OXYGEN EFFECT WITH X-RAYS 271 



quency obtained in oxygen might result from an effect of oxygen itself 

 on the recovery process, such that when oxygen is present new reunions 

 of broken ends are favored as opposed to restitution. It seemed possible, 

 for example, that such an effect could result from the stimulation of 

 chromosome movement by oxygen. Another and perhaps more likely 

 possibility appeared to be that the effect of oxygen itself is an indirect 

 one, such that in the presence of dissolved oxygen x-rays produce in 

 cells a certain substance or substances which increase the yield of aberra- 

 tions. In this event, such an intermediate substance could produce an 

 effect by way of either the recovery or the breakage mechanism. In the 

 former instance, the x-ray breakage of chromosomes would still be con- 

 sidered a direct effect; in the latter, however, the breakage would have 

 to be considered an indirect effect, at least in part. 



It appeared feasible to attack some of these problems experimentally 

 in Tradescantia, since in this organism the recovery process extends over 

 a considerable period of time (the average time between production of a 

 break and restitution or reunion being at least 4 min). Thus it is pos- 

 sible to separate to a considerable degree the two processes of breakage 

 and recovery and to test the effect of oxygen on each. To do this, in- 

 florescences were exposed to a single dose of 300 r of x-rays in 1 min, 

 either in the presence of pure oxygen or in the absence of oxygen (in 

 vacuo). Immediately after the irradiation oxygen was either removed 

 (by evacuation) or introduced (to a positive pressure of 1500 mm of Hg). 

 The exchange of gases, as recorded by the manometer, could be effected 

 quite rapidly, and in this fashion it was possible to have the breakage 

 process occurring in oxygen and recovery largely in its absence, or the 

 reverse. In addition, other experiments were performed in which oxygen 

 was either introduced or removed during part of the x-ray exposure. 

 The results of such comparative exposure are reported in the paper of 

 Giles and Riley (12). Certain other experiments of a similar type have 

 been performed, and these results will be presented here. In the original 

 experiment in which a single exposure of 1 min to 300 r was made in 

 vacuum followed by the immediate introduction of oxygen, no effect on 

 the recovery process was noted. It was decided to increase the possi- 

 bility of detecting such an effect by fractionating the dose so that a rel- 

 atively larger portion of the recovery period would take place in oxygen. 

 The following procedure was used. A set of inflorescences was exposed 

 in vacuum for 20 sec at 300 r per min; after the irradiation, oxygen was 

 immediately introduced into the chamber to a positive pressure of 1500 

 mm of Hg and allowed to remain for 8 min; the chamber was then re- 

 evacuated and another 20-sec exposure made, followed by the reintroduc- 

 tion of oxygen as above; this procedure was repeated 5 times to give a 



