Timber as used in woodwork is the product of the felled body of the tree, stripped of its outer covering of bark.

If the transverse section of a log of, say oak, in its original condition as cut down is examined, it will be seen to consist of three broadly marked divisions: the bark, the sapwood next to it, which resembles in some degree the inner main portion, or heartwood, and in or near the middle of the tree is the section of the pith, or medulla.

Let us first consider the large inner mass of wood in detail. Closer examination will at once reveal a succession of thin concentric rings of darker colour than the remainder across the whole surface, the smallest ring enclosing the medulla, and the largest forming the margin of the central wood next to the bark.

Confining our attention for the present to these rings, we shall observe outside the largest a thin ring, dividing the sap-wood from the bark.

This ring, the cambium, consists of a thin sheet of cells, which, by their growth and subsequent development, form on one side wood, and on the other, in a much less degree, cortex, or bark. The lateral growth of the tree, with which we are immediately concerned, occurs, then, next to this cylindrical plate of cambium cells.

These cells are of rectangular prismatic form, with tapered ends. In the spring, when the quickening influence of sunlight and other less-known causes stimulate the growth of vegetation into renewed activity, the cells on the inner side of the cambium split up radially, expand, and grow rapidly into tracheides or fibres.

Though they do not radically alter in shape, these tracheides become provided with a curious and little-understood series of valves in their sides, enabling them to obtain and communicate moisture. While this lateral growth is going on, a slowly flowing current of 'sap,' obtained principally by the roots, ascends the tree, and the wood-forming materials contained in it feed and thicken the walls of the tracheides.

As the summer advances, the increasing bulk of the wood, by the formation of this ring of cellular tissue, presses on the cortex, but the latter exercises a restraining influence on the growing tracheides.

These, continuing to form as rapidly as before, are then unable to grow as fast radially, and from a nearly square shape in the spring wood, the tracheides become very much flatter radially in the autumn wood.

The upward-flowing sap pauses as it were for a short time at the height of the summer, and then commences to slowly descend.

The resulting benefit of the various elements taken up by the roots and leaves of the tree is now more marked, and the sap is more fully charged with wood-forming materials. The . walls of the tracheides consequently grow faster in the autumn and become thicker than in the spring wood, causing still greater contrast in the density of their appearance. Indeed, it is probable that, if the degree of vital activity in the spring were as great as that attained by the tree in the late summer and autumn, the tracheides of the spring wood would resemble very much more closely those of the autumn wood than under existing conditions of growth.

Another circumstance which must be noted as tending to make the spring wood still looser, as compared with that formed in the autumn, is the presence of a large number of air tubes, or pores, which will be noticed in examining the end section of a piece of elm or oak. These are formed in the tracheides of the spring wood, but are much fewer and smaller in the autumn wood.

These pores will accordingly be seen grouped in bands, as it were, in the earlier portion of the spring wood of elm, to take a good case, though much less distinct, and frequently invisible, in some woods.

When winter arrives, the flow of sap and the formation of cells cease, and the growth of wood with it. A complete annual ring has then been formed - in its first and greater portion light-coloured, porous, and comparatively loose fibred, and in its latter denser, tougher, and darker in colour.

The newer rings of tracheides are those in which the sap chiefly flows, and these in the course of years have gradually less sap supplied to them, and the channels down which it formerly flowed become filled simply with air and water. The walls of these older tracheides are then thoroughly lignified, and become much tougher and have more cohesion than those of the sap-wood, making the most perfect form of wood. This is the heart-wood, or true woodwork timber.

By counting the annual rings, the age of a tree may be discovered.

Referring again to the end section of a log, a number of fine, sharply delineated streaks of lighter colour than the rest of the wood will be seen running, usually, but not always, from the medulla completely through the heart-wood, sap-wood, and cambium, to the cortex.