WOOD (AS. wudu, widu. 013G. /ritn, wood ; connected with Orr. /id, Welsh gnid,gwydd. Corn. guider, Bret. gine:Awn, wood, tree). Popularly considered, the aggregated tougher portions of the vascular bundles of seed and fern plants. Only the seed-plants yield wood which is of any value as lumber, and among them nearly all the wood of commerce is produced by the conifers and the dicotyledons, the monocotyledons pro ducing comparatively little. In the latter group only the larger bamboos and a few of the palms yield important timber. In the monocotyledons the bundles are not arranged in concentric cylinders, but are scattered through the pith in such a way that there is no 'grain.' In a typical dicotyledon or conifer a transverse section of the stein a short distance below the growing point shows a well differentiated epidermis and a zone of vascular bundles surrounding the pith, the bundles being separated from each other by plates of parcn eln•ma (pith rays o• medullary rays). A section a little farther from the growing point shows the cambium, consisting of actively dividing cells between the wood and bast of the bundles, and also stretching from one bundle to another, so that there is a complete circle of cambium. With the establishment of this circle of cam bium, the outer cells of which become differen tiated into bast and the inner into wood, the zone of wood or mechanical tissue (q.v.) becomes
nearly complete, being interrupted only by the narrow secondary medullary rays.
The wood cells formed in the spring are of larger calibre and often have thinner walls than those formed later in the season. and the larger duets of many dicotyledons occur in the spring wood. The small-celled tissue of the autumn wood abutting against the tissue of the spring wood results in the distinct appe:u• ance of the growth rings. In the great majority of eases only one ring is found each year, but if a long Indian summer succeeds a period of rather cool weather a second ring may be formed, just as a second flowering may occur ill fruit trees under similar conditions. so that the number of growth rings Cantina] rings') may not al ways indicate lice age of the tree. As trees grow older the inner portion of the W(11111 (heart wood or durainen) assumes a pharacteristie color, as white hi holly, yellowish in many pities, brown in walnut, black in ebony, etc. The young est few years' growth, surrounding the heart-wood and usually lighter in color, is called sap-wood or alburnum.