chromosome aberrations in Tradescantia 749 



chromosomes in the resting stage, a part of the mitotic cycle where there 

 is relatively little variation in radiosensitivity (Sax, 1938; Roller, 1946). 

 It also appears highly improbable that the introduction of oxygen during 

 irradiation could have modified nucleic acid synthesis rapidly enough to 

 have affected chromosome radiosensitivity if, in fact, the course of such 

 synthesis does influence radiosensitivity (Darlington and La Cour, 1945). 

 The possibility must also be considered, especially in Tradescantia, 

 that the effect of oxygen is on the recovery mechanism — the reunion, or 

 restitution of broken chromosome ends — rather than on the initial 

 breakage mechanism. Experiments have been performed (Giles and 

 Riley, 1950) which appear to eliminate the possibility that oxygen itself 

 may be influencing the recovery process. In these experiments, buds 

 were X-irradiated in a vacuum, and oxygen was introduced immediately 

 following the exposure. There was no increase in aberration frequency 

 over control experiments in which buds were irradiated in vacuum and 

 maintained in this condition for ca. 15 minutes following the exposure. 



These experiments do not completely rule out the possibility that the 

 oxygen effect may still operate by way of the recovery process, since as 

 noted in possibility (3) above, it is conceivable that some substance pro- 

 duced in the cells when oxygen is present during X irradiation might 

 influence the reunion of broken chromosome ends or that broken ends 

 produced in the presence and absence of oxygen might be qualitatively 

 different with respect to their subsequent behavior during the recovery 

 process. Evidence against this interpretation is provided by intensity 

 experiments in oxygen and in nitrogen (Riley, Giles, and Beatty, 1952), 

 in which the restitution times for breaks giving rise to chromatid 

 exchanges in the presence and absence of oxygen are found to be 

 essentially identical. If the oxygen effect were operating by way of the 

 recovery process, a difference in the average restitution time might well 

 be anticipated. Further general support of the view that a major effect 

 on the recovery process is not involved comes from the comparative oxy- 

 gen effect with different types of radiations. It is found that the effect 

 of oxygen on aberration frequencies is inversely correlated with the 

 specific ionization of the radiation used (X rays, fast neutrons, and alpha 

 particles). There appears to be no reason to assume that breaks pro- 

 duced by these radiations, especially by X rays and fast neutrons, should 

 differ qualitatively with respect to a possible oxygen effect on the recovery 

 process. It is possible, however, to provide a reasonable hypothesis to 

 explain these observations on the basis of differential initial breakage, 

 as will be discussed later. It should be noted that, despite the evidence 

 just presented, the possibility cannot yet be excluded that the oxygen 

 effect, even though operating primarily on the breakage mechanism, may 

 also exert some influence on the recovery process. 



The preceding evidence is taken to indicate that the effect of oxygen in 



