. The cereals in America. Grain. IMPROVEMENT OF FIELD CROPS 21. Influence of crossing as a cause of variation. yield in grains of I 00 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 âxâ (After Hays.) 42. Crossing.âCrossing 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
. The cereals in America. Grain. IMPROVEMENT OF FIELD CROPS 21. Influence of crossing as a cause of variation. yield in grains of I 00 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 âxâ (After Hays.) 42. Crossing.âCrossing 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 character- istic 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.âPlants 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 ;^ 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 â ^ Willet M. Hays. Plant Breeding. Division of Vegetable Physiology and Pathology, U. S. Department of Agriculture, Bui. 29, p. 21. 2 Henry L. De Vilmorin. E. S. R., Vol. XI, p. Please note that these images are extracted from scanned page images t
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