282 MUTATION AND PLANT BREEDING 



frequency obtainable by acute irradiation. For some plants, e.g., 

 barley, we know now that this does not seem to be possible (84). Also, 

 the results of Cuany, et al. (25) do not point to any conspicuous 

 advantage of chronic irradiation over acute. 



Obviously even growing plants are able to eliminate a certain 

 fraction of the induced changes. Therefore, as pointed out by Sand, 

 et al. (97), the "pertinent dose", i.e., the dose corresponding to the 

 number of recovered mutations, is "considerably less than that admin- 



7 J 



istered to the whole plant as chronic irradiation during its develop- 

 ment". Even though further studies on the possible differences in 

 efficiency between chronic and acute exposures are required, there 

 are no results yet to indicate any disadvantage of the more convenient 

 method of acute or semi-chronic irradiation. 



Consequences of Chimaeric Structure of Somatic Mutations 



There are certain consequences of this fact that most of the 

 induced and, probably to a still higher extent, the spontaneous 

 somatic mutations are propagated as periclinal chimaeras (7). 



One practical consequence is that some of the induced muta- 

 tions will be more or less unstable, more so perhaps if the new geno- 

 type has a reduced competitive ability compared with the original 

 one. Another consequence, also mentioned earlier in this paper, is 

 that most of these mutations when used in crosses will turn out not 

 to transmit the changes to the offspring unless they also comprise 

 the subepidermal cell layer giving rise to the gametes. 



However, irradiation seems to constitute a very efficient tool for 

 bringing about tissue recombinations so that di-ectochimaeras or 

 tissue-homogeneous plants are produced. If this succeeds, we may 

 then expect these new types to be both more stable and perhaps 

 more pronounced in phenotypical expression. As far as the geno- 

 typical change itself does not impair the fertility or viability of the 

 subepidermal mutants, they should then also become accessible for 

 further breeding work by means of crossing. The spontaneous muta- 

 tions isolated hitherto present in this way a very attractive and com- 

 pletely unexplored raw material for further irradiation experiments. 



Still another consequence of this chimaeric structure is that it 

 permits the use of such changes that, when tissue-homogeneous, they 

 would lead to complete sterility or complete inviability of the plant. 



