812 AQUATIC PH YCOM YCETES 



tube, after about five minutes suddenly coming to rest and encysting, 

 the cysts 2-2.5 u, in diameter; other stages unknown. 



Parasitic in Chlorodendron subsahum, Dangeard (he. cit.); J. and G. 

 Feldmann (he. cit.), France. 



Since no zoospores were seen by Dangeard, whether or not his fungus 

 and that of the Feldmanns were identical cannot be determined. 



PYTHIELLA Couch 



Mycologia, 27: 160. 1935 

 (Fig. 63 A-B, p. 824) 



Thallus endobiotic, holocarpic, without specialized vegetative system, 

 spherical, subspherical, or (in Pythiella besseyi) ellipsoidal and irregu- 

 larly tubular, causing hypertrophy of the host ; sporangium with one or 

 more tubes; zoospores formed after a sequence of protoplasmic changes 

 similar to that found in mycelial members of the order, primary zoo- 

 spores upon emergence encysting at the orifice of the discharge tube, 

 each cyst giving rise after a period of quiescence to a laterally biflagellate 

 secondary zoospore; oogonium a spherical cell bearing a single egg; 

 antheridial cell single, adnate to the oogonium, forming a well-defined 

 fertilization tube; oospore endobiotic, thick-walled, sexually formed, 

 with distinct periplasm, lying loose within the oogonium; germination 

 not observed. 



Parasites of aquatic Phycomycetes. 



The type species (Pythiella vernalis) is an endobiotic holocarpic par- 

 asite in the hyphae of algae-inhabiting species of Pythium. 



Pythiella is of great interest because its members resemble in habitat 

 and appearance those of Olpidiopsis, a genus composed primarily of 

 parasites of other Phycomycetes. In Olpicliopsis the zoospores exhibit 

 no well-defined diplanetism and the sex organs are less highly special- 

 ized, that is, the protoplasm of the contributing thallus is conveyed into 

 the receptive structure through a pore. The resting-spore wall consists, 

 in part at least, of the wall of the receptive thallus and there is no dif- 

 ferentiation at maturity (except in O. oedogoniarum) between the spore 

 and its container. In Pythiella, on the contrary, there is well-defined 

 diplanetism (Fig. 63A), a fertilization tube is formed by the contributing 



