CHYTRIDIALES 105 



who, from studies of a number of different chytrid parasites of algae, 

 concluded that many of them were at most only facultative parasites. 

 Serbinow's views have received support from numerous subsequent 

 observations, and it is probable that they hold true for a large number 

 of the Phlyctidiaceae and Chytridiaceae. Certain members of these 

 families appear, however, to be truly parasitic and have never been 

 induced to live saprophytically. 



The Chytrid Epidemic 



One feature of the chytrids which has been noted by many students 

 of the group is the evanescent nature of their occurrence. Many in- 

 stances (Cohn, 1853; Zopf, 1888; Dangeard, 1889b; Wager, 1913; 

 Reynolds, 1940, and others) are on record of the sudden appearance 

 of these fungi in ponds, as well as in the restricted habitat of the lab- 

 oratory. Once established, they enjoy a brief period of rapid multi- 

 plication, which is followed by a decline, often involving the formation 

 of resting spores, and ultimately by total disappearance. The rapid 

 rate of establishment and the intensity of the epidemic are no doubt 

 due to the virulence of the fungus, as well as to the degree of host-plant 

 susceptibility, which in turn may possibly be strongly influenced by en- 

 vironmental factors. Random samples of collections of algae brought 

 into the laboratory and examined for chytrids may show little or no 

 evidence of their presence. It is a common experience, however, to 

 find that this material, if left in the laboratory (which is generally warm- 

 er than the natural habitat) under conditions which allow for a maxi- 

 mum oxygen supply, will within a few days become infested with vari- 

 ous chytrids. At the time of the first examination the invaders may 

 have been present in the water as zoospores, resting spores, or in- 

 conspicuous immature thalli. The sudden change in the environment 

 no doubt works to the advantage of the fungi and to the disadvantage 

 of the algae. 



The reason for the brief period of rapid multiplication of chytrid 

 individuals once they are established under favorable conditions is 

 readily understandable, as Nowakowski (1876b) long ago indicated 

 for Polyphagia euglenae. The progress of an epidemic of this chytrid 

 can be easily followed in the laboratory. Under favorable conditions 



