4: Cell Structure
The wood cell structure is set out below.
- Middle lamella (mostly lignin)
- Primary wall (randomly orientated microfibrils)
- Outer wall (microfibrils oriented in 2 directions not parallel to cell axis)
- Middle wall (microfibrils orientated almost parallel to cell axis, also thickest layer)
- Inner wall (same as 3)
- Pore (or lumen)
All of the common timbers used for building applications have very similar cell structures and compositions. The differences in properties of wood can be mainly accounted for by their varying densities. The strength of timber is also dependent on the amount of moisture in the wood.
In hardwoods the elongated cells are relatively short. Diameters are usually 0.1mm and lengths are usually 1mm. There are longitudinal pores that carry water through the tree. In softwoods the cells, also referred to as tracheids, are longer than in hardwoods. The lower strength and weight of softwoods compared to hardwoods is due to the greater percentage of void space within the tree cells.