used on snapdragons by Noethling and Stubbe in 

 their experiments in 1936. All these experi- 

 ments indicate that rays with voltages from 8 to 

 500 kilovolts, i. e. , the entire range of the X- 

 ray spectrum, induce numerous mutations. 



The results of experiments that attempted to 

 establish the threshold of action on the ultra- 

 violet end of the spectrum are less clear cut. 

 The negative results obtained in experiments 

 with snapdragons and Drosophila indicate that 

 ultraviolet rays can be effective only when they 

 are not absorbed by the surrounding tissues. 



The results obtained by Noethling and Stubbe 

 indicate that pollen grains of snapdragons are 

 particularly suitable for studying the effect of 

 ultraviolet light (Table 34). ^ 



In their experiment, the authors divided the 

 spectrum into the smallest parts possible. 

 Pollen grains transmit a significant percentage 

 of both visible and ultraviolet light and it can be 



'Table 34 does not refer to the effect of ultraviolet 

 light. 



stated with certainty that they also absorb a 

 definite amount of the rays. In comparing the 

 effects of unfiltered rays with monochromatic 

 ultraviolet rays it was discovered that the fre- 

 quency of mutations rises sharply when light 

 from the 280 to 285 millimicron line is used, 

 whereas irradiation of pollen by rays of the 366 

 millimicron line has no particular effect in 

 increasing the frequency of mutations. From 

 this we can conclude that the threshold of action 

 in the ultraviolet area, which induces mutation 

 in the pollen grains of snapdragons, lies in the 

 vicinity of the 313 millimicron line, although it 

 is not possible to establish any effect for the 

 265 millimicron line. In this fashion these 

 experiments on the pollen grains of snapdragons 

 established the thresholds for action of ultra- 

 violet rays in both directions; that is, for 

 longer as well as for shorter rays. 



Stadler and Sprague (1936) showed that when 

 they irradiated the pollen grains of corn with 

 unfiltered ultraviolet rays the frequency of mu- 

 tations rose sharply. 



At the same time we get an answer to the 

 first question posed by Stubbe (see above) 

 "concerning the threshold of wavelength at 



Table 34 



The relationship between dosage and frequency of gene mutations 

 after irradiation of the male germ cells of Antirrhinum majus L. 

 with X rays of various qualities. 

 (based on the data of Noethling and Stubbe, 1934)^ 



Not in 1934 reference cited in list of references. 



99 



