Over time the centre of the innermost layers of the stem gets deposited with organic compounds like tannins, raisins, oils, gums and other aromatic substances. This deposition gives the secondary xylem a dark brownish colour and makes it durable and resistant to the microorganisms and insects. This dead inner wood with highly lignified walls is known as the hardwood. They are not involved in the conduction and transport of water but gives mechanical support to the stem. The sapwood is the outermost portion of the secondary xylem lighter in colour and involved in the conduction of water and minerals.