Part 72. Wood in general. In economic importance woods rank next to vegetable fibers. Just as the great use of fibers is for clothing, which is almost as necessary to us as food, so the great use of wood is for buildings, which are scarcely less needful than clothing. Both materials serve us mainly by their mechanical strength, but with this difference, that whereas a fiber offers but little resistance except to stretching, a piece of wood maintains its form but little changed against severe mechanical strains of whatever sort. Hence the great use of wood for support in structures for shelter, storage, transportation, and repose; and its wide application to innumerable minor uses. The ready separation of vegetable fibers and the facility with which they may be twisted and interlaced is matched by the comparative ease with which wood may be shaped and joined.

The great importance of the wood-working trades, carpentry, joinery, turnery, and carving indicates something of the extent of our dependence upon the material in which they work. A further idea of the usefulness of this material may be gained from a brief review of the more important classes of things which are made wholly or in part of wood, and of the qualities they especially require in the material used.

Buildings require different qualities in the frame, the exterior and the interior finish. Strength, ease of working, and availability in large dimensions are the main needs for the framing timbers; resistance to weather or adaptability to paint, for exterior finish; while hardness, as little shrinkage as possible, and an attractive appearance when polished are most desirable for interior finish.

Furniture has needs similar to interior finish and at the same time demands special strength.

Domestic utensils have no such need for beauty of material but generally require considerable strength and hardness.

Boxes, including crates, need to be strong and when used for transportation, as light as possible.

Cooperage, whether "dry," as flour barrels, or "wet," as casks and tanks, or "white," as tubs and pails, calls for wood which is stiff yet elastic and not liable to irregular twisting or warping even when in contact with fluid on only one side.

Vessels, including all sorts of water-craft, present in the hull somewhat similar requirements to wine casks, the chief difference being that the fluid must be prevented from leaking in instead of leaking out. As regards the spars, uniform stiffness through considerable length together with lightness are most important.

Vehicles, in their running parts, require great toughness together with elasticity in order to meet the very severe and frequent shocks and wrenchings to which they are subjected; while on the body lightness and stiffness are especially desirable.

Harness though made mostly of other material may consist largely or wholly of wood, as with certain saddles, stirrups, hames, and yokes; and then, as being subject to much the same strains as parts of vehicles, needs scarcely less stiffness, lightness, and elasticity than they.

Road materials, including wooden pavements and railway ties, require blocks or logs of exceptional strength and durability to stand satisfactorily the heavy loads and the alternate drying and wetting to which they are subjected.

Fences require wood as durable under similar exposure but without the same mechanical strain.

Poles, as for flags and wires, need similarly to resist decay and also to meet about the same requirements as spars.

Trestlework, as for bridges and the like, needs especially stiffness with durability under exposure to weather.

Piling, as the foundation for bridges, wharfs, and so forth, needs not only to be stiff but to be durable under water or in contact with moist soil.

Mine timbering must be equally strong and at the same time able to resist decay under conditions of dampness much more trying than those of entire submergence.

Industrial implements, machines, and weapons, mostly require wood of especial toughness to serve for handles, cogs, spindles, gunstocks, and the like.

Canes and umbrellas call for fancy woods of attractive appearance and considerable stiffness, small dimensions being no drawback.

Surgical appliances such as splints, crutches, and artificial limbs are best made of wood that is both stiff and light.

Recreational appliances such as tennis-rackets, base-ball or cricket-bats, hockey-sticks, golf-clubs, croquet-mallets and balls, nine-pins, balls and bowling alleys, billiard cues, checkers, and chessmen, are made mostly of wood that is especially tough or hard.

Musical instruments such as violins, guitars, and pianos depend for their quality of tone mainly upon the resonance of the wood used in their construction.

Toys are made generally of woods which are most easy to work, and the same consideration largely influences the selection of woods for various minor articles such as spools, button-molds, shoe-pegs, toothpicks, and matches. Other uses of wood apart from its value as constructive material will be referred to later.

To the plant which produces it, as to us who use it, wood serves mainly for mechanical support. In large trees the trunk must be a column of great strength in order to hold up the immensely heavy crown especially when loaded with snow and ice, and severely strained by wind. So also must the branches be joined with great firmness to the trunk, and be stiff enough to hold the foliage well in place. Even the leaves require a woody framework or skeleton to keep their soft, green parts spread open to the sunshine. The woody parts of leaves are continuous with the new wood of the stem which in turn connects with the new wood of the root. What is absorbed by the root is conducted as crude sap mostly through the new wood of root and stem to the food-making parts of the foliage. When as in many trees the new wood is formed next to the bark in successive layers it is distinguished as sap-wood so long as it retains its power of conducting sap. After a certain number of years, varying greatly in different kinds of trees, the wood is no longer useful in this way, but becomes more useful mechanically because of increased dryness, compactness, and strength. It is then known as heart-wood and is commonly distinguished from the sap-wood by a marked change in color. The color is due to the presence of substances formed as by-products of the plant's activities but of no further use to it, and therefore best accumulated in wood which has ceased to be a channel for sap. The sap-wood is also used by the tree to some extent for the storage of food substances, which have but little color, as for example the sweet sap of the sugar-maple. Such food makes the sap-wood a particularly good feeding ground for wood-boring insects and other parasites which injure or destroy the wood. Its greater liability to the attacks of these destructive agents, together with its inferiority to heart-wood in strength lead commonly to the rejection of sap-wood for constructive purposes; while for ornamental uses as well, heart-wood is furthermore preferred on account of its more attractive coloring. A still further advantage of heart-wood for economic use is the much larger masses of it which may be obtained from large trees. Thus we see that wood, especially heart-wood, is the great massive and resistant material of plants. In slender parts it is, as we have seen, either replaced by fibers or shares with them more or less the service of mechanical support. Viewed broadly, it may be said that wood corresponds to the bony skeleton of animals in contrast with their tendons and hairs to which we may liken internal and external vegetable fibers respectively.