LIFE CYCLE OF SAPROLEGNIA 



29 



cut off contains many nuclei. Vacuoles appear in various parts of 

 this cell and by growing and coalescing they gradually cut up the 

 enclosed protoplasm into a number of small cells, which develop 

 cell walls and finally two flagella each. Thej^ are the zoospores, and 



ATM 



Fig. 23. Sporangia of Saprolegnia with zoospores in various stages of 



development. 



the cell in which they are formed is a zoosporangium (Fig. 23). The 

 zoospores are pear-shaped, with the two flagella inserted at the apex. 

 After swimming about for a time they come to rest, lose their flagella, 

 and become rounded. After a resting period, these rounded cells give 

 rise to a new sort of zoospore, bean- 

 shaped, with the two flagella inserted on 

 the side. These then swim about and, if 

 they reach a suitable substrate, come to 

 rest and germinate, forming the coeno- 

 cytic mycelium. 



Sexual spores are not formed so fre- 

 quently. They are produced by the con- 

 jugation of two morphologically unlike 

 cells, a large oogonium and one or more 

 small tube-like antheridia. These gen- 

 erally develop from neighboring branches 

 of the same filament. Both the oogonium 

 and the antheridium are at first multi- 

 nucleate like the coenocytic mycelium 

 from which they are derived. They become separated from this my- 

 celium by crosswalls. The protoplasm of the oogonium becomes 

 divided into cells containing each a single nucleus. The antheridia 

 penetrate the oogonium and their nuclei fuse two by two with those 



Fig. 24. An oogonium of 

 Saprolegnia. It has been 

 fertilized by two antheridia 

 and contains six oospores. 



