554 STUDIES IN FOSSIL BOTANY ,. .• 



may be due to their common origin rather than to a 

 direct connection in later periods. 



Returning for a moment to the anatomical characters, 

 it may be pointed out that the general structure of 

 the leaf in Cordaiteae agrees so closely in essential 

 points with that of Poroxylon, that we can scarcely 

 doubt that the two groups were nearly allied. Poroxylon 

 retained centripetal wood in the stem, a primitive 

 character which some of the Cordaiteae had lost. 

 The Poroxyleae appear thus to combine the characters 

 of Lyginodendreae or Calamopityeae with those of 

 Cordaiteae, though their horizon is too late for us 

 to regard this family as an actual link with the 

 Pteridosperms. The Pityeae belong to the earliest 

 Carboniferous times, but, to judge from the few 

 characters available for investigation, were already far 

 advanced on the Cordaitean line of descent. 



The affinities of the Cordaiteae and of the Cordaitales 

 generally are, as we have seen, extraordinarily complex, 

 and cannot be cleared up until our data are much more 

 extensive than at present. The great step which has 

 been made in the last few years is the full confirmation 

 of the affinity between Cordaitales and the Pteridosperms, 

 which the discpvery of the seeds of the latter class has 

 afforded. The relations to Cycadophyta, Ginkgoaceae, 

 and Coniferae have long been recognised, and thus a 

 strong presumption is established that the whole of 

 the Gymnosperms sprang originally from the same 

 ancient stock of Fern-like plants (see Chap. XIV.). 



