Wood

Wood is a hard fibrous material which is a natural composite of cellulose fibres embedded in a matrix of lignin. It forms the stem of trees and transfers water and nutrients to the living, growing tissues, for instance in the leaves, and supports the tree .

The exact structure of wood is different in different species of tree but also in the same species which have grown in different environments and conditions, or indeed in different parts of the tree.

The complexity of the internal structure of wood means that the biological and material properties (such as strength) can vary along different directions. This property means that wood is an intrinsically anisotropic material.

Such anisotropy can result in variations in properties which is often undesirable to those who wich to use the wood industrially.


The photographs below show cross, radial and tangential sections of a sample of Trochodendron aralioides (the wheel tree). The differences in the structure along these three directions within the wood illustrate the anisotropic nature of wood. 

References:

Kramer, Kramer and Groves etc: see for example Wood Grain Pattern Formation