'584 MUTATION AND PLANT BREEDING 



ent workers' findings and the data were very largely ignored. Added 

 to this was the fact that, at the times these studies were conducted, 

 the test materials used in most radiobiological studies were not suit- 

 able for demonstrating the influence of environment on radiosensi- 

 tivity and there was general skepticism concerning reports that the 

 damage to biological systems could continue after the cessation of 

 irradiation. Indeed, this particular opinion was expressed very 

 recently, as far as a post-irradiation effect of oxygen is concerned (3). 



These two reasons undoubtedly account for the fact that little 

 consideration was given to the work of Kempton and Maxwell (17), 

 who demonstrated that maize seeds X-rayed at either the temperature 

 of liquid air or at 61° C evidenced less radiosensitivity than seeds 

 X-rayed at room temperature. 



Now that some of the environmental parameters which affect 

 radiosensitivity can be controlled, and it is established beyond any 

 reasonable doubt that this applies to damage to the chromosome, 

 speculation is warranted concerning the biophyiscal mechanisms that 

 are involved. 



Of the possibilities that might be examined, three seem worthy 

 of mention. First, that all damage to the chromosome results from the 

 direct absorption of energy. This does not impugn the possible sig- 

 nificance of intramolecular energy transfer mechanisms. Second, that 

 all damage to the chromosome results from absorption of energy by 

 the ambient layer, the effect on the chromosome being initiated by 

 active radicals or some other mechanism for transferring energy. The 

 following discussion would appear to rule out the need for further 

 consideration of this possibility. Third, that damage to the chromo- 

 some results from a combination of the first two kinds of events. 



For the reasons indicated (loc. cit.), it seems reasonable to con- 

 clude that the energy released in regions of high specific ionization 

 is more than sufficient to break a chromosome at the initial center of 

 absorption. This particular type of event would not likely be modi- 

 fiable by the changes in the physical structure of the chromosome that 

 could conceivably result from high temperature treatment or the 

 other conditions that have been imposed on seeds during the course 

 of this work. Data supporting this statement have been obtained in 

 the study relating to the nature of the interchange-dose relationship 

 (Figure 5) when seeds were hydrated in the presence of oxygen and 



