12 SPHENOPHYLLALES [CH. 



its peduncle. The suggestion that the Devonian fossils known 

 as Pseudobornia may represent the foliage shoots of a plant 

 closely related to Cheirostrobus has still to be proved correct. 

 Although we may find justification in the highly complex and 

 peculiar structure of Cheirostrobus for the recognition of the 

 genus as a type of still another group of Pteridophytes, it would 

 be unwise to take this step without additional knowledge. 



The undoubted similarity between Cheirostrobus and 

 Sphenophyllum coupled with striking points of difference favours 

 the inclusion of the two genera in distinct families placed, for 

 the present at least, in the group Sphenophyllales. 



Group SPHENOPHYLLALES. 



Sphenophylleae : genus Sphenophyllum. 

 Cheirjpstrobeae : genus Cheirostrobus. 



It has recently been proposed to include the family 

 Psilotaceae, comprising the two recent genera Psilotum and 

 Tmesipteris, as another subdivision of the Sphenophyllales. 

 This proposal had been made by Professor Thomas^ primarily 

 on the ground that the sporophylls of Tmesipteris and Psilotum 

 appear to afford the closest parallel among existing plants to 

 the peculiar form of sporophyll characteristic of the Spheno- 

 phyllales. The morphological interpretation of the bporophylls 

 of both Sphenophyllum, and Cheirostrobus has been the source 

 of considerable discussion^. If we regard each sporophyll as a 

 leaf with two lobes, one fertile and one sterile, except in the 

 case of Sphenophyllostachys fertilis in which both are fertile, an 

 obvious comparison may be made with the fern Ophioglossum ; 

 but the difference between a single fern frond, consisting of a 

 comparatively large sterile lamina bearing a fertile branch 

 composed of a long axis with two rows of sporangia embedded 

 in its tissues, and the whorled sporophylls of Sphenophyllum is 

 considerable. 



A brief reference may be made to the principal reasons which 

 have led to the suggestion that the Psilotaceae should be included 



1 Thomas, A. P. W. (02) p. 350. ^ Bower (04) p. 227 ; (08) p. 424. 



