BARK. See BARQUE.
BARK (rortex), in phanerogamous or flowering plants, is the external covering of the stem. It is composed of layers of cellular tissue, whilst the woody stem, to which it forms a sort of sheath, is vascular. In endogenous plants (palms, etc.), there is not, in General, a very marked line of separation between the B. and the vessels or vascular bundles of the stem, so that these plants are generally, but incorrectly, said to have no i hark. It is in exogenous plants, and especially in perennial woody stems. that the development of B. is most perfect, and the distinction between wood and B. most rteu:ked. The outermost layer of the B. of exogenous plants is the epidermis (q.v.), ' whit . however, is in general only to be seen in annual stems, and in the youngest parts of IN laxly stems: peeling off as the stem becomes older along with the outer lavers of the true bark. Beneath the epidermis is the true B., of which the outer layer isicalled the eph m pum (Gr. outer bark), and consists of cells, usually rectangular and flattened, with thick walls. The inner layer of the true B. is called the mesophla,uut (Gr. middle bark), i and is generally formed of a cellular tissue of roundish cells with thin walls. These layers are sometimes very distinctly separated from one another, and sometimes pass gradually dually into one another; sometimes there is merely a continuous cellular tissue.
thin the true B. is a very distinct layer, the inner B., .fiber (Lat.) or endophlawm (Gr. inner bark), also frequently called bast, which is composed of bundles of woody fiber or vascular tissue mixed with cellular tissue. The layer of cambium (q.v.) is often regarded as belonging to the inner B., but rather belongs to the vascular part of the stem, In the inner B. are sometimes cells containing a milky juice, as in the apocynaceo, or vessels for a milky juice, as an the common fig. The combined strength and flexibility of the fibers of the inner 13. render it in many cases useful for various purposes. See FIBER and BAST. In the true 13., the peculiar juices and most characteristic substances elaborated by the plant are very generally found, for which reason that part is often of i the greatest importance in medicine and the arts. The 13. of many trees abounds in
tannin or tannic acid (q.v.).
The B. of a stem or branch of not more than one year old exhibits only a cellular integument or epidermis with an interior lining of woody fiber—the inner 13.; but new layers are added from year to year, the 13. as well as the woody stein being increased from the cambium, the mucilaginous layer which is interposed between them, and which particularly abounds is spring, when the separation of the B. from the stem is most easy. The annual layers, however, cannot long be distinctly recognized in the B. as in the wood; and in the older portions of woody stems, the outermost parts of the 13. become desiccated and lifeless, and are in general gradually thrown off. On this account, those mosses, lichens, and other plants which attach themselves only to the outermost layer of the B. of trees, and derive their nourishment from it, cannot be regarded as true parasites, as they are in no degree supported by the juices of the stem, but only consume and remove external matter already destitute of life. The II. of some trees is remark able for the thickness which it acquires, as that of the cork-tree, in which the epiplikum is formed of many layers of cells. The outer parts of thick barks vcry often crack, to admit of the expansion of the stem within; in the lace-bark tree of the 'West Indies, the fibers of the inner 13. become partially separated as it is distended, forming lozenge shaped 'fleshes arranged with beautiful regularity.
The connection between the cellular tissue of the B. and that of the pith in the center of the tree is continually maintained by means, in exogenous stems, of the medullary rays. See EXOGENOUS PLANTS and PITII. The B. is a protection to the young and tender wood; it appears also to exercise functions analogous to those of the leaves, which, when young, it resembles in its color, and which are regarded as dilatations of it, so that it has been called the " universal leaf " of a plant.