chromosome aberrations in Tradescantia 741 



In these experiments the X-irradiation of microspores during the resting 

 stage did not increase the subsequent sensitivity of the chromosomes in 

 these same cells when they were X-rayed in prophase. In view of these 

 results it appears improbable that infrared treatments following X 

 irradiation would be more likely to produce additional true breaks. 



It is possible that these difficulties may be resolved and a better under- 

 standing of the mechanisms of the infrared effect be achieved as a result 

 of future research based on experiments briefly reported by Swanson and 

 Yost (1951). These authors find that a heat shock applied after infrared 

 exposure, and before X irradiation, will remove the infrared effect in 

 Tradescantia microspores. They suggest that infrared may act to pro- 

 duce a metastable state in the chromosome which renders it more sensitive 

 to X-ray breakage, and that this state is replaced by the normal state 

 following heat shock. They also state that a similar effect is obtained 

 when the order of treatment is X rays, heat, infrared and suggest that 

 X rays, too, may produce, in addition to the usual breaks, incipient breaks 

 or metastable states that are acted upon by infrared. 



TEMPERATURE 



As might perhaps be expected, temperature was the first modifying 

 factor whose effect was studied in Tradescantia microspores. Sax and 

 Enzmann (1939) demonstrated that the yield of chromatid aberrations of 

 all types was greater at low than at high temperatures for a given X-ray 

 dose. They interpreted these results as indicating an effect of tempera- 

 ture on the recovery process, such that restitution is favored at high and 

 reunion at low temperatures, since it was supposed that at low tempera- 

 ture a break would remain open longer allowing the broken ends a greater 

 chance, following chromosome movement, of undergoing reunion as 

 opposed to restitution. Although there was evidence in the early experi- 

 ments of an effect of postirradiation temperature changes, later experi- 

 ments failed to demonstrate this effect, and Sax (1947) concluded that 

 the temperature of the cells at the time of irradiation was the factor of 

 major importance. The general observations of Sax and Enzmann were 

 confirmed in independent investigations by Faberge (1940) on X-ray- 

 induced " fragments" and by Catcheside, Lea, and Thoday (1946b) on 

 chromatid aberrations, although the latter investigators failed to find 

 much temperature effect on chromatid deletions. Darlington and 

 La Cour(1945) have maintained that most or all of the temperature 

 effect in modifying aberration frequency is an indirect one resulting from 

 an influence of this factor on the timing of the nuclear cycle, and thus on 

 chromosome sensitivity to radiation. This objection has been met by 

 Sax (1947) and especially by Catcheside (1948), who showed that the 

 relatively brief exposures to high or low temperatures at the time of 

 irradiation had little or no effect on the relative temporal positions of the 



