DEVELOPMENT OF PLANTS 173 



that definite environmental conditions caused motile bodies to be 

 produced that are lacking in some essential substance and conse- 

 quently do not have the energy for growth, or at least can only 

 develop into feeble plants. This weakness is overcome by the 

 fusion of two gametes which results in the formation of a cell 

 or gametospore with renewed energy for growth. 



The gametospore germinates after the period of rest in a rad- 

 ically different manner from previous cases. The contents of 

 the spore, instead of giving rise directly to a new plant, divides, 

 forming several zoospores (Fig. 112, E, F) which after a period 

 of motility become attached to some object and grow into a new 

 plant. The cause of this peculiar behavior of the germinating 

 gametospore is not known. Some changes must have occurred 

 in it that was lacking in the gametospores previously noted, and 

 as a consequence, it is not able to develop directly into a new 

 plant. This variation in the growth of the gametospore marks 

 the beginning of one of the most important departures in the 

 evolution of plant life and it will become more and more signifi- 

 cant as the work proceeds. It is evident that the formation of 

 several zoospores, each of which produces a new plant, is a de- 

 cided advance over those types in which the gametospore develops 

 directly into but one new plant. Note also that in Ulothrix there 

 are two kinds of zoospores, one formed from the Ulothrix plant 

 and the other from the gametospore, the former being frequently 

 called zoogonidia to distinguish them from the latter. This dis- 

 tinction is of great importance, because most plants have two dis- 

 tinct stages or generations in their life history. One is repre- 

 sented by the plant that bears the sexual cells or gametes and 

 therefore called the sexual plant or sexual generation, or, in short, 

 the gametophyte. The real nature of this plant is to produce 

 gametes, although it may for a time produce zoospores. The 

 other stage is derived from the gametospore and is called the 

 asexual plant, asexual generation or sporophyte, since this plant 

 can only produce zoospores. Among the green algae this dis- 

 tinction is not very manifest, the Ulothrix plant for example being 

 the sexual plant or gametophyte and the gametospore is the asex- 

 ual plant or sporophyte, in this case being merely a single cell. 



