ANGIOSPEEMS 333 



mature produces asexual spores. This gives rise to alterna- 

 tion of generations, in which a gametophyte by means of an 

 egg and a sperm produces an oospore ; the oospore upon ger- 

 mination produces a sporophyte ; the sporophyte produces 

 asexual spores, which germinate and produce new gameto- 

 phytes. In some bryophytes (mosses and Anthoceros) the spo- 

 rophyte bears chlorophyll, but in all it is wholly or largely 

 dependent upon the gametophyte for nourishment. The game- 

 tophyte is the chief chlorophyll-working generation. Indeed, 

 the most complex nutritive structures in any gametophyte of 

 the plant kingdom are in the bryophytes. 



305. Pteridophytes. In most pteridophytes both generations 

 bear chlorophyll. In the true ferns the gametophyte is smaller 

 and less complex than in most bryophytes. It looks like a 

 simple liverwort. The sporophyte has a heavy woody stem 

 and large leaves. The stem and leaves possess fibrovascular 

 tissues, which aid in giving support and in conducting food 

 material. The introduction of fibrovascular tissue represents 

 an epoch of very great significance in the plant kingdom. It 

 makes possible the development of large upright plants and 

 giant tree trunks, which can expose chlorophyll tissue high in 

 the air. It must be kept in mind that in so far as bryophytes 

 develop special chlorophyll structures, these are found in the 

 gametophyte and not in the sporophyte. 



In the true ferns a gametophyte may bear both sex organs 

 and produce an oospore. The sporophyte produces many spores, 

 any of which may produce gametophytes. The asexual spores 

 are of one kind (homosporous). In other fern-like plants 

 (Sdaginella) two kinds of asexual spores (heterosporous) are 

 produced. The small spores produce male gametophytes, which 

 produce the antheridia and sperms, and the large spores pro- 

 duce female gametophytes, which produce archegonia and eggs. 

 The egg is fertilized within the female gametophyte, which 

 has not escaped from the megaspore wall. The oospore pro- 

 duces an embryo sporophyte, which grows out of the old 

 megaspore wall, and becomes established as a new plant. 



