1. Kinds of trees used for lumber. - (A.) The classifi cation of trees here considered is based upon the method by which the trunk, or stem, of a tree is formed. The term exogenous is applied to outside growers, around which a layer of wood grows each year, and from which is cut the lumber of commerce. As the wood-worker is interested mainly in trees which grow by this method, we will do no more than mention the endogenous, or inside-growing, trees or plants of the nature of palm trees, cornstalks, etc., in which the woody fiber is formed upon the inside of the stem.

(B.) The new wood formed each year upon exogenous trees is known as the annual layer, or ring; the separate layers being more prominent in open-grained woods, such as oak, ash, and chestnut, than in close-grained woods, such as maple, cherry, poplar, and birch. It is the dif ference in the character and structure of these layers which makes some woods hard and others soft, some with open and others with close grain, and which also, with the coloring matter peculiar to each kind of wood, causes its individuality and adaptability to certain uses.

(The color and odor of wood are caused by chemical com binations, and are not part of the substance of the wood.) 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 impos sible 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.

Fig. 1. - Section of Yellow Pine.

a, soft, cellular grain; b, hard, compact grain.

The coniferous or evergreen trees furnish the cedars, pines, hemlocks, spruces, firs, redwood, tamarack, cy press, 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 heart wood, 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 season ing, but the sapwood of most trees used for building pur poses 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.:

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 charac teristic of the wood.

(C.) The sapwood (c), or alburnum, is generally light colored, and in most building woods its presence is con sidered 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.