RADIATION STUDIES ON FUNGI 



437 



direct irradiation of the cells and irradiation of the medium, however. 

 The former is so much more effective, dose for dose, that it seems hardly 

 possible to interpret the difference solely in terms of greater absorption by 

 the cells. Since activation of the medium is produced only by lamps with 

 a quartz envelope, it appears that very short wave lengths, producing high 

 yields of peroxide, may play an important role. Action spectra for 

 mutagenesis by direct cell irradiation are clearly unrelated to the yield of 

 peroxide. It is not yet possible to state to what extent the results pro- 

 duced by irradiating the cells directly are brought about by the same 

 mechanisms which operate when the medium is irradiated in the absence 

 of cells, but these mechanisms are most likely of secondary importance. 



Table 11-4. Types of Survival Curves Obtained after 

 Ultraviolet Irradiation 



Survival and Mutation Kinetics. There appears to be no clear sepa- 

 ration of lethal and mutagenic effects on the basis of wave-length depend- 

 ence, although nonmutagenic wave lengths can be lethal at very high 

 doses. On the other hand, the dose-response curves for the two effects 

 show marked differences. Table 11-4 lists the types of ultraviolet sur- 



