Wood is composed mostly of hollow, elongated, spindle-shaped cells that are arranged parallel to each other along the trunk of a tree. The characteristics of these fibrous cells and their arrangement affect strength properties, appearance, resistance to penetration by water and chemicals, resistance to decay, and many other properties.
Just under the bark of a tree is a thin layer of cells, not visible to the naked eye, called the cambium. Here cells divide and eventually differentiate to form bark tissue to the outside of the cambium and wood or xylem tissue to the inside. This newly formed wood (termed sapwood) contains many living cells and conducts sap upward in the tree. Eventually, the inner sapwood cells become inactive and are transformed into heartwood. This transformation is often accompanied by the formation of extractives that darken the wood, make it less porous, and sometimes provide more resistance to decay. The center of the trunk is the pith, the soft tissue about which the first wood growth takes place in the newly formed twigs. See Stem
In temperate climates, trees often produce distinct growth layers. These increments are called growth rings or annual rings when associated with yearly growth; many tropical trees, however, lack growth rings. These rings vary in width according to environmental conditions.
Many mechanical properties of wood, such as bending strength, crushing strength, and hardness, depend upon the density of wood; the heavier woods are generally stronger. Wood density is determined largely by the relative thickness of the cell wall and the proportions of thick- and thin-walled cells present. See Wood properties
In hardwoods (for example, oak or maple), these three major planes along which wood may be cut are known commonly as end-grain, quarter-sawed (edge-grain) and plain-sawed (flat-grain) surfaces (see illustration).
Hardwoods have specialized structures called vessels for conducting sap upward. Vessels are a series of relatively large cells with open ends, set one above the other and continuing as open passages for long distances. In most hardwoods, the ends of the individual cells are entirely open; in others, they are separated by a grating. On the end grain, vessels appear as holes and are termed pores. The size, shape, and arrangement of pores vary considerably between species, but are relatively constant within a species.
Most smaller cells on the end grain are wood fibers which are the strength-giving elements of hardwoods. They usually have small cavities and relatively thick walls. Thin places or pits in the walls of the wood fibers and vessels allow sap to pass from one cavity to another. Wood rays are strips of short horizontal cells that extend in a radial direction. Their function is food storage and lateral conduction. See Parenchyma, Secretory structures (plant)
The rectangular units that make up the end grain of softwood are sections through long vertical cells called tracheids or fibers. Because softwoods do not contain vessel cells, the tracheids serve the dual function of transporting sap vertically and giving strength to the wood. The wood rays store and distribute sap horizontally.
The principal compound in mature wood cells is cellulose, a polysaccharide of repeating glucose molecules which may reach 4 μm in length. These cellulose molecules are arranged in an orderly manner into structures about 10–25 nm wide called microfibrils. The microfibrils wind together like strands in a cable to form macrofibrils that measure about 0.5 μm in width and may reach 4 μm in length. These cables are as strong as an equivalent thickness of steel.
This framework of cellulose macrofibrils is cross-linked with hemicelluloses, pectins, and lignin. Lignin, the second most abundant polymer found in plants, gives the cell wall rigidity and the substance that cements the cells together. See Cell walls (plant), Plant anatomy, Tree