In a woody stem, due to secondary growth, secondary xylem constitutes the bulk of the stem and is commonly called wood. The secondary xylem consists of vessels, tracheids, wood parenchyma and wood fibres. Width of secondary xylem increases continuously with the age of the plant. In older stems, the wood (or secondary xylem) gets differentiated into two zones - the outer light coloured younger sapwood and the inner darker older heartwood. The xylem vessels and tracheids of heartwood become non-functional because they get filled with resins, tannins, gums etc. produced by adjacent living parenchymatous cells and due to tyloses. Changes in the nature of sapwood occur with the death of parenchyma cells and with the completion of these changes, heartwood is formed. Thus, the conversion of sapwood into heartwood is a progressive phenomenon which continues throughout the life of the plant. Initially, both sapwood and heartwood increase in size but as a tree reaches a certain age, amount of heartwood increases continuously and the amount of sapwood remains almost constant.