628 AQUATIC PHYCOMYCETES 



certain others were induced to do so only after very special treatment. 

 He points out that on this physiological basis we have a series in both 

 Allomyces arbuseula and A.javanicus grading from those which regularly 

 produce a sexual stage to those which produce one only after manipu- 

 lation. The reason for this variation from the long-cycled type is not 

 known but Wilson (1952) postulates an inhibiting factor (p. 627). Sorgel 

 and Emerson both speculate that it may be linked up with environmen- 

 tal conditions, particularly with the amount of nutriment immediately 

 available to the planont on its emergence from the resting spore. No 

 evidence for the fusion of these planonts with each other has ever been 

 observed. 



A third departure, but by no means uncommon, is the formation of 

 the dark, pitted resting spores on the sexual as well as on the asexual 

 thallus. Sorgel (1937b) attempted to prove by volumetric studies of the 

 nuclei of plants showing this peculiarity that there are mixtures of gameto- 

 phyte and sporophyte nuclei within the hyphae. Emerson believes, 

 however, that until there is definite cytological evidence from chromo- 

 some counts to confirm the existence of such "mixed hyphae" it is better 

 to assume that both sexual and asexual plants may form resting spores 

 rather than that such structures are restricted to the asexual generation 

 alone. Although Hatch (1935) stated that the planonts from these rest- 

 ing spores produce sexual thalli, Emerson feels that this is still a matter 

 of doubt. 



Beneke and Wilson (1950) found that in Allomyces macrogynus so- 

 dium nucleate treatment of germinating zygotes increased the frequency 

 with which resting spores from resultant sporophytes gave rise at 

 germination to planonts which developed into sporophytes as well as 

 gametophytes. Colchicine appeared to have a similar effect (see Sost, 

 1955). These authors suggested that such treatment of the zygotes 

 induced some nuclei to become polyploid and the resulting asexual 

 thalli mixoploid. Hence, at reduction division in the germinating resting 

 spores, some of the resulting planonts would be haploid and some 

 diploid or even polyploid. Only the first-named would give rise to sex- 

 ual thalli, the others would be asexual. 



Whiffen (1951) noted that when cycloheximide (Acti-dione), an anti- 

 biotic substance, was added to media on which Allomyces arbuseula 



