Imágenes de páginas
PDF

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

[ocr errors]

also resulted in adapting corn to regions far north of its former habitat. Wheat perhaps has not been so generally improved 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 ineat, 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 may be 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 slowers. Possibly the stigma is usually not receptive to foreign pollen.

In artificial cross-fertilization, self-fertilization must be prevented 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 withi 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. To 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 flowers 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 distinctly manifest until the second generation, when they appeared in a remarkable degree. Some races of wheat may differ 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 composite 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 limited to the few very best wheats, for a fairly large number of varieties can be used profitably for special characteristics.

The great advantage of hybridization is shown in three effects, all of which aid in accomplishing more rapidly the results aimed at in selection. It makes it possible immediately and directly to combine the qualities of two different plants in one; it immensely increases that variation which alone makes selection possible; and it imparts greater vigor to the offspring. Hybridizing does not always give a progeny im

Pedigree White Hunter's White

Hung;

Hybrid 3 Hybrid 5 Hybrid 4.
Hybrid 3 Hybrid 4

Hybrid 6 Hybrids

Hybrid 7
DIAGRAM SHOWING PEI) IGREE OF GARTON'S HY BRID.

mediately averaging better than the parents. In many cases the first progeny will average much poorer than either parent. Its great value lies in throwing together qualities and multiplying variations, both of which may be developed by selection. This greatly increased variation has been explained on the ground that “the wheat plant being so closely self-fertile, there is within it, lying dormant, a wonderful power to vary (a power far greater than in plants cross-fertilized in nature), which is thrown into action when different varieties are artificially crossed.” As to these varieties Hays says: “The further they have departed from ancestral characteristics and formed diverse qualities, the more likely will their progeny exhibit new characteristics made up by combining those which have become so radically different in the two parents.’’ ” There also arise characteristics so new in kind and degree that they can hardly be considered as a mere combination of any characteristics found in the parents. All of the so-called “botancal” classes of wheat have been produced by hybridizing two varieties, a fact which “certainly indicates blood relationships between the classes of wheats.’’ Since the qualities originated by hybridization must be developed by selection, it may take years before the value of the hybrid can be determined. The advantage in large numbers lies in the fact that “only one individual in several thousand has marked power to produce a valuable strain.” These individuals have been called the “Shakespeares of the species,” and the labor of eliminating by selection all of the other individuals is 99 times that of producing the hybrids.' By the usual method, the seeds to be tested and selected are planted individually in rows 4 or 5 inches apart. If any promising plants develop, 100 seeds from each are again planted. These groups of plants from single parents are called centgeners. By means of selection, crossbred wheats can thus be reduced in four or five generations to a type so uniform that little or no variation will occur among plants in the field. Whether they will retain their acquired characteristics has been questioned. Hybrids originated by Hays and Saunders seem to do so. The question as to whether wheat will deteriorate under self-fertilization is still an open one. In breeding a variety of wheat, the ideal to be held in mind constantly is “that it yield the largest possible amount of grain of the best quality for the purpose desired under given conditions.’’ ” In such a course botanical appearances seemingly will take care of themselves. Historical.—The sexuality of plants was proved experimentally by Camerarius (1691). The first recorded hybrid was produced by Thomas Fairchild (1719), an English gardener, who crossed the carnation with the sweet william. The publieations of Koelreuter (1761) paved the way for the work of Thomas Andrew Knight (1800), the eminent English plant physiologist, who has been called the father of plant breed

1 Carleton, Toasis for Improv. of Amer. Wheats, p. 73. * I’lant Breeding, p. 37. _ – 1 Hays. Yearbook U. S. Dept. A gr., 1901, p. 229. * Scofield, Algerian Durum Wheats, p. 7.

[graphic]
[graphic]
[graphic]
« AnteriorContinuar »