Wood

Wood derives from woody plants, notably trees but also shrubs. Wood from the latter is only produced in small sizes, reducing the diversity of uses.

In its most common meaning, "wood" is the secondary xylem of a woody plant, but this is an approximation only: in the wider sense, wood may refer to other materials and tissues with comparable properties. Wood is a hygroscopic, cellular and anisotropic material. Dry wood is composed of fibers of cellulose (40%–50%) and hemi cellulose (15%–25%) held together by lignin (15%–30%)

Wood has been used for millennia for many purposes, being many things to many people. One of its primary uses is as fuel. It is also used as a material, for making artworks, boats, buildings, furniture, ships, tools, weapons, etc. Wood has been an important construction material since humans began building shelters, and remains in plentiful use today. Construction wood is commonly known as timber in International English, and lumber in American English. Wood may be broken down and be made into chipboard, engineered wood, hardboard, medium-density fiberboard (MDF), oriented strand board (OSB), paper or used to make other synthetic substances.

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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 absent.

Within a growth ring it may be possible to see two more or less well-defined parts. The part nearest the center of the tree is more open textured and almost invariably lighter in color 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

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Knots are portions of branches included in the wood of the stem or larger branch. Branches generally originate at or near the pith (central axis) of a stem, and the living portion will increase in size through 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 fiber direction is at right angles or oblique to the grain of the stem, thus producing local cross grain. A small knot may also be the result of a dormant bud.

During the development of a tree the lower limbs die, but may 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 sawn. 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 (cracking) 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 fiber, 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.

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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 some purposes, e.g. wall paneling, knots are considered a plus as they add visual texture to the wood, giving it a more interesting appearance.

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