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].
CHAPTER XIII.
PSILOTALES.
The two recent genera Psilotum and Tmesipteris are usually spoken of as members of the family Psilotaceae which is included as one of the subdivisions of the Lycopodiales. It is probable, as Scott[36] first suggested, that these two plants are more nearly allied than are any other existing types to the Palaeozoic genus Sphenophyllum.
We may give expression to the undoubted resemblances between Tmesipteris and Psilotum and the Sphenophyllales by including the recent genera as members of that group, originally founded on the extinct genus Sphenophyllum; this is the course adopted by Thomas[37] and by Bower[38]: or we may emphasise the fact that these two recent genera differ in certain important respects from Lycopodium and Selaginella by removing them to a separate group, the Psilotales. The latter course is preferred on the ground that the inclusion of Psilotum and Tmesipteris in a group founded on an extinct and necessarily imperfectly known type, is based on insufficient evidence and carries with it an assumption of closer relationship than has been satisfactorily established.