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XXV. — The Prothallus of Tmesipteris Tannensis. By Professor A. Anstruther 

 Lawson, D.Sc, F.L.S. (With Three Plates.) 



(MS. received May 9, 1916. Read June 4, 1916. Issued separately April 21, 1917.) 



The Psilotacese 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 Psilotacese 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 Psilotacese 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 

 Psilotacese 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 Psilotacese is the only group of Pteridophytes in which the gameto- 

 phyte 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. 



Our knowledge of the gametophyte generation of the Psilotacese consists entirely 

 of a description of a structure supposed to be the prothallus of Psilotum. This 

 supposed prothallus was described by Lang in 1904, and his account of it is based 

 upon the study of a single specimen. It was found embedded among the adventitious 

 roots of a tree-fern and associated with a Psilotum sporophyte which had already 

 [produced aerial shoots and rhizomes. It was more or less cylindrical in form, about 

 TRANS. ROY. SOC. EDIN., VOL. LI, PART III (NO. 25). 114 



