488 RADIATION BIOLOGY 



recently, Muller, Valencia, and Valencia (unpublished data) have found 

 that in this material 8-kvp (1.5 A) and 200-kvp X rays produce not only 

 translocations but also individual chromosome breaks, as represented by 

 chromosome losses, with the same frequency. The equivalence of these 

 different types of radiation shows that the individual breaks are similar 

 to individual gene mutations in that they are not produced in appreciable 

 measure by the cooperation of ionizations or activations derived from 

 separate electron tracks but are caused either by the single ionizations 

 and/or activations or by the natural clusters of them occurring in 8 rays 

 and in the ends of electron tracks in general. 



Similarly in Tradescantia microspores, Faberge (1940b) found X rays 

 from 0.44 down to 0.077 A to be equally effective in the production of 

 chromosome fragments. In the same material Kotval is reported by 

 Lea (1946) to have found X rays and y rays to be about equally effec- 

 tive; the changes looked for by Kotval were chromatid breaks, isochro- 

 matid breaks (breaks detected in each of a pair of sister chromatids in 

 like positions), and chromatid interchanges. It is true that Kirby-Smith 

 and Daniels (1952) have reported that chromosome aberrations are pro- 

 duced with approximately half as much efficiency by irradiation of Trades- 

 cantia pollen with high-energy y and /3 rays as by irradiation with 250-kvp 

 X rays. This may be due to the more dispersed ionizations and acti- 

 vations, caused by the y rays, producing breaks that are likewise more 

 dispersed spatially. If this interpretation is correct, many of the aberra- 

 tions produced by X rays must have had both their breaks arising in the 

 same electron track. This situation would be difficult to reconcile with 

 a frequency which, for short or constant exposure times, varies with the 

 square of the dose. 



3. NUMBER OF IONIZATIONS NECESSARY FOR BREAKAGE 



The question arises : can a single ionization or activation cause a break, 

 or, if not, how large a natural cluster of ionizations is necessary for pro- 

 ducing this effect? Observations of Catcheside and Lea (1943) on the 

 effectiveness of X rays ranging from the extremely soft rays of 8.3 and 

 4.1 A through 1.5-0.15 A have been regarded as bearing on this question. 

 The changes studied were chiefly chromatid and isochromatid breaks pro- 

 duced by irradiating germinating Tradescantia microspores at a stage 

 when chromatids have presumably formed but still lie very close to 

 each other. The examinations were conducted at the subsequent meta- 

 phase, about 14)^^-17 hours after irradiation, the chromosomes having 

 been held at that stage for an indefinite period by application of acenaph- 

 thene. The data showed that, in the production of both these changes, 

 0.15-A radiation was nearly as effective as 1.5 A, but that 4.1 A gives a 

 pronounced peak, with a frequency of each type of change about 60 per 



