CLUB MOSSES 



427 



facts about Selaginella which forecast the habits of seed 

 plants and throw light upon their evolution. Not only 

 are there two kinds of spores 

 produced, but these spores are 

 produced separately in two kinds 

 of 'sporangia. And not only are 

 there these two kinds of spo- 

 rangia, but these sporangia are 

 produced separately on two kinds 

 of sporophylls. (See Figure 220.) 

 And these facts are also true of 

 seed plants. 



In what way are these two 

 kinds of spores different from 

 each other? They are different 

 in two ways. They are differ- 

 ent both in size and in what they 

 produce. (Study Figures 221, 

 222, and 22 3.) 



The larger spores are called 

 megaspores and they produce 

 gametophytes which bear eggs, 

 but not sperms. The smaller 

 spores are called microspores 

 and they produce gametophytes 

 which bear sperms but not eggs. 

 So heterospory gives us not only 

 two kinds of spores, but also two 

 kinds of gametophytes. The 

 megaspores produce female ga- 

 metophytes and the microspores 

 produce male gametophytes. 



Fig. 219. — Selaginella. A delicate 

 kind of club moss whose repro- 

 ductive structures and habits are 

 very important in indicating the 

 evolution of seed plants. The 

 cone- like structures at the tips are 

 strobili. Each of the leaves which 

 compose them bears a single spo- 

 rangium upon its inner face. — 

 After Coulter. 



