THE LIFE-SPAN OF MUTAGENS PRODUCED IN Cl'LLS BY IRRADIATION 



after irradiation, approaching a limit after two weeks' post-storage. In 100 

 per cent oxygen the total damage continues to increase even after four weeks 

 of post-storage, while in nitrogen no post-storage effect is shown. 



Standard errors are not shown on the figures. These increase with oxygen 



10 20 30 



Days of post -storage —^ in air 



Figure 7. The effect of increasing the post-storage period in air on 



irradiation damage measured by frequency of cells with chromosome 



aberrations, showing the continued breakage in the two weeks following 



irradiation (2 • 4 kr X irradiation) 



concentration. Treatments with oxygen show a large variance both within 

 and between seeds. In general, the variance increases with the mean 



100 % O2 



2 U 



Weeks of storage following irradiation - — — 



Figure 8. The effect of post-storage in xarious oxygen-nitrogen 

 mixtures for two and four weeks after irradiation, showing that the 

 increased breakage is dependent on oxygen concentration (1-2 kr 



X irradiation) 



breakage but at a faster rate so that the ratio of the variance to the mean 

 increases in the series nitrogen, 20 per cent oxygen, 100 per cent oxygen and 



198 



