106 AQUATIC PHYCOMYCETES 



a single zoospore, possibly originating in the sporangium formed by 

 a germinating resting spore, will come to rest, germinate, and eventually 

 produce a sporangium within which develop perhaps twenty zoospores. 

 If alter the swarmers are liberated only half of them— probably a 

 very low figure— ultimately give rise to new thalli, the first nonsexual 

 generation will have been multiplied tenfold within a short time. At 

 least part of these new thalli will produce new plants and new zoospo- 

 rangia, and thus enormously increase the number of units capable of 

 causing new infections. Furthermore, in such a form as Polyphagus, 

 the effect of this multiplication of individuals on the attacked algae 

 is intensified by the fact that the thalli are strongly polyphagous, each 

 of the tips of the numerous profusely branched and widely spread ele- 

 ments of the rhizoidal system being potentially capable of penetrating 

 a different Euglena. In this way the original zoospore has produced a 

 tremendous number of structures able to seek out and infect new host 

 cells, itself and each of its progeny being capable, by means of rhizoids, 

 of invading up to fifty or more Euglena individuals. The optimum rate 

 of multiplication does not last long, no doubt being gradually dimin- 

 ished by various adverse factors in the environment, chief of which 

 are probably the shrinking food supply, decrease of oxygen, increase 

 of C0 2 , increase of by-products of metabolism, and competition. 

 Natural enemies of the fungus, such as Protozoa which consume the 

 zoospores, develop in the culture, and there appears to be a resistance 

 to attack on the part of a certain percentage of potential host cells. 

 All these factors, as well as others, combine to produce a decline in 

 the number of new parasites. It is during this period that resting-spore 

 formation often occurs, usually followed by the complete disappearance 

 of the chytrid. Those individuals of the host which have survived the 

 epidemic may then reoccupy the culture, or it may be taken over by 

 other, different, algae. Attempts to maintain these parasitic chytrids 

 in gross cultures are usually unsuccessful, and it even seems that they 

 cannot be induced to reappear, once the epidemic has subsided. 



With respect to chytrid epidemics in nature. Canter (1946-51) 

 and Canter and Lund in recent years have gathered information in the 

 English Lake District concerning the parasites of phytoplankters (see 

 Figs. 10-11 pp. 108, 110) and their influence on the phytoplankton popu- 



