Bark, the outer covering of trees and plants. It is found in its complete form only in the exogenous and gymnospernious classes, in which it consists of three portions, often quite distinct, but generally closely blended: the liber or inner bark (endophlczum), the cellular tissue or green layer (mesophlceum), and the corky envelope (epiphlceum). The liber, or fibrous bark, consists of bast cells, long, with thick walls, formed of cellulose; liber cells, thin-walled, of ordinary parenchyma, marked with reticulated spots, and seldom if ever absent from the liber; and laticiferous tubes, containing various secretions. The cellular envelope, which usually disappears after the second year, is formed of loose parenchyma, giving the bark its green color. The suber, or corky envelope, consists of cork, formed of parenchymous cells with thin walls and rectangular section, soon dead and empty; and periderme, of flat, thick-walled cells united in layers. The epidermis or outer skin is not permanent, but breaks away as the layers beneath it expand. The bark serves as a channel through which the sap elaborated by the leaves descends to feed the cambium layer, with which the bark is continuous, and by which it grows in annual rings, as does the wood itself.

The medullary rays also connect the bark and wood and afford channels for the deposit of the solid contents of the wood cells. From this it follows that while the youngest part of the wood is on the outside, the youngest part of the hark is on the inside; and when the newly formed cells are gorged with sap in the spring the hark may be readily separated from the wood; the newly formed cells are also the first to decay in the dead wood. The course of the sap is seen by cutting horizontally through the bark, when the upper edge of the cut will be moistened with the oozing sap, while the lower is nearly dry. Cutting off entirely the circulation of sap, as in girdling, destroys the tree. Bark may be reduced to extreme thinness, as in the grape vine, which sheds its liber annually, or be very thick, as in the sequoia gigantea, where it attains a thickness of two feet. The fibres, usually called hast (see Bast), are sometimes wanting, and are sometimes found in the woody portion of the stem. When present they are frequently limited to the young plant. They are of use when tenacious for cordage, many barks well supplying the place of ropes even in the construction of bridges.

The leatherwood (dlrca palustris), and the inner bark of the white cedar, are used in this country in place of hempen cordage, and the fibres may be soaked and felted into a cloth or paper, as in the tapa of the Pacific islanders. In the West Indies a remarkably tough bark called miha-gua is in general use for a great variety of purposes, and the hibiscus fibres are well known throughout the tropics. The corky envelope occurs on many trees, but attains a remarkable thickness on certain species of the oak. (See Cork.) Bark contains many of the secretions of the sap, and thus has many economic uses as a reservoir of vegetable products. The Peruvian bark (see Cinchona) is the source of quinine; the Angostura bark (galipea officinalis), canella bark (from C. alba), cascarilla (proton cascarilla), and other species, are well known drugs. Cinnamon is the bark of cinnamomum Ceylonicum, a lauraceous tree, native of Ceylon. Quercitron bark is the yellow dyestuff of quercus tinctoria. From the tannin which barks contain, especially oak and hemlock barks, arises their importance in the making of leather.