CHYTRIDIALES 399 



vegetative system, this fungus was a chytrid allied to Polyphagus and 

 Saccomyces. In all three genera the sporangium is an outgrowth of a 

 prosporangium, but Rhizosiphon crassum differs from the other two in 

 having a discharge tube and an endobiotic, apparently asexually formed, 

 resting spore. Its method of development is unlike that in Polyphagus. 



Canter (1951) gives a clear account of the development of Rhizosiphon 

 crassum on Anahaena. According to her, the zoospore penetrates the 

 mucilage sheath of the host, encysts, and produces a fine thread, the 

 tip of which enters the host cell. The contents of the encysted spore 

 flow through the tube and form a walled globose structure. The latter 

 enlarges and from opposite sides two broad undulate blunt extensions 

 protrude which pass through several host cells. The central spherical 

 part of the thallus is the rudiment of the prosporangium. At maturity 

 of the thallus, a small bud from the prosporangium emerges from the 

 host cell just beside the point of penetration of the germ tube. This bud, 

 the incipient sporangium, enlarges as the contents of the prosporangium 

 and its outgrowths pass into it. The sporangium, which is external, as 

 it matures becomes somewhat flask-shaped due to the development of 

 a broad apical discharge tube. Its contents, at first granular, at maturity 

 bear many irregularly placed, minute, refractive globules. The apex of 

 the discharge tube becomes filled with a clear gelatinous material and, 

 upon maturity, bulges out, expands, and deliquesces. The zoospores 

 then emerge rapidly and individually. 



How the resting spores develop is not yet clear. Not until a late stage 

 are they distinguishable from the prosporangia. Pairs of encysted 

 zoospores that are attached, in some instances, by long slender pene- 

 tration tubes to the resting spore strongly suggest some sort of sexual 

 process, but there is no definite evidence for it in Rhizosiphon crassum. 

 The mature resting spore has a smooth, thickened wall and many 

 globules. According to Scherffel (1926a) at germination it functions as 

 a prosporangium. 



Canter regards Whiffen's (1944) assignment of Rhizosiphon to the 

 Polyphagoideae as hardly tenable, because the thallus arises at the tip 

 of a germ tube. Rather, she says, in Whiffen's scheme it clearly belongs 

 in the Diplophlyctoideae. 



