404 AQUATIC PHYCOMYCETES 



at least the tips are endobiotic, and a reproductive rudiment which 

 may be converted into a sporangium, prosporangium, gametangium, 

 or resting spore; sporangium inoperculate; zoospores posteriorly 

 uniflagellate (except where aplanospores are formed), often with a 

 single globule; resting spore asexually formed or sexually by fusion of 

 iso- or anisogamous apian ogametes which are never liberated into the 

 outside medium, upon germination functioning as a sporangium or a 

 prosporangium. 



The family is characterized by the formation of an extensive, richly 

 branched often polyphagous rhizoidal system and a reproductive 

 rudiment, which ordinarily is not intimately connected with the sub- 

 stratum. It includes many of the most interesting and bizarre parasites 

 of fresh-water algae, the curious group of species inhabiting the exuviae 

 of aquatic insects, and many ubiquitous soil fungi. In some forms the 

 zoospore bears a brightly colored globule. Where sexuality occurs, it 

 takes place by the fusion of aplanogametes. The sexual process in 

 Polyphagus (see under "Sexual Reproduction," p. 75) has been studied 

 both morphologically and cytologically and is perhaps the best known 

 among the chytrids. A type involving anastomosis of the rhizoids is 

 found in Rhizidium and Siphonaria. 



In Whiffen's (1944) classification all monocentric chytrids, whether 

 operculate or inoperculate, are included in this family. 



KEY TO THE SUBFAMILIES AND GENERA OF THE RHIZIDIACEAE 



Body of encysted zoospore or aplanospore forming the rudiment of 

 the sporangium; sexuality, where known, by conjugation of thalli 

 by means of rhizoidal anastomosis . . . Subfamily Rhizidioideae, p. 405 

 Vegetative system consisting of an unbranched double-contoured 



tube; aplanospores formed Sporophlyctidium, p. 406 



Vegetative system of richly branched rhizoids; zoospores formed 

 Rhizoidal system arising predominantly from a single axis on the 

 sporangium 

 Sporangium wall smooth, persisting after spore discharge 

 Main rhizoidal axis predominantly prolonged; resting spores 

 asexually formed or sexually by rhizoidal anastomosis 



Rhizidium, p. 407 



