558 RADIATION BIOLOGY 



growth in Vicia faba rooi tips is positively correlated with the amount 

 of oxygen present during (but not after) exposure to X rays. That 

 X-ray damage to survival is so correlated had been made known long 

 before by Holthusen (1921) working with Ascaris eggs (see, however, 

 Dognon, 1925, who considered this as w^ell as the damaging influence of 

 water and of temperature with X rays, found by him, as indirect effects 

 of the heightened vital activities in the Ascaris eggs) and was demon- 

 strated by many others afterward in other material (e.g., Crabtree and 

 Cramer, 1933; Mottram, 1935a, b; Smith, Sieburth, and Norby, 1950; 

 Hollaender, Stapleton, and Martin, 1951). 



Thoday and Read (1949) next showed that with a rays the oxygen 

 effect is much less. This result, as they pointed out, was to be expected 

 if hydrogen peroxide (or, it should be added, its derivatives) is a cause of 

 chromosome breakage, since the production of peroxide is much less pro- 

 moted by the presence of dissolved oxygen under irradiation with a than 

 with X rays. This is because with a radiation, OH radicals, derived from 

 water, are produced so close together that they often interact to form 

 hydrogen peroxide and its derivatives. With X rays, on the other hand, 

 in the absence of oxygen, each OH radical freed into the water by the 

 radiation is usually so far away from any other OH radical that (in the 

 absence of interferences such as can be caused by some metaUic ions) it 

 reunites with a freed hydrogen atom. However, if oxygen molecules are 

 present, many of the hydrogen atoms unite with those of oxygen to give 

 rise to HO2 and then H2O2, while more of the OH radicals, being left 

 as a surplus, will eventually find each other, giving more H2O2. In corres- 

 spondence with this interpretation, X-ray-induced chemical reactions 

 which are promoted by oxygen are relatively independent of oxygen 

 when a radiation is used instead. 



Unfortunately for the rigorousness of this argument in its application 

 to the findings on chromosome changes, there is a further reason why 

 oxygen would promote these changes to a lesser degree with a than with 

 X rays. This is that each a track crossing a chromatin thread has such 

 a high chance of causing an effective break in the thread anyway (and 

 often in its sister thread also) merely because of the large number of ions 

 in the track that, even if oxygen did promote breakage, this influence, 

 being largely superfluous, would cause relatively few additional breaks 

 that were separately visible (see pp. 490 491). Despite this objection to 

 the argument from a rays, the influence of oxygen in connection with 

 ionizing radiation is thoroughly established as a general phenomenon. 

 Moreover, this influence must in any case be exerted through the forma- 

 tion of atom groups, such as hydrogen peroxide and its derivatives and 

 potential antecedents, which carry oxygen in highly reactive form. 



Giles and Riley (1949, 1950; Riley and Giles, 1949), in observations on 

 the frequencies of exchange aberrations and interstitial deletions induced 



