Bark, the outer covering of the stems, branches, and roots of woody plants. It consists of all tissues outside of the vascular cambium, the layer of cells that encloses the inner stem. It is composed mainly of secondary phloem, cortex, cork cambium, and cork cells. (For illustration,
The secondary phloem, or inner bark, consists of living tissues. It conducts food, manufactured in the leaves, to the branches, trunk, and roots. If a complete ring of bark is removed from the trunk, the tree will die because the roots no longer obtain food from the phloem. The cortex, a layer of primary tissue beneath the epidermis, protects and strengthens and stores food. As the tree grows and the cork cambium develops, the cortex is pushed outward and eventually disappears. The cork cells, produced in the cork cambium, a layer of secondary growth cells that originates in the cortex, form the outer bark. The cells die upon reaching maturity. Their cell walls contain suberin, a fatty substance that acts as waterproofing. The outer bark forms a tough covering that reduces water loss and helps prevent damage from insect and animal pests, fungi, and bacteria.
The outer bark is constantly being shed and replaced as cork cells mature, die, and are pushed outward. As the tree increases in size, the outer bark is stretched until it usually splits or cracks. Some bark, such as that of the birch tree, peels off in strips. Other types of bark flake or shred. Some trees, such as the beech tree, always have smooth bark. Their outer bark grows so slowly and is so thin that fissures do not develop.
Bark at one time was used to make boats and canoes and baskets and other containers. Many kinds of bark have stringy inner fibers that have been used to make cloth, rope, and nets. Dyes are extracted from the bark of oak and many tropical trees. Tannin, used in tanning leather, comes from birch, oak, hemlock, and willow bark. Cinchona bark is the source of the drug quinine. Cinnamon comes from the inner bark of a tropical evergreen. Wild cherry bark is pleasantly flavored and is used in cough remedies. Cork is one of the most useful barks.