i8o TENDENCIES IN THE CHLOROPHYCEAE 



117). The advance of this type is indicated not only by the 

 localization of growth at definite points, but also by the formation 

 of the zoospores and gametes in definite regions, i. c, in special 

 cells which are usually located at the end of the filaments. The 

 zoospores are produced singly from such cells and from smaller 

 pear-shaped cells single male gametes are formed. The female 

 gametes are developed singly in large flask-shaped cells, access 

 to which is afforded by an opening that appears at the end of 

 the long neck of the flask (Fig. 118, A). The gametospore de- 

 velops a cell wall and becomes enveloped by the adjoining cells 

 of the filaments (Fig. 118, B). In this condition the winter is 

 passed and in the spring it germinates, forming neither a new 

 plant directly as in Spirogyra and Vaucheria or zoopores as in 

 Ulotlirix and Oedogonium, but instead, the gametospore forms 

 a number of cells. This growth ruptures the coat of the spore 

 and finally from each cell a rather irregular zoospore (Fig. 118, 

 c) is derived that develops into a small plant. The plants thus 

 formed multiply solely by zoospores until finally, after several 

 generations, larger plants are produced that bear gametes, thus 

 completing the life history. 



72. Noteworthy Features of the Chlorophyceae. — Two fea- 

 tures in the study of green algae should be kept clearly in mind, 

 because they are closely connected with the tendencies that will 

 appear in the development of the mosses. First : The gameto- 

 spore is essentially a dormant or resting cell that tides the life 

 of the plant over the conditions unfavorable for growth. Second : 

 In passing from lower to higher types, the gametospore tends 

 to vary in its nature and possibilities of growth. At first it 

 forms directly a sexual or gamete-bearing plant as in Spirogyra 

 and Vaucheria. In higher types, it develops a generation of 

 zoospores from which the sexual plant is derived as in Ulotlirix 

 and Oedogonium. Finally in Colcochacte, we find a further ad- 

 vance in that several non-motile cells are formed by the gameto- 

 spore. These cells, to be sure, are essentially like the zoospore 

 because they develop directly into zoospores, but this condition 

 emphasizes the fact that the nature of the gametospore is steadily 

 departing from that of the gamete-bearing plant. 



