382 MUTATION AND PLANT BREEDING 



with this, it was postulated that the free electrons resulting from 

 absorption of X-rays, because they can produce a range of ion densi- 

 ties within a sensitive volume, often only break one chromosome at a 

 time. Accordingly, the probability of having two breaks simultane- 

 ously present for exchange would increase at a greater rate than the 

 first power of the dose, with the result that the dose interchange 

 relationship would be exponential. It has since been demonstrated 

 (8) that, with barley seeds, the relation between dose and interchange 

 frequency is linear when the seeds are subjected to both X-rays and 

 thermal neutrons. It is now known that the linear relation obtained 

 with X-rays only occurs in seeds with a moisture content above 10 

 per cent at the time of irradiation or in seeds with water contents 

 below that level when they are hydrated anaerobically immediately 

 after irradiation (see following discussion). From these earlier results 

 (8) it was concluded that only the densely ionizing tails of the X-ray- 

 induced free electrons cause immediate chromosome breakage and 

 that then two chromosomes are usually broken "simultaneously". 



This conclusion was disputed by Wolff and Luippold (30) on 

 the grounds that the scoring of dicentric bridges in root-tip cells was 

 a poor means of determining interchange frequencies. They pointed 

 out that in their analyses there w T as actually an exponential relation 

 between interchange frequency and dose when decentrics were scored 

 at metaphase and a linear relation when they were scored at ana- 

 phase. This particular consideration ignored the fact that the inter- 

 change frequency as detected at the first metaphase division of micro- 

 sporogenesis was also linear (8). A more pertinent and convincing 

 argument should have included data on neutron bombardment, the 

 water content of the seeds at the time of irradiation, the conditions 

 under which the seeds were hydrated after irradiation, and the 

 frequencies of interchanges at microsporogenesis. 



What appears to be a more likely answer to the problem is that 

 the two workers were probably using test materials that had non- 

 uniform water contents and, therefore, differential sensitivities to 

 post-irradiation modification. To develop the significance of this 

 consideration, it will be necessary to diverge briefly to examine one 

 possible explanation as to why post-irradiation effects were observed 

 with X-rays and not with neutrons (8, 12). 



It has been postulated that post-irradiation sensitivity to oxygen 



