THE PTERIDOPHYTES 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. 882), though most ferns have all their spores 
alike. The highest of all plants (seed-plants) are hetero- 
sporous, producing a smaller kind of spore, the pollen-grain, 
and a larger kind, the embryo sac (Sect. 883). 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 Age), and a large part of the bulk of 
1 See Bergen and Davis’ Principles of Botany, Chapter XXV. 
