The main chemical components of wood are the polymers cellulose, hemicellulose and lignin and the extractives.   The extractives are normally a minor component but tend to determine the characteristics of wood in terms of colour, smell and bio-resistance. Wood has many different extractives so they are often classified by how they are extracted. Common solvents include water, ethanol and ether. Each extracts different groups of components.   An example of commercial use of extractives is turpentine, made by steam distillation of resins collected from wounded trees.   Pure cellulose is made of glucose. More accurately, cellulose is a linear polymer of cello-biose units and cello-biose is two glucose molecules joined 'end to end'. Consequently, the two ends of the cello-biose units have different chemical properties and this is true of cellulose too, because of the repetition of cello-biose units throughout its length. The number of cello-biose units found in a cellulose molecule varies between species.   Hemicellulose is also made from sugar molecules and so is a polysaccharide, like cellulose, but the hemicelluloses are more complex because they consist of different sugars and often have a branched structure. The hemicelluloses of softwoods tend to have higher proportions of mannose and galactose, which are hexoses, ie sugars with six carbon atoms, than hardwoods. Hardwoods have more xylose, which is a pentose.   Lignin too is a complex polymer made of three basic units which can join together in a variety of ways leading to the complexity of lignin. The resultant macromolecule is a high-strength 3-D amorphous polymer. Lignin in the cell walls of woody plants allows them to grow high and support branches. It is also responsible for the gradual discolouration of wood in sunlight. Thus lignin must be removed during the manufacture of paper for books.   This simple introduction to wood's chemical components shows its complexity and helps to explain its variety and why it is not like other materials.