270 CHROMOSOME ABERRATION PRODUCTION 



suited in a marked decrease in aberration frequency. Similar results 

 were obtained by Hay den and Smith (15) in experiments with barley 

 seeds. Experiments on the effect of oxygen have also been performed 

 with Tradescantia, and the results of some of these will now be discussed. 



The Effect of Oxygen on X-Ray-Induced Chromosomal 

 Rearrangements in Tradescantia 



The experimental methods used to expose Tradescantia inflorescences 

 to x-radiation while in atmospheres containing various percentages of 

 oxygen have been described in some detail by Giles and Riley (11, 12) 

 and will be outlined only briefly here. Inflorescences were placed in an 

 appropriate holder inside an airtight Lucite exposure chamber. This 

 chamber was placed inside the x-ray machine and connected through 

 ports by pressure tubing and appropriate valves to a vacuum pump, a 

 gas pressure cylinder, and a mercury manometer. Air in the chamber 

 could be evacuated and replaced by the appropriate gas or gas mixture 

 from the cylinder. The chamber could also be maintained under vac- 

 uum, or under pressures up to 3 atm above normal atmospheric. Rapid 

 introduction or removal of gas could be effected with the apparatus. 

 All these manipulations could be carried out before, during, or after ir- 

 radiation, depending on the experimental conditions desired. The x-ray 

 intensity for each exposure was determined by means of a Victoreen 

 thimble ionization chamber which could be inserted into the box in the 

 same position as that normally occupied by the inflorescences. 



A series of dosage curves was obtained for inflorescences exposed in 

 air, in oxygen, and in nitrogen. The yield of both interchanges (di- 

 centrics and rings) and interstitial deletions was markedly reduced when 

 nitrogen replaced air in the chamber, and increased somewhat when 

 oxygen replaced air. Additional comparative exposures were made in 

 other gases, such as helium and argon, and also under vacuum. In all 

 instances, reduced aberration frequencies similar to those obtained in 

 nitrogen resulted, indicating that the absence of oxygen was responsible 

 for the decrease in radiosensitivity (11). No chromosomal effects were 

 noted in control experiments in which similar exposures to nitrogen and 

 oxygen were made without irradiation. It was clear from these resuhs 

 that the presence of oxygen resulted in a marked increase in aberration 

 frequency. 



The next problem was to determine the reason for this effect of oxygen. 

 If it is assumed that all breakage is the result of direct-hit effects, the 

 same number of breaks should be produced in the presence or absence 

 of oxygen by a given x-ray dose. Thus the increased aberration fre- 



