General Conclusions 



In a book of this limited size it is impossible to describe 



in detail all the fossil plants that are known. Accordingly 



several major groups of vascular plants, many minor groups 



and a large number of genera have had to be omitted. Thus, 



there has been no mention of the Noeggerathiales, nor of the 



Pseudoborniales, on the grounds that they occupy isolated 



positions in the classification and throw almost no Hght at 



all on the evolution of modern plants. For details of these 



strange plants the reader is referred to textbooks of paleo- 

 botany, i. s. i*. 22 



There are, also, several fossil genera of fronds and trunks 

 of which no mention has been made, since they seem to 

 stand midway between pteridophytes and gymnosperms 

 (e.g. Aneurophyton, Eospermatopteris, Tetraxylopteris, Proto- 

 pitys, Pitys, Archaeopteris, Callixylon and Archaeopitys), 

 and might appropriately be described in a text-book of 

 gymnosperms. However, there has been a recent suggestion^® 

 that these plants, while indeed ancestral to the gymno- 

 sperms, were, nevertheless, still at the level of pteridophytes 

 in their mode of reproduction. Brief mention of them must, 

 therefore, be made here. This suggestion arose out of the 

 discovery, in Upper Devonian rocks near New York, of 

 large fern-Hke fronds, known as Archaeopteris, actually in 

 organic connection with Callixylon, a large tree whose mas- 

 sive woody trunks were at least 20 m tall and more than i -5 m 

 across. The fronds of one species oi Archaeopteris are known 



175 



