EVOLUTION AS SHOWN BY PLANTS 



345 



stand the strobilus. The pine cone furnishes perhaps the 

 most familiar example of a strobilus or "aggregation of 

 sporophylls." 



The other important step which is introduced by a few 

 members of the fern and club-moss group has to do with 

 the spores. Up to this point spores are all alike. They all 

 produce just one kind of gametophyte, that gametophyte 

 bearing both kinds of sex organs — the antheridia which 

 contain sperms and the archegonia which contain eggs. 

 But now, in the few members of the fern group referred to 

 (selaginella, a common foliage plant in greenhouses is one 

 of them), two distinct kinds of spores are produced. One 

 of these is relatively very large and is called the megaspore. 

 The other is relatively very small and is called the micro- 

 spore. The megaspore produces a gametophyte which 

 bears only archegonia, and is therefore called a female 

 gametophyte. The microspore produces a gametophyte 

 which bears only antheridia, and is therefore called a male 

 gametophyte. This condition, in which two kinds of spores 

 are produced, is called helerospory. Heterospory is just as 

 important to understand as the strobilus, for this habit is 

 the forerunner of seed formation, and the seed cannot be 

 understood without understanding heterospory. 



When heterospory appears the gametophytes become 

 small and parasitic, just as the sporophytes were small and 

 parasitic when they first appeared. In fact the gameto- 

 phytes of heterosporous plants are parasitic within the 

 spores that produce them. We see herein some explanation 

 of the size of the megaspores. Not only do they have to 

 have food stored up to provide for their own germination, 

 they must also provide food for the whole development of 



