YESTERDAY AND TO-DAY
Although to most people the study of fossil botany may appear to be an
uninviting pursuit, there can be no question as to the importance of the
science. It is only in this way that we are able to appreciate the
changes which have led up to the existing types of plants. Now the
question of the past history of the Vascular Cryptogams is of very
special interest in more ways than one. It is, of course, most
fascinating to be able to discover what kinds of Ferns flourished, for
instance, at the period when the coal deposits were being formed. But,
even in a cursory description, it will be quite impossible to allow the
matter to rest there. The story of the past, in the case of the Vascular
Cryptogams, is closely interwoven with some of the most absorbing phases
in the evolution of the Flowering Plants which are such a dominant
feature on the earth at the present time.
Quite recently we have had to alter our views materially on the matter of
the past history of the Ferns. Within the last few years it has been
proved that a huge number of the fossil remains, belonging to the
Palæozoic formation at any rate, are not Ferns at all. They belonged to a
very distinct race of plants altogether, known as Pteridosperms, even
though they had a superficial resemblance to Ferns. After a large amount
of patient research it has been demonstrated that these plants bore
seeds. The method of flowering and seed-production was vastly different
from that which is to be observed in the flowering plants of to-day. The
male or pollen-bearing organs were produced straight on the foliage in
much the same way as the sporangium of a true Fern is developed. In a
similar manner the seeds were borne straight on to the leaf. In some
general points these plants also bore a strong resemblance to the Ferns,
and it was this which misled the early observers. Without a doubt these
Pteridosperms were related to the Ferns, and probably at some remote
period the two groups had a common ancestry. There is good reason for
believing that at the same time these Pteridosperms were flourishing true
Ferns were also well represented.
Now the interesting point about these Ferns is that they were not vastly
different, in many ways, from the species which exist on the earth at the
present time. Of course certain types, of which we have living examples,
were more fully represented than is the case nowadays; on the other hand,
some of our most widely distributed families seem to have been at rather
a low stage in their history. As well, the remains evidence a large
number of very simple species, which perhaps give us an idea of what the
original Ferns were like. But on the whole there is nothing to show that
our living Ferns are any more developed than the highest types which grew
in the coal forests. In these far-away times there were Tree Ferns; but
so there are, of course, at the present time.
The early botanists who strove to prove that Ferns were, so to speak, the
last development before the Flowering Plants were not, perhaps, very far
from the truth. It has been seen that during the period when the coal
deposits were being formed there flourished side by side races of true
Ferns and Fern-like plants which bore seeds—the Pteridosperms. The point
has also been suggested that in all probability these two groups of
plants had a common origin. With the coming of later times (the
Cretaceous and Jurassic periods) there appeared the Bennettiteæ. These
remarkable plants seem to have entirely taken the place of the
Pteridosperms, and were an enormous advance towards the Flowering Plants
of the present day. Only recently have the Bennettiteæ been properly
described and their interesting features fully understood. We have a few
representatives of this important group in the Cycads, plants bearing a
superficial resemblance to Palms, but actually very different in all
other ways. By the manner in which the reproductive organs are produced,
and the way in which the scheme is carried out, these Bennettiteæ appear
to be a half-way house between the cryptogams and the advanced flowering
plants. The stamens bearing the pollen are produced on the fronds very
much like the sporangia of Ferns. On the other hand, the seed-bearing
structures are collected together into a sort of pistil. This was borne
at the tip of the branches and ended their growth, just as happens in the
case of Flowering Plants. For some reason which we cannot understand
these Bennettiteæ seem to have fallen back in the race for supremacy, for
the group is but poorly represented in our modern Cycads and a few allied
plants. In all the world there are perhaps not more than about a dozen
species, the sole survivors of a race which at one time dominated the
world. There seems every reason for thinking that the Flowering Plants
arose as an offshoot of the Bennettiteæ, and in some way secured an
advantage which enabled them to arrive at their present position.
When we come to consider the past history of the Club Mosses the record
is of a different nature to that of the Ferns. Nowadays the Club Mosses
are not of great importance in the world, even though, as will be shown
later, the number of species is considerable. But when we travel back to
Palæozoic times, particularly in the coal period, it is evident that
these plants were represented by a number of very large and dominant
families. Some of these early Club Mosses certainly came very near to
rivalling the Flowering Plants. Probably the tendency of the world to
become drier has had something to do with the decline, seeing that in all
cases the fertilization is carried out under water. We may gather some
idea of the importance of the Club Mosses in Palæozoic times from the
fact that in every part of the world where coal deposits have been
examined great numbers of the fossil remains of these plants are always
discovered. Many of these grew into large trees which were a hundred or
more feet in height, sending out great branching shoots above and an
enormous root system below.
From a botanical point of view there is no doubt that some of the Club
Mosses, particularly those belonging to the family _Selaginellaceæ_, have
approached very nearly to the Flowering Plants. At the present time the
existing species, the Selaginellas, bring us up to the very threshold of
the dominant group. The lowest division of the Flowering Plants is the
_Gymnospermæ_ (which includes the Conifers), and it is interesting to
note the points of similarity between a typical Gymnosperm and a
Selaginella. To start away with, the Selaginella bears two kinds of
spores, each of which in its development has a definite sex character.
The smaller ones (microspores) are in their manner of production
analogous to the pollen-grains of the Flowering Plant. The prothallus and
the male organ (antheridium) are comparable to the special cell-group in
the pollen-grain, whilst the spermatozoids approximate to the generative
cells. In the larger spores (megaspores) these represent the embryo sac,
and the sporangium in which they are produced closely approximates to the
part containing the embryo sac in the Flowering Plant. The prothallus
which arises from the megaspore in the Selaginella closely resembles the
endosperm—a special tissue formed to feed the embryo in the case of
flowering plants. The female organ (archegonium) and the cell which it
produces are practically identical in both cases. Fossil remains have
shown that some of the plants like Selaginella which flourished in
Palæozoic times seemed to have come very near to the production of seed.
Thus one species which has been described shows a megaspore which was
permanently within the sporangium, and which in its general development
greatly resembled a fruit. It is, of course, impossible to give more than
a very brief outline of some of the chief points in this highly important
comparison between the Gymnosperm and the Selaginella. The author trusts
that those of his readers who are interested will pursue the study in the
admirable textbooks which are now available.
The Horsetails, like the Club Mosses, have had a very important past.
Although they are few in number, as far as the species are concerned,
they still retain many striking characteristics. Without a doubt the
Palæozoic Horsetails grew into giant plants, sending out branches and
developing trunks which in some ways are comparable to those possessed by
our trees at the present time. These great stems seem to have arisen from
rhizomes which travelled about in the mud of the coal jungles. It is
usual to refer to these Palæozoic Horsetails as Calamites, owing to the
fact that they were originally supposed to bear a resemblance to a reed
(_Calamus_). In the later rocks, such as those which belong to the
Jurassic and Triassic periods, occur the Equisetites, plants which were
still of great size, but already in some respects showing signs of that
decline which has culminated at the present day in the little group of
plants which, were it not for a certain robustness of growth, would find
it hard to maintain their position at all.
To complete our brief survey of the Vascular Cryptogams it is now
necessary that we should review the position of these plants at the
present time. Of course in number the Ferns are enormously in advance of
all the other plants put together. In the whole world, there are not far
short of seventy distinct genera, which include anything between three
and four thousand species. The Ferns of the United Kingdom number not far
short of fifty, and there are certain variations from the type which some
folk are tempted to include as species. For some reasons which we cannot
well understand, the Ferns alone amongst the Vascular Cryptogams have
been able to hold their own in the world. It is probable that there are
quite as many species, and that these are as varied, to-day as has ever
been the case. The size of Ferns, as we have already seen, varies
enormously. In the tropics and in Australasia there are Tree Ferns eighty
feet in height, whilst with many of the Filmy Ferns the size is scarcely
larger than that of Mosses. Owing to the fact that it is so necessary in
the scheme of reproduction, the majority of Ferns are lovers of moisture.
None the less, a few specimens have adapted themselves marvellously to
drier conditions. Thus the Bracken will grow on the exposed hillside or
cliff-top even where its rhizomes cannot carry the roots to a great
amount of moisture. Some of the most interesting species of Ferns are
those which grow on walls and rocks, where there is little dampness,
during the summer at any rate. Many of these have adopted special devices
to cope with drought, such as are to be seen in the Scaly Spleenwort.
Here the underside of the frond is covered with hairy scales, and in dry
weather the leaves roll up so that the well-protected underside is alone
exposed to the sun. After all, however, Ferns are most at home where
there is a comparatively deep shade with abundance of moisture. Many
species which will grow in somewhat dry situations attain a much finer
development under happier conditions.
Luckily many kinds of Ferns are still very common in the United Kingdom.
Of course, in much-visited localities the ravages of the trippers have
practically exterminated some interesting species in these particular
districts. Naturally, one hardly expects to find the Royal Fern
flourishing to any extent in the popular holiday haunts—none the less,
there are still any number of places where this noble plant “grows like a
weed.” The wise man does not talk about such things to his friends. Many
of our most beautiful Ferns are saved even in much-frequented places on
account of the fact that they grow out of reach. No doubt the graceful
Trichomanes of South Ireland would long ago have been stamped out in the
Killarney district, were it not for the fact that it often grows in
situations which it is almost impossible for anyone to reach.
As far as number is concerned, the living Club Mosses represent a
comparatively insignificant group when compared with the Ferns. In all
the world there are probably not more than five or six hundred species.
These are very widely distributed, and there is hardly any part which
cannot offer at least a few species. We have five species of Lycopodium
in the United Kingdom. All are rather local, though often enough they
occur in great abundance in special localities. Only one (_Lycopodium
inundatum_) ever occurs in lowland districts; all the rest must be looked
for on highland moors. A few exotic Lycopodiums grow to a fair size,
though this is largely due to the fact that their creeping stems straggle
along the ground for a considerable distance. The Selaginellas are a much
more important group as far as the world generally is concerned. There
are certainly as many as four or five hundred species, and some of these
assume almost a shrubby habit. A species from Borneo (_S. grandis_) is
said to attain the height of two feet. In the United Kingdom we have but
a single species of Selaginella—_S. spinosa_, an insignificant little
plant. Many exotic kinds are frequently grown in greenhouses, so that a
variety of species is within the reach of everybody.
Authorities vary as to the exact number of species which belong to the
only genus of the Horsetail—_Equisetum_; the estimate is never higher
than forty. Nearly all these plants are striking in appearance, and some
are quite large. A tropical American species is said to attain the height
of thirty feet, though this is not so remarkable when one considers that
the plant has a climbing habit. In the United Kingdom we have at least
eight distinct species. Some of these are exceedingly common, and owing
to their vigorous growth will often hold their own against all comers.
Indeed, the existing Horsetails are, to use a common expression, “putting
up such a good fight” that it is certain they will continue to hold their
own for many a long day. Unlike the Club Mosses, the Horsetails seem to
be quite happy in the vicinity of towns, and are often seen at their best
on railway embankments and in similar situations.
Although the Vascular Cryptogams played an important part in helping to
build up our vast stores of coal, it is astonishing to note of what
little direct economic value they are to mankind at the present time. In
a few parts of the world, where the native races make little or no
attempt at agriculture, the root-stocks of Ferns—often rich in starch—are
eaten. Thus the Maoris of New Zealand and some of the South Sea Islanders
secure a poor kind of sago from some of the Tree Ferns which grow in
their districts. The Japanese use the growing tips of the Bracken as
food. Years ago, Ferns used to be burnt for potash in this country, and
their astringent properties naturally attracted the old-world pharmacist.
They are practically useless as fodder on account of their bitter taste,
and no animal—except, perhaps, the goat—would think of eating Ferns. In
many cases, however, Bracken is used regularly as a bedding-down material
for cattle.
Coming to the Club Mosses, it is even more difficult to find that they
are of any direct benefit. Some kinds in South America are said to yield
a blue dye. Our Common Club Moss is in its huge quantity of spores
responsible for the “Lycopodium Powder” which at one time was employed in
the making of fireworks. The powder is said to be highly inflammable, but
when shaken straight out of the cones it does not always ignite very
readily. One of the Horsetails, the Dutch Rush (_Equisetum hyemale_),
was—and perhaps is still—used in polishing, owing to the large amount of
silica which is present in its stems. In addition, those who are
interested in coast erosion say that the stronger-growing species of
_Equisetum_ should be planted to keep clay cliffs from falling. From an
ornamental point of view the Ferns and some of the Club Mosses are, of
course, of great value. They are widely cultivated in garden and
greenhouse, and we may say that these plants make up in æsthetic value
what they lack from an economic point of view.