destroyed the outer skin, even if only for a very small space, decay begins at the wound, and spreads till it destroys the branch, unless warmer weather quickens growth in the weakened plant, when the process comes to a stop.
The most common protective armor of the higher and woody plants is the cork-bark. The corky substance is of itself extraordinarily tough, and even when dead is only very slowly destroyed by molds. Tree-barks also appear generally to contain chemical substances that operate as poisons upon the lower organisms. The most commonly diffused of such substances are tannin and other coloring-matters allied to it. Many barks also contain strong bitter matters and alkaloids, like salicin, pinipicrin, quercitrin, sesculin, chinin, aricin, strychnin, berberin, etc. The most persistent waxes also reside in the bark, and ethereal oils in individual cases, as in the laurels. The general diffusion of these substances in the bark is the more remarkable because they are rarely found in the wood or the annual leaves.
The subterranean parts of plants need the same protection as the stem. Swamp-plants in particular, which grow in a soil always undergoing decomposition, would fall a prey to decay very quickly without some especial defense. All plants whose organization does not insure them against the action of swamp-soil, perish alike, even under exposure in the winter, at any other time than during the growing season. The under-ground parts of plants growing in such soils are protected in part by the hard epidermis, partly by coloring-matter, as in the alder, comarum, and sanguisorba; or by bitter constituents, as in menyanthes; or by ethereal and aromatic substances, as in valerian and acorus; or by acrid matter, as in frangula and the Ranunculaceœ. Antiseptic substances, such as tannin, saponin, and phloridzin, are also found in the underground organs of plants that do not grow in swamps, and the strong essences of the rhizoma of ferns and of the punica-root belong to the same class.
Evergreen leaves, besides requiring means of defense against the lower organisms, need protection against the higher animals, which would consume them during the winter if they were quite accessible and enjoyable. Ulex and Ruscus, therefore, have thorny limbs, smilax and the evergreen brambles and roses have spines on their leaf-nerves, juniper and the holly-leaved plants have thorny leaves. The foliage of yew, arbor-vitæ, ledum, rhododendron, oleander, and laurocerasus is poisonous; and the palatableness of the leaves of pine and spruce, of laurel, ivy, and box, is at least very limited. Only the common underwood, consisting mostly of plants of the heath family, which are to a great extent covered during winter by leaves and snow, contain food for animals in their leaves and twigs. These leaves are likewise defended against rots by the poison in the poisonous kinds, by the hard, bright epidermis in the hollies, and presumably by chemical qualities; the heaths contain a coloring-matter.