I 



ORDER SAPROLEGNIALES 105 



author the extensive and rapid formation of transverse and obHque walls 

 until the hypha was- cut up into innumerable angular segments, under 

 the influence of exceedingly minute amounts of copper in the water. 

 Growth of the external hyphae is terminal and by the limited formation 

 of branches. Unlike the Lagenidiales the mycelium does not become all 

 converted into reproductive organs but these are mostly terminal or 

 subterminal on the external hyphae and their branches. 



The Saprolegniales are saprophytic on dead plant or animal matter 

 in the soil or in water (fresh water, rarely brackish) or parasitic in algae 

 or small animals or even fish, and in some cases in the roots of plants in 

 the soil. Although spoken of usually as water molds perhaps the majority 

 are inhabitants of moist soil. 



The zoosporangia are of the same diameter as the hyphae or somewhat 

 enlarged, cylindrical or ovoid. At their inception the protoplasm from 

 the lower portions of the hyphae crowds into the terminal portions 

 destined to become the zoosporangia until these are filled with dense, 

 multinucleate protoplasm with a much reduced central vacuole. At the 

 base of each a septum is formed setting it apart from the supporting 

 hypha. Within this zoosporangium cleavage of the protoplasm begins 

 next to the wall and progresses inwardly to the central vacuole, cutting 

 out uninucleate naked portions of protoplasm which round up somewhat 

 and eventually become the zoospores. These are usually pyriform, with 

 two anterior flagella, one of the tinsel type and the other of the whiplash 

 type. Such zoospores are called primary zoospores. Escape of the zoospores 

 occurs through the softened tip and sometimes through a lateral papilla. 

 In all cases the opening is inoperculate. After the zoosporangium is 

 emptied the basal septum may arch up into the empty space, filling it 

 and becoming a second zoosporangium. This formation of zoosporangia 

 by proliferation may occur repeatedly. Instead of proliferating, the hypha 

 just below the emptied zoosporangium may branch out laterally, the 

 branch quickly turning upward to form a new zoosporangium parallel to 

 the empty one. This sympodial formation of zoosporangia is characteristic 

 of some genera. 



The primary zoospores may escape and swim away some distance 

 before encysting. In some species of Pythiopsis this encysted primary 

 zoospore eventually germinates by a slender tube and forms a new plant, 

 or the encysted zoospore may escape from its cell wall as a primary 

 zoospore again. More often in other genera the encysted primary zoospore 

 escapes from its cyst after a few minutes or hours in a different form, the 

 secondary zoospore. It is kidney-shaped or resembles a grape seed with 

 the two flagella arising in the groove, usually nearer the more pointed 

 end than the base. One flagellum, of the tinsel type, is directed forward 

 while the whiplash type flagellum is directed posteriorly. This zoospore 



