In the spring and summer, our cells grow. In the autumn, they slow down. And in winter, they don't grow at all. But every year, the growth of our cells produces our rings, or tree rings.
The darker bands are the spring wood and the pale bands are the summer wood. So each pair of one light band and one dark band is one year's growth. The summer wood is wider than the spring wood in fast-growing trees, and miniscule (very thin) in slow-growing trees. The width of our growth rings is affected by many factors, including water, nutrients, sunlight, temperature, and location. (Move your mouse over the top cross section graphic for labels).
 Our cells in the older rings become inactive and harden. This is our heartwood. It is our strength and the part used to make lumber.
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