112 AQUATIC PHY COM YCETES 



ance of the spores always corresponds with a period of intense parasitic 

 activity and their number builds up to a maximum near the close of 

 an epidemic. Since Asterionella, whenever collected, is rarely free of 

 the sporangial or zoospore-cyst stage, the authors conjecture that it 

 is the germination of the resting spores in abundance which initiates 

 the full-scale epidemic. Actually, however, no one is sure just what 

 factor or group of factors is primarily responsible. Temperature, light, 

 lake level, dissolved substances, antibiotics, abundance of host cells, 

 predators, and the possibility that zoospores from germinated sexually 

 formed resting spores possess unusual vigor and infective capacity 

 all have been considered. Evidence in regard to any one of them is 

 inconclusive. 



Although Canter and Lund pose many unsolved problems, they do 

 give us the first clear-cut picture and analysis of a chytrid epidemic, 

 open a new approach to the study of Phycomycetes, and demonstrate 

 beyond question that these organisms are significant factors in the c>cle 

 of disintegration in an aquatic environment. 



Saprophytism and Facultative Parasitism 



A great number of chytrids apparently lead a purely saprophytic 

 existence. These forms occur wherever appropriate substrata are avail- 

 able in the natural habitat. Moreover, they readily appear when gross 

 laboratory cultures containing vegetable trash from an aquatic site 

 are baited with a variety of organic materials. During the temper- 

 ate seasons such fungi are probably constantly at work in the sub- 

 merged decaying parts of plants and animals. 



There have also been reported a relatively large number of species 

 which seem capable of living either as saprophytes or parasites (see 

 especially Serbinow, 1907). Unfortunately, it is far easier to demon- 

 strate that a chytrid may live as a saprophyte than it is to prove un- 

 questionably that it can attack a living healthy organism. Sapro- 

 phytism may be shown, for example, by the simple expedient of boiling 

 the prospective substratum until it is killed and then placing it in a 

 culture dish in which the chytrid is growing. If the fungus attacks this 

 material there is no question of its ability to utilize a dead substratum 

 as a source of food. On the other hand, living microscopic organisms 



