CHAPTER XX.
FILICALES.
This division of the Pteridophyta includes both the true ferns (Filicineae) and the less familiar water-ferns or Hydropterideae. The almost complete absence of satisfactory evidence in regard to the geological history of the latter renders this group of secondary importance from a palaeobotanical standpoint, but, on the other hand, we possess a wealth of material bearing on the past history and relative antiquity of the true ferns.
The study of extinct types has so far rendered no substantial help towards bridging the wide gap between the Filicales and the lower plants. As Mr Tansley[675] says in his admirable lectures on The Evolution of the Filicinean Vascular System, “The biggest gap in the plant kingdom at the present time is undoubtedly that which separates the Pteridophytes from the plants definitely below them in organisation, and directly we try to step behind the ferns we tumble into this abyss.” Resemblances long ago recognised between certain ferns and the cycads, a section of the Gymnosperms, were regarded by a few botanists as indications of blood-relationship, and the results of recent researches into the morphological characters of extinct Palaeozoic types are generally held to confirm these surmises. Prof. Chodat[676] of Geneva has recently challenged the validity of the arguments on which the affinity of cycads and ferns has been accepted by the great majority of botanists. Whether or not his criticisms stand the test of unbiassed examination, they must at least lead us to substitute a critical consideration of the facts for a mere repetition of conclusions which appeal to our imagination. Despite Prof. Chodat’s warning, we may still quote with confidence a phrase used in another connexion—ferns “are links in a chain and branches on the tree of life, with their roots in a past inconceivably remote[677].”
PTERIDOSPERMS
Transitional forms which are regarded as pointing to a common origin for ferns and cycads are known in abundance; other types have also been discovered which lead some authors to go so far as to derive the whole of the seed-bearing plants from an ancestry the descendants of which are represented by existing ferns. While hesitating to allow the ferns or fern-like plants the peculiar position of universal ancestors, we must admit that there is no group of plants with a history of greater importance from an evolutionary standpoint than that with which we are now concerned.
There are, however, some difficulties to face in attempting to decipher the history of the Filicineae as recorded in the earth’s crust. Few fossil plants are so familiar as the well-preserved carbonaceous impressions of compound leaves on the shales of our Coal-Measures, which were referred by older authors to recent genera and species of ferns and accepted by later writers as undoubted examples of Palaeozoic ferns. The common belief in the dominance of ferns in Palaeozoic floras is reflected in the novelist’s description of the Carboniferous period, “when the forms of plants were few and often of the fern kind[678].” We now know that very many of these Carboniferous leaves belonged to plants differing widely in morphological characters from the modern genera to which they exhibit so deceptive a resemblance. These pseudo-ferns, recently christened Pteridosperms or seed-bearing fern-like plants, are dealt with in a later chapter. The discovery of this extinct group has added enormously to our knowledge of plant-evolution and at the same time has rendered much more difficult the task of unravelling the past history of the true ferns. As soon as it was demonstrated that many familiar Palaeozoic “ferns” are not ferns, some authors went far towards concluding that however close might be the agreement between fossil and recent leaves suspicion of close relationship must be set aside. Like the earlier writers who described fossils as lusus naturae fashioned by devilish agency to deceive too credulous man, the discovery of seed-bearing plants with the foliage of ferns threatened to disturb the mental balance of palaeobotanists. The fact is, we cannot in some cases determine from leaf-form alone whether or not a fossil is a true fern; we may, as Professor Bower[679] suggests, regard all fern-like fossils as ferns until they are proved to be Pteridosperms, or in a spirit of scientific scepticism, we may at once admit that many Palaeozoic fern-like leaves must await further evidence before their true position can be determined. It is impossible, as Zeiller[680] says, in the present state of our knowledge to range fern-like Palaeozoic plants in two groups, one referred to Filicineae and the other to the Pteridosperms.
The following classification of the Filicales is based on that adopted by Prof. Engler in the latest edition of his Syllabus[681] and on the results of Bower’s[682] excellent work on the spore-bearing members of recent ferns.
The members of the Filicales are characterised by the same well-marked physiological division of labour in their vegetative parts as are the Lycopods; the plant is the asexual generation (sporophyte), while the sexual generation (gametophyte) is small and inconspicuous, either an independent green prothallus or a tissue more or less completely enclosed in the spore. The large size of the leaves, which in the young state are usually coiled like a crozier ([fig. 220], A), is a striking characteristic of the ferns; they are megaphyllous in contrast to the microphylly of the Lycopods.