We must first understand how a wound heals on a tree. Note any wound,--knot-hole on the trunk, place where wood has been removed. The exposed wound itself does not heal; it is covered and inclosed by tissue built out from the edges or periphery of the wound. This tissue is like a roll. It is the callus. Eventually the tissue meets in the center, and the lid is thereby put on the place, and it is sealed. The exposed wood has died, if it is the cross-section of a branch or a deep wound, and it remains under the callus a dead body. If the wood has not started to decay in the meantime, the place is safe, but too often invasion has begun before the process is complete, the rot disease finally extends to the heart of the tree, causing it to become hollow. If the center of the wound falls in, the callus cannot cover it, and an open sore remains. In these cavities birds may sometimes build. Therefore there are two points for the surgeon to consider in respect to the wound itself--whether it is so placed on the tree that the callus forms readily; whether the wound is kept healthy during exposure. All ragged tissue being removed, deep-wound surfaces should be kept aseptic. For ordinary cases, white-lead paint with plenty of linseed oil is a good protective from the germs of decay. On old wood, no longer active, creosote is good, perhaps followed by coal-tar. Usually, however, paint is quite sufficient. Small exposures usually receive no dressing. When the fresh surface wood is exposed by removal of bark, it is necessary to keep the tissue from drying out, and antiseptics are usually not applied. Bandaging with cloth is the usual practice, after the wound is cleaned and trimmed. The repairs fall into two classes,--those that require merely removal of injured parts and treatment of the wounds, and those that demand the ingrafting of new wood. We have learned, in the discussion of pruning, that long projecting ends of severed branches do not heal. The branches to be removed should be cut back close to the larger branch or to the juncture with another. In repairing injured trees, all projecting parts that do not have life in themselves must be removed. All wounds should be left smooth, without splinters or hanging bark. Decaying wood is to be removed, and the area cleaned out and disinfected. The nature-lover may find much to interest him in the observation of knot-holes as he comes and goes. Every knot-hole has a history; this history usually can be traced by one whose eye is keen and who becomes practiced in connecting cause with final result. One prides oneself on the ability to work out the obscure cases. An old neglected apple orchard thereby affords much entertainment. If a very large branch breaks off, the remaining part is cut back to fresh hard wood; antiseptic is applied; the other part of the tree may be shortened-in to aid in restoring the proportion or balance.