SUMMARY 281 



to this situation would appear to be sensitivity tests of cells that vary 

 in water content. This is not possible with developing microspores of 

 the type used in these experiments. It might be feasible with mature 

 pollen grains, however. There are in fact many data which indicate 

 that dry seeds of plants show increased radioresistance, both with re- 

 spect to killing and to induced genetic changes, genie as well as chro- 

 mosomal, as compared with hydrated seeds [Gustafsson (14)]. However, 

 it is not always possible from such comparisons to conclude that the 

 sensitivity changes are directly correlated with water content, and do 

 not result from changes in the mitotic state of the nucleus, Avhich are 

 known to affect radiosensitivity. 



Regardless of the degree of importance of the direct effect, it appears 

 probable that in Tradescantia, at least, the magnitude of the indirect 

 effect is considerably greater. However, it is still possible to interpret 

 the results in terms of target theory, as indicated by Thoday (27). 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 H2O2, 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. 

 Such a localized distribution would appear to be essential, as has been 

 pointed out by Zirkle (31), in order to explain the striking quantitative 

 differences among various radiations, as, for example, the shapes of the 

 dosage curves for interchanges induced by x-rays and alpha particles. 



Summary 



Recent experiments have demonstrated that oxygen has a marked ef- 

 fect in increasing the sensitivity of chromosomes in Tradescantia and 

 other plants to x-rays as measured by the frequency of cytologically 

 detected aberrations. It has been shown that this is not an effect of 

 oxygen itself on the behavior of broken ends of chromosomes. The ef- 

 fect apparently arises from the production by x-rays, as a result of the 

 radiodecomposition of water in cells containing oxygen, of some sub- 

 stance which causes an increase in aberration frequency. Several inde- 

 pendent lines of evidence indicate that this substance may be H2O2. It 

 appears likely that the increased frequency of aberrations arises from an 

 increased production of chromosome breaks when oxygen is present dur- 

 ing irradiation, rather than from a modification of the recovery process. 

 Thus a major fraction of the radiation effect on Tradescantia chromo- 



