Q.Central part of the wood is always dark. Why?

Step 1. Identify which wood zone sits at the centre. In an older stem's secondary xylem, the oldest wood - laid down earliest by the cambium, now pushed furthest from the currently active cambium - occupies the very centre. This oldest, central zone is the heartwood (duramen), distinct from the paler, younger sapwood (alburnum) that surrounds it nearer the bark.

Step 2. Explain what happens to wood as it becomes heartwood. As sapwood ages into heartwood, its vessels stop conducting water, and their lumens become blocked by tyloses - balloon-like ingrowths that bulge in from neighbouring living parenchyma cells through pits in the vessel walls. At the same time, the wood becomes infiltrated with various organic substances - tannins, resins, gums and similar compounds.

Step 3. Connect this to colour. It is specifically this infiltration of tannins, resins and gums (along with the tylose blockage) that darkens the wood - the older and more heavily infiltrated the wood is, the darker it appears, which is why the wood gets progressively darker moving from the pale, young sapwood at the periphery toward the dark, old heartwood at the very centre. …

Identify the wood at the centre as heartwood and explain that tylose blockage plus tannin/re …

• Attributing the dark colour simply to 'age' without naming the actual cause - tylose blockage and infiltration by tannins/resins/gums. …