BARK, in Vegetable Physiology, is the external coating of the stem and branches of plants, unsheathing the wood. In woody Exogens it separates spontaneously from the wood in spring and Bummer, and in herbaceous plants of the same class it may be easily removed with a little care ; but in Endogens and derogens it is so continuous with the central part of the stem that it can never be divided except by violence, and by lacerating the tissue which lies immediately below it. This difference arises from the manner in which the plants of these three great natural' classes respectively grow. Exogene add annually new matter to the inside of their bark and the outside of their wood, which renders it necessary that a spontaneous separation of wood and bark should take place in order to make room for the newly-generated substance ; but Endogcns, which grow by addition to their centre, and Acrogens, by elongation of their point, require no such separation. [Exooc?ra ; ErrnooExs ; AeuOOExe.] Bark may be considered to originate thus :---When a plant is in the state of embryo, that part which finally develops into a stem and root, or, as botanists say, into the axis of growth, is something like two cones applied to each other by their bases, but it will simplify our ideas if we consider it as a cylinder. In a dormant state it con sists of nothing but cellular substance ; but in Exogens, as soon as tho cotyledons, or seed-leaves, are roused into growth, woody matter is generated in the form of a number of little bundles, which are arranged in a circle (a a) about half way from the centre to the circum ference, thus forming a sort of hollow cylinder within the first. The cylinder so commenced cuts off the cellular sub stance into two parts : one central (b), which finally becomes pith, and the other external (c), which becomes bark ; the two maintaining their connection by means of the passages (d d) between the woody bundles (a a). These pas sages ultimately become the medullary processes. The direction thus given in the beginning to the several parts in the interior of an exogenous stem is never afterwards departed from, but all the additions which are subsequently made are moulded, as it were, upon this original form. The woody bundles (a a) increase in size by growing outwards, and consequently the medullary. processes are extended ; the bark continues to grow and give way to the pressure of the wood from within, till at last a year's increase has been accom plished. Up to this time no separation between the wood and the bark has taken place ; but in a second year, as it is necessary for the new matter to be added to the outside of the wood and to the inside of the bark (at d d), a spontaneous separation of the two takes place over the whole surface of the wood, the medullary processes softening, stretching, and growing externally, in order to admit of such a sepa ration. But Endogens and Acrogens always retain their bark in the
same connection with the wood as it is in Exogens at the end of the first year, there being no necessity for a separation between the two in order to admit of subsequent growth.
In its anatomical structure bark consists of a mass of cellular tissue pierced longitudinally by woody matter, which is composed entirely of woody tubes without any trace of vessels, but which is sometimes accompanied by long fistnlar cavities, in which resinous, or milky, or juicy, or other secretions are lodged.
In the first year of its existence bark is a cylinder, the woody matter of which is a continuation of that of the wood itself. In Endogens and Acrogens it undergoes no material increase or alteration subsequently, unless it be that the parts are increased in quantity without shifting their position. But in Exogens, in consequence of their wood being annually augmented by external additions, as before stated, the bark undergoes annual changes. Corresponding with the annual additions to the wood are annual additions to the inside of the bark, consisting of a cellular layer overspreading the whole of the inside, and then a layer of woody matter, which answers to the spaces of wood included between" the medullary processes. These annual additions, which are called the liber (whence books which were written upon such layers, properly prepared, were called libri), must therefore be exactly the same in number as the annual layers of wood, and would be arranged with equal regularity if the bark were not affected by any disturbing cause. But in consequence of the wood's perpetual increase in diameter there is an incessant lateral strain upon the liber. so that after the first year there is little trace of regularity to be discovered in the structure of the bark. It soon becomes a mere confused mass of woody tubes and cellular tissue, in which all trace of annual concentric formation has disappeared. The manner in which it was originally generated is however said to be detected in some plants by the facility with which the bark will peel into layer after layer; but it may be doubted whether this phenomenon is not more connected with the original arrangement of the tissue of which the bark is composed than with the annual formations. These layers are sometimes so numerous that as many as 150 have been separated on a single tree.