The question of phylogeny is really inseparable from that of the truth
of the doctrine of evolution, for we cannot have historical evidence
that evolution has actually taken place without at the same time having
evidence of the course it has followed.
As already pointed out, the progress hitherto made has been rather in
the way of joining up the great classes of plants than in tracing the
descent of particular species or genera of the recent flora. There
appears to be a difference in this respect from the Animal record,
which tells us so much about the descent of living species, such as the
elephant or the horse. The reason for this difference is no doubt to be
found in the fact that the later part of the palaeontological record
is the most satisfactory in the case of animals and the least so in the
case of plants. The Tertiary plant-remains, in the great majority of
instances, are impressions of leaves, the conclusions to be drawn from
which are highly precarious; until the whole subject of Angiospermous
palaeobotany has been reinvestigated, it would be rash to venture on
any statements as to the descent of the families of Dicotyledons or
Monocotyledons.
Our attention will be concentrated on the following questions, all
relating to the phylogeny of main groups of plants: i. The Origin of the
Angiosperms. ii. The Origin of the Seed-plants. iii. The Origin of the
different classes of the Higher Cryptogamia.
i. THE ORIGIN OF THE ANGIOSPERMS.
The first of these questions has long been the great crux of botanical
phylogeny, and until quite recently no light had been thrown upon the
difficulty. The Angiosperms are the Flowering Plants, par excellence,
and form, beyond comparison, the dominant sub-kingdom in the flora of
our own age, including, apart from a few Conifers and Ferns, all the
most familiar plants of our fields and gardens, and practically all
plants of service to man. All recent work has tended to separate the
Angiosperms more widely from the other seed-plants now living, the
Gymnosperms. Vast as is the range of organisation presented by the great
modern sub-kingdom, embracing forms adapted to every environment, there
is yet a marked uniformity in certain points of structure, as in the
development of the embryo-sac and its contents, the pollination
through the intervention of a stigma, the strange phenomenon of double
fertilisation (One sperm fertilising the egg, while the other unites
with the embryo-sac nucleus, itself the product of a nuclear fusion, to
give rise to a nutritive tissue, the endosperm.), the structure of
the stamens, and the arrangement of the parts of the flower. All these
points are common to Monocotyledons and Dicotyledons, and separate the
Angiosperms collectively from all other plants.
In geological history the Angiosperms first appear in the Lower
Cretaceous, and by Upper Cretaceous times had already swamped all other
vegetation and seized the dominant position which they still hold. Thus
they are isolated structurally from the rest of the Vegetable Kingdom,
while historically they suddenly appear, almost in full force, and
apparently without intermediaries with other groups. To quote Darwin's
vigorous words: "The rapid development, as far as we can judge, of
all the higher plants within recent geological times is an abominable
mystery." ("More Letters of Charles Darwin", Vol. II. page 20, letter
to J.D. Hooker, 1879.) A couple of years later he made a bold suggestion
(which he only called an "idle thought") to meet this difficulty. He
says: "I have been so astonished at the apparently sudden coming in of
the higher phanerogams, that I have sometimes fancied that development
might have slowly gone on for an immense period in some isolated
continent or large island, perhaps near the South Pole." (Ibid, page
26, letter to Hooker, 1881.) This idea of an Angiospermous invasion from
some lost southern land has sometimes been revived since, but has not,
so far as the writer is aware, been supported by evidence. Light on the
problem has come from a different direction.
The immense development of plants with the habit of Cycads, during the
Mesozoic Period up to the Lower Cretaceous, has long been known. The
existing Order Cycadaceae is a small family, with 9 genera and perhaps
100 species, occurring in the tropical and sub-tropical zones of both
the Old and New World, but nowhere forming a dominant feature in the
vegetation. Some few attain the stature of small trees, while in the
majority the stem is short, though often living to a great age. The
large pinnate or rarely bipinnate leaves give the Cycads a superficial
resemblance in habit to Palms. Recent Cycads are dioecious; throughout
the family the male fructification is in the form of a cone, each scale
of the cone representing a stamen, and bearing on its lower surface
numerous pollen-sacs, grouped in sori like the sporangia of Ferns. In
all the genera, except Cycas itself, the female fructifications are
likewise cones, each carpel bearing two ovules on its margin. In Cycas,
however, no female cone is produced, but the leaf-like carpels, bearing
from two to six ovules each, are borne directly on the main stem of the
plant in rosettes alternating with those of the ordinary leaves--the
most primitive arrangement known in any living seed-plant. The
whole Order is relatively primitive, as shown most strikingly in its
cryptogamic mode of fertilisation, by means of spermatozoids, which it
shares with the maidenhair tree alone, among recent seed-plants.
In all the older Mesozoic rocks, from the Trias to the Lower Cretaceous,
plants of the Cycad class (Cycadophyta, to use Nathorst's comprehensive
name) are extraordinarily abundant in all parts of the world; in
fact they were almost as prominent in the flora of those ages as the
Dicotyledons are in that of our own day. In habit and to a great extent
in anatomy, the Mesozoic Cycadophyta for the most part much resemble the
recent Cycadaceae. But, strange to say, it is only in the rarest
cases that the fructification has proved to be of the simple type
characteristic of the recent family; the vast majority of the abundant
fertile specimens yielded by the Mesozoic rocks possess a type of
reproductive apparatus far more elaborate than anything known in
Cycadaceae or other Gymnosperms. The predominant Mesozoic family,
characterised by this advanced reproductive organisation, is known
as the Bennettiteae; in habit these plants resembled the more stunted
Cycads of the recent flora, but differed from them in the presence of
numerous lateral fructifications, like large buds, borne on the stem
among the crowded bases of the leaves. The organisation of these
fructifications was first worked out on European specimens by
Carruthers, Solms-Laubach, Lignier and others, but these observers had
only more or less ripe fruits to deal with; the complete structure of
the flower has only been elucidated within the last few years by the
researches of Wieland on the magnificent American material, derived from
the Upper Jurassic and Lower Cretaceous beds of Maryland, Dakota and
Wyoming. (G.R. Wieland, "American Fossil Cycads", Carnegie Institution,
Washington, 1906.) The word "flower" is used deliberately, for reasons
which will be apparent from the following brief description, based on
Wieland's observations.
The fructification is attached to the stem by a thick stalk, which,
in its upper part, bears a large number of spirally arranged bracts,
forming collectively a kind of perianth and completely enclosing the
essential organs of reproduction. The latter consist of a whorl of
stamens, of extremely elaborate structure, surrounding a central cone
or receptacle bearing numerous ovules. The stamens resemble the fertile
fronds of a fern; they are of a compound, pinnate form, and bear
very large numbers of pollen-sacs, each of which is itself a compound
structure consisting of a number of compartments in which the pollen
was formed. In their lower part the stamens are fused together by their
stalks, like the "monadelphous" stamens of a mallow. The numerous ovules
borne on the central receptacle are stalked, and are intermixed with
sterile scales; the latter are expanded at their outer ends, which are
united to form a kind of pericarp or ovary-wall, only interrupted by the
protruding micropyles of the ovules. There is thus an approach to
the closed pistil of an Angiosperm, but it is evident that the ovules
received the pollen directly. The whole fructification is of large size;
in the case of Cycadeoidea dacotensis, one of the species investigated
by Wieland, the total length, in the bud condition, is about 12 cm.,
half of which belongs to the peduncle.
The general arrangement of the organs is manifestly the same as in a
typical Angiospermous flower, with a central pistil, a surrounding whorl
of stamens and an enveloping perianth; there is, as we have seen, some
approach to the closed ovary of an Angiosperm; another point, first
discovered nearly 20 years ago by Solms-Laubach in his investigation of
a British species, is that the seed was practically "exalbuminous," its
cavity being filled by the large, dicotyledonous embryo, whereas in all
known Gymnosperms a large part of the sac is occupied by a nutritive
tissue, the prothallus or endosperm; here also we have a condition only
met with elsewhere among the higher Flowering Plants.
Taking all the characters into account, the indications of affinity
between the Mesozoic Cycadophyta and the Angiosperms appear extremely
significant, as was recognised by Wieland when he first discovered the
hermaphrodite nature of the Bennettitean flower. The Angiosperm
with which he specially compared the fossil type was the Tulip tree
(Liriodendron) and certainly there is a remarkable analogy with
the Magnoliaceous flowers, and with those of related orders such as
Ranunculaceae and the Water-lilies. It cannot, of course, be maintained
that the Bennettiteae, or any other Mesozoic Cycadophyta at present
known, were on the direct line of descent of the Angiosperms, for there
are some important points of difference, as, for example, in the great
complexity of the stamens, and in the fact that the ovary-wall
or pericarp was not formed by the carpels themselves, but by the
accompanying sterile scale-leaves. Botanists, since the discovery of the
bisexual flowers of the Bennettiteae, have expressed different views as
to the nearness of their relation to the higher Flowering Plants, but
the points of agreement are so many that it is difficult to resist the
conviction that a real relation exists, and that the ancestry of the
Angiosperms, so long shrouded in complete obscurity, is to be sought
among the great plexus of Cycad-like plants which dominated the flora
of the world in Mesozoic times. (On this subject see, in addition
to Wieland's great work above cited, F.W. Oliver, "Pteridosperms
and Angiosperms", "New Phytologist", Vol. V. 1906; D.H. Scott,
"The Flowering Plants of the Mesozoic Age in the Light of Recent
Discoveries", "Journal R. Microscop. Soc." 1907, and especially E.A.N.
Arber and J. Parkin, "On the Origin of Angiosperms", "Journal Linn.
Soc." (Bot.) Vol. XXXVIII. page 29, 1907.)
The great complexity of the Bennettitean flower, the earliest known
fructification to which the word "flower" can be applied without forcing
the sense, renders it probable, as Wieland and others have pointed
out, that the evolution of the flower in Angiosperms has consisted
essentially in a process of reduction, and that the simplest forms
of flower are not to be regarded as the most primitive. The older
morphologists generally took the view that such simple flowers were to
be explained as reductions from a more perfect type, and this opinion,
though abandoned by many later writers, appears likely to be true when
we consider the elaboration of floral structure attained among the
Mesozoic Cycadophyta, which preceded the Angiosperms in evolution.
If, as now seems probable, the Angiosperms were derived from ancestors
allied to the Cycads, it would naturally follow that the Dicotyledons
were first evolved, for their structure has most in common with that of
the Cycadophyta. We should then have to regard the Monocotyledons as
a side-line, diverging probably at a very early stage from the main
dicotyledonous stock, a view which many botanists have maintained,
of late, on other grounds. (See especially Ethel Sargant, "The
Reconstruction of a Race of Primitive Angiosperms", "Annals of Botany",
Vol. XXII. page 121, 1908.) So far, however, as the palaeontological
record shows, the Monocotyledons were little if at all later in their
appearance than the Dicotyledons, though always subordinate in numbers.
The typical and beautifully preserved Palm-wood from Cretaceous rocks
is striking evidence of the early evolution of a characteristic
monocotyledonous family. It must be admitted that the whole question of
the evolution of Monocotyledons remains to be solved.
Accepting, provisionally, the theory of the cycadophytic origin of
Angiosperms, it is interesting to see to what further conclusions we
are led. The Bennettiteae, at any rate, were still at the gymnospermous
level as regards their pollination, for the exposed micropyles of the
ovules were in a position to receive the pollen directly, without the
intervention of a stigma. It is thus indicated that the Angiosperms
sprang from a gymnospermous source, and that the two great phyla of
Seed-plants have not been distinct from the first, though no doubt the
great majority of known Gymnosperms, especially the Coniferae, represent
branch-lines of their own.
The stamens of the Bennettiteae are arranged precisely as in an
angiospermous flower, but in form and structure they are like the
fertile fronds of a Fern, in fact the compound pollen-sacs, or synangia
as they are technically called, almost exactly agree with the spore-sacs
of a particular family of Ferns--the Marattiaceae, a limited group,
now mainly tropical, which was probably more prominent in the later
Palaeozoic times than at present. The scaly hairs, or ramenta, which
clothe every part of the plant, are also like those of Ferns.
It is not likely that the characters in which the Bennettiteae resemble
the Ferns came to them directly from ancestors belonging to that class;
an extensive group of Seed-plants, the Pteridospermeae, existed in
Palaeozoic times and bear evident marks of affinity with the Fern
phylum. The fern-like characters so remarkably persistent in the highly
organised Cycadophyta of the Mesozoic were in all likelihood derived
through the Pteridosperms, plants which show an unmistakable approach to
the cycadophytic type.
The family Bennettiteae thus presents an extraordinary association
of characters, exhibiting, side by side, features which belong to the
Angiosperms, the Gymnosperms and the Ferns.
ii. ORIGIN OF SEED-PLANTS.
The general relation of the gymnospermous Seed-plants to the Higher
Cryptogamia was cleared up, independently of fossil evidence, by the
brilliant researches of Hofmeister, dating from the middle of the
past century. (W. Hofmeister, "On the Germination, Development and
Fructification of the Higher Cryptogamia", Ray Society, London, 1862.
The original German treatise appeared in 1851.) He showed that "the
embryo-sac of the Coniferae may be looked upon as a spore remaining
enclosed in its sporangium; the prothallium which it forms does not come
to the light." (Ibid. page 438.) He thus determined the homologies on
the female side. Recognising, as some previous observers had already
done, that the microspores of those Cryptogams in which two kinds of
spore are developed, are equivalent to the pollen-grains of the higher
plants, he further pointed out that fertilisation "in the Rhizocarpeae
and Selaginellae takes place by free spermatozoa, and in the Coniferae
by a pollen-tube, in the interior of which spermatozoa are probably
formed"--a remarkable instance of prescience, for though spermatozoids
have not been found in the Conifers proper, they were demonstrated
in the allied groups Cycadaceae and Ginkgo, in 1896, by the Japanese
botanists Ikeno and Hirase. A new link was thus established between the
Gymnosperms and the Cryptogams.
It remained uncertain, however, from which line of Cryptogams the
gymnospermous Seed-plants had sprung. The great point of morphological
comparison was the presence of two kinds of spore, and this was known to
occur in the recent Lycopods and Water-ferns (Rhizocarpeae) and was
also found in fossil representatives of the third phylum, that of the
Horsetails. As a matter of fact all the three great Cryptogamic classes
have found champions to maintain their claim to the ancestry of the
Seed-plants, and in every case fossil evidence was called in. For a long
time the Lycopods were the favourites, while the Ferns found the least
support. The writer remembers, however, in the year 1881, hearing the
late Prof. Sachs maintain, in a lecture to his class, that the descent
of the Cycads could be traced, not merely from Ferns, but from a
definite family of Ferns, the Marattiaceae, a view which, though in a
somewhat crude form, anticipated more modern ideas.
Williamson appears to have been the first to recognise the presence, in
the Carboniferous flora, of plants combining the characters of Ferns and
Cycads. (See especially his "Organisation of the Fossil Plants of the
Coal-Measures", Part XIII. "Phil. Trans. Royal Soc." 1887 B. page 299.)
This conclusion was first reached in the case of the genera Heterangium
and Lyginodendron, plants, which with a wholly fern-like habit, were
found to unite an anatomical structure holding the balance between
that of Ferns and Cycads, Heterangium inclining more to the former
and Lyginodendron to the latter. Later researches placed Williamson's
original suggestion on a firmer basis, and clearly proved the
intermediate nature of these genera, and of a number of others, so far
as their vegetative organs were concerned. This stage in our knowledge
was marked by the institution of the class Cycadofilices by Potonie in
1897.
Nothing, however, was known of the organs of reproduction of the
Cycadofilices, until F.W. Oliver, in 1903, identified a fossil seed,
Lagenostoma Lomaxi, as belonging to Lyginodendron, the identification
depending, in the first instance, on the recognition of an identical
form of gland, of very characteristic structure, on the vegetative
organs of Lyginodendron and on the cupule enveloping the seed. This
evidence was supported by the discovery of a close anatomical agreement
in other respects, as well as by constant association between the seed
and the plant. (F.W. Oliver and D.H. Scott, "On the Structure of the
Palaeozoic Seed, Lagenostoma Lomaxi, etc." "Phil. Trans. Royal Soc."
Vol. 197 B. 1904.) The structure of the seed of Lyginodendron, proved to
be of the same general type as that of the Cycads, as shown especially
by the presence of a pollen-chamber or special cavity for the reception
of the pollen-grains, an organ only known in the Cycads and Ginkgo among
recent plants.
Within a few months after the discovery of the seed of Lyginodendron,
Kidston found the large, nut-like seed of a Neuropteris, another
fern-like Carboniferous plant, in actual connection with the pinnules
of the frond, and since then seeds have been observed on the frond in
species of Aneimites and Pecopteris, and a vast body of evidence, direct
or indirect, has accumulated, showing that a large proportion of the
Palaeozoic plants formerly classed as Ferns were in reality reproduced
by seeds of the same type as those of recent Cycadaceae. (A summary
of the evidence will be found in the writer's article "On the present
position of Palaeozoic Botany", "Progressus Rei Botanicae", 1907, page
139, and "Studies in Fossil Botany", Vol. II. (2nd edition) London,
1909.) At the same time, the anatomical structure, where it is open to
investigation, confirms the suggestion given by the habit, and shows
that these early seed-bearing plants had a real affinity with Ferns.
This conclusion received strong corroboration when Kidston, in 1905,
discovered the male organs of Lyginodendron, and showed that they were
identical with a fructification of the genus Crossotheca, hitherto
regarded as belonging to Marattiaceous Ferns. (Kidston, "On the
Microsporangia of the Pteridospermeae, etc." "Phil. Trans. Royal Soc."
Vol. 198, B. 1906.)
The general conclusion which follows from the various observations
alluded to, is that in Palaeozoic times there was a great body of plants
(including, as it appears, a large majority of the fossils previously
regarded as Ferns) which had attained the rank of Spermophyta,
bearing seeds of a Cycadean type on fronds scarcely differing from the
vegetative foliage, and in other respects, namely anatomy, habit and the
structure of the pollen-bearing organs, retaining many of the characters
of Ferns. From this extensive class of plants, to which the name
Pteridospermeae has been given, it can scarcely be doubted that the
abundant Cycadophyta, of the succeeding Mesozoic period, were derived.
This conclusion is of far-reaching significance, for we have already
found reason to think that the Angiosperms themselves sprang, in
later times, from the Cycadophytic stock; it thus appears that the
Fern-phylum, taken in a broad sense, ultimately represents the source
from which the main line of descent of the Phanerogams took its rise.
It must further be borne in mind that in the Palaeozoic period there
existed another group of seed-bearing plants, the Cordaiteae, far more
advanced than the Pteridospermeae, and in many respects approaching the
Coniferae, which themselves begin to appear in the latest Palaeozoic
rocks. The Cordaiteae, while wholly different in habit from the
contemporary fern-like Seed-plants, show unmistakable signs of a common
origin with them. Not only is there a whole series of forms
connecting the anatomical structure of the Cordaiteae with that of the
Lyginodendreae among Pteridosperms, but a still more important point is
that the seeds of the Cordaiteae, which have long been known, are of
the same Cycadean type as those of the Pteridosperms, so that it is not
always possible, as yet, to discriminate between the seeds of the two
groups. These facts indicate that the same fern-like stock which gave
rise to the Cycadophyta and through them, as appears probable, to the
Angiosperms, was also the source of the Cordaiteae, which in their turn
show manifest affinity with some at least of the Coniferae. Unless the
latter are an artificial group, a view which does not commend itself to
the writer, it would appear probable that the Gymnosperms generally,
as well as the Angiosperms, were derived from an ancient race of
Cryptogams, most nearly related to the Ferns. (Some botanists, however,
believe that the Coniferae, or some of them, are probably more nearly
related to the Lycopods. See Seward and Ford, "The Araucarieae, Recent
and Extinct", "Phil. Trans. Royal Soc." Vol. 198 B. 1906.)
It may be mentioned here that the small gymnospermous group Gnetales
(including the extraordinary West African plant Welwitschia) which were
formerly regarded by some authorities as akin to the Equisetales, have
recently been referred, on better grounds, to a common origin with the
Angiosperms, from the Mesozoic Cycadophyta.
The tendency, therefore, of modern work on the palaeontological record
of the Seed-plants has been to exalt the importance of the Fern-phylum,
which, on present evidence, appears to be that from which the great
majority, possibly the whole, of the Spermophyta have been derived.
One word of caution, however, is necessary. The Seed-plants are of
enormous antiquity; both the Pteridosperms and the more highly organised
family Cordaiteae, go back as far in geological history (namely to the
Devonian) as the Ferns themselves or any other Vascular Cryptogams. It
must therefore be understood that in speaking of the derivation of the
Spermophyta from the Fern-phylum, we refer to that phylum at a very
early stage, probably earlier than the most ancient period to which
our record of land-plants extends. The affinity between the oldest
Seed-plants and the Ferns, in the widest sense, seems established, but
the common stock from which they actually arose is still unknown; though
no doubt nearer to the Ferns than to any other group, it must have
differed widely from the Ferns as we now know them, or perhaps even from
any which the fossil record has yet revealed to us.
iii. THE ORIGIN OF THE HIGHER CRYPTOGAMIA.
The Sub-kingdom of the higher Spore-plants, the Cryptogamia possessing a
vascular system, was more prominent in early geological periods than at
present. It is true that the dominance of the Pteridophyta in Palaeozoic
times has been much exaggerated owing to the assumption that everything
which looked like a Fern really was a Fern. But, allowing for the fact,
now established, that most of the Palaeozoic fern-like plants were
already Spermophyta, there remains a vast mass of Cryptogamic forms
of that period, and the familiar statement that they formed the main
constituent of the Coal-forests still holds good. The three
classes, Ferns (Filicales), Horsetails (Equisetales) and Club-mosses
(Lycopodiales), under which we now group the Vascular Cryptogams, all
extend back in geological history as far as we have any record of the
flora of the land; in the Palaeozoic, however, a fourth class, the
Sphenophyllales, was present.
As regards the early history of the Ferns, which are of special interest
from their relation to the Seed-plants, it is impossible to speak quite
positively, owing to the difficulty of discriminating between true
fossil Ferns and the Pteridosperms which so closely simulated them.
The difficulty especially affects the question of the position of
Marattiaceous Ferns in the Palaeozoic Floras. This family, now so
restricted, was until recently believed to have been one of the
most important groups of Palaeozoic plants, especially during later
Carboniferous and Permian times. Evidence both from anatomy and from
sporangial characters appeared to establish this conclusion. Of late,
however, doubts have arisen, owing to the discovery that some
supposed members of the Marattiaceae bore seeds, and that a form of
fructification previously referred to that family (Crossotheca) was
really the pollen-bearing apparatus of a Pteridosperm (Lyginodendron).
The question presents much difficulty; though it seems certain that our
ideas of the extent of the family in Palaeozoic times will have to be
restricted, there is still a decided balance of evidence in favour
of the view that a considerable body of Marattiaceous Ferns actually
existed. The plants in question were of large size (often arborescent)
and highly organised--they represent, in fact, one of the highest
developments of the Fern-stock, rather than a primitive type of the
class.
There was, however, in the Palaeozoic period, a considerable group of
comparatively simple Ferns (for which Arber has proposed the collective
name Primofilices); the best known of these are referred to the family
Botryopterideae, consisting of plants of small or moderate dimensions,
with, on the whole, a simple anatomical structure, in certain cases
actually simpler than that of any recent Ferns. On the other hand the
sporangia of these plants were usually borne on special fertile fronds,
a mark of rather high differentiation. This group goes back to the
Devonian and includes some of the earliest types of Fern with which we
are acquainted. It is probable that the Primofilices (though not the
particular family Botryopterideae) represent the stock from which the
various families of modern Ferns, already developed in the Mesozoic
period, may have sprung.
None of the early Ferns show any clear approach to other classes of
Vascular Cryptogams; so far as the fossil record affords any evidence,
Ferns have always been plants with relatively large and usually compound
leaves. There is no indication of their derivation from a microphyllous
ancestry, though, as we shall see, there is some slight evidence for the
converse hypothesis. Whatever the origin of the Ferns may have been it
is hidden in the older rocks.
It has, however, been held that certain other Cryptogamic phyla had
a common origin with the Ferns. The Equisetales are at present a
well-defined group; even in the rich Palaeozoic floras the habit,
anatomy and reproductive characters usually render the members of this
class unmistakable, in spite of the great development and stature which
they then attained. It is interesting, however, to find that in the
oldest known representatives of the Equisetales the leaves were highly
developed and dichotomously divided, thus differing greatly from the
mere scale-leaves of the recent Horsetails, or even from the simple
linear leaves of the later Calamites. The early members of the class, in
their forked leaves, and in anatomical characters, show an approximation
to the Sphenophyllales, which are chiefly represented by the large genus
Sphenophyllum, ranging through the Palaeozoic from the Middle Devonian
onwards. These were plants with rather slender, ribbed stems, bearing
whorls of wedge-shaped or deeply forked leaves, six being the typical
number in each whorl. From their weak habit it has been conjectured,
with much probability, that they may have been climbing plants, like the
scrambling Bedstraws of our hedgerows. The anatomy of the stem is simple
and root-like; the cones are remarkable for the fact that each scale or
sporophyll is a double structure, consisting of a lower, usually sterile
lobe and one or more upper lobes bearing the sporangia; in one species
both parts of the sporophyll were fertile. Sphenophyllum was evidently
much specialised; the only other known genus is based on an isolated
cone, Cheirostrobus, of Lower Carboniferous age, with an extraordinarily
complex structure. In this genus especially, but also in the entire
group, there is an evident relation to the Equisetales; hence it is of
great interest that Nathorst has described, from the Devonian of Bear
Island in the Arctic regions, a new genus Pseudobornia, consisting of
large plants, remarkable for their highly compound leaves which,
when found detached, were taken for the fronds of a Fern. The whorled
arrangement of the leaves, and the habit of the plant, suggest
affinities either with the Equisetales or the Sphenophyllales; Nathorst
makes the genus the type of a new class, the Pseudoborniales. (A.G.
Nathorst, "Zur Oberdevonischen Flora der Baren-Insel", "Kongl. Svenska
Vetenskaps-Akademiens Handlingar" Bd. 36, No. 3, Stockholm, 1902.)
The available data, though still very fragmentary, certainly suggest
that both Equisetales and Sphenophyllales may have sprung from a
common stock having certain fern-like characters. On the other hand the
Sphenophylls, and especially the peculiar genus Cheirostrobus, have in
their anatomy a good deal in common with the Lycopods, and of late years
they have been regarded as the derivatives of a stock common to
that class and the Equisetales. At any rate the characters of the
Sphenophyllales and of the new group Pseudoborniales suggest the
existence, at a very early period, of a synthetic race of plants,
combining the characters of various phyla of the Vascular Cryptogams.
It may further be mentioned that the Psilotaceae, an isolated epiphytic
family hitherto referred to the Lycopods, have been regarded by several
recent authors as the last survivors of the Sphenophyllales, which they
resemble both in their anatomy and in the position of their sporangia.
The Lycopods, so far as their early history is known, are remarkable
rather for their high development in Palaeozoic times than for any
indications of a more primitive ancestry. In the recent Flora, two
of the four living genera (Excluding Psilotaceae.) (Selaginella and
Isoetes) have spores of two kinds, while the other two (Lycopodium and
Phylloglossum) are homosporous. Curiously enough, no certain instance
of a homosporous Palaeozoic Lycopod has yet been discovered, though
well-preserved fructifications are numerous. Wherever the facts have
been definitely ascertained, we find two kinds of spore, differentiated
quite as sharply as in any living members of the group. Some of the
Palaeozoic Lycopods, in fact, went further, and produced bodies of the
nature of seeds, some of which were actually regarded, for many
years, as the seeds of Gymnosperms. This specially advanced form of
fructification goes back at least as far as the Lower Carboniferous,
while the oldest known genus of Lycopods, Bothrodendron, which is found
in the Devonian, though not seed-bearing, was typically heterosporous,
if we may judge from the Coal-measure species. No doubt homosporous
Lycopods existed, but the great prevalence of the higher mode of
reproduction in days which to us appear ancient, shows how long a course
of evolution must have already been passed through before the oldest
known members of the group came into being. The other characters of
the Palaeozoic Lycopods tell the same tale; most of them attained
the stature of trees, with a corresponding elaboration of anatomical
structure, and even the herbaceous forms show no special simplicity.
It appears from recent work that herbaceous Lycopods, indistinguishable
from our recent Selaginellas, already existed in the time of the
Coal-measures, while one herbaceous form (Miadesmia) is known to have
borne seeds.
The utmost that can be said for primitiveness of character in Palaeozoic
Lycopods is that the anatomy of the stem, in its primary ground-plan, as
distinguished from its secondary growth, was simpler than that of most
Lycopodiums and Selaginellas at the present day. There are also some
peculiarities in the underground organs (Stigmaria) which suggest the
possibility of a somewhat imperfect differentiation between root and
stem, but precisely parallel difficulties are met with in the case of
the living Selaginellas, and in some degree in species of Lycopodium.
In spite of their high development in past ages the Lycopods, recent
and fossil, constitute, on the whole, a homogeneous group, and there is
little at present to connect them with other phyla. Anatomically some
relation to the Sphenophylls is indicated, and perhaps the recent
Psilotaceae give some support to this connection, for while their
nearest alliance appears to be with the Sphenophylls, they approach the
Lycopods in anatomy, habit, and mode of branching.
The typically microphyllous character of the Lycopods, and the simple
relation between sporangium and sporophyll which obtains throughout the
class, have led various botanists to regard them as the most primitive
phylum of the Vascular Cryptogams. There is nothing in the fossil record
to disprove this view, but neither is there anything to support it, for
this class so far as we know is no more ancient than the megaphyllous
Cryptogams, and its earliest representatives show no special simplicity.
If the indications of affinity with Sphenophylls are of any value
the Lycopods are open to suspicion of reduction from a megaphyllous
ancestry, but there is no direct palaeontological evidence for such a
history.
The general conclusions to which we are led by a consideration of the
fossil record of the Vascular Cryptogams are still very hypothetical,
but may be provisionally stated as follows:
The Ferns go back to the earliest known period. In Mesozoic times
practically all the existing families had appeared; in the Palaeozoic
the class was less extensive than formerly believed, a majority of the
supposed Ferns of that age having proved to be seed-bearing plants. The
oldest authentic representatives of the Ferns were megaphyllous plants,
broadly speaking, of the same type as those of later epochs, though
differing much in detail. As far back as the record extends they show no
sign of becoming merged with other phyla in any synthetic group.
The Equisetales likewise have a long history, and manifestly attained
their greatest development in Palaeozoic times. Their oldest forms show
an approach to the extinct class Sphenophyllales, which connects them
to some extent, by anatomical characters, with the Lycopods. At the
same time the oldest Equisetales show a somewhat megaphyllous character,
which was more marked in the Devonian Pseudoborniales. Some remote
affinity with the Ferns (which has also been upheld on other grounds)
may thus be indicated. It is possible that in the Sphenophyllales we
may have the much-modified representatives of a very ancient synthetic
group.
The Lycopods likewise attained their maximum in the Palaeozoic, and
show, on the whole, a greater elaboration of structure in their early
forms than at any later period, while at the same time maintaining a
considerable degree of uniformity in morphological characters throughout
their history. The Sphenophyllales are the only other class with which
they show any relation; if such a connection existed, the common point
of origin must lie exceedingly far back.
The fossil record, as at present known, cannot, in the nature of things,
throw any direct light on what is perhaps the most disputed question in
the morphology of plants--the origin of the alternating generations of
the higher Cryptogams and the Spermophyta. At the earliest period
to which terrestrial plants have been traced back all the groups of
Vascular Cryptogams were in a highly advanced stage of evolution, while
innumerable Seed-plants--presumably the descendants of Cryptogamic
ancestors--were already flourishing. On the other hand we know
practically nothing of Palaeozoic Bryophyta, and the evidence even for
their existence at that period cannot be termed conclusive. While
there are thus no palaeontological grounds for the hypothesis that the
Vascular plants came of a Bryophytic stock, the question of their actual
origin remains unsolved.