4th--Wounds, and the attacks of insects, may be considered more in the light of mechanical injuries by a loss of substance, hence they belong rather to the department of surgery, and can scarcely be considered as disease. The breaking of a branch, or the removal of a portion of the bark, may inflict a serious injury, but it is one which, under ordinary circumstances, will be recovered from, without any impairment of the health of the tree--unless where the wound is so large that the new growth will not soon cover it over, in which case exposure to the moisture of the atmosphere may result in decay of the woody tissues, or, if the sap exudes, at certain seasons, it may produce canker and fungous growths. In some varieties of our cultivated fruits, wounds of this character are often attended by an effusion of gummy matter; this is particularly the case with those that are known as stone-fruits, and in these the excision of a large limb is seldom followed by a deposit of woody matter in the way of healing over the wound, which is always desirable; hence in such cases particularly, it is well to cover the exposed surface with something to exclude atmospheric moisture, whether this be paint, varnish, of shellac, or common grafting wax. Insects, by eating the foliage extensively, very materially injure the healthy condition of a tree--even the minute aphides that suck the sap from the leaves and tender bark, will seriously impair the health of our plants; but the borers that mine under the bark, extensively consuming the vital cambium, and even burrow into the solid wood, reducing it to a honeycomb, cannot fail to affect the healthy condition of the tree materially, and often cause its premature death. Some knowledge of the habits of these little creatures is considered of so great importance, that the subject will be brought before the reader's notice more at length in another part of this volume. There is no doubt, however, that many unhealthy conditions of our trees, that might be traced to other causes, but which are not manifestly dependent upon a want of care on the part of the orchardist, nor upon a deficiency in the constitution of the soil, are often attributable to the inroads of these minute foes, which, in some cases at least, are made the scape-goats upon which is laid the blame that should be applied to our own neglect, or want of forethought and care. * * * * *