Page:Popular Science Monthly Volume 77.djvu/353

Rh interpretations need not bother us. Suffice it to say that most plant breeders have accepted the explanation that the recessive character is simply the lack or absence of the character in question, while the dominant character is its presence. This is simply a slightly different interpretation of the same facts and simplifies some of the more complex results of crossing. Instead of 50 per cent, of the germ cells bearing the flint character and 50 per cent, bearing the sweet character when sweet corn is crossed with flint corn, one should think of all of the germ cells bearing the ability to produce the wrinkled sweet corn seeds, but that 50 per cent, of them contain in addition the presence of a flint or starch producing character. In other words, the "starchy" character is superimposed upon the "sweet" character. The dominant and recessive characters in such a cross, then, are simply the presence and absence of the starchy character.

When several character pairs differentiate the two parent plants in a cross, all possible recombinations are formed, the relative frequency with which the combinations occur being simply the algebraic product of as many of the simple ratios as there are character pairs.

The importance of these Mendelian facts to the commercial plant breeder is great. In crossing plants differing in several simple characters that segregate after hybridization he may rest assured of two things. First, that with a sufficient number of progeny in the second hybrid generation, every possible recombination of the characters present will be represented by at least one pure specimen. Second, that these pure specimens when selfed, or pollinated with their own pollen, will breed true. It should be remembered, however, that one may have to self a number of plants to get the combination desired with all characters pure, for if any dominant characters are concerned, their purity can be ascertained only by breeding for another generation. As an illustration we may take the snap dragon, Antirrhinum. There is a long series of colors that segregate. There is also a type called the "Delilah," where the tube of the corolla is uncolored. Starting with this form in only one color, the whole color series of Delilah forms may be reproduced by crossing with the self colored strains. Or, one may combine the dwarf habit of growth of the Dwarf Champion tomato, with any of the various colors and shapes now on the market which have the ordinary tall habit of growth. Sometimes a very simple recombination is of very great commercial value. The so-called Havana type of wrapper tobacco grown in the Connecticut River valley has large leaves and a short stocky habit of growth. It produces from nineteen to twenty-one leaves. There is another type grown under cheese cloth shade which has a tall habit of growth with about twenty-six smaller leaves. The tall slender habit of growth makes it an undesirable type to grow in open fields where it is apt to be blown down. Mr.