HOW PLANTS BUSTS T DECAY. 525 



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 alka- 

 loids, like salicin, pinipicrin, quercitrin, sesculin, chinin, aricin, strych- 

 nin, 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 be- 

 cause 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 under- 

 going 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 expos- 

 ure 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 men- 

 yanthes ; or by ethereal and aromatic substances, as in valerian and 

 acorus ; or by acrid matter, as in frangula and the Ranunmlacem. An- 

 tiseptic 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, smi- 

 lax 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-vitaB, ledum, rhododendron, oleander, and lauro- 

 cerasus is poisonous ; and the palatableness of the leaves of pine and 

 spruce, of laurel, ivy, and box, is at least very limited. Only the com- 

 mon underwood, consisting mostly of plants of the heath family, which 

 are to a great extent covered during winter by leaves and snow, con- 

 tain food for animals in their leaves and twigs. These leaves are like- 

 wise defended against rots by the poison in the poisonous kinds, by 

 the hard, bright epidermis in the hollies, and presumably by chemical 

 qualities ; the heathsxcontain a coloring-matter. 



