PTERIDOPIIYTES 333 



In this case two gametophytes are involved, one pro- 

 ducing a sperm, the other an egg, which fuse and form the 

 oospore, which in germination produces the sporophyte, 

 which produces two kinds of asexual spores (megaspores 

 and microspores), which in germination produce the two 

 gametophytes again. 



One additional fact connected with heterospory should 

 be mentioned, and that is the great reduction of the gam- 

 etophyte. In the homosporous ferns the spore develops 

 a small but free and independent prothallium which pro- 

 duces both sex organs. When in heterosporous plants this 

 work of producing sex organs is divided between two gam- 

 etophytes they become very much reduced in size and lose 

 their freedom and independence. They are so small that 

 they do not escape entirely, if at all, from the embrace of 

 the spores which produce them, and are mainly dependent 

 for their nourishment upon the food stored up in the spores. 



