If the ringing is done on a young and vigorous branch of an old tree, the lower lip may receive supplies from the leaves of branches below the wound, or from shoots which spring from adventitious buds close to it, and the wound may heal over normally. Such healing may be rendered more certain by keeping the wounded surface moist--_e.g._ by means of damp moss, and so encouraging the formation of callus-bridges from the medullary rays. If on ringing a tree or a branch the young wood is removed as well as the cambium and cortical layers, the death of the parts above the wound is almost certain, owing to the stoppage of the transpiration current: the exceptions to this rule depend simply on the existence of other channels of communication, such as internal phloems, very thick sap-wood, and so forth. _Bruises._--If a branch or woody stem is struck sharply, with a hammer, for instance, the bruised cortex, phloem and cambium are killed by the blow, and the general effect is as if these tissues had been removed at that spot by the knife, but with the following complications. The bruised cortical tissues rapidly dry as they perish, and may adhere to the wood below. Consequently the still sound parts bordering on the wound are not released from pressure, but, on the contrary, have to advance towards each other over the surface of the wood under still greater pressures, in part due to the tightening of the whole cortex as the dead parts dry and contract, and in part due to the above-mentioned adherence of the latter to the wood. It results from this that such wounds heal very slowly and badly, and when the killed patch at last ruptures, wound-fungi, insects, and other injurious agencies may get in and do irreparable damage, as has been found to occur in cases where such wounds have been made in striking trees to shake down insects, fruit, etc. NOTES TO CHAPTER XXI. The essential facts regarding wounds and healing by occlusion are given in Marshall Ward, _Timber and some of its Diseases_, 1889, chapters viii. and ix., and in Laslett, _Timber and Timber Trees_, 1894, chapters iv. and v. More detailed treatment will be found in Frank, _Krankh. d. Pflanzen_, B. 1. cap. 2, where the special literature is collected. The reader may also consult Hartig, _Diseases of Trees_, Engl. ed. 1894, pp. 225-269. CHAPTER XXII. NATURAL WOUNDS. _Burrows and excavations. Bark-boring--Wood-boring--Wood fungi--Leaf-miners--Pith flecks--Erosions. Skeleton leaves-- Irregular erosions--Shot holes. Frost cracks--Strangulations-- Spiral grooving._