486 



General Botany 



gates and pushes into the soil ; (3) a leaf of very simple structure, 

 which soon rises above the prothallus and forms the first photo- 



FiG. 301. The life history of a fern. The prothalkis (.-I) produces egg cells and sperms 

 in organs on the lower surface. One of the sperms set free from B unites with an egg 

 cell (shown in C) and produces an oospore. This germinates and produces the leafy 

 fern plant (D), which in turn produces spores in sporangia {F and G) on the lower 

 side of the leaves. By the bursting of the walls of the sporangium {H) the spores are 

 set free. They then germinate on the soil (in some species on rocks or trees) and 

 produce a new generation of prothalli like the one shown in A . The prothallus is here 

 shown about four times its natural size. 



synthetic organ of the sporophyte; and (4) a stem tip, which 

 extends more slowly and gives rise to the successive leaves and 

 adventitious roots. The sporophyte is thus at first parasitic on 

 the gametophyte, but it soon becomes independent and the pro- 

 thallus dies and disappears. 



The embryo develops into the mature sporophyte, which has 

 already been sufficiently described. 



Alternation of generations. Among the algae both the oo- 

 spores and the spores formed from vegetative cells usually re- 

 produce the plant directly. In CEdogonium the oospore, when 

 it germinates, produces an enlarged cell {sporangium), in which 

 four swimming spores are formed, and these reproduce the fila- 

 mentous plant. 



Among the Bryophytes the spores, formed asexually, develop 



