CHAPTER VIII 

 OOMYCETES 



In the Oomycetes there appears, instead of the uninucleate and 

 centrally organized thallus of the Chytridiales, a well-developed poly- 

 energid mycelium with characteristic hyphae adapted to independent 

 existence. This independence in some families (e.g., the Saproleg- 

 niaceae) goes so far that hyphal fragments may grow to new individuals. 

 Furthermore, under unfavorable conditions, smaller portions of hyphae 

 can thicken their walls and become gemmae. 



In the lower aquatic forms, asexual reproduction occurs exclusively 

 by zoosporangia with zoospores which may be uni- or biflagellate. In 

 Ectrogella and Saprolegnia, the zoospores are diplanetic, and in some 

 other genera can shed their membranes. Diplanetism is the separation 

 of the swarm period into two morphologically different phases, always in 

 the same sequence, separated from each other by a resting period, while 

 at the shedding of the membrane both swarm phases appear morphologi- 

 cally similar. In diplanetism, the zoospores in the first period may be 

 terminally flagellate, in the second laterally. At the shedding of the 

 membrane, e.g., of a laterally flagellate zoospore, however, the naked 

 protoplasm assumes its previous form, moves out of the membrane and 

 begins a swarm stage (Fig. 39, 3 to 5). 



Even in these aquatic forms, at different parts of the system and 

 independent of each other, e.g., Ancylistes, Aplanes, Araiospora and 

 Pythium, there is a tendency to allow the zoosporangia to germinate 

 with coenocytic germ tubes intead of with zoospores. As tube ger- 

 mination, in contrast to zoospore germination, can occur in damp air, 

 some of these forms were able to migrate to dry land, where they 

 developed as parasites on phanerogams. Since the highest of these 

 forms have lost their ability to form zoospores, the coenocytic 

 zoosporangia develop directly to coenocytic mycelia. W* 



Along with the decline of differentiation of the sporangial content 

 and the discharge of the unaltered protoplasm into a germ tube, there 

 is a proportional increase in external differentiation of the sporangia 

 themselves. In the lower forms, they are always connected with_ the^ 

 sporiferous hyphae and morphologically almost indistinguishable frDm 

 them, while in the higher forms, they are separated from the hyphae 

 before spore formation, and either fall off and are independently dis- 

 seminated, or become spores and conidia. 



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