Page:Popular Science Monthly Volume 77.djvu/350

nished the pollen and half from the plant to which it has been applied. If one studies the characters of several such hybrids, he finds many surprising facts. It usually makes no difference which plant is the mother plant, the result is the same. Certain characters are found in the hybrid that are identical with those possessed by the male parent and other characters the same as those possessed by the female parent. Other characters appear to have resulted from the blending of those of the two parents, while still others appear to be entirely new. The plant may be sterile if the cross is between widely differing species, but if it is fertile and the flower of the hybrid is self-fertilized, the plants resulting from this seed present still more surprises. For example, if one has crossed a pear-shaped yellow tomato with a round red tomato, in the second hybrid generation he will find individual plants bearing fruit of four kinds, pear-shaped yellow and round red, as were the two parents, and in addition pear-shaped red and round yellow. In other words all possible combinations occur and in definite proportions. Stated as a principle it may be said that where either of the parent plants possesses characters absent from the other, the potential characters remain pure in the germ cells of the hybrid and recombine as if by chance. This is the most important feature of the only law of heredity of which there is any exact knowledge—the law of Mendel. Let us illustrate the action of the law. Such a character as starchiness, as shown in "flint" maize, is either present or it is not present. The flinty appearance of the seed is due to the possession of some character that causes the maturation of plump starch grains. When this character is absent, the seeds dry up without maturing their starch grains, and present the wrinkled appearance common to sweet maize. Pairs of characters such as these, affecting a certain plant structure, are called contrasted or allelomorphic pairs. When a sweet maize is crossed with a flint maize, the resulting seeds are all flint like. That is, the dominant character or the character that calls for the presence of the structure or compound in question, manifests itself in the first hybrid generation. Complete dominance, however, is not a general phenomenon in crosses and as its importance is slight as compared with the second law, that of segregation of the pure characters (potentially) in the germ cells of the hybrid, we will discuss it no further. The second law predicts that in the generations succeeding a cross, plants grown from the selffertilized seeds of the hybrid reproduce both contrasted characters in the proportion of three of the dominant or "presence" characters to one of the recessive or absent character. Furthermore, inbred or self-fertilized plants bearing the recessive character continue ever after to breed true, while of those plants bearing the dominant character one third are pure and breed true while two thirds are hybrids and again throw the recessive character in one fourth of their offspring.