Stem

sb. vapour, ray of light, flameVariants: steemEtymology: Anglo-Saxon stéam
The stem consists of a central portion, either made up of long bundles of woody fibre running side by side, as in the endogenæ, or deposited in rings and having a central cylinder of pith, as in the exogenæ. In these the wood at the central part (or the oldest) is called "duramen," or "heart-wood," while that at the outer part or nearest to the bark (the newest), is called "alburnum" or "sap-wood." The former is the harder and the latter the softer portion. From the pith in the centre, through the woody part, rays or laminæ of cellular tissue similar to that which constitutes the pith itself, are sent outwards through the woody rings to the inner part of the bark, which they form; these are called medullary rays, and may be seen in wood which has been cut across the grain, they are often called the "silver grain," and are very evident in oak, beech, and elm, the inner part of the bark is called the "liber." The bark itself is made of cellular tissue dried and hardened by age; the outer portion (called the "epidermis") is in many cases shed, and may be constantly seen hanging loosely from the bark of the birch and other trees in loose white silvery portions. The outer part of the bark of some trees is so largely developed as to be of considerable thickness, this is especially the case in the Cork oak (Quercus suber ), which is the cork of commerce. The bark cannot stretch as the circumference of the tree increases, it is therefore split up and cracked, which accounts for the rough state of it on those trees which do not shed the outer part.