Most species grown in temperate climates (having four seasons yearly) produce well defined annual growth rings, which are formed by the difference in density and color between wood formed early and wood formed late in the growing season.  The inner part of the growth ring formed first is called "springs wood," and the outer part formed later in the growing season is called "summer wood."  Spring wood is characterized by cells having relatively large cavities and thin walls.  Summer wood cells have smaller cavities and thicker walls, and consequently are more dense than spring wood.  The growth rings, when exposed by conventional methods of sawing, provide the grain or characteristic pattern of the wood.  The distinguishing features of the various species are thereby enhanced by differences in growth ring formation.  By counting the growth rings you can determine a trees age as one ring is formed each year.

Sapwood contains living cells and performs an active role in the life processes of the tree.  It is located next to the cambium and functions in sap conduction and storage of food.  Sapwoods size varies by species and where the tree is growing.

Heartwood consists of inactive cells formed by changes in the living cells of the inner sapwood rings, presumably after their use for sap conduction and other life processes of the tree have largely ceased.  The cell cavities of heartwood may also contain deposits of various materials that frequently provide a much darker color.  All heartwood, however, is not darker.  The infiltrations of material deposited in the cells of heartwood usually make lumber cut form heartwood more durable when exposed to weather.  Sapwood is not weather resistant in any species.

Medullary rays extend radially from the pith of the pith of the log toward the circumference.  The rays serve primarily to store food and transport it horizontally.  They vary in height from a few cells in some species to four or more inches in the oaks, and produce the flake effect common to the quarter sawn lumber in these species.