Biological Effects of Ioni::ing Radiations 131 



per fx) to X-radiation (A := 80 ions per |.i), the efficiency of the 

 radiation has generally been found to increase by about 50%. 



Zirkle and Lampe ^^ compared the inhibition of growth of 

 both the shoot and root of wheat seedlings, when irradiated by 

 neutrons, for which A -=z 400 ions per ii, with that produced 

 by X-rays (A = 80 ions per ^). The neutron radiation was 

 about 3 times as efficient as the X-radiation, making a total 

 increase in efficiency of 4.5 as the ion density is raised from 11 

 to 400 ions per \v. Very similar results were obtained by Gray, 

 Read and Mottram,^- who investigated the lethal effect of gamma 

 rays. X-rays, neutrons, and alpha particles on the roots of 

 Vicia faba. Their results are shown in figure 3. The wheat 

 seedling results fall almost on the same curve. 



The primary injury is evidently very sensitive to changes in 

 ion density over the range 100 to 1,000 ions per micron. This 

 is just the region of ion density in which, as we have already 

 seen, there is a rapid increase in the efficiency of ionizing par- 

 ticles in breaking the chromosomes of a variety of materials 

 including Vicia faba. Experimental data for two types of chromo- 

 some break observed in Tradescantia pollen are also shown in 

 figure 3, since corresponding data for J^icia faba are not yet 

 available. The trend of one of the curves is .similar, suggesting 

 that the inhibition of growth may arise from chromosome struc- 

 tural changes produced in the meristematic cells. 



This hypothesis has been tested in a variety of ways, one of 

 which is of special interest from the point of view of ion- 

 density studies (Gray and Scholes, unpublished). It will be 

 recalled that, whereas some types of structural change require 

 the joint action of two ionizing particles when produced by 

 X-rays, and, therefore, increase as the square of the dose and 

 decrease with duration of exposure, all types produced by alpha 

 particles increase in direct proportion to the dose and are 

 independent of duration of exposure. Methods have been evolved 

 of estimating the proportion of cells in the root tip which are 

 injured by exposure to lethal and sublethal doses of radiation, 

 and it has been found that this proportion does, in fact, increase 

 linearly with dose in the case of alpha radiation, and is not 



