118 AQUATIC PH YCOM YCETES 



in other members of the Chlorophyceae and in Heterokontae, as well 

 as in the phycomycete Allomyces (Scherffel, 1925b; Dangeard, 1937; 

 Foust, 1937; Couch, 1938b; Sparrow, 1939a; Karling, 1948c). Certain 

 flowering plants react in a similar manner when attacked by chytrid 

 zoospores (see especially Kusano, 1936). The formation and function- 

 ing of these protective plugs or calluses have been described at some 

 length by Dangeard in vigorous individuals of Closterium ehrenbergii 

 attacked by Phlyctochytrium desmidiacearum. In this instance the zoo- 

 spore of the chytrid, after encysting on the surface of the host, produced 

 a delicate tube which penetrated the algal wall in its effort to make con- 

 tact with the living cytoplasmic membrane. In the short period during 

 which penetration was taking place the host had responded to the in- 

 cursion by the formation of a sheath of wall material which surrounded 

 the infection tube. As the latter elongated, the sheath too became ex- 

 tended, at all times insulating, as it were, the fungus from the living 

 cytoplasm of the alga. During this process of elongation the chytrid 

 germling was necessarily using up its own food supply in an effort to 

 reach the nutritive contents of the alga. The chytrid, with no nour- 

 ishment forthcoming and its own supply exhausted by the tube for- 

 mation, was literally "starved out." But Dangeard also noted that 

 when host cells were growing under unfavorable conditions or when 

 they had been weakened by the attacks of other parasites they lacked 

 this capacity for callus formation and succumbed in great numbers 

 to the chytrid. 



Hyperparasitism 



Instances of one chytrid parasitizing another have been reported 

 by various investigators. Serbinow (1907) recorded the invasion of the 

 prosporangia of Saccomyces by another chytrid, called by him "Phlyc- 

 tidium dangeardii." No endobiotic system was found and it is possible 

 that he was dealing with a species of Rozella parasitic in the sporangium 

 rather than the prosporangium of the host. Sporangia of the marine 

 species Rhizophydium discinctum and Chytridium polysiphoniae have 

 been found to be attacked by other chytrids, the first by Pleotrachelus 

 paradoxus (H. E. Petersen, 1905) and the second by Rozella marina 

 (Sparrow, 1936b). Similarly, sporangia of the fresh-water Rhizophydium 



