632 AQUATIC PHYCOMYCETES 



the laboratory; and the chromosome numbers of natural isolates range from 

 13 to 21, just as those of the F 2 and F 3 from artificial crosses vary, though 

 over a somewhat higher and wider range. 



The genetical data given above are of special interest, as Emerson 

 has pointed out, in connection with the location of the reduction di- 

 vision in the life cycle of the Euallomyces group. Kniep (1930) had 

 approached the question by way of volumetric studies of the nuclei of 

 the sexual and the asexual plants. He found that in Allomyces javanicus 

 a well-marked ratio of 1 :2.12 exists between the average volumes of 

 the nuclei of the sexual and the asexual phases and from this concluded 

 that meiosis occurs in the resting spore. 1 His observations were abun- 

 dantly confirmed by Sorgel (1936) for A. arbuscula {"A. kniepii"). 

 Indeed, even in plants of this species which Sorgel considered to be 

 heteroploid, he obtained the same constancy in ratio of nuclear volume 

 between the two phases. Sorgel mentions that there are six chromosomes 

 in the gametophyte and twelve in the sporophyte. The only cytological 

 study dealing with the reduction division in Allomyces prior to the 

 reports of Emerson and Wilson (1949, 1954) and Wilson (1952), was one 

 by Hatch (1938). From his investigations on nuclear behavior in the 

 zygote, Hatch came to the conclusion that meiosis takes place at the 

 first division of the fusion nucleus, at which time the chromosomes 

 are reduced from twelve to six. 



Wilson's (1952) and Emerson and Wilson's (1954) studies, utilizing 

 the aceto-orcein smear technique, have yielded definitive information 

 on meiosis, as well as on chromosome numbers, not only in members 

 of Euallomyces but of Cystogenes. They have also afforded light on the 

 nuclear situation in Brachyallomyces. The haploid chromosome count 

 for the Euallomyces group shows that hypogyny (Allomyces arbuscula) 

 is associated with a basic chromosome number of 8 or of simple multi- 

 ples of 8, that is, 16, 24, and 32. Since in most natural isolates of A. 

 arbuscula the gametophyte has 16 chromosomes and the sporophyte 

 32, they are considered tetraploid. Among the epigynous forms the one 

 later termed A. macrogynus (A. javanicus var. macrogynus= A. javanicus 

 var. japonensis Indoh) seems to possess the basic number of chromo- 



1 See, also, Sost's (1955) work on the induction of polyploidy, in which he uti- 

 lized nuclear volume as a criterion. 



