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 Hays' 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.

Influence of crossing as a cause of variation.

Yield in grains of 100 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 -x- (After Hays.)

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 .ndividual. 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

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

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

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 character. istics 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

common expression that this animal is a good breeder or that animal a poor breeder.

At the Ohio State University in 1902, fourteen ears of maize of a given variety were selected, and two rows of fifty hills each were planted from each ear. The smallest ear, containing next to the smallest weight of maize, produced the heaviest yield of maize. This ear weighed 14 per cent less than the average weight of the fourteen ears and yielded 32 per cent more than the average yield of the same fourteen ears. This testing of the power of plants to transmit their characteristics is painstaking work, and will form a large part of the work of the successful plant breeder.

45. Importance of Large Numbers.-If a thousand persons stand in a row it will be found that most of them are nearly the height of the average, while a few are considerably shorter and a few considerably taller than the average. The length or weight of a number of ears of maize will vary in the same manner as shown in wheat. (42) In fact this seems to be a universal law of organic beings. Most of them tend to breed true to type: a few vary considerably from the type. In order, therefore, to make progress in breeding it is necessary to find the organisms that have the tendency to vary as desired. Among a million organisms there may be only one that possesses the required characteristics. The chances of finding the desired individual increase as we increase the number from which selection is made. The chances of securing satisfactory results are increased many fold if 5000 seeds are planted instead of 500.

46. The Plant Breeder's Advantage. It has been shown that the breeder of animals has the advantage of the breeder of plants in that he can more easily control the mating of the parents. The breeder of plants has a distinct advantage in being able to work with large numbers.

In the case of livestock only the inferior females can be discarded, because in working with adults the expense of discarding

the adults cannot be afforded. Indeed the number of sires that are to be found in the upper end of the curve is so small that the sires are apt to be but little if any better than the average. In the breeding of animals in practice it is the few inferior animals represented by the lower end of the curve that are discarded. In the case of plants, however, embryo plants (seeds) are produced in such abundance and at so small expense that only the few at the upper end of the curve which are distinctly superior need be saved. Instead of discarding the poorest ten per cent, as in the case of animals, only the best five, or even one, per cent may be saved in the case of plants.

Practicums.

47. TO DEMONstrate the LAW OF VARIATION FROM TYPE.-Take one hundred ears of maize of one variety. Take weight of each ear in grams, or ounces, and mark with gum label. Arrange ears in order of weight. Furnish each student with a sheet of cross section paper, five inches square, with twenty sections to the inch, or five by ten inches, ten sections to the inch, and have each plot the curve indicated by the weight of the hundred ears. If necessary to save time, the instructor may have ears weighed and marked in advance of the class exercise. Variations in the length of one hundred ears may be shown in the same way.

Variation in the weight of grains of wheat may be shown if facilities for accurate weighing are at hand. The larger the number of grains used the better.

48. ORGANS of ReproduCTION.-In order to become familiar with the floral parts of wheat and other cereals, furnish each student with several heads of wheat in different stages of inflorescence:

I.

Describe ovulary and state changes in size at different stages of maturity. 2. Describe stigmas, state number of styles and position at various stages of maturity.

3. Describe length and position of filaments at different stages of maturity and note manner and mode of attachment of filament to anther.

4. Describe method by which anthers open and discharge their pollen. Describe the pollen grain.

For a portion of this work a high power microscope will be desirable. A twoinch, two-thirds-inch and one-sixth objectives will be found suitable. With a large class specimens may be prepared by the instructor and placed under one or more microscopes and each student allowed to inake examination by turn.

To show that rye is cross-fertilized, while wheat is generally self-fertilized, a similar study of rye may be made. The large anthers and abundant pollen of the rye will be found to be the most striking contrast.

49. TIME AND Manner of BLOOMING.-The student may be required to watch the opening of the wheat flower and the discharge of the pollen. Hays has shown that this whole process may take place in less than an hour in spring wheat and that it usually occurs in the early morning hours.

The opening of wheat flowers. (After Hays.)

50. COLLATEral Reading.-Selection and Its Effect on Cultivated Plants. Henry L. De Vilmorin. Experiment Station Record, Vol. XI, pp. 3-19.

Plant Breeding. Willet M. Hays. Division of Vegetable Physiology and Pathology, U. S. Department of Agriculture, Bul. 29, pp. 7-24.

The Station for Plant Breeding at Svalöf, Sweden. By David G. Fairchild. Experiment Station Record, Vol. XIII, pp. 814-819.