(a) : Heartwood or duramen is the dark coloured wood near the centre of the axis formed after many years of secondary growth of stem. A small outer region, however, remains light coloured. It is known as sap wood or alburnum. The heartwood is formed due to changes in the elements of the secondary xylem. As secondary growth proceeds most of the older elements of secondary xylem lose water and become filled with organic compounds such as oils, gums, resins, tannins, and aromatic and colouring materials. The wood becomes dark coloured due to accumulating of these substances and is also termed as duramen. The sap wood is the light coloured region of the secondary xylem. Cells of this region are functionally active. The elements of the secondary xylem added by cambial activity are those of sap wood. But gradually most of these elements get transformed into heart wood. Thus the amount of heart wood increases as the tree grows older. The amount of sap wood, however, remains almost constant.