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 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 cion and of the stock must come together. Therefore the cion is set in the side of the stock. There are many ways of shaping the cion and of preparing the stock to receive it. These ways are dictated largely by the relative sizes of cion and stock, although many of them are matters of personal preference. The underlying principles are two: securing close contact between the cambiums of cion 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 cion is firmly inserted. Fig. 165 shows the cion; Fig. 166, the cions 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 practiced in spring, as growth begins. The cions are cut previously, when perfectly dormant, and from the tree which it is desired to propagate.'' The cions are kept in sand or moss in the cellar. Limbs of various