C. p. SWANSON 



Table II. Percentage Reduction in Total Breakage in Tradescantia as the Result of Exposure in the 



Absence of Oxygen 



* Reduction of 58 per cent at 250-kvp X-radiation was extracted from Riley, Giles and Beatty (1952, Table 1 

 at 150 r). The neutron and alpha ray reductions were derived from data in Giles, Beatty and Riley {1952, 

 Table 2 at 10 n). Thoday and Read (1949) indicate that some reduction is obtained with alpha rays when ex- 

 posure is made in nitrogen. 



of restitution in such a manner that restitution, which follows the actual 

 breakage of chromosomes, is favoured by anoxia. As will be pointed out, 

 neither hypothesis is satisfactory in its present form. If we adopt the point 

 of view of Gray- (1953) that a variety of primary events are produced in 

 or near the chromosome by ionizing radiations and that the relative import- 

 ance of these events, when translated in terms of breakage, is determined 

 by the physiological condition of the cell during irradiation and up to the 

 time of final rupture of the chromatin strands, it becomes clear that breakage 

 and restitution can be effected in a variety of ways and at various levels. 

 In what follows, a more detailed analysis of possible events will be made, 

 and the two schemes to be presented {Figures 1 and 2), although obviously 

 tentative, will at least serve a useful purpose as points of departure for dis- 

 cussion and possibly for future studies. 



Before proceeding with an explanation of Figures 1 and 2, it will be well 

 to point out why the two previous hypotheses are inadequate. Insofar as 

 the breakage hypothesis is concerned, increases in any type of aberration — 

 for example, the chromatid deletions induced by 50-kvp X-ra}s in nitrogen 

 as compared with air ( Table 1) — cannot arise by a reduction in breakage 

 alone. Partial repair or restitution, which converts one type of aberration 

 into another, as explained earlier, must be postulated as an additional pro- 

 cess. Also, if breakage alone is affected by oxygen tension, it becomes 

 equally difficult to explain how aberrations such as chromatid and isochro- 

 matid deletions, which have somewhat similar relations to dose, vary quite 

 differently with stage of division (Swanson and Schwartz*, 1953). 



Similar awkwardness is encountered if an attempt is made to explain the 

 data in Table 1 in terms of the restitution hypothesis. It can be seen that 

 as the ion density of the radiation increases the chromatid deletion /isochro- 

 matid ratio decreases. With the more densely ionizing particulate radia- 

 tions it would be expected, on the restitution hypothesis, that this ratio 

 would be even lower in value than that obtained with 50-kvp X-rays, and 

 that the values for the air/nitrogen ratios would be lower for chromatid 

 deletions and higher for isochromatid deletions than those obtained with 

 50-kvp X-rays. That this expectation has not been realized has been 



257 s 



