Walnut tree trunk. Cross-section of the trunk of a walnut tree (Juglans regia), showing its different rings. On the outside is the cork, which exhibits the characteristic pattern of cork layers that are laid down at angles to one another. The cork is an external, secondary tissue impermeable to water and gases and protects the wood beneath. The wood is divided into an outer layer of sapwood (living wood in a growing tree) and a central layer of dead wood (heartwood) that is impregnated with tannins and resins. Tannins and resins deter fungi, bacteria and insects, and so make it resistant to decay. Walnut is one of the finest woods for cabinet makers because of its beautiful grain.