GENERAL RESULTS— PTEROPSIDA 643 



sperms are so ancient that we can have no direct 

 knowledge of their ancestry, but one naturally looks 

 to the oldest known types of Ferns for indications of 

 what it may have been. 



It has been suggested that the more complex 

 Botryopterideae, such as Asterochlaena, may have been 

 related to polystelic types of Pteridosperms, such as 

 Medullosa. A breaking up of the deeply lobed stele 

 into separate meristeles is quite conceivable, and may 

 well have led on to the structure of Cladoxyleae, in 

 which the numerous steles tend to a radial arrangement 

 (p. 497). 1 No light, however, is thus thrown on the 

 history of the Medulloseae themselves, for, since the 

 discovery of Sutcliffia, it seems clear that polystely arose 

 within this Pteridospermous family, and was not derived 

 as such from any ancestral stock. 



It has already been pointed out that the Pterido- 

 sperms may probably have arisen from some early 

 Fern-like type with a protostelic stem (pp. 464 and 473). 

 In this respect, and in the structure of the tracheae, 

 the simpler Botryopterideae may resemble the hypo- 

 thetical ancestors of the Seed-plants, but there are not 

 sufficient grounds for assuming any near relationship. 



Professor F. W. Oliver's interesting discovery of a 

 sporangium (Tracheotheca), similar to that of certain 

 Botryopterideae, containing tracheides in the inner 

 layers of its wall, 2 suggests an early approach to 



1 M. Paul Bertrand has quite recently come to the conclusion that 

 Clepsydropsis, a Zygopteridean petiole, was borne on the stem of a 

 Cladoxylon, " Sur les Stipes de Clepsydropsis;' Comptes Rendus, Nov. 16, 



1908. 



2 F. W. Oliver, "On a Vascular Sporangium from the Stephanian of 

 Grand Croix," New Phytologist, vol. i. 1902, p. 60. 



42 



