nybom: vegetatively propagated species 275 



As pointed out already by Asseyeva and de Mol and also verified 

 in later experiments, especially those of Howard (58) and Pratt (87), 

 irradiation seems to be a potent means of producing such tissue 

 recombinations, not only "stripping off" external mutated tissue, but 

 also giving rise to solid mutants (Figure 12). Therefore, we have this 

 year irradiated some ectochimaeric mutations in order to make them 

 available for future breeding work, namely, the blackberry sport 

 Thornless Evergreen and a colchicine-induced ecto-chimaera in 

 Prunus cerasifera. The latter was irradiated as summer buds, whereas 

 the blackberry was irradiated as leaf-bud cuttings propagated under 

 mist. 



Part of the anatomical background for such tissue recombina- 

 tions has been treated by Crockett (24). 



Finally, I should like to mention another case when such tissue 

 recombinations might be the true purpose of an irradiation treat- 

 ment, and that is in order to make a single layer chimaera more stable 

 by transforming it into a diectochimaera or into a solid mutant. We 

 have irradiated material of a spontaneous, but unfortunately not very 

 stable nor very intensively, red sport of the James Grieve apple. If we 

 could make it both more stable and more intensely colored, that 

 should be a great advantage. I believe that the secondary, stable types 

 of Delicious, like Starkrimson which has originated from the unstable 

 Starking, very well might be examples of such tissue recombinations 

 (1, 28). It is not clear, however, whether Starking is an ecto- or a 

 meso-chimaera. 



Reduced fertility is another kind of genetical change that may 

 turn out to be an undesirable by-product in some cases but that might 

 even be desired in other cases. We have been noting that the fruit-set 

 among the irradiated trees sometimes seems to be reduced. In fact, 

 some branches with aberrant foliage never have set any fruit. We 

 have also found that there is a rather high frequency of reduced 

 pollen fertility among apple trees that were irradiated 5 years earlier. 



Buds from fertile and partially sterile branches have been 

 brought over onto new rootstocks, and in time it will be possible to 

 see whether this reduction in fertility is also reflected in reduced fruit- 

 set and perhaps in increased fruit size. In some fruit varieties of 

 apples, plums, and grapes, too many fruits are normally formed with 

 reduced size and quality as a consequence. In such varieties a slight 



