254 MUTATION AND PLANT BREEDING 



eye excision method, she was able to unveil the genotypical constitu- 

 tion of the deeper lying tissues. She also published some work on 

 radiation-induced changes (3, 4). X-ray treatment was found to be 

 "a powerful and reliable means" for bringing about tissue recom- 

 binations similar to those obtained by eye excision. Not all changes 

 were such tissue recombinations, however. Some behaved like origi- 

 nal mutations. Unfortunately, her promising experiments were dis- 

 continued. It was 20 years before they were taken up again (51, 54, 

 55, 58). 



Heiken's own results were supplemented by a survey of the 

 reported spontaneous mutations in the potato. His list of "tuber skin 

 color aberrations" contains more than 50 cases, and to this list may 

 be added changes in skin structure, shape and color of tuber, stem, 

 foliage and flowers, as also the very characteristic potato mutations 

 "bolters" and "wildings". 



The author also presents the results of his own extensive studies 

 on the occurrence of similar aberrations in untreated material. In 

 addition to 1 per cent bolters, he found 25 cases of such spontaneous 

 aberrations in a material of several million plants, i.e., somewhat 

 more than 1 in 200,000 plants. 



For irradiation, Heiken divided each tuber into two parts. One 

 half was irradiated and the other was sown as a control. In this w r ay 

 it was possible to detect and remove all cases of contamination and 

 tuber-carried diseases. The irradiations were carried out at different 

 times. With regard both to the lower lethality and the considerably 

 higher mutation frequency, the best results were obtained just at the 

 beginning of germination, during February to March. A certain 

 number of primary tubers then yielded six times more mutations 

 than if irradiated during dormancy, i.e., during November to Decem- 

 ber. After germination had started, April to May, a drop in mutation 

 frequency was noted. The most suitable X-ray dose was around 

 4,000 r. 



All tubers from the surviving Xi plants (plants growing from the 

 irradiated tubers) were harvested and sown the next year to form 

 the X 2 families, the plants in which then carried the X 3 tubers. 



In the first year, irradiation gave rise to typical "primary effects", 

 viz., increased fleshiness of leaves, disturbances in vein branching, 

 deformation of leaves, etc. However, only a very small portion of 



