PLANT BREEDING. The first object of plant breed ing is to produce new varieties superior to one or more of the old ones. Superiority means agricultural superiority and is measured by net monetary return per acre. It may be sought in many direc tions, of which yielding capacity and quality of produce are chief. In many countries and for a diversity of crops, resistance to disease has a great importance. Rapid growth is sometimes a transcendent consideration because it ensures escape from an intensely hot summer or from early frosts.
The earliest efforts in crop improvement consisted of the simple practice, many centuries old, of picking out the finest plants as the source of seed for the next season. During the 19th century the two methods of breeding now universally used—selection of single plants and artificial hybridization—were brought to a rela tively high pitch. But they were entirely empirical and, unin formed by scientific knowledge, appeared incapable of any sub stantial modification. Then, swiftly, with the opening of the 20th century, came Mendel's laws of heredity, Johannsen's theory of pure lines, and the mutation theory of de Vries. These great advances, while recasting biological science, gave to plant breed ing an orderly basis and a new outlook.
An ordinary field crop, especially in a backward or agricul turally young country, often contains plants of several different varieties. By selecting and separately propagating single plants it is possible to produce seed stock of these several varieties. Those which, on test, prove best, may be introduced into general culti vation. This procedure, the selection method of breeding, is widely used. Chevallier and Goldthorpe barleys and also Potato and Sandy oats, all of great value in their day, arose from selected single plants. Dr. E. S. Beaven's Archer and Plumage barleys are other notable selections. The earlier exponents sought to carry improvement further by restarting from the best seed from the best plant among the progeny of a single plant. Johannsen demon strated the futility of this, showing that, in self-fertilizing plants, the descendants of a single plant were normally a "pure line," i.e., all alike save for purely fortuitous differences and such as to breed true on producing seed.
Normal constancy in reproduction may, in some instances, be varied by the production of a "sport" or new type. The biological processes responsible for this happening—or mutation—are still imperfectly understood. But de Vries demonstrated that a muta tion, once produced, remains constant in subsequent generations. Thus a valuable mutant type is a permanent asset to agriculture and de Vries' discoveries gave fresh stimulus to the selection method of breeding. Mutations may arise either in the reproduc tion of plants by seed or as buds. In citrus fruits mutant buds arise with marked frequency and it is by finding and propagating them that Californian growers of the navel orange have developed a considerable number of new types, some of all-round superiority to the original navel orange which was introduced from Brazil.
Artificial hybridization or crossing, now the chief method of breeding, may be illustrated by a simple hypothetical example. Suppose it were desired to breed a wheat having short straw and earliness in ripening (short-early) ; that the only short straw variety adapted to the country concerned was late ripening (short late) ; and that the only available early ripening variety had tall straw (tall-early). The actual half-bred or first-hybrid ob tained by hybridizing the two chosen varieties would be about intermediate in straw and maturity. If allowed to self-fertilize it would form seeds from which would grow the second hybrid generation plants. Mendel's laws make it possible to pre dict (i.) That among the F2 plants would be four types, viz., short-late : short-early : tall-late : tall-early. (ii.) That the four types would occur in certain definite proportions (in other crosses, involving different plant characters, the proportions might be dif ferent from these but they would always be constant and systemati cally explicable for any one case) (iii.) That only a proportion of the plants of any one type would breed true to type.
By raising separate progenies from the seed of all short-early F2 plants it would be possible to select those lines which were fixed or true breeding.
Unfortunately yield and almost all attributes of agricultural consequence do not behave with this hypothetical simplicity. They are, indeed, not single units of inheritance but combinations of many. Thus the breeder cannot pick the ideal F2 plant and by simple seed propagation build up a stock of an ideal new variety. In cereals, for instance, 1 o to 12 years of continual selection must follow on the act of hybridizing before it is possible to obtain a uniform true breeding type having the desired combination of characters from the two parent forms. Not even the keenest eye, the ripest experience or the utmost resource of science is able to detect, in the early stages of breeding, which is best of the new types that hybridization has brought into being. It is necessary by successive years of study to eliminate types one by one until a few remain. These must then be systematically tested for yield and the other characters of importance. The test methods asso ciated with plant breeding represent a specialized field of applied science.
The older hybridists frequently sought to cross widely different parents, as for instance cabbage by turnip or two markedly diverg ent wheats. Complication or failure was the usual result. Mor gan's chromosome theory of heredity has now furnished the breeder with both a test and an understanding of the limits of cross-compatibility.