chromosome aberrations IN Tradescantia 



745 



elusions which have emerged from these studies may be summarized as 



follows. 



1. There is a pronounced effect of oxygen in increasing the frequency 

 of X-ray-induced aberrations of all types. Results of experiments with 

 chromosome interchanges are shown in Fig. 10-9. The possibility that 

 other gases are responsible for the effect has been excluded. 



90 180 



X-RAY DOSAGE, r 



270 



360 





EXPOSED IN 2 

 EXPT. I 

 EXPT. E 



EXPOSED IN AIR 



A EXPOSED IN N 2 

 Fig. 10-9. X-ray dosage curves for chromosome interchanges induced in atmospheres 

 of oxygen, air, or nitrogen. (Giles and Riley, 1949. Figure reproduced by permission 

 of the authors and the editor of the Proceedings of the National Academy of Sciences.) 



2. Oxygen must be present during the actual X irradiation to be 

 effective. Pre- and posttreatment exposures of cells to pure oxygen or 

 to anaerobic conditions for periods of time up to 1 hour have no effect. 

 The results with pure oxygen in microspores contrasts with the effects 

 observed by Conger and Fairchild (1951), who found that oxygen alone 

 can produce aberrations in pollen grain chromosomes. 



3. Even in the complete absence of oxygen, in so far as this gas can 

 be completely removed from microspore cells, there is still an appreciable 

 aberration frequency. 



