Biological Effects of Ionising Radiations 117 



of tissue has been estimated with reasonable accuracy. From 

 such experiments, we learn that biological effect is not in gen- 

 eral uniquely determined by the total number of ions, but that 

 it is also conditioned by the spatial distribution of these ions ; 

 the effect of a small number of particles, each producing a large 

 number of ions, is not necessarily the same as that of a large 

 number of particles, each producing few ions. 



To take a concrete example, consider the effect of equal doses 

 (25 rontgen) of beta radiation and alpha radiation on the 

 meristematic cells in the root tip of the broad bean, Vicia faba. 

 The total ionization produced in a nucleus lOji in diameter is, 

 in each case, 23,400 ions. In the first case, the total is made 

 up of the contribution from 500 beta particles, each producing 

 on an average 7 ions per micron of path. In the second case, 

 the whole ionization is produced by the transit of a single alpha 

 particle producing ions at the rate of 3,500 per micron. The 

 beta radiation w'\\\ produce an appreciable diminution in mitotic 

 activity 3 hours after irradiation, but the effect on the subse- 

 quent growth of the root will be scarcely detectable. The alpha 

 radiation has no detectable immediate effect on mitosis, but six 

 days later the average growth rate of the roots will be less than 

 a third of its normal value, and a small proportion of the roots 

 will cease to grow altogether. 



The contrast between the effects of beta and alpha rays is 

 sometimes striking, as in the example just given, because these 

 two radiations lie almost at the opposite extremes of the known 

 radiations in regard to the density of the ionization along the 

 tracks of the particles. Even in this case, however, the differ- 

 ences are quantitative and not qualitative. A sufficiently large 

 dose of alpha radiation has an immediate effect on mitosis, and a 

 sufficiently large dose of beta radiation will kill the roots. Radia- 

 tions intermediate between beta rays and alpha rays are not 

 always intermediate in the effectiveness of a given amount of 

 ionization, since there may be an optimum linear ion density 

 for any given biological effect which is not at either extreme, 

 but in the cases so far studied it has almost alwavs * been found 



An exception is noted on p. 129. 



