Growth rings in a wych elm tree, Ulmus glabra. Each year the outer, living part of a tree trunk lays down a new layer of water-conducting xylem cells. The older cells dry out and die, forming the hard mass of wood in the core of the trunk. Each ring corresponds to a single year, so the tree's age can be determined by counting the rings. Climatic fluctuations from year to year result in distinctive patterns of wide and narrow rings, which are repeated in different trees. These patterns can be used to determine the age of ancient timbers in buildings of uncertain origin. The rings in this sample correspond to the years 1900 (top right) to 1940 (bottom left).