chromosome aberrations in Tradescantia 753 



effects to the immediate vicinity of particle paths, a point which will be 

 discussed later. 



It will be recalled that in the absence of oxygen, i.e., in X irradiations 

 performed in other gases such as nitrogen or helium or in a vacuum, there 

 is still an appreciable frequency of aberrations. The question may be 

 raised as to whether this residual frequency is due largely to indirect 

 radical effects even in the absence of oxygen, as has been assumed in the 

 immediately preceding discussion, or whether this effect arises from the 

 direct absorption of the radiation energy by the nucleoprotein structure 

 of the chromosome. The whole problem of distinguishing between direct 

 and indirect effects, particularly in a situation such as this one, becomes 

 exceedingly difficult. The distinction may in fact become largely mean- 

 ingless in instances where such complex structures as chromosomes are 

 involved, in which, for example, water molecules giving rise to active 

 radicals upon irradiation may occur within the volume of the chromo- 

 some and probably even to some extent bound to it. 



Despite these difficulties, it appears worth while to consider the avail- 

 able evidence bearing on the question of the mechanism of the radiation 

 effect in the absence of oxygen. On a priori grounds, it seems reasonable 

 to expect that an indirect effect by way of OH radicals, and possibly H 

 atoms, would occur in an aqueous system such as a cell in the absence of 

 oxygen. The difficulty of demonstrating such an effect experimentally is 

 considerable, however. In the first place it is difficult to prove that all 

 the oxygen has been removed from cells by the evacuation procedure 

 employed. If it is assumed that the oxygen is in fact removed, then the 

 existence of an indirect effect in the absence of oxygen can best be demon- 

 strated by the efficacy of some protective substance in decreasing this 

 effect by reacting with the intermediate active radicals or atoms before 

 they can produce their biological result, in this instance, a chromosome 

 break. A major difficulty in testing for such an effect arises from the 

 problem of ensuring that the protective substance is actually penetrating 

 the cell. For Tradescantia inflorescences, penetration by gases has been 

 shown to be effective, hence an attempt was made to test for a protective 

 effect using H 2 , which is known on radiochemical evidence (Allen, 1948) 

 to promote the back reaction to form H 2 in X-irradiated water by com- 

 bining with the OH radical [reaction (7)]. However, no clearly sig- 

 nificant decrease in aberration frequencies was found following irradiation 

 in hydrogen at normal pressure or at three atmospheres above normal, 

 compared with irradiation in a vacuum or in nitrogen at three atmos- 

 pheres above normal pressure (Giles and Beatty, 1950). 



Evidence that a hydrogen effect can be detected, at least in a chemical 

 system possibly similar to the one in this biological experiment, is indi- 

 cated by the experiments of Scholes and Weiss (1950) who found a 

 decreased effect of X rays in disrupting nucleic acid (as measured by the 



