Fossil plants, Vol. 2: A text-book for students of botany and geology

by A. C. Seward

CHAPTER XXV.

Coenopterideae.

I. II.

Botryoptereae. Zygoptereae.

The term Botryopterideae, first used by Renault, has been applied to a group of Palaeozoic ferns ranging from the Lower Carboniferous to the Permian and containing several genera, the distinguishing features of which are supplied by the anatomical structure of the stems or, in many cases, by that of the petiolar vascular strand. Scott[1109] subdivides the Botryopterideae into the Botryopteris and the Zygopteris sections. In an admirable monograph recently published by Paul Bertrand[1110] considerable changes are proposed in current nomenclature; he substitutes the name Inversicatenales for Botryopterideae, a designation, which as Scott remarks, is “probably too technical to command general acceptance.” A more serious criticism is that the name Inversicatenales has reference to a character (the inverse curvature of the leaf-trace in relation to the axis of the stem) which is by no means universal in the group[1111].

In the following account, necessarily incomplete, the generic terminology of Bertrand is adopted, but this decision does not carry with it any obligation to accept the name Inversicatenales. We may speak of the types of Palaeozoic ferns dealt with in the following pages as members of a group differing in many respects from any existing genera of the Filicales, and exhibiting the characteristics associated with generalised plants. Williamson, as early as 1883, spoke of Renault’s Botryopterideae as comprising “altogether extinct and generalised” types[1112]. For these generalised Palaeozoic ferns I propose to use the name Coenopterideae[1113]. This term may be adopted in a wider sense than Renault’s name Botryopterideae. The name Primofilices proposed by Arber[1114] might be employed, but the implication which it carries is an argument against its adoption. We have not yet reached a stage in the investigation of extinct types at which we are able to recognise what are actually primary or primitive ferns. The search for origins will continue; as new discoveries are made our point of view shifts and the primitive type of to-day may to-morrow have to take a higher place. The epithet primitive or primary is in reality provisional: to adopt such a name as Primofilices suggests a finality which has not been, or is likely to be, achieved. The true ancestral type—the Urform—which we strive to discover eludes the pursuer like a will-o’-the-wisp.

Seeing that the number of true ferns of Palaeozoic age has been recently considerably reduced and is likely to suffer further reduction, the consideration of such undoubted Carboniferous and Permian examples of the Filicales as are left acquires a special importance. In the first place it is natural to ask whether the Palaeozoic ferns include any types which, if not themselves ancestral forms, may serve to indicate the probable lines of evolution of existing families. It is probable that in the near future our knowledge of the Coenopterideae will be considerably extended; as yet we possess meagre information in regard to those characters on which most stress has generally been laid in the classification of recent ferns, namely the structure of the spore-bearing organs. The sporangia of Diplolabis and Stauropteris (figs. [309], A; [322]) are exannulate; in the former genus they occur in sori or synangia consisting of a small number of sporangia, while in the latter they are borne singly at the tips of ultimate ramifications of a highly compound leaf. The resemblance of the synangium of Diplolabis to that of Kaulfussia ([fig. 245], C) is not shared in an equal degree by the sporangia of Stauropteris, which are in some respects comparable with those of the Ophioglossaceae. 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. In the sporangia of Kidstonia we have a similar though not an identical type ([fig. 256], E, p. 340). 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.

The investigation of the vascular system of the petioles has afforded results which in the hands of P. Bertrand have led to conclusions in regard to inter-relationships. We must, however, not overlook the danger of attributing can excessive importance to this single criterion and of neglecting the facts of stem anatomy.