« AnteriorContinuar »
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.
47. To Demonstrate The Law Of Variation From Type.—Take oiu 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:
1. 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 make 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.
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, Bui. 29, pp. 7-24.
The Station for Plant Breeding at Svalof, Sweden. By David G. Fairchild. Experiment Station Record, Vol. XIII, pp. 814-819.
51. Relationships.—Wheat belongs to the family of true
grasses (Gramineae). The Gramineae are characterized by , ^ 1 having hollow steins with closed joints, alternate leaves with their sheaths split open on the side opposite the blade. Wheat is included under the tribe Hordeae, in which the spikelets are one to many-flowered, sessile and alternate, thus forming a spike. (59) To this tribe belong also rye and barley, as well as the cultivated rye grasses (Loltum perenne L. and L. italicum Beam .). This tribe also includes some troublesome weeds. CoucJ>—^rass (Agropyron repens Beauv.), a perennial, was formerly included in the same genus as wheat. Because of its underground stems, or rhizomes, couch grass is difficult to eradicate and thus becomes a very troublesome weed in cultivated fields. Darnel (Lolium temulentum L.) is common in wheat fields in Europe and on the Pacific coast in this country. A related species (L. remotuvi) occurs in flax fields.
52. Roots.—When a grain of wheat germinates, it throws out a whorl of three seminal or temporary roots. The coronal or permanent roots are thrown out in whorls from the nodes. The distance between the temporary roots and the first whorl of permanent roots will depend somewhat upon the nature of the soil, but principally upon the depth of planting. The depth at which the first whorl of permanent roots occurs will vary with the soil, but is usually about an inch from the surface, irrespective of the depth of the grain or of the temporary roots. There is nothing in the nature of a tap root in any of the grasses such as is found in the legumes. Any node under the soil, or even near the soil, may throw out a whorl of roots. When wheat is planted under ordinary field conditions the roots curve slightly outward and then descend almost vertically. The more unoccupied soil about a wheat plant the more the roots curve outward. As soon as the available surface soil is occupied the roots'descend. An abundance of roots has been observed at a depth of four feet, and under favorable conditions they doubtless go much deeper. Schubart traced the roots of a winter wheat plant seven feet deep.1 Webber found that if the roots of one wheat plant were placed end to end they would reach 1,704 feet.2 Near the surface the roots branch and re-branch abundantly, filling the soil with a mass of roots, the ends of which are covered with root hairs. The Minnesota Station found about eight branch - to the inch on the main roots to a depth of eighteen to twenty inches, varying in length from one-half inch to twenty inches. Below this distance few or no branches were found, suggesting that the purpose of these deep roots was to secure water.3
53. Culms.—Like the majority of the plants of the grass family, wheat has usually hollow culms, but in some varieties this space is more or less filled with pith. The greatest variation is found in the upper internode, which should be examined in describing a variety. The walls of the culm also vary in thickness, and the surface varies in color, and may be whitish, yellow, purple or brownish. Just below the spike the surface of the culm is more or less furrowed. The length varies with type and variety. The same variety is variable on different soils, with different fertilizers, and in different seasons. The variation in length of stem and yield of straw is greater than in size or yield
1 Agricultural Botany. M. C. Potter, p. 170.
8 Minn. Bui. 62 (1899), p. 405.