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 Carriére 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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