The bark of a one-year-old twig consists of three concentric zones: (1) The inner is the phloem (q.v.), or bast, whose work is the con (q.v.) of foods; (2) the outer is the skin (epidermis) and the layers of cork begin ning to develop near it: (3) between these lies the green zone, whose cells are still capable of making food. See PHOTOSYNTHESIS.
The green zone does not usually exhibit further growth; the others increase in thickness. The inner bark receives additions to its inner face season after season from a layer of actively dividing cells, known as the cambium (q.v.), which lies between it and the wood. The outer zone increases because a layer of similar cells, the phellogen (q.v.), develops from the epider mis (q.v.), or some of the cells of the cortex. This cork cambium, as it is otherwise known, gives rise by division of its cells, chiefly in a tangential plane. to layers of cork and other tissues. The corky layers, being almost im pervious to water, cut off all tissues outside them from nutriment: these tissues consequently die and dry up. If the cork cambium continues its activity year after year, the tissues it pro duces become thicker and thicker. By drying
and weathering, the outside becomes rough and irregularly seamed, the internal increase in di ameter tending to make the longitudinal furrows and ridges the more conspicuous. In this case the green layer loses its color wholly, or retains it only beneath the deep furrows.
if the activity of the first cork cambium ceases and new phellogen layers are formed, deeper por tions of the green zone are continually involved, and finally the whole of it comes to lie outside the newest cork; wherefore it perishes. The new phellogen layers may next invade the phloem, so that the bark may come to consist only of the phloem and the tissues produced by the cork cambium. The corky layers are lines of weak ness. Warping. clue to drying and wetting, cracks these layers. and the outermost sheets or flakes of bark are loosened and fall off. If the corky layers are very close together and concen tric, a bark such as that of the paper birch or sycamore results; but in most barks the flakes are irregular.