72 ORIGIN OF CORMOPHYTA [ch. 



Our next historical reference, founded chiefly on the palaeo- 

 botanical evidence, is to the view of Dr Scott^ published 

 some years ago in regard to the descent of Pteridophyta along 

 three main lines: the Sphenopsida, Pteropsida and Lycopsida. 



Lastly we have Halle's ^ recently expressed conclusions founded 

 on the evidence of the Psilophyton flora. Apart from his views 

 on the antiquity of the sporogonium and Bryophyta to which 

 we have already referred (p. 44), Halle regards Arthrostigma as 

 a microphyllous Pteridophyte possessing leaf-bearing stems and 

 Psilophyton as possibly of the same nature. Our interpretation 

 is that they are Thallophytes possessing emergences. We, how- 

 ever, agree with Halle in regarding the vascular structure as 

 primitive in these genera. 



Halle also discusses the question whether megaphyllous 

 Pteridophyta occurred in Lower Devonian times. 



With this brief review of previous opinion, we proceed to 

 point out how recent work on the Devonian floras appears to 

 us to establish even more securely the conception of the early 

 existence of three distinct lines of descent, the Sphenopsida, 

 Pteropsida and Lycopsida. In the present chapter we shall 

 confine our attention to questions concerning the origin of the 

 external morphology of these groups. 



The Cormophytic Habit. 



We regard it as probable that the Psilophyton habit (Figs. 1 

 and 2, pp. 15 and 16) was primiti^'e for all three lines of Cormo- 

 phytic descent. That is to say, there was an epiterrafieous or subter- 

 raneous limited, erect or horizontal axis, fixed by rhizoids or roots, 

 and giving off one or more vertical erect branches. This type of 

 habit persists throughout the earlier Sphenopsida (e.g. Calamites), 

 Pteropsida (Pteridosperms) and hycopsida, (Lepidodendron). 



These three groups, however, differed essentially as regards 

 the morphology of the erect shoots and lea%-es, and it was. the 

 evolution of these distinct lines of modification of types met 

 with among the Psilophyton flora — as is already foreshadowed 

 in the Archaeopteris flora — ^that originally called these three 

 groups into being. In some cases the main erect axis was trans- 



1 Scott (1909), p. 616, and (1910).- ' Halle (1916), pp. 35^0. 



