Heartwood is wood that has died and become more resistant to decay as a result of deposition of chemical substances (a genetically programmed process). It appears in a cross-section as a usually colored circle, usually following the annual rings in shape. Heartwood is usually much darker than living wood, and forms with age. Many woody plants do not form heartwood, but other processes, such as decay, can discolor wood in similar ways, leading to confusion. Some uncertainty still exists as to whether heartwood is truly dead, as it can still chemically react to decay organisms, but only once. //Shigo 1986, 54