CHAPTER XXI 

 MUTATIONS IN PLANT BREEDING 



Discontinuous heritable variations have appeared very frequently in 

 cultivated plants under conditions such that they could not be attributed 

 to hybridization. The selection of these variations has produced new 

 varieties in the same way that the early color varieties of the sweet pea 

 arose. By means of breeding experiments many such variations have 

 been proved to follow the -Mendelian principles of inheritance. The 

 general conformity of varietal crosses with the Mendelian principles is 

 sufficient reason for asserting that the vast majority of cultivated varieties 

 arise either directly or indirectly through factor mutations. A few have 

 originated through chromosome aberrations, but, so far as is known, none 

 which are of importance to agriculture. It is especially clear that in 

 self-fertilized species the production of new varieties from single plant 

 selections is made possible by the occurrence of factor mutations. 

 The successes of Le Couteur, Shirreff and Hallet, and the achievements of 

 Vilmorin, Nilsson, Hays and Johannsen with individual plant selections 

 find their explanation in the existence of genetically diverse forms within 

 the species or varieties with which they worked. These pure lines must 

 have originated through changes in specific factors. Similarly with the 

 genotypes of maize and other cross-fertilized species, although new 

 combinations of factors are continually arising through natural inter- 

 crossing, yet entirely new factors can arise only by means of changes in 

 existing factors. These factor mutations do not necessarily induce pro- 

 found somatic changes, and slight morphological variations may hardly 

 be distinguished from modifications due to environment. When physio- 

 logical characters alone are affected, as is sometimes the case, the most 

 careful tests of many individuals may be required to discover a desirable 

 mutation. But once a mutation arises, the normal range of fluctuation 

 in the character or characters affected is different from that of the parent 

 form, and new material has been provided for man's selection if he desires 

 to use it and can isolate it. When these facts are realized the funda- 

 mental importance of mutations to breeding will be appreciated. 



Bud mutations, especially when strikingly different from the parental 

 type, have long been known. Bailey states that Carrie're in 1856 enu- 

 merated over 150 bud-varieties or sports of commercial importance in 

 France and he estimated that no fewer than 300 named horticultural 



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