140 



HADIATION mOLOGY 



shown l)y tlu* hctciokaryoii inctliod (Atwood, l!)5()) tliat sufliciciit reces- 

 sive lethal mutations are induced in the nuclei of macroconidia to distort 

 noticeably the exponential survival curve if these mutations were induced 

 in micronidia according to the same dose-effect relation as in macro- 

 conidia. It is possible that an inteiaction of unknown nature between 

 the nuclei of the multiiuideate cells may be the explanation t'oi' this 

 discrepancy. 



Data on ('. (ilohosKm have been interpreted as signifying a re(|uirement of 

 two quantum hits at 2804 A to cause lethal mutation (Ford and Kirwan, 



120 



37401 



37102 



64001 



100 200 300 400 



ULTRAVIOLET OOSE, seC 



Fig. 1 1-5. Relation between ultraviolet dosage and frequency of reverse mutations in 

 three inositolless mutants (all in c m f stocks). The dotted line for nmtant 89(501 

 is added to indicate the exceedingly low reverse-mutation rate for this mutant. 

 {Giles, 1951.) 



1949; McAulay et al., 1949), and earlier work with yeast (Oster and 

 Arnold, 1934) suggested that different numbers of hits are required to 

 bring about different degrees of ultraviolet action, as judged by the cri- 

 terion of the amount of budding prior to cessation of growth. Such 

 observations may be indicative of a manifold inactivation unit allowing 

 budding to continue until mutually complementary surviving parts have 

 been segregated from one another. Detailed studies of cell lineage in 

 irradiated yeast might be very instructive. 



Modifying Effects on Ultraviolet Action. Various factors have been 

 studied which modify the effects of ultraviolet radiation on fungi; some of 

 these are listed in Table 11-5. Perhaps the most important is the marked 



