wood

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Hard tissue beneath the bark of many perennial plants; it is composed of water-conducting cells, or secondary xylem, and gains its hardness and strength from deposits of lignin. Hardwoods, such as oak, and softwoods, such as pine, have commercial value as structural material and for furniture.

The central wood in a branch or stem is known as heartwood and is generally darker and harder than the outer wood; it consists only of dead cells. As well as providing structural support, it often contains gums, tannins, or pigments which may impart a characteristic colour and increased durability. The surrounding sapwood is the functional part of the xylem that conducts water.

The secondary xylem is laid down by the vascular cambium which forms a new layer of wood annually, on the outside of the existing wood and visible as an annual ring when the tree is felled; see dendrochronology.

Commercial wood can be divided into two main types: hardwood, containing xylem vessels and obtained from angiosperms (for example, oak); and softwood, containing only tracheids, obtained from gymnosperms (for example, pine). Although in general softwoods are softer than hardwoods, this is not always the case: balsa, the softest wood known, is a hardwood, while pitch pine, very dense and hard, is a softwood. A superhard wood is produced in wood–plastic combinations (WPC), in which wood is impregnated with liquid plastic (monomer) and the whole is then bombarded with gamma rays to polymerize the plastic.

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