426 



THE VASCULAR PLANTS 



the Selaginellas alone. But the fact is that all seed plants 

 also produce two kinds of spores. It is evident that seed 

 plants are descended from ancestors which had two kinds 



of spores, and Selagi- 

 nellas are important to 

 us because they are 

 two-spored forms which 

 throw much light upon 

 the evolution of seed 

 plants. 



This habit of pro- 

 ducing two kinds of 

 spores is called heter- 

 ospory. (The word 

 means different spores.) 

 The habit of producing 

 one kind of spore, the 

 habit with which we 

 are already familiar, 

 is called homospory. 

 (The word means same 

 spores.) 



The spore-beariftg 

 leaves (sporophylls) of 

 Selaginella are found at 

 the tips of the stems 

 as in the other club 

 mosses. The clusters 

 of sporophylls (strobili) are evident, but the sporophylls 

 themselves are not very different in their general appear- 

 ance from the ordinary leaves. (See Figure 2ig.) 

 In addition to heterospory, there are other important 



Fig. 2i8. — Lycopoiium. A variety which bears 

 distinct strobili of the kind which gave rise to 

 the name club moss. 



