wood

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.

The heartwood is what is used as timber for making wooden products. Once cut and sliced, the timber is left to season. This allows the wood to dry out a little (10-20%) and can take up to two years. Softwoods are obtained from narrow-leaved, evergreen trees. The grain of the softwood is not too compact and this makes it easy to cut. Softwoods include firs and pines such as Douglas fir, spruce, Parana pine, and red cedar. Hardwoods come from broad-leaved, deciduous trees. The grain of hardwood trees is generally tightly packed, making them more difficult to cut. Hardwoods include oak, ash, beech, mahogany, teak, and elm. Hardwood trees are slow growing; once cut they take a long time to replace. This is why hardwoods are often expensive to buy. Softwood trees are usually faster growing (about 25 years to maturity); they can be replaced more easily and form the basis of the sustainable forestry industry.

Wood can be processed to make composite timbers such as laminates, particle boards, and fibreboards.