MANNER OF PRODUCTION OF MUTATIONS 575 



produced by radiation, such effects are not surprising on the view that 

 the process of radiation mutagenesis involves this pathway. This was 

 pointed out by Rapoport in 1943 in his prophetic paper reporting that 

 iron promotes radiation mutagenesis in Drosophila, whereas lead in his 

 experiments had no appreciable effect. This method of action, as well 

 as the specific absorption by the heavy element, must be borne in mind 

 in considering any results obtained in work of this kind. Thus, the 

 findings by D'Amato and Gustafsson (1948) that uranyl nitrate and 

 ferric sulfate impregnations of barley seeds cause a rise in their X-ray- 

 induced mutation rate probably belong in the same category. 



The pH of the intracellular medium also greatly affects the mode of 

 disposition of the dissociation products of water and might thereby affect 

 the frequency of mutations and chromosome breaks induced by ionizing 

 radiation. Thus Marshak (1938a, b, c) found that ammonia, when pres- 

 ent during irradiation, decreased the frecjuency of aberrations in root-tip 

 mitoses, whereas carbon dioxide seemed to be without an effect on them. 

 Similarly, D'Amato and Gustafsson (1948) found that, when barley seeds 

 were presoaked in alkaline buffer (of pH 10), the frecjuency of sterility, 

 indicative of chromosome aberrations, was distinctly decreased but that 

 it was probably also decreased somewhat by acidity (pH 3). With these 

 agents the frequencies of induced gene mutations seemed to run parallel 

 with those of the aberrations, but these data were not so significant. 

 Since attempts to change the hydrogen-ion concentration produce so 

 many indirect effects in the complicated medium which protoplasm con- 

 stitutes, it is not easy to determine the pathway by which such changes 

 in aberration and mutation rate are brought about. 



In view of this situation it is not surprising that some agents have a 

 different effect on the frequencies of the aberrations and the point muta- 

 tions produced by ionizing radiation. Results of this kind for infrared in 

 Drosophila and for heat shocks in barley seeds have already been men- 

 tioned. Prior to the work with heat shocks a similar differentiation of 

 effects had been obtained on barley seeds by Kaplan (1950c, 1951) by the 

 use of chemical treatments before X irradiation. As in the earlier work 

 by Marshak on directly treated root tips and of D'Amato and Gustafsson 

 on barley seeds, presoaking with ammonium hydroxide (0.063 per cent) 

 was found to cause a reduction in the frequency of aberrations. This 

 was evidenced both in the data on the sterility of the plants which grew 

 from the irradiated seeds and in the numbers of chromosome aberrations 

 observed in their root tips. At the same time, however, the frequency 

 of visible recessive mutants obtained as segregates was not reduced but 

 was raised slightly, although not significantly. Carbon dioxide satu- 

 ration of the water in which the seeds were presoaked likewise lowered 

 the aberration frequency distinctly, and again the mutation frequency 

 instead of being reduced appeared to be slightly higher. On the con- 



