Group SPHENOPHYLLALES.

It has recently been proposed to include the family Psilotaceae, comprising the two recent genera Psilotum and Tmesipteris, as another subdivision of the Sphenophyllales. This proposal had been made by Professor Thomas[24] primarily on the ground that the sporophylls of Tmesipteris and Psilotum appear to afford the closest parallel among existing plants to the peculiar form of sporophyll characteristic of the Sphenophyllales. The morphological interpretation of the sporophylls of both Sphenophyllum and Cheirostrobus has been the source of considerable discussion[25]. If we regard each sporophyll as a leaf with two lobes, one fertile and one sterile, except in the case of Sphenophyllostachys fertilis in which both are fertile, an obvious comparison may be made with the fern Ophioglossum; but the difference between a single fern frond, consisting of a comparatively large sterile lamina bearing a fertile branch composed of a long axis with two rows of sporangia embedded in its tissues, and the whorled sporophylls of Sphenophyllum is considerable.

PSILOTACEAE

A brief reference may be made to the principal reasons which have led to the suggestion that the Psilotaceae should be included in the Sphenophyllales. The shoots of Tmesipteris bear simple foliage leaves spirally disposed on a slender axis, and in association with these occur sporophylls consisting of a short axis bearing a pair of small lobes and a bilocular synangium[26] ([fig. 120], B). The synangium is seated on a very short stalk given off from its sporophyll at the base of the pair of laminae: the synangium with its short stalk may be spoken of as the sporangiophore. In most cases the synangium appears to be sessile on the sporophyll, but occasionally the much reduced stalk is prolonged and forms an obvious feature. Dr Scott[27] suggested that the Tmesipteris synangium with its axis may correspond to the ventral lobe (or sporangiophore) of Sphenophyllum. In the latter genus the whorled sporophylls consist in most species of a dorsal and a ventral lobe, the latter serving as a sporangiophore bearing one or more sporangia; in Tmesipteris the sporophylls are spirally disposed and each consists of a bilobed sterile portion bearing a septate sporangium or bilocular synangium on a very short ventral lobe. Professor Bower[28], in his account of the development and structure of the sporophylls of Tmesipteris, drew attention to the comparatively frequent occurrence of abnormal sporophylls and spoke of the plant as unstable. More recently Professor Thomas[29] of Auckland has carefully examined living plants, with the result that variations of different kinds are proved to be exceedingly common. He finds that sporophylls occur which exhibit repeated dichotomy of the axis ([fig. 120], D, F) and thus each may bear four instead of two leaf-lobes and three synangia, one at the first fork and one at each of the forks of the second order[30].

Other abnormalities occur in which the synangium is raised on a distinct stalk instead of being more or less sessile at the point from which the leaf-lobes diverge. A third form of departure from the normal is that in which there is no synangium on the bilobed sporophyll, its place being taken by a leaf-lobe. The deduction from the occurrence of these abnormalities is that the synangium of Tmesipteris represents a ventral leaf-lobe, as Scott suggested. Professor Thomas draws attention to the resemblance between Tmesipteris sporophylls and the foliage-leaves of Sphenophyllum, which are either simple with dichotomously branched veins or the lamina is deeply divided into two or more segments. In some types of Sphenophyllostachys the bracts are simple (S. Dawsoni), but in others (Sphenophyllum majus, [fig. 113], C) they are forked like the foliage-leaves and bear a close resemblance to the abnormal sporophylls of Tmesipteris. Moreover, in Sphenophyllostachys Römeri ([fig. 113], A) each ventral lobe of a sporophyll bears two sporangia, a condition almost identical with that represented by the occasional occurrence of a synangium on a comparatively long stalk in Tmesipteris. Similarly the more elaborate sporophylls of Cheirostrobus may be compared with the branched sporophylls of Tmesipteris ([fig. 120]). This agreement between the sporophylls of the Palaeozoic and recent genera acquires additional importance from the very close resemblance between the exarch stele of Sphenophyllum and that of the genus Psilotum, which conforms to the Palaeozoic type not only in the centripetal character of the primary xylem and in its exarch structure, but also in the occasional occurrence of secondary xylem[31], and in the stellate form of its transverse section. The occasional mesarch structure of the stele of Cheirostrobus finds a parallel in the mesarch xylem groups in the stem of Tmesipteris. It is thus on the strength of these resemblances that Thomas and Bower would remove the Psilotaceae from the group Lycopodiales and unite them with Sphenophyllum and Cheirostrobus in the Sphenophyllales. While admitting the validity of the comparison briefly referred to above, I prefer to retain the Psilotaceae as a division of the Pteridophyta including only Psilotum and Tmesipteris.

SPHENOPHYLLUM

In his recent book on The Origin of Land Flora, Prof. Bower raises objection to the use of the term ventral lobe in speaking of the sporangium-bearing stalk or sporangiophore borne on the sporophyll of Sphenophyllum. He points out that the use of this term implies the derivation of the sporangiophore by metamorphosis of part of a vegetative leaf, an opinion untenable in the absence of proof. The designation sporangiophore is no doubt preferable to that of ventral lobe as it carries with it no admission of particular morphological value; as a further concession to a non-committal attitude we may provisionally at least regard a sporangiophore as an organ sui generis “and not the result of modification of any other part[32].”

The view put forward by Prof. Lignier[33] that the Sphenophyllales are descendants of primitive ferns is not convincing, and his comparison of Sphenophyllum with Archaeopteris lacks force in view of our ignorance as to the nature of the reproductive organs of the latter genus. That the Sphenophyllales are connected with the Equisetales and with the Psilotales by important morphological features is clear; but the comparison between the sporophylls of the extinct genera with those of the existing genus Tmesipteris, though helpful and possibly based on true homology, cannot be considered as settling the morphological value of the sporangiophores of Sphenophyllum and Cheirostrobus.

I do not propose to discuss at length the different views in regard to the morphological nature of the sporangiophore of Sphenophyllum. The comparison, which we owe in the first instance to Scott, with the synangium of the Psilotales with its short stalk, though not accepted by Lignier as a comparison based on true homology, is one which appeals to many botanists and is probably the best so far suggested. The further question, whether these sporangiophores are to be called foliar or axial structures is one which has been answered by several authors, but it is improbable that we shall soon arrive at a decision likely to be accepted as final. Discussions of this kind tend to assume an exaggerated importance and frequently carry with them the implication that every appendage of the nature of a sporangiophore can be labelled either shoot or leaf. 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 homogeneous 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].” Admitting the probability that this view is correct, our faith in the importance of discussions on the morphological nature of sporangiophores is shaken, and we realise the possibility that our zeal for formality and classification may lead to results inconsistent with an evolutionary standpoint[35].