The central section of a mature tree trunck, which is usually darker in colour. It is often harder and denser than the surrounding sapwood and no longer transports water, because it becomes saturated with resin and tannin.
Heartwood is wood that has become more resistant to decay as a result of deposition of chemical substances (a genetically programmed process). Once heartwood formation is complete, the heartwood is dead. Some uncertainty still exists as to whether heartwood is truly dead, as it can still chemically react to decay organisms, but only once.
Usually heartwood looks different; in that case it can be seen on a cross-section, usually following the growth rings in shape. Heartwood may (or may not) be much darker than living wood. It may (or may not) be sharply distinct from the sapwood. However, other processes, such as decay, can discolour wood, even in woody plants that do not form heartwood, with a similar colour difference, which may lead to confusion.