Wounds. - All the parts of plants are exposed to the danger of wounds, from mechanical causes such as, wind, falling stones or trees, hail, etc., or from the bites of animals such as rabbits, worms, and insects, and although such injuries are rarely in themselves dangerous, they open the way to other agencies-water, fungi, etc., which may work great havoc; or the loss of the destroyed or removed tissues is felt in diminished nutrition, restriction of the assimilative area, or in some other way.
We have seen that living cells die when cut, bruised, or torn; and that the cells next below in a layer of active tissue are stimulated by the exposure to increased growth and division, and at once pro duce a layer of cork, the impervious walls of which again protect the living cells beneath. This is found to occur in all cell-tissues provided the cells are still living, and it matters not whether the wound occurs in the mesophyll of a leaf, the storage parenchyma of a Potato-tuber, the cortex of a root or stem, or in the fleshy parts of a young fruit, the normal effect of the wound is in all cases to call forth an elongation of the uninjured cells beneath, in a direction at right angles to the plane of the injured surface, which cells then divide by successive walls across their axis of growth: the layers of cells thus cut off are then converted into cork, by the suberisation of their walls. Further changes may then go on beneath the protective layer of wound-cork thus produced, and these changes vary according to the nature of the cells beneath: the cambium forms new wood, the medullary rays similar rays, cortex new cortex, and so on.