All living organisms are composed of cells. In the growing tree different cell tissues perform different functions; some store and convey food manufactured in the leaves, some convey liquids, while others provide strength and elasticity to the tree. Most of the conducting and supporting cell tissue is arranged vertically. This is evident in the grain direction of timber. Food storage and conduction across the tree is largely a function of narrow ribbons of weak cells, known as rays, which run across the grain.
Wood substance is composed almost entirely of organic matter. The structural components consist of cellulose, hemicellulose and lignin. Lignin acts as a bonding and stiffening agent, while the cellulose and hemicellulose provide elasticity and strength. Other chemicals are often present as part of the cell wall structure or in the cell cavities, while pigments determine the colour of wood.
The growth and structure of the tree has an effect on the properties of the timber. Some of the natural features which are easily visible in timber include:
The trunks of many trees show a darker coloured area in the centre; this is the heartwood. Its function is almost entirely to provide mechanical support. Around this is a ring of sapwood which conducts water from the roots to the leaves. Not all trees show a difference in colour between the sapwood and heartwood, but both exist in mature trees.
Sapwood generally has low resistance to fungal or insect attack; the resistance of heartwood varies considerably depending on the species. Trees with richly coloured heartwood can provide highly decorative timber. Many of the softwoods used for structural purposes, eg European redwood, will contain a significant amount of sapwood as the trees are harvested whilst relatively small in diameter.
Knots are the remains of branches. Normal branches originate from the centre of the tree and become embedded in the main trunk, appearing as a knot in the converted timber. A branch sawn off close to the trunk or shed naturally, is rapidly covered by normal wood. These branches produce sound, live or tight knots. Broken branch stubs may remain for some time before new growth surrounds them, producing a loose or dead knot in the timber.
A growth ring is the layer of wood produced in one growing season. Trees which grow in areas of the world with distinct seasonal patterns exhibit rings corresponding to an annual period of growth. These can be used to judge the age and rate of growth of felled trees. Earlywood is the less dense wood formed during the earlier stages of growth of each growth ring, eg in spring or the rainy season. Latewood is denser wood formed during the later stages of growth, as in summer, autumn or the dry season.
Grain refers to the general direction of the fibres relative to the axis of the tree. Grain may be straight, spiral, diagonal, interlocked or wavy.
Figure refers to markings seen on the cut surface of timber, formed by the structural features of the wood.
Texture depends upon the structural character of the wood; small celled species produce fine textured timber, large cells produce a coarse texture. This can be important in the selection of decorative timber and in the choice of finishing techniques.
