2 AQUATIC PHYCOMYCETES 



body (thallus) may be unspecialized and entirely converted into a re- 

 productive organ, or it may bear tapering rhizoids, or be mycelial and 

 very extensive. In any event, the outstanding characteristic of the thallus 

 is a tendency to be nonseptate and, in most groups, multinucleate, 

 cross walls being laid down in vigorously growing material only to 

 delimit the reproductive organs. The unit of nonsexual reproduction, 

 the spore, is borne in a sporangium, and, in the aquatic and semi- 

 aquatic orders, is provided with a single posterior flagellum, a single 

 anterior flagellum, or two laterally or terminally attached ones. Sexual 

 reproduction is accomplished in one great group, the "Zygomycetes," 

 by conjugation of the tips of two mycelial branches, which results in 

 the formation of a thick-walled zygospore; only nonmotile spores are 

 formed. In the other group, generally spoken of collectively in the older 

 literature as the "Oomycetes" (to which many students assign the forms 

 treated in the present work), there is great diversity in the method of 

 sexual reproduction and, as in the green algae, all gradations from 

 isogamous planogametic to oogamous aplanogametic types occur. 

 Moreover, the character of the sexual reproduction is not necessarily 

 linked with the degree of thallus development. Thus planogametes 

 (isogamous) are found in the endobiotic holocarpic genus Olpidium, a 

 form with a simple thallus, and planogametes (anisogamous) in Allo- 

 myces, a eucarpic form with a well-developed mycelium. On the other 

 hand, in Sapromyces, a member of the Leptomitales, although the 

 thallus resembles that of Allomyces, sexual reproduction is by oogamy 

 of a high type and, in general, involves, as in the better-known genera 

 Saprolegnia and Achlya, the formation of an oosphere contained in an 

 oogonium and of an antheridium. Fertilization in Sapromyces is ac- 

 complished by the transference of antheridial material into the oogo- 

 nium through a specialized tube produced by the male gametangium, 

 and a single thick-walled oospore is formed from the fertilized egg. 

 This type of sexual reproduction is characteristic of the higher aquatic 

 and semiaquatic orders. Other variations occur, but these are sufficient 

 to illustrate the diversity found in the Oomycetes. 



