130 



RADIATION BIOLOGY 



ii«o to K mutations in this material. X rays wen^ found not to produce 

 the K saltation (Ford and Kirwan, 1949; Ford, 1918; McAulay et ai, 

 1949), hut other morpholofjjical changes were produced. So-called "lethal 

 mutations" were found with all wave lengths of ultraviolet examined and 

 with X rays (Ford and Kirwan, 1949; Ford, 1948; McAulay et ai, 1949). 

 These were, in fact, spores which germinated, produced a small amount of 

 mycelium, and then stopped growing; it is impossible to say whether the 

 cessation of growth is attributable to genetic mutation in the usual sense. 

 Since C. (jlobomni is apparently unsuitable for ordinary genetic experi- 

 ments, the basis for the K saltation remains (Mnially uncertain. In view 



2500 3000 3500 



WAVE LENGTH, A 



(a) 



4000 



2500 



3000 3500 



WAVE LENGTH, A 



(b) 



Fig. 11-2. Curve a sho\v.s the ratio of percentage K saltants to the percentage "oilier" 

 saltants at different wave lengths. Curve h shows the optimum doses in joules per 

 square centimeter for saltant production. Both curves are plotted against wave 

 length: curve a, natural scale; curve b, logarithmic scale. Doses read downward to 

 bring out the similarity between the curves. {McAulay and Ford, 1947.) 



of these uncertainties and of the general finding in other material of action 

 spectra suggesting nucleic acid rather than protein excitation it seems 

 reasonable to suppose that the interesting findings in Chadomium require 

 an ad hoc explanation, and are not to be regarded as serious obstacles to 

 the hypothesis that nucleic acid excitation is the most effective primary 

 action of ultraviolet. 



It should be noted that the finding of a marked effect (in terms of 

 killing and mutation) of ultraviolet-irradiated medium (Wagner et al., 

 1950) raises some question as to the complete propriety of any interpre- 

 tation based solely on a consideration of certain absorptive characteristics 

 of the organism (Bacq, 1951). The experiments show that toxic and 

 mutagenic substances are formed by the action of ultraviolet on nutrient 

 broth. Similarly, treatment of nutrient broth with hydrogen peroxide 

 produces mutagenic activity. There are important differences between 



