A cross-section of an exogenous tree is shown in Fig. 10.1. It shows almost distinct concentric rings called annual rings. Pith is the innermost central portion consisting of cellular tissues. The inner annular area surrounding the pith is called heartwood. Heartwood is the one which is used for all engineering works. The portion between the cambium layer and the heartwood is called the sapwood, which can not be of any use for engineering purposes since it contains large quantities of water. A very small amount of cells that grow in the horizontal direction occurs as groups or bundles of cells known as medullary rays.
Figure 10.1 Typical cross-section of wood
Freshly cut trees contain large quantities of moisture. If the percentage of moisture is more than about 20%, then the cut trees are subjected to different types of fungus attacks and attacks by insects. Hence, the amount of moisture has to be reduced.
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