616 - Heredify and Evolution 



is very small. Indeed, in higher forms (gym- 

 nosperms and angiosperms) both male and 

 female gametophytes are microscopic (Fig. 

 12-19). The protection and nurturing of the 

 embryonic sporophyte is exceptionally elfec- 

 tive. 



Three large classes, each containing a wide 

 variety of living species, have been evolved 

 among the Pteropsida. These are: 

 Class 1. The Filicineae — ferns 

 Class 2. The (iymnospermae — cone- 

 bearing [slants 

 Class 3. The Angiospermae — flower- 

 ing plants 

 Among the Pteropsida, only the last two 

 classes (Gymnospermae and Angiospermae) 

 produce seeds. Formerly these seed plants 

 were considered to represent a separate 

 phylum (the Spermatophyta). However, ac- 

 cumulating paleontological evidence has 



made it necessary to modernize the classifica- 

 tion. 



The Ferns (Class Filicineae). The dis- 

 tinctive reproductive cycle and the general 

 characteristics of this familiar group of plants 

 •were described in an earlier section (Chap. 

 12). During the Carboniferous period a num- 

 ber of large treelike ferns existed and these 

 played some role in the formation of coal. 

 Today, however, ferns seldom exceed knee 

 height, although a few resemble small trees 

 (Fig. 31-21). Among living ferns there are 

 about 250 genera (10,000 species). 



The ferns tend to be restricted to environ- 

 ments where water is abundant, at least 

 periodically. The gametophyte (Fig. 12-5), 

 although green and nutritively independent, 

 is small (seldom more than 14 inch wide) and 

 not well adapted to land conditions. More- 

 over, the flagellated sperm (Fig. 12-4) must 



Fig. 31-24. Young tree-ferns, Jamaica. Few modern Filicineae grow to such a 

 size; but many ancient forms were much larger. (From The Plant World.) 



