MANNER OF PRODUCTION OF MUTATIONS 553 



1938). On the other hand, Marqiiardt and Ernst (1940) obtained a much 

 higher frequency of mutations by X irradiation of immature than by 

 X irradiation of mature pollen of Antirrhinum. Knapp (1938) found the 

 meiotic divisions more susceptible to mutation production than the com- 

 pleted germ cells of Sphaerocarpus. D. Lewis (1948, 1949), investigating 

 mutations of the locus for self-sterility, in pollen mother cells of Oenothera 

 and of Prunus, found that both the spontaneous- and the X-ray-induced 

 mutabilities vary greatly with the stage of meiosis and are especially high 

 in the neighborhood of metaphase. 



In addition, some significant although usually slight differences in 

 induced mutability were long ago found between comparable species and 

 between stocks of the same species (Chap. 7). Large differences of this 

 kind, some of which have been found to arise by a sudden genetic change, 

 have been reported in bacteria (see, e.g., Witkin, 1947; Bryson, 1947), 

 but some of these may be only apparent changes in mutability, caused 

 by a diploidization which conceals the mutations. Considerable intra- 

 specific differences of genetic origin in regard to resistance to the killing 

 effect of ionizing radiation have been noted in widely distinct types of 

 organisms. There is evidence of the Mendelian nature of some of these 

 differences, e.g., in a case found by Smith (1942) in wheat. 



Another genetic influence on X-ray mutability, reported by Stubbe 

 (1935), was apparently exerted by the plasmagenes or plastogenes of the 

 fireweed Epilobium luteum on the chromosomal genes of Epilobium hir- 

 sutum. In plants in which this combination had been brought into exist- 

 ence by a succession of backcrossings of hybrids having an E. luteum 

 ancestor in the all-maternal line to E. hirsutum pollen, the induced- 

 mutation rate was higher by an amount which verged on the significant. 

 Belgovsky (1934) found a probably significant increase in the frequency 

 of induced mutation occurring in the normal allele of white derived from 

 Drosophila simulans when this was present in the optic anlagen of D. 

 melanogaster-simulans hybrids, as compared with its mutability in "pure" 

 D. simulans. Such effects, no matter whether or not attributable exclu- 

 sively to chromosomal genes, fit in with the interpretation that disturb- 

 ances in normal functioning, regardless of their cause, would be expected 

 oftener to raise than to lower the radiation-induced-mutation rate, just 

 as they appear more often to raise than to lower the spontaneous-muta- 

 tion rate; for low rates would, on the whole, reciuire a more highly organ- 

 ized system than high rates and would, on the whole, have been more 

 favored by natural selection (see Chap. 7). 



It had very early been observed that tissues in which there was more 

 mitotic activity were more damaged by radiation (e.g., Bohn, 1903; 

 Apolant, 1904; Bergonie and Tribondeau, 1904, 1906; Levy, 1906; 

 Mottram, 1913). Moreover, various early observers had noted that 

 radiation resulted in abnormal mitoses, including fragmentation, lagging. 



