The structure of timber

When a tree trunk is sawn through one can see first the outer layer or bark and then a thin growth layer. It is this layer that is responsible for the growth in thickness of the trunk or branch of a tree, for it forms bark outside and wood inside. A new growth ring is added every year. During the spring, large thin-walled cells are formed which transport water, while later in the summer the cells formed have walls that are thicker, and often darker. While the tree is growing the cells in the outermost annual rings are living and it is here that the transport of liquids from roots to leaves takes place. This part of the wood is called the sapwood, while the inner part, the heartwood, consists of dead cells. Right in the centre is the pith.

There are certain structural differences between coniferous trees, e.g. pine or spruce, and deciduous trees, such as oak or beech, and it is often difficult to distinguish these two types on external features. The surest method of distinguishing them is on the presence or absence of true vessels. These are present in deciduous trees, but absent in conifers. These vessels are specially adapted for the transport of water. In some species these are visible to the naked eye in a cross section. Oak and ash, for example, have large vessels.

In some deciduous trees, such as birch and beech, the vessels are more uniform in size throughout the season, and are generally quite small. Normally the darker colour of the heartwood is due to waste products, such as tannins, and to some extent these serve as a protection against fungal infections and the attacks of various animals. On the other hand, the heartwood is not receptive to impregnation with timber preservatives.