736 RADIATION BIOLOGY 



to the square of the dose. If the dose is extended over a considerable 

 period (at a low intensity), restitution is favored over reunion and the 

 exchange yield is reduced. 



With fast neutron and a-particle experiments, in which many fewer 

 ionizing particles traverse the nucleus than do electrons in comparable 

 X-ray exposures, all aberrations exhibit a linear relationship with dose. 

 Thus exchange aberrations apparently arise as the result of the simul- 

 taneous production by the same ionizing particle of the two breaks taking 

 part in the exchange. Confirmation of this fact comes from the evidence 

 that there is no effect of neutron intensity on aberration yield. 



The interpretation of chromosome aberration production as just out- 

 lined has been quite generally successful in accounting for most of 

 the quantitative results of radiation experiments with Tradescantia. 

 Recently, however, experimental data of two sorts have been obtained 

 which indicate that this theory in its simplest form is not entirely ade- 

 quate. The first evidence was that obtained by Kotval and Gray (1947) 

 in their studies with a particles. On the basis of comparative ionization 

 distribution and particle numbers, the theory predicts that a given 

 amount of ionization produced by a particles should be considerably less 

 efficient in producing chromosome breaks than an equal ionization dose 

 produced by fast neutrons; whereas, the experimental results indicate 

 that, for equal ionization doses, a particles are somewhat more efficient. 

 It was concluded that a proportion of the breaks produced by a particles 

 arise from ionization produced in the immediate vicinity of, but not 

 within, a chromosome, thus suggesting the involvement of an indirect as 

 well as a direct mechanism. The second, and even more striking, evi- 

 dence was that obtained by Thoday and Read (1947), who noted a pro- 

 nounced effect of oxygen on the frequency of X-ray-induced aberrations 

 in the root-tip mitosis of the broad bean, Vicia faba. Their experiments 

 indicated that the absence of oxygen during irradiation resulted in a 

 marked decrease in aberration frequency and consequently supplied 

 additional evidence for the probable presence of indirect mechanisms 

 responsible for chromosome breakage. Extensive studies of the oxygen 

 effect on aberration production in Tradescantia microspores have also 

 been performed and the results of these experiments will be discussed in 

 one of the next sections on modifying factors. In that discussion an 

 attempt will be made to evaluate the significance of all these more recent 

 findings in relation to the mechanism of aberration production as pre- 

 viously described. 



EFFECTS OF MODIFYING FACTORS 



The effects of various modifying factors applied before, during, or after 

 exposures to radiations have been extensively studied in attempts to 



