88 ORIGIN OF CORMOPHYTA [ch. vi 



primitive, are, in our opinion, reduction features, the reduction 

 being correlated with the semi-saprophytic habit of these plants. 

 As primitive features we would point to the mierophyllous 

 habit and pericaulome, characters which as we have already 

 seen, appear to have arisen in different groups quite independ- 

 ently. The forking of the sporophylls is another such feature, 

 originating as in other groups from dichotomous thalloid branches. 

 In this feature the Psilotales may resemble Sphenophylls rather 

 than Lycopods as has often been pointed out, and it may be 

 that in this group the leaf was originally a branch and not an 

 emergence as in the Lycopods. But even if this is the case, it 

 is more than likely, in our view, that these features were quite 

 independent in origin from the similar organs of Sphenophylls, 

 especially since the latter race was in all probability entirely 

 defunct long before the Psilotales appeared on the scene. 



The vascular structure of the Psilotales represents a primitive 

 stage passed through in the history of the evolution of the stele 

 in many, if not all groups, and here either not developed further 

 or regained by reduction in correlation with the semi-saprophytic 

 habit. 



Finally as regards Psilophyton we see no real point of contact 

 with the Psilotales except in habit, a character which of course 

 is worse than useless as a guide to affinity, if considered by itself. 

 In the first place the fructification and the manner in which it 

 is borne are entirely distinct. Tmesipteris is a Cormophyte 

 without emergences, Psilophyton is a Thallophyte possessing 

 emergences, and Psilotum is in all probability a reduced semi- 

 leafless Cormophyte. Finally the only comparison as regards the 

 stele of Psilophyton is confined to the tip of the shoot in the 

 Psilotales, near the growing point. In the mature stems of the 

 latter the stele has well-developed primary wood, arising from 

 several protoxylem groups (unknown in Psilophyton) and has 

 altogether reached a higher stage of development than anything 

 known in Psilophyton. At the same time, as we shall show in the 

 following chapter, this being a stage passed through in the 

 evolution of the steles of most groups, has no obvious phyletic 

 bearing. 



