DEVELOPMENT OF PLANTS 175 



to the filaments. _A_sexual reproduction is effected by very large 

 zoospores which are developed in the enlarged extremities of the 

 filaments (Fig. 113, B). These tips are cut off by a transverse 

 wall and a single zoospore escapes through an opening in the 

 tip of the sporangium thus formed. The entire surface of the 

 zoospore is clothed with cilia arranged in pairs, each pair being 

 associated with a nucleus so that the zoospore resembles the motile 



FIG. 113. Structure and asexual reproduction of Vaucheria: A, portion 

 of a plant showing the branching tubular filament and colorless root-like 

 outgrowth, r. B, end of a filament enlarging to form a zoospore. C, 

 zoospore. D, germination of a zoospore. 



colonies previously noted (Fig. 113, C). After a very short 

 motile period, the zoospore comes to rest and grows into the 

 characteristic tubular plant (Fig. 113, D). When the plants are 

 exposed to too dry conditions, the tips of the filaments often 

 enlarge and finally become detached as motionless spores that 

 germinate when conditions are again favorable. 



The male and female gametes are produced in gametangia that 

 are formed from short branches. The males are developed in 

 large numbers in curved branches that become cut off from the 

 filament by a cross wall (Fig. 114, A) and the gametes finally 

 escape through an opening that forms at the apex of the branch. 



