wood

wood

[wood]
wood, botanically, the xylem tissue that forms the bulk of the stem of a woody plant. Xylem conducts sap upward from the roots to the leaves, stores food in the form of complex carbohydrates, and provides support; it is made up of various types of cells specialized for each of these purposes. Among them are tracheids, elongated conduction and support cells; parenchyma (food storage) cells, some of which form rays for transverse conduction; xylem vessels, formed of hollow cells joined end to end; and fiber cells that reinforce these tubes. In the conifers the xylem is made up mainly of tracheids, thus presenting a uniform, nonporous appearance; their wood is called softwood. Deciduous trees have more complex xylem, permeated by vessels, and are called hardwoods, although the description is sometimes inaccurate.

The xylem is formed in the growing season by the cambium; in temperate regions the cells formed in the spring are larger in diameter than those formed in the summer, and this results in the annual rings observable in cross section. The new cells lose their protoplasm as they form the various tissues; the older, nonfunctional cells become plugged up, darken in color, and often accumulate bitter or poisonous substances (tannins, dyes, resins, and gums). This inner wood (the heartwood, as opposed to the functional sapwood) is valued for outdoor construction because of its resistance to moisture and to decay-producing organisms.

Commercial Uses

Freshly cut wood contains much moisture and tends to warp and split as it dries. Lumber is therefore seasoned before use—dried either slowly in the sun and air or more quickly by artificial means (kiln drying). Seasoning increases wood's buoyancy, strength, elasticity, and durability. Although synthetic materials have supplanted wood in many of its former uses, it is still widely employed for furniture, floors, railway ties, paper manufacture, and innumerable other purposes. Wood distillation yields methyl alcohol, wood tar, acetic acid, acetone, and turpentine; charcoal is made by burning or heating wood in insufficient air to consume it.

The wood of different species of trees varies considerably in weight, strength, and appearance. Softwood is normally uniform in grain (texture) and color; hardwood, in which the rays are more prominent and the arrangement of tissues is variable, produces lumber in which the grain may run vertically or horizontally and be coarse or smooth. The manner in which a log is cut results in lumber with thin or wide ray markings. A log cut horizontally shows the concentric annual rings; lengthwise cuts through the center are marked by thin vertical ray lines; and lengthwise cuts through the outer sections show the wood's characteristic wavy grain and wider ray markings, prized for their beauty. The rarer decorative woods may be cut in thin layers and glued to other wood structures (see veneer). Plywood, made of thin layers of wood glued so that the grains alternate in direction, makes an especially strong construction material. For some applications composition board offers another inexpensive substitute. Pressure-treated wood is lumber that has had a preservative forced into it under pressure.

Bibliography

See H. Cone, Wood Structure and Identification (1979); H. Bucksh, Dictionary of Wood and Woodworking Practice (2 vol., 1986).

Wood or à Wood, Anthony, 1632-95, English antiquary. His painstaking researches into the history of Oxford resulted in two great works, The History and Antiquities of the University of Oxford (in Latin, 1674; in English, tr. by him but not published until 1792-96), and Athenae Oxoniensis (1691-92; rev. and enl. ed. 1721) containing biographies of noted Oxford graduates. The second work included statements about the 1st earl of Clarendon that were adjudged libelous and for which he was expelled from Oxford. Wood's own Life and Times (comp. from his papers by Andrew Clark, 5 vol., 1891-1900) was abridged by Llewelyn Powys (1932).
Wood, Clement, 1888-1950, American writer, b. Tuscaloosa, Ala., grad. Univ. of Alabama, 1909, LL.B. Yale, 1911. Among his many works are books on the craft of poetry; biographies, including a critical one of Amy Lowell (1926); novels; and a rhyming dictionary (1943). His most famous poem is the title piece of his collected poems, The Glory Road (1936).
Wood, Edward Frederick Lindley: see Halifax, Edward Frederick Lindley Wood, 1st earl of.
Wood, Fernando, 1812-81, American politician, b. Philadelphia. He became a successful shipping merchant in New York City and a leader of Tammany Hall. Wood was elected mayor in 1854 and was reelected in 1856, but he displeased the other Tammany leaders in dispensing patronage and was ousted in 1857. He formed Mozart Hall, a rival organization, and won reelection in 1859. Pro-South, Wood suggested in Jan., 1861, that New York establish itself as an independent city. He was defeated for reelection in that year. During the Civil War he was a leading Peace Democrat. As a Congressman (1841-43, 1863-65, 1867-81) he reflected the views of the city's moneyed interests.

See biography by S. A. Pleasants (1948).

Wood, Grant, 1891-1942, American painter, studied at the Art Institute of Chicago and in Paris. In Munich in 1928 he was decisively influenced by German and Flemish primitive painting. Subsequently in the 1930s he created his "American scene" works in which stern people and stylized landscapes offer rigid, decorative images of the rural Midwest. He taught at the State Univ. of Iowa and was director of WPA art projects in Iowa. His American Gothic (Art Inst., Chicago) and Daughters of Revolution have been many times reproduced; other works include Stone City (Joslyn Art Mus., Omaha, Nebr.) and a series of murals at Iowa State Univ.

See D. Garwood, Artist in Iowa (1944, repr. 1971).

Wood, Mrs. Henry, 1814-87, English novelist whose maiden name was Ellen Price. Her melodramatic and sensational novel East Lynne (1861) was dramatized and became a permanent stock piece for more than a generation. Most of her work appeared first in the magazine Argosy, which she bought in 1867.
Wood, Jethro, 1774-1834, American inventor, b. either in Dartmouth, Mass., or in Washington co., N.Y. In 1814, while a farmer in Cayuga co., N.Y., he patented a cast-iron plow in which he later embodied improvements (patented 1819). He used in the improved model replaceable cast-iron parts and a curved plate called a moldboard from which the shape of the modern moldboard is derived. Litigation concerning his patent rights impoverished him.
Wood, John, 1704-1754, English architect, called Wood of Bath. When he went (1727) to Bath from Yorkshire to begin his career as a road surveyor, the city was at its height as a center of fashion. Wood devised civic layouts on a grand scale. His executed schemes exhibit entire streets and terraces (groups of row houses) formally arranged in continuous rows, curves, or circles. He designed Queen's Square, North and South Parade, and the Circus. Wood of Bath also designed the mansion of Prior Park, near Bath, his most handsome detached building. His work, by its charm and imagination, set a standard for the architects who later worked at Bath, and it remains an inspiration for modern city planners. His son, John Wood, Jr., 1728-81, completed the Circus and also built the Royal Crescent and the Assembly Rooms.
Wood, Leonard, 1860-1927, American general and administrator, b. Winchester, N.H. After practicing medicine briefly in Boston, he entered the army in 1885 and was made an assistant surgeon; in 1891 he was promoted to captain. At the outbreak of the Spanish-American War he joined with his friend Theodore Roosevelt in organizing a volunteer cavalry unit—the Rough Riders—and as their commander he participated in the attack on Santiago de Cuba. He was military commander of Santiago (1898-99), and as military governor (1899-1902) of Cuba until the republic was formed, he cooperated in improving sanitary conditions on the island. Sent (1903) to the Philippines as governor of Moro prov., he was promoted (1903) to major general. He helped crush the opposition to U.S. occupation there, although he was criticized for his ruthlessness. From 1906 to 1908 he commanded U.S. military forces in the Philippines. Returning to the United States, he served (1910-14) as U.S. army chief of staff. He was commander (1914-17) of the Dept. of the East and after the outbreak of World War I in Europe led the movement for preparedness in America. He advocated the creation of civilian training camps, which brought him into conflict with the neutralist position of President Wilson, and incurred the President's displeasure. After the U.S. entry into World War I, Wood was refused a commission on the European front. He failed to win the Republican nomination for President in 1920, but he was appointed (1921) governor-general of the Philippines. Distrusting the natives' capacity for self-government, he reversed the lenient policy of his predecessor, F. B. Harrison. Wood liquidated the economic enterprises of the Philippine government, assumed wide powers of control, allowed little prerogative to the legislature, and surrounded himself with military advisers. Until Wood died in 1927, unrest was widespread among the Filipinos, and in 1925 the Philippine senate unanimously voted to hold a plebiscite on independence. The report of the Thompson Commission, sent to the islands in 1926, sharply criticized Wood's rule.

See biography by H. Hagedorn (1931, repr. 1969).

Wood, Robert Williams, 1868-1955, American physicist, b. Concord, Mass., grad. Harvard (B.A., 1891). After studying abroad he became associated with Johns Hopkins as professor of experimental physics in 1901, professor emeritus in 1938, and later research professor. Internationally known for his work in optics and spectroscopy, he made important researches in resonance radiation and in the use of absorption screens in astronomical photography and devised a vastly improved diffraction grating. He also developed a color-photography process, originated the method of thawing street mains by passing an electric current through them, and studied the biological and physiological effects of high-frequency sound waves. He wrote Physical Optics (1905) and Researches in Physical Optics (2 parts, 1913-19). Wood was also the author of The Man Who Rocked the Earth (with Arthur Train, 1915) and nonsense verse, How to Tell the Birds from the Flowers (rev. ed. 1917).

See biography by W. Seabrook (1941).

Wood, Sarah: see under Kemble, Roger.

Design printed from a plank of wood incised parallel to the vertical axis of the wood's grain. One of the oldest methods of making prints, it was used in China to decorate textiles from the 5th century. Printing from wood blocks on textiles was known in Europe from the early 14th century but developed little until paper began to be manufactured in France and Germany at the end of the 14th century. In the early 15th century, religious images and playing cards were first made from wood blocks. Black-line woodcut reached its greatest perfection in the 16th century with Albrecht Dürer and his followers. In the late 19th and early 20th centuries, artists such as Edvard Munch, Paul Gauguin, and the German Expressionists rediscovered the expressive potential of woodcuts. Woodcuts have played an important role in the history of Japanese art (see ukiyo-e).

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or methyl alcohol or wood alcohol

Simplest of the alcohols, chemical formula CH3OH. Once produced by destructive distillation of wood, it is now usually made from the methane in natural gas. Methanol is an important industrial material; its derivatives are used in great quantities for making a vast number of compounds, among them many important synthetic dyes, resins, drugs, and perfumes. It is also used in automotive antifreezes, in rocket fuels, and as a solvent. It is flammable and explosive. A clean-burning fuel, it may substitute (at least in part) for gasoline. It is also used for denaturation of ethanol. A violent poison, it causes blindness and eventually death when drunk.

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or pack rat

Any of 22 species (genus Neotoma, family Cricetidae) of rodents that are nocturnal vegetarians of North and Central American deserts, forests, and mountains. Wood rats are buff, gray, or reddish brown, usually with white undersides and feet. They have large ears and are 9–19 in. (23–47 cm) long, including the 3–9-in. (8–24-cm) furry tail. The nest, up to 3 ft (1 m) across and usually built of twigs or cactus, is placed in a protected spot (e.g., under a rock ledge). Wood rats are sometimes called pack rats because they collect material to deposit in their dens.

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or wood mouse

In general, any mouse that normally lives in fields; more strictly, any of about seven species of small, long-tailed mice in the genus Apodemus (family Muridae). Field mice in this genus are found in fields, woodlands, and mountain meadows in the warm and temperate parts of Eurasia. They are grayish or light or reddish brown and are 2–5 in. (6–12 cm) long excluding the tail. They generally live in burrows and build nests of grass and other plants. They eat seeds, roots, and other plant material, occasionally damaging crops or young trees.

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Drake wood duck (Aix sponsa)

North American duck (Aix sponsa, family Anatidae); a popular game bird. Wood ducks, 17–21 in. (43–52 cm) long, nest in a tree cavity up to 50 ft (15 m) off the ground; they have long-clawed toes for perching. Both sexes have a head crest in winter. The beautifully coloured male has a purple and green head, red-brown breast flecked with white, and bronze sides; the female has a white eye ring and duller colouring. Ducklings eat aquatic insects and other small organisms; adults prefer acorns or other nuts. Hunted nearly to extinction for its flesh and feathers, it has been restored to healthy populations by strong conservation efforts.

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Cross section of a tree trunk. Wood is secondary xylem produced by growth of the vascular cambium elipsis

Hard, fibrous material formed by the accumulation of secondary xylem produced by the vascular cambium. It is the principal strengthening tissue found in the stems and roots of trees and shrubs. Wood forms around a central core (pith) in a series of concentric layers called growth rings. A cross section of wood shows the distinction between heartwood and sapwood. Heartwood, the central portion, is darker and composed of xylem cells that are no longer active in the life processes of the tree. Sapwood, the lighter area surrounding the heartwood, contains actively conducting xylem cells. Wood is one of the most abundant and versatile natural materials on earth, and unlike coal, ores, and petroleum, is renewable with proper care. The most widely used woods come from two groups of trees: the conifers, or softwoods (e.g., pine, spruce, fir), and the broadleaves, or hardwoods (e.g., oak, walnut, maple). Trees classified as hardwoods are not necessarily harder than softwoods (e.g., balsa, a hardwood, is one of the softest woods). Density and moisture content affect the strength of wood; in addition to load-bearing strength, other variable factors often tested include elasticity and toughness. Wood is insulating to heat and electricity and has desirable acoustical properties. Some identifying physical characteristics of wood include colour, odour, texture, and grain (the direction of the wood fibres). Some 10,000 different wood products are commercially available, ranging from lumber and plywood to paper, from fine furniture to toothpicks. Chemically derived products from wood and wood residues include cellophane, charcoal, dyestuffs, explosives, lacquers, and turpentine. Wood is also used for fuel in many parts of the world.

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Fossil formed by the infiltration of minerals into cavities between and within cells of natural wood, usually by silica (silicon dioxide, SiO2) or calcite (calcium carbonate, CaCO3). Often this replacement of organic tissue by mineral deposits is so precise that the internal structure as well as the external shape is faithfully represented; sometimes even the cell structure may be determined.

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(born Oct. 9, 1860, Winchester, N.H., U.S.—died Aug. 7, 1927, Boston, Mass.) U.S. army officer. He studied medicine and became a contract surgeon with the U.S. Army. In the Spanish-American War he and his friend Theodore Roosevelt recruited and commanded the volunteer Rough Riders. Promoted to brigadier general, Wood served as military governor of Cuba (1899–1902) and organized a modern civil government. After service in the Philippines, he was chief of staff of the U.S. Army (1910–14). Though he had advocated preparedness for war, as a Republican he was passed over for a command post in World War I by the Democratic administration. He later served as governor general of the Philippines (1921–27).

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(born Feb. 13, 1891, near Anamosa, Iowa, U.S.—died Feb. 12, 1942, Iowa City, Iowa) U.S. painter. He was trained as a craftsman and designer as well as a painter. On a visit to Germany in 1928, he was strongly influenced by the sharp detail of 15th-century German and Flemish paintings, and he soon abandoned his Impressionist manner for the detailed, realistic manner for which he is known. His American Gothic caused a sensation when exhibited in 1930. A telling portrait of the sober, hardworking Midwestern farmer, it has become one of the best-known icons of U.S. art, though it is often misinterpreted: the woman is not the man's wife but rather the unmarried daughter designated to stay on the farm to assist her widowed father.

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Park, western Canada. Situated between Athabasca and Great Slave lakes, it was established in 1922; it occupies an area of 17,300 sq mi (44,807 sq km). The world's largest park, it is a vast region of forests and plains, crossed by the Peace River and dotted with lakes. The habitat of the largest remaining herd of wood buffalo (bison) on the North American continent, as well as of bear, caribou, moose, and beaver, it also provides nesting grounds for the endangered whooping crane.

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(born Oct. 9, 1860, Winchester, N.H., U.S.—died Aug. 7, 1927, Boston, Mass.) U.S. army officer. He studied medicine and became a contract surgeon with the U.S. Army. In the Spanish-American War he and his friend Theodore Roosevelt recruited and commanded the volunteer Rough Riders. Promoted to brigadier general, Wood served as military governor of Cuba (1899–1902) and organized a modern civil government. After service in the Philippines, he was chief of staff of the U.S. Army (1910–14). Though he had advocated preparedness for war, as a Republican he was passed over for a command post in World War I by the Democratic administration. He later served as governor general of the Philippines (1921–27).

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(born April 16, 1881, Powderham Castle, Devonshire, Eng.—died Dec. 23, 1959, Garroby Hall, near York, Yorkshire) British statesman. He was elected to Parliament in 1910. As viceroy of India (1925–31), he worked on terms of understanding with Mohandas K. Gandhi and accelerated constitutional advances. His tenure as foreign secretary (1938–40) in Neville Chamberlain's government was controversial because of Chamberlain's policy of appeasement toward Adolf Hitler, but Halifax kept the post into Winston Churchill's ministry. As ambassador to the U.S. (1941–46), he greatly served the Allied cause in World War II, for which he was created earl of Halifax in 1944.

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(born Feb. 13, 1891, near Anamosa, Iowa, U.S.—died Feb. 12, 1942, Iowa City, Iowa) U.S. painter. He was trained as a craftsman and designer as well as a painter. On a visit to Germany in 1928, he was strongly influenced by the sharp detail of 15th-century German and Flemish paintings, and he soon abandoned his Impressionist manner for the detailed, realistic manner for which he is known. His American Gothic caused a sensation when exhibited in 1930. A telling portrait of the sober, hardworking Midwestern farmer, it has become one of the best-known icons of U.S. art, though it is often misinterpreted: the woman is not the man's wife but rather the unmarried daughter designated to stay on the farm to assist her widowed father.

Learn more about Wood, Grant with a free trial on Britannica.com.

(born April 16, 1881, Powderham Castle, Devonshire, Eng.—died Dec. 23, 1959, Garroby Hall, near York, Yorkshire) British statesman. He was elected to Parliament in 1910. As viceroy of India (1925–31), he worked on terms of understanding with Mohandas K. Gandhi and accelerated constitutional advances. His tenure as foreign secretary (1938–40) in Neville Chamberlain's government was controversial because of Chamberlain's policy of appeasement toward Adolf Hitler, but Halifax kept the post into Winston Churchill's ministry. As ambassador to the U.S. (1941–46), he greatly served the Allied cause in World War II, for which he was created earl of Halifax in 1944.

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Wood is hard, fibrous, lignified structural tissue produced as secondary xylem in the stems of woody plants, notably trees but also shrubs. In a living tree it conducts water and nutrients to the leaves and other growing tissues, and has a support function, enabling plants to reach large sizes. Wood may also refer to other plant materials and tissues with comparable properties.

People have used wood for millennia for many purposes, primarily as a construction material, for making tools, weapons, furniture, packaging, artworks, paper, and as a fuel. Wood can be dated to make inferences about when a wooden object was created and the climate at that time.

Formation

A tree increases in diameter by the formation, between the old wood and the inner bark, of new woody layers which envelop the entire stem, living branches, and roots. Where there are clear seasons, this can happen in a discrete pattern, leading to what is known as growth rings, as can be seen on the end of a log. If these seasons are annual these growth rings are annual rings. Where there is no seasonal difference growth rings are likely to be indistinct or absent.

Within a growth ring it may be possible to see two parts. The part nearest the center of the tree is more open textured and almost invariably lighter in colour than that near the outer portion of the ring. The inner portion is formed early in the season, when growth is comparatively rapid; it is known as early wood or spring wood. The outer portion is the late wood or summer wood, being produced in the summer. In white pines there is not much contrast in the different parts of the ring, and as a result the wood is very uniform in texture and is easy to work. In hard pines, on the other hand, the late wood is very dense and is deep-colored, presenting a very decided contrast to the soft, straw-colored early wood. In ring-porous woods each season's growth is always well defined, because the large pores of the spring abut on the denser tissue of the fall before. In the diffuse-porous woods, the demarcation between rings is not always so clear and in some cases is almost (if not entirely) invisible to the unaided eye.

Knots

A knot is a particular type of imperfection in a piece of timber, which reduces its strength, but which may be exploited for artistic effect. In a longitudinally-sawn plank, a knot will appear as a roughly circular "solid" (usually darker) piece of wood around which the roughly parallel fibres (grain) of the rest of the "flows" (parts and rejoins).

A knot is actually a portion of a side branch (or a dormant bud) included in the wood of the stem or larger branch. The included portion is irregularly conical in shape (hence the roughly circular cross-section) with the tip at the point in stem diameter at which the plant's cambium was located when the branch formed as a bud. Within a knot, the fibre direction (grain) is up to 90 degrees different from the fibres of the stem, thus producing local cross grain.

During the development of a tree, the lower limbs often die, but may persist for a time, sometimes years. Subsequent layers of growth of the attaching stem are no longer intimately joined with the dead limb, but are grown around it. Hence, dead branches produce knots which are not attached, and likely to drop out after the tree has been sawn into boards.

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. This firmness is affected by, among other factors, the length of time for which the branch was dead while the attaching stem continued to grow.

Knots materially affect cracking (known in the industry as checking) and warping, ease in working, and cleavability of timber. They are defects which weaken timber and lower its value for structural purposes where strength is an important consideration. The weakening effect is much more serious when timber is subjected to forces perpendicular to the grain and/or tension than where under load along the grain and/or compression. The extent to which knots affect the strength of a beam depends upon their position, size, number, direction of fiber, and condition. A knot on the upper side is compressed, while one on the lower side is subjected to tension. If there is a season check in the knot, as is often the case, it will offer little resistance to this tensile stress. Small knots, however, may be located along the neutral plane of a beam and increase the strength by preventing 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 necessarily 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 fiber than upon defects in the beam. The effect of knots is to reduce the difference between the fiber 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.

For purposes for which appearance is more important than strength, such as wall panelling, knots are considered a benefit, as they add visual texture to the wood, giving it a more interesting appearance.

The traditional style of playing the Basque xylophon txalaparta involves hitting the right knots to obtain different tones.

Heartwood and sapwood

Heartwood is wood that has died and become resistant to decay as a result of genetically programmed processes. It appears in a cross-section as a discolored circle, following annual rings in shape. Heartwood is usually much darker than living wood, and forms with age. Many woody plants do not form heartwood, but other processes, such as decay, can discolor wood in similar ways, leading to confusion. Some uncertainty still exists as to whether heartwood is truly dead, as it can still chemically react to decay organisms, but only once (Shigo 1986, 54).

Sapwood is living wood in the growing tree. All wood in a tree is first formed as sapwood. Its principal functions are to conduct water from the roots to the leaves and to store up and give back according to the season the food prepared in the leaves. The more leaves a tree bears and the more vigorous its growth, the larger the volume of sapwood required. Hence trees making rapid growth in the open have thicker sapwood for their size than trees of the same species growing in dense forests. Sometimes trees grown in the open may become of considerable size, 30 cm or more in diameter, before any heartwood begins to form, for example, in second-growth hickory, or open-grown pines.

The term heartwood derives solely from its position and not from any vital importance to the tree. This is evidenced by the fact that a tree can thrive with its heart completely decayed. Some species begin to form heartwood very early in life, so having only a thin layer of live sapwood, while in others the change comes slowly. Thin sapwood is characteristic of such trees as chestnut, black locust, mulberry, osage-orange, and sassafras, while in maple, ash, hickory, hackberry, beech, and pine, thick sapwood is the rule.

There is no definite relation between the annual rings of growth and the amount of sapwood. Within the same species the cross-sectional area of the sapwood is very roughly proportional to the size of the crown of the tree. If the rings are narrow, more of them are required than where they are wide. As the tree gets larger, the sapwood must necessarily become thinner or increase materially in volume. Sapwood is thicker in the upper portion of the trunk of a tree than near the base, because the age and the diameter of the upper sections are less.

When a tree is very young it is covered with limbs almost, if not entirely, to the ground, but as it grows older some or all of them will eventually die and are either broken off or fall off. Subsequent growth of wood may completely conceal the stubs which will however remain as knots. No matter how smooth and clear a log is on the outside, it is more or less knotty near the middle. Consequently the sapwood of an old tree, and particularly of a forest-grown tree, will be freer from knots than the heartwood. Since in most uses of wood, knots are defects that weaken the timber and interfere with its ease of working and other properties, it follows that sapwood, because of its position in the tree, may have certain advantages over heartwood.

It is remarkable that the inner heartwood of old trees remains as sound as it usually does, since in many cases it is hundreds of years, and in a few instances thousands of years, old. Every broken limb or root, or deep wound from fire, insects, or falling timber, may afford an entrance for decay, which, once started, may penetrate to all parts of the trunk. The larvae of many insects bore into the trees and their tunnels remain indefinitely as sources of weakness. Whatever advantages, however, that sapwood may have in this connection are due solely to its relative age and position.

If a tree grows all its life in the open and the conditions of soil and site remain unchanged, it will make its most rapid growth in youth, and gradually decline. The annual rings of growth are for many years quite wide, but later they become narrower and narrower. Since each succeeding ring is laid down on the outside of the wood previously formed, it follows that unless a tree materially increases its production of wood from year to year, the rings must necessarily become thinner as the trunk gets wider. As a tree reaches maturity its crown becomes more open and the annual wood production is lessened, thereby reducing still more the width of the growth rings. In the case of forest-grown trees so much depends upon the competition of the trees in their struggle for light and nourishment that periods of rapid and slow growth may alternate. Some trees, such as southern oaks, maintain the same width of ring for hundreds of years. Upon the whole, however, as a tree gets larger in diameter the width of the growth rings decreases.

There may be decided differences in the grain of heartwood and sapwood cut from a large tree, particularly one that is mature. In some trees, the wood laid on late in the life of a tree is softer, lighter, weaker, and more even-textured than that produced earlier, but in other species, the reverse applies. In a large log the sapwood, because of the time in the life of the tree when it was grown, may be inferior in hardness, strength, and toughness to equally sound heartwood from the same log.

Different woods

There is a strong relationship between the properties of wood and the properties of the particular tree that yielded it. For every tree species there is a range of density for the wood it yields. There is a rough correlation between density of a wood and its strength (mechanical properties). For example, while mahogany is a medium-dense hardwood which is excellent for fine furniture crafting, balsa is light, making it useful for model building. The densest wood may be black ironwood.

Wood is commonly classified as either softwood or hardwood. The wood from conifers (e.g. pine) is called softwood, and the wood from broad-leaved trees (e.g. oak) is called hardwood. These names are a bit misleading, as hardwoods are not necessarily hard, and softwoods are not necessarily soft. The well-known balsa (a hardwood) is actually softer than any commercial softwood. Conversely, some softwoods (e.g. yew) are harder than most hardwoods.

Wood products such as plywood are typically classified as engineered wood and not considered raw wood.

Color

In species which show a distinct difference between heartwood and sapwood the natural colour of heartwood is usually darker than that of the sapwood, and very frequently the contrast is conspicuous. This is produced by deposits in the heartwood of various materials resulting from the process of growth, increased possibly by oxidation and other chemical changes, which usually have little or no appreciable effect on the mechanical properties of the wood. Some experiments on very resinous Longleaf Pine specimens, however, indicate an increase in strength. This is due to the resin which increases the strength when dry. Such resin-saturated heartwood is called "fat lighter". Structures built of fat lighter are almost impervious to rot and termites; however they are very flammable. Stumps of old longleaf pines are often dug, split into small pieces and sold as kindling for fires. Stumps thus dug may actually remain a century or more since being cut. Spruce impregnated with crude resin and dried is also greatly increased in strength thereby.

Since the late wood of a growth ring is usually darker in colour than the early wood, this fact may be used in judging the density, and therefore the hardness and strength of the material. This is particularly the case with coniferous woods. In ring-porous woods the vessels of the early wood not infrequently appear on a finished surface as darker than the denser late wood, though on cross sections of heartwood the reverse is commonly true. Except in the manner just stated the colour of wood is no indication of strength.

Abnormal discolouration of wood often denotes a diseased condition, indicating unsoundness. The black check in western hemlock is the result of insect attacks. The reddish-brown streaks so common in hickory and certain other woods are mostly the result of injury by birds. The discolouration is merely an indication of an injury, and in all probability does not of itself affect the properties of the wood. Certain rot-producing fungi impart to wood characteristic colours which thus become symptomatic of weakness; however an attractive effect known as spalting produced by this process is often considered a desirable characteristic. Ordinary sap-staining is due to fungous growth, but does not necessarily produce a weakening effect.

Structure

Wood is a heterogeneous, hygroscopic, cellular and anisotropic material. It is composed of fibers of cellulose (40% – 50%) and hemicellulose (15% – 25%) impregnated with lignin (15% – 30%).

In coniferous or softwood species the wood cells are mostly of one kind, tracheids, and as a result the material is much more uniform in structure than that of most hardwoods. There are no vessels ("pores") in coniferous wood such as one sees so prominently in oak and ash, for example.

The structure of the hardwoods is more complex. They are more or less filled with vessels: in some cases (oak, chestnut, ash) quite large and distinct, in others (buckeye, poplar, willow) too small to be seen plainly without a small hand lens. In discussing such woods it is customary to divide them into two large classes, ring-porous and diffuse-porous. In ring-porous species, such as ash, black locust, catalpa, chestnut, elm, hickory, mulberry, and oak, the larger vessels or pores (as cross sections of vessels are called) are localized in the part of the growth ring formed in spring, thus forming a region of more or less open and porous tissue. The rest of the ring, produced in summer, is made up of smaller vessels and a much greater proportion of wood fibres. These fibres are the elements which give strength and toughness to wood, while the vessels are a source of weakness.

In diffuse-porous woods the pores are scattered throughout the growth ring instead of being collected in a band or row. Examples of this kind of wood are basswood, birch, buckeye, maple, poplar, and willow. Some species, such as walnut and cherry, are on the border between the two classes, forming an intermediate group.

If a heavy piece of pine is compared with a light specimen it will be seen at once that the heavier one contains a larger proportion of late wood than the other, and is therefore considerably darker. The late wood of all species is denser than that formed early in the season, hence the greater the proportion of late wood the greater the density and strength. When examined under a microscope the cells of the late wood are seen to be very thick-walled and with very small cavities, while those formed first in the season have thin walls and large cavities. The strength is in the walls, not the cavities. In choosing a piece of pine where strength or stiffness is the important consideration, the principal thing to observe is the comparative amounts of early and late wood. The width of ring is not nearly so important as the proportion of the late wood in the ring.

It is not only the proportion of late wood, but also its quality, that counts. In specimens that show a very large proportion of late wood it may be noticeably more porous and weigh considerably less than the late wood in pieces that contain but little. One can judge comparative density, and therefore to some extent weight and strength, by visual inspection.

No satisfactory explanation can as yet be given for the real causes underlying the formation of early and late wood. Several factors may be involved. In conifers, at least, rate of growth alone does not determine the proportion of the two portions of the ring, for in some cases the wood of slow growth is very hard and heavy, while in others the opposite is true. The quality of the site where the tree grows undoubtedly affects the character of the wood formed, though it is not possible to formulate a rule governing it. In general, however, it may be said that where strength or ease of working is essential, woods of moderate to slow growth should be chosen. But in choosing a particular specimen it is not the width of ring, but the proportion and character of the late wood which should govern.

In the case of the ring-porous hardwoods there seems to exist a pretty definite relation between the rate of growth of timber and its properties. This may be briefly summed up in the general statement that the more rapid the growth or the wider the rings of growth, the heavier, harder, stronger, and stiffer the wood. This, it must be remembered, applies only to ring-porous woods such as oak, ash, hickory, and others of the same group, and is, of course, subject to some exceptions and limitations.

In ring-porous woods of good growth it is usually the middle portion of the ring in which the thick-walled, strength-giving fibers are most abundant. As the breadth of ring diminishes, this middle portion is reduced so that very slow growth produces comparatively light, porous wood composed of thin-walled vessels and wood parenchyma. In good oak these large vessels of the early wood occupy from 6 to 10 per cent of the volume of the log, while in inferior material they may make up 25 per cent or more. The late wood of good oak, except for radial grayish patches of small pores, is dark colored and firm, and consists of thick-walled fibers which form one-half or more of the wood. In inferior oak, such fiber areas are much reduced both in quantity and quality. Such variation is very largely the result of rate of growth.

Wide-ringed wood is often called "second-growth", because the growth of the young timber in open stands after the old trees have been removed is more rapid than in trees in the forest, and in the manufacture of articles where strength is an important consideration such "second-growth" hardwood material is preferred. This is particularly the case in the choice of hickory for handles and spokes. Here not only strength, but toughness and resilience are important. The results of a series of tests on hickory by the U.S. Forest Service show that:

"The work or shock-resisting ability is greatest in wide-ringed wood that has from 5 to 14 rings per inch (rings 1.8-5 mm thick), is fairly constant from 14 to 38 rings per inch (rings 0.7-1.8 mm thick), and decreases rapidly from 38 to 47 rings per inch (rings 0.5-0.7 mm thick). The strength at maximum load is not so great with the most rapid-growing wood; it is maximum with from 14 to 20 rings per inch (rings 1.3-1.8 mm thick), and again becomes less as the wood becomes more closely ringed. The natural deduction is that wood of first-class mechanical value shows from 5 to 20 rings per inch (rings 1.3-5 mm thick) and that slower growth yields poorer stock. Thus the inspector or buyer of hickory should discriminate against timber that has more than 20 rings per inch (rings less than 1.3 mm thick). Exceptions exist, however, in the case of normal growth upon dry situations, in which the slow-growing material may be strong and tough."

The effect of rate of growth on the qualities of chestnut wood is summarized by the same authority as follows:

"When the rings are wide, the transition from spring wood to summer wood is gradual, while in the narrow rings the spring wood passes into summer wood abruptly. The width of the spring wood changes but little with the width of the annual ring, so that the narrowing or broadening of the annual ring is always at the expense of the summer wood. The narrow vessels of the summer wood make it richer in wood substance than the spring wood composed of wide vessels. Therefore, rapid-growing specimens with wide rings have more wood substance than slow-growing trees with narrow rings. Since the more the wood substance the greater the weight, and the greater the weight the stronger the wood, chestnuts with wide rings must have stronger wood than chestnuts with narrow rings. This agrees with the accepted view that sprouts (which always have wide rings) yield better and stronger wood than seedling chestnuts, which grow more slowly in diameter."

In diffuse-porous woods, as has been stated, the vessels or pores are scattered throughout the ring instead of collected in the early wood. The effect of rate of growth is, therefore, not the same as in the ring-porous woods, approaching more nearly the conditions in the conifers. In general it may be stated that such woods of medium growth afford stronger material than when very rapidly or very slowly grown. In many uses of wood, strength is not the main consideration. If ease of working is prized, wood should be chosen with regard to its uniformity of texture and straightness of grain, which will in most cases occur when there is little contrast between the late wood of one season's growth and the early wood of the next.

Monocot wood

Structural tissue resembling ordinary 'dicot' wood is produced by a number of monocot plants, and these are also usually called wood. Of these, the wood of the grass bamboo has considerable economic importance, larger culms being used in the manufacture of engineered flooring, panels and veneer. Other plant groups that produce woody tissue are palms, and members of the Liliales, such as Dracaena and Cordyline. With all these woods, the structure and composition of the structural tissue is quite different from ordinary wood.

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 8-16% 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 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 5 cm 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.

Uses

Fuel

Wood is burned as a fuel mostly in rural areas of the world. Hard wood is preferred over softwood because it creates less smoke and burns longer. Adding a woodstove or fireplace to a home adds ambiance and warmth.

Construction

Wood has been an important construction material since humans began building shelters, houses and boats. Nearly all boats were made out of wood till the late 19th century, and wood remains in common use today in boat construction. New domestic housing in many parts of the world today is commonly made from timber-framed construction. In buildings made of other materials, wood will still be found as a supporting material, especially in roof construction, in interior doors and their frames, and as exterior cladding. Wood to be used for construction work is commonly known as lumber in North America. Elsewhere, lumber usually refers to felled trees, and the word for sawn planks ready for use is timber. Wood is also commonly used as shuttering material to form the mould into which concrete is poured during reinforced concrete construction.

Wood unsuitable for construction in its native form may be broken down mechanically (into fibres or chips) or chemically (into cellulose) and used as a raw material for other building materials such as chipboard, engineered wood, hardboard, medium-density fiberboard (MDF), oriented strand board (OSB). Such wood derivatives are widely used: wood fibers are an important component of most paper, and cellulose is used as a component of some synthetic materials. Wood derivatives can also be used for kinds of flooring, for example laminate flooring.

Wood is also used for cutlery, such as chopsticks, toothpicks, and other utensils, like the wooden spoon.

In the arts

Wood has long been used as an artistic medium. It has been used to make sculptures and carvings for centuries. It is also used in woodcut printmaking, and for engraving.

Certain types of musical instruments, such as the xylophone and marimba, are made mostly or entirely of wood.

See also

Notes

References

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