SAPROLEGNIALES 797 



its tip. The plant body presumably is always haploid, the only diploid 

 structure being the oospore. 



Couch (1926b) established the existence of heterothallism or, better, 

 dioecism in the ubiquitous water mold Dictyuchus monosporus} When 

 the oogonial and the antheridial plants bearing sexual organs were 

 isolated and grown separately oospore formation did not occur. When, 

 however, the two strains were grown together sex organs were formed 

 and mature oospores were produced wherever the hyphae of the two 

 intermingled. Contact of the two strains was necessary and seemed to 

 be the only type of stimulation concerned in the process. Environmental 

 conditions had only a secondary and variable effect. Intercrosses be- 

 tween the four species of the genus then recognized were successful, and 

 the resulting variations in morphological characters, hitherto supposed 

 to be of specific significance, were so marked as to invalidate all but the 

 type species, D. monosporus. Male, female, and neutral strains were 

 found to occur in nature, as well as one which formed its oospores 

 parthenogenetically. Oospores of this last strain were germinated and 

 the mycelium thus produced was crossed (1) with a male strain, which 

 was, as a consequence, stimulated to the formation of (functional?) 

 antheridia that applied themselves to the oogonia produced by the 

 parthenogenetic strain; (2) with an oogonial strain, in which experiment 

 oogonia were formed on both strains and, in addition, functional anther- 

 idia were also produced on the parthenogenetic strain. Thus the latter 

 strain was shown to be inherently monoecious. Germination of the 

 oospores formed by dioecious strains disclosed an interesting fact, 

 namely, that parts of the mycelium arising from the germ hypha were 

 male, parts female, and parts mixed. This suggests that sexual segrega- 

 tion took place early in the process of germination. If the germ hypha 

 gave rise to a sporangium, the segregation probably occurred in the 

 cleaving-out of the zoospores, since some of the latter produced male, 

 some female, and some mixed mycelia. 



Homothallism or monoecism has been definitely proved in Lepto- 

 legnia and in certain species of Saprolegnia, Achlya, and Protoachlya 

 (Schlosser, 1 929). Schlosser also gave some interesting details of regenera- 

 tion and "reversal" in the isolated diclinous sex organs of certain homo- 



1 Also termed "gynandromixis" (Raper, 1940b). 



