40 AQUATIC PHYCOMYCETES 



tures; zoospore usually bearing a single conspicuous oil glob- 

 ule, germination monopolar Chytridiales, p. 42 



Thallus nearly always differentiated into a well-developed veg- 

 etative system, often hypha-like, on which are borne num- 

 erous reproductive organs; zoospore without a conspicuous 

 globule, germination bipolar 

 Thallus usually having a well-defined basal cell anchored in the 

 substratum by a system of tapering rhizoids; resting 

 structure an asexually formed thick-walled often punc- 

 tate resting spore; sexual reproduction by means of isog- 

 amous or anisogamous planogametes; alternation of 

 generations present in some species 



Blastocladiales, p. 604 

 Thallus without a well-defined basal cell ; composed of delicate 

 much-branched hyphae; resting structure an oospore; 

 sexual reproduction oogamous, the male gamete always 

 free-swimming, the female devoid of a flagellum 



MONOBLEPHARIDALES, p. 696 



Zoospores anteriorly uniflagellate, usually formed outside or 



inside the sporangium Hyphochytriales, p. 743 



Zoospores (at least the secondary ones) with two oppositely di- 

 rected flagella, formed either inside or outside the sporangium 

 Zoospores with a very short anterior flagellum and a long poste- 

 rior one; thallus wholly endobiotic, holocarpic, forming a 

 zoosporangium or a sorus of zoosporangia or resting spores 



Plasmodiophorales, p. 768 

 Zoospores with two flagella of approximately equal length, 

 which are oppositely directed in the secondary zoospores; 

 thallus endobiotic and holocarpic or eucarpic and both 

 intra- and extramatrical ; sexual reproduction oogamous 

 Zoospores always cleaved out within the sporangium, dipla- 

 netic, the primary zoospore motile or encysting immedi- 

 ately after discharge, or completely suppressed (i.e. only 

 secondary zoospores formed) 

 Thallus holocarpic or eucarpic; the hyphae in eucarpic 

 forms without constrictions or cellulin plugs, not aris- 

 ing from a well-defined basal cell; oogonium with one 



or more eggs Saprolegniales, p. 792 



Thallus eucarpic; the hyphae bearing constrictions accom- 

 panied by cellulin plugs, usually arising from a more or 

 less well defined basal cell anchored in the substra- 

 tum; oogonium (with the exception of Apodachlyelld) 

 with a single egg Leptomitales, p. 852 



