PAST HISTORIES OF PLANT FAMILIES
VI. The Ferns
Unfortunately the records in the rocks do not go back so far as to touch
what must have been the most interesting period in the history of the
ferns, namely, the point where they diverged from some simple ancestral
type, or at least were sufficiently primitive to give indications of
their origin from some lower group.
Before the Devonian period all plant impressions are of little value, and
by that early pre-Carboniferous time there are preserved complex leaves,
which are to all appearance highly organized ferns.
To-day the dominant family in this group is the _Polypodiaceæ_. It
includes nearly all our British ferns, and the majority of species for
the whole world. This family does not appear to be very old, however, and
it cannot be recognized with certainty beyond Mesozoic times.
From the later Mesozoic we have only material in the form of impressions,
from which it is impossible to draw accurate conclusions unless the
specimens have sporangia attached to them, and this is not often the
case. The cuticle of the epidermis or the spores can sometimes be studied
under the microscope after special treatment, but on the whole we have
very little information about the later Mesozoic ferns.
A couple of specimens from the older Mesozoic have been recently
described, with well-preserved structure, and they belong to the family
of the Osmundas (the so-called “flowering ferns”, because of the
appearance of special leaves on which all the sporangia are crowded), and
show in the anatomical characters of their stems indications that they
may be related to an old group, the _Botryopterideæ_, in which are the
most important of the Palæozoic ferns.
In the Palæozoic rocks there are numerous impressions as well as fern
petrifactions, but in the majority of cases the connection between the
two is not yet established. There were two main series of ferns, which
may be classed as belonging to
I. Marattiaceæ.
II. Botryopterideæ.
Of these the former has still living representatives, though the group is
small and unimportant compared with what it once was; the latter is
entirely extinct, and is chiefly developed in the Carboniferous and
succeeding Permian periods.
The latter group is also the more interesting, for its members show great
variety, and series may be made of them which seem to indicate the course
taken in the advance towards the Pteridosperm type. For this reason the
group will be considered first, while the structure of the Pteridosperms
is still fresh in our minds.
The Botryopterideæ formed an extensive and elaborate family, with its
numerous members of different degrees of complexity. There is,
unfortunately, but little known as to their external appearance, and
almost no definite information about their foliage. They are principally
known by the anatomy of their stems and petioles. Some of them had
upright trunks like small tree ferns (living tree ferns belong to quite a
different family, however), others appear to have had underground stems,
and many were slender climbers.
[Illustration: Fig. 86.—Stele of _Asterochlaena_, showing its deeply
lobed nature]
In their anatomy all the members of the family have monostelic structure
(see p. 62). This is noteworthy, for at the present time though a number
of genera are monostelic, no family whose members reach any considerable
size or steady growth is exclusively monostelic. In the shape of the
single stele, there is much variety in the different genera, some having
it so deeply lobed that only a careful examination enables one to
recognize its essentially monostelic nature. In fig. 86 a radiating
star-shaped type is illustrated. Between this elaborate type of
protostele in _Asterochlaena_, and the simple solid circular mass seen in
_Botryopteris_ itself (fig. 88) are all possible gradations of structure.
[Illustration: Fig. 87.—The Stele of a Botryopteridean Stem, showing
soft tissue in the centre of the solid wood of the protostele.
(Microphoto.)]
In several of the genera the centre of the wood is not entirely solid,
but has cells of soft tissue, an incipient pith, mixed with scattered
tracheids, as in fig. 87.
In most of the genera numerous petioles are given off from the main axis,
and these are often of a large size compared with it, and may sometimes
be thicker than the axis itself. Together with the petiole usually come
off adventitious roots, as is seen in fig. 88, which shows the main axis
of a _Botryopteris_. The petioles of the group show much variety in their
structure, and some are extremely complex. A few of the shapes assumed by
the steles of the petioles are seen in fig. 89; they are not divided into
separate bundles in any of the known forms, as are many of the petiole
steles of other families.
[Illustration: Fig. 88.—Main Axis of _Botryopteris_ with simple solid
Protostele _x_. A petiole about to detach itself _p_ and the strand
going out to an adventitious root _r_ are also seen.
(Micro-photograph.)]
[Illustration: Fig. 89.—Diagrams showing the Shapes of the Steles in
some of the Petioles of different Genera of Botryopterideæ
A, _Zygopteris_; B, _Botryopteris_; C, _Tubicaulis_; D,
_Asterochlaena_.]
In one genus of the family secondary wood has been observed. This is
highly suggestive of the condition of the stele in _Heterangium_, where
the large mass of the primary wood is surrounded by a relatively small
quantity of secondary thickening, developed in normal radial rows from a
cambium.
Another noteworthy point in the wood of these plants is the thickening of
the walls of the wood cells. Many of them have several rows of bordered
pits, and are, individually, practically indistinguishable from those of
the Pteridosperms, cf. fig. 81 and fig. 90. These are unlike the
characteristic wood cells of modern ferns and of the other family of
Palæozoic ferns.
The foliage of most members of the family is unknown, or at least, of the
many impressions which possibly belong to the different genera, the most
part have not yet been connected with their corresponding structural
material. There are indications, however, that the leaves were large and
complexly divided.
The fructifications were presumably fern sporangia of normal but rather
massive type. Of most genera they are not known, though in a few they
have been found in connection with recognizable parts of their tissue.
The best known of the sporangia are large, in comparison with living
sporangia (actually about 2.5 millimetres long), oval sacs clustered
together on little pedicels. The spores within them seem in no way
essentially different from normal fern spores.
The coexistence of the Botryopterideæ and Pteridosperms, and the several
points in the structure of the former which seem to lead up to the
characters of the latter group, are significant. The Botryopterideæ, even
were they an entirely isolated group, would be interesting from the
variety of structures and the variations of the monostele in their
anatomy; and the prominent place they held in the Palæozoic flora, as the
greatest family of ferns of that period, gives them an important position
in fossil botany.
[Illustration: Fig. 90.—Tracheæ of Wood of Botryopteridean Fern in
Longitudinal Section, showing the rows of pits on the walls.
(Microphoto.)]
The other family of importance in Palæozoic times, the Marattiaceæ, has
descendants living at the present day, though the family is now
represented by a small number of species belonging to but five genera
which are confined to the tropics. Perhaps the best known of these is the
giant “Elephant Fern”, which sends up from its underground stock huge
complex fronds ten or a dozen feet high. Other species are of the more
usual size and appearance of ferns, while some have sturdy trunks
above-ground supporting a crown of leaves. The members of this family
have a very complex anatomy, with several series of steles of large size
and irregular shape. Their fructifications are characteristic, the
sporangia being placed in groups of about five to a dozen, and fused
together instead of ripening as separate sacs as in the other fern
families.
Impressions of leaves with this type of sorus (group of fern sporangia)
are found in the Mesozoic rocks, and these bridge over the interval
between the living members of the family and those which lived in
Palæozoic times.
In the Coal Measure and Permian periods these plants flourished greatly,
and there are remains of very numerous species from that time. The family
was much more extensive then than it is now, and the individual members
also seem to have reached much greater dimensions, for many of them had
the habit of large tree ferns with massive trunks. Up till Triassic times
half of the ferns appear to have belonged to this family; since then,
however, they seem to have dwindled gradually down to the few genera now
existing.
On the Continent fossils of this type with well-preserved structure have
long been known to the general public, as their anatomy gave the stones a
very beautiful appearance when polished, so that they were used for
decorative purposes by lapidaries before their scientific interest was
recognized.
The members of the Palæozoic Marattiaceæ which have structure preserved
generally go by the generic name _Psaronius_, in which there is a great
number of species. They show considerable uniformity in their essential
structure (in which they differ noticeably from the group of ferns just
described), so that but one type will be considered.
In external appearance they probably resembled the “tree ferns” of the
present day (though these belong to an entirely different family), with
massive stumps, some of which reached a height of 60 ft. The large
spreading leaves were arranged in various ways on the stem, some in a
double row along it, as is seen by the impressions of the leaf scars, and
others in complex spirals. On the leaves were the spore sacs, which were
in groups, some completely fused like those of the modern members of the
family, and others with independent sporangia massed in well-defined
groups. In their microscopic structure also they appear to have been
closely similar to those of the living Marattiaceæ.
The transverse section of a stem shows the most characteristic and
best-known view of the plant. This is shown in fig. 91, in somewhat
diagrammatic form.
The mass of rootlets which entirely permeate and surround the outer
tissues of the stem is a very striking and characteristic feature of all
the species of _Psaronius_. Though such a mass of roots is not found in
the living species, yet the microscopic structure of an individual fossil
root is almost identical with that of a living _Marattia_.
Though these plants were so successful and so important in Palæozoic
times, the group even then seems to have possessed little variety and
little potentiality for advance in new directions. They stand apart from
the other fossils, and the few forms which now compose the living
Marattiaceæ are isolated from the present successful types of modern
ferns. From the _Psaronieæ_ we can trace no development towards a modern
series of plants, no connection with another important group in the past.
They appear to have culminated in the later Palæozoic and to have slowly
dwindled ever since. It has been suggested that the male fructifications
of the Bennettiteæ and the Pteridosperms show some likeness to the
Marattiaceæ, but there does not seem much to support any view of
phylogenetic connection between them.
[Illustration: Fig. 91.—Transverse Section of Stem of _Psaronius_
_v_, Numerous irregularly-shaped steles; _s_, irregular patches of
sclerenchyma; _l_, leaf trace going out as a horseshoe-shaped stele;
_c_, zone of cortex with numerous adventitious roots _r_ running
through it; _sc_, sclerized cortical zone of roots; _w_, vascular
strand of roots.]
Before leaving the palæozoic ferns, mention should be made of the very
numerous leaf impressions which seem to show true fern characters, though
they have not been connected with material showing their internal
structure. Among them it is rare to get impressions with the _sori_ or
sporangia, but such are known and are in themselves enough to prove the
contention that true ferns existed in the Palæozoic epoch. For it might
be mentioned as a scientific curiosity, that after the discovery that so
many of the leaf impressions which had always been supposed to be ferns,
really belonged to the seed-bearing Pteridosperms, there was a period of
panic among some botanists, who brought forward the startling idea that
there were _no_ ferns at all in the Palæozoic periods, and that modern
ferns were degenerated seed-bearing plants!
[Illustration: Fig. 92.—Impression of Palæozoic Fern, showing _sori_ on
the pinnules. (Photo.)]
These two big groups from the Palæozoic include practically all the ferns
that then flourished. They have been spoken of (together with a few other
types of which little is known) as the _Primofilices_, a name which
emphasizes their primitive characters. As can be seen by the complex
organization of the genera, however, they themselves had advanced far
beyond their really primitive ancestors. There is clear indication that
the Botryopterideæ were in a period of change, what might almost be
termed a condition of flux, and that from their central types various
families separated and specialized. Behind the Botryopterideæ, however,
we have no specimens to show us the connection between them and the
simpler groups from which they must have sprung. From a detailed
comparative study of plant anatomy we can deduce some of the essential
characters of such ancestral plants, but here the realm of fossil botany
ceases, to give place to theoretical speculation. As a fact, there is a
deep abyss between the ferns and the other families of the Pteridophytes,
which is not yet bridged firmly enough for any but specialists, used to
the hazardous footing on such structures, to attempt to cross it. Until
the buttresses and pillars of the bridge are built of the strong stone of
fossil structures we must beware of setting out on what would prove a
perilous journey.
In the Coal Measures and previous periods we see the ferns already
represented by two large families, differing greatly from each other, and
from the main families of modern ferns which sprung at a later date from
some stock which we have not yet recognized. But though their past is so
obscure, the palæozoic ferns and their allies throw a brilliant light on
the course of evolution of the higher groups of plants, and the gulf
between ferns and seed-bearing types may be said to be securely bridged
by the Botryopterideæ and the Pteridosperms.