74 ORIGIN OF CORMOPHYTA [ch. 



Lycopsida — his Phylloideae — but in place of his PhyUineae, two 

 distinct hnes of descent are recognised, and the Sphenopsida 

 are regarded as primitively microphyllous and not megaphyllous. 

 In this and other features these conclusions differ from any 

 previously expressed, and since our interpretation oiPsilophyton 

 and Arthrostigma differs from that to which Halle has inclined 

 (though in some cases he admits that we may here be dealing 

 with emergences and not leaves) so our conclusions as regards 

 phylogeny are quite distinct from his. 



We now propose to trace the early stages of the evolution of 

 each group in some detail. 



The Evolution of the Sphenopsida. 



It seems probable that the Sphenopsida took their origin from 

 Algae possessing a whorled habit like that of the living fresh 

 water Red Alga — Batrachospermum. In this ancestor we should 

 expect to find some form of primitive vascular system, at least 

 as far advanced as in Psilophyton, and a distinct alternation of 

 generations. 



As regards the vegetative habit, we should expect to find that 

 the whorled branches gradually took on the leaf function, 

 without any very radical change in external morphology. They 

 remained either simple or dichotomously forked microphyllous 

 elements, the branching of the primitive lamina being the most 

 general method of increasing the lamina area, in view of the 

 new functions which these organs had assumed. As regards the 

 cones, the primitive whorled habit was again retained. These 

 were originally made up of whorled branches, the branches being 

 sometimes simple and sometimes forked several times. Some of 

 the branches of the whorl or their segments took the path of 

 remaining protective organs (i.e. the bracts) and in some cases 

 became coalescent to a considerable degree in order to fulfil this 

 function more effectually. Other segments were metamorphosed 

 partly into sporangiophores, partly into sporangia. 



Such represents our conception of the main course of evolution 

 in this group. We now have to enquire what fossil evidence we 

 have for the support of these contentions. 



With regard to the primitive whorled algal type there is little 



