bark (plant)

bark outer covering of the stem of woody plants, composed of waterproof cork cells protecting a layer of food-conducting tissue—the phloem or inner bark (also called bast). As the woody stem increases in size (see cambium ), the outer bark of inelastic dead cork cells gives way in patterns characteristic of the species: it may split to form grooves; shred, as in the cedar; or peel off, as in the sycamore or the shagbark hickory. A layer of reproductive cells called the cork cambium produces new cork cells to replace or reinforce the old. The cork of commerce is the carefully harvested outer bark of the cork oak ( Quercus suber ), a native of S Europe. The phloem (see stem ) conducts sap downward from the leaves to be used for storage and to nourish other plant parts. "Girdling" a tree, i.e., cutting through the phloem tubes, results in starvation of the roots and, ultimately, death of the tree; trees are sometimes girdled by animals that eat bark. The fiber cells that strengthen and protect the phloem ducts are a source of such textile fibers as hemp, flax, and jute; various barks supply tannin, cork (see cork oak ), dyes, flavorings (e.g., cinnamon), and drugs (e.g., quinine). The outer bark of the paper birch was used by Native Americans to make baskets and canoes.

bark Outer protective covering of a woody plant stem. It is made up of several layers. The cork layer, waxy and waterproof, is the thickest and hardens into the tough, fissured outer covering. Lenticels (pores) in the bark allow gas exchange between the stem and the atmosphere. See also cambium