556 RADIATION BIOLOGY 



1942; Kaplan, 1951; D'Amato and Gustafsson, 1948), and in some of 

 these cases the interval from the beginning of soaking until irradiation 

 was too short for much change in the developmental stage, morphologi- 

 cally considered, to have occurred. On the other hand, soaking seeds of 

 Antirrhinum or barley after irradiation was found by Knapp and Kaplan 

 (1942) and by Kaplan (1951), respectively, to reduce the frequency of 

 chromosome aberrations (but not with certainty that of point mutations), 

 an effect which Kaplan presumed to consist in a promotion, under the 

 conditions of water content propitious for vital activities, of restitutional 

 as opposed to recombinational unions of broken ends. 



Soaking of pollen of Antirrhinum in 10 per cent sodium chloride for 

 8 hours just before irradiation also raised the frequency of X-ray-induced 

 dominant mutations, recessive mutations of certain types (called the "Q" 

 types), and possibly chromosome aberrations, in experiments of Kaplan's 

 (1939, 1940a, 1946). If the pollen was dried again for 8 hours after soak- 

 ing, before irradiation, the influence disappeared. Moreover, a 33-hour 

 soaking was much less effective than an 8-hour one, presumably because 

 of the intervention of developmental processes of some kind. At the 

 same time, one phenotypic group (called the "R" group) of recessive 

 mutations — which, however, seemed to have a higher spontaneous than 

 induced frequency and w^ere subject to a considerable variance — did not 

 show these effects. Although Kaplan suggests that the two groups of 

 recessives, Q and R, may have differed in the mutational responses of 

 their respective genes or may have represented chromosome changes (Q) 

 vs. gene mutations (R), it would seem premature to base conclusions on 

 their apparent differences in mutational pattern. For the considerable 

 irregularities in the data, together with the existence of so large a pro- 

 portion of noninduced cases in the R group, would •tend in this group to 

 hide such differences as the Q group showed ; this also applies to the chro- 

 mosomal changes, evinced by partial sterility. In AspergillHs spores, 

 Stapleton and HoUaender (1952) found that the frequency of morpho- 

 logical mutants, as well as the damage to survival, caused by X irradi- 

 ation was at all doses (but more especially at the lower ones, where the 

 mutants must have consisted in larger measure of point mutations) con- 

 siderably greater in wet than in dry spores. This difference persisted, 

 although to a reduced extent, even when oxygen was removed. 



All these observations indicate that the presence of water, especially 

 when it contains dissolved oxygen but also without it, is conducive to 

 mutagenesis. This is in line with the ideas originally suggested by Fricke 

 (e.g., Fricke, 1934, 1935; Fricke and Hart, 1936; Fricke, Hart, and Smith, 

 1938), who regarded much of the biological as well as chemical influence 

 of ionizing radiation as due to the formation of active oxygen-containing 

 groups (e.g., OH, H2O2) from water, and from the oxygen dissolved in 

 water. Thus, in the experiments in which Fricke and Demerec (1937) 



