Imágenes de páginas
PDF

A good illustration of unconscious improvement is to be found in cabbage, kale, collard, palm borecale, Brussels sprouts, kohl-rabi, ruta-baga and cauliflower. These all come from a single, somewhat woody, branching perennial (Brassica oleracea Z.) which is to be found growing wild on limestone bluffs in southwestern Europe. Some are a modification of the leaf, as in the cabbage and kale, others of the stem, as kohl-rabi, still others of the root, as ruta-baga, while in the cauliflower it is the selection of the inflorescence that has caused the peculiar modification. Some of these types have twenty or thirty varieties, so that there are probably over one hundred distinct forms from this one wild type. All of these forms are the result of long and patient selection of variations that were considered desirable by the gardener without any conscious attempt to produce these specific forms.

37. Examples of Definite Improvement.—The sugar beet is an illustration of systematic breeding to bring about a definite improvement. In less than a hundred years of systematic selection of individuals of known excellence, and by testing their ability to reproduce the desired characters, the common garden beet, with 6 per cent of sugar, has been transformed into the sugar beet, which often contains from 15 to 20 per cent of sugar and is otherwise improved.

By similar methods, wheat, flax, timothy and other farm crops are being systematically bred for definite characters. The proper method to be employed will be discussed under the crop in question. Much greater advance has been made with vegetables and other horticultural crops than with field crops.

"At the present day species that have been cultivated for many years have become, so to say, like wax in the hands of special growers, who mold them and fashion them to their taste, obtaining the various modifications of shape, size, flavor, etc., demanded by their patrons and the caprices of fashion."1

The time will doubtless come when there will be many breeders of pure strains of maize, wheat, timothy and other field

» Henry L. De Vilmorin. E. S. R., Vol. XI, p. &

crops, just as there now are many breeders of pure strains of domestic animals.

38. Methods of Improvement. — There are three steps or methods in the improvement of plants or animals, viz.:

A. Inducing variation.

B. Selection of forms having desired characteristics.

C. Testing the power of specific forms to reproduce themselves.

39. A. Inducing Variation.—Variation is the basis of selection. Plants must vary or they could not be selected. There are two general methods of producing variations, viz.:

1. Environment, such as soil, climate, space, cultivation, etc.

2. Crossing.

40. The Influence of Environment.—The causes of variability cannot be discussed here, but the following facts should guide the breeders of plants.

1. Horticulturists do take advantage of a superabundance of food in causing modification or multiplication of parts, such as the development of petals from stamens. After this habit becomes fixed it will be transmitted in some measure even in poor soil.

2. Nevertheless the most important value of cultivation in the case of most plants is to allow the plant breeder or cultivator to study individual forms. It enables him to select the desirable forms and reject the undesirable ones. By milking the cow and testing her milk we are able to select the best milkers. By trotting horses we are enabled to breed those best able to trot. Whatever influence milking or trotting may have, the fact remains that it makes possible intelligent selection.

3. The variations selected should be those induced under the environment in which we expect to continue to grow the crop. If we expect to grow three stalks of maize to the hill in general field culture, it is desirable to select the ears for planting from maize grown in a similar manner, rather than from ears where but one stalk is grown in a hill. In the latter case the size of the ear will not be a criterion of the size of the ear where three stalks are grown in a hill. Where it is not possible to make selection under field conditions, care should be taken to select from among plants under like environment and subsequently subject to field conditions.

"In selecting sugar beets," says Vilmorin,l "those roots are sought for that are straight, long, and free from lateral branches. This is right, for those that are branched are more difficult, and hence more expensive, to gather. Now, certain growers of beet seed in the north of France once formed the idea—thinking, no doubt, in this way to improve their varieties—of growing the plants which were to be used as seed stocks in very rich deeply worked soil where they were very much crowded together; so much so that 16 to 20, or even more, grew on one square meter of ground. The result was that the beet assumed the form, and later the length of a whipstock. They were not branched because the roots were very closely crowded together. Their sugar content was abnormally high as a result of their growing so close together, and the conclusions drawn from the form of the roots and their sugar content, as determined in the laboratory, were tainted with error because they did not represent qualities truly acquired, but modifications accidentally imposed by external conditions. Thus these beets which were declared to be of good shape and composition in the laboratory yielded seed which when sown in the open field, produced branched roots of only moderate sugar content, because the descendants had reassumed their true characters when they were released from the restraint which had been artificially imposed upon the parent plants."

41. Change of Seed.—A frequent change of seed is not necessarily a good thing; certainly it is not necessary to obtain seed from distant parts of the country for a region whose soil and climate are well suited to the crop. If the region is not well adapted to the crop frequent new supplies of seed may be helpful and even essential. Probably no part of the world is better adapted to maize than is much of the central Mississippi valley. There would seem to be no good reason for changing seed of maize in this region. Much of this same region is not equally well suited for the oat crop. The climate is too hot and dry. The oats are much lighter than those produced in more moist and cool regions. Obtaining seed oats from regions where the crop does better may be good business management .

[ocr errors]

42. Crossing.—Crossing two unlike forms or two varieties may not be a fundamental cause of variation. Some other cause must have operated to have produced the two unlike forms. In practice, however, crossing is a means of inducing variation, so as to enable the breeder to select forms more nearly suited to his ideal. This is shown by Hays1 in the case of a hybrid between Fife and Blue Stem wheat.

Some of the plants of hybrid wheat yielded more and some less than any of the plants of either the Fife or of the Blue Stem. If the yield is the characteristic desired, then a few plants of the hybrid were better than either of the present varieties.

Crossing is also employed not only to induce variation but to combine two or more desirable qualities in one plant.

43. B. Selection.—Plants having varied either through the efforts of the breeder or otherwise, the next step is to select plants having the characteristics desired. "Selection is the surest and most powerful instrument that man possesses for the modification of living organisms."2

The unit of selection is the individual. In the case of wheat the unit is not the seed, nor even the head of the wheat, but it is the stool containing several heads and many seeds which have been produced from a single seed. In the case of the potato it is the single hill and not the single potato. However, in plants, unlike higher animals, portions may be used for the purpose of

1 Willet M, Hays. Plant Breeding. Division of Vegetable Physiology and Pathology, U. S. Department of Agriculture, Bui. 29, p. 21.

* Henry L. De Vilmorin. E. S. R., Vol. XI, p. 19.

[graphic]

Influence of crossing as a cause of variation.

Vield in grains of l00 plants, showing greater variation in yield of hybrid wheat than of either parent form. The yield of the hybrid is indicated by the line marked —%— (After Hays.)

reproduction and the inheritance of variations in these parts is recognized as possible.

Only useful characters should be selected, because two characters are more difficult to develop than one; three more difficult than two, and so on. Some characters are mutually antagonistic, as extreme earliness and either great size or productiveness. To select wisely requires deep study and good judgment. Varieties frequently deteriorate on account of unwise selection. This is especially true of maize, although it is the field crop which it is the easiest to select .

44. C. Testing Power of Specific Forms to Reproduce Themselves.—Having selected a desired form, it is next necessary to test its ability to transmit its characters. Even though the sire (plant furnishing the pollen) may be known, there is no certainty that the plant will transmit the characters which it possesses. Different grains from the same head of wheat are known to yield unequally. Some variations are easily fixed: others require generations of selection before the characters can be depended upon. Under ordinary farm conditions the ability of individuals to reproduce themselves is not tested, and furnishes a very important reason why little progress has been made in the improvement of field crops. Take timothy, for example. A casual inspection of a field of timothy will show that there is a great variation in the length of head, the length of stem, the amount of leaves and number of stalks per stool. Under the usual method no selection is exercised, and no test of the power of the transmission of characters is possible. A few experimenters have selected plants (stools) having different characteristics and by planting 100 seeds from each plant in rows, one seed at a place, have obtained remarkable results. After the ability of the plant to transmit its characters has been demonstrated, the seed can be rapidly multiplied for field purposes.

It is well understood by livestock breeders that the best individual does not always produce the best progeny. It is a

« AnteriorContinuar »