MANNER OF PRODUCTION OF MUTATIONS 567 



age itself was being affected. The same effect might also have been 

 suspected from the finding by Catcheside, Lea, and Thoday (cited by 

 Catcheside, 1948) that the frequency of aberrations shows a strong 

 negative correlation with the temperature existing during the exposure 

 time, even when that time is kept so short that the frequency is little 

 influenced by changes in the time-intensity relation. 



With the demonstration by Thoday and Read of the oxygen effect on 

 aberration production by X rays and the parallel work of Stone and 

 co-workers on ultraviolet mutations, the question arose: To what extent 

 might the temperature effects on high-energy-induced genetic changes be 

 exerted through the influence of temperature on oxygen concentration? 

 The work of Baker and Sgourakis (1949, 1950) and that of Haas et at. 

 (1952), bearing on this matter in so far as the production of lethals in 

 Drosophila was concerned, have already been cited. Evidence regarding 

 the same ciuestion, in its relation to the prodviction of chromosome aberra- 

 tions in Tradescantia pollen, was obtained by Faberge. He had found 



(1948) that ripe Tradescantia pollen survived freezing in liquid air 

 ( — 192°C) and that at this temperature a given dose of X rays produced 

 only about a fifth as many chromosome breaks (as scored at a later 

 stage) as it did at 25°C. This is quite in contrast to the increase observed 

 when the temperature at irradiation is lowered from 25° to 2°C. Faberge 



(1949) further noted that, as the temperature is lowered from a little 

 above freezing, there is a distinct dip in mutation production at 0°C and 

 that further lowering of the temperature causes a gradual decrease in 

 mutation production. He called attention to the correspondence between 

 his curve relating temperature to breakage frequency and the curve 

 observed by Bonet-Maury and Lefort (1948) relating temperature to 

 hydrogen peroxide production in water containing oxygen. He took 

 these results to mean that breakage had been influenced through an 

 effect involving the oxygen present and that the atomic changes responsi- 

 ble for the affected breakages had been initiated in material other than 

 that which finally underwent breakage, in opposition to the target 

 hypothesis in its strictest form. At the same time, he noted that there is 

 still a residuum of breaks even at temperatures so low (below — 116°C) 

 that hydrogen peroxide production in water has completely stopped. 



A similar pronounced decrease in the killing effect of X rays on digitalis 

 and lupin pollen and fern spores, when irradiated at liquid-air tempera- 

 ture, was noted by B. Rajewsky (cited by Howard, 1950). Strangely, 

 Rajewsky found that, when the spores were at the same time dried beyond 

 a certain degree, the effect was again somewhat increased. On the other 

 hand, Faberge (1950) reported that at liquid-air temperature the substi- 

 tution of nitrogen for oxygen during X irradiation did not affect the fre- 

 quency of breaks, although, as expected, it decreased their frequency at 

 temperatures above freezing. 



