120 AQUATIC PHYCOMYCETES 



animals. A few are found in marine waters on marine algae. Some 

 species of otherwise primarily aquatic genera, as well as species of 

 Synchytrium and Physoderma (including Urophlyctis), occur as obli- 

 gate parasites of terrestrial and aquatic vascular plants. 



The order as defined here is restricted to "chytrids" with posteriorly 

 uniflagellate zoospores formed in the sporangium. There are recog- 

 nizable within the group two parallel series in which specialization of 

 the thallus has achieved equal complexity; in one the sporangium 

 opens by the deliquescence or rupturing of the tip of the discharge 

 tube (Inoperculatae), in the other, by the dehiscence of a very definite 

 and discrete operculum (Operculatae). 



KEY TO THE FAMILIES OF THE CHYTRIDIALES 



Sporangium opening by the deliquescence or rupturing of one or 



more papillae Series Inoperculatae, 1 p. 121 



Thallus holocarpic, without specialized vegetative struciures, 

 endobiotic 



Thallus forming a single sporangium Olpidiaceae, p. 121 



Thallus forming more than one sporangium 



Thallus converted into a linear series of sporangia 



Achlyogetonaceae, p. 182 

 Thallus converted into a prosorus or sorus surrounded by 



a common soral membrane Synchytriaceae, p. 190 



Thallus eucarpic, i.e. differentiated into a vegetative system and 

 leproductive organs, monocentric or polycentric, epi- and 

 endobiotic, endobiotic, interbiotic, or extramatrical 

 Thallus always monocentric 



Thallus epi- and endobiotic, or entirely endobiotic, repro- 

 ductive organ epi- or endobiotic . . Phlyctidiaceae, p. 207 

 Thallus interbiotic; sporangium, prosporangium, or resting 

 spore formed from the enlarged body of the encysted 

 zoospore Rhizidiaceae, p. 403 



1 Whiffen (1944) does not consider inoperculate and operculate sporangial dis- 

 charge to be of fundamental importance above generic level. Rather, she prefers 

 as indicated earlier (p. 50) to group the monocentric chytrids on the basis of whether 

 the sporangium develops from the encysted zoospore body or from its tube. The 

 writer does not accept this view, particularly since recent work has shown much 

 variation in types of development within one fungus, whereas not a single authentic 

 instance of a chytrid being both inoperculate and operculate has been recorded. 

 See Willoughby, Trans. Brit. Mycol. Soc, 41 : 309. 1958, for support of this view- 

 point. 



