138 INTRODUCTION TO PLANT GEOGRAPHY [cHAP. 



organs. But, having become structurally modified for life in swamps, 

 with extensive tissues for gaseous exchange and very little for water 

 conduction, they were apparently unable to adapt themselves to the 

 more adverse climates following the Carboniferous. Meanwhile 

 the small herbaceous types survived and gave rise to those persisting 

 at the present time, even though they apparently represent another 

 evolutionary end-line. Like the Psilophytineae and Equisetineae, 

 this group is sometimes given the rank of a division, as the Lycopsida ; 

 for it is now realized that these groups of so-called ' fern-allies ' 

 represent separate lines of development as far back as it is possible 

 to trace them. 



Of Filicineae, the Ferns, characterized by large complicated leaves 

 and gaps in the vascular cylinder, there are plentiful fossils — which 

 go well back into the Devonian in the case of the long-extinct 

 Coenopteridales. The earliest of these ' Primofilices ' were only 

 partly distinct from their presumable psilophytinean forebears, but 

 others soon became characteristic and prominent elements of the 

 flora. Further groups arose towards the end of the Palaeozoic era 

 and persist to the present day — in the case of the relatively primitive 

 ones such as the Marattiales apparently in decreasing numbers, 

 but in the case of the more orthodox and modern Filicales still 

 plentifully. Many even in early times were much like those, living 

 nowadays, that are described in Chapter II and illustrated in Fig. 17. 



Fossil Seed-Plants 



Probably more important than true Ferns in the Carboniferous, 

 and certainly accounting for a large proportion of the fern-like 

 foliage of that and the following periods up to the mid-Jurassic, were 

 the Pteridosperms or Seed-ferns, also called Cycadofilicales. As 

 their names imply, these were fern-like (often tree-fern-like, but with 

 secondary thickening) in some respects. But they bore seeds, as may 

 be seen from Fig. 33, which shows a reconstructed plant and part 

 of a frond of difi^erent types. The seeds were more or less naked, 

 this group belonging to the Gymnosperms ; but although there are 

 abundant difterences, there remain sufficient deep-seated similarities 

 to suggest that they may have given rise to the groups dealt with in 

 the next paragraph, even as they themselves probably arose in pre- 

 Carboniferous times from Ferns or fern-like stock that had not 

 advanced far beyond the psilophytinean stage. Apparently allied 

 or belonging to this group are the Caytonialcs, which in some respects 



