SEXUAL REPRODUCTION . 9S 



In Achlya, Protoachlya, Sommerstorffia, Aphanomyces, and 

 Plectospira the first swimming stage is almost completely sup- 

 pressed. The spores, on discharge, come to rest at the mouth of 

 the sporangium and encyst, and the reniform or "secondary" 

 zoospores escape from the cysts. 



In Dictyuchus encystment of the zoospores occurs within the 

 sporangium, and each reniform zoospore escapes individually 

 through its own pore in the sporangial wall. 



In Thraustotheca, Brevilegnia, and Geolegnia the spores, hav- 

 ing encysted within the sporangium, are gradually liberated by 

 a dissolution of the sporangial wall. In some species of Brevileg- 

 nia and in the genus Geolegnia zoospores are not formed as such. 

 The spores are non-motile and may even be multinucleate. This 

 phenomenon has been interpreted [Hohnk (1935)] as an adapta- 

 tion for life in the soil. The occurrence of two distinct types 

 of zoospores in the Saprolegniales is a feature for which no satis- 

 factory explanation has yet been offered. 



Sexual reproduction. Sexual reproduction, involving the for- 

 mation of antheridia and oogonia, generally occurs in water cul- 

 tures upon the exhaustion of the nutrient supply. Except in 

 occasional instances the sex organs are not readily produced on 

 solid media. When a single hypha gives rise to both oogonia 

 and antheridia, the species is said to be androgynous (homo- 

 thallic). To other species, in which antheridia and oogonia may 

 be formed on hyphae at some distance from each other, the term 

 diclinous has been applied. In Dictyuchus monosporiis [Couch 

 (1926)], Achlya biseximlis [Raper (1936)], and A. mnbisexiialis 

 [Raper (1940)] true heterothallism has been demonstrated. 

 Working with A. aiJibisexualis, Raper has clearly shown the 

 existence of relative sexuality; that is, a strain which produces 

 antheridia in one cross may produce oogonia instead when 

 crossed with a different isolate. 



The young oogonium, which generally appears as a lateral 

 branch of a main hypha, is multinucleate. After nuclear division 

 there occurs a degeneration of many nuclei and a reorganization 

 of the oogonial contents into a number of uninucleate eggs 

 (oospheres). In a number of genera, including Leptolegnia, 

 Aphanomyces, Dicrv^uchus, Brevilegnia, and Geolegnia, only a 

 single Qgg is formed \\ithin the oogonium. In most of the re- 

 maining genera more than one egg is produced, but the number 



