Fig. 164.—Currant Cutting.
The Graft.—When the cutting is inserted in a plant rather than in the soil, it is a graft; and the graft may grow. In this case the cutting grows fast to the other plant, and the two become one. When the cutting is inserted in a plant, it is no longer called a cutting but a scion; and the plant in which it is inserted is called the stock. Fruit trees are grafted in order that a certain variety or kind may be perpetuated, as a Baldwin or Ben Davis variety of apple, Seckel or Bartlett pear, Navel or St. Michael orange.
Plants have preferences as to the stocks on which they will grow; but we can find out what their choice is only by making the experiment. The pear grows well on the quince, but the quince does not thrive on the pear. The pear grows on some of the hawthorns, but it is an unwilling subject on the apple. Tomato plants will grow on potato plants and potato plants on tomato plants. When the potato is the root, both tomatoes and potatoes may be produced, although the crop will be very small; when the tomato is the root, neither potatoes nor tomatoes will be produced. Chestnut will grow on some kinds of oak. In general, one species or kind is grafted on the same species, as apple on apple, pear on pear, orange on orange.
The forming, growing tissue of the stem (on the plants we have been discussing) is the cambium (Chap. X), lying on the outside of the woody cylinder beneath the bark. In order that union may take place, the cambium of the scion and of the stock must come together. Therefore the scion is set in the side of the stock. There are many ways of shaping the scion and of preparing the stock to receive it. These ways are dictated largely by the relative sizes of scion and stock, although many of them are matters of personal preference. The underlying principles are two: securing close contact between the cambiums of scion and stock; covering the wounded surfaces to prevent evaporation and to protect the parts from disease.
On large stocks the commonest form of grafting is the cleft-graft. The stock is cut off and split; and in one or both sides a wedge-shaped scion is firmly inserted. Fig. [165] shows the scion; Fig. [166], the scions set in the stock; Fig. [167], the stock waxed. It will be seen that the lower bud—that lying in the wedge—is covered by the wax; but being nearest the food supply and least exposed to weather, it is the most likely to grow: it will push through the wax.
Cleft-grafting is practised in spring, as growth begins. The scions are cut previously, when perfectly dormant, and from the tree which it is desired to propagate. The scions are kept in sand or moss in the cellar. Limbs of various sizes may be cleft-grafted,—from a half inch up to four inches in diameter; but a diameter of one to one and a half inches is the most convenient size. All the leading or main branches of a tree top may be grafted. If the remaining parts of the top are gradually cut away and the scions grow well, the entire top will be changed over to the new variety.
| Fig. 165.—Scion of Apple. | Fig. 166.—The Scion Inserted. | Fig. 167.—The Parts Waxed. |
Another form of grafting is known as budding. In this case a single bud is used, and it is slipped underneath the bark of the stock and securely tied (not waxed) with soft material, as bass bark, corn shuck, yarn, or raffia (the last a commercial palm fibre). Budding is performed when the bark of the stock will slip or peel (so that the bud can be inserted), and when the bud is mature enough to grow. Usually budding is performed in late summer or early fall, when the winter buds are well formed; or it may be practised in spring with buds cut in winter. In ordinary summer budding (which is the usual mode) the “bud” or scion forms a union with the stock, and then lies dormant till the following spring, as if it were still on its own twig. Budding is mostly restricted to young trees in the nursery. In the spring following the budding, the stock is cut off just above the bud, so that only the shoot from the bud grows to make the future tree. This prevailing form of budding (shield-budding) is shown in Fig. [168].