278 MUTATION AND PLANT BREEDING 



basal leaf, then when the tuber is formed on the stolon, and finally 

 when the eyes are formed on the X 2 tubers. 



However, in other plants, like fruit trees, the irradiated bud 

 may grow vigorously, several feet long into a shoot where the mutated 

 sectors are probably found only at the basal buds. These buds are 

 usually sitting close together, and normally they never take up growth 

 again due to the apical dominance. These conditions, which have 

 already been briefly touched upon in connection with the survey 

 of the fruit tree experiments, are no doubt the main reason 

 for the differences in mutation yield between different mutation 

 experiments. 



Genetic Background of Somatic Mutations 



With few exceptions, the genetic background of stable somatic 

 mutations have not been the object of closer studies. This certainly 

 is due to the fact that most of the plants concerned are very incom- 

 pletely investigated genetically. An exception is formed by the studies 

 of Blakeslee and Avery (14) on spontaneous changes in Datura. These 

 were practically all found to be due to chromosome changes, not to 

 gene mutations. Also in chrysanthemum spontaneous mutations have 

 been found often to be associated with changes in chromosome 

 number (32, 96). 



The studies on radiation induced somatic aberrations in the 

 endosperm of maize carried out by Dollinger (31) showed that most 

 of these were losses from a genetical point of view, and that they were 

 mostly associated with chromosome structural changes and deletion 

 of certain chromosome segments. Some of them, however, were not 

 cytologically detectable. This information on the cyto-genetic nature 

 of these changes is also consistent with the repeated finding that most 

 of them are eliminated at meiosis (97). 



Although we know very little from other plants, it seems reason- 

 able to assume that conditions would be about the same, namely, 

 that most, though not all, of the induced somatic changes are made 

 up of gross cytological changes with phenotypical effects, most of 

 them, perhaps, simply being losses or duplications of gene material. 



By experience we know that most of the radiation-induced muta- 

 tions studied in seed-propagated plants are recessive, only quite few 

 clear-cut dominant or semi-dominant induced changes having been 



