If a suitable mass of living cells is neatly cut with a sharp razor the first perceptible change is one of colour: the white "flesh" of a potato or an apple, for instance, turns brown as the air enters the cut cells, and the microscope shows that this browning affects cell-walls and contents alike. The cut cells also die forthwith; and the oxygen of the air combining with some of their constituents forms the brown colouring matter which soaks into the cell-walls. The uninjured cells below them grow longer, pushing up the dead débris, and divide across by walls parallel to the plane of the wound, and so form series of tabular cells with thin walls, which also soon turn brown and die, the cell-walls meanwhile undergoing changes which convert them into cork. The living cells deeper down are now shut off from the outer world by a skin, of several layers, of cork-cells, which prevent the further free access of air or moisture. During the period of active cell-division which initiates the cork, the temperature of the growing cells rises: a sort of fever (wound-fever) is induced, evidently owing to the active respiration of the growing cells. This healing by cork occurs in any tissue of living cells exposed by a cut--leaf-tissue, young stem or root, fruit, cambium, etc.; and the same applies to any other kind of cutting or tearing injury--such as a prick with a needle or the proboscis of an insect, a stripping, or even a bruise. Such healing is prepared for and carried out very thoroughly in the case of falling leaves and cast branches, the plane of separation being covered by a cicatrix of cork. If the cell-tissue under the wound is actually growing at the time, however, a further process is observed when the wound-cork has been formed. The uninjured cells below go on growing outwards more vigorously than ever, the pressure of the overlying tissues taken off by the cut having been removed, and, lifting up the cork-layer as they do so, they rapidly divide into a juicy mass of thin-walled cells which is of a cushion-like nature and is termed a _Callus_. This callus is at first a homogeneous tissue of cells which are all alike capable of growing and dividing, but in course of time it undergoes changes in different parts which result in the formation of tracheids, vessels, fibres and other tissue-elements, and even organs, just as the embryonic tissues of the growing points, cambium, etc., of the healthy plant give origin to new growths. Such wound-wood, however, is apt to differ considerably in the arrangement, constitution and hardness of its parts as compared with normal wood, and its peculiar density and cross-graining are often conspicuous.