DEVELOPMENT OF PLANTS 189 



Ulothrix, has been deferred to this point because it presents 

 several interesting departures from previous types which indi- 

 cate that a higher point has been attained in some respects by 

 this plant than by any other of the green algae. The filaments of 

 Coleochaete have a pronounced apical growth and are usually 

 associated together in a radiate manner, forming small discs or 

 cushion-like masses on the stems and leaves of water plants (Fig. 

 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. e., 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 nor zoospores as in / 



Ulothrix and Oedogonium, but instead, the gametospore forms (jTUA/-^^''^*'^ 

 a number of cells. This growth ruptures the coat of the spore ^ ' ^fyi^l 

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

 c) is flerived that develops into a small plant. The plants thus oXiJ)^ 

 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 



