MANNER OF PRODUCTION OF MUTATIONS 559 



by X rays in Tradescantia microspores, showed that these frequencies 

 are strongly affected by the amount of oxygen present during exposure 

 but not by the amount present before or after exposure. The curve 

 expressing the relation of frequency as ordinate to oxygen concentration 

 as abscissa was found by Giles and Riley to rise steeply at first but to 

 become convex as the oxygen concentration in air (21 per cent) was 

 approached and, after that, to show only a very slow rise so that, even at 

 several atmospheres pressure of pure oxygen, it was not much above the 

 value for air. It did not matter what pressure of inert gas was used, or 

 what inert gas (nitrogen, helium, or argon) was present in place of or in 

 addition to oxygen (Giles and Beatty, 1950a, b), or even whether a 

 vacuum was used instead of an inert gas when oxygen was absent; the 

 result depended only on the pressure of the oxygen itself (i.e., on its 

 partial pressure). In independent work Hayden and Smith (1949) had 

 also found that the frequency of chromosome aberrations induced by 

 X rays (in their case, by treatment of barley seeds) was much higher 

 when oxygen was present during exposure than when irradiation was con- 

 ducted in a vacuum. 



Viewing hydrogen peroxide produced in the protoplasmic medium in 

 which the chromosomes lie as an intermediary in the main pathway lead- 

 ing to the aberrations which arise under the influence of oxygen, Giles 

 and his co-workers raise the question whether those aberrations which 

 arise even without oxygen represent the direct action of the radiation on 

 the chromosomes themselves or an indirect action, attributable to the 

 OH radicals produced in the water content of the cell. Since according 

 to Allen (1948) hydrogen molecules, when present in pure water at least, 

 quickly combine with these radicals, Giles and Beatty (1950a) tried 

 irradiation in an atmosphere of hydrogen but found no significant reduc- 

 tion of the yield of aberrations below that produced in other inert gases or 

 in a vacuum. This result would appear to favor a direct action but, as 

 these authors point out, it is quite conceivable that in the intracellular 

 fluid, in contrast to pure water, hydrogen peroxide and related groupings 

 may be formed by radiation even in the absence of oxygen. 



The lack of effect of a difference in oxygen pressure before or after 

 exposure strongly indicates that oxygen promotes the process of breakage 

 itself instead of influencing the junction of the broken ends. However, 

 since it was still conceivable that oxygen, when acting in conjunction with 

 radiation but not without it, somehow alters the chromosomes in such a 

 way as to affect their later joining, a new series of observations was 

 carried out by Riley, Giles, and Beatty (1951). They made a study of 

 the frequency of one-hit aberrations of the chromatid and isochromatid 

 types which vary linearly with dose and of two-hit aberrations of the 

 exchange and interstitial deletion types which vary as a higher power than 

 1 of the dose, the power depending on the time-intensity relations of the 



