As a tree grows older, which of the following increases more rapidly in thickness?

Explanation

Heartwood or duramen is the dark-coloured wood near the centre of the axis formed after many years of secondary growth of the stem. A small outer region, however, remains light-coloured. It is known as sapwood 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 the 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 the accumulation of these substances and is also termed duramen. The sapwood 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 sapwood. But gradually most of these elements get transformed into heart wood. Thus the amount of heartwood increases as the tree grows older. The amount of sapwood, however, remains almost constant.