In the one-year-old seedling the stem is composed wholly of live or sap wood. But with increased age the older layers or rings of growth are buried by the newer ones. The newer layers with light color on the outside are called sapwood or alburnum. As the tree gets old the inner wood becomes drier, darker, and more solid. This interior darker wood is not alive. If kept from the air by the live wood and perfect bark it may remain sound for a century or more, but if the air is admitted by cutting or accident it will soon make a rotten spot in the stem. Such rotten spots often result in the interior States from sunscald of the stem on the south side. Premature darkening of the wood or "black heart" may also result from feeble growth from any cause after transplanting. But it more frequently comes in the prairie States from severe winter freezing. This winter injury often extends to near the cambium layer, and the tree may survive feebly at first, but soon the injury is covered by new growth. But in such cases the heartwood never assumes its proper color.