CHAPTER IX 



SEED PLANTS (SPERMATOPHYTA) (GYMNO SPERM 'JET) 



ONE of the most notable peculiarities of the higher 

 Pteridophytes is the extreme reduction of the gameto- 

 phyte and the corresponding specialization of the 

 sporophyte. This culminates in the various hetero- 

 sporous types, where the gametophyte may lack all 

 power of independent growth and serve merely to de- 

 velop the reproductive organs and nourish the embryo- 

 sporophyte until it is self-supporting. In Selaginella 

 the gametophyte is partially developed within the 

 spores while they are still included within the sporan- 

 gium, and is nourished directly from the sporophyte 

 through the sporangium wall, which serves thus not 

 only to protect the spores, but also to nourish them 

 during the early stages of germination. Finally, how- 

 ever, the spores are discharged from the sporangium, 

 and the gametophyte completes its development away 

 from the sporophyte. 



In the highest of all plants, the seed-bearing, or, 

 as they are commonly called, the "flowering plants," 

 heterospory is carried one step further, and the macro- 

 spore remains permanently within the sporangium. Not 

 only is the germination of the spore completed within the 

 sporangium, but the fertilization of the archegonium is 

 effected and the development of the embryo-sporophyte 



168 



