nybom: vegetativelv propagated species 261 



Fruit Trees and Small Fruits 



Most of the mutation experiments with vegetatively propagated 

 plants fall within this group. For the sake of surveyability, the various 

 mutation projects known through publications have been collected 

 in Table 1. 



A special kind of mutation of great practical interest is repre- 

 sented by the self-fertility mutations of Lewis, who has given detailed 

 descriptions of their induction (66, 67). Irradiation was done during 

 the resting stage before meiosis. Irradiation of mature pollen grains 

 had no effect as the substances responsible for the incompatibility 

 reaction are obviously by then already formed. In cherries, 800 r on 

 an average gave one seed after self-pollination (or incompatible cross- 

 pollination) of 130 flowers. Ten per cent of the seedlings so formed 

 turned out to be completely self-fertile and to give rise to self-fertile 

 offspring in their turn. 



The artificial induction of self-fertile types must be said to be of 

 very great importance, not only from the point of view of fruit pro- 

 duction but also with regard to the possibility of obtaining individu- 

 als homozygous for rare recessive genes, leading, e.g., to extreme 

 earliness (67). 



The method most commonly used for the induction of somatic 

 mutations in fruit trees has been to irradiate dormant scion wood 

 during winter or early spring and then to graft these scions into other 

 trees. In order to ensure a better union between stock and scion, 

 Grober (45) and Zwintzscher (112) recommend that only the upper 

 part of the scions be irradiated. Granhall (42) and Zwintzscher (112) 

 also overcame the same difficulty by irradiating complete young trees 

 already growing on an understock. Granhall used 1 -year-old "year- 

 lings", while Zwinzscher used somewhat older trees, the crowns of 

 which had been pruned back in order to form new shoots. 



Other methods of irradiation described, e.g., by Hough and 

 Weaver (57), involve continuous exposure of growing trees at a 

 stationary cobalt 60-source. This method of irradiation has also been 

 practiced in Sweden, although we are not prepared to judge whether 

 it is more efficient than acute irradiation. We have also, like Grober 

 (45), irradiated bud sticks of apples and pears during August and 

 inserted the irradiated buds into suitable rootstocks. As far as we 

 can judge, this method seems to offer certain advantages. 



