82 AQUATIC PHYCOMYCETES 



when grown alone gave rise only to zoosporangia. When certain strains 

 were paired, however, either with or without actual contact of the hyphae 

 of their respective hosts, abundant resting spores were formed. The 

 type of sexuality involved has evidently not as yet been discovered. 

 With respect to the saprophytic R. rosea the following situation has 

 been observed. Plants of all but one collection were found incapable 

 of forming resting spores. This one collection, however, produced 

 them in abundance. Individual sporangia from it were isolated in sepa- 

 rate dishes of sterile water baited with appropriate substrata. Of the 

 twenty monosporangial cultures thus started four showed growth of 

 new plants. In none were resting spores ever formed. When all possible 

 crosses had been made between the four available strains it was found 

 that whenever a certain strain was paired with the others resting spores 

 were produced. It was concluded from this that three of the four 

 strains were of the same "sex," and that the fourth was of the opposite 

 "sex." As in Pringsheimiella, the type of sexuality was not determined. 

 In his summary on sexuality in the chytrids, Emerson (1950) has 

 this to say concerning sex determination: 



Suggestions regarding possible instances of genotypic sex determination 

 have been made by various investigators and schemes, such as that recently 

 proposed by Karling [1945d] for Siphonaria have been drawn up, but, to 

 the writer's knowledge, there is not yet a single well-established example 

 in the chytrids of a differentiation of sexes clearly attributable to segregation 

 of genetic factors at the time of meiosis in the life cycle. In view of the wide- 

 spread and very general occurrence of hermaphroditism and phenotypic sex 

 determination in the higher aquatic fungi, and the very few instances in 

 which a genetic difference between the sexes is indicated, perhaps we should 

 not expect to find more than scattered examples of genotypic sex determina- 

 tion in the lower forms. 



The Resting Spore 



The resting spore or "resting sporangium" may be produced either 

 sexually, from the zygote, as previously indicated, or, more often, 

 asexually, from a modified reproductive rudiment or prosporangium 

 or from an apophysis. With the exception of Chytridium and such 

 genera as Blyttiomyces, the asexually developed resting spore is formed 

 either outside or inside the substratum, according to the position of 

 the sporangium. Outstanding features are its thickened wall and its 



