SEXUAL REPRODUCTION. 



271 



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

 These resting spores, after a dormant period, germinate and 

 develo]) into new ijlanis. 



In Ulothrix, tlierefore, the gametes are imperfectly sexual. 

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

 new individuals. A con- 

 sideration of the apjjearance 

 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, y 

 complete sexuality was ac 



Fir.. 302. — .Sporelings of I'U-'tJirix zoiiata. 

 quired by certain cells, while «. a young plant" from a large zoospore. 



/', young plants from small zoospores whiclt 

 others were more COmpletelv germinated without leaving the mother cell. 



INIagnihed 4S2 diam. — After Dodel-Porl. 



specialized as non-sexual re- 

 productive bodies. The latter ha\e already been discussed 



(1l3°4ff.)- 



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 irorn the cell pro- 

 ducing them. From this point two lines of development 

 diverge. 



375. I. 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 



