Structure

The physical features of timber

Wood is the product of a class of plants called 'exogens', which grow by the addition of new wood on the outside of the trunk and branches. It is composed of minute, tubelike cells packed closely together and joined end to end or overlapping obliquely. The cells are made up of cellulose and lignin, a complicated organic material, which is a combination of carbon, hydrogen and oxygen. Inside the cells is a watery composition of starches, sugars, minerals, gums and resins. As the cells age, lignin is deposited in and between the cell walls, making the walls more rigid and the wood more durable.

There are two botanical classes of trees:

• Angiosperms - seeds in fruit, broad leaves - hardwoods (pored)

• Gymnosperms - naked seeds in cones, usually narrow leaves (needles) - softwoods (non-pored)

How the tree grows

A tree makes its food through the process known as 'photosynthesis', which is the formation of sugar from carbon dioxide and water in the presence of sunlight. It takes place only where there is chlorophyll - the green colouring matter in leaves.

Water and minerals in solution are absorbed from the soil by the fine hairs on the roots and transported by the sapwood, which is immediately beneath the cambium layer and bark, to the leaves. Here, a large proportion of the water evaporates, leaving the minerals and some water behind. This process is called 'transpiration'. Carbon, a basic element in all plants, is obtained from carbon dioxide (CO2) in the air. Entering through minute apertures called 'stomata' on the underside of the leaves, the carbon dioxide is broken down by chlorophyll, with the aid of sunlight. The carbon combines with the water from the roots, while oxygen is returned to the air.

The tree's food, which is manufactured in the leaves, is transported down the inner layers of the bark cells (phloem), thus providing the living cambium cells with materials for the formation of new wood and bark. On its way down some of the food material also reaches the living wood cells immediately inside the cambium layer by means of special cells called 'medullary rays' or 'ray parenchyma', which are produced for this purpose (and also for food storage). A ring-barked tree will eventually die as the food from the leaves cannot pass down through the inner layers of the bark to the roots. In addition, if the sapwood also is severed, the tree will generally die more quickly.

A tree grows in two ways:

1. in height

2. in girth or diameter.

Growth in height is due to the division and growth of numerous special cells at the extreme tips of the trunk and branches. These special cells are thin-walled and do not, in themselves, produce woody tissue. 

A short way back from the growing tip the inner cells form the pith or medulla, while the outside cells form the cambium layer.

Parts of the tree

The pith (or medulla) is the heart or centre of the tree and stores food in the young tree. It is usually quite soft, darker in colour than the other wood in the tree and frequently rots out, sometimes causing 'piping'.

The sapwood (or xylem), which is usually light in colour, contains living cells, some of which conduct water and mineral salts from the roots to the leaves. Other living cells form the medullary rays (or ray parenchyma), whose purpose is to store plant food, mainly in the form of starch. Sapwood has three main functions in the living tree: support, conduction and food storage.

The heartwood (sometimes referred to as Truewood) is the fully matured wood that surrounds the pith (see Fig. 2.2). It is frequently darker in colour than the sapwood due to the deposits of tannin, resins and gums in the dead cells. The heartwood's natural resistance to decay and fungi is mainly due to the presence of these 'antiseptic' inclusions. Also, because it contains very little starchy food, the heartwood is not as susceptible to insect or fungi attack as sapwood. However, sapwood cells are more open and can be treated more readily with penetrating preservative fluids, which makes sapwood as durable as, or even more durable than, heartwood. In some cases, sapwood may be a superior material. For instance, sapwood from Canadian ash is selected for the manufacture of laminated tennis racquet frames because it is slightly lighter and less brittle than heartwood. Heartwood has practically no other function in the growing tree than that of mechanical support.

The cambium is entirely responsible for the tree's growth in girth through the development of growth rings. These concentric rings of alternating light and dark wood tissues, which are visible in the cross-section of most trees, are sometimes called 'annual rings'. However, some trees, particularly in the tropics, have more than one period of growth each year and, consequently, form more than one ring each year. Many other tropical and subtropical trees, in which growth is more or less regular throughout the year, produce no visible growth rings, while trees grown in areas with greater climatic variation produce growth rings that are more pronounced. These rings are true annual rings and provide a means of determining the age of the tree. In spring, the cambium forms a layer of light-coloured, thin-walled cells commonly called 'springwood'.

Smaller, darker-coloured cells with thicker walls are formed later in the year, and are called 'summerwood '. However, this term is misleading because summerwood may be formed in autumn, so the terms 'earlywood' and 'latewood ' are now widely accepted as being more precise.

Rays (or ray parenchyma) are specialised cells in the xylem that grow radially from the pith or intermediate growth rings to the cambium layer. Their function is to store sap food and convey it to the inner living parts of the tree. They also serve to bind the growth rings together. The bark (or cortex) varies considerably in thickness and appearance between species, but always contains some dead, corky material, which is good protection for the tender living inner bark (phloem). The phloem carries the plant food from the leaves to the cambium layer.

The structure of wood

According to its botanical features, wood is classed as either 'hardwood' (pored wood) or 'soft wood' (non-pored wood). Hardwood trees  have  leaves, are generally deciduous and have covered  seeds; for example, gum nuts. Softwood trees usually have needle-like foliage, and are generally from the pine/conifer family with uncovered seeds that fall from the pine cone as it opens. Hardwoods do not necessarily have harder wood than softwoods; balsa wood, which is the lightest and softest commercial wood in the world (used in model airplane construction), is a hardwood. 

Softwood

Softwoods or non-pored woods have a simpler and more uniform cell structure than hardwoods. This is due to the fact that  the bulk of the wood is made up of long, thin cells called 'tracheids', so thin that they are generally visible only under a microscope. These cells perform two functions: (1)  they provide mechanical support and (2) they carry water and mineral salts from the roots to the leaves. Connection between the cells is by 'pits', which are minute holes in adjacent cell walls. Rays, which are very small when compared with those in some hardwoods, radiate from the centre of the tree and at right angles to the tracheids, and store food and carry food materials from the phloem inwards to the living cells in the wood. All non-pored woods come from coniferous or pine trees (e.g. Oregon pine, radiata pine, hoop pine, cypress pine). 

Hardwood

Hardwoods or pored woods are characterised by large, tubelike vessels or pores. These consist of short stubby cells varying considerably in size and joined together to provide, when newly formed, a continuous means of conducting solutions from the roots to the leaves. The vessels frequently occur in groups and have pits in the side walls to allow the passage of solutions. The pores in some hardwoods such as pacific maple are visible to the naked eye, whereas a magnifying glass is necessary to see those in coachwood. Smaller, thick-walled cells (called 'fibres') act simply as mechanical support for the tree, while the medullary rays have the same function as the rays in softwoods. Some of the more important hardwoods are eucalypts, blackwood, maple, oak, walnut, ash, beech, willow, red cedar, mahogany, basswood and hickory.