The New Student's Reference Work/Heartwood

Heart′wood. As the trunks of trees increase in diameter, new layers of wood are laid down each year upon the outside of the older wood. As the ascending sap moves along through the newer wood, it gradually abandons the older, more interior wood. This wood gradually changes in character the most obvious feature usually being a change in color. The color of the heartwood differs in different trees, and gives the characteristic appearance to the various kinds of lumber. For example, walnut, oak or cherry lumber is recognized by and valued for the characteristic color of the heartwood of each.