nybom: vegetatively propagated species 279 



found (80). Although most of the endosperm changes studied by 

 Dollinger turned out to be losses, changes in recessive direction, he 

 also found indications of dominant changes. In apples we know that 

 gain in color as well as loss of color both do occur (9, 12). It is, how- 

 ever, not necessarily so that the former changes should be dominant. 

 They might as well be caused by losses of suppressor factors. Cases of 

 obvious dominant mutations are known, however. The deep-red 

 mutant of the Williams pear, Max-Red Bartlett, for example, seems 

 to be a really dominant subepidermal factor mutation as it gives rise 

 to 50 per cent seedlings with the increased anthocyanin content in 

 the shoot, typical also for the Max-Red Bartlett. 



The Technique of Mutagenic Treatment 



As is evident from Table 2, most of the various types of diaspores 

 used in vegetative propagation also have been exposed to mutagenic 

 treatment, including scions, buds, cuttings, bulbs, tubers, etc. The 

 range of doses that have been used is, however, narrow compared with 

 the wide range of doses tolerated by seeds (50, 82) or by continuously 

 growing plants (81, 105, 106). The most common doses lie between 

 2,000 and 4,000 r units of X-rays or gamma rays. For especially sensi- 

 tive, herbaceous material these doses may be halved, while for many 

 lignified or dormant structures they may be doubled. 



Table 2. — Examples of Radiation Doses that Have Been Used in Order to Induce 

 Mutations in Various Kinds of Vegetatively Propagated Plants. 



Apples and Pears: 



Dormant scions, 3,000 to 6,000 r, X-rays and gamma rays, Bishop, 1955; Granhall, 



et. al. 1949; Grober, 1959; Zwintzscher, 1959; 4-6xl0 12 Nth/cm 2 , Bishop, 1958 

 Summer buds, 2,000 to 4,000 r, Grober, 1959 

 Growing trees, 25 to 50 r/day, several years; 50-100 r/day, 5 months, Granhall and 



Nybom, unpublished 

 Flower buds before meiosis, 1,000 to 3,000 r, Lewis and Crane, 1954 



Cherries: 



Dormant scions, 2,000 to 4,000 r, Zwintzscher, 1955 

 Flower buds before meiosis, 800 r, Lewis and Crane, 1954 



Plums: 



Dormant scions, 2,000 to 6,000 r, Zwintzscher, 1955 



Growing trees, 25 to 50 r/day, several years, Granhall and Nybom, unpublished 



Peaches: 



Growing trees, 10 to 60 r/day, 8 to 20 months, Hough and Weaver, 1959 



