576 RADIATION BIOLOGY , 



trary, acetic acid (0.5 per cent) markedly raised the aberration frequency, 

 nearly doubling it, unlike what had been reported for acidity by D'Amato 

 and Gustafsson, while, as before, the mutation frequency was raised but 

 only slightly (in this case by an amount which was probably significant). 

 Since, as Kaplan points out, the differences in aberration frequency may 

 have been brought about through effects on the process of union, these 

 results cannot yet be used as evidence of a difference between the proc- 

 esses causing gene mutation and those causing chromosome breakage. 



In the work by D'Amato and Gustafsson (1948) on barley seeds, one 

 of the agents used had shown a difference in the direction of the effects 

 on mutation and aberration; this was potassium cyanide. It is true that, 

 when this substance was used in low concentrations (0.0001 and 0.001 M) 

 for presoaking the seeds, the frequencies of both the chromosome aber- 

 rations and the visible mutations produced by X rays were altered in the 

 same direction, i.e., increased. This result is more or less in Une with 

 the expectation based on the inactivation of cytochrome and other heavy- 

 metal-carrying enzymes by cyanide, although it may be recalled that, in 

 Neurospora, cyanide did not show synergism with ultraviolet but did act 

 as a mutagen by itself. However, when more concentrated potassium 

 cyanide (0.01 M) was used on barley seeds, although the aberration 

 frequency was further increased, there was an actual decrease of the fre- 

 quency of visible mutations below the value obtained with X rays alone. 

 As King et at. (1952) have pointed out, this decrease corresponds with the 

 strong alkalinity (pH > 10) caused by so high a concentration of potas- 

 sium cyanide inasmuch as other experimental series of the barley seeds 

 had shown that their visible-mutation rate decreased with alkalinity. 

 This consideration does not, however, answer the question as to w^hy the 

 chromosome-aberration frequency was increased with this same treat- 

 ment. Possibly the answer to this, as to Kaplan's similar results, is to 

 be found in an effect on the union of broken ends, but this can by no 

 means be taken for granted as yet. 



Various other chemical influences on the frequency of production of 

 point mutations by ionizing radiation have been reported from time to 

 time. Thus Stubbe (1940), who had found an effect of undernourish- 

 ment (through an undersupply of either phosphorus, sulfur, or nitrogen) 

 in raising the spontaneous rate of visible mutations in Antirrhinum, 

 reported a similar effect in connection with the X-ray mutation rate. 

 Dotterweich (1939, 1940) and Dotterweich and Schmidtke (1941a, b) 

 reported that various substances, namely, pilocarpin, caffeine, the follicle 

 hormone progynon, and even sea water, if fed to Drosophila over one or 

 more generations, increased the X-ray mutation rate, but there were 

 peculiarities in the data which open them to question, and a repetition of 

 the experiment with pilocarpin by Rapoport (1943) and of that with 

 progynon by Kanellis (1943) gave negative results. Such work in 



