WOOD

hard, fibrous substance, technically known as xylem, that is the principal strengthening and water-conducting tissue found in the stems and roots of many plants, including trees and shrubs. It is mainly the wood derived from the trunks of forest trees that has economic value. As such, wood has been in service since humans appeared on Earth and has contributed to survival and to the development of civilization. In contemporary times, in spite of technological advancement and competition from metals, plastics, cement, and other materials, wood maintains a place in most of its traditional roles, and its serviceability is expanding through new uses with the result that its consumption is steadily increasing. The long list of present wood uses includes products in which its natural texture is retained and others in which the wood is mechanically and chemically modified to the extent that its presence cannot be recognized. In addition to well-known products, such as lumber, furniture, and plywood, wood is the raw material for wood-based panels, for pulp and paper, and many other products, especially chemical derivatives of cellulose and lignin. Finally, wood is still an important fuel in much of the world. The versatility of wood is basically attributable to its structure, chemical composition, and properties. Produced by many botanical species, it is available in various colours and grain patterns. In relation to its weight, wood has high strength. It is insulating to heat and electricity and has desirable acoustical properties. Further, wood imparts a feeling of warmth not possessed by competing materials, such as metals, and is relatively easily worked. Cellulose is mostly obtained from wood. Wood is found throughout the world and is a renewable resourcein contrast to coal, ores, and petroleum, which are gradually exhausted. Wood has certain undesirable characteristics, however. It may burn and decay. It is hygroscopic (moisture absorbing), and in gaining or losing moisture it changes dimensions. As a biological product, wood is variable in quality. hard, fibrous substance, technically known as xylem, that is the principal strengthening and water-conducting tissue found in the stems and roots of trees and shrubs. Wood is one of the most abundant and versatile natural materials on Earth, and unlike such resources as coal, ores, and petroleum, it is renewable. It has been utilized by man since the earliest days of human existence, first as a source of fuel and later for an enormous variety of uses. Despite the widespread use of metals and synthetics in modern society, the demand for such wood products as lumber, plywood, paper, and chemical derivatives of cellulose continues to increase annually. The most widely used woods come from two groups of trees: the conifers, or softwoods, such as pine, spruce, or fir; and the broadleaves, or hardwoods, which include such trees as oak, walnut, and maple. Trees classified as hardwoods are not necessarily harder than softwoods: balsa, for example, is a hardwood but is one of the softest woods. Hardwoods and softwoods differ, rather, in their cellular form and structure. The cells of hardwoods consist of vessel members, fibres, and parenchyma; those of softwoods are tracheids and parenchyma. The three basic parts of a treethe bark, wood, and pithcan easily be seen in cross section; the fourth, the cambium, is a thin layer of living cells between the bark and the wood where tree growth takes place. The wood forms around the pith (the central core) in a series of concentric layers, called growth rings, which usually represent the yearly or seasonal cycle of growth. Each ring is composed of two parts: the inner part, called earlywood, or springwood, is lighter and softer and is produced in the spring; the outer part, called latewood, or summerwood, is added later in the growing season. Another characteristic of wood evident in cross section is the distinction between heartwood and sapwood. Heartwood, the central portion, is darker in colour and composed of inactive cells that do not participate in the life processes of the tree. Sapwood, the lighter area surrounding the heartwood, contains cells that conduct water and dissolved minerals up to the leaves and store food after photosynthesis. The wood in most trees exhibits a radiating pattern from pith to bark, with rays varying in size from species to species; many softwoods also possess visible resin canals. Other identifying physical characteristics of wood include colour, varying from the white of a holly tree to the red of a redwood or the black of an ebony; odour, especially noticeable in such trees as the cedar but indiscernible in others; texture, the uniformity of a wood's surface; and grain, the direction of the wood fibres. Wood is also hygroscopic (moisture-absorbing) to varying degrees among different species. The water content in harvested wood causes shrinkage and swelling and affects weight, volume, density, hardness, strength, heat-producing capacity, and resistance to decay. The mechanical properties of wood include strength, stiffness, relative lightness in weight, and flexibility. Different woods vary considerably in their degrees of strength, and the strength of an individual piece will change according to the direction in which a load is applieda board, for example, is strongest along the grain (axially) but when subjected to bending is strongest perpendicular to the grain (transversely). Commercial producers evaluate these basic properties to determine the best use for a particular wood. Two important steps in wood processing are drying and preservation. Wood is dried in the open air or in kilns in order to reduce shrinkage, swelling, and weight and to better prepare it for finishing. Wood must also be protected from deterioration and decay, which is caused by such agents as insects and fungi. Chemical preservatives, including oils and water-soluble salts, are most commonly used; the methods of application include spraying, brushing, and immersion. After a tree is felled, the wood is processed for roundwood products (such as poles and posts) or as sawnwood (sawed lumber). Logs for roundwood are ready for use after simply being debarked and treated with preservatives. Sawnwood is treated and cut at a sawmill and then transported to other manufacturing facilities to be fashioned into particular wood products. Some basic wood products include veneer, a thin layer sliced from a larger piece of wood; plywood, a laminated panel made of a wood core that is glued between layers of veneer; particleboard, a processed board or panel made by gluing chunks, shavings, and splinters of wood together; and fibreboard, a panel formed from wood-pulp fibres. There are an estimated 10,000 different wood products commercially available, ranging from fine furniture to toothpicks. Wood and wood residues are also the basic materials used in the manufacture of many chemically derived products, including cellophane, charcoal, dyestuffs, explosives, lacquers, turpentine, and yeast. Wood pulp is also the chief constituent used in the manufacture of paper and packaging materials. Pulp is produced by either mechanically grinding up wood, cooking it with chemicals, or pretreating it with chemicals and then reducing it mechanically. Finished products range in quality from fine stationery to newsprint and corrugated cardboard boxes. Additional reading The structure and properties of wood are treated in F.W. Jane, The Structure of Wood, 2nd ed., rev. by K. Wilson and D.J.B. White (1970); George Tsoumis, Wood as Raw Material: Source, Structure, Chemical Composition, Growth, Degradation, and Identification (1968), mainly on structure, with keys for identification of North American and European commercial woods; A.J. Panshin and Carl De Zeeuw, Textbook of Wood Technology: Structure, Identification, Properties, and Uses of the Commercial Woods of the United States and Canada, 4th ed. (1980); Franz P. Kollmann and Wilfred A. Ct, Jr., Principles of Wood Science and Technology, vol. 1, Solid Wood (1968), on structure, chemical composition, and biologic deterioration, with detailed coverage of properties; Alfred J. Stamm, Wood and Cellulose Science (1964), fundamental information on properties; H.E. Desch, Timber: Its Structure and Properties, 6th ed., rev. by J.M. Dinwoodie (1981), including a discussion of drying, preservation, and grading; and Jozsef Bodig and Benjamin A. Jayne, Mechanics of Wood and Wood Composites (1982). John G. Haygreen and Jim L. Bowyer, Forest Products and Wood Science (1982), provides basic information concerning physical and chemical properties of wood and the nature of major wood products. H.A. Core, W.A. Ct, and A.C. Day, Wood Structure and Identification, 2nd ed. (1979), contains photomicrographs of wood structure.Basic principles of tree felling and timber haulage are covered in Steve Conway, Logging Practices: Principles of Timber Harvesting Systems, rev. ed. (1982); George Stenzel, Thomas A. Walbridge, Jr., and J. Kenneth Pearce, Logging and Pulpwood Production, 2nd ed. (1985); and Roelof A.A. Oldeman (ed.), Tropical Hardwood Utilization: Practice and Prospects (1982). Methods and machines evolve so rapidly that recent issues of a trade periodical, such as World Wood (bimonthly), should be consulted for current practice and equipment. An interesting historical overview of the industry is provided in Kenneth L. Smith, Sawmill: The Story of Cutting the Last Great Virgin Forest East of the Rockies (1986).Works on the commercial utilization of wood include A.J. Panshin et al., Forest Products: Their Sources, Production, and Utilization, 2nd ed. (1962), which provides general information on all products; Franz P. Kollmann and Wilfred A. Ct, Jr., Principles of Wood Science and Technology, vol. 2, Wood Based Materials (1975); Darrel D. Nicholas (ed.), Wood Deterioration and Its Prevention by Preservative Treatments, 2 vol. (1973); Food and Agriculture Organization of the United Nations, Plywood and Other Wood-Based Panels (1966); and Kenneth W. Britt (ed.), Handbook of Pulp and Paper Technology, 2nd rev. ed. (1970). Certain products (and properties) are discussed in the U.S. Department of Agriculture, Wood Handbook: Wood as an Engineering Material, rev. ed. (1987); and German Gurfinkel, Wood Engineering, 2nd ed. (1981), containing data for use in design and specification.