The cycle of growth in a temperate tree can be clearly seen when the tree is cut down. Each year the new cells that are produced under the bark create a new ring of tissue, visible in a cross section of the trunk. Each ring has light and dark sections. The light tissue is less dense and is made up of cells produced in the spring when the tree is growing fastest. The dark part of the ring is composed of cells laid down in the summer when the rate of growth has slowed. These rings are known as growth rings. By counting them it is possible to work out the age of a tree.