Ch. XII] THE PTERIDOPHYTES 511 



carbon dioxide was reduced to a point where new growth and 

 decay nearly balance, as at present. The steps in the 

 conversion of plant tissue to coal are well known, and have 

 already been indicated in brief (page 99). 



Ecologically, the Pteridophytes are typically terrestrial 

 plants, mostly mesophytes, though some are water plants, 

 some are epiphytes, and a few are xerophytes. While 

 undoubtedly they formed the first forest vegetation of the 

 earth, they have been largely superseded by the more highly 

 developed Spermatophytes, and manage to linger only in 

 the undergrowth of tropical forests or as creeping forms in 

 our own woods. Phylogenetically their relations are some- 

 what uncertain. No one of the three classes seems to have 

 been derived from any of the others, but all three must have 

 had independent but contiguous origins from lower forms. 

 It is not likely that they came directly from the Bryophytes, 

 though this view was long held, but apparently Bryophytes 

 and Pteridophytes came anciently from one stock, which 

 evolved from the higher Green Algae (Fig. 275). 



