SEXUAL REPRODUCTION. 



27I 



may round itself off and, alone, become a resting spore. 

 These resting spores, after a dormant period, germinate and 

 develop into new plants. 



In Ulothrix, therefore, the gametes are imperfectly sexual. 

 Failing to conjugate, as many do, they may still develop into 

 new individuals. A con- 

 sideration of the appearance 

 and behavior of the gametes 

 leaves little doubt that they 

 are merely small zoospores 

 which have acquired imper- 

 fectly the habit of conjuga- 

 tion and retained partially 

 the power of independent 

 growth. 



374. Further develop- 

 ment. — The perfecting of 

 reproductive methods fol- 

 lowed the two lines just sug- 

 gested. On the one hand, 

 complete sexuality was ac- 



Fig. 302. — Sporelings of Ulothrix zonata. 

 quired by Certain Cells, While a, a young plant from a large zoospore. 



b, young plants from small zoospores which 

 Others Were more Completely germinated without leaving the mother cell. 

 1 Magnified 482 diam.— After Dodel-Port. 



specialized as non-sexual re- 

 productive bodies. The latter have already been discussed 



(i 304 ff.)- 



Tracing now only the line of sexual development, it is 

 probable that the first step in this differentiation was the 

 failure of some of the zoospores to escape from the cell pro- 

 ducing them. From this point two lines of development 

 diverge. 



375. 1. Isogamy. — Along one of these lines, the zoo- 

 spores ceased to form cilia, and became non-motile sex 

 cells, in some cases similar in form and function, and in others 



