748 RADIATION BIOLOGY 



the possible identity of the intermediate substances involved, it is neces- 

 sary to consider the evidence against other possible explanations for the 

 oxygen effect. The three other principal explanations for this effect on 

 chromosome aberration production, especially in Tradescantia, appear to 

 be the following: (1) an effect of oxygen, as compared with oxygen lack, 

 on some metabolic system or systems in the cell resulting in increased 

 radiosensitivity ; (2) an effect of oxygen itself on the recovery process 

 such that the reunion of broken ends, which results in aberrations, is 

 favored over restitution, which restores the original conditions; and (3) an 

 effect of oxygen, when present during irradiation, not on the breakage 

 process, but on the subsequent recovery process. Such a situation could 

 arise either from the production in the presence and absence of oxygen of 

 qualitatively different types of breaks with respect to their subsequent 

 behavior in restitution or reunion, or from the effect on the recovery pro- 

 cess of some product arising when oxygen is present during irradiation. 



There would appear to be a distinct possibility that oxygen lack might 

 influence radiosensitivity by modifying certain aspects of cellular metabo- 

 lism, thus causing a decrease in chromosome aberration frequency. For 

 example, there is good evidence that radiosensitivity varies with different 

 stages in the mitotic cycle (cf. Sparrow, 1951), and an effect of oxygen (as 

 compared with nitrogen) in modifying the timing of this cycle might 

 result in an effect on aberration frequency (cf. Gaulden, Carlson, and 

 Tipton, 1949). Further, it seems possible that modifications in the rate 

 of cellular respiration might influence radiosensitivity. Other effects on 

 cellular metabolism (e.g., on nucleic acid synthesis) of a lowering or 

 increase in oxygen tension might also be anticipated. The major evi- 

 dence against the view that the oxygen effect operates by way of dis- 

 turbed cellular metabolism comes from the comparative studies of this 

 effect with different kinds of radiations, as emphasized by Thoday (1950). 

 Although there is a marked oxygen effect with X rays on chromosome 

 aberration frequency in Vicia root tips, there is little or no effect with 

 a particles. Similar observations have been made in Tradescantia (Con- 

 ger, unpublished) and, in addition, the oxygen effect with fast neutrons 

 has been found to be intermediate (Giles, Beatty, and Riley, 1952) 

 between that for X rays and a particles. If the effect of oxygen were a 

 metabolic one, there is no reason to suppose that the resulting modifica- 

 tion in radiosensitivity would vary with different kinds of radiations. 



Further evidence against the possibility that a disturbance of the 

 mitotic cycle can explain the oxygen effect comes from the experiments 

 (Giles and Riley, 1950; Giles, 1952) in which it was shown that this 

 effect is an immediate one, since the introduction of the gas during irradi- 

 ation results in a marked increase in aberration frequency. That oxygen 

 could have influenced the timing of the mitotic cycle in these experiments 

 is clearly impossible. Furthermore, the experiments were performed on 



