Natural wounds are produced in a variety of ways during the life of the plant, and, generally speaking, are easily healed over by the normal process if the area destroyed is not too large, and the parts remaining uninjured are sufficiently provided with foliage, or with supplies of food-materials stored up in the roots, rhizomes, medullary rays, etc., to feed a vigorous callus. The nature of such wounds and the mode of healing are explained by what we know of artificial wounds, and it only remains to point out that the principal danger of ordinary wounds is not so much the direct traumatic action, because the simpler organisation of the plant does not involve matters connected with shock, loss of blood, etc., as in animals; the danger consists, rather, in their affording access to other injurious agents, especially fungi, and the treatment of wounds frequently resolves itself into cutting or pruning in order to get clean surfaces which can heal readily. Wounds on leaves imply loss of foliar surface--_i.e._ of chlorophyll action--and the remarks on page 193 apply. _Burrows_ may be taken as comprising all kinds of tunnel-like excavations in the various organs of plants, including those cases where insects burrow into hollow stems of grasses, etc., as indicated by the perforations they make in the outer tissues. _Bark-boring_ is done by many species of beetles, especially _Scolytidae_, which excavate characteristically formed branching passages tangentially in the inner bark of Conifers and other trees. Some of them also bore down to the surface of the sap wood (_e.g._ _Tomicus bidentatus_) or even burrow right into the latter (_e.g._ _T. lineatum_). It commonly happens that the external apertures show up clearly, owing to the brown dust and excrement, sometimes accompanied by turpentine, which exude from them. Many of these Bark beetles only attack trees which are already injured by fire, lightning, etc.; possibly they cannot bore through a cortex which swamps them with sap, as a vigorous one might do. _Wood-boring_ is also done by many of the bark-beetles as well as by Longicorns, _e.g._ _Saperda_ in Poplars and Willows, the young shoots of which often show characteristic swellings with lateral holes indicating the points of exit. From the external apertures comminuted wood, like saw-dust, is frequently ejected in quantity and betrays the presence of the insects. Certain wood-wasps (_Sirex_) and the larvae of moths (_Cossus_) also make large perforations in the wood of Willows and other trees, often destroying it completely. In the case of these larger borers, whose tunnels may be as broad as the little finger, the foul smell as well as abundant "saw-dust" betray the evil.