268 MUTATION AND PLANT BREEDING 



the roots. Somewhere between, we then have the optimal dose, which 

 may vary from time to time, depending on kind of fruit, chromosome 

 number, condition of the trees, and so on. Through repeated pruning, 

 first in order to remove all growth from the lower portion of the tree 

 leaving only the uppermost three living buds, and then again the 

 next year back to the basal buds of the surviving shoots, we try to 

 "dissolve" the tissue having got the most suitable irradiation dose. 



The technique described might perhaps be improved by iso- 

 lating the basal buds of the first year's growth over onto clonal root- 

 stocks. The trees that have been exposed to irradiation or into which 

 a lot of irradiated scions have been grafted usually will be rather irreg- 

 ularly developed, and it is extremely difficult to identify these minor 

 changes that may be of the greatest practical interest, such as changes 

 in color, period of ripening, size or form of fruit, etc. When raising 

 new trees on clonal rootstocks by taking buds from previously irradi- 

 ated material, one might expect more uniform tree material among 

 which it should be easier to trace slighter changes. 



We have also irradiated summer buds of apples and pears which 

 have then been budded onto suitable rootstocks. Figure 4 shows such 

 a shoot coming from an irradiated bud. About the same result would 

 be obtained by irradiating dormant winter wood and grafting it into 

 a rootstock just above soil level. After pruning the next year, such a 

 mutated bud would result in a more or less completely changed tree 

 without primary effects and other disturbances in growth habit. We 

 have some such material with irradiated Williams pears among which 

 numerous minor changes have been noted. However, the deviating 

 trees must be tested further, budded over onto new rootstocks, togeth- 

 er with control material, before we can be sure of the true nature of 

 the changes. Some drastic aberrations have been recorded, however, 

 like the irregularly corrugated mutants in Figure 5 and the "seedless" 

 mutant in Figures 6 and 7. 



The irregularly furrowed fruits obviously are homologous to the 

 spontaneous sport Corrugated Bartlett (101) and probably also to the 

 irregular apple types described by Gilmer and Einset (41). A mutant 

 in Cortland, quite similar to the neutron-induced one of Einset 

 and Pratt (37), has been found in the same variety after gamma- 

 irradiation, Figure 8. 



The seedless Williams type also deviates by its more elongated 



