Sapwood and Heartwood

Wood is also classified by age and function into sap wood and heart wood. Sap wood (also called alburnum) is the pale-coloured, physiologically active outer zone of the secondary xylem - the youngest, most recently formed wood, whose vessels remain open and still conduct water and dissolved minerals up the plant. Heart wood (also called duramen) is the dark-coloured, inner and older zone of secondary xylem, formed as sapwood ages and stops functioning in conduction; the lumens of its xylem vessels become blocked by tyloses - balloon-like ingrowths that bulge in from neighbouring living parenchyma cells through pits in the vessel wall - and the wood becomes infiltrated with tannins, resins, gums and other substances that darken it and make it markedly more resistant to decay and insect attack than sapwood. Because heartwood no longer conducts water, its only remaining role is mechanical - providing rigid su …