How can the age of a tree be estimated from the analysis of the rings present on a cross section of its stem?

For the growth of the tree it is essential to have formation of new vessels within the stem, a task performed by the vascular cambium. The vascular cambium is more active in hot seasons (spring and summer) forming a lighter band made of large calibered vessels. During winter and fall the vascular cambium makes the opposite, so small calibered vessels and a darker band appears outside the previous lighter band. Therefore a two ring bands are made yearly, one lighter and the other darker. By a direct count of these band pairs one can estimate the age of the tree.