260 MUTATION AND PLANT BREEDING 



sports White Sim and Pink Sim back to the original red William 

 Sim, obviously are tissue recombinations. However, some other new 

 colors were also obtained as well as changes in flower morphology. 

 These studies are being continued by Mehlquist (71). 



In Saintpaulia, Sparrow and his co-workers have reported very 

 interesting irradiation experiments (107 and 108). In the white- 

 flowered and semi-double variety Dwight's White mutations with 

 violet and lavender-colored flowers were induced as also types with 

 single and double flowers and a series of leaf changes. 



After irradiating leaf-petioles with 2,000 and 3,000 r X-rays, 14.2 

 and 25.2 per cent, respectively, mutated plants were recorded, in all 

 154 cases. A feature of special interest concerning these mutants is 

 that they are mostly homogeneous, i.e., not chimaeric. This is also 

 to be expected as the new plants are derived from single cells of the 

 primary, irradiated leaf-petiole. 



In this connection there are some unpublished Swedish results 

 with roses that I have been allowed to mention here. Doctor Gelin 

 (39) at Weibullsholm Plant Breeding Institute irradiated 1957 

 summer buds of five varieties of roses with 2,500 to 10,000 r gamma- 

 rays from cobalt 60. After irradiation these were budded into com- 

 mon rootstocks. About half of them took and developed into new 

 shoots next year. In order to isolate the possible induced changes, 

 buds were again taken from these new shoots in 1958. 



In addition to the primary effects the first year, the isolation the 

 next year also gave rise to persistent changes. Besides changes in 

 thorniness (increased thorniness), leaf color (increased anthocyanin 

 content), and leaf shape, there was also found, after 5,000 r, a darker 

 colored mutation in the variety Peace, somewhat similar to the 

 spontaneous "Pink Peace," which is now under propagation for 

 further tests (40). 



Thus, these first experiments with roses did give at least one 

 mutation that might become of direct commercial importance if 

 further trials show it to deviate in a positive way from the other types. 



It mieht be added here that similar-looking mutations should 

 always be worth further trial and comparison, as varying pleiotropic 

 changes seem to be a rule rather than an exception. Some of the red 

 Delicious mutations in apples, for example, at the same time show a 

 distinctly different and, from some points of view, an improved mode 

 of growth and fruit production (1, 64). 



