a, soft, cellular grain; b, hard, compact
grain.
Each of these annual layers is composed of two parts, the formation being shown in Fig. 1, in which the grain of yellow pine is depicted. The soft, cellular, or open, grain, a, is formed as the sap moves upward in the spring, and the hard, compact grain, b, is formed later in the year. In soft woods the open grain predominates, while in hard woods the compact grain is more in evidence.
The age of a tree may be determined by counting these annual rings upon the stump, though a drouth during the growing season may have at some time so affected its growth as to make some layers indistinct, rendering it impossible to be absolutely sure of the count. In a young tree the annual layers are thicker than when the tree becomes more mature.
(C.) The different kinds of timber which the carpenter uses are cut from deciduous, or broad-leaf, trees, and from coniferous, or needle-leaf, trees. This classification of trees is based upon the difference in the forms of their leaves, the former kind furnishing the ash, oak, walnut, beech, birch, and other woods that are hard to work, as well as poplar or white wood, linn or bass wood, and others which are called hard woods, not in the sense that they are hard to work, but because their method of growth is the same.
The coniferous or evergreen trees furnish the cedars, pines, hemlocks, spruces, firs, redwood, tamarack, cypress, and a few other woods of the same nature. These woods have a resin which is always present, no matter how old or dry the wood may be, which explains their superior weather-resisting qualities.
2. The formation of wood.—(A.) In the spring the sap begins to flow upward, mainly through the open cells of the cambium (the new growth in the stem, by which the diameter of the tree is increased), and to some extent through the sapwood. As it comes in contact with the air by means of the leaves and the green shoots of the tree, (B.) it gives off water and oxygen, and absorbs carbonic acid gas from the air, (C.) which is formed into starch, sugar, oil, etc.; this in turn becomes part of the sapwood, and forms the compact part of the annual ring, completing the layer for that year.
(D.) The wood is formed by a deposit of matter from the sap, which gradually thickens the cell walls until the cells are filled, when that layer becomes a part of the heartwood, or that part of the tree which is inclosed within the annual layers of sapwood.
(E.) Both the fibers and the cells of the sapwood are filled with water or sap, which may be removed by seasoning, but the sapwood of most trees used for building purposes is not as good lumber as the heartwood, as it is always susceptible to moisture. The exceptions to this will be mentioned elsewhere.
(F.) In all unseasoned lumber from 20 to 60 per cent of its weight is moisture, which must be evaporated before the lumber has its highest commercial value. This may be done by weather drying or by artificial means, the lumber being treated in a specially constructed kiln.
3. Tissues.—(A.) A tree trunk is composed of four different tissues, viz.: