194 NATURE OF THE GAMETOSPORE 



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 

 distinct 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 or the 

 little plant derived from it (Fig. 112, F] is the asexual plant or 

 sporophyte, in this case being merely a single cell. It is very 

 manifest, however, that this single cell or plant is decidedly 

 different from the Ulothrix plant because it cannot develop into 

 the Ulothrix plant, but can only produce zoospores. So we see 

 that there are two generations in the life history and that they 

 differ radically in their natures and possibilities of growth. In 

 higher types of plant life it will be seen that the gametospore 

 tends to develop into a more and more complex body or plant 

 and it will be one of the most intresting features of the work to 

 watch the evolution of this asexual plant. 



The higher members of the Chaetophorales illustrate the same 

 gradual differentiation of the gametes, as has been noted in the 

 motile forms of the Volvocales. The female gametes become 

 distinguishable because of their larger size and shorter period of 

 motility, while the male gametes are small, owing to the large 

 numbers that are found in a cell or because of the small size of 

 the cells in which they are developed. Finally, in several of the 

 genera we find but a single female gamete which remains motion- 

 less in the cell. This condition is very well illustrated in one of 

 the genera of the next order. 



