BLASTOCLADIALES 627 



vation, failed to produce sexual plants. Recalling the situation found 

 in Blastocladia pringsheimii by Blackwell (1940), in which precisely the 

 same life cycle was observed, Emerson suggested that there are "brachy-" 

 or "short-cycled" forms in nature in which the asexual thallus alone 

 is produced. 



Wilson (1952), who examined the situation in Allomyces anomalus 

 cytologically, determined that, although there is an increase in nuclear 

 size during maturation of the resting spores (thus resembling prophase 

 stages of other species before meiosis), the chromosomes either com- 

 pletely or partly fail to pair. Ensuing divisions in the resting spores are 

 consequently mitotic and not meiotic and the zoospores eventually 

 formed are diploid. They only give rise, of course, to sporophyte 

 plants; hence, a gametophyte phase is lacking. Wilson concludes from 

 this that an inhibiting factor is operative which may, by some as yet 

 unknown mechanism, suppress meiosis in long-cycled forms over long 

 periods. The potentiality for production of a haploid generation con- 

 tinues to exist, however, and is expressed whenever such "inhibition" 

 is released. 



Besides the three types of life cycles that are a regular and character- 

 istic feature of certain isolates, variations or "departures" from the 

 normal have been described in some isolates of species belonging to 

 the Euallomyces type. One such departure is the capacity of the female 

 gamete to develop parthenogenetically. Kniep (1929, 1930) had ob- 

 served that the female gamete in Allomyces javanicus could, on occasion, 

 come to rest, germinate, and form a new sexual thallus, thus simulating 

 a zoospore in function. Sorgel (1937b) and Emerson (1941) have con- 

 firmed this both for Kniep's species and for A. arbuscula. Sorgel found 

 that in some instances the female gamete might also give rise directly 

 to asexual plants, a fact verified by Emerson (1941) and by Emerson 

 and Wilson (1954). No instances of germination of the male gamete, 

 however, have ever been observed nor have such gametes ever been 

 seen to fuse with each other. 



A second departure noted by Sorgel and Emerson in both the long- 

 cycled species mentioned is the formation of asexual plants by planonts 

 from the resting spores. Emerson found that certain of his strains did 

 not regularly produce sexual plants under aquatic conditions and that 



