PTERIDOPHYTES 153 



alternation of generations (Bryophytes and most Pterido- 

 phytes) was given as follows (§ 62) : 



Gizg> 0— S— 0— G=8> 0— S— o— Gz:8> 0— S, etc. 



In the case of heterosporous plants (some Pteridophytes 

 and all Spermatophytes) it would be modified as follows : 

 g=8>o— Sizg=g=8>o— S=:g=g=8>o— S, etc. 



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 

 (Figs. 140, 141). As the spore is produced by the sporo- 

 phyte, heterospory brings about a condition in which the 

 gametophyte is dependent upon the sporophyte, an exact 

 reversal of the condition in Bryophytes. 



The relative importance of the gametophyte and the 

 sporophyte throughout the plant kingdom may be roughly 

 indicated by the accompanying diagram, in which the 



shaded part of the parallelogram represents the gameto- 

 phyte and the unshaded part the sporophyte. Among the 



