OXYGEN EFFECT WITH X-RAYS 



275 



of the determinations. A graphical summary of the results is presented 

 in Fig. 1, together with the averages of points obtained at higher per- 

 centages of oxygen in previous experiments. It is clear that there is still 

 a substantial yield of aberrations even in the complete (or nearly com- 

 plete) absence of oxygen. When oxygen is present in irradiated cells, 

 there is a rapid rise in aberration frequency above this base level. This 



10 20 30 40 50 60 70 80 90 



Percentage of oxygen in exposure chamber 



(normal atmospheric pressure) 



Fig. 1. Reproduced by permission from Science, 112: 643, 1950. 



increase is linear between and 10 per cent oxygen, after which the rise 

 is apparently more gradual. 



Certain additional experiments (Giles and Beatty, unpublished) pro- 

 vide further evidence that the amount of dissolved oxygen present in the 

 cells is an important factor in determining aberration frequency. In 

 these experiments, a constant percentage of oxygen was used in the ex- 

 posure chamber, but irradiations were carried out with the inflorescences 

 under pressures up to 3 atm above normal. The data obtained for ex- 

 posures at 0, 1, 2, and 3 atm above normal pressure (approximately 740 

 mm of Hg) in 10 per cent oxygen (plus 90 per cent helium) have been in- 

 dicated in Fig. 2, on the assumption that the amount of effective dis- 

 solved oxygen in the cells is directly proportional to the pressure. Con- 

 trol experiments in which comparable exposures were made in helium 

 under pressure indicated that pressure alone did not change the aberra- 

 tion frequency. As can be seen from the graph (Fig. 2) there is good 

 agreement with previous exposures in different percentages of oxygen 



