Wood is composed of:
Cellulose: Cellulose is a polymer (C6H10O5) that can crystallise to form very strong fibres. Cellulose is the primary strengthening material in wood.
Lignin: Lignin is also a polymer but is typically in an amorphous form. The lignin acts as the matrix or binder for the cellulose.
Hemicellulose: Hemicellulose is a partly crystalline polymer that also acts as a matrix or binder.
Extractives: Extractives are the organic impurities that are responsible for the wood’s colour, smell, and, in some cases, resistance to rot, fungus, and insects.
The high strength of wood is derived primarily from its structure at the microscopic level. Wood is composed of long cells, in the axial direction, and thin cells in the radial and tangential directions.
The smallest microstructural component of wood is called a microfibril. Microfibrils are bundles of cellulose chains covered first by hemicellulose and then by lignin.
The long thin wood cells are composed of many layers of microfibrils. The microfibrils are arranged in separate differently oriented layers. The changing orientation of layers within the structure gives the wood cells better strength in more directions. The wood structure, however, is still considered to be very anisotropic.