THE PTEEIDOPHYTES 293 



spores from spore-leaves which are sometimes grouped in 

 a cone at the end of the stem and sometimes scattered 

 along the stem. No gametophytes of our species of the 

 northern United States, have been found, and therefore re- 

 production in these must be by division of the stem or by 

 certain buds specialized for this purpose. Selaginella pro- 

 duces two sorts of asexual spores. Of these the smaller 

 kind develops into male and the larger into female gameto- 

 phytes. From the fertilized female gametophyte a young 

 (asexual) spore-plant is produced. 



The term heterospory is applied to the condition of 

 plants which (like Selaginella) bear two kinds of asexual 

 spores from which are developed two kinds of gametes. 

 Heterospory begins among the pteridophytes with certain 

 ferns (Sect. 382), though most ferns have all their spores 

 alike. The highest of all plants (seed-plants) are hetero- 

 sporous, producing a smaller.kirid of spore, the pollen-grain, 

 and a larger kind, the embryo sac (Sect. 888). An impor- 

 tant result of heterospory is that the gametophyte becomes 

 extremely small, remaining partially or almost wholly en- 

 closed in the spore * from which it is developed. This 

 dwarfing of the gametophyte is one of the most noteworthy 

 steps which mark the rise of the highest of the pterido- 

 phytes and the seed-plants from all lower spore-plants. 



375. Additional Notes on the Pteridophytes As has 



already been suggested in regard to the horsetails (Sect. 

 371), the pteridophytes in general are of much less impor- 

 tance at present than they were in earlier geological ages.^ 

 Tree-like club-mosses were extremely abundant during the 

 age when most of the earth's coal deposits were formed 

 (the Carboniferous ^go)-, and a large part of the bulk of 



1 See Bergen and Davis' Principles of Botany, Chapter XXV. 



