PLANT BREEDING 



367 



375. Mutations. — If a variety selected as just described 

 would never change — that is, if its seeds grew year after 

 year into plants which, on the average, when raised under 

 similar conditions, were like the plants that bore the seeds — 

 then the work of selection for any particular quality might be 



done once for all. But as a matter of 



fact, varieties, no matter how pure they 

 may be at the start, do not remain un- 

 changed. In each generation, the indi- 

 vidual plants of a variety differ from one 

 another in many ways. Most of the 

 differences result from differences in the 

 conditions under which the plants grow ; 

 such differences are not passed on through 

 the seeds to the offspring, and so do not 

 affect the purity of the variety. But 

 now and then a plant differs from its 

 parents or from any of its immediate 

 ancestors in a way that is not caused, 

 at least directly, by external conditions. 

 Such a nmtation, as a difference of this 

 kind is called, is due to causes that we 

 do not understand ; unlike the differ- 

 ences that result from external condi- 

 tions, it is likely to be passed on to the 

 offspring. A mutation may therefore be 

 the starting point of a new race. By the 

 appearance of new races in this way, a pure variety in the 

 course of time becomes a mixed variety. Some of the races 

 resulting from mutations are sure to be undesirable. There 

 may occur, for instance, in a large-headed variety of wheat a 

 plant that not only itself bears small heads, but which, if 



Fig. 207. — A stool 

 of rye grown at the 

 Wisconsin Experi- 

 ment Station from a 

 single seed of an im- 

 proved variety. 



reference to certain important qualities. Such a race, while not strictly speaking 

 pure, is thoroughbred in the same sense that a closely inbred race of cattle or horses 

 is thoroughbred. 



