114 AQUA TIC PH YCOM YCETES 



exhibit well-defined distinctive structural features and which do not 

 depend upon the nature of their host for their specific identity. On the 

 other hand, species of Rozella and Olpidium, which also have apparent 

 host specificities, are poorly defined morphologically, and many of 

 them have been separated from already described species chiefly on 

 the basis of the host. Little or no evidence exists in most instances to 

 support this segregation. 



Inoculation experiments conducted by Johnson (1955a) using Ro- 

 zella achlyae, an olpidiaceous form originally discovered parasitizing 

 a species of Dictyuchus, demonstrate that this fungus is capable of in- 

 fecting two species of Achlya as well as a second species of Dictyuchus. 

 His success in this respect is important for it indicates a wider host 

 range for a single species than has generally been believed to be true. 

 Earlier, Karling (1942a) had reported a Rozella in Cladochytrium 

 that parasitizes three species each of Cladochytrium and Nowakows- 

 kiella but does not infect certain monocentric chytrids. Two other 

 species of Rozella that he studied, however, were found to be confined 

 to their respective hosts. 



Effect of Chyirid on Host Cell 



Parasitic chytrids differ in their destructive effect on the invaded 

 host cell. Almost always, except perhaps in the case of protozoans at- 

 tacked by certain species of Sphaerita and said to extrude the fungus, 

 the cell is ultimately killed. Braun (1856a: 71) noted that whereas 

 chytrid infection produced the death of a one-celled alga, it usually 

 killed only the infected cell in multicellular forms. Filaments of multi- 

 celled Ulothrix zonata parasitized by Phlyctidium laterale were ex- 

 ceptions. In organisms attacked by Sphaerita and Nucleophaga (Dan- 

 geard, 1889b, 1894— 95b) actual death may be delayed for a considerable 

 period after infection, usually until the sporulation of the parasite. 

 It has also been observed that motile chlamydomonads attacked by 

 Rhizophydium transversum may continue to swim about while the 

 thallus of the invader is maturing, and succumb completely only at 

 the time of sporulation of the chytrid. 



By reason of their simple body plan, semitransparency, color, and 

 well-defined internal organization, the reaction of green algae to at- 



