. Textbook of botany. Botany. 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 resu
. Textbook of botany. Botany. 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 Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these
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