nybom: vegetativelv propagated species 277 



anesulphonate (EMS), which had been shown by Heslot, et al. (56) to 

 give up to 40 to 50 per cent mutations per spike progeny. The pri- 

 mary effects on fruit trees were more pronounced with EMS than 

 with the other chemicals. Even white sectors were found in Striped 

 Williams, so this chemical seems to offer definite possibilities and 

 should be tested further. Treatments with 0.5 to 1 per cent gave the 

 best results; 2 per cent killed most of the buds. 



Mode of Origin of Somatic Mutations 



If we look back at this survey of mutation experiments we shall 

 find certain features that are common to vegetatively propagated 

 plants. In the first place, I think of the mode of origin and the dis- 

 tribution of mutated tissue. When a mutation originates at a growing 

 point, it will be localized in a single cell. When the tissue grows, the 

 mutation will normally be delimited to a certain one of the histogenic 

 layers at the growing point. There are usually supposed to be three 

 (or four) such histogenic layers, or germ layers, the dermatogen, the 

 periblem, and the plerome, the innermost one (98). These layers are 

 also called L I, L II, and L III, respectively (29, 30, 36). 



As the mutation is only very rarely induced exactly in the central 

 part of the growing point but more often at the side of it, it will form 

 a more or less narrow sector at the base, or at the side, of the growing 

 shoot (Figure 12). The shoot will become a mericlinal chimaera, and 

 when side buds are to be formed, these may get several alternative 

 constitutions, as shown in Figure 12. They may become completely 

 normal again, they may retain a sectorial constitution, or they may 

 also become complete, periclinal chimaeras. Only in the last case do 

 we get a stable mutant more or less relieved of the competition, i.e., 

 the intrasomatic elimination, taking place in the growing plant. In 

 the side shoots, the mutated sectors usually will be broader, but also 

 more rare than in the original, central shoot. Many of the induced 

 mutants will probably be lost right in the beginning of the shoot 

 growth and never be included in any side buds. 



In some plants there is normally provision for the formation of 

 enough basal buds in a way that effectively conserves the mutations. 

 In the potato plant, for example, the X 2 eyes on the tubers formed 

 by the irradiated X l tubers have gone through three such bud forma- 

 tions rather rapidly, first when the stolon is formed in the axis of a 



