756 RADIATION BIOLOGY 



derived from them, produced in the aqueous medium of the cell. Despite 

 this indicated modification of earlier views on the basis of the recent 

 results, it must be recalled that the major emphasis in the theory of 

 radiation-induced chromosome changes in Tradescantia has been that a 

 particular chromosome break results from several ionizations produced by 

 one particular particle (whether an electron, a proton, or an a particle) 

 and not from a single ionization or from a cumulative effect of several 

 particles. Thus an essential feature of this theory, for which the major 

 evidence has been derived from comparative experiments with X rays 

 and fast neutrons, has been the localization of the biological effect along 

 particle tracks, and the influence on this effect of differences in the pat- 

 terns of ionization distribution along such tracks with various radiations 

 (Lea, 1946). Since the original experiments were all performed in the 

 presence of oxygen (in air), the evidence that chromosome breakage 

 stems from events produced by single particle tracks remains valid, 

 whether such events result from direct ionization of the molecules of the 

 chromosome or from indirect effects produced by radicals arising in the 

 water along the particle track. The types of dosage curves for inter- 

 changes induced by X rays and fast neutrons in the absence of oxygen are 

 similar to those found when oxygen is present, again indicating a localized 

 breakage mechanism involving the passage of single particles, regardless 

 of whether the breakage in the absence of oxygen arises from direct or 

 indirect effects. Thus the major modification required of earlier opinions 

 is simply concerned with the nature of the chemical events involved in 

 chromosome breakage. Consequently, it is only necessary to replace the 

 concept of characteristic columnar patterns of ionization along various 

 particle tracks, with one of columns of active radicals having similar 

 patterns (or columns of their immediate products arising from radical 

 interactions or from reactions with other solute molecules, such as 

 oxygen). As emphasized by Thoday (1950) and Allsopp and Catcheside 

 (1948), the principal point at issue is one of the relative localization of 

 effects to the immediate vicinity of particle tracks. Even though 

 indirect effects involving radical formation are involved, the evidence 

 indicates that the effective diffusion of such substances is very limited. 

 As a consequence, although a particle involved in producing a chromo- 

 some break by way of indirect radical effects may not actually traverse 

 the chromosome thread, it seems clear that it must at least pass in the 

 immediate vicinity of the site of breakage. 



REFERENCES 



Allen, A. O. (1948) Radiation chemistry of aqueous solutions. J. Phys. & Colloid 

 Chem., 52: 479-490. 



Allsopp, C. B. (1944) Radiochemistry. A review of recent progress. Trans. Fara- 

 day Soc, 40: 79. 



