SUMMARY OF SELAGIXELLA 337 



which when carefully studied is essentially the same as the 

 general life-history formula of bryophytes and pteridophytes 

 (Sees. 285, 310), namely, 



-S-sp-G, etc. 



The differences lie in the fact, above mentioned, that there are 

 two forms of spores, microspores and megaspores, which develop, 

 respectively, male and female garnetophytes, a complication which 

 was introduced with heterospory and which is present, as will 

 appear later, in the life histories of seed plants (Sec. 356). 



327. Summary of Selaginella. Selaginella is the highest of the 

 pteridophytes and the most important because of the evolution- 

 ary principles which it illustrates, leading up to the seed habit. 

 The first three of these principles are also illustrated by Mar- 

 silia and Isoetes, but the fourth and fifth are new. They are 

 (1) the establishment of heterospory resulting in the separation 

 of male and female gametophytes ; (2) the reduction or degen- 

 eration of the gametophyte, which becomes dependent upon 

 food stored in the microspores and megaspores ; (3) provision 

 in the rnegaspore for the nourishment of the embryo of the suc- 

 ceeding sporophyte generation ; (4) the early parasitic relation 

 of the female gametophyte within the megasporangiuin upon the 

 sporophyte ; (5) the occasional habit of developing the young 

 sporophyte while the megaspore is still retained within its 

 parent megasporangiuin. 



328. Isoetes. The species of Isoetes are known as quillworts. 

 Their position among the pteridophytes is a matter of dispute, 

 and some botanists place them with the ferns (Filicinece), but 

 the anatomy of the sporophytes is more like that of the club 

 mosses than any other group. They have a peculiar rush-like 

 habit of growth, the long leaves arising in clusters around a 



o o o 



short stem (Fig. 291,^4). Some forms are aquatic, growing on 

 mud at the bottom of ponds, while others are usually found out 

 of water. 



