nybom: vegetatively propagated species 255 



these effects persisted into the next generation. The frequency of 

 transmitted aberrations was very much the same in those families that 

 had passed the X x germination without primary effects. Observations 

 on the X 3 plants, on the other hand, showed that most of the induced 

 changes had already been found in X 2 . 



As far as distinct, easily observed changes are concerned, selec- 

 tion work may therefore be concentrated in the second year only. 

 However, a breeder looking for less drastic changes might prefer to 

 postpone selection to X 3 , when it can be based on more or less pure 

 tuber lines. 



In the second year, changes turned up in certain families. Only 

 changes in the above-ground plant parts were looked for. The dose 

 response was evidently linear. Thus, 2,000 r gave 4.9 per cent mutated 

 X 2 families, 4,000 r 10.5 per cent, and 8,000 r 20.8 per cent. However, 

 due to the differences in lethality, the absolute number of mutated 

 families always was higher after 4,000 r. The 640 tuber-halves irradi- 

 ated with 4,000 r gave 409 surviving Xi plants and the same number 

 of X 2 families. Of these, 364 were normal while 45 contained muta- 

 tions. The "segregation" ratio in X 2 was about 2 aberrant plants to 8 

 normal plants per family. 



By means of eye excisions Heiken demonstrated that most, 

 though evidently not all, e.g., none of the "wildings", of the isolated 

 changes were periclinal chimaeras, having normal tissue below the 

 mutated. All isolated types were also grafted on virus-free stocks in 

 order to test the presence of any sap-transmissible principles, but no 

 such indications were found. 



Even though these studies were intended as a "preliminary exer- 

 cise" for mutation breeding work in the potato, like most other 

 studies on induced mutation hitherto they were not planned as a 

 mutation breeding project. The author wanted, in the first round, 

 to determine the range of variability induced and concentrated mainly 

 on distinct and easily identifiable mutations. Thus not less than 55 

 of the isolated 109 aberrations consisted of various malformations, 

 17 of them were dwarf types, and 27 were different flower color vari- 

 ations. Strange enough, no bolters were found after irradiation. 



The average yield of these isolated, drastic changes was, as one 

 could expect, considerably reduced. Most of them gave between 70 

 to 80 per cent of the original clone, whereas some were rather 

 similar to the mother variety in vigor. 



