OOMYCETES 



63 



empty part. Thus the formation of zoospores is entirely suppressed 

 and the undifferentiated sporangium germinates by a mycelial filament. 

 Wherever this comes in contact with a Closterium it pierces the wall and 

 grows into the interior. A. Closterii is generally dioecious. As in Myzo- 

 cytium, there are few nuclei in the antheridium but many in the oogonium. 

 They divide in the mature sexual organs. Then the antheridia form 

 copulation tubes (Fig. 37, 2 and 3) through which the whole content 

 migrates into the oogonium. The details of development and the 

 germination of the oospores are not yet known, it is particularly uncertain 

 whether nuclear fusion occurs between all nuclei or, as in Myzocytium, 

 between privileged nuclei. 



§ • 



>, A': ■ . 



Fig. 38. — Saprolegnia. Development of zoospores. A, Young zoosporangium whose 

 peripheral protoplasm has a large number of nuclei; B, (section) uninucleate spore initials 

 are formed in the peripheral layer by radial cleavage; C, D, spore initials contract, separate 

 and round up. (After Davis, 1903.) 



Saprolegniaceae. — This family is mostly saprophytic, rarely parasitic 

 on plant and animal substrates in water and soil. The mycelium is 

 tubular, generally differentiated into slender, ramose, intramatrical 

 hyphae and into thick, less-branched extramatrical hyphae, up to 200/j 

 in diameter. The vacuolate protoplasm lies next the wall. Under 

 favorable conditions, the resting hyphae change to gemmae. The hyphal 

 tip swells to a short clavate organ stored with protoplasm, is abjointed, 

 rounds off and thickens its wall. This process is repeated basipetally, 

 so that a moniliform chain results. 



When the extramatrical hyphae have attained a certain age, their 

 ends become clavate or spherical, filled with thick protoplasm and 

 numerous nuclei, and are abjointed as zoosporangia (Rothert, 1892). 

 The peripheral protoplasm splits without further nuclear division into 



