If instead of a simple tissue, the cut or other wound lays bare a complex mass such as wood, the resultant changes are essentially the same to start with. The living cells bordering the wound form cork, and then those deeper down grow out and form a callus. The exposure of the wood however, entails alterations in its non-living elements also. The lignified walls of tracheids, fibres, etc., turn brown to a considerable depth, and this browning seems to be--like all such discolorations in wounds--due to oxidation changes in the tannins and other bodies present: the process is probably similar to what occurs in humification and in the conversion of sap-wood into heart-wood in trees. Such wood is not merely dead, but it is also incapable of conveying water in the lumina of its elements, which slowly fill with similarly dark-coloured, impervious masses of materials termed "wound-gum," the nature of which is obscure, but which slowly undergoes further changes into resin-like substances. The exposure of wood by a wound results also in another mode of stopping up the vessels and so hindering the access of air, loss of water, etc., for the living cells of the medullary rays and wood-parenchyma grow into the lumina of the larger vessels through the pits, forming _thyloses_, again a phenomenon met with in heart-wood. In Conifers the stoppage of the lumina is increased by deposition of resin, which also soaks into the cell-walls and the wounded wood becomes semi-translucent owing to the infiltration. Every living cell in an active condition is irritable, and one of the commonest physiological reactions of growing tissues is that of responding to the touch of a resistant body, as is vividly shown by the movements of the Sensitive plant, _Dionaea_, etc., and by those of tendrils, growing root tips, etc., on careful observation. We have reason for stating that if a minute insect, too feeble to pierce the cuticle, cling on to one side of the dome-shaped growing point of any shoot, the irritation of contact of its claws, hairs, etc., would at once cause the protoplasm of the delicate cells to respond by some abnormal behaviour; and, as matter of experiment, Darwin showed long ago that if a minute piece of glass or other hard body is kept in contact with one side of the tip of a root, the growth on the side in contact is interfered with. Moreover we know from experiments on heliotropism, thermotropism, etc., that even intangible stimuli such as rays of light, etc., impinging unsymmetrically on these delicate cells cause alterations in their behaviour--_e.g._ arrest or acceleration of growth.