The third structural component, hemicelluloses (polyoses),
has an intermediate degree of complexity and is made
up of different pentose and hexose residues, which are often
acetylated, and generally form branched chains. Typically,
hemicelluloses in softwood are glucomannans, whereas those
in hardwoods are mainly xylans together with variable percentages
of galactose, arabinose, rhamnose and methylglucuronic
acid units, and acetyl groups. Other non-structural components
of wood include compounds extractable with organic solvents
(the so-called extractives) which can be either polar (e.g. phenols
and tannins) or apolar (e.g. fats and sterols), water-soluble
compounds (e.g. sugars and starch), as well as proteins and
ashes. These components together generally represent less than 5% of the dry weight of wood but can reach 20% in some
softwoods (e.g. in some Cupressaceae)