DISCUSSION 



291 



100 



80 



g 60 



S-. 40 



20 



20 



100 



120 



40 60 80 



X-ray dose, kiloroentgens 



Fig. 3. Data from Fig. 2 recalculated for "target" determination. See discussion 



in text. 



Discussion 



The data from Fig. 2 were recalculated on the basis of the target 

 theory [see Lea (17)] and are given in Fig. 4. A purely physical interpre- 

 tation on the basis of the target theory of the effects of x-rays on E. 

 colt leads one to the conclusion that it is possible to vary the number 

 of "targets" at will, by means of adjustment of growth conditions or 

 conditions during irradiation. Whereas this type of physical interpre- 

 tation may be a helpful tool in the study of radiation effects, the chemi- 

 cal approach appears more promising at the present time. The hypothe- 

 sis that hydrogen peroxide formation induced by radiation is responsible 

 for radiation effects on chromosomes is in many ways attractive, as 

 Giles has so well shown. However, this hypothesis is less attractive in 

 the interpretation of lethal effects on bacteria and fungi for the following 

 reasons, (o) Bacteria suspended in a medium which has been irradiated 

 with x-rays (60,000-80,000 r) will not be killed by this medium. The 

 half life of hydrogen peroxide in buffer solution is of sufficient length 

 that it should show a residual effect. (6) Hydrogen peroxide added to 

 the suspension medium becomes toxic to bacteria only if concentrations 

 are used which are far in excess of the ones produced by the usual 

 amounts of radiation (> 200,000 r). (c) Organic peroxides, as well as 

 hydrogen peroxide, should be produced by irradiation in broth suspen- 



