220 MUTATION AND PLANT BREEDING 



Variety-Board in Germany. From the experience lie has in his cross- 

 material, v. Rosenstiel is fairly sure that the unusual strawstiffness 

 obtained was derived from the erectoides parent. 



Another example that may be given is the mildew-resistant 

 mutant of barley found by Freisleben and Lein (22). This mutant 

 is resistant against all race groups isolated in Germany. However, 

 the older plants have strongly chlorotic spots and, in turn, a reduced 

 yield. Using this mutant as cross parent, Vettel (personal communi- 

 cation) was able to "separate" the resistance from the leaf spots. He 

 selected high-yielding lines carrying the mutant resistance. 



There are further possibilities for a combination of cross- 

 breeding with mutations. Thus, induction of mutations in F 2 seeds 

 of a hybrid population may be useful because it means an additional 

 increase of variability. Up to date there is little experimental evi- 

 dence of such an approach. Likewise, the possibilities of using mutants 

 for heterosis breeding are almost completely unexplored. In the 

 latter case there is, however, the interesting instance of the com- 

 mercial use of a tobacco mutant (121, 122) which has been over- 

 looked in the older literature concerning mutations and plant breed- 

 ing. This (lightgreen) chlorina mutant of Vorstenland tobacco was 

 produced in Java. The leaves had an attractive color and quality. 

 Since the homozygous chlorina mutant produced too few leaves, its 

 Fi hybrid, with the ancestral type, was used, and had to be produced 

 again every year. In the second half of the 1930's these hybrid plants 

 were grown extensively in the Netherlands East India, but the 

 mutant was lost during the war as far as is known. This seems to be 

 the first case of an induced mutant that has ever been used in prac- 

 tice (8, 83). 



Induction of Mutations in Cross-pollinating Populations 



There is no experiment known to me which proves crucially 

 the use of mutations in cross-pollinating species. However, the new 

 varieties of oil rape and white mustard in Sweden already mentioned 

 were selected from X-rayed material. There are also a number of 

 other reports where variants with valuable characters have been 

 selected after application of mutagens, either with or without self- 

 ing of the Mj generation. This is, for example, the case in Trifolium 

 pratense (10, 89), in PJialaris arundinacea (53), in Alopecurus. pra- 

 tensis (128), and in Melilotus albus (90). It may be expected that 



