retarded development, growth proceeded quite 

 normally. From the irradiated seeds 8230 were 

 selected; 27. 42% turned out to be mutants. The 

 irradiated sprouts^ gave 29. 17% mutants. 



As has already been pointed out, Stubbe 

 [1930b] was dealing with an extraordinary object, 

 since these snapdragons had for a great number 

 of years been under Baur's care. The latter 

 developed many pure strains of this plant and 

 studied many genes which determined its char- 

 acteristics and properties. Baur's long experi- 

 ments revealed, incidentally, that each strain 

 has its own coefficient of mutation. Strain "50, " 

 which Stubbe used for his experiments, had a 

 relatively low coefficient of mutation, only 1%. 

 By the use of high doses of X rays this coeffi- 

 cient was raised up to 300%. 



In these same experiments Stubbe established 

 the dependence of the mutation frequency on the 

 size of the dose. The table presented below 

 proves this, since for all doses except 400 r the 

 number of mutations increases as the dose is 

 raised (Table 21). 



In a subsequent article Stubbe [1932] solved 

 the problem of which developmental stage is 

 most sensitive to Xrays. With this aim in mind 

 he studied the X3 generation and discovered that 

 the majority of the gene mutations occurred in 

 those buds which at the time of irradiation had 

 finished reduction-division. From 80 mutations, 

 58 descended from such flower buds and only 22 

 from buds which were undergoing reduction - 

 division [at the time of irradiation] or had not 

 yet undergone it. 



Already in his first experiments Stubbe dis- 

 covered that the frequency of mutations after 

 irradiation is not identical for both sexes of 

 snapdragons. This caused him eventually (in 

 1933) to set up experiments with irradiation of 

 mature male sex cells. He thought that if this 

 stage of male haplophase showed the same high 

 X-ray sensitivity as immature germ cells, it 



^Ed. note: 3438 irradiated seedlings were selected. 



would be of considerable advantage to the 

 experimenter. In the first place, a desired 

 uniformity in the stage of development for 

 irradiation would be obtained. In the second 

 place, there would be no need to worry about the 

 absorption of soft rays by somatic tissues. In 

 the third place, an exact count of the frequency 

 of gene mutations could be made. 



The methodology of the experiments consisted 

 of the following: pollen from flowers that had 

 just bloomed was placed in a thin layer on 

 parchment paper and, immediately after irradi- 

 ation, was transferred to the stigmas of emas- 

 culated plants. For irradiation he used both 

 soft (30, 50, and 70 kv with filters) and hard 

 (125 and 175 kv with filters) rays. The results 

 of this experiment were amazing: in both parts 

 of the experiment completely unexpected, but 

 identical curves for the frequency of mutations 

 were obtained. The curves rose up to 400 r, 

 then fell to 1600 r, and after 3200 r rose again. 

 From this it seems clear that even such great 

 differences in voltage as 10 and 175 kilovolts 

 do not have any effect on mutation frequency, 

 that everything depends on the quantity of 

 roentgens. These curves are the result of two 

 completely independent biological processes. 

 On the one hand, they are an expression of the 

 relationship existing between doses of irradia- 

 tion and frequency of mutation; on the other 

 hand, they are due to the gradual injury and 

 killing off of the pollen grains by increasing 

 doses of X rays. 



The explanation of the drop in the curve be- 

 tween 400 and 3200 r, according to Stubbe, lies 

 in the fact that the radiation affects areas of 

 different sensitivity within the nucleus of the 

 cell — some of which give mutations with low 

 doses (up to 400 r), while others produce muta- 

 tions only with doses of around 3200 r. This 

 explanation by the author is confirmed by the 

 fact that labile loci have been found in all of the 

 fully investigated objects ( Drosophila , corn, 

 etc). Consequently, it is no surprise that they 

 should vary more frequently than other areas 



Table 21 



The relationship between dose and frequency 



of gene mutations after irradiation of buds. 



(based on Stubbe' s data, 1932) 



45 



