XL 



INDUCED MUTATIONS IN PLANTS 



L. J. Stadler 



United States Department of Agriculture and 

 University of Missouri, Columbia 



Introduction. What radiations are effective? Species differences in mutability. 

 The physical nature of transmutation by X-rays. References. 



INTRODUCTION 



The discovery by Muller (31) in 1927 of mutation induced by X-rays 

 in Drosophila greatly stimulated interest in the possibilities of the experi- 

 mental modification of heredity in plants. The partial control of heredity 

 which the results of MuUer's experiments seemed to promise probably 

 would have its chief application in the breeding of plants, for economic 

 plant breeding can make much more rapid and extensive use of advances 

 in genetic knowledge than can economic animal breeding. The effects 

 of penetrating radiations on genetic variability have now been investi- 

 gated in a wide variety of species among the higher plants, and the results 

 in general confirm those obtained by Muller with Drosophila. 



In plants, as in the fruit fly, it is found that chromosomal irregularities 

 of various types, as well as gene mutations, are a characteristic result 

 of irradiation. This chapter is concerned primarily with the evidence 

 regarding induced mutation; the grosser chromosomal variations occur- 

 ring in irradiated plants are discussed separately in Papers XLI and XLII. 

 The distinction, however, is in some degree arbitrary. "Mutation" 

 and "chromosomal aberration" are not coordinate classes. Mutation 

 implies a hypothetical change within the individual gene, or at any rate 

 a change affecting no more than a single gene. But since the individual 

 gene is invisible, the identification of a germinal variation as a mutation 

 is a matter of inference — we assume from its genetic behavior that it is 

 due to a change in a gene. A chromosomal variation, on the contrary, 

 may be identified by direct examination of the chromosome, and there 

 are instances of variations which from their genetic behavior were taken 

 to be mutations and which on direct examination have turned out to be 

 chromosomal variations. Thus in effect a mutation is simply a mendeliz- 

 ing variation that has not been proven to be the result of something 

 other than a change in a single gene. It is the task of future investiga- 

 tion to determine to what extent such variations represent changes 

 in the genes themselves. 



1263 



