286 FACTORS MODIFYING THE SENSITIVITY OF CELLS 



studies of radiation effects on chromosomes, which have been so well 

 demonstrated by Giles, and for most genetical studies, which will be 

 discussed by Muller. 



Typical killing curves which have been obtained with fungi in our 

 laboratory are given in Fig. 1. They refer to a comparative study of 

 x-rays, and alpha particles and protons, on Aspergillus terreus [Stapleton 

 and Martin (18)]. We are, however, not too enthusiastic about the use 

 of these curves as the only basis for interpretation of the mechanism 

 of lethal action of radiation, since "death" could be caused by a variety 

 of mechanisms which are difficult to untangle. 



There are many treatments which in themselves have little or no ef- 

 fect on the radiation sensitivity but, if given in supplement to ionizing 

 radiation, produce striking effects. In general, these supplementary 

 treatments have increased the sensitivity of cells to radiation rather 

 than protected them; however, there are a few exceptions to this. 



Heat and Visible Light 



The first two factors to be discussed are the effect of heat and light 

 in the treatment after irradiation. Cells are more sensitive to heat after 

 irradiation; this applies to x-rays as well as ultraviolet [Gaulden (8)]. 

 I should like to refer you also to the work of Curran and Evans (5) 

 and of Anderson and Duggar (2). E. H. Anderson (1), in our laboratory, 

 found that a certain strain of Escherichia coli can be reactivated after 

 exposure to ultraviolet several hundred-fold, if incubated at 40° C, as 

 compared with incubation at 30° C. This effect can be found to a shght 

 degree after x-raying (about five-fold). However, this heat recovery 

 has been recognized wdth only strain B of E. coli. In this connection 

 it should be mentioned that Hayden and Smith (9), in their work with 

 maize, found heat reactivation of seeds after x-ray exposure. Unfortu- 

 nately, no repetition of this work has been reported. A careful test in 

 our laboratory has given negative results [Suskind (20)]. 



Photoreactivation 



In 1949 Kelner (14) and Dulbecco (7) first reported "photoreactiva- 

 tion" after exposure to short ultraviolet. Most of the work reported 

 until now has dealt with primary irradiation of 2537 A. Some work by 

 Carlson and McMaster (4) in our laboratory has shown that, in the 

 nucleolus of the grasshopper neuroblast, the photoreactivation declines 

 after exposure to wave lengths shorter than 2537 A. The photoreactiva- 

 tion itself is limited to wave-length range of 3650-4500 A, given im- 



