CHROMOSOME RUPTURE 205 



4_to-5-day interval following irradiation. Initial comparative ex- 

 posures made in air, in nitrogen, and in oxygen demonstrated that 

 aberration frequencies were markedly reduced in nitrogen and increased 

 in oxygen as compared with air (cf. 139). It was also shown that the 

 effect of oxygen in increasing aberration frequency was exerted on the 

 initial breakage mechanism rather than on the recovery process. Similar 

 behavior was shown by Drosophila irradiated in the presence or absence 

 of oxygen (89). 



The frequency of aberrant cells in Vicia faha is much higher when a 

 particular dose of x-rays is administered in the presence of oxygen than 

 it is when the oxygen is replaced by nitrogen. On the other hand, 

 oxygen has little, if any, influence on the effectiveness of alpha rays (73). 

 That the number of structural changes produced by a given dose of 

 alpha radiation can be much larger than that produced by the same 

 dose of x-rays, gamma rays, or neutrons has been repeatedly observed. 

 For example it is true for Tradescantia hradeata (75). A possible in- 

 terpretation of these observations is the following. The primary action 

 responsible for radiation lesion is due to the effect of H2O2, or products 

 formed from H2O2. Whereas H2O2 is formed in alpha-ray-irradiated 

 water even in the absence of oxygen, its formation under the effect of 

 x-rays requires the presence of oxygen in the water (74). 



That ionizing radiation can disrupt bonds in the absence of all meta- 

 bolic influence follows from the above-mentioned experiments in which 

 irradiation in vitro was found to depolymerize desoxy ribonucleic acid. 

 Simultaneous treatment of Tradescantia with x-rays (250 r) and sonic 

 energy (9100 cycles per second) increases the yield of x-ray-induced 

 chromosomal aberrations by approximately 1.3 times the yield obtained 

 with the same amount of x-rays (76). It also follows from the fact that 

 chromosome breakage is observed after irradiation at low temperature 

 and under other conditions of almost imperceptible metabolic activity. 

 Faberge (77) observed chromosome breaks in Tradescantia pollen grains 

 at —192° C. Finally, chromosome breakage was shown very spectacu- 

 larly in experiments where hemocyanine was irradiated with alpha rays 

 at the temperature of liquid air. Under these conditions a marked de- 

 polymerization of the highly polymerized molecule was observed (78). 



In the above discussions we considered mostly the mitotic effects of 

 ionizing radiation. We do not lack indications, however, that other than 

 mitotic effects may be decisive for the histological response of tissue to 

 radiation. The lymphoid tissue and intestinal epithelium, which are 

 among the most radiosensitive tissues in the body, show less interference 

 of mitotic activity after exposure to x-rays than do the less radiosensi- 

 tive skin and adrenal gland (79). We may attempt to make a less pro- 



