Imágenes de páginas

now climate, wheat shows an unusual tendency to vary, it changes, and if the better yielding plants are crowded out the change results in a lower yield. Virgil' mentions selection of seed before the time of Christ, and noticed its advantages. A Scottish agriculturist, Shireff, made discoveries pertaining to the selection of wheat as early as 1819. The Belgian horticulturist, Van Mons, scientifically practiced selection before 1835. The works of Le Couteur, the English breeder, show that selection in wheat was early practiced, but never long continued or repeated. One of the early experiments in selection of wheat was that of Hallett” in England, begun in 1857. He selected the best heads and kernels. The following table gives his results..

[blocks in formation]

Thus by means of repeated selection alone, the length of the ear was doubled, the number of grains per head was nearly trebled, and the tillering power was increased over fivefold. It is only within recent years that wheat experiments of this nature have been carried on in America. The most extensive and successful of these were begun in 1892 at the Minnesota experiment station under the direction of Prof. W: Mo Hays. From 1891 to 1896 experiments were made in Kansas with light, common and heavy seed, and seed from selected heads. The light seed uniformly gave a lower yield, but common seed gave the highest yield during three years. At the Minnesota station from 1895 to 1898, No. 169, a wheat selected on principles similar to those of Hallett, gave an average yield of 28.3 bushels per acre, while during the same years the un

1 Georgics I., lines 286-288.
2 Neb. Bul. 32, p. 91.
3 Kan. Buls. 20, 33, 40 and 59.

selected parent sort yielded only 22.5 bushels, an increase during four years of 5.8 bushels per acre. In ten years nearly 25 per cent in yield was gained.'

Ninety-six tests of selected wheat seed during the years 1900 to 1902 at the Canada experiment farms gave an average gain of about 3.6 per cent in favor of selection. Principles differing somewhat from those usually followed in selection were

[graphic][subsumed][subsumed][ocr errors]

CROSSING AS A CAUSE OF VARIATION: Yield in grain of 100 plants, showing greater variation in yield of hybrid than

of parents. Yield of hybrid shown by x line. (After Hays.)

utilized by Lyon. His selections were for quality rather than quantity. He experimented with the smallest and lightest kernels on account of their high nitrogen content. Heavy seed planted at the rate of 1.5 bushels per acre gave a greater yield of wheat the first year than light seed sowed at the same rate. Selecting heavy seed grown from the heavy wheat and light from the light wheat, the difference in yield in 3 or 4 years was small. After the first year of the separation, the light seed gave much the greater amount of proteids per acre. Lyon points out, however, that proteid nitrogen is no index to the amount of gluten, which is the better basis for improvement. It is not yet decided whether selection should be for plants with large heads or for plants with a large number of medium-sized heads. In general, the results of many experiments seem to favor the selection of large seed.'

1 Hays, Plant Breeding, p. 10.
3 Evidence of Wm. Saunders, 1903, p. 48.
• U. S. Dept. Agr., Bu. of Plant Indus., Bul. 78 (1905).
• Hunt, Cereals in Amer. (1904), pp. 87-89.

While there may be a few slight or questionable exceptions to the general rule, it can be said that enough results of scientifically conducted experiments are now at hand to prove, conclusively that by means of selection alone the yield of wheat can be materially increased, even in a few years. The gain from the increased yield is much greater than the cost of making the selection, if the work is carried on systematically through a series of years.

A method frequently used is the selection of plump kernels from grain in the bulk. While there is doubtless some advantage in this method, it cannot give the best results, for many of the plump kernels may come from imperfectly-filled heads, or from plants having few or weak suckers. Selection is a choosing of the individual, which, in the case of wheat, is a stool with several spikes and many seeds. When mutually antagonistic characters are desirable, such as earliness and productiveness, selection is very difficult and requires good judgment. By proper selection, not only may yield be increased, but all the other variations above mentioned may be influenced. Prolificacy of races may be fixed. Another important quality to be considered is vigor. Indeed, it has been held that the vigor and productiveness of the parent are far more important than its mere size.

The chance of improvement by selection increases as the number from which individuals may be chosen grows larger. The plant breeder has a great economic advantage over the animal breeder, for the expense of producing seeds for individual plants is so small that only a few of the best seeds are kept, while in animal breeding expense ordinarily forbids disregarding more than a small per cent as poor specimens. Prodigious variations may be induced by a long continuation of the selective process. Rigid selection systematically and scientifically practiced on a large scale by European seed growers in the last century has increased the sugar content of sugar beets more than 100 per cent. Six years of selection at the Minnesota station increased the length of flax fiber over 20 per cent. Hays estimates that the farmers have increased the yield of corn 20 per cent by annually selecting the largest and best formed ears from among many thousands. The process has

1 Thorpe, Harper's Mag., 15:302.
3 Yearbook U. S. Dept. Agr., 1901, pp. 217-218.

also resuited in adapting corn to regions far north of its former habitat. Wheat perhaps has not been so generally im-proved by selection as corn has, but the wide practice of seed grading through the use of the fanning mill must have similar results. This is a slow process, however, and no great changes are effected at any one time.

Natural selection is also continually operative, especially in connection with such qualities as rust resistance and hardiness against heat, drought or cold. Thus wheat naturally tends to adapt itself to its environment. Such crude methods of seed selection as have been practiced in conjunction with natural selection have been the factors in evolving Turkey wheat so that it is more drought resistant than formerly, and has improved in hardiness so that it can be grown much farther north. Quality of the grain in any respect, yield, earliness in ripening, and non-shattering, in addition to the qualities just named above, are some of the most important characteristics that may be readily increased on any farm by selecting seed from those plants which exhibit these qualities in the highest degree. As these things cannot be properly determined after harvest, all selections for seed should be made in the field. Marked variations or sports possessing improved characters are occasionally met with in the fields. These are often carefully developed into valuable races by seed selection. Fultz, some of the Fife wheats, and many other well-known races have been originated in this way.

Hybridization consists in cross-fertilization. This simple, the fertilization of one race with another, resulting in a hybrid of two bloods, or it may be composite, the fertilization of a hybrid with another race or hybrid, resulting in a hybrid containing the blood of three or more races or species. Hybridization may be natural or artificial. Natural hybrids rarely occur. This is shown by growing different varieties of wheat side by side. Why varieties do not cross under these circumstances has not been fully explained. It is claimed that over half of the pollen from an anther is deposited into the air, and it would seem that it could readily find its way to adjacent flowers. Possibly the stigma is usually not receptive to foreign pollen.

may be

In artificial cross-fertilization, self-fertilization must be prerented by removing the male organs, the anthers, from the flower before the plumes are open and the pollen shed. A good spike of wheat is prepared for hybridization by removing witli sharp scissors all but one or two dozen strong flowers in the center of the spike. From these the anthers are removed while they are still green, or slightly tinged with yellow. Tɔ prevent accidental introduction of foreign pollen, the emasculated spike is wrapped about with tissue paper, tied above and below. Neighboring spikes of the same age show when the flɔwers are fully developed, usually in one or two days. Pollen brought from the variety chosen for the male parent is then inserted into the emasculated florets, and the crosspollinated spike is again wrapped to exclude other pollen and to afford protection against pilfering birds and insects. The hybrid produced partakes of the characters of both parents. Saunders found that the crossbred kernel closely resembles that of the female plant, and that the modifications were not Cistinctly manifest until the second generation, when they appeared in a remarkable degree. Some races of wheat may cffer so widely that they cannot be successfully crossed. If it is desired to combine the characteristics of the two, it can be done by first crossing each with an allied variety, when comrosite hybridization will succeed between the two hybrids produced.

The operation of cross-fertilization is by far the easiest part of the process for attaining results desired. We began with a "good spike." To secure this requires a ready knowledge and judgment of wheat. In hybridization, as in selection, any quality may serve as an ideal for the operator. To attain success, he must know for which qualities to seek and he must have the judgment which enables him to recognize these qualities and to select a foundation stock which possesses them in a high degree. This is truly a case of well begun, half done. Certain desirable or necessary qualities may be entirely lacking in a variety, which must then be improved by breeding into it the desired characteristics from some other variety possessing them to an unusual extent. In selecting a stock with which to begin, it is advantageous to draw from a variety already improved by selection, but the breeding of wheat should not be

« AnteriorContinuar »