chromosome aberrations in Tradescantia 737 



elucidate further the mechanisms of chromosome breakage and reunion. 

 In certain instances, for example centrifugation effects, such studies have 

 served primarily to support previous hypotheses as to the process 

 involved. In other cases (e.g., infrared effects) the investigations have 

 revealed pronounced effects which may require modification of original 

 views, but for which no clear explanations are as yet available. Finally, 

 certain studies (the oxygen effect) have yielded results of paramount 

 significance for any interpretation of the biophysical and biochemical 

 mechanism of aberration production and appear to necessitate a definite 

 revision of certain aspects of older hypotheses. In view of the past and 

 probable future importance of studies of modifying factors, it appears 

 desirable to devote a rather considerable amount of space to a considera- 

 tion of these effects. Since it is felt that the oxygen effect is of special 

 significance, a major portion of the discussion will be devoted to the 

 available evidence relating to this effect. It should be borne in mind, 

 however, that many of the conclusions arrived at in this latter discussion 

 must be, of necessity, tentative ones, since such investigations have only 

 recently been initiated and much work remains to be done to clarify 

 various aspects of the problem. 



CENTRIFUGATION; SONIC VIBRATION 



Experiments of Sax (1943) have shown that centrifugation (2080 rpm) 

 during X irradiation approximately doubles the yield of chromosome and 

 chromatid interchanges as well as of isochromatid aberrations. When 

 centrifugation followed irradiation (the interval between cessation of 

 irradiation and initiation of centrifugation being 5 minutes) there was no 

 increase in aberration frequency. In control experiments, centrifugation 

 alone produced no effect. In analogous experiments in which sonic 

 energy (9100 cycles/second) was applied during X irradiation (Conger, 

 1948), an increased frequency of both chromosome interchanges and dele- 

 tions resulted, although the magnitude of the effect was less than that 

 found with centrifugation. In control experiments, no effect of this 

 treatment alone was detected, although chromosome breakage following 

 supersonic treatment (400,000 cycles/second) alone has been reported in 

 other plant material (Wallace, Bushnell, and Newcomer, 1948). These 

 experiments are interpreted as indicating that factors such as mechanical 

 stresses or vibrations can cause movement of broken ends produced by 

 irradiation, and thus promote new reunions yielding aberrations, as 

 opposed to restitution. 



COLCHICINE 



Brumfield (1943) has studied the effect of pretreatment with colchicine 

 on the frequency of X-ray-induced aberrations in Allium root tips. The 

 experimental roots were placed in a 0.05 per cent solution of colchicine for 



