Steps in tongue grafting. a, the two branches to be formed; b, a tongue cut in each; c, fitted together; d, method of wrapping.
Grafting.—Of much the same nature is grafting. Here, however, a small portion of the stem of the closely allied tree is fastened into the trunk of the growing tree in such a manner that the two cut layers just under the bark will coincide. This will allow of the passage of food into the grafted part and insure the ultimate growth of the twig. Grafting and budding are of considerable economic value to the fruit grower, as it enables him to produce at will, trees bearing choice varieties of fruit.[34]
Other Methods.—Other methods of plant propagation are by means of runners, as when strawberry plants strike root from long stems that run along the ground; layering, where roots may develop on covered up branches of blackberry or raspberry plants; slips, roots developing from stems which are cut off and placed in moist sand; from tubers, as in planting potatoes; and by means of bulbs, as the tulip or hyacinth. All of the above means of propagation are asexual and are of importance in our problem of plant breeding.
Plant breeding plots. (Minnesota Experiment Station.)
Illustration of Mendel's Law.
The Work of Gregor Mendel.—Fifty years ago, an Austrian monk, Gregor Mendel, found in breeding garden peas that these plants passed on certain fixed characters, as the shape of the seed, the color of the pod when ripe, and others, and that when two pea plants of different characters were crossed, one of these characters would be likely to appear in the offspring of the second generation in the ratio of three to one. Such characters as would appear to the exclusion of others in the first crossing of the plants were called dominant, the ones not appearing, recessive characteristics. When these seeds were again sown the ones bearing a recessive characteristic would produce only peas with this recessive characteristic, but the ones with a dominant characteristic might give rise to a pure dominant or to offspring having partly a dominant and partly a recessive character; pure dominants being to the mixed offspring in the ratio of 1 to 2. The pure dominants if bred with others like themselves would produce only pure dominants, but the cross breeds would again produce mixed offspring of three kinds in the ratio of one dominant to two cross breeds and one recessive. The feature of this work that interests us is that unit characters are passed along by heredity in the germ cells pure, that is, unchanged, from one generation to another, and independently of each other.
Determiners of Character.—A child then resembles his parents in some definite particulars because certain determiners of characters have been present in the germ cells of one of the parents. If the determiner of a certain character is absent from the germ cells of both parents, it will be absent in all of their offspring.
