68 AQUATIC PHYCOMYCETES 



known chytrids with pigmented globules in their zoospores in which 

 no phototactic response has ever been noted (Cladochytrium replicatum, 

 Rhizoclosmatium aurantiacum, Siphonaria variabilis, and so on). 



Muller (1911) has observed that the zoospores of Rhizophydium 

 pollinis-pini and of R. sphaerotheea are positively chemotactic to gen- 

 uine protein. The latter fungus also shows a positive response to the 

 products of regressive protein metamorphosis and allied N compounds. 

 The reactions of the swarmers were always greatly inhibited by poison- 

 ous materials in the atmosphere and by the lack of oxygen. H and OH 

 ions exerted a negative effect. Similarly, Kusano (1932) has found the 

 swarmers of Olpidium trifolii and O. viciae to be positively chemotactic 

 to juices of their respective phanerogamic hosts. The active substances 

 here were potassium compounds. 



It seems probable, then, even from the limited information now 

 available, that the zoospore is greatly aided in seeking out available 

 sources of food in nature by its tactic responses to external stimuli. 



Germination of the zoospore. — After the period of swarming the zoo- 

 spore comes to rest and encysts. During this process the flagellum either 

 contracts and is absorbed into the body or, sometimes, apparently 

 drops off; its precise fate is difficult to follow. At the disappearance 

 of the flagellum the spore body is surrounded by a delicate but rigid 

 wall. Further development of the encysted zoospore is dependent upon 

 the proximity of the substratum. If the source of food cannot be reached 

 the spore may degenerate at once or after the formation of a rudimen- 

 tary rhizoidal system. A curious fusion of zoospores of Chytridium iagen- 

 aria in drops of water lacking food has been noted (Sparrow, 1936a). 

 Some of the motile spores settled down in contact with quiescent or 

 even with germinated spores. The contents of the contacting and, by 

 now, encysted spore then flowed into the other one. The receptive 

 body subsequently enlarged, and there ensued during the next five 

 days a remarkable development of the rhizoidal system. This was of 

 far greater extent and complexity than that formed by single germinated 

 spores in the same medium. Such unconjugated spores gave rise to only 

 a feebly developed rhizoidal system and disintegrated within twelve 

 hours. It seems probable that this fusion was an attempt by the chytrid 

 to rejuvenate and prolong its vegetative activity under poor nutritional 



