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.
Today 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
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.
In their anatomy all the members of the family have monostelic structure. 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.
In several of the genera the center 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.
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.
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.
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!
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.