248 FACTORS INFLUENCING CELL RADIOSENSITIVITY 



tion (Fig. 5). On chains of giant cells apparently doomed to die, cells of 

 normal appearance often arise from which normal colonies then develop. 

 In yeasts this ability to recover from radiation injury is thus linked 

 to polyploidy. 



Haploid Diploid 



Fig. 5. Aspect of colonies and injured cells of S. cerevisiae after x-irradiation. 



Action of Temperature 



When temperature influences the production of a lesion, it is difficult 

 to know whether this influence takes place during the primary effect, 

 or later, during the "dark reactions" (chemical reactions initiated by 

 the primary effect). However, in certain cases, an experiment may 

 answer this question. 



Radiochemical reactions are usually considered insensitive to tem- 

 perature. Although rather general, this affirmation should not be ac- 

 cepted in all cases. Thus Schreiber (26), in suppressing the motility 

 of Sphaerocarpus donelUi spermatozoa by monochromatic ultraviolet 

 rays (2650 and 3025 A), obtained the following results: with 2650 A, 

 the efficiency of irradiation remains constant between and 15° C, then 

 increases regularly above 15°; with 3025 A, the efficiency increases 

 steadily with temperature from 0° on. The temperature coefficient of 

 the phenomenon therefore depends on the wave length ; this proves (a) 

 that temperature acts on the primary effect rather than on the dark 

 reactions; (6) that this primary effect is a photochemical reaction with- 

 out intermediate; (c) that this photochemical reaction is sensitive to 

 temperature. (In this case, a relatively simple experiment informs us 

 of the nature of the primary effect.) 



