572 RADIATION BIOLOGY 



alone would cause breaks, and/or X rays without infrared would cause 

 breaks, the frequency of which would be proportional to a power of the 

 X-ray dose higher than 1. Again, the assumption, required on the break- 

 age view, that infrared promotes breakage but does not affect gene muta- 

 tion, seems less plausible than that it affects union of ends but not break- 

 age or gene mutation (even though it is known that some conditions, e.g., 

 sperm maturity in Drosophila, as shown by Liining, can affect breakage 

 but not gene mutation). A third difficulty for the breakage view arises 

 from the finding of Yost (1950) that whether chromosome or chromatid 

 aberrations are produced depends solely on the stage at which the X rays 

 were applied and not on the stage at which infrared was applied. Finally, 

 the conclusion that very few broken ends remain open in Tradescantia 

 microspores for many hours after treatment has, as previously noted, 

 been challenged by Lane; if this conclusion should not hold, then the 

 chief argument for breakage, based on the long time after X irradiation 

 during which infrared can still be effective, would lose its cogency. 



In any case, however, the promoting effect of infrared on aberrations 

 is undoubtedly a long persisting one, and in that sense at least it consti- 

 tutes a metastable state, as does also the preventing effect of heat shock 

 on aberrations noted by Caldecott and Smith. Whether this effect of 

 heat is related to the one dealt with by Swanson and Yost and whether 

 either of these effects or both are on breakage or on union are matters 

 which cannot yet be regarded as definitely settled. 



Brief consideration may now be given to attempts to influence the pro- 

 duction of mutations by ionizing radiation by means of radiation other 

 than infrared. Some studies of ionizing radiation in combination with 

 ultraviolet are reviewed on pp. 533-534 and others are mentioned on 

 pp. 578-579. The possible reparative action of visible light on mutagene- 

 sis by ionizing radiation has been studied by Kimball and Gaither (1949) 

 in P. aurelia, using X rays, and by Newcombe (1950) in E. coli, using y 

 rays. No reparative effect was discovered, contrary to what these 

 investigators had found for ultraviolet mutagenesis in the same organ- 

 isms. On the other hand, Watson (1950) found that light, given as a 

 posttreatment, does reduce to some extent the killing effect of X rays 

 on bacteriophage, although not nearly as much as it reduces the killing 

 effect of ultraviolet. This suggests that a very searching study might 

 show a small amount of reparation by light to be possible, even for the 

 mutagenic effect of ionizing radiation. As for the effect of radiation of 

 wave length longer than infrared on mutagenesis by ionizing radiation, 

 the only experiments thus far reported are those of Pickhan, Timofeeff- 

 Ressovsky, and Zimmer (1936), who employed very intense radio waves, 

 6 meters in length, without influencing the frequency of sex-linked lethals 

 induced by X rays in Drosophila spermatozoa. 



Mechanical stress in the form of intense sonic vibrations at a rate of 



