XXV.—The Prothallus of Tmesipteris Tannensis

The Psilotaceæ constitute one of the most interesting of existing Pteridophyte groups. This interest is mainly due to their phylogenetic isolation. They are perhaps the most isolated of existing types—showing no close affinity to other Pteridophytes. They are very highly specialised in their anatomy, habit, and habitat, and limited to a comparatively narrow geographical distribution. They include two probably monotypic genera—Tmesipteris and Psilotum—which are closely related. Although both genera are limited to the tropics and sub-tropics, Psilotum has a much wider distribution than Tmesipteris. The latter is confined to the South Sea Islands, Australia, New Zealand, and parts of Polynesia. Up till recent years they have been classed with the Lycopodiales, but this has been more a matter of convenience than an indication of close relationship, and has only served as a temporary classification until more knowledge has been obtained regarding them. The peculiar nature of these plants, as shown in their structure and in their habitat, suggests that they are highly specialised remnants of a much larger group which has practically become extinct. Recent inquiry into the structure of the sporophyte of both plants has made this much more than a suggestion. As a result of the careful investigations of Scott (1908), Boodle (1904), Bower (1894-1908), Ford (1904), and others, there has been a marked tendency to remove the Psilotaceæ from their temporary position among the Lycopodiales and associate them with the extinct Sphenophyllales. Scott (p. 631) states that it seems best to regard the Psilotaceæ as forming a class of their own, the Psilotales, under the main division Sphenopsida—their closest affinity under this class being the Sphenophyllales. This probable affinity of the Psilotaceæ to such an ancient and long extinct race as the Sphenophylls, and the great uncertainty of their relationship to other existing types, have awakened a great interest in these two specialised and isolated genera. When, in addition to this, we consider that the Psilotaceæ is the only group of Pteridophytes in which the gametophyte generation and embryo are not yet definitely known, our interest in these plants becomes twofold, and any new information throwing additional light on their life-histories will be welcomed.