274 CHROMOSOME ABERRATION PRODUCTION 



rays, when oxygen is present, of some substance which increases the 

 frequency of aberrations. As indicated previously, this effect of such a 

 substance might operate by way of either the breakage or the recovery 

 mechanism. The most Hkely hypothesis seems to be that such a sub- 

 stance would cause an increased production of chromosome breaks. 

 However, the alternative possibility, that the recovery process is mod- 

 ified, cannot be immediately excluded on the basis of the data just pre- 

 sented. This problem must be attacked by other methods. Experi- 

 ments designed to determine whether the recovery time is different for 

 breaks produced in the presence and absence of oxygen are under way 

 (Giles and Riley, unpublished). The results to date are compatible with 

 the view that the recovery mechanism is essentially the same under 

 these two conditions. The previously reported differences in the slopes 

 of dosage curves obtained at a constant intensity of 45 r per min in air, in 

 oxygen, and in nitrogen, which at first appeared to be due to an effect 

 on restitution time, can be explained on the basis of an intensity effect 

 resulting from a lesser production of breaks per unit time in nitrogen. 

 It has been shown by Sax (26) that the exponents of dosage curves from 

 chromosome interchanges decrease with decreasing radiation intensity. 

 Evidence has also been obtained (Giles and Beatty, unpublished) that 

 the effect of temperature, which has been previously interpreted as in- 

 fluencing the recovery process (25), is actually, at least to a considerable 

 extent, an indirect effect on oxygen availability. The experiments of 

 Baker and Sgourakis (4) with Drosophila have demonstrated that oxygen 

 has a marked effect in increasing the frequency of other types of x-ray- 

 induced genetic changes, sex-linked lethal mutations, where there is no 

 evidence that a recovery process is involved. All these results suggest 

 very strongly that the oxygen effect is on the breakage and not on the 

 recovery mechanism in Tradescantia. 



In order to provide additional evidence concerning the mechanism of 

 the oxygen effect, it appeared desirable to determine the relationship 

 between the amount of oxygen present during irradiation at a constant 

 dosage and aberration frequency. Consequently, a series of exposures 

 has been made of inflorescences in atmospheres containing different per- 

 centages of oxygen (mixed with helium) at a single x-ray dose of 400 r. 

 Some data on this point have already been published; see Giles and 

 Riley (12). Another experiment (Giles and Beatty, unpublished) has 

 been carried out to determine this relationship more precisely. In this 

 instance special attempts were made to free the helium used of any 

 residual oxygen. The percentages of oxygen (2, 5, and 10 per cent) in 

 the other gas mixtures used were accurate to within ±0.2 per cent, and 

 a larger number of cells was scored to increase the statistical reliability 



