The diagram shows the layers of the cambium, from the primary xylem, outward to the cork cambium that forms new bark. The layers in between produce different types of tissue for conducting water and minerals through the trunk.
Each layer of new wood that is added to a tree forms a recognizable ring. The cambium produces large cells in the spring when water is abundant and growing conditions are generally good. As it becomes drier later in the summer and the growing season comes to a close in the fall, the cambium produces smaller cells that have thicker walls. In the late autumn, tree growth stops.
The new, large cells that are produced the following spring are easily distinguished from the previous year's tree growth as a distinctive ring. A ring composed of a light part (spring growth) and a dark part (late summer/fall growth) represents each year's growth.