282 CHROMOSOME ABERRATION PRODUCTION 



somes is to be considered indirect rather than direct. In the absence 

 of oxygen, however, there is still an appreciable aberration frequency 

 and some evidence indicates that this entire fraction may be the result 

 of direct radiation action. 



Since the previous hypothesis explaining chromosome breakage in 

 Tradescantia assumed that all the effect of the radiation resulted from the 

 direct ionization of the molecules of the chromosome, the demonstration 

 of an indirect effect necessitates a revision of this interpretation. How- 

 ever, it is still possible to interpret the results in terms of target theory. 

 The essential requirement is that the action of ionizing particles, whether 

 direct or indirect, be relatively localized. If the effect is principally in- 

 direct, it appears that a substance, such as II2O2, must be produced 

 along the track of an ionizing particle and must have a relatively limited 

 effective diffusibility (or short half life). In fact, it seems necessary that 

 its effective distribution within the nucleus must correspond in pattern 

 rather closely to that of ionization distribution along particle tracks. 



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