BARK, the more or less easily separable layers of tissue surrounding the woody cylinder of trees and shrubs; also, by extension, the analogous part (cortex) of textile plants such as hemp, jute, ramie, flax, etc., and other annual stems. The layers are divided into three groups which may be readily seen in a yearling stem: (I) The phloEtn, bast, the inner food-conduct ing tissue annually thickened from the cam bium (q.v.) layer which separates it from the wood; (2) the green zone which generally does not increase in thickness but which in young twigs assists in food elaboration (see Piforo SYNTHESIS) ; (3) the epidermis or external layer with contiguous cork cells which increase from the phellogen, or cork 'cambium, a layer of epidermal or cortical cells. These cork cells, which develop mainly at right angles to the direction of the stem, die and become more or less weather-beaten and seamed from cracking and give the characteristic appearance to tree trunks. Many trees can be identified by their bark alone.
The bark of many trees and shrubs is of economic use mainly in tanning, dyeing, medi cine and cookery. In tanning (q.v.) such barks
as are rich in tannic acid are most in demand; oak, hemlock and chestnut (qq.v.) are general favorites in America and Europe; eucalyptus and acacia in Australia. Larch and willow bark are used for special work. To obtain these barks the trees are felled after the sap has started to flow in the spring, the rough exterior layers removed, the bark of the trunk and main limbs peeled off in lengths of about two feet with specially made tools; the bark of the smaller branches, in equal lengths, is loosened with mallets and slipped off. After removal the bark is loosely piled in open sheds to dry or stacked on end in the open air, the larger pieces being placed on the outside to protect the smaller inner ones f rom rain and sun, which together with mildew are the important agencies.that may injure the quality of the prod uct. The barks used in medicine, cookery, etc., are treated under individual titles. See CASCA