The formation and accumulation of tannins is linked to the conversion of sapwood into heartwood. Sapwood is the outer and youngest part of the wood, it lies nearest the bark. It contains the working xylem vessels (that conduct the water to the leaves) and living non-lignified cells. The concentration of tannins in sapwood is generally low, so it is prone to decay. The heartwood has no living cells but it is richer in tannins, which make it much more resistant to decay. The tannins are made by the living, non-lignified cells as the sapwood transforms into heartwood - just before they die.

The formation and accumulation of tannins is linked to the conversion of sapwood into heartwood.   Sapwood is the outer and youngest part of the wood, it lies nearest the bark.  It contains the working xylem vessels (that conduct the water to the leaves) and living non-lignified cells. The concentration of tannins in sapwood is generally low, so it is prone to decay. The heartwood has no living cells but it is richer in tannins, which make it much more resistant to decay. The tannins are made by the living, non-lignified cells as the sapwood transforms into heartwood - just before they die.