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Wood is a material found as the primary content of the stems of 'woody plants', especially trees, but also all shrubs. These perennial plants are characterised by stems that grow outward year after year, and that are composed of cellulose and lignin based tissue. Plants that do not produce wood are called 'herbaceous'; this group of plants includes all annual plants, many perennial plants, and most submerged and floating aquatic plants.

The woody tissue is formed by the plant for structural purposes, and because it is an effective and efficient structural material, it is useful to humans. Wood is made of cellulose fibers, held together with lignin.

When cut down and dried, wood is used for many different purposes. Wood that is broken down into fibers is called pulp, which may then be made into paper. Artists and craftsmen shape and join pieces of wood with special tools, which is called woodworking or carpentry. Wood has been an important construction material since humans began building shelters, and remains in plentiful use today.

In modern times, many of the traditional uses of wood may be filled by metal and plastics.

Wood is commonly classified as either hardwood or softwood. The wood from conifers (e.g., pine) is called softwood, and the wood from broad-leaved trees (e.g., oak) is called hardwood. This classification is sometimes misleading, as some hardwoods (e.g., balsa) are actually softer than most softwoods.

Additionally, woods from different types of trees have different colors and grain densities. Because of these differences, and the fact that some woods take longer to grow than others, wood from different kinds of trees have different qualities and values. For example, while mahogany is a dark, dense hardwood which is excellent for fine furniture crafting, balsa is light, soft, and almost spongelike, making it useful for model building.

Water content

Water occurs in living wood in three conditions, namely: (1) in the cell walls, (2) in the protoplasmic contents of the cells, and (3) as free water in the cell cavities and spaces. In heartwood it occurs only in the first and last forms. Wood that is thoroughly air-dried retains from 8 to 16 per cent of water in the cell walls, and none, or practically none, in the other forms. Even oven-dried wood retains a small percentage of moisture, but for all except chemical purposes, may be considered absolutely dry.

The general effect of the water content upon the wood substance is to render it softer and more pliable. A similar effect of common observation is in the softening action of water on rawhide, paper, or cloth. Within certain limits the greater the water content the greater its softening effect.

Drying produces a decided increase in the strength of wood, particularly in small specimens. An extreme example is the case of a completely dry spruce block two inches in section, which will sustain a permanent load four times as great as that which a green block of the same size will support.

The greatest increase due to drying is in the ultimate crushing strength, and strength at elastic limit in endwise compression; these are followed by the modulus of rupture, and stress at elastic limit in cross-bending, while the modulus of elasticity is least affected.

Knots

Knots are portions of branches included in the wood of the stem or larger branch. Branches originate as a rule from the central axis of a stem, and while living increase in size by the addition of annual woody layers which are a continuation of those of the stem. The included portion is irregularly conical in shape with the tip at the pith. The direction of the fibre is at right angles or oblique to the grain of the stem, thus producing local cross grain.

During the development of a tree most of the limbs, especially the lower ones, die, but persist for a time--often for years. Subsequent layers of growth of the stem are no longer intimately joined with the dead limb, but are laid around it. Hence dead branches produce knots which are nothing more than pegs in a hole, and likely to drop out after the tree has been sawed into lumber. In grading lumber and structural timber, knots are classified according to their form, size, soundness, and the firmness with which they are held in place.

Knots materially affect checking and warping, ease in working, and cleavability of timber. They are defects which weaken timber and depreciate its value for structural purposes where strength is an important consideration. The weakening effect is much more serious where timber is subjected to bending and tension than where under compression. The extent to which knots affect the strength of a beam depends upon their position, size, number, direction of fibre, and condition. A knot on the upper side is compressed, while one on the lower side is subjected to tension. The knot, especially (as is often the case) if there is a season check in it, offers little resistance to this tensile stress. Small, knots, however, may be so located in a beam along the neutral plane as actually to increase the strength by tending to prevent longitudinal shearing. Knots in a board or plank are least injurious when they extend through it at right angles to its broadest surface. Knots which occur near the ends of a beam do not weaken it. Sound knots which occur in the central portion one-fourth the height of the beam from either edge are not serious defects.

Knots do not materially influence the stiffness of structural timber. Only defects of the most serious character affect the elastic limit of beams. Stiffness and elastic strength are more dependent upon the quality of the wood fibre than upon defects in the beam. The effect of knots is to reduce the difference between the fibre stress at elastic limit and the modulus of rupture of beams. The breaking strength is very susceptible to defects. Sound knots do not weaken wood when subject to compression parallel to the grain.

See also: tree, list of woods, forestry

External links

• WoodBin info on different kinds of wood
• how trees are made into paper