1 ir> RADIATION niol.oOY 



rntiis, l(';i\iii^ tlic initially diploid cell in a ('ondition more nearly' appioxi- 

 inating the liaploid. Kxpcrimonts with genet ically marked dij)l()i(ls 

 would he especially informative on tliis point. 



There has been consideruhle confusion in interpreting sigmoidal sur- 

 vival cur\-es, particularly with regard to dctcriniiiing tlic number of 

 events necessary to kill. The dithculty arises largely from failure to 

 recognize that one must distinguish between two completely difTerent 

 meanings of the luimber of events: (1) number of hits that are necessary, 

 and (2) number of units that must be inactivated. The difference 

 between these interpretations and the reasons for preferring the latter in 

 most cases have been discussed by Atwood and Norman (1949). 



A study of the survival kinetics in material in which the mitotic stage, 

 the ploidy, or the number of nuclei per cell can be systematically varied 

 will be most profitable. The lack of such criteria precludes, in our present 

 rudimentary state of knowledge, any satisfactory interpretation of sur- 

 vival data, and for this reason most of the experimental observations in 

 Tables 11-4 and 7 must be simply noted without comment. 



Curves relating mutation production to radiation dosage have been 

 obtained for X rays with A. terreus (Stapleton and ]\Iartin, 1949), N. 

 crassa (Sansome et al., 1945), and P. notatum (Hollaender and Zimmer, 

 1945), with morphological mutations as the criterion, mutation being 

 essentiall}^ proportional to dose in all cases. Streptomyces flaveolus 

 differed in that the curve broke consistently downward between 100,000 

 and 200,000 r (Kelner, 1948). A critical study of mutation frequency is 

 that recently carried out with Neurospora (Giles, 1951) using reversions 

 to inosital independence as assay procedure. A linear relation between 

 dose and frequency of mutations was obtained with X rays, as shown 

 in Fig. 11-7. 



Modifying Factors. Certain environmental or cultural conditions have 

 been found to exercise a considerable effect in modifying results obtained 

 with ionizing radiations (Table 11-8). Rather striking effects have been 

 obtained, both in augmenting and in decreasing the magnitude of the 

 results expected from a given dosage. Pretreatment with infrared radi- 

 ation has been found to increase significantly the morphological mutation 

 rate with A. terreus and Trichophyton mentagrophytcs (Swanson et al., 

 1948; Hollaender and Swanson, 1947). The range reported as active 

 was 7000-18,000 A, with a maximum at 10,000 A. 



Heat had no saltating effect on Chactomium cochliodes (Dickson, 1932). 

 That temperature may play an important role is suggested by the obser- 

 vation with Saccharomyces ellipsoideus of a reduction in killing from 37 to 

 19 per cent at 13,000 r by holding the cells on ice for 10 days after irradi- 

 ation (Latarjet, 1943). This observation led to the suggestion that the 

 radiation damage may actually be caused in large part by the secondary 

 chemical reactions occurring after irradiation, rather than the primary, 



