CYCADOFILICALES AND CYCADALES 



293 



The present and the following chapter will deal with the repre- 

 sentatives of the Archigymnospermae. 



Our knowledge of the Archigymnospermae, from the fact 

 that many of the group are extinct, is necessarily incomplete. 

 It will accordingly be convenient in the present chapter to focus 

 attention on the best-known fossil group and the nearly related 

 family which possesses living representatives. The extinct aggrega- 

 tion of forms which 

 here present the 

 strongest claim to 

 attention are the 

 C ycadofilicales, 

 characterized by a 

 habit so closely re- 

 sembling that of the 

 true ferns that it is 

 only since the very 

 end of the last cen- 

 tury that they have 

 been recognized as 

 seed plants. The 

 type of seed con- 

 nected with the veg- 

 etative structures of 

 the Cycadofilicales 

 has been diagrammatically indicated in an earlier chapter, to which 

 the reader is here referred. In the present connection only ana- 

 tomical features of the group need be discussed. The antiquity 

 of the Cycadofilicales is vouched for, not only by their Paleozoic 

 occurrence, but also by the occasional presence of protostelic 

 stems, axes of this type not being known for any other group 

 of seed plants living or extinct. Fig. 207 illustrates the organiza- 

 tion of the protostelic axis of the genus Heterangium from the 

 English Carboniferous. The transverse section shows the presence 

 of an external ribbing of sclerenchymatous strands, a feature com- 

 mon to many older representatives of the Gymnospermae. The 

 fibrovascular apparatus consists of protostelic primary wood sur- 

 rounded by a thin layer of secondary xylem. 



CO. 



FIG. 207. Transverse section of the stem of Heter- 

 angium (after Scott). 



