chromosome aberrations in Tradescantia 



747 



8. In agreement with previous studies on the temperature effect, an 

 increased aberration yield at low temperatures is observed when irradi- 

 ations are performed in the presence of oxygen in the range between 

 approximately 1° and 35°C (see Fig. 10-7), but this result cannot be 

 attributed entirely to an effect of temperature on oxygen solubility. 



Table 10-2. Experiments Demonstrating that the Effect of Oxygen on the 



Yield of X-ray-induced Chromosome Interchanges in Tradescantia Is Confined 



to the Period of X-ray Exposure. (All Doses: 300 r at 300 r/Minute) 



(Data from Giles and Riley, 1949) 



9. In the absence of oxygen, a reversal of the temperature effect is 

 noted, aberration frequencies being higher at high than at low temper- 

 atures (see Fig. 10-8). 



10. When fast neutron radiation is used, an oxygen effect is also found, 

 but the magnitude of the effect is considerably less for all types of aber- 

 rations than it is with X radiation. 



11. Experiments on the intensity effect with X rays in oxygen and in 

 nitrogen, using dosages of equivalent biological effect, have shown similar 

 decreases in the yields of chromatid exchanges in the two gases at low 

 intensities. 



As a working hypothesis, to explain the oxygen effect, the view will be 

 taken that this effect results from an increased frequency of initial chro- 

 mosome breakage as a consequence of the action of certain active sub- 

 stances produced in the aqueous cellular medium when oxygen is present 

 during irradiation. Before discussing the evidence for this view and for 



