[024 AQUATIC PH YCOM YCETES 



PYTHIACEAE 



Microscopic aquatic, amphibious, or terrestrial saprophytic or par- 

 asitic fungi; thallus a richly branched hyphal complex, cross walls in 

 vigorously growing parts normally formed only to delimit reproductive 

 organs; thick-walled chlamydospores and gemmae sometimes formed; 

 zoosporangia either undifferentiated portions of the mycelium, or an 

 irregularly expanded complex of lobulate elements and an evacuation 

 tube, or an ovoid, spherical, or bursiform structure with or without a 

 more or less prolonged evacuation tube, formed singly, or in catenulate 

 series, or in whorls, sometimes internally proliferous; zoospores of 

 the reniform laterally bifiagellate type, either formed outside the spo- 

 rangium in a vesicle, or free in the water, or within the sporangium, 

 capable of repeated emergence ; oogonia terminal or intercalary, spher- 

 ical or cylindrical, smooth or spiny-walled, usually containing a single 

 egg which is often differentiated into ooplasm and periplasm ; antheridia 

 terminal or intercalary, rarely lacking, monoclinous or diclinous, pa- 

 ragynous, hypogynous or amphigynous, each forming a well-defined ferti- 

 lization tube; oospore lying loosely in the oogonium or completely 

 filling it, smooth or rough-walled, upon germination producing a 

 germ tube or zoosporangium. 



Parasitic in algae, flowering plants, and microscopic animals ; sapro- 

 phytic on both plant and animal substrata, in water and soil. 



KEY TO THE AQUATIC GENERA OF THE PYTHIACEAE 



Zoosporangium an undifferentiated portion of the mycelium 



Hyphae bearing short lateral branches adapted to the capturing of 



rotifers Zoophagus, p. 1025 



Hyphae not bearing such branches Pythium \ p. 1031 



Zoosporangium differentiated from the mycelium, consisting either 

 of a complex of lobulate elements and an evacuation tube or of a 



1 FulMescriptionsand figures of species of Pythium may be found in the mono- 

 graphs by Middleton (1943) and Matthews (1931). A comprehensive key (based on 

 Middleton) to the species occurring in soil and water is furnished (p. 1033). Only the 

 ones restricted to higher plants are omitted. Even though the latter doubtless exist 

 in soil, they have not as yet been proven to be "free-living." 



