Wood is, in fact, the secondary xylem formed by cambium (layers of tissues) during secondary growth. As the tree grows old, the wood at the centre ceases to perform the function of conduction and is blocked with organic compounds like oil, resins, and tannins, and develops a dark colour. This dark coloured portion of the wood is called heartwood. The peripheral, light-coloured, better water-conducting wood is called sapwood. Heartwood is much more durable and resistant to micro-organisms, insects and pests.
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