172 MUTATION AND PLANT BREEDING 



place to the same extent as when they are initiated immediately after 

 irradiation. Thus the potential mutations have been stabilized by the 

 amino acids and they have neither been fixed nor have they disappeared. 

 Furthermore, if one interrupts the process of mutation fixation at some 

 intermediate level and then subjects the cells to conditions causing 

 mutation frequency decline, no decline is obtained but neither is any 

 further mutation fixation. Similarly, if at some intermediate level of 

 mutation frequency decline one interrupts this process and introduces 

 conditions favorable for mutation fixation, no increase in the frequency 

 of mutants fixed above this level is obtained and neither is there any fur- 

 ther mutation frequency decline. It is quite apparent that in the former 

 case, where mutation fixation was being measured, the mutations already 

 fixed were not subject to processes causing mutation frequency decline, 

 while the fraction remaining unfixed was equally susceptible to either 

 mutation fixation or mutation frequency decline. In the latter case, 

 when mutation frequency decline was being measured, the potential 

 mutants that have disappeared cannot be made to reappear by intro- 

 ducing conditions favorable to fixation, and the fraction which has 

 not disappeared is as susceptible to mutation frequency decline as to 

 mutation fixation. 



