Wood cells form in the cambium layer of the tree, which is the single layer of cells found between the wood and the bark which can divide to make new wood cells.

The wood formed in the spring consists of light-colored, thin-walled cells; toward the end of the season smaller cells are formed—their walls are heavier and darker, and thus summer wood is formed. This alternation of spring wood and summer wood causes the concentric structure of the tree trunk known as annual rings; they are seen clearly on the cross section of a tree. By counting the annual rings of a tree, one can determine fairly closely its age. When growth conditions are favorable and food and water are abundant, the rings are wide. When drought occurs, the growth slows down and the rings are narrow. By reading a cross section of an old tree, one can determine what growth conditions prevailed during any particular year of the past.