=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 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=. =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). [Illustration: FIG. 64.--ERRONEOUS PRUNING.] [Illustration: FIG. 65.--KNOT IN A HEMLOCK LOG.] =Large wounds should be protected= with a covering of paint, melted wax, or other adhesive and lasting material, to keep out the germs and fungi. A covering of sheet iron or tin may keep out the rain, but it will not exclude the germs of decay; in fact, it may provide the very moist conditions that such germs need for their growth. Deep holes in trees should be treated by having all the decayed parts removed down to the clean wood, the surfaces painted or otherwise sterilized, and the hole filled with wax or cement. [Illustration: FIG. 66.--A KNOT HOLE, and the beginning of a hollow trunk.] =Stems and roots are living=, and they should not be wounded or mutilated unnecessarily. Horses should never be hitched to trees. Supervision should be exercised over persons who run telephone, telegraph, and electric light wires, to see that they do not mutilate trees. Electric light wires and trolley wires, when carelessly strung or improperly insulated, may kill trees (Fig. 67). [Illustration: FIG. 67.--ELM TREE KILLED BY A DIRECT CURRENT FROM AN ELECTRIC RAILROAD SYSTEM.]