which mutations can be induced. " The answer 

 is that the threshold is determined by the ability 

 of the rays to penetrate [the tissues] ; and that 

 in order to induce mutations any source of rays 

 may be used, beginning with ultraviolet rays. 



Stubbe considers that the second question 

 concerning rays of different qualities is also 

 answered, since no statistically significant 

 differences in results were obtained when iden- 

 tical doses were used in the region of X rays, 

 from softest to the hardest rays. In plants this 

 was confirmed by experiments with snapdragons, 

 in which equal doses of soft and hard X rays 

 (as well as gamma rays of radium) induced 

 identical frequencies of mutations. In other 

 words, it can be considered established that 

 the inductive action of X rays does not depend 

 on wave length. This conclusion is splendidly 

 confirmed by the table taken from Stubbe 

 (Table 35). 



This table shows that when equal doses of 

 radiation are used (3000 r in one case and 6000 

 r in another), hard X rays (506 kv) and soft 

 ones (180 kv) produce equal increases in the 

 number of mutations as compared with the 

 control. It should be noted that the number of 

 mutations induced by 6000 r is nearly 3 times 

 as great as that produced by 3000 r. 



Now let us turn to the question raised in 

 1928 by Ancel as to whether administration of 

 doses in fractions affects the appearance of 

 mutations. The most clear cut experiment set 

 up for clarification of this point was also that of 

 Stubbe who tested the difference between the 

 action of rays of different qualities in concen- 

 trated doses and in doses which had been di- 

 vided into three fractions and administered at 

 24 -hour intervals (Table 36). 



All these experiments indicate, despite the 



indications of earlier experiments to the con- 

 trary, that neither the quality of the rays nor 

 the fractionation of doses has any effect on the 

 number of mutations but that the only signifi- 

 cant factor is the size of the dose (i. e. , the 

 number of r). 



This was confirmed by our experiments with 

 rye, which revealed that there is no difference 

 between hard and soft rays in relation to stim- 

 ulation or inhibition (Breslavets, Medvedeva, 

 and Afanas'eva, 1935). 



One more very interesting question remains, 

 namely, the nature of gene mutations obtained 

 by experimental means. In his very first work, 

 Muller noticed the identical nature of spontane- 

 ous mutations and those obtained by experi- 

 mental means. This is confirmed for all the 

 objects examined. The numerous gene muta- 

 tions obtained through the action of X rays in 

 snapdragons, corn, and Drosophila had all been 

 encountered previously among spontaneous mu- 

 tations. They possess the same properties, 

 affect all the organs and the physiological 

 properties of the organism, and are likewise 

 usually recessive, very few being dominant. 



The first indication of differences in the 

 nature of spontaneous and experimentally ob- 

 tained mutations crops up in Stubbe' s experi- 

 ments (1932*- 1933). The first experiments 

 showed that after irradiation of the buds the 

 percentage of gene mutations in the vegetative 

 organs rose sharply, in comparison with the 

 controls, whereas the percentage of mutations 

 that cause changes in the shape of the flowers 

 did not rise. In another experiment, after 

 irradiation with soft and semi -hard rays, it 

 was found that as the dose was increased the 

 percentage of previously encountered mutations 

 diminished, while the number of new, i.e., not 

 previously encountered, mutations increased. 



Table 35 



The action of equal doses of radiation 

 of various qualities that induce mutations 

 (based on Stubbe' s data, 1937) 



100 



