The Columbia Encyclopedia, Sixth Edition. 2001-07.
heartwood
the central, woody core of a tree, no longer serving for the conduction of water and dissolved minerals; heartwood is usually denser and darker in color than the outer sapwood. Before the synthesis of aniline dyes, the heartwood of several tropical trees (sold collectively under the commercial name brazilwood) was used to produce blue, purple, and red dyes. As a tree becomes older, the heartwood increases in diameter, whereas the sapwood remains about the same thickness. See wood.
