408 AQUATIC PHYCOMYCETES 



As understood here, Rhizidium includes all monocentric chytrids 

 that develop free in the medium, form their sporangium from the 

 enlarged body of the encysted zoospore, and have a definite taproot-like 

 main rhizoidal axis of variable length from which arise most of the 

 rhizo'ds. No stress is laid upon the question of how many cells compose 

 the whole plant. 



The only authentic account of sexuality in Rhizidium comes from 

 Canter (1950c). In R. windermerense, according to her, resting spores 

 are formed after conjugation by means of rhizoidal anastomosis. As 

 Canter describes the process, two spherical bodies (resembling encysted 

 zoospores and regarded as sexual thalli) give rise to fine threads (con- 

 jugation tubes) that meet at their tips. Contents of the bodies pass from 

 the sexual thalli along the tubes and a swelling is formed at their 

 juncture. This is the incipient resting spore or zygote and the contained 

 protoplasm is homogeneous and encompassed by a thin, colorless wall. 

 No constant difference in size between the fusing sexual thalli is 

 evidenced; when empty, they vary from 2.6 to 5.9 jx in diameter. At an 

 early stage, sometimes even before the thalli lose their contents, a few 

 fine branches are formed on the conjugation tubes which may extend 

 to a host cell or into the surrounding mucilage. After protoplasmic 

 fusion, a large rhizoidal system, similar in nature and almost in extent 

 to that of the sporangium, is formed on the rudiment of the resting 

 spore. The mature zygote is more or less spherical (14-17 [i) and has a 

 thick, smooth wall which may become yellowish; the contents consist 

 of numerous small globules. 



The sexual process in Rhizidium windermerense resembles that in Po- 

 lyphagus, especially P.euglenae Nowiik. sensu recent. Bartsch, in which 

 the resting spore develops subterminally (see Bartsch, 1945, figs. 9, 

 12). In Polyphagus, however, the conjugating thalli have already grown 

 to considerable size before they fuse. 



Karling (1944c), who observed resting-spore formation in several 

 species of Rhizidium, makes no mention of their being sexually formed. 



KEY TO THE SPECIES OF RHIZIDIUM 



Sporang'um predominantly with a single rhizoidal axis 



Sporangium appearing as a lateral outgrowth of the apex of the rhi- 

 zoidal axis 



