SAPROLEGNIALES 833 



SAPROLEGNIACEAE 



Thallus eucarpic, mycelial, without constrictions, of unlimited growth, 

 bearing numerous reproductive organs, homo- or heterothallic; zoo- 

 sporangia, varied in character, usually terminal ; zoospores mono- or 

 diplanetic (mono- or dimorphic), the secondary zoospores sometimes 

 capable of repeated emergence; antheridia from one to many, hypogy- 

 nous, androgynous, or diclinous, sometimes nonfunctional or entirely 

 lacking; oogonium with smooth or pitted wall, eggs from one to many, 

 formed from the entire contents of the oogonium ; oospores sexually 

 or apogamously formed, thick-walled, partly or nearly completely filling 

 the oogonium, upon germination forming hyphae or a hyphal stalk 

 bearing a zoosporangium. 



Primarily saprophytic on plant and animal debris in fresh water or 

 soil; some species parasitic on microscopic animals or on fish, amphibia 

 and their eggs, and on the roots of higher plants. 



The following key to the genera is based mainly on Coker and Mat- 

 thews (1937). 



KEY TO THE GENERA OF THE SAPROLEGNIACEAE 

 Zoospores of the primary type only, monoplanetic (monomorphic); 



sporangia spherical to somewhat elongate Pythiopsis, p. 835 



Zoospores exhibiting varying degrees of diplanetism (dimorphism) 

 Zoospores normally emerging from the sporangium without previous 

 encystment 

 Primary zoospores bearing flagella at discharge and swimming 

 away 

 Zoospores in more than one row in the sporangium 

 Sporangia renewed primarily by internal proliferation 



Saprolegnia, p. 835 

 Sporangia renewed by cymose branching as well as by internal 



proliferation Isoachlya, p. 838 



Zoospores in one row in the sporangium 



Sporangia for the most part unbranched, eggs borne singly in 



a distinct oogonium Leptolegnia, p. 839 



Sporangia usually branched; resting structures asexually 

 formed in undifferentiated portions of the hyphae, 



numerous Leptolegniella, p. 840 



Primary swimming stage partly or wholly suppressed, the primary 

 zoospores after emergence mostly at once encysting 



