744 RADIATION BIOLOGY 



certainly a possible interpretation (to be discussed later), then the 

 temperature effect may result from an influence on either the production 

 or, more likely, the effectiveness of the OH radicals. The discovery of 

 this reversal of the temperature effect in the absence of oxygen makes it 

 unlikely that the effect of temperature is to be attributed solely to a 

 modification of the behavior of broken chromosome ends during the 

 recovery process. Hollaender et al. (1951) have reported preliminary 

 experiments indicating a similar reversal of the temperature effect with 

 X-ray-induced killing of bacteria. 



Faberge (1948, 1950) has investigated the effect of X irradiation of 

 pollen at very low temperatures and finds that the breakage frequency 

 at — 192°C is only about one-fifth that at +25°. Furthermore, the sensi- 

 tivity curve for intermediate temperatures resembles, in general character, 

 that for hydrogen peroxide production plotted against temperature when 

 water containing oxygen is X-rayed (Bonet-Maury and Lefort, 1948). 

 Experiments have been performed also to investigate the combined effects 

 of temperature and of nitrogen (Faberge, 1950 and unpublished). At 

 + 25°C the aberration frequency is reduced in nitrogen, as compared with 

 air. At — 192°C there is a reduction in aberration frequency in air com- 

 pared with +25° as previously observed; and in nitrogen, sensitivity is 

 still further reduced by about one half. Faberge feels that nitrogen and 

 very low temperature both reduce the sensitivity by the same amount, 

 but through independent mechanisms. 



OXYGEN 



The experiments of Thoday and Read (1947), using root tips of Vicia 

 faba, demonstrated that oxygen has a marked effect on chromosome sensi- 

 tivity to X radiation as measured by induced aberrations visible at 

 anaphase. The absence of oxygen during X irradiation reduced the 

 aberration frequency to about one-third that observed when the irradi- 

 ation was performed in the presence of air. Subsequent studies by the 

 same investigators (1949) showed that the oxygen effect was markedly 

 less when a-particle radiation was used. 



A series of investigations (Giles and Riley, 1949, 1950; Giles and 

 Beatty, 1950; Giles, Beatty, and Riley, 1951; Riley, Giles, and Beatty, 

 1952; Giles, Beatty, and Riley, 1952; and Giles, 1952, and unpublished) 

 has been carried out utilizing Tradescantia inflorescences in order to con- 

 firm and extend these observations. In most of the experiments special 

 exposure chambers have been used to make possible the rapid removal or 

 introduction of various gases either before, during, or after irradiation 

 and to control the temperature of the inflorescences during exposures. 

 Most of the cytological observations have been confined to chromosome 

 aberrations — interchanges and interstitial deletions. However, some data 

 are also available for chromatid effects. The major experimental con- 



