A. C. Seward

“ We treat the question from an academic standpoint and run a risk of ignoring the fact that the conception of stem and leaf is based on morphological characteristics, which have been evolved as the result of gradual differentiation of parts of one originally 16homogeneous whole. There is much that is attractive in the view recently propounded by Mr Tansley that a leaf is not an appendicular organ differing ab initio from the axis on which it is borne, but that it is in phylogenetic origin a “branch-system of a primitive undifferentiated sporangium-bearing thallus [34] .” ”

“ 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. ”

“ The highly compound tri- or quadripinnate leaves of such species as Pteris aquilina, Davallia and other genera stand for the central type of fern frond; others exhibit a well-marked dichotomy, e.g. Lygodium, Gleichenia, Matonia, etc., a habit in all probability associated with the older rather than with the more modern products of fern evolution. Before attempting to determine specifically fossil fern fronds, it is important to familiarise ourselves with the range of variability among existing species and more especially in leaves of the same plant. ”

“ When we are concerned with the study of extinct plants it is essential to be familiar not only with the commoner recent types, but particularly with exceptional or aberrant types. The vascular system of many ferns consists of strands of xylem composed of scalariform tracheae associated with a larger or smaller amount of parenchyma, surrounded either wholly or in part (that is concentric or bicollateral) by phloem: beyond this is a pericycle, one layer or frequently several layers in breadth, limited externally by an endodermis, which can usually be readily recognised. ”

“ The Marattiaceae, the single family of ferns included in the Marattiales, comprise the genera Angiopteris, Archangiopteris, Marattia, Danaea, and Kaulfussia, which are for the most part tropical in distribution. These genera are characterised by eusporangiate sori or synangia, the presence of stipules at the base of the petioles, and by the complex arrangement of the vascular tissue. In view of the fact that many fossil ferns show a close resemblance to the recent Marattiaceae, the surviving genera are briefly described. The prothallus is green and relatively large. ”

“ The danger which should be borne in mind, in adopting as a matter of convenience the term herbaceous as a sectional heading, is that it should not be taken to imply a complete inability of the so-called herbaceous types to make secondary additions to their conducting tissues. The specimens on which the species of Lycopodites and Selaginellites, (genera which may be designated herbaceous,) are founded are preserved as impressions and not as petrifications; we can, therefore, base definitions only on habit and on such features as are shown by fertile leaves and sporangia. ”

“ 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. ”

“ In specimens which have lost more of the cortical tissues the surface is characterised by spirally disposed, discontinuous vertical grooves representing portions of leaf-traces precisely as they appear in similar casts of Lepidodendron. There is no direct evidence of the existence of secondary wood in the stem, but, as Potonié has pointed out, the greater transverse elongation of the leaf-scars in the lower part of a cast (fig. 134, A) points to the production of some secondary tissue either in the vascular cylinder or cortex, or possibly in both regions. ”

“ The roots are constructed on a simple plan, having in most cases only one strand of spiral protoxylem elements (monarch structure) . In Lycopodium, in which stem and root anatomy are more nearly of the same type than in the majority of plants, several protoxylem strands may be present. The sporangia are axillary or, more frequently, borne on the upper surface of sporophylls, which are either identical with or more or less distinct from the foliage leaves; in the latter case the sporophylls often occur in the form of a well defined strobilus (cone) at the tips of branches. ”

“ Below the small-celled epidermis are a few layers of thinner-walled cells which are not arranged in radial series. The structure of the outer part of the cortex is similar to that in the petiole of recent species of Angiopteris (fig. 243, p. 319) and Marattia, in which a more delicate hypoderm is succeeded by a band of mechanical tissue.
The rachis of this type of frond gives off two rows of lateral branches from the vascular axis, the plane of symmetry being at right angles to the primary rachis. Each pinna bore at its base two aphlebiae supplied with vascular strands from the leaf-traces. ”

“ So far, then, as the evidence afforded by sporangial characters is concerned, it points to comparison with the Ophioglossaceae, the Osmundaceae, and the Marattiaceae. When we compare the steles of the stems we find a wide range of structure. All the genera agree in being monostelic; in Tubicaulis and Grammatopteris the protoxylem is exarch, in Botryopteris it is internal, while the foliar strand of Stauropteris and the stele of Ankyropteris corrugata are mesarch. The axillary branching of species of Ankyropteris suggests comparison with the Hymenophyllaceae. ”

“ In the Zygoptereae, or at least in the case of such fertile fronds as are known, and in Botryopteris (fig. 319) , the sporangia occur in groups, and the pedicel of each sporangium is supplied with vascular tissue as in Helminthostachys. Another characteristic of the sporangia of the extinct types is the possession of an annulus several cells in breadth, a peculiarity which supplies a point of contact with the Osmundaceae. ”

“ 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] .” ”

“ Several leaf-traces are seen in transverse section in the secretory zone (black dots in fig. 148, A, sc; fig. 154, C, lt) : each trace consists of a strand of narrow tracheae accompanied by a few encircling layers of small parenchymatous cells. As a trace continues its steeply ascending course through the secretory zone, it becomes associated with a strand of that tissue and assumes the form of a collateral vascular bundle, the outer part of which does not consist of typical phloem but of shorter elements derived from the secretory zone. ”

“ The surface of the vegetative organs bears large multicellular hairs. The leaf-traces resemble those of B. forensis, but the projecting teeth which terminate in protoxylem elements are less prominent than in the French species; the petioles were named by Felix Rachiopteris tridentata [1124] . As a leaf-trace passes into the stele of the stem the three protoxylem strands unite and take up an internal position in the solid stele. The stele may, therefore, be described as endarch. The small tracheae at the edge of the stele supply the xylem strands of adventitious roots. ”

“ There can be no doubt as to the significance of the morphological resemblances between the Psilotales and the genera Sphenophyllum and Cheirostrobus, but the position of Tmesipteris and Psilotum in the plant-kingdom may probably be best expressed by adopting the group-name Psilotales rather than by transferring the recent genera to the Sphenophyllales. One of the most striking differences between the Psilotales and the genus Lycopodium is in the form of the sporophylls and sporangia ”

“ The vascular tissue of the stem usually assumes the form of a single axial strand (stele) (fig. 125) , but the shoots of some species of Selaginella often contain two or more distinct steles (fig. 131) . The group as a whole is characterised by the centripetal development of the xylem composed almost entirely of scalariform tracheids: secondary xylem and phloem of a peculiar type occur in Isoetes, and the production of secondary xylem elements in a very slight degree has been noticed in one species of Selaginella (S. spinosa) [84] . ”

“ The form of frond represented by certain species of Gleichenia, characterised by a regular dichotomy of the axis and by the occurrence of arrested buds, is on the whole a trustworthy character, though Davallia aculeata (bearing spines on its rachis) (fig. 232) and Matonia sarmentosa have fronds with a similar mode of branching and also bear arrested radius-buds. A limited acquaintance with ferns as a whole often leads us to regard a certain form of leaf as characteristic of a particular species, but more extended enquiry usually exposes the fallacy of relying upon so capricious a feature. ”

“ If a Lepidodendron stem loses its superficial layers of outer cortex and in this condition is embedded in sand or mud, the cast is distinguished from that of a perfect stem by the absence of the leaf-scars and by other features. It may, however, still show spirally disposed areas, corresponding approximately to the original leaf-cushions, which are characterised by a small depression or pit either at the apex or near the centre of each oval area: the pit marks the position of the leaf-trace and its parichnos strand. ”

“ By the partial transformation of the central xylem region into parenchymatous tissue and the concentration of water-conducting elements in the peripheral region the style of Ankyropteris corrugata was developed. The vascular strand of the older plant, which is of the Diplolabis type, may be regarded as a more primitive style than that of the H-form of petiole strand represented by Ankyropteris corrugata. A further stage in evolution is seen in the stem stele of Ankyropteris Grayi and A. scandens, both of which have the H-form of meristele. ”

“ The middle cortex is frequently characterised by the presence of spaces and by the hyphal or trabecular structure of the tissue, a feature which, as Bower [117] pointed out, is common to many recent and fossil members of the Lycopodiales. In some cases, e.g. S. erythropus, from tropical America, the cortex of the creeping stem consists entirely of thick-walled cells. Selaginella grandis (fig. 130) has “a short decumbent stem rooted at close intervals [118] ,” from which thick erect aerial shoots rise to a height of one foot or more. ”

“ In several species the stem stele is endarch. Our knowledge of the leaves is very meagre: in B. forensis they were repeatedly branched and apparently bore small fleshy pinnules; the sporangia, though differing from those of recent ferns, may be compared with the spore-capsules of Osmundaceae as regards the structure of the annulus. The abundance of hairs on the stems and leaves of some species, the tracheal sheath in the sporangium described by Oliver [1131] as Tracheotheca (= Botryopteris?) ”

“ It is, however, important not to overlook the differences between Psaronius and recent genera of Marattiaceae, or the resemblances between the extinct genus and the Cyatheaceae. In habit Psaronius agrees closely with recent tree-ferns; in the vascular system and in the sequence of events connected with the production of leaf-traces, there are striking resemblances between Psaronius and the Cyatheaceous fern Saccoloma adiantoides (= Dicksonia Plumieri Hook.) as described by Mettenius [1088] . ”