leaf f\ and the apex
of the axis lies between them. A and B X 200. CX3J. Z>X2oo. (After Treub.)
their position relatively to the foot and to the suspensor not being constant
(Fig. i86c). This is ascribed by Bruchmann1 to inconstancy of the foot:
lL.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 ; /F2 = 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; vt—punctinn
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; r2 = second root;
*^ = stem apex, x 150. (After Campbell.)
faces the cotyledon : the very exiguous apical cone lies between them
(Fig. 191 G). And so the shoot is established, bearing successive leaves
with spiral arrangement, and successive roots, of which the second is below
the base of the second leaf.
Comparing this apparently divergent embryogeny with that of the
Lycopods above described, if the whole embryo be imagined inverted
in its orientation relative to the archegonial neck, and their suspensor
be imagined entirely away, then the two embryonic tiers may be com-
360 LYCOPODIALES
pared with those seen in Isoetes. Here, as in L. Selago and L. Phlegmaria,
the hypobasal tier forms the suctorial organ only, and takes no direct
part in the establishment of the plant. The epibasal tier is like that of
L. Selago as regards the parts which it initiates and in the positions
which they severally hold, but differs in its growth in length being
stunted, and in the early ascendancy of the cotyledon, which condition it
shares, however, in some measure with L. Phlegmaria : it differs also in
the late definition of the apex. But the position of the latter relatively
to the whole embryo is the same, for the stem originates in close
relation to the centre of the upper tier of the embryo, as it does in
all the Lycopods where the embryogeny has been exactly followed. The
apical cone is small in bulk and late in appearance, these being probably
correlative consequences of the early advance of the cotyledon. It is
thus possible to see even in the embryo of Isoetes some clear relation
to the plan which, with such curious modifications, underlies the embryo-
geny of the Lycopods.
i
We are now in a position to enunciate a comparative view of the
embryogeny as known in the Lycopodiales, and to state it so as to place
the several curiously divergent types in what is believed to be their
natural relation to a probable primitive embryogeny. In so doing pro-
minence is given to the more constant features, while only a subsidiary
place is given to those characters which are less stable.
In those Lycopods in which the embryogeny has been exactly followed,
the embryo consists of a suspensor and two tiers of four cells each
composing the embryonic body : the two tiers are separated by a wall
which may be called the "basal" wall (Fig. 182 b, /;). This seems to be
a general condition, subject only to minor modifications : in Isoetes,
however, the suspensor is entirely wanting. In the very various develop-
ments which follow, the most constant feature is undoubtedly the close
relation of the stem-apex to the point of intersection of the octant-walls
in the epibasal tier. In the simplest cases the axis of the embryo is
thus defined at once as lying between that point and the base of the
suspensor. The whole embryo is thus primarily a spindle-like body, and
this may be held to have been the primitive condition for them all.
But this simple form is subject to early modifications, which disguise
the position of the axis by delaying its apical growth, and by distorting
the form : so much so that the position and identity of the apex is liable
to be lost. The least distorted types are those of L. Selago (Figs. 183, 184)
and Phlegmaria (Fig. 185), and of S. spitiulosa (Fig. 190), all plants which
are relatively primitive in their genera as recognised by the characters
of the mature sporophyte. In L. Selago and Phlegmaria no haustorial
swellings exist. The early development of the single cotyledon at first
throws the apex of the axis to one side, but this is rectified later when
the second leaf appears on the side opposite to the first. The apex thus
EMBRYOLOGY 361
" righted " is then carried up together with the two leaves by the elong-
ating hypocotyl, while the first root appears laterally at its base. The
whole arrangement is relatively simple, but illustrates a slight degree of
distortion of the apex, which is, however, temporary only.
In the c/avatum-type (Figs. 170, 186) the hypobasal tier is enlarged,
and curved to one side, with correlative late differentiation of the epibasal
region, and absence at first of localised intercalary growth. The orien-
tation of the foot is not constant, but it is directed towards the chief
source of nutriment, a fact which indicates its opportunist character. In
the epibasal region the apex of the axis is clearly of central origin,
between the two small but equal cotyledons, which appear relatively late.
The root originates in a position corresponding to that of the former
type. The whole embryo may be held to be a biologically intelligible
modification of the Se/ago-type consequent on the underground habit of
the large mycorrhizal prothallus. The enlargement of the haustorial foot
leads correlatively to slow development of the epibasal region, while the
first leaves, having no nutritive function, are not hurried on in their
development so as either to distort or to produce correlative reduction of
the apical cone.
In the cernuum-type (Figs. 187, 188) the tuberous swelling is not in the
hypobasal but in the epibasal region, and it profoundly disturbs its develop-
ment. The biological cause of the swelling, which is extra-prothallial and
liable to repetition, may be the intrusion of the symbiotic fungus which is
present, or there may be some other reason for the tuberous development,
associated as it is with the late origin of the root. But whatever the
cause, the form is such as might be expected in a secondary tuberous
modification of a green leafy shoot of a young Lycopod. The bulky
development below, and the rapid enlargement of the assimilating leaves
act correlatively in keeping the stem-apex inconspicuous. Its identity
throughout the embryogeny of L. cernuum itself is not yet demonstrated ;
but in Phylloglossum the definitive apex of the shoot has been seen to
coincide with the apex of the tuber (Fig. 189) ; it is therefore probable
that in the embryogeny of L. cernuum the apex of the axis is present
in the very young embryo in the usual position, but has escaped recog-
nition owing to its correlative diminution. The protophylls would then
be leaves of the normal type, altered in relation to the gouty habit of the
axis which bears them, and disposed in an apparently irregular and isolated
fashion upon the swollen axis. The root is long deferred, perhaps in
relation to the mycorrhizal habit; but when it does appear, its relation to
the leafy shoot is like that which it has in the other types of the genus.
The normal leafy shoot and the root-system, thus delayed by the gouty
interlude called the " protocorm," ultimately continue their development
as in other Lycopods. Thus the different types of the genus appear to
start alike, and when established in the soil continue alike, but show
divergent intermediate phases. The simple development of Z. Selago and
362 LYCOPODIALES
Phlegmaria is believed to be primitive, the turgid developments of Z.
clavatum, cernuum and inundatnm, and also of Phylloglossum, are believed
to be secondary.
The embryogeny of Selaginella (Fig. 190) corresponds in all essentials to
that of Lycopodium, and shows only minor distortion or swelling. In the
simple case of S. spinulosa the apex of the axis originates as before from
the centre of the upper tier : the active growth of the first leaf throws
the apex of the axis to one side ; but it is " righted " again on the
appearance of the second, and the identity of the apex is clearly
maintained throughout. The whole epibasal tier is then carried upwards
by intercalary growth of the hypobasal region, but the hypocotyl thus
formed is without lateral swelling, and the first root originates laterally
at its base. As regards distortions, this case is quite similar to that of
Z. Selago or Phlegmaria. But in other Selaginellas, as exemplified by
6". Martensii, the cotyledons arise equally, as indeed they sometimes
do in *$". spinulosa, and the temporary distortion of the apex does not
appear ; but a lateral swelling, absent in S. spinulosa, constitutes the
" foot." Thus Selaginella shows only minor and inconstant deviations
from the simple type.
The embryogeny of Isoetes is less easily compared, but the following
tentative suggestion is given (Fig. 191). The suspensor is entirely absent,
and the embryo, composed only of the two tiers corresponding to those of
other Lycopods, is usually orientated so that its apex is from the first
directed towards the neck of the archegonium. That the rotation necessary
to bring this about may occur is indicated by the differences of position
of the basal wall noted by Campbell. The product of the hypobasal tier is
the haustorial foot only : the upper tier hastens at once to form the large
cotyledon, with the effect that the stem-apex is delayed, and remains
minute : it only becomes clearly recognisable after the appearance of the
second leaf opposite the first. In relative position, however, these parts of
the shoot correspond to those of S. spinulosa or Z. Phlegmaria. The
first root originates from the epibasal tier as in Lycopodium rather than
Selaginella, and unlike Z. Phlegmaria and S. spinulosa on the side opposite
to the cotyledon ; but the orientation of the root relatively to the cotyle-
don has been seen to vary within the Lycopodiales, so no great importance
need attach to this discrepancy. The primary embryogeny of Isoetes may
thus be held as related to that of the other Lycopodiales, but without a
suspensor, and greatly abbreviated, and with the apex of the axis correlatively
reduced and delayed in its development, owing to the early production of
the cotyledon and the root Nevertheless, its position at the centre of the
epibasal tier is maintained.
It is thus seen that the embryos of all the Lycopodiales may be held
as variants on a single type, and fundamentally of spindle-like form.
SUMMARY 363
SUMMARY OK THE COMPARATIVE EXAMINATION OK THE
LYCOPODIALES.
The sporophyte of the Lycopodiales has now been studied compara-
tively as regards its external form, its spore-producing members, its
anatomy, and embryology. The conclusion arrived at from all these
quarters is favourable to a strobiloid origin, with subsequent specialisation
along lines variously divergent. By the comparison of known representa-
tives of the Lycopodiales, living and fossil, certain characters have been
recognised as relatively primitive, others as derivative : and thus a general
idea has been obtained of a primitive type of Lycopod-sporophyte, which
forms the basis of a theory how such a sporophyte came into being. In
form this primitive sporophyte was probably a simple, unbranched, radially
constructed shoot, endowed with unlimited apical growth, while local
intercalary growth might also occur. The axis bore undifferen dated leaves,
each of which had one sporangium associated with it in a median position.
It was rooted at its base, but the origin of the root may be held
to have been accessory in evolution, as it is seen to be late and variable
in the individual development. The internal construction of the shoot
showed a non-medullated monostele, continuous as a cauline column to
the apex of the axis, while the foliar strands were inserted with but slight
local disturbance upon its periphery. Its sporangia were kidney-shaped,
and not greatly extended radially. The primitive body thus sketched in
its broad outlines was derived from a spindle-shaped embryo, without
any haustorial swelling, or tuberous protocorm. The theory of the
strobilus, as enunciated in Chapter XL, would adequately account for
the origin of so simple a sporophyte as this, from a still more primitive
body, with sterile base and fertile apical region, by segregation of the
fertile tissue into separate sporangia, and by enation of sporophylls.
The nearest living representative of such a sporophyte which has been
adequately investigated is Lycopodium Selago ; but it is to be remembered
that this is the only one of 39 species of the &&<£&-section of the genus
so examined, and there are indications, derived as yet from external
characters only, that other and more primitive types than L. Selago exist
among them : these await, further investigation. The first leaves formed
on the embryo of L. Selago are lateral in origin, and become aerial and
green, but are sterile : sporangia were noted by Bruchmann,1 as first
appearing after the second branching of the axis, which, however, is
early as compared with the other European species, though not as
compared with the large Andean forms. Their early appearance, as
well as the similarity of the sterile and fertile leaves, coupled with
the evidence of abortion of sporangia in the upper region, all point
to the conclusion that originally all the leaves were sporophylls, while
all arose laterally upon the axis.
'/..f., p. 100.
364 LYCOPODIALES
From such a starting-point various lines of elaboration may be traced,
open often to ready biological explanations : and these appear to have
run in some degree parallel in the ligulate and non-ligulate series. The
steps which may be traced on a basis of comparison are as follows :
First, the progressive sterilisation by abortion of sporangia increased the
vegetative region: this led to more definite specialisation of the strobilus:
in the more advanced forms the sporophylls are no longer nutritive, but
only protective in function, so that the differentiation of the nutritive
from the vegetative tract has become clearly marked. The vegetative shoot
once distinct from the propagative strobilus was susceptible of various
specialisation. In the dendroid fossils it attained large size, with secondary
increase of its tissues, both stelar and extra-stelar, but still it maintained
its radial symmetry. In the smaller forms, the straggling or climbing
habit led not uncommonly to dorsiventral development, which occasionally
extended to the more conservative strobilus itself. Such advances were
accompanied by various elaboration of the vascular tissues, such as
medullation, disintegration into separate strands, or even into meristeles.
But these are all referable back in origin to the primitive monostele, just
as the variations of external character are referable by comparison to the
primitive strobilus.
The sporangia all conform to one general fan-shaped type, with
singular constancy of number and position relatively to the leaves. But
the dimensions vary, and at least in Lycopodium there is a relation
between the size of the sporangium and the definition of the strobilus :
where the shoot is undifferentiated, as in L. Se/ago, the sporangium is
radially compressed : where the strobilus is clearly defined, and the
vegetative region more specialised, as in L. clavatum or alpinum, it is
radially elongated. The most extreme cases of this are found among the
ligulate forms, as in the dendroid fossils with their ample vegetative
system. But, on the other hand, this relation is not constant, for
the sporangia of Isoetes are radially elongated, though there is no
differentiation of the strobilus, while the sporangia of Selaginella are
compressed, though the strobili are clearly defined. One of the most
interesting points in these large sporangia is the partial sterilisation of
their sporogenous tissues, probably to meet mechanical and nutritive
requirements : sterile trabeculae are thus formed in the sporangia of
Isoetes, and in certain Lepidostrobi. This leads towards a condition
of septation, but in the Lycopods the step is never taken to complete
partition of the sporangium. Finally, the heterosporous differentiation is
probably a condition assumed after the character of the sporangium was
already defined, and it has not greatly affected the general morphology
of the shoots where it has occurred.
In the eligulate series the embryo is simple and spindle-shaped. In
L. Selago. which on other grounds is regarded as a primitive type, it
grows directly and without complications into the seedling, with its green
SUMMARY 365
assimilating leaves. In other cases it shows various modifications. Where
the thallus is buried deeply underground, as in L. clavatum, the lower tier
of the embryo enlarges as an haustorial foot, while the first leaves are
modified into colourless protective scales, evidently a secondary condition.
In the iw;//////«-type and in Phylloglossum a distinct extra-prothallial
swelling appears in the upper tier of the embryo, disturbing the position
and even the arrangement of its parts. Since the first stages of this
embryo resemble those of other Lycopods, and since the normal shoot, when
ultimately established, is also of the usual Lycopod type, it is concluded
that the swollen stage, styled the "protocorm" by Treub, is a gouty
interlude, introduced secondarily into the normal development, and not a
stage of general significance. In the ligulate series, Selaginella spinulosa,
which is held as a relatively primitive type on comparison of its mature
sporophyte, the seedling is very similar to that of Z. Selago, notwithstanding
the striking difference of their prothalli. But the simple spindle-form
which it shows is departed from in other species, by the lateral formation
of a swollen haustorium : this "foot" is again held to be a secondary
development. The apparently divergent ernbryogeny of Isoetes is carried
out without a suspensor, but the position of the parts in relation to the
greatly abbreviated axis is essentially similar to that in Selaginella. It thus
appears that in both series the most primitive type has an embryo in
the form of a simple spindle : it forms its first leaves as normal green
foliage leaves, and those species in which this is departed from are held
as the result of secondary modification. The first foliage leaves in these
simple forms differ in no essential respect from the subsequently formed
sporophylls, except in the absence of the sporangium. Hence the observed
facts support the view that all the leaves were originally sporophylls, and
the whole plant originally a simple strobilus.
It has thus been seen that a strobiloid theory is applicable to all
known types of the Lycopodiales. This matter has been dealt with at
:onsiderable length because, in the first place, this phylum of Vascular
Plants dates back fully as far as any other in the Palaeontological record.
Secondly, because it is represented by many living species susceptible ot
minute investigation throughout their life-cycle : and, thirdly, because these
and the fossils together £how gradual, and at the same time considerable
divergence of detail in the one uniform scheme. They thus provide a
better basis for comparison than any other series of Pteridophytes of equal
age. The conclusions arrived at will be susceptible of comparison with
those relating to other phyla of Vascular Plants. But though the applica-
tion of the theory of the strobilus may be extended to other phyla, it
must be remembered that the arguments and conclusions relative to the
Lycopodiales stand by themselves, and would still be equally cogent if no
other Vascular Plants existed on the earth's surface.
CHAPTER XXVII.
SPORANGIOPHORIC PTERIDOPHYTES.
I. EQUISETALES.
UNDER the common designation of the " Sporangiophoric Pteridophytes "
may be grouped together those forms whose sporangia are disposed, either
singly or in larger numbers, upon more or less elongated vascular stalks,
which are enlarged as a rule at their distal ends. The existence of the
sporangiophore clearly distinguishes these plants from the Lycopodiales,
though it may for the present remain an open question whether any
genetic connection existed between the latter and the sporangiophoric types.
Under this designation are included the Equisetales and the Sphenophyllales
(incl. Psilotaceae), while, according to the view which will be developed
below, the Ophioglossales will also appear as an outlying group sharing
the same character, though in a more elaborated form. It will be a
matter for later discussion how far the existence of the sporangiophore
as the immediate sporangium-bearing member will supply a valid basis
on which to trace affinity : the decision must rest on the degree of
correspondence of the sporangiophoric types in other characters, such as
the external morphology and anatomy of the vegetative organs, and the
details of the gametophyte. Unfortunately these are often so imperfectly
known that we are thrown back in great measure upon the spore-producing
members : but on grounds previously explained these are held to be the
most important of all.
The Equisetales, which are taken first of the sporangiophoric types,
are distinguished from the rest by the fact that their sporangiophores are
inserted directly upon the axis, not on appendicular parts : in some
cases they show a definite relation to the bracts which subtend them : in
others no such relation exists. Other less distinctive characters of the
vegetative organs are the constantly radial construction of the shoot :
the elongation of the internodes which are longitudinally striated, the
verticillate arrangement of the leaves, a high degree of branching, and a
structure of the stele with a ring of isolated vascular strands ; these
EQUISETALES
367
FIG. 192.
Equisctiun pratcnse, Ehrh. Rhizome with unbranched fertile shoots (a), a fertile
shoot which has begun to form branches (b), and a young sterile shoot (c). Natural size.
(After Duval-Jouve, from Rabenhorst's Krypt. Flora.)
368
EOUISETALES
collectively characterise the group as a definite one. As regards its past
history, the evidences of the existence of the Equisetales extend back to
the Devonian period, where they already showed a high degree of
elaboration. But these plants formed a more conspicuous feature in the
Carboniferous Flora, where they attained their maximum development in
point of numbers as well as in
size. Subsequently the type
became less prevalent, till at
the present day it is repre-
sented only by the cosmo-
politan genus Equisetum, with
its twenty-four species, showing
remarkable uniformity of type.
The essential characteristics of
the living genus will be taken
first, as it is susceptible of more
complete study than the fossils :
these will be worked in on a
basis of comparison with what
is seen in Equisetitm itself.
EXTERNAL CHARACTERS.
It will be unnecessary to
describe the characters of the
shoot in Equisetum in full
detail, or the comparatively
slight modifications of it upon
which the species are distin-
guished : a brief account will
suffice to indicate the essential
features, for beneath them all
lies a general unity of plan
which is closely followed,
whptVipr trip srinnr HP nnHpr
WHCtHCr
ground 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 ; b1 , 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 Goebel1 that even in so pronounced a case of the absence of lateral
branches as the fertile axis of E. arvense the development of green lateral
branches could be induced : this was done by culture of the lower
internodes in a moist chamber, when green assimilating branches were
put out from the nodes, as in the vegetative shoot. The apparently
branchless fertile shoot was thus brought into line with the ordinary
branched type prevalent in the genus. Such facts indicate that the
branched condition was probably common for the genus, but in certain
cases a late differentiation has arisen between the colourless fertile shoots
where the branches are dormant, and the branched green assimilating
shoots.2
The fertile strobilus of Equisetum is normally terminal on the axis,
and is usually borne on the relative main axis only. Many cases exist,
however, of the development of the strobili on lateral branches : this may
be normal for certain species, such as E. myriochaetum, Cham, of the
sub-section Pleiostachya, Milde, well shown in Engler and Prantl, Pflanzen-
familien, i., 4, Fig. 343, p. 547 ; but it also occurs occasionally in others,
where a single terminal strobilus is normally present ("forma polystachya"}?
In the case of Equisetum sylvaticum polystachyum (Fig. 194), where
numerous lateral branches normally sterile bear small strobili, Luerssen
has been able to correlate the change with external conditions : 4 this is
the next step to bringing its determination within the limits of experiment.
On the other hand, numerous cases have been recorded of the continued
growth of the strobilus, at its apex, with a return to the ordinary vegetative
characters. Such facts show that the lateral branches are not essentially
different from the relative main axis, as regards the final end of spore-
production : also, that there is no absolute barrier between the vegetative
and the fertile regions in Equisetum. Speaking generally, the fertile strobilus
is not restricted to axes of any definite order. Thus it requires no great
effort of imagination to see in the shoot-system of Equisetum the result
of amplification of a simple unit, the shoot, composed of axis and successive
^ Ber. d. Deutsch. Bot. GeselL, 1886, p. 184.
2 For an interesting discussion of the biological relations of the sterile and fertile shoots
in living species of Equisetum, see Goebel, Organography , vol. ii., p. 501.
3 For records of such developments in European species, see Rab. Krypt. Flora, iii.,
p. 622, etc.; and especially Luerssen, " Beitr. z. Kenntn. d. Flora, W. and Ostpreussens,"
Bibl. Bot., 1894, Heft 28.
*L.c., p. 13.
(iKXKRAL MORPHOLOGY
leaf-sheaths, and capable of spore -production
by a terminal strobilus. The branching,
however complex, may be held as accessory,
as also the formation of roots so closely
associated with the branches. The funda-
mental idea of the plant is thus carried
back to the first shoot which originates
with the embryogeny. It may be held that
from this, by successive accessory branch-
ings, the complex shoot-system arose, while
the spore-production was deferred to the
later branchings : it is on these that the
fructification ultimately appears in the living
species, while the primary axis and earlier
branchings are normally sterile.
The strobilus itself consists of a con-
tinuation of the axis which bears it, and
upon this the sporangiophores are disposed,
but often with less regularity than rules in
the case of the leaf-sheaths. The whole
strobilus is normally occupied by the spor-
angiophores, without any intervening bracts
(Fig. 195 A). The sporangiophore itself
consists of a central stalk supporting a
polygonal distal end : from the margin of
this the sporangia hang in variable number,
forming a series surrounding the stalk (Fig.
195 B). The spores are all of one type
(Isosporous). At the base of the strobilus
a ring-like structure is found— the annulus
— which is like a reduced leaf-sheath, and it
has usually been held to show a transitional
stage between the vegetative leaf-sheaths
and the first whorl of the sporangiophores,
these being recognised as" equivalent parts.
Reasons will be advanced below for not
accepting this apparently simple view. The
strobilus of Equisetum is liable to variations
of development, which have their importance
in relation to certain fossil forms. The most
notable of these is proliferation, the apex of
the strobilus being continued as a vegetative
shoot : the effect is thus gained of a fertile
zone bearing sporangiophores, threaded upon
an axis, or of a succession of such zones,
371
FIG. 194.
Equisetnm syfoaticum, L. , forma poly-
stachya, Milde. Plant with 18 secondary
strobili, in three whorls of branches which
are normally sterile. Natural size. (After
Luerssen.)
372
EOUISETALES
separated by leaf-sheaths (Fig. 196). It is thus seen that the strobilus of
Equisetum is not always that circumscribed terminal body which is typical
for the living species.
The Equisetum-^^ has been recognised, though with some uncertainty,
and only in few specimens, as far ^back as the Middle Coal Measures ; l
but it is seen represented more commonly,
and by large forms, in the Mesozoic rocks.
Related to it are two other fossil forms :
the genus Phyllotheca of Permian age
resembles Equisetum in the general features
of the shoot, with its cup-like leaf-sheaths
webbed at the base, but differing in the
form of the leaves and in the fertile
region : this is constructed on the general
plan of Equisetum, but with the strobilus
interrupted at intervals by sheaths of
sterile leaves, as in some abnormal con-
ditions of Equisetum (Fig. 197). Some
specimens of Phyllotheca have, however,
been described by M. Zeiller as having
strobili like those of Annularia, that is,
of the Calamostachys-type? The other
genus is Schizoneura, of Triassic age,
characterised by the whorled leaves being
associated in webbed sheaths, which may,
however, be slit longitudinally to the base.
They thus form leaf-like lobes which stand
off at a considerable angle from the axis
(Fig. 198). The axis is marked by longi-
tudinal grooves, which are continuous
longitudinally from internode to internode,
thus showing that the leaves of successive
whorls did not alternate. The fructifi-
cation is unknown.
Most of the older Equisetal fossils,
however, belong to the Calamarian type.
These plants were often of dendroid habit,
with secondary thickening of the stem, but with a similar primary
construction of the shoot to that seen in Equisetum. The leaf-whorls
are frequently webbed at the base, though often only slightly, as in
Annularia ; but in Asterophyllites, which is traced back to the Devonian
period, the leaves appear quite separate, in widely divergent whorls.
1 Kidston, "On the occurrence of the genus Equisetum, etc.," Annals Mag. Nat.
Hist., ix., p. 138, 1892.
2 Zeiller, Palaeobotanique, p. 164.
FIG. 195.
Equisetum maximum, Link. A, the
upper part of a fertile axis, with the lower
half of the strobilus. Natural size, b — the
leaf-sheath. n; = annulus. x — stalks of spor-
angiophores cut off. y = transverse section of
axis. j9 = sporangiophores in various posi-
tions, slightly enlarged. s/ = stalk. sg—
sporangia. 5 = enlarged distal end. (After
Sachs.)
GENERAL MORPHOLOGY
373
The leaves themselves were usually simple, as in Ec/uisetum, though of
greater dimensions, and accordingly more effective as assimilating organs;
but among the earliest forms, such as Asterocalamites (Schimper), from
the Culm, the leaves were branched in repeated dichotomies (Fig. 199).
In the very early Pseudobornia, from the upper Devonian of Bear Island,1
the foliage was forked in a fan-like fashion, and of considerable dimensions.
Another feature, in which certain of the earliest forms differed from the
later, was in the fact that the members of successive whorls were super-
posed, and did not alternate (e.g.. Asterocalamites). Such forms have been
FIG. 196.
Equisetum pratense, Ehrh. Shoots showing recurrent whorls
of sporangiophores and of bracts. (After Milde.)
B
FIG. 197.
Pltyllotheca. Zigno. A, Ph. eqnisetiforntis
from Rovere di Velo, near Verona. B, inflores-
cence from Siberia, placed by Schmalhausen with
Phyllotheca. (After Solms.)
associated by Potonie as" a family of " Protocalamariaceae." The facts
would seem to indicate then a primitive construction of the Equisetoid
shoot as having relatively large \vhorled and superposed leaves, effective
as assimilating foliage : these were also separate from one another, and
liable to bifurcation. The condition, as seen in the present Equisetum,
might be understood as attained by reduction of the coalescent and simple
leaves, which became also alternate instead of superposed, while the
assimilatory function was relegated almost entirely to the axis. But there
is no certain proof that the actual evolution of Equisetum itself was along
such a line as this.
' Nathorst, Z. Foss. Flora d. Folarliinder. i., Lief. 3, Taf. 7, 8.
374
EQUISETALES
The Calamarian strobili were terminal on the axes, but they had a more
elongated form than is usual in Equisetum (Fig. 200) : sometimes they
FIG. 198.
Schizoneura Godwanensis. Two-thirds the natural size. (After O. Feistmantel, from
Engler and Prantl, Nat. Pflanzenfain.}
extended to a length of 30 cm. (Potonie). They differed also in their
construction : the nearest to the Equisetum-typQ is the ancient Archaeo-
calamites (Bornia)?- characteristic of the oldest
Carboniferous strata, and of the upper Devonian
(Fig. 201). Its strobilus is essentially like that
of Equisetum, having no sterile bracts intervening
between the whorls of eight to ten sporangio-
phores. These whorls did not alternate, but
neither did the whorls of branched leaves in this
early type. Here it would appear that there is a
more complete differentiation of the reproductive
from the vegetative region than is the case where,
as in other Calamarians, sterile bracts are dis-
tributed throughout the strobilus.
Asterocaiamites scrobicuiatus. ^ }atter was the more prevalent type
bchlotneim (sf) from the culm.
Fragment of a leafy shoot, re- amOng the early Equisetales : in them the sterile
duced to hall its natural size.
(After Stur, from Zeiiier, Paieo- leaf-whorls and the whorls of sporangiophores
botanique.)
regularly succeeded one another, as it is seen
in Calamostachys, and is well shown in C. Binneyana, which is the
best known type (Fig. 202). The sterile whorls are commonly composed
1 Renault, Bassin Hoitiller et Per mien d? Autir.i et tfEpinac, vol. ii., p. 80, Plate 42.
FIG.
GKNKRAI, MORPHOLOGY
375
of twelve coherent Jeaves, but thkteen have been counted : the sporangio-
phores are usually six, that is, half the usual number of the leaves of the
sterile whorls; but seven and eight have been seen in a single whorl of
them, while no whorl of sixteen bracts has been seen. Hence it is clear
that the sporangiophores bear no strict
numerical relation to the sterile bracts.
The position of the bracts in successive
whorls of them alternates : the successive
whorls of sporangiophores, on the other
hand, do not alternate, " but are placed one
above the other in vertical rows. Hence it
is evident that their position can bear no
constant relation to that of the bracts."1
This absence of a strict relation of the
sporangiophores to the bracts comes out
also in C. Ludwigi, described in detail by
Weiss.'2 He remarks of this species that
the number of leaves in the sterile whorl is
evidently variable : he made several count-
ings, and concludes, " accordingly it may
be accepted that there were sixteen leaves
in the whorl, but that they might be re-
duced to twelve (or thirteen ? ) by abortion
of some of them." The leaves of the
neighbouring whorls certainly alternated.
Of the sporangiophores he says, the number
in each whorl is six, and the successive
whorls of sporangiophores stand vertically
above one another ; but he notes slight
deviations from this, perhaps due to torsion.
A still further step is depicted by Weiss,3
in the case of Calamostachys germanica,
where apparently the narrow bracts are
approximately three times the number of
the sporangiophores ; but this is not speci-
fically stated to be the case in the text.
On the other hand, it has been shown
clearly in the case Palaeostachya vera that
the number of bracts approximately corresponded directly to the number
of sporangiophores, though possibly in some cases they somewhat exceeded
1 Williamson and Scott, " Further Observations on the Organisation of Fossil Plants, etc.,''
part i., Phil. Trans., 1894, B, pp. 902-3. See also Scott, Studies, p. 47, etc.
~ Abhandl. z. Geol. Spezialkarte, vol. ii., part i. , p. 38.
A L.c. , vol ii., part i., Taf. xvi., Fig. 3 K.
FIG. 200.
Palaeostachyapedunculata. Specimen
from the coal-shales, showing a fertile
shoot bearing about a dozen cones, and a
few leaves. >t = stem. About two-thirds
natural size. (After Williamson, Phil.
Trans. Will. Coll., 1060.) From Scott's
Sttuiics in Fossil Botany.
376
EQUISETALES
it. In fact it is to be recognised that, speaking of the bracts, " a tendency
to multiply the number of appendages in each whorl seems to have been
Archaeocctlajiiites. Part of
cone showing the axis (ax)
in surface view, bearing
superposed verticils of peltate
sporangiophores (sp) without
bracts, sm — sporangia. (After
Renault.) From Scott.
FIG. 202.
Calamostachys. Diagram- of
cone in radial section. a.r = axis,
which bears successive vercicils of
bracts (br), and peltate sporangio-
phores (sp). sm — sporangia borne
on the sporangiophores. As the
bracts are alternate with one
another their upturned tips are
only shown in every alternate
verticil. (After Scott.)
FIG. 207.
Palaeostacliya. Diagram of cone
in radial section. ax — axis, which
bears verticils of bracts (l>r) with
peltate sporangiophores (s/>) in
their axils, sm = sporangia. (After
Renault.) From Scott.
a characteristic Calamarian feature."1 There appears, consequently, to
have been no constant relation either of number or of radial position
between the bracts and the
sporangiophores.
The relation of these
two parts as regards vertical
position is also variable
within the fossil Equise
tales ; for, as is well known,
the sporangiophores occupy
in Palaeostachya a position
at the base of the internode
(Fig. 203), in Calamostachys
p. 882.
Cingularia, typica, Weiss. From the Westphalian. Diagrammatic
drawing of part of a shoot. X about 2. After Weiss.
1 Hickling, Ann. of Bot. , 1907,
SPORE-PRODUCING MEMBERS
377
in the middle of the internode^(Fig. 202), and in Stachannularia or
Cingularia at the top of the internode (Fig. 204). Such facts as these,
here only briefly sketched, must be taken into account in discussing the
morphology of the strobilus of the Equisetales, and in deciding the true
chararacter of the sporangiophores. But before this is entered upon their
detailed structure and development must be examined.
SPORE-PRODUCING MEMBERS.
Naturally the development of the spore-producing members can only
be followed in the living genus, though from the similarity of their mature
features to those seen in the fossils it
is probable that there was substantial
similarity in these also. In Equisetum
the axis, which is about to produce
a strobilus, ceases active growth in
length, retaining a conical form : the
sporangiophores arise upon it in aero-
petal order, as convex swellings (Fig.
205). The details show some varia-
tion in different species : they are here
described for Equisetum arvense and
Itmosum* In the first stages the spor-
angiophores are not unlike the sterile
leaf-sheaths, involving, as seen in
longitudinal section, some six cells,
which grow out with a fan-like tracery
and repeated anticlinal walls (Fig.
206 A). This similarity has been used
as an argument favouring the view that
the sporangiophore and the bract-leaf
are results of " metamorphosis " of
essentially the same part, a point
which will be taken up later. Single superficial cells near the margins
of the convex outgrowths' are early recognisable as the parent cells which
give rise to all the essential parts of the sporangia, though adjoining
cells also grow out together with these* to form the sporangial body : the
origin of the sporangium is thus of the eusporangiate type (Fig. 206 A, B).
At an early stage there is active growth in the middle region of the
sporangiophore, which results in an inversion of the young sporangia, so
that they come to point with their apices towards the axis. Each parent
cell first divides periclinally (Fig. 206 A) : the inner cell gives rise only
to a portion of the sporogenous tissue, the outer undergoes further division,
first by anticlinal, later by periclinal walls (Fig. 206 B, c, D). The inner
1 Studies, i. , p. 496, etc.
FIG. 205.
Half-developed strobilus of Equisetum arvense,
in longitudinal section, taken at end of October.
X 50. (After Hofmeister.)
78
EOUISETALES
products thus formed share with the product of the inner cell already
described in constituting the large sporogenous tissue, which, though entirely
derived from the single parent cell, is not defined by its first periclinal
wall : it is indicated by shading in the figures, while the products of the
subsequent periclinal divisions are marked with a cross. Transverse sections
at the stage represented in Fig. 207 A show the sporogenous tissue in a
central position surrounded by several rather irregular layers forming the
sporangial wall (Fig. 207 B). The size and construction of the sporangia,
even of those in near juxtaposition, may vary greatly : this has been
especially seen in the case of E. limosum. As the sporogenous group
enlarges a layer of cells immediately adjoining it externally becomes glandular
FIG. 206.
Equisetum arvense, L. A, radial longitudinal section of part of young strobilus,
showing two sporangiophores in a very young state. B, C, £>, individual sporangia, in
older states, cut in median section. X 200.
in appearance, and develops as the tapetum (Fig. 208 A). Later the cells
of the sporogenous tissue itself separate, and round themselves off as spore-
mother-cells ; but it is only about two-thirds of these cells which undergo
the tetrad-division, about one-third of them shrivel, and become disorganised,
their substance mingling with that of the tapetum, which becomes intrusive
as a multinucleate plasma into the interstices between the spore-mother-
cells (Fig. 208 B) : the fertile cells which remain are nourished by this
as they develop into the mature spores. Finally the superficial cells of
the wall become indurated and spirally thickened, while those within it,
excepting at the base of the sporangium, are disorganised. The mature
sporangium, consisting thus of a single layer of cells of the wall, and
containing the ripe spores which are all alike, dehisces along a longitudinal
line facing inwards towards the stalk, which line had previously been defined
by the cell-structure.
SPORE-PRODUCIXG MEMBERS
379
A.
Each of the sporangiophores, ' from which the sporangia thus depend,
is traversed from the stalk upwards by a vascular strand, which branches
in the enlarged head, and each branch terminates immediately below the
base of one sporangium. The sporangiophores are in close juxtaposition
while young, and thus the sporangia are effectively protected. At the base
of- the strobilus lies the annulus, which completes the investment of the
lowermost series of sporangiophores : it has as a rule no vascular supply
(Fig. 209). Goebel has pointed out the
protective biological use of the annulus ; l
also that at the apex the highest spor-
angiophores may be imperfectly developed
and concrescent, thus forming a terminal
cap : the protection of the young sporangia
is thus very complete.
The number of sporangiophores in the
Eguisetum-strobi\us is not strictly defined,
while the number of sporangia on each
sporangiophore is also variable : it is usually
larger in Equisetum than in the Cala-
marians: this raises the question of evidence
of variability of number of sporangia. There
is in Equisetum no structural evidence of
the septation of sporangia such as might
lead to their increase in number, nor is
there any interpolation of later sporangia
between those first formed. In some of
the larger cones, such as E. maximum,
branched sporangiophores are commonly
found, which appear to indicate a possible
increase in their number by fission : the
irregularity of their number and arrange- £?1tisftum *„„„„, L. A, section ua-
ment in these large cones would seem to ^f^^^^ii^h^b!
support this (compare Fig. 195.) Excepting f^^^^gZZ™^^
for such indications there is no evidence first. Periclinal division. />•, a similar spor-
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.
38o
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. 2Z..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 describes1 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, are4 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 Scott2 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, that3 "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. 2Z.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
out2 that the earliest forms had narrowly
linear, branched leaves, those of later
occurrence had larger, more broadly wedge-
shaped, and unbranched leaves : thus the
size of the leaf increased in the rising
geological scale, while the branching of it
fell off. But, on the other hand, a striking
feature illustrated in the well-known S. cunei-
folium was the heterophyllous character.
Here on the same plant finely cut leaves
may be found below and broader, wedge-
shaped leaves above, while in the strobilus
the leaves are again finely cut (compare
Fig. 215). Commonly the members of one
whorl were equally developed, but in the
forms from the Glossopteris Flora, named
Trizygia, they were unequal. Examples of the leafage of different types of
Sphenophylls are shown in Fig. 216, A, B, c, D. The plants were fixed in
the soil by diarch roots, which appear to have been borne on the nodes ;
but the details regarding them are imperfectly known." The whole plant
seems to have been of a weak, straggling character.
The internal structure possessed greater distinctiveness than the external
form, and showed a marked secondary thickening : this originated very
1 Scott, Studies, p. 82. - Engler and Prantl, i., 4, p. 516. a Scott, Studies, p. 92.
Fu;. 215.
Sphenophyllitm s/>., branched stem, bearing
linear and cuneate whorled leaves on different
parts. The branch (a) terminates in a long
and slender cone. Half natural size. (After
Stur, from Scott's Stjidies in Fossil Botany.)
400 SPHENOPHYLLALES.— A. SPHENOPHYLLEAE
early, so that the unaltered primary state is seen only in small twigs.
Here a protostelic structure is seen, without any pith or conjunctive
parenchyma. The primary xylem is of triangular form, the groups of
protoxylem, either single or double, being at the projecting angles; or
the angles may be duplicated, and a hexarch form be attained. The
vascular system is strictly cauline : it passes through the nodes without any
appreciable change of structure, a point of interest for comparison with
the Equisetal structure as interpreted by Gwynne-Vaughan.1 A peculiarity
of some importance for further comparison is shown in the primary wood
D
FIG. 216.
A = a leaf-whorl of Sphenophylhun ctmeifolium, and one leaf of it somewhat enlarged.
B — a. leaf- whorl of Sphenophllynm tenerrimum. C — Sphenophyllum verticillatum.
(From Potonie's Lehrbuch der Pflanzenpalciontologie.) D = " Trizygia " speciosa.
Royle, from the Glossopteris-facies, (after O. Feistmantel.)
of the ancient species, S. insigne, from the calciferous sandstone : here a
canal is formed at each of the three angles of the primary wood, pre-
sumably by disorganisation of the protoxylem, as in the Equisetales2
(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,
'7v 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.) Jn ,£ 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.
4o6 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.
1A second specimen, belonging also to Mr. Kidston, to whom the original discovery
was due, shows only eleven protoxylems.
2Z.r., p. 113.
SPORE-PRODUCING MEMBERS
407
Comparing this with Fig. 225, Which is from the axis of Calamostachys
Binneyana, .there is a similarity in outline of the primary stele ; but as
this is not constant in the species it cannot bear weight in the comparison.
The points of importance are, the similar canals, of like position to those
of 6". insigne, and like them showing the position of the protoxylem ; the
continuous medullary rays, and the similarity of the tracheides. These
Fit;. 223.
Chcirostrobns Pettycurensis. Diagram. The upper part in transverse, the lower in
radial section ; the position of the organs corresponds in the two sections, i. Transverse
section. Six complete sporophylls, each with three segments, are shown ; Sp. a = section
passing through sterile segments; S^.6 — ditto through fertile segments, or sporangio-
phores ; j/=lamina of sterile segment ; ^j = downward outgrowths of sterile laminae cut
transversely ; $/» = their apices, transverse ; _/== peltate sporangiophores ; ^. in — sporangia.
Note that in Sp.a each peltate lamina,/", is seen in two distinct lobes, with the sterile lamina
between ; v.b\, -v.ba — vascular bundles of two whorls. 2. Radial section. The sporophylls
are separated from one airbther for clearness' sake, in nature they are in close contact.
,*/.r = axis of cone; fj^—its stele; ^/* = base of sporophyll. Other lettering as in trans-
verse section. The diagram is true to nature as regards proportions of parts, as well as
their relative position. X about 2. (From Scott's Studies in Fossil Botany.)
features appear to indicate a real structural resemblance, and it is important
to note that the nearest approach is between the oldest of the Sphenophylls
and the strobilar structure of a Calamite : for according to the views here
advanced, it is in the strobilus that the more primitive structure might be
anticipated.
A special interest in relation to the strobiloid theory attaches to
Sphenophyllum majus, with its ill-defined cone. It is important to note
that this state, so prominent in Lycopodium, is found in that species
408
SPHENOPHYLLALES. B. PSILOTACEAE
of Sphenophyllum in which the arrangement of the sporangia is in a group
disposed radially around a central attachment — a condition comparable
with that of the Calamarians, and which was
probably a relatively primitive state. Finding these
two features combined in the same plant gives to
both additional weight. But they are also combined
in that other series which, following the suggestion of
Thomas, are here included with the Sphenophyllales,
viz. the Psilotaceae. These will now be described,
and the general discussion of the morphology of the
sporangiophoric Pteridophytes will be reserved till it
can be illuminated by the facts which these living
genera supply.
FIG. 224.
B. PSILOTACEAE.
Diagram cf the vascular
supply to the sterile lobes
(st), and to the sporangio-
phores (f) in Cheirostrobus.
The genera Tmesipte?-is and Psilotum are the only
living representatives of this peculiar and somewhat
isolated family, while there is nothing known among
Fossils which can with any certainty be ascribed to
it. They have commonly been classed with the Lycopodiales, and, as
we shall see, there are many undoubted points of resemblance in that
FIG. 225.
Calamostachys Binneyana. Transverse section of axis of cone, showing stele and part
of cortex. Surrounding the pith there are six bundles, in groups of two, with secondary
wood. /jr = protoxylem groups. X about 60. Phil. Trans. W. andS. Will. Coll., 1016.
(From Scott's Studies in Fossil Botany.')
direction. But increasing knowledge of the Sphenophylleae, as well as
of the Psilotaceae themselves, has indicated a more natural position of
both together in the phylum of the Sphenophyllales. The two genera of
the Psilotaceae are so similar in their general characters that there is no
doubt of their close affinity : on the other hand the differences of detail
CiKNKRAL MORPHOLOGY
409
between them, as well as the variations in the individuals in either genus,
afford an important basis for
comparison with other forms,
throwing light upon fluctuations
of structure which would other-
wise be more puzzling than
they now appear to be.
Both genera are rootless.
The green, more or less shrubby
shoot, is established in the
substratum, which is usually of
humus character, by means of
a plexus of leafless rhizomes
invested with rhizoids, and
penetrated by a mycorhizic
fungus. The nutrition of these
plants is thus of a mixed char-
acter, partly saprophytic, partly
by photosynthesis. The aerial
shoots bear appendages of two
sorts, described as foliage leaves,
which are simple, and sporo-
phylls, which are forked. These
may be associated together
irregularly on the same shoot
which thus takes the character
of a lax, undifferentiated stro-
bilus.
In Tmcsipteris, of which the
single species T. tannensis is
native in Australasia, though
extending northwards to the
Philippines, the structure is
more simple than in Psilotum.
Its habit is peculiar, the plant
being established on the trunks
of tree-ferns, though occasion-
ally it has also been found
growing upon the ground.
The rhizome, which fixes it in
the substratum, is repeatedly
branched in a dichotomous
manner and is without appen-
dages other than rhizoids. Branches of this system turn upwards to the
light, and develop as the aerial shoots : these are usually themselves
D
FIG. 226.
Tinesipteris tanncnsis, Bernh. A = Habit-figure of a whole
plant (pendent form), snowing a dichotomy. Natural size.
B-E, sporophylls, with synangia ; J5, seen from the side ; C,
from above ; D, after dehiscence ; E, from the under (dorsal)
side, all X about 3. F— rhizome Vz 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 : ndtv 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 show2 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 thusv traversed. A truly radial section of a
young synangium is shown in Fig. 232 B, the arrow indicating the direc-
tion of the main axis : the cell ( x ) is a conical cell, which is commonly
though perhaps not constantly found occupying the centre of the apical
surface of the synangium. The cell shaded is believed to be the arche-
sporial cell for one of the loculi, but after comparison of a large number
FIG. 232.
Psilotuin triquctruin, Sw. Various stages of development of the synangium and
sporangium. In C the sporogenous group is shaded. D shows the differentiation of its
cells, the fertile cells being shaded. E shows the disorganisation of the remaining cells
without forming spores. X 100.
of sections I am still uncertain whether the whole of the sporogenous tissue
in each loculus is really referable to a single parent cell, for just the same
difficulty arises here as in Tmesipteris in recognising the exact limits of
the sporogenous masses.
The subsequent stages of development are illustrated by Figs. 232 c, D, E,
and it will be seen from these how the sporogenous masses assume large
dimensions, and are at first composed of uniform cells. The wall of the
synangium meanwhile becomes multiseriate, and the cells of the outermost
layer assume a deep and prismatic form, while the inner layers are narrow.
2D
4i 8 SPHENOPHYLLALES. B. PSILOTACEAE
The same is the character of the more superficial cells of the sporogenous
mass (Fig. 232 D), so that it is almost impossible to recognise the limit
between the tissue of the wall and of the sporogenous mass : the superficial
portions of the latter become disorganised without the formation of spores,
but there is no clearly defined tapetum. Such is also the fate of a
considerable proportion of the more central cells : for as the synangium
develops, irregular groups of cells of the sporogenous masses assume
dense granular contents, and subdivide, while the others remain paler,
with more watery contents, and do not divide : the former undergo the
final tetrad-division and form spores, while the latter become disorganised.
The actual state of partial disorganisation is shown in Fig. 232 E: thus a
partial sterilisation of cells of the sporogenous tissue, essentially similar to
that in Equisetum^ is seen also in Psilotum, and, as above stated, it occurs
also in Tmesipteris.
In both genera the lines of dehiscence at maturity are defined
structurally. In Psilotum the lines radiate from the centre, and it has
been seen that a similar condition may be recognised in the sporangio-
phores of Sphenophyllum majus. Thus, in the broad outlines of
structure, in function, and in position the sporangiophore of the Psilotaceae
is the correlative of that in the Sphenophylleae. It remains to see how
far there is a correspondence also in the anatomical structure of these
plants.
ANATOMY.
The internal structure of the Psilotaceae is relatively simple, in
accordance with their outward form. The axis is traversed by a stele,
which is fundamentally of the protostelic type, and limited by an
endodermis which is more definite in Psilotum than in Tmesipteris. The
broad cortex which surrounds the stele is variously differentiated in the
rhizome and in the aerial shoot : in the former it consists of starchy
parenchyma, with endophytic mycorhiza in the outer layers, while the
superficial cells grow out into rhizoids : in the aerial shoot the stele is
surrounded successively by thin-walled parenchyma, sclerenchyma, and
assimilating tissue, while peripherally is an epidermis with stomata. Such
characters, however, present little that is of comparative value ; it is in
the vascular tissue that a better basis for comparison is found.
The structure of the stele in Psilotum^ varies according to the position
and size of the part : in the rhizome there is often no protoxylem, but
the xylem consists of a somewhat irregular, and exiguous group of
scalariform tracheides, surrounded by an ill-developed phloem, while peri-
pherally there is a definite endodermis. At the base of an aerial shoot
the xylem increases in bulk, with interspersed parenchyma cells, and
1 These statements are based partly on personal observations, but also on the writings
of Bertrand (Arch. hot. du Nord., i.), of Boodle (Ann. of Bot., xviii., p. 505), and of
Miss Ford (Ann. of Bot., xviii., p. 589).
ANATOMY 419
protoxylem makes its appearance :x where the protoxylem is clear it is
evident that the differentiation of the primary xylem is centripetal, as it
is, with local exceptions, throughout the aerial shoot. Passing upwards
along the aerial shoot, the peripherally projecting protoxylems increase
in number, the whole stele enlarging : finally, in transverse section the
xylem appears as a hollow, many-rayed star, while the centre is occupied
by sclerotic tissue. Peripherally as before lie the phloem and the endo-
dermis (Fig. 233). An examination of the lower part of the aerial shoot,
and of some adjoining parts of the rhizome, shows a feeble secondary
formation of xylem : there is no definite cambium, but the additional
tracheides which arise from the tissue outside the primary wood often
show signs of a radial arrangement. The secondary development fades
off as the stellate structure of the upper region is attained. Thus
FIG. 233.
Psilotutn triqitetrutn. Part of a transverse section of the central stele. X 100.
structurally the base of the stem of Psilotum recalls the stem of Spheno-
phylhtm, though with very feeble secondary growth, while the upper part
of the stem resembles the axis of the cone of Cheirostrobus, though on a
simple scale, and with fewer xylem-rays.
In the stem of Tmesipteris no secondary development has hitherto
been found. The rhizome exhibits much the same structure as that of
Psilotum, with a solid group of tracheides at the centre, or in weaker
branches an irregular xylem-ring, surrounded by phloem, and a very ill-
defined endodermis. Often there is no distinct protoxylem ; but, as the
passage is made to the aerial stem, protoxylem may appear : its position
in all the upper region is mesarch. In weaker shoots, and especially in
the upper regions, the cauline tissue of the stele fades out : the leaf-
traces become the main feature of the vascular system, which in trans-
verse sections is then represented by a, ring of separate strands : each
of these has its mesarch protoxylem corresponding to that seen in the
leaf-trace (Fig. 234) : in point of fact this becomes a phyllosiphonic type
of structure. In strong axes, however, the xylern may still form a
420 SPHENOPHYLLALES. B. PSILOTACEAE
coherent ring surrounding a central pith, but with the protoxylem
mesarch.
The chief anatomical difference between the two genera appears thus
to be in the position of the protoxylem. But Boodle points out that
locally a mesarch position may be found in Psilotum also, and he con-
cludes that both genera might be referred to a common parent form,
in which the aerial stem had a rayed mesarch xylem-mass, the suppression
of leaf-traces having caused the loss of centrifugal wood in the one genus,
and the influence of the leaf-traces in the other genus having broken up
FIG. 234.
Tinesipteris tannensis. Transverse section of the sterile region, high up. The proto-
xylem {pr. -xy>) is mesarch. The xylem of the stele is fading out, and being replaced
by parenchyma ; three of the tracheides (/. tr.~) show incomplete development ; there
is no longer a complete ring, and the leaf-trace bundles (/. t.~) enter the gaps which result,
in much the same way as in a phyllosiphonic type. There is no definite endodermis.
Xi 50.
the xylem into distinct bundles.1 He further recalls the fact that in
Cheirostrobus there are indications of a mesarch structure, while paren-
chyma is present among the tracheides towards the centre of its stele :
such cells in response to mechanical requirements might readily be
converted into mechanical tissue, as in Psilotum. It thus appears that
the Sphenophylleae and Psilotaceae show uniformity in the general type
of their vascular construction, though the details are subject to consider-
able fluctuation. This result adds point to the comparisons already
based upon the external characters and the spore-producing parts. At
the same time, it is to be remembered that a structure resembling that
of Psilotum and Cheirostrobus is seen in certain of the Lycopodiales ; in
1 L.c., p. 515.
ANATOMY 421
particular it is found in LepidostKolnis Brownii, where also the leaf-trace
bundles are of the mesarch type. The comparison has also been made
by Miss Ford with Bothrodendron rnundum : in this case the corre-
spondence seems to be rather with the rhizome of Psilotum than with
its aerial shoot.
EMBRYOLOGY.
Of the embryology of the Psilotaceae nothing is at present known.
Even the prothallus has not been recognised with certainty, though
Dr. Lang1 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.
424 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 comparative evidence to the contrary.
Accordingly the existence of the complex and definite cone of Cheirostrohus
as the earliest of the Sphenophyllales cannot be held as invalidating the
conclusion above stated.
In all these sporangiophoric Pteridophytes the axis is the dominant
part of the shoot, and takes the lead in its development. This is shown
anatomically by tthe typically protostelic structure of Sphenophyllum, while
the foliar strands insert themselves marginally upon it. In the Psilotaceae
the type of stem-structure is the same, but the xylem becomes hollow
and medullated in the upper region, and in Tmesipteris it may break
up into separate strands. Even in the apparently divergent case of the
Equisetales it has been shown that the vascular system is referable to
a primitive type of monostele, though greatly reduced in accordance with
a semi-aquatic habit. Notwithstanding such changes the structure of the
axis in all these forms indicates origin from a type in which the axis is
predominant over the appendages, the vascular supply of these being
inserted with the minimum of disturbance upon the cauline stele (Clado-
siphonic type of Jeffrey). This confirms the theory of a strobiloid origin,
with pre-existent axis and subsidiary appendages.
The differences in number of the sporangia on the individual
sporangiophore call for remark. In the Sphenophylleae they have been
observed to vary from six in Sphenophyllum majus to one only in
S. Dawsoni, while Psiiotum and Tmesipteris take a middle position with
three and two respectively. In the Equisetales the number in the
fossil form appears to be commonly four, but in recent species of Equisetum
the number may be much larger and variable.1 In the Sphenophyllales
the variations present some points of interest : the number six occurs
only occasionally in the one species named, while four is the usual
number in that species, which it shares with Cheirostrobus. But in the
species of Sphenophyllum with compact strobili the number may be two,
or only one. These low numbers go along with a larger number of the
sporangiophores, which may be twice (S. Dawsoni}, or three times
(S. Romeri] the number of the subtending bracts. In the former species
the frequent juxtaposition of the stalks, and the insertion of the vascular
supply of the stalks upon ' the strands supplying the bracts, suggests that
fission has been operative, as in the chorisis of stamens : and it seems
probable from the facts that with an increase of number of the sporangio-
phores, however brought about, there has gone a decrease in the number
of the sporangia which each bears. Accordingly S. Dawsoni and S. Romeri
may be held to bear sporangiophores of a type reduced from the original :
and a central type of sporangiophore would appear to be one with about
four sporangia.
The typical position which such a sporangiophore holds in the
Sphenophyllales is one of attachment in a median position to the upper
] In Calamites paleaceus the pendulous sporangiophore bears a solitary sporangium.
426 SPORANGIOPHORIC PTERIDOPHYTES
surface of the sporophyll. This is illustrated by the living Psilotaceae,
and by some species of Sphenophyllum, notably S. ma/us, which shows
other characters held to be primitive. But it is departed from in
S. Dawsoni and S. Romeri, where the number of the sporangiophores
is in excess of the sporophylls, while the leaf-whorls are deeply webbed
into a cup : Cheirostrobus is also an exception, but there the three sporangio-
phores correspond in position and number to the lobes of the tripartite
sporophyll : this condition, together with the vascular connections, suggests
a parallel amplification of the sporophyll and of the sporangiophore, to
which we shall see modern correlatives later among the Ophioglossaceae.
Thus, though liable to modifications, the characteristic position of the
sporangiophore in the Sphenophyllales is in a median position on the
upper surface of the subtending bract.
Here I must enter my dissent from certain " interpretations " which
have been given of the leaf-borne sporangiophore. In cases where it
is inserted on the upper surface of the leaf, as in the Sphenophyllales, it
has been designated a "ventral lobe." If "ventral lobes" were of
common occurrence on the vegetative leaves of these or of other Pterido-
phytes, there might be some meaning in the term. It lies with those
who use this expression to show that such " ventral lobes " exist normally,
other than these spore-producing bodies which they so designate. If they
do not normally exist, then calling a leaf-borne sporangiophore a " ventral
lobe" merely leads to confusion, and provides no explanation of its real
nature. It introduces the idea that the sporangiophore is a result of
" metamorphosis " of some pre-existent vegetative structure, of the nature
of a " ventral lobe," an opinion untenable in the absence of proof that
such bodies existed in the vegetative state.
But, on the other hand, it has been shown above that in the Equisetales,
a series undoubtedly related to the Sphenophyllales, parts similar to the
sporangiophores of the Sphenophyllales in structure and in function are
borne upon the axis and have no constant relation to the bracts : for
reasons assigned above (p. 382, etc.) these are not themselves to be held as
foliar. Study of such sporangiophoric types, not separately but collectively,
thus leads to a conception of the sporangiophore as a non-foliar structure,
which may be inserted either on the axis or on the leaf, though in certain
groups it shows a regular relation to the latter. It is, in fact, a part sui
generis as much as the sporangium is, and not the result of modification
of any other part.
The history of individual development of the sporangiophore, as traced
in Tmesipteris and Psilotum for leaf-borne types, and in Equisetum where
they arise directly from the axis, gives a clue to their nature. The sporan-
giophore first appears as a broad cushion of tissue, in the peripheral parts
of which the sporangia are early initiated : these are from the first orientated
outwards from the centre of the outgrowth. In the Psilotaceae (as also
in S. majus) they maintain this, which may probably have been their
SUMMARY 427
primitive position till maturity. ,ijut in the Equisetales the pendulous
jition is gradually assumed, the inversion of the sporangia being
>ught about by active growth of the middle region of the sporangio-
>hore. This inverted position was probably a derivative state, as indeed
gradual appearance in the development of the individual would seem
indicate. The result presumably of a similar inversion is seen in
Iheirostrobus and in S. Romeri, while it appears to have been general
the Equisetales.
This discussion leads naturally to the question whether in the cases
;fore us the synangial state, as seen in the Psilotaceae and in S. majus,
the more primitive, or that with separate sporangia, as seen in Equisetum
in Chrirostrobus. So far as individual development can serve as a
lide it would point to the former, for in their first stages all sporangio-
lores are synangial, and the state as seen in the Psilotaceae is merely
the consequence of maintaining to maturity the relation of the loculi as
first initiated. The condition seen in Equisetum, on the other hand,
a consequence of the individual projection of each developing sporangium.
rhen this is considered together with the inversion which goes along with
it would appear probable from the story of development that the erect
'nangial condition was relatively primitive, and the condition with separate
iverted sporangia a derivative state.
In view of the initial synangial condition of all young sporangiophores,
further question arises of the origin of the whole structure in descent.
[t has been designated a placental growth : is there any clue from develop-
icnt how it came to be? It has been pointed out (p. 414) that in the
)ung sporangiophore of Tmesipteris the origin of the tissue of the sterile
septum which separates the sporangia is similar to that of the sporogenous
tissue, while in certain reduced sporangiophores the septum may itself be
sporogenous. These facts point, in the simple case of Tmesipteris, to the
conclusion that the septum is not essentially different from fertile tissue,
and suggest that the whole body arose from the subdivision of a single
sac, together with upgrowth of the adjoining tissues. In fact, that the
sporangiophore is really a form of sorus, resulting from septation of a
primitive fertile loculus, together with upgrowth of its receptacle or
placenta: the separate loculi would thus be carried outwards with its
growth. The facts of development as well as of distribution of the sporan-
giophores readily coincide with this view of its probable origin. There
is a biological probability that this mode of progression to a more complex
state should occur, for the nourishment of separate loculi is more readily
carried out than that of one large one, while the scattering of the ripe
spores is more certain where the sporangia project. Lastly, there is
precedent for the conversion of sporogenous tissue into sterile in Isoetes,
and apparently also in Lepidodendron, while numerous Angiosperms show
septa in the anther, formed by sterilisation of fertile tissue in the manner
here suggested.
428 SPORANGIOPHORIC PTERIDOPHYTES
On the grounds thus stated it appears probable that in the Sporangio-
phoric Pteridophytes the sporangiophore is a non-foliar structure, arising
either on axis or on leaf: that it is of placental origin, and bears outwards
as it grows the sporangia, which may be regarded as the results of
disintegration of a single primitive loculus : that the synangial condition
of these was the prior state, but that in most cases the sporangia have
shown independent growth, and now project as separate sacs, often
becoming inverted during development, a change which brings advantages
of protection while young.1
The general features of the shoot common to the Equisetales and
Sphenophyllales may be summarised as follows :
1. The axis is the predominant part of the shoot: it is traversed by a
monostele, which frequently shows the protostelic state, with solid xylem-
core. All known types of vascular system in these phyla are referable in
origin to that primitive structure.
2. The lower part of the plant is vegetative : there is a more or less
definite and compact strobilus above, usually terminal : but in some there
is an indefinite " Selago " condition, characterised by being more lax, and
often also more effective for assimilation.
3. The leaves are simple or branched, in whorls (Equisetales and
Sphenophylleae), or alternate (Psilotaceae).
4. The sporangia in variable number are disposed radially on the
sporangiophores, which may be inserted either on the axis (Equisetales),
or on sporophylls (Sphenophyllales), but still have an essentially uniform
character in both types. They are held to be organs sui generis, of the
nature of placental growths.
5. The sporangia are eusporangiate, and dehisce by slits radially
disposed and structurally defined.
6. In all cases which have been examined developmentally a consider-
able portion of the sporogenous cells distributed through the mass are
disorganised without undergoing tetrad-division.
The plants thus characterised lend themselves readily to interpretation
on a hypothesis of a strobiloid origin. The predominance of the axis in
the embryology of Equisetum (the only sporangiophoric type in which it
has been followed), as well as in the mature shoot of them all, is very
striking, while the sporangiophores and leaves figure only as minor
appendages : the primitive monostelic structure of the axis, with more or
less definite cauline xylem-core, and insertion of the leaf-traces upon its
periphery, also supports a strobiloid hypothesis from the point of view of
internal structure. The existence of the undifferentiated " Selago " con-
dition brings these plants into line with the Lycopods : the facts showing
the relation of the sterile to the fertile regions would here be insufficient
1 With the above paragraphs, compare Scott, Studies, pp. 496-500 ; also Progressits,
vol. i., p. 163, etc.
SUMMARY 429
for consecutive argument, but their correspondence with those in
Lycopodium, where the argument can be more fully developed, points to a
clear analogy. Accordingly the facts may be held to indicate a probability
that here also a progressive differentiation of sterile and fertile regions
from an indifferent shoot which performed both functions has been
effective, and that abortion of sporangia has played its part. That the
strobilus which we see is the residual and now specialised fertile tract,
while the vegetative system below has been initiated, or at least extended,
by abortion of sporangia : this results in deferring the spore-production to
a later period. Appendages of two sorts are recognised throughout, viz.
the leaves and the sporangiophores : the former remain effective for
assimilation or for protection in the lower vegetative region : in the
strobilus the two may exist together, and even show intimate relations :
or the sporangiophores only may be present, as in Egiiisetum, the true
javes being absent, perhaps by abortion.
As regards spore-output, continued apical growth and branching have
>rved as a set-off against the progressive sterilisation in the region below.
Jut in addition these plants illustrate a probable amplification of the
spore-producing members themselves, by septation and upgrowth of the
/ascular placenta, resulting in the sporangiophore itself: this is a more
fective spore-producing member than a single sporangium. Another
lethod of advance has probably been by fission, which increases the
mmber of sporangiophores ; but this appears to have brought in its train
corresponding reduction in the number of the sporangia, as seen in
Sphenophyllum Dawsoni. Thus both evidences of increase and of decrease
in number of sporangia are illustrated in this, as in most other phyla of
rascular Plants.
Some idea of the probable origin and nature of the sporangiophore
las been gained by the study of the Equisetales and Sphenophyllales.
[t is a member attaining a considerable size, and endowed with a vascular
system, while it carries out the important function of spore-production.
There is no prima facie reason why such a member should show any
trict limitation of size. The larger it grew, the greater would be its
ipacity for producing fresh germs : there would thus be an inherent
)robability of its increase', rather than the reverse. When the question
is asked whether such increase is illustrated in any forms of Vascular
'lants, a debatable ground is reached in which the Ophioglossales are
ie subject of dispute. It is only by a careful study of their details
that an opinion can be formed : this will now be taken up.
CHAPTER XXX.
OPHIOGLOSSALES.
THE Ophioglossales include three l genera of living plants : Ophioglossum,
with ten species as described in Hooker's Synopsis Filicum, though
Prantl distinguishes twenty-nine : Botrychium with about six, or according
to Prantl fifteen species : and Helminthostachys with only one. The three
genera have well-marked characters in common, so that there is no doubt
of their natural affinity. The most distinctive is the fertile spike, a
process which rises from the adaxial surface of the leaf, and serves as
a basis for insertion of the sporangia: these are of the eusporangiate
type, and are without any annulus. There is no early fossil that
can be attributed with any certainty to this family, and thus, notwith-
standing that the appearance of these plants is commonly held to
be archaic, there is no direct evidence of any great antiquity. They
have usually been classed with the Ferns, of which thay have been
held to be an outlying group. Other authors recognise certain
characters as linking them with the Lycopodiales. A careful consider-
ation of the evidence leads to the conclusion that they are best in
place as an independent phylum of the Ophioglossales, and the justifi-
cation of this will appear from the account of them now to be given.
Any decision on the point of affinity is closely related to the
question whether the organisms constitute an upgrade or a downgrade
sequence. In the description which follows the various types of the
family will be traced from the simpler to the more complex, and the
discussion of their relationships will be left over to the conclusion,
when the facts necessary for forming an opinion shall be before the
reader.
1The foundation of a fourth genus " Sceptridiuvi " has been suggested by H. L. Lyon
(Bot. Gas., Dec., 1905). It is based mainly upon embryological detail. I prefer for the
present to suspend any decision as to the validity of this proposal, awaiting the detailed
statement of the facts.
GENERAL MORPHOLOGY 431
X
EXTERNAL CHARACTERS.
The plants of this cosmopolitan family are all perennials, and are for
the most part underground-growing organisms, though some few are
epiphytic. The method of their perennation is closely connected with
their external form. Given a leafy shoot in an underground-growing
organism, there are two ways in which it may be specialised so as to
icure perennation, and often the arrangements are such as to fit in
mveniently with alternating seasonal periods of activity and of rest. The
is by elongation of the internodes, accompanied by repeated branching :
this case the terminals of certain branches themselves appear above
>und in the active season, and may die off at its close, the perennation
sing effected by the branching stock which remains in the soil : such
loots are usually small-leaved, and examples are seen in Equisetum, and
some of the more specialised species of Lycopodium and Selaginella,
/here the primitive upright habit of the main shoot has been discarded,
ic other method is by enlargement of the individual leaf, while the stock,
rhich is sparsely branched or even unbranched, remains protected below :
lis is exemplified by Isoetes in a less pronounced form, but in its most
rtreme type by the Ophioglossaceae, and by some Ferns of such habit
Pteris aquilina. The stock itself in such plants is provided with
ifficient storage-tissue, and may in some species be specially distended
id tuberous (O. crotalophoroides, Walt., and O. opacum, Carmich.). This
>e tends to become monophyllous, with only one large leaf expanded
each season. The chief biological advantage in the monophyllous
ibit in a plant with a perennial stock lies in the fact that the soil
;nts an obstacle to the upgrowth of the tender young leaf : the difficulty
overcoming this is minimised by the production of only one leaf in
ich season, and that a large one. This would apply equally to the case
Pteris, and to that of the Ophioglossaceae.
It is then as organisms showing a peculiar specialisation for a perennating
habit that the Ophioglossaceae are to be studied. There is one further
point on which it is necessary to be clear at the outset : the Lycopods
and the Horse-tails are small-leaved forms and show a similar method of
perennation : but still they' are held to represent distinct phyla. Similarly,
though the Ophioglossaceae and the Ferns may show in common another
mode of perennation, accompanied by large foliar development, still this
does not in itself indicate any near relationship : for clearly leaf-enlargement
is not the prerogative of one phylum only.
Taking first the genus Ophioglossum, the well-known species O. vulgatum
occupies a middle position in the genus (Fig. 235) : it consists of a short
upright stock, covered externally by the scars of leaves expanded in previous
years : thick roots, which are commonly unbranched (though occasionally
showing dichotomy), and hairless, radiate from it, one being inserted as
a rule below the base of each scar; but this arrangement is not rigidly
432
OPHIOGLOSSALES
FIG. 235.
Ophioglossutn vulgatum, L. A— old plant sprung as an adventitious bud from the
root (a-b) ; from its stem have sprung further roots (c-d and e-f), from one of which again
an adventitious bud (h) has arisen : g— the leaf for the next vegetative period, still un-
folded ; /=an abortive spike attached to expanded leaf. (After Stenzel.) Z? = an old
plant with one sterile and one fertile leaf. C and D show form of leaf, with spike, and
K the venation. F, G, H — details of spike. /= Ophioglossum vulgatum var. polyphylla
A. Br. The figures A-D and J are half the natural size. From Rabenhorst's Krypt.
Flora.
GENERAL MORPHOLOGY 433
maintained here, and is departed from in other species. The apex of the
stock is occupied by a bud, and according to the season the outermost
leaf (or sometimes two or more of them) may be extended above ground,
or it may be still enveloped by the ochrea-like stipule of the preceding
leaf (Fig. 236. i, 2, 3). The bud on dissection shows that the apex
of the axis is buried deep down among the successive leaves of the bud :
each of these is provided with a large stipular sheath, which covers the
bud, including all the succeeding spirally arranged leaves. There is no
circinate venation. Each leaf develops slowly in the bud for three years,
and expands in the fourth year. In spring the young leaf of the year,
•bursting the sheath of the preceding leaf, extends with an elongating
petiole upwards, forcing its way through the soil: and the broadly ovate
sterile lamina finally unfolds as a fleshy, undivided expansion, with
reticulate venation. From its upper surface, at the point of junction
with the lamina springs the fertile spike, a body which is stalked, and
bears on either lateral margin of its upper part a dense row of sunken
sporangia (Fig. 235 B, c, F, G) : the tip of the spike is sterile. Terminal
branching of the shoot is exceedingly rare : a case is recorded by Poirault.
But that deficiency is made up by the frequent formation of adventitious
buds : these may appear in relation to the axis (Fig. 236. 8), but more
frequently upon the roots, where they arise in close proximity to the apex
(Figs. 235 A, 236. 7).
These external characters of the mature plant of O. vulgatum represent
typically the salient features of the Adder's Tongues ; but to obtain a
conception of the genus as a whole, it is necessary to examine other
species, and they will here be taken in a sequence which is held to
illustrate a morphological progression. The species are not all habitually
monophyllous : several small species are found to be polyphyllous, showing
constantly that condition which is exceptional in O. vulgatum (Fig. 235 B, j).
Conspicuous among them is O. Bergianum : this rare little plant differs
externally from other species in the fact that the fertile spike is inserted
very low down upon the narrow linear sterile leaf, of which three or four
are commonly expanded at once (Fig. 237). The number of sporangia on
each spike may also be very small ; but notwithstanding these differences,
the general disposition of the parts is that usual for the genus. The
polyphyllous condition which it shows is shared also by O. bulbosum^ Michx
( = O crotalophoroides, Walt.), and especially by O. nudicaule^ L. fil., where
it appears to be common, and even habitual, four to six leaves being
simultaneously expanded, and most of them bearing fertile spikes. In
O. lusitamcum also, as well as in several other species, a plurality of leaves
simultaneously expanded is the rule. That condition is most frequent in
the smaller-leaved forms, and it may be held to connect the monophyllous
habit as seen in the Ophioglossaceae with the polyphyllous strobiloid type
common in other Pteridophytes.
But the genus shows a capacity for amplification of the parts of the
2 E
fitfl.
*» _r.f Af- v
' r N/f-r
. I
--^r
;.A
-
FIG. 236.
Fig. i. Adult plant towards end of autumn ; // = leaf of succeeding summer; s/t = \l<
sheath ; /g^second leaf; /s = thircl leaf; <f=debris of dead leaves ; r=parent root.
Fig. 2. Longitudinal section of a very young bud. /., //., ///., leaves; c = cortex
s/t = sheath ; .r-xylem ; ph — phloem.
Fig. 3. Longitudinal section of an older bud, where the first leaf (/) is expanded
s/t, s/ii, shn — sheaths of successive leaves.
Fig. 4. Central cylinder of a very young bud prepared by maceration ; r— parent root
/,-/4 = traces of successive leaves ; ryr± = successive roots.
Fig. 5. Central cylinder of an adult stem ; /!-/15 = the traces of successive leaves.
Fig. 6. Enlarged apex of a root ; the first phase of appearance of a bud.
Fig. 7. Bud slightly developed, where the first leaf has just pierced the sheath.
Fig. 8. A false "branching.
GENERAL MORPHOLOGY
435
FIG. 237.
Ophioglossnm Bergianutn,
Schlecht. Whole plant,
slightly reduced.
individual leaf beyond what is typically seen in O. vulgatum, though
characters which are usual in such species as O. pendulum or palmatum
appear as occasional abnormalities in O. vulgatum
and other species. The large series of examples in
the Herbaria of Kew and the British Museum have
been examined in order to elucidate these amplifica-
tions, and among the specimens compared gradual
steps of progression are illustrated from the con-
dition with a single spike to the most complex types
of O. palmatum. Some of these are here illustrated.
Fig. 238 A shows a specimen in which a single fertile
spike rises from the adaxial surface of the frond,
and it may be seen that the vascular bundles directly
below its insertion continue upwards, and supply the
centre of the sterile frond; the position appears to
be exactly median, as in O. vulgatum. The specimen
shown in Fig. 238 c also has a single fertile spike,
but its position relatively to the two-lobed sterile frond
is not so clearly median as in Fig. A. Fig. 238 D
shows two fertile spikes of equal size, inserted
almost symmetrically on the adaxial face of the
four-lobed sterile frond ; such a specimen, when looked at alone, might
be thought to support the view suggested by Roeper, and adopted
by others, that the fertile spike is the result of coalescence of two
lateral lobes or pinnae ; but a comparison of other specimens shows
that no such view can be consistently supported, and Fig. 238 E shows
a case which it would be difficult to bring into harmony with it ;
for here there are three fertile spikes of almost equal size, all inserted
clearly on the adaxial surface of the sterile frond. The next specimen
(Fig. 238 F) shows a larger number of fertile spikes, eight in all;
every one is inserted well within the margin, on the surface of the
frond, and in close relation to vascular bundles which supply the central
part of it. Of the eight spikes, six are associated in pairs upon a common
stalk, a character which is frequent in specimens where the number of
spikes is large. Fig. 238 G shows one of the most elaborate specimens
in the whole series, with 14 fertile spikes, of which only one is really
marginal. Here again certain of the spikes are associated together,
especially the lowest group of three, which have a common stalk of inser-
tion. Sometimes, however, the fertile spikes are distributed with some
nearer approach to regularity than in the above samples, and it is doubtless
upon such specimens as that shown in Fig. 238 B that the descriptions
of previous writers have been based. But it is to be remarked that such
specimens are by far the least common among the herbarium plants
examined. I was permitted to soak out the specimen shown in Fig. 238 B,
preserved in the British Museum, and to arrange it so that the position
436
OPHIOGLOSSALES
and insertion of the parts could be accurately drawn. Now it is to be
noted that not one of its spikes is actually marginal, but each is inserted
upon the upper surface, just within the margin ; that is most clearly
so in the lower spikes, while the two lowest are seated near to the
median line, and with their stalks so near to one another as to be even
slightly united at the base. From the above specimens it will be sufficiently
II G
FIG. 238.
Ophioglossum palmatum, L. Drawings, slightly reduced, of specimens in the Kew
Herbarium (excepting B, which is in the British Museum), showing the various arrange-
ments of fertile spikes, and their insertion as a rule intra-marginal.
clear that though the fertile spikes may occasionally be marginal, the
large majority of them are inserted upon the upper surface of the sterile
frond, while the lowest are commonly most near to the median line.
There is a rough, though not exact, parallelism between the number of
fertile spikes on a frond and the number of lobes of the sterile portion.
In Fig. 238 c there are two lobes of the latter, and a single fertile spike;
in Fig. 238 D, four lobes of the sterile (two incompletely separate), and
two fertile spikes ; in Fig. 238 E, two lobes of the sterile frond, and
three fertile spikes ; in Fig. 238 F, seven ill-defined lobes of the sterile
and eight fertile spikes ; in Fig. 238 G, eight lobes of the sterile frond,
GENERAL MORPHOLOGY 437
and fourteen fertile spikes. Of 70 specimens examined in Kevv and the
British Museum, ranging from those with a single sterile lobe to eleven,
and from one fertile spike to seventeen, the totals came out as follows :
Specimens observed, - 70
Sterile lobes, - - 328
Fertile spikes, 373
When these figures are taken together with observation of special cases
as illustrated in the drawings, they demonstrate a substantial parallelism
between the number of sterile lobes and of fertile spikes, though this
parallelism cannot be pursued into exact numerical detail. It is plain,
;o, as illustrated by the above figures, that the leaves with most lobes
re those which are broadest and have the largest assimilating surface ;
thus, speaking generally, the number of fertile spikes increases with the
icreasing leaf-area.
It has already been pointed out that spikes in a truly marginal position are
ire ; they do, however, occur, and Fig. 238 H shows one, together with its
jcular connection with the marginal bundle of the sterile frond. The
idividual spikes correspond in form and general structure to the single
>ike of O. vulgatum. But many of them show various stages of branching.
"he following drawings (Fig. 239) illustrate such steps as may be seen
Ophioglossum pahnatum : In Fig. 239 A are two spikes, each with an
iperfect lateral branch, but in both the series of sporangia is continuous
)ver the lateral protuberance. At the apex of each of the spikes of
rig. 239 D is an indication of branching of the same nature. The branching
be more elaborate, as in Fig. 239 B, where there are three borne upon
me stalk, the series of sporangia along the margins of them all being
iterrupted, while it may also be noted that the vascular bundles are
lited below in the common stalk. But in other cases the series of
)orangia may be interrupted (Fig. 239 c), so that the two branches now
ippear as two distinct spikes seated upon a common stalk, though the
central vascular bundles unite below into a common bundle before their
insertion on the vascular system of the sterile frond. Figs. 239 D and E are
substantially similar, but show a more complete separation of the vascular
supply for the two spikes ;' while Fig. 239 F shows two spikes in which the
stalks are completely separate to the base, though the two are inserted
close to one another, and in the same relative positions as the branches
in Figs. 239 c, D, and E.
The above series thus illustrate successive stages leading up to complete
branching of the fertile frond. It has been suggested by Bitter1 that the
simpler examples are really young plants of O. palmatum, and it seems
.not improbable that this may actually be the case, and the progression
be illustrated in the advancing life of the individual. However that may
be, it is by comparison of O. pendulum and of abnormal cases of
1 Engler and Prantl, i., iv., p. 456.
438
OPHIOGLOSSALES
O. vulgatum that an understanding may be arrived at as to the true
morphology of O. palmatum. The fertile spike in O. pendulum is commonly
simple, and its insertion is very constant at a median point on the upper
'/ill/' D
FIG. 239.
A -F= Various spikes of O. palmatuin, showing details of branching and insertion.
G, //, 7=spikes of O. pendulum. J, A'= abnormalities of O. vulgatum. .Z, -/* = abnor-
malities of Helminthostachys. O and P are from drawings by Prof. Goebel. A -N are
one half natural size.
surface of the frond : it appears that there is no close parallel in this
species between the lobing of the sterile and the branching of the fertile
portions. The mode of branching of the spike, when it occurs, is sub-
stantially similar to that in O. palmatum. Figs. 239 G and H illustrate
branchings in which the series of sporangia is almost or completely
continuous over the branches, but in Fig. 239 i the series is interrupted,
GENERAL MORPHOLOGY 439
and the three separate spikes ate inserted by sterile pedicels upon a
common sterile stalk. Thus the branching, though less common, appears
to be similar in kind to that in O. palmatum.
Somewhat similar branchings, though less complete, are not uncommon
O. vulgatum. In the Kew Herbarium there are certain abnormal speci-
icns which are of some interest in this connection. Fig. 239 j represents
plant of O. vulgatum taken in wet fields at Farnham, Surrey; from the
ipper surface of the sterile frond arise three fertile spikes, one of which
branched, while the point of insertion of another is at some little distance
rom the remaining two, which are seated close together. Though the
letails of insertion are not identical, this may be compared with the Fig.
238 E of O. palmatum, or as regards insertion of the spikes with Fig. 238 D.
Another, and much larger specimen, showing a somewhat similar abnormality
of O. vulgatum, is seen in Fig. 239 K ; there are two leaves from the same
)lant, each bearing three fertile spikes, which have, however, a common
insertion. Somewhat similar monstrosities are mentioned, as occurring
irely, by Luerssen.1
In the Kew collection specimens of O. reticulatum also show abnor-
lalities of a similar nature, though the branching is less complete : and
these specimens will serve to show that such abnormalities cannot be used
support the view that the fertile spike is a result of fusion of two
pinnae. One specimen from the Society Islands (Bidwell, Herb, Hook)
shows an equally bifurcated fertile spike, with a long sterile stalk : this
might appear to support the hypothesis of coalescence; but another speci-
men from Java (Lobb, Herb, Hook) shows three branches, of which the
central one is the strongest ; comparison should also be made of Figs. 239
j, K of abnormalities in O. vulgatum ; such cases as these would be entirely
inconsistent with the theory of coalescence as supported by abnormalities.
It must therefore be concluded from the genus, as we should already have
judged from the cases of O. palmatum and O. pendulum, that the forms
which the fertile spike occasionally assumes, gives no constant support to
the hypothesis of coalescence of lateral pinnae. This being so, and taking
also into account generally the facts of branching and insertion of the
fertile spike or spikes in the genus, the hypothesis that the fertile spikes
are of the nature of pinnae or leaf-segments appears to receive no consistent
support. On the other hand, all the facts are consistent with an hypothesis
of chorisis of a single original spike, holding a median adaxial position :
and it may be concluded that in Ophioglossum a fission, occasionally seen
in such species as O. vulgatum, has become habitual in O. palmatum, and
in less degree in O. pendulum. This is interesting for comparison with
what is seen in certain of the Sphenophyllales, where fission of the
sporangiophore appears to have occurred.
But besides such probable amplifications within the genus, there is
also a line of probable simplification : it is seen in the new species,
1 Rab. Krypt. Flora, vol. iii., p. 544.
440
OPHIOGLOSSALES
FIG. 240.
Botrychium simplex, Hitchc. Developmental series of forms in alphabetical suc-
cession ; a-f— forma shnplicissima, Lasch, that is young stages of development ; g-k —
forma incisa, Milde ; and I— transition to forma subcomposita, Lasch, m and n ; in has an
enlarged fertile basal segment of the sterile leaf; o-r=forma composita, Lasch ; rwith
four primary segments of the sterile part. The description is from Luerssen in Raben-
horst's Krypt. flora, and the drawings were from specimens in his herbarium. Natural
size.
GENERAL MORPHOLOGY 441
O. simplex, Ridley.1 This ground-growing mycorhizal plant has tall fertile
spikes, without any sterile lamina. Anatomically as well as in form it
resembles O. pendulum • but more especially in its external characters and
habit it resembles the rare O. intermedium, Hook, which is also a
Mind-growing species. For reasons explained at length in the paper
>ve quoted, it is thought that O. simplex forms the end of a series of
iuction of the vegetative system consequent on a mycorhizal habit and
laded habitat : that as O. intermedium, when compared with O. pendulum,
shows a relatively large spike but only a reduced lamina, so in O. simplex
the reduction having proceeded further has resulted in the complete
elimination of the sterile blade.
In the genus Botrychium the construction of the upright stock is
essentially similar to that of . Ophioglossum, and the plants are habitually,
though not always monophyllous. The main external difference lies in the
branched form both of the sterile leaf and of the fertile spike : these parts
show a similar parallelism of ramification to that which is present though
less regular in O. palmatnm. According to the complexity of the two
parts the species may be arranged, starting from those very small and
simple forms included under the name Botrychium simplex. These are
held by Luerssen not to be actual varieties, but rather plants of various
ages, and therefore in different stages of development which pass into
one another, a point which greatly increases their interest (Fig. 240). The
sterile leaf in the smallest of these may be entirely unbranched, as in a
small Ophioglossum, while the fertile spike is also unbranched, and bears
a very small number of sporangia (Figs. 240 A-F) : these appear in the simplest
cases as individual lateral projections from the spike, but here, as in the
whole genus, they are disposed along its lateral margins, in the same
relative position as in Ophioglossum. The steps from this simple condition
are clearly shown in Luerssen's drawings (Figs. 240 G-L), lobation of the
sterile leaf progressing in marked parallelism with branching of the fertile
spike : first a simple pinnation, and then an incipient double pinnation.
The condition is thus attained which is seen in the common B. Lunaria
(Fig. 241), where the pinnation in its different forms may be single or
double. And so onwards through the species, the sterile leaf may be
three (B. daucifolium\ or even four times pinnate (B. virginianum), the
fertile spike showing a corresponding complexity. The whole genus from
the simplest to the most elaborate, shows such gentle gradations of change
that the unity of type throughout is unmistakable.
Various abnormal modifications have been described for Botrychium,
some of them involving the formation of accessory parts, such a
doubling of the sterile leaf, or increase in number of the fertile spikes, as
in Ophioglossum ; but no species of Botrychium is recognised in which this
is established as a permanent character. The abnormalities involving dis-
tribution of the sporangia are the most important : all stages of vegetative
1 See Ann. of Bot., 1904, p. 205.
442
OPHIOGLOSSALES
FIG. 241.
Botrychiuin Lunaria, Sw. a— forma, normalis, Roeper. b— var. incisa, Milde.
<: = var. subincisa, Roeper. All of natural size, depart of the fertile spike, with open
sporangia, enlarged. ^ = two open sporangia somewhat bent asunder, to show their
attachment, enlarged. From Luerssen in Rabenhorst's Krypt. Flora.
development of the fertile region have been described, even up t(
its complete replacement by a sterile structure quite like the normal
sterile leaf: this will rank as "phyllody." But, on the other hand, it is
GENERAL MORPHOLOGY
443
not an uncommon thing for sporangia to appear upon the sterile leaf: an
example of this is shown for B. simplex in Fig. 240 M, but it is more
clearly shown in specimens of B. Lunaria (Fig. 242). Moreover, not a
part only, but even the whole of the normally sterile lamina may be thus
occupied, and Goebel quotes a locality on the Ostsee where this condition
has become constant.1 The importance of this from a theoretical point
of view will be discussed later.
FIG. 242.
Botrychium Lunaria. Sterile laminae, which occasionally produce sporangia (j/) 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.)
The third genus, Helminthostachys, differs from the others in having a
creeping rhizome, which is markedly dorsiventral, bearing the leaves in two
rows on its upper surface, while the roots spring from its flanks and under
surface (Fig. 243). The individual roots are not definitely related to the
leaves either in number or position, a condition comparable with
Botrychium rather than with Ophioglossum : they branch monopodially, and
are hairless. The rhizome is normally unbranched'2 and perennial, serving
1 Schenk's Handbiich, vol. iii., p. 1 1 2.
2 Farmer (Ann. of Bot., xiii., p. 423) found that adventitious branches were frequentlv
seen on old, almost decorticated parts of the rhizome of helmmthostachys. Gwynne-
Vaughan (Ann. of Bot., xvi., p. 170) has described how in the axil of each leaf, and
even of the leaves of young seedlings, a narrow oblique invaginated channel leads
through the cortex to a point just outside the stele, at the upper limit of the leaf-gap.
A mass of parenchyma, covered in except at its apex by an extension of the endodermis,
and terminated by a small, obliquely truncated, conical projection extends outwards from
the stele to meet this invaginated channel. He suggested that these structures represent
vestigial axillary buds, and that possibly the ancestors of Helminthostachys branched
more copiously than the present plant. Gwynne-Vaughan's recognition of their bud-
character received its full justification by the discovery of similar bodies in Botrychiicn
Lunaria by Bruchmann (Mora, 1906, p. 226), which actually develop into lateral
branches. He found them present chiefly upon young plants, and traced their origin
each from a single superficial cell of the rhizome : they occur especially where the axis
444
OPHIOGLOSSALES
as a storage-body. The leaves are inclined right and left of the mediai
line in acropetal succession, one as a rule but sometimes more rising
above ground in each season. The leaf shows a similar stipular structui
to that seen in others of the family : upwards it consists of a stout petiole,
with a large lamina usually ternate, each of the divisions being again
subdivided. From their point of junction rises the fertile spike, which
adaxial as in other Ophioglossaceae. But the chief distinctive point is in
the structure of it ; for numerous sporangiophores each bearing several
sporangia are disposed in dense serried ranks right and left — that is, in
positions corresponding to the rows of sporangia in Ophioglossum (Figs.
244 and 83).
-R
FIG. 243.
H elminthostachys zeylanica, Hook. Rhizome. Natural size, /^flap; .ft = root;
Z- = leaf ; P — petiole ; LS — leaf scar. (After Farmer and Freeman.)
The spike of Helminthostachys not uncommonly shows irregular accessory
branchings, such as those seen on Figs. 239 L, M. These may be combined
as in Botrychmm with correlative vegetative growth where sporangia are
absent (Fig. 239 N), while the details of these show in a beautiful manner
becomes elongated as an internode : the initial cell is gradually overarched by upgrowth
of the surrounding tissue, while by its sunken position it remains in close relation to the
vascular system of the rhizome. The single cell meanwhile divides into a cell-group, and
may continue to grow, forming a leafy bud. Bruchmann compares this bud-formation
with that of lateral buds in many Ferns. The comparison may also be made with the
lateral buds of Equisetum : the deeply sunken position compares not only with these, but
more particularly with that seen in the tuber of Phyttoghssum ; in fact, the sunken
character in the Ophioglossaceae has probably, as in these plants also, been assumed in
relation to the underground habit.
GENERAL MORPHOLOGY
445
the balance which may subsist between the vegetative and sporangial
' development (Fig. 239 o, P). Such changes are in line with those
observed in other Ophioglossaceae, and will with them have to be considered
in relation to general questions below.
It is easy thus to arrange the Ophioglossaceae in sequence, starting from
simple beginnings and proceeding to those which show greater complexity,
A
FIG. 244.
A. B, Ophioglossutn paiinatuHi, L. A—z. single fertile spike with sporangia still
closed. B — part of the same with sporangia ruptured. C-E^Botryckium Lunaria,
Sw. C = a fertile spike. Z> = a branch of the same with ruptured sporangia, seen from
within. .£ = the same seen from without. F, G = Helminthostachys zeylanica. Hook,
/^sterile and fertile regions of the leaf. G = branch of the latter with a group of spor-
angia, and at the apex the lamina-like terminals of the fertile appendage. (After Bitter
in Engler and Prantl, Nat. Pjlanzenfam. D after Luerssen. F, G after Hooker- Baker.)
whether of the vegetative or reproductive parts. It will remain to be seen
whether such sequences have any probable relation to truly phyletic lines
when the internal structure and development have been considered,
together with the comparison of the details with those of other Pterido-
phytic types. But, meanwhile, it appears certain that the three genera
form a natural group : the sterile leaf and the fertile spike are homologous
throughout, so also is the stock, notwithstanding that it is upright and
radial in Ophioglossum and Botrychiuw, but creeping and dorsiventral in
446 OPHIOGLOSSALES
Helminthostachys. Such a difference is of common occurrence within near
limits of affinity. For reasons given in Chapter XVI. the upright radial
type will be held to be the primitive, and the dorsiventral as seen in
Helminthostachys the derivative : it is interesting to note that this goes
along with a large and heavy leaf-development.
While, however, there may be no doubt of the homology of the spike
in all the three genera, it is to be noted that the level of its insertion
upon the adaxial face of the sterile leaf is not constant. In most species
of Ophioglossum* as also in Helminthostachys, it is at the base of the
sterile lamina ; but in O. Bergianum, where there is no differentiation of
petiole and lamina, it is but a short distance above the insertion of
the leaf itself upon the axis. In (9. palmatum it has been seen that the
numerous spikes may be distributed over a considerable length of the
basal region of the lamina. In Botrychium the differences in respect of level
of insertion are more marked : in B. Lunaria it is usually a short distance
below the lowest pair of pinnae : in B. ternatum it may be about four
inches below them, and about two inches from the base of the frond :
in B. daucifolium the insertion may be close to the base of the frond ; but
in B. virginianum, on the other hand, it may be above the second pair of
sterile pinnae. The chief question in the morphology of the Ophio-
glossaceae will be as to the real nature of this member, which shows so
variable a level of insertion, though it maintains in a remarkable degree
its constancy of character, as well as its position upon the upper face of
the leaf. A knowledge of its development and internal structure will be
essential before arriving at any definite conclusion.
Lastly, in comparing the shoot of the Ophioglossaceae as a whole
with that of the strobiloid types, the essential relation of leaf to axis is
the same. The nearest resemblance as regards general proportion is with
Isoetes, both having the stunted axis and spiral arrangement of the relatively
large leaves : one main difference lies in the tendency to the monophyllous
habit in the Ophioglossaceae, which may be held to be a consequence of
its perennation underground. It has been seen that in Isoetes all the
leaves of the mature plant show evidence of being potentially fertile, but
that an early abortion of the sporangia leaves some of them sterile. A
similar abortion is seen in the Ophioglossaceae : in O. vulgatum a rudi-
mentary spike is often to be seen on apparently sterile leaves, as a small
peg-like growth in the place where the normal spike would be inserted:
it is shown in Fig. 235 A, letter i. In other cases it may be found
that no vestige of the spike remains. Similar abortive spikes have been
seen in O. reticulatum and pendulum. In Botrychium Lunaria and simplex
extraordinarily small plants are found to bear fertile spikes, porportional
in size to the sterile lamina ; but in some cases of small, weak plants
the fertile spike appears to be entirely absent. In Helminthostachys Lang
observed that abortive fertile spikes are commonly found, subtended in
each case by a fully developed sterile lamina. It thus appears that the
SPORE-PRODUCING MEMBERS
447
fertile condition of the leaf is normally the rule in the family ; but that
the fertile spike of the Ophioglossaceae behaves like the sporangiophore of
the Psilotaceae, or the sporangium of Isoetes or Lycopodium in the matter
of its abortion : this is complete in some leaves, while in others a vestigial
structure remains to show what has occurred. Further, though their
tendency towards a monophyllous .habit may make such a comparison less
obvious, the Ophioglossaceae show essentially a " Selago " condition of the
shoot, that is, an imperfect differentiation of the vegetative and reproductive
functions. Their condition would, in fact, be consistent with a strobiloid
origin, modified in further development by enlargement of the appendages,
all of which were originally fertile. This matter
will be referred to again in connection with the
early appearance of the fertile spike in the
young seedling plant.
SPORE-PRODUCING MEMBERS.
The development of the fertile spike has been
traced in Ophioglossum from its first beginnings.1
The leaf itself originates very close to the initial
cell of the deeply depressed apex of the axis.
The sheathing stipule which envelopes the growing
point as well as all the later leaves, is formed
early : the spike appears above it in a median
position on the adaxial face of the leaf, but near
to its base (Fig. 245). The outgrowth is at first ophwgiossum vuigatum. The
, . lower drawing shows a longitudinal
bluntly rOUnded, but It SOOn becomes more median section of a young leaf, with
, . . the spike (sp) arising about half-way
acute and turned upwards : it consists of several up its adaxial face. The upper
j f , , .... drawing shows a rather older leaf in
cells, and of these the uppermost, which is frontal view. x35.
already the largest (x in the Figs. 246 A, B, c, E),
undergoes further segmentation with some degree of regularity : its
segments go to form the bulk of the free portion of the spike. The
form of the initial cell is that of an irregular four-sided pyramid, but in
some cases at least its identity is soon lost, and the type of construction
passes over to that with four initials (Fig. 246 F, G). As a consequence
of further subdivisions, whichever be the type of the initial segmen-
tation, the spike comes to be composed of four quarters, separated
by walls at right angles, as seen in the transverse section : its form
is that of an elongated cone, slightly flattened on its adaxial and
abaxial sides. Sections of it, transverse, radial, and tangential, are shown
in Figs. 247 A-D : from these it appears that a special band of cells, the
sporangiogenic band, runs along the lateral margins of the slightly flattened
1 The account here given is condensed from the full statement (Studies, ii., pp. 10-27),
where the literature is quoted. The development has been worked out for three species :
O. rulgatinn, reticu/atu/n, and pendulum.
FIG. 245.
448
OPHIOGLOSSALES
spike, in the position ultimately to be occupied by the series of sporangia :
it is derived from two regular rows of cells, which form part of the two
abaxial quarters of the spike ; but the usual regularity of their arrangement
is liable to interruptions. It is from this band that the sporangia ari<
constituting when mature the continuous, linear series of them seen in th<
drawings of the mature spike. But they are not always regular, an<
exceptions may be seen where the sporangia are imperfectly partitiom
or of anomalous outline. This is not surprising in bodies so nearly relat
to one another from the first.
The two series of superficial cells composing the sporangiogenic ban<
soon divide periclinally, as well as in other directions, and form a broac
FIG. 246.
Ophioglossum vulgatum, L. A = median-radial section through a very young spike
showing an initial cell (x). B = similar section of an older spike. C~ transverse section
of a leaf, as along a line (tr) in A, traversing the young spike. Ophioglossum reticu-
laturn, L. D = tangential section of leaf (/) traversing the young rudiment of a fertile
spike. E — another section from the same series, including the outer surface of the
projecting spike. F, G = transverse sections from the apex of a young spike of O. vul-
gatum, showing a construction with four initials. All Figs. X 100.
and deep tract of tissue from which the sporangia are differentiated. Ii
position and origin they compare with those superficial cells which ii
other Pteridophytes give rise to the essentials of the sporangia. The
differentiation shows various successive steps leading to the final definitior
of those cells which are to form the spores. It will be readi<
understood from the structure seen in the large spike of O. pendulum
(Figs. 248, 249). Here certain cell-groups derived from the inn<
products of the sporangiogenic band soon begin to show more den<
protoplasmic contents : these are recognised as sporogenous groups, am
are seen in transverse section in Fig. 248 A, in radial section in Fig. 248
and in tangential section in Fig. 248 c. The result is that the inne
product of the band is segregated into alternate blocks of sterile an<
SPORE-PRODUCING MEMBERS
449
fertile tissue, while the outer tissue- begins to constitute the protective
wall. But the whole of the cells composing these sporogenous groups do
not become matured into spores ; for a peripheral part of each group takes
the character of tapetum, and becomes disorganised as the development
FIG. 247.
Ophioglossmn vulgatmit. ,4= par), of a
longitudinal section including the apex of the
fertile spike, and traversing the sporangiogenic
band longitudinally. B = tangential section,
following the sporangiogenic band, and
showing the regularity of its cells. C = lateral
part of a transverse section of a spike ; the
cells shaded are recognised as the sporangio-
genic band. D — a similar section showing an
older state. X 100.
FIG. 248.
Ophioglossum (pphioderma) pendulum, L. A =
transverse section of spike showing sporangiogenic
band. B = longitudinal section following it, and
showing it already differentiated, with sporogenous
groups. C= tangential section of a spike of like
age, also showing two sporogenous groups. X 100.
of the spores proceeds. The outline of the definitive sporogenous group
which remains is very variable : sometimes it is relatively regular, as
in Fig. 249 D ; but in other cases, which may even occur in the same
sections as the regular ones, the disposition of the cells is less regular.
These have probably arisen from parts of the sporangiogenic band which
2 F
450
OPHIOGLOSSALES
were of irregular construction from the first, as above described. It is
thus seen that in O. pendulu?n only a comparatively small residuum of the
original sporangiogenic band finally remains as fertile tissue. Meanwhile, in
the broad bands of sterile tissue which have thus been initiated between
the sporangia, vascular bundles make their appearance, connected as
branch-bundles with the general system of the fertile spike (v. B, Fig. 249
D, and Fig. 250) : in those cases where the arrangement is regular they
may occupy a definite position, corresponding very nearly to the point of
FIG. 249.
Ophioglossum (Ophiodenna) pendulum, L. A, .5— transverse sections of spikes of
different ages to show sporangia and vascular bundles, slightly enlarged. C = a single
sporangium, older than in Fig. 248, seen in longitudinal radial section ; the tapetum lightly
shaded surrounds the darker sporogenous mass. D = tangential section of corresponding
age, showing one sporogenous mass shaded ; the smaller shaded groups will form the
vascular strands. -£"=part of a transverse section of an older sporangium of O. reticu-
latum. X loo.
intersection of the lines limiting the cell-groups of the original sporangio-
genic band. In certain cases, where the segmentation is regular it appears
that one sporangium is referable in origin to two of those cell-groups, but
it cannot be said that it is always so : frequently the arrangement is
irregular, and in any case the single sporangium cannot readily be referred
in origin to a single parent-cell.
Examination of the young spikes of O. vulgatum and reticulatnm
shows that in all essentials the development is the same, though naturally
with differences of detail and proportion in those less bulky species. In
them it is also impossible to refer each sporangium to a single parent cell.
Further, it has been shown in them that the archesporium is not hypo-
dermal in the strict sense, that is, that it is not cut off once for all by
one periclinal wall or walls, but that successive additions may be made
SPORE-PRODUCING MEMBERS
to it, as in certain other Pteridophytes, by successive periclinal divisions.
The structure of the sporangium as it approaches the stage of separa-
tion of the spore-mother-cells and of the tetrad-division is shown in
Figs. 249 and 250 : in the latter the relation of the vascular strands to
the sporangium is already clearly indicated, especially of those which pass
outwards in the thickness of the septum. The tapetum appears to be
variable ; in O. vulgatum it consists of several ill-defined layers of cells.
This is seen in Fig. 251 A, B, which illustrates the steps leading to tetrad-
division in that species : the cells of the tapetum first lose their identity,
their protoplasts fusing into a continuous plasmodium surrounding the
sporogenous cells, while the nuclei
persist, and apparently increase in
number by fragmentation. The
plasmodium penetrates between
the sporogenous cells, the whole
mass being first broken up into
irregular blocks (Fig. 251 A), and
later into individual cells (Fig.
251 B). Normally all these spore-
mother-cells undergo tetrad-divi-
sion, and form spores.1 When
ripe each sporangium bursts by
a horizontal slit, already defined
structurally in the tissue of the
wall : it gapes as the tissues dry
up, but there is no mechanical
annulus.
The origin of the leaf in
as Well as that Of
FIG. 250.
Ophjoglossum reticulatuin, L/. Longitudinal section
through a sporangium before separation of the spore-
mother-cells ; the walls of the vascular tissue are drawn
the fertile Spike Which it bears, in rather more heavily. Xioo.
has been described by Bruchmann
for B. Lunaria!2- He found that the fertile spike originates in the same
position relative to the sterile leaf as in Ophioglossum, but much
nearer to its apex; indeed, at the period when they may first be
recognised by their respective initial cells, these are in close proximity to
1 The statement of Rostowzew that a large number of spore-mother-cells are dis-
organised, and contribute with the tapetum to the nutritive plasmodium, was adopted
by me in Studies, ii., p. 20, as it appeared to accord with my own rather limited
observations. A re-examination of the question, for which a number of slides of
O. vitlgatum were lent by Professor Farmer, has shown that this is an error ; as had
been already noted for O. reticulatum by Burlinghame (Bot. Gaz., July, 1907, p. 34).
Occasional cells may be disorganised (Fig. 251 B), as may happen in any large sporangia;
but normally there appears to be no systematic disorganisation, such as is seen in
or the Psilotaceae.
2 Flora, 1906, p. 213.
452
OPHIOGLOSSALES
one another, that of the spike probably originating from one of the latest
adaxial segments of the leaf-initial (Bruchmann, I.e., Fig. 55). Though this
close proximity of origin of the fertile body to the apex of the sporophyll
differs from what has been seen in Ophioglossum vulgatum, still it has its
parallel in the case of Tmesipteris, as already described : the details of
segmentation are not the same, but the relation to the whole leaf is
similar. Both parts in B. Lunaria retain their active initial cells till
about the time of origin of the lateral pinnae : and it is specially to be
FIG. 251.
Ophioglossum yulgatum, L. Portions of sporangia showing the sporogenous tissue in
two stages of disintegration. In A the tapetum (t\ evidently derived from more than a
single layer of cells, has formed a plasrnodium with many nuclei, which is beginning to
penetrate the sporogenous tissue, in which an occasional cell (sf) is seen disorganised.
B shows a more advanced state, where the sporogenous cells (sp) appear in small clusters,
or isolated, embedded in the tapetal plasmodium (t) ; iv = sporangial wall. X 100.
noted that the origin of these in Botrychium is by a process quite distin<
from that of the fertile spike : the latter appears in a median position wit
a definite apical cell from the first : the pinnae arise in acropet*
succession by marginal growth.1
The disposition of the sporangia on the fertile spike of Botrychium is
essentially similar to that in Ophioglossum, but they differ in being further
apart, and not laterally coalescent, except in individual cases. The
similarity is most readily recognised in the simplest examples (Figs. 252 A,
B, c), in which the number of sporangia may be very small : these are
disposed in lateral rows, obliquely facing the sterile frond : their position
lZ.r. >. 218.
SPORE-PRODUCING MEMBERS 453
is shown by comparison of Fig. 25* A and n. The similarity of these
simple fertile spikes to those of the smaller species of Ophioglossum, such
as O, Bergianuin, is plain enough : if we imagine the sporangia in this
plant to be somewhat more prominent, less bulky, and their position
slightly altered, so as obliquely to face the sterile frond, the result would
be such a type as is seen in the simplest forms of Botrychium. From
these simple forms to the more complex, even to those in which the
spike attains its largest development, is a progression which may be
traced by very gradual steps : the first of these steps is illustrated by
the Figs. 252 A, is, c: in the first figure (A) two lateral rows of simple
sporangia are seen: in (B) the place of
one sporangium is taken by two coherent
together, while in the third (c) there are
at the base of the spike, which is simple
above, two " branches," the one with two
sporangia and the lower one with three.
These specimens will illustrate the gradual
steps towards branching of the spike which
are to be found in the simplest types of
Botrychium, Sections also bring out some
interesting points: Fig. 252 E represents
in outline under a low power a transverse
section of a spike of Botrychium Lunar ia
traversing two of its lower branches longi-
tudinally, and following the series of their Botrychium £«««««, L. .*,£,£= three
' t very simple spikes. A shows no branching,
Sporangia: this Shows the acropetal SUCCCS- but only two rows of sporangia, of arrange-
ment like those of Ophioglossnm, but pro-
SlOn of development Of the Sporangia, while jecting further. B and C show simple cases
of branching enlarged. D, E show trans-
it Will be noted that the tWO lowest On the verse sections of spikes; D close to the
. . apex, E lower down ; the latter follows two
right are Coherent tO form a SynanglUm a branches longitudinally, and traverses their
_ sporangia. Note the synangium on the
matter of common occurrence, and corre- right-hand branch, x about 20.
spending to what is seen in Fig. 252 B.
Such simple observations as the above, which might readily be extended
into further detail, will suffice to show that it is possible to illustrate,
from simple though otherwise normal specimens, how a transition may
have taken place from the' condition of the spike similar to that of a
small Ophioglossunt) through the simpler types of Botrychium to the more
complex branched spikes.
But it is in the large B. daurifolium that a better opportunity has
been found of observing intermediate steps between the single normal
sporangium and a twin pair of them : the series Figs. 253 A-G illustrate this :
Figs. A, c show sections of young sporangia of normal type, with sharply
curved contour : others are of broader form, and show a considerable
mass of tissue on either side of the sporogenous group (Figs. D, E) ;
in these there may be seen cells laterally adjoining the latter, and
obviously of similar origin and position (x in Fig. D), which, though
454
OPHIOGLOSSALES
corresponding in every other respect, do not assume the dense
protoplasm of sporogenous cells. These more bulky sporangia lead
on to such as that shown in Fig. 253 E, in which it is possible
that the whole sporogenous group is referable to a single parent cell,
though the proportions of the whole group are quite different from those
of the typical sporangia ; the sporogenous cells appear, however, to form
two groups, and probably originated from two similar cells side by side.
The interest of this lies in the fact that these broad sporangia approach
FIG. 253.
Botrychium daucifolium, Wall. A and C, successive stages of the most common type
of sporangium. B = a small sporangium of narrow form. D = a very broad sporangium; the
cells marked (x) correspond to the sporogenous group, but showed no signs of developing
further as such. £=a still broader sporangium with wide sporogenous group, referable
to two parent cells, possibly ultimately to one. F, G = synangia cut transversely and
vertically. X 200.
in their form and bulk to the synangia which, as above noted for
Botrychium Lunaria, are not unfrequently found ; one of these, cut
through transversely, is shown in Fig. 253 F ; here there is a zone of three
layers of sterile tissue forming a septum between the two sporogenous
groups. The septum is, however, commonly broader than this : if a
comparison be made between this and the young synangia of Tmesipteris
the similarity will be readily seen. Such examples as these, which can
easily be found in sections of the fertile spike, illustrate the gradual
transitional forms which may be traced between the simpler and more
complex spikes of the genus. Whether these steps will bear an evolu-
tionary interpretation, as illustrating the manner of advance from
;
SPORE-PRODUCING MEMBERS
455
simpler, or reduction from a mors complex type, may be a question
for discussion ; but it is clear that the gradually transitional forms do
exist.
As regards the first appearance of the sporangium, the essential parts
of it, though not the whole body, are normally referable to a single
parent-cell, and the first periclinal division delimits the sporogenous
tissue (see Figs. 43, 44, p. 88). It appears that here all the sporogenous
cells undergo the tetrad-division, and the nourishing tapetum, which is
entirely derived from the surrounding tissue, makes
its way inwards between the fertile cells.1 As
regards vascular supply, a strand extends to within
two cells of the base of the cavity, and there
terminates. Finally, the dehiscence is as in
Ophioglossum. It thus appears that the whole
sporangium of Botrychium is of the Ophioglossum
type, but it is more definite and specialised in its
characters, and this goes naturally along with its
smaller size, which is most marked in B, virginianum.
It has already been noted that the position
of the fertile spike in Helminthostachys is similar
to that of Ophioglossum : it may further be added
that the origin of it is similar, and its structure
in early stages not unlike. It appears first as an
outgrowth on the adaxial side of the sterile frond,
and it is curved over while young, so that the
actual apex is pointed downwards : the whole of
the spike is at first covered and protected by
the segments of the sterile frond, which again are
protected by the stipular sheath. Since then, as
regards position, and the main facts of origin,
the whole spike of Helminthostachys may be
regarded as homologous with the whole spike of
Ophioglossum, a special interest will attach to the
origin and development of those bodies which directly bear the sporangia
in this genus, viz. the spora'ngiophores.
Transverse sections of the fertile spike show at the lateral regions
corresponding to the sporangiogenic bands in Ophioglossum, a fan-like
tracery of the cell- walls, while the surface is covered by a rather regular
series of deep cells : it is from these that the sporangiophores originate,
as outgrowths of very irregular size and arrangement (Fig. 254 A). Growing
first deeper, these cells divide by periclinal and anticlinal walls ; the
growth, however, is not uniform, but is localised at points so that
rounded processes, often of very unequal size, make their appearance
1 Holtzman, Bot. Gaz., xvii., p. 214; Cardiff, Bot. Gaz., xxxix., p. 340; also
-Studies, v., p. 197.
-.
FIG. 254.
Magnified' (After
456
OPHIOGLOSSALES
(Fig. 255 A). There seems to be no fixed type of segmentation of the cells
which leads to the formation of these sporangiophores, and though, as
they increase in size, their apex may sometimes be occupied by a
wedge-shaped cell, more frequently no such cell can be found (Fig. 255 P-).
The subjacent tissue may sometimes increase to a slight degree below
a young sporangiophore, but it is obvious from the drawings, both of
transverse and of longitudinal sections, that the sporangiophores are
derived essentially from the superficial cells. Now this is also the case
with the sporangia of Ophioglossum, in which the sporangiogenic band is
FIG. 255.
Helminthostachys zeylantca, Hook. A — early ; phases of sporangiophores. Note the
variability of size, shown also in the tangential section F. B and C = older sporangiophores.
C, D, £ and Gshow stages of development of the sporangium, with the sporogenous cells
shaded. X 200.
in the first instance composed of superficial cells ; thus there appears to
be correspondence as regards the place of origin of the spore-producing
members of the two genera.
Fig. 255 B illustrates the most regular type of these very variable organs;
already in the central part longitudinal divisions are taking place which
indicate the position of the central vascular bundle. The great differences
in size which they show when young are suggested by the tangential section
shown as Fig. 255 F. As they develop further the sporangiophores may
become irregularly lobed or branched. Thus, though disposed with some
regularity along the margins of the fertile spike, they are themselves very
variable in size, form, and mutual arrangement.
It has already been noted also that the position and number of the
sporangia which they bear is inconstant. In early stages it is impossible
SPORE-PRODUCING MEMBERS 457
distinguish the cells which will give rise to the sporangia (Fig. 255 B),
from rather older stages it appears that the sporogenous group,
>ther with the superficial cells which cover it, is referable in origin to the
segmentation of a single superficial cell (Figs. 255 c, D, E). Moreover,
the first periclinal division of that cell defines the whole of the
sporogenous tissue from the protective wall. As the sporangia grow older
they project from the surface of the sporangiophore; the sporogenous
mass increases rapidly in bulk, while the cells surrounding the sporo-
genous mass, to the extent of several layers, assume the character of a
tapetum (Fig. 255 G), which gradually becomes disorganised; finally the
sporogenous cells separate, and divide into tetrads.1 As the sporangia
approach maturity, the upper part of the sporangiophore may grow out
into an irregular rosette of laciniae of vegetative tissue. These are seen
in Fig. 244 G.
Comparing the development of the sporangia in the three genera, it
appears that with the larger size goes less definite segmentation, while greater
definiteness is seen in the smaller types. It has been shown that the
essential parts of the sporangium of Ophioglossum cannot be referred in
origin to a single cell, while those of Botrychium and Helminthostachys
can in normal cases. Also, that the large sporogenous mass of Ophio-
glossum throws off its superficial tissues as tapetum, which is of variable
bulk : - in the other two genera the tapetum originates from the adjoining
tissue, entirely outside the sporogenous mass. Further, when the definitive
sporogenous tissue is clearly marked off, there is reason to believe that
all the cells normally undergo the tetrad-division in all the three genera.
The Ophioglossaceae may in fact be arranged in sequence, from those with
large and indefinite sporangia to those with smaller and more definite,
Helminthostachys taking the middle position.
The same sequence emerges also from the comparative study of their
fertile spikes. The least elaborated type is that of Ophioglossum, with its
two series of sunken sporangia. Its spikes are liable in many species to
occasional bifurcation, or even complete fission, and in O. pendulum and
palmatum this may become habitual : but these are only cases of repetition
of the same unelaborated part. In Helminthostachys the external form as
well as the development show that the sunken sporangia of Ophioglossum
are replaced by sporangiophores, with separate and smaller sporangia,
which at the same time are more numerous. In Botrychium the elabora-
tion of the spike takes a different line : the occurrence of synangia has
been shown to be closely related to the branching of the spike, which
extends to a high degree, while the numerous separate and relatively
small sporangia continue to hold the same relative position as in
Ophioglossum.
It is possible, as in the case of almost all organic sequences, to
regard this series as either an upward one of progressive elaboration or a
1 Beer, Annals of Bot. , xx., p. 177. 2 Stevens, Ann. of Bot., vol. xix., p. 472.
458 OPHIOGLOSSALES
downward one of reduction. Before discussing these two alternatives, it
will be necessary to look into the anatomy of these plants, since arguments
on that ground have been held to be very material to a decision.
ANATOMY.
The roots of the Ophioglossaceae show a wide range of internal
structure.1 In Helminthostachys they are tetrarch to heptarch, and most
commonly hexarch, with central pith, alternating phloem, a large-celled
pericycle, and endodermis. In Botrychium the number of protoxylems
FIG. 256.
Ophioglossuin Bergianujn, Schlecht. Transverse sections of the stele of a root, the one
showing two unequal groups of xylem, the other only one. X 200.
varies a good deal, common numbers being two and three, but it has
been shown in B. Lunaria that roots which are diarch distally may be
monarch near the base. In Ophioglossum, also, there is some variety, for
in O. pendulum diarch, triarch, and tetrarch roots have been described,
while in this species also a monarch structure has been seen at the base
of a rootlet : O. decipiens has triarch structure : O. 'palmatum is diarch.
Most of the species of Euophioglossum have monarch roots, and this is
conspicuously so in O. vulgatum. In O. Bergianum the structure may be
diarch or monarch, the latter having been observed in roots close to their
base (Fig. 256). With the monarch structure goes bifurcate branching,
while monopodial branching is seen where the structure is more complex,
as in O. pendulum, Helminthostachys, and Botrychium. Thus both dicho-
tomous and monopodial branching are found in the same genus. Possibly
dichotomy . is restricted to the monarch roots : this was suggested by Van
Tieghem, who remarks that, if the monarch root divides, we know
beforehand that it will dichotomise.2
It is the fashion of the time to hold that all monarch roots are
1 Compare Boodle, Ann. of Bot., xiii., p. 377, where the literature is fully quoted.
"-Ann. Sci. Nat., V. Serie, T. xiii., p. 108.
r
ANATOMY 459
uced from some more complex structure : l in the observations relating
o the Ophioglossaceae there is no necessity to adopt this view, which
does not readily accord with the fact that the monarch condition appears
at the very base of the root, both in Botrychium and in Ophioglossum.
I am disposed to regard the monarch state as primitive. But whether
they be primitive or reduced does not materially affect comparison :
the family is clearly one with great instability of root-structure, and
there are in a number of cases monarch roots which dichotomise. In
these respects the Ophioglossales find their nearest correlatives among the
Lycopodiales. Comparison should also be made with the Sphenophyllales :
but the Psilotaceae are rootless, and the knowledge of the root-system of
Sphenophyllum is still very obscure : apparently they were diarch, with
secondary thickening,2 a condition not far removed from that described
'or the roots of Ophioglossum^ but still more clearly seen at the base of
e roots of Botrychium • for here it has been shown by Boodle that
ondary thickening of the root may occur.3 Thus in the Ophioglossaceae
ere are unmistakable points for comparison with the Lycopodiales and
phenophyllales. On the other hand, the larger polyarch roots in the
mily show structure reminiscent of certain Ferns, and especially of the
rattiaceae.
The stock of the Ophioglossaceae originates directly from the embryo,
it may be formed indirectly as a result of adventitious budding. The
ung axis has been examined in all three genera, and in the first
stance the vascular tissue is found to be centroxylic, either with a
ite solid core, as in some seedlings of Helminthostachysf or in others
may have a central pith from the first, and this seems to be the case
Botrychium.^ In Ophioglossum the axis of the embryo, as described
Bruchmann in O. vulgatum? is very short, and no facts are at hand
to its stelar structure. But Bruchmann states that the development of the
bryo coincides in all its later particulars with that of the adventitious
uds, and these have been described and figured by Rostowzew.7 The
vascular tissue, on entering one of these buds from the parent root, "forms
a central cylinder, which dilates and becomes concentric (Fig. 236, No. 4):
higher up it takes the form of a funnel, which is filled with parenchymatous
pith : higher again the cylinder produces on one side a mesh from the
lower angle of which the strand of the first leaf arises." This description,
together with the drawings (Fig. 236, Nos. 2, 3, 4), indicates at the start a
protostelic state, or at least a stele with only small medulla. It thus appears
1 It will suffice here to mention Boodle's Ideological theory that the monarch structure
of Ophioglossum is an adaptation for favouring the growth of adventitious buds on the
roots.. He himself quotes cases which do not bear out his view: his theory, moreover,
seems to confuse cause with effect.
2Seward, Fossil Plants, i., p. 399. 3Z.r., p. 388 and Fig. 14.
4 Lang, Ann. of Bot., vol. xvi., p. 42. 5 Jeffrey, /.r., p. 21, Fig. 61.
6 Bot. Zeit., 1904, p. 240. 7Z.r., PI. i, Figs. 2, 3, 4.
460
OPHIOGLOSSALES
that all the three genera show either a solid xylem-core or a slightly
medullated stele in the young axis.
Proceeding to the upper part of the shoot the medulla increases, while
the vascular tissue forms a more or less interrupted cylinder surrounding
it : the interruptions are the leaf-meshes, for above the exit of each leaf-
trace there is a gap in the cylinder. Jn Ophioglossum the meshes are large,
•and as the leaves are arranged in a compact spiral, the whole system assumes
a form clearly shown in Rostowzew's drawings (Fig. 236, Nos. 4, 5). In
Botrychium a similar arrangement is found ; but as the proportion of the
leaf-meshes to the whole surface of the cylinder is less, it approximates
more nearly to a continuous tube. This is still more clearly seen in
Helminthostachys, where the shoot is
dorsiventral ; for there the foliar gaps
are disposed obliquely upon the
upper side only of the cylinder, while
the lower side of it is uninterrupted
(Fig. 257). It would thus appear
that the vascular system of the axis
is essentially similar in them all, and
is referable in origin to the amplifi-
cation of a primitive stele, with a
distending pith, and perforation of
the vascular cylinder by foliar gaps.
As regards the tissues themselves,
the most important of them for
comparison are the xylem and the
endodermis. The latter shows
curious irregularity of occurrence in
this family. In Botrychium there
is a well-marked outer endodermis
throughout the length of the stock :
there is also an inner endodermis in the pith, but it is found only at
the base of the stock (Poirault). In most species of Ophioglossum
there is no endodermis in the stock at all; but in O. Bergianum,
capense, and ellipticum, all small species, an outer and inner endodermis
are 'both present, though at the base of the stalk only : passing upwards
they fade gradually away, the inner disappearing first.1 In Helmintho-
stachys, curiously enough, the converse is seen : here there is throughout
the stock a well-marked outer endodermis, as in Botrychium, but the inner
appears only in the older stems, the young plants being quite destitute of
it.2 It is difficult to draw any definite conclusions from such discordant
facts : it must suffice for the moment to remark that, on the one hand,
there is want of constancy of the endodermis also in the Psilotaceae, and
on the other, that in the Marattiaceae the endodermis is present in
FIG. 257.
Helminthostachys zeylanica^ Hook. The upper
figure represents the vascular skeleton, dissected out.
L = leaf- trace ; R — root-strand ; Fg — foliar gap. The
lower figure shows the rhizome-stele giving off a leaf-
trace, L.T., which breaks up above into separate
petiolar bundles. /?= root-trace. (After Farmer
and Freeman.)
1 Poirault, I.e. , p. 169.
Farmer and Hill, Ann. of Bot. xvi., p. 401.
ANATOMY
461
the stem of the young plant, but fades off in the upper regions, as in
Ophioglossum.
In transverse sections of the stock of Botrychium, in which the leaf-
gaps are limited in area and not so closely placed as in Ophioglossum,
the vascular ring is often seen to be complete, or where a leaf-trace issues
it may be interrupted : the xylem is endarch. Much importance has
been accorded to the secondary thickening seen in both stem and root
of Botrychium. A sluggish cambium appears between the phloem and
xylem, and may even be seen to be active close below the apex before
either of those tissues are differentiated : it adds fresh tracheides to the
xylem, but little or nothing to the phloem, while the radially seriated wood
A B
Ophioglpssnm Bergianum, Schlecht. A— transverse section ot the stock, showing a
large semilunar stele, with wide foliar gap into which a small leaf-trace strand is entering.
£=another section, showing probably the result of overlapping of the foliar gaps. xaoo.
is traversed by parenchymatous rays. The secondary activity extends also
into the basal region of the root, but it does not extend far along it. In
Ophioglossum the transverse section of the stock shows an interrupted
ring of xylem-bands, the interruptions representing the closely grouped
and overlapping leaf-gaps, as will be readily understood by comparison
)f Fig. 236, Nos. 4, 5. But in simple cases, and especially near to the
base of the stock, the ring may appear more complete (Fig. 258). The
development is endarch, and there is no process of secondary thickening
except that a few tracheides may occasionally be added peripherally to
those primarily formed. In the root also there may be a feeble formation
of secondary wood, especially in the neighbourhood of the insertion of
an adventitious bud (Boodle). In Hehninthostachys the vascular ring is
interrupted only on the oblique upper side, by the leaf-gaps. The xylem
is, however, mesarch, while the secondary thickening is altogether absent
(Farmer and Freeman).
462 OFHIOGLOSSALES
The facts thus stated relating to the vascular structure of the stock in
the three genera are all consistent with a theory of origin from a type
with primarily a solid protostele, and subsequently a medullated monostele : •
for the structure of the vascular system in the mature shoots of them all
is in point of fact a hollow cylinder perforated by the leaf-gaps : in
Botrychium and Helminthostachys, where these are less closely grouped
than in Ophioglossum, the fundamental structure as a vascular cylinder is
plainly seen. The opening of the cylinder to give exit to the leaf-trace
is a characteristic of that type designated by Jeffrey " phyllosiphonic," and
he distinguishes it from the " cladosiphonic type," in which the leaf-trace
passes off from the axial system without any opening. It has already
been pointed out that these two types are the anatomical expression of
the relative prevalence in the whole shoot of the axis in the cladosiphonic,
and of the leaf in the phyllosiphonic type. Supposing in any phyletic
series there should be an increasing dominance of the leaf, it would be
reasonable to expect evidence in the individual of a transition from the
one vascular type to the other. In the young plants of^ the Ophioglossaceae
themselves there is no indication of any such transition, for the young
plants are phyllosiphonic from the first. It will, however, be shown
later that on comparative grounds there is reason to think the origin
of the phyllosiphonic state in the Ophioglossaceae was from the clado-
siphonic, following upon an increase of proportion and importance of the
leaf.
The leaf-trace itself is typically a single strand of the collateral type.
This is seen in Botrychium and Helminthostachys, and in most species of
Ophioglossum. The collateral strand may have its margins curved together
on the adaxial side, so that in the petiole of large leaves it may approximate
to a concentric structure, as in B. virginianum ; but this is merely a
modification of the collateral structure. Even in the large-leaved Helmin-
thostachys the leaf-trace comes off as a single strand, though it branches
very soon, in fact before the cortex is traversed, to form the numerous
strands of the petiole (Fig. 2573). The condition seen in some species
of Ophioglossum is interesting for comparison with this, forming as it does
an exception to the rest of the family. In the section Euophioglossum
the leaf-trace comes off, as in other Ophioglossaceae, as a single strand,
which soon branches into three ; and this fact is embodied in Prantl's
diagnosis as amended' by myself.1 But in the section Ophioderma the
numerous strands of the petiole are not united into a single strand at
the base : they are inserted as separate strands upon the vascular system
of the stock. It is still uncertain whether or not ^Cheiroglossa shares this
character. A comparison with other forms of Ophioglossum shows this con-
dition to be exceptional, and it is probably a derivative state, the separation
of the strands shown in other species only in the upper leaf having been
continued in ^Ophioderma down to their actual base of insertion on the
1 Ann. of Bot., xviii., p 215.
ANATOMY
463
system of the axis : comparison with the Ferns shows that in them the
concrete leaf-trace is characteristic of the primitive types, and that its
separation into many distinct strands is a feature of those which are later
and derivative. This analogy strongly supports
the view that the state of the leaf-trace in
>j Ophiderma is not primitive.
Passing up the petiole the vascular strands
undergo branchings, which vary in extent
according to the dimensions of the fully formed
leaf. The strands arrange themselves in an
approximate circle in the transverse section,
lile those on the adaxial side pass out into
ic fertile spike. The details are various : the
iplest is in the small O. Bergianum, where
ic single leaf-trace strand may long remain
idivided, giving off two lateral strands which
ise on the adaxial side to form the supply
>r the spike : further up the strands of both
jrile lamina and of fertile spike may branch
tin. In the larger species of Ophioglossum
ic plan is the same, but with the difference
it the branching is more profuse, and takes
lace before the lateral supply is given off
right and left for the fertile spike ; in the
rger species the latter consists not of a single
ind but of several. The same is the case
O. pendulum, and even for O. palmatum
the case of the lowest spike, though in the FlG- 259-
ipper spikes the supply is less regular in N.os- J4-i6 successive transverse
. *• sections of leaf of O. palmatum,
ordance with the indefinite positions which showing the origin of the vascular
supply to the lowest of its spikes.
y hold (Fig. 259, 14-17). In OphiogloSSUm 17 = transverse section of the stalk of
that spike. 18-23 = successive sections
re is a strong median strand in the leaf, higher up on the same leaf, showing
.. iii-i- tne origin of the vascular supply to
frequently holds itS OWn throughout the second and third spikes. X4.
ie complicated reticulations of the expanded
blade. In Botrychium, however, the broad strap-shaped leaf-trace forks
early; and from the adaxial margins of each limb branch-strands are
given off, which form the supply of the fertile spike : subsequently
both systems may branch further, showing dichotomous characters, and an
ultimate " Neuropteris " venation. In Helminthostachys the first branchings
of the leaf-strand are described as dichotomous ; the resulting strands
arrange themselves in a ring, and traverse the petiole with occasional
anastomoses. Where the leaf branches complex anastomoses occur,
resulting in a fairly regular vascular supply passing into each branch.
The spike receives four or five strands, arranged in a circle, in its
transverse section. Further branchings occur in both sterile and fertile
464 OPHIOGLOSSALES
regions, those in the former being ultimately forked, giving a " Neuropteris "
venation, as in Botrychium. All these arrangements are clearly variants
upon one plan, of which the essential point is that the vascular supply
of the spike is of marginal origin, right and left from that of the whole
leaf. It was upon this that Roeper based the anatomical support for
his theory of the spike as a result of fusion of lateral pinnae. The facts
would accord, it is true, with Roeper's theory ; but it is to be borne in mind
that a marginal origin of vascular strands from the main system is much
more usual in leaves than any antero-posterior branching. On the other
hand, the origin of the vascular supply of the spike from both sides of
the foliar system gives no support to the theory that it is essentially
lateral pinna which has taken a median position.
Lastly, the relation of the vascular system to the sporangia deserves
notice. In Ophioglossum lateral branches from the anastomosing strands
of the spike pass between the sporangia, traversing the septum and expanding
toward the periphery into a tuft of tracheides, an arrangement which
doubtless efficient in the case of deeply sunk sporangia (compare Fig. 250)
But in Botrychium and Helminthostachys the ultimate strands terminal
immediately below the base of each sporangium. The condition seen ii
Ophioglossum does not appear to accord well with a theory of sporangia
fusion : it points rather to an upward process of progressive septation.
Summarising the results of this anatomical examination of the shoot
in the Ophioglossales, the facts are consistent with the origin of the axial
system from a protostelic state, with amplification of the stele, followed b)
formation of a leaf-gap at the exit of each leaf-trace : the latter is typicalb
a single strand: as it passes upwards it branches, with prevalent bifurcation ii
Botrychium and Helminthostachys^ but not in Ophioglossum : these facts are
consistent with an origin of the leaf from a simpler source by enlargement
and branching/ The vascular supply to the fertile spike originates from
the lateral margins of the foliar system, and with this the simpler states
of O. palmatum coincide, though not the more complex. The bearing of
these facts, as indicating the probable origin and relationships of the
Ophioglossales, will be discussed later.
EMBRYOLOGY.
Until recently the prothallus and the development of the embryo in
the Ophioglossales were very imperfectly known, though observations
upon them date back to the writings of Hofmeister and of Mettenius.
But during the last ten years the prothalli and embryos have been dis-
covered in a number of cases where they were previously unknown, s<
that it is now possible to give some approach to a comparative account
of the embryology of the family.1
1 The account here given is based upon the works of Hofmeister, Higher Cryptogamia,
1862 ; Mettenius, Filices Horti Lipsiensis, 1856 ; Campbell, Mosses and Ferns, 1895 am
EMBRYOLOGY 465
The prothallus throughout the ^Ophioglossaceae is subterranean, and
without chlorophyll, excepting some traces observed by Campbell in the
early germination of Botrychium, while Mettenius, and later Campbell, have
noted in O. pedunculosum that some branches of the thallus extended
above ground, and became flattened and green. But with such exceptions
as these the gametophyte is massive and colourless, and is buried under-
ground. Its nutrition is holosaprophytic, with an endophytic mycorhiza,
which is located especially in the lower region. In Botrychium its form is
that of a flattened cake, with the slowly growing apex in a lateral position :
but in Ophioglossum and Helminthostachys there is a definite apical growth
associated with the formation of one or more upward or lateral conical
processes, and it is upon these that the sexual organs are borne. In
Botrychium they appear upon the surface of the cake, where the mycorhiza
is absent. The antheridia are deeply sunk in the tissue of the gameto-
phyte : the archegonia, which have the early segmentation as in Ferns,
are deeply sunk in Ophioglossum, but in Botrychium and Helminthostachys
the neck of the archegonium is elongated and projecting. The orientation
of the archegonia does not appear to be constant, but in Ophioglossum
and Helminthostachys its axis appears to be horizontal, while in Botrychium
it is oblique or vertical. The spermatozoids are spirally coiled, and bear
numerous cilia.
The development of the embryo of the Ophioglossaceae follows slowly
on fertilisation, and shows peculiarities which may be held as concomitant
on the subterranean habit, while the mycorhizic state may affect not only
the prothallus, but in some cases the young sporophyte also. The most
marked peculiarity is the delay in the actual growth of the apical bud,
while there is a very precocious development of the root-system. Also,
it will be seen that there is considerable variety in detail in the different
representatives of the family, and even within the generic limits. This
will make it desirable to describe them separately.
In most species of Ophioglossum fertilisation seems to be of rare
occurrence, and few embryos have therefore been available for study. The
first division of the zygote is transverse to the axis of the archegonium :
though Campbell specially points out that it is not regularly so in O.
pendulum * : this first segmentation separates the epibasal from the hypo-
basal region ; but it has been difficult to follow the details of further
segmentation owing to the scanty material, and reference of the parts to
any definite relation to the initial cleavages is made specially uncertain
by the fact that the embryo attains considerable size before any differen-
tiation occurs (Fig. 260). Bruchmann states, however, for O. vulgatum, that
the hypobasal half gives rise to the first root and the foot ; the latter is
1905; Jeffrey, Gametophyte of Botrychium^ Toronto, 1898; Lang, Ann. of Bot., xvi.,
1902 ; Bruchmann, Bot. Zeit., 1904, and Flora, 1906 ; Lyon, Bot. Gaz., Dec., 1905; and
of Campbell, Ann. Jard. Bot., Buitenzorg, 1907, p. 138.
lZu-., p. 171.
2 G
466
OPHIOGLOSSALES
never large, but appears only as a slight swelling which remains in close
relation with the prothallus. The root rushes forward in its development, and
forming its apical cell early (perhaps it is rightly recognised in cell "w"
in Fig. 260), it attains a considerable length : it bursts freely through the
prothallus before there is yet any definite trace of the apex of the axis
or of the cotyledon (Fig. 260 bis). Up to this time the embryo is stored
with nutritive substances, but it contains no endophytic fungus. It appears
that the development up to this stage occupies several vegetative seasons.
The differentiation of the shoot which is thus long delayed accompanies
the origin of the second root, which is formed endogenously close to the
proximal end of the vascular strand of the first. Immediately above this.
FIG. 260
Ophioglossuin vulgatum, L. The central figure shows an archegonium, at period of
fertilisation. X225- The left-hand figure shows the first division of the zygote. X225-
To the right a more advanced embryo. /, /=basal wall ;• ep — epibasal ; /y=hypobasal
hemisphere ;'/~=the region of the foot ; w = root. X 225. (After Bruchmann.)
and opposite the neck of the archegonium, the cotyledon and the apex
of the axis appear simultaneously, the cotyledon being on the side of th(
axis next to the first root : surrounding both axis and cotyledon is the
first sheath (Fig. 260 bis, hl^}. The cotyledon remains quite rudimentary :
it is followed by a second leaf, which may develop as a small sterile leaf
expanded above ground, up to which time the embryo has been growing
some eight to ten years. The third leaf, expanded in the following year,
may, under favourable circumstances, bear a fertile spike. The further
development then follows as in the mature plant.
Campbell, having examined several tropical species, recognises three
types of embryogeny in the genus, that of O. vulgatum, above described :
that of O. moluccanuni, described by Mettenius and by himself; and that
of O. pendulum^ on which he has added largely to the observations of
Lang, and finds that the embryo is variable within the species. According
to his statement, the first type is characterised (as we have seen) by an
EMBRYOLOGY
467
early formation of the root, and late development of the axis and leaf:
in the second, leaf and root only are developed, in the third roots only :
the definitive sporophyte in both O. moluccanum and pendulum is
'" formed as an adventitious bud upon the root of the embryo sporophyte."
It would thus appear that the genus Ophioglossum shows almost equal
variety in its embryogeny to that seen in Lytopodium. It has been seen
FIG. 260 bis.
Ophioglossum vulgatum, L. 58 =
longitudinal section through a young
seedling. <w± = first root with evident
apical cell; /=the foot only slightly
projecting; t' = the epibasal region of
the embryo ; p = rudiment of pro-
thallus. X35. 61 = larger, three-
rooted seedling in longitudinal section ;
/> = prothallus ; «'i = first root; h —
entering fungal filament; £« = endo-
pnytic fungus. «zi> = insertion of and
and 3rd roots; s^apex of rhizome:
b\, b%, &j = leaves; c, c = canal ; hl\-
hl\ = sheath of first leaf; hl.rhL = sheath
of second leaf; /z/3 = sheath of third
leaf, x 35. (After Urucbmann.)
in that genus how the different forms are referable to variation of a single
type, and it seems probable that the same should be the case also for
the embryos of Ophioglossum. In Lycopodium the variants arise through
tuberous swellings and delay of root-formation : here the variants arise in
relation to the precocity of the root, a feature related in all probability
to mycorhizic nutrition. We have seen that the development of the axis
is delayed in O. vulgatum, which may be held to be a less specialised
type, though still with precocious root : in O. moluccanum, also, the root
emerges early from the prothallus and projects downwards, but the
468
OPHIOGLOSSALES
cotyledon, which is itself green and expanded, emerges upwards, while
the axis is still further delayed than in O. vulgatum : it may be suggested
that the latter has been slipped out from the prothallus owing to the
early elongation of the base of the cotyledon, and so its original genetic
connection is not easily followed, and its appearance comes to be like
FIG. 261.
Botrychium virgiuianu/n, Sw. The
upper figures show two embryos ; the
arrows show the direction of the
archegonial neck, x — shoot ; y = root ;
_/=fpot; « = initial cell of shoot; b =
initial cell of root. X 250. The lower
figure shows an embryo more advanced ;
j Astern ; r=root ; c = cotyledon; cal=
calyptra. X 50. (From Jeffrey.)
that of an adventitious bud. Or possibly in this, as apparently in the type
of O. pendulum^ the primary axis may be arrested completely (a step in
advance on the vulgatum-\y^e\ and the adventitious shoots described as
originating on the root be actually such : in fact, early representatives of
those so often found on the roots of the genus. But from the facts, as
presented by Campbell, which are far from giving the complete develop-
mental story, there does not appear to be sufficient reason to regard the
EMBRYOLOGY
469
peculiarities he describes as other than extreme modifications along the
lines already indicated by the less specialised embryos of the family.1
The first detailed description of the embryo in any of the species of
Botrychium was given by Jeffrey for B. virginianum, and it has been
verified in many points by Campbell. The very large prothallus bears
its archegonia on the upper surface : after fertilisation the zygote enlarges,
and divides first vertically to the axis of the archegonium, and in the
hypobasal and epibasal hemi-
spheres the usual octant
divisions appear ; but these
segmentations are obscured
by the less regular divisions
which follow. The embryo
thus appears as an ellipsoid
body, in which no apical
cells are at first defined.
Jeffrey states that the whole
hypobasal hemisphere goes
to form the foot, while the
stem-apex and the root
originate from the epibasal
half: and his drawings cer-
tainly seem to bear this out
(Fig. 261). The apical cell
of the stem (a) is defined
before the cotyledon appears :
this is formed on the side
of the axis next to the root
(/>), and Jeffrey records that
it is derived from the shoot-
meristem. It grows rapidly, and finally becomes expanded above ground
as the first assimilating leaf. The root is, however, the first part of the
embryo to emerge, and a second and third root may make their appearance
before the cotyledon unfolds : subsequently successive spirally arranged
leaves are formed on the 'axis, but the earliest fertile spike observed in
this species was borne on the ninth leaf.
1 This interpretation of the data of Campbell differs widely from his own. It is
impossible here to enter into any full discussion of the question. It should be stated,
however, that Campbell's own view is that the type of embryo of 0. moluccanum is
probably the most primitive, and shows an embryo in which no axis exists at first ; he
regards the definitive sporophyte as a secondary structure developed as a bud upon the
primary root. In O. pendulum, also, the leafy sporophyte is secondary, neither stem-apex
nor leaf being produced from the embryo itself (I.e., p. 183). In fact, Campbell takes
as the most primitive forms those which are most divergent from the type of embryo
which is usual in other Pteridophytes. It would seem more satisfactory, however, in so
specialised a case as this, to start from the least divergent, such as 0. vulgatum.
FIG. 262.
rychiitm Lunaria, L. 36 = fertilised archegonium; 37 =
:, showing the first segmentation ; 38 = embryo of four cells ;
Botrychi
zygote, sho „
39-40 embryos cut in direction of the axis of the archegonium ;
42 = an embryo breaking out of the prothallus; 36-40X225;
42X150. (After Bruchmann.)
470
OPHIOGLOSSALES
The account given by Bmchmann for B, Lunar ia corresponds in
all essentials to the above, though it differs in certain details. The octants
appear as usual, and are followed by less regular divisions which disguise
them in the resulting ellipsoid body. The limits between the epibasal
and hypobasal parts are lost, and owing to the late origin of the several
parts of the embryo, Bruchmann found it impossible to refer them
FIG. 263.
Botrychium Lunaria, L. The lower figure represents an old embryo, with well-
developed foot (f) \ Wi = apex of the first root ; j = apex of the rhizome, with the second
root, w<i. The endophyte (en) is already in the cells. X52. The upper figure is a
diagrammatic section of a seedling, with six to eight roots, of which three are in plane of
section, /"—foot ; ivi = first root ; w — roots ; s — apex of rhizome ; ^ — ^3 developing leaves.
X6. (After Bruchmann.)
strictly to one or the other source (Fig. 262) The root, which is
organised early, grows first in a horizontal direction, and bursts laterally
out from the prothallus, but the remainder of the embryo rests within the
prothallus, where a distended foot is formed. On this ovoid cellular
body, and opposite to the neck of the archegonium, there arises the apical
cell of the axis : it is immediately overarched by a small growth
(apparently on the same side of the axis as the root), which Bruchmann
takes for a rudimentary cotyledon. Up to this time the embryo has a
predominant root — more so than in B. virginianum — while the foot serves
' "~" x
EMBRYOLOGY 471
both for storage and as a haustorium (Fig. 263 A). Even at this early
stage the embryo may contain an endophytic fungus. The formation of
a succession of roots may then follow, while the growth of the bud
remains almost quiescent, though it forms a succession of small leaves
(Fig. 263 B) : of these about the eighth appears above ground, the rest
serving only for protection to the bud. It is interesting, however, to note
that a rudimentary fertile
spike may be found even
on some of these rudimentary
scale-leaves. From this point
onwards the development is
as in the mature plant. Com-
paring this development with
that in B. virginianum, the
relative position of the several
parts is essentially the same :
the chief differences are in
their proportion. The root
and foot are larger, and the
axis later in definition : also
there is the fact that the first
few leaves are scale-leaves,
whereas in B. virginiannm
the first leaf is itself ex-
panded above ground. The
same difficulty holds here as
before in defining whether the
root is hypobasal or epibasal
in origin. It is from such
differences as these existing
within a narrow circle of
affinity that a true estimate
of the value of embryonic
characters may be derived.
But these differences are of small account compared with the divergence
from the general type of the genus shown by another species, B. obliquum,
Muhl. H. L. Lyon has described how its zygote does not develop directly
into the embryo as in other species, but first gives rise to a suspensor,
which burrows into the tissue of the gametophyte in the manner characteristic
of certain Lycopods. The embryo itself is formed at the tip of this
suspensor, and its parts are differentiated relatively early (Fig. 264). The
parts themselves are essentially similar to those of other species of the
genus : the first leaf (cotyledon) appears on the side of the axis directed
upwards, and it breaks through the upper surface of the prothallus : the
root originates on the side directed downwards, and it emerges on its
FIG. 264.
Botrychium (Scefitridiuiti) obsjgttuni, Muhl. Photo micro-
graph of a section through a gametophyte and young sporophyte.
The root is already protruded from the under side of the game-
tophyte. a =archegonium ; s = suspensor ; ^ = stem-tip; /=first
leaf; r = root. x6o. (After H. L. Lyon.)
4/2
OPHIOGLOSSALES
under side. Hitherto only a preliminary account of this strange anomaly
within the genus Botrychium has been published, and it will be necessary
to await the detailed description which will
supply the materials for an exact comparison.
But meanwhile Mr. Lyon has most generously
lent slides showing not only some advanced
stages, but also the earliest stages of embryo-
geny, from which the following facts and
drawings have been derived.1 Transverse
sections of an embryo corresponding to that in
Fig. 264, showed the suspensor (s), cotyledon
(c), and apex of the axis (ap] in the relative
positions ascribed to them by Lyon, and
demonstrated the correctness of his interpre-
tation of the longitudinal section (Fig. 265).
But what is more important is that sections traversing archegonia, shortly
after fertilisation, showed that the zygote, while still undivided, grows
FIG. 265.
Embryo of Botrychium obliqumn, in
transverse section at the level of the
stem -apex («/). cc — cotyledon ; -y = sus-
pensor. From a preparation lent by H.
Lyon.
FIG. 266.
Botrychium obliquum. First stages in the embryogeny ; before the first segmentation
the zygote grows into an elongated tube (the suspensor), which burrows its way
irregularly into the tissue of the prothallus. X 150. From sections lent by H. Lyon.
into an elongated tube, which takes an irregular course downwards into
the tissue of the prothallus (Fig. 266) ; its nucleus settles down to the
1 Mr Lyon's action in this matter deserves special recognition. When circumstances
delayed the completion of his own statement, knowing the importance which the main facts
bore in embryological comparison, he forwarded a set of slides to me in Glasgow, with
permission to use the facts they showed in whatever way I found necessary. F. O. B.
,
KM BRYOLOGY
473
base, and maintains that position^ as the growth proceeds. This is
susceptible of no other interpretation than that a suspensor is formed, and
that the whole embryogeny is inverted, as compared with that of other
Ophioglossaceae where a suspensor is absent. The importance of this
lies in its bearing on the general comparison of embryos, and on the
estimate of the weight to be attached to some of those differences which
have hitherto been made to bear a burden of comparative and phylogenetic
argument. If we see that within a narrow circle of affinity the suspensor
may be present or absent, and the apex of the embryonic axis be
lirected either towards the archegonial neck or away from it, then
ich characters become suspect. This will find
special application in the comparative study of
Lycopodiales and of the Ophioglossales.
In the third genus, Helminthostachys, the
mngest stages have not yet been seen : but the
Id embryo resembles that of Botrychium virgi-
inum.1 It has a large foot derived from the
lypobasal region, while the primary root, first leaf,
id stem-apex appear to be referable to the
>ibasal half. The first 'leaf has a ternate lamina,
id reaches the light, but the young plant remains
ttached to the prothallus till several leaves have
jn formed : one root lies below each of the
irliest leaves, but in the older plant this regularity
lost (Fig. 267). An endophytic fungus is present
the first few roots, though the adult plant is
)rmally free from mycorhiza.
The character of the prothallus, and perhaps
the position of the archegonium upon it, have
be taken into account when making comparison
of the embryology of the Ophioglossaceae. All their prothalli are
typically underground and saprophytic, and the embryos show differing
degrees of adjustment to the peculiar conditions thus imposed upon them.
In these facts the dominating features of the embryogeny may be found,
and they must be borne in- mind not only in any comparison with other
Pteridophytes, but also as regards the minor differences which they them-
selves show. The most obvious points relate to the development of the
earlier leaves : in Botrychium virginianum, Ophioglossum pedunculosum and
Moluccanum, and in Helminthostachys the cotyledon itself may be expanded
above ground ; but in both of the larger genera there are species which bear
the first leaves as rudimentary underground scales : this is seen in O.
vulgatum, where the second leaf only is effective for assimilation, and in
JB. Lunaria, where a succession of scale-leaves appear, and the eighth
leaf is the first that is expanded above ground. The scale-leaves can
1 Compare Lang, I.e.
FIG. 267.
Helminthostachys zeylanica.
Young plant attached to pro-
thallus. Natural size. (After
Lang.)
474 OPHIOGLOSSALES
only be held as the representatives, secondarily reduced, of leaves primitively
expanded above ground.
Another feature for comparison is the balance between the root and
the shoot. Owing to the saprophytic mycorhizal habit of the prothallus
— and in some of the species even of the sporophyte itself — there is no
immediate need for leaf-expansion, though an effective root-system is
wanted, especially where it is itself mycorhizic. This finds its realisation in
the embryogeny ; for the root-development in the Ophioglossaceae is liable
to be hurried forward, and the development of the shoot to be postponed.
That is seen in O. vulgatum, where the first root may already have freely
emerged from the prothallus even before the shoot is clearly initiated.
B. Lunaria also shows the first root as predominant, and the shoot relatively
backward in development, with its succession of scale-leaves. Both these
familiar plants are thus relatively specialised types of their respective genera.
But the case of O. pendulum shows a still more- extreme type; and it
seems not improbable that the precocious development of the root has
completely upset the balance of parts in the embryo, with the result that
the primary axis and cotyledon are difficult to locate, or may be even
entirely arrested. Comparing the embryos of the family as a whole,
it would seem probable that the primitive prothallus was above ground,
and that in the original state of the seedling even the first leaf was an
effective assimilating leaf, while those with one or more ineffective scales
show a more advanced adjustment to their underground habit. The
deferring of the period of functional activity of the shoot carries its
reflection back to the early steps of the embryogeny ; the relatively
late appearance of the axis with its appendages is thus explained, as weL
as the apparently precocious development of the root. The differences ii
these respects shown by the various representatives of the family indical
their unusual capacity for adjustment of such details. It is through con-
siderations of this nature that we may bring these embryos into relatioi
with those of other Pteridophytes where the embryo shows differentiatioi
at an earlier stage.
• The late differentiation of the parts of the embryo in the Ophioglossaceae
brings with it a difficulty in their exact location relatively to the primary
segmentations of the zygote. There is no doubt that in the types under
consideration (excluding the type of B. obliquuni\ the apex of the axis
arises in them all from the epibasal hemisphere, and allowing for distortions
due to unequal growth, it appears to be coincident with, or in near
proximity to, the intersection of the primary octant-walls. Thus as regards
the initial polarity the Ophioglossaceae resemble other types of Pterido-
phytes. The -cotyledon .appears in close relation to the apex of the axis,
both in time and space, and it usually lies between the apex of the axis and
the first root : but it will be remembered that in Isoetes^ which offers some
other points of analogy, the root is on the opposite side of the axis to the
cotyledon. As to the exact point of origin of the first root there is some
EMBRYOLOGY 475
degree of uncertainty. In B. virginianum it is traced by Jeffrey from the
epi basal hemisphere, and his drawings seem to bear this out. But in
'.ilgatum Bruchmann indicates a cell in the hypobasal region as the
probable initial cell. It seems not improbable that in the Ophioglossaceae,
as also in the genus Equisetum and among the Lycopodiales, the origin of
the root is not uniform in position, but may in this relatively large embryo
be at a point either above or below the primary segment-wall.
A very striking feature in the young seedling is the early appearance of
the fertile spike. In O. viilgatum it may appear upon the third leaf, while it
may be seen even on the first leaf of the adventitious buds of this species.
In Botrychium Lunaria its minute representative may be found on the
rudimentary underground scales of the embryo. In these cases the body
actually seen does not seem to differ either in position or in origin from those
produced on the later leaves. Such facts will have their bearing on the
question of the morphological nature of the spike. Taken in relation to
the general theory of sterilisation they indicate that the plants are but little
removed from a condition where the very first leaves were fertile. On the
other hand, Jeffrey figures several fairly advanced plants of B. virginianum as
having no spikes ; but this species is one of advanced leaf-complexity. In
Helminthostachys also, in which the leaves are large and complex, Lang
has depicted young plants with expanded leaves, but without spikes. One
is disposed to conclude from these scanty facts that the simpler-leaved forms
of this family are more early fertile than those with more complex leaves,
an indication of their more primitive state : but further data are necessary
to substantiate the point.
Lastly, there remains the case of B. obliquum, with its suspensor and its
complete inversion of the polarity of the embryo. It is difficult to see how
this is to be brought into relation to its biological surroundings. As the
details of this aberrant embryogeny are not yet to hand, it must for the
present be accepted as an objective fact, the chief interest of which lies in
the demonstration that such differences as presence or absence of a
suspensor, and consequent inversion of the initial polarity of the embryo,
are possible within near circles of affinity : this will have its important
bearing upon the comparison of Isoetes, where as in most Ophioglossaceae
there is no suspensor, with other Lycopodiales, in which a suspensor is
present and the embryogeny inverted as in B. obliquum.
CHAPTER XXXI.
COMPARATIVE DISCUSSION AND SUMMARY FOR THE
OPHIOGLOSSALES.
THE Adder's Tongues cannot yet be considered as located in a definil
position in relation to other groups of Pteridophytes. Their tradition;
place among the Ferns was accorded to them somewhat light-heartedly, am
before the details of their anatomy or development were adequately knowi
They share two external characters with the Ferns, viz. that they are large
leaved, and that the sporangia are distributed over a considerable extei
of the foliar organ. But to use these in themselves as a ground for ranking
them as Ferns involves the assumption that the origin of a large sporoph]
only occurred once in Descent, an assumption that is not warranted. Oi
the other hand, a relationship with the Lycopodiales has been ascribed
them : this has been based in the first instance upon the position of theii
peculiar spore-bearing member, the spike, as it is called; and it has
urged that the insertion of this part is the same as that of the sporangiui
of the Lycopodiales or of the sporangiophore of the Psilotaceae, while tl
function of these parts is also alike. This argument, like the first,
its cogency from an assumption, that all the appendages holding a venti
position on the leaf were of common origin. But parallel development ii
distinct phyletic lines may account for this common feature, as it doe
for so many others in the plant-body. The day is past when single
characters such as these can be accepted as defining relationships, and
is in the study of all the characters that an indication of the natun
position of any family is to be found. Certain recent writers have indicate
a specially primitive position for the Ophioglossaceae, comparing them directb
with the Anthocerotales,1 while V. Wettstein2 gives them the first positioi
in his treatment of the Pteridophyta, with the remark that "the Ophic
glossales are the only living Pteridophytes from which the rest of the
Pteridophytes can be derived." With such divergent opinions before
1 Campbell, Mosses and Ferns, 1905, p. 600.
- Handbuch d. Syst. Bot., p. 52, etc.
COMPARATIVE DISCUSSION 477
a general revision of the characters of the Ophioglossales has seemed
advisable ; and any such revision should involve not only their comparison
with other types, but also, what is perhaps more important, a comparison
of their different genera and species among themselves.
The whole question of the character and relations of this family turns
upon whether they be regarded as an ascending or a descending series.
The former view, that they are a series of reduction, is entertained by many
botanists, but without, as far as I am aware, any full or detailed statement
of the grounds for this opinion : their " saprophytic habit " has, however,
been advanced as one source of their modification.1 As regards this
saprophytic habit the following considerations may be brought forward.
Mycorhiza has been observed in Qphioglossum vulgatum? in the mature
plant : Bruchmann states, however, that it is absent from the young plant.3
It is present in the mature plants of O. pendulum^ and simplex? and is
specially prevalent in the peculiarly modified embryo of the former species
with its unusually precocious root.6 It has been seen in twelve species
of Botrychium by Grevillius, but in varying abundance,7 and Kiihn had
previously described it for B. Lnnaria : s Bruchmann 9 showed that
mycorhiza is present in the young plant of the Moonwort, and that since
the eighth or ninth leaf is the first to be expanded above ground,
the plant is saprophytic in its nourishment up to its eighth or ninth
year. In Helminthostachys the fungus is present in the first three or four
roots of the young plant, but absent in the roots later produced.10 It is
thus seen that mycorhiza is not distributed with constancy in the family :
in some it may be present in the young plant but absent in the mature :
in others the converse ; while some are distinctly saprophytic, none have gone
so far as to discard entirely the chlorophyll-function throughout life : in
Botrychium Lunaria, however, the mycorhizic habit appears to be obligatory.11
The most peculiar case, as it is also instructive in another point, is
O. simplex, in which the presence of mycorhiza goes along with the
apparently complete absence of the sterile leaf; here it would seem that
the mycorhiza makes the nutrition of the large spike still possible in the
dense wet forest in which the plant grows, notwithstanding that the usual
assimilating organ is functionally absent. Reduction is, however, not
apparent in the large spike itself, for provided nutrition be kept up from
whatever source, it would still retain its character, being essentially a spore-
bearing organ. Thus O. simplex teaches what is also seen elsewhere, that
it is the vegetative rather than the propagative system which is primarily
1 Scott, Studies in Fossil Botany, p. 511. 2 Russow, Vergl, Unters., p. 122.
3 Hot. Zeit., 1894, p. 241. 4Janse, Ann. Jard. Suit., xiv., p. 64.
5 Bower, Ann. of Bot., xviii., p. 207.
6 Campbell, I.e., Plate XVII., Figs. 128, 129.
7 Flora, 1895, P- 445- % Flora, 1889, p. 494. 9 Flora, xcvi., 226.
10 Farmer, Ann. of Bot., xiii., p. 421, and Lang, Ann. of Bot., xvi., p. 42.
11 Stahl, Prings. fahrb., xxxiv. , p. 574.
478 OPHIOGLOSSALES
affected by disturbed nutrition. This may be presumed to have been the
case in Psilotum, where the large size of the synangium proportionally to
the small bifurcate leaf indicates reduction of the latter, but not of the
former in the same degree.
In estimating the effect of mycorhiza in any family as a whole in which j
it occurs it is necessary to take into account in the first place its constancy; I
and it is seen that it is not constant in the Ophioglossaceae. Secondly, ]
it is a matter of general observation that plants with an endotrophic
mycorhiza commonly show a structure in accordance with a limited tran-
spiration stream : their root-system is compact, and hydathodes are absent
from their rather leathery foliage.1 The Ophioglossaceae show such
structure, which should not be confused with the results of saprophytic
reduction. It may have been primitive for them, and in this connection \
it is to be noted that mycorhiza has been described in plants from the
Lower Coal Measures, so that it is no newly adopted manner of life.'2
The condition of ffelminthostcuhys, with its mycorhizal embryo and non- '
mycorhizal mature plant, would be consistent with a primitive mycorhizal j
state, from which the plant had broken loose and passed to an autotrophic
condition. But, thirdly, it is important to note that among plants at large j
many species in which it is present show no sign of reduction either in |
their vegetative or in their reproductive parts. This may be well illustrated .;
among the Pteridophytes themselves. Thus Lycopodium inundatum is found ,
to be mycorhizic, but its general habit, especially in the large American
forms, does not suggest reduction as compared with other species.
L. cernuum is mycorhizic in the young plant, but it is one of the most j
elaborate of Lycopods. Selaginella spinulosa is mycorhizic, but S. helvetica
is not ; and yet S. spinulosa cannot be held as relatively reduced. How
little the occurrence of mycorhiza may be found to affect the morphological
character of Ferns is shown in the Marattiaceae. According to Kiihn 3
a fungus is found in the roots of Kaulfussia, of Angiopteris, and of
Marattia alata, but not in those of Marattia fraxinea ; but no reduction
is to be noted as the result in the former Ferns as compared with the
latter. Again, Cyathea is stated to have mycorhiza, while Asplenium nidus
has not.4 Such facts as these clearly indicate that in Pteridophytes the
presence of a mycorhiza does not spell reduction. Accordingly it cannot
be justly assumed that the somewhat inconstant occurrence of the mycorhizic
habit in the Ophioglossaceae has been a source of general reduction in the
family, though reduction may have played its part in certain species. The
argument in favour of their being a reduction series as a whole will have
to be based on other evidence.
Pending the statement of such evidence, the Ophioglossaceae may be
treated, without any preconceived i'dea of general reduction, in the same
way as any other family of plants. The natural comparison of them
1Stahl, I.e. 2 Weiss, Ann. of Bot., xviii., p. 255.
* Flora, 1889, pp. 491-497. 4Janse, I.e., p. 64.
COMPARATIVE DISCUSSION 479
among themselves — and not giving undue weight to the species which
happen to be the commonest — leads almost inevitably to their seriation
in the way indicated above (pp. 431-446). The upright radial, unbranched
shoot is the central type, and the only departure from it is in the
large-leaved Helininthostachys^ where the dorsiventral rhizome may be held
as illustrating a secondary condition ; the primitive stock was probably
upright and radial for them all. It was also polyphyllous, as in most
other Vascular Plants, while each leaf bore the characteristic spike, which
-sentially identical in them all, whatever its actual nature may be
held to be. Within the family it is probable that the three genera
illustrate three distinct lines of descent from some common source, already
provided with a body of the nature of the spike. In Ophioghssum the
original polyphyllous state is still seen in various smaller species : and it
is worthy of remark that the nearest similarity to other strobiloid types is
seen in those species in which the appendages are simplest and smallest.
But, as pointed out above, the monophyllous habit has biological advan-
tages in plants with an underground stock, and with its adoption followed
enlargement of the individual leaf, and of the spike, the two parts showing
some degree of parallelism of dimensions. Thus the ordinary type of
O. vulgatum is attained. Fission or chorisis of the spike is an occasional
occurrence in O. vulgatum and other species, but it became a fixed
character in O. palmatum. It appears probable, however, that it is only
attained in this species in fully matured plants : thus the individual of
this species may be held to illustrate in its own life the origin of its
more complex form. Here again a parallelism exists between the irregular
lobing of the sterile lamina and the number of spikes .which it bears. It
would be difficult to explain these characters in any other way than as an
ascending series involving chorisis. A probable line of reduction does,
however, occur : it is illustrated by the series Q. pendulum, intermedium,
and simplex, the latter having no functional representative of the fertile
lamina.
A distinct line, also of progression, is seen in Botrychium, but with
different details. The series of forms seen in B. simplex (Fig. 240), and
in the young plants of B. Lunaria, link on by their simplest forms with
the condition of a small .Opliioglossum with simple sterile lamina and
unbranched spike : by very gentle gradations they lead on to the branched
sterile lamina and fertile spike characteristic of the genus, the branching
of the spike being closely connected with the enlargement and fission of
the sporangia. There is reason to believe, as Luerssen has indicated,1
that these forms illustrate progress in the life of the individual, from the
simpler to the more complex : and the suggestion lies near to hand that
the individual in this respect " climbs up its own evolutionary tree." The
continuation of this method of advance would lead onwards to the most
complex forms, the spike and lamina preserving a parallelism as before.
1 Rab. Krypt. Flora, iii., p. 579.
480 OPHIOGLOSSALES
On this view the two genera, starting from a common source, would be
held to illustrate two distinct lines of progression to a more complex state.
The third genus, with its single highly elaborate species, gives no such
suggestion of its origin by comparison of nearly allied species. It stands
as the most isolated member of the family : but its normal spike is
evidently similar in plan to that of a large Ophioglossum, supposing its
marginal rows of sunk sporangia were replaced in Helniinthostachys by
serried ranks of sporangiophores. There is a biological probability that
such an advance should occur in a large spike bearing many spores, for
thereby the advantage is gained of more ready nutrition of the subdivided
sacs, and more easy dissemination of the spores when mature.
The progressive advance thus suggested for the Ophioglossaceae is in
accord with biological probability, in a series with a marked tendency to
a monophyllous state, and consequent enlargement of the individual leaf.
Provided the nutrition be available, an increase in numbers of spores is
an advantage in any homosporous form. But an indefinite increase in
size of individual sacs raises difficulties of nutrition : subdivision is thus
to be anticipated in any progressive series, and that is seen in Ophio-
glossum. A projecting position of the individual sporangium is an
advantage in dissemination of the spores. This is ill provided for in
Ophioglossum, and in this respect Botrychium and Helniinthostachys show
a more effective state. It appears to me difficult, without special reasons
assigned, to recognise this family as a series of reduction, for it would
be in opposition to these biological considerations. On the other hand,
comparisons within the family clearly indicate an upward rather than a
downward progression, while in any case those who hold a theory of
reduction would find peculiar difficulty in explaining the condition seen
in Ophioglossum palmatum.
The next step will be to discuss the morphology of the fertile spike,
and to see what are its nearest correlatives among the members of other
Vascular Plants. The spike in all the representatives of the family is
clearly the same part : it is in fact truly homologous, or homogenous in
the strict evolutionary sense. This follows from the high degree of
constancy of position and function which it shows in normal cases.
Various theoretical explanations of its morphological nature have been
given by different writers. It has been suggested by Braun1 that the
sterile frond is a foliage leaf, and the fertile spike the only developed
leaf of a bud seated in its axil, and coalescent with it. Somewhat later,
Roeper (1843) published the opinion that the sterile spike and fertile leaf
are equivalent — that is, borne by the same axis — but coalescent together.
Subsequently he substituted for his old view the opinion that the fertile
spike is the result of coalescence of two lateral, lower, fertile pinnae of a
frond, of which the remainder is usually sterile.2 Lastly, Goebel has put
forward the opinion that the fertile spike is the lowest pinna of the
1 Flora, 1839, p. 301. * Bot. Zeit., 1859, p. 271.
COMPARATIVE DISCUSSION 481
sterile frond, which however arises vnot laterally, but in a median position.1
Of these various views, which all coincide in attempting to refer the
spike in origin to a leaf or part of a leaf of the ordinary vegetative
sequence, only that of Goebel may be considered to hold the ground at
the present time.2
The arguments advanced by Goebel in support of his theory were
primarily a comparison of malformations, especially in Botrychium Lu'naHa.
It was pointed out that here the normally sterile leaf shows most various
intermediate steps of fertility till, in extreme cases, it may be wholly
represented by a fertile sporophyll (Fig. 242, p. 443). It is recognised that
in these cases there has occurred a true metamorphosis of a foliage leaf into a
-characteristically formed sporophyll, which differs in a striking manner from a
foliage leaf. The inference which Goebel drew was "that this is also the
case in the normal and usual sporophyll, i.e. that this is produced from
a part of a foliage leaf." This argument has been dealt with at length
elsewhere.3 But more recently Goebel has strengthened his position by
observations on the young leaf of Helminthostachys.^ He does not give
any account of the first origin of the spike, which in Botrychium has
been traced and found to be different from that of the sterile pinnae,5
but lays stress upon its similarity of form to the sterile lobes, upon its
dorsiventral character, and upon the development of the lateral wings in
abnormal cases, like those of the foliage leaves. He concludes : " We can
therefore trace back the sporophyll to a specially far-reaching transformation
of the vegetative leaf."
In his admirable discussion on teratology in Schenk's Handbuch*
>fessor Goebel has drawn attention to the fallacious conclusions
rhich may be arrived at, on the assumption that malformations
illy afford evidence of the morphological nature of the parts con-
irned ; he has quoted as an example the malformations of the ovule, of
lich specimens may be selected, so as to illustrate the gradual steps
abortion of the nucellus and integuments, together with correlative
relative growth of the funiculus, till the result is reached that the
rhole ovule is replaced by a simple leaflet ; 7 but he concludes that this
inal result shows how little justification there is in accepting the vegetative
Schenk's Handbuch, vol. iii., p. ill; Organography, vol. ii., p. 481.
2 Sufficient reasons for setting aside Keeper's theory have been given in Studies, ii.,
46. The position of those who speak of the spike as a " ventral lobe " of the leaf
irs somewhat obscure : if by this is meant a body which may exist in the vegetative
ite, then either it must have been marginal or ventral in origin : if the former, the
is that of Goebel : if the latter, then it lies with them to show that such vegetative
parts exist in these or other plants. But the term may connote a ventral part which has
been fertile throughout descent : in that case the view is coincident with my own
advanced in 1891 (Proc. A'.S., Dec. 17, p. 270), and now submitted again in a modified
form.
* Studies, ii., p. 8. 4 Organography, vol. ii., p. 481-485.
5Bruchmann, !.<:,, p. 218. 6Vol. iii., pp. 114-125. 7 Loc. cif., p. 121.
2 H
482 OPHIOGLOSSALES
development of spore-bearing parts as phenomena of reversion. This is
precisely the view which I take with regard to the fertile spikes of the
Ophioglossaceae ; the fact that when spore-production is suspended in
them a correlative vegetative growth may result, in form like that of the
sterile leaf, or some portion of it, is to be compared with the similar
cases of those ovules which are replaced by leaflets. In the case of the
Ophioglossaceous spike, as in that of the ovule, its replacement by a
body resembling a foliage leaf or leaflet does not demonstrate its
homology with, or its origin from, such a part : nor does the formation
of a lateral vegetative wing in place of the marginal sporangia, or
sporangiophores, show that the latter were in descent the result of trans-
formation of the former.
There is also the inherent objection to Goebel's view, that it ascribes
the origin of the sporophyll to transformation of a vegetative leaf. It is-
doubtless possible, by assuming a megaphyllous plant with elaborate
vegetative structure as pre-existent, to imagine its reduction and modifi-
cation to produce such forms of spore-bearing parts as we see in the
Ophioglossaceae. But to those who hold consistently to a theory of
antithetic alternation, with sterilisation as one of its most important
features, this assumption is not admissible : to them sporophylls are not
modified foliage leaves (compare Chapter XIII.).
All the theories which would refer the spike in origin to some foliar
part, modified or altered, start from the more elaborate types of the
family, and assume reduction. But if the converse line be taken, quite
different views emerge. And there have not been wanting those who
have already approached the question of the morphology of the spike in
this way, which is certainly the most direct.1 It seems more probable
that a sound view of the morphological nature of the spike should be
obtained through comparison of its simpler forms than of the more
complex with what is seen in other Pteridophytes : " and it is naturally
with the microphyllous forms that the closest correspondence may
accordingly be expected.
1 Mettenius, Hot. Zeit., 1867, p. 98; Celakovsky, Pringh. Jahrbs., 1884, p. 291.
2 An interesting passage from GoebeFs Organography may here be quoted, which is
specially applicable to the present case (Engl. ed., vol. i., p. 60) : " Most of our phylo-
genetic series are reduction-series, that is to say, are those in which the changes are
brought about by arrest. There is a simple psychological explanation for this. If we have
a definite ' type ' we obtain through it a fixed starting-point for our comparison. But
this is wanting when our comparisons deal with an ascending and not with a descending
series. It is specially necessary to refer to this, because arrests have frequently been
assumed upon the subjective grounds above indicated without definite proof of their being
existent. ... It is only our synthetic necessity which forces us always to the assumption
of reduction-series, of which, however, many can only claim to be fictions, imparting
the aesthetic pleasure of bringing a series of facts into connection with one another.**
The "synthetic necessity" in the present case has been to bring the large-leaved
Ophioglossaceae into line with the definite large-leaved type of the Ferns : the latter
have been constituted a fixed starting-point chiefly because they are well known.
COMPARATIVE DISCUSSION 483
Among the microphyllous Pterfdophytes there is sometimes no strict
relation between the spore-bearing members and the bract-leaves, but in
the majority of them some constant relation is found. The common
type is for the former to be in the median position relative to the latter,
though the level of insertion may vary. In the Sphenophyllales this
position is seen in the simpler forms, such as S. majus and trichoma-
tosurn ; but it is departed from in others where, probably owing to fission,
the number of sporangiophores is larger than that of the subtending
bracts. In the Psilotaceae the radial position is maintained, but the
insertion is very close to the apex of the sporophyll. This local relation
of the two parts is so constantly seen in these groups, which include
some of the most ancient plants, that it suggests for them a fixed
morphological character rather than a mere result of independent adap-
tation. The existence of a like constant relation in another group
compels the exact comparison of the parts thus similarly placed and
functionally identical. The criterion whether this is a mere analogy, or
whether some deeper significance underlies it, will be found in the
degree of correspondence which the plants show in other characters than
the one in question. To apply this test a comparison will be made
between the Sphenophyllales and the Ophioglossales, first as regards their
spore-producing members, and afterwards in respect of the other characters
of the sporophyte.
Both in Ophioglossum and Botrychium species occur with small spikes
bearing few sporangia, and they are specially found in the young plants
(O. Bergianum, B. simplex, and Lunaria) : it is in these that the closest
similarity exists between the two genera, while from them by gradual
steps the two types diverge. These small spikes consist of a simple stalk
with vascular supply, bearing towards its distal end a few sporangia
marginally disposed : the insertion is median on the sporophyll : the
essentials of form, position, and function are here similar to those of
the sporangiophores of the Psilotaceae, and especially of those stalked
sporangiophores of Tmesipteris described by Thomas.1 The differences
lie in the forked leaf of the Psilotaceae, while that in these simple types
of the Ophioglossaceae is usually entire, and in the exact position of the
sporangia.
But in both groups there may be a departure from the exact numerical
and local correspondence of sporangiophores and sporophylls : and these
lead to an interesting comparison. The case of O. palmatum has been
referred to chorisis of the single spike, which seems the only explanation
of the plurality of irregularly branching spikes seen in an intra-marginal
position in that species. In the Sphenophylls the frequent close juxta-
position of the pedicels of the sporangiophores points to a similar chorisis,
as accounting for the condition seen in ,5". Dawsoni, and possibly also in
the imperfectly known S. Roemeri. Moreover, the vascular connections of
1 Proc. R.S., vol. Ixix., p. 345.
484 OPHIOGLOSSALES
the sporangiophores in the more isolated Cheirostrobus point to a similar
origin of its very complex state. It would thus appear probable that the
sporangiophore of the Sphenophyllales and the spike of the Ophioglossales
are parts not only similar in position and function, but also susceptible
of chorisis. This may be held by some to be only a distant analogy :
but such analogies have a way of developing into convincing evidence when
they prove to be cumulative.
The first appearance of the spike in Ophioglossum is upon the upper
surface of the sporophyll, in a median position some distance from the apex,
but in Boirychium it is close to the apex (Bruchmann, I.e., Fig. 57). A com-
parison of the latter with what has been seen in Psilotum and Tmesipteris
(Figs. 230, 232), shows a striking similarity in the position of the spore-
bearing parts relatively to the apex of the sporophyll. If this similarity
also be only one of analogy, it is at least a singularly close one. In the
Psilotaceae only two lateral leaf-lobes are subsequently formed, but in
Botrychium a considerable number. The pronounced apical growth of the
Ophioglossaceous spike is also a point of difference from the sporangiophore
of the Psilotaceae ; but it naturally accords with the more elongated form
when mature.
The details of development of the sporangium afford material for
further comparison. It has been shown how in the Psilotaceae there is
special difficulty in recognising the exact limits of the sporogenous masses
at an early stage of the sporangium, and that there is no definite
tapetum. In Ophioglossum the condition is similar : there is the same
indefiniteness of the sporogenous masses when young, and the same
absence of a definite tapetum. On the other hand, the Ophioglossaceae
themselves show interesting gradations : for while Ophioglossum has the
indefinite characters noted above, Botrychium and Helminthostachys show
a more definite specialisation of the sporangia, which goes along with
their smaller size; for here the tapetum is a definite one, and originates
outside the sporogenous tissue. There is thus an approach in the
Ophioglossaceae to the condition seen in the Eusporangiate Ferns. The
indefinite condition of the sporangium is exceptional among the Pterido-
phytes : of living forms it is most prominently seen in Ophioglossum,
Psilotum, and Tmesipteris. This similarity materially strengthens the
comparison between the spore-producing members of the Psilotaceae and
Ophioglossaceae.
From the development of the individual spikes of the latter some idea may
be formed of the steps which probably led from the simple structure on which
the comparison has so far rested to the more complex. In Ophioglossum
the lateral rows of sporangia arise from the sporangiogenic band : its cells,
originally alike, differentiate into sporangial wall, septa, and sporogenous
groups : in point of origin the latter are all alike, and the structural details
are in accord with a theory of progressive septation, that is, a conversion
of part of the potentially fertile tissue into sterile septum in the enlarging
COMPARATIVE DISCUSSION 485
part. Such sterilisation, amounting" even to the formation of permanent
septa, has been shown to take place in other plants, and the theory is
not open, therefore, to a priori objection (see Chapters VII. and X.).
The frequent absence of sporangia from the tip of the spike is probably
due to abortion : they cover the apex in some species, which also show
abortion of sporangia lower down (e.g. O. pendulum}. The presence of
vascular strands in the septa themselves shows how the physiological supply
followed the structural advance : on a theory of lateral fusion of sporangia
such a position of the vascular strands would be, to say the least,
improbable. Lastly, imperfect or irregular septa have sometimes been
seen. Thus the structure, so far as it goes, readily coincides with a
theory of extension, and progressive septation to produce the spike of
Ophioglossum from some simpler beginning.
The case of Botrychium is similar, though less obvious, owing to the
isolation of the sporangia, while it is complicated by the fact that
branching of the spike frequently accompanies septation. That a structure
compatible with progressive septation exists is shown by Figs. 253, and
its relation to the branching, which brings such conspicuous results in
the spike of Botrychium, appears in its simplest form in Figs. 252. It
only requires the repetition of the processes, which are thus illustrated
in the individual, to lead from the simplest to the most complex spikes
in the genus.
Lastly, in Helminthostachys the ranks of sporangiophores correspond
in position to the rows of sporangia in Ophioglossum. An upgrowth from
the sporangiogenic band, less regular, but of the same nature as that
seen in the branching of the spike of Botrychium, would give the sporangio-
phores of Helminthostachys, while the individual development directly
represents what this progressive theory demands. This, indeed, is the
foundation upon which the present view of elaboration of the spike in
the Ophioglossaceae is primarily based : without any preconceptions
involving reduction or modification, the theory is founded directly upon
the simple facts of individual development.
The anatomical structure of the shoot in the Ophioglossaceae with
its rare dichotomy, which compares rather with the microphyllous than
with the megaphyllous Pteridophytes, may next be considered. It has
been seen above (p. 464), that the facts observed are compatible with
an origin of the axial system from a protostelic state. The stele of the
seedling or adventitious bud, is either a protostele or slightly medullated
monostele : passing upwards along the shoot there is an amplification of
the stele, with swelling of the central pith. In the lower region there is
usually a well-marked endodermis : this may be continued throughout
the length of the rhizome, but in some cases it fades out upwards, as
the stele distends. The xylem in the upper region forms a hollow
cylinder or funnel, more or less interrupted by leaf-gaps, where the
single strands of the several leaf-traces pass off. The protoxylem is not
486 OPHIOGLOSSALES
always well marked : it is next the pith in Ophioglossum and Botrychium,
but mesoxylic in Helminthostachys. The central protoxylem in the stele
of the seedling is in a position corresponding to that in the medullated
stele of the older stem of Ophioglossum and Botrychium ; consequently,
the mature state appears to be a natural amplification of the centroxylic
protostele.
The mesarch xylem of Helminthostachys presents a difference from
the rest, and it raises a question as to the importance attaching to the
exact position which the protoxylem holds, for purposes of comparison.
The stele of Tmesipteris is mesarch also in its upper region (Fig. 268), and
this is stated to be so also locally in Psilotum, though the position of the
protoxylem in both is peripheral in the lower parts. Again, in Selaginella
spinulosa it fluctuates in the individual stem (Fig. 173): in the seedling all
conditions from the endarch below, to the mesarch, and finally to the exarch
above, may be seen in sections taken successively from the same plant. The
Psilotaceae and Ophioglossaceae thus show a similar instability within
their respective families, and in less degree in the individual plants also,
and this instability is shared by Selaginella. This deprives comparisons
based on the exact position of the protoxylem of much of their cogency,
so far as they relate to these families. Too much weight has been
attached to the position of the protoxylem in the comparative study of
the Pteridophytes. It is a well-known principle of taxonomy that
diagnostic characters which may be good in one alliance may be so
fluctuating as to be useless in another. This appears to be so in
respect of the position of the protoxylem in many of the strobi-
loid Pteridophytes. Accordingly, a prevailing, though not constant
central position of the protoxylem in any given family cannot be held
as in itself invalidating comparisons on other grounds with types where
the protoxylem is usually though not always peripheral. The conditions, in
point of fact, overlap within certain families, or even in the individual ;
the position of the protoxylem as a comparative or diagnostic character
must therefore be held as suspect. In the present case the prevalent
centroxylic state of the Ophioglossaceae cannot in itself be held to
dissociate them anatomically from the strobiloid Pteridophytes (and particu-
larly from the Psilotaceae), since both meet on common ground in showing
at times a mesoxylic condition.
The stele of the Ophioglossaceae, amplified as described, does not
remain a closed cylinder: its continuity is interrupted by foliar gaps, the
vascular ring opening at the point of exit of each leaf-trace. The structure
is that described as phyllosiphonic by Jeffrey, and distinguished by him
from the cladosiphonic type, where the leaf-trace passes off from the stele
without any opening. Jeffrey has laid this distinction down as separating
his Lycopsida from his Pteropsida. According to their structure, the
Ophioglossales would then fall into the Pteropsida. Jeffrey remarks x that
1 Phil. Trans. , vol. cxcv., p. 144.
COMPARATIVE DISCUSSION
487
these two great stocks appear \o have been separate back to the
ginning of the period when the palaeontological record begins." If
lis were so, the anatomical difference would, in all cases, indicate a true
lyletic distinction. It is necessary to obtain a clear idea whether or
>t this will hold good.
The structural difference is based upon the greater or less dominance
the leaf in the whole shoot ; the phyllosiphonic type going, as a rule,
rith a megaphyllous state. But megaphylly may have been attained along
more than one line of descent. If it arose in more than one phyletic
FIG. 268.
Ttnesipteris tannensis. Transverse section of the sterile region, high up. The proto-
xylem (;»?-. 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.
line, then the phyllosiphonic state, which is its internal structural expres-
sion, will also have originated more than once. If this were so, then the
phyllosiphonic structure would not necessarily indicate affinity, and the
Pteropsida, as based on the structural point, could not be held to be a
natural group. The question will therefore be whether there is any evidence
of the origin of a phyllosiphonic from a cladosiphonic state. It might be
expected either in a shoot, with increasing proportion of the leaves, or of
decreasing proportion of the axis. The latter is the state of the distal
region of the shoot of Tmesipteris, and Fig. 268 shows the condition there
seen : the two larger tracts of xylem are separate ; but isolated elements
showing imperfect lignification link them together : the cauline stele is here
seen in course of disintegration into mere leaf-traces : these enter the
488 OPHIOGLOSSALES
axial system by foliar lacunae, after the manner of the phyllosiphonic type.
But in its lower parts, Tmesipteris is typically cladosiphonic : it is thus
seen that a phyllosiphonic structure may arise in a cladosiphonic stock,,
and the illustration is taken from that group of plants which show analogies
with the Ophioglossaceae in other respects : on comparison of Fig. 268
with Fig. 258 of O. Bergiamun, the essential similarity of the two cases
is evident. But in the Ophioglossaceae the structural dominance of the
leaf is on our hypothesis a consequence of the advance of the leaf towards
megaphylly, combined perhaps with weakening of the axis which bears
it. It matters little how the balance between the parts of the shoot is
disturbed : the progression would be essentially the same in either case.
These considerations show, in the first place, that it cannot be assumed
that all phyllosiphonic plants are necessarily derived from a distinct and
primitive phyllosiphonic stock, such as Jeffrey assumed for his Pteropsida :
and secondly, that analogy of their structure with Tmesipteris indicates a
possible origin of the phyllosiphonic type in the Ophioglossaceae, phyletically
quite distinct from that shown by the true Ferns.1
The leaf-trace in the Ophioglossaceae is typically a single strand, which
branches within the cortex into strands which vary according to the pro-
portions of the leaf which they serve : 2 these facts accord with a theory of
origin of the leaf from the simpler type. It is interesting to note that
" the branching of the leaf-traces within the cortex is very characteristic
of Sphenophyllum"z As regards the structure of the collateral strands of
the leaf, those of the larger forms show similarity to the Eusporangiate
Ferns, the smaller correspond rather to those of the larger-leaved strobiloid
Pteridophytes.
The occurrence of secondary thickening in the Ophioglossaceae is
occasional rather than typical of them. In Helminthostachys it is absent : in
Ophioglossum a feeble growth has been occasionally seen ; but in Botrychium
it is a marked feature, and extends from the axis onwards to the base
of the root. This inconstant occurrence of secondary activity, sometimes
feebly shown, has its parallel in other affinities, both of Filicales on the
one hand, as in the Marattiaceae,4 and of the Psilotaceae, where a develop-
ment very like that of Ophioglossum has been seen in Psilotum? The
JIt will be seen below that certain Ferns, for instance the Botryopterideae, are not
phyllosiphonic : thus the anatomical distinction of Jeffrey breaks down on both sides.
- The only known exceptions to this are in O. pendtdum and simplex, where the leaf-
trace is inserted on the cauline system as several distinct strands. These species belong,
however, to a section of the genus believed to be highly specialised rather than primitive
types : and this character itself must, by analogy with the similar cases in the Ferns, be held
to be derivative (see Ann. of Bot., xviii., pp. 209, 215).
3 Scott, Studies, p. 91. 4 Farmer, Ann. of Bot., xiii., p. 440.
5 Boodle, I.e. Scott (Journ. Roy. Micr. Soc., 1906, pp. 519-521) has described
under the name of Botrychioxylon a new genus from the Lower Coal Measures, with
"radially seriated wood, apparently of secondary character." It is related anatomically
to Zygopteris somewhat as Botrychium is to Ophioglossum.
COMPARATIVE DISCUSSION , 489
more active development in Botrvfliium finds its correlative in the more
active thickening seen in Sphenophyllum. These again may be mere
analogies, but they are cumulative, in that they run parallel with others.
Lastly, it has already been shown that as regards structure of the
roots there are unmistakable points for comparison of the Ophioglossales
with the Lycopodiales on the one hand, and on the other with the Fili-
cales, especially the Marattiaceae : the former comparison is in respect
of the simpler, monarch types, the latter as regards the more complex:
the latter branch monopodially as a rule, behaving thus like the roots
of the Filicales : the monarch roots, however, show dichotomous branching.
Unfortunately, the Psilotaceae, which show so many other points for com-
parison, are rootless, while the roots of Sphenophyllum are so imperfectly
known as to give little help. Though the facts relating to the roots are
not in any way decisive, they indicate, what emerges from so many other
comparisons, that Ophioglossum shows characters approaching the strobi-
loid Pteridophytes, while Helminthostachys compares rather with the Filicales,
and Botrychium takes a middle position.
In the embryology two distinct types have been recorded for the
Ophioglossaceae, the one with and the other without a suspensor. That
without a suspensor corresponds in its essentials to the type prevalent
in those Pteridophytes which have the usual octant division. But there
are modifications here in accordance with the underground origin from
a large mycorhizic prothallus, which nourishes itself saprophytically : the
chief of these is the deferring of the period of functional activity of the
shoot : consequently it is differentiated late, and though the root is not
initiated early, as compared with other embryos, it very markedly precedes
the appearance of the axis and cotyledon in Ophioglossum, and in less
degree in Botrychium. This appears in an extreme form in those species
described by Campbell, and especially in O. pendulum, where it is possible
that the primary shoot is permanently replaced by adventitious root-buds,
similar to those common in the genus. These modifications in time of
development make the reference of the parts to definite positions in the
embryo somewhat difficult. But it seems certain, nevertheless, that in the
less extreme forms the axis arises from the epibasal hemisphere, in close
proximity to the intersection of the primary octant-walls. The cotyledon
appears between the stem-apex and the root, but it is late in origin. In
0. vnlgatum it appears simultaneously with the axis, and the relation is
so close in B. virginianum that Jeffrey states that the cotyledon, like any
other leaf, is derived from the shoot meristem. This is interesting in
its bearing on the theory of the cotyledon, which has been held to be
simply a leaf of the shoot showing anticipatory development (see p. 186-7).
The foot which is not largely developed originates from the hypobasal
hemisphere. The position of the first root appears to be indeterminate,
as it is in some other embryos, a fact which is interesting as upholding
the view that it is a mere accessory to the shoot. It is referred by
490 OPHIOGLOSSALES
Bruchmann to the hypobasal hemisphere in O. vulgatum, but to the epi-
basal by Jeffrey in B. virginianum, where the whole hypobasal hemisphere
goes to form the foot. In B. Lunaria Bruchmann found it impossible
to refer it with any certainty to either. These facts, taken together with
a similar uncertainty in the embryos of Equisetum, and the demonstration
in the Lycopods that the root is variable in its point of origin, show
that its indeterminate position is a frequent feature in the embryos of
the strobiloid Pteridophytes, however constant it may appear to be in
the Ferns.
Regarded as a whole, the Ophioglossaceous embryos without suspensor
consist of a simple shoot, of which the polarity becomes apparent relatively
late, but it is of the same nature as that seen in Isoetes, in Equisetum,
and in the Ferns. The apex of the axis, arising in close relation to the
intersection of the epibasal octant walls, is directed to the neck of the
archegonium : the foot occupies the opposite pole, and the root appears
as a lateral, accessory part, of indeterminate position, but of relatively
early origin, and precocious growth.
The other type of embryogeny seen in B. obliquum shows an exactly
inverted polarity : the condition appears to be comparable to that of the
Lycopodiales (excl. Isoetes] : the pole directed towards the neck of the
archegonium becomes the suspensor, while the opposite pole develops
the embryo, having parts quite comparable in position to those, for
instance, of Selaginella spinulosa?- but with an early and strong assertion
of the first root. The importance of this lies in the relaxation which
.such a fact brings from any rigid view of embryonic development : it
seems completely to disprove any morphological predestination attaching
to the primary cleavages of the zygote in the Pteridophytes.
The materials of this discussion may now be drawn together into a
general hypothesis of the morphology of the sporophyte, as it is seen in
the Ophioglossaceae. At the outset it has been, concluded that the some-
what inconstant occurrence of mycorhiza in the sporophyte is not a sufficient
reason for assuming that the family has undergone general reduction : in
the absence of any such preconception the family may be treated com-
paratively as an ascending series, though with the recognition of occasional
reduction. The facts before us are in accord with the following account
of it. The embryo sporophyte achieves an early polarity, marked by the
definition of the stem-apex : the base of the shoot thus initiated is
represented by the foot, or in B. obliquum by the suspensor. The
primary axis thus defined continues its growth, with rare bifurcation,
throughout the life of the stock ; but adventitious or axillary buds (usually
arrested) may be formed, which simply repeat the development of the
primary shoot. The axis bears leaves in spiral or dorsiventral succession,
and they are all of one primitive type, though liable to differentiation.
1 Compare Bruchmann, I.e., Taf. iii., Fig. 63.
SUMMARY 491
The roots appear early : the first root (sometimes precocious and
inordinately developed in accordance with the mycorhizic habit) is
essentially lateral upon the slowly developing axis, and the indefiniteness
of its position, above or below the basal wall, indicates its accessory
character. The whole shoot is, in fact, a rooted strobilus, which remains
usually simple ; but its strobiloid character is disguised by the abbreviation
of the axis, and by the slow succession and relatively large size of its
leaves.
The first leaf of an adventitious bud of O. vulgatum, or the third leaf
in the sexually produced plant, may be fertile : in Botrychium Lunaria
the ninth leaf has been seen to be fertile. Such data, limited as they
are, show a record of early appearance of spore-producing members
unequalled elsewhere. They indicate a high probability that all the
leaves are of the nature of sporophylls, while abortion of the spike, so
frequently seen in various degrees in later leaves, would account for its
absence in those first formed. These may be expanded above ground
(HelminthostachyS) O. pedunculosum, B. virginianuw\ or may be arrested,
and appear as mere scale-leaves. The latter is clearly a consequence of
the underground and saprophytic habit of the prothallus, which diminishes
the necessity of early self-nutrition of the sporophyte, and thus leads to
reduction of the first leaves of the shoot as a purely secondary condition.
On the other hand, the underground habit leads, as already explained,
towards a monophyllous development, with enlargement of the individual
leaf. This is imperfectly realised in the smaller species of Ophioglossum^
which on our hypothesis would be the more primitive; but it appears
typically, though not universally, in the larger-leaved forms. Comparison
combined with biological reasoning indicates, then, that leaf-enlargement
has taken place. The anatomical facts accord with this : the solid or
slightly medullated xylem of the stock widens out upwards into a funnel
or cylinder, with foliar lacunae, where the single leaf trace-strands pass
out : the dilating of the stele follows the increase in size of the leaves in
the individual : this may be held to prefigure that of the race. Probably
the original foliar supply was here, as in the strobiloid forms, a single
strand, and this is still represented by the single bundle of the leaf-
trace. In O. Bergianum the single strand may be seen continued without
branching some distance upwards into the leaf. The branchings which
appear in other species early in its course may on our theory have
followed upon the enlargement and elaboration of the leaf. The Ophio-
glossaceae are phyllosiphonic from the first : but the case of Tmesipteris
has been adduced as showing that a transition may occur from the
cladosiphonic to the phyllosiphonic type : this may occur in any case
where the balance between the axis and the appendage is disturbed, so
as to increase the preponderance of the leaf. On our hypothesis of a
strobiloid origin for the Ophioglossaceae this has been the result of the
stunted development of the axis consequent on the subterranean habit,
492 OPHIOGLOSSALES
and of the enlargement of the leaf culminating in monophylly : both these
factors will have tended towards the dominance of the leaf, and so it is
not surprising that the structure of the shoot should be phyllosiphonic
from the first.
Reasons have been advanced above for not accepting the view of the
fertile spike as a modified pinna, holding a median position. The
alternative is that it is a substantive part not referable in origin to any
vegetative structure previously present. Such substantive parts are seen
in the Psilotaceae and Sphenophylleae, occupying a position corresponding
to that of the Ophioglossaceous spike, viz. the sporangiophores. The
smallest spikes of Ophioglossum or Botrychium are little in advance of
these. From them, by seriation of specimens of the same species of
different ages, and by further seriation of different species, the steps
leading to the most complex forms of spike may be represented : while
its branching, where present, is matched by the increasing complexity of
the sterile leaf. The advance thus contemplated in the spike involves
continued apical growth, and branching, together with growth and septation
of the sporangia. Apical growth of limited duration already exists in the
sporangiophore of Psilotum : the structure of the young spike in Ophio-
glossum, and less clearly that of Botrychium, is such as to be perfectly
compatible with septation, a process for which there are demonstrated
precedents elsewhere. Further, it has been shown that with the growth
and septation of the sporangium the simplest branching of the spike of
Botrychium is very closely allied. In Helminthostachys a further elaboration
is present, which may be referred to the replacement of the sunken
sporangia of Ophioglossum by dense ranks of sporangiophores : and it has
been shown that this mode of origin is reflected in the individual develop-
ment of the sporangiophore. Lastly, the spike, like so many other parts,
is liable to fission or chorisis. The numerous sporangiophores of the
Sphenophyllales seen in some species probably owe their origin to such
chorisis. In Ophioglossum it appears occasionally in common species, such
as O. vulgatum : branching or fission of the spike occurs not uncommonly
in O. pendulum, but in O. palmatum it has become habitual, though there
is reason to think that it is only attained in that species when the plant
becomes fully mature. The various types of spike in the family thus
readily lend themselves to interpretation as an upgrade series
As regards the development of the sporangium the Ophioglossaceae
form a series, from Ophioglossum with its large, ill-defined sporangia to
the larger-leaved Botrychia and Helminthostachys with smaller and more
definite sporangia. It has been shown that in the indefiniteness of
limitation of the sporogenous tissue, and in the absence of a marked
tapetum, Ophioglossum, Tmesipteris, and Psilotum agree more nearly than
other Pteridophytes. It seems highly improbable that such indefinite
characters would be the result of specialisation along parallel lines in two
distinct series. This similarity may more probably be held as indicating
SUMMARY 493
some degree of real affinity, and strongly confirms the initial comparison
of the spike with the sporangiophore of the Sphenophyllales. Lastly, the
anatomical comparison of the Psilotaceae with the Ophioglossaceae has.
shown not only the interesting transition from the cladosiphonic to the
phyllosiphonic structure, but also that in the upper region the wood of
Tmcsipteris is mesoxylic, as it is also in Hdminthostachys, while feeble
secondary development, analogous to that in Ophioglossum and stronger in
Rotrychium, is seen both in Tmesipteris and in Psilotum. These several
characters form a cumulative body of evidence, confirming the comparison
of the shoot and of the sporangiophore in the Sphenophyllales with those
-of the Ophioglossales : the nearest approach among living plants being
between the Psilotaceae and Ophioglossum}
It would thus seem probable that the Ophioglossaceae sprang from
some offshoot of the sporangiophoric Pteridophytes, allied in some degree
to the Sphenophyllales, and possessing early a saprophytic habit of the
underground prothallus. That this encouraged a peculiar specialisation of
the sporophyte, which shared occasionally, though not generally, in the
mycorhizic habit, but not so far as to lead to the cessation of self-
nutrition. That the exigencies of the underground habit were met by an
enlargement of the leaves, culminating finally to the. monophyllous state.
A parallel enlargement of the sporangiophore with that of the leaf was a
natural consequence, since in homosporous forms, as comparison shows,
the spore-output usually marches with the vegetative development. If this
rere so, then the spike would never in its descent have been anything
ier than it is now normally seen to be, viz. a spore-producing part,
riginally of the nature of a sporangiophore, and seated in a median position
the adaxial face of the sporophyll.
Referring in conclusion to the theory of the strobilus, the Ophio-
glossaceae readily conform to it. The shoot, with its rare dichotomous
branching,' appears as a simple strobilus, while the indeterminate position
of the root in the embryo bespeaks the accessory nature of that part
upon it. The axis bears leaves, which are of one order only. The
spore-producing parts appear earlier in the individual life than in any
other group of Pteridophytes, and this indicates a probability that all the
*I wish to state quite explicitly that the homology of the Psilotaceous synangium
•with the Ophioglossaceous spike is no new opinion on my part, though additional and
more detailed evidence is here adduced to support it. It was accepted by me in 1891
(Prof. Roy. Soc., p. 270) and more fully stated in 1893, on the basis of developmental
•evidence (Proc. Roy. Soc., vol. liii., p. 22) : this view has never been relinquished. I
emphasize this here because a passage recently published appears to suggest that I do
not uphold that homology (Scott, Progress us Rei Botanicae, i., p. 163). My position is
unchanged, except in so far as I now include the Sphenophylleae also in the comparison :
the suggestion of this came from Dr. Scott (On Cheirostrobus, Phil. Trans., vol. clxxxix.,
1897, p. 27), and it greatly strengthens the comparison originally drawn by Celakovsky.
There may be differences of opinion as to what morphological rank these parts hold,
•or how ultimately they came into being : these are, however, separate questions from the
recognition of their homology.
494 OPHIOGLOSSALES
leaves were originally fertile. Abortion of the spike, partial or complete,
accounts for its occasional absence, just as in Isoetes. These two types,.
so similar in their embryology, are similar also in the " Selago " condition
seen in their stunted stocks. The one, however, bears a simultaneous
brush of leaves, the other, for reasons biologically intelligible, tends to the
monophyllous habit : this difference is only one of time, not of form or
of relation, and accordingly both types are equally referable to a strobiloid
origin, with enlargement of the leaf, and of the spore-producing part which
it bears.
As regards factors of increase or decrease in number of sporangia,
there may be some difference of opinion according to the view taken of
the family as a whole. In accordance with the conclusion that the
spore-producing spike illustrates an upgrade of development, there would
be recognised as factors of increase, septation with continued apical
growth of the spike, its branching and occasional fission : and in the case
of Helminthostachys a further disintegration of sporangia and enation of
sporangiophores. But there is no interpolation of sporangia so common
a factor in Ferns. As factors of decrease there appear abortion of the
whole spike, abortion of sporangia at the apex, and sometimes also at
points lower on the spike, while a factor to be considered in addition is
the reduction down to one in number of leaves simultaneously expanded.
The factors of increase may in this case be held to have successfully
counterbalanced those of decrease, and the net result is a spore-output
that appears numerically to meet the requirements of the plants, though
their ultimate success in propagation is limited by the exacting conditions
necessary for their germination.
CHAPTER XXXII.
FILICALES.
INTRODUCTION.
the Pteridophytes only the Ferns now remain to be examined. They
mstitute a larger and more varied series than any of those which have
>ne before, and are especially prominent among those living at the
resent day. This, together with the fact that in them the observation of
complete life-cycle was first carried through, and is of all the
'teridophytes most easily followed, has given to them a peculiar position,
'he present-day Ferns have undoubtedly been appraised beyond their
leserts as factors in the story of descent. It will be well at the outset
consider how they stand at the moment in the light of such knowledge
we possess of the vegetation of the past, and to compare their present
>sition with the former estimates.
We have seen that the recognition of the main incidents of the life-
:le in a Leptosporangiate Fern was completed by Suminski in 1848,
id it was found shortly after by Hofmeister, that the same scheme
)incided in essentials with that of other Pteridophytes. Further com-
parison of the organs of propagation, and especially of the sporangia,
disclosed the fact that those of the Leptosporangiate Ferns were structurally
the simplest. In accordance with evolutionary views which became
prevalent about the same, time, the general assumption was made that
the simplest organisms were those which were also earliest in descent,
and that from them all the more complex were derived. On this founda-
tion a superstructure of phylogeny was raised. In accordance with these
views it became necessary to express the large and complex sporangia
of the Lycopods or Ophioglossaceae in terms of those of the Leptosporangiate
Ferns : this was effected through the theory of the sporocyst.1 It was
held that by fusion of numerous small sporangia, and elimination of their
individual identity the large sporangia of the Ophioglossaceae were
produced : by reduction of the whole spike the Lycopod sporangium ;
1 Strasburger, Bot. Zeit., 1873, No. 6.
496 FILICALES
and finally by contraction of the whole Lycopod strobilus the synangium
of the Psilotaceae. On the other hand, the origin of the simple Lepto-
sporangiate sporangium was traced on the theory of Prantl, through the
Hymenophyllaceae, directly from the Moss-sporogonium ; the sorus of
Hymenophyllum was held to correspond to the opened capsule.1 These
were doubtless extreme opinions of the time, but they show the position
assigned to the Leptosporangiate Ferns in the discussions of a generation
ago. These plants were regarded as the primitive Pteridophytes, and other
forms as having been derived from them, while reduction was held to
have been a general factor in the process.
The revolt against this position was initiated by Campbell,2 who
definitely gave precedence to the Eusporangiate types. Over and above
the difficulties of comparison already felt, there loomed large the
impossibility of harmonising a belief in the Leptosporangiate Ferns as
primitive with the growing knowledge of Palaeophytology. The dearth
of evidence, even of the existence of true Leptosporangiates comparable
to those of the present day in Palaeozoic times, was pointed out : at
the same time the existence of numerous fossils then believed to be
rightly referred to the Marattiaceous affinity, indicated a priority of the
Eusporangiate type. The comparative study of development of the vegeta-
tive organs and of the sporangium had meanwhile been actively pursued :
on the basis of such facts it came to be held as probable that the more
delicate structure seen in the- Leptosporangiate Ferns was not itself
primitive, but resulted from progressive specialisation.3 With the adoption
of such a view the theories of Strasburger and of Prantl fell away, and
the ground was open for recognising the Eusporangiate type, whether of
Ferns or of other Pteridophytes, as of prior existence.
As a consequence, the Marattiaceous type of Ferns was believed to
be the prevalent megaphyllous constituent of the Palaeozoic Flora. The
sporangial structure, as well as the construction of the sori in many forms,
agreed more or less nearly with that conclusion. The Lycopodiales,
Equisetales, and Sphenophyllales, however, were held to constitute separate
Eusporangiate phyla, there being no need to refer them to a Filicineous
origin. The next step affecting the early history of the Fern-phylum was
the discovery that certain of those fossils which had been held to be
true Ferns of the usual homosporous type were in reality Seed-bearing
Plants, the male sporangia of which had been taken for the fructifications
of a homosporous nature. The removal of such forms to the newly con-
stituted Class of Pteridosperms has perhaps only commenced, and it is
still impossible to say for certain how many of the fossils bearing like
fructifications may follow. The question is thus raised, what is the
residuum of true Ferns that actually remains among the Palaeozoic
1 Die Hymenophyllaceen, Leipzig, 1875.
*Bot. Gaz., Jan. 1890, and Dec. 1891. See also Bower, Ann. of Bot., 1891, p. 127.
3 Ann. of Sot., 1889, p. 305, and 1891, p. 127.
INTRODUCTION 497
fossils? It will be shown in detail" below that at least three types, which
may reasonably be held to have been true Ferns, were represented in the
Primary rocks, viz. the Botryopterideae, the Pecopterids of the group
Cyatheites of Goeppert,1 and also certain forms allied to some of the
lowest Leptosporangiates, though there is some room for doubt how
nearly they coincided with these.
On the question of detailed proof of the homosporous nature of these
plants the reply for a given case has been supplied by Scott. He has
found in the sporangia of Stauropteris Oldhamia, a fossil referred to the
Botryopterideae, that the spores may germinate within the sporangium,
just as they may be found to do in Todea, Trichomanes, and some other
living homosporous Ferns. This leaves little room for doubt that the
mode of reproduction of Stauropteris Oldhamia was essentially that of a
true Fern.2 But it is not to be expected that such evidence will be
available in every case : nor indeed should it be considered necessary.
The fact that such proof is accessible, even in a single instance, comes
as a wholesome corrective to that tendency, which followed on the first
discovery of Pteridosperms, to regard all Palaeozoic Ferns as potential
Seed-Plants. The converse will, however, be the more natural position
for those who view the new facts calmly, viz. to hold all Fern-like fossils
as true Ferns until their character as Pteridosperms is proved. The
question is mainly one of the state of advancement reached by any given
fossil, for it may be presumed that the Pteridosperms sprang ultimately
from a homosporous Fern-like ancestry. The onus probandi lies with
those who are disposed to accord to any given fossil the more advanced
position, however readily others will accept the proof as it becomes
available. On this footing the Pecopterids, as limited above, together
with the Botryopterids, and some others, may for the present be held
to be Palaeozoic Ferns of the homosporous type, of which the life-history
was in all probability essentially the same as that seen in modern
Ferns. The early existence of homosporous Ferns, which evolutionary
theory would suggest, or even demand, appears on the basis of Palaeo-
phytological evidence to be beyond any reasonable doubt. But they
are now recognised as bulking less largely in the early Flora than was
once believed to be the case.
According to the arrangement and succession of development of their
sporangia the homosporous Ferns have been divided into three series : 3
the Simplices, in which the sporangia of a sorus are produced simultaneously :
the Gradatae, in which there is a definite succession in time and space
in their production : and the Mixtae, in which there is a succession in
time, but no regular succession in space. These three types appeared
successively in geological time : the Simplices were the characteristic Ferns
of the primary rocks, though many of that type still survive : the Mixtae
1 Syst. Filic. Foss., 1836, p. 319. -New Phytologist, vol. v., p. 170.
3 "Studies," iv., Phil. Trans., Vol. 192 (1899), p. 122.
2 I
498
FILICALES
are the dominant Ferns of the present day, while the Gradatae take a j
middle place. This succession will be maintained in the detailed account <
of the several families, and consequently the description will follow in
the main, though not in exact detail, the order of appearance of the
several families of Homosporous Ferns upon the earth's surface. The
order in which they will be taken up will be as follows :
Simplices •<
Botryopterideae.
Marattiaceae (together with many Pecopterids).
Osmundaceae.
Schizaeaceae [Marsiliaceae] ?
Gleicheniaceae.
^Matonineae.
fLoxsomaceae.
Hymenophyllaceae.
Gradatae { Dicksonieae (excluding certain genera).
Dennstaedtiinae.
^.Cyatheaceae [Salviniaceae] ?
IDennstaedtia-Davallia series. '
Onoclea-Woodsia series.
Matonia-Dipteris series.
Pterideae and other Polypodiaceae
Mixtae
BOTRYOPTERIDEAK.1
The organisms grouped under this name occur as Palaeozoic fossils
extending upwards to the Permian.2 Though they are distinct frorr
any other known family of Ferns, still there is no reason to doubt theii
Fern-nature : its recognition is based not only upon the external characters
of the shoot, with the usual circinate vernation of the leaves, but als(
upon the anatomical details of axis and leaf, and upon the fact that tl
numerous sporangia are borne upon the distal region of the repeatec
pinnate sporophylls. Finally, in Stauropteris Oldhamia Scott has sh<N
that the spores possessed the capacity for germination within
sporangium, as in some modern Ferns.
The plants had an erect shoot of radial construction : it was sometii
short, with closely aggregated leaves, as in Grammatopteris RigolL
1 The materials for this description have been derived in the main from Rena
Bassin Houiller et Permien d'Autnn et d'Epinac, ii., p. 33, etc. ; Scott, Studies, p.
etc. ; Stenzel, Bibliotheca Botanica, 1889, No. 12 ; Scott, Progresses Rei. Bot., i., p. i
I have also had the advantage of comparing specimens, chiefly those belonging to
Kidston.
-Mr. Kidston has shown me a Botryopterid (B. antiijna) from the Petticur P>ed
with axis and leaf bases showing structure. This he reg;uds as probably the earlie
record of a Botryopleris.
BOTRYOPTKRIDEAE 499
B.R. (Fig. 269), a condition comparable with that seen in a modern
Osmunda : but in other cases the axis was more elongated, and the
leaf-arrangement less dense, as in Zygopteris Grayi, where there is a two-
fifths divergence (Fig. 270), or in species of Botryopteris, where the
leaf-arrangement appears to have been more lax still. From the axis,
which was often thin in proportion to the more robust leaves which it
bears, sprang also numerous adventitious roots (r, Fig. 270): these seem
to have acted as oblique prop-like supports where the axis was elongated.
In some cases at least axillary buds arise in the axils of the leaves, very
much as they do in some modern Hymenophyllaceae.
The leaves themselves were repeatedly branched, the pinnae arising
alternately from the rachis, and being themselves further branched.
FIG. 269.
Botryopteris Rigolloti, B. Renault. Transverse section of the central part of a stem :
within the axis lies the solid vascular cylinder (a) surrounded by a continuous band of
bast ; i he cortex (/>) is traversed by vascular strands (c) passing to the leaves ; d~ petioles
surrounding the stem. Communaux de Saint-Martin. (After Renault.)
Aphlebiae have been described on the leaves of both British and
Continental specimens. The leaves were of a finely divided Sphenopterid
type: in the sterile leaves the ultimate segments widened out into fan-
like expansions : in the fertile regions the segments remained narrow, and
upon the ultimate branchlets the large, pear-shaped sporangia were borne
in distally directed tassels, or in some cases solitary.
Fortunately the internal structure is fairly well known in several distinct
types of the family, and generic characters have been based upon the
differences recognised. The simplest, and for comparative purposes pro-
bably the most important type, is that shown by Grammatopteris Rigolloti
(Fig. 269), where there is in the axis a solid xylem-core, with the smallest
tracheides at the periphery. Round this is an exiguous phloem, and a
broad outer cortex. In the latter are embedded numerous leaf-trace
bundles on their way out to the crowded leaves : their structure is simpler
than in others of the family, the prominent feature being a strap-shaped
500 FILICALES
xylem band, flattened on its inner and outer faces. These strands branch
off from the central stele with the minimum of disturbance, after the
manner of the strobiloid Pteridophytes. A similarly simple origin of the
leaf-trace has been seen also in species of Botryopteris, in which the stele
is as little differentiated as in Grammatopteris : there is indeed an entire
absence of well-marked protoxylem in the stele of Botryopteris. In
Zygopteris also the origin of the leaf-trace is essentially the same, though
here the matter is complicated by the curious differentiation of the
xylem of the stele : there is an outer band consisting of larger, scalariform
DC I
see
FIG. 270.
Zygopteris Grayi. Transverse section of stele, showing wood and remains of phloem.
1-5 the' five angles of the wood, from which leaf- traces are given off, in order of the
phyllotaxis, No. 5 belonging to the lowest of the series, x, principal ring of xylem ;
jrz, small tracheides of internal xylem ; j;e, small trachetdes at periphery ; //;, phloem ;
r, base of adventitious root. X 14. Will. Coll., 1919, B. (From Scott's Studies in Fossil
Botany.)
tracheides, and a central core consisting of parenchyma together with a
system of smaller tracheides : both of these contribute to the strand of
the leaf-trace, which is abstricted off from the ray-like projections of the
cauline stele (Fig. 270). A new genus from the lower Coal Measures
has recently been described by Scott,1 which is characterised by radially j
seriated wood, apparently of a secondary character : in other respects it
had much in common with Zygopteris. This is the first evidence of
secondary thickening in the Botryopterideae : the fossil has been named
Botrychioxylon \ but as the sporophylls have not yet been described,
this very allusive name must be understood only to convey the fact that i
it is a Botryopterid showing secondary growth, just as Botrychium is an
ljonrn. fi. Micr. Soc., 1906, p. 519.
BOTRVOPTERIDEAE 501
Ophioglossaceous plant showing secondary thickening. Other axes are
known, which are probably of this affinity, such as Tubicaulis, Anachoropteris,
and Asterochloena : they show various modifications of the protostelic
state. From these, as well as from the better known Botryopterideae, it
is clear that a considerable series of Ferns existed in the Palaeozoic
period which had a solid protostele, or some slight modification of it :
their leaf-traces consisted of a single strand, and were given off without
those profound disturbances of the cauline system characteristic of the
" phyllosiphonic " type of Jeffrey.
In Grammatopteris the vascular strand of the petiole was simple in
outline, as seen in the transverse section. But in other Botryopterids it
assumed highly complex forms, showing in some cases a tendency to
radial organisation (Stauropteris) : it is upon these that generic distinctions
have been based. It is unnecessary here to follow out the structural
details : it suffices to state that the relatively bulky petioles were cylindrical
in form, and gave off pinnae laterally ; while the upper regions have in
some cases been seen to have the circinate vernation, and to be covered
while young by a felt of peculiar hairs, as is the case in the more
primitive types of modern Ferns.1
The sporangia are known in Botryopteris, Zygopteris, Grammatopteris^
and Stauropteris : the latter genus is now recognised as a member of
the Botryopterideae, and it will be taken first.2 Its sporangia have been
found connected with the petiole known as Rachiopteris Oldhamia, Will,
and are borne terminally on the finest branches of the rachis (Fig. 271).
Their form is nearly spherical : the wall consisted of a superficial layer of
larger cells, succeeded internally by several layers of smaller cells : no
annulus has been observed, and the dehiscence is by a pore at the
distal end. The spores are numerous : a moderate estimate, based upon
the sections, would be 500 to 1000 for a single sporangium. It was in
1 Since the above was written the publication of Tansley's Lectures on the Evolution
of the Filicinean Vascular System has commenced (New Phytologist, 1907). He advocates
a theory of origin of the leaf in Ferns by differentiation of a dichotomous branch-system
to conslitute axis and leaf, and adduces in connection with it many anatomical facts
relating to the Botryopterideae. It is impossible here to review these facts in detail :
it may, however, be stated that there appears to me to be nothing in them inconsistent
with the leaf having been throughout a lateral member. If such a lateral member
developed to a large size, it is to be anticipated that it should assimilate structurally to
the axis in its lower parts, as it is seen to do in the Botryopterideae. There is no need
to assume that it should retain constantly its dorsiventral character : the tendency to radial
organisation seen in Stauropteris and some others is interesting, but not in any way
decisive in the absence of all evidence how the leaf actually developed in relation to its
axis in these ancient forms. Positively, however, we know that in the nearest living
relatives (Hymenophyllaceae and Osmundaceae) the leaf does originate laterally on the
axis. The question will be whether surmises based on observation of the mature structure
in certain imperfectly known fossils are to take precedence of direct observations of
development in living plants.
2 See Scott, New Phytologist, 1904, p. 18, 1905, p. 114, and 1906, p. 170.
502
FILICALES
this species that the incipient stages of germination have been observed
within the sporangium, a fact held to show its Fern-nature, while its
other characters relate it to the Botryopterideae.
The sporangia have been successfully observed in Zygopteris by Renault
(Fig. 272). They were borne in groups on the ends of the pinnules,
and were pear-shaped and slightly curved. The stalk, though elongated,
was fairly robust, and widened gradually into the sporangial head. The
latter was composed of at least two layers of cells, the inner of which was
FIG. 271.
Stauropleris Oldhamia, Binney. A— sporangium in nearly median section, attached
terminally to an ultimate branchlet of the rachis ; $£ = stomium. Scott. Coll., 2213.
B = sporangium in tangential section attached to a short piece of a branchlet. Scott.
Coll., 2207. C= sporangium with wall burst attached as before. / = palisade tissue of
branchlet. Scott. Coll., 2219. All figures X about 50. (From sketches by Mrs. D. H.
Scott. The specimens are from Shore, Littleborough, Lanes.).
transient, while the outer remains as the mature sporangial wall. This
is differentiated to form the annulus, which appears as a broad band
composed of several rows of deep cells, with indurated walls, and ran
along either side of the sporangium from base to apex; the remainder
of the wall is composed of smaller, elongated cells. The mechanical
annulus thus composed of several rows of cells, forming a broad marginal
band or hoop, resembles the similar structure seen in the sporangium
of Angiopteris (see below, pp. 515-16). The spores are numerous : a rough
estimate from the transverse and longitudinal sections drawn by Renault
points to an output of 500 to 1000 spores in each full-sized sporangium :
but there is considerable variation in the dimensions of the sporangia.
BOTRYOPTERIDKAE
503
The spores are of approximately uniform size, and the plant appears to
have been homosporous.
The sporangia of Botryopteris forensis have also been observed :
they are of smaller size, and have the broad annulus on one side only.
But in other respects they resemble those of Zygopteris.
FIG. 272.
Zygopteris, sj>. i, group of four sporangia on a common pedicel (a). X 10. 2, two
sporangia on pedicel. The upper shows the annulus (c) in surface view, with spores
exposed at/"; the lower in section. X2o. 2 bis, sporangium cut in plane of annulus.
3, group of sporangia in transverse section. X2O. Lettering common to the figures,
a, common peduncle ; b, sporangial wall ; c, annulus ; e, tapetum (?) ; f, spores ; m,
pedicel of individual sporangium ; «, probable place of dehiscence. All after Renault.
(From Scott's Studies in Fossil Botany.)
A particularly interesting fructification attributed to this affinity is
that described as Corynepteris (Fig. 273). Hitherto it is known only
in the form of impressions. The leaves were of the Sphenopteroid, or
Pecopteroid type, and the pinnules bore each a single sorus, of five
to ten sporangia, grouped round a common centre. The annulus is
here again a broad lateral band, consisting of several cell-rows : as the
sporangia are grouped in the sorus the annulus of each sporangium is
in juxtaposition with that of its next neighbour, a condition not unlike
504 FILICALES
that of the group of sporangia of Zygopferis shown in transverse section,
which thus appear to constitute a radiate sorus. (Figs. 272-3). Whether or
not this is a constant feature in the latter Fern, it is clear that Corynepteris
shows a sorus strongly suggestive of the arrangement in the Marattiaceae,
a comparison already suggested by Scott.1
The Botryopterideae have been recognised as a synthetic group,
combining the characters of several known series of Ferns. The reasons
for this opinion are to be found, first in their anatomical structure, and
secondly in their sporangia. Though the leaves were relatively large,
and much branched, and the petiolar structure often complex, the
regularly radial axis remains relatively small, and its vascular structure
exceedingly simple. The common occurrence of the undifferentiated
protostele is regarded as a primitive character : added to this is the
ir
FIG. 273.
A — Corynepteris Essenghi, Andrae (sp\ from the Westphalian. Fragment of a fertile
pinna. X6. B = Coryn. coralloides, Gutbier (sp\ from the Westphalian. Fragment of
a fertile pinna. X 4. Bl = sorus of the same species seen laterally. X 28. (After Zeiller.)
simple origin of the leaf-trace from it. In both these characters the
Botryopterideae compare with the simpler, strobiloid Pteridophyta, rather
than with the more advanced Ferns. The sporangia are clearly of the
Eusporangiate type, as evidenced by their massive stalk, relatively thick
wall, broad non-specialised annulus, and the large spore-output. But
their arrangement has not usually been recognised as being in definite
sori ; it is possible, however, that the terminal tassels of sporangia may
have had some degree of regularity of orientation, such as is certainly
suggested by Fig. 272-3. A slightly closer grouping of them together,
coupled with a definite orientation such as that seen in Corynepteris,
leads naturally to the type of sorus prevalent among the Marattiaceae.
These Ferns diverge, however, very widely from the Botryopterideae in
their mature anatomy. It will be seen later that on this point interesting
comparisons may be drawn between the Botryopterideae and the
Osmundaceae and Hymenophyllaceae, which are held to be relatively
primitive groups, while they occur very early in the geological record.
1L.c., p. 291.
CHAPTER XXXIII
MARATTIACEAE.
THIS family is represented by five genera of living homosporous Ferns,
viz. AngiopteriS) Archangiopteris, Marattia, Danaea, and Kanlfussia. The
characters of all the five genera are now well known, so that they form
a sound basis for comparison with the fossils. A number of these, dating
back to the Palaeozoic period, show strong similarity to the modem
forms, both as regards anatomy and the characters of the sorus. Their
existence indicates that the Marattiaceous type has been a very ancient
one. The natural course will be first to consider the living Marattiaceae :
we shall then proceed to compare with them their fossil correlatives.
EXTERNAL CHARACTERS.
The erect stock in Angiopteris, Marattia, and Archangiopteris is
relatively short, massive, and unbranched : it is of the radial type, and
is entirely covered by the persistent bases of the crowded leaves
(Fig. 274). It continues directly the radial symmetry initiated in the
embryo, which is probably a primitive condition. Some species of
Danaea have also an erect radial shoot (e.g. D. simplicifolid) : others
show at first an erect position and radial construction, but it passes
over gradually to an oblique position, with distichous arrangement of
the leaves (D. alata, Fig. 275). Kaulfussia, on the other hand, is
strongly dorsiventral, its rhizome showing distinct internodes, and being
horizontal, while the leaves alternate obliquely on its upper surface
(Fig. 276). It seems a natural interpretation of the facts to hold that
in the last named Ferns the primitive radial and erect type of the shoot
has been relinquished in favour of the derivative creeping habit, which
goes along with its diminished bulk and greater elongation: in fact, the
case is similar to that seen in the Ophioglossaceae, where also in Helmin-
thostachys the primitive shoot with its massive stock has given way to a
more elongated but creeping rhizome. (See Chapter XVI.)
5o6
FILICALES
The leaves are produced in the usual acropetal order, and show great
diversity of outline, though conforming to a common type. The leaf-base
in all cases bears stipular enlargements laterally, which are connected
across the adaxial face of the petiole by a transverse commissure (Fig. 276).
Though these are characteristic for all the Marattiaceae when mature,
they are absent from the first, and often from the second leaf of the
seedling. They remain persistent after the upper leaf decays, in close
relation to the smooth scar which marks its attachment.
FIG. 274.
Angiopleris Teysmanniana, de Vriese. A= habit of a small plant, reduced to one-
twentieth ; 5 = part of a pinna, natural size. (From Bitter, in Engler and Prantl, Nat.
PJlanzenfam.}
The upper leaf of the living genera varies considerably. The base
of the leaf-stalk, and often the bases of the pinnae also, bear fleshy
swellings or pulvini : here the stalk breaks on decay, leaving a clean scar,
as above noted. The texture of the leaf is usually leathery, but Danaea
trichomanoides shows a thin and almost filmy character of the foliage,
in obvious adaptation to its moist habitat. The leaf may be simply
ovate, with marked midrib and acuminate apex, as in D. simplicifolia
(Fig. 277): or it may be simply pinnate, as in D. alata (Fig. 275),
or Archangiopteris : or the pinnation may be repeated, as in Angiopteris
(Fig. 274), or Marattia. In large plants the leaf may in the latter attain a
high complexity of branching, while its length may be as much as fifteen
feet. In Kaulfussia the outline of the leaf differs from all the rest :
MARATTIACEAE
507
the long petiole bears five palmatelyk disposed lobes, of broadly lanceolate
form, with a general similarity of outline to the leaf of the Horse Chestnut
(Fig. 278, D). The venation, which is simple in other genera, of the
Neuropterid, Pecopterid, or Taeniopterid types, is more complex in
Kaulfussia^ approaching that of the Drynaria-type.
The roots originate internally close beneath the growing point of the
stem (Fig. 279): in simple cases there may be one root to each leaf,
FIG. 275.
A small plant of DaAnea alata. X \. st ''= stipules. (After Campbell.)
but in strong plants the roots are more numerous. They take a course
obliquely downwards through the tissue of the stock, finally issuing as
robust roots which branch monopodially.
Among Palaeozoic fossils radially constructed stems of greater length,
but showing strong analogies with the stems of modern Marattiaceae,
have long been known under the name of Caulopteris, when the external
surface is seen in the form of impressions ; or of Psaronius when the
internal structure is preserved. Among other points of similarity which
they show, the roots may be found traversing the cortex of these stems
508
FILICALES
f
in the same way as in Marattia or Angiopteris. Their reference to
a Marattiaceous affinity has been further confirmed by the proof of their
relations with Pecopterid foliage, bear-
ing characteristic fructifications.1 Such
stems were not only of arborescent
stature, but also of considerable thick-
ness. The leaves were in some cases
distichous (Megaphyton), in others
tetrastichous, in others again spiral
on a more complex plan : the latter
correspond more nearly to the leaf-
arrangement seen in the living genera.
The general character of such stems
is suggested by Fig. 280. The con-
clusion seems clear that certain Fern-
FIG. 276.
Diagrammatic representation of the end of a
rhizome of Kaulfussia. w = wings of stipule;
co m = transverse commissure. (Af
Vaughan. )
fter Gwynne-
like plants, of Marattiaceous affinity,
but of more aborescent habit than the
living Marattiaceae, existed in Palaeozoic times, a conclusion which
borne out by the study of the leaves and fructifications.
s
A.
B.
FIG. 277.
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 Angiopteris evecta
with young sporophyte. (A, B, after Brebner ; C, after Farmer, from Campbell's Mosses
and Ferns. )
The latter in the modern genera are always intra-marginal, on the
lower surface of the leaf : the sori are distinct, seated each upon a vein.
1 Zeiller, Element 's, p. 1 20.
MARATTIACEAE
509
FIG. 278.
Pinnae of the five genera of the Marattiaceae, all of them lateral pinnae. A — Angiop-
teris crassipes, Wall. ; B^Archangiopteris Henryi, Christ and Giesen. ; C = Marattta
fraxinae, Sm. ; D = Danaea (esculifolia, Bl. ; E — Danaea elliptica, Sm. A, C, D, JS
after Bitter ; B after Christ and Giesenhagen. (From Engler and Prantl, Nat PJfanzenfant.)
In all of them, except in Kaulfussia, and occasionally in Danaea, they
are disposed in a single series on either side of the midrib : they may
be short and compact, and be seated near to the margin, as in Marattia
5io
FILICALES
and Angiopteris (Fig. 278 A and c) : or they may be more extended along
the veins, as in Archangiopteris (Fig. 278 B) : or they may occupy the
whole space from margin to midrib, as in Danaea (Fig. 278 E). But in
all these oases the disposition of the sorus is essentially the same, and
the differences those of detail only. In Kaulfussia, however, the sori are
dotted over the broad under-surface without apparent order, a condition
FIG. 279.
Vertical longitudinal section of the stem of a young plant of A ngiopteris evecta. b = the
youngest leaves still quite covered up by the stipules, nb ; si — stalk of an unfolded leaf
with its stipula, nb ; «, everywhere the leaf-scars on the basal portions/^ from which the
leaf-stalks have separated ; c, c, the commissures of the stipules in longitudinal section ;
•w, zy, the roots. Natural size. (After Sachs, from Goebel's Outlines.')
which appears widely different from the rest (Fig. 278 D) : but comparison
of leaves of Danaea, and especially of those which are only partially
fertile, gives the clue to an explanation ; for there the normally elongated
sori are found to show occasional fissions, and the partial sori, with
circular outline like those of Kaulfussia, appear isolated upon the enlarged
leaf-surface (Fig. 281 a, b, c). It seems probable that the condition of
Kaulfussia was acquired during descent in some such way as this, on the
gradually broadening leaf. The frequent occurrence of sori showing similar
MARATTIACEAE 511
fissions in Kaulfussia supports this vtew (Fig. 281 a-e, lower series). Thus
the apparently aberrant genus may be brought into line with the rest. It
will be seen later that this spreading
of the sori over an enlarged leaf-
surface has its parallels among the
Leptosporangiate Ferns also.
The normal arrangement with
one row of sori on either side of
the midrib corresponds to what is
frequently seen in the fossils which
are referred to this affinity : as
examples Asterotheca, • Scolecopteris,
and Ptychocarpus may be quoted
(Fig. 282, also Fig. 288 A). In
all of these, though the pinnules
are small, the arrangement of the
sori is on the same plan. But FlG 28o
among the Fern-like plants of the
Diagrammatic view of a trunk of a tern from the
PalaeOZOlC age many Other arransre- Coal> showing above the external cortex with petiolar
scars (Caulopteris), and below the woody cylinder
meiltS OCCUr which appear tO have with scars corresponding to the foliar strands, and
their sclerotic sheaths (Pty 'chop tens). Reduced to
no near Correlative among living J natural size. (After Zeiller).
Ferns.
The examples here chosen from among the fossils are those in which
there is at present no reason to doubt the homosporous Fern-character.
It is possible that some of them may ultimately be shown to be Seed-
FIG. 281.
a, 6, c (above), Danaea data, Smith. a = a fertile pinna with many normal sori: the
arrow indicates an abnormal fission ; b, c, show more numerous abnormal fissions, resulting
in irregularly formed sori, distributed over a slightly enlarged leaf-surface. X2. a, c
(below), sori of Kaulfussia at-sculifolia, Blume, showing states of partial or complete"
ahstriction.
Plants, and the sori to represent groups of pollen-sacs. But even if they
were, the structural similarities would remain, and they would then only
strengthen the opinion that the Pteridosperms had an ultimate origin in
a Fern-like ancestry.
512
FILICALES
SPORE-PRODUCING MEMBERS.
The sorus of the modern Marattiaceae is strictly circumscribed, and
has no definite indusium : it is true that certain hairs round its periphery
in Angiopteris have been thus described, but they hardly deserve such
recognition (Fig. 283 u, D): also in Danaea the tissue of the leaf grows up
between and partly envelops the sori where they are in close juxtaposition,
and the growth has been called an' indusium, but this use of the term
is open to question (Fig. 283 K). The sori are all constructed on a plan
which may be described as radiate, and uniseriate, for a single series of
sporangia are disposed in a radiate fashion round a central attachment.
When the sorus is circular, as in Kaulfussia, the attachment is at a central
point : when elongated, as
in Danaea, the attach-
ment is linear. All the
sporangia of a sorus ori-
ginate simultaneously, a
character which is general
for the Simplices, The
sporangia themselves may
be separate, or united into
synangia : they are massive,
with a broad base of in-
sertion, and each produces
FIG. 282.
B, Scolecopteris elegans, Zenker, from the low*- Permian. a large Output of
A= transverse section of a fertile pinnule enlarged (after Zenker).
morpha, Brongn, from the Stephanian, longi-
fertile pinnule enlarged (after Grand' Eury).
B^Scolecopteri
tudinal section
The dehlSCCttCe IS in all
.
C, D, E, Asterotheca. C = Asterotheca Miltoni, Astis, from the CaSCS by a Silt Or pore,
Westphalian : fertile pinnules. X2. Z? = synangium of Asterotheca. . ,.
X about 6. £ = longitudinal section of a pinnule of A sterotheca, in a median position at
traversing three synangia, enlarged. (After Grand' Eury, from ,, j- . i j -u
7jt\Vitx'& Paiaebotanique.) the distal end, or on the
oblique inner face^ of eacl
sporangium : there may be differences of the opening mechanism, but th<
plan of dehiscence is the same in them all.
The structure of the mature sori of the five genera is illustrated ii
Fig. 283. Figs. A and E represent the sori of Angiopteris and Marattia
the plan of them is clearly the same, the difference being that in tl
former the sporangia are separate, in the latter they are fused into
synangium, which is of firm, almost woody texture. Each sporangium ii
either case opens by a slit on the oblique inner face. The questioi
will be considered later whether the synangial condition or that witl
separate sporangia is probably the more primitive : meanwhile, as regards
the general character of the sorus, these genera may be regarded as centi
types in the family, while the rest of the -genera are probably derivative
Thus an elongation of the sorus of Angiopteris, so that it occupies
considerable length of each vein, would give the condition seen ii
Archangiopteris (Fig. 283 c, D). It is probable that this is the correct
MARATTIACEAE
513
view of its origin, since its sporangial structure is closely similar to that
of Angiopteris, while such elongated sori are absent from the corresponding
fossil types. A similar elongation of a synangial sorus of the Marattia-
type, so as to extend the full length of the vein, would give the condition
seen in Danaea (Fig. 283 j, K, or better in Fig. 286 A) ; here also the
sporangia are disposed as in Marattia, but differing in the minor fact
,t-
G +5
K
FIG. 283.
Sori and sporangia. A, B — Angiopteris crassipes, Wall, A= sorus. .5 = two
sporangia, one in surface view from without, the other cut longitudinally. C, D = Arch-
angioptcris Henryi, Christ et Giesen. C = sorus. D = two sporangia in section. /?, f=
Marattia fraxinea, Sm. E — synangium ; ^"^thesame in section. G, H = Kaulfussia
aesculifolia, Bl. G — part of 'the lamina seen from below, with three synangia ; the
numerous small circles on the leaf-surface are stomata. // = section through a synangium.
y, K= Danaea elliptica, Sm. y = two synangia; K= section through a synangium.
(A, B, E, G, J after Bitter. C, D after Christ and Giesenhagen. F, H, K after
Hooker-Baker. From Engler and Prantl, Nat. PJJanzenfam.').
that the valley between the rows of them is almost levelled up, and the
sorus thus forms a solid cake. It will be shown shortly that developmental
details support the view that such elongation has actually taken place,
while the fact is worthy of note that the length of the sorus varies greatly
in different species of Danaea. And lastly, in Kaulfussia the synangial
association of the sporangia together in the circular sorus is on the plan of
Marattia and Danaea (Fig. 283 G, H). The origin of the numerous sori
by fission from the Da/mea-type as the lamina expanded has already been
2 K
514 FILICALES
suggested, and structurally the sorus itself offers no difficulty (compare
Figs. 283 H and K). It is thus seen that the general type of sorus is
constant in the family : the chief differences lie in the mode of associa-
tion of the sporangia, and in the extent and fission of the sori.
As the development of the individual sporangium has been found to
be essentially the same in the several genera, notwithstanding the difference
that exists between the synangial state and that where the sporangia are
separate, it will suffice to describe it for one only, and Angiopteris, with
which Archangiopteris shows close similarity, may be selected as being
the most familiar.1 At an early stage the sporangia begin to project as
separate upgrowths; but it is impossible, from a study of superficial sections,
to detect any regular system of segmentation which is maintained in all
sporangia; a comparison of the four sporangia, shown in Fig. 284 A ii
surface view from above, discloses no regular sequence of segmentations
and the cell-groups which will develop into the sporangia appear con-
sequently ill-defined. Of the sporangia a, b, c, d, shown, that mark(
(b) is believed to be the most regular and usual type ; and the cell
shaded in it are evidently sister cells, derived from a single parent cell
which, as we shall see, gives rise to the central and essential part of
the sporangium ; we may call this, as in the other genera, the superficial
parent cell. If a section were taken along a line x — x through such a
sporangium, after it had grown more convex, it would appear as Fig. 284 B,
in which the cells shaded are believed to correspond to those shaded
in Fig. 284 A. It becomes apparent from such sections as these that
a single cell, the central cell, had divided periclinally to form an inner
cell and a superficial one ; the former is the archesporium, and has in
Fig. 284 B already divided into two ; the superficial cell has also divided
repeatedly. Though we may thus select sections so as to represent a
reasonably regular and typical structure of the young sporangium, it is
clear, from Fig. 284 A, that what has been described is only a central
type, and as a matter of fact hardly any two sporangia show exactly the
same details of segmentation. As development proceeds, growth and
cell-division often continue with sufficient regularity to allow the genetic
grouping of the tissues to be clearly followed (Fig. 284 c). Meanwhile,
certain cells at the apex enlarge to form the crest-like annulus. The
relation of this to the main lines limiting the product of the superficial
parent cell is variable ; a common case is that shown in the figure, where
the middle line (x) coincides with the limit of the annulus ; but this is
by no means constant : from this point (x) downwards, on the central
side of the sporangium, the de'hiscence will take place. The sporogenous
group is now clearly defined by the character of its protoplasmic body,
and it forms a definite block of cells, referable to a single parent. Next
follows the change of the cells immediately surrounding the sporogenous
1 Compare "Studies," iii., Phil. Trans., 1897, where more full details are given for
this and other genera.
MARATTIACEAE
515
group, to the character of tapeturhx (Fig. 284 E), and the rule appears
to be that the whole tapetum is extra-archesporial in its origin. The
figure represents in radial section a sporangium which has arrived at the
FIG. 284.
Angiopteris ezwcta, Hoffm. A =part of a young sorus seen in surface view from with-
out. B = vertical (radial) section of a sporangium such as would be seen on cutting the
sporangium (6) in Fig. A along the line indicated. C = vertical section of an older
sporangium, showing genetic grouping of cells. Z> = apex of an almost mature sporangium
seen from above ; such a section as along the line .r, x, in Fig. E. E— vertical section of
a sporangium with spore-mother-cells ; the tapetum is marked .r. F— transverse section
of an almost mature sporangium. All X2oo.
stage of complete division of the sporogenous mass, and in which the
spore-mother-cells are about to separate and round themselves off, prior
to the tetrad-division.
From such sporangia it is possible to compute the average number of
spore-mother-cells, and for this purpose a number of countings have been
made. In radial sections the average of the countings gave 59, say in
516 FILICALES
round numbers 60, spore-mother-cells traversed. Tangential sections show,
on the average, about six such layers of cells to be present, and the
average number of spore-mother-cells in each sporangium would thus be
6 x 60 = 360, while the average potential output of spores per sporangium
would be 360x4=1440 spores. It may be noted that, in the cases
observed, all the spore-mother-cells undergo the tetrad-division, and none
have been seen to be disorganised, though this, doubtless, may occur
occasionally.
Reverting to the tissues outside the sporogenous group, the tapetum is
not a strictly denned layer, and is often irregularly doubled by periclinal
divisions ; but this is by no means always the case. It remains recognis-
able as distinct, enlarged, glandular cells, often with several nuclei, up to
the period of the tetrad-division. The wall of the sporangium outside the
tapetum consists of two or more layers, commonly three, of which the
outermost is the firmest and most differentiated ; the inner layers are less
regular, and are composed of more or less tabular cells. I have nevei
seen the tapetum in direct juxtaposition with the superficial layer, as it
has been figured and described by Campbell.1 Of the external wall three
essential parts may be recognised, and they will be best seen in Fig. 284 D,
which represents the apex of the sporangium, cut off in such a plane
as x, x in Fig. 284 E. The first part (a, Fig. 284 E) is found on the
peripheral side of the sporangium, and consists of large turgid cells,
with moderately thin walls^ and granular contents, which stain deeply
with Bismarck brown. A second region (b) consists of deep prismatic
cells with thick lignified walls, which may be recognised as the annulus ;
it extends as a narrow bridge across the apex of the sporangium (Fig. 284 D),
and widens out on either side, as the apex is left, into a broader band
of cells with lignified walls (Fig. 284 F). The third region (c) consists
of thinner-walled cells, of elongated form, which constitute a narrow
band running down the anterior (ventral) side of the sporangium. This
is the tissue which defines the fissure of dehiscence.
The structure of the sporangial wall being as thus described, we
may now consider how it works in connection with dehiscence. The
annulus, together with the two broader lateral extensions of it, constitute
a firm resistant arch, of which the apex is the narrowest part, being
only about three cells wide (Figs. 284 D, E). If the thinner- walled posterior
region (a) were to contract, as we may presume it does by drying as the
sporangium matures, the two sides would be pulled backwards, while the j
thin bridge of the annulus at the apex would act as a sort of semi-rigid
hinge ; the line of dehiscence on the ventral face, having been structurally
defined, would thus, on fission, be caused to gape widely. It is not
probable, however, that this hinge-like action is very considerable, and
the gaping of the slit may be mainly due, as in other Marattiaceae, t(
mere drying up of the cells in the neighbourhood of the rupture.
1 Mosses and Ferns, p. 297, and Fig. 164 C.
MARATTIACEAE
517
At maturity the more or less^ indurated superficial layer of cells of
the sporangial wall is the most conspicuous part, but the thinner-walled
cells lying within, though they may shrink, do not entirely disappear.
The essential parts of the sporangium of Angiopteris, and especially
the archesporium, are thus seen to be referable in typical cases to a
single parent cell : this also is the case typically for all the other genera.
FIG. 285.
Marattiafraxinea, Smith. A = section transversely through a sorus : the sporogenous
cells shaded, the tapetum marked (-r, x) ; the left-hand sporangium is the most usual
arrangement of sporogenous' tissue, the other two less frequent. B and C show in similar
section irregular groupings not referable to a single parent cell. X 200.
But in Angiopteris and Marattia, and notably in Danaea, this is not
always so, and there is much individual divergence of detail. Not only
is the segmentation conformable to no strict plan, but the size of the
sporogenous groups varies greatly, while in non-typical cases it is not
always possible to refer the sporogenous group of one sporangium to a
single parent cell (Fig. 285). These irregularities are usually associated
with considerable differences in size of the sporangia. In no genus
does this irregularity appear more pronounced than in Danaea : an
almost exact uniformity in size and segmentation of sporangia is so
5.8
F1LICA1 ES
marked a feature in l-'erns at large thai those irregularities eomuund the
greater attention. Fig. 286 A represents a tangential section traversing
three sori in A///,/™ e//iptiui, of an age prior to the tetrad division : the
varying size of the sporangia is striking, as also their frequent grouping
in pairs, separated by a thin, or even by a partial septum. Sonu
Ifll
ft Oli
•f*t*tff»fltffllft1
ft ft! Hi
»*»»•!••» "
ftwMMM *Uiftic*, Smith. Drawings illustrating partial septations of the :
I, tangential section through three sori, showing the loculi in ground plan : i
ftcn thin, so that pairs of loculi are in close juxtaposition ; the loculi
_
.4. tausential section through three sori, showing the loculi in ground plan : the sepia we
often thin, so that pairs of loculi are in close juxtaposition ; the loculi marked (.r)
are large, and show one or more partial septa, x ao. 0, C, />, /t, show such locuti with
jvmiaf septa in greater detail : in /> and E it is difficult to decide whether the cells
marked (?) will develop as tapetum or as spore-mother-cells. X 150.
n
these partially septate sporangia are represented more in detail
Figs. 286 B-E, From these drawings it is dear that the identit
sporangium is not maintained: that where the initial sporogenous group
is large, some of its cells may develop as transitory tapetum, o;
permanent cells of septal tissue : and thus various intermediate step-
completion of a septum may be observed. Somewhat similar eond
jig
Lave been noted also in other genera, so that A;//</<w docs not stand
,loiu\ though ihe demonstration is most effective ill that genus.
In putting a morphological interpretation on these lads it is to be
cmcmbcrcd that the genus A///,/,,/ has in many of its specie's vei \
long sori compared with those of .!/<;/v///</. Kxiernal comparison had
\ suggested an elongation of the type of the latter genus to produce'
a sorua of Aw/<w. The internal structure is now seen to be conform-
le to such a progression, tor the partial scptations are commonly
found in those sporangia which are larger than the normal: they thus
appear to have followed upon expansion of the sporangia. Their existence
Parl -I .1 fi.'ii'l wiili tli- M.iitix. Reduced after a
piioi,.-iapii hv MI. \\ . ii. mini army, From So • i A'O.VA// AW<i«y.
here, as well as occasionally in other genera, raises the question whether
or not septation has been effective in the primary production of the
sorus: this will he considered again laler.
The synangial sori differ from those with separate sporangia in the
mechanism which accompanies dehiscence, though the dchiscence itself is
uniformly 1>\ a radial slit. The annulus represented in . ///.v/','/>/Vm by a
hroad indurated hand or hoop, is absent: clearly it would be useless in
a sxnangium, for it could not possibly be effective unless the sporangium
were free from mutual relations with others. In KavlfuSSia and Dannca
the radial slit of each sporangium may widen as the neighbouring cells
drv up, to form an almost circular pore. In Af,trnffin there is in addition
to this a change of lorm of the whole sorus at maturity: its two sides,
originally in dose juxtaposition so that the distal ends of thcii sporangia
almost touched, move apart like the opening of a hook. In /)ti/itn'ti
and Mtirnttiii the whole external wall of the synangium is composed of
deep indurated cells: the condition of Angwptcri* appears accordingly
520
FILICALES
a,
as a slight modification of this, by localisation of the sclerotic thickening
so as to form a hoop-like band, while the remaining walls are thin.
It is thus seen that the sporangia of the living Marattiaceae conform
essentially to a single type : but that that type is less definite in its
detailed characters than is the case in other Ferns : and this goes along
with their larger size, and the
high output of spores, which is
its natural concomitant. For
on computation in round num-
bers, the sum of the spores
potentially present in a spor-
angium of Angiopteris is about
1450: of Danaea about 1750:
of Marattia about 2500 : and
of Kaulfussia about 7850. It
is thus seen that the synangial
forms have the largest number.
This, with various other con-
siderations, will have weight
in the discussion whether the
state with synangia or that with
separate sporangia is probably
the more primitive.
Passing now to the fossil
Ferns having fructifications
which may be ascribed to a
Marattiaceous affinity, many of
them had foliage of the Pecop-
terid-type (Fig. 287), though this
in itself cannot be held as any
clear indication of relationship :
it is the soral structure which is
distinctive. A few of the best
FIG. 288
Ptychocarpus unitus. Fructification. A, part of a fertile
jinnule (lower surface), showing numerous synangia. B,
synangia in side view. (A and B X about 6.) (After Grand'
lum in section parallel to the surface of
/en confluent sporangia, a, bundle of
receptacle ; b, its parenchyma ; c, tapetum ; d, spores ; <?,/j
common envelope of synangium. X about 60. (After
Renault.) From Scott's Studies in Fossil Botany.
Eury.) C, a synangium in section parallel to the surface of kllOWtt examples will be de-
the leaf, showing seven confluent sporangia, a, bundle of
scribed, with a view to their
comparison with the fructifica-
tions of the living Marattiaceae.
One of the most striking is Ptychocarpus (Pecopteris) unita : T here, on the
lower surface of the pinnules of a Pecopterid leaf, the sori are disposed
on either side of the midrib : each is a solid synangium, composed of
about seven sporangia united upon a common receptacle. Each synangium
is attached by a short and narrow pedicel, so that it may be removed
bodily, and the synangia are frequently found lying free. The form is
that of a truncated cone, with a slight terminal dimple. The sporangia
1 Renault, Bassin Houiller d'Autun, ii., p. 9.
MARATTIACEAE 521
are surrounded by a rather delicate common wall, about four layers of
cells in thickness, of uniform structure, and without trace of any annulus.
Centrally there is a vascular strand connected with the system of the
leaf. The dehiscence appears to have been by terminal pores. The
number of spores in a single sporangium was very large : to judge from
Renault's detailed drawings it was probably equal to the output of the
modern Kaulfussia (Fig. 288). It is evident that the correspondence with
this genus was very, close indeed : the plan of the sorus is the same :
its form also, for the terminal depression in Kaulfussia is often less
marked than is represented in the drawings usually quoted. The two
are alike also in the thin parenchymatous tissue of the sporangial wall :
in the absence of any indurated annulus, and in the presence of a
vascular connection, which I have shown to extend in Kaulfussia also,
upwards into the receptacle : 1 though it is not so long or so coherent
a strand in the living Fern as in the fossil. The differences are of degree
only, and the similarities are most convincing.2
Another genus which conforms in type of its fructification to modern
Marattiaceae is Scolecopteris. This genus includes plants with sessile or
shortly pedunculate sori, of three to six sporangia : they are united
below, but separate above, and extended into a more or less elongated
beak (Fig. 289 D). The Marattiaceous characters of this fructification
are unmistakable, and it has been pointed out by Strasburger3 that
Scolecopteris elegans, Zenk, shows features connecting it with Marattia as
regards the form of the sporangia, and with Kaulfussia in their circular
disposition in the sorus, while the outline of their upper free portion
would point to Angiopteris: in dehiscence it compares with all three,
but especially with Marattia. In fact it is a type which unites in itself
characters of various living genera. It may be noted that the number
of sporangia in the sorus of Scolecopteris is small and variable, but that
four and five seem to be the most frequent numbers.
A genus of very early occurrence, and bearing sori of similar
character is Asterotheca : here the three to eight sporangia are in close
apposition while young almost up to the apex, but they separate
and diverge radially when mature : the peripheral wall is strongly
convex, and shows no annulus : the mode of dehiscence was by radial
slits (Fig. 289 F).
1 Studies, iii., Fig. 42, p. 46.
2 Mr. D. M. S. Watson (Journ. R. Micr. Soc., 1906, p. i) has described a "Fern"
synangium from the Lower Coal Measures, well preserved, but detached from the part
which bore it. It resembles Ptychocarpus unitus in its synangial state, but differs in its
more elongated form, its hollowed apex, and in the vascular* supply being widened into
a cup of tracheides : in these latter characters it resembles Kaulfussia. There is no
evidence to show . whether it was truly Filicinean or Cycadofilicinean. It is named
Cyathotrachus altus.
3 [enaischc Zeitschrift, 1874, p. 87.
522
FILICALES
As a further example Hawlea x may be mentioned, which is also of
very early occurrence. Here the sorus is of the circular form, as before :
the sporangia are sessile and
elongated-ovoid in form, and radi-
ate outwards from the centre of
attachment, so that the slit of
dehiscence on the inner side is
directed upwards, and the spor-
angium when open has the form
of a boat (Fig. 289 B). Stur de-
scribes a rudimentary apical annulus
in these sporangia, but it is not
clearly shown in his figures. The
sorus of Hawlea appears most
nearly comparable to that of the
modern Angiopteris, but this com-
parison would be accepted with
greater confidence if the micro-
scopic details were better known
by a study of sections.
Lastly, under the name of
Danaeites certain Ferns have been
described, which, so far as can be
judged from the study of im-
pressions, conform to the Danaea
type of sorus (Fig. 290). It may
be a question whether the rela-
tion of the sporangia together, and
the mode of their dehiscence were
exactly as in the modern Danaea,
but at least the plants appear to
FIG. 289.
Fructifications from the carboniferous formation.
fructifications Irom the carbomterous formation. UOT7fl K^™ T\/Tavnffi'o/-^r>nc a.-i/-l f-^
A =Senftenberzia ophidermatica,: to the right _the posi- nave Deen MarattiaCCOUS, andtO
have had elongated sori. They
come from the Coal Measures,
Keuper, and Lias.
Having regard to the fact that
the comparisons above sketched
do not relate to one or two, but
to several different genera, it
appears that there is ample evi-
dence of the early existence of the
Marattiaceous type. It would in fact be difficult to find clearer evidence
of affinity between a recent and a fossil group of plants, while, as we shall
1 Mr. Kidston suggests to me that Asterotheca is really identical with Hawlea, the
apparent difference being due to conditions of preservation.
tion of the sporangia on both sides of the median nerve
of the pinnule : to the left a single sporangium, seen
from above. B — Hawlea Miltoni : to the right a pinna
with the sori on the extremities of the lateral nerves : to
the left a single sorus more highly magnified. C —
Oligocarpia lindsaoides, showing position of the few-
membered circular sori on the nerves of the pinnule.
D=-Scolecopteris polymorpha, Brongn : to the left a
pinnule showing the position of the sori in transverse
section : to the right a longitudinal section of a sorus
in which the sporangia are united below into a columnar
receptacle. Ji= Asterotheca sternbergii : to the left the
pinnule with sori : to the right a side view of a sorus,
and a sorus in radial section. D and E diagram-
matically represented. (All Figures after Stur. From
Solms-Laubach's Fossil Botany.)
MARATTIACEAE
523
A C B
FIG. 290. x
Danaeites saraepontanus, Stur. From the upper car-
boniferous of the Saar district. A =a fertile segment of
the last order. B — transverse section through two adjoin-
ing sori, or the hollow impression of them. C = below a
sorus of sixteen sporangia ; above the impression of it.
(After Stur. From Engler and Prantl, Nat, PJIanzenfani.}
see, the comparison is confirmed by reference to the petrified stems known
as Psaronius. This consideration will justify our drawing together the
modern and the fossil forms into a comparison with a view to tracing
probable phyletic changes in the structure of the sorus, and a recognition
of an original type.1 The definitely circumscribed sorus appears to be
a characteristic of the Marattiaceae, both ancient and modern. The form
of the sorus varied from circular to elongated, both in the fossils and in
living forms : there is no distinctive stratigraphical evidence to show which
type was the prior, but in the majority of the early fossils the sorus is
circular, with a small number of
sporangia. Further, the Pecop-
terid is a relatively narrow-leaved
type, while the leaves of Danaea
and Kaulfussia are broad : if a
widening of the leaf took place,
followed by extension of the
sorus, the result would be as in
Danaea or Danaeites : if abstric-
tion of the elongated sori followed
also, the result would be as in
Kaulfussia, The evidence of the
partial septa in Danaea, and the irregularity of size and segmentation of
the sporangia throughout the family, accords with the suggested extension
of an originally circular sorus with few loculi to produce the more or
less elongated sori of the living forms with more numerous loculi.
A further point for discussion is the original relation of the sporangia
to one another in the sorus. Among both ancient and modern Marattiaceae
various gradations may be seen between such as have their sporangia
quite separate, and those in which they are synangially united. On this
point the palaeontological evidence would be consistent with either view,
for neither the synangial nor the polysporangiate state is distinctly the
prior in stratigraphical sequence. It becomes thus a question of comparison,
rather than of demonstration. As a matter of fact, all Marattiaceous sori
are synangia in the first phaxses of their ontogeny : many of them remain
so to maturity. It is only as the individual development proceeds that
the sporangia project as individual outgrowths in such a case as that of
Angiopteris, So far then as individual development bears on the question,
it would indicate the synangial state as the more primitive. Reasons have
already been shown for holding that a progressive septation accompanies
the extension of the sorus in the type of Danaea : a similar septation of
an enlarging initial spore-sac would produce the type of sorus seen, for
instance, in Ptychocarpus, Such an origin would consistently carry back to
an initial point that process of septation which is seen to be effective in
Danaea, From the synangial state thus produced the polysporangiate state
JA more full statement of the arguments is given in Studies, iii., p. 67-77.
524 FILICALES
of Hawlea or Angiopteris would readily result from individual growth of
the sporangia already initiated. This seems more probable than a fusion
of sporangia originally separate, of which there is no structural evidence in
the synangia themselves.
An indirect argument that the synangium was the primitive type is to
be found in comparison of the spore-output. It is much larger from the
single sporangium of the synangial types, such as Kaulfussia or Ptychocarpus,
than from the separate sporangia, such as Angiopteris. It will be shown
below that in the Ferns at large a progressive reduction of spore-output
from the single sporangium has. accompanied specialisation. If the experience
from comparison of other Ferns hold good for the Marattiaceae, then the
larger output per sporangium in the synangial types would show them to
be the more primitive, while the polysporangiate type with its smaller
output would be the more advanced. The question is one incapable of
present demonstration, but the comparative and developmental evidence
supports the view as stated here.
In conclusion, it is impossible to avoid the comparison of the Maratti-
aceous sorus with the sporangiophores of other Pteridophytes : the vascular
stalk or receptacle, the arrangement of the sporangia upon it, the relations
of the sporangia, their radial dehiscence — all find their correlatives else-
where. The chief differences are in the number of the sori, and their
position relative to the parts of the shoot which bear them. But in
view of the various positions which the sporangiophores hold in the
strobiloid Pteridophyta this cannot be held as invalidating the comparison
of them with these primitive sori. It may be that the similarity is a
result of parallel development; but if that be so, it would still appear
probable that the evolutionary progressions which produced them were
of a like kind. . It will probably be objected that many of the early Ferns
show isolated sporangia of large size, and that this precludes any general
application of a primitive soral state for Ferns of the Palaeozoic Period.
In reply to this, it may be remarked that the genus Sphenophyllum illustrates
how a "monangial" condition may probably arise from an originally soral
state. The sporangiophores with four or more sporangia are seen in
Cheirostrobbs and in S. majus : and smaller numbers in other species
lead to the solitary sporangium of S. Dawsoni (see p. 425). A reduction
of like nature is seen in the sori of Gleicheniaceae, and may probably
explain also the solitary sporangia of the Schizaeaceae, as indicated by
Prantl. Senftenbergia is itself an early example (Fig. 289 A). These early
forms must be given full consideration in elucidating the Fossils : they
indicate the probability that in early Pteridophytes a monangial state
may have been derived from a polysporangiate sorus or sporangiophore.
ANATOMY.
. The vascular system of the shoot in certain of the Marattiaceae is
well known to be among the most complicated of all the Pteridophytes.
M. \RATTIACEAE
525
It will be unnecessary to describe "St in detail here: our object will be
rather to bring it into relation with the less complex systems of other
Ferns, and with the cognate fossils. This is most readily done by reference
to the seedling-structure, and to those genera which are less complicated
in their mature state ; for there is considerable variety of complexity in
the different living genera of the family. It is found that Kaulfussia and
Archangiopteris are relatively simple, while Angiopteris is the most
complicated of all, Marattia and Danaea
taking a middle position.
In the seedlings of them all the axis
is traversed by a monostele : in Danaea
simplidfolia it has a solid xylem-core,
which is maintained till several leaf-traces
have been given off from it, naturally
without any leaf-gap : x it then becomes
crescentic, and expands into a dictyostele
with leaf-gaps, while a central strand or
commissure arises from the concavity of the
dictyostele, and pursues an upward course
with occasional fusions at the successive
leaf-gaps.'2 The same type of structure is
closely followed in the mature stem of
Archangiopteris? though on a simpler scale ;
in fact, this stem still retains at maturity a
stage rapidly passed through by the young
plants of other more complex genera. A
similar vascular system, consisting of a
cylindrical dictyostele, with normally a
single central strand, is found also in the
mature axis Of KaulfuSSia', but it is dorsi-
ventral, and rather more elongated between
the leaf-gaps, in accordance with its creeping habit.4 In Angiopteris and
Marattia the final structure is more complex, though the initial steps are
similar. There is in their seedlings also a solid protostele : in the central
part of its xylem-core certain cell-rows cease to differentiate as tracheides,
but give rise to a parenchymatous pith : the siphonostele thus formed
becomes broken up by leaf-gaps, thus giving rise to a dictyostelic cylinder5
(Fig. 291). Subsequently, as the stem passes to maturity, there arise
^ut Jeffrey (Phil. Trans., 1892, B, vol. cxcv., p. 120, etc.) states that in several
species of Danaea the stele is tubular in the seedling, and that it is interrupted by leaf-gaps.
That may be so in older conditions, and Jeffrey's material does not appear to have
been young enough to decide the question for the earliest stages.
2=Brebner, Ann. of Bot., xvi., p. 524.
3 Gwynne-Vaughan, Ann. of Bot., xix., p. 259. 4 Kiihn, Flora, 1889, p. 475-
:> Farmer and Hill, Ann. of Bot., xvi., p. 371.
FIG. 291.
(After Farmer and
526 FILICALES
within the cylinder of Angiopteris three or four crescentic-meshed zones
of vascular tissue, and it has been stated that there is here again a
single central strand.1 Marattia resembles Angiopteris, but does not obtain
so high complexity.
As regards the attachment of the appendages, the vascular supply to the
mature leaves originates as many distinct strands from the dictyostele : this
is obvious enough in the simpler cases, but it appears to hold also for the
more complex : here the leaf-trace is stated to spring from the outermost
zone only. The roots, on the other hand, originate even in the simpler
forms, in close relation to the central strand, while in the more complex
they mostly spring from various points in the internal system, but some
also from the outer zone.
It is thus seen that the ontogeny opens in all cases with a monostelic
state, with a solid xylem-core. This gives a basis for comparison with
other types of Fern, where the monostele is permanent. It is in the
later phases of the individual life that the complications arise, and it will
be recognised that these vary in rough proportion to the size and complexity
of the whole shoot, and are most complex in the large plants of
Angiopteris.
Comparing the structure of the fossil Marattiaceous stems with that
of the living genera there are marked differences, though the points of
similarity suffice to indicate a true relationship. The casts show on their
smooth leaf-scars that the leaf-trace was habitually a continuous vascular
band (Fig. 280), while that of all the modern Marattiaceae is composed of
numerous independent strands : the latter are, however, disposed in series,
of which the outermost corresponds in outline to one of those continuous
bands, as though it had been broken up. This greater coherence of
the vascular tracts is characteristic also of the stem of Psaronius : for the
centre of these fossils is occupied by numerous broad band-like plates,
disposed in concentric series, which show differences in relation to the
phyllotaxis. These series of vascular plates are doubtless the correlatives
of the meshed zones seen in the mature stems of Angiopteris, the former
being disintegrated in the modern Ferns, in conformity with the disintegrated
leaf-traces with which they are connected.2
1 Mettenius, Abhandl. Konigl. Sachs. Ges. d. Wiss., vi. ; Miss Shove, Ann. of Bot.,
xiv., p. 497.
2 It is interesting to compare this disintegration of vascular strands seen in the modern
Marattiaceae while the related fossils show connected vascular bands, with the analogous
cases seen in other Ferns. It will be shown below that most of the Simplices have a
single vascular band of the leaf-trace, while the larger Gradatae have a leaf-trace composed
of many smaller strands. A parallel is also seen in the Ophioglossaceae : it has been
shown that in Euphioglossum^ which is held to be the more primitive section of the
genus, the leaf-trace is a single broad strand : in Ophioderma, which is held to comprise
derivative forms, the leaf-trace consists of several distinct strands. It seems probable
that a progressive disintegration of a primitively simple leaf-trace has been a wide-spread
phenomenon in the evolution of large-leaved types.
MARATTIACEAE
527
The cortex which surrounds the central region in the fossil stems was
traversed by large numbers of downward-growing roots, having characteristic
Marattiaceous structure : the cortex with its contained roots was often
distended to great bulk, while outside it a mass of densely woven roots
is sometimes seen. It seems probable that these specimens represent
the basal region of arborescent stems, which, though greater in stature
than the living forms, were Marattiaceous in their characters. Not only
is this so as regards general structure, but also in the details of the tissues,
into which, however, it is impossible to enter fully here ; l the recognition
of their affinity with the modern Marattiaceae is thus further confirmed.
EMBRYOLOGY.
The embryology of the Marattiaceae shows features distinct from that
of other Ferns. The prothallus is of the normal flattened Fern-type,
though of larger size and more massive
construction. The sexual organs are
borne on its under side, and are deeply
sunk in its tissue, not projecting from its
surface, as in the Leptosporangiates.
The first segmentation of the zygote is
by a basal wall transverse to the axis
of the archegonium, whereas in most
Ferns it is nearly coincident with it
(<£, by Fig. 292 A). The basal wall is
followed by segmentation into octants,
and the relation of the parts of the
embryo to these is essentially similar to
what is seen in other Ferns : the epibasal
hemisphere, here turned away from the
archegonial neck, forms the cotyledon
and the apex of the stem : the hypobasal
hemisphere contributes the root and
foot (Fig. 292 A). But here the segmen-
tation proceeds further befofe the several
parts are defined than is the case in the
Leptosporangiate Ferns (Figs. 292 B), and
especially the foot is not clearly limited
at first, nor does it develop later to any considerable size. The
cotyledon and the axis grow directly upwards, surrounded by prothallial
tissue forming a calyptra, which .projects upon the upper surface of the
prothallus : this is finally ruptured, and the cotyledon emerges upwards.
Meanwhile the root developing from the hypobasal half emerges downwards,
and the prothallus is thus transfixed by the young sporophyte (Fig. 277).
1 Rudolph, Psaronicni uud Murattiaceen, Wien, 1905.
FIG. 292.
Marattia Douglasii. A — longitudinal
section of a young embryo. X225- b, £ = the
basal wall ; the arrow points to the neck of the
archegonium. B = z. similar section of an older
embryo, showing its position in the prothallus.
j/ = stem ; /r = prothallus ; ar-neck of arche-
gonium. Xy2. (After Campbell.)
528 FILICALES
The late definition of the parts of the embryo is in accord with the
indefiniteness of the apical segmentation of these Ferns. There are some
though inconstant signs of a single initial cell in the apex of root and stem of
the embryo ; but the apices of the mature parts of the Marattiaceae, whether
axis, leaf, or root, show as a rule a more complex structure, three or
more commonly four initial cells being recognised. This arrangement of
the meristems is in accord also with the Eusporangiate character of these
Ferns. Many years ago 1 I showed that a parallelism exists in the Filicales
between their sporangial origin and the meristems of all the vegetative parts;
that in the Leptosporangiate Ferns, where the whole sporangium originates
from a single parent cell, the apical meristems of stem, leaf, and root are
referable also to the segmentation of a single initial cell ; but that in the
Eusporangiate Marattiaceae the apical meristems are more complex, with
no single initial. With this goes also the deeply sunk character of the
sexual organs on the prothallus. Thus the general conclusion must be
that in all its parts the Marattiaceous type differs from the Leptosporangiate
type in its greater robustness of construction.
The account given in the preceding pages includes facts which show
good reason for holding to the early existence of plants of a Maratti-
aceous type. Not only does this follow from the detailed comparison of
Pecopterid-sori with those of the living Marattiaceae, but also from the
structure of the Psaronius-stems. From evidence of comparative structure
and association it appears certain that the Psaromus-stems bore the foliage
of Pecopteris of the same nature as the leaves on which various synangic
fructifications have been found. Thus we have to do with a group of
Palaeozoic fossil-plants showing affinity with the Marattiaceae alike in their
anatomical structure and in their reproductive organs. But certain fructi-
fications previously classed as Marattiaceous have lately been shown to be
the pollen-apparatus of Spermophytes, e.g. Crossotheca and Pecopteris (Dick-
sonites) Pluckeneti. Considering the anatomical evidence, however, it seems
impossible to doubt that Palaeozoic Marattiaceae actually existed, for the
Psarom'us-type of stem is altogether Fern-like in structure, and presents
none of those anatomical features by which the Cycadofilices were recog-
nised long before the evidence of fructification led to the foundation of
the class Pteridospermeae. For the present, therefore, we must continue
to accept the existence of a certain number of Marattiaceous Ferns,
especially in the later Carboniferous and Permian periods, though we may
not always be able to distinguish their fructifications from the pollen-
bearing organs of Fern-like Seed-Plants. It seems not improbable that
Marattiaceae and Pteridospermeae may .have owed their synangic fructi-
fications to some common descent from a primitive group of Filicales in
which that character had already appeared.'2
1 Ann. of Bot., 1889, vol. iii., p. 305.
2 This paragraph is taken almost verbally from Scott, "Present position of Palaeozoic
Botany," Progressus Rei Botanicae, 1907, pp. 187-189.
MARATTIACEAE 529
The shoot of the Marattiaceae, && a whole, being of a usual Fern-type,
it will be considered in relation to the theory of the strobilus at the con-
clusion of the Filicales. Meanwhile a comparison of the characters of the
known representatives, modern and fossil, may be made, and this not
only of the mature structure, but also of the details seen in the seedlings
of the living forms. Such a comparison gives some foundation for an
opinion as to the nature of the stock from which the family may have
sprung. In the first place it had an upright, radially constructed shoot, as
indicated by the fact that no dorsiventral fossil stem of this affinity has
been described, while those which exist among the living genera are probably
derivative : moreover the embryos are all upright, and radially constructed.
Presumably it had a protostelic structure of the axis, as indicated by the
simple anatomy of the young seedlings of the living genera. From this pro-
tostele sprang the leaf-traces, at first without leaf-gaps, as is still seen in some
living seedlings : the formation of leaf-gaps probably followed early as the stele
dilated and became medullated. Each leaf-trace itself was a single strand,
as is seen in the fossils even in the mature shoot, but only in the seedlings
of the living genera : this would suggest some simple form of leaf in the
ultimate parentage. The arrangement of the leaves was on a radial plan,
but was probably simpler than in the living forms : this is indicated by the
occurrence of early fossils with regular orthostichies of leaves. The root-
system was endogenous, and after traversing the cortex with a more or
less lengthy course, the roots emerged at the surface, forming sometimes
a supporting external felt. The arrangement of the sori on the leaf was
on the plan of a simple row on either side of the midrib, and the form of
the sorus circular. The relation of the sporangia was probably synangial,
and their number in each sorus small, or liable to be reduced to a solitary
one. The individual sporangia were large, the spore-output numerous, and
the mechanism of dehiscence simple, or altogether absent.
These characters, which comparison would indicate as those of the
Marattiaceous ancestry, show convergence in many points of form and
structure towards the apparently distinct series of the Botryopterideae. As
regards the reproductive organs also, it is to be remembered that certain
Ferns referred to a nearer relation with the Botryopterideae have a more
or less pronounced groupirig of the sporangia into sori : this is suggested
in Zygopteris itself (Fig. 272), and it is a marked feature in Corynepteris
(Fig. 273). Thus the two families are not absolutely distinct even in this
respect. It is probable that they represent divergent branches from some
common primitive stock.1
1 As regards relation to Pteridosperms, see Kidston, Phil. Trans., 1906.
2 L
CHAPTER XXXIV.
OSMUNDACEAE.
THE Osmundaceae are represented by the living genera Osmunda and
Todea, while certain species of the latter are sometimes separated under
the generic name of Leptopteris, The number of species is ten. The order
is of wide geographical distribution, but outside the limits of the ice-cap
of the glacial period. The plants are all perennial, with an upright, but
usually short stock, which bifurcates occasionally. The axis is covered by
the persistent and winged bases of the leaves, which are disposed upon
it in a dense spiral. The attachment to the soil is maintained by numerous
stout and darkly-coloured roots, which originate in close relation to the
leaf-bases. The leaves expanded in the current year form together a shuttle-
cock-shaped group, the outermost of which are often sterile, and the inner
fertile ; but some of the leaves never attain full development, their lamina
being abortive : these lie at the outside of the winter bud, and their basal
region, which remains persistent, acts as a scale-like protection to those
within. The leaves show the usual circinate vernation, and are covered
while young by mucilage-secreting hairs, which take the place of ramenta.
These hairs fall off as the leaf expands, leaving a smooth surface. The
leaves themselves are singly or doubly pinnate. In Todea there is no
marked difference between the fertile and the sterile regions, but in
Osmunda the sporangia are localised on various parts of the leaf, which
then show a considerably smaller expansion of surface (Fig. 293). There
is a difference of texture of the leaves which has given the basis for the
recognition of the third genus, Leptopteris : while Osmunda and Todea
barbara have leaves of a leathery character, those species from Australasia
and the South Sea Islands which are grouped under Leptopteris show a more
or less thin and pellucid structure of the pinnules, an approach to the
" filmy" character seen in the Hymenophyllaceae. It is, however, a question
whether this difference deserves generic recognition. It is probably a
relatively direct and recent adaptation to life under conditions of excessive
moisture. The leaves of Ferns are typically winged structures throughout
OSMUNDACEAE 531
their length :l in the upper regions this is more obvious than in the lower,
where modifications of reduction and of special development are seen.
A = leaf of Osmunda Presliann, J. Sin. B — young plant of Osinunda regalis, L. C-
?ler
FIG. 293.
n. B = yo'
leaf of mature plant of Osnninda regalis. (From Engler and Prantl, Nat. T^flanzenfam.)
In the Osmundaceae this results in the formation of the broad protective
leaf-base. In Osmiuida the wings are traceable as continuous down to the
^See Phil. Trans., 1884, p. 573.
532
FILICALES
flattened expansion : and developmentally it is found that this arises by
transverse growth of the superficial tissues, especially at the wings them-
selves. The result in Osmunda is a mere flattened leaf-base ; but in Todea
superba the development extends also as a
transverse commissure across the adaxial
face of the leaf-stalk. This development,
so exceptional in Ferns, may be compared-
with the stipular development in the
Marattiaceae, which is also extended as
a commissure across the leaf-stalk. The
two cases, though differing in detail, appear
alike in morphological nature.
The sporangia of Todea are borne
only on the under surface of the leaves,
but in Osmunda they are inserted in-
differently on both sides of the much-
contracted part, so that at maturity it
appears to be completely covered by them.
There is no protective indusium. The
sporangia themselves are relatively large,
of pear-like form, and thick-stalked. They
consist at maturity of a single layer of
cells forming the wall, but with a few
tabular cells within : a group of polygonal,
thick-walled cells in a lateral position,
but nearer the distal end, is recognised
as the annulus : it is related to the slit
of dehiscence so that the latter passes
from the centre of the annulus, over the
distal end, and approaches the stalk
on the opposite side of the sporangium
(Fig. 294). The line of dehiscence, de-
fined structurally by narrow thin-walled
cells, gapes widely at ripeness ; this
arrangement requires elbow-room, which
the lax arrangement of the sporangia
readily allows. Those sporangia which
are in near proximity to one another
originate simultaneously : there is no
interpolation, nor any marked sequence of their origin. Nor is there
any regularity in their orientation : in Osmunda the sporangia face in
the most various directions : and even in Todea, where their arrangement
has some reference to the nerves of the leaf, the sporangia upon a
single nerve show no common rule of orientation. These Ferns are thus
non-soral.
FIG. 294.
Todea barbara, Moore.
Sporangium.
B seen from
A, in side view, closed.
behind. C from in front, in both cases after
dehiscence ; the annulus is darkly shaded.
X 80. (After Luerssen.)
OSMUNDACEAE
533
Fructifications with unmistakable Osmundaceous characters have been
traced back to the Jurassic period. Several species, referred by Raciborski
even to the genera Osmunda and Todea^ have been found, bearing sporangia
having the characteristic distribution, form, and structure. But, as Raci-
borski remarks,1 they are there so highly differentiated that their origin
probably dates back earlier still. I have myself pointed out that certain
sporangia even from the Coal period show a detailed correspondence
of section with those of Todea.- Without wishing to urge this similarity
too far, such comparisons of the propagative organs suggest an early
origin of the Osmundaceous stock, which is fully borne out by the existence
of stems, with structure, having characters both external and internal
comparable with those of the Osmundaceae. Such are the sterns described
from external characters as Chelepteris by von Eichwald (Lethaea Rossica),
from the Permian : while these lead towards Grammatopteris, a form
referred to a Botryopterid affinity. These all share with the modern
Osmundaceae the general characters of an upright radial stock, with
closely disposed leaves, the bases of which persist. It will be shown
below that the anatomical details support the recognition of these stems
as a probable phyletic sequence.
SPORE-PRODUCING MEMBERS.
The development of the sporangium in the Osmundaceae differs from
that of most Ferns in the variety of its details in different individual
sporangia, even when they may be in
:lose juxtaposition on the pinnule. The
sporangia fluctuate between two types,
shown by the details both in
unda and in Todea : these are
illustrated by Figs. 295, which were
drawn from actual sporangia of Todea
barbara, and are not diagrams. They
represent extreme types, the one with
segmentation resulting in 'a square-
based archesporium as seen in the
Eusporangiates : the other showing the conical type characteristic of Lepto-
sporangiate Ferns : the latter is the commoner in the Osmundaceae.
The differences of individual detail start from the very first, as is seen
from F"ig. 296 A, in which two sporangia are shown already projecting as
convex outgrowths, but the segmentation is not uniform : it is further
1 Englers Jahrb. , vol. xiii. , p. 7. For further data see also Seward and Ford, Linn.
Trans., vol. vi., p. 250, etc.
-Annals of Botany, vol. v., 1891, p. 109. Renault has also described as Todeopsis
primaeva a fern sporangium from the Culm of Sanost, having Osmundaceous characters,
Cites Mineranx, Paris, 1896, p. 21.
FIG. 295.
Young sporangia of Todea barbara in longi-
, showing different modes of
X365.
534 FILICALES
to be noted that the cells marked (x) do not compose the whole outgrowth,
but that adjoining cells also contribute in a certain degree, so that
strictly speaking the whole sporangium is not referable in origin to the
single cell. The massive stalk is partly formed from surrounding cells :
this is a further feature for comparison with Eusporangiate types. How
various the subsequent segmentations may be will be seen from the
Figs. 296 A, E ; these show that the large cell in the centre of the growing
sporangium may be of prismatic or of conical form, while they also prove
Todea barbara, Moore. .4=small part of section of pinnule showing two young
sporangia (-r, x). B, C, D = examples of variety of segmentation, as seen in vertical
sections. E = older sporangia in transverse section, showing differences in sporangia in
juxtaposition. F= vertical section of sporangium of like age, with square-based sporo-
genous cell. £ = similar sporangium with triangular sporogenous cell. //, /—vertical
and transverse sections of older sporangia. The central figures show two unequal
sporangial stalks, in transverse section. All X 200.
that adjoining cells take part in the formation of the sporangial outgrowth ; a
comparison of Figs. 296 c, D, also brings out clearly the very great difference
of bulk occasionally seen in sporangia of the same age. The large cell
in the centre divides usually by three anticlinal walls corresponding to
those seen in the usual Leptosporangiate type, though the cell which
remains in the middle may still be either truncate or pointed at the
base. But sometimes it appears that four lateral cells may be cut off by
anticlinal walls, as in the largest sporangium in Fig. 296E; thus the central
cell after periclinal divisions in Todea, may sometimes have the form
which is characteristic rather of the Eusporangiate sporangia, though this
is exceptional. Then follows the periclinal division to separate the cap-cell
from the archesporium : the cell which thus undergoes periclinal division
OSMUNDACEAE
535
is believed to be truly comparable to the " central cell " of the Marattiaceous
sporangium.
The archesporial cell thus surrounded by the tissue which will form
the sporangial wall undergoes segmentation to form the tapetum (Fig. 296 E
and F), and the irregularity seen in other segmentations is fully maintained
in this also. From a comparison of Figs. 296 F and G it will be seen that
sometimes the segmentation of the tapetum is almost according to the
ordinary Leptosporangiate type (Fig. 2960); the most interesting cases are,
however, those in which the archesporium is truncate at the base ; in
these it appears that the cell or cells below it contribute to the completion
of the tapetum (Fig. 296r) ; this is a point of some special interest, for one
of the most constant differences between the Leptosporangiate and
Eusporangiate Ferns has been in the origin of the tapetum : but Todea
occasionally shows an intermediate condition. The division of the tapetum
into two or partially three layers follows
(Figs. 296 F to i), together with the sub-
division to form the sporogenous group ;
beyond this point it will be unnecessary
to trace the development of Todea barbara.
In conclusion the drawings not lettered in
the centre of the group are added, as
showing differences of thickness and seg-
mentation of the sporangial stalk; these
are both from sporangia, of the age of
Fig. 296 H.
An abnormality, which has been ob-
served occasionally in Todea barbara, has
been found not unfrequently on certain
fronds of Osmunda regalis, viz. the occurrence of synanagia : one of these
is represented in Fig. 296 bis: each half shows, except on the side where
it is coherent with the other, the ordinary sporangial structure. Such
developments are no very surprising consequence of the origin of two
sporangia close side by side, with a deeply seated central cell; normally,
each develops separately, but the obliteration of the groove between them
would give a synangial structure, and clearly this is not a very great
modification. . But what interpretation is to be put upon such cases,
whether they may be counted as retrogressions or the reverse, must for
the present be left open.
The more robust and massive origin and structure of the sporangia
of the Osmundaceae than of ordinary Leptosporangiate Ferns is to be
connected with the greater number of spore-mother-cells produced, .and
consequently the greater potential output of spores. In Osmunda Russow
long ago estimated the number of spores in the single sporangium as over
500, and assumes therefore the number of spore-mother-cells to be I28.1
1 Vergl. Unters., p. 87.
Osmunda regalis. A synangium, in
vertical section. X2oo.
536 FILICALES
To form an estimate, the number of spores produced in a single
sporangium may be actually counted, or an estimate of the potential
number may be based upon 'the number of spore-mother-cells as shown
in sections. By the latter method, as applied
to Osmunda regalis, since the number of spore-
mother-cells in a median section is 30 to 32
(Fig. 297), and the sporogenous mass is
approximately spherical with the diameter of
each cell about Jth that of the whole sphere,
the total number would approximate to 128,
though probably somewhat below that number.
Actual countings of spores showed figures
about midway between 2.56 and 512, distinctly
below the estimate of Russow. In Todea
barbara, while some sporangia approximate to
those of Osmunda, others fall far short, and
approximate to 256 ; but in T. superba and
hymenophylloides, which are "filmy" in habit,
the out?ut is sti11 lower> approximating in the
last species to 128. The bearing of these
facts will be discussed below; the results are such as coincide readily
with the relatively robust structure and variable details of the sporangial
development.1
ANATOMY.
The vascular structure of the axis in the Osmundaceae has long been
an object of interest, on the ground of its distinctness from that of other
Ferns, and its apparent analogy with that commonly seen in Dicotyledons.
But in the form of the leaf-trace, and the way in which it breaks up as
it passes into the leaf, the Osmundaceae present features which are more
readily matched among known Filicineous types. Starting from the peri-
phery of the leaf, the numerous forked veins fuse, on passing downwards
from pinnule and pinna to leaf-stalk, into a single half-cylindrical strand
with its concave channel directed adaxially. This strand consists of a
continuous band of metaxylem, with numerous protoxylem-groups at its
concave limit, which alternate in position with groups of% mucilage-sacs.
Peripherally is a mantle of phloem, thicker on the adaxiaL side, and
surrounding the whole is a continuous endodermis. As it passes down to
the base of the petiole the strand contracts, and the protoxylems unite
into a single one in a median adaxial position, while in transverse section
it assumes a deep U-shaped outline (Fig. 298 A, B). It is in this neighbour-
hood that the vascular supply to the roots is given off laterally from the
strand of the leaf-trace (Fig. 298 A HI.). The strand thus contracted, after
entering the axis, takes its place in a ring of similar traces surrounding a
1 For a more full statement see Stiidies, iv. , pp. 38-42.
OSMUNDACEAE
537
central pith : as it does so its endodermis opens, and becomes continuous
with that which completely surrounds the stelar system (Fig. 298 A, B, C).
The latter, as seen in transverse section, is composed of several layers
of parenchyma at the periphery : then follows a band of phloem which
is continuous, but may be uneven in width, extending inwards at the
medullary rays. Within this are the xylem-strands, which vary greatly in
number. O. claytoniana may have as many as 40, O. regalis about 15
(in Fig. 298 A there are 14). Todea barbara 8 or less (in Fig. 2986 there
are 8, in c there are only 3), while in T. superba the xylem may form an
unbroken cylinder. The position of the protoxylem also varies : in Osmunda
it is nearly on the inner edge of the metaxylem, but in Todea the xylem
is mesarch, or in T. hymenophylloides the strands are almost exarch.
B
FIG. 298.
A— diagram showing the arrangement of the vascular tissue in the axis of Osmunda
reg-alts (after Zenetti). B and C — transverse sections of the stele of Todea barbara, with
leaf-trace (after Seward and Ford), showing the greater continuity of the xylem than in
Osmunda. s — phloem. s/=sclerotic tissue.
Centrally lies the pith : in some cases an internal endodermis is present
(O. dnnamomea, T. hymenophylloides}, while in the former species some
internal phloem has been found locally in several specimens examined
by Jeffrey1 and by Faull/2
If the course of the several strands be followed they are found to fuse
downwards according to a regular scheme, so that they form a cylindrical
network, of which the meshes are very long and narrow : the number and
proportions of these vary in different cases, but in all the inter-communication
of the whole system is exceedingly close and effective. The scheme is
represented for Osmunda in Fig. 299 A, as flattened into a single plane,
and for Todea, where the number of the strands is less, in Fig. 299 B.
A comparison of these diagrams with the figures of transverse section
will explain the main features of the system of the axis.
There are two possible views as to the nature of this system of the
axis, as seen in the living Osmundaceae : either that it is a result of reduction
1 Phil. Trans., vol. cxcv., p. 119, etc.
2 ".Anatomy of the Osmundaceae," Bot. Gaz., 1901, p. 381.
538
FILICALES
from a more complex condition, or that the system is itself in the upgrade,
and an indication, as seen in the living examples, of the approximate
limit which development had attained in the group. The former opinion
has been elaborated by Jeffrey l and by Faull : 2 they hold the Osmundaceous
stele to be a reduced form of " amphiphloic siphonostele," and in support
of their opinion they adduce the presence of an internal endodermis
(O. cinnamomea and T. hymenophylloides), and the occasional presence in
some specimens of O. cinnamomea of internal phloem also, locally in the
FIG. 299.
A =0. representation of a portion of the xylem-ring of Osmunda regalis seen from
without; #=cut end of a departing leaf-trace; /^ = leaf-gap. (After Lachmann, from
Kidston and Gwynne-Vaughan.) B = a representation of a portion of the xylem-ring of
Todea. barbara, seen from without. Lettering as above. (After Seward and Ford, from
Kidston and Gwynne-Vaughan.)
neighbourhood of the branchings of the axis. There are good grounds
for doubting whether the local and inconstant occurrence of internal phloem
and endodermis will bear the weight of a far-reaching theory of reduction :
the question has been argued sufficiently elsewhere,8 on grounds of anatomical
comparison of living forms, and without acceptance of the reduction theory.
Even on grounds of physiological probability it would appear less likely
that a robust and large-leaved phylum of Ferns should show a reduced
vascular system in its stock than that the stock should retain a primitive,
though perhaps imperfectly efficient system.
Apart, however, from such questions of probability, a good basis for
1 Phil. Trans, vol. cxcv., p. 119, etc.
2 " Anatomy of the Osmundaceae," Bot. Gaz.t 1901, p. 381.
3 Scott, New Phytologist, vol. i., p. 209; Seward, I.e., p. 255; Boodle, Ann. of
Bot., 1903, p. 518; Chandler, Ann. of Bot., 1905, p. 406.
OSMUNDACEAE 539
• X
an opinion opposed to a theory of reduction is to be found in the ontogeny
of the living plants : for in the seedlings there is at first a protostele without
internal complications, which expands later, and becomes medullated ; but
at first the passing out of a leaf-trace does not necessarily interrupt the
continuity of the xylem-ring : leaf-gaps aje not found till later.1 Thus
the ontogeny suggests a progressive evolution of the complex structure
from the protostele.
But still more cogent evidence is derived from the study of the structure
seen in the related fossils, examined successively according to their strati-
graphical succession. This work has lately been carried out by Kidston
and Gwynne-Vaughan, and the demonstration is a very convincing one.2
If the present Osmundaceous structure be reduced, the fossil correlatives
should show a progressively more complex structure as they are followed
to earlier strata, but the reverse is found to be the general trend. Five
salient stages of complexity of the stele are involved in the series recognised
by Kidston and Gwynne-Vaughan: they are these: (i) the condition with
interrupted xylem-ring, and internal endodermis and phloem; (2) an
interrupted xylem-ring surrounds pith only ; (3) a continuous xylem-ring
surrounds the pith ; (4) a solid xylem is present, without pith, but
heterogeneous in structure (?) ; (5) a solid homogeneous xylem.
In the modern Osmundaceae the usual condition is (2), but with
indications of (i) in O. cinnamomea, and less clearly in T. hymenophylloides.
Among the fossils Osmundites Dowkeri, Carr, from the Eocene, shows
the condition (2). Osmundites Skidigatensis, Penhallow, from the
Cretaceous, shows internal phloem, and is in fact the most complex
Osmundaceous structure known. If no other fossils were available than
this, there would appear to be some support for a reduction theory; but
other fossils preclude this conclusion : 3 thus Osmundites Chemnitziensis,
Unger, from the Tertiary Quartz of Hungary, shows the condition (2).
From the Jurassic comes Osmundites Gibbeana, Kidston and Gwynne-
Vaughan, which shows the structure of type (2), but with narrow leaf-gaps :
also Osmundites Dunlopi, Kidston and Gwynne-Vaughan, with a continuous
ring of xylem surrounding a central pith (3). From the Permian of Russia
Chelepteris grarilis, Eichwald, which shows type (3), with continuous
xylem-ring : also Chelepteris Zalesskii, Kidston and Gwynne-Vaughan,
which appears to conform to type (4), showing a protostelic state, but with
the central region of the xylem differentiated from the peripheral. This
condition approaches very near to type (5), with solid homogeneous proto-
stele, a state which is seen in Grammatopteris, from the Permian of Autun.
It has already been remarked above (p. 499) that in general habit and
1 Seward and Ford, /.<-., p. 241.
2 Kidston and Gwynne-Vaughan, Trans. Roy. Soc., Edin., 1907, vol. xlv., p. 759.
3 An example such as this, leading to a possible conclusion which wider knowledge
of the fossils shows to be erroneous, exemplifies one of the many dangers of argument
from fossil evidence.
540 FILICALES
structure certain fossils attributed to the Botryopterideae have pronounced
Osmundaceous characters : in view of the sequence of fossils above quoted,
it seems probable that the Osmundaceous structure is referable in origin,
with upward differentiation of the stele, to some type of the nature of the
Botryopterideae (Kidston and Gwynne-Vaughan). It thus appears that a
study of the related fossils in their stratigraphical sequence lends no serious
support to a theory of reduction of the stele from an " amphiphloic siphono-
stele " : it indicates rather an upward development from a protostelic state.
Taken with the comparative considerations already advanced, the evidence
against Jeffrey's view appears very strong indeed.
Zenetti l had already regarded the stele of Osmunda as being in the
up-grade of development, and had compared it with the structure seen in
certain of the Lycopodiales. With these a very interesting parallel may
be drawn, and especially with that series illustrating a progressive elaboration
of the stele, and its disruption into separate strands, which Kidston has
recently demonstrated by a stratigraphical sequence of fossils as cogent as
this in the Osmundaceae (see above, pp. 230, 337). The fact that such
parallels have been shown to exist in distinct phyla is in itself a support of
the views above indicated.
EMBRYOLOGY.
The primary embryology of the Osmundaceae being on the same
plan as that of the Leptosporangiate Ferns as a whole, it need not be
described in detail. The Leptosporangiates all differ from the Marattiaceae
in the position of the basal wall : in the latter it is transverse to the axis
of the archegonium, in the former it is parallel with it : in relation to this
the epibasal half, which gives origin to the axis and leaf, is here directed
laterally, and ' the cotyledon originates from its lower quadrant. The
consequence is that, as in all the other Leptosporangiates, the cotyledon
of the Osmundaceae emerges on the lower side of the prothallus, not from
the upper as in the Marattiaceae. Comparing the embryo itself with that
of other Leptosporangiate Ferns, it will suffice to remark that in the
segmentation there is somewhat less regularity in the later divisons, and \
that the external differentiation of the members appears later, the embryo
retaining longer than in them its spherical form. These are but minor |
differences; they indicate, however, for the Osmundaceae an intermediate
place between the typical Leptosporangiates and the Eusporangiate Ferns.2
A similar intermediate character comes out also from comparison of
the meristems of the Osmundaceae with those of the Marattiaceae on the
one hand, and of the typical Leptosporangiate Ferns on the other. I
have shown at length elsewhere,3 that in respect of the apices of root, stem,
1 Bot. Zeit.^ 1895, pp. 72-76. '2For details see Campbell, Mosses and Ferns, p. 356.
3 Quart. Journ. Micr. Sci., vol. xxv., 1885, p. 75, etc. ; Phil. Trans., 1884, part ii, p.
565; Ann. of Bot., vol. iii., p. 305. This- matter will be taken up again later, when
the general comparison of Ferns is made, and also in Part iii.
OSMUNDACEAE 541
and leaf, and even in the segmentation of the wings of the leaf, the condition
of Osmttnda and Todea is less regular and more bulky than is habitual in
the Leptosporangiates : and in particular, in the segmentation of some of
their roots, where four prismatic initials take the place of the single initial of
the Leptosporangiates, there is a near approach to the structure seen in the
Marattiaceae : also in the apex of the leaf of Osmunda and Todea there is
a three-sided initial cell, as against the usual two-sided type of the Lepto-
sporangiates. When these facts are put in relation with what has been
demonstrated for their sporangia, where there is so strange an oscillation
between the Eusporangiate segmentation and that typical of Leptosporangiate
Ferns, it becomes clear that the Osmundaceae hold a transitional place
as regards their embryonic, and meristematic structure. This harmonises
readily with their mature characters, and with their probable early origin as
shown by palaeophytological enquiry.
Thus an examination of the Osmundaceae, living and fossil, leads to
the recognition of the following characters as probably existent in the stock
from which the family sprang. It had an upright, radially constructed shoot,
as shown both by the living species and by the related fossils ; for though the
embryo has the prone position in living forms, this is only a temporary juvenile
phase (see pp. 213-215). The axis was protostelic, as indicated by the
seedling structure, as well as by that of the earlier fossils : and though the
stele tended to be disintegrated in the more recent types there is still no
proof that the state of typical dictyostely was ever reached. The absence of
leaf-gaps in the early condition of the seedlings, and in the early fossils, as well
as the fact that the leaf-trace in all consists of a single strand, indicates an
ultimate origin from a stock in which the leaf had not attained the ascendant
in the shoot. The young parts were protected by mucilaginous hairs,
ramenta being absent. The disposition of the relatively bulky sporangia
was non-soral, either uniformly on both sides or margin of the leaf, or on
the lower surface : the individual spore-output was relatively large, and
the opening mechanism simple. These characters all point towards the
Botryopterideae among known early forms, and make it appear probable
that the source of the Osmupdaceae is to be found in some near relation
to that early family of Ferns.
CHAPTER XXXV.
SCHIZAEACEAE.
THIS family includes Lygodium, Schizaea, Aneimia, and Mohria of living
genera, with about 80 species, of wide distribution, but chiefly within the
tropics. The fossil genera Senftcnbergia Klukia, and perhaps Kidstonia,
referred to this affinity, indicate that the Schizaeaceous type was of early
occurrence. Whereas in the Osmundaceae, and in the Marattiaceae with
few exceptions, the radial type of shoot prevails, in the living Schizaeaceae
there is a pronounced leaning towards a dorsiventral habit. The radial
type of construction appears in Schizaea, in Mohria, and in most species of
Aneimia : frequently, however, the stock is not upright in position, but more
or less oblique, while in Aneimia (§ Aneimiorrhiza] the stock is a creeping
one. The extreme case is in Lygodium, which has a creeping underground
rhizome with bifurcate branching, and it bears the leaves inserted in a single
row, or it may actually be two nearly coincident distichous rows, upon its
upper side. The arrangement of the leaves is, however, in a dense spiral in
those cases where the axis is upright or oblique, while in the creeping
Aneimias it is in two alternating rows. It is probable in this family, as
in others, that the dorsiventral is the derivative and the radial the primitive
type ; but it will be seen that Lygodium, which departs most markedly from
the radial construction, is in certain other respects relatively primitive.
The leaves show great diversity of detail in the different genera. In
Schizaea there is a very marked and repeated dichotomy (Fig. 300) : the
branches may be more or less completely webbed together below, and they
bear the fertile segments on their distal ends. In Lygodium also the leaf-
architecture is traced by Prantl to repeated dichotomy,1 but complicated by
the continued apical growth and sympodial development of the branches : the
leaf may attain a length of 100 feet or more. This extraordinary foliar
structure acts as a prehensile climber, and the fertile segments are seated on
the distal ends of the branched pinnae which it bears at intervals. In
Aneimia and Mohria the leaves are less complex, and the ultimate reference
1 Die Schizaeaceen, Leipzig, 1881.
SCHIZAEACEAE 543
to a dichotomous branching is not so -dear. In the former the lowest pair of
pinnae are usually fertile, in the latter genus the sporangia may be distributed
over the whole length of the leaf.
Hairs are present in all the genera, and in all except Mohria they
are filamentous, as in the Botryopterideae, Marattiaceae, and Osmundaceae,
A
FIG. 300.
Upper parts of fertile leaves of the genus Schizaea. A =Sck. fiennula, S\v. B-Sch.
bifida, Sw. C, D — Sch. elegans, J. Sm. In D the ultimate segments are more strongly
magnified. (After Diels, from Engler and Prantl, Nat. Pflanzenfain .)
and are sometimes glandular. In Mohria they are no longer filamentous,
but flattened as scales : this condition, which is characteristic of most Ferns
of a more advanced type, is readily referable in origin to lateral widening
accompanied by longitudinal cell-divisions.
The sporangia are not arranged in sori, but solitary, a number of them
being borne on each fertile segment. In Schizaea and Aneimia they
appear when mature disposed in regular rows, one on each side of the
midrib, on the lower surface of the fertile segments. They may be protected
544
FILICALES
by curling over of the margin of the pinnule, as in Mohria and Schizaea,
and in some degree in Aneimia : or there may be a special protective
growth, comparable to the indusium of the Hymenophyllaceae, which
completely covers each separate sporangium, as in Lygodium (Fig. 301).
FIG. 301.
Disposition of the sporangia of the Schizaeaceae. A — Sch. dichotoina, J. Sm., part of
a fertile segment (sorophore). B, C = Lygodium japonicum, Sw. .Z> = apex of a young
fertile segment. C — mature fertile segment, at (x) the sheaths have been removed, so as
to display the sporangia. /? = Mohria caffrorum (-L.) Desv., segment of a fertile pinna.
E, F= Aneimia Phyllitidis, Sw. /t=side view of a young fertile segment. F= fertile
segment from below (A, B, E after Prantl. C, D, Rafter Diels, from Engler and
Prantl, Nat. Pftanzenfani.'}.
SPORE-PRODUCING MEMBERS.
These in the Schizaeaceae are simply the solitary sporangia, each of
which Prantl recognised as constituting a "monangial sorus." He accurately
worked out their development, and found them to arise in acropetal order
on each fertile segment : he ascribes to them all an origin from cells of
the marginal series, with a terminal position on the fertile vein. Con-
sequently the protecting flanges must, according to his account, be accessory
growths from the adaxial surface of the leaf. As Diels remarks, however,
extended and renewed investigations are desirable before this is finally
accepted.1 It seems improbable for Schizaea, and still more so for certain of
the related fossils : from their mature position in these it would appear
1 Nat. Pflanzenfam., i., iv. , p. 360.
SCHIZAEACEAE
545
more likely that the sporangia are originally of surface origin. Nor does
it appear unlikely that there should be inconstancy in this respect within
the family, when it is remembered that the sporangia may be either
marginal or superficial in the Osmundaceae.
The sporangia are large and sessile, or in Lygodium shortly stalked,
and are annulate. The annulus in the living forms is usually uniseriate,
FIG. 302.
Sporangia of the Schizaeaceae. A, B — Schizaea pennula, Sw. A seen laterally. B
the tip seen obliquely from above. C = Lygodium japonicmn, Sw., seen laterally. D,
E = Mohria caffrorum (L) Desv. Z> = seen from above. E, laterally, f, G = Ancimia
Phyllitidis, Sw. ^=view from midrib, G from margin of pinnule. H — Z.=spores of
the Schizaeaceae. H — Schizaea pennula, Sw. J ' = Lygodium japonic-urn, Sw. K=
Moliria caffrorum (L) Desv. L=-Aneintia fuhia, Sw. (All but K after Prantl.
K, after Diels — from Engler and Prantl, Nat. Pflanzenfam.')
though occasionally it is more complex : it is contracted towards the apical
end of the sporangium, and there is a definite stomium ; but however
contracted it may appear, there is in its centre an apical group of thin-walled
cells (or it may be only a single one in Lygodium and Sc/itzaea), designated
by Prantl the " plate." It is important to note its existence for comparison
with sporangia of other Ferns. The rest of the sporangial wall is thin.
2 M
546
FILICALES
The sporangia of Mohria are radially constructed, the apex and base
being opposite (Fig. 302 D, E). In the other genera the sporangia are more
or less curved, so as to be dorsiventral
this curvature is slight in Aneimia an<
Schizaea (Fig. 302 A, B, F, o), but ven
marked in Lygodium (Fig. 302 c).
Here it will be well to introduce
a brief notice of certain fossil sporangia
which have been referred to this affinity,
Senftenbergia (Pecopteris) elegans, Corcla.
A— a small piece of sporophyll (|). £ = a spor-
angium (-3r5-). (After Zeiller, from Engler and
Prantl, Nat. Pflanzenfam.')
the structure of those of living forms.
The best known of these is Sen/ten
bergia (Pecopteris) elegans, Corda, fro
the upper Carboniferous (Fig. 303) : i
corresponds to Schizaea both in th
disposition of the solitary sporangia
and in their form and mode of dehi-
scence; but the annulus is composed of several cell-rows, and the terminal
"plate" has not been observed. Zeiller points out, however, that this is not
an absolute difference from living forms, for various species of Lygodium
(a genus which has itself been traced back
to the Cretaceous Period), have a partially
double series of cells of the annulus, while
the " plate " in living forms is often so
small that a similar one in a fossil-
impression might well escape detection.1
A second example is the genus Klukia,
the fructification of a Pecopterid from the
Jurassic, of which several species have
been described by RaciborskL2 Here the
arrangement of the sporangia, their struc-
ture, and line of dehiscence are as in
Schizaea, there being only a single series
of cells of the annulus (Fig. 304). In
both of these genera of fossils it is to be
noted that the sporangia are intra-marginal,
11 r r i • i i
On the lower Surface Of the pinnule, but
without any indusial protection, while there
is no specialisation of the fertile pinnules.
From such comparisons it would appear probable not only that the
Schizaeaceous type is an ancient one, but that it sprang from plants with
a Pecopterid type of frond, without differentiation of specialised fertile
pinnules, and that the sporangia were intra-marginal, on the lower surface.
^ Bull. Soc. Bot. de France, T. xxiv., p. 217.
2 Engler'' s Jahrb, , xiii., p. I, Taf. I.
FIG. 304.
Ki«ki\ exiiis (PhiiiPPs).
Fertile pinnule of last order, seen from
below(^). From the Jurassic of Krakau.
SCHIZAEACEAE
547
)bably they had originally a more complex annulus than those of the
isent day.
The development of the sporangium follows in its main features the
lal Fern-type, each being referable to a single parent-cell, with rectangular
It is specially noted by Prantl,1 that in all the Schizaeaceae the
>t segment-wall extends from the outer to the inner periclinal wall of
lis cell : such segmentation is a feature characteristic of the more robust
js of Fern-sporangium (Fig. 305). This is followed by two other anticlinal
ivisions, as seen in section, and then comes the
riclinal division which separates the cap-cell from
archesporium. The former gives rise to the
reater part of the sporangial wall, while the lower
rments complete the wall, and form the short
ilk. In the archesporium the usual tapetum and
>rogenous group are formed : the latter consists,
>wever, of a larger number of spore-mother-cells
in usual in the Leptosporangiate Ferns : in
[neimia Prantl figures 16 spore-mother-cells as seen
)m one side only of the sporogenous mass, and
have seen the same number in Mohria actually
traversed in a single section. These observations led to an enumeration
of the spores actually produced from the sporangia of the Schizaeaceae :
in Lygodium japonicum and dichotomum the number approximated to 256,
but in L. pinnatifidum to only 128: there is thus a difference between
species of the genus, as in Todea and elsewhere. The lower figure is
shared also by Schizaea? Mohria and Aneimia. The numbers are thus
larger than are seen in ordinary Leptosporangiate Ferns, and they approach
those seen in the Osmundaceae. The largest number is seen in Lygodium :
it will be seen that its anatomical characters also mark this genus out as
more archaic in structure than the rest of the family.
FIG. 305.
Diagram of segmentation of
a sporangium of the Schizae-
aceae ; the first segment wall
meets the periclinal (basal)
wall of the parent cell ; but
the second (_r, JT) meets the
first, and does not extend to
the base of the parent cell.
ANATOMY.
The Schizaeaceae show ' diversity of habit, varying from those with
creeping rhizome and laxly disposed leaves to those with ascending or
upright stock, and leaves densely spiral : the internal structure of the
shoot also shows marked differences, which follow these differences of
habit.3 In Lygodium the simplest vascular structure is found, for there
the rhizome is traversed by a protostele with solid xylem consisting of
tracheides and parenchyma, surrounded by phloem, pericycle, and
endodermis. There is no typical proto-xylem : the first formed tracheides
lL.c., p. 49. 2Tansley and Chick, Ann. of Bot., 1903, p. 495.
3 See Boodle, Ann. of Bot., 1901, p. 359, and 1903, p. 511; Jeffrey, Phil. Trans. ,
B, 1902, p. 128; also Tansley and Chick, Ann. of Bot., 1903, p. 493.
FILICALES
are finely scalariform, and are scattered round the periphery of the xylem-
core, which is itself composed of tracheides, intermixed with parenchyma
(Fig. 306). The petiole is traversed by a single strand, which comes off
from the protostele with only superficial disturbance of it : in fact the
mature plant maintains the simple relation of the protostele and trace
which is seen in its seedling (Fig. 307). The foliar strand in the climbing
petiole is an almost cylindrical body, with bays of phloem protruding
into the xylem : it is probably a derivative form of the more usual flattened
type, contracted in accordance with the climbing habit. In Schizaea the
obliquely ascending or erect stock shows a medullated stele : endodermal
FIG. 306.
Transverse section of the rhizome of Lygodium dichotomum.
ph = phloem; x = xylem. (After Boodle.)
6'=endodermis
pockets are often present at the nodes, or an isolated internal endodermis
is occasionally seen, but no internal phloem. Internal tracheides occur
in the medullary region, sometimes isolated, sometimes almost bridging
across the central pith (S. molluccana). It has been clearly demonstrated
that as the stele in the seedling expands, no internal phloem appears :
these facts favour a theory of amplification of the stele in Schizaea rather
than one of reduction. In Aneitnia Phyllitidis and most other species,
as also in Mohria, the mature stem is dialystelic, having a hollow reticulate
vascular cylinder similar, except for the leaf-traces being only a single
strand, to that of Nephrodium filix mas : each mesh is a leaf-gap and the
leaf-trace is inserted at its base. But in A. mexicana and other creeping
species the dialystelic state is replaced by closed vascular ring or solenostele.
The simpler type of Lygodium is probably the more primitive, and as
SCHIZAEACEAE 549
bearing on this the development of Its seedling does not suggest reduction
from any more complicated type. In the seedling of Aneimia there is
at first a solid stele, which is converted into a dialystelic one by gradations
similar to those for instance in Pteris : the ontogeny may here be held
to indicate the probable evolutionary progression. In the case of Schizaea
FIG. 307.
Transverse section of axis of seedling of Lygodium japonic-urn, below the first leaf, p — one
of the xylem-parenchyma cells. X 390. (After Boodle.)
the facts indicate that its middle position is due to amplification of the
stele leading up towards the solenostelic state, though the opinion is also
a tenable one that the genus illustrates phases of reduction.
EMBRYOLOGY.
This is described as being similar to that common for most Ferns in
the case of Schizaea pusilla.1 It may be noted, however, that the fila-
mentous prothallus in this species shows very close analogies with that of the
Hymenophyllaceae.
In discussing the probable phyletic relations of the living Schizaeaceae,
Prantl 2 remarks that it is impossible to derive any one genus from any other :
they have obviously similar soral and sporangial characters, but they differ
so greatly one from another in other respects that we can only regard them
as derived from some simpler type, which may be held as a common
1 Britton and Taylor, I.e., p. 2. 2 L.c., p. 148.
550 FILICALES
ancestor. If the attempt be made to .sketch the characters of that ancestry
they would be as follows : Probably like other primitive Ferns the early
Schizaeaceae had an upright, dichotomously branching stock (retained until
after the leading soral characters were established), with radially disposed
leaves, which also branched dichotomously : a protostelic structure (retained
till after Lygodium had assumed its creeping habit), and a relatively simple
leaf, as indicated by the single strand of the leaf-trace. On the surfaces were
simple filamentous hairs. The monangial sori were probably superficial, as
indicated by Senftcnbtrgia and Klukia, with a tendency towards the margin
realised in the more modern forms. The sporangia were relatively large,
with the annulus consisting of more than a single series of cells.
Of the living forms Lygodium represents structurally the most primitive
type, being protostelic. Subsequently the stele dilated, perhaps to accommo-
date the enlarging leaf-traces,1 as seen in the genus Schimea ; and became
even dialystelic, as in Aneimia and Mohria ; but the section Aneimiorrhiza
probably assumed its prone habit before the solenostele became dialystelic.
On this view Aneimia and Mohria would be anatomically the most advanced
types. This harmonises with the facts relating to spore-output : for on this
ground also Lygodium would be the most primitive, and the other genera
would have proceeded further towards reduction in number of the sporo-
genous cells. It is in Lygodium also that Zeiller recognised that more
complex structure of the annulus which corresponds to that of the earlier
fossils.
There is, however, another, and from its entire independence of the
characters compared above, a most important feature, which marks off
Aneimia and Mohria as advanced genera in the family. Heim,2 in
selecting organs which are typical for the divisions of the Ferns and
recur under altered cultural conditions, lays great stress upon the structure
and mode of dehiscence of the antheridium, of which he recognises two
types : Type A, in which at maturity the cap-cell breaks away ; this includes
the Osmundaceae, Gleicheniaceae, Hymenophyllaceae, Cyatheaceae, Dick-
sonieae, and Lygodium ; it is, in fact, characteristic of those Ferns which are
usually held as primitive. Type B, in which the antheridium has a star-like
dehiscence, includes Aneimia a"hd Mohria, and the whole body of the Poly-
podiaceae : thus these genera share with the later and presumably derivative
Ferns3 a character by which they differ from Lygodium. Accordingly, on
their anatomy, on their spore-output, and on the mode of dehiscence of the
antheridium Aneimia and Mohria appear relatively advanced, and Lygodium,
which itself goes back to the Cretaceous Period, is relatively primitive.
Any converse view will have to meet not only one, but all of these
lines of evidence.
1 Boodle, Ann. of Bot., 1903, p. 530. -Flora, 1896, p. 329, etc.
3 Heim notes also other characters of the gametophyte in which Aneimia and Mohria
differ from Lygodium : so that the distinction is not based merely on the antheridial
dehiscence, but is more general.
MARSILIACEAE 551
• x
It is then specially through Lygojlium that the nearest connection may
be sought with ancestral Fern-types, which should have a protostelic stock,
and show dichotomy both in axis and in leaf: large sporangia, with the
annulus not necessarily uniseriate, and with a relatively large spore-output.
As these characters are less decisive than those seen in the preceding
families, the difficulty in locating the Schizaeaceae will be correspondingly
greater.
MARSILIACEAE.
It is probably in near relation to the Schizaeaceae that the Marsiliaceae
find their most natural position. This has been argued by Campbell,1 and
the data relating to the sporangia appear specially convincing. But as these
heterosporous plants constitute a peculiarly specialised line, which has
probably never advanced further, the discussion of them, however interesting
in itself, does not bear directly upon the problem in hand. Accordingly no
detailed account will be given of the Marsiliaceae.
There is, however, one striking feature in their morphology which calls
for notice here, since it provides an apparent analogy with the Ophio-
glossaceae : viz. the position of the "sporocarp." This curious and complex
body may be stalked or sessile and be borne singly attached to the leaf-
stalk, or in considerable numbers as in M. polycarpa : finally it may itself
be branched. From its position and structure, as well as from the way in
which the sporangia are produced, a foliar character is probable, notwith-
standing that the form is far removed from that of any ordinary leaf-segment :
and this is the conclusion to which study of the development has clearly led.
Johnson '2 found that both in Marsilia and Pilularia the origin of the
sporocarp is from a cell of the marginal series of the leaf: he concludes
that the capsule is the equivalent of a branch of a leaf in which the
marginal cells have been devoted to the formation of sporangia instead of
a lamina. Goebel obtained a somewhat similar result from the investigation
of M. polycarpa : 3 here the numerous sporocarps arise in acropetal succession
from the margin of the leaf, but from one margin only : they assume upon
the leaf-primordium the same position as the sterile pinnae. But the arrange-
ment of the cells is different : the sporophylls have a two-sided initial, while
the sterile pinnae show from the first a marginal growth. This, however,
need not preclude the recognition of the sporocarps as the correlatives of
pinnae.
The analogy of these bodies with the spikes of the Ophioglossaceae is
too obvious to escape remark, and some have seen in them and their
pinna-character strong evidence that the same must be the nature of the
Ophioglossaceous spike. The objections to this facile conclusion are two :
^American Naturalist, 1904, pp. 761-775.
2 Ann. of Bot., xii., p. 119; and Bot. Gaz., xxvi., p. i.
3 Organography, vol. ii., p. 479.
552 FILICALES
first, that the latter are not marginal, excepting very occasional examples
in O. palmatum, which is held as a derivative and outlying species ; but
secondly, the general comparison of characters, morphological and anatomical,
of the sporophyte, of the sporangia, and of the gametophyte, indicates a quite
separate position for the Ophioglossaceae. It suggests that the most that
can be said is that some analogy exists between the Marsiliaceae and
Ophioglossaceae in the position of their spore-producing parts ; but the
way in which this analogy has been attained is quite a different question.
It seems probable that they represent quite distinct evolutionary sequences :
a well-founded hypothesis has been stated above of an origin of the Ophio-
glossaceous spike quite distinct from that of the sporocarp in the Marsiliaceae.
CHAPTER XXXVI.
GLEICHENIACEAE.
THIS family is represented by about twenty living species, all of which
are referred by some systematists to the single genus Gleichenia, though
others separate off the monotypic genera, Platyzoma^ Br., and Stromatopteris,
Mett. The living species are distributed throughout the tropics, whence they
extend far southwards, but only in less degree north, and they are absent from
the northern temperate zone.
FIG. 308.
Gleichenia, S\v. § Mertensia, Willd. Scheme of branching of the leaf in the four sections
of the genus. (After Diels, from Engler and Prantl, Nat. PJianzenfam.')
Among these Ferns an upright shrubby axis is occasionally found (Stroma-
topteris), but usually there is a creeping rhizome, which sometimes takes an
ascending position. Upon it the leaves are solitary, often with long inter-
nodes, but sometimes more closely arranged (Platyzoma\ The leaves are
occasionally simply pinnate (Stromatopteris, Platyzoma) : but usually they
show higher degrees of branching, together with a peculiar straggling habit.
The branching of the leaf has frequently been described as dichotomous ;
but according to Goebel no species of Gleichenia has a dichotomous leaf,1
the branching is always a monopodial pinnation ; the appearance of "forking"
is the consequence of the two pinnules below the circinate but temporarily
1 Goebel, Organography, vol. ii., p. 319, footnote.
554 FILICALES
arrested leaf-tip developing equally, and so strongly as to exceed the actual
apex which lies between them. But on the other hand, as the result of
comparison apparently of mature specimens, Tansley refers the leaf-architec-
ture ultimately to dichotomy. He states that "a bud normally arises from
the angle of the primary dichotomy." l In face of such diametrically opposite
statements the accurate observation of the ontogeny is most desirable;
hitherto the details of development of the Gleicheniaceous leaf have never
been worked out.
The degrees of branching of the leaves have been made the basis of
subdivision of the genus into four sections (Fig. 3o8).2 Goebel has described
the mode of protection of the resting bud seen in some species : the pinnules
which stand nearest to the apex form protec-
. ^ tive scales, and they have been mistaken for
adventitious or aphleboid growths.3 As a
.« matter of fact, the whole structure can be
^J referred to a normal pinnate development of
'#%'^ the leaf, altered by temporary arrest of the
%£/ / •''•?• \ :V apex, and by precocious development of
certain pinnae. Hairs and paleae are found
'.'«/& on the surface both of rhizome and leaf,
i #tf^ The sori are always superficial, disposed
, fc :•&* in a single row on either side of the midrib
c^ H.
;fc of the fertile segment (Fig. 309). Typically
they are radiate-uniseriate, the sporangia
being attached in a ring round a central
receptacle : they are without indusium. The
number of sporangia varies in different
Gleicheniaflabellata, Br. Midrib and . . .
three pinnules, showing the arrangement SpCClCS, tWO tO five being COmniOn numbers ;
and constitution of the sori, with variable , _ ,
number of sporangia. but the sorus may often be represented by
a solitary sporangium . (monangial sorus),
especially towards the distal end of the segment, a fact pointing in the
direction of the Schizaeaceae : or the number may be larger than five
or six, as in G. pectinata and dichotoma (Fig. 310, a-h\ and this points in
the direction of the Cyatheaceae.
The existence of the fossil Fern with fructification designated Oligocarpia
has been held as evidence of the existence of Gleicheniaceous Ferns as
early as the Palaeozoic period. But the fact that the Gleicheniaceous and
Marattiaceous sori are of the same type throws the burden of proof upon
the sporangial structure, on which point it may be admitted that there is
some doubt.4 But the Gleicheniaceous habit of frond is seen in the
Palaeozoic genus Diplotmema and other types, while certain Carboniferous
stems had an anatomical structure like that of the Gleicheniaceae.5 But
1 Ann. of Bot., xix., 1895, p. 479. 2 Diels, Nat. PJJanzenfam., i. 4, p. 352.
3 Goebel, I.e. , p. 318. 4 See below, p. 560.
5 Scott, Studies, p. 263.
GLKICHENIACEAE
555
whatever may be the doubt as to the proof of Palaeozoic Gleicheniaceae,
their existence in the Mesozoic seems clear : certain of the fossils of that
age have even been referred to the sub-genera of Gleichenia, as represented
by living species. It will be seen that a detailed examination of the living
species supports on comparative ground an early origin of the family, such
as the fossil evidence suggests.
SPORE-PRODUCING MEMBERS.
The naked sorus consists of a low circular receptacle bearing a variable
number of sporangia. The sporangia are commonly quite separate from
one another, though instances of synangia which resemble a fusion of two
FIG. 310.
a-h sori of Gleichenia dichotoma, Willd. a-c show sori of radiate type, but with one or
more sporangia in the centre of the sorus, usually in this genus vacant, f, g, h show
degrees of fission of the sorus. a-h X about 14. z, /, k = sporangia of Gleichenia circinata,
Sw., seen respectively from the side, from the distal end showing the line of dehiscence
(-r), and from the proximal end showing the stalk. X 50. /, m, n = sporangia of
Gleichenia dichotoma, Willd, seen respectively from the side, presenting the perigheral
face. Note the difference in size from G. circinata. X 50.
sporangia, are not uncommon. The sporangia usually form a single row
round the receptacle ; their orientation is in this case constant, the longi-
tudinal slit of dehiscence facing directly towards the centre of the rosette-
like sorus. Where the number of sporangia in the sorus is more than five,
single sporangia may be displaced, perhaps by lateral pressure, and point
obliquely upwards. But in GA dichotoma, in which the number of sporangia
in the sorus may be as high as ten, or even more, the central area of the
sorus, which is usually vacant in other species, may also be occupied by
sporangia. Figs. 310, a-e, show cases of the insertion of sporangia on the
apex of the receptacle ; the number of these sporangia may vary from
one upwards, and they form a second tier above the basal rosette. When
556 FILICALES
one of these only is present it usually occupies a central position. The
orientation of these central sporangia is not constant. By the presence
of these supernumerary sporangia the gap is bridged over within a single
genus, between two well-marked types of sorus ; on the one hand are the
Marattiaceae, and most of the Gleicheniaceae, representing the "radiate
uniseriate " type, with a single linear series of sporangia, surrounding the
periphery of the low receptacle ; on the other hand are the Cyatheaceae,
Dicksonieae, Loxsomaceae, and Hymenophyllaceae, with a more or less
elongated receptacle covered to its apex with numerous sporangia.
As in other genera where the sorus is circumscribed, so also in Gleichenia,
fissions of the sorus may be found, chiefly in conjunction with branching
of the veins. Examples of this are shown in Fig. 310^ g, Ji.
The sporangia have an annulus, consisting typically of a single row of
cells : it is complete round the head, with the exception of the region of
dehiscence, which is on the side directed away from the lower surface
of the leaf (Fig. 310 i-n). The position of the annulus is oblique, so
that of the two thinner areas of the sporangial wall which lie on either
side of it, the one faces obliquely towards the centre of the sorus, and
away from the leaf-surface, the other obliquely away from the centre, and
towards the leaf-surface. The former may be styled the acroscopic or
central, the latter the basiscopic or peripheral face of the sporangium.
There is considerable variation in size of the sporangia in the genus
Gleichenia. Those species which have a small number of sporangia in the
sorus, such as GL rupestris and tircinata, have relatively large sporangia
(Figs. 310 z, y, k}; those which have more numerous sporangia in the
sorus have them of smaller size, e.g. GL dichotoma (Figs. 310 /, m, n\ Taking
first the sporangia of the larger type, as seen in GL circinata, the form
is almost that -of a kettledrum ; the " peripheral " face is almost flat, and
lies in apposition to the leaf-surface, while the annulus runs round its
margin ; the " central " face is very convex. The stalk is short, and con-
sists of a central group of cells, surrounded by a peripheral series; it is i
thus thicker than in ordinary Leptosporangiate Ferns. The sporangium
of Gleichenia dichotoma is of much more elongated form, the stalk is
thinner, and has no central group of cells : the annulus rises more
obliquely from the surface of the leaf. Gleichenia flabellata holds a
middle position between these two types as regards size and shape of the ,
sporangium, but in the number of spores produced in each sporangium
it is, as we shall see, an extreme type.
Tracing the development in G. flabellata^ the sorus first appears in the
still tightly circinate pinnule ; it arises as a* smooth outgrowth opposite a
nerve (Fig. 311 a), a considerable number of cells being involved in its
origin. Having grown to a height almost equal to the thickness of the
pinnule, it becomes flattened at the apex ; in those cases where the sorus |
is to be a simple rosette (Figs. 311 b, g), the convex margin begins to
grow out as rounded processes, which develop into the sporangia. There
GLEICHENIACEAE
557
is some variety of detail, according to the size of the future sporangium ;
in the larger type of GL cirdnata or Gl. flabellata each process under-
goes segmentation, resulting in a conical sporangial cell (x) ; in this
successive obliquely inclined divisions follow, the earlier of which con-
tribute to form the relatively massive stalk (Figs. 311 />, c). These divi-
sions do not appear to be uniform, as will be seen on comparison of the
four corners of Fig. 311 c, and of vertical sections (Fig. 311 b) ; the
FIG. 311.
«, b, c = sori of Gleicheniajlabellata, a, b in vertical, c in horizontal section ; d, e,f=
sporangia of GL cirdnata showing central cell and tapetum ; g, h, z = sori of Gl.
dichotoma ; in g the centre is vacant, in h and i young sporangia appear in the vacant
space ; /, k = sporangia of Gl. flabellata with spore-mother-cells formed, and very
numerous, a - i X 200 ; j, /£ X 100.
latter also show in the case, of these more massive sporangia that the peri-
clinal division, which cuts off the cap-cell, takes place at a time when
the sporangial head projects but slightly from the surface of the receptacle.
From this description, and from the figures it is apparent that the whole
sporangium is from the first of more massive construction, and results
from more numerous segmentations than that of ordinary Leptosporangiate
Ferns, though the last segmentations which define the central cell follow
the usual sequence.
In the more attenuated type of Gl. dichotoma the sporangium is from
the first of more elongated form, and its stalk less massive (Fig. 311 g) ;
the formation of the cap-cell takes place at a time when the sporangial
head is more clearly in advance of the adjoining tissue, and the central
558 FILICALES
cell is thus never actually immersed in the tissue of the receptacle as
is the case in Gl. flabellata. In this feature again, Gl. dichotoma approaches
the ordinary Leptosporangiate type. The central sporangia, above noted
as occurring in this species, arise, as far as can be seen, simultaneously
with • the rest, and actually occupy the central area of the sorus from the
first (Figs. 311 /z, i) • this area is usually vacant in other species, and is
sometimes vacant also in Gl. dichotoma (Fig. 311 g)'. Since the sporangia
originate in this central position, their presence there cannot be accounted
for by displacement due to pressure ; it is to be ascribed rather to extra
development, or interpolation of one or more accessory sporangia, which
arise in a position usually unoccupied in the genus.
The divisions in the sporangial head to form the lateral cells of the
wall, the cap-cell, the tapetum and definitive archesporial cell, follow with
slight deviations the type general for Leptosporangiate Ferns (Figs. 311 b—g) :
the archesporial cell is usually of tetrahedral form, but from the first it
is of relatively small size, while the tapetum, which soon divides periclinally
into two layers, grows rapidly. The outer wall remains a single layer of
cells, but the cells divide freely by anticlinal walls so that in the mature
state the sporangia! wall consists of very numerous cells (Figs. 311 d, j\ k).
The annulus soon becomes differentiated, and it appears that part of the
annulus owes its origin to the cap-cell, but the larger part to the lateral
segments. The outer layer of the tapetum with occasional supernumerary
cells near the attachment of the stalk remains small, and forms a narrow
inner investment of the wall ; it is permanent for a considerable time,
and traces of it may be found even in the mature sporangium. The inner
tapetal cells enlarge greatly, and often become polynucleate ; their pro-
toplasm becomes aggregated, with the nuclei in close proximity to the
sporogenous mass (Figs. 311 /', k\ while the cell-walls become absorbed.
The definitive archesporial cell in G. flabellata undergoes successive
divisions (Figs. 311 d,j\ but the divisions are continued beyond the limited
number usual for Leptosporangiate Ferns ; the result is a very considerable
cell-mass, so that a single vertical section through a sporogenous group of
an average sporangium may traverse as many as 46 spore-mother-cells
(Fig. 311 j) • 45 was found to be the mean of countings in sections
through eight different sporangia. A section through a sporangium parallel
to the surface of the leaf may traverse even a larger number, as in the
sporangium of Fig. 311 /£, where 66 are shown in section. This difference
may be in part due to the section traversing the curved sporogenous
mass obliquely, but this explanation will not account completely for the
variation in number. Any one section will only traverse about one-eighth
of the whole number of sporogenous cells, thus there will be about
45 x 8 = 360 spore-mother-cells in a single sporangium, and the potential
output of spores may be estimated at about 360x4=1440. Comparing
this with the case of ordinary Leptosporangiate Ferns, it is plain that the
potential productiveness of an average sporangium -of GL flabellata is far
GLEICHENIACEAE 559
in advance of the latter. G. dichdidma shows individual fluctuations in
size of the sporogenous group, while the number of spore-mother-cells is
considerably below that in G. flabellata. Subsequently, the spore-mother-
cells separate, becoming rounded off, and undergo the usual tetrad
division. Prior to this, the tapetal nuclei make their way in among the
developing spore-mother-cells, as has been described for other sporangia.
In order to test the results obtained from sections, and the estimates
of potential spore-production based on them, countings of the actual spores
produced from single sporangia have been made in various species of
Gleiche?iia, with the following results :
Gl. flabellata, 794, 695, 838, 634.
GL cird?iata, 241, 242.
GL rupestris, var. glaucescens, 220, 232, 244.
GL hecistophylla, 265, 272.
GL dichotoma, 251, 319.
From the figures it appears that the output is very irregular, but consider-
ably in excess of that in most Leptosporangiate Ferns ; that the high
estimated number in GL flabellata is not actually attained ; and that though
in the four latter species the numbers approximate to 256, that figure is
liable to be exceeded. That the actual figure in GL flabellata falls below
the estimate may be accounted for partly by the abortion of some spore-
mother-cells, or young spores, of which there is evidence ; partly by errors
in counting such large numbers ; but it may also be due to the number
of spore-mother-cells being inconstant, or being actually not so large as
the estimate, which is necessarily only a rough one ; another reason for
the deficiency is the frequently incomplete division of the spores of single
tetrads. There is no exact proportion between the size of the individual
sporangium and its output of spores in this genus as a whole. GL rirrinata,
with its large sporangium, has a smaller output than GL flabellata^ of
which the sporangium is a medium size. It is, however, to be noted that
the spores in the latter species are smaller than in the former.
The dehiscence of the sporangium takes place by a slit in the median
radial plane ; the annulus, which is continuous all round, except along
the line of rupture, becomes gradually straightened on drying, or even,
everted, the whole sporangium thus widening laterally so as to elbow
aside the other sporangia in cases where these are numerous. Then a
sudden jerk on both sides of the slit throws the spores out, right and
left. Plainly, this mode of dehiscence requires lateral space, to allow of
the widening before the jerk, and it is thus ill-suited for a crowded sorus.
Its existence here indicates that the Gleicheniaceae are in the upgrade,
not in the downgrade, of soral complexity. The facts point to the radiate-
uniseriate type of sorus as being the primitive state, while the spore-numbers
would indicate that of the species examined, G. flabellata, in which that
type of sorus is represented in its most regular form, is probably the most
FILICALES
primitive. It will be seen that this species has a stelar structure of the axis
of a type which also indicates its relatively primitive character in the genus.
The sorus of Oligocarpia from the upper Carboniferous corresponds
in its arrangement to that of Gleichenia (Fig. 312). O. Gutbieri and
Hndsaeoid.es show uniseriate sori with varying number of the sporangia,
as in G. flabellata ; but O, Brongniartii has accessory sporangia occupying
FIG. 312.
I. Oligocarpia Brongniartii, Stur. A =a sterile ; Z?=a fertile pinnule (y) ; C = a sorus
more strongly magnified. (After Stur.) 1 1. = two sori of the same species. Xss- (After
Zeiller.) III. Oligocarpia Giitbieri, Gopp. .4 = position of the sori on a segment of
the last order. XSQ. B = a sorus. X6o. (After Stur.) IV. ^Oligocarpia lindsaeoides
(Ett.), Stur. -4 = position of the sori on a segment of the last order. X3O. £ = a sorus.
C—a. sporangium. x6o. (After Stur, from Potonie's Lehrbuch.)
the centre of the sorus, as in G. dichotoma. There is, however, a difference
of opinion as to the annulus, and it is upon this that the ultimate deter-
mination must rest : Zeiller recognises an annulus similar to that of the
• Gleicheniaceae; but Solms Laubach * does not assent to this, asserting
that the supposed annulus is due to an effect of lighting of the specimen
under observation. Zeiller nevertheless adheres to his opinion.2 Whatever
1 Fossil Botany, p. 146.
2 Potonie, Lehrbuch, p. 102. Mr. Kidston has shown me specimens of Oligocarpia
Gutbieri in which the line of dehiscence was clearly seen running radially down the
central face of the sporangium. The annulus could not be reduced to a single row of
cells. Probably the type had a pluriseriate annulus like other Palaeozoic Ferns (compare
Kidston, Phil. Trans., Ser. B, vol. 198, p. 188 ; also Scott, Progresses Rei Botanicae,
vol. i., p. 184). It may, however, be remarked that a division of the cells of the annulus
appears as an occasional irregularity in sporangia of living species of Gleichenia, a distant
suggestion of a pluriseriate annulus in the ancestry.
GLEICHENIACEAE
561
be the final decision on this point, it is clear that sori of the same type
as those of Gleichenia existed at the Carboniferous period, and that in size
and form the constituent sporangia were like those seen in the living
species.
ANATOMY.
There is greater uniformity of anatomical structure in the Gleicheniaceae
than in the Schizaeaceae ; but still there are marked differences within
the family which have a probable phyletic bearing when placed in relation
FIG. 313.
A — diagram of the tissues of the rhizome of Gleichenia Jlabellata. X 8. B = section of
the stele (somewhat diagrammatic) of G. pectinata. X 26. C = part of the stele of G.
dichotoma. X 350. (All after Boodle, from Campbell's Mosses and Ferns.")
to other characters.1 In the majority of species the rhizome shows in
;he internodes a centrally placed, solid stele (protostele), consisting of a
central mass of xylem composed of tracheides and parenchyma, and
surrounded by a continuous ring of phloem, pericycle, and endodermis
'Fig. 313 A). There is thus a general resemblance to the structure of the
rhizome of Lygodium ; but a point of difference is that whereas in Lygodium
there is no typical protoxylem, in Gleichenia the protoxylem is represented
ay several distinct groups of spiral elements, which are mesarch. The
tracheides of the xylem are arranged in chains and groups separated by
parenchyma : in fact the structure as seen in G. flabellata is strikingly
ike that of Lygodium, except in the matter of the protoxylem. In several
*The data here embodied are chiefly derived from Poirault, Ann. Set. Nat. Bot.>
7 Serie, T. xviii., p. 170, etc., and from Boodle, Ann. of Bot., vol. xv., p. 703.
2 N
562 FILICALES
species the xylem-core is fluted, the protoxylems being mesarch in its
slightly projecting ridges. The only other widely different type of structure
found in the genus is the solenostelic, which has been observed in G.
pectinata alone : here the stele is larger than in any other species which
have been examined : in addition to the structure as described the centre
of the fluted xylem is replaced by a mass of sclerenchyma, surrounded by
a ring of endodermis, pericycle, phloem, and conjunctive parenchyma
(Fig. 3136). Another type which takes an intermediate position as
compared with those already mentioned is seen in G, (Platyzomd) micro-
phyllum, in which the leaves are densely crowded and polystichous on the
rhizome. Here there is also an inner endodermis surrounding a central
sclerenchyma, but there is no internal phloem between the xylem and
endodermis.
In this last-named species the leaves are small, and the leaf-trace
separates as a small collateral strand from the periphery of the stele
without disturbance of the underlying tissues. This appears also to be
the mode of origin in the seedling of the more complex G. circinata\.
but in these larger-leaved species the leaf-trace of the mature leaf takes
in the petiole an almost cylindrical form bounded by an endodermis,
with (§ Mertensia} or without (§ Eugleiche?iid) an involution on the adaxial
side (Fig. 314). G. dichotoma is exceptional in § Mertensia in having
no involution. The whole petiolar bundle may be regarded as a single
flat ribbon widened laterally, but closely compressed and crumpled so as
to take a cylindrical form : in that case the condition of § Mertensia with
the endodermal involution would be more primitive than § Eugleichenia
where there is none. But G. dichotoma is an exception in the latter
section, showing the more advanced state.
The node of insertion of these larger leaf-traces may be marked by
complications, islands of tissue (composed of phloem, endodermis, and
sclerenchyma) appearing in the xylem of the stele as cut transversely :
these correspond actually to pocket-like encroachments of those tissues,
extending down from the centre of the petiolar trace into the stele of the
axis. Such pockets are only slightly developed in Eugleichenia, but more
so in Mertensia, and especially so in G. dichotoma, which leads suggestively
on towards the continuous solenostely seen in G, pectinata. They have
their relation to the theory of stelar structure, and on the facts two views
are possible : either that the protostelic condition of most Gleichenias is
primitive, and that the solenostelic type has been derived from it, or that
the protostelic Gleichenias might be regarded as showing the reduced
remnants of a previous solenostelic structure. The former view appears
the more probable : in the first place the seedling is protostelic, and offers
no suggestion of reduction to produce that primitive state : analogy with
Lygodium corroborates this. Further, the nodal pockets may naturally^
be held to be local complications of the stele, directly connected with
the insertion of the peculiarly complicated leaf-trace of an unusually
GLEICHENIACEAE 563
developed leaf : the formation of the more bulky pockets, and their
continuation throughout the internode would give the solenostelic structure.
Lastly, the most complex stelar state is seen in G. dichotuma and pectinata,
species which in the character of the leaf, as well as of the sorus and
sporangium, are aberrant from the rest of the genus, and have been
recognised as showing advance towards the Cyatheaceous type. These
several grounds indicate that an evolutionary progression rather than a
retrogression is illustrated in the genus, from a protostelic to a solenostelic
structure.
FIG. 314.
Transverse section of the base of the petiole of Glcichenia dicarpa, showing the
pseudo-stelar structure resulting from contraction of the horse-shoe-like xylem, till its
margins fuse. Photograph by R. Kidston, from section by Gwynne-Vaughan.
But lastly, there is the case of G. (Plafyzoma) microphylla : Boodle
suggests that this is a xerophytically reduced form, in which the leaf-
traces have become small and crowded, and that it is probably derived
from a solenostelic form by obliteration of the leaf-gaps and disappearance
of the internal phloem. But his alternative suggestion, that it may have
been derived from a protostelic Gleichenia, and its structure be due to
the new formation of a pith and internal endodermis, appears the simpler
as well as the more probable, in the case of an upright plant with closely
crowded leaves. For it must be remembered that this was the condition
of the shoot common for primitive Pteridophytes.
564 FILICALES
EMBRYOLOGY.
The development of the embryo appears to follow the type usual for
Leptosporangiate Ferns, but the details are not adequately known.1
The Palaeophytological evidence coupled with the anatomical and soral
characters indicates for the Gleicheniaceae a position among relatively
primitive Ferns. The comparative examination of the living species leads
to the recognition of G. flabellata as a central type. This is not so much
suggested by the external form, as by the sorus, the stelar structure, the
relatively simple insertion of the leaf-trace, and the non-involuted strand
of the petiole. This species also shows the largest spore-output per
sporangium observed in the family. There has probably been a line of
diminution of the individual pinnules to produce the condition seen in
§ Eugleichenia, together with a reduction in number of the sporangia in
the sorus, leading to a type of monangial sorus similar to that of the
Schizaeaceae. A line of probable advance has been to such forms as
G. pectinata and dichotoma • for not only do these species show interpo-
lation of extra sporangia in the sorus, together with smaller sporangia
and diminished output per sporangium, but also they are anatomically
more complex. This is specially shown by the large nodal pockets of
G. dichotomy and ultimately by the continuous solenostely seen in G.
pectinata. In both respects these species indicate changes from the central
type in the direction of Cyatheaceous characters.
MATONINEAE.
This family,2 is represented by only two species of living Ferns, Matonia
pectinata and M. sarmentosa, both of limited distribution in the Malayan
region. But Ferns referred to this affinity on the characters of leaf and
fructification played a prominent part in the vegetation of the Secondary
Rocks, and have been traced back as far as the Rhaetic period : this
fact accords with the unmistakable analogies which they show to the
Gleicheniaceae.
The two living species differ in habit : M. pectinata is a stout, ground-
growing species, with elongated creeping rhizome, covered with filamentous
hairs, and branching in an apparently dichotomous manner. It bears
solitary leaves at considerable distances apart on its upper surface.
These grow to a height of 6 to 8 feet, and have a very characteristic pedate
construction of the lamina, which is referable to a dichotomous system
of branching (Fig. 315): even the "middle lobe," which often appears
1 Rawenhoff, Arch. Neerl., T. xxiv., p. 223.
2 The chief sources of information have been Natur. Pflanzenfain . , i. 4., p. 343;
Seward, Phil. Trans., vol. 191, p. 171 ; Tansley and Lulham, Ann. of Bot., vol.
xix., p. 475, and my own Studies, iv., p. 44.
MATONINEAE
565
to hold a terminal position, has been recognised as the inner branch of
the second dichotomy. The segments themselves are pinnatifid, and the
solitary sori are borne on their wings at points near to the midrib. The
other species, M. sarmentosa, grows on rocks or on the branches of trees,
with straggling, pendent leaves : at first sight the branching of the leaf
seems quite different from that of M. pectinata ; but this is due partly to
the unequal development of the dichotomies, certain of the branches being
represented only by arrested buds : partly it is due to their sympodial
concatenation : but still the dichotomous branching appears to hold for
FIG. 315.
Matonia pectinata, R. Br. Leaf drawn from a specimen in the British Museum
Herbarium, by Mrs. Seward. £ natural size.
Doth. An interesting feature in this species is that the sori are borne in
arger numbers, forming a row on either side of the distal part of the
Dinnule : an arrangement more closely resembling that in Gleichenia than
hat of M. pectinata. The , structure of the sorus is, however, precisely
ike that in M. pecti?iata^ and there is no doubt of the close alliance of
he two species.
SPORE-PRODUCING MEMBERS.
The general structure of the mature sorus is well known ; the sporangia,
commonly six to nine in number, form a simple ring-like series round the
receptacle, and are covered till maturity by the thick and leathery hemi-
spherical indusium, which is ultimately deciduous* The orientation of
the sporangia is not exactly uniform ; that of the majority is as in Gleichenia,
3ut many have the annulus inclined, a consequence probably of crowding ;
this is seen also in the fossil Laccopteris. The annulus is incomplete at
566 FILICALES
one side, an ill-defined lateral stomium being present, while the rupture
is by a ragged lateral slit, opened by the straightening annulus (Fig. 316).
i The sorus originates as a smooth upgrowth from the lower surface of the
pinnule, opposite a nerve, a considerable number of cells being involved
from the first ; no definite mode of segmentation
has been recognised (Fig. 317 F). As develop-
ment proceeds, the margin of the upgrowth
extends all round, as the overarching indusium
(/, /') ; this, undergoing a somewhat regular seg-
mentation by anticlinal walls, curves so as to
cover in the sporangia which arise below (s., Fig.
317 F) ; the indusium thus precedes the appear -
§to«^^ ance of the sporangia, as in many other indusiate
Ferns. The sporangia originate from single cells,
FIG. 316. i'ii i
which have commonly a square base, though it
may be a question whether this is always so.
Sewnar3a)turally detached' (Mter The segmentation is by walls inclined to one
another; the first wall is usually on the side next
to the leaf-surface, and meets one of the lateral walls of the parent cell ;
then follow three other inclined walls, and the segments thus produced
surround a central triangular wedge-shaped cell, from which finally the
cap-cell is cut off in the usual way (Fig. 317 E).
The further segmentation of the central cell follows the course usual
for Leptosporangiate Ferns; a double tapetum is formed (Fig. 317 D) of
which the inner cells become greatly enlarged, and their nuclei, clustering
round the sporogenous group of cells, and undergoing fragmentation,
present an appearance very like that in Gleichenia ; the archesporium
divides into 1 6 spore-mother-cells, and the typical number of spores seems
to be 64 : countings of mature spores gave figures between 48 and 64 as
the produce of single sporangia. Sections of sporangia, when cut so as
to traverse the annulus throughout its course, show the wall as a single
layer, but composed of more numerous cells than is the case in many
of the Leptosporangiate Ferns (Fig. 317 D) ; this is also brought out
plainly in views of the mature sporangia from without (Figs. 317 A, B, c).
It may be noted further that the stalk, which remains very short, is
rather massive, and consists of a peripheral series of six or seven cells,
surrounding a central cell (Fig. 317 A), which corresponds to the structure
of the stalk in the massive sporangia in Gleichenia and Osmunda.
The mature sporangium is a body of rather irregular and variable
form, owing apparently to pressures in the developing sorus. The annulus
is incomplete and variable in position ; it consists of a series of large
cells, 20 or more in number, which takes an oblique and sinuous course,
corresponding in the main to that in Gleichenia. The sporangia are
liable to be tilted right or left, as shown in Fig. 317 B, which represents
two sporangia in situ, as seen from the side facing the indusium.
MATONINEAE
567
Sporangia in which the annulus is not tilted are shown in Figs. 317 A, c;
from these it will be seen that the annulus starts close to the stalk; it
first curves downwards towards the basiscopic side of the sporangium,
then circling round it, curves upwards, the highest point being reached
at the distal end of the sporangium ; passing this it again curves down
wards towards the basiscopic side, and stops short at some distance from
the stalk ; it is at this point that the dehiscence takes place, but though
the cells immediately beyond the end of the annulus may show some
regularity of division, there is in Matonia no highly specialised stomium
as is the case in most Leptosporangiate Ferns. Fig. 317 (the central
FIG. 317.
Malonia. pectinata. A, £, C and the central figure represent the mature sporangia in
various aspects. ,F=young sorus ; /, z = indusium ; s = sporangium. _£= sporangium
with cap-cell; « = acroscopic, b = basiscopic side. D = sporangium with tapetum
doubled. A-Cx$o. D-Fx.2oo.
figure) shows the rupture ; x it also shows a case of the annulus stopping
short of the stalk on either side, and that there is not here any
continuous series of non-indurated cells, such as that seen in Loxsoma.
Thus Matonia has a sporangium with a short and massive stalk and
a large head, in which the annulus is not of a highly specialised, nor
even of a constant type, though in its main features it corresponds to
that of the Gleicheniaceae. It differs here, however, in its variability,
its lateral dehiscence, and in the comparatively small output of spores.
Of the fossil Matonineae the sori are best known in Laccopteris, which
is practically identical with Matonia in the size, disposition, and structure
of the sporangia and spores, but differs in having apparently no indusium
(Fig. 318). Probably, however, the sori of Matonidium and of Microdictyon
568
FILICALES
were indusiate, as in Matonia itself. The difference does not seem to
be an essential one, and in face of the correspondence of the Ferns in
question in other respects it
c cannot be held to invalidate
the reference of these, and of
certain other Mesozoic Ferns
to the family of the Maton-
meae.
ANATOMY.
FIG. 318.
A= pinnule of Laccopteris Woodward i from the inferior
oolite of Yorkshire : the hemispherical bosses show the
position of the sori (No. 217, Brit. Mus.). B = pinnule of
Laccopteris polypodioides with sori and soral impressions.
Upper shale, Gristhorpe Bay (No. 2522, Brit. Mus.). C=
pinnule fragment from the inferior oolite of Stamford (No.
52867, Brit. Mus.). (After Seward, from drawings by Miss
G. M. Woodward.)
The mature rhizome of
Matonia shows the most com-
plicated solenostelic structure
known in Ferns : in the young
stem, however, simpler condi-
tions are found which suggest
how the final condition was probably arrived at. In the most complex
rhizomes three concentric vascular rings may be found embedded in
parenchyma, and each showing the typical solenostelic structure. Each is
limited externally and internally by an endodermis and pericycle, while
between these in each is a continuous ring of xylem, with phloem on either
side of it. The arrangement of this solenostelic structure is represented
diagrammatically in Fig. 319, together with its connections with the leaf-
trace. The latter is in these Ferns one continuous band, with involuted
margins, which are shown in Fig. 319 c: this drawing also indicates that
foliar gaps occur, and shows how the leaf-trace is directly continuous with
the outer and middle of the concentric rings at the node. There may also
be a connection with the inner ring ; but this occurs at some little distance
from the actual node, and so is not shown in the drawing. The result
is that the whole system is connected, but only at intervals of its whole
length, while there is also connection through the leaf-gaps between the
parenchymatous tracts in which the cylinders are embedded.
The ontogeny gives the suggestion how this complicated structure is
to be placed in relation to that of other Ferns. The young axis contains
at first a slender protostele ; but this simple stele soon expands, and a
strand of phloem appears in the midst of the xylem. This internal phloem
appears to be a phloem-pocket decurrent from the adaxial surface of the
second leaf, but there is as yet no true leaf-gap. The stele soon widens
into a solenostele with internal endodermis and central parenchyma.
Meanwhile at the nodes a ridge of xylem projects internally, which becomes
more prominent at subsequent nodes, and is continued forwards into the
internode further and further at successive nodes, till that of one node
eventually connects with a similar xylem-dilatation of the next node
(Fig. 3 19 A). A continuous central strand is thus produced, which is
connected at the nodes with the outer cylinder.
MATONINEAE
569
central
which
strand :
is still
The process thus described may then be repeated in that
it becomes cylindrical, forming the second vascular ring,
connected at the nodes with the foliar
system (Fig. 3196), and a fresh central
strand originates internally from it :
this in its turn becomes cylindrical
in the most advanced types, but still
maintains its connection with the
middle and outer rings in the neigh-
bourhood of the nodes. The whole
development is in fact an extreme
type of the progression described by
Gwynne-Vaughan in other solenostelic
Ferns.1 He showed how an internal
vascular system may arise by progres-
sive elaboration from a local thickening
of the margin of the leaf- gap of the
original solenostele. In Matonia this
development is the same, but it may
be twice repeated.
It is indicated by the palaeophy-
tological evidence that while the
Matonia-type is an ancient one it is
not among the earliest. This accords
with the soral and anatomical char-
acters ; for the sorus, though of the
Gleicheniaceous type, and still form-
ing its sporangia simultaneously as in
other Simplices, shows an advanced
feature in the indusium, as also in
the lateral dehiscence, and small
spore-output. Anatomically the indi-
cations are of the same nature :
•\f , • i • i_ /^» j.' ' M atonia pectinata, drawings from wax models of
Matonia aCCOrds With G. pectinate in the stelar system. ^=froma young stem showing
the solenostelic structure, but carries JJ^"
FIG. 319.
an older stem, showing node seen
that line of elaboration much further. Luihaml*25' *X'2' Cx'°' <AfterT»MleyMld
Finally, in the rhizomic habit and in
the branching of the leaf there appears to be further similarity : there is
indeed sufficient reason to regard the Matonineae as a family related to
the Gleicheniaceae, but advanced in several respects beyond that type,,
in directions which are represented more fully in other series of Ferns.
1 Ann. of Bot., xvii., p. 703.
CHAPTER XXXVII.
GRADATAE.
THE Ferns so far described, however different in detail, all correspond in
producing those sporangia that are in near juxtaposition simultaneously :
the sporangia themselves are of large size, with short, usually massive
stalks. The output of spores per sporangium is commonly in excess of
that in other Leptosporangiate Ferns. Matonia is, however, an exception
to this, having not more than 64 spores : as also in its dehiscence, which
is lateral, while in all the rest it is in a median plane. But notwithstanding
these discrepancies, in the fact that the sporangia are simultaneously
produced, as well as in other features, the Matonineae find their natural
place with those Ferns which have been styled the Simplices.
We shall now proceed to types in which the sporangia appear not
simultaneously, but in basipetal succession : these have been styled the
Gradatae. In them the position of the sorus may vary, as indeed it does
in those with simultaneous sporangia ; while the Marattiaceae, Gleicheniaceae,
and Matonineae have superficial sori, the sporangia of the Schizaeaceae
may be marginal : in the Osmundaceae the sporangia may be on the lower
surface only (Todea) or cover both surfaces and margins (Osmundd). So
also we shall find similar variations of position in the basipetal sori : the
Loxsomaceae, Hymenophyllaceae, Dicksonieae, and Dennstaedtiinae all have
marginal sori, while in the Cyatheaceae they are superficial. Such difference
of position may serve as a useful character separating the tribes, but need
not in any way vitiate our comparisons. In other words, the method of
internal arrangement of the sorus is to be estimated as a more important
character than the exact position which the sorus holds upon the leaf
which bears it. It will be seen that while the basipetal succession in the
sorus is taken as the defining character of the Gradatae, other characters
indicate a higher position, but none with the same distinctness, and in
many features these Ferns resemble the Simplices. There is reason to
think, therefore, that they originated from some similar common stock,
but adopted the basipetal succession of sporangia at a relatively late stage.
LOXSOMACEAE 571
This is biologically probable, since the successive development has the
advantage of producing a large spore-output, while the physiological drain
would thus be spread uniformly over a long period of time.
LOXSOMACEAE.
This family is represented only by the single species Loxsoma Cunning-
hami, Br., native in New Zealand. It unites in itself characters of several
distinct tribes of Ferns, and as a consequence its systematic position has
been difficult to fix. It has the habit of a coriaceous Dicksonia or of a
Davaltia. and a sorus like Trichomanes; but it differs from all of these
in having a dehiscence of the sporangia in a median plane. This combina-
tion of characters has led to its being variously placed by different
systematists. It is best regarded as the sole representative of a distinct
tribe, and its natural position appears to be about the limit between the
Simplices and the Gradatae, in a phyletic line which leads towards the
Hymenophyllaceae and Dicksonieae.
L. Cunninghami is an elegant Fern, with elongated, creeping rhizome,
bearing irregularly disposed roots, and at intervals of about an inch firm
coriaceous leaves, one to two feet high, which are glabrous, twice or
thrice pinnate, and glaucous beneath. The sori are marginal, each seated
upon the ending of one of the simple or branched veins. There is a basal
•cup-shaped indusium, with an entire rim : it surrounds the receptacle,
which is columnar, and bears numerous fluffy hairs interspersed among
sporangia, which originate in a basipetal succession. The whole appearance
of the Fern is very like some of the creeping species of Dicksonia or
Davallia.
I am not aware of any fossils having been attributed to this family.
SPORE-PRODUCING MEMBERS.
A vertical section through a sorus of medium age shows, as in Fig. 320 E,
the short receptacle, sporangia, and hairs, all of which are included within
the cup-like indusium ; the/e is an obvious basipetal sequence of the
sporangia. The orientation of the sporangia relatively to the centre of
the sorus is constant, on the Gleicheniaceous type. The pear-shaped
sporangia, which rise obliquely upwards, have a complete annulus, as is
shown in Fig. 3200, which represents the "peripheral" face; but though
the complete series of cells of the ring can usually be traced, the induration
of the walls is very unequal ; commonly the cells of the distal half are
enlarged, and their walls thickened ; these are mechanically functional,
while the lower part may be composed of thinner-walled cells, sometimes
slightly or irregularly thickened, but usually not differing from the rest
of the cells of the wall, except in their form and arrangement. This is
shown in side view in Fig. 3200. If we compare Figs, c and D with drawings
572 FILICALES
of Gleichenia, it is plain that the sporangia are of the same type, as regards-
the position of the annulus, though differing in the details ; or the comparison
might be extended to the Schizaeaceae on the one hand, or the Hymeno-
phyllaceae on the other, as regards the position of the annulus.
The longitudinal slit of dehiscence traverses the distal part of the
annulus, following the median plane of the sporangium, and may extend
some distance down its peri-
A B pheral side, so that it faces
outwards from the receptacle.
The orientation of the spor-
angia being strictly upon the
Gleicheniaceous type, it ap
pears that the main difference
is that, while maintaining the
same position of the annulus,
the slit gapes towards the
peripheral face of the spor-
angium rather than on the
central side of it. There is no
differentiation of a stomium,
but the rupture occurs regu-
larly at the distal end (JVT,
Fig. 3200). The portions of
the annulus on either side of
the slit straighten as they dry
and curve outwards in the
usual way ; they may even
become reflexed, tearing away
irregularly from the rest of
the wall, or carrying frag-
ments of it outwards ; in this
state the two flaps may appear
like the covers of an opei
book. As the induratioi
stops short about half-wa^
Cand"z>x5o. down the side, the genei
form of the sporangium
not altered by the dehiscence, so as to press upon or displace neighbouring
sporangia ; in fact, no elbow-room is required, as in GUichtnia, and this
is a distinct advantage in a sorus where sporangia are numerous ; in this
we may perhaps see the rationale of the incomplete annulus. No suddei
jerks of the annulus have been observed, nor would such jerks be vei
efficient, since the majority of the spores lie below the flaps of the annulus
the shedding of the spores seems to be mainly on the principle of th<
pepper-box.
FIG. 320.
Loxsoma Cunninghami, Br. E~ young sorus with sporangia
still protected by indusium (/, z). ^4=sorus rather older, with
sporangia (s, s) carried up on the elongated receptacle (/.$•).
B = vertical section of the base of the young receptacle (r)
showing sporangia (s) in basipetal sequence. C, D = mature
sporangia showing the incompletely indurated annulus, and
distal point of dehiscence (x). A and Ex about 20. .5x250.
LOXSOMACEAE
573
The receptacle performs an important part in connection with dispersal.
At first it is short, so that the sporangia are all included within the indusium
'(Fig. 320 E), and this is so till the oldest sporangia are mature; an inter-
calary growth then takes place at the base of the receptacle, the thin-
walled cells above the terminal mass of tracheids (tr.) become greatly
elongated (Fig. 320 A), forming a sort of pseudopodium (ps.\ upon which
the sporangia are raised so as to project beyond the lip of the protective
indusium, and are thus free to scatter their spores. The arrangement is
-similar to that seen in the Hymenophyllaceae, but in Loxsoma the pseudo-
podium is formed independently of the long-continued formation of a
series of sporangia.
The sporangium makes its first appearance as a massive deeply sunk
cell, near the base of the groove between the receptacle (r) and the
indusium (ind.) (Fig. 320 B) : the first segmentation in it passes down to
the base of the cell, as in the Schizaeaceae and some other Simplices ; the
later ones cut the previous wall obliquely, and thus a three-angled conical
cell is surrounded by three lateral segments. The cap-division, and
segmentations forming the tapetum take place in the usual way ; the inner
series of tapetal cells enlarge considerably, and become polynucleate, thus
resembling other large sporangial types. The definite sporogenous group is
composed of 16 spore-mother-cells, which undergo a tetrad division to give
typically 64 large spores.
ANATOMY.1
The chief point of anatomical interest is the structure of the stele of
the stem : a transverse section of an internode shows a typical solenostele,
with phloem, pericycle, and
^ndodermis, both outside
and inside of the continu-
ous ring of xylem. The
protoxylem elements are all
scalariform, and are not
localised into groups, but
-are distributed around the
periphery of the solenostele.
Where a leaf-trace is given
off the tube of the stele
opens, forming a foliar gap
on the acroscopic side.
The leaf-trace itself consists
of a single vascular strand,
showing the horse-shoe outline in transverse section (Fig. 321). An unusual
feature is the occurrence of islets of parenchyma in the sclerenchymatous
masses of the stem, a peculiarity shared with certain species of Dicksonia.
FIG. 321.
Loxsoma Cunninghami. Diagram showing the form of the
vascular system at the node of the rhizome. jj = solenostele ; lt =
departing 1 eaf- trace : lg = leaf-gap. The arrow points toward the
apex of the rhizome. (After Gwynne-Vaughan.)
Gwynne-Vaughan, Ann. of Bot., vol. xv., p. 71.
574 FILICALES
These features indicate that Loxsoma is more nearly related anatomically
to the Dicksonieae and Dennstaedtiinae than to any other family of
Ferns ; but a reasonable analogy is also to be found with the more
advanced species of Gleichenia : the solenostelic structure seen in G.
pectinata as well as the origin of its foliar trace are points for comparison,
while structural affinities of a more remote nature are also indicated with
the Schizaeaceae and Hymenophyllaceae.
It thus appears that Loxsoma is a generalised type, while its rare
and local occurrence countenances this view. In habit it shows similarity
to such genera as Dennstaedtia, Microlepia, and Davallia, a comparison
which finds support in the anatomy of the vascular system : not only do
the habit and anatomy support this, but also the. form of indusium and
receptacle, and the basipetal succession and orientation of the sporangia.
An affinity with the Hymenophyllaceae is also unmistakable, but probably
not so close as has often been assumed : against it are the texture of the
leaf, the mode of dehiscence and the structure of the sporangium,
and the low output of the very large spores : in any case the affinity
is with the less specialised types (e.g. Hymenophyllum dilatatum] rather
than the more specialised (e.g. Trichomanes). The sporangium, and its
annulus and dehiscence point clearly towards the Gleicheniaceae and
Schizaeaceae ; and though the habit of the leaf is different from these
Ferns, the structure of the creeping rhizome shows a certain resemblance.
The similarity of position of the annulus, and constancy of orientation
are important, especially when taken with the very peculiar facts of induration.
For, as we have seen, the distal side of the annulus is indurated, while
the proximal can still be followed, though it is commonly thin-walled ;
but occasionally single cells, or groups of cells, of the proximal side are
also indurated : these cannot be functionally active, since they do not
form a connected series. In them I think we can only see a decadent
vestige of a completely indurated annulus, and conclude that Loxsoma was
derived from ancestors with a complete oblique annulus, probably with
a median dehiscence. Such ancestry might be found in the neighbourhood
of Gleichenia. In Gl. dichotoma we have a type in which the sporangium
and the sorus are similar in their main character. If we imagine these
sori to be marginal (as they are in Lygodium], surrounded by a cup-like
annulus which is already suggested in some Gleichenias, with the annulus
modified as explained above to suit the more crowded sorus, and with a
smaller number of spores, balanced by a larger number of sporangia
produced in basipetal order, the sorus of Loxsoma would be before us. It
is not suggested that any living Gleichenia was a progenitor of Loxsoma,
but Loxsoma appears to be a link connecting the Gleichenia Schizaea
affinity with the type of Dennstaedtia and Microlepia. It should be
regarded as the sole representative of a distinct tribe : the attempt should
not be made to force it into any other tribe of living Ferns.
HYMENOPHYLLACEAE
575
HYMENOPHYLLACEAE.
This family includes only the two genera, Hymenophyllum and Tricho-
manes, but each is represented by a large number of species, distributed
FIG. 322.
Habit of Hymenophyllum. A = H. cruentunt, Cav. B = H. dilatatutn, Sw. C = //.
australt, Willd. (After Sadebeck, from Engler and PrantI, Nat. P^fanzen/am.) _. fl;jM .*« *
chiefly in moist and shaded spots throughout the tropics : they extend
as stragglers northwards, though more freely to the south, and there is
a special centre of their distribution in New Zealand.
576 FILICALES
The shoot is sometimes upright and radial, with leaves showing fths
phyllotaxis, as in some species of Trichomanes\ or more commonly
creeping and dorsiventral, with the leaves arranged distichously, with
elongated internodes, as irt many species of Trichotnanes, and all of
Hymenophyllum. From the axis numerous scattered roots arise in most
species, but in some, and especially in the section Hemiphlebium of
Trichomanes, no adventitious roots are formed, leafless branches of the
rhizome serving as substitutes. These are covered by root-hairs, which
resemble, however, the hairs which are normally found on axis and leaf
in the rooted species also. The hairs are filamentous, and ramenta are
absent, but peculiar scales are found in some species of Trichomanes, and
in some of Hymenophyllum of exposed habit the leaf is covered with
a hairy felt.
The leaves in some of the larger species (If. dilatatum^ australe) conform
in outline to ordinary branched Filicinean types, and are winged structures
to the base (Fig. 322). But in other cases the leaf may appear as a
widened expansion of simple form, with or without a leaf-stalk (Fig. 323), as
in H. cruentum, or T. reniforme, and meuibranaceum. It would appear probable
that the latter are specialised and derivative forms, and they occur more
freely in the genus Trichomanes, which there is good reason to believe to
be the more specialised genus. The leaves are translucent or "filmy"
in texture, a feature that will be considered at length below.
Axillary branches occur very generally in the Hymenophyllaceae, bu
at many nodes the rudiment of the axillary bud remains undeveloped.
The sori are marginal in all cases : the receptacle upon which the
sporangia are inserted in strictly basipetal sequence is traversed by the
direct continuation of one of the veins of the lamina ; it is surrounded
by the cup-like indusium, which is entire in Trichomanes^ but two lipped
in Hymenophyllum. The sporangia vary greatly in size and productiveness,
but have uniformly an oblique annulus and lateral dehiscence : it will
be seen that these characters are closely related to the regular basipetal
sequence in which they are produced upon the receptacle.
Sori and sporangia of corresponding type have been traced back to
early geological formations. From the upper Carboniferous, or perhaps
even earlier, come the doubtful sporangia of Hymenophyllites, which will
considered in detail below. Meanwhile it may be noted that there i
reason to believe the type to have been a very ancient one.
SPORE-PRODUCING MEMBERS.
The " comparative study of the sorus of the Hymenophyllaceae leads
to the conclusion that while these Ferns show the highest complexity
of the receptacle, the sporangia themselves are related in character
to more massive types, and that this will justify a systematic positioi
near to the Loxsomaceae, Dicksonieae, and Gleicheniaceae on the
;
m
Cf
FIG. 323.
Halnt of Trichomanes, A-T. renifornte, Forst. B—T, ntembranaceum. L.
a, sterile: /'. fertile. C=T. Lyaltii, Hook, a, sterile; b, fertile. D=T. spicatnm,
Hedw. (After Sadeheck, from Engler and Prantl, Nat. Pflanzenfain.')
2 O
OF THE
UNIVERSIT
578
FILICALES
one hand, and on the other to the Osmundaceae and certain of the
early fossils. The general construction of the Hymenophyllaceous sorus
is well known. Many satisfactory drawings are given by Presl, and
other descriptive writers, which show how the
sporangia with their oblique annulus are disposed
with regularity of orientation upon the elongated
receptacle, so that they overlap one another like
the shields of a Roman testudo. The orientation
for each single sporangium corresponds essentially
with that seen in Gleichenia or Loxsoma. This is
well shown for Trichomanes in the drawing of
Goebel (Fig. 324). The sporangia are produced in
basipetal succession upon the more or less elongated
receptacle. This fact is demonstrated in Fig. 324^/5-,
which represents the young sorus of Hymenophyllum
Wilsoni already bearing the young sporangia (s) near
the apex of the receptacle, while below there are clear
indications of the active intercalary growth. The
extent of the intercalary growth of the receptacle is
greater in Trichomanes than in Hymenophyllum,
and the genera were by early writers distinguished
on this ground, those with the receptacle exserted
being ranked with the former, while those with it
included fell into Hymenophyllum. Though this
generic distinction will not hold accurately, still the
general statement is correct that intercalary growth
of the receptacle, and basipetal succession of the
sporangia are longer continued in Trichomanes', it
is the extreme example of this mode of development
of the sorus among Ferns, but none the less is it
similar in kind to that described for other Gradatae.
The sporangia in the Hymenophyllaceae differ
greatly in size, between a large, almost spherical
type, such as that of Hymenophyllum dilatatum (Fig.
325, Nos. 95, 96, 97, 98), and small compressed
bodies such as are formed in many species of
Trichomanes (Fig. 325, Nos. 99, 100, 101, 102);
it will be shown that with this goes a very wide
difference in the output of spores. The large
sporangia of Hym. dilatatum are produced in relatively small numbers
upon the short, but rather broad, receptacle ; in size and form they are
comparable to those of Gleichenia circinata (compare Fig. 325, No. 95,
with Fig. 310 i of Gl. circinata\ while the annulus shows a similar degree
of obliqueness. Externally there is close similarity, excepting in the dehis-
cence, which is lateral in Hymenophyllum, a position which we shall see
FIG. 324.
Trichomanes tenerum.
Sorus in surface-view ; the
placenta bearing radially
distributed sporangia issues
from the two-lobed beaker-
like indusium. The annulus
is visible on the several
sporangia. Magnified. (After
Goebel.)
HYMENOPHYLLACEAE
579
FIG. 324 bis.
Hymenophyllum Wilsoni, Hk. Sorus
in longitudinal section showing the
receptacle with divisions indicating inter-
calary growth, and the first sporangia
originating near the apex. X 100.
FIG. 325.
— Sporapgia of Hymenophyllum dilatatum. Swartz, seen respectively
(Nos. 95, 96), and from the central (No. 97), and peripheral (No. 98)
faces. Nos. 90, 100, 101, 102 = similar figures, to the same scale of Trichomanes
Nos. 95, 96, 97,
from the two sides (Nos. 95, 96), and from the
radicans, Swartz. Nos. 99 and 100 show the lateral views. No. 101 shows the central,
and No. 102 the peripheral faces. All X 50.
the annulus varying between 20 and 25. There is also a greater simplicity in
the region of the stomium, which in Trichomanes is represented by two cells
only. No. 102 represents the "peripheral" face, the thin-walled region being
surrounded by the annulus, which takes the form of a twisted hoop. This
twisted form is clearly shown in No. 99, which demonstrates also the relation
580 FILICALES
of the sporangium to the receptacle, and that the distal part of the annulus
is directed obliquely towards its apex. This being the case for all the
sporangia, the free action of the annulus on dehiscence is assured for each
individual sporangium; this may be compared with No. 95 of Hym. dilatatum.
The stomium in Tr. radicans is of very simple construction : in the last of
the drawings it is shown in surface view, with the slit of dehiscence gaping
between the two cells. Comparing this with No. 96 of Hym. dilatatum, we
see again that Trichomanes
^~~~~^ is constructed on the same
general plan, of which it ap-
pears as a simplified edition.
The origin of the spor-
angium has been followed
by Prantl 1 in Trichomanes
^\\ speciosum, with which that
of Hymenophyllum agrees in
essentials. The parent cell
has a square base, and grows
out into a papilla, with seg-
mentation according to the
usual Leptosporangiate type;
but it is important to notice
that the first segmentation
strikes the basal wall of the
parent cell (Fig. 326), a con-
FIG. 326. dition which is seen in the
Trichomanes speciosum, Willd. (=7\ radicans), transverse massive Sporangia of the
section of the receptacle, showing early segmentation of the Cimrii:~~c nfh^r rhan in fVi^
sporangia. (After Prantl.)
more advanced Ferns.
The comparison thus suggested with the Fern-types which have large
sporangia is borne out by the facts which follow from enumeration of the
spores produced. The Hymenophyllaceae show among their species a wider
range of number of spores per sporangium than has been noted for any
other family of Ferns : while certain of their species approach, or even equal
the high numbers of the Gleicheniaceae, Schizaeaceae, and Osmundaceae,
in others the number is distinctly low : it will be seen that the species of
Hymenophyllum give for the most part a higher output per sporangium than
Trichomanes.^ The typical number for H. Tunbridgense was found to be
256-512, and for H. sericeum 256, while that of H. dilatatum and IVilsoni
was 128; but for six species of Trichomanes examined the typical numbers
varied from 32 to 64. Trichomanes reniforme, however, which is in many
respects an isolated and peculiar species, has the typical number of 256,
thus corresponding to Hymenophyllum rather than to its own genus.
Seeing that in Trichomanes the usual output per sporangium is lower
} Hymenophyllaceen, p. 38. 2 For full details see Studies, iv. , p. 64.
HYMENOPHYLLACEAE
581
than in Hymenophyllum, while on the other hand the receptacle is usually
longer, and has more continued intercalary growth, the question arises whether
the larger number of sporangia will approximately compensate for their lower
individual output. A computation was made of the output per sorus in
H. Timbridgenst and dilatatum, and compared with a similar computation
in T. reniforme and radicans, with the result that, notwithstanding the great
variations in spores per sporangium, the output per sorus appears approxi-
mately uniform for the cases quoted. Thus the increased length of the
receptacle and higher number of the sporangia tends to compensate the
smaller output per sporangium which is seen
in its extreme form in some species of
Trichomanes.
It has been seen that in Gleichenia, where
there is a median dehiscence of the sporangium,
elbow room is required for the process of
mechanical ejection of the spores, and that this
is only possible where the sporangia are loosely
arranged. In a crowded sorus such as that of
the Hymenophyllaceae the necessary space is
not available. In Loxsoma this difficulty is met
by the incomplete development of the annulus,
which then merely opens the distal end of the
sporangium and allows the spores to be shaken
out. But in the Hymenophyllaceae the whole
mechanism is altered by the adoption of the
oblique lateral dehiscence : as this is usual in
the basipetal sori of the Gradatae it demands
the greater attention. The sporangia are placed
relatively to one another as seen in the diagram
(Fig. 327), in which the cells of the annulus
traversed in the median section of the sporangium are indicated by
heavier lines. It is plain that on dehiscence taking place laterally, the
distal side of each annulus has freedom to alter its form independently
of the adjoining sporangia:1 the oblique position of the annulus thus
finds a practical explanation, and is even a necessity where the sporangia
are short-stalked and imbricate. The dehiscence is, however, aided by its
occurring usually in strict basipetal sequence in the Hymenophyllaceae :
after the lateral slit is formed, the annulus is first of all reflexed, and then
recovers with a sudden jerk, which often dislodges the whole sporangium,,
and scatters the spores. The result in Trichomanes is that the distal
end of the receptacle may remain bare of sporangia, an indication of the
extent of the intercalary growth, while young sporangia may still be found
around its base.
FIG. 327.
Diagram illustrating the relative
position of the sporangia on the
receptacle in the Hymenophyl-
laceae. It was constructed from
Prantl's section of a mature spor-
angium of Trichomanes Speciosnm.
1 As a matter of fact the freedom is greater than is shown in the diagram, for the sporangia
alternate, and are not disposed in closely consecutive orthostichies.
582 FILICALES
Various fossil fructifications from early geological formations have been
referred to the Hymenophyllaceae with more or less reason. The Devonian
Palaeopteris hibernica may be put aside as insufficiently known, while it
shows no distinct evidence of Hymenophyllaceous affinity : moreover, the
presence of its stipules is against it. In Hymenophyllites Weissii, Sch.,
figured by Schimper from the coal of Saarbrucken, the reference is chiefly
upon the sorus as a whole, while the sporangia themselves were not
distinguished, or described in detail. Perhaps the best authenticated case
is that of Hymenophyllites delicatulus ( = H. quadridactylites (Gutb. Zeiller),
described and figured by Zeiller.1 Here the sori were borne on the distal
ends of the pinnules : he was also able to recognise and draw the sporangia,
with an oblique annulus, which corresponds very closely with that of a
modern Filmy Fern. The original specimens were examined by Solms-
Laubach, who concluded that the fact is correctly stated, and no other
interpretation of the figures is possible.2 Scott, however, remarks3 that
the evidence as to the position of the reproductive organs on the leaf is
not sufficient to place the affinities of these fossils beyond doubt.
It must be admitted that the evidence of existence of Hymenophyllaceous
fructifications from the Primary rocks is insufficient. It does not, however,
appear essential to the position to be ascribed to the Hymenophyllaceae
below that their early existence should be established, though their occurrence
even in the primary rocks would readily accord with the view here enter-
tained of their origin from the protostelic Simplices.
ANATOMY.4
The most obvious structural peculiarity of the Hymenophyllaceae is the
" filmy " texture of their leaves : the lateral wings throughout are pellucid
owing to the absence of intercellular spaces, while there are no stomata
upon them. Usually the flattened region consists of only a single layer of
cells, though in some cases, such as T. reniforme and H. dilatatum^
there may be three or four layers, but still without intercellular spaces.
The question naturally arises whether this filmy character is primitive or
the result of special adaptation. In Prantl's view it was held to be primitive,
and indicated an affinity with the Mosses ; but there are many reasons
for rejecting this, and seeing in the simple structure a specialised and
reduced character. It is plainly suited to the moist habitats in which the
Hymenophyllaceae mostly live : while on the other hand quite a number
of Ferns living in moist surroundings, but of divers affinity, show the same
character in varying degree : it is seen in Danaea crispa, Endres, and in
D. trichomanoides. Spruce, MMS. ; but in these several layers of cells are
present, with intercellular spaces : it is seen in various degrees in the
Leptopteris section of Todea, and most clearly in T. superba : also in
1 Ann. Sci. Nat., Ser. 6, vol. xvi. 2 Palaeophyfology, p. 157. s Studies, p. 264.
4 See Boodle, Ann. of Bot., xiv., p. 455, where the literature is fully referred to.
HYMENOPHYLLACEAE 583
.
Asplenium resectum, Sm., and obtusifolium. Linn. : the existence of abortive
stomata observed in some of these (e.g. A. resectum} indicates their reduced
character. In A. obtusifolium two varieties have been recognised, one in
which the filmy habit is fixed, and another in which stomata and intercellular
spaces occur in the larger specimens, but are sometimes quite absent in
the smallest, the filmy forms growing in the dampest localities. Giesenhagen 1
compares the condition of the Hymenophyllaceae with that of the fixed
ilmy variety of A. obtiisifoliitm; he holds that as these plants have been
lapted to an extremely damp habitat, so also have the Hymenophyllaceae,
id the character has become hereditary, partially so in the Asplenium
id wholly in the Hymenophyllaceae. Finally, it has been shown
experimentally that a thinning of the leaf can be produced by cultivation
ider moisture and shade, even in some ordinary species of Ferns
(Scolopendrium vulgare, Pteris aquilina\ as is so frequently the case also
plants of other affinity.2 From all this it may be concluded that the
Imy habit is secondary and adaptive.
It would appear from their structure that H. dilatatum and T. reniforme
ire among the less specialised of the Hymenophyllaceae, for in them both
leaf-expansion is more than a single layer in thickness, a condition
cceptional in the family. And in this connection the facts of segmentation
>f the young wings are interesting : in the development of the wings of
leaf in ordinary Leptosporangiate Ferns the marginal cells segment
>y alternating oblique walls, but in the Hymenophyllaceae the segmentation
)f the marginal cells is as a rule repeatedly transverse. Now, in the lower
irt of the leaf of T. reniforme, and occasionally also in H. dilatatum the
^mentation is by oblique alternating walls, as in the ordinary Lepto-
sporangiate Ferns, while in Todea superba, which is also held as filmy by
luction, there is instability between the two types, though with a pre-
mderance of the oblique segmentation.3 These facts are further evidence
mt the filmy habit of the Hymenophyllaceae has been secondarily acquired,
rhile they indicate an intermediate position for Todea superba, and for
L dilatatum and T. reniforme.
The filmy character is accompanied by structural reduction of other
irts : thus in certain leaves pseudo-veins are present 4 which can hardly
be anything else than the vestigial remains of true veins no longer functional.
Cognate with this is the fact that the root-system is reduced, and even
entirely absent in some species. It may then be expected that the vascular
system of the axis and leaf will also show signs of reduction as compared with
other Fern-types : an examination of them shows that this surmise is correct.
The stem of the Hymenophyllaceae is monostelic, and one leaf-trace
passes off to each leaf, while the vascular supply to the axillary bud is
1 Flora Ergiinzungsband, 1892, p. 174.
2 Boodle, Linn. Jonrn., vol. xxxv., p. 659; J. H. M'llroy, Trans. Roy. Phil. Sot:,
Glasgow, vol. xxxvii., p. 136.
3 Bower, Ann. of Bof., vol. Hi., pp. 340-360. 4 Prantl, I.e., p. 24.
584 FILICALES
attached to that of the leaf-trace. The stele varies considerably in
its construction in different species : in Hymenophyllum there is less
variation than in Trichomanes. In all cases the stele lies centrally,
and is delimited by the endodermis, followed by a broad pericycle, while
the phloem surrounds the centrally-lying xylem. In species of Hymeno-
phyllum with large rhizomes, such as H. scabrum or dilatatum, the
metaxylem forms a ring enclosing the protoxylem together with some
parenchyma : the metaxylem often has the form of two bands, their definition
having relation to the origin of the strands which pass to the roots (Fig.
328; Boodle, Fig. 10). In species with small rhizomes the metaxylem
FIG. 328.
Transverse section of a node of Hytnenophyllum dilatatum v. Forsteriamtm. Stele
of rhizome to the right, leaf- trace to the left, ph — phloem ; p.r — protoxylem ; /= lower
xylem- bands ; « = upper xylem-band. X2oo. (After Boodle.)
forms a small band or mass, and the protoxylem is peripheral to it on
the lower side : these two types are bridged by transitional forms. In
Trichomanes the structure of the rhizome of T. reniforme corresponds in all
essentials to that of H. scabrum, but with a considerable mass of parenchyma
accompanying the central protoxylem. In others the parenchyma is scanty
( T. radicans], while in T. trichoideum the stele is sub-collateral : these types
find their analogues in Hymenophyllum. But other species of Trichomanes
diverge along lines of their own : thus the stele is collateral in T. muscoidesy
with the xylem downwards : others ( T. labiatum and Motley 7) may have
only a single tracheid, or none, and no phloem : these are plainly reduced
types. Others again may show a solid mass of xylem, with scattered
indefinite protoxylem (T. spicatum), or with the protoxylem peripheral
HYMENOPHYLLACEAE 585
( T. scandens) : there is reason to think that these, being more specialised
in their general habit, have undergone secondary modification also of
their stelar structure.
The petiole receives in all cases a single strand, which passes off from
the stele without a leaf-gap, and widens out upwards into a collateral
structure with more or less clearly curved xylem.
The comparison of this structure with that of other living Ferns leads
to the recognition of no near relation. Perhaps the nearest is with
the ancient family of the Botryopterideae, and especially with the genus
Zygopteris. It will be seen that the correspondence is closest between
the structure seen in Zygopteris Grayi (Fig. 270, p. 500), and that seen in
T. reniforme and H. scabrum, that is, with the species which show the
nearest resemblance to one another of the living forms of Hymenophyllaceae.
The chief features of resemblance are in the structure of the stele, and
the mode of origin of the leaf-trace, and of the supply to the axillary
bud. As to the former, if the small central tracheides of Zygopteris are
protoxylem, as seems highly probable, the agreement with T. reniforme,
or the larger Hymenophyllums is very close, the differences being such as
would be due to a fths arrangement of the leaves on the one hand, and
distichous on the other. The mode of origin of the leaf-trace without
any foliar gap, and the insertion upon it of the supply to the axillary bud
are also points of similarity. This resemblance to a very ancient form
appears to confirm the recognition of the living species named as being
probably primitive, while from that central point the remaining species
of Hymenophyllum diverged slightly in one direction, but those of Tricho-
manes diverged much more strongly along their own lines, either of reduction
or of other specialisation. Such a conclusion appears to emerge clearly
from the anatomical comparison.
The Hymenophyllaceae have undergone vicissitudes of classification :
Brongniart first separated them as a special family, though the name of
Endlicher is usually connected with their recognition as ranking on an
equal footing with the Cyatheaceae or Polypodiaceae. Presl regarded them
as a connecting link between Mosses and Liverworts, and somewhat far
removed from the Filicineae ; but the actual separation of them from the
Filicineae was opposed by Mettenius. The comparison of their vegetative
structure with that of certain Bryophytes was, however, maintained, and
strengthened by fresh observations : it was pursued subsequently by Prantl,
with regard to the sporophyte, and especially to the sorus, and by Goebel
as regards the gametophyte. Consequently they were held to illustrate
the phylogenetic connection between Mosses and Vascular Cryptogams,
and to bridge over the gap between these circles of affinity in the sequence
of development of the Archegoniateae. Further, the Hymenophyllaceae have
been held to be at least as near to the Polypodiaceae as to any other family
of the homosporous Leptosporangiateae : in accordance with such views we
586 FILICALES
find them placed first in the system of Christ,1 and removed far from
those Ferns with oblique annulus- with which they were associated in
Hooker's Synopsis Filicum. I have elsewhere expressed my dissent from
the view that the Hymenophyllaceae are the most primitive type of Ferns :
an alternative view is here adopted as to the relationship of this interesting
family, based not only on comparison of the living forms, but also upon
the rapidly increasing knowledge of related fossils.
It has now been seen that the two genera of the family may be separately
recognised as forming natural sequences, and that there is greater uniformity
in the genus Hymenophyllum than in Trichomanes. In the former genus
the usual characters of the larger types are, a creeping rhizome containing
a stele with metaxylem surrounding the protoxylem : with much branched
leaves, sometimes of more than a single layer of cells in the wings : with
sori, having a short receptacle, bearing large sporangia with large spore-
output. Such a species as H. dilatatum may be taken as a central non-
specialised type of the genus. In the case of Trichomanes, the species
T. reniforme is isolated from the rest, and shares with Hymenophyllum
the character of a creeping axis, containing a stele of similar construction :
it has leaves several layers in thickness, short receptacle, and relatively
large sporangia, with large spore-output. All these characters place it in
close relation to Hymenophyllum. The rest of the genus represents various
lines of specialisation : in some the creeping axis appears to have resumed
an upright position, with modifications of the stele which may be held as
secondary : 2 this is found in species with advanced leaf-differentiation, thin
leaf-texture, elongated receptacle, relatively small sporangia, and low spore-
output per sporangium, such as T. spicatum. In others there are found
in varying degree diminution of size and complexity of leaf-form, reduction
of stelar structure in the creeping axis, and even complete absence of roots :
these characters are accompanied by thin leaf-texture, elongated receptacle,
small or often minute sporangia, and a spore-output varying from 64 to
as low as 32. A general conspectus of the family, bearing these characters
in mind, leads to the conclusion that the species in which its two genera
are most nearly alike are themselves the most primitive, and that it is
through them that comparisons may best be instituted with a view to
determining the evolutionary relations of the family. The rest may be
held to form sequences of specialisation, which will accordingly possess
less direct interest for comparison with other Ferns.
Taking, then, the characters of the sporophyte, as seen respectively in
H. dilatatum and in T. reniforme^ they may be compared seriatim with
those of other Filicales. The creeping habit is already seen in such early
Ferns as the Schizaeaceae and Gleicheniaceae, which are also protosteiic ;
but the. peculiar structure of the stele of the above species finds its nearest
correlative not in these families, but in the Botryopterideae, and especially
in Zygopteris : allowing for the differences which follow on upright habit
1 Farnkrduter der Erde, p. I. 2 Boodle, /. r. , p. 487.
HYMENOPHYLLACEAE 587
and radial construction in Zygopteris and the creeping dorsiventral rhizome
in these Hymenophyllaceae, the structural resemblance is very close : and
with this go the strikingly similar facts of structure and insertion of the
leaf-trace, and of the mode of supply to the axillary buds.
Next, as to the leaf-texture, H. dilatalum and T. reniforme are both
species with the lamina composed of several layers, and occasionally showing
in their development the alternate segmentation seen in the leaves of
ordinary Leptosporangiate Ferns. According to the argument advanced
above, the filmy texture is an adaptive character shown in various families
of Ferns : the peculiarity of the Hymenophyllaceae is that they show it in
the highest degree. But the species named appear less specialised in the
hygrophilous direction than others of the family, and thus they serve to
connect it with the ordinary types.
The marginal position of the sorus is shared with the Schizaeaceae, while
it is to be remembered that the Botryopterideae and Osmundaceae may also
bear sporangia in their leaf-margins. But the Hymenophyllaceae differ from
any of these in the basipetal sequence of the sporangia, the elongated
receptacle, and the cup-like indusium. The basipetal sequence may be
held to be a secondary condition, bringing with it the advantage of spreading
the drain of spore-production over a longer period than if all were developed
simultaneously : the elongation of the receptacle, a consequence of intercalary
growth, is almost a necessary condition of its adoption. The basal cup-like
indusium, imperfectly represented in the Schizaeaceae, has probably been
a new formation : its efficacy in protecting the youngest sporangia at the
base of the sorus amply justifies its existence. It is thus possible to conceive
of the origin of the Hymenophyllaceous sorus from some Fern-type with
marginal sporangia, by initiation of a basipetal sequence, and establishment
of a protective indusium. The type from which they might have originated
would probably be found among some protostelic types with large sporangia
marginally produced, of which the Botryopterideae, Osmundaceae, and
Schizaeaceae are the known representatives.
A comparison of the sporangia themselves confirms this reference to
the Ferns with large sporangia, rather than to simpler forms such as the
Polypodiaceae. For there is 'an oblique annulus corresponding in position
on the one hand to that of the other Gradatae, but also to that of certain
of the Simplices. A comparison of Fig. 325 of Hymenophyllum with Fig. 310
of Gleichenia shows plainly the close similarity of the sporangia : and it
has been shown that if the peripheral face of the Gleicheniaceous sporangium
be reduced the Schizaeaceous type is the result, both being variants of the
same form. If finally the point of dehiscence were shifted from the median
plane to the side— a practical necessity where there is a basipetal sequence
— the Hymenophyllaceous sporangium would be the result. Further, in the
spore-output certain of the Hymenophyllaceae approach the Simplices : it has
been shown that in H. Tunbridgense the output per sporangium is 256-512 :
in T. reniforme and sericeitm it is typically 256, while other species of
588 FILICALES
Hymtnophyllum yield 128 as the typical number. These are figures which
find no correlative in ordinary Leptosporangiate Ferns, but only among
the Simplices, a fact which strongly supports the view above expressed.
On the other hand, certain species of Trichomanes show only low spore-
output, but they are on other grounds regarded as specialised, and their
small sporangia and low output are further indications of their derivative
character.
It would be impossible to close any comparative account of the Hymeno-
phyllaceae without some reference to the gametophyte, for it has figured
largely in previous discussions. Trichomanes is the simpler type of the
family in its prothallus : while that of Hymenophyllum consists of a broad
ribbon-like expansion, that of Trichomanes is usually filamentous, with more
massive archegoniophores. The archegonia of these Ferns do not show
distinctive features, but Heim,1 who has drawn attention to the value
of antheridia for comparative purposes, specially notes the similarity of
those of the Hymenophyllaceae to those of the Gleicheniaceae. This is
a fact of importance when taken with the data of spore-output, for
it is thus seen that features of the reproductive organs of both generations
indicate a similar affinity.
The result of a general comparison of the Hymenophyllaceae with other
Ferns is then to recognise that they approach most nearly to certain of
the Simplices, with which they agree in many points, both of the sporophyte
and the gametophyte. The structural peculiarities of the gametophyte
apart from the sexual organs are probably in large measure the result of
secondary adaptation : a comparison of the antheridia, however, points to
certain of the Simplices. The characters of the sporophyte are more
distinctive : they point, in one feature or another, to all the known protostelic
families of the. Simplices, but to no one family in particular : so that it is
impossible at present to locate the origin of the family with any degree i
of exactitude. The Hymenophyllaceae are to be looked upon as of early
origin, but ending as a blind line of descent, characterised by specialisation
of both generations to a hygrophilous habitat, which has taken the form \
of simplification ; in both generations Trichomanes shows the greater
simplicity, and is on that account to be held as more removed from the
original source.
^ Flora, 1896, p. 363.
CHAPTER XXXVIII.
GRADATAE (Continued}.
THYRSOPTERIDEAE.
Tin; rare monotypic genus Thyrsopteris, which is endemic on the Island of
Juan Fernandez, was at once placed with Dicksonia, which appears to be its
natural position, though it is better, perhaps, to make it the sole repre-
sentative of a separate family. It is a Fern with an upright axis, three to five
feet high, covered by the scars of leaves : these have thick stalks, bear
a lamina three to four times pinnate : the upper pinnae are sterile and of
leathery texture : the lowest pairs of pinnae are fertile but slender : they
are as highly branched as the sterile pinnae, but with the surface undeveloped :
each pinnule is terminated by a sorus, the whole giving the appearance of
a complicated thyrsus. There is some evidence that Ferns of this type
existed as early as the Jurassic period.
The sori have a cup-like basal indusium, surrounding a receptacle which
bears numerous sporangia. As in the Hymenophyllaceae, and on the other
hand as in Dicksonia, the receptacle is the actual apex or margin of the
pinnule ; it appears at first, while the pinna is still tightly coiled, as a smooth
cone, slightly flattened in the plane of the leaf. Below this, before the spor-
angia make their appearance, the indusium begins to be formed, as a massive
outgrowth : a transverse section at this stage often shows that the indusium
is slightly two-lipped, and here we may trace an indication of correspondence
with Dicksonia (Cibotium}, or, on the other hand, with Hymenophyllum :
but this two-lipped character is only slight, and is not obvious at later
stages. The formation of sporangia soon follows, and their succession is
basipetal : the first appear at the extreme margin, of which one is shown
in Fig. 329 A, the section being perpendicular to the surfaces of the leaf:
others then appear in lower positions. The marginal sporangium thus seen
is only one of a series which arise along the edge of the flattened receptacle :
thus the receptacle is a flattened lobe developed from the margin of the
pinnule, as in Dicksonia, while the indusium originates as a growth within
the margin, on either side of the pinnule.
590
FI LIC ALES
The form of the parent cells of the sporangia is not constant ; commonly
the cell has a square base, and the first segment-wall passes obliquely to
the basal wall (Fig. 329 A and c), the next segment-wall being inserted
obliquely on the first : the result is from the outset a sporangium with a
short massive stalk, as shown in Fig. 329 D. In other cases the parent
cell is more wedge-shaped, and the first segment-wall cuts an oblique lateral
wall (Fig. 329 B) : thus the segmentation in Thyrsopteris wavers between
two types — the one characteristic of larger, the other of smaller spore-output.
The further segmentation of the sporangial head follows the type usual for
Leptosporangiate Ferns. Very soon, however, the sporangium takes an
FIG. 329.
Thyrsopteris elegans, Kze. A = longitudinal section through the young sorus, showing
the two-lipped indusium /, i, and sporangia s, s, seated on the receptacle, the oldest
being at the distal limit of it. C=two young sporangia. B— one rather more advanced.
D = a. sporangium with tapetum and sporogenous group shaded. E, F — mature
sporangia. /4-Z>X2oo. E, Fx$o.
unsymmetrical form, the more strongly growing side being that directed
towards the apex of the receptacle (Fig. 329 D) : the oblique annulus, which
in this genus presents peculiar characters, makes its appearance early, and
occupies a position comparable to that in Gleichenia : the orientation of the
sporangia is thus on the Gleicheniaceous type, which, when repeated and
associated with lateral dehiscence, offers the advantages for spore-dissemi-
nation noted in the Hymenophyllaceae, and effective in other Gradatae.
The central cell undergoes the usual segmentation to form a tapetum of
the usual type, and 12 to 16 spore-mother-cells. From countings of the J
spores it has been concluded that the typical number for each sporan-
gium is from 48 to 64.
The sporangia when mature are of large size and rather peculiar form :
seen from without they present some rather unusual features. Fig. 329 F
shows one presenting its central face : the annulus, starting from the base,
DICKSONIKAE 591
runs round two sides, but at the point (*) it appears to stop, but it does not
do so actually : it merely curves round upon the peripheral face, and dis-
appears behind the body of the sporangium. That the annulus is really
a continuous one is seen from Fig. 329 E, which represents the peripheral
face : it is very irregular, especially at the base of the sporangium, and
consists of a large but not definite number of rather narrow cells ; together
they form a twisted hoop, so disposed that the distal end of the hoop is
curved in the direction of the apex of the receptacle, and this curvature
makes the sporangium a peculiarly difficult object to represent in a drawing.
The thickening of the walls is greater at the distal part of the annulus than
at the proximal, while at one side or the other is a part not strictly defined,
where rupture will take place. There is no definite stomium, and though
rupture usually occurs about the region to the right in Fig. 329 E, the
actual point of dehiscence may vary.
The features thus noted mark off the annulus of Thyrsopteris as one of
the least specialised among Leptosporangiate Ferns : the inequality of its
thickening suggests a comparison with Loxsoma. It seems probable that,
while showing clear points for comparison with Dicksonia, it has been derived
from a type with a completely indurated annulus and median rupture : that
this became modified in consequence of the close packing of the sporangia
in the sorus, which would interfere with a median dehiscence : that a lateral
rupture had been adopted, but the sporangium not definitely specialised for it.
The anatomy of Thyrsopteris is very imperfectly known. The leaf shows
a vascular system, with a few separate bands arranged in the usual horse-
shoe outline ; in fact, the structure suggests similarity with Dicksonia. There
are no data as to the internal structure of the axis.
From the known characters it would appear that the relationship of
Thyrsopteris is primarily with Dickso?iia, but in some remote degree also
with the Hymenophyllaceae : there appears little reason to relate it to the
Cyatheaceae, as has been suggested by various writers. It is, however,
best placed as a separate family, on account of the peculiar characters of
the sporangium and annulus : these show less perfect differentiation than
of the Ferns named, while an archaic character is indicated by the numerous
cells of the annulus, and / the imperfect localisation of the point of
rupture.
DICKSONIKAE.
The family of the Dicksonieae included, as arranged in Hooker's Sy?wpsis
Filicu m, six genera ; but of these the affinity of Onoclea appears to be rather
with the Cyatheae, while Hypoderris, Woodsia, Sphaeropteris, and Deparia
differ from Dicksonia itself not only in the position of the sori, which are
superficial (except in Deparia 1), but also in the fact that the various ages
1 'Deparia Aloorei, a fern in which the sori are mostly marginal, shows occasional
superficial sori also : but these are upon the upper surface, and the case is perhaps com-
parable with that of Aspidiuin anomaium. Hk. and Arn : see p. 117.
592 FILICALES
of the sporangia are intermixed. The probable position of these genera
will be considered later. There remains, then, only the old comprehensive
genus Dicksonia. This was divided in the Synopsis Filicum into three
sections — Cibotium, Eudicksonia, and Patania ( = Dennstaedtid). While
Cibotium and Eudicksonia have obvious relations to Thyrsopteris, Patania
(Dennstaedtid) clearly approaches the genus Davallia, and especially to that
section of the old genus which has been separated as the independent genus
Microlepia : these relationships will now receive the support of develop-
mental and anatomical evidence. I shall follow Prantl 1 in separating
Dennstaedtia and Microlepia from the position given them in the Synopsis
Filicum, and recognise them as constituting, perhaps with certain other genera,
a natural sub-tribe under the name of the Dennstaedtiinae Prantl, having a
position between Dicksonia and Davallia, and constituting with them a
natural sequence. It will be shown that following this series from Thyrso-
pteris to Davallia we shall pass from a type with basipetal sequence of the
large, short-stalked sporangia, with oblique annulus, to forms with a mixed
sorus, smaller, long-stalked sporangia, and a vertical annulus. The receptacle,
which is a prominent feature in the former, is reduced, or represented only
by a flat surface in the latter types. The gradual nature of these parallel
steps seems to indicate that the whole series is one of natural affinity, as
indeed has always been recognised by systematic writers.
DICKSONIA (EXCL. § Patania}.
The genus Dicksonia includes some large Tree Ferns, and others of smaller
stature but with prevalent radial construction. Some of the smaller species
closely resemble small plants of Thyrsopteris : like it they have leaves
repeatedly pinnate, with numerous sori borne at the margins, but without
any differentiation of sterile and fertile pinnae. The sori are protected by
a two-lipped indusium, but the lips are unequal, and their character has
been used as a basis of division of the genus. It will be shown that the
receptacle itself is marginal, and that the lips of the indusium are develop-
mentally outgrowths from the surface of the pinnule, just as in the
Hymenophyllaceae and in Thyrsopteris. Thus there is essential corre-
spondence with these Ferns, and the differences are rather of habit and
size than of the more fundamental features of the sorus.
SPORE-PRODUCING MEMBERS.
The sorus in this genus has already been investigated by Gliick,2
who points out that the receptacle arises from the original leaf-margin,
while the two lips of the indusium spring from the upper and lower
leaf-surfaces. The structure of the young receptacle, as seen in Dicksonia
(Cibotium) Schiedei, Baker, is like that of a leaf-margin, with a marginal
1 Arb. Konigl. Bot. Cart, zu Breslatt, 1892, p. 18. 2 Flora, 1895, Heft 2.
D1CKSONIEAE
593
series of actively dividing cells. In sections perpendicular to the leaf-
surface the young sorus appears as in Fig. 330 A, the cell marked (m)
being one of the marginal series. A section of a similar sorus in a
plane (x, x) appears as shown in Fig. 330 D (the chain of 10 cells
superposed on the lip of the indusium), while at the lower level (y, y)
it is. as shown between the indusial lips. It is thus seen that the receptacle
is structurally like n normal leaf-margin, a conclusion again supported by
Fig- 330 B, which shows a similar sorus traversed in a plane (z, z)
H
FIG. 330.
Dicksonia Schicdci, Baker. A= section through a young sorus perpendicular to the
leaf-surface ; z", 2 = indusium ; /« = cell of marginal series. .5 = section of sorus parallel to
the leaf-surface as along a line z, /', in Fig. A, showing receptacle bearing sporangia s, s.
C = a similar section bearing older sporangia. /^ = transverse section of a young sorus
showing the two lips of the indusium (znd), and receptacle between them, as along a
plane y, y, in Fig. A. A section of the receptacle as in plane -t",.f, in A, is superposed on
the lower indusial lip. The central figure shows sporangial stalks cut transversely. A-D
X 200. E) f, G, H sporangia of Dicksonia Menzicsii from four different points of view.
Xso.
indicated in Fig. 330 A. It is important to note the inequality in size
of the two lips of the indusium. Here it is only slight, but in forms to
be subsequently described an increasing inequality, both in area and in
substance, makes its appearance. The formation of the sporangia has
begun in the marginal cells, and it is seen that their appearance is
almost simultaneous, a point better shown in Fig. 330 c, which repre-
sents a slightly more advanced stage. These marginal sporangia are
succeeded by others produced in a basipetal sequence, but the succession
is not long continued, and in D. Culdta, L'Herit, it is not clearly marked ;
there is, however, no evidence of intercalation of younger sporangia
between those already present.
^ 2 P
594 FILICALES
As seen from Figs. 330 B, c, the sporangial mother-cells are deeply
sunk, and the first segment-walls may insert themselves upon the basal
wall, as is shown in sporangium (3), Fig. c ; in other cases the segment
walls may cut the lateral walls of the mother-cell (e.g. sporangium 4).
But, however this may be, the first segmentations are those characteristic
of bulky sporangia.
The sporangia themselves are of large size, and vary somewhat in
form, being in D. Culcita of an almost pear-like shape with very oblique
ring, while in D. Menziesii the ring is more longitudinal. The stalks
are relatively thick though elongated, and show in transverse section a
rosette of six or seven cells (Fig. 330, central drawing). As the publishec
drawings of Dicksotiia sporangia are not altogether satisfactory, I have
represented those of D. Menziesii, Hook and Baker, from four different
points of view. Fig. 330 H shows how, on the peripheral face, the con-
tinuous ring of the annulus surrounds the relatively large thin-walled area
the indurated part of the annulus is shaded, and of the rest, four smallei
cells (connective cells) are seen on either side of the group of five cells
which form the stomium. The central face is shown in Fig. 330 F
but in this case the stomium is composed of only four cells, while five
connective cells are seen on either side of it. Fig. 330 E shows a latera
view of the side on which the annulus is completely indurated, and Fig
330 G shows the stomium as again a group of five cells, while two pairs
of " connective " cells are seen on either side of it. From these drawings
the form and large size of the sporangium, and the position of the annulm
are clearly seen, while we also conclude from them that the number o
cells of the stomium and connective is not constant.
The orientation of the sporangia has been examined in D. Schiedei
Chamissoi and Menziesii '; at the margin of the flattened receptacle it i$
not uniform, but the sporangia seated on the sides nearer its base shov
in the majority of cases, though with no strict uniformity, an orientatior
on the Gleicheniaceous plan.
Notwithstanding the large size of the sporangia, the output of spore*
in Dicksonia is not a high one. Enumerations were made for D. Menziesi
with 62, 63 as the result, and in D. antarctica, 64. Clearly 64 is the typica
number for these Ferns.
ANATOMY.
The vascular system of Dicksonia can only be properly understooc
in the light of the simpler forms, and especially of the Dennstaedtiinae
Nevertheless it will be described here, and referred to again later. I
has been examined in D, Barometz and Culcita by Gwynne-Vaughan,
who finds the system of the axis to consist of a cylindrical dictyostel<
surrounding a large central pith. The meshes of the dictyostele are th<
foliar gaps, and from the lower limit of each arises a leaf-trace in th<
1 Ann. of Bot., xvii., 1903, p. 708.
DICKSONIEAE 595
form of a broad ribbon of tissue, with its margins folded inwards (Fig. 331).
Very shortly after its departure from the axial stele this ribbon breaks
up into a number of isolated strands arranged in horse-shoe fashion as
seen in the transverse section. The point of disintegration varies from
one leaf to another, and sometimes it does not occur until the free petiole
is reached. There are no accessory strands in the pith nor in the leaf-
stalk, as are seen in some other related Ferns.
The structure, even in the large dendroid species, such as D. squarrosa,
antarctica, and Schiedei, appears to be the same as that in D. Barometz,
but on a larger scale. The relation of the whole to a solenostelic type
is obvious : where a leaf is inserted a gap occurs in the solenostele ; but
the leaf-gaps are very small and close up rapidly : if these leaf-insertions
be close together, as they are in an upright radial stock, the foliar gaps
FIG. 331.
Dicksonia Karometz. Portion of the vascular system of the stem, seen from within,
and showing the departure of three leaf-traces. (After Gwynne-Vaughan.)
will overlap, and give to the stele a reticulate character. On the other
hand, the leaf-trace is originally a single strand, and is clearly seen to
be so at its base in D, Barometz ; but as it passes up the petiole it is
disintegrated so as to form a number of separate strands. It will be
seen later that this structure is in principle the same as that seen in
the genus Dennstaedtia, allowance being made for difference in size, and
in the elongation of the rhizome with its isolated leaves.
Both structurally and in the characters of the sorus and sporangium
the Dicksonieae as now limited occupy a position between Thyrsopteris and
the Dennstaedtiinae, while in soral condition, though not in spore-output,
the family shows analogies with the larger species of Hymenophyllum.
DENNSTAEDTIINAE.
This sub-tribe was founded by Prantl,1 to include the genera Dennstaedtia,
Microlepia, Leptolepia, Saccoloma^ and Hypolepis. The most important of
1 Arbeiten Kbnigl. Bot. Garten zu Breslau, vol. i., p. 18.
596 FILICALES
these genera are the two first, which were placed apart in the Synopsis
Filicum : Dennstaedtia (as § Patania} was included in Dicksonia, while
Microlepia was included as a section of Davallia. But evidently systematists
had reason to know how closely allied these two were, since the synonyms
have been numerous. The fact is that Dennstaedtia is not very nearly related
to Dicksonia', the form of sorus is different, and the details of the sporangium;
these characters should weigh more strongly than any similarity of habit.
On the other hand, Microlepia, while it resembles Dennstaedtia in its sorus,
differs in some essential points from Davallia. Certain new facts have
confirmed the soundness of Prantl's systematic method of founding the
sub-tribe : it will be seen that the Ferns included in it occupy a peculiarly
interesting position, as connecting links between the basipetal type of sorus
seen in the Dicksonieae, and that of the Davallias.
MICROLEPIA = (Davallia, § Microlepia}.
This genus includes Ferns with creeping rhizome, on which solitary
leaves are borne, which are not articulated at the base. The surfaces bear
hairs, not ramenta. The leaves are repeatedly pinnate, and bear sori with
the indusium unequally lipped ; the upper lip appears as a continuation of
the leaf-surface, the lower lip as a membranous half-cup-shaped outgrowth :
the result is that the whole sorus appears to be intra-marginal.
The sorus has been examined in Microlepia speluncae (L.), Moore, hirta
(Kaulf), Presl, strigosa (Thunb.), Presl, and platyphylla (Don), J. Sm. In
all these it shows in the main a basipetal succession. This is seen in
M. speluncae, in Fig. 332 A, in which the two lips of the indusium appear
with the characteristic inequality, the superior lip (s.) having the aspect of
a continuation of the leaf-lobe, while the inferior lip (i.) is smaller, and has
some similarity in position to the indusium of Cystopteris. The receptacle
is conical, but the sporangia are not very numerous, and it is not greatly
elongated ; it is traversed by a band of tracheides (//*.). The order of
appearance of the sporangia is in a strict basipetal succession, but this is
not long maintained. The sporangia themselves are on short, rather thick,
three-rowed stalks, and the head shows a slightly oblique form, the peripheral
face being the more convex. The annulus is almost longitudinal, but still
it shows a slight degree of obliquity, such as will be described below in
Dennstaedtia apiifoiia ; the orientation of the sporangia shows a considerable
regularity on the Gleicheniaceous type.
In Microlepia hirta there is general correspondence to M. speluncae as;
regards structure of the sorus and sporangium, though the succession of
sporangia is longer, and the receptacle accordingly more elongated ; but
cases occur occasionally in which the strict basipetal succession is not
-maintained. One of these is represented in Fig. 332 B, in which there is<
a larger sporangium at the tetrad stage, and below a small one in which
the spore-mother-cells have not yet expanded. Fig. 332 c shows another
DENNSTAEDTIINAE
597
example of departure from the strict basipetal succession ; still the receptacle
is seen to be elongated and traversed for a considerable distance upwards
by a strand of tracheides. Such exceptions occur in about one in every
five sori cut, and are therefore not excessively rare. These irregularities
are of interest for comparison with those to be described for Dennstaedtia.
In M. platyphylla and M. strigosa the sorus was found to be basipetal,
the receptacle conical, and the sporangia of the same type as in the other
species. Thus it may be concluded for Microlepia that the sorus is typically
one showing basipetal succession of rather short-stalked, slightly oblique
FIG. 332.
A = Microlepia spelioicac, Baker. Sorus showing unequal lips of the indusium, and
bttipeUl succession of sporangia. X 100. B, C = Microlepia hirta, Kaulf. Similar
.sections to A, but showing departures from the strict basipetal succession. X zoo.
sporangia, with reasonable regularity of orientation, inserted up.on a conical
receptacle, and protected by two unequal indusial flaps. But that in some
species occasional departures from the strictly basipetal succession occur,
younger sporangia being found inserted between those which are more
advanced.
DENNSTAEDTIA = (Dicksonia, § Patanid}.
This genus includes numerous species of more delicate habit than
Dicksonia, and with creeping rhizomes, the solitary leaves are non-articulated,
a character in common with Microlepia, and ranked as an important one
by Smith. Hairs are borne, not ramenta. The habit is like Microlepia,
but also very like Davallia. The small sori are marginal, and Prantl had
598
FILICALES
already noted1 that the receptacle is derived from the leaf-margin, while
the cup-like indusium originates as two flap-like outgrowths of the lower
and upper surfaces ; excepting that the indusium is not two-lipped, and
that the receptacle is cylindrical instead of flattened, it resembles in its
main features the sorus of Dicksonia. The relations of the sporangia within
the sorus have been examined in Dennstaedtia apiifolia, Hook., with the
result that the basipetal succession has been found to be much more
marked than in Dicksonia. The receptacle is cylindrical and elongate
and traversed by tracheides for a considerable distance (Fig. 332 bis, A)
upon it the sporangia arise, as a rule, in strictly basipetal order. Ttu
sporangia themselves undergo the usual segmentation; when mature the]
FIG. 332 Us.
A = Dennstaedtia apiifolia, Hook! Sorus showing basipetal succession throughout.
C = dehiscent sporangium of the same showing very slightly oblique annulus. B =
Dennstaedtia rubiginosa, Kaulf. Sorus in vertical section showing that it has been at
first basipetal, but with a mixed character supervening. D = dehiscent sporangium of the
same, seen from the base, showing that the annulus stops short on either side of the
insertion of the stalk (st~). All X 100.
have long stalks composed of three rows of cells; the head is not
bilaterally symmetrical, but one side is more strongly convex than the other;
and it will be seen from the figure that the more convex side is the
peripheral one. The annulus is not exactly vertical, but running round the
margin of the sporangium to the attachment of the stalk, it is there slightly
diverted to one side (Fig. 332 bis, c) ; usually the sequence of cells of the
annulus is not wholly interrupted by the insertion of the stalk, but its cells
are more or less in contact with one another, and the annulus is in such
cases actually continuous at the base, as it is in those sporangia where it is
more conspicuously oblique. In fact, the sporangium of D. apiifolia shows,
though in a less obvious degree than in Dicksonia, the oblique annulus.
The nearer side of the sporangium shown in Fig. 332 bis, c is the less
convex one, the more convex side is the peripheral face as regards the
lLoc. cit., p. 19.
DENNSTAEDTIINAE 599
whole sorus, and it is on this side that the annulus may be seen to show
the last traces of continuity at the base. The orientation of the sporangia,
as described, is not always maintained in the mature state; transverse
sections of the mature sorus show some latitude in this respect ; but this
may be due merely to a twisting of the long thin stalk. This seems
the probable explanation, since the young sporangia show a fairly accurate
orientation.
Sections of sporangia show that the number of spore-mother-cells in
each sporangium is variable : eight, twelve, and sixteen have been observed.
Countings of mature spores have shown that in certain cases the full
number of 64 may be produced.
Deviations from the basipetal succession have been observed in D.
davallioides (Br.), Moore, and in D. rubiginosa (Kaulf), Moore. Even in D.
apiifolia isolated cases have been seen of a sporangium seated near the apex
of the receptacle, apparently arrested in its development, and representing
a less advanced state than those surrounding it. In D. davallioides, cases
of this sort are of fairly common occurrence towards the apex of the
rather elongated conical receptacle. These may be held to be transitional
forms to what is seen in the allied species D. rubiginosa, the sorus of
which is represented in longitudinal section in Fig. 332 bis, B. Here, upon
a relatively short receptacle, and between indusial flaps which are also
short, the sporangia are disposed with no definite succession ; the
persistent stalks of two old sporangia are near the apex of the receptacle,
and the younger sporangia below, with spores and tetrads, give a slight
indication of a basipetal succession, but still younger sporangia are disposed
irregularly among them. The sorus, though showing some slight signs
of a basipetal succession at first, is clearly of that type which we shall
designate "mixed," that is, with the sporangia produced in no definite
succession, but the younger interspersed irregularly among those which are
more advanced ; correlated with this we find the receptacle short, but
wide. The sporangium also shows a difference from D. apiifolia, the
annulus being definitely interrupted at the insertion of the stalk as is shown
in Fig. 332 bis, D, which represents a dehiscent sporangium seen from
below, (st.) indicating the insertion of the round stalk. We shall subse-
quently see that, by these gradations in Dennstaedtia, a transition is indicated
between two fundamental types of soral arrangement, the basipetal
succession of sporangia, and the mixed, where the various ages are irregularly
interspersed. Dennstaedtia is thus found to correspond to Microlepia in
the occasional departure from the typical basipetal succession of the
sporangia in the sorus : this condition leads on to that seen in the
genus Davallia.
The other genera included in the Dennstaedtiinae by Prantl are
Leptolepia, Saccoloma, and Hypolepis. The first two of these are closely
related to Microlepia, as their numerous synonyms show : many of
their species have indeed been included in that genus. Hypolepis is
6oo
FILICALES
mentioned by Prantl himself as an uncertain member of this sub-tribe.
An examination of its sorus shows the sporangia of various ages intermixed,
and without definite orientation. On these grounds the affinity of this
genus would appear to be elsewhere than in the Dennstaedtiinae.
ANATOMY.
It has long been known that the axes of Dennstaedtia and of Microlepia
show the structure now recognised as solenostelic ; but the detailed know-
ledge has lately been extended by Mr. Gwynne-Vaughan,1 and made the
subject of important comparisons, of which the following paragraphs are
a brief abstract. The solenostele is itself held to be a relatively primitive
state : all the species of Dennstaedtia that have hitherto been examined
FIG. 333 A.
Dennstaedtia (Dicksonia) fiunctiloba. Dia-
gram of vascular system of rhizome, including
a node and the base of a leaf-trace. The upper
surface of the rhizome would face the observer.
(After Gwynne-Vaughan.)
FIG. 333 B.
Dennstaedtia ( Dicksonia) adiantoides. Diagram of
vascular system of rhizome, including a node and the
base of a leaf-trace, l.sh. — lateral shoot arising from
basiscopic margin of leaf-trace: i.s. Bridge upon
internal surface of solenostele. The upper surface
of rhizome would face the observer. (After Gwynne-
Vaughan.)
prove to be essentially solenostelic : similarly all the reputed species of
Microlepia that have been examined are also typically solenostelic, with
two exceptions only : one of these is Davallia (Microlepia) hirsuta, Hk.,
which is dorsiventrally dictyostelic, and this appears structurally out of
place among the Microlepias : an examination of its sorus, however, shows
that the plant is one of the Mixtae : thus both the anatomy and the state
of the sorus show that its proper place is elsewhere, probably with Davallia,
The other exception is Davallia (Microlepia) pinnata, Cav., which appears
to have relations anatomically rather with Lindsaya. Excluding these it
may be said then that solenostelic structure is typical for Dennstaedtia
and Microlepia.
The vascular relations of leaf and axis are indicated by the Figs. 333
A, B, c. The leaf-trace is in all cases an undivided ribbon-like strand :
where it is inserted upon the tubular solenostele the latter opens, forming
the foliar gap, which is here only short, and soon closes (Fig. 333 A).
1 Ann. of Bot., xvii., p. 689.
DENNSTAEDTIINAE 60 1
This simple vascular structure holds for most species, with minor modifica-
tions of form of the leaf-gap, and of insertion of the supply for lateral axes.
There is, however, a further com-
plication in Dennstacdtici adiantoides
and rubiginosa : in the former a
local thickening appears internally
at the margins of the leaf-gaps in
the ordinary stelar cylinder : this
becomes extended to form an in-
wardly projecting ridge, continuous
from one leaf-gap to another (Fig.
333 B). In D. rubiginosa this ridge
is represented by a separate strand, FIG. 333 c.
Which Still maintains itS Connection Denmtaedtia (Dicksonia) rubiginosa. Diagram of
with each leaf-gap-margin, but may
divide into several distinct rods
(Fig. ITI c) This peculiarity is the rhizome would face the observer. (After Gwynne-
of importance for comparison with
what is seen in the stems of the Pterideae on the one hand, and of the
Cyatheae on the other, while it also has its bearings in the elucidation of
the complex structure already seen in the Matonineae.
These facts of vascular anatomy, coupled with those relating to the
sorus appear to indicate for the Dennstaedtiinae a very interesting inter-
mediate position. On the one hand their confirmed solenostely is evidence
of a relatively primitive state, derived probably from a protostelic source ;
but it shows only slight indications of passing onwards to the more
advanced state of dictyostely. Moreover, the constant condition of the
leaf-trace as an undivided, ribbon-like strand is a clear index of their
primitive position. Similarly, the uniform occurrence of hairs and the
absence of ramenta points to the simpler rather than to the more advanced
Filicales.
On the other hand, the soral conditions are characteristically those
of the Gradatae : the basipetal sequence of sporangia is quite as marked
as in Dicksonia or Loxsoma, with which they share the basal indusium,
here, as in Dicksonia, two-lipped. But in certain species occasional
departures from the strict basipetal sequence occur : these are most
prominent in Dennstaedtia mbiginosa, a species which shows also the
vascular strands within the solenostele. With the loss of basipetal succession
and the advent of the elongated stalk the sporangium loses its regularity
of orientation and its markedly oblique annulus. But this is what might be
expected, since there is no longer any mechanical reason for the regularity.
In both of these characters, of anatomical structure and of sorus, the
Dennstaedtiinae approach the Pterideae. On the other hand their relation
to Dicksonia and to Loxsoma is clear : it is based primarily upon habit
602 FILICALES
and leaf-form ; but also upon the characters of the marginal sorus, and
lastly upon the vascular system ; for even the dendroid Dicksonias show
a stelar system but little in advance of the solenostelic Dennstaedtias,
allowance being made for the closer leaf-arrangement in their upright
shoot.
It is thus seen, not from one character alone but from several, that
the Dennstaedtiinae take a transitional position between certain types
of the simpler Gradatae and those Mixtae which have marginal or approxi-
mately marginal sori. But lastly, the inequality of the lips of the indusium,
and the obvious lopsidedness of the sorus, both in Dicksonia and in the
Dennstaedtiinae, has its interest in relation to what is seen in some of
the more advanced Ferns. The origin of the receptacle is still strictly
marginal, but already there is a leaning towards the lower surface, and it
will be seen that, in forms belonging to the Mixtae which appear to be
related, this becomes more pronounced, till finally a superficial position
of the sorus is fully attained.
CYATHEAE.
In all the Gradatae hitherto considered the sorus is of marginal origin,
though in some of the most advanced there is a tendency towards the
lower surface. But in the Cyatheae, in which the sorus is also basipetal,
its position is superficial, being thus comparable with that of the Gleicheni-
aceae or Marattiaceae. There is no comparative ground for referring this
in the Cyatheae immediately to any transition from a marginal position :
there is indeed good reason for believing that the superficial sorus was
of very early occurrence, for it is exemplified in some of the most primitive
types of Ferns.
The Cyatheae as now limited include the dendroid genera Alsophila,
Hemitelia, and Cyathea, though it will be seen that certain other genera
of Ferns of smaller stature are probably related. In habit they are occasion-
ally creeping (A. blechnoides), but mostly of tree-like habit, the columnar
stem being covered by the scars of the tufted leaves : these may be simple
(C. sinuata\ or singly pinnate (C. Brunonis\ but usually repeatedly pinnate.
Broad superficial scales are present generally, but hairs commonly accompany
the sorus. Thorn-like outgrowths are not uncommon upon the surface,
especially about the base of the petiole : these must be held as new
formations, by enation from surfaces previously untenanted in descent :
they show^hat such origin of new appendages existed among very early
vascular Plants. Adventitious roots are numerous, and form a dense felt
investing the lower part of the erect trunk to a thickness often far beyond
its own bulk.
The three genera named form a very natural group, separated from
one another technically by the character of the indusium, which is absent
in Alsophila^ incomplete and scale-like in Hemitelia, while in Cyathea it
CYATHEAE
603
is cup-like, and may form a complete investment for the sorus while young.
The disposition of the sori is fundamentally as in Gleichenia, in a single
series on either side of the midrib of the pinnule ; but the regularity of
the series is liable to be disturbed according to the mode of growth of
the leaf. Moreover, the identity of the sorus is not always maintained,
fissions (or fusions) being frequently seen.
The development of the sorus has been followed in A. atrovirens : an
early condition of it is shown in Fig. 334 A, in which the sporangia near
the apex of the receptacle have the cap-cell already formed, while in the
lower sporangia on either side it is not yet cut off. The succession is
thus basipetal, though not markedly so, since their number is not very
large in this species. The sporangia are from the first more robust than
those of Cyathea, but less so than those
of Gleichenia, to which they correspond
in the oblique annulus, and in their
position in the sorus : moreover, the
first segmentations take a middle place :
the parent cell of the sporangium has
frequently a wedge-shaped base, and
the first segmentation-wall cuts one of
its oblique lateral walls : this type is
thus intermediate between that of the
more robust Simplices and that of the
smaller sporangia of the Polypodiaceae.
In point of its convex form and actual
size the sporangium of A. atrovirens
does not differ widely from that of
Gleichenia dichotoma, but the stomium Too?' r£=VSTrf° ^S^Br'
is lateral as against the median dehi- S;re •<£ a transverse section of a sporangial
scence of Gleichenia : still it is obvious
that the cells of the stomium form part of the very regular series of the
annulus (Fig. 334 c). The regularity of the orientation is according to the
type of Gleichenia in the basal part of the sorus, but it is not strictly
maintained by the sporangia at the apex of the receptacle : in this also
Alsopliila corresponds to Gl. dichotoma. The typical number of spores
per sporangium appears to be 64.
It is thus seen that Alsophila shows certain points of interest for
comparison with Gleichenia : notwithstanding the difference of habit it
corresponds in the position of the sorus and in the absence of the indusium,
while the number of the sporangia in the sorus in A. atrovirens is not
greatly in excess of that in Gleichenia dichotoma. There is also some
similarity in the form of the sporangium with its oblique annulus, and in the
orientation of the sporangia, at least in the lower part of the sorus, though
in both there is irregularity towards the apex of the receptacle. But there are
important differences in the position of dehiscence and in the number
FIG. 334.
A— A Isophila atrovirens^ Prest. A young
sorus cut vertically, and showing a slight indica-
tion of basipetal succession of the sporan
celsa,
sporangial
604
FILICALES
of the spores produced from each sporangium. If in a sorus of the type
of Gleichenia dichotoma the receptacle were elongated to receive a basipetal
succession of sporangia, which retained their form, but showed a diminished
spore-output, and lateral dehiscence in accordance with their basipetal
sequence, the sorus of Alsophila would be
the result.
Such changes are inherently probable,
and it has been seen in the sorus of
the Hymenophyllaceae how the greater
number of sporangia goes along with a
fall in their individual productiveness.
This is carried further in Cyathea than in
Alsophila, for there the sporangia are
smaller, and the output in C. dealbata
may fall as low as 16, or even 8 spores
per sporangium, though in C. medullaris
the number may remain at 64. The
development of the sorus in this genus
has also been followed : it differs in no
shows a very young sorus, with receptacle CSSCntial point from that of AlsopJllla,
excepting in the presence of the basal
indusium, which appears before any of the
sporangia (Fig. 335). The inconstancy of
occurrence of the indusium in a group of closely related plants indicates
clearly that, however large, or early in appearance, or biologically important
it may be, it is not to be held as an essential part of the sorus, nor trust-
worthy as a phyletic character.
ANATOMY.
Anatomically the Cyatheae show very great complexity of structure,
though it can be referred, even in the most complex examples, by comparison
to a simpler source : the conclusions are, however, rendered less certain
by the lack of graded intermediate conditions. A relatively simple state
was found by H. Karsten x in the western species Alsophila pruinata, a
Fern which grows with an upright stem some three feet or more in height.
In transverse sections of the axis a solenostelic structure is seen, which
opens here and there with a foliar gap, from the margin of which the
leaf-trace is given off, apparently as a simple strand, with the usual horse-
shoe-like transverse section r after leaving the axis the leaf-trace soon breaks
up into a number of strands. As the internodes are of perceptible length
the leaf-gaps do not overlap, and the stele often appears as a complete ring
(Fig. 336). A peculiar feature is seen in this Fern in the leafless
runners, which originate below the leaf-bases, and grow like roots downwards
into the soil : it is interesting to note that they have at first a solid stele,
FIG. 335-
Cyathea dealbata, Sw. The upper figure
lows a very young sorus, with receptacle
and indusium already indicated. The lower
shows the indusium (z) more advanced, and
the sporangia s, s, arising in basipetal
succession. X 200.
1 Vegetationsorgane d, Palmeii., p. 123.
CYATHEAE
605
FIG. 336.
Alsophila pruinata, Kaulf. Transverse sections ot stem, leaf- bases, and bud (a) in
Figs. 3 and 4 ; it is apparent that the axis is solenostelic, that the solenostele opens at
the exit of each leaf, and that the leaf-trace is a continuous horse-shoe at its base. (After
H. Karsten.)
•f a s
FIG. 337 A.
Cyathca Iinrayana, Hook. A = axile longitudinal section. Natural size. The section
was about ~\ mm. thick, and semi-transparent. The black stereom and paler vascular
strands drawn as superficial do not all lie exactly in the same plane. # = vascular strands
of the main cylinder; s = outer ; s' = inner plates of its sclerotic sheath ; internally to s', is
the pith with its medullary strands, outside s, the cortex with cortical strands ; r =
cortical bundles; b = leaf-scars ; £/=strands passing into leaves; w = insertions of roots ;
w = a foliar strand running into the pith ; above .r a blind ending of a medullary bundle.
(After De Bary, from Engler and Prantl, Nat. Pflanzenfam.)
6o6
FILICALES
which, however, soon develops a pith. The anatomy of this Fern, which
deserves a thorough reinvestigation, appears to be comparable with that
of a solenostelic Dennstaedtia : it suggests the structure from which the
more complicated Cyatheaceous stems may have arisen. Another species
with exceptionally simple structure is stated to be A. blechnoides, which is
peculiar in having a trailing axis.1
In the vast majority of the Cyatheae the vascular system consists
essentially of a dictyostele, with accessory strands in the pith, and in
some cases in the cortex also. The leaves being closely disposed on the
massive axis, the leaf-gaps, which
are narrow, overlap, and accord-
ingly several are traversed in a
single transverse section (Fig.
337 B). The dictyostele is thus
represented by several broadly
strap-shaped tracts, with their
margins turned outwards, and
guarded on either side by bands
of brown slerenchyma : this is
the correlative of the solenostele
of simpler types. The leaf-
trace, composed from the first
of numerous distinct strands,
springs from the margin of the
leaf-gap, the strands being dis-
posed in the usual horse-shoe
series. But over and above
this fundamental vascular system
accessory strands are found in
Cyatkca Imrayana, Hook. Transverse section of stem. the pith (FigS. 337 A, B) I these
Natural size. At b, c, d, foliar gaps ; all the black bands and
g
spots are stereom, all the paler bands are vascular strands
'
nricrirmrp from rhp foliar
Originate Oliar
and traverse the pith as a
branched system with occasional
blind endings. In origin and nature they would appear to be comparable
to the accessory rods seen within the solenostele in Dennstaedtia rubiginosa
(compare Fig. 333). 2 In the cortex also an accessory series of strands,
related to the leaf-trace, is found : it is well shown in C. Imrayana
in Fig. 337 c, and is reported also for other species, both of Cyathea
and of Alsophila. This cortical system has no recognised correlative in
other Ferns.
Young plants of Alsophila excelsa have been examined by Gwynne-
Vaughan,3 with a view to tracing the ontogeny of the vascular system, and
especially the origin of the medullary strands : his results are illustrated
1 Mettenius, Ueber Angiopteris, p. 524, note 3.
2 Gwynne-Vaughan, /.c., p. 709. 3 L.c., p. 709.
CYATHEAE
607
by a diagrammatic figure (Fig. 338), but with the reservation that the rapidity
with which the successive stages are passed through varies considerably : it
is believed, however, that the diagram will serve to represent the course
of development of the vascular system, not only in the Cyatheae, but also
in most of the solenostelic and dictyostelic Ferns, up to the particular
stage that they retain when mature. The following description is taken
almost verbally from Mr. Gwynne-Vaughan's memoir.
The young plant of Alsophila excelsa has its leaves arranged radially
all round the axis. At the very base of the stem the single central
FIG. 337 C.
Cyathea Itnrayana, Hook. Pieie of stem with four leaf-bases, after removal of the
outer layers of cortex, seen from without. The margins of the four leaf-gaps, the bundle?
which spring from them and pass into the leaves, the roots inserted on them (black), and
the bundles which run down within the cortex are exposed. The cortical bundles and
root bases are quite free, the rest are covered by semi-transparent parenchyma. Natural
size. (After De Bary, from Engler and Prantl, Xat. Pflanzenfatn.')
cylinder possesses a small central strand of xylem, usually with a few
xylem-parenchyma cells intervening between the tracheides. The first leaf-
trace may depart without in any way altering the structure of this stele,
or of its xylem-strand, but usually the phloem on the adaxial surface of the
leaf-trace is prolonged a short distance downwards into the substance of
the central xylem. At the departure of the subsequent leaves this feature
is much more pronounced, and the phloem thus decurrent runs down
through the whole length of the internode to meet with that decurrent
from the leaf below. In the second leaf, however, it often falls short of
the point of departure of the first leaf, and ends blindly in the internode.
6o8
FILICALES
From this point, therefore, up to the third or fourth leaf, the centre of
the xylem-strand is occupied by a core of phloem. At the departure of
about the third or fourth leaf the pericycle follows the phloem down into
the internode below, so that a few pericyclic cells are now to be found in
the centre of the core of phloem. At the fifth leaf
(or sometimes at the fourth) the endodermis also
is decurrent, giving rise at first to a few cells only
in the centre of the pericycle, which usually dis-
appear before the node below is reached. Higher
up it is continuous from node to node, and sur-
rounds a progressively increasing amount of ground-
tissue, which is now decurrent with it. The vascular
system has, in fact, become a solenostele. This
stage, however, does not last long, for the leaf-gaps
begin to overlap after the departure of about the
eighth leaf, and above this point the system
becomes more and more dictyostelic, although at
first a complete vascular ring is occasionally to be
met with. The leaf-trace of the first five or six
leaves consists of a single curved strand. Above
this point two or three separate strands are given
off to each leaf, and at about the tenth leaf four
such strands are present, two arising from each
side of the leaf-gap.
The first indication of the internal steles that
occur in the mature plant is to be found at about
the tenth leaf. Just below one or both of the
two upper (adaxial) traces of this leaf the xylem
of the stem-stele is seen to project slightly inwards,
so as to form a small ridge on its internal surface,
which is often continued as such for some distance
down the stem. Sometimes, however, it separates
off completely so as to produce a small xylem-
strand lying free within the phloem of the stele,
which either ends blindly below or eventually fuses
up again with the main xylem-strand. These free
xylem-strands are always present at the subsequent
leaf-gaps, and, although still remaining enclosed by
the same endodermis, they become more and more distinct from the main
xylem-strand of the stele. Later on they may even separate off from the
stele altogether in the upper part of their course, only fusing with it
again at a point lower down. The separation of the small xylem-strands
from the main stele finally becomes complete throughout, and from
their starting-point they run as small independent vascular strands ending
blindly in the central ground-tissue, and having no further communication
FIG. 338.
Alsophila 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.)
CYATHEAE 609
with the main stele, except sometimes by a small branch near their point
of origin.
It seems, therefore, that the internal vascular strands of Alsophila
xcelsa owe their existence to the same initial phenomena as do those of
Dennstaedtia rubiginosa ; that is to say, they are probably derived from
lie elaboration of a local thickening of the xylem-ring at the margins of
leaf-gaps in the ordinary stelar cylinder ; but they do not appear at
11 until the ordinary stelar cylinder has become dictyostelic.
The ontogeny thus disclosed for a complex Tree-Fern may be held as
valid suggestion of the way in which the mature condition was achieved
n descent. It starts from a protostelic state, which is, however, brief, and
)asses to the solenostelic by intrusion of outer-lying tissues into the xylem-
ore ; but this again passes into the dictyostelic by reason of the overlapping
•f the leaf-gaps : and lastly, by intrusion of vascular growths from the
margin of the leaf-gaps, the medullary system is produced. All these
teps, so quickly passed over in the individual life, are readily intelligible,
,nd even probable, in the evolutionary story of plants with a massive
xis, bearing large and closely disposed leaves. •
The protostelic state, here so short, is the permanent condition in
most of the Gleicheniaceae. But the most advanced species of Gleichenia
G. dichotoma and pectinata) show signs of solenostely, while in Alsophila
mrinata the solenostelic state appears to be permanent. But in other
pecies of Alsophila it also is a phase quickly passed through to the
iictyostelic state, which is then permanent. Finally, the medullary
ystem absent in A. pruinata, as it is also in Dicksonia, but developed
n Cyathea, is clearly a late accessory, probably consequent upon the
enormous distension of the pith in relation to the wide leaf-bases.
The leaf-trace also presents features of comparative interest : in the
plant it consists of a single strand, as it is in all the Gleichenias :
n A. pruinata it appears to be so at the base even of the mature leaf,
though it soon breaks up into separate strands as it . passes up into the
eaf-stalk : but in most of the Cyatheae the leaf-trace in the mature
shoot is from the first composed of a number of distinct strands. These
uccessive steps again indicate a probable phyletic progression, the young
plant showing a condition similar to that seen in simpler types, and
especially in the Gleicheniaceae.
If the facts derived from the characters of the sorus be put into
relation to these from anatomy, a substantial parallelism' emerges, point-
ng in both cases towards the Gleicheniaceae as a probable indication
of the genetic source. In soral characters Alsophila is the nearest to
Gleichenia, and especially to those species in which the sorus is no
Conger uniseriate, but consists of a large number of relatively small
sporangia (G. dichotoma and pectinata). It is in these very species that
there is a definite advance towards a state of solenostely not very far
removed from that actually seen in A. pruinata. From such a
2 Q
610 FILICALES
vascular type to that seen in the more complex Cyatheae, the probable
progress has been as suggested in the ontogeny of A. excelsa, while in
the sorus the basal indusium appears as a new structure, and the individual
sporangia are liable to diminution in size and spore-output, as is exemplified
in the extreme form in Cyathea dealbata. Thus there seems good reason
to see in the Cyatheae a series having probable genetic relations with the
Gleicheniaceae, but advanced on the one hand to the basipetal succession
of the sori, and on the other to a high complexity of the vascular system.
This conclusion is in agreement with the palaeontological facts, for
representatives of the Cyatheae have been recognised as present from
Jurassic times onwards. It is, moreover, specially interesting to note that
the genus Alsophila is among the earliest of the fossils referred with certainty
to this family, as exemplified by A. polonica, described by Raciborski
from the fire-clay of Krakau.1
Of Ferns in which evidence of a basipetal sequence of the sporangia
in the sorus has been observed there remain Onoclea, Sphaeropteris, andj
Diacalpe, all genera in which the position of the sorus is superficial and
the indusium 'basal. The natural place for these genera appears accordingl
to be in relation to the Cyatheae.2 The annulus in these Ferns is almos
vertical : in Sphaeropteris it is slightly oblique, and may be traced a
continuous past the insertion of the stalk of the sporangium, as i
characteristic of the Gradatae ; but in Diacalpe and in Onodea the annulu
is interrupted at the insertion of the stalk. These genera appear tc
illustrate how, when the basipetal succession is not long continued, an<
the orientation of the sporangia not strictly maintained, the annulus is n<
longer markedly oblique, but passes over into the vertical position, and maj
even be discontinuous at the base. This will be noted also in the
Dicksonia-Davallia series.
SALVINIACEAE.
It is impossible to leave the Gradatae without mention of the peculia
little group of heterosporous water Ferns, of the genera Salvinia anc
Azolla. They have been so exhaustively described elsewhere that it wil
be unnecessary to give any detailed account of them here, especially a:
they are in all probability a side branch from the main series. Examinatior
of their sori shows resemblances to the sorus of the Gradatae : it is, however
difficult to attach them on these, or on more general grounds to any actua
genus of living ferns. It would seem probable that the type from whicl
they sprang was homosporous, having an elongated receptacle upon whicl
arose a basipetal succession of sporangia, with short thick stalks, and eacl
containing 16 spore-mother-cells. That with the differentiation of the spore
followed certain other modifications, such as a decrease in number of the femal
sporangia, and perhaps an increase of the male sporangia : the former i
1 Abhandl. Akad. Wiss. Krakau, xviii., 1894. 2See Studies, iv., p. 55-58.
SALVINIACEAE
61
particularly exemplified in the female sporocarp of Azolla, where the number
has sunk to a single one : the latter in the male sorus of Salvinia, which
shows the unusual phenomenon of branching of the pedicels. Since the
annulus is absent, there is no ready clue to the orientation of the sporangia,
and it may be a question whether in itself the basipetal succession of origin of
the sporangia is a real index of affinity : it is one of those characters which
might readily appear in several distinct evolutionary lines. But taken with
the other characters of the sorus, and the fact that in these plants the
basipetal succession is not always strictly maintained, and does not appear to
be of any great practical importance, its existence in the Salviniaceae may
be regarded as a survival of an ancestral character. The soral characters
I would all harmonise with the view that the Salviniaceae are a series of
organisms related to the Gradatae, but subjected to modification consequent
upon their aquatic habit, and upon their assumption of the heterosporous
state.
CHAPTER XXXIX.
MIXTAE.
THERE still remain to be considered the great majority of genera and species
of living Ferns. It is not proposed here to enter fully into the characteristics
or the classification of them : it must suffice to indicate certain features
only which they show, and to place them in general relation to those of
the other Filicales which the Palaeontological evidence indicates as prior
to them in time.
It has been found, as the result of examination of representatives of all
the remaining genera of living Ferns, that the sorus is of the type which
is designated " mixed " : that is, that sporangia of different ages are aggre-
gated together without any definite sequence: in fact, that promiscuous
interpolation of younger sporangia between those already present is th<
rule.1 This is accompanied by an absence of any definite orientation o
the sporangia', such as has been seen especially in the Gradatae : also there
is commonly an elongation of the sporangial stalk, which is often reducec
to a single row of cells at its base. With this there is a vertical position
of the annulus, which is interrupted at the point of insertion of the stalk
The numerical output of spores per sporangium has never been seen in
these Ferns to exceed 64, while lower numbers are frequent. These
characters are general for the remaining Ferns exclusive of those alread)
described, and they are accordingly designated collectively the " Mixtae.'
There may, however, be very great differences in the number, position
and extent of the sori, and in the presence or absence of an indusium
and it is upon these characters that their classification has principally beer
founded. But before such classification can be held as more than pro
visional the criteria will have to be extended to include the results of wide
anatomical study, and of comparison of the gametophyte.
It is improbable that the Mixtae constitute one single phyletic line
evidence will be adduced that in more than one distinct line of descent th«
mixed type of sorus was arrived at, and that it was probably derived in mos
1 See Stttdies, iv. , pp. 78-87.
DENNSTAEDTIA-DAVALLIA SERIES 613
cases from the type of the Gradatae, but might also be produced directly
from the sorus of the type of the Simplices. It must suffice here to trace
some probable lines of phyletic origin which have so far emerged, though
others may eventually be recognised.
DENNSTAEDTIA-DAVALLIA SERIES.
The best accredited case is seen in Ferns with marginal sori, and it
has been found that among them there are forms which lead from the
Gradatae towards those genera with mixed sori which have been grouped
by Prantl as the Pterideae.1 It has already been seen that though the
sorus of Dennstaedtia is typically basipetal (see Fig. 332 bis, A), occasional
departures from the strict sequence exist in D. apiifolia, while in D. rubi-
ginosa the sorus retains some signs of the basipetal sequence, but younger
sporangia occur interpolated without order among those pre-existent, while
the receptacle is more flattened (Fig. 339 A). It will be seen that these
characters approach those seen in Davallia.
The genus Davallia, as it stands in the Synopsis Filicum, is a compre-
hensive one. Sir William Hooker remarks 2 of the Davalliae : " No two
authors are agreed as to the limits of this group, nor of the genera which
compose it ; and no wonder, seeing how gradually the genera seem to
run one into another. To me the genera seem to have been need-
lessly multiplied, upon very insufficient grounds, so that in many cases
I cannot even adopt them as sub-genera." The result of Sir William
Hooker's view, as thus expressed, has been that he grouped several genera
of other authors under the comprehensive genus Davallia. But the
tendency has since been to reinstate some of his sub-genera as substantive
genera, the most prominent case being that of Microlepia : the characters
derived from the sorus have been described above, and justify the removal
of Microlepia from the genus Davallia.
From the § Eu-Davallia of the Synopsis Filicum, observations have been
made on D. griffithiana, Hook, pyxidata, Car., canariensis, Smith, solida,
Swartz, and divaricata, Blume. In all of these the sorus shows various
ages of sporangia intermixed, while they are inserted upon a wide, flat
receptacle. The sporangia themselves have long stalks when mature, so
that the ripe sporangial head is raised far above those of the younger
sporangia, and thus scattering of the spores is ensured without an elongated
receptacle.
The development has been specially studied in D. Gi'iffithiana. Fig. 339 B
shows a young sorus with the first sporangia appearing. It may be noted
that on the flat receptacle the first sporangium is in a median position,
and this may be taken as a slight trace of basipetal succession ; but even
this is not constant, and as the development proceeds any superficial cell
1 Arb. K. Bot. Gart. Breslau, vol. i. (1892), p. 17.
2 Species Filicitni, i., p. 150.
614
FILICALES
of the receptacle may grow up into a sporangium, developing as such in
any order whatever, and without any regularity of orientation. The confused
mass which results is shown in Fig. 339 c, and this also illustrates how, as
the sporangia grow older, their stalks, composed in the lower part of but
a single row of cells, become elongated. The vascular strand runs upward
to a point immediately below the surface of the sorus, and there widens out
FIG. 339.
A =sorus of Denn staedtia rnbiginosa. Cut vertically and showing mixed condition in
a sorus originally basipetal. B = Davallia. Griffith.ia.na, Hk. Young sorus in section,
showing first formation of sporangia. C = old sorus of the same, showing sporangia of
different ages intermixed. All X 100.
into a considerable mass of tracheides, surrounded by a thin band of paren-
chyma, and limited by a brown layer, which is doubtless of the nature of
an endodermis.
Examination of representatives of all the other sections of the genus
Davallia led to similar results, and it is thus seen that, with the exception
of Microlepia, which had already been removed on other grounds by Prantl,
and accorded a separate place by Christ,1 the genus Davallia shows
1 Farrnkrauter, p. 10.
DENNSTAEDTIA-DAVALLIA SERIES
615
FlG' 34°'
(After Hooker, from Christ's
r arrnkrauter.)
uniformity of structure of the sorus ort the mixed plan, with flat receptacle,
and with no definite rule of orientation of the long-stalked sporangia.
The connection of the Dennstaedtia-Davallia series
with such genera as Lindsay a, Pteris, Pellaea, and
Adiantum, where the sori are marginal, seems beyond
question, and it is strongly supported by the anatomical
evidence. But, on the other hand, the sori are liable
to move from the marginal position : this phyletic
change is illustrated by very gradual steps. An
inequality of the lips of the indusium is apparent in
Microlepia (Fig. 332 A): it appears often in greater
degree in the various forms of Davallia, and is
specially marked in § Leucostegia (Fig. 340), where
the upper lip appears as the continuation of the leaf-
lobe, the lower as a cup-shaped indusium apparently
some distance from the margin. Leucostegia has long
been recognised as closely related to Cystopteris, which
also has a mixed sorus, without regular orientation
. '
of its sporangia, while it is protected by an indusium
of similar form to that of Leucostegia (Fig. 341). These examples will
.serve as illustrating a feature which has probably been widely effective
in the descent of the Leptosporangiate
Ferns, viz. the retreat of the sorus from
the margin to the under surface of the
leaf. From Cystopteris the sequence
may with probability be traced on to
the Aspideae. It would thus appear
that a considerable proportion of the
Mixtae are referable in origin to forms
with a marginal position of the sorus.
A further illustration of probable
relationship, in this case to Polypo-
dium, is seen in Hypolepis. This
genus was included by Kiihn and by
Prantl * in the Dennstaedtiinae, though
by others it has been placed in the
Pterideae. The marginal sori, covered
by the reflexed margin of the leaf,
consist of a slightly convex receptacle,
upon which the sporangia of various
ages are inserted in no definite order:
the annulus is definitely interrupted
at the insertion of the stalk. It is clearly one of the Mixtae, but the
affinity with the Dennstaedtiinae is indicated by the position and character
*L.C. . 1 8.
FIG. 34i.
Cystopteris fragiiis. Pinnule of the form from
Tasmania, and its sorus enlarged. (After Hooker,
from Christ's Farrnkrciuter.)
6i6 FILICALES
of the sori and the habit, as well as by certain anatomical features. The
genus appears to hold an intermediate position between the Dennstaedtiinae
and some Ferns referred to Polypodium : the relationship to the latter
has already been the subject of remark ; for instance, in the Synopsis
Filicum, p. 130, Dr. Griesebach is quoted as writing of H. Purdieana, Hk. :
" Not to be distinguished from P. rugtdosum but by the specially transformed
involucral appendages, and probably passing into that widely-ranging species."
Again,1 under Polypodium (Pheg.) punctatum, Thunb. (which Hooker
regarded as including P. rugulostim, Labill), he remarks : " Very closely
related to Euhypolepis" All this seems to indicate a probable sequence
which would consist of (a) some Dennstaedtiinous Ferns with basipetal
sori, (b] some type with mixed sorus, and receptacle within the margin
which is curved over as an indusium, as in Hypolepis : (c) such -a type as
Polypodium punctatum, Thunb., with
its definitely superficial, unprotected
sorus, having sporangia with ages inter-
mixed, and no regularity of orientation.
It may next be enquired how far
the anatomical data will support the
results of examination of the sori in
this series. It cannot be assumed that
characters so distinct as those of the
sorus and of the vascular system
FIG. 342. must necessarily run parallel ; but if
r Pteris elata, \. Karsteniana. Diagram showing they do, it is a Strong Support of the
the arrangement of the vascular tissue at the _ . .
insertion of a leaf. A piece is supposed to be cut Correctness OI recognition OI a phyletlC
out of the side of the solenostele, so as to show the •,- .-^ -vruur iu
internal vascular system. Note that a small strand, Une. UWVline- VaUghan has lOUnd that
lying within the second vascular ring, is also present. • • r L u j. *i_
(After Gwynne-Vaughan.) m CVCry SpCClCS of the SUb-tribC
Dennstaedtiinae in which the anatomy
is known the same type of primitive vascular system, the solenostelic, is
seen : this indicates the primitive nature of Prantl's sub-tribe as a whole.
The statement applies for all species of Dennstaedtia examined, but in
D. rubiginosa the solenostele is not quite typical, additional vascular
strands being also present : this is, however, the very species in which
an approach to a mixed sorus is found : thus, the two characters indicate
that plant as an advance upon the rest. The approach is towards a
condition seen in species of Pteris, where with a mixed sorus there is a
still more elaborate accessory vascular system within the original soleno-
stele (Fig. 342).
Turning to the Hypolepis-Polypodium line above noted, the anatomy
again supports the relationship. Hypolepis is solenostelic : so is P. punctatum :
in fact this species stands structurally isolated among the Polypodieae, and
is evidently related closely to Hypolepis, which in turn is related to the
other Dennstaedtiinae.2
1 Syn. Filic., p. 312. - Gwynne-Vaughan. I.e., p. 735.
ONOCLEA-WOODSIA SERIES 617
In the case of Litufsaya the matter is not so clear. The marginal sorus is
of the mixed type, but the vascular structure is less advanced than in the
Dennstaedtia-Davallia series : it is characterised by possessing in addition to
the external phloem-mantle a strand of phloem completely embedded in the
xylem.1 In this, however, there is no serious discrepancy : it appears that
the soral and anatomical characters do not always march abreast : in
Lindsaya the anatomical advance has lagged behind that of the sorus.
Lastly, there is abundant evidence to show that an ultimate state of
dictyostely has been achieved in the vast majority of the Mixtae : it
appears already in Davallia and in Cystopteris, among the series now under
discussion. It may be held as a final modification of the solenostelic
structure, consequent upon the overlapping of the leaf-gaps. And so it is
seen that in the Dennstaedtia-Davallia series the anatomical advance is
in the main parallel with that of the soral characters, though exact parallelism
is not always maintained. There is thus good reason for holding that the
series represents a true line of phyletic advance, leading from the condition
of the Gradatae to that of the Mixtae.
ONOCLEA-WOODSIA SERIES.
A phyletic line of progression from a basipetal to a mixed sorus, possibly
distinct from the last though of much less certainty, may be traced through
genera where the sorus is already superficial : viz. from Onoclea and Diacalpe
on the one hand to Woodsia and Hypoderris on the other. These genera
have been grouped together in most of the leading systems, and are all
included under the Woodsieae by Diels.- But an examination of their
sori shows that in Onoclea and Diacalpe there is a basipetal succession of
the sporangia : this has been demonstrated developmentally in Onoclea,
and the result may be summed up in the statement that the sorus is
characteristic of the Cyatheae ; but the sporangium is characteristic of the
Polypodiaceae, having a long stalk, and the annulus is definitely interrupted
at the insertion of the stalk, while there is no regularity of orientation. In
Diacalpe — as in Sphaeropteris, as well — there is also evidence of basipetal
succession (see Studies, iv., pp. 55-60).
But in Woodsia and Hypoderris, where also the sori are superficial and
the indusium basal, the case is different : in Hypoderris the sorus is clearly
of the mixed type, with flattened receptacle : the same appears to be the
case in Woodsia, though the small number of sporangia makes the decision
less certain. Full anatomical data are not at hand for comparison, though
Onoclea at least appears to have already an advanced type of dictyostele.
The evidence, such as it is, appears to indicate that a line of advance from
a basipetal to a mixed sorus has existed among the Ferns with superficial
sorus and basal indusium, of Cyatheaceous affinity. But these forms require
^ansley and Lulhani, Ann. of Bot., xvi., p. 157.
2 Engler and Prantl, i., 4, p. 159.
618 FILICALES
a careful revision, with special reference to their anatomical characters,
before this progression can be regarded as established.
MATONIA-DIPTERIS SERIES.
The genus Dipteris, Reinvv., so long merged in the comprehensive genus
Polypodium on account of its sorus being naked and superficial, has recently
been restored to its independent position, and is now held to be the sole
representative of the family of the Dipteridinae.1 There is little doubt
that this position is justified, while among relatively primitive types the
family finds near allies among the Matonineae.
The genus is represented by four living species from the Indo- Malayan
Flora, which illustrate an interesting progression in leaf-architecture. They
all have creeping rhizomes, showing occasional dichotomy, the type of shoot
being closely similar to that of Matonia. The axis and the bases of the
leaves alike are invested with a dense covering of hairs, which are, however,
flattened into elongated scales, an advance upon the filamentous hairs of
Matonia. The leaves of the different species vary in area, but are alike
in plan : upon the end of a long petiole is borne a lamina which is repeatedly
branched in a dichotomous manner. The branches may remain narrow, with
a marked midrib and lateral flanges of no great area, as in D. Lobbiana,
Hooker, and D. quinquefurcata, Baker : or they may be broader, and be
more or less webbed into a lamina, which is, however, still divided by a
median sinus into two symmetrical halves : this is seen in D. conjugata,
Reinward (Fig. 343 A), and D, Wallichii, Hook, and Grev. This leaf-
structure is comparable with that of Matonia, in which also the outline of the
lamina is referable to a dichotomous branching, and as in that genus, so
here also the sori are in the narrow-lobed species disposed upon the flanged
wings on either side of the midrib. Their relation to the area of the leaf-
surfaces within this very natural genus is instructive for comparison with
other Ferns. In the narrow-leaved D. Lobbiana, and especially near to
the bases of the several lobes, the sori form a regular linear series on
either side of the midrib (Figs. 344 and 343 E). In D. quinquefurcata the
lamina is larger and the segments broader than in D. Lobbiana, and the
areolae within the veins on either side of the midrib are larger, and contain
more sori : these illustrate various degrees of fission, and thus they become
spread over the enlarging area (Fig. 345). D. Wallichii appears to occupy
.a middle position between these species and the large D. conjugata ; for
it is described as having the ultimate segments linear in form, and the
sori as being similar to those of D. conjugata, but more numerous than
in D. Lobbiana or quinquefurcata. Lastly, in the large-leaved D. conjugata
the bifurcate lamina is broadly webbed, and the very numerous small
sori, which are distributed over the wide expanse, may be circular or oval,
.and not always distinct from one another: they may vary much both in
1Seward and Dale, Phil. Trans., vol. cxciv., p. 487.
MATONIA-DIPTERIS SERIES
619
size and shape, and their individuality is often lost, so that nearly the whole
of the lower surface of the frond appears as though densely covered with
a mass of sporangia (Figs. 343 A c, and 346).
FIG. 343.
Dipteris, Reinw. A-C = D. conjugata (Kaulf), Reinw. ^=leaf of a mature plant.
.5=habit of a young plant. C = part of a fertile leaf with venation and sori. D =
sporangia and paraphyses enlarged E = D. Lobbiana (Hook.), Moore. Part of a fertile
segment with venation and sori. (A, C, D after Kunze. B, E after Diels, from Engler
and Prantl, Nat. PJlanzenfain.}
There seems to be only one probable way of reading these facts
phyletically. Comparison points to Matonia and Gleichenia as primitive
620
FILICALES
types of leaf, to which that of Dipteris is related by D. Lobbiana', but
from this simple narrow-leaved type, with its single row of sori on either
side of the midrib, the broader-leaved Dipteris has broken away as its leaf-
area enlarged, and the sori have been spread over the extended surface,
while the absence of their individuality gives the key to the way in which
FIG. 344.
Dipteris Lobbiana (Hook.), Moore.
Parts of two pinnae, showing narrow form,
venation, and regular disposition of the
sori. Natural size.
FIG. 345.
From a specimen collected by Capt. Hope, R.N.,
on the " China Station," but without exact locality:
recognised as Dipteris qidnquefurcata, Baker. Pinna
showing greater width, and sori arranged below in two
lateral series as in D. Lobbiana, but spreading out
upwards, with many fissions, over the more extended
surface. Natural size.
the result has been brought about, viz. by fission. This process, so clearly
seen in the few species of this very natural genus, has probably occurred
also in other types of Ferns. It is suggested by Kaulfussia among the
Marattiaceae, but much more obviously in various lines of the Polypodiaceae.
It will have to be reckoned with in any general conception of the phylogeny
of the leaf in Ferns.
Examining the sorus itself, it is composed of a number of sporangia,
and numerous glandular hairs are associated with them. The sporangia
MATONIA-DIPTERIS SERIES 621
show no regularity of position or of orientation, such as is seen in Matonia:
there is also an absence of any projecting receptacle. The sporangia of
the same sorus have been found to arise simultaneously in D. Lobbiana,
which may in this respect compare with Matonia. But in D. conjugata
they are formed successively, while those which appear later are distributed
without order amongst those first formed. The sorus, in this respect,
compares with that of the Mixtae, but the succession is not long main-
tained. When the individual sporangia are examined an essential
difference is found from the Polypodiaceous sporangium, with its vertical
ring ; for here the annulus is not only oblique, but also twisted : the series
of cells of the annulus can be traced laterally past the insertion of the
stalk, but the induration of their walls is interrupted at that point : the
>
FIG. 346.
Dipteris conjugata, Rein. Portion of leaf, showing the extended surface, the webbing
between the pinnae, the venation, and the numerous sori spread over the surface.
Natural size. Figs. 344-346; after drawings by Mr. A. K. Maxwell.
dehiscence is lateral, but there is no clearly defined stomium. The
sporangium itself is small, and the spore-output has been found both in
D. Lobbiana and in D. conjugata to approach the typical number of 64.
Comparing this sporangial' structure with that of other Ferns, it is actually
most like that of the Cyatheae, though the interrupted induration of the
annulus points a further departure from the primitive type, such as may
with reasonable probability be found in the sporangia of Matonia, and
ultimately of Gleic'henia.1
Turning to the anatomical characters, they bear out the above com-
parison ; for the rhizome contains a simple solenostele, while the leaf-trace
comes off as a single ribbon-like strand, opening a leaf-gap which soon
closes again. The margins of the petiolar strand curve inwards to form
the usual horse-shoe curve, which only breaks up at a point close below
1 See Miss Armour, New Phytologist, 1907.
622 FILICALES
the lamina. These are all relatively primitive characters, and unusual in
Ferns showing a mixed sorus : they direct the line of comparison down-
wards to Matonia and Gleichenia. The former has a vascular system of
the same type as Dipteris, but it has run into greater complications,
with its concentric solenosteles. Both genera, however, are considerably
in advance of the most complex Gleichenias, Yet all these Ferns appear
to conform in their various degrees of elaboration to the same vascular
type.
There is, however, no exact parallelism in the soral and vascular
characters. Gleichenia is the most primitive in both respects ; while
Matonia is the most advanced of all in vascular structure, its sorus is still
that of the Simplices, though it has only a small spore-output per sporan-
gium, and a protective indusium is present ; but as this is apparently
absent in Laccopteris, it has probably been in Matonia a special generic
feature. Dipteris, with its vascular system taking a middle place, has the
most advanced soral condition, as shown by their distribution on the leaf,
by the flat receptacle, and by the mixed aggregation of the sporangia in
D. conjugata. But still it proclaims its origin by the absence of indusium,
the oblique annulus, and the imperfectly differentiated stomium. The sum
of characters justifies the conclusion that in Dipteris we see a genus of
origin from a stock included in the Simplices, in which at least one species
has passed, apparently without the intermediate state of a basipetal sorus,
directly to the condition of the Mixtae. There is, moreover, good reason
for holding that this phyletic line has proceeded quite independently of the
other progressions to a mixed sorus which have been traced elsewhere.
Finally, the palaeophytological data harmonise with this conclusion ;
for representatives of the Dipteridinae figured largely in the Mesozoic
Flora, as far back as the Rhaetic, with sori agreeing in form and distribution
with these of Dipteris; but the annulus is described as probably complete.1
This point may be considered doubtful ; but if it were confirmed it would
fall in readily with the phyletic position suggested for the Dipteridinae.
The conclusion of Seward seems fully justified that Matonia and Dipteris
are linked together as remnants from a bygone age. They have advanced
independently, the one to higher vascular complexity, the other to a distri-
bution and construction of the sori characteristic rather of the more recent
Ferns than of its own progenitors.
It has now been seen that the condition of sorus characteristic of the
Mixtae is absent from the Ferns which Palaeophytology tells us were the
most primitive, but that it is the prevailing feature in the Ferns of the
present day. It has also been seen that steps leading from the more
primitive condition of the Simplices and Gradatae to the mixed type of
sorus exist in certain Ferns : and further, that there is a probability that this
end has been achieved by progression along more than one phyletic line :
1 Seward, I.e., p. 507.
MIXTAE A HETEROGENEOUS GROUP 623
these conclusions have been shown to be supported by the facts of anatomy.
It must, therefore, be allowed that those Ferns which are associated under
the general heading of "Mixtae" are relatively late derivative forms,
and that they do not constitute a natural group, any more than do those
plants which are heterosporous or those which produce seeds. It would
then seem desirable to proceed at once to divide this heterogenous group
into true phyletic sequences. But to do this requires much greater com-
mand of facts, and especially of those of anatomy, than is at present
available. Here it must suffice to recognise the unsatisfactoriness of the
present position, and at the same time to give a very few general indications
of the form the future system may take.
It would appear probable that the main bulk of the Mixtae have been
derived along a line where the sori were marginal, with the Dennstaedtiinae,
the Davalliinae, and Pterideae of Prantl as early representatives of it. This
was accompanied by transition through the solenostelic to the dictyostelic
structure of the stem. A gradual shifting of the sorus to the under surface
of the leaf also occurred, till the condition was reached as seen in the
Aspidiinae and Aspleniinae of Prantl. Certain forms allied to these, losing
their indusium altogether, constituted one section of the old comprehensive
genus Polypodium. All through the more advanced members of this
sequence the dictyostelic structure of the stem was maintained. Another
contingent, with very similar final result, probably arose from forms with
superficial sori and basal indusium, allied to the Cyatheae : in this also
the dictyostelic structure is seen. A third series, also with superficial sori,
is represented by the Gleichenia-Mato?iia-Dipteris line : it is true that
Dipteris is at present the only recognised representative of this sequence
which has attained to the rank of the Mixtae, and it has consequently
been removed from its old position in Polypodium. It seems, however,
not improbable that future investigations may add fresh contingents from
the ranks of Polypodium, and possibly from some other genera, and one
useful criterion will be found in the stem-structure, for in the recognised
forms it is persistently solenostelic.
The attempt will not be made at present to assort all the remaining
forms of Polypodiaceous Ferns into probable phyletic sequences : they are
left to be dealt with as knowledge increases. Meanwhile the general view
of them will be as of a brush of diverging phyletic lines, which have proved
blind. In fact, the ultimate fulness of development of the Homosporous
Ferns is that which is before us to-dav.
CHAPTER XL.
GENERAL COMPARISON OF THE FILICALES.
THE burden of evidence in the comparative study of the Ferns has
habitually been laid upon the sporophyte ; indeed, this was a matter of
necessity to the older Pteridologists, since the prothalli were then practically
unknown. But subsequent investigation has largely justified what was at
first a matter of circumstance rather than of choice : it has been shown
that for very many Ferns there is a dead level of form of the gametophyte,
while it has been proved to be possible, by varying the conditions of growth,
to elicit great differences of development even in individuals of the same
species. It is true that while some groups of Ferns have habitually a robust
prothallus, as in the Marattiaceae, others show habitually a delicate and
sometimes a filamentous type, as in the Hymenophyllaceae or Schizaeaceae,
while the same appears also in Vittaria. But though in some measure such
characters may be held as useful evidence, the very slight positive features
that the vegetative development of the prothallus presents, and their liability
to modification, will always derogate from its importance in comparison.
Turning to the sexual organs, they vary in their level, being either sunken
or projecting ; and an interesting parallel may be drawn between them and
the sporangia in this respect, for they are habitually sunken in Eusporangiate
and projecting in Leptosporangiate forms. The archegonia are singularly
uniform in structure throughout the Ferns ; but the antheridia show two
distinct types as regards dehiscence : the one, in which a cap-cell breaks
away at maturity, is characteristic of all Ferns with an oblique annulus, with
the exceptions of Aneimia and Mohria : the other, in which there is a star-
like dehiscence, includes Aneimia and Mohria, together with the whole
body of the Polypodiaceae. Such facts are interesting as a confirmation
of the results of study of the sporophyte, for they group together on the
basis of a gametophyte character those Ferns on the one hand which
comparison of the sporophyte indicates as primitive, and on the other
those which are held to be later and derivative. It is in this way that
the characters of the gametophyte may be used, as ancillary rather than
COMPARISON OF EXTERNAL CHARACTERS 625
.
dominant in our comparisons ; and the burden of the argument must still
rest upon the facts derived from the sporophyte generation. We shall then,
excepting for an occasional reference, leave the gametophyte aside in the
present discussion, and review the characters of the Fern-plant in its relation
to the general theory of the sporophyte.
EXTERNAL CHARACTERS.
The Ferns are the characteristic megaphyllous members of the Pterido-
phyta, and thus differ markedly in habit from the smaller-leaved strobiloid
types. It is necessary first to inquire what are their probable relations to
these series. In point of time the distinction of habit dates back as far as
the earliest known fossils, and accordingly it is only by comparison that any
opinion can be formed as to their origin by descent, and then only as a
probability, not as a demonstration. The similarity of life-history shows,
however, that the sporophyte of the Fern as a whole corresponds to that
of the strobiloid types : the further question will then be as to the
correspondence of the parts, especially the axis and leaf.
The chief difference lies in the proportion of leaf to axis, and in the
branching of the leaf, not in the fundamental relations of those parts as
regards origin or position : this is specially obvious in upright growing
species, with radial symmetry of the shoot. In the Ferns, as in other
Pteridophytes, there is reason to regard the radial type of the shoot as
primitive, notwithstanding the fact that a very large proportion of living
Ferns are dorsiventral. Among the Ferns of the Primary Rocks no dorsi-
ventral type of shoot has been described, unless it be the Permo -Carboniferous
genus GlossopteriS) the relation of which to the true Ferns is still a matter
for discussion. It is possible that a creeping rhizome may have existed as
the base of insertion of some of the unattached fronds, but still in the
absence of demonstration of this the evidence points to the radial type as
having been prevalent. This is the case with the various stems designated
Caulopteris, in many of which the leaf-arrangement is on a spiral plan :
even those designated Megaphytum, where the leaves are distichous, were
of radial character, and all eyidence indicates that their position was upright.
Among the best known of the early forms are the Botryopterideae, which
had relatively thin axes with leaves in some cases closely aggregated, in
others more laxly disposed : both types are of radial construction. Thus
the evidence, so far as it goes, indicates that the radial type of shoot was
prevalent, if not indeed exclusive, for the early Ferns. It is exemplified
by the Botryopterideae, the Marattiaceae, and the Osmundaceae, all early
types.
That large-leaved forms would be mechanically unstable structures is
obvious, especially where the stem is thin and the internodes of appreciable
length. There is an inherent probability that such axes should become
oblique or prone, with a dorsiventral development as a natural consequence.
2 R
626 GENERAL COMPARISON OF THE FILICALES
Examples illustrating that this has actually occurred have already been seen
in the living Marattiaceae ; while Angiopteris and Marattia have upright
and radial stocks,, that of Danaea becomes oblique or even prone as it
grows older, and Kaulfussia, with its longer internodes, is a creeping form.
In all of these, however, where the embryo is known, the shoot is in the
first instance erect. It seems plain that there has been a transition from
the upright and radial to the prone and dorsiventral type.
In the living representatives of those sequences of Ferns which culminated
in the Leptosporangiate group dorsiventrality is more common, and it is
already seen to be prominent in such early types as the Schizaeaceae,
Gleicheniaceae, and Matonineae, though the Cyatheae and Dicksonieae are
strongly radial. There is some reason on anatomical grounds for thinking
that the living Hymenophyllaceae show in their radial types a recovery of
FIG. 347.
Portion of the leaf surface of a seedling of asplenium serpentini, showing how
dichotomy t accompanies the marginal growth. X 190. To the left a diagrammatic
representation of the same. (After Sadebeck.)
the upright shoot from the creeping rhizome, and this may have occurred
in others of the Leptosporangiate Ferns. However this may be, the
Leptosporangiate Ferns show radial and dorsiventral development so
intimately intermixed that it is more difficult in them to trace the probable
evolutionary relations than in those groups which are clearly indicated as
the most ancient. But taking the facts over all, it appears reasonably
probable that the primitive shoots of Ferns were radial, and that dorsi-
ventrality was here as elsewhere derivative.1
In some Ferns the axis remains unbranched, as in the Marattiaceae.
In others dichotomous branching of the axis is seen to occur, and there
is reason to recognise this as a primitive mode of increase, since it occurs
characteristically in relatively early forms, such as in Lygodium, in the
1 Mr. Tansley remarks very pertinently that "dorsiventrality is not very common in
fern steles, in spite of the prevalence of creeping rhizomes" (New Phytologist, 1907, p. 112).
To those who hold that vascular structure follows rather than dominates development this
is important evidence in favour of a primitively radial construction of the Fern-shoot.
EXTERNAL CHARACTERS
627
Osmundaceae and Matonineae, and occasionally in other genera, for instance
in Cyathea and in Pteris. On the other hand, axillary branches are found
with a high degree of constancy in the Botryopterideae and Hymeno-
phyllaceae. In other Ferns buds are found in varying relation to the
leaf-bases, and at various other points upon the leaves : all these appear
to be different in their nature
and origin from the terminal,
dichotomous branches above
mentioned.
The architecture of the
leaves of Ferns, with their
complex and variable out-
lines, presents features which
are important for comparison.
For long the developmental
interest centred in the apical
segmentation, as exhibited in
the Leptosporangiate Ferns
with their single initial cell.
It was not till 1874 that
Sadebeck extended that in-
terest to the marginal growth
of the ultimate pinnules, and
showed in the case of Asplen-
ium Shepherdi that the last
branchings of the veins are
true dichotomies (Fig. 347).
The same was shown later by
Prantl in the Hymenophyl-
laceae : such dichotomy may
be held to be wide-spread in Ferns, and its results are apparent in the
external outline of many young leaves ; for instance, it cannot be missed
in the case of Allosorus crispus, quoted by Goebel (Fig. 348) : here the
successive pinnules are produced as branches of a dichotomy, and are
successively relegated to a lateral position right and left : the whole pinna
is thus a sympodial development of a dichotomous system, though when
mature it presents an appearance of pinnation.
Dichotomous branching is a very obvious feature even in the mature
leaves in some of those Ferns which are believed to be relatively primitive.
For instance, in the Schizaeaceae, and especially in Schizaea itself, while
the same is seen with modifications in the pinnae of Lygodium.1 Again,
in Matonia and Dipteris it is very obvious, though with sympodial develop-
ment of the branches; and in the Hymenophyllaceae, especially in the
1 See Prantl, Unters. z. Morph. d. Gefasskryptogamen. I. Die Hymenophyllaceen
Die Schizaeaceen.
FIG. 348.
Allosorus crispus. Outline of a leaflet. The branching is
clearly dichotomous. The apex has divided into lobes i and 2,
of which i is the stronger and continues the growth, 2 forms a
lateral lobe. Below we have lobes 3 and 4 which have been
similarly formed. The leaf-spindle (rachis) S, is only a narrower
portion of the lamina which is subsequently mechanically
strengthened. Magnified. (After Goebel. )
628 GENERAL COMPARISON OF THE FILICALES
distal branchings. Moreover, the prevalence of dichotomy in the venation
of Ferns at large is to be taken into account in this connection. Lastly,
dichotomy is a common feature in the first leaves of Fern-seedlings, and
is seen, probably as an occasional reversion, abnormally in the later leaves
of many Ferns, being sometimes a persistent character of varietal forms.
These facts suggest the enquiry as to the branching of the leaves of the
early Ferns : it has been pursued by Potonie, who finds among the early
fossils, and even among those of Pecopterid-type, evidences of dichotomy
which lead him to conclude that the truly pinnate type of leaf-construction
in all its parts originated phylogenetically from the true dichotomy.1
Potonie strengthens his position by noting certain palaeontological facts.
The Archaeopterids from the Devonian, Culm, and lower Carboniferous
have no midrib in their ultimate pinnules, but are characterised by parallel
veins, forked in a fan-like manner. In higher strata, however, a midrib
with lateral veins is found. Reticulate venation was apparently absent from
the Ferns of the Culm, and appeared in the Middle Carboniferous, while
the higher type of reticulation, with areas of smaller meshes filling up the
meshes of a larger reticulate system, occurs first in the Mesozoic period.
The fossil record would thus support the early prevalence of dichotomy,
so far as venation is concerned.
Before accepting Potonie's conclusion an examination of the development
of the apparently pinnate type in living Ferns is necessary. In 1875 Kny
showed that in Ceratopteris the lower pinnae arise alternately below the
leaf-apex, the branching being monopodial, and without individual relation
to the segments of the apical cell.2 This origin of the lower pinnae has been
verified also in other Ferns, and may be taken as the usual type where the
leaves are elongated and the lateral parts numerous. But it is to be noted
in such cases .that the pinnae themselves may branch dichotomously, that
towards the apex of the leaf there may be a gradual transition to a dichoto-
mous branching, the pinnae being then produced sympodially after the
scheme seen in the pinnae of Allosorus (Fig. 248) ; and that in all cases
the pinnae arise in strict relation to the lateral wings or flanges of the leaf.
For, however much disguised by special developments at the base of the
leaf, or by the bulk of the leaf-stalk in proportion to the wings, still every
Fern-leaf is essentially a dorsiventral structure, with margins which may or
may not be developed as projecting wings, but can commonly be traced even
down to the base of the leaf-stalk ; and it is upon these that the pinnae
originate. The general facts may be summed up thus : that the branches
arise marginally on the flattened leaf; that where the leaf is massive and
greatly elongated the lateral parts are laid down monopodially, but where
the surface-growth predominates there is dichotomous branching without
the formation of a strong midrib ; but the one type may pass into the other
in the length of a single leaf.3
1 PalaeophytologiC) pp. 110-121. 2 Compare Kny, Parkeriaceen, Taf. xxiv.
3 Compare Goebel, Organography , p. 317-
EXTERNAL CHARACTERS 629
In 1884 I formulated a theory of origin of the " phyllopodium," or rachis
of the leaf, chiefly based upon comparative study of the leaves of Ferns.1
It was pointed out how in an enlarging dichotomous system a main rachis
asserts itself as a supporting organ among parts similar in origin and structure
to itself. This theory of genesis of the Fern-leaf may now be restated as
follows : the Fern-leaf was originally a limited structure of flattened form,
endowed with growth at its distal end : this growth was conducted with fan-like
segmentation, but it was apt to be localised at points which diverge dichoto-
mously. Sometimes the margin remained entire, and the fan-like expansion
is then traversed by dichotomising veins : it is not improbable that this is
really a secondary condition of condensation of a branch-system. More
commonly the margin grows out dichotomously, the veins following, and
a fan-like forking is produced such as is actually seen existent in many
Ferns. But frequently with the enlargement of the branch-system the
equality of the forking was not maintained : certain branches took the
lead, and a sympodial development resulted in a rachis being produced,
as a strong support for the rest, though primarily it was of similar origin
with them. It is but a slight modification which would establish the rachis
thus initiated as the definite "phyllopodium," upon which the earlier, and
sometimes also the later branches would arise monopodially, being lateral
from the first : and thus a Pecopterid-type of leaf would result. The facts
certainly indicate that such a transition has been effective in descent, though
it may be a question whether all pinnate types, for instance the Marattia-
ceae, originated in this way. Lastly, it may be noted that the winged
structure, so prevalent in Fern-leaves as lateral lines leading even to the
base, still indicates the ultimate origin from a flattened expansion : the
margins may often still be traced in this way even where the petiole is
almost cylindrical in form.
A theory of the whole shoot based upon similar progressions was also
suggested in the same Memoir in 1884, viz. that just as the phyllopodium
gradually asserts itself as a supporting organ among structures of similar
origin and structure to itself, so also the stem may have gradually acquired
its characters by differentiation of itself as a supporting organ from other
members similar to itself in origin and development. A similar idea has been
subsequently expanded into Potonie's theory of origin of the Fern-shoot.2
There seems to be no sufficient foundation in fact for its acceptance. In
the first place, there is in Ferns no known case where the axis and leaf
appear as the two branches of a dichotomy, so that the suggestion is purely
hypothetical : it is based only on analogy with what is learned from the
comparative study of the leaf. The strength of the argument referring
the branching of Fern-leaves to an original dichotomy rests on the fact
that that mode of branching commonly appears at the apex, and is specially
apparent in the lateral branchings. There are no such examples showing
1 Phil. Trans., 1884, part ii., pp. 604-5.
2 Palaeophytologie, pp. 156-159.
630 GENERAL COMPARISON OF THE FILICALES
leaf and axis as branches of a dichotomy. The leaves always originate
monopodially. Secondly, other Pteridophytes, such as the Sphenophylls
and early Calamarians, exist with bifurcate leaves, but without any
suggestion of an origin of axis and leaf from a common dichotomous
system. These grounds, over and above the inherent improbability of
the comparisons with Fucoids introduced by Potonie, or with the game-
tophyte of Liverworts by other writers, should suffice to show that the
suggestion which I threw out in 1884 is untenable, as I very soon realised
it to be. All developmental evidence shows that the axis in Ferns, as
in other plants, was throughout descent a part of distinct origin from
the leaves which it bears.
The dichotomous theory of origin of the whole shoot, including axis
and leaf, has been supported also by Tansley on the basis of anatomy ; and
it has been pointed out that analogies exist between the structure of axis
and of leaf in certain early fossils.1 Especially it has been shown that there
is an approach to a radial type of construction of the lower region of the
leaf in certain cases. It need be no surprise that such similarities to
the structure of the axis should exist in an appendage which is a part
of the same shoot as the axis; as the leaf became larger and more
important its requirements would become similar to those of an axis : to
meet these a structure analogous to that of the stem would then be
probable, such as is actually seen. In the facts adduced I see nothing
stronger than structural analogies : this class of evidence carries little
weight as against the objective fact that in living Ferns the leaf is always
seen to arise monopodially. Thus the dichotomous theory, which is based
on analogies, appears to break down in the absence of developmental
fact.
It is possible now to institute a comparison of the shoot of Ferns with
that of other Pteridophytes, and to consider its relation to the theory of the
strobilus. In its original radial structure, with derivative dorsiventrality,
and in its occasional dichotomous branching it corresponds to other
strobiloid types. The genetic relation of leaf to axis as actually observed
is the same, and in point of fact it is in the proportion of leaf to axis
and in the architecture of the leaf that the chief difference lies. But among
strobiloid types, and especially among their fossil representatives, the leaf
is not always small or simple : the leaves of certain living Lycopods
(L. serratum and Isoetes} are relatively large, as were also those of some
of the fossils, notably Sigillaria. The branched leaves of the Spheno-
phylleae and Psilotaceae, and even of some of the Calamarians, such as
Archaeocalamites, and notably of Pseudobornia, are instances of branching
of leaves in strobiloid forms. Again, in our view a great leaf-enlargement
in a fundamentally strobiloid type has resulted in the Ophioglossaceae. Thus
variety in size and complexity of the leaves existed in other Pteridophytes
besides the Ferns. Even the dichotomy which is so frequent in the first
1 New Phytologist, 1907.
EXTERNAL CHARACTERS 631
leaves of young Ferns, and lies at the basis of the architecture of the mature
leaves, finds its counterpart in the dichotomy seen in certain strobiloid
Pteridophytes.
It is true that the Leptosporangiate Ferns show a very distinctive mode
of segmentation both of apex and margin of the leaf. But it has been
shown that in this character the Osmundaceae form an intermediate step
from them to the Marattiaceae, while the latter diverge clearly from the
Leptosporangiate type. It is thus seen that the definite segmentation of
the Leptosporangiate type is no essential character of the Fern-leaf at large.
Such considerations point to the justness of the view that the Fern-leaf,
however different in size, in continued apical growth, and in its segmentation,
is essentially comparable with the smaller and simpler leaves of the strobiloid
forms. We shall therefore accept the conclusion that in the evolution of
Ferns some such leaf-enlargement as is faintly indicated in certain strobiloid
Pteridophytes, and notably in the Ophioglossaceae, was carried out to a
higher degree than in any other Archegoniate Plants. It would appear
probable that the Ferns, developing early towards megaphylly, worked
out to the fullest such methods of leaf-enlargement as are outlined in
some other early types ; in fact, that they were ultimately derived from a
smaller-leaved ancestry, with a strobiloid shoot not unlike that which
remained in the rest persistently small-leaved. .
As regards the differentiation of their leaves, Ferns show a comparatively
low position. In a very large proportion, in which are included most of
the types which are held as primitive, the leaves are general-purposes leaves :
each serves at first for protection of the apical bud, and on unfolding is
at once an organ of assimilation and of propagation. The differentiation
of trophophylls and sporophylls is usually marked by a reduction of the
assimilating surface where the sporangia are borne : examples are seen in
Struthiopteris and Blechnum, in Acrostic/mm and Platy cerium, and the
distinction is to be held as a morphological advance which had, however,
already made its appearance in the Ferns of the Carboniferous Period. A
good example of this is seen in the Hymenophyllaceae, where the leaves
are undifferentiated in Hymenophyllum ; but in certain species of Trichomanes
(§ Feea.\ the genus which on other grounds is held to be more specialised
than Hymenophyllum, a distinction of sporophylls from trophophylls is seen.
Sometimes the differentiation may be between parts of the same leaf, as
in Osmunda, and the fact that within this genus the relative position of the
sterile and fertile parts may vary indicates that the distinction is not very
deep-seated. Innumerable middle-forms between the sterile and fertile
conditions further indicate how imperfect the differentiation actually
is. A further specialisation of certain leaves as protective scales is seen
in Osmunda and in some elongated rhizomes : in such cases the rudimentary
leaf-apex shows that these are potentially normal leaves diverted to the
protective duty. It thus appears that the differentiation of the leaves in
Ferns is not on a high scale : that they are all essentially of one type, and
632 GENERAL COMPARISON OF THE FILICALES
that that type was probably the tropho-sporophyll ; in fact, the Ferns show
a Selago condition of their shoot.
As in other Pteridophytes, so here the question is an interesting one
at what period fertility of the plant may begin. In most Ferns the period
jis late, especially in the larger forms, and, as in the strobiloid types, this
may be ascribed to a progressive sterilisation of the earlier leaves. But in
other cases the formation of sori may happen relatively early, and these
serve to direct attention to what was probably a more primitive condition.
As examples where an early fertility is seen there may be quoted Blechnum
lanceolata, Swartz, in which, however, no exact record was kept of the leaf
on which sporangia were first seen. In specimens of Pteris heterophylla,
L. var. internata, supplied by Messrs. Hill, the fourth or fifth leaf of the
seedling already produced sori ; but an extreme case is that quoted by
Prantl,1 of Lygodium subalatum, in which not only are the normal leaves
fertile to the base, but it was noted also that the sub-primordial leaves,
and even the primordial leaves bear " sorophores," so that completely
sterile leaves are hitherto quite unknown in this species. Such an example
points clearly to the conclusion that in Ferns, as in other Pteridophytes,
all the leaves of the sporophyte are potential sporophylls.
A minor character too little used in comparison as yet is to be found
in the superficial appendages. These may be filamentous or scale-like :
the latter are readily recognised by their development as flattened expansions
of the former. Speaking generally, the hair is characteristic of relatively
primitive types, such as the Botryopterideae, Hymenophyllaceae, and
Osmundaceae, while the scale or ramentum is found very generally among
the Leptosporangiate Ferns, though it is also present in some of the Gradatae
and Simplices. It is to be noted that in a considerable number of cases
ramenta accompany a dictyostelic structure, though there is no constant
coupling of the two characters. A good example of their phyletic signifi-
cance is to be found in the Schizaeaceae, in all of which, excepting Mohria^
the appendages are of the primitive filamentous type ; but in Mohria,
which is regarded on various other grounds as a relatively advanced genus,
and has a dictyostelic stock, the appendages are flattened scales. It is
possible that such characters may never acquire the systematic importance
claimed for them by Kiihn,2 but they certainly must not be overlooked as
evidence having some degree of phyletic value.
SPORE-PRODUCING MEMBERS.
The sporangia of Ferns are usually grouped in sori; but apparent
examples occur even among the most primitive types, as well as among
those which are more recent, of their nori-soral disposition. The first
question will therefore be, what was the mode of disposition of the
sporangia in the first instance?
1 Schizaeaceen, p. 14. 2 Prantl, I.e., p. 13.
SPORE-PRODUCING MEMBERS 633
At first sight it might seem probable that the non-soral state was
primitive, since it is seen apparently in such early forms as Botryopteris
and Myriotheca, and in Osmunda. But there are objections to this as a
generalisation ; for, in the first place, definitely soral types, such as the
Marattiaceae, are quite as well represented in the primary rocks as any
non-soral forms : secondly, while Botryopteris itself appears to be non-soral,
there is a distinct indication of a disposition of the sporangia around a
central point in Zygopteris (Fig. 272, p. 503), while in Corynepteris there
are very definite sori : thirdly, there are among living Ferns clear indica-
tions that the soral may pass into the non-soral state : such a progression is
suggested in the species of Dipteris (p. 620), while the condition of Acrosti-
chum and of Platy cerium can hardly have been produced in any other way
than by spreading of the sporangia of some soral type over an enlarged
surface, as is indeed suggested by such genera as Gymnogramme and
HemionitiS) etc. This is exactly what would be expected as a consequence of
indefinite multiplication of parts closely aggregated together, in cases where
no biological check determined their exact position. Thus it would seem
probable that the soral state is the original condition and the non-soral
the derivative, notwithstanding its early appearance.
But the sorus, whether marginal or superficial, does not always maintain
its identity, even in those cases where it is habitually circumscribed. In
many Ferns, and especially in those in which there is an enlarged leaf-area,
sori may be found of unusual size, elongated, and constricted in the middle ;
and from these it is a slight step to complete fission, two smaller sori being
then seated close together (Figs. 281, 310). In such changes from the normal
as these there lies a capacity for increase in number of sori, and there is
hardly room for doubt that in such cases as Kaulfussia and Dipteris, as
well as in many of the broader-leaved Polypodiaceae, where the sori con-
stitute more than a single row on either side of the midrib, the more complex
condition has been brought about in this way ; in fact, the statement seems
fully justified that the primitive disposition of the sori was in a single
marginal or intra-marginal row : all more complex arrangements in Ferns
are secondary and derivative.
A further matter for disgussion is the position which the sorus holds
relative to the leaf which bears it. Two positions are common, marginal
and superficial, the latter almost always on the lower surface of the leaf:
both of these are of very early occurrence, the superficial being characteristic
of the Marattiaceae and Gleicheniaceae, and the marginal of the Botryo-
pterideae and Schizaeaceae, while both types are continued upwards into the
Gradatae and Mixtae. It would be important to know which of these
positions was the more primitive in Ferns. Here, again, an indirect
indication may be obtained by comparison of more recent types : among
the Leptosporangiate Ferns there is ample evidence to show that the
marginal sorus has shifted by gradual steps to the lower surface. This
is clearly proved by comparison within the Dennstaedtia-Davallia series :
634 GENERAL COMPARISON OF THE FILICALES
Prantl had already noted it, and held that the translocation had occurred
along several distinct phyletic lines within the Polypodiaceae.1 There is
no evidence at hand of the converse progression from the surface to the
margin. But though a probability is thus established of progression of
the sorus from the margin to the lower surface, this does not prove that
the former position was prior for the Ferns at large. It must be remembered
that Marattiaceous types with sori intra-marginal are recorded as far back as
the Culm ; and it is quite possible that they may have originated from forms
.with sori superficial from the first. The question is accordingly an open one
whether all Ferns sprang from types with marginal sori, though it seems
certain that in some the superficial position has been secondarily acquired.
The sorus itself consists of a receptacle upon which the sporangia are
inserted, and of the sporangia themselves, while various accessory growths
may be present also, and are called by the collective name of indusium.
According to the construction of their sori the F"erns have been seen to
fall into three main groups : the Simplices, Gradatae, and Mixtae. In the
Simplices the sporangia are all simultaneous in origin : in the Gradatae
there is a basipetal succession of the sporangia, but there is no intercalation
of younger sporangia between those already initiated : in the Mixtae new
sporangia are intercalated without order between those first forme.d. There
is reason to believe the Simplices to be the most primitive type, the Gradatae
to occupy a middle position, and the Mixtae to be the most advanced, and
that either of the latter might be derived phyletically from the first. The
differences in order of origin of the sporangia in these three types have
entailed variety in adjustment of the sorus, especially in regard to the
protection and nutrition of the sporangia, and the distribution of the spores
when ripe.
Taking the- receptacle first, it is required as a means of transfer of nourish-
ment to the sporangia and as a basis for their support in such a position
that the spores can be scattered when mature. These requirements will
have to be considered separately in the case of each of the three groups.
In the Simplices the sporangia are usually produced in such moderate
numbers that there is room for them all in a single radiating series round
the centre of attachment : there appears to be little need in them for a
raised receptacle. It is true that in some cases, such as Kaulfussia and
Ptychocarpus unitus, there may be a massive receptacle with a vascular
extension into it, while in Marattia Kaulfussii it may be elongated into a
stalk below the sorus; but in many of the Simplices the receptacle is
hardly developed as such, the sporangia receiving their supplies directly
from the leaf through their own massive stalks. The Gleicheniaceae show
certain points of interest for comparison : those species which have few
sporangia in the sorus have only a comparatively small receptacle; but in
G. dichotoma, where the sporangia are more numerous, the receptacle is
slightly elongated, while it is well provided with vascular tissue.
L.C., p. 12.
SPORE-PRODUCING MEMBERS 635
This elongation and structural adaptation of the receptacle becomes much
more pronounced where there is a continued succession of sporangia, as
in the Gradatae. The basipetal sequence seen in the Cyatheae, Dicksonieae,
Dennstaedtiinae, Hymenophyllaceae, would hardly be possible without a recep-
tacle having intercalary growth : the continual moving upwards of the more
mature sporangia, so that they can freely shed their spores, leaves space
for the initiation of younger ones below, while the latter are in their early
stages close to the source of supply, and protected by the basal indusium.
This seems to be the raison d'etre of the elongated receptacle in such
cases : given a basipetal succession, its origin has been a response to the
need of space, by upgrowth from the base, not the result of " metamorphosis "
of any pre-existing vegetative part such as a lamina of a leaf (Prantl).
An indirect support for this opinion is found in the fact that the
receptacle disappears in those Ferns where the mixed sorus is acquired ;
for here the receptacle is commonly flat, though some exceptions do
occur. A peculiar interest attaches to those closely allied Ferns of the
Dicksonia-Davallia affinity, in which a transition from the basipetal sequence
to the mixed sorus is believed to have taken place. In Dennstaedtia and
Microlepia the receptacle is clearly conical, though it does not show an
elongation at all comparable to that of the Hymenophyllaceae (Fig. 332).
Occasionally in Denn. davallioides and in Microlepia hirta, but more com-
monly in Denn. rubiginosa, the strict basipetal succession which is seen in the
first stages of the sorus is departed from, though the receptacle still maintains
in some degree its conical form. But in Eudavallia^ which is without doubt
closely allied to the above, the sorus is a mixed one, and the receptacle
is almost flat, though still provided below with a considerable mass of
tracheides (Fig. 339). Here, since the basipetal succession is not maintained,
an elongated receptacle is not required, and since this difference occurs
between Ferns which are certainly of a common stock, it is probable that
a reduction has taken place. Accordingly, an elongated receptacle is not
in itself an important morphological feature; it is to be correlated with a
continued basipetal succession of sporangia, and it is this last which is the
essential morphological feature in such cases.
There are, however, instances where there is an enlarged receptacle on
which the sporangia are borne not in basipetal succession, but in mixed
order ; as an example, the familiar sorus of Nephrodium may be quoted,
with its large receptacle and internal mass of tracheides (Fig. 4). It may
be urged that these facts are inconsistent with the view expressed in the
last paragraph, but the large receptacle may here be a survival, which,
though the sorus has become a mixed one, may still be a convenience ;
as it certainly is in Nephrodium, where its size allows space for the
sporangia between the indusium and the leaf-surface. The general view
may then be formulated thus : the Simplices have as a rule a receptacle
of small size; the Gradatae have a more or less elongated receptacle, and
it may attain very considerable dimensions where the succession is long
636 GENERAL COMPARISON OF THE FILICALES
continued. The Mixtae have commonly (though not always) a flat re-
ceptacle. It is not a constant morphological feature, as shown by the
fact that a line of transition from an elongated receptacle to a flat one
has been demonstrated in the genera Dennstaedtia and Microkpia, and it
is possible that other transitions have also taken place elsewhere.
The term indusium has been applied to bodies of the most various
form, position, and structure borne in relation to the Fern-sorus, and
serving for the protection of the sporangia. It is hardly necessary to
point out that these, however similar in their function, cannot be regarded
as homogenetic throughout : they are often only examples of homoplasy.
We call the basal indusium of Cyathea by the same name as the umbrella-
like indusium of Matonia or Polystichum, or the marginal flap of an
Adiantum. It may be possible, by regarding the indusium as having had
a high degree of plasticity, to accept some of its different forms as being
modifications of one another, and a reasonable case can be made out
sometimes, such as that in the transition from the Dennstaedtiineae to the
Pterideae. But such cases as Cyathea and Matonia seem incompatible
with any opinion of homogeny of the two structures, especially when it is
remembered that in Alsophila and Gleichenia, genera which are respectively
allied to the above, an indusium may be entirely absent : and this is in
fact the usual condition among the Simplices. We shall then hold the
indusium to be an inconstant body, varying in occurrence and in position,
and the term will be used to designate outgrowths protective of the sorus,
whatever their position and whatever their evolutionary history may have
been.
It has been demonstrated in many cases that the indusium is formed
before the earliest sporangia appear : this is probably a case where
physiological opportunism, rather than any recapitulatory habit, determines
the order of succession. It is impossible to believe that those constant
bodies, the sporangia, are of later evolutionary origin than the less
constant body, the indusium. In this respect the indusium and the
embryonic haustoria are probably alike. It is important to recognise
such cases as these, for they go far to break down the dogma, that the
prior existent parts in the development of the individual were necessarily
prior existent in the evolution of the race.
The views of Prantl have already been quoted, which involve a
comprehensive shifting of sori from the margin to the surface. This has
brought about great modifications of the indusium. Starting with a
strictly basipetal sorus, with cup-like indusium, slightly two-lipped it may
be, as in Loxsoma (Fig. 320), we have seen that the type in Dennstaedtia
apiifolia (Fig. 332) is similar in position and structure. The indusial lips,
which are equal in the above plants, become unequal in Microlepia
speluncae (Fig. 332), the superior lip (s) being both longer and thicker than
the inferior (/'), and taking on itself the structural characters and appearanc
of a continuation of the leaf; this is repeated in Davallia Griffithiant
THE SPORANGIUM
637
(Fig. 339), and is still more pronounced in Cystopteris and Lindsaya.
Thus the equal lips may be differentiated, the one taking on the structure
of the leaf-margin, the other becoming a mere appendage of the surface.
There is reason to believe that a reduction of the indusium has taken
place along more than one line of descent ; one such probable series of
reduction may be traced from Cyathea to the very interesting conditions
seen in flemitelia with its one-sided indusium, and in Woodsia and
Hypoderris, in which there is an exiguous, fimbriated indusium. It is
but a slight step from these to some forms of the comprehensive genus
Poly podium, in which, with a similarly superficial sorus, the indu-sium is absent.
Another line of possible reduction may be traced from the Dennstaedtiinae,
through Hypolepis, to certain types of Polypodium. The probability is
that there is here a progression from a type with basipetal succession of
sporangia protected with a basal indusium, to a mixed type in which the
indusial protection is less essential, and the indusium is accordingly abortive.
THE SPORANGIUM.
The morphological equivalence of the sporangia of Ferns at large will
be generally admitted, whatever their modifications of detail may be. It
has been customary to distinguish the Leptosporangiate from the Eusporan-
giate types, on the basis of the origin respectively from one or from several
Diagrams illustrating the segmentation of Ferns. rt = Polypodiaceae (compare Kny,
Wandtafeln XCIV.) b = Ceratopteris (compare Kny, Parkeriaceen Taf. XXV., Fig. 3).
c—Alsophila (compare Fig. 334). d~Schizaect (compare Prantl, Taf. V., Fig. 69), or
Thyrsopteris (compare Fig. 329), or Trichomanes (compare Prantl, Taf. V., Fig 92).
e,f= Todea (compare Fig. 295). g= A ngiopteris (compare Fig. 284).
parent cells. But comparative observation shows that this distinction is
based not on any difference of kind, but only of degree. The transition
from one to the other is illustrated by the diagrams (Fig. 349 a-g\ which
show the initial segmentations of the sporangia of various types of Ferns,
from the Marattiaceae to the Polypodiaceae. Taking the Simplices first,
638 GENERAL COMPARISON OF THE FILICALES
so as to follow roughly the probable phyletic sequence, the massive
sporangium of the Marattiaceae has its archesporium deeply sunk : the
walls all cut at right angles, and since the outer surface is but slightly
convex, the walls are almost parallel, and the archesporial cell approximately
cubical (Fig. 349 g). The segmentation in the Osmundaceae is variable,
and it has been observed to be so even in sporangia on the same plant
in Todea barbara. In some cases the archesporium is still square-based,
and may be square in its transverse section : but as the outer surface
becomes early convex, the lateral walls converge, and the archesporium
has the form of a four-sided truncated pyramid (Fig. 349 /). In other
sporangia of the same plant the lateral walls limiting the archesporium
converge more strongly, the outer surface being more convex, and one of
them inserts itself upon another : consequently the archesporium takes the
form of a three-sided pyramid (Fig. 349 e). It is to be noted that in
this figure the wall (x, x) is inserted on an inner periclinal • but in
Fig. 349 d> which represents the segmentation in Schizaea, or in Tricho-
manes or Thyrsopteris, the wall (x, x) cuts another anticlinal. This marks
another step in attenuation of the sporangium, though only a slight one,
and in other essentials the segmentation is as in the simplest of the
sporangia of Osmunda or Todea. Figs. 349 b, c, show the segmentation
seen in various Gradatae : (c) corresponds to the condition of Alsophila
and Cyathea, and (b) is a slight variant upon it which is sometimes found :
it is seen also in Ctratopteris. In the Polypodiaceae, however, where the
sporangium may often be long-stalked, the wall cut by the wall (x, x) may
be no longer inclined, but transverse. From this series of diagrams it is
seen how gradual are the steps from the segmentation typical of the
Eusporangiate Fern to that of the most advanced Leptosporangiate. The
unity of the scheme cannot naturally be divided by any distinction of
origin from a single cell or from more. The difference of type thus gently
graded over is an index of the progressive attenuation of the sporangium
seen in descent, and it will be shown to go along with progressive reduction
of the individual productiveness.
Closely related to the segmentation of the young sporangium is the
structure of its stalk when mature. Putting synangia aside, the stalk
varies from the short massive type of Angiopteris^ through various types
such as the Osmundaceae, Gleicheniaceae, Schizaeaceae, and Hymeno-
phyllaceae with relatively thick stalks, to the Polypodiaceae, where the
stalk is commonly attenuated and long. It may, in extreme cases, be
reduced to a unicellular filament, as in Scolopendrium. These steps again
show a general parallelism with the individual spore-output, the thickness
of the stalk being roughly proportional to the stream of nourishment
required.
But it is the sporangial head, with its relatively thin wall surrounding
the cavity filled with spores, that is the most distinctive part, and as the
character of the opening mechanism, or annulus, has been made the chief
THE SPORANGIUM 639
diagnostic feature in Ferns, it requires special notice, and will be taken
first. In the synangial Marattiaceae and Pecopterids the opening mechanism
is very simple : a slit-like rupture is formed, and it gapes by drying up of
the adjoining cells, while the firmer region of the wall stands rigid. This
may be complicated by changes of form of the whole sorus, as in Marattia.
But in those early forms of Ferns in which the sporangia were separate,
there is commonly a band of mechanical tissue, composed of several rows
of indurated cells : this band varies in extent and in position in different
sporangial types. It has been stated by Scott,1 referring to the annulus
of certain primitive Ferns which is more than one cell wide, that
" this was perhaps a general character of the annulate Fern-sporangia of
Palaeozoic age : at least no clear case of a uniseriate annulus has yet been
demonstrated." In fact, it seems that in the Primary rocks the distinctive
Leptosporangiate annulus was at least rare, if indeed it existed at all.
It may be a question whether the more complex mechanism thus seen
in Eusporangiate Ferns is really the true correlative of that in the Lepto-
sporangiate type. A comparison of the indurated tissues in the sporangia
of Angiopteris and Gleichenia shows certain points of essential similarity,
though the details do not correspond. The firm resistant arch of indurated
tissue described in Angiopteris consists of cells of a similar nature to
those which form the annulus of Gleichenia ; its position is in the main
the same, though it does not stretch as a continuous hoop round the
back of the stalk, but stops short on either side of its base. When we
consider the similarity *of the sorus in these two genera, and of their
sporangia with the same orientation and dehiscence, the comparison of
these two bands seems inevitable, notwithstanding that the one is usually,
though not always, a single row, and the other a broad and ill-defined
band. But a further important fact is that among early Ferns of reputed
Leptosporangiate affinity the annulus is not always a single row of cells ;
this is seen in the Schizaeaceae, for Senftenbergia has an annulus of several
rows : Zeiller has shown that living species of Lygodium may have a
double-rowed annulus, which is an intermediate step to the type with
a single row.2 Again, in living species of Gleichenia occasional cells of
the annulus have been found to be divided, showing thus a reminiscence
of a pluriseriate state. Such evidence favours the opinion that the simple
annulus is the correlative of the pluriseriate, and that a simplification of
its structure has accompanied the reduction in size and spore-output of
the sporangium in the course of descent. In the Leptosporangiate Ferns
the homogeny of the annulus seems the only view which is in accord
with the constancy of its occurrence in plants which are so clearly related
to one another.
The position of the annulus and of the point of dehiscence appears to
have undergone change in the progressive evolutionary course. In the
1 Progress us Rei Botanicae, i., p. 184. Compare Kidston, Phil, Trans. , Ser. B,
vol. 198, p. 188. -Bull. Soc. Bot. de France, xliv., p. 214.
640 GENERAL COMPARISON OF THE FILICALES
archaic types the annulus was vertical or oblique, and the dehiscence
was mainly, though not exclusively,1 in a plane including the axis of the
sporangium : this is seen in all the surviving Simplices, excepting the
Matonineae, and also in Loxsoma. But in the Gradatae the annulus was
oblique and the dehiscence lateral, while in the Mixtae the annulus is
again vertical, but the dehiscence transverse. If we contemplate
a derivation by descent of Ferns with a lateral dehiscence from
those with median dehiscence, we shall have to enquire whether
there are any evidences of shifting of the annulus itself as well as
of the point of dehiscence. One material point is that the formation
of the annulus in Ferns at large does not stand in any constant relation
to the segmentation of the sporangium-mother-cell, though that segmentation
itself shows so singular a constancy. This fact leaves the question of
a shifting of the annulus more open than it would otherwise appear. The
more primitive type of complete annulus is that seen in the Gleicheniaceae
and Schizaeaceae, with oblique position and median dehiscence : Loxsoma
maintained the oblique position and median dehiscence, but part of its
annulus is incompletely indurated. In others, while the complete oblique
ring was maintained, the point of dehiscence was shifted laterally, the
result being as in the Cyatheaceae, Hymenophyllaceae, and others with
a basipetal sorus. With the transition from the basipetal sorus to the
mixed came also a further change of the annulus : maintaining the
lateral dehiscence, the annulus became vertical, stopping short on either
side of the stalk, which interrupts it. But in many cases a slight
obliquity was retained, as seen in Dennstaedtia apiifolia (Fig. 333 c)
and Diacalpe, the two sides being so far dissimilar that it is possible
still to distinguish the "central" from the "peripheral" face: this is
also the case in Davallia, Lindsaya, Nephrodium (Fig. 6), and many
others. But there are other outstanding cases of an oblique annulus among
Polypodiaceae which have been the subject of discussion, and have even
been considered a sufficient reason for rejecting the oblique or vertical
positions of the annulus as characters which are not dependable : for
instance, the genus Lomaria, in which the § Plagiogyria has a well-marked
continuously oblique ring of the annulus, somewhat similar to that seen
in the Dicksonieae. So far from looking upon such cases as these as
being subversive of views based on the character of the annulus, they are
exactly what might have been anticipated if the types with a vertical annulus
were derived from forms in which the annulus was oblique : it is hardly
to be expected that the transition would be carried out completely in all
cases : these exceptions may be regarded as being occasional survivals of
the earlier oblique type. _
It would appear thus probable that the simple annulus of the Leptosporan-
giates is prefigured by the vertical, many-rowed hoop of the Eusporangiates.
That in the course of descent, as the bulk of the sporangium was reduced,
1 Scott quotes a porous dehiscence for Stanropteris (Progressus Rei Bot., i., p. 186).
THE SPORANGIUM 641
.
this hoop became simplified to a single cell-row ; and that it changed its
position in accordance with the soral arrangements, being oblique in some
Simplices and in all Gradatae, first with median but subsequently with lateral
dehiscence ; and that finally it swung again into the vertical position in the
Mixtae, but with the stomium still remaining lateral.
Passing to the contents of the sporangium, these are derived from the
archesporium, which in all Ferns, with occasional exceptions in abnormal
Eusporangiates, consists of a single cell. From this in the Leptosporan-
giatae the tapetum is first cut off peripherally, and the central cell which
remains divides up into cells which are commonly found to number some
power of two. This is a consequence of the successive divisions occurring
as a rule simultaneously within the sporogenous group, and the result is
that the spore-mother-cells number 2, 4, 8, 16, 32, 64, etc. Since each of
these undergoes tetrad divisions, the numbers of spores produced may as
a rule be 4, 8, 16, 32, 64, 128, 256, 512, etc. These may be styled the
typical numbers.
There are two ways of computing the output of spores from a single
sporangium : either by examining preparations of sporangia with spore-
mother-cells, and estimating their number, or by actual counting of the
spores produced. Where the output of spores is small a reasonable
degree of precision is to be expected by either method ; but where the
numbers are large there are various sources of error, and the results must
be taken as mere approximations. The results of such computations will
now be given for various Ferns : the estimates for the Marattiaceae have
been made from study of sections traversing the sporogenous group before
tetrad-division, but in all the rest the results are those from direct
counting of the mature spores (see Table, p. 642).
In estimating the value of these results it is first to be noted that the
largest numbers are associated with complete synangial fusion, a smaller
number being found in Angiopteris where the sporangia are separate than
in any of the synangial genera of living Marattiaceae. The potential
number estimated for Gleichenia flabellata is nearly equivalent to that of
Angiopteris, though the actual countings run considerably below ; still they
are in excess of those for any other Leptosporangiate Fern observed, and
as this is seen in a Fern in why:h the type of sorus is the same as in
Angiopteris, it strengthens the affinity between these two genera, notwith-
standing that the one is, as regards the mode of segmentation of the
young sporangium, a typical Eusporangiate, while the other shows
essentially the segmentation of the Leptosporangiates.
From Gleichenia, as we pass through the table, successively lower
numbers are seen, and it is obvious that the larger numbers are
associated with those Ferns which on other grounds are held to be
relatively primitive. Of the Simplices examined none have a lower typical
number than 128: and in this connection it may be noted that a rough
estimate of the spore-output from a sporangium of Zygopteris, after
2 S
642 GENERAL COMPARISON OF THE FILICALES
Renault's drawings, would give a figure of 500-1000, while drawings of
other early Ferns, such as Stauropteris and Pteridotheca, plainly show
Name.
Result of Countings.
Typical Number.
Kaulfussia, -
7850
Marattia,
2500
Danaea,
175°
Angiopteris, -
—
H50
Gleichenia flabellata, -
838, 794, 695, 684
5I2-IO24
,, dichotoma, -
319, 251
256 or more
,, hecistophylla,
265, 272
256 „
,, circinata,
241, 242
256
,, rupestris, var. glaucescens, -
244, 232, 220
256
Osmunda regalis, -
476, 462, 396, 373
256-512
Todea barbara, - ...
478, 445, 442 ; 225, 238
256-512
, , superba,
206, 306, 342
256 or more
,, hymenophylloides,
112, 115, I2O, 124, 2O4
128 „
Lygodium dichotomum,
232, 246
256
,, javanicum, -
237, 238, 245
256
,, pinnatifidum,
128, 127
128
Aneimia phyllitidis, - ...
114, III, 1O4
128
Mohria caffrorum,
107, 107, ioi
128
Hymenophyllum Tunbridgense,
Trichomanes reniforme,
413, 4l6, 42I
247, 243
256-512
256
Hymenophyllum sericeum, -
216, 239
256
dilatatum, -
121, 127, 127, 127
128
Wilsoni,
IIQ, 121
128
Trichomanes crispum, -
Si/S2, 59
64
rigidum, -
32, 48, 56
32-64
radicans, -
46, 58, 62
48-64
javanicum,
38, 42, 48
32-48
spicatum,
48
48
pinnatum,
32, 48, 32
32-48
Loxsoma Cunningham!!,
64, 62, 63
64
Alsophila excelsa, ....
64, 60
64
,, atrovirens, - - - -
57, 62
64
Cyathea medullaris,
57, 61
64
,, dealbata,
16, 8, 8, 16
8-16
Dicksonia antarctica, -
64
64
,, Menziesii, -
62, 63
64
Dennstaedtia apiifolia, -
61, 62
64
Thyrsopteris elegans, -
—
48-64
Davallia speluncae,
64, 64
64
Polypodiaceae generally,
48-64
Notochloena sinuata,
24, 32, 32
24-32
Sadleria cyatheoides, -
16
16
Ceratopteris thalictroides,
32 (i 6, Kny)
16-32
that the spore-output was large. It may then be held that in the more
primitive forms, and especially in those types which are represented in
the Primary Rocks, the number of spores produced by the individual
SPORE-ENUMERATIONS 643
sporangium was uniformally large. Passing from these to the prevalent
Leptosporangiate Ferns of the present day, the output falls with some
degree of consistency, and the striking fact emerges that, so far as
observation goes, no Polypodiaceous Fern has a larger output than 64,
while in many cases it is smaller. The evidence points to a general
reduction in the course of Descent.
But variations occur within near circles of affinity, and in accordance
with the above generalisation these have a special interest in those
families which hold an intermediate position. This is seen within
moderate limits in the Gleicheniaceae, Osmundaceae, and Schizaeaceae,
in which the numbers approximate for the most part to the typical
numbers 512, 256, 128. In some cases it is difficult to see any circum-
stances which serve as an explanation ; thus the variation between
Gleichenia flabellata (512-1024) and GL dichotoma (256 or more), is not
susceptible of ready explanation, unless it be connected with the more
numerous sporangia in the sorus of the latter. Nor is that within the
genus Lygodium. In other cases, however, the conditions of life and
the structure of the assimilating apparatus throw some light on the
matter ; thus Todea barbara, with its thick assimilating leaves, gives
higher numbers per sporangium than T. superba and hymenophylloides,
with their thinner pellucid leaves. But the most interesting series in
this connection are the Hymenophyllaceae, for the limits of variation in
number are here very wide, ranging between such numbers as 421 and
32. Here the variation in number of spores per sporangium runs fairly
parallel with the size of the sporangia, the smaller number of spores
being contained in smaller sporangia (compare Tr. reniforme and
Tr. pinnatum). Further, there is a correlative elongation of the recep-
tacle, on which there is thus space for a larger number of the smaller
sporangia to be inserted and produced in succession. On these characters
the Hymenophyllaceae may be laid out as a series, extending from types
chiefly of the less specialised genus Hymenophyllum with short receptacle,
large sporangia, and large output of spores from each, to types of the
more specialised genus Trichomanes, with longer receptacle, smaller spor-
angia, and smaller output 'of spores from each. The former link on
naturally to the Gleicheniaceae in the characters named, as well as in
the general appearance of the sporangia : it seems not improbable that
in the Hymenophyllaceae we may see a series of specialisation in which
the " filmy " character is the most salient vegetative feature, and that
this has carried with it, as in the genus Todea, a decrease in size of
the sporangia, and in the number of the spores produced from each.
Taking the facts thus acquired from the Simplices and Gradatae they
show that within circles of near affinity there may be a wide margin of
variation in output of spores per sporangium, such as a theory of phyletic
reduction of the sporangium would demand : and this goes parallel with
the increase in number of the sporangia in the sorus, a decrease in size
644 GENERAL COMPARISON OF THE FILICALES
of the sporangium, and in the thickness of its stalk and of the sporangial
wall, and a progressive change from the early segmentation characteristic
of the Eusporangiate type to that of the Leptosporangiate.
As a consequence of such phyletic changes in the constitution of the
sorus, it is noteworthy how often the output of spores per sorus is
similar in Ferns which are systematically remote from one another : for
instance, Marattia fraxinea (45,000) and Polypodium aureiim (57,600);
Angiopteris evecta (14,500) and Hymenophyllum dilatatum (11,500);
Alsophila excelsa (3,200) and Gleichenia flabellata (3,000). These
examples show how a similar result may be obtained by various means,
a large number of small sporangia balancing a smaller number of large
ones. The similarity of output in such cases may be merely a conse-
quence of similarity in the powers of the underlying nutritive mechanism.
The real interest, however, arises when in nearer circles of affinity, with
varying size of sporangia, and of output per sporangium, the result per
sorus is kept approximately constant by converse variation of the two
factors. This is illustrated in the genus Gleichenia^ and in a less
precise way in Alsophila excelsa and Cyathea dealbata. But the best
demonstration of it is seen in the Hymenophyllaceae. undoubtedly a
very natural series, in which the sorus has a uniform type of con-
struction, though the size and number of the sporangia, and the length
of the receptacle are variable. In illustration of this, estimates have been
made with such accuracy as possible, with the results which are given
in the subjoined table :
Name.
Sporangia per
sorus.
Spores per
sporangium.
Output per sorus.
Hym. Tunbridgense,
2O
42O
8,400
Trichomanes reniforme, -
40
256
10,240
Hym. dilatatum, -
QO
128
11,500
Trich. radicans,
140
64
8,960
It thus appears that notwithstanding the great variations of sporangial
output, the result per sorus is approximately uniform for the cases
quoted from that very natural family of Ferns. This suggests a true
biological progression, and it probably does not stand alone, but
illustrates a principle which has been of wide application in the Fern
phylum.
The production of numerous spores is a drain upon the resources of
the plant. That drain may be relieved within the sorus by the development
of a succession of sporangia, the demand being thus spread over an extended
period. In the Simplices the sporangia of a single sorus arise simultane-
ously : the physiological drain thus comes at one time : this method, which
is, physiologically speaking, a simple and probably a primitive one, is
DISPERSAL OF SPORES 645
specially prevalent among the earliest fossils. A definite succession of
sporangia in time and in space is found in the sori of the Gradatae : here
the physiological drain is not sudden and severe, but it is spread over a
long time : in Trichomanes it may be over a period of years. The basal
position of the youngest sporangia gives them the further advantage of
being near to the source of supply at the time when they are most in
need of it, while those which are approaching maturity are successively
removed from it to a position where they can readily shed their spores.
In the prevalent type of the Mixtae there is a succession of sporangia in
time but not in space. The physiological drain is spread as before over
a long period, so that in this respect the Gradatae and Mixtae appear
equally practical ; but in the mixed sorus the receptacle is found to be
wide and flat : this has the double advantage of enlarging the surface
from which nutriment can be derived, and of shortening the distance through
which it must be transferred. In point of protection those sporangia which
are more advanced give an adequate protection to those which are younger :
there are, however, multitudinous minor adaptations to this end which
cannot be entered upon here. It thus appears that each of these types
of sorus, the simultaneous, the basipetal, and the mixed, which Palaeon-
tology shows to have been of successive appearance in the course of descent,
was a biological amendment upon its predecessor.
Lastly, the facilities for dispersal of the spores when mature remain
to be considered. There are three main types of dehiscence : by a slit
in the median plane, which is characteristic of the Simplices ; by a slit in
an oblique lateral plane, seen in the Gradatae ; and by a slit laterally in a
transverse plane, characteristic of the Mixtae. Dehiscence can only be
properly carried out when free movement of the mechanical tissue is allowed :
where, as in the Leptosporangiates, there is a definite annulus and a free
sporangium, the whole body alters its form on dehiscence : accordingly
the grouping of the sporangia in the sorus is a determining factor in the
position of the mechanical ring if it is to be effective. In the case of
median dehiscence, as in Gleichenia, the sporangium widens laterally, requir-
ing free elbow-room on either side before the sudden springing of the
annulus throws the spores ' out right and left. It thus appears that the
median dehiscence of an annulate sporangium can only be a practical
working arrangement where the sporangia are not in close lateral juxta-
position. Loxsoma,) with its basipetal sorus, is an exception in having the
median dehiscence ; but the sporangium is so constructed with its incomplete
ring that though the dehiscence is median, still the sporangium as a whole
does not widen on dehiscence ; it is, in fact, a compromise, the annulus
being so modified as to be still compatible with the basipetal sorus.
With the exception of Matonia^ Dipteris, and Plagiogyria, all the Ferns
showing the oblique dehiscence have basipetal sori. The sporangia overlap
one another like the shields of the Roman testudo, while all are so closely
packed together that no space is given, as in the former case, for lateral
646 GENERAL COMPARISON OF THE FILICALES
expansion before the sudden jerk. But some space is necessary for the
straightening of the annulus before its quick recovery : the free space
available is found obliquely upwards, towards the apex of the receptacle.
In that direction the annulus is free to straighten itself out, dehiscence
taking place at one side, near to the attachment of the stalk : it can then
execute without obstacle the sudden jerk by
which the spores are scattered (Fig. 350).
The dehiscence by a lateral transverse slit,
worked by. a vertical annulus, is the prevalent
type of the Mixtae. The ripe sporangia usually
have long stalks, and show no regularity of
orientation. The vertical annulus with trans-
verse dehiscence is a mechanical arrangement
which makes use of the free space immediately
above the surface of the sorus for the straighten-
ing of the annulus prior to the jerk of ejection :
a bias to either side is quite unnecessary, and
may be a positive disadvantage. As the young
sporangium grows in a mixed sorus, for instance
of a Polypodium, its stalk elongates, carrying the
head vertically upwards from the receptacle : it
is thus lifted above the crowd of younger spor-
angia, and the space directly above it is free
for the movement of ejection. The vertical
annulus thus satisfies the conditions of the
mixed sorus.
It has already been shown how the different types of annulus charac-
teristic of the three types of sorus pass phyletically one into another; and
it is now seen that there are biological reasons for this in the exigencies
of the mechanism of dehiscence ; in fact, the details of the method of
dehiscence in the more specialised Ferns appear to have been determined
by the mutual relations of the sporangia.
FIG. 350.
Diagram illustrating the relative
position of the sporangia on the
receptacle in the Hymenophyll-
aceae. It -was constructed from
Prantl's section of a mature spor-
angium of Trichomanes Speciosnm.
ANATOMY.
It has been shown from comparison of the external characters of Ferns
that they were probably in the first instance strobiloid types, with a radial
construction of the shoot, and that their present condition was probably
attained by advance from a smaller-leaved state to megaphylly : with this
went frequent assumption of a dorsiventral development. This matter must
now be considered from the point of view of comparative anatomy, and
especially of the vascular system. If the Fern-shoot were primitively
stfobiloid and radial, we should expect the fact to be reflected in the
vascular construction of those Ferns which are held on comparative or on
palaeontological evidence to be primitive ; and also that it would be
ANATOMY 647
supported by the structure of the individual plant when young. In both
cases a prevalence of a cylindrical protostelic state, with comparatively
slight disturbance of the axial system on departure of the leaf-traces, would
be expected if the shoot were primitively strobiloid. Further, the leaf-trace
would be relatively simple.
Leaving aside for the moment the Marattiaceae, which are anatomi-
cally a peculiarly specialised series in themselves, a comparison of the
early types of Ferns points clearly to origin from a protostelic state with
a leaf-trace consisting of a single strand, which comes off with but slight
local disturbance from the periphery of the stele. This, with certain
variants, is the typical condition in the Botryopterideae : a near approach
to it is found also in the earliest Osmundaceae, though those of later
epochs depart from the simple type by elaboration of the stele, as
described above (p. 539). The close correspondence of the Hymeno-
phyllaceae with certain of the Botryopterideae stamps their structure as
relatively primitive also, though it shows some variants upon the simple
protostelic state. Lygodium also, recognised as the most primitive genus
of the Schizaeaceae, is protostelic, and the same is the case with the
simpler species of Gleichenia ; in fact, those early stocks of Ferns which
are recognised by comparison of other characters, as well as by their
geological history, as forming the phyletic basis of the Leptosporangiate
series, show the protostelic structure, or a condition very little removed
from it.
The Ferns thus mentioned are all included in the Simplices, except the
Hymenophyllaceae. These are exceptional among the Gradatae in showing
a protostelic structure of the axis : most of the Gradatae have a more
elaborate stem-structure, which may be held to be derivative from the pro-
tostele, just as the basipetal sorus is probably derivative from the type of
the Simplices. The probable steps towards a solenostelic state are illustrated
in Lindsay a and Dennstaedtia, and suggested also in Gleichenia ; but the
solenostelic structure is typically seen in Dipteris and Loxsoma, as well as
in the Dennstaedtiinae. Here at each leaf-insertion the vascular tube opens
by a foliar gap. Where the internodes are long and the gap itself short,
as in the rhizomic species,/ the structure is easily intelligible. It is but a
slight step to the dictyostelic type, as seen in Ferns with short axis and
overlapping leaf-gaps : the transition is illustrated in the Dennstaedtia-
Davallia series, and has probably occurred also in the Alsophila-Cyathea
series, and elsewhere. It seems probable that the progression from a
protostelic to a solenostelic or dictyostelic state has been effected in
several distinct phyletic lines, while the dictyostelic, with or without internal
accessory strands, is the most elaborate system of all.
It usually accompanies an advanced soral condition : that this is, how-
ever, no obligatory parallelism is shown by the comparison of Matonia with
Dipteris. The latter retains a simple solenostelic structure of the axis,
though its sori have progressed to the condition of the Mixtae : the
648 GENERAL COMPARISON OF THE FILICALES
former, though its soral structure has remained virtually unaltered, shows
the highest condition known of the solenostelic development.
Parallel with such structural progression of the axial system goes an
elaboration of the leaf-trace. In protostelic, and usually in solenostelic
forms, it is represented by a single strand, which may, however, be widened
into a broad strap, and curved in transverse section into a horse-shoe outline:
and this may again be contracted into a pseudo-stelic condition (compare
Fig. 98, p. 194). But with dictyostely comes usually a division of the
single strand into many. It is interesting in Dicksonia to see a middle
condition illustrated ; for in D. Culrita and D. Barometz the leaf-trace at
its base is still a single strand, but at a point above the base, varying in
different leaves, it breaks up into many separate strands : it thus represents
the various stages of the probable phyletic sequence (Fig. 97). The
complete subdivision is seen in the larger species of Dicksonia and in
Cyathea^ as well as in most of the Mixtae, and it is held to be an
advanced and derivative state.
The seedling structure gives a strong support to the view of elaboration
here put forward : in all observed cases the stele of the axis is either
protostelic or very closely related to that structure, and the leaf-trace is a
single vascular strand. In the primitive forms this may remain permanently
so ; but in others there may be a quick transition to the more complex
and presumably derivative state. The example of Alsophila excelsa
(p. 608) shows that the individual life, after the first stages are past,
reflects the probable story of development of the complex adult condition
in the race.
It is in this way, through the seedling, that the Marattiaceae may best
be approached. They have in the mature stem a still more complicated
system of vascular strands than other Ferns ; but in their seedlings the
ontogeny opens in all cases with a monostelic state, with a solid xylem-core.
Complications soon arise : in Kaulfussia and Archangiopteris a cylindrical
dictyostele is formed, not unlike that of other Ferns ; but in Angiopteris
there may be as many as three or four concentric, meshed zones in the
stock, while the leaf-trace is also disintegrated into numerous strands. It
is important to note, however, that in the related fossils the leaf-trace is
habitually a single connected strand, while greater coherence is also seen
in the vascular tracts of the axis in the fossils than in the living species.
These facts all indicate that in the evolution of Ferns there has been
a progressive amplification and disintegration of the vascular tissues ; and
they lead back towards a type, which seems to have been a common one :
the original type was characterised by a radial shoot traversed by a protostele,
from which the successive leaf-traces came off each as a simple strand, and
with the minimum of disturbance of the axial stele. If this were the
original type of shoot in the Filicales, it is plain that the foliar gap, to
which Jeffrey attaches so much importance as the distinctive character of
his phyllosiphonic type, must have been a secondary development : it is
EMBRYOLOGY
649
absent as a matter of fact in many of the Simplices in their mature state,
and apparently from all Ferns in the first condition of the seedling.
The vascular structure thus held to be primitive and typical for Ferns
is that characteristic of strobiloid plants, and it seems reasonable to read
the anatomical data as indicating that the Ferns also are essentially
strobiloid, but have progressed to a condition of megaphylly, while the
anatomical characters that ultimately accompany that habit lagged behind
in the several evolutionary series, and only become apparent in the later
representatives of each.
EMBRYOLOGY.
The primary embryology of Ferns shows so nearly a dead level of
uniformity that it has not seemed necessary to describe the details for
the successive families. There are, however, two distinct types : that of
Transverse sections through growing point of root in Osmundaceae. A—Osmunda
regalis ; the section shows a three-sided initial (.r), but the segments are not regular,
^^shows transverse section immediately below the root-cap; three initials (x, x) are
present ; the dotted line is the cell-division in the root-cap, seen on focussing deeply into
the hand-cut section ; this shows that the section has traversed the initials and is not
below them. C= Todea barbara, showing a very regular meristem of the root, with four
initials (x, x). X 144.
the Marattiaceae, and that of the main series of the Leptosporangiate Ferns,
and these must be compared. The most obvious difference lies in the
fact that the seedling of the Marattiaceae perforates the prothallus, and
emerges with an upright axis through its upper surface (see Fig. 292, p. 527) :
that of the Leptosporangiate series emerges on the lower surface, and the
axis is at first prone (see Figs. 14, 15, pp. 30, 31). This difference may
be referred back to the first segmentation of the zygote, and appears to be
closely related to the difference of polarity then initiated ; for in the
Marattiaceae the first wall appears at right angles to the axis of the
archegonium, but in the Leptosporangiates it is approximately in a plane
including the axis. The further segmentation, and the relation of the
parts of the embryo, to the first divisions are substantially alike in both :
except that in the Marattiaceae, as also in the Osmundaceae, there is a
less regularity in the later divisions than is the case with the other Lepto-
sporangiate Ferns. In this respect Osmunda occupies an intermediate
position between the latter and the Marattiaceae. The parts formed in
650 GENERAL COMPARISON OF THE FILICALES
either case are the same in nature and relative position, though they
differ in their relation to the prothallus, and the foot is less fully
developed in the Marattiaceae.
It does not appear that the facts of the primary embryology have any
very direct bearing on the present problem. It is interesting, however,
to note that the axis is vertical
from the first in the Marattiaceae,
as it is also in most other Pterido-
phytes, and this may be held to
be the more primitive state for
Ferns : the prone position char-
acteristic of the Leptosporangiate
Ferns is exceptional among Pteri-
dophytes at large, and is probably
derivative.
A comparative study of the
meristems which carry on the"
continued embryogeny of the Ferns
has brought interesting results.1
Examination of the apical regions
of axis, leaf, and root indicates
that in all of these parts the
Marattiaceae show a relatively
complex state, the whole part
being referable in origin as a rule
to a group of some three or four
initial cells, usually of prismatic
form. It has also been seen in
them, in sections of the thick
\
FIG. 352.
A= apex of stem of Angiopteris evecta, seen from
above ; apparently there are four initials (x, -r). X 83.
7? = apex of stem of Osmunda regalis, seen from above,
with initial cell (x) of regular form and segmentation ;
/, /^leaves, the youngest (/) shows also a three-sided
initial cell. X 83.
marginal wings of the leaf, that
at least two and possibly more
cells appear as initials. In the
typical Leptosporangiate Ferns a
single initial cell of conical form
is present at the apex of stem,
leaf, and root : and in the case
of the wings of the leaf there is a single marginal series, so that in any
transverse section only one initial appears. A similar examination of the
Osmundaceae shows that structurally they hold an intermediate position :
for while a single initial may be found in stem, leaf, and root, deviations
from this are common. In the roots of Osmunda and Todea various
^ee Bower, Annals of Bot., iii., p. 305. Though this paper of 1889 was written
from the point of view then current, that the Leptosporangiate Ferns were more primitive
than the Eusporangiate, the facts are now equally available in their bearing on the contrary
view at present held. (Compare Ann. of Bot., vol. v., p. 109.)
APICAL MERISTEMS
651
irregular and intermediate conditions have been found between the
Marattiaceous type with four prismatic initials and that of the Lepto-
sporangiate Ferns, where there is only one (Fig. 351). The axis has
usually a single conical initial, but it is doubtful whether this is
always so, and irregularities certainly occur (Fig. 352) The leaf of the
Osmundaceae is alone among Ferns in showing the more complicated
three-sided initial in place of the two-sided common for the Lepto-
sporangiates, while the wings of the leaf have a segmentation of the
Marattiaceous rather than of the Leptosporangiate type (Fig. 353). In
D
FIG. 353.
Sections transversely through young wings of the leaves of various Ferns. A=Angi-
opteris evecta, showing that there is no single marginal cell, but a small-celled meristeni
at margin of the wing. B=a. similar section of Todea barbara, with like structure.
C— similar section of Scolopendrium vulgare, showing regular segmentation of a marginal
cell (m) by alternating cleavages. D = similar section of Trie ho manes radicans, showing
marginal cell (m) with transverse segmentation. X2i6.
point of fact, all the meristems of the Osmundaceae show nearer relation
to those of the Marattiaceae than do any other Leptosporangiate Ferns
that have been examined. '
These facts are in obvious accord with the segmentation of the sporangia
(see above, p. 637) : it appears, therefore, that in stem, leaf, root, and
sporangium those types of Ferns which are held as more primitive
commonly show a more bulky initial construction, while the true Lepto-
sporangiates, which are held as derivatives, show in all their parts a less
bulky type of segmentation. It has been seen above that the spore-
output per sporangium follows a similar sequence, and indeed the numbers
might be taken as a rough indication of the complexity of organisation
for the plant at large. The conclusion follows that in the Filicales
there has been a progressive simplification of the meristic plan : in the
more ancient forms the meristems are not referable to a single initial,
652 GENERAL COMPARISON OF THE FILICALES
but in the Leptosporangiates this became the rule, and with it is associated
the more definite segmentation of the projecting and delicate sporangium,
and a diminution of the output of spores. In fact, the character of the
sporangium may be taken as an index of the meristic character of all
the embryonic tissues.
PHYLOGENY OF FERNS.
The comparative study which has now been made of the Filicales
provides a basis for views as to their phylogeny. Several different lines
of comparison have been adopted, such as that on external form, on
anatomy, on the spore-producing members, and on spore-output: and
the results have been found to be substantially parallel along them all,
though with some exceptions. Moreover, these results are found to harmonise
with the geological record : from this it is learned that, speaking generally,
the Ferns with simultaneous formation of the sporangia in the sorus
(Simplices) were those present mainly, or perhaps exclusively, in the
Palaeozoic period : the Ferns of the Mesozoic Period included, in addition
to these, a large proportion of those with basipetal sequence of the sporangia
(Gradatae), while the bulk of the more recent Ferns are characterised
by the mixed type of sorus (Mixtae). While these three types are
found to coincide generally with three great periods of time, it must not
be assumed that every phyletic line ran through them all : at the moment
it is impossible to demonstrate in any clearly defined family of Ferns
that the sorus has passed successively through all the three phases. What
is recognised is a general trend of development, carried out in many
phyletic sequences, rather than any single progression. It may indeed
be said that no connected phyletic tree of the Filicales can at present
be constructed ' with any degree of certainty : it must suffice to give a
provisional arrangement of the Ferns, divided in the first place according
to their soral condition, which also tallies approximately with their geolo-
gical age (Fig. 354). The three recognised grades are limited by horizontal
lines in the graphic table. The several sequences of Ferns are indicated
by vertical or oblique lines so placed as to indicate relationships, but
disconnected so as not to convey necessarily a common descent. Where
one of these lines crosses the limit between the three grades of soral
construction it is intended to convey the idea of the derivation of the
higher from the simpler plan. Further, the arrangement is such as to
give some general idea of the position of the sorus : the forms with marginal
sori are placed to the left side of the plan, and those with superficial
sori to the right. Finally, no attempt has been made to represent separately
the numerous sequences of the Mixtae : the unravelling of the lines will
have to be deferred till a better knowledge is gained of their anatomy,
and of other details. They have been represented conventionally in the
scheme by overlapping areas, the one to the left stands for those
with marginal sori, that to the right for those with superficial sori, while
PHYLETIC RELATIONS
653
FIG. 354.
Scheme to illustrate approximately the relations of the several families of Ferns, as
concluded from the discussions in the text. It will be noted that the distinctions as
Simplices, Gradatae, and Mixtae traverse several of the probable evolutionary- lines:
the grouping is so disposed that Ferns with marginal sori lie to the left, with superficial
sori to the right. No attempt is made to connect the lines into a phyletic tree.
654 GENERAL COMPARISON OF THE FILICALES
the overlap represents those which are in course of transition, or are
believed to have recently passed from the one state to the other. With
these explanations the scheme may be taken as representing the con-
clusions arrived at in the preceding pages.
Of the Simplices the Marattiaceae stand somewhat isolated from modern
Ferns, both anatomically and sorally. They are approached most nearly
by the Gleicheniaceae in point of the sorus, but in anatomy and in habit
they stand widely apart from them. Their stock probably never progressed
beyond its present state. If they had any near relation with other living
Ferns it must have been very far back.
The other very ancient family, the Botryopterideae, shows obvious
relations with the Osmundaceae, both in type of shoot, in anatomy of
the earlier species, and in sporangial characters. There appears to be
an anatomical resemblance, on the other hand, to the Hymenophyllaceae,
which extends to the external characters of the shoot also ; but the sorus
of the Hymenophyllaceae is widely apart, having passed to the basipetal
type. It is perhaps in relation to their consequent increase in number
that reduction of size of the sporangium is here shown, and especially
illustrated in Trichomanes. None of these three related families appear to
have progressed further than their living representatives : they are held
to be blind branches of descent.
The Schizaeaceae appear as an isolated family, though nearest to the
Gleicheniaceae : their solitary marginal sporangia show analogies of structure,
but not of position, with those of Gleichenia on the one hand and of
Osmunda on the other : it is, however, probable that their monangial
sorus is a result of reduction from a radiate type, such as is seen in
some species of Gleichenia. Anatomically also their simpler types approach
Gleichenia, but the upright genera, Aneimia and Mohria, appear to have
progressed along a line of their own to a dictyostelic structure. The
Schizaeaceae also appear to have been a blind branch.
The Gleicheniaceae are somewhat isolated, from the fact that they
show cross characters : their sori compare most nearly in position and
structure with those of the Marattiaceae, but their type of shoot and their
anatomy correspond rather with the Schizaeaceae. Thus among the Ferns
which show their primitive character by their large spore-output per
sporangium, including the Simplices and the Hymenophyllaceae, there
appear to be several fairly distinct lines : it is possible to link these
together by hypothesis as to a common ancestry, but there is no distinct
evidence of their common descent from any known Fern-type. This is
indicated in the scheme by the convergent but disconnected lines, the
longer lines of the Botryopterideae and Marattiaceae indicating their
priority in the fossil record.
From the Simplices, though with uncertainty as to their definite
reference to any exact origin, at least two other main lines in addition
to the Hymenophyllaceae appear to have proceeded to the basipetal type
PHYLETIC RELATIONS 655
of sorus : the one with marginal sori, the other with superficial. They
are both characterised by having their spore-output restricted to the typical
number of 64 as the upper limit. It may be a question whether these
two were truly distinct in phyletic origin, but they appear to have pro-
gressed independently. The one comprises the Dennstaedtia-Davallia series,
leading onwards to various families of the Mixtae : the other is the Cyathe-
aceous series. Besides these the genus Dipteris seems to have taken a
line of its own towards the mixed sorus, and may perhaps be not alone
in having done so.
The most complete phyletic story has been made out for the Denn-
staedtia-Davallia series, to which Dicksonia itself may probably be a collateral
line, with unusual elaboration of the structure of the stock. The scheme
suggests a somewhat independent origin for this series, together with such
genera as Loxsoma, Thyrsopteris, and Dicksonia, from a source probably
between the Botryopterideae and Schizaeaceae, though not directly from
either of them. They all show a structure of the axis advanced to a
solenostelic or even a dictyostelic state, and a basipetal sorus, derived pre-
sumably as an amendment on the simple type : Loxsoma and Thyrsopteris
have a peculiarly archaic structure of the sporangium. The further pro-
gression, which harmonises reasonably with the palaeontological record,
has been traced both anatomically and sorally, and leads with advance
in both respects, through the Dennstaedtia-Microlepia-Davallia sequence
from Ferns with a solenostelic structure and basipetal sorus to those
with a dictyostelic stock and mixed sorus. Several side-branches, or it
may be concurrent lines, also exist, such as Lindsaya, Pteris, and Adiantum,
thus giving rise to the bulk of those Ferns which Prantl grouped together
as the Pterideae.
But in several branches from this line a transition is found from the
marginal to the superficial position of the sorus. One line is direct from
Dennstaedtia through Hypolepis, a genus with mixed sorus, to some forms
with superficial sorus reckoned as Polypodium : another line may have been
as illustrated by Deparia, where the mixed sori are sometimes marginal,
sometimes superficial. Another, and a more decisive line is through such
forms as Cysfrptcris, and certain sections of Davallia such as § Leucostegia,
in which there can be little doubt that the mixed sorus has been shifted
from the margin to the surface. And so by a number of phyletic sequences,
by no means exhausted by the examples quoted, it seems probable
that there has been progression to that prevalent and final type of the
Mixtae which has an intra-marginal sorus on the lower surface of the
leaf. The Ferns thus derived are characterised by their advanced
dictyostely.
But there is some reason to think that a similar result has been reached
also by a more direct route. The series of the Cyatheaceae is linked by
Alsophila, and especially by the solenostelic A. pruinata, with the Glei-
cheniaceae : it is a very slight transition, already indicated by G. dichotoma,
656 GENERAL COMPARISON OF THE FILICALES
and pectinate^ to the basipetal sorus of Alsophila : the addition of a basal
indusium, together with increased vascular complexity, gives the full type
of Cyathea. In relation with these genera, though on a minor scale of
structure, is probably Onoclea, and as possible last terms leading to the
mixed sorus may be Woodsia and Hypoderris. But this line requires more
full investigation before it can be accepted with assurance.
A third line, which is however more certain, is the Matonia- Dipteris
series. Again the sori are superficial, and the relation of Matonia is
clearly with the Gleicheniaceae. There can be no doubt of the close
relation of Dipteris to Matonia, as shown by external form, as well as
by anatomy : but in place of the simple sorus of Matonia and of Dipteris
Lobbiana that of Dipteris conjugata shows the mixed condition. It seems
clear that there has been a direct transition in this case from the simple
to the mixed sorus, leading in fact to a form long included in the genus
Polypodium • but without the intermediate basipetal condition, though this
has been found usual in other cases.
It would thus appear that along several distinct phyletic lines the
condition with mixed sori may have been attained. This is the most
advanced state of the present day among the Filicales. It will remain
for future workers, by anatomical and other enquiry, to disentangle more
fully the confused phyletic lines of the Polypodiaceae : the present work
will at least have served to show where the tangle actually lies, and
some of the probable lines which have led up to it.
Returning now to the base of the series of the Filicales, as represented
in the scheme, it remains to consider what idea can be formed of a
primary type for the group. As there is no clear evidence of the nature
of the Fern-stock prior to the known types of Simplices, it is only on
a basis of comparison of these with one another, and with other early
Pteridophytes, that a conception can be formed of the probable origin
of the sporophyte in the Filicales. Comparison, checked by the Palaeonto-
logical facts as stated at length above, has led to the recognition of the
following characters as primitive for Ferns : the shoot consisted of a radial
upright stock, showing occasional dichotomy, with protostelic structure,
.bearing radially disposed leaves, and supported by adventitious roots:
the leaves were primitively flattened, of relatively simple outline, in which
dichotomy was a prevalent, but perhaps not an exclusive feature : the
leaf-trace consisted of a single strand : the meristems of axis, leaf, and
root were not dominated by a single apical cell, but showed a group of
initials : all the leaves were potentially sporophylls : the sori were probably
definite, marginal or superficial, disposed in a single row on either side
of the midrib : the sporangia of each sorus were simultaneous in origin,
sessile, and of large size, with high individual output of spores.
The characters thus summarised indicate an essentially strobiloid type
not very much removed from some of those which Palaeophytology has
brought to light. Among the Sphenophyllales and Equisetales are forms
ESSENTIALLY A STROBILOID TYPE 657
which share many of the characters thus set down. The chief difference
lies in the extent of the development of the leaves, and the number and
position of the sori upon them. These are, however, matters of degree
rather than of kind. A dichotomous leaf like those seen in Sphenophyllum
or in Pseudobornia is in point of construction not unlike that type which
is found in certain primitive Ferns, where dichotomy was also prevalent. It
has been seen that the sporangiophores may be multiplied on the single leaf
of the Psilotaceae or on the leaf-sheath of the Sphenophylls, while a plurality
of them is a character of Chcirostrobus. But sporangiophores are held as
correlatives of Fern-sori, though probably not homogenetic with them : it
is thus seen that precedents are present for their plurality on the single
sporophyll in the strobiloid types. On the view of the Ophioglossaceae
given above (pp. 490-494), this family is held to represent a series in which
megaphylly has been achieved from a strobiloid origin : the spore-producing
members have there been shown to follow the leaf-enlargement, in size,
and sometimes even in number. This series, though probably a quite
separate megaphyllous phylum, shows an interesting parallel to the Ferns, and
suggests how spore-producing members may be spread over an enlarging part.
Lastly, the position of the sorus is seen to have varied in Ferns from the
margin to the lower surface, and occasionally to the upper surface : what is
thus liable to change within the Filicales as now denned may probably have
been equally liable to change at the inception of the phylum : therefore
the habitual position of the sori on the lower surface of the sporophyll
must not be held to be a vital point of difference from other Pteridophytes.
Accordingly, there appears to be reason for regarding the fundamental
plan of the sporophyte in the Filicales as being essentially strobiloid, like
that in the other phyla of Pteridophytes, but specialised to a greater extent
than in any of them in the direction of megaphylly, while a cognate
spread of the sori has followed the enlargement of the sporophylls.
2 T
PART III.
CONCL USION.
CHAPTER XLI.
ALGAE AND BRYOPHYTA.
THE general theory which may be based on the occurrence of anti-
thetic alternation in Archegoniate plants has been expounded in the
First Part of this book : the Second Part has been devoted to the
examination of those facts which specially bear upon the theory, as
they are seen in the several groups of Bryophytes and Pteridophytes.
It now remains to draw these facts together into a collective statement,
and to see how far they uphold the hypothetical position : at the same
time, the attempt may be made to formulate some general morpho-
logical and phyletic conclusions. It must be remembered, however,
while doing so, how fragmentary the series of genera and species, living
and fossil, actually is, and how incomplete the knowledge of the details,
especially in the fossils, in which developmental facts can rarely be
observed. These considerations will restrain any tendency to dogmatism,
and make such statements as are offered rank rather as tentative con-
clusions than as matters susceptible of ultimate demonstration under
present conditions of knowledge.
It must be admitted at the outset that the theory of initiation of the
sporophyte — by amplification of the zygote, by repeated cell-division in its
products, by sterilisation of some of them so as to form vegetative tissue,
and consequently by deferring of the tetrad-division, with its concomitant
reduction of chromosomes— is not fully demonstrated by comparison of the
representatives of any one series of living organisms : there is no known
phylum which exemplifies all of these several steps ab initio. Nor is it
likely that there should be, if the biological advantage following on the
multiplication of spores in land-growing organisms were such as has been
suggested in Chapter VI. ; for it is not probable that those land-growing
organisms in which the sporophyte was nascent would have stood per-
manently still in the- earlier phases of it : the probability would be that
all surviving forms would have proceeded some considerable length in the
direction of that biological advantage which follows upon a multiplication
ALGAE AND BRYOPHYTA 659
of germs. But at least the existence of post-sexual divisions in certain
Algae associated more or less definitely with reduction show that the initial
structure postulated by our theory does actually exist. At the same time,
the Thallophytes present no facts which directly disprove the hypothesis
for land-growing plants : they do illustrate, however, other types of cytologi-
cally distinct generations, both algal and fungal, analogous, no doubt, to
that seen in the Archegoniatae, but probably produced along phyletic lines
which were distinct, and subject to quite different external conditions
during their development.
A general objection to the whole theory of antithetic alternation was
raised some years ago on the ground of the assumed improbability that
new parts should appear in the life history.1 It was pointed out that
nature is conservative, and it was stated that when a new organ is formed
it is almost always fashioned out of some pre-existing organ. The adage
was quoted "ex nihilo niliil fit" \ the same objection to the whole anti-
thetic position has recently been reiterated from the Continent. The reply
to this general objection is a very simple one : it is, that the zygote from
which our hypothesis starts is not " nothing " : it is a cell, with all the
powers and possibilities of a complete — and in point of fact a diploid —
cell. It has already been concluded generally as regards the sporophyte
(p. 100) that a living cell which is capable of growth has not a specific
and unalterable function : this we may conceive to have been the initial
condition of the zygote and of its early products. The hypothesis involves
a development of the potentialities of a living cell : the zygote is actually
seen in each normal ontogenetic cycle to give rise by gradual steps of
development to the whole sporophyte : the theory contemplates a cognate
development as having proceeded gradually in the course of descent. In
face of the ontogenetic facts the initial objection does not appear to be valid.
Passing to the more special question of the origin of members, it is
necessary to examine the inherent improbability which is assumed to
surround their appearance as new structures. It is plain that the difficulty
lies in their phyletic not in their ontogenetic origin : for it is a fact which
anyone may observe that in the individual development new parts do
appear where previously there were none : new axes, new leaves, hairs,
emergences, roots, all are originated in this way, each growing out from a
spot previously in the individual unoccupied. The position, then, of those
who entertain this objection appears to be that what is the rule in the
development of the individual is inherently improbable in the evolution of
the race. This is surely a new principle in morphology. The practice
of the science has been hitherto to hold the exact converse; the onus
probandi lies with those who declare that the origin of organs in the
1 Dr. Scott. Presidential Address to Section K (British Association Report, 1896,
996). The position there taken up was substantially that ot Pringsheim (Gesammelte
Abhand. ii. p. 370). It was criticised in my address to Section K (British Association
Report, 1898, p. 1032).
66o CONCLUSION
evolution of the sporophyte was essentially different from that so constantly
seen in their individual development Here it is held that unless good
reason be found to the contrary, the development of the individual will
probably reflect in some degree the evolution of the race ; but it is
recognised that the principle is not directly applicable in all cases (Chapter
XIV.).1 Accordingly the ontogenetic facts would support a view involving
the appearance of new structures in the course of descent.
We have seen that the first steps in the organisation of a sporophyte
are suggested by the post-sexual divisions in certain Algae which there is
good reason to believe were associated with a reduction of chromosomes.
Passing from these initial stages of the sporophyte, of which the post-
sexual stage in certain Algae cannot be held as more than suggestive of
what actually occurred, to those where it appears as a more or less
extensive tract of tissue, it has been shown that the sporogonia of Bryo-
phytes provide numerous examples of sterilisation, and that the result has
been to defer the event of reduction, and in various ways to increase the
means of nutrition and dispersal of the spores (see pp. 258-286). The
facts of sterilisation and their biological results have been accepted by
other writers, and though they do not actually demonstrate that the
sporogonium of the Bryophyte did originate by intercalation of a new
phase in the life-cycle, nevertheless the observed facts harmonise with that
view : it is difficult, without having recourse, as some have done, to purely
hypothetical preliminary phases in the descent of this phylum, to read the
facts in any other way.
One important point on which the Bryophyta differ markedly in their
individual development from all Vascular Plants, is that in them, as a rule,
the whole sporophyte originates by a primary embryogeny : it is initiated
directly from the zygote with the minimum of apical or intercalary growth,
and with entire absence of appendages.2 There is no continued embryogeny,
with secondary initiation of fresh parts, as in Vascular Plants. This simple
1 There are two leading features of development to which a theory of recapitulation
will not apply, and both are open to a biological explanation. The one is where those
gouty developments occur in the embryogeny, especially in the Lycopods (p. 351), the
other is the apparent priority of the vegetative system over the spore-production in the
individual life. In both cases the recapitulation of the sequence of developmental events
may be held to have been overruled by physiological requirement : the latter is fully
explained on the basis first of sterilisation of individual cells, and secondly of abortion
of the spore-producing parts : the consequence is that the vegetative system appears before
the spore-production begins : though the latter was the prior function of the sporophyte,
the overruling requirement was for early nutrition. The former has its origin in the
demands of early nursing of the embryo, and it has been shown that it has arisen
along two distinct lines within the genus Lycopodiiim. Such responses to biological
requirement are readily intelligible ; but they must not be held to invalidate the whole
doctrine of recapitulation, they show rather that it applies within limits only, and that
the evolutionary story which the individual may tell is liable to secondary disturbance.
2 An exception is seen in Eriopiis. in which rhizoids appear at the base of the seta :
this appears to be a good example of the origination of new organs not fashioned out
of pre-existing organs (Goebel, flora, 1906, pp. 66, 68).
ALGAE AND BRYOPHYTA 66 1
origin falls in readily with antithetic theory, under which it would be held to
be a primitive, not an acquired condition. Moreover, it accords with the
relatively simple form and structure of the sporogonium when mature :
this simplicity has made the recognition of the part played by sterilisation
easier in the Bryophytes than it is in plants where continued embryogeny
leads to a more complex state.
But the details of this primary embryogeny are carried out differently
in Mosses and in Liverworts : in the former, after the first division which
separates the hypobasal cell, apical growth appears in the epibasal hemis-
phere with regular segmentation of a two-sided initial cell : in the latter
the segmentation in the epibasal hemisphere is not localised apically, but
after division into octants the growth is intercalary. Both of these types,
so distinct in their details of segmentation, present points of interest for
comparison with the more complex embryogeny of Pteridophytes : but the
analogies offered by the Liverworts are the more instructive. In some of
them (Ricciaceae) there is no distinction of apex and base : it may be a
question whether this absence of polarity is primitive or acquired. In
others (Marchantiaceae) there is definite polarity, the whole hypobasal
hemisphere serving functionally as a foot and seta, while the epibasal is
reproductive. In others again (Jungermanniaceae) the hypobasal hemis-
phere develops into a unicellular appendage of small size ; the epibasal
hemisphere after octant division undergoes intercalary growth, with repeated
transverse segmentation : the seta is derived from the lower tiers of cells
thus produced, and it may be only the uppermost tier that remains fertile
(Figs. 124, 125). The interest here lies in the deferring of the propagative
function, as compared with the previous cases : the part which is in them
an absorptive seta is here a small body, with probably a minor or
temporary function, while the lower part of the epibasal region, which is
elsewhere propagative, takes up the duty of the hypobasal. The propa-
gative function is relegated to the apical tier, and thus, on a basis of com-
parison along the Liverworts, an example is established of that process of
deferring of the event of spore-production which is an essential feature in
the theory here put forward. A somewhat similar process has been traced
in the Mosses ; and in the Pteridophytes there is reason to believe that it
has been very prevalent. The presence of such evidence from phyla which
have probably been distinct from one another at least in the later phases
of descent, illustrates what is believed to have been a progressive
development which owes its prevalence to the fact that it was dictated
by biological advantage.
The similarity of the small hypobasal appendage in the Jungermanniaceae
(Fig. 125) to the suspensor in certain Pteridophytes is a further point
for comparison ; but it is doubtful whether this is in reality anything more
than a very distant analogy. In either case the body in question represents
a part of the zygote which takes no active part in the further embryonic
development : both owe their origin to a form of meroblastic segmentation.
662 CONCLUSION
Another point of interest in the Bryophytes for comparison is the
establishment of a central sterile tract — the columella. In the Liverworts
this is incompletely carried out in Aneura (Figs. 127, 129), and in Pellia
(Fig. 128), the final end here being a more effective distribution of the spores:
it is more completely organised in Anthoceros, where it probably serves for
nutrition as well as for distribution (Fig. 130 E) ; . but its more definite
character is established in the Mosses, where it is probably effective in
water-storage as well as in nutrition. However different these several parts
may be in origin or in function, they all illustrate that process of relegation
of the spore-production, originally central, to a more superficial position.
It has been pointed out above (p. 286) that in sporogonia of no great
bulk, which dehisce by apical pores or by lateral slits, the superficial
position of spore-production is not a point of biological moment in the
same way as it is in larger plants, with separate sporangia, and with a
larger proportion of sterile to propagative tissue ; doubtless here again the
tendency to a superficial position of the spores, so imperfectly carried out
in the Bryophytes, shows only a distant analogy to the more pronounced
condition in Vascular Plants, as seen in their superficial sporangia.
So also with the assimilatory system, imperfectly represented in most
Bryophytes, though better developed in some few (Splachnum, Buxbaumia,
Anthoceros] ; however similar these tissues may be to the functionally cor-
responding tissues in Vascular Plants, the similarities cannot with certainty
be held as more than points of analogy. The facts point to a wide-spread
" homoplasy " as having been effective in the Bryophytes and Pteridophytes ;
at the same time the similarity of the consequent characters seen in the
simpler organisms, throws suggestive light upon the origin of those of the
more complex. Nevertheless the similarities cannot safely be held to lead
further than to the recognition of certain methods of morphological
advance : they indicate that the origin of the sporophyte was probably the
same in both classes ; it may be traced from a primitive body, initiated
by the post-sexual complications involving chromosome-reduction. The
requirements of both in respect of increasing spore-production, and con-
sequently of nutrition under subaerial conditions, were essentially alike ;
independently each has probably worked out its own evolution ; and they
have independently arrived at results which show points of analogy such as
those above recognised. The mere existence of those analogies, with the
differences both of general scheme and of detail which they show, appear
to lend probability to the recognition of the general biological conditions
under which they are believed to have arisen. They were briefly these1
that in land-growing forms which maintained the aquatic type of fertilisation
by a spermatozoid motile in water, a premium was put upon multiplication
of germs : and that multiplication of germs necessitates increased facilities
for their nutrition and dissemination. It appears probable that these offices
were carried out by tissues which originated ultimately by sterilisation of a
proportion of the potential germs.
1 Compare Chapter VI. where the biological aspect of alternation has been discussed.
CHAPTER XLII.
EMBRYOGENY OF THE PTERIDOPHYTES.
No great difficulty is experienced in recognising the sporogonium of the
Bryophyta, in its various forms, as the result of the working out of the
requirements in respect of increasing spore-production and consequently
of nutrition, under conditions of sub-aerial life. They are believed to
present a sequence of forms for the most part caught in the up-grade
of evolution, though showing occasional evidences of reduction.1 But in
the more complex Pteridophytes the case is different : they have, according
to our hypothesis, proceeded so far in the elaboration of the sporophyte
that the steps of earlier evolution are less easily grasped : and as the
area of fact involved is very much greater than in the Bryophytes, and
the application of the theory of antithetic alternation, with sterilisation as
a leading feature, has never till now been fully formulated for them, it
will be necessary to summarise the evidence which has been derived
from the comparative study of their sporophyte generation. This summary
will be arranged in order of the events of the individual life, starting
with the embryology, and proceeding to the vegetative, and finally to the
propagative system.
From the criticisms of the older methods of comparative embryology
advanced in Chapter XIVV it will be gathered that at the moment the
study of the earliest phases of the individual, as an avenue to an opinion
on the morphology and phylogeny of Vascular Plants, stands in a dis-
credited position. Modern analysis has disproved the conclusions drawn
from the primary segmentation, and shown that there is no constant
relation between cell-cleavages and the genesis of the several parts.
1 It is possible to make out a case for the converse view of the Bryophytes as a
series in which the dependence of the sporophyte has been secondarily acquired, and
reduction widely effective ; but that idea is not seriously entertained here, as it is not
based upon observation o any actually existent organisms indicating that such a progression
took place : nor has any physiological ground been advanced as a sufficient reason that
the presumed reduction should have been carried out.
664 CONCLUSION
On the other hand, the examination of the embryos of various types of
Pteridophytes has shown that the occurrence of a suspensor is variable
within near phyletic limits, that the form of the embryo itself is in high
degree plastic, and that a certain correspondence can be traced between
biological requirement and the proportions, or even the actual position
of the parts relatively to one another, and to the parent prothallus. Thus
the haustorium or foot is found to be inconstant in position, and may
be present or absent in plants of near affinity ; the root may be entirely
absent, or vary in its position ; the cotyledons also may vary in number
and in position as well as in form and dimensions. Such irregularities,
together with a certain degree of physiological reasonableness which may
often be seen to underlie them, led not unnaturally to the conclusion
drawn by Goebel 1 that " root, shoot, and haustorium are laid down in the
positions that are most beneficial for their function." This implies that
all parts are opportunist growths. To those who accept this thesis
as true, embryology cannot form a secure basis for general comparisons
or for phylogenetic argument. For them comparative embryology would
be little better than a study of the more or less immediate biological
adaptations of the embryos themselves : there would be no common ground
from which the comparison could start.
But it may be questioned whether this extreme position is fully
justified. The endeavour must be made to recognise and isolate those
characters of the embryo which are variable, and to see whether there is
not some element of constancy in shape or in construction which underlies
the fluctuating features, and runs through all the different forms. This
has been greatly facilitated by recent discoveries; for now the embryos
of all the leading types of living Pteridophytes are fairly well known, with
the exception of the Psilotaceae — though possibly these are, for comparative
purposes, the most important of them all.
A revision of the embryology in the whole series of Pteridophytes
described above leads to the conclusion that the form is not so inchoate
or immediately plastic as Goebel's statement implies : comparison shows
that there is one point comparable in them all (where fully investigated)
which does not appear susceptible of disturbance on a basis of opportunism,
viz. the position of the apex of the axis relatively to the primary
segmentation ; or, expressed in other words, the relation of the polarity of
the embryo to its first cleavages.
Of this primary segmentation there are two types, according as a
suspensor is present or absent; otherwise it shows that remarkable constancy
of cleavage which led earlier writers to construct the theory of octants,
now no longer to be upheld. It has been shown that these two types
may appear in the same phylum (Lycopodiales, Ophioglossales) and even
in the same genus ' (Botrycftiwri) ; and there is accordingly reason to
believe that, however important biologically, they do not mark such
1 Organography, ii. p 246.
EMBRYOGENY OF THE PTERIDOPHYTES 665
a difference of initiation of the embryogeny as will serve for a safe
taxonomic guide. Where a suspensor is formed, the first segment-wall
(i, i) divides the zygote, separating the parent-cell of the suspensor from
what has been styled the embryonic cell (Fig. 355 i.). As the position of
the first segment-wall in all Pteridophytes where a suspensor occurs is
approximately at right angles to the axis of the archegonium, the mother-
cell of the suspensor is directed towards the archegonial neck, and the
practical effect of biological moment is that the embryonic cell is thrust
downwards into the tissue of the nourishing prothallus. While the
suspensor is thus recognised as biologically important, it may, on the
other hand, be regarded as a means of deferring the actual constitution
FIG. 355.
Diagrams illustrating the segmentation of embryos. I. = where a suspensor is formed,
which is cut off by the first wall, /, /; the suspensor is cross-hatched ; £, -5 = basal wall,
separating the hypobasal hemisphere (dotted) from the epibasal (clear). II. is the same
seen from above, x marking the pole. III. =an embryo where no suspensor is formed,
and the segmentation resembles that in the embryonic cell where the suspensor is present ;
the lettering corresponds ; x, y indicate the polarity. Each hemisphere divides into
four by quadrant walls {Q, Q in II.) and octant walls o, o.
of the definitive embryo, which is entirely derived from the remaining
portion of the zygote. The formation of a suspensor is in fact a form of
meroblastic segmentation, comparable generally, though not in detail, with
that seen in many Gymnosperms. But a further analogy is to be found,
as already pointed out, in the sporogonia of the Jungermanniaceae (Fig.
125): here, however, it is the segment furthest from the neck of the
archegonium which takes no part in the constitution of the definitive
sporogonium.1 In either case a part of the product of the zygote,
which has some more or less obvious biological use, may in certain
forms be set aside from partaking directly in the formation of the definitive
embryo.
Passing now to the embryonic-cell in the Pteridophytes which have
a suspensor, it has been shown in several well-investigated cases that it
1 It is interesting to note that this body is absent from the Marchantiaceae ; and the
inconstancy in the Liverworts may be compared with that of the suspensor in the
Pteridophyta.
666 CONCLUSION
undergoes octant-division : l the succession of the divisions is not always
the same, but as a rule there is first a basal wall (B, B) parallel to the
wall i, i, which divides the embryonic cell into hypobasal and epibasal
tiers, and this is followed by quadrant and octant walls at right angles
(Fig. 355 Q, Q; o, o), which divide each of those tiers into quarters. The
result is a body which shows in many cases, by its elongating form, that
there is a distinct polarity : its form and constitution are illustrated by
diagrammatic figures (Fig. 355 i. u), in which the suspensor is cross-
hatched, the hypobasal tier dotted, and the epibasal tier left clear. Such
a scheme will serve for all Pteridophyte embryos with suspensor which
have been fully elucidated.
Turning to embryos without a suspensor, the segmentation of the whole
zygote into octants is similar to that seen above in the embryonic cell,
where a suspensor is present, but with the suspensor completely omitted
(compare Equisetum^ Fig. 214; Ophioglossaceae (excl. Botr. obliquum],
Figs. 260, 261, 261; Isoetes, p. 350; and all Filicales). It is represented
diagrammatically in Fig. 355 in., where again the hypobasal region is dotted
and the epibasal left clear. Without attaching undue importance to the
cell-cleavages themselves (for they resemble those in certain quite distinct
bodies, such as capitate hairs), they may be held as indications of the
growth, and, what is more important, of the polarity already denned in
the body of the embryo. The first indication of the existence of this
polarity is given by the position of the first segment-wall (i, i), or B, B
in cases where a suspensor is absent ; and it may be shown that in all
fully investigated cases the apex of the axis has a definite relation to that
first wall. It appears at the centre of the epibasal hemisphere, that is,
in close relation to the intersection of its octant walls : the point is
marked (x) in the diagrammatic Figures 355 i., n., in.
It should be clearly understood that however constant the orientation
of the embryo may be in cases where a suspensor is present, the orienta-
tion is not constant in the type without a suspensor : in these the apex
of the axis bears no necessary or constant relation to the axis of
the archegonium, either for Archegoniate plants at large, or for the
several phyla of them, or yet for genera or even for individuals. As a
matter of observation, the orientation of the definitive shoot is initiated
sometimes with its apex towards the neck of the archegonium (compare
Fig. 214 of Equisetum, and Figs. 260-262 of the Ophioglossaceae, with
the diagrammatic Fig. 356 in.); or obliquely to one side, e.g. Lepto-
sporangiate Ferns (compare Figs. 14, 15 with diagrammatic Fig. 356 11.) ; or
away from it (as in Marattiaceae, compare Fig. 292 with the diagrammatic
1 Compare especially Fig. 190 ot Sel spiniilosa ; also, though less clearly, Pfeffer's
drawings of S. Martensii, Hanstein's Abhandl., vol. i. Taf. 2, 3 ; Treub's drawings of
Lye. Phlegmaria (Fig. 185), but more fully in Ann. Jard. Bot. Buit., vol. v. Taf. xxiii.,
xxiv., and Bruchmann's drawings of Lye. clavatum and annotinuni (Fig. 1 86) ; but more
fully in Bruchmann's own memoirs quoted above.
EMBRYOGENY OF THE PTERIDOPHYTES 667
Fig- 356 i.) ; it has been shown that the latter type exists initially in all
cases where a suspensor is present, e.g. Lycopods (Figs. 183, 186, 187),
and presumably in Botrychium obliquum (Fig. 264). In Isoetes the
orientation may vary between wide limits even in the same species.1 But
a still more interesting case is that of the genus Botrychium : in
B. Lunaria and virginianum the orientation of the primary axis is
towards the archegonial neck (Figs. 261, 262, 263). In B. obliquum,
however (Figs. 264, 266), where a suspensor is present, it is at first
turned away from the archegonial neck, as in other embryos with a
suspensor. Thus within the old genus Botrychium there are two types
of opposite orientation. An inversion of polarity must have occurred in
descent. Probably in more than one case such an inversion of polarity
FIG. 356.
Diagrams to show the relation of the basal wall, B, B, and hypobasal (dotted) and
epibasal (clear) hemispheres to the archegonial neck, which is indicated by an arrow ; x,y
shows polarity, x being the apex; 6" = stem ; Z, — leaf; ^? = root; f=foot. I. shows
the orientation seen in Marattiaceous Ferns. II. that for Leptosporangiate Ferns.
III. that for Equisetum and Ophioglossaceae.
has taken place, not by any rotation of the embryo, but by change in
the way in which the zygote has itself initiated its organisation. It is
necessary in this connection to realise that the zygote is at first without
any determinate polarity : that this may be initiated in various relation
to the axis of the archegonium, in different types of plant or even in
different individuals ; and that its position is controlled, not by external,
but by internal causes at present unknown.- But whatever those causes
may be, and whatever the orientation, a comparative study of embryos
shows that when the direction of polarity is once indicated, as it is
by the first segment-wall, the apex of the axis of. the first shoot
is initiated in a definite position relatively to it: occupying, in fact, the
epibasal pole.
1 Campbell, Mosses and Ferns, pp. 545'547 J compare Fig. 191 B above, p. 359.
2Goebel, Organography, i. p. 219, and ii. p. 246.
668
CONCLUSION
This general principle may be illustrated by comparison of certain of the
figures quoted in Part II. Thus in Fig. 185 A, B (p. 348) of Lycopodium
Phlegmaria the apical point T coincides very nearly with the intersection of
octants, though it appears unsymmetrical owing to unequal growth caused by
the precocity of the cotyledon (c), but it is righted by the appearance later of
the second leaf (Figs. 185 c, D). It cannot be doubted that the case of Z.
Selago is very similar, though the
^ detailed study of cleavages is not
yet to hand (Fig. 183, p. 346).
Fig. 1 86 (p. 349) of L. annotinum
shows the coincidence of the
apex (s) with the cleavage-wall
(n) very plainly indeed. As
the cleavages have not yet been
traced in the more aberrant
Z. cernuum, it is impossible to
say more than that the ob-
served facts do not preclude a
similar origin of the axis, which
comparison with 'Phylloglossum
makes probable (p. 353). In
Selaginella spi?iulosa (Fig. 190,
P- 357) a comparison of the
stages A, c, D clearly shows
that the small-celled tissue of
the apex of the axis includes
the intersecting octant walls. A
similar origin of the axis to that
in Lycopodium and Selaginella
spinulosa may be traced for
Isoetes, notwithstanding the ab-
sence of a suspensor and the
small size and late definition of
the apex (Fig. 191, pp. 359-
360). The case of S. Martensii
is interesting for comparison,
since there is a single initial
cell, a condition which is probably derivative as compared with that of S.
spinulosa, with its small-celled meristem. Pfeffer's drawings1 demonstrate
how this originates with the octant wall forming one of its lateral faces ;
in fact, at the nearest point to a central position compatible with its
existence as a single initial cell. The embryo of Equisetum shows this
even more plainly : if a single initial cell is to be carved out of an
epibasal hemisphere of four octants so as to be as near to the centre of
1 Hanstein's Abhandl., i., Taf. iii. iv.
Diagrams to show in view from above and in section how
growth with a single three-sided initial cell may be estab-
lished in an epibasal hemisphere divided into octants. The
quadrant wall, Q, Q, and the octant wall, o, o, are the first
of the series of cleavages, continued by the walls 2, z, //, zV,
etc. The result is that the initial cell (x) is formed at the
nearest possible point to the centre, consistent with the
sequence of its segmentation.
EMBRYOGENY OF THE PTERIDOPHYTES 669
it as possible, it could not be done more exactly than is shown in Sade-
beck's drawings (Fig. 214, p. 393): one octant enlarges and thrusts the
less active octants aside ; and its central angle immediately becomes one
of the angles of the pyramidal initial, which then continues to segment in
a sequence of which the original octant walls were the first terms. The
succession of the cleavages is shown diagrammatically in Fig. 357 A and B.
The necessary consequence is an appearance in section accurately shown
in Fig. 358 A, in which it will be seen that the apical segmentations
conform with great exactitude to those shown in the diagram.
ivh
FIG. "358.
Drawings of embryos. A, of Equisetum (after Sadebeck). B, of Marsiiia (after
Hanstein). C, of Adiantum (after Atkinson). They all illustrate with accuracy the origin
of the apical cell of the axis, according to the scheme shown in Fig. 357.
Even in. Leptosporangiate Ferns, notwithstanding the influence of a
large and precocious cotyledon, the same relation of the apical cell of the
axis to the octant segmentations may be observed. It is accurately shown
in Hanstein's drawing of the embryo of Marsiiia salvatrix (Fig. 358 B),
where the apical cell with, its first segment directly adjoins the octant-wall.
It is equally clear in Campbell's Fig. 178 F1 for Onoclea sensibilis, while
Fig- 358 c, after Atkinson, showing the embryo of Adiantum^ indicates the
same cleavages there also. Thus, even in embryos where there is a single
initial cell, that cell is carved out so as to be in the point nearest the centre
of the epibasal hemisphere that is consistent with their mode of segmen-
tation. In the Marattiaceous Ferns, 'where there is no constant single initial
at the apex of the stem, the matter is not so clear ; but Fig. 292 leaves no
room for doubt that the position of the apex of the stem is substantially
the same. In the Ophioglossaceae the segmentation in the embryo has
not been accurately made out, but sufficient is known to show that in
1 Mosses and Ferns, p. 322.
6;o CONCLUSION
Ophioglossum vulgatum (Figs. 260 and 260 bis, p. 466) and in Botrychium
(Figs. 261, 262, p. 468) the apex originates from approximately the centre
of the epibasal hemisphere, and notwithstanding that there is an early
displacement owing to the precocious development of the first root. The
facts thus suffice to support the general statement, that whatever the other
fluctuations of form of the Pteridophyte embryo may be, all the exactly
investigated types show the apex of the axis to originate in close relation
to the intersection of the epibasal octant-walls.
It is accordingly recognised that the very first step that can be
observed in the embryogeny involves the definition of its polarity, and
that the apex of the shoot bears a constant and close relation to the
centre of the epibasal hemisphere. The base of the primitive shoot thus
defined is. the suspensor where that part occurs ; where it is absent the
centre of the hypobasal hemisphere may be held to mark the base of the
primitive axis.1 The whole embryo thus appears from the first as a
radially constructed spindle upon which appendages may be borne : these
are of the nature of leaves and of accessory roots, and they may vary in
number, and in position and time of origin, causing thereby marked
variations in the early structure, which are for the most part open to
biological explanation. The embryo is, however, subject also to early
distortion in various ways, through the formation of swellings of the nature
of haustoria, or of storage tubers : or it may be that modifications of
form arise in relation to the precocious development of some one
appendage and the correlative delay or diminution of another, or even
of the axis itself. Though such modifications are probably secondary,
yet they have produced such peculiarities of form and aspect in the
embryos in which they appear that the originally radial form of the shoot
is disguised, and its morphology has consequently been misunderstood.
Examples will now be quoted illustrating these various points.
The origin of the cotyledon in Ferns is constant in time and place :
this is probably related to the prone position of the embryo, and to its
importance for early nutrition in replacing the supply derived from the
small and evanescent prothallus. But in other cases there is less constancy :
in Equisetum there may be sometimes two, though usually three cotyledonary
leaves in the first whorl. In L. Selago and Phlegmaria one cotyledon takes
precedence, soon followed by a second leaf (Figs. 183, 184, 185), but
in L. davatum two equal cotyledons are formed (Fig. 186). Again, in
Selaginella Martensii two equal cotyledons appear very early ; in S.
spinulosa Bruchmann specially notes that though the two cotyledons may
1 Some previous writers have held the primary axis to run from the stem apex
obliquely to the apex of the first root. Reasons will be shown below for regarding the
root as an accessory part, commonly lateral, and not determinate in position. Its
growth may in certain cases approximate to the original axis of polarity of the shoot,
as it does in the embryos of Ferns ; but this is held to be an occasional and accidental
rather than an inherent character, as is indicated by a comparison with the more bulky
embryos of Equisetttm and the Ophioglossaceae.
EMBRYOGE'NY OF THE PTERIDOPHYTES 6; i
sometimes be equal (Fig. 190 c) they are usually unequal (Fig. 190 D), and
that the second may be long delayed, and only make its appearance after
the shoot issues from the spore : nor is there any constancy in the
position of the first relatively to the suspensor and first root (compare
Bruchmann, Figs. 62, 63 of Sel. spinu/osa). These examples will serve
to show the inconstancy of time and place of origin of the first leaves in
the Pteridophytes at large, notwithstanding the constancy seen in Ferns.
There is, however, one feature that appears constant : it is their orientation
relatively to the axis : they all appear to present towards the axis or to
that point where the axis will ultimately make its more obvious appearance,
a surface that may be recognised as more or less characteristically " adaxial " :
even in the extreme cases of Lycopodium cernuum and of Phylloglossvm,
where the number of protophylls is most irregular (Figs. 101, 188, 189),
they are not disposed at haphazard, but face towards the point where the
apex of the definitive axis appears. This constancy of orientation of the
first leaves resembles that of the later leaves, and supports the conclusion
already arrived at, that cotyledons and protophylls are essentially of the
same category as the later foliage leaves, and are essentially appendages
of the axis (pp. 186-7).
Here it may be well to mention cases of that precocity of the cotyledon
which carries with it a correlative delay in development of other parts, but
especially of the axis (pp. 183-4). It is seen in Ferns, where the cotyledon
is hurried forward to supply a nutritive need, and a correlative delay of
the axis is the consequence (Fig. 15). The same is seen in Isoetes. with
a similar result (Fig. 191 G). But perhaps the most remarkable examples
are seen in the Ophioglossaceae, plants which show greater adaptive
plasticity of the embryo than any others. It has been shown that in
certain forms, Oph. vulgatum (Fig. 260, 261), Botrychium Lunaria (Fig. 262,
263), the cotyledon is small, and probably reduced in accordance with
the underground habit : in others, Helminthostachys (Fig. 267), Botrychium
virginianum (Fig. 261) the cotyledon appears above ground as an
expanded green leaf, and though the apex of the axis is correlatively
delayed, it is still recognisable. But in others again the cotyledon is
precociously developed to 'such a degree that it is difficult or impossible
to recognise the apex of the axis ; x this may be held to be an extreme
case bearing with it correlative consequences which have completely upset
the balance of parts in the embryo.2
The time and place of origin of the first and subsequent roots is open
to variation. In Ferns it arises in the hypobasal hemisphere, and this is
the case also in certain types of Equisetum (Fig. 214), though in E. hiemale
it is apparently higher up (p. 392) : but in any case it is clearly lateral
in Equisetum, and the condition in Ferns appears to be only a less bulky
variant on the same type. The apparent difference in exact point of
1 Oph. inohiccannni, Campbell, I.e., p. 189, and PI. X.
- Compare p. 469, where Campbell's alternative view is mentionecl.
672 CONCLUSION
origin of the first root is again fully illustrated within the Lycopods :
in Selaginella (Fig. 190) the first root originates laterally from the hypo-
basal tier, and near to the suspensor : in Lycopodium (Figs. 183, 185,
1 86, 1 88) and in Isoetes (Fig. 191) it springs from the epibasal tier, and
is thus necessarily in a lateral position upon the whole embryo. Its
orientation relatively to the cotyledon also varies : in Isoetes it is opposite
to the cotyledon (Fig. 191), in Lycopodium and Selaginella it is frequently
on the same side of the axis as the cotyledon, but this is not constant
in S. spinulosa : in Ferns it is on the same side as the cotyledon (Fig.
15). It thus appears that the root is not definite in level or in orienta-
tion relatively to the other parts ; while in point of time, its extreme
delay in L. cernuum and its absence in Salvinia are cases too well
known to require remark. It is notable that though the root in Seed-
Plants directly faces the suspensor, this is not the case in any Pteri-
dophyte : in them it is always a lateral appendage, however nearly it may
sometimes approach the centre of the hypobasal hemisphere. Accordingly
it cannot be held to be itself the continuation of the primitive axis.
Though the root may appear late in the embryology of certain Lycopods,
the converse is seen in the Ophioglossales ; in them the precocity of the root
upsets the balance of parts usual in other embryos. This is seen in moderate
degree in such types as Botrychium virginianum (Fig. 261) or B. obliquum
(Fig. 264), in which, though the embryo differentiates slowly, the root soon
takes a prominent place; but in Botrychium Lunaria (Fig. 263) and
Ophioglossum vulgatum (Fig. 260, 260 bis) it is clear that the root, rushing
forward in its development, outstrips the other parts, and becomes the
prominent feature of the embryo. The extreme is, however, found in
Oph. pendulum, and so prominent is the root here that Campbell has
described the embryo as consisting of "roots only."1 This may probably
be held as the consequence of precocity of the root carried to a greater
degree than in other species : and the precocity finds a ready biological
meaning in its mycorhizic function. It may be held that the embryo
hurries it forward as an accessory aid to nutrition, and the parts of the
shoot are correlatively delayed till sufficient store is at hand to justify
their development above ground.
Though the balance of parts in the embryo may be thus disturbed by
the precocity of certain parts, still more profound disturbances appear
associated with parenchymatous swellings of the nature of haustoria or of
storage tubers, and these are usually accompanied by considerable curvature,
and distortion of the axis. Such swellings are of two sorts : intra-prothallial
haustoria, to which the name " foot " has commonly been applied, and
extra-prothallial tubers, known under the name of "protocorm." In simple
types of embryo with suspensor the hypobasal tier of the embryo may
remain small, though functionating as an haustorium (L. Selago, Fig. 183,
and L, Phlegmaria, Fig. 185): but in others it may enlarge in the direction
1 Ann. fard. Buit., vol. xxi., p. 189. See remarks on p. 469 above.
EMBRYOGENY OF THE PTERIDOPHYTES 673
of the greatest nutritive supply and take a strong curvature, as in L. clavatum
(Fig. 1 86). In others, again, it may provide the basal part of the embryo
and root, without any swelling (Sel. spinulosa^ Fig. 190), or an haustorial
swelling may be formed, with convex curvature, on the side next to the
food supply (Sel. Martensii}. In embryos without suspensor the hypobasal
tier may maintain this same function, but it is usually only slightly enlarged
(Equisctum, Fig. 214; Fern, Fig. 15; Isoetes, Fig. 191; Ophioglossum vulgatum,
Fig. 260 bis; Botrychium Lunaria, Fig. 263). It would appear from the
inconstancy of their development, and their position in relation to their
obvious uses when present, that these haustorial growths are of the nature
of relatively late and direct adaptations at or near to the basal region of the
axis of the embryo, and it is significant that there is no special haustorial
growth in Lye. Selago or in Selag. spinulosa, both of them species believed to
be primitive types of their respective genera.
The extra-prothallial swellings, of the nature of protocorms, differ in
origin and in function from the intra-prothallial haustoria (Figs. 101, 178,
1 88): they spring from the epibasal tier, and do not serve as suckers.
It has been argued at length above (p. 351, etc.) that there is good reason
to believe them to be secondary in their origin : however greatly these
gouty interludes may affect the form and appearance of the embryo, their
effect is temporary, and the shoot ultimately settles down into a normal
Lycopodinous type. If this view of the protocorm as a special secondary
development be accepted, then it may be put on one side as not directly
affecting the bearings which embryogeny may have on the theory of
origin of the shoot.
The various types of embryogeny observed among Pteridophytes have
now been reviewed, and it remains to attempt to separate the characters
which are secondary, special, and fluctuating, from those which are primary
and constant, with a view to some general estimate of the embryogeny
as an aid to a morphological conception of the shoot. Following the
reasoning contained in the preceding pages, the occasional swellings of
the nature of a protocorm or of a haustorium, together with the curvatures
and distortions which these often produce, may be set aside as secondary ;
similarly, the precocious developments of root or of leaf, which sometimes
upset the balance of parts in the embryo, may be set aside as special
biological adaptations ; for even where a cotyledon or a root appears
early and anticipates apparently the other parts, still in all accurately
observed cases the relation of the axis to the primary segmentations is
found to be maintained. Further, the position of the first root is always
lateral ; its orientation and level of origin varies, as well as the time of
its appearance : these facts point to its being an accessory part upon the
embryonic body. The first foliar development is inconstant in position
and time and number of the leaves, but it is constant in the fact that
the protophylls are always lateral with respect to the point where the
axis will appear, and orientated with regard to it, so that more or less
2 u
6/4
CONCLUSION
definite " adaxial " surface is presented towards it. It thus appears that the
most constant features of the embryo in Pteridophytes are: (i) the origin
of the axis in relation to the initial polarity of the embryo, and (2) the
orientation of the first leaves relatively to it. These facts once recognised
must needs take a premier place in Pteridophyte embryology. The
embryo is thus presented to the mind as consisting essentially of an axis
or spindle, liable in the different types to varying proportions of length
FIG. 359.
Diagrams of embryos : the suspensor is cross-hatched ; the hypobasal hemisphere
dotted, and the epibasal clear. A=Selaginella spinulosa. B = Selaginella Martensii.
C =• Lycopodium Selago. D = Lycopodium clavatum. E — Lycopodium cernuum. F—
Isoetes. G — Equisetum. H=Adiantum. c — cotyledon ; ap — apex of axis ; r— root ;
Aj^ = hypocotyl; ;/=foot ; ,y = suspensor. These diagrams place various of the divergent
types described in the text in juxtaposition, and thus bring into prominence their points
of similarity and of difference.
and breadth, upon which the other parts are inserted as appendages :
the leaves with a more regular relation, the roots with less regular
relation, and the haustoria or tubercles being occasional. In cases
where a suspensor is present this constitutes the organic base, while
the stem-tip forms the organic apex of the spindle, which is itself
built up from the suspensor, the hypobasal, and the epibasal tiers. In
cases where no suspensor is formed the relations of parts are still the
same, but the base is formed from the centre of the hypobasal tier;
often, however, the latter is specially developed as an haustorial foot,
or disguised by the early origin of a root. The spindle thus defined is
EMBRYOGENY OF THE PTERIDOPHYTES 675
held to be the primitive axis, which in virtue of its constancy and of its
early development is regarded as the fundamental factor in the embryonic
shoot. The prior existence of the axis in the normally developing shoot,
and the origin of the leaf laterally upon it, have been held as the basis
of the enation theory of the leaf (Chapter XL) : since there is reason to
recognise the existence of that polarity of the embryo which defines the
axis, prior to the origin of the leaf in all the varying forms of the embryo,
the same arguments will apply even to the earliest phases of the ontogeny.
In fact, the embryo itself is from the first segmentation a shoot showing
polarity : the appendages appear later. Such results from the comparative
study of embryos greatly strengthen the strobiloid theory of the shoot,
as enunciated in Chapter XI. : at the same time they indicate that the
embryo is not a thing apart from the later developed shoot, but merely
its initial phase, modified in various ways to meet biological needs,
but preserving essentially the same relations of prior-existent axis and
of leaf produced in lateral relation to it.
A question remains as to the relation of the embryos with suspensor to
those in which there is none. Is it possible to recognise either of these as
the prior state ? The two types indicate different modes of prothallial nursing :
that with a suspensor is characteristic of stocks having relatively bulky
prothalli, often underground, and at the present time carrying on as a rule
a saprophytic nutrition : the type without suspensor is characteristic of stocks
with less bulky prothalli, usually above ground, and self-nourishing. The
determining factor would appear to have been the bulk of the nourishing
prothallus, rather than the exact way in which it obtained its nourishment.
The question will therefore be, whether the evidence points to a bulky
prothallus and embryo with suspensor as the prior condition, or a less
bulky prothallus and embryo without suspensor. There are two phyla from
which comparative evidence on this point may be drawn, viz. the Lycopo-
diales and the Ophioglossales. In the Lycopodiales, in view of the upward
curvature commonly seen in their embryos (Figs. 183, 186, 188, 190),
and the necessity of their bursting through the tissue of the prothallus
at some point apart from the archegonium to gain their freedom, the
complete inversion of the embryo, and its emergence in the neighbourhood
of the archegonial neck would be a simplification of an awkward and
inconvenient process. Such a simplification is found in Isoetes, which there
is good reason to look upon as a more specialised type of the Lycopodiales,
and in which the indeterminate position of the first segmentation of the
zygote suggests how the inversion may have come about. Moreover,
the condition with suspensor is found in its simplest form, and without
any tuberous complications, in such species as L. Selago and Sel. spinulosa,
both of which are believed to be relatively primitive forms. The facts
supply no proof, but they suggest a reasonable probability that within the
Lycopodiales there has been a progression from the state with suspensor,
and apex directed to the base of the archegonium, to the state without a
676 CONCLUSION
suspensor, and with the apex directed to the archegonial neck. A similar
probability may be recognised in the Ophioglossales, and Botr. obliquum
may be held to illustrate the more primitive embryogeny; and it shows
also that an awkward curvature during development is entailed on the
young embryo (Fig. 264) : the type common for the rest, without suspensor,
and with the apex directed to the archegonial neck would be the derivative,
and in them the awkward curvature is avoided. As regards other phyla, such
as the Equisetales and Filicales, where a suspensor is absent, the question
must remain open; but there is nothing apparently to oppose the view
that they also may have sprung from a stock with a suspensor, and
that, as suggested for Isoetes^ and for most of the Ophioglossales, they
also may have broken away from a development which had ceased to be
practically useful. The evidence from the Ferns, such as it is, indicates
a probable progressive reduction of the prothallus on passing from the
Eusporangiate to Leptosporangiate types : this would accord with a
general opinion that the primitive Pteridophyte prothallus was generally
a massive structure, and the primitive embryo which it nursed of the
type with a suspensor.
A comparison of the spindle-like embryonic axis of the Pterido-
phytes which these observations have disclosed with the young sporo-
gonium of Bryophytes, and especially of some of the Jungermanniaceae,
is inevitable : it would, however, be an error to press this comparison
closely. In both cases a segmented body of radial symmetry is
recognised, endowed with growth. But there is no sufficient reason to
believe that any living sporogonium really prefigures any early Pterido-
phyte : the similarity may well have had its evolutionary origin along
distinct phyletic lines, but subject to somewhat similar biological
requirements. On this point the difference in position of apex and base
has its interest; while the suspensor of Pteridophytes points to the neck
of the archegonium and the apex towards the nutritive prothallus, in
Bryophytes the apex is towards the neck of the archegonium and the
foot, or in Jungermanniaceae the basal appendage, grows into the tissue
of the gametophyte. There would appear to have been an essential
difference of method here : in the one case leading to the direct establish-
ment of an ephemeral sporophyte, deriving its nourishment from a
perennating gametophyte, and demanding early dissemination of its spores :
this is characteristic of the Bryophytes. On the other, the tardy establish-
ment of a perennating sporophyte deriving its nourishment at first from
the gametophyte, but eventually achieving a power of self-support, and
producing its spores relatively late : this is characteristic of the Pterido-
phytes, and extended with modifications to the whole Vascular Vegetation.
From the above pages it will appear that the evidence to be drawn
from comparative embryology as bearing on the morphology of the shoot
is by no means to be neglected. When the fluctuating characters and
features of more immediate adaptation are removed, there remains a sub-
EMBRYOGENY OF THE PTERIDOPHYTES 677
stratum of constant fact, which gives no uncertain support to the strobiloid
theory of the shoot. For it appears that from the very first segmentation
of the zygote the polarity of the embryo is defined, and the position of
its axis may thereafter be recognised with certainty. The embryo is in
fact from the first a shoot with pre-existent axis, not a congeries of parts
which are ultimately related to an axis of later origin. While we recognise
thus the importance of the facts of development in indicating the shoot-
character as initiated at, once, it is necessary always to bear in mind the
critical position of the young embryo until it is self-supporting: the
urgent need of nutrition is the chief influence which has contributed
to its biological specialisation, and to the assumption of those aberrant
forms which tend so strongly to disguise its real nature as a simple
and primitive, but from the earliest stages a leafy shoot.
CHAPTER XLIII.
THE VEGETATIVE SYSTEM OF VASCULAR PLANTS
ANALYSED.
A MOST effective factor in the higher development of the sporophyte is
the continuance of apical growth. In some few cases this is absent, as
in the sporogonium of certain Liverworts, and the development is then but
small ; or intercalary growth may intervene as in the Jungermanniaceae,
and be continued for a long period, as in the Anthoceroteae ; but in all
the more elaborate cases, including the Mosses and all Vascular Plants,
localised apical growth is effective, though it is usually associated with
intercalary growth. This localised and continued apical growth is
taken up early by the apex of the axis in the young embryos of
Vascular Plants, and is persistent through life : it is by reference to the
simpler cases where it does not exist that its importance as a factor in
the organisation, of the plant-body will be duly appreciated. In presence
of the sporophytes such as those of the Liverworts it becomes evident
that apical growth is not a general factor in the neutral generation : it
seems probable that in the first instance it did not exist, and that the
whole sporophyte owed its origin to a primary, intra-archegonial embryo-
geny : that localised apical growth, and as a consequence continued
embryogeny, was acquired as a secondary development, though it has
become a dominating influence in all the more elaborate sporophytes.
The mode of segmentation which accompanies apical growth provides
important material for comparison, according as it is conducted with a single
initial cell or with many, and according as the meristem is stratified or
not. In certain cases comparison leads to the conclusion that the more
definite segmentation with a single initial is a derivative state in the
sporophyte, and that with several initials the more primitive. Among the
Bryophyta there is no distinctive evidence on this point : the sporogonia
of the Musci have as constantly a single initial cell as those of the Hepaticae
have none. But among the Pteridophyta evidence of value comes from
the Filicales, and also, though less clearly, from the Lycopodiales. A
THE VEGETATIVE SYSTEM 679
comparative study of the meristems of root, stem, and leaf in Ferns indicates
that the most complex meristic condition is found in the Marattiaceae, a
series of Ferns known to have been well represented in Palaeozoic times
(see p. 650, etc.). The Osmundaceae are now being more and more firmly
established in relation to the ancient Botryopterideae : their characteristic
structure is recorded back certainly to early Mesozoic times, and possibly
earlier : they show in their meristems an intermediate condition, while
that of the roots is variable : Todea often has the structure characteristic
of the Marattiaceae : Osmunda has sometimes a single initial in the
root, but often more, with curious irregularities of the segmentation.
Its stem-structure shows a similar state, while the leaf in the Osmun-
daceae is alone among Ferns in possessing a three-sided initial
with regular segmentation : the leaf (except in the filmy Todeas)
also shows a structural complexity of the wings similar to that of
the Marattiaceae. All other Ferns, including even such early forms as
Schizaeaceae and Hymenophyllaceae, have the single initial in all their
parts, while the wings of the leaf also have a single marginal series of cells
with definite segmentation. From this it is concluded that in the Filicales
there has been a progression from types which were more primitive — where
the meristic structure was more complex, with the centre of construction
more deeply seated, and as a consequence with a plurality of initials of
prismatic form — to those characteristic of more modern times, where the
meristic construction is less complex, the centre of construction less deeply
seated, and as a consequence with a single initial having the form of a
three-sided, or even a two-sided, pyramid. The progression has been from
a more massive to a less massive construction, and from less definite
to more definite segmentation. It has been shown above (p. 637) that
a similar progression may be traced in the sporangial character.
A parallel progression, though less definitely indicated, is to be traced in
the Lycopodiales. At the apices of stem and root in the ancient genus
Lycopodium the tissues are not referable to a single initial cell. In
Selaginella spinulosa also, that species which on grounds of its radial shoot
and its anatomical structure we have recognised as a relatively primitive
type of the genus (pp. 300^332), there is from the first stages of the embryo
a small-celled meristem, without any single initial in stem or root. But in
the dorsiventral species, which on grounds of form and structure are held
to be derivative types, there may be a single initial both in axis and root,
though variable in the details of form.1 This mode of apical growth makes
its appearance in the very first stages of the embryo (p. 356). It seems
therefore probable that here again, as in the Filicales, there has been a
phyletic progression from a less definite segmentation with several initials
at the apex of stem and root to a more definite segmentation with a single
initial cell.
1 De Bary, Comparative Anatomy, p. 15. Treub, Selaginella Martensii, Leide, 1877,
PI. i., n", in.
68o CONCLUSION
Such examples, showing a parallelism of progression raise the question
whether in the Pteridophytes generally the apical segmentation with a
small-celled meristem and several initials was not the more primitive state,
and that with the more definite segmentation of a single initial the derivative.
It is not possible in the present state of knowledge to come to a definite
conclusion on this point ; and in the sporangiophoric Pteridophytes, including
also the Ophioglossales, the evidence is less clear than in the cases above
quoted ; for in them there is a prevalence in the very isolated living genera
of a definite segmentation with a single initial : there is, it is true, nothing
to preclude the view that they also were derived from forms with several
initials : certainly their eusporangiate sporangia, and deeply sunk antheridia
and archegonia, which usually go with the less precise segmentation at
the apex, would suggest that this was so.
Closely associated with the continued apical growth of the shoot is
the formation of the appendages, leaves, emergences, and hairs. The
leading fact with regard to the leaves is that in all cases they are found
to originate normally in the same way, by enation from the pre-existent
axis, and in acropetal succession : the embryological comparison given
above shows that this holds even for the protophylls. It applies equally
for the small leaves of the strobiloid types and for the larger and more
complex leaves of the Ophioglossales and Filicales : moreover the leaves
are dorsiventral, and show a constant orientation to the axis which bears
them. The high degree of persistence of their relation to the axis,
notwithstanding the differences in size, form, and number, indicates that
the parts large or small are substantially of similar nature throughout the
Pteridophytes, though not necessarily homogenetic. The discussion in
Chapter XI. has led to the conclusion that the leaves originated in descent
as they are seen to do now in the normal course in all Pteridophytes,
viz. by enation from the apically growing axis. It is held as probable
that the process of leaf-formation which appears in every normal ontogeny,
should represent the mode of their phyletic origin.
The view that there is an inherent improbability in this mode of
phyletic origin of the leaves has already been alluded to (p. 659).
But leaves are not the only appendages of the simple shoot : emergences
and hairs must also be considered, and from these some light
may be derived as to the origin of appendages at large which may
illuminate the probable origin of leaves. Emergences occur in isolated
genera and species of Vascular Plants, both in Pteridophytes, and Seed-
Plants : they sometimes contain vascular tissue, and in early stages of
development may closely resemble leaves. Both emergences and hairs
arise ontogenetically by enation from the plant-surface, and both are often
irregular in their position. Is there any reason to believe that these
sporadic appendages of the shoot were fashioned out of some pre-existing
organ.? The very irregularity of their position in the individual, and
of their occurrence in the race precludes such a view for them : the conclusion
THE VEGETATIVE SYSTEM 68 1
seems unavoidable that these minor organs arose phyletically by enation,
as new outgrowths, from a previously smooth surface. If it be admitted
for emergences and hairs that new organs, not pre-existent in the race, can
originate by enation, are we to take a different view for leaves, notwith-
standing that the facts of individual development by enation are alike in
both cases? Is the leaf to stand alone among the appendages of the
shoot in having been fashioned from some pre-existing organ? It may
well be asked whether this view has any other foundation than in pre-
conception apart from fact. The ontogeny is against it. The phylogeny
does not show it to be a necessary view. Comparison with other
appendages of the shoot gives it no support. And, finally, its acceptance
has led its adherents into theoretical difficulties involving hypothetical
organisms such as " Archegoniate Algae " ; or a " Prohepatic " type has been
assumed. These appear as unnecessary as they are non-existent to those
who accept the guidance which the individual development gives with so
great constancy. It may, on the other hand, be urged that leaves are
essentially different from emergences and hairs : that they are more constant
in occurrence, and more regular in position, as well as physiologically
more important, as they were also prior in descent. But such differences
do not indicate a radical difference in their mode of origin : the early
phyletic appearance and physiological importance of the leaf would rather
lead one to expect that just such priority and regularity should rule in
their organisation as distinguishes them from the other appendages of the
shoot. On these grounds it is held that the phyletic origin of the leaf by
enation, like that of emergences and hairs, is more probable than any theory
under which it would be fashioned from some pre-existing organ, hitherto
undefined, and wholly hypothetical.
The other appendages— the roots— bear no direct relation to the
continued apical growth of the axis. This fact, together with the great
diversity of their position and time of origin indicates them as accessory
parts — as they have already been held to be in the primary embryogeny.
Thus whether from the primary embryogeny, or from the plant showing
continued apical growth, the conception of the simple shoot emerges ; it
is composed of a pre-existent axis defined in relation to the first
cleavage of the zygote ; upon this axis leaves are produced laterally, by
enation in acropetal order, also, though less constantly, emergences and
hairs; while the roots, and even the first root of the embryo, are
accessory organs.
The simple shoot thus constituted, forms the unit upon which the
vegetative region of all Vascular Plants is built. Comparison indicates
that the radial construction of the shoot was primitive for the sporophyte,
and that where dorsiventrality occurs, it has been secondarily acquired
(Chapter XVI.). Such a shoot, developed as it is directly in the embryogeny,
may sometimes remain entirely unbranched : this is seen in some of the
simplest species of Lycopodium (e.g. L. Trendlla) or Selaginella (S. pumila
682 CONCLUSION
in its simplest forms) : it is habitual, though with occasional exceptions,
in Isoetes and the Ophioglossaceae, and it is seen in many Ferns, and
especially in such early types as the Marattiaceae, and Osmundaceae.
The whole plant in these cases consists of a single upright radial shoot,
and there is reason to believe that this is itself a primitive condition ;
such a view accords with the generally primitive character of the plants
in which it is seen. It may, however, result also from reduction, as may
often be seen in starved seedlings of annual flowering plants.
But in all the more advanced types, branching of the shoot occurs,
resulting in multiplication of shoots, and ramification often of a high
order. It is necessary to put this in relation to the simple unbranched
state. The terminal dichotomy of the shoot was probably a primitive
mode of branching. It is characteristic of those species of Lycopodium
and Selagintlla, which are held as primitive; (viz. the Selago section of
Lycopodium, and in Selaginella spmulosa) : it is seen occasionally in Isoetes,
in the Psilotaceae, and in the Ophioglossaceae as a rare occurrence, also
in the Osmundaceae, and in some other Ferns. Gradual transition from
the dichotomous to the monopodial branching may be traced by comparison
of the more primitive with the more specialised species of Lycopodium
and Selaginella, while in some cases the change may be traced through
unequal development of the branches of the dichotomy in passing from
the earlier to the later branchings of the individual life.1 This makes it
appear probable that the monopodial is a later and derivative mode of
branching.
It is a question what the relation of these terminal branchings of the
shoot may be to such lateral branchings as are seen in Equisetum, and
Sphenophyllum, or in those Ferns where axillary branching occurs. It seems
not improbable that these are in origin quite distinct modes of amplification
of the vegetative system from those brought about by terminal branching,
and that they are to be regarded rather as regularly recurring and early, but
nevertheless accessory developments. A reason for this. distinction is to be
found in what is seen in Equisetum, for here terminal fissions of the strobilus
are occasionally to be found, and are quite different in nature and origin from
the formation of branches normal for the genus. Again, in the Ophioglos-
saceae, in which family dichotomous branching has been seen as a rare
occurrence, and in many Ferns such as Pteris, which show occasional
dichotomy, buds arise at points remote from the apex of the shoot, in the
former case upon the roots, in the latter commonly near to the bases of the
leaves : these are clearly adventitious. All of these are probably of distinct origin
and nature from the terminal branching which is fundamentally dichotomous.
Moreover there is a structural difference between terminal ramifications and
branchings which are accessory : the former carry on the vegetative con-
struction with amplified stele, and fully formed leaves arranged as in the
1 This is believed by Bruchmann to be the rule throughout the genus Selaginella,
their first branching being regularly dichotomous. L.c., p. 18.
THE VEGETATIVE SYSTEM 683
region below the branching : the latter commonly start afresh from simple
beginnings, analogous to those of the seedling, with a contracted stele, and
leaves of smaller size, and simpler form and arrangement. These facts seem
to mark a distinction between terminal and accessory ramification.
By either, or by both of these modes of branching, there is ample
provision for extension of the shoot-system, over and above its own apical
growth. The branchings, whether terminal, axillary, or adventitious result
in the repetition of the original unit, modified, it may be, in certain minor
respects, but retaining the essential characters of the primary shoot. But
the upright position so common for the latter is not habitually maintained by
the later derivatives, which show a tendency to run off into plagiotropic and
dorsiventral modifications : not uncommonly they may take an underground
course. And thus, primarily from its own apical growth, but secondarily
from repetition of the primitive unit as a result of branching, the diverse
vegetative systems of vascular plants are built up.
There are certain analogies between the branching of the axis and
that which is seen in the leaf of many vascular plants. In not a few
cases the leaf is unbranched, and this — as in the case of the unbranched
axis — may be held as a primitive condition, though very many cases where
simple leaves exist have probably been derived by reduction from more
complex types with branched leaves. But just as the axis may dichotomise
in primitive forms, so also is dichotomy seen to be widely existent in the
leaves of early vascular types, and examples 'come from all the phyla
excepting the Lycopodiales. In the Equisetales, the ancient Asterocalamites
had leaves repeatedly dichotomous (Fig. 199); and a somewhat similar
branching of the large leaves existed in Pseudobornia : these show that
though many of the fossil Equisetales, and all the living ones have simple
leaves, the capacity for their dichotomy existed in the race. In the
Sphenophyllales the dichotomy of the leaf is an outstanding feature, and it
is represented in the modern Psilotaceae : in the latter Tmesipteris is
specially interesting, since, though normally the sporophylls dichotomise
but once, repeated dichotomies occur occasionally in the middle of the
fertile region; this suggests that the leaves possess capacities for branching,
normally unrealised, but brought into existence where the nutrition is most
effective. In the Ophioglossales branching of the leaf is also seen; some-
times it is clearly dichotomous ( Ophioglossum palmatum)^ but in Botrychium
and Helminthostachys it is modified in the direction of a monopodial
branching. It is, however, in the Filicales that branching of the leaf
attains its climax; and the prevalent dichotomy, and transition to a
monopodial branching show interesting analogies to what is seen in the
shoot itself.1
The roots, which have been recognised as adventitious and accessory
parts upon the shoot, also show a branching similar to that of axis and of
leaf. In the Lycopods the roots are sometimes unbranched, as is usual
1 See p. 627, etc., where the literature is quoted.
684 CONCLUSION
in Phylloglossum ; but in Lycopodium, Selaginella and Isoetes there is
dichotomous branching, often with unequal development of the shanks.
Ophioglossum also shows dichotomy of the roots. But in Equisetum and
in Ferns the branching is definitely monopodial, the lateral roots originating
apart from the apex of the main root ; a condition comparable with the
origin of the lateral buds in Equisetum^ or of the axillary buds in the Hymeno-
phyllaceae. The similarity of these conditions to what is seen in axis and
leaf is unmistakable.
It is thus seen that in the axis, leaf, and root provision is made for
amplification of each several part by branching, and the methods of
branching seen in them all are essentially alike : each type of part may
remain unbranched, or it may dichotomise, or show monopodial branching :
it is also seen that dichotomous branching is prevalent in those forms which
comparison or palaeontological evidence shows to have been primitive. It
is natural that such analogies should exist between parts of the same
individual plant ; but there is no reason to see in them anything more
than parallel modes of amplification of parts which were throughout their
descent distinct in their origin, and in their nature.1
An analysis of even the most complex types of the vegetative system
in Vascular Plants involves only the factors thus disclosed, viz. the shoot
consisting of axis and leaves, with occasional emergences and hairs, and
the accessory roots. The apical growth of the shoot may be continued
indefinitely, with indefinite repetition of its several appendages ; or it may
itself be duplicated either by terminal or by lateral branchings, with or
without accessory roots. In fact, the whole vegetative system of the plant-
body, however complex, is built upon the simple shoot as the unit : its
apex, initiated in the first definition of polarity in the embryo, shows
continued apical growth with formation of an indefinite succession of
appendages : it may fork at its distal end : or new shoots may be initiated
below the apex: but still the whole plant-body is derived from the
extension or it may be the forking or repetition of that fundamental
unit — the shoot.
1The fact that these structural analogies exist cannot rightly be held to show any
common origin of those parts, unless examples of dichotomy can be brought forward in
which one limb develops as one type of part, the other as another type ; or unless a gradual
transition from dichotomy to monopodial branching, such as is seen in the branching of the
leaves of Ferns, smooths over the transition from branchings which produce parts of the
same category to those which produce those of different category. Such direct evidence can
easily be found indicating a common origin of rachis and pinna in the leaves of Ferns ; but
it has never yet been produced in support of the views of Potonie or of Tansley as to the
common origin of axis and leaf, already alluded to above (pp. 628, 630). All the evidence
adduced by them is indirect ; though the structural analogies are interesting, they carry little
weight against the positive fact that in all observed cases the leaf originates normally as a
lateral appendage of the axis.
CHAPTER XLIV.
THE VASCULAR SKELETON.
PASSING from the characters of external form to the internal arrangement
of tissues, the Vascular system provides by far the most constant
structural characters; and, as it is naturally the best preserved tissue in
the fossils, it gives a basis for comparison of both ancient and modern
Pteridophytes. But in dealing with anatomical facts it must be
remembered always that in any progressive evolution vascular structure
follows, and does not dictate external form : all the evidence which it
yields is necessarily ex post facto evidence. On the other hand, the
structural effect of a certain development may persist even after the
formal characters with which it was primarily bound up may have been
altered or even wholly removed. In fact, anatomical characters are apt
tardily to follow evolutionary progress, and to thereafter persist; they
possess what may be described as a sort of phyletic inertia.
It has already been shown in Chapter XV. that the prevalence of a
central stele in the axis of Vascular Plants is in direct accord with a
strobiloid theory of the primitive shoot : and that the strictly cauline origin
of the central region of the stele, and the insertion of the leaf-traces upon it
with but slight disturbance, as seen especially in the smaller-leaved forms,
are also features which harmonise with a strobiloid theory : the facts were
held to suggest a primitive condition in which the axis was the dominant
factor, and the appendages of subordinate importance. This position
receives additional support from the demonstration given above in
Chapter XLII., that the axis is the first part to be structurally defined
in the initiation of the embryo. But it will be necessary further to
show how far the Vascular structure of the larger-leaved types will accord
with a strobiloid origin. The leading anatomical facts required for this
are contained in the special descriptions of the several groups in Part II. ;
they may now be drawn together into a short collective statement.
By a general consensus of opinion, the non-medullated monostele is
recognised as the primitive stelar type, and it has been shown severally
686 CONCLUSION
in the case of Lycopodiales (p. 337), Equisetales (p. 391), Sphenophyllales
(p. 418), Ophioglossales (p. 464), and Filicales (p. 646), how the stelar
structure, however various, is uniformly referable in origin to the monostele :
for it is seen in the young plant either to show a solid xylem-core, or a
medullated state not far removed from that condition. The frequent
occurrence of a like structure even in the mature axis of the early fossils
has also been shown : and from such observations it becomes apparent
how fully justified the opinion is that for the various types of the
Pteridophytes the non-medullated monostele was the original vascular
structure in the axis.
It will probably be objected that in many of the Pteridophytes the
embryogeny does not bear this out; and that what is apparent, especially in
the larger-leaved types, is that the vascular tissue of the shoot is initiated
by a simple foliar strand, which descends from the first leaf continuously
to the root, and in fact that the axial system is in its origin little more
than a sympodium of leaf-traces. But before this objection is allowed to
have weight the condition in the smaller-leaved forms must be taken into
account, and the question examined as a whole rather than from one aspect
only. A comparison of those Lycopods, which are held to be relatively
primitive, shows that the cauline stele is initiated in the first stages of the
embryonic development ; this is seen with particular clearness in Fig.
190 c, D, E of Selaginella spinulosa, where the tissue formative of the
stele can be recognised as extending up to the broad apex of the axis
before any foliar strand is initiated. The same is the case in Lycopodium
Phlegmaria (Fig. 185 c, D) and L. annotinum^ and it is indicated also
in the imperfectly known embryology of L. Selago (Fig. 183). In
these plants the vascular condition from the very first establishment
of the embryonic shoot is the same as in the continued embryogeny
(compare Fig. 172, p. 331): the stele is essentially cauline, and the
foliar strands insert themselves upon its periphery. This appears to
be the normal condition of small-leaved forms ; according to our
hypothesis these are themselves primitive, and the result of a com-
parison of the embryogeny in the two types would be that in larger-leaved
forms the cotyledon bulks more largely at first; that the axis in the first
instance is correlatively reduced in size, and the cauline vascular core is
reduced with it. But, nevertheless, the examination of the embryogeny
has shown with constancy that the axis is pre-existent to all the other
parts of the embryo, though it may often be correlatively reduced, or its
development deferred where the cotyledon or the root is precociously
developed. The same view will hold also for the constituent tissues of
the axis, including the cauline vascular core. The condition where this
xylem-core is present is accordingly held to be the primitive state of the
embryo, that where it is reduced and even absent is held as the secondary
and derivative. But even in the latter cases, the stelar tissue asserts
1 Bruchmann, I.e., PI. 4, Fig. 17.
THE VASCULAR SKELETON 687
itself as the individual shoot develops : so that the absence of it in the
young embryo is only an apparent condition secondarily due to correlative
reduction.
A protostelic state will functionally serve only a limited vegetative
system. Starting from relatively small beginnings, as that system enlarges —
either by continued growth of the axis and multiplication of small leaves,
or by increase in size of a more limited number of larger leaves — the
size of the stele becomes proportionally increased : and this may be seen to
be the case either in the individual life, or it may be illustrated by com-
parison of different related species or genera. But there is a limit to the
size which a solid protostele may attain with functional advantage, and as
a matter of fact when large size is approached the protostelic character is
sacrificed, and amplification begins, which may take several distinct forms.
The simplest of these, as it is also the most general, is medullation. It
is illustrated in many of the dendroid Lycopods. While certain of the
early species of Lepidodendron have a solid protostele (L. rhodumnense),
Lepidodendron selaginoides (Fig. 176, p. 336) has the centre of its stele
composed of parenchyma and tracheides intermixed : others again, and
especially later species, show a parenchymatous medulla (L. Harcourtii,
Fig. 174), derived by conversion of the central region of the wood into
pith (Fig. 175). The result of a similar change is seen in Sigillaria, but
with a further progression to the breaking up of the ring of xylem sur-
rounding the pith into separate strands (p. 337). This condition is very
nearly approached in Lepidostrobus Brownii (Fig. 175), and finds an
interesting parallel also in the upper part of the shoot in Tmesipteris
(Fig. 234) : in the latter a sclerotic tissue takes the place of the pith in
the lower regions of the axis, but is replaced by thin-walled tissue above.
Such cases prepare the way for the view of the stelar structure adopted
above for Equisetum (pp. 386-392); the condition there seen appears to
be the result of carrying the medullation of the stele to an extreme.
Turning to the larger-leaved forms, the condition seen in the Ophio-
glossaceae (p. 459) may be referred in origin to a centroxylic protostele ;
it appears in fact as a medullated monostele with opening of the xylem
at departure of the leaf-traces. Lastly, the series of Osmundaceous fossils
described by Kidston and Gwynne-Vaughan (p. 539) shows most convincingly
how their vascular structure is also referable in first instance to the
medullation of a protostele, with ultimate breaking of continuity of the
xylem-ring. It is thus seen that in a number of Pteridophytes, and probably
along quite distinct phyletic lines, a progression may be traced from a
primitive protostele to a state of medullation, and in some cases even to
the disintegration of the remaining xylem-ring into distinct strands. This
progression may even be followed in the successive stages of the individual
life, which are accordingly held as further evidence of the phyletic story.
Another modification of the protostele, which probably has an importance
in interrupting the continuity of an enlarging mass of xylem, is seen in
688 CONCLUSION
the modern Lycopods, but it is quite different in origin from medullation.
Intrusive bands of phloem invade somewhat irregularly the central xylem,
giving it sometimes the form of a fluted column, or of a series of
plates connected at intervals, or of a continuous xylem-sponge (Fig. 171,
p. 329). Such conditions, which are characteristic of modern Lycopods,
are probably secondary derivatives of the simple protostele, since they are
absent in the early fossils, as well as in the early condition of the plants
that show them when adult.
A somewhat similar intrusion of tissues from without leads, in many
Ferns, to the condition which is described as the solenostelic. But here
it is regularly at the point just above the exit of the foliar strands from the
stele that the intrusive tissues enter; it thus comes about that phloem
and endodermis and ground parenchyma come to occupy continuously
the centre of the stele, which accordingly takes the form of a hollow
tube, with openings opposite each leaf-base (Figs. 95, 97, 100). This
formation of a solenostele has probably occurred along more than one
phyletic line, and it lies at the base of those complex types of dictyostelic
structure of the stem seen in Leptosporangiate Ferns. These follow upon
the overlapping of the foliar gaps, which results in dictyostely formerly
described as a polystelic state (p. 190). A similar condition in some
species of Selagindla, though phyletically quite distinct, shows interesting
analogies ; but its origin appears to be in relation to the departure of
supplies to axes, not to leaves; these are, however, referable also by origin
to a primitive monostelic structure.
Still further complications occur in certain Ferns which are associated
with the formation of accessory vascular tracts ; these arise in relation to the
foliar gaps as described on pp. 568, 600, and lead to a doubling or even
trebling of the solenostele (Figs. 319, 342), or accessory strands may arise
in pith or cortex (Fig. 339). The condition of the modern Marattiaceae
and of the fossil Psaronius may also be mentioned as extreme cases of
complexity of vascular structure based probably on a scheme allied to
those above noted (p. 525). Into these details it is not necessary to enter
further here, they concern us chiefly as illustrating some of the extreme
methods of amplification of the vascular system seen in the axes of
Pteridophytes.
In some degree parallel with this progressive dilatation and disintegration
of the stele goes also the disintegration of the foliar trace. In all the
smaller-leaved, and in many of the larger-leaved forms, the leaf-trace consists
of a single strand ; in the Lycopodiales this is uniformly so, with exception
of certain Sigillarias described by Kidston.1 It is a single strand also in
Isoetes? and in the Equisetales. In the Sphenophyllales and Ophio-
glossales (except § Ophioderma, and perhaps § Cheiroglossa), the leaf-trace
comes off always as a single strand, but branches frequently while still
1 Proc. Roy. Soc., Edin., vol. xxvii., part iii., p. 203.
z Studies, ii., Fig. 105.
THE VASCULAR SKELETON 689
within the cortex (Cheirostrobus), giving sometimes a median bundle
(Ophioglossuni), sometimes a paired trace (Botrychium). All the more
primitive types of Ferns, including the fossil Psaronius, have a single
more or less horseshoe-shaped trace ; but the modern Marattiaceae and
the bulk of the Polypodiaceous Ferns have a trace composed of many
strands : these are, however, arranged in series corresponding to the
horseshoe outline of the undivided trace. The facts indicate with no
possible uncertainty that there has been a disintegration of the leaf-trace
by fission : it finds its origin in branching of the strands in an enlarged
upper region of the leaf, and has been phyletically progressive from a region
lying above towards the base. A comparison of Fig. 97 will make this clear :
leaf-traces are there shown each of which consists at the base of a broad
strap-shaped strand : this breaks up distally into numerous strands. But in
Cyathea, which is structurally a more advanced type, the breaking up has
been continued down to the base, and the leaf-trace comes off initially as
numerous separate strands (Fig. 337). The same has probably happened
in the modern Marattiaceae as compared with Psaronius; in most Mixtae
as compared with the Gradatae (p. 648), and in the section Ophioderma as
compared with Euophioglossum (p. 462). Thus in several distinct phyla
it is shown that a progressive disintegration of the leaf-trace has been
effective; and goes always with leaf-enlargement just as disintegration of
the axial stele has followed on expansion of the axis. But in both cases
the enlargement has phyletically preceded the consequent disintegration.1
The present interest in these complex structures in axis and leaf-stalk
does not lie in their detailed study, so much as in the fact that in all
cases they appear only in the plant when advanced towards full develop-
ment In the young seedling a stelar structure, little removed from or, in
most cases, actually showing a protostelic state, is constantly found; and
from it the various steps may be traced to the more complex condition.
So far as the development of the individual can be held to reflect the
1 In certain Pteridosperms and Gymnosperms a double leaf-trace has been found to be
prevalent, and it has been suggested that it finds its origin in the bifurcation of the leaf.
Arguments based on the existence of a double leaf-trace have recently been extended to
Flowering Plants (Miss Thomas/^V^w Phytologist, 1907, p. 77). It is not proposed here to
criticise those arguments, but merely to point out from the side of the Pteridophyta that
there is no constant relation between foliar dichotomy and a double leaf-trace. In SigiHaria,
Kidston (Proc. R.S., Edin., vol. xxvii., p. 203) has shown that the double leaf-trace,
already recognised by Renault, exists in a leaf of simple-form ; on the other hand, the
bifurcate sporophyll of Tmesipteris has only a simple leaf-trace. In the Ophioglossaceae,
Euophioglossum and Helminthostachys have a simple leaf-trace, which soon branches,
Botrychium has a double leaf-tree, Ophioderma a trace of several strands, not arranged
in any binary scheme (Ann. of Bot., xix., PI. xv., Figs. 6-29). Lastly, in many primitive -
Ferns, where dichotomous and other branching of the leaf is prevalent, the leaf-trace is a
single strand. Such facts suggest the propriety of extreme caution in applying arguments
based on the vascular structure at the base of the leaf. It would seem not improbable that
a double leaf-trace might appear in any broad flattened organ which is bilaterally sym-
metrical, whether branched or not. This may very well have been the case in Sigillaria.
2 X
690 CONCLUSION
evolution of the race, the evidence is quite clear : it indicates that the
large-leaved forms, in which solenostelic or dictyostelic structure rules,
originated from a smaller-leaved ancestry, with protostelic structure and a
single strand of the leaf-trace. This is in full accord with probability,
according to the antithetic theory of origin of the leafy sporophyte ; for on
that theory smaller-leaved would necessarily have preceded larger-leaved
types.1
Another mode of amplification of the stele, which often accompanies
the first but is not necessarily associated with it, is by secondary thickening.
The stem of Sphenophyllum (Fig. 217), and of Ltpidodendron Petticurensis?
are examples of how a secondary development of vascular tissue may
surround a solid protostele : this shows that medullation does not neces-
sarily precede secondary thickening, but commonly the secondary thickening
occurs where medullation is present : and indeed in some cases the two
are in a sense complimentary, the secondary vascular tissue taking the
place functionally of the primary tissue reduced by medullation ; this is
exemplified in the Calamarians (Fig. 225) and in Sigillaria? as also in
some forms of Stigmariaf and it is seen with special clearness in Lygino-
dendron, Poroxylon, etc. In other types structurally more advanced, the
secondary development may be held to have completely replaced the
centripetal wood of the original stele.
The distribution of secondary vascular development among the Pteri-
dophyta indicates clearly that it is a phyletic afterthought, originated in
relation to the increasing size of the vegetative system consequent upon
continued apical growth, repeated branching, and leaf-enlargement, either
separate or in combination. Enlargement of the primary stele, with or
without attendant medullation, may meet the demand in some degree ;
but it is a fixed and limited scheme compared with that of secondary
thickening, which is capable of increasing the conducting tract in proportion
to the demand. In some cases, however, it appears that a phyletic decrease
of the secondary development has occurred, and it is probable that the
feeble cambial activity in the nodes of Equisetum, and locally in the
Psilotaceae may be vestigial remains of a more active increase in their
predecessors, allied respectively to the ancient Calamarians and Sphenophylls.
1 This is, however, quite contrary to the opinions of Dr. Jeffrey, who holds that the
large-leaved and small-leaved stocks were "separate back to the beginning of the period
when the palaeontological record begins." This view would recognise no transition from
the structure characteristic of the smaller-leaved forms (cladosiphonic) to that characteristic
of the larger-leaved (phyllosiphonic). But, as a matter of fact, this can be demonstrated
to have occurred in the individual life of Ferns, and probably it has occurred also in other
forms in the passage from small-leaved youth to large-leaved maturity. It has been
pointed out repeatedly in Part II. how cladosiphony is the anatomical expression of the
dominance of axis, phyllosiphony that of the leaf in the shoot : and the balance may
be altered in the individual life. (See Jeffrey, Phil. Trans., 1902, vol. 195, p. 144.)
2Kidston, Proc. Roy. Soc., Edin., 1906-7, p. 208.
3 Scott, Studies, Figs. 77-78. 4/£., p. 234.
THE VASCULAR SKELETON 691
It is naturally the primary developments, however, which are of im-
portance in the present comparisons : and sufficient has been said to show
that the anatomical evidence, combined with that from embryology, has
a very direct bearing on the theory of the strobilus. The uniform reference
of the stelar structure to a protostele, and the actual existence of this
structure in the young seedlings of the most diverse types, points clearly
to its early existence in the race. Its continuity up to the apex of the
axis in the more primitive of the living, small-leaved types is a further
fact of importance : while the attachment of the foliar traces to the outer
surface of the cauline core indicates not only the priority of the latter,
but also the subsidiary character of the former. Lastly, the correlative
reduction of the axis in the embryos of the larger-leaved forms, consequent on
their precocious development of the first leaf accounts on well-known prin-
ciples for their structure : it explains the fact that in them the evidence of
early existence of the cauline core is not so prominent as it is in the
smaller-leaved forms, which are on our hypothesis the nearer to an
original type. The general conclusion from comparative study of the
vascular skeleton, combined with the facts of embryogeny, will therefore
be that it supports the priority of the axis over the leaf: it shows that
the axis was from the first traversed by a conducting core, upon which
the conducting strands of the leaves became attached. But that both the
stele and the leaf-trace were susceptible of amplification and disintegra-
tion as a consequence of the enlargement of axis and leaf, and of the
increasing proportional influence of the latter : in fact, the leaf in certain
forms became at last the dominating feature of the shoot, and conse-
quently its influence also controlled the internal vascular structure of the
whole shoot. This condition, which is that characteristic of those forms
which have been designated " phyllosiphonic," is believed to have been
of secondary origin, and the structural progress shown in the individual
life would appear to indicate with special clearness that it was so.
CHAPTER XLV.
THE SPORE-PRODUCING MEMBERS.
So far only the vegetative organs have been considered in this summary
of results ; the reason for this is that they appear the first in the individual
life of Vascular Plants, and it is only after the vegetative system of the
sporophyte has been established that spore-production supervenes. The
relation of the sterile to the fertile region from the point of view of
descent has, however, been discussed at length in Chapter XIII. : the
conclusion was there reached that in vascular plants the sterile
tract, which is prior in the individual life, is itself from the evolutionary
point of view, the consequence of a secondary change, since the foliage
leaves are themselves held to be sterilised sporophylls. In Chapter XIV.
it was further concluded (p. 186) that there existed initially only one
type of leaf — the sporophyll, and that even the protophylls are the
result of their transformation. Moreover, justification for this is found
in the positive fact that spore-production occurs very early in certain
plants (Ophioglossaceae and some Lycopods), while in Lygodium subalatum
the extreme condition was actually observed by Prantl, viz. that the
primordial leaves are themselves fertile sporophylls. With these facts, and
this general conclusion before us, we may now proceed to consider the
morphology of the spore-producing members and their relation to the
other parts of the shoot.
On an antithetic theory of origin of the sporophyte we contemplate
an initial condition of a simple body having a coherent group of spore-
mother-cells, provided, in fact, with a simple spore-sac. The Bryophytes,
with their concrete archesporium, retain this state even in their more
advanced forms ; but the Vascular Plants, with their discrete sporangia,
have diverged from it very widely. The two types of construction are
not connected by any living intermediate links, nor is there any direct
proof that the one type is phyletically related to the other. But both
provide evidence suggestive of how a segregation of spore-mother-cells
into distinct sporogenous masses, such as appear in the separate sporangia,
THE SPORE-PRODUCING MEMBERS 693
may have come about. In Chapter VII., which deals with sterilisation,
examples have been brought forward showing how widespread is the
conversion of individual cells, and even tracts of tissue from the fertile
to the sterile state, and that in some cases septation of spore sacs has
actually been the result. It was concluded (p. 102) 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. Further,
in Chapter VIII. (p. 112) it was shown that commonly the archesporium
of Vascular Plants is not strictly circumscribed, but that the sporogenous
groups have often ragged edges : this suggests on the basis of structure
that each fertile tract is a residuum left by advancing sterilisation; in
fact, the sporangia may in the simpler cases be regarded as islands of
fertile tissue which have retained their spore-producing character. In
Chapter XL (p. 140), on the theory of the strobilus, it was shown how
the disposition of the parts in some of the simplest Pteridophytes suggests
as a prototype, prevalent though perhaps not general, an upright, radial,
strobiloid structure, consisting of a predominant axis showing continued
apical growth, and bearing relatively small and simple appendages formed
from it by enation. Associated with these are sporangia each containing
as its essential feature an island of fertile tissue. It is impossible to
bring proof how a simple strobilus such as this actually originated ; but
it can be claimed that all the structural and developmental facts
described in Part II. accord readily with a theory of origin by septation
from a continuous spore-sac and enation of appendages. So also is
physiological probability, for the sporangial types are better fitted for the
mechanical protection, the nutrition, and the dispersal of the numerous
spores than those with the non-septate sac : and in homosporous forms,
which all the most primitive types were, the larger the number of germs
the greater the probability of survival and of spread.
Passing, however, from such hypotheses, which are not susceptible of
actual proof under present conditions, to matters of direct observation, a com-
parison of the fertile shoots of all the known homosporous Pteridophytes
shows them to be composed of three constituent parts : (i) the axis, which
the embryological comparison as well as the facts of development in
the growing shoot have shown to be the pre-existing part; (ii) the bracts
or sporophyllS) which are appendages produced by outgrowth from the pre-
existent axis ; and (iii) the spore-producing-members, under which general
term are included sporangia and sporangiophores, with their phyletic products.
These may be inserted either on the axis or on the sporophyll. It is
believed that (ii) and (iii), though they show commonly a local relation to
one another, have actually been distinct organs throughout descent : neither
has been the result of metamorphosis of the other. In further support of
this it will be shown that they do not bear any obligatory relation one to
another : either may exist without the other : while either may show fission
independently of the other, though in some forms both are alike in this.
694
CONCLUSION
In point of the positions which they hold on the shoot the two types of
spore-producing members, the sporangia and sporangiophores, show some
degree of uniformity : in either case the insertion may be directly upon
the axis or in the axil of the sporophyll, or they may be inserted further
outwards upon the surface of the sporophyll. Leaving for the moment
the Ferns on one side, illustrations may be taken from the strobiloid
types. As regards the simple sporangia, these may originate from the
axis quite apart from the subtending leaf, as in Selaginella (Fig. 360 A) : in
Lycopodium the sporangium arises from the upper surface of the leaf close to
its base (Fig. 360 B, c) ; but in Spejicerites it is borne at a point far removed
from the leaf-base, though otherwise in accordance with the Lycopod-type
(Fig. 360 D). Similarly with the sporangiophores, the insertion may be
FIG. 360.
Diagrams illustrating the varying positions respectively, of sporangia (upper row) and
of sporangiophores (lower row). It is seen that a parallelism exists. For details
see Text.
on the axis or on the sporophyll, but they show rather more latitude of
detail : thus in Equisetum and in the ancient Archaeocalamites they are
seated upon the axis, showing no association with bract-leaves (Fig. 360 E) :
or in Calamostachys whorls of bracts may intervene between the successive
whorls of sporangiophores, but without individually subtending them (Fig.
360 F) : or the sporangiophore may, possibly by a secondary displacement, be
apparently axillary, as in Palaeostachya (Fig. 360 G) : or, again, the sporangio-
phore may arise from the upper surface of a sporophyll, in which case it
necessarily has a subtending position, as in the Psilotaceae (Fig. 3601):
a similar position is seen in Sphenophyllum majus (Fig. 360 H), but
in other species of the genus there are more complex arrangements
probably resulting from fission.1 From such examples as these it appears
1 Scott mentions a species (S. emarginatum) which appears to have borne its sporangio-
phores separately, so that they "have left their own distinct scars on the axis above
the bracteal node" (Progresstis, i. p. 153). This arrangement closely approaches that
of Palaeostachya, or of Calamostachys.
THE SPORE-PRODUCING MEMBERS
695
that the spore-producing members, whether sporangia or sporangiophores,
have been susceptible of considerable differences of position in the radial
plane, and that in this the sporangia show a parallelism with the sporangio-
phores which it is important to recognise in parts which are identical in
their function.
The position of the leaves relatively to the spore-producing members
in strobiloid forms is usually, but not constantly, a subtending one : there
is reason to believe that a constant relation was a usual condition in
primitive forms, while the exceptions may be held to be secondary in their
origin. In the Lycopodiales the subtending relation of leaf to sporangium is
FIG. 361.
Diagrams illustrating the relation of spore-producing members and sporophylls or
bracts, and the results of their respective fissions, as seen in surface view from the
adaxial side. A=Lycopodium. B = Isoetes. C ~ Lepidostrobus. The dots in B and C
show the trabeculae. D^Tmesjfiteris. E = Psilotum. P'=Palaeostachya. G = Calamo-
stachys Binneyana. H=Calamostachys germanica. I = Sphenophylhwt ntaj'us.
J=Sphenopliyllnin Daivsoni. K=Cheirostrobus. L = Ophioglossum. M= abnormal
case of Ophioglossum (see Fig. 359 j, K). N^Helminthostachys.
constant (Fig. 361 A, B, c), as it is also in those of Sphenophyllales wherever
there is a single sporangiophore to each bract-leaf (Fig. 361 D, E. i), but
it is departed from in those cases where more than one is associated
with each bract (S. Dawsoni, Romeri, p. 402, Fig. 361 j). The con-
dition seen in Cheirostrobus strongly suggests chorisis of both bract and
sporangiophore (Fig. 361 K), and their mode of insertion upon the bract-
whorl in other species of Sphenophyllum above quoted indicates it as
probable that some similar chorisis of the sporangiophores has been
effective in them also (Fig. 361 j). In the Equisetales the relation is
generally less exact : it seems still uncertain whether or not the sporangio-
phores were actually subtended by the bracts in Palaeostachya (Fig.
361 F):1 in C alamo stachys they may be somewhat irregularly subtended by
aSee Hickling, I.e., pp. 372, 377.
696 CONCLUSION
two (Fig. 361 G), or by three bract-leaves (Fig. 361 H) : or in Archaeo-
calamites and Equisetum the sporangiophores may be borne quite inde-
pendently of any bracts. It thus appears that the most usual condition
is clearly that where the bract subtends the spore-producing member,
whether sporangium or sporangiophore ; but this arrangement is liable to
be disturbed by chorisis of either bract or sporangiophore, or by the
entire absence of the bract.
These presumable fissions, which comparison indicates as having
occurred in both sterile and fertile parts, deserve attention : it appears
that they may affect either class of parts separately or both together.
In the simple condition of the Lycopodiales fissions of the appendages
are very rare ; but examples have been recorded where two sporangia
appear side by side in Lycopodium?- and an occasional case of a bifid
protophyll has been observed in the young plant of Lycopodium and
of Phylloglossum ; 2 but these characters have never become permanent
for any race of Lycopods. In the Equisetales the result of fission is seen
frequently in the bracts (Fig. 361 G, H), though not in the sporangiophores
of the ancient types ; but in the modern genus Equisetum fission of
sporangiophores appears to have been effective. An examination of the
very numerous sporangiophores of Equisetum maximum shows frequent
cohesion of their stalks, while a comparison of the simpler species, such
as E. palustre and of the Calamarians, leaves little doubt that with
enlargement fission of the appendages has occurred (Fig. 195). Forking
is a marked feature of the leaves in the Sphenophyllales (Fig. 361 i), but
not necessarily accompanied by fission of the sporangiophore. In some
forms, but not in all, there is, however, such a collocation of the sporangio-
phores, in number and position as well as in vascular connection, as would
indicate that an increase by fission has occurred to produce them : but
this may occur independently of any fission of the bract (Fig. 361 j). In the
very complex cone of Cheirostrobus it is highly probable that fission has
been effective in both parts, as the vascular connections appear to indicate
(Fig. 361 K). Lastly, the branching of leaf and spike, described at length
for the Ophioglossaceae (pp. 435-439), can best be understood as the
consequence of similar progressive fissions (Fig. 361 L, M, N). It thus
appears that fission has probably been a frequent feature in producing
the condition of the appendages in the strobili actually observed in the
more complex sporangiophoric types, and that such fission may occur
independently in either sporophylls or spore-producing members, or
coincidently in both. On the other hand, the condition usual in the
Lycopods may be regarded as a type which has remained on the simpler
basis without fission.3
^•Annals of Botany, vol. xvii., p. 278.
2 Treub, Ann. Jard. Suit., vol. viii., taf. v., fig. 2 A.
3 By the term "fission," as here used, is to be understood a chorisis which dates
from the initiation of the primordium : the fission is . not a branching of a part which is
THE SPORE-PRODUCING MEMBERS 697
The recognition of the spore-producing members as a category of
parts, probably distinct in origin from the bracts, though often supported
on them, having a uniform function, and showing, whether as simple
sporangia or as sporangiophores, similarities of position, raises the question
whether the two types of spore-producing members are genetically connected ;
it is necessary to enquire whether there is any structural indication of an
evolutionary progression having taken place from the simple sporangial
sac to a septate state, and thus of the origination of the stalked sporangio-
phore with vascular supply from the single sporangium. In the first
instance it is to be recognised that such a progression cannot rightly
be negatived on d priori grounds ; for - it has been shown that septation
of sporangia has occurred in well-authenticated cases (p. 120), while
biological probability would favour such amplification in homosporous
forms (p. in). The structural evidence showing that septation has Igken
place may be derived both from the septate and from the non-septate forms,
but no consecutive demonstration is to be obtained from comparison of the
representatives of any one phylum. On the one hand the occurrence of
sterile cells and tissue-tracts has been described at length in simple
sporangia, and it is specially worthy of note that it is in the largest
of them (Isoetes, p. 318, Lepidostrobus, p. 323) that the nearest approach
to a septate state is found : in the megasporangium of Isoetes the
sporangium is technically septate, for each spore-mother-cell may be
completely partitioned off by tracts of sterile tissue (Fig. 320). Such a
condition, which only appears relatively late in the individual development
of Isoetes, is comparable with that of a young synangium of Equisetum
or of Kaulfussia^ inasmuch as in these also the archesporial cells are
found isolated in sterile tissue (Fig. 206 A) : the fact that the condi-
tion of isolation is seen earlier in the individual development of these
sporangiophores is in complete accord with their greater morphological
advance : a less advanced state is, however, seen in Tmesipteris (Fig. 230 B),
in which the septum and sporogenous groups are at first indistinguishable
from one another, but differentiate after the tissue has attained a consider-
able bulk. If the individual development be rightly held as an indication
of the evolutionary progression in the race, then the sporangiophore
in the cases quoted would find its evolutionary prototype in larger
non-septate sporangia, such as those seen in the Lycopods, from which
the condition in Tmesipteris would be less far advanced than that of
Equisetum or of Kaulfussia. Such a comparison comes with special
force in those cases where, as in the Psilotaceae and Sphenophylls, the
position of the sporangiophore is identical with that of the Lycopod
sporangium.
already existent in the individual, but the substitution of two related centres of initiation
in place of one, while their near proximity may lead to a more or less common
upgrowth with consequent cohesion at the base.
1 Studies, iii. pi. viii., and Fig. 37.
698 CONCLUSION
But the objection may be raised that the vascular supply has also to
be accounted for. It is a general experience in the plant-body that vascular
development follows demand: and many examples might be quoted both
from vegetative and propagative organs. It appears that similarly a
vascular supply extended into the synangium ; a first indication of such a
development is seen occasionally in the sporangia of Lycopodium (Fig. 161),
while it is a common feature in the megasporangia of Seed-Plants. Thus
any objection to a theory of origin of the sporangiophore by a process of
septation and outgrowth on the ground of the presence of vascular tissue
does not appear to be valid. Moreover, such vascular extension is seen
in less full development in those sporangiophores where the sporangia are
obliquely erect and synangial, as in the Psilotaceae, Kaulfussia^ and
Ptychocarpus (Fig. 288), but further developed where they are inverted
and separate, as in the Equisetales. It has already been argued that the
former ane the less advanced, and those with separate and inverted
sporangia the more advanced types (pp. 426-7).
It is thus seen that there is coincidence between sporangia and
sporangiophores in their leading function of spore-production : that there is
commonly a similarity of position of the two : that either may undergo fission
independently of the subtending bract, that in . certain sporangia there are
indications of partial septation, and occasionally a technically complete
septation : also that the facts of development of the synangial sporangio-
phores harmonise in varying degree with a theory of origin from a non-
septate sporangial sac. The conclusion therefore seems justified that they
are essentially comparable parts, the one being the simpler, the other the
more complex terms of a category of phyletically uniform organs.1 That
the non-septate sporangium was the more primitive there can be little
doubt. So far as palaeontological evidence bears upon the question,
Lycopodinous types with their non-septate sporangia appear to have been
fully as early as any of the more elaborate forms.
Turning now to the Ferns, which had been temporarily put aside while
discussing the strobiloid types. It has been accepted as probable that the
soral condition was the original state in Ferns, and the non-soral
derivative (p. 633), while it was left an open question whether the sori
were originally marginal or superficial in their position upon the
sporophyll (p. 634). It has also been pointed out how close the structural
similarity is between certain synangial sori and the sporangiophores of the
smaller-leaved types (pp. 151, 524). It may have been the fact that this
striking similarity was a result of parallel development, but still it would
appear probable that the evolutionary progressions which produced them
were of a like kind. There is ample evidence also of fission of sori in
Ferns (pp. 511, 555, 620), essentially like that of the sporangiophores of
the strobiloid types. It would therefore appear probable that the condition
1 The designation of sporangiophores -as ventral or other lobes of the sporophyll has
been objected to on a previous page, and reasons given for its rejection (p. 426).
THE SPORE-PRODUCING MEMBERS 699
seen in Ferns is similar in kind to that of strobiloid types, but modified
in accordance with the great amplification of the sporophyll, with its
continued apical growth and often profuse branching : this was accom-
panied by increase in number of the sporangiophores (sori), fission being
one prominent source of that increase, and also by a tendency for the
sorus to diffuse itself as scattered sporangia over the enlarged surface,
producing thus the non-soral state as a secondary condition : moreover,
the position of the sori shows frequent tendency towards the lower leaf-
surface. From this point of view the Fern-type does not stand apart
from the rest in the essentials of its morphology, but only in the fact
that it has proceeded to a larger-leaved state, and that this has brought
with it secondary changes of the spore-producing members.
There is a considerable bulk of evidence to show that, apart from
fissions, the sorus or sporangiophore has also been capable of extension
in the course of descent : this is to be found in certain points of structure
which have not been satisfactorily accounted for on any other footing.
It has been noted that in the fossil Equisetales the number of sporangia
on each sporangiophore is commonly four (p. 425), but that modern
Equiseta have usually more. In the Psilotaceae and Sphenophylleae the
number may be from one to six, the lowest number being probably in
some cases due to reduction : thus fluctuating numbers are a common
feature in the simpler types. In the Ophioglossaceae the fluctuations are
within wider limits, and the larger numbers of sporangia are associated
with an apical growth of the sporangiophore, which is either of very short
duration or entirely absent in other cases. The result is in Ophioglossum
the elongated spike, with its lateral rows of sporangia partitioned some-
times imperfectly from one another (Fig. 361 L). The structure bespeaks
a progressive condition in which septation has played a leading part
(p. 404). In Botrychium, profuse branching parallel to that of the
sterile leaf, occurred, and it is very closely related with septation of the
individually projecting sporangia (p. 454) ; lastly, in Helminthostachys
the rows of sporangia of Ophioglossum are replaced by dense ranks of
sporangiophores (pp. 455, 485), and their origin is believed to have been
virtually a repetition of that process of septation and upgrowth above
recognised in the origin of the sporangiophore from a simple sporangium
(Fig. 361 N). All these amplifications of the sporangiophore are consistent
with physiological probability, as shown in Chapter XXXI.
In the Ferns also similar extension of the sporangiophore (or sorus)
is seen, but it has taken a different form in accordance with the expansion
of the leaf-surface to which it remains attached. It is exemplified in the
simplest form in the Marattiaceae, in which the structural condition of
Danaea seems plainly to be the result of elongation and progressive septation
of a sorus of the same type as that of Marattia (Fig. 278 c, E) ; the
partial septations are themselves specially convincing evidence of how the
highly septate state has been acquired (p. 518). The progression has
700 CONCLUSION
been similar to, though phyletically quite independent of, that in Ophio-
glossum j and the results are, in the former an elongated sorus attached
to the leaf-surface, in the latter an elongated sporangiophore which is
attached to the sporophyll only at its base. In many other Ferns there
is evidence of amplification of the sori, whether by intercalary elongation
of the receptacle and a basipetal succession of sporangia, as in the Gradatae,
or by marginal extension, as in the Lindsay a- Pteris series, or by superficial
spread so as to produce the conditions seen in Gymnogramme, Acrostichum,
or Platycerium : associated with these is the profuse interpolation of new
sporangia characteristic of the Mixtae. It is thus possible to picture how
even the most complex and divergent types of spore-production in large-
leaved forms may be referred back in their ultimate origin to elementary
types, and to recognise how they conform to that general scheme of
construction which obtains among the simpler strobiloid Pteridophytes.
It remains to consider the distribution of the spore-producing members
on the plant as a whole. We have recognised the shoot or primitive
strobilus as composed of (i) axis, (ii) leaves or bracts, and (iii) spore-
producing members. It has also been seen to be probable that originally
all the leaves were sporophylls. The primitive shoot appears to have been
a general-purposes shoot, in which vegetative and propagative regions were
not segregated. But it is evident that two other conditions are possible,
that is a shoot bearing (ii) alone, and one bearing (iii) alone ; both of
these states are known in living forms, and both may be held to be
secondary and derivative.
The former case, where leaves without spore-producing members are
present, is by far the commoner condition of the two, and it appears
in the early stage of the ontogeny in almost all Vascular Plants. But it
also appears in successive intermediate zones higher up in various plants,
and notably in Lycopodium Selago, from which it is called the " Selago "
condition (Frontispiece) (Chapter XIII.). It has been shown that this
condition would result from abortion of the spore-producing members, and
the fact that this has taken place is clearly indicated by the occurrence
of imperfect sporangia about the limits of the region which has remained
fertile (p. 162). The converse evidence, that in certain cases (Z. Selago,
Botrychium^ and Ophioglossum) the spore-producing members appear very
early in the individual life, and that in Lygodium subalatum the very
first leaf may be fertile, further strengthens the view that the whole plant
was originally fertile (p. 186), and that the sterile regions, whether basal or
intermediate, are so by abortion of the spore-producing members.
The second case above mentioned, in which spore-producing members
are present but no leaves, is less common ; it is seen in Archaeocalamites
and in the modern Equisetum. It has been argued at length above
(pp. 382-4, and p. 429) that the leaves and sporangiophores in these
plants are parts of distinct nature and origin, and that the condition of
their strobili is due to abortion of the leaves, of which in Equisetum the
THE SPORE-PRODUCING MEMBERS 701
annulus is the last representative. A somewhat similar condition appears
in Ophioglossum simplex, where the spike is present, but the subtending
leaf absent (p. 441); in both cases the structure seen appears to be based
upon the persistence of the sporangiophore, while the leaf is abortive— in
fact, the converse of the process which brings the "Selago" condition
into existence.
There remain, however, certain instances where the distinction between
the leaf and the spore-producing members appears to break down, and
middle forms appear with the characters of leaves bearing sporangia ; the
annulus of Equisetum sometimes bears sporangia, grouped as upon mal-
formed sporangiophores (p. 382) ; or sporangia may appear upon the sterile
leaf of Botrychium (Fig. 242, p. 443) ; or, as in Sphenophyllum fertile
(p. 404), the bract bears sporangia as well as the sporangiophore which
it subtends. I do not think that these occasional exceptions suffice to
prove that leaves and sporangiophores belong to the same category of
parts, any more than the substitution of a foliage leaf for an ovule, in
certain well-known cases, proves that the ovule is really an organ of the
same category as the leaf. What they really appear to show is, that in
certain cases a primordium is not always of clearly defined character at
its initiation, and consequently that the characters pertaining to members
of distinct category may occasionally be intermingled. Accordingly, not-
withstanding the exceptions quoted, the distinction of leaves and sporangio-
phores may be upheld for the early forms of Pteridophytes.
It thus appears that the whole plant-body, as seen in the simpler
Vascular Plants, is referable to the simple shoot or strobilus, of radial
construction, as a unit ; that it consisted, in its most primitive form, of
an unbranched axis, simple leaves, and unilocular spore-producing members,
all of which were distinct in their character and in their phyletic origin,
and none the result of metamorphosis of another part; that the whole
plant-body of the known Pteridophytes may be regarded as derived from
some such simple source, by continued apical growth, and terminal and
adventitious branching of the axis, and by branchings and fissions of the
appendages; by adoption of a dorsiventral in place of the primitive radial
habit ; by abortion of certain of the spore-producing members, which
differentiated the sterile regions from those which remained fertile ; and
in these sometimes by abortion of the leaves themselves, thus leaving the
spore-producing members as the sole appendages. Such an origin is fully
in accord with the details of individual development ; for the ontogeny
often demonstrates those very progressions from the simpler to the most
complex state which the phyletic development of the more elaborate forms
from so simple a source would require
Combining the results which thus follow from the detailed examination
of Vascular Plants with the conclusions from comparison of the Bryophytes,
there appears to be very strong support for our general theory of
origin of the sporophyte, as the essential constituent of the Flora of the
702 CONCLUSION
Land. The theory presupposes in the first instance post-sexual complica-
tions involving reduction : by deferring that event through sterilisation
of individual cells, a neutral cell-group is established : this shows con-
tinued growth, and further progressive sterilisation as it is seen exemplified
in the Bryophytes, and leading to their complete state with a vegetative
system of considerable extent and a concrete fertile tissue. Such
sterilisation of individual cells or cell-groups is also seen among
Vascular Plants, and has been in them a source of vegetative increase.
But in them, with their discrete sporangia another, and a more effective
factor arises, viz., the abortion of whole sporangia and sporangiophores.
This leads in a more rapid and wholesale fashion towards the same
end, viz., the establishment of a vegetative system, by separation of
the function of nutrition from that of propagation in a shoot primitively
constructed to serve both purposes. Such an early state is seen in
every plant which shows the "Selago" condition; it has been shown
above that this exists in more or less obvious form among the representatives
of all the main phyla of Vascular Plants : and that it figures among their
early fossil forms. There is less certainty about the earlier steps of origin
of the sporophyte in the poly-sporangiate type, and analogy with the
Bryophytes has to serve in place of more direct observation. But the
later steps, by abortion of spore-producing parts, are more secure, even
though the observations are frequently of the negative fact established by
comparison, viz., that certain parts are not present, having been com-
pletely obliterated, so that not even a vestige remains to show what has
happened.
In the nature of things this theory of the origin of the sporophyte,
and of its establishment as the leading factor in the Flora of the Land,
is not susceptible of direct or full proof under present conditions. But
it offers a coherent account of how the sporophyte may have arisen : it
is based on a wide comparative study of known forms from the point
of view of their individual development, their external morphology, their
anatomy, spore-producing members, and embryology : it does not assume
wide-spread reduction, nor does it postulate any imaginary types, but
proceeds by comparison of those forms of which there is evidence
actually existing either in the living or the fossil state. On these
grounds the theory is put forward with some degree of confidence, though
in the full knowledge that it has not been, and indeed that it cannot
be, proved.
CHAPTER XLVI.
HETEROSPORY AND THE SEED-HABIT.
THE theme of this book has been the origin of a Land-Flora, not the
examination of its ultimate developments : and accordingly the detailed
study has related to the homosporous Archegoniatae, with only occasional
allusion to those which are heterosporous, and hardly any to the Seed-
bearing Plants. The reason for this lies in the high degree of certainty
that the homosporous state was the pre-existent, and the heterosporous
the derivative condition from it : any study of origins will therefore relate
primarily to the former. But the upward evolution of Vascular Plants has
been intimately connected with the differentiation of the spores according
to sex, and the establishment of the Seed-Habit, changes which have
brought with them biological advantages conducing to increased precision
in the establishment of new individuals. The earlier step was the
introduction of heterospory, which results only in minor reflex effects on
the parent : the later adoption of the Seed-Habit has in certain cases
been followed by a profound modification not only of the immediate
spore -producing members themselves, but also of the parts which bear
them. It will be necessary then to, compare the condition of homosporous,
heterosporous, and Seed-Bearing Plants, especially with respect to questions
of amplification and reduction, such as have been treated of in Chapter XIX.
It was there concluded (p. 241) that the production of the largest number
of effective germs was the end of all development of the sporophyte : that
any increase in their number involves amplification not only of the
propagative system but also of the nutritive : and consequently, other
things being equal, there is a probability that homosporous plants as a
whole should illustrate lines of amplification rather than lines of reduction.
While admitting that reduction may occur in homosporous forms, the
homosporous types have for the most part been recognised as constituting
natural series of ascending complexity.
The innovation of heterospory does not appear to have brought with
it any general reduction of parts, but changes rather of the contents
704 CONCLUSION
themselves of the sporangia. It is well known to have been initiated along
several distinct phyletic lines : well-ascertained cases are seen in the ligulate
Lycopods (Figs. 23, 24; 165, 166; and 170), in the Calamarians (Fig. 210),
and in the Hydropterideae, while it is quite possible that the heterospory
which preceded Seed-formation in the Pteridosperms may also have been
independently initiated. The innovation is closely connected with the
sacrifice of a proportion of the potential germs for the better nutrition
of the rest : this has already been seen to occur in various homosporous
types such as the Psilotaceae (p. 417), and Equisetum (p. 380), though
the spores produced in these plants show no differentiation in size, or
apparently of sex. The condition seen in Calamostachys Casheana (p. 381)
is but little removed from this : here, however, heterospory is clearly present,
but not far advanced from that homosporous state where sacrifices for
nutritive purposes are seen : the megaspores appear relatively small and
numerous, as they are also in some of the heterosporous Lycopods, such
as Lycopodites Sm'ssei, with 16 to 24 in each sporangium. In Selaginella
itself the number of the megaspores is smaller, and may vary from 8 in
S. apits, through the common number of 4, to sometimes a single one,
as in S. rupestris. The latter condition is found also in the Hydropterideae,
and it is the state which is commonly seen in all the higher Seed-Plants.
The facts indicate with no possible uncertainty that a progressive reduction
in number of the spores, which prove on germination to be female, has
taken place, till finally a single, large, well-nourished spore is the sole
product of each megasporangium.
Such changes, however effective they may be in the successful establish-
ment of the new individual, through the concentration of the nutritive
store conveyed from the parent plant in a few enlarged megaspores, or
in only a single 'one, are nevertheless intra-sporangial : they rarely affect
other parts. It is true that in Azolla abortive primordia of microsporangia
accompany the megasporangium, as though their correlative diminution
followed on the great enlargement of the megasporangium; but this case
is exceptional among heterosporous plants, and thus it is seen that the
introduction of heterospory does not necessarily bring far-reaching effects,
but involves a readjustment of the available nutritive material within the
single sporangium, and its concentration round few centres, or only a
single one, in place of many.
It is different, however, with the other, and much more effective
innovation, viz., the Seed-Habit. This also was initiated along more than
one line of descent, though it may still be a matter of doubt whether
it became permanently effective in more than one distinct phylum. It
will suffice here to quote the cases of incipient seed-like habit of the
Lycopodiales, seen in Lepidocarpon Lomaxi, and in Miadcsmia, in which
the megasporangium, with its single megaspore retained within it, is covered
in by an integument, leaving a micropylar slit or pore : the whole
structure, together with the sporophyll to which it is related after the
HETEROSPORY AND THE SEED-HABIT 705
type of the sporangia of other Lycopodiales, falls away, but the details
of fertilisation and of embryogeny which follow are still unknown.1 The
nearest approach to a seed-like habit among the living Lycopods is seen
in Selaginella apus and rupestris? Here fertilisation occurs while the spores
are unshed, and the sporangia still attached to the strobilus : in S. rupestris
the connection is maintained with the parent plant until the embryo has
produced cotyledons and a root. Thus the Lycopodiales, both fossil and
modern, show approaches to a seed-habit, though it is doubtful whether
that habit was ever firmly established among them, or persists in the
form of any of the Seed-Plants of the present day.3 The condition now
so fully demonstrated for the Pteridosperms shows that a Seed-Habit was
definitely acquired along another quite distinct phyletic line.4 These
large-leaved types, bearing their large seeds of Cycad-like character dis-
tributed on fronds effective also for assimilating purposes, probably sprang
from the same stock as the Ferns, and it is especially with the Botryo-
pterideae and the Osmundaceae that they show the nearest analogies.
Thus the Seed-Habit appears to have been initiated certainly in two
distinct phyla, and it is not improbable that it may have been repeatedly
initiated within either or both of them.
The establishment of a Seed-Habit does not necessarily bring immediate
reduction of the supporting system in its train : but it has frequently
happened that such reduction follows. The fact that the large seeds of
Neuropteris heterophylla are borne on a rachis bearing characteristic
vegetative pinnae shows that a correlative reduction is not obligatory.
But on the other hand, a reduced state of the sporophylls does usually
accompany the seed-habit : in Lyginodendron the female fructification is
described as being borne on the rachis of fertile fronds which differed
from the sterile foliage in the reduced leaf-area : and this applies also in
some degree to the male sporophylls as well. From such minor degree
of reduction of the megasporophyll to that condition seen in Cycas is
no great step, and from this the sequence through the Cycads gives
very convincing evidence of further reduction.5 It seems not im-
probable that in Cycadeoidea a still further step in reduction has been
taken, so that while many of the sporophylls appear as minute sterile
scales, those which are fertile exist merely as radio-symmetric pedicels,
each bearing a single terminal ovule.6 The microsporophylls show a
series of reductions in less prominent degree, but without any strict
parallelism with the megasporophylls : thus in Cycadeoidea where the
1 See Scott, Progresses Rei Bot., i., p. 171.
-Miss F. Lyon, Bot. Gaz., vol. xxxii., pp. 182-3.
3 See SeMjard and Ford, "The Araucariaceae, Recent and Extinct," Phil. Trans.,
Series B, vol. 198, p. 305, etc.
4 See Scott, Progressus Rei Bot., i., pp. 190-212, where the literature is quoted.
5Engler and Prantl, Nat. Pflanzen., II. i., Fig. 7.
tj This is the opinion of Wieland, American Fossil Cycads, p. 230, etc.
2 Y
706 CONCLUSION
megasporophylls are the simplest of all, the microsporophylls are still
of considerable size, while those of the modern Cycads are much smaller,
though their megasporophylls show less extreme reduction. Such com-
parisons appear to indicate with unmistakable clearness that in the course
of descent a reduction of sporophylls has followed upon the establishment
of the Seed-Habit, but that it came gradually, and is not to be held as
a simple or direct example of correlation.
The essential point in the Seed- Habit is the retention of the megaspore
within the tissues of the parent plant till after fertilisation : on this has
followed, in the first place, the ultimate achievement of a higher degree
of independence as regards fertilisation ; and secondly, the opportunity of
continued nutrition of the embryo by the parent plant up to an advanced
age. Both of these are important steps in the establishment of a Land-
Flora, and must be briefly considered. Taking first the question of method
of fertilisation, it may be remarked that the differentiation of sex as
evidenced in heterospory is in itself no point of adaptation to a land-
habit : while it brings an advantage in the superior nutrition of the female
spore, it imposes a fresh difficulty in fertilisation, viz., the necessity during
germination of a near juxtaposition of the microspores and megaspores,
bodies which have a distinct source of origin : the more distinct the origin
in space, and in time of production, the larger will be the number of
microspores requisite to ensure a reasonable probability of fertilisation. As
a matter of observation the number of microspores in the Pteridophytes is
habitually maintained according to the plan of the original homosporous
sporangium, and it may be in Seed-Plants also, so long as their distribution
is by no specialised method, and so long as juxtaposition with megaspores
is only a matter of chance. This is exemplified in the Pteridosperms and
in Cycadeoidea^ and in less complete degree in the Cycads. But in the
higher forms of Seed-Plants the specialised methods of transfer of micro-
spores, and especially those by animal agency, have led to economy, so
that a reasonable certainty of fertilisation is secured with a smaller output
of microspores. This theme may be so fully illustrated by well-known
examples from the Flowering Plants that it requires no further explanation
here. But as against the difficulty of securing juxtaposition of the mega-
spores and microspores during germination may be set the adoption of
siphonogamy, which followed ultimately upon the Seed-Habit. A precision
previously unknown was thus introduced into the act of fertilisation, so
that once juxtaposition of spores was secured, fertilisation followed with
a high degree of certainty. This reduced and finally abolished the motile
stage, and so removed the critical process of fertilisation from its primitive
dependence on the presence of external fluid water. The adoption of
siphonogamy was the last adaptive step of prime importance in the
establishment of a Land Flora upon a permanent basis of suitability to
external circumstance : and the high degree of certainty of the resulting
fertilisation still further favoured economy of pollen-production.
HETEROSPORY AND THE SEED-HABIT 707
A second consequence of the adoption of the Seed-Habit was the
continued nutrition of the embryo by the parent plant : not only was
accurate fertilisation secured, but the embryo was far advanced in its
development, and supplied with a large nutritive store before being isolated,
and becoming dependent on its own resources. This, together with the
mechanical protection of the seed-coat, .brings a highly increased certainty
of establishment of each germ as a new individual. Economy will again
follow on the increased chance of success of each individual germ, and
the general tendency of these precise and certain arrangements must
have been in the direction of reduction : evidence of this is to be recognised
generally in the floral construction of Seed-Plants. Amid all the fluctuations
of detail of the floral mechanisms they show, as compared with the
Pteridosperms or Cycadales, evident traces of that reduction which the
adoption of the Seed-Habit would on biological grounds lead us to expect.
The higher terms of the series of Vascular Plants show more exact
differentiation of the vegetative and reproductive systems than the lower.
Each appears to have taken independently its own line of specialisation.
But there is good reason to hold these advances as mere changes of detail
in a plan substantially the same, however important may be the biological
effects thus gained. The general plan of the shoot of Flowering Plants,
whether vegetative or propagative, and the characters of its several parts
remain the same as in the more primitive Vascular Plants, though subject
to an infinity of modifications ; and the conclusion which is forced upon the
mind in contemplating the construction of Vascular Plants at large is, the
unity of the general scheme underlying them all. It is based, as we have
seen, on the individual shoot, consisting of an apically-growing axis with
appendages borne in acropetal succession, and accessory roots. The
general-purposes shoot, as seen in its essentials in the earliest homosporous
Pteridophytes, is the pattern : from this, by segregation of the vegetative
and propagative regions, and subsequently by their independent specialisation,
even the highest terms of the Flora of the Land may be held to have been
derived. And in the course of this evolution there is evidence of two
main progressions as regards the size of the appendages, and their prq-
pagative capacity. In the first and more primitive phase, which was
characterised by being homosporous, there are comparative reasons which
have been explained at length above for recognising a very general ampli-
fication, though subject in special cases to reduction. This is in accordance
with the obvious biological advantage in homosporous forms of producing
as large a spore-output as possible. It involved in some cases profuse
branching of the shoot, while the individual appendages remained small,
as in the microphyllous Lycopodiales. In other cases the axis was not
greatly extended, nor the appendages numerous, but the latter made up for
these deficiencies by their extensive individual growth and ramification.
This is exemplified in the megaphyllous Ophioglossales and Filicales,
while the sporangiophoric Pteridophytes take an intermediate place. Thus
;o8 CONCLUSION
in various ways, though probably from somewhat similar beginnings, the
various primitive homosporous phyla may be held to have worked out
the line of biological advantage which follows on direct increase of numerical
output of spores. This may be regarded as the upward limb of a curve
of morphological complexity.
But with heterospory and especially with the Seed-Habit and more
precise methods of fertilisation following on siphonogamy, the line of
biological advantage was diverted from mere numerical increase of germs
to their individual nurture, with, as a consequence, a higher degree of
certainty of their final establishment in life. This brought in various
ways reduction as against the previous amplification. The most conclusive
evidence of this is to be found in the sequence from the Ferns, and
Fern-like Pteridosperms, through the Cycads. It seems impossible to see in
these any other story than one of simplification of appendages following
on increased precision of propagative method; it may be represented as
the downward limb of a curve of morphological complexity. It is not at
present possible to indicate any other phyletic lines with the same degree
of certainty as this ; but the condition of the floral construction in other
Gymnosperms, and in the Angiosperms themselves is certainly such as to
harmonise with an origin in which reduction has played a prominent
part.1 The biological probability that such an homosporous amplification
should be succeeded by a reduction following on the adoption of a Seed-
Habit is in full accord with the evolutionary course which the facts
themselves appear to indicate. ,
1 1 leave entirely open the question of phyletic origin of certain Gymnosperms from
the Lycopodiales. If that were their true origin the reduction would in that case be in
restriction of the number of sporophylls and sporangia rather than in size of the
sporophylls themselves : in fact a reversal of their homosporous amplification, analogous
to but not coincident with that of the larger-leaved Filicales, Pteridosperms, and Cycads.
CHAPTER XLV1I.
RESULTS, PHYLETIC AND MORPHOLOGICAL.
IT remains to state the chief phyletic and morphological conclusions
which may be based upon the facts and the reasoning contained in what
has gone before. They are derived primarily from the sporophyte generation,
and the reason for this is that it supplies the most distinctive features.
Nevertheless, the characters of the gametophyte have not been ignored.
The method adopted, wherever it is possible, has been to start from the
detailed comparison of forms within a near circle of affinity : to lay these
out in short series which there is reason to believe were progressive, and
then to compare the more primitive types of each with a view to gaining
some idea of a prevalent original type for the whole group, or phylum.
A comparison may then follow of the original types of different groups or
phyla, with a view to the recognition of characters which are common in
them : and so a conception may be formed of some of those general
features which ruled in the remote ancestry, and even in the early
foundation of the distinctive Flora of the Land, as expressed in the rise of
the sporophyte generation.
It has already been seen that no definite Algal form now living can
be held to have been a direct progenitor of any known Archegoniate type.
Certain Algae suggest in their post-sexual phase how the initiation of a
sporophyte may have occurred, but there is no sufficient reason to hold them
as being in the actual line of descent of Archegoniate forms. The probable
relation of the Bryophytes to the Pteridophytes is somewhat similar: for
though the comparison of their sporogonia with the sporophytes of the
Vascular Plants shows many points of similarity, still it cannot be held
that there is sufficient evidence to assume a phyletic relation between the
non-vascular and the vascular Archegoniates. Both Mosses and Liverworts
may with probability be held to be blind branches of descent, which illustrate
nevertheless phyletic progressions that illuminate the origin of sterile tissues
from those potentially fertile, and the establishment of a self-nourishing
system in the sporophyte. With these few words the Algae and
;io CONCLUSION
Bryophyta may be dismissed as side issues, and the special phyletic
interest will centre round the vascular Archegoniatae, as the forerunners
of all the higher vegetation of the Land.
The method above described may be first applied in the case of the
GAMETOPHYTE of the homosporous forms of Pteridophytes. A comparison
of the prothalli of various species of Lycopodium (pp. 340-345) points
towards a massive body, probably exposed above ground and capable
of assimilation, with its sexual organs sunk in the massive thallus : the
form seen in L. Selago is held to be not far removed from the original
type. Probably the filamentous condition seen in L. Phlegmaria is a
specially attenuated development in accordance with saprophytic habit,
while the colourless condition of the underground prothalli, where depend-
ence is entirely upon saprophytic nutrition, can hardly have been anything
else than secondary. The same opinion applies also for the prothalli of
the Ophioglossaceae as regards their colour, and the deeply sunken sexual
organs (p. 465), while their massive construction compares with that usual
in Lycopodium. The female prothallus of Equisetum is of essentially a
similar type, but it shows less massive structure, especially in the upward-
growing lobes, which are not unlike those of L. cernuum. The male
prothallus is, however, of a simpler type : the antheridia are sunk as
before, but the archegonial neck projects, as it does also in some species
of Lycopodium. Turning to the Ferns, the delicate prothallus of the
Leptosporangiates, and especially the simple filamentous forms of the
Hymenophyllaceae, suggests at first sight that they are of an essentially
different type from the more massive forms previously considered. But
comparison within the Fern-phylum shows that the prothallus of the most
ancient living type, the Marattiaceae, is more massive in construction :
and in the Osmundaceae the same is seen, though in less degree. These
facts strongly suggest that the Fern-phylum has undergone a progressive
simplification of the prothallus, and indicate an origin like the rest from
a massive source. The sexual organs also are deeply sunk in the Euspor-
angiate types, but show a successively more projecting position in the
Leptosporangiates, just as their sporangia also project more than in
Eusporangiate Ferns. Thus the propagative organs of the two generations
march parallel in respect of their relation to the surface of the part which
bears them. // may accordingly be concluded as probable that the prothallus
of early Pteridophytes at large was a relatively massive green structure,
with deeply sunk sexual organs*
Turning now to the comparison of the SPOROPHYTE, the phylum of the
Lycopodiales, in which it is of the simplest construction among the
Pteridophyta, is certainly as ancient as any of the rest : the two constituent
series, the Ligulate and the Eligulate, illustrate parallel progressions, but
their similarity of plan shows that they are closely allied. On the basis
of comparison of the known forms a primitive type of Eligulate Lycopod
has been sketched out, and it is nearly approached by what is actually
RESULTS, PHYLETIC AND MORPHOLOGICAL 711
seen living in L. Selago (see frontispiece, also p. 363) : perhaps it may
ultimately be found to be even better represented by some others of the
thirty-eight less fully known species of the Se/ago-Section of the genus.
The undifferentiated "Selago" condition, which is seen in them, is no
recent characteristic, for it appears also in certain Palaeozoic Lycopods : from
this state the various living forms illustrate the achievement of a more clear
segregation of sterile and fertile tracts, initiated by abortion of sporangia
in the sterile regions : along with this goes more adequate protection of
the sporangia, and their change to a broader form : there is also a greater
complexity of the stelar structure, and a greater specialisation of the
embryogeny : the essential parallelism of these progressions indicates that
they constitute true phyletic lines, the advance having been from the
primitive condition of the "Selago" Section to the more specialised state
of the rest of the genus. The Ligulate series, which includes the most of
the fossil genera and the modern Selaginella and Isoetes, has as a rule
more definite heterosporous strobili, though the " Selago " condition is
again seen in Isoetes. In this respect the Ligulate Lycopods are more
advanced than the Eligulate. The highest type of propagative organs in
the whole phylum are the seed-like structures in Lepidocarpon and
Miadesmia, which show an advance parallel to that found in the Pterido-
sperms. Both the living and the fossil forms are in their simplest types
protostelic, but there has been advance to medullation, and finally to
disintegration of the xylem of the stele and to secondary thickening in the
dendroid forms. Selaginella Spinulosa has been recognised among living
species as a relatively primitive Ligulate type, on the ground of its
radial construction, its branching, and its anatomy : in these characters,
as also in point of the embryogeny, S. Spinulosa resembles L. Selago,
notwithstanding its heterosporous state ; this fact has a special interest, and
the convergence in many features between the two species confirms the
correctness of their recognition as primitive in their respective genera.
The Lycopodiales stand by themselves in the simplicity of their
sporangial arrangement, and constitute a type of extreme antiquity, which
has come down practically unaltered to the present day. Their comparative
study may be conducted independently of other phyla: for there is no
reason to think that they were derived from any other known vascular
type. It has been shown that the several lines of comparison converge
downwards : the condition actually seen in the "Selago" type may be held as
truly primitive, and Lycopodium Selago, with its imperfectly differentiated
shoot, is in fact a near approach in a living species to the ideal primitive
form which emerges from wide comparative study of the phylum as a whole.
There are two further characters seen occasionally in the Lycopodiales
which call for special remark. In the very early fossil, Lycopodites Stockii
(p. 298), the leaves are arranged in whorls, as they are also in certain
living species of Lycopodium (p. 291). In others the leaf-arrangement is
irregular. Sometimes, however, whorled and spiral arrangements may be
7i2 CONCLUSION
found at different heights on the same plant : or the plane of the whorls
may be set obliquely to the axis. It would appear probable from such
facts that the original type had whorled leaves, and that the spiral
arrangement was acquired by secondary disturbance of it, a point of some
considerable interest for comparison with the sporangiophoric Pteridophytes.
The other character is seen in Isoetes (p. 318), and in Lepidostrobus Brownii,
both of which had very large sporangia (p. 322). In these a partial
sterilisation of sporogenous tissue producing trabeculae meets a mechanical
and nutritive requirement following on their large size, and the structure
thus approaches a state of septation : such septation is indeed technically
completed in the megasporangia of Isoetes, but no Lycopod shows a septate
state of the sporangium as a permanent character. The interest in this is
in comparison of these sporangia with the similarly placed synangia of the
Psilotaceae and Sphenophyllaceae.
These two series, together with the Equisetales, have been included
under the general designation of the Sporangiophoric Pteridophytes (p. 423).
Though differing in detail, the main plan of their sporophyte is similar to
that in the Lycopodiales, as regards axis and leaves, branching, and
anatomical structure ; but the sporangia of the latter are replaced by
sporangiophores, while the relations of these to the bracts is not
uniformly so regular as that of the sporangia in the Lycopodiales. More-
over, both bracts and sporangiophores show evidences of fission, sometimes
independently, sometimes together. These relations have been considered
above (p. 694-5), together with the similar variations of exact position of
the sporangia and sporangiophores relatively to the axis : such facts, com-
bined with the arguments already advanced in Chapter XXVIII. , lead to
the conclusion that the functionally identical parts designated sporangiophores
and sporangia are cognate parts ; it appears probable that the sporangiophore
is itself a consequence of elaboration of a simpler type of spore-producing
member, of which the sporangium of Lycopodium is an example, while the
trabeculae in Isoetes and Lepidostrobus Brownii suggest a mode of origin of
the septate state. If this were so, then the sporangiophore would have been
distinct in its phyletic origin from the bract-leaves, which habitually subtend
the spore-producing members, whether they be sporangia or sporangiophores.
The Sporangiophoric Pteridophytes (which include the " Articulatae " of
Lignier together with the Psilotaceae) are primarily characterised by the
presence of the sporangiophore. The fact that the leaf-arrangement is often
whorled, which is a leading feature of the Articulatae, while that in the
Psilotaceae is alternate, is here regarded as a point of secondary moment.
The reasons for this are, first, that the leaf-arrangement varies from the
whorled to the alternate in the very natural phylum of the Lycopods, and
secondly, that a similar change appears from the ancient Sphenophylleae
to the modern Psilotaceae — groups clearly related to one another. It
seems probable that the whorled arrangement was initially general for the
strobiloid types, but that the regularity has been secondarily abandoned. The
RESULTS, PHYLETIC AND MORPHOLOGICAL 713
character of the sporangiophore once acquired appears to have been more
constant, and affecting as it does the production and dispersal of the spores,
it is of much more biological moment than details of leaf-arrangement :
consequently it deserves a prior place in our comparisons. The designation
of the Equisetales and Sphenophyllales, including the Psilotaceae as
sporangiophoric Pteridophytes, is to be preferred to any separation of the
" Articulatae " on the ground of leaf-arrangement.
The essential unity of the characters of the Sporangiophoric Pteridophytes
is becoming more apparent as the knowledge of them widens : this indicates
that they constitute a brush of phyletic lines sprung probably from a
common source : the original characters of the common stock appear to
have been not unlike those of a primitive Lycopodinous type where the
whole shoot was fertile ; but here the spore-producing members proceeded
early to a more elaborate structure, the sporangiophore replacing the simple
sporangium, while a capacity for fission of the leaves supervened, and
often of the sporangiophores also. The stelar structure in many cases so
closely resembles that of the more primitive Lycopodiales as to lend material
support to this suggestion. Starting from such a central type as Spheno-
phyllum majus, in which a " Selago " condition is seen, a departure from the
whorled disposition of the leaves, such as the Lycopods show within the
genus Lycopodium, would give the type of the modern Psilotaceae : a
transition to a higher differentiation of the sterile and fertile regions, with
fission of the sporangiophores and reduction of the number of sporangia
borne by each would give the more complex state of S. Dawsoni and
Roemeri: a similar fission of both bracts and sporangiophores would lead
towards the type of Cheirostrobus. It is not suggested that the species
named were thus grouped in actual phyletic lines, nor would these accord
with stratigraphical sequence ; the intention is rather to indicate morpho-
logical relationships of the different known forms to a probable primitive
type, a primitive type to which Sphenophyllum majus retained a high degree
of similarity.
On the other hand, the structure seen in Sphenophyllum emarginatum
(p. 694, footnote) connects the Sphenophyllaceous-type of strobilus with the
usual Calamarian type : it has been shown above how the various other types
of the Equisetales are related to this (pp. 694-6; also chapter XXVIII.).
The analogy of the Lycopodiales, together with the facts seen in the
sporangiophoric Pteridophytes themselves, points to their origin also from
a strobiloid type with a general-purposes shoot, in which the axis was
dominant and protostelic, the leaves were whorled, and in which the spore-
producing members early attained to the sporangiophoric structure. The
phyletic relationship of the Sphenophyllales and Equisetales has undoubtedly
been a very close o?ie ; the distinguishing features are not to be found in the
primary plan or construction of the shoot, so much as in the secondary modifi-
cations of number and relation of the appendages, and of their branching,
together with changes in the originally protostelic structure of the axis. Such
;i4 CONCLUSION
considerations support the conclusion that the Sporangiophoric Pteridophvtes
constitute a brush of naturally related phyletic lines.
It has been argued at length above (Chapter XXXI.) that the Ophioglos-
sales are an upgrade sequence, a view which accords with their homosporous
state : also that their spike illustrates various steps in the increasing complexity
of a body of the nature of the sporangiophore. The elaboration of the
subtending leaf runs parallel with it, while both leaf and spike show
branchings and fissions comparable with those recognised in the sporangio-
phoric Pteridophytes, but carried out here on a larger scale. On this view
the ivhole unbranched shoot is a simple strobilus bearing leaves, of which all
are potentially fertile, and the great majority actually so. But the large
size of the leaves, and their isolation in point of -time (commonly only one
being expanded at once), disguises the real nature of the strobilus. All
the three genera have attained to great complexity, but in Ophioglossum,
and more clearly in Botrychium, the gradually increasing complexity of the
leaf in the individual life indicates what has probably occurred also in the
race. Along one line, that of Ophioglossum penduhim, intermedium, and
simplex, it seems probable that reduction of the vegetative system has
occurred ; but with this exception the Ophioglossaceae appear to have been
an upgrade sequence, sprung from some Sporangiophoric stock, and bearing
no near relation to the large-leaved Ferns. The anatomy here again points
to an origin from a protostelic structure, while the single leaf-trace strand
in all the simpler forms indicates a primitively simple structure of the leaf.
The Filicales constitute a more isolated phylum than any of the smaller-
leaved forms. Their general comparison among themselves has been fully
discussed in Chapter XL., and the relations of their leading families graphically
indicated on p. 653. It is now recognised that true Ferns were represented
in the Primary Rocks by relatively few forms, while their derivative families
increased in number and extent in later periods. The Leptosporangiate
type is essentially modern : it is indeed doubtful whether any of the
Palaeozoic Ferns had an annulus composed of a single row of cells : on the
other hand, though Eusporangiate Ferns still survive, they were the leading
type of the Palaeozoic Period. Accordingly, it is in the latter and not in
the former that the features of interest for comparison with other phyla of
Pteridophytes are to be found.
It has been shown that the construction of the shoot of the primitive
Eusporangiate Ferns is essentially strobiloid, maintaining constantly the same
relations of axis and leaf as in smaller-leaved forms : the axis is in some of
them permanently protostelic (Botryopterideae), while in the rest a protostelic
structure figures in the early seedling of the forms still living. The leaf-trace
is a single strand in primitive forms, though in the modern Marattiaceae it
may be broken up into separate strands. In addition to this the outcome
of anatomical comparison of the Ferns at large has been to show that the
axial structure is constantly referable in origin to a primitive protostele, a
construction which is held to be typical and primitive for strobiloid plants ;
RESULTS, PHYLETIC AND MORPHOLOGICAL 715
this indicates that they are themselves essentially strobiloid types which
have progressed to a condition of megaphylly. That is also the conclusion
which comparison of their external morphology with that of other phyla
suggests, while the absence of differentiation of the sterile and fertile regions
is the same as is seen in the " Selago " condition of the strobiloid types. On
the general biological ground that in homosporous forms there is direct
advantage in enlarged spore-output, there is reason to regard amplification
as probable. The amplification of the appendages has been more extensive
here than in any other phylum, but there are many points of similarity with
what is seen in certain of the strobiloid Pteridophytes, and especially in the
Sphenophyllales and Ophioglossales. Accordingly, it is held that the Filicales
were ultimately of strobiloid origin, but have undergone amplification of their
leaves analogous to, but phyletically quite distinct from what is seen in other
Pteridophytes, and carried to a higher degree.
One chief reason for regarding the lines of the Filicales and Ophioglos-
sales as distinct lies in the difference of position of the spore-producing
members. It has been argued above (p. 633) that the soral condition was
primitive for Ferns, and that the sorus is a body similar in kind to the
sporangiophore, the two being alike in function, in structure, and in capacity
for fission and extension (p. 699) : the number and position are poi?its of
difference. An increase in number of sporangiophores (or sori) is a natural con-
comitant of increase in size and nutritive capacity of the leaves ; in the Ferns
a process of fission similar to that suggested in the Sphenophyllales probably
played a part, rather than elaboration of the single sporangiophore as seen
in the Ophioglossales. The disposition of the numerous sori upon the leaf
in Ferns differs from that in other Pteridophytes : but it must be remembered
that in large-leaved forms this necessarily became a matter of biological
adaptation in the absence of the protection afforded by a compact strobilus.
The Filicales are thus a phylum showing fundamentally the strobiloid
characters, but secondarily modified in relation to their pronounced
megaphyllous habit. This was adopted very early by them, as the fossil
story as well as their general morphology clearly show. Accordingly, the
Filicales appear as the most divergent phylum of homosporotts Pteridophytes.
The prevalence of a whorled arrangement of the leaves has already
been noted among early strobiloid types, but it was seen to have been
departed from in many of the Lycopods, and in the modern Psilotaceae.
In the Filicales, however, as also in the Ophioglossales, alternate leaf-
arrangement is the rule. This difference from early strobiloid types is a
very natural one in megaphyllous shoots : for the whorled arrangement is
mechanically inconvenient where the leaves are large. The alternate
leaf-arrangement in the megaphyllous types may be held as a natural
though not an inevitable consequence of the large size of the appendages.
If this is itself secondary in the Filicales it is quite possible that their
alternate arrangement was also secondary in descent. But on this point
there is no clear evidence.
;i6 CONCLUSION
// thus appears that comparison of the several phyla, as represented
both by their fossil and their modern representatives, leads in each case
towards the recognition of a primitive type, and that its construction in
the several phyla has certain features in common. The chief of these
are the definition of axial polarity in the first initiation of the embryo :
the continued apical growth : the radial construction of the shoot : the
origin of the appendages laterally from the axis by enation, and in strictly
acropetal order : a protostelic structure of the conducting system of the
axis, and a leaf-trace composed of a single strand, which comes off from
the protostele with the minimum of disturbance of its structure. The
appendages were from the first of two kinds which were closely associated
together : bracts or leaves, and spore-producing members : the structure
of these, and their relations to one another and to the axis, varied in
the different phyla, and gave them their distinctive characters : but a
whorled arrangement of the bracts was prevalent in early small-leaved
forms, while they commonly held a subtending relation to the spore-
producing members. A body such as that sketched appears to have been
common for all the early Pteridophytes, and constituted the primitive shoot.
There is no clear indication, beyond comparison based on the facts of
embryology and of mature structure, how such a body was in the first
instance produced ; but this leads to the hypothesis put forward in
Chapter XL The sporophyte, thus constituted, probably arose originally
as a structure of limited size, and unbranched, upon a prothallus of
considerable dimensions, and producing Homosporous Spores. From it, by
branching of the axis, by differentiation of vegetative and propagative
regions, by amplification of the leaves and spore-producing members, by
adoption of an alternate leaf-arrangement as the leaves enlarged, and
by expansion of the vascular system to meet these additional require-
ments, all the known homosporous types may be understood to have
originated. But as explained in Chapter XLVL, the adoption of
Heterospory, and of the Seed-Habit supervened later. This, while it has
led to the final independence of the Land-Flora as regards external fluid
water for the completion of its Life-Cycle, has brought as a secondary
consequence a wide-spread reduction.
The final gt>al of all organic development is the establishment of
new individuals. The evolutionary story of the sporophyte illustrates this
in two distinct ways. In the prior and non-specialised homosporous forms
large numbers of germs are produced: those are individually small, and
ill provided with nourishment, but they make up for deficiency of method
by their large numbers. The larger their number the better the chance
of survival and spread of the race : consequently amplification of the whole
sporophyte is the leading characteristic of these earlier and simpler types ;
it was carried out either by multiplication of appendages individually small,
as in the microphyllous types, or by enlargement of individual appendages,
as in the megaphyllous types. It was in these homosporous forms that
RESULTS, PHYLETIC AND MORPHOLOGICAL 717
the vegetative system was established and amplified, while it tended
to become differentiated from the propagative system. In the later and
more specialised heterosporous forms, and particularly in the Seed-Plants
with their more refined methods, individual precision supersedes mere
numbers : and reduction of the propagative system has been its usual con-
comitant. The vegetative system which became fully distinct from the
propagative, often retained or even increased its dimensions and complexity.
Taking an evolutionary course of its own it diverged more and more
in character from the propagative system. The final result is seen
in the Angiosperms which are now dominant : here the flowers differ
widely from the vegetative shoots, though the plan of each resembles that
of the primitive shoot from which both sprang. But whatever the modern
complications may be, comparison along lines which have been pursued in
this volume indicates that the sporophyte, which is the essential feature
in the Flora of the Land, is referable back in its origin to post-sexual
complications : it appears to have originated as a phase interpolated between
the events of chromosome-doubling and chromosome-reduction in the primitive
life-cycle of plants of aquatic habit.
INDEX.
Abortion of spore-producing members, 120,
127, 161, 700.
Abortive spikes in Ophioglossaceae, 446 ;
sporangia in Lycopodium, 163, 292 ; in
Isoetes, 165, 307.
Acacia, 235 ; seedlings, 185.
Achy la, 69,
Aconitum, 128.
Acrostichum, 631.
Actaea, 128.
Adaptation to Land Habit, 3, 81, 245.
Adiantum concinnum, 30, 31 (Figs. 14, 15);
Edgworthi, 183 (Fig. 94).
Aglaozonia, 66.
Akrogynous Jungermanniaceae, 264.
Albugo, 68.
Alchemilla, apogamy in, 101.
Allosorus crispus, 627 (Fig. 348).
Alsophila atrovirens sorus, 603 (Fig. 334) ;
excelsa stem -structure, 198 (Fig. 100) ;
anatomy of young plant, 606 (Fig. 338) ;
phyletic position of, 655 (Fig. 354) ;
prinnata, 604 (Fig. 336).
Alternating generations, balance of, 33 ;
inversion of balance of, 45 ; cytological
distinction of, 46, 61.
Alternation, biological aspect of, 79.
Amphibious habit, 81 ; organisms, 3, 244.
Amphithecium, 272, 278, 285.
Amplification, 233 ; progressive in homo-
sporous forms, 236, 717 ; of leaf in
Ophioglossaceae, 433.
A nachoropteris , 501.
Anakrogynous Jungermanniaceae, 264.
Anatomical evidence, 188.
Anatomy of Filicales, 646 ; characteristic of
strobiloid plants, 649.
Andreaea, 275 (Fig. 133, 134).
Andreaeales, 275.
Anemone nemorosa, 127 (Fig. 70).
Aneura, 266 (Fig. 127); 267 (Fig. 129);
ambrosioides , 90 (Fig. 46) ; 161 (Fig. 86).
Angiopteris, 505 (Fig. 274); sorus of, 512
(Figs. 278, 283, 284) ; anatomy of, 525
(Figs. 279, 291); embryo of, 508 (Fig. 277).
Annularia, 372.
Annulus, 23, 104 ; of Equisetum, 382 ;
in Ferns, change of position of, 639.
Aneimia, 543 (Figs. 301, 302); anatomy, 548.
Anthocerotales, 267.
Anthoceros, 268 (Fig. 130).
Anthoceroteae, self-nutrition of sporogonium,
237.
Antithetic alternation, 32, 47, 80 ; theory,
general objection to, 659.
Apex of axis, of constant origin in embryo,
181, 664, 673.
Apical cell of axis, origin in embryo, 668
(Figs. 357, 358) ; cone of L. Selago, 331
(Fig. 172); segmentation, 678; with
small-celled meristem, 679 ; with definite
initials, 679.
Apogamy, 51, 52 (Fig. 33).
Apophysis, 281.
Apospory, 53 (Fig. 37).
Appendages, classification of, 145.
Appendicular organs, origin of, 86.
Archaeopteris hibernica, 228.
Archangiopteris , 505; sorus of, 512 (Figs.
278, 283) ; anatomy of, 525.
Archaeocalaniites, 374 (Fig. 82) ; whorled
leaves, 230.
Archegoniate series, origin of, 82.
Archesporium, 88, 106 ; of Anthoceros, 268 ;
of moss, 278, 285.
Archidium, 277, 284.
INDEX
719
Arisanun, 127.
Aspidium acrostichoidcs, 24 (Fig. 8).
Asplenium obttisifoliwn, 583 ; resectum, 583 ;
shepherdi, 626 (Fig. 347).
Assimilatory system in Bryophytes, 662.
Astelic state, 192.
Asterocalamites, 373 (Fig. 199).
Asterochloena.) 501.
Asterophyllites, 372.
Aslerotheca, 511 (Fig. 282), 521 (Fig. 289);
Stcrnbergii, $22 (Fig. 289).
Athyrinni filix-foemina v. clarissima apos-
pory 55 (Fig. 37)-
Atnc/inm, 35 (Fig. 19).
Auricula polystely, 192, 193.
Axillary branching in Ferns, 627.
Axis pre-existent, 141.
Azolla, 176, 610.
Bartramia,) 281.
Basal wall, 666 ; indeterminate in Isoetes,
358.
Basipetal sorus, 635.
Bilateral construction, 201.
Blechmun, 631 ; B. lanceolata, 632.
Blechnum boreale differentiation of leaves,
167.
Bornia, 150, 154.
Bornia radiata, 384.
Bothrodendron Kiltorchense^ 228.
Botrychioxylon, 500.
Botrychitim, external characters, 441 ; spore-
producing members, 452 (Figs. 244, 252,
253); anatomy, 458, etc. ; prothallus, 469 ;
embryo, 269 (Figs. 261-266) ; daucifolium
(Figs. 43, 44) ; simplex ', 441 (Fig. 240) ;
Lunaria, 441 (Fig. 241); abnormal fer-
tility, 1 60 (Fig. 85) ; daitcifoliuin, 441 ;
virgi 'nianui/i, 441 ; obliqitutn, 182, 471
(Figs. 264-266).
Botryopterideae, 498, 501 footnote ; phyletic
position of, 654 (Fig. 354).
Botryopferis, sporangia of, 501, 503.
Bracts or sporophylls, 693.
Branching of shoot, dichotomous primitive,
682 ; transition to monopodial, 682 ;
lateral branchings, 682 ; of spikes in
Ophioglossaceae, 438 (Fig. 239).
Braun's criticism of Phytonic theory, 138.
Bryales, 277.
Bryophyta, 257 ; balance of alternating
generations in, 35.
Buxbaunria, 281 (Fig. 137).
Calami tes petty curensis, 390.
Calamostachys, 150 (Fig. 80) ; 372, 374, 376
(Fig. 202) ; morphology of cone, 384 and
footnote; Binneyana, 374, 380; German -
ica, 375 5 Casheana, 381 (Fig. 210) ;
Zeilleri, 392 ; Binneyana, 408 (Fig. 225).
Cambial activity, 335.
Carinal strands of Equisetnm, 388.
Casuarina, 97 (Figs. 55, 56).
Catharinea, 35 (Fig. 19).
Cauline bundles, 195 ; stele, 195.
Cauloids (Lignier), 136.
CaulopteriSy 507, 511 (Fig. 280), 625.
Ceratodon purpuren s, 277 (Fig. 135).
Ceratopteris, leaf development, 628.
Cheirostrobus, 230, 404, 424 (Fig. 223, 224).
Chelepteris, 533, 539.
Chemiotactic action, 30.
Chlorophyceae, 70.
Chorisis of sporangiophores, 695 (Fig. 361);
of bracts, 695 (Fig. 361).
Chromatin, 47.
Chromosome-cycle, irregularities of, 58.
Chromosomes, 47 ; their numbers, 48.
Cingularia, 376 (Fig. 204).
Cladosiphonic structure, 189, 198.
Clarkia, 96 (Fig. 54).
Cleistocarpae, 282.
Closterittm, 70 (Fig. 40).
Coleochaete, 73 (Fig. 42), 260.
Columella of Anthoceros, 268; of Sphag-
num, 273.
Columella-less forms of Notothylas, 270.
Common bundles, 195.
Comparative morphology, 5.
Conjugatae, zygotes of, 70.
Cordaiteae, early existence of, 228.
Corsinia, 263.
Corynepteris, 503 (Fig. 273), 529.
Cotyledon, variable in time and place of
origin, 670; orientation constant, 671.
Cotyledons, constant orientation of, 182.
Crossotheca, 528.
Cutleria, 66.
Cyatheae, 602 ; phyletic position of, 656
(Fig. 354) ; dealbata, sorus, 604 (Fig. 335) j
Imrayana, 606 (Fig. 337) ; sorus, 602 ;
anatomy, 605 (Figs. 336, 337, 338).
Cyathodium, 237 (Fig. 116), 263 (Fig. 123).
Cyathotrachus altus, 521.
Cycadeoidea, 705.
Cycads, reduced sporophylls of, 154.
Cyclanthera, 126.
720
INDEX
Cynosurus, 128.
Cystopteris, phyletic position of, 655 (Fig.
354) ; bnlbifera, 19 (Fig. 3).
Cystopus, 68.
Cytological distinction of alternating genera-
tions, 61.
Cytopterisfragilis, 615 (Fig. 341).
Danaea, 94 (Fig. 49), 505 (Fig. 275) ; sorus
of, 512 (Figs. 278, 281, 283, 286) ; anatomy
of, 525 ; embryo of (Fig. 277) ; alata,
symmetry of, 212 (Fig. 106).
Danaeites, 523 (Fig. 290).
Davallia, 613 ; phyletic position of, 655
(FiS- 354); Griffithiana (Fig. 66), 613
(Fig- 339) 5 hymenophylloides, 615 (Fig.
340).
Decentralisation in Mosses, 286.
Dennstaedtia, 613 (Fig. 332 bis) ; D.
rulnginosa (Fig. 333 c), (Fig. 65), 601,
6l6> 597 (Fig. 332 bis) ; irregular arrange-
ment of sporangia of, 598 ; solenostely in,
600 (Figs. 333 A-C) ; apiifolia (Fig. 65) ;
Davallia series, 613 ; phyletic position of,
655 (Fig. 354)-
Dennstaedtiinae, 595.
Deparia, phyletic position of, 655 (Fig. 354).
Dermatogen, 178.
Desmids, 70.
Diacalpe, 617.
Dichotomous branching of stem in Ferns,
626 ; theory of origin of shoot, 630.
Dichotomy in Fern leaves, 627, 628.
Dicksonia, 592 (Figs. 330, 331); phyletic
position of, 655 (Fig. 354) ; Barometz,
J93 (Fig. 97) ; punctiloba, 190 (Fig. 95).
Dicksonieae, subdivision of the family, 591.
Dictyostele, 190.
Dictyostelic state in Ferns, 647.
Diclyota dichotoma, 66, 81.
Dipkyscium, symmetry of, 205 (Fig. 104).
Diploid phase, 47, 52.
DipZotmema, 554.
Dipteridinae, 618, solenostely in, 621.
Dipteris, 618 (Figs. 343-346).
D. conjugata, mixed sorus, 621 : phyletic
position of, 656 (Fig. 354).
Dispersal of spores, 645.
Divergent series, 10.
Dorsiventral construction, 201.
Dorsiventrality of shoot, 208 ; derivative in
Ferns, 626.
Double leaf-trace, 689, footnote.
Equisetales, 366 ; external characters, 368 ;
spore-producing member, 377 ; anatomy,
385 ; embryology, 392 ; summary, 395.
Equisetum, 94 (Fig. 50) ; anatomy of, 191
(Fig. 96) ; reduced leaves of, 239 ; spor-
angial development, 377 ; sterilisation in,
378; stelar structure of, 386 (Figs. 211,
212, 213) ; maximum, 149 (Fig. 79) ;
368 (Fig. 193) ; 370 (Fig. 194) ; pratense,
367 (Fig. 192) ; 373 (Fig. 196) ; scirpoides,
176 (Fig. 91); sylvaticum, polystachyum,
370 (Fig. 194) ; hiemale, 369 ; anatomy
of seedling, 391 ; root apex (Fig. 92) ;
limosum, 369 ; arvense, 370 ; sylvaticum,
370 ; myriochaehim, 370.
Elaterophore, 90, 266.
Elaters, 262.
Eligulatae, 291 ; embryology of, 340.
Embryo, biological study of, 181 ; dependent
on prothallus, 238 ; of Equisetnm, 392
(Fig. 214).
Embryology, 173, 251; initial and continued,
174; primary in Bryophytes, 660; con-
tinued in Vascular Plants, 678 ; of Pterido-
phytes, 663 ; segmentation of embryos,
664 (Fig. 355) ; of Ferns, 649 ; of
Filicales, 649 ; of Lycopods, 340 ; of
Ophioglossales, 489 ; of Ophioglossum
vtilgatum, 466 (Figs. 260, 260 bis) ; of
0. moZuccanum and O. pendulum, 466 ;
o&Botryckiummrginianum, 469 (Fig. 261 );
of B. Lunaria, 470 (Figs. 262, 263) ; of
B. obliquum, 471 (Figs. 264-266) ; of
Helminthostachys, 473 (Fig. 267).
Enation of leaf, 141 ; of leaves from axis,
680; objections answered, 681.
Endothecium, 272, 278, 285.
Enumerations of spores, 641.
Ephemerum, 208.
Ettckaridium, 96 (Fig. 54).
Eu- Davallia, mixed sorus, 613.
Eusporangiate Ferns, relatively primitive,
496.
Epibasal tier, 666.
Exogenous roots, 219.
Experimental Morphology, 6.
External characters of Filicales, 625.
Extra-prothallial swellings, 673.
Factors of advance, 85.
Fegatella (Conocephalus), 260.
Fern, life history of, 14 ; vascular skeleton
of, 15 ; sorus of, 20 ; spores of, 20 ;
INDEX
72
sporangium, development of, 22 ; spore-
mother-cells of, 23 ; spore tetrads of, 23 ;
sexual organs of, 27 ; antheridium of, 27 ;
spermatozoids of, 28 ; archegonium of, 28 ;
fertilisation of, 29; embryo of, 30, 31;
life cycle of, 32 (Fig. 1 6).
Fern leaf, origin of, 630, 631 ; differentiation
of, 631.
Ferns, resistance to drought, 18 ; vegetative
propagation of, 19.
Fern spores, dispersion of, 24.
Fertile and sterile regions, their relations,
156, 251.
Fertile spike of Ophioglossaceae, general
morphology of, 432-447, 480 ; development
of, 447, etc.
Filicales, 495 ; general comparison of, 624 ;
external characters of, 625 ; spore-pro-
ducing members of, 632 ; sporangium of,
637 ; anatomy of, 646 ; embryology of;
649 ; phylogeny of, 652 ; essentially
strobiloid, 657 ; of strobiloid origin, 715.
Film}- Ferns, classification of, 585 ; structural
specialisation of, 586 ; reduction of spor-
angia of, 587.
Fission of spike in Ophioglossaceae, 479.
Florideae, 67.
Flower, symmetry of, 207.
Foliar trace, disintegration of, 668.
Foot, 672 ; intra-prothallial of Lycopodiuni,
225 ; in Lycopods, 348 ; in Selaginella,
356.
Formative regions of Hanstein, 178.
Free-living sporophyte, establishment of, 21 8.
Frullania, 264 (Fig. 124).
•Fitcns, 66.
Funaria, 278 (Figs. 136, 137); symmetry
of, 204 (Fig. 103) ; hygrometricat 91.
Fungi, alternation in, 68.
Gametophyte, 32.
Gametophytic budding, 27, 61.
General comparison of Filicales, 624.
Geographical distribution, 5.
Geothallus, 263.
Germinal layers, theory of, 175.
Germs, distribution of, I.
Gleicheniaceae, phyletic position of, 654
(Fig. 354); external characters, 553; spore-
producing members, 555 ; anatomy, 561 ;
spore-enumerations, 559.
Gleichenia, 553 ; cim'nata, 555 (Fig. 310),
557 ; spore-enumerations, 559 ; dicarpa
(Fig. 98) ; anatomy, 563 (Fig. 314); dicho-
toma (Figs. 63, 64), 554 (Fig. 310), 557
(Fig. 311); anatomy, 561 (Fig. 313);
circinata (Figs. 63, 64) ; flabellata (Fig.
64), 554 (Fig. 309), 557 (Fig. 311);
anatomy, 561 (P^ig. 313) ; a central
type, 564 ; pectinata, 554 ; anatomy, 561
(Fig. 313).
Glossopteris, 625.
Gradatae, 117, 497, 498, 470, 634 ; transition
to Mixtae, 602.
Grammatopteris Rigolloti, 498 (Fig. 269),
SOT, 532, 539-
GnetuDi gnenion^ 97.
Gunner a polystely, 192.
Gymnosperms, early existence of, 228.
Gymnosporangitim, 69.
Hairs of Ferns, 632.
Haploid phase, 48, 52.
Hawlea Miltoni, 522 (Fig. 289).
Haustoria, 181, 672.
Haustorium, intra-prothallial, 347.
HelmintkostachyS) 151 (Fig. 83), 443 (Figs.
243, 244) ; external characters, 443 (Figs.
243, 244) ; spore-producing members, 455
(Figs. 254, 255) ; anatomy, 458, etc. (Fig.
256) ; embryo, 473 (Fig. 267).
Hepaticae, 257.
Heterosporous condition, 114.
Heterospory, 703, 716; in Selaginella, 317 ;
in Isoetes, 318; in Lepidostrobus, 324; in
Calamostachys, 381.
Hippuris, 178.
Hofmeister's Vergleichende Untersuchungen,
14, 33-
Homogeny, 76.
Homologous alternation, 76, 79.
Homologous theory of alteration, 47.
Homosporous condition, 113; ferns in Palaeo-
zoic period, 497.
Horsetails, 366.
Hydrodictyon, 64.
Hymenophyllaceae, phyletic position of, 654
(Fig. 354) ; external characters, 575 (Figs.
322, 323) ; spore-producing members, 576
( Figs. 324, 327) ; anatomy, 582 (Fig. 328) ;
classification, 585.
Hymenophy lliles delicatnlus, 582 ; IVeissii,
582.
llymawphylliim, habit, 575 (Fig. 322) ;
sorus, 579 (Fig. 324 bis} ; sporangia, 579
(Fig. 325) ; spore-enumerations, 580 ;
2 Z
722
INDEX
filmy structure, 582 ; stock, 584 (Fig.
328) ; dilatatum (Fig. 68).
Hypobasal appendage of Jungermanniaceae,
analogy with suspensor, 66 1.
Hypobasal tier, 666.
hypoderris, 617.
Hypolepis, 615/616; phyletic position of, 655
(Fig. 354).
Hypothetical archegoniate algae of Tansley,
137, 216.
Imperfectly developed parts, 162.
Indusium, 636 ; reduction of, 637.
Initiation of sporophyte not demonstrated in
any one phylum, 658.
Intercalation of sporophyte, 260.
Interpolation of sporangia, 612.
Irregularities of chromosome-cycle, 58.
Isoetes, 95 (Figs. 52, 53), 307 (Fig. 155);
sporangia of, 318 (Figs. 165, 166) ; ana-
tomy of, 337 (Fig. 177) ; embryology of,
358 (Fig. 191) ; sporophytic budding, 57 ;
stele of, 337 ; secondary thickening of,
338 ; echinospora, 319 ; hystrix, 337
(Fig. 177).
Jungermanniales, 264.
funiperus communis, 127 (Fig. 69).
Kaulfussia, 151, 505 (Fig. 276); sorus of,
512 (Figs. 278, 281, 283) ; anatomy of,
525.
Khtkia, 546 (Fig. 304-).
Laccopteris, 565, 622.
Lastraea pseudo-mas^ v. cristata, 60.
Leaf, "free-living," 183; its vascular supply,
192 ; wings of in ferns, 651 (Fig. 353).
Leaf-formation, in Liverworts, 133 ; in
vascular plants, 134.
Leaf-trace, 193 ; of Ophioglossaceae, 462,
488 ; in ferns, 648.
Leaves, sterile and fertile, 87 ; polyphyletic
origin of, 133.
Lepidocarpon, 704.
Lepidodendron fuliginostim, 338 ; Har-
courtii, 334 (Fig. 174) ; rhodumnense,
334 ; saalfeldense, 334 ; petticurensis,
334; selaginoiaes, 336 (Fig. 176).
Lepidophloios, 304 (Fig. 152).
Lepidostrobus, 305 (Fig. 153) ; Brownii, 95,
322 ; anatomy of, 335.
Lepidoslrobus Veltheimianus, 324 (Fig. 170).
Leptopteris, 530.
Leptosporangiate Ferns, symmetry of, 213 ;
not primitive, 496.
Leucostegia, 615 (Fig. 340).
Ligulatae, 291, 299; embryology of, 356;
" Selago" condition, 700 ; truly primitive,
711.
Lily, pollen-mother-cells of, 49 (Fig. 32).
Lindsay a, 617 ; phyletic position of, 665
(Fig. 354)-
Lornatophloios macrolepidotus, 305.
Loranthiis, 126.
Loxsoma, systematic position of, 574 ; phyletic
position of, 655 (Fig. 354).
Loxsoma Cunninghami, 105 (Fig. 60).
Loxsomaceae, 571 ; spore-producing mem-
bers, 571 ; anatomy, 573.
LoxsomopsiS) see addendum, p. xii.
Lycopodiales, progressive disintegration of
stele, 231 ; general morphology of, 290;
spore-producing members of, 311 ; com-
parative anatomy of, 328 ; embryology
of, 340 ; summary on, 363.
Lycopodites Stockii, 298 (Fig. 147, 321) ;
whorled leaves, 230 ; Gutbieri, 301 ;
pnmaevus, 301 ; Suissei, 301 ; ciliatits, 305 ;
Reidii, 305.
Lycopodiuni) origin of sporangium, 146 (Fig.
75) ; leaf arrangement of, 291 (Fig. 141) ;
section Urostachya, 294, 313 ; section
Rhopalostachya, 294, 314; subgenus
Lepidotis, 296 ; subgenus Diphashim, 296 ;
Selago, 292 ; Subselago, 292 ; alpinnm,
sporangia, 314 (Fig. 161); annotinum,
anatomy of, 329 (Fig. 171); prothallus
of, 341 (Fig. 179) ; embryo of, 347 (Fig.
186) ; cernuwH) 296 (Fig. 143) ; pro-
thallus of, 341 (Fig. 178) ; embryology of,
351 (Fig. 187), 188, 101 ; gametophyte
of, 37 (Fig. 21 ) ; sporophyte of, 38 (Fig.
22); detached leaf-traces, 199 (Fig. 101);
chamaecyparissus, 125 (Fig. 67) ; clavatum,
296; sporangium of, 314; prothallus of,
343 ; embryo of, 347 ; reduced scales of
seedling, 239 (Fig. 117); compaction, 292;
Trencilla, 292 ; firmutn, 292 ; rigidum^
292 ; Dalhousiaeamnn, 292 ; carinainin,
292 ; gnidioides, 292 ; squamosum, 292 ;
Phlegmaria, 293; (Fig. 142); varium,
294 ; subulatum, 294 ; munmulari folium,
294 ; ophioglossoides, 294 ; pinifolium,
294 ; inundatum, 294 ; Drunimondii, 294 ;
cernuum, 295 (Fig. 148) ; clavattiw, 296 ;
i
INDEX
725
carolinianuni) 296 (Fig. 144) ; coni-
planatnni, prothallus of, 334; dichotomnni,
sporangial wall, 325 ; imtndatum, 294 ;
sporangium of. 313 ; prothallus of, 340 ;
embryo of, 351 ; phlegmaria, 293 ; habit
of (Fig. 142) ; sporangium of, 313 (Fig.
158) ; prothallus of, 342 ; embryo of, 346
(Fig. 185); salakense, prothallus of, 340;
Selago, frontispiece ; form of, 292 ; spor-
angium of, 311; anatomy of, 328;
prothallus of, 343 (Figs. 180, 181) ;
embryo of, 345 (Figs. 183, 184) ; com-
parison of, 363.
Lycopods, symmetry of, 210.
Lycopsida, 486.
L.yginodendront 705.
Lygodiitm, 542 (Figs. 301, 302); anatomy,
547 (Figs. 306, 307) ; sttbatatum, fertile
primordial leaves, 187 ; early fertility, 632.
Male shield fern, 15.
Malformations, 481.
Marattia, external characters, 505 ; sorus,
513 (Figs. 278, 283, 285) ; anatomy of,
525 ; embryology of, 527 (Fig. 292).
Marattiaceae, symmetry of, 21 1 ; external
characters, 505 ; spore-producing mem-
bers, 512; anatomy, 524; embryology,
527 ; phyletic position of, 654 (Fig. 354).
Marchantiales, 257.
Marsilia, 511 ; Drununondii, 59.
Marsiliaceae, 551.
Matonia, see Matonineae ; phyletic position
of, 656 (Fig. 354) ; Dipteris series, 618.
Matonidiunt) 567.
Matonineae, 564 (Fig. 315); spore-producing
members, 565 (Figs. 316, 317) ; anatomy,
569 (Fig. 319).
Medullation in Lepidodendron, 334.
Megaphylly, secondary in Ferns, 657.
Megaphyton, 508, 625.
Megasporangia, of Selaginella, 3 1 7 ; of Isoetes,
320.
Meristele, 190.
Meristems of Ferns, comparative study of,
650.
Meroblastic segmentation, 66 1, 665.
Mesarch xylem, of Helininthostachys, 486 ;
of Tmesipteris, 486 (Fig. 268).
Metamorphosis, 157, 151.
Metzgeria, 266.
Miadesmia^ 704; membranacea, 301.
Microdictyon, 567.
2Z 2
Microlepia, 596 (Fig. 332), 613, 614.
Microsporangiaof6V/</^V/f//a, 317; Rhodes*
3I9-
Migration from water to land, 83.
Mixtae, 117, 497, 498, 612, 634.
Mohria, 542 (Figs. 301, 302); anatomy, 548.
Monarch roots in Ophioglossaceae, 458 (Fig.
256) ; in Lycopods. 259.
Monocka, 262 (Fig. 122) ; symmetry of, 204.
Monophyllous habit in Ophioglossaceae, 431-
Monostele, 190.
Moss, cauline stelar column, 195.
Musci, 272.
Mycorhiza, in Cyathea, 240 ; in Neottia
and SarcodeS) 240 ; in Psilotaceae, 241 ;
in Ophioglossales, 241, 477; in Lycopods,
478 ; in Ferns, 478.
Mycorhizic symbiosis, its relation to reduction,.
240.
) 128.
Najas, 127.
Nanomitrium^ 283 (Fig. 140).
Nephrodium dilatatum, apogamy, 53 (Fig.
35)-
hemalion, 67.
Nematophycus, 228.
Nephrodium Hlix-?nas, 15-25 (Figs. I, 2, 4,.
5, 6, 9, 10, n); pseudo-mas, v. cristata,
apogamy and apospory, 56 (Fig. 88) ;
\.polydactylum, 57, 58 (Fig. 39).
Neuropteris, 705.
Non-medullated monostele, 339.
Non-soral state in Ferns, 633.
Notothylas, 269 (Fig. 131).
Nuclear division, 47, 48 (Fig. 31).
Nutrition of sporophyte, 242.
Nutritive cells, 263.
Octants, theory of, 179.
Oligocarpia, 554, 560 (Fig. 312); lindsaeoidest
$22 (Fig. 289).
Onagraceae, 96.
Onoclea, 617 ; sensibilis, 29 (Fig. 13) ;
Strut 'hiopteris, differentiation of leaves,
169 (Fig. 89).
Ophioglossales, symmetry of, 212, 430;
external characters, 431 ; spore-producing
members, 447, 484 ; anatomy, 458 ; em-
bryology, 464, 489 ; comparative discus-
sion of, 476 ; mycorhiza in, 477 ; not a
reduction series, 477 ; origin from sporan-
giophoric Pteridophytes, 493.
724
INDEX
Ophioglossum, external characters, 431 ;
spore-producing members, 447 ; spore-
mother-cells, 451 (Figs. 250, 251); ana-
tomy, 458 (Figs. 256, 258, 259) ; embryo,
466 (Fig. 260); prothallus, 464; crotalo-
phoroides, 431 ; opacuin, 431 ; vtilgatum,
431 (Fig. 235); Bergiamim, 433; bulbo-
sum, 433 ; nudicaule, 433 ; lusitanicum,
423 ; pendulum, 435 ; palinatuin, 435
(Figs. 238, 239) ; simplex, 441 ; inter-
medium, 441 ; reticulatnin, 439, 448 (Fig.
246); 451 (Fig. 250).
Orientation of embryo variable, 666.
Origin of members as new structures, 659 ;
objections answered, 680.
Osmwtda, 530 (Fig. 293) ; sporangia of,
532> 535 (Figs. 296 bis, 296) ; anatomy
of, 536 (Figs. 298, 299) ; embryology of,
540 ; reduced leaves of, 239 ; regalis and
javanica, 169 (Fig. 90).
Osmundaceae, external characters, 530 (Fig.
293) ; spore-producing members, 533 ;
anatomy, 536 ; embryology, 540 ; phyletic
position of, 654 (Fig. 354).
OsmunditeS) 539.
Overtopping, 135, 136.
Pachytheca, 228.
Palaeophytology, evidence of, 227 ; its
limitations, 229.
Palaeopteris .hibernica, 582.
Palaeostachya, 150 (Fig. .81); vera, 375
(Fig. 203) ; morphology of cone, 384
footnote.
Parts, independent origin of, 183.
Pecopteris, 528; (Dicksonites) Phickeneti,
528; dentata, 519 (Fig. 287); unita, 520.
Pellia, 266 (Fig. 128).
Periblem, 178.
Periodic reduction, 84.
Peronospora, 68.
Phascum, 282 (Fig, 139).
Phragmidium, 69.
Phyllanthus, 126.
Phylloglossum, 297 (Figs. 145, 146) ; spor-
angium of, 315 ; embryology of, 352, 355
(Fig. 189) ; detached leaf-traces, 199 ;
protoconn of, 225.
Phylloids (Lignier), 136.
Phyllopodium, 629.
Phyllosiphonic structure, 139, 198; state,
may be derived from cladosiphonic, 487-8;
secondary, 648.
Phyllotheca, 150, 167, 372 (Fig. 197), 384.
Phylogeny of Filicales, 652.
Physcomitrella patens, 36 (Fig. 20).
Physcomitrium , 280 (Fig. 137).
Physiological experiment, 6 ; a check on
phyletic speculation, 236.
Phy tonic theory, anatomical aspect of, 188 ;
of Delpino, 135.
Picea exceha, ovule of, 41 (Fig. 27).
Pilularia, 551.
Pinakodendron musivum, 304.
Pinus Laricio, germination of pollen, 42
(Fig. 28).
Platycerium, 631.
Platyzoma, 553.
Plerome, 178.
Pleitromoia, 220 (Fig. 114), 302 (Fig. 151);
strobilus of, 304 (Fig. 154).
Podostemaceae, symmetry of, 201.
Polarity. 203 ; of embryo variable, 666 ;
inversion of, 675.
Pollen-mother-cells, 49 (Fig. 32).
Polygomun, ovary of, 44 (Fig. 30).
Polyphyletic development, n.
Poly podium, 628 ; phyletic position of, 656
(Fig. 354) ; punctatum, 616; vulgare, 23,
28, 214 (Figs. 7, 12, no); symmetry of
seedling, 214 (Fig. no).
Polysiphonia, 67, 81.
Polysporangiate state, 113.
Polystelic type, 189.
Polystichum angulare, v. pulcherriiiniiu ;
apospory in, 55 (Fig. 37).
Polytrichaceae, stem-structure, 195-6.
Polytrich urn, 281.
Porella, 265 (Fig. 126).
Precocity of cotyledon, 670, 671 ; of root,
672.
Primitive shoot, 716.
Progressive metamorphosis of Goethe, 157,
251.
Prohepatic type of Lignier, 137, 216.
Prothalli of Lycopodium, 340 ; saprophytic,
342 ; subterranean, 343.
Prothallus of Fern, 25.
Protocalamariaceae, 373.
Protoconn, 181, 223, 253, 672 ; in Phanero-
gams, 224; of Lycopods, 351.
Protostelic state in primitive Ferns, 647.
Protoxylem, peripheral in Lycopodinm, 328;
central in Selaginella, 332.
Psaronius, 507, 526, 528.
Pseudobornia, 373, 424.
INDEX
Pseudosteles, 193.
Psilotaceae, 398, 408.
Psilotttm, 88 (Fig. 45) ; sporangiophore,
147 ; spore-producing members of, 416
(Fig. 232); anatomy of, 418 (Fig. 233);
408, 412 (Fig. 229).
Pteridophyta, 288 ; balance of alternating
generations, 36.
Pteridosperms, their discovery, 496.
PtcHs, phyletic position of, 655 (Fig. 354) ;
elata, 616 (Fig. 342) ; heterophylla, 632.
Pteropsida, 486.
Ptychocarpiis, 511, 520 (Fig. 288); unitus,
151 (Fig. 84).
Rachiopteris Oldhamia, 501.
Radial construction, 201, 252.
Radula, 264 (Fig. 125).
Recapitulation, theory of, 173; applicable
within limits, 185 ; exceptions to its
applications, 159, 636, 660.
Receptacle of sorus, 634 ; not a result of
" metamorphosis," 635.
Red Seaweeds, 67.
Reduction, 233, 253 ; its prevalence in
phyletic speculation, 235; of leaf, 139;
in moss-sporogonia, 238 ; in Ophioderma,
241 ; follows on seed-habit, 717 ; of chro-
mosomes, 50 (Fig. 32) ; phyletic delay
in, 77.
Reduction-series, synthetic necessity of, 482.
Rhi~ophora, 96, 142 (Fig. 72).
Rhizophores, of Selaginella, 219.
Rhopalodia, 71 (Fig. 41).
Ricda, 33, 34 (Fig. 17) ; absence of polarity,
203; archegonium of, 257 (Fig. 118).
Ricciocarpus, 34 (Figs. 18, i8A) ; sporo-
gonium of, 257 (Figs. 119, 120).
Riella, 263.
Root of embryo, variable in time and place
of origin, 671, 672; origin of, 216;
exogenous, 219; capless, 219.
Root-apex of Osmundaceae, 649 (Fig. 351).
Rootless sporophytes, 218.
Roots, "free-living," 183.
Root-structure in Ophioglossaceae, 458 (Fig.
256), 489.
Sahinia, 176, 610.
Salviniaceae, 610 ; related to Gradatae, 611.
Schizaea, 543 (Figs. 300, 301, 302) ; anatomy,
549-
Schizaeaceae, external characters, 542; spore-
producing members, 544 ; anatomy, 547 ;
segmentation of sporangium, 547 (Fig.
305) ; phyletic position of, 654 (Fig. 354).
Schizoneura, 372 (Fig. 198).
' Schizostelic state, 19.?.
J Scolopendriiini rulgare (Fig. 93) ; apogamy,
52, 54 (Figs. 34, 35).
Scolecopteris, 511 (Fig. 282), 521 (Fig. 289) ;
polymorpha, 522 (Fig. 289).
Secondary thickening, 690 ; in Lepidoden-
dron, 334 ; in Ophioglossaceae, 488.
Seed-habit, 703, 716; often leads to re-
duction, 705.
Seed-plants, balance of alternating genera-
tions, 43.
Segmentation, 176; of embryo, 179; of
zygote in Lycopods, 345.
Selaginella, origin of sporangium, 146 (Fig.
74); symmetry of, 211.
Selaginalla apus, microsporangium of, 39
(Fig. 23); megasporangium of, 40 (Fig.
24) ; microspore of, 40 (Figs. 25, 26).
Selaginella sanguinolenta, 299 ; Martensii,
299; apus, 317; rupestris, 317; helvetica,
316; Wallichii, 316; Kraussiana, 316;
inaequalifolia, 334 ; Willdonovii, 334 ;
laevigata, 334; spinulosa, 299 (Fig. 51);
basal knot of, 220 (Fig. 113); general
morphology of, 300 (Figs. 148, 149) ;
sporangia of, 316 (Figs. 163, 164); anatomy
of, 332 (Fig. 173) ; embryology of, 356
(Fig. 190).
" Selago" condition, 164; in Lycopods,
164 ; in Isoetes, 165 ; in Psilotaceae, 165 ;
in Ophioglossaceae, 166 ; in Ferns, 167.
Senftenbergia, 546 (Fig. 303) ; Ophioder-
matica, $22 (Fig. 289).
Septa, origin of, 97, no.
Septum in Tmesipteris, 411, 415.
Series of progression, 10 ; of reduction, 10.
Sexual cycle, 75.
Sexuality, a constantly recurring feature, 9.
Sigillaria, stelar structure, 231 ; fructifi-
cations of, 325 ; elongata, 337 ; elegans,
337 ; Menardi, 337 ; spinulosa, 337.
Sigillariostrobus Crepini, 325.
Simplices, 117, 497, 498, 634.
Small-leaved types primitive, 139.
Solenostelar structure, of Gleichenia, 562
(Fig- 313); of Matonia, 569 (Fig. 319);
of Loxsoina, 573 (Fig. 321) ; of Dcnu-
staedtiinae, 600 (Fig. 333) ; of Pteris, 616
(Fig. 342).
726
INDEX
Solenostele, 190 ; in Ferns, 647.
Somatic expansion, 77.
Soral state in Ferns, 633.
Sorus, a sporangiophore, 151 ; fission of,
633 ; primitive position of, 633 ; shifting
of position of, 636 ; extension of, in
Ferns, 699.
Speculative morphology, 6.
Spencerites, 146 (Fig. 76) ; insignis, 321
(Fig. 167).
:Spermatozoids, fertilisation by, 2, 244.
Sphaerocarpus% 92, 263.
Sphaeropteris, 617.
Sphagnales, 272.
Sphagnum, 93 (Fig. 48), 272 (Fig. 132).
Sphenophylhtm, vegetative system, 399 ;
anatomy, 400; strobilus, 401.
•Sphenophyllales, 398 ; summary for, 423.
Sphenophylleae, 230, 398.
.Sphenophyllum cuneifolium, 400 (Fig. 216)
\ = S. Dawsoni), 402, 425 (Fig. 219);
S, tenerrimtim, 400 (Fig. 216) ; S. verti-
cillatum, 400 (Fig. 216) ; majus, 147
•(Fig. 78); insigne, 400 (Fig. 217); S.
.trichomatosum, 402 (Fig. 218); S. angusti-
folittm, 402 ; tenerrimum, 402 ; Romeri,
402, 425 (Fig. 220) ; majus, 402, 424
(Figs. 221, 222) ; fertile, 404.
.Splachnum, 281 (Fig. 138) ; luteum, 203
(Fig. 102).
-Sporangia, 693 ; positions of, 694 (Fig. 360) ;
increase and decrease of, 86 ; uniformity
of dimensions of, '114; indefiniteness of
number, 115; relation to axis, 115; in-
dividual identity of, 117; simultaneous
•or successive, 117; variations in number
-of, 119, 129, 249; increase in number of,
120, 249 ; decrease in number of, 120, 249;
septation of, 120, 249; interpolation of,
1 20, 121, 249 ; interpolation restricted to
certain groups, 130; fusion of, 120, 126,
130, 249 ; abortion of, 120, 127, 161, 249.
..Sporangiophore, 144, 250, 693 ; of Tmesip-
teris, 409, 410, 414 ; of Psilotum, 412,
416 ; number of sporangia, 425 ; position,
425 ; development, 426 ; a part sui generis,
153, 426; amplification of, 699; positions of,
694 (Fig. 360); of Helminthostachys, origin
of, 455 (Figs. 254, 255) ; of Eqitisetum,
37i, 377, 379; morphology of, 382.
.Sporangiophoric Pteridophytes, 366 ; sum-
mary for, 423 ; a brush of related phyletic
lines, 712-714.
Sporangium defined, 103, 1 12; individuality
of, no; septation of, no; of Ferns,
segmentation of, 637 (Fig. 349) ; stalk of,
638 ; head of, 638 ; annulus of, 638 ; pluri-
seriate annulus, 639 ; contents of, 641 ;
succession of, 644 ; of Filicales, 637.
Sporangiogenic band, 447, 449 (Figs. 247,
248).
Spore-enumerations, 641 ; variation in num-
ber in near affinities, 643 ; in Botryopteri-
deae, 502; in Marattiaceae, 516, 520; in
Osmundaceae, 536 ; in Schizaeaceae, 547.
Spore-output of Male Fern, 23.
Spore-producing members, 693 ; of Filicales,
632.
Spore-production a constantly recurring
event, 9.
Spores, dispersal of, 645 ; in Simplices, 645 ;
in Gradatae, 646 (Fig. 350) ; in Mixtae,
646.
Sporogonia, symmetry of, 203 ; of Mosses,
general comparison of, 285.
Sporogenous group, 87 ; tissue, segregation
of, 85 ; hypodermal origin of, 109 ; not
strictly circumscribed, 112; time of dis-
tinctive development, 116; disintegration
of, 142.
Sporophyll converted to foliage leaf (Goebel),
171 ; of Tmesipteris, 409, 410, 414 ; of
Psilotum, 411, 416.
Sporophyte, 32.
Sporophytic budding, 20, 61.
Sporophylls, 144.
Spross-glied-lehre of Celakovsky, 135.
Stachannularia, 377-
Stauropteris oldhaniia spores germinate in
sporangium, 497, 498 (Fig. 271), 501.
Stigmarian trunks, 220 (Fig. 112); 302
(Fig. 150).
Stegocarpae, 277.
Stelar theory, 189.
Stele, 189 ; non-medullated monostele pri-
mitive, 685 ; medullation, 687 ; disinte-
gration, 687 ; xylem-sponge of Lycopods,
688 ; intrusion of outer tissues leads to
solenostele, 688 ; of Lycopods, 328 ; of
Selaginella, 332 (Fig. 173); of Lepidoden-
dron, 333 (Fig. 174).
Stem-apex of Angiopteris and Osmunda, 650
(Fig. 352).
Sterile and fertile regions, their relations,
156, 251.
Sterile region secondary, 161.
INDEX
727
Sterilisation, 84, 87, 161, 246; in Pterido-
phyta, 89, 93 ; in Bryophyta, 90 ; in
Seed-Plants, 96, 97 (Fig. 57) ; in Mar-
chantiales, 263 ; in Jungermanniales, 267;
in Anthocerotales, 269 ; in Hepaticae
generally, 271; in Mosses, 286; in
Bryophytes, 660.
Stock of Ophioglossaceae, structure of, 459,
etc. (Fig. 236) ; of O. fiergianum, 460
(Fig. 258) ; of Hdminthostachys, 460 (Fig.
257). "
Stomata, functionlcss in Sphagnum, 274.
Storage-tuber of Phylloglossum, 352.
Strobili, symmetry of, 208.
Strobilus, theory of, 132, 138, 248; of
Equisetuni) 370.
Stromatopteris, 553.
Stnithiopteris, 631.
Subtending position of bract, 695 (Fig. 361).
Suppression, 162.
Suspensor, its variability, 182; in Botrychium
obliquum, 472 (Figs. 264-266) ; present or
absent, 675.
Symmetry, 201, 252; radial primitive for
sporophyte, 203, 217, 252.
Synangia, in Botrychium Lunaria, 453 (Fig.
252) ; in Botrychium daucifolium, 454
(Fig. 253); of Marattiaceae, 512.
Synapsis, 50 (Fig. 32).
Synthetic types, 230.
Tapetum, 104.
Thyrsopterideae, 589.
Thyrsopteris, 589 (Fig. 329); phyletic
position of, 655 (Fig. 354).
Tetrad-division, 49, 50 (Fig. 32), 87.
Tetraspores, 66, 67, 68.
Thallophytes, alternation in, 63.
Tmesipteris, 144 (Fig. 73) ; 408 (Figs. 226,
227, 228) ; spore-producing members of,
413 (Figs. 230, 231); anatomy of, 419
(Fig. 234); sporangiophore, 147 (Pig. 77);
Tannensis, septum fertile, 99 (Fig. 58).
Todea, 530, sporangia of, 532 (Fig. 294),
533 (Figs. 295, 296) ; anatomy of, 536
(Fig. 298) ; superba, 582.
Trabeculae of hoetes, 95, 318 ; of Lepido-
strobus, 323.
Tradesfantia% pollen grain of, 43 (Fig. 29).
Trichomanes, habit, 575 (Fig. 323) ; soius,
578 (Fig. 324) ; sporangia, 579 (Figs. 325,
327) ; spore enumerations, 580 ; filmy
structure, 582; stock, 584; $Feea, 631:
alatum, apogamy and apospory, 56.
Tubicaulis, 501.
Uebergipfelung theory of Potonie, 135.
Ulex seedlings, 185.
Ulothrix, 73.
Uredineae, 69.
Vaccinium Myrtillus, symmetry of, 209.
Radial type of early Ferns, 625.
Vascular shoot, symmetry of, 206.
Vascular skeleton, 685.
Vaucheria, 64.
Vegetatio languescens, 1 5 J-
Vegetative region of Pteridophytes, sym-
metry of, 209.
Velum, origin of, 319.
Venation in Ophioglossaceae, 463.
Ventral lobe, 153, 426, 481.
Water-relation, 2.
Welwitschia, floral symmetry, 208.
Whorled leaves, probably primitive, 711.
Wings of leaf in Ferns, comparative study
of, 651 (Fig. 359).
Woodsia, 617-
Working hypothesis, summary of, 244.
Xylem-core, 334.
Xylem-islands, 330.
Xylem-sponge in Lycopodium, 330.
Zygopteris, 585, sporangia of, 501, 508 (Fig.
272) ; 529, Grayi, 499 (Fig. 270).
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