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. The genus _Tmesipteris_ (fig. 120, A) is represented by a single species _T. tannensis_ Bertr.[39] which usually occurs as an epiphyte on the stems of tree-ferns in Australia, New Zealand, and Polynesia. _Psilotum_, with two species _P. triquetrum_ Sw. (fig. 118) and _P. complanatum_ Sw., flourishes in moist tropical regions of both hemispheres, growing either on soil rich in organic substances or as an epiphyte. Both genera are considered to be more or less saprophytic. [Illustration: FIG. 118. _Psilotum triquetrum_ (½ natural size). A. Synangium. B. Sporophyll after removal of the synangium. (M.S.)] _Psilotum._ The common tropical species _P. triquetrum_ (fig. 118) is characterised by an underground rhizome which forms a confused mass of dark brown branches covered with filamentous hairs as substitutes for roots and gives off erect repeatedly forked aerial shoots. In _P. complanatum_[40] the habit is similar to that of the more abundant and better-known species, but the pendulous shoots are characterised by their broader and flatter form. In both species the function of carbon-assimilation is performed by the outer cortex of the green branches, as the small size of the widely-separated foliage leaves renders them practically useless as assimilating organs.