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portion of this chapter is exclusively devoted to the plant, the treatment of which naturally comes first.

Variation. It has been recognized for at least a century that wheat is capable of variations. These may be peculiar to the plant itself, and may occur although the environment remains constant. Variation in this sense became established only with the theory of evolution, and refers to those changes which tend to become permanent through inheritance. Such variations are assumed to be the manifestations of a natural tendency inherent to all organic life.

The theory of common descent for all living beings found its first great advocator in Lamark at the beginning of the nineteenth century. Fifty years later Darwin assembled enough evidence in support of the theory to enable it to gain general acceptation. Darwin assumed that the great variation involved in the theory proceeded in the main by slow and gradual changes. He recognized, however, that species may also originate in nature by leaps and sports. The theory that all variation occurs by sudden mutations has been held by a minority of scientists. Cope and De Vries' are among those who have most recently increased the evidence in this direction. A defence of discontinuous evolution has also been made by various other scientists, such as the paleontologist Dollo, the zoologist Bateson, and the botanist Korshinsky. In general, it may be said that if the followers of Darwin have been open to the criticism of under-emphasizing sudden change, the supporters of the theory of mutations have certainly erred more widely in the opposite extreme.

Variations may also be induced. In this process two different methods may be used, hybridization and change of environment. Only those variations which may occur or be induced independently of environment are considered in this chapter. Others are treated in subsequent chapters. Variations may include differences in habit of growth, chemical composition, periods of development, appearance, form, yield, prolificacy, vigor, hardiness and stability of type. Whatever his concep

1 An able criticism of the theory of mutations has been made by Prof W. F. R. Weldon, "Professor De Vries on the origin of species," Biometrika, 1:365, 1902. A study of this theory is interesting in conjunction with the more elaborate theory of homotyposis developed by Prof Karl Pearson in his work at University College, England, but space forbids a discussion of the matter here.

the most powerful influences from the very f wheat, although men were not aware of its ever protection or cultivation early man be cereal plants was naturally bestowed upon wheats which produced the most food in retu those comparatively less important as food. of the importance attached to wheat has alw yielding quality. It is a perfectly sound inf varieties of wheat which had this quality i gree had an advantage which aided them to rieties. This, however, is only the operati factor of selection, or, as Darwin calls it, preservation of the favorable individual diffe tions, and the destruction of those which are Selection and cultivation, in the ordinary processes of domestication. After domesticat tinue to be propagated in a similar manner. been attained none the less advantageously account of the fact that man was unconsciou agent. To this force of artificial selection w natural selection in early development, which the coincidence that the quality of wheat as a the reproductive functions of the plant were seed. The plant producing the greatest num most apt to survive, not only because man w give it his fostering care, but also because chances of reproduction. In wheat artifici must be exercised lest this force of natural disadvantageously, for fewer seeds are no long in reproduction. If for any reason, such as b

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that selection in wheat was early practiced, but never lo continued or repeated. One of the early experiments in sel tion of wheat was that of Hallett in England, begun in 18 He selected the best heads and kernels. The following ta gives his results.

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Thus by means of repeated selection alone, the length of ear was doubled, the number of grains per head was nea trebled, and the tillering power was increased over fivefold. is only within recent years that wheat experiments of t nature have been carried on in America. The most extens and successful of these were begun in 1892 at the Minnes experiment station under the direction of Prof. W. M. Ha From 1891 to 1896 experiments were made in Kansas w light, common and heavy seed, and seed from selected hea The light seed uniformly gave a lower yield, but common se gave the highest yield during three years. At the Minnes station from 1895 to 1898, No. 169, a wheat selected on pr ciples similar to those of Hallett, gave an average yield 28.3 bushels per acre, while during the same years the

1 Georgics I., lines 286-288.

2 Neb. Bul. 32, p. 91.

3 Kan. Buls. 20, 33, 40 and 59.

The substances of which wheat flour is composed may be divided into three classes: (1) Nitrogenous, which include mainly gluten, fibrin, albumen, casein, cerealin, and modifications of some of these; (2) non-nitrogenous, embracing sugar and dextrin, but chiefly starch, fat and cellulose; and (3) the minerals, for the largest part alkaline phosphates and silicates, especially phosphate and silicate of potash.

WATER.-Wheat ordinarily contains from 10 to 15 per cent of moisture. Changes in the moisture content of the air cause corresponding variations in wheat, and consequently in its weight. Usually such fluctuations in weight do not exceed 6 per cent, but they may be as much as 25 per cent, and an increase of 9 per cent in 24 hours has been observed. When wheat is shipped, especially if it is transported long distances, this may be a matter of great commercial importance. Wheat transported from the dry atmosphere of the inland of California to ordinary temperate regions will invariably gain from 5 to 15 per cent in weight. In a voyage from San Francisco to Liverpool, the increase in weight due to the moisture absorbed en route may be sufficient to pay all expense of transportation. Every portion of the wheat grain is so susceptible to influence from hydroscopic conditions that all of the products of wheat exhibit similar oscillations in weight. Two days equalized the moisture content in samples of flour varying from less than 8 to over 13 per cent.1

ASH.-Lawes and Gilbert observed the composition of the ash of wheat grown on unmanured ground during 20 years. The average results are given in the table below.2

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1 Hunt, Cereals in Amer. (1904), p. 38; Mich. Bul. 191, p. 159-164. 2 Hunt, Cereals in Amer. (1904) p. 39.

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