IMPLICATIONS OF THE RESULTS WITH ULTRAVIOLET 311 



or gamma rays are applied a single ionization, if favorably placed, 

 would be sufficient to give rise to a mutation, that is, that an ion cluster 

 would not be required. As regards the effectiveness of the scattered ac- 

 tivations produced by x-rays or gamma radiation, the conclusion to be 

 drawn is less clear, however. If the matter could be regarded purely 

 quantitatively, it would seem that one activation had little or no chance 

 of being effective unless its energy reached the relatively high level of 

 two 2537 A ultraviolet quanta, or (supposing the activation to be weaker) 

 unless its effect were combined with that of one or more other activa- 

 tions. However, judging by the infrequency with which such combina- 

 tions are effective in producing mutations even in the case of ultraviolet 

 quanta, we should hardly expect the far less abundant activations arising 

 from x-rays or gamma rays at the doses ordinarily used to have an ap- 

 preciable mutagenic influence. (For, as noted above, the x-ray or gam- 

 ma-ray doses are, in terms of total energy absorbed, several orders of 

 magnitude lower than the ultraviolet doses.) This purely quantitative 

 way of judging the problem represents an oversimplification, since most 

 of the activations produced in protoplasm in the tracks of ionizing radi^- 

 ation are of much more varied types than those arising from 2537 A 

 ultraviolet (which are in effect confined to the purine and pyrimidine 

 groups), and their potentialities would therefore be less limited. But 

 whether they would, on the average, be more or less effective than 

 those of ultraviolet in producing mutations it is at present impossible 

 to know. 



The discussion in the above paragraph concerns itself with the scat- 

 tered activations produced in the tracks of ionizing particles. There are, 

 however, clusters of activations produced at the ends of the tracks (and 

 of their branches, notably in delta rays), chiefly in the same regions as 

 the clusters of ionizations. If we are right in judging that ultraviolet 

 mutagenesis usually involves the cumulative effect of several near-by 

 activations, then these clusters of activations produced in the tracks of 

 ionizing particles after they have slowed down should give a much higher 

 chance of mutation than the same number of scattered activations would. 

 However, the reckoning of how great that chance actually is, in com- 

 parison with that of mutation produced by ionization, could not be 

 carried out without far more detailed knowledge than is now available. 



It should also be observed that the ionizations themselves result in ac- 

 tivations, in fact in clusters of activations of varying size, depending in 

 part upon the amount of energy transfer the ionization has involved. 

 The comparatively high ionization energy must become degraded, and in 

 this process by no means all of it is translated simply into kinetic energy. 

 Thus even those mutations that result from ionizations may, in part at 



