Timber

The fundamental difference between softwoods and hardwoods is best understood if we imagine a number of the wider "water fibres" very greatly widened and strung end to end with their terminal walls absorbed, so that long continuous tubes, some times yards in length, are produced. These water-conducting tubes are termed wood-vessels: in cross-section they are often visible to the naked eye as "pores." All commercial hardwoods have wood-vessels, which in the sap-wood contain air with or without liquid sap. In addition to the vessels, the different hard woods have "fibres" of various forms.

Finally medullary rays are present, and in many hardwoods some of them are much thicker than in softwoods and visible to the naked eye in cross-section, producing the so-called "silver grain" on the radial side view of the wood of, for instance, oaks, beech, plane, etc. Their contents in the sapwood are identical with those of softwoods.

Sap-wood and Heart-wood.---The

change of sap-wood into heart-wood is a vital one, occurring only in the living tree, and thus is very different from the process of seasoning of timber which takes place post mortem. The sap-wood is living inasmuch as its brick-shaped cells contain living protoplasm : the heart wood is dead, as the protoplasm dies and the cells lose their nutritive contents, which are replaced by air and more indigestible substances such as "tannin," gums and resin. The heart-wood contains less water than the sap-wood, and its water-conducting elements ("fibres" and vessels) become more or less plugged by microscopic bladders or gums, or the pits are closed. Conse quently sap-wood is more easily impregnated with wood-preserva tive liquids than is heart-wood of the same tree. In different trees the age at which sap-wood first changes to heart-wood differs : for instance the change takes place earlier in larches than in true pines, so that the former have fewer annual rings of sap-wood outside the heart-wood and produce valuable durable heart-wood earlier in the life of the tree, and contain a smaller proportion of the perishable sap-wood (see DRY ROT).

Grain of Timber.

Most frequently the fibres and other struc tural elements of knotless timber run parallel with the long axis of the trunk or branch, and the wood is described as being straight-grained. If a plank or post be sawn in a direction not parallel to the axis of the trunk, the grain of the sawn article is not parallel to the sides of this and the plank or post is described as cross-grained. But not infrequently the grain of the trunk (or

branches) runs in a spiral direction as if the trunk had been twisted round its long axis : the grain is then described as spiral or torse, and the timber when cut up is inevitably cross-grained. There are yet other, especially tropical, woods in which the grain more or less swings from a left-handed to a right-handed spiral direction, so that the wood when cut into plain boards shows a double cross-grain or interlocked grain, and when cut along the radii of the trunk ("on the quarter," rift-sawn) shows roe-figure, for instance in mahogany.

In addition to these deviations from the straight grain the structural elements of the wood may pursue a wavy or sinuous course, for instance in so-called rammy-ash. When the waviness is exaggerated the grain is said to be curly.

Knots, being the basal parts of branches that have become embedded in the thickening trunk, are naturally associated with a change of direction of the fibres. The wood of large excrescences on the trunk of certain trees known as burrs exhibits structure similar to that which would be produced by many crowded branches : and a somewhat similar structure is shown by bird's eye maple.

The fibres of most timbers overlap and dovetail with one another at their ends, which are at different levels, and the medul lary rays as seen in circumferential or tangential view are arranged apparently irregularly or in spirals. But some woods when exam ined from the same view-points show lines running across the grain and producing what is known as ripple-marking: this is caused by either contiguous fibres ending at the same levels with little or no overlapping or medullary rays placed at the same levels, so that the structure of the wood is tiered. Such structure is often shown by true American mahoganies.

Chemical Composition.

The wood-substance composing the walls of the hollow structural elements that constitute the solid frame-work or skeleton of wood may, for the present purpose, be regarded as always being composed of two main constituents, lignone and cellulose, together with smaller quantities of additional more or less gum-like bodies. The lignone can be removed from the wood by steam or warm weak acids (and is so removed in the manufacture of chemical wood-pulp), the cellulose remain ing.