320 



GENERAL BOTANY 



stem, root, and foot surrounded by food material previously stored 

 in the gametophyte within the spore. The further growth of the 

 embryo (e) is accomplished by the elongation of the young stem 

 (or hypocotyl) and the root, which bore their way out of the 

 gametophyte and become adjusted to soil and air in response to 

 gravity and light. 



Life history. The life history of Selaginella (Fig. 185) pre- 

 sents certain new and important features. The spores are of 

 two kinds: small microspores, resembling pollen grains, which 



Spores 



Spores 



Gametophyte Sporophyte Gametophyte 



FIG. 185. Life history of Selaginella represented graphically 



are easily carried by air currents ; and large megaspores, which 

 develop a nutritive gametophyte and female gametes. Spore 

 germination takes place while the spores are still in the spo- 

 rangia, and consequently the gametophyte plants thus formed 

 within the spore coats are nourished by and are parasitic upon 

 the mother sporophyte. The male gametophytes are reduced 

 to gamete-producing structures and are composed of a single- 

 celled plant body and an antheridium bearing motile male 

 gametes. The female gametophytes are richly stored with food 

 which supports the embryo and young sporophyte until the latter 

 becomes adjusted to its environment and is self-supporting. The 

 heterosporous selaginellas thus approach the conditions found 

 in the higher seed plants, in that they bear two kinds of spores, 

 one of which, the megaspore, is stored with food reserve derived 

 from the mother plant. 



