Tree

In a typical, sawed-off sector of a tree trunk, one encounters layers of different cells and a series of concentric, annual growth rings going from the outside toward the inside.

The cork of bark is on the external surface of the trunk, and consists of dead cells which are impregnated with suberin, a waxy substance which inhibits evaporation of water through the bark. The cork cambium lies just inside the cork. It produces cork cells on its outside face and secondary cortex on its inside face. Growth and division of the cork cambium differs among tree species, and this gives the bark of each species its own characteristic appearance.

Phloem cells lie just inside the secondary cortex. Phloem cells are elongated cells specialized for the transport of plant nutrients, such as the carbohydrates made during photosynthesis. The vascular cambium lies just inside the phloem cells. It produces phloem cells on its outside face, and xylem cells on its inside surface. Xylem cells are elongated cells specialized for transport of water and dissolved ions throughout the tree. Trees growing in places with a strongly seasonal climate typically contain thick layers of xylem cells with readily apparent, concentric growth rings. This thick layer of functional xylem cells is referred to as the sapwood.

Finally, the heartwood is in the center of the tree. This layer is typically darker than the sapwood and consists of dead cells that are very stiff and serve to strengthen the tree. The heartwood may also have readily apparent growth rings.