n When certain trees and other plants are wounded, either by natural accident or by tapping, they exude liquids that harden or partially harden upon exposure to air to form resinous or balsam-like products. Deposits of such secretions undergo chemical changes – oxidation and polymerization – when buried for long periods, forming solid or semisolid products that are soluble in oils and organic liquids but insoluble in water. Such water-insoluble resins are generally known as the natural resins, sometimes called varnish resins. Those that are water-soluble are known as gums or essential oils. Examples of natural resins are accroides (acaroid resin), amber, Canada balsam, Congo copal, dammar, elemi, Kauri copal, and sandarac. They are used in varnishes and lacquers, also as modifiers for plastics (Langenheim JH (2003) Plant resins: chemistry, evolution ecology and ethnobotany. Timber Press, Portland, OR).