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Revision as of 09:06, 22 April 2004
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.
See also: tree, list of woods, forestry