Most woody plants increase in diameter by the addition of an annual layer or “ring” on the outside of the woody cylinder, underneath the bark. The monocotyledonous plants comprise very few trees and shrubs in temperate climates (the palms, yuccas, and other tree-like plants are of this class), and they do not increase greatly in diameter and they rarely branch to any extent.
Bark-bound Trees.—If, for any reason, the bark should become so dense and strong that the trunk cannot expand, the tree is said to be “bark-bound.” Such condition is not rare in orchard trees that have been neglected. When good tillage is given to such trees, they may not be able to overcome the rigidity of the old bark, and, therefore, do not respond to the treatment. Sometimes the parts with thinner bark may outgrow in diameter the trunk or the old branches below them. The remedy is to release the tension. This may be done either by softening the bark (by washes of soap or lye), or by separating it. The latter is done by slitting the bark-bound part (in spring), thrusting the point of a knife through the bark to the wood, and then drawing the blade down the entire length of the bark-bound part. The slit is scarcely discernible at first, but it opens with the growth of the tree, filling up with new tissue beneath. Let the pupil consider the ridges which he now and then finds on trees, and determine whether they have any significance—whether the tree has ever been released, or injured by natural agencies.
Fig. 63.—Proper Cutting of a Branch. The wound will soon be “healed.”
The Tissue covers the Wounds and “heals” them.—This is seen in Fig. [63], in which a ring of tissue rolls out over the wound. This ring of healing tissue forms most rapidly and uniformly when the wound is smooth and regular. Observe the healing on broken and splintered limbs; also the difference in rapidity of healing between wounds on strong and weak limbs. There is a difference in the rapidity of the healing process in different kinds of trees. Compare the apple tree and the peach. This tissue may in turn become bark-bound, and the healing may stop. On large wounds it progresses more rapidly the first few years than it does later. This roll or ring of tissue is called a callus.
Fig. 64.—Erroneous Pruning.
The callus grows from the living tissue of the stem just about the wound. It cannot cover long dead stubs or very rough broken branches (Fig. [64]). Therefore, in pruning the branches should be cut close to the trunk and made even and smooth; all long stubs must be avoided. The seat of the wound should be close to the living part of the trunk, for the stub of the limb that is severed has no further power in itself of making healing tissue. The end of the remaining stub is merely covered over by the callus, and usually remains a dead piece of wood sealed inside the trunk (Fig. [65]). If wounds do not heal over speedily, germs and fungi obtain foothold in the dying wood and rot sets in. Hollow trees are those in which the decay-fungi have progressed into the inner wood of the trunk; they have been infected (Fig. [66]).
Fig. 65.—Knot in a Hemlock Log.