A growth ring/tree ring/annual ring is a layer of wood (xylem) produced during a tree's growing season. Each tree ring marks a line between the dark late wood that grows at the end of the previous year and the relatively pale early wood that grows at the start of current year. One annual ring is composed of a ring of early wood and a ring of late wood. The early wood and late wood differ distinctly in the cell size and cell types that they are composed of. Both the cell types contribute to tree's growth in diameter. Depending upon the function that the xylem elements perform in a growth ring, two types of woods are formed in a growth ring. These are known as sap wood and heart wood. Sap wood consists of xylem elements that are actively involved in water transport and food storage. Heartwood consists of xylem vessels that are no longer involved in water transport. Such vessels are often blocked by substances like resins and tannins.By counting the tree rings we can estimate the age of the tree. The branch of science that deals with the study of tree rings, and thereby helping to know the age of a tree, is called dendrochronology.