Wood or timber forms the greater part of the stems and branches of trees and shrubs.

To examine the inner structure of a tree-stem, a section may be made at right angles to the direction of its length, i.e., a transverse or cross section; or from the pith to the bark in the line of one of the radii and parallel to the direction of the length, i.e., a radial section; or a third section may be made at right angles to both the preceding as a tangent to the circumference, i.e., a tangential section.

A tree-stem in section.

On examining the cross-section of a stem we find an outer ring, the bark, consisting of a corky layer, the outer bark, and the inner bark or bast; next comes the wood, constituting the chief portion of the stem, and in the central part is a canal, filled with soft cellular tissue called the pith or medulla. Between the wood and the bast lies a narrow, light-coloured ring, the Cambium. This consists of a layer of embryonic cells, from which are developed on the one side wood, and on the other bast, and it is here that the growth of the tree takes place.

The Cambium forms the soft, moist, spongy mass which may be seen under the bark in-spring when the sap begins to rise. It consists of microscopic cells, some of which are long, prismatic, and pointed at the ends, while others are shorter and have ends which terminate abruptly. The inner bark and wood are developed chiefly from the long cells, the medullary rays from the short ones.

Fig. 1. Three sections of a tree-stem, at right angles to one anotker.

T. cross section, A", radial section, Tg. tangential section. M. medulla or pith, B. Bark, C. Cambium, aa. Concentric annual layers, mm. Medullary rays, ab. thickness of medullary rays, cd. height of medullary rays, 11. vessels.

Wood Cells.

The young cells from which wood is developed have at first very thin walls. They are filled with sap, the fluid which nourishes the growing tree, and which circulates with ease from one thin walled cell to another, and thus permeates the whole of the tissue. Gradually the walls of the cells become thicker; the cell contents solidify; the sap flows less and less freely; the whole tissue assumes the characteristics of wood, and ceases to take part in the circulation and assimilation of the sap.

The formation of wood.

The cellular tissue consists chiefly of cellulose, the chemical constituents of which are carbon, hydrogen, and oxygen.

Wood Fibres.

The cells from which wood is developed are principally the long-pointed cells. They lie close together and overlap one another at the ends, thus forming minute tubes or fibres. The zone of wood in any stem consists of these fibres massed together, and extending in the direction of the length of the stem. The connection between separate fibres is often very slight, as is shown by the ease with which they may be separated.

In trees of regular growth the fibres are straight and parallel. Wood of this kind is called "straight fibred." It is easily split. This is not the case with wood in which the fibres are crooked, or twisted about one another, as in gnarled or mis-shapen trees. The fibres in the root, the lower part of the stem, knotty branches and rough excrescences are always crooked, and sometimes they are twisted and involved in the most remarkable way. This gives rise to the peculiar speckled and veined appearance which is so highly prized in some kinds of wood.

The bast also consists of fibres, but they are longer and usually tougher than wood-fibres.

Concentric Annual Layers.

A new layer of bast and a new layer of wood are formed annually. This new formation goes on rapidly in spring and early summer, when vital activity in the tree is at its height. The cells are then large, and the wood formed from them, i.e., spring wood, is soft and loose in texture and light in colour. After the tree has budded the formation of wood goes on for a time, but less actively. The cells diminish in size and in diameter, and are more closely packed together. The wood formed at this period - autumn wood - is generally darker in colour and closer in texture than spring wood. There are fewer vessels (see p. 31) in autumn wood; in spring wood, on the contrary, they are numerous and quite visible as pores. In consequence of the characteristics of autumn wood, the boundary line between two periods of vegetation is clearly defined, and it is easy to distinguish the concentric annual layers which mark each yearly increase in growth.

Distinctness of the concentric annual layers.

These layers are most sharply defined in needle-leaved trees and in some broad-leaved trees, e.g., the oak, the ash, and the elm.* They are less conspicuous in the birch, the aspen, the alder, etc., and in some cases it is even difficult to distinguish them at all. As a new layer of wood is formed every year, the age of a tree may be determined by the number of layers.

Determination of *he aye of a tree.

In the tropics, where vegetation goes on during almost the entire year without any well-marked period of rest, the concentric annual layers disappear entirely.

The breadth of the concentric layers varies in different trees. In some cases they are more than 1 inch broad, in others scarcely -^ inch. Their breadth may vary even in the same stem, depending on the more or less favourable weather of successive seasons. The layers on the side exposed to the south are often broader than those on the north. In old needle-leaved trees we usually find very narrow layers nearest the pith; beyond these the layers widen for the greater portion of the stem, and then contract once more until the outermost ones are often so narrow that they can with difficulty be distinguished by the naked eye. See Fig. 2.