Growth in the diameter of plants is due to the cell divisions in the cambium, an extremely thin cylinder of meristermatic tissue found just under the bark. New cells are formed on both sides of the cambium each year. Those to the inside make up the xylem, which conducts water and nutrients; and those to the outside make up the phloem, which transports sugars, amino acids, vitamins, hormones, and stored food. In the xylem, the fibers provide strength and the vessels allow water and nutrient flow to the leaves.
The annual rings found in tree stems are a result of variations in growth rate and in the type of wood produced early and late in the growing season. Within each ring, the lighter wood is springwood, formed early in the season with larger, thin walled cells; the darker, thick walled cells of the summerwood are formed later in the year. When counting the rings to determine the age of a tree, both of these bands are included in one year. The environmental conditions of an individual tree, most notably the amount of moisture and light available, are recorded each year in its rings. The width of these rings may be used as a measure of the health and vigor of the tree.
A cross-section of a tree stem reveals differences in its basic structure. Heartwood is found at the center of the tree. It is composed of old xylem tissue that is no longer living, but still retains structural strength and infection resisting ability. Sapwood is the living xylem inside the cambial layer that is actively involved in fluid transport. Researchers have found that the number of annual rings still living at any time is highly variable, ranging from one to 20 rings depending on the species. The living phloem cells just to the outside of the cambium, the inner bark, provide nutrient transport. The outer bark is composed of dead phloem cells that are pushed to the outside, and sloughed off by the tree over time.