(1) Menders law of hybrids.
The preceding discussion represents fairly well the general understanding of hybrids until about 1900, when DeVries and Correns rediscovered what is now termed "Mendel's law of hybrids." While Mendel's laws or principles may not be of great value from an economic standpoint, they have proved of the greatest scientific interest, and the general fundamental principles of the law or laws should be thoroughly understood by every practical breeder of plants. It has been known for many years that a splitting up and redistribution of parental characters occurs in hybrids, and it is on this fact largely that the practical application of hybridization in plant-breeding depended. Ordi narily, careful plant-breeders would plan to hybri dize varieties or races having a definite combi nation of characters in view, as, for example, the combining of the fruit quality of one parent with the hardiness or drought-resistance of the other. Until Mendel's law was discovered, however, we had no understanding of why or how such a com bination could be made, and it was necessary to experiment extensively in order to determine what could be accomplished.
Mendel's law includes several important features which must be thoroughly understood before its important bearings can be comprehended. One re quisite for the application of the law is that the two parents shall possess certain characters that are opposed to each other. These two opposing qualities or characters are termed a " character pair." As illustrations of such character-pairs, may he cited bearded and bald heads in wheat, sweet and starchy kernels in corn, fuzzy and smooth seeds in cotton, and stringy and stringless pods in beans. When parents possessing these opposed or contrasted characters are crossed, the hybrid contains a combination of the potentialities representing both characters, and the first-gene ration hybrid will thus show an intermediate form of the particular character under consideration in case the two characters are of equal strength or potency. If, however, as sometimes occurs, one of the characters is very strong or dominant, only this character will show in the first-generation hybrids, the other character remaining recessive or masked, although present. For example, in
crossing a race of wheat having bald heads with a race having bearded heads, all of the first-generation hybrids, or at least the major ity of them, will have bald heads, this character being strong or dominant over the bearded char acter. In some instances where the potentialities of these two characters appear to be of nearly equal strength or potency, the beards seem to be produced in the first- generation hybrids but are reduced in length, being intermediate between the bald and the bearded state. A number of inter mediate cases of this kind were shown to the writer by Dr. C. E. Saunders, of the Canadian Experimental Farms. Frequently, in crossing flow ers of different colors, the resulting hybrids will show a blend of the two colors, being light pink, for example, when the parents crossed are a white and a red. In other cases, however, one color or the other becomes the dominant character, and the first -generation hybrids show the color of one parent only.
The second important principle of Mendel's law is what is termed the purity of the germ-cell. It seems certain from the researches that have been conducted that, when the germ-cells of the first generation hybrids are formed, the potentialities which represent the two different characters under consideration, and which were united by the hybri dization, ordinarily segregate again in the cell divisions, which lead to the formation of the germ cells, so that certain germ-cells include the poten tiality of one only of the two characters. We have thus two kinds of germ-cells formed with respect to this one character-pair. Taking as an illustra tion a hybrid of wheat having bald heads with one having bearded heads, when the germ-cells were formed a segregation of the two potentialities representing the two opposed characters would take place, and we would have germ-cells of one kind containing the bald-head potentiality and of a second kind containing the bearded-head potential ity. This segregation, it must be understood, takes place in the formation of both the egg-cells and the sperm-cells or pollen-grains.