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442 PTERIDOPHYTES (FERN PLANTS) 



diums the spores are alike as to size, although in some cases the; 

 differ in the kinds of gametophytes produced. Other Pterido- 

 phytes differentiated spore-bearing and vegetative tissues, but 

 the Selaginellas have differentiated spores both in size and func- 

 tion. The larger spores, which are many times larger than the 

 smaller ones, produce only female, while the smaller ones produce 

 only male gametophytes. The two kinds of spores are borne 

 in separate sporangia which also differ in size. The prefixes, 

 micro, meaning little, and mega or macro, meaning large, are used 

 to designate these spores. and also the sporangia and sporophylls 

 which bear them. Thus we speak of microspores and megaspores, 

 miarosporangia and megasporangia, and microsporophylls and mega- 

 sporophylls (B and C, Fig. 394). 



This habit of producing two kinds of spores in regard to size is 

 called heterospory (meaning different spores), while the habit of 

 producing spores alike in size is called homospory (meaning same 

 spores). The introduction of heterospory by Selaginella is a 

 significant feature because all Seed Plants are heterosporous. In 

 Seed Plants the pollen grains are microspores and within the 

 ovules occur the megaspores. 



Gametophytes. — The second notable feature which Selagi- 

 nella presents is that the gametophytes are so much reduced that 

 they develop within the spores, where food and protection are 

 provided. Thus in Selaginella there are no green independent 

 gametophytes as we have been used to in other Pteridophytes 

 and in Bryophytes, but the gametophyte now lives on the sporo- 

 phyte just as the sporophyte of the Bryophytes lives on the 

 gametophyte. This, also, is a feature that is characteristic of 

 Seed Plants. 



The male gametophyte is extremely simple, consisting of one 

 vegetative cell and a simple antheridium containing only a few 

 sperms, each of which nas two slender cilia {C, Fig. 395). In 

 developing, the male gametophyte breaks the spore wall, so that 

 a crack is produced through which the sperms escape. 



The megaspores germinate and form the female gametophytes 

 while still in the sporangium, and this is a third feature that is 

 characteristic of Seed Plants. The female gametophyte is much 

 larger than the male gametophyte (A and B, Fig. 395). Its much 

 larger size is permitted by the greater size of the megaspore and 

 is also necessary because the female gametophyte must support 



