256 MUTATION AND PLANT BREEDING 



The dormancy period of the tubers was also studied in special 

 experiments. Some of the isolated mutants had increased periods of 

 dormancy while others started germination earlier than the normal 

 clones. 



After the last, main irradiation experiment a series of less marked 

 and probably more interesting changes were isolated, but as they 

 required another year for verification and testing, they were not 

 included in the report. Summarizing his results, Heiken states that, 

 "the somatic aberration frequencies obtained have been high enough 

 to make continued research in this field very desirable". He suggests 

 that a systematic selection of induced mutations could be combined 

 with the present virus-testing routines. 



One of the keys to Heiken's success was the fact that he had the 

 privilege of working at an institute specializing in virus diseases of 

 potatoes and in growing virus-controlled stocks of the varieties used. 

 I should like to stress this fact, as it is of general importance for muta- 

 tion studies in vegetatively propagated crops. Virus diseases often 

 simulate genetical changes and may, thereby, cause serious trouble. 



Chrysanthemum 



A very interesting and promising piece of work on induced muta- 

 tions in Chrysanthemum indicum was published some years ago by 

 Jank (59) in Germany. Chrysanthemum is also a plant where spon- 

 taneous mutations, especially concerning flower color, have often 

 been observed and also have been of great practical importance in 

 the breeding work. 



Jank also combined his work with an extensive survey of the 

 reported spontaneous color variations. Not less than 318 such "sports" 

 have been described in 170 different varieties. It turned out that 

 rose-colored varieties especially often give rise to mutations with 

 varying new colors. White varieties usually are somewhat less mutable, 

 and the same is true of the bronze-colored ones. There are also violet, 

 red, orange, yellow, brown, and other colors which seem to be more 

 stable. 



Three varieties were selected for the main experiment, Day 

 Dream, Vogue, and Berta Talbot, all with different shades of rose. 

 Cuttings were taken on February 2. On March 18 the rooted and 

 potted plants were decapitated in order to give rise to side shoots, and 

 a few weeks later they were X-rayed. The suitable doses had been 



