Q.Central part of wood is dark. Why?

The dark, hard, water-conduction-incapable inner core of secondary xylem in an old dicot stem is the duramen (heartwood); the lighter, still-functional outer wood is the alburnum (sapwood).

In an old, woody dicot stem that has undergone many years of secondary growth, the secondary xylem (wood) differentiates into two visibly and functionally distinct zones:

• Alburnum (sapwood) — the outer, physiologically active region of the wood. It is lighter in colour, and its vessels and tracheids remain open and functional, actively conducting water and dissolved minerals from root to shoot. …

The balloon-like ingrowths that block the lumen of xylem vessels in many dicots are called tyloses, and they are a normal feature of heartwood (duramen) formation.

As a dicot stem ages, secondary xylem in the centre of the stem stops conducting water and becomes heartwood (duramen). During this transition, living xylem parenchyma cells adjacent to a vessel push balloon-like outgrowths through the pits in the vessel wall, and these outgrowths bulge into and progressively block the lumen (cavity) of the vessel. These structures are called tyloses. Tylosis formation, along with deposition of tannins, resins, and gums, is what makes heartwood darker, denser, and more decay-resistant than the surrounding sapwood (alburnum).

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