1. The pith (Fig. 2, a), a cellular tissue: this is worthless and its presence in lumber is considered a defect.
2. The wood, which includes the heartwood (b), the sapwood (c), and the medullary rays (g). From this part of the tree the commercially valuable lumber is taken.
3. The cambium (d), which is a thin layer between the sapwood (c) and the bast (e).
4. The bark, which includes both the bast (e) and the outer bark (f). The bark of most trees is worthless, but that of chestnut, oak, hemlock, and other trees which are rich in tannic acid is used in large quantities by tanneries.
(B.) The heartwood (b), or duramen, of most trees is the part generally used by carpenters upon the best work. It is firm, compact, and of the color and qualities characteristic of the wood.
(C.) The sapwood (c), or alburnum, is generally light-colored, and in most building woods its presence is considered a defect, though not in hickory, ash, maple, or yellow pine, and a few other woods; in fact, in these woods it is often preferred to the heartwood for many kinds of work on account of its color. The alburnum is filled with the active elements of the sap, which are deposited as the sap passes through the trees, and in time becomes part of the duramen, or heartwood. The time required for sapwood to attain maturity ranges from thirty to one hundred years, according to its kind and age.
Fig. 2.—Section of Oak Tree Trunk.
a, pith; b, heartwood, or duramen; c, sapwood, or alburnum; d, cambium; e, bast, or inner bark; f, outer bark, or corky layer; g, medullary rays, or silver grain; po, plain oak; qo, quartered oak.
(D.) Outside of the sapwood (Fig. 2, c) is the cambium (d), which furnishes the substance upon which the life of the tree depends. Here, nourished by the richest sap, new cells are formed, which become either sapwood or a part of the bast. (E.) At (e) is shown the bast or inner bark, which is composed of a woody fiber combined with a tissue of cells. This is elastic, which allows it to expand as the wood grows beneath it. Outside of the bast is the bark (f), or outer covering, which is of a corky nature, and protects the delicate vital parts of the tree.