In old trees, the greater part of secondary xylem is dark brown and resistant to insect attack due to the deposition of organic compounds like tannins, resins, oils, gums, aromatic substances, and essential oils in the central or innermost layers of the stem (often referred to as statement b). These substances are deposited in the lumen of vessels (often referred to as statement a), making the wood hard, durable, and resistant to microorganisms and insects. This region is known as heartwood.