264 MUTATION AND PLANT BREEDING 



back close to the originally irradiated scion so that adventitious buds 

 are forced to develop. The basal buds on the detached shoot may be 

 expected to contain induced, sectorial changes and are, therefore, 

 budded over on new rootstocks, either a certain number beginning 

 from the base, e.g., five, or only those that sit in the axis of a leaf 

 showing primary effects. The primary effects can certainly not be 

 called "mutations" but may perhaps be taken as an indication that 

 the tissue in question was in a sensitive condition during irradiation 

 and that it has not had the chance of getting rid of the changes to 

 the same extent as the buds sitting with normal leaves further to the 

 tip of the growing shoot. The originally irradiated wood may be kept 

 for several seasons as a reservoir of induced changes. 



Grober (45) is using a similar pruning method. After the end of 

 the first growing season, all shoots coming from irradiated buds are 

 cut back on three or four buds, which are then allowed to break next 

 year. 



The types of mutations induced are also briefly indicated in 

 Table 1. On the whole, these mutations are very similar to those 

 known to occur spontaneously in fruit trees, as listed, e.g., by Shamel 

 and Pomeroy (102). From the genetically better explored organisms 

 we know that most mutations, whether induced or spontaneous, con- 

 sist of destructive changes, deformations with more or less reduced 

 vigor, etc. The same is, of course, true of mutations found among the 

 fruit trees. 



From almost all mutation projects, however, there are also 

 reported mutations that may be of practical importance, e.g., the solid 

 red sports of Cortland apple (11); the high-quality, late-ripening 

 Elberta peach (57); mutations with later flowering period in the black 

 currant (6); with earlier ripening in grapes (18); the "self-thinning" 

 changes in the seedless Perlette Grape (85); and certainly the self- 

 fertile cherry mutations (67). 



Most of the mutation breeding experiments in these time- 

 consuming plants have been going on for a relatively short time. We 

 are undoubtedly still in the beginning of the exploration of the muta- 

 tion method, and even if the positive achievements are far from 

 definite, this is of course still less true of failures. Preliminary results 

 are also reported from other mutation experiments with vegetatively 

 propagated plants (16, 23, 69, 70, 73, 88). Of special interest is the 



