74 PHYCOMYCETEAE 



branching and tapering to fine points. Numerous fusiform 

 swellings which may become large or small zoosporangia or 

 resting spores, the latter acting as prosporangia when they 

 germinate. Zoosporangia not proliferating. 



Septochytrium 

 Extra- and intramatrical, rhizomycelium rather thick and undulate, 

 branched, not tapering into fine points. Zoosporangia ter- 

 minal and intercalary, rarely proliferating. Resting spores 

 intercalary, on germination acting as prosporangia. 



Megachytrium 

 Exit tubes of zoosporangia operculate near base (endo-operculate). Rhizo- 

 mycelium moderately stout. Catenomyces 

 Parasitic epibiotically and endobiotically on higher plants, the epibiotic stage 

 monocentric, consisting of a proliferating, slipper-like 

 zoosporangium with a tuft of rhizoids in the epidermal cell, 

 the endobiotic stage being a slender branching rhizomy- 

 celium with numerous centers which give rise to further 

 rhizomycelium and to thick-walled resting sporangia, mostly 

 flattened on one side. Family Physodermataceae 

 Endobiotic stage not causing warts or galls on the host. 



Physoderma 

 Endobiotic stage causing warts or galls. Urophlydis 



Appendix to the Operculate Cladochytriaceae, but with relationship doubtful. 

 Stalk forked and bearing terminally the operculate zoosporangia. Sexual re- 

 production by formation of thick-walled zygospore by a 

 mucoroid process of conjugation. Zygochytrium 



Stalk branched and bearing at its tips operculate gametangia producing 4 

 gametes in each. No thick-walled resting spores known. 



Tetrachytrium 



Key to the Families and Genera of Order Hyphochjrtriales 



{Based upon Karling, 1943) 



Holocarpic, monocentric, endobiotic. Family Anisolpidiaceae 



Vegetative stage provided early with a cell wall and becoming a round or 

 ellipsoidal zoosporangium with 1-3 exit tubes. Sexual reproduction unknown. 

 Parasitic in Phaeophyceae. Anisolpidiuni 



Vegetative stage naked and amoeboid almost until maturity of the rounded 

 zoosporangium. Exit tube single. Sexual reproduction by the union of uni- 

 flagellate isogametes. Parasitic in Lemnaceae. Reesia 

 Vegetative structure becoming elongated, sometimes septate, to form one 

 elongated or several rounded zoosporangia with thick walls. Zoospores 

 escaping through a median aperture. Sexual reproduction unknown. Para- 

 sitic in roots. Cystochytrium 

 Eucarpic, monocentric, epibiotic. Family Rhizidiomycetaceae 

 Zoosporangia with or without apophysis, zoospores undergoing cleavage within 

 a vesicle at the apex of the exit tube. Parasites or saprophytes. 



Rhizidiomyces 

 Zoosporangia without apophysis, zoospores fully developed within the 

 zoosporangium. Parasites in oospores of Vaucheria. Latrostium 



Eucarpic, polycentric, hypha-likc, without rhizoids, with terminal and inter- 

 calary zoosporangia. Family Hyphochytriaceae 



