PTERIDOPHYTES 109 



heterospory (" different spores "), and such plants are 

 heterosporous. 



Heterospory is extremely important, for it is the condition 

 that leads to seed-formation ; that is, heterospory is the pre- 

 cursor of the seed. Pteridophytes in general are not heter- 

 osporous, but heterospory began among Pteridophytes, and 

 when it reached the formation of seeds, then Seed-plants 

 (Spermatophytes) began. 



Not only are the spores of heterosporous plants different, 

 but the two kinds are produced by different sporangia (Fig. 

 89). Therefore the sporangia that produce megaspores are 

 called megasporangia, and those that produce microspores 

 are called microsporangia. In Selaginella (the heterosporous 

 Club-mosses) the megasporangia are usually in the lower 

 part of the strobilus, and the microsporangia in the upper 

 part. The difference in the size of the spores involves a 

 difference in the number of spores produced by the two kinds 

 of sporangia. In Selaginella, for example, a megasporan- 

 gium usually contains four megaspores, while a microspo- 

 rangium contains hundreds of microspores. Since the two 

 kinds of sporangia are approximately of the same size, this 

 difference in the number of spores will give some idea of 

 their difference in size (Fig. 89). 



It is necessary also to recognize the fact that the sporo- 

 phylls that produce the two kinds of sporangia may become 

 different ; in fact, among the Seed-plants they become very 

 different. In order to distinguish them, the sporophylls 

 producing megasporangia are called megasporophylls ; while 

 those producing microsporangia are called microsporophylls. 



A little experience with these terms will make them recall 

 easily the structures they stand for, especially if their rela- 

 tions are remembered as follows : a megasporophyll bears one 

 or more megasporangia, which contain megaspores; and 

 when megaspores germinate, they produce female gameto- 

 phytes, that is, gametophytes that bear only archegonia 



