nvbom: vegetativelv propagated species 263 



On the branches formed from the irradiated scions various effects 

 may be observed, primary changes like shoot bifurcations, disturb- 

 ances in the leaf spiral, deformed leaves, and so on, but also fruit 

 changes that may be genetical in nature, like sectors with deviating 

 over-color. Such sectors may be rather common at first, but it is a 

 general experience that if the shoots are allowed to grow undisturbed 

 for several years these sectors eventually disappear. They may get 

 broader later on but at the same time more rare, and only in a very 

 few cases thus far have "stable" or "pure" changes been observed 

 affecting whole branches (10, 12, 45). 



German workers especially have paid much attention to the 

 problem of recovery and isolation of induced changes. A piece of 

 work of great interest in this connection is that of Bauer (6) on black 

 currants. According to his experiences, the first primary shoots com- 

 ing from the irradiated buds only very rarely reveal any mutational 

 changes. But if these shoots are pruned back to the originally irradi- 

 ated stock and new shoots thereby forced to develop, a certain propor- 

 tion of these will show such changes. These second-year shoots may 

 then be removed, transplanted, and new shoots forced to develop 

 again. This was repeated for several years, and after five years of 

 selection in this way not less than 324 aberrant plants had been 

 isolated from the originally irradiated 343 shoots. 



The reason why the unpruned primary shoot (like the Xi potato 

 plant) only exceptionally gives rise to any changes may be explained 

 by intrasomatic elimination which is supposed to take place in an 

 irradiated multicellular organism and leads to an elimination of the 

 mutated cells and a "normalization" of the plant (34, 63, 83). In 

 barley, e.g., the primary roots may contain up to 100 per cent cyto- 

 logically disturbed cells immediately after irradiation; but still, after 

 completed ontogenesis, most of the cells in the spikes and the roots 

 may look quite normal again. When mature buds are irradiated, the 

 changes will form narrow sectors along the primary shoot. When the 

 shoots grow, these sectors run a great risk of being eliminated due to 

 intrasomatic competition. This must be part of the reason for the 

 meagre results of many mutation breeding projects in fruit trees. 



Zwintzscher (112, 113) has adopted a special system in order to 

 unravel the irradiated tissue and to isolate the induced changes in 

 pure condition. The shoot developed from an irradiated bud is cut 



