The trunk of a tree owes its woody girth to a phenomenon called secondary growth. In secondary growth, a plant grows wider. Contrast this to primary growth, in which a plant grows taller. Secondary growth occurs within a thin layer of actively dividing cells, called the vascular cambium, which lies between the plant's xylem and phloem.
Some stems and roots remain slender and show little or no growth in diameter, but in many eudicots, stems and roots thicken considerably. In the accompanying animation, we study the process of secondary growth in the stem of a woody eudicot.
The vascular cambium is a meristematic tissue that develops between the stem's primary phloem and primary xylem. As cells within the vascular cambium divide, they produce secondary phloem and secondary xylem.
These cell divisions increase the stem's thickness. After a cell in the vascular cambium expands, it divides and produces two new cells. One of these cells differentiates into a new xylem or phloem cell, and the other remains undifferentiated as part of the vascular cambium. This undifferentiated cell can then continue to divide to produce additional cells of secondary xylem or phloem.
The secondary xylem cells make up the wood of a plant's stem (or roots). As the plant grows, the wood increases in girth, gaining an outer layer of new secondary xylem cells. In temperate zones, the growing periods are seasonal, and the new layers appear in cross-section as concentric rings in the wood, called annual rings.
Of the flowering plants, only eudicots are capable of secondary growth. The eudicots, but not the monocots, have a vascular cambium, which produces wood, and another meristem, called the cork cambium, which produces bark.
In the accompanying animation we studied the process by which vascular cambium cells in a woody eudicot divide to produce secondary xylem cells (wood) toward the interior of the stem and secondary phloem cells toward the exterior. During this process, the cells of the vascular cambium grow larger and then divide. One of the daughter cells from each division remains in this meristematic tissue, but the other then differentiates into either a secondary xylem or a secondary phloem cell.
The trees of temperate-zone forests produce secondary xylem in characteristic annual rings. Each year, the growth during the spring produces secondary xylem cells that are relatively large. During the summer, when water is less abundant, the new secondary xylem cells are relatively small and thick-walled. For this reason, the summer wood appears darker and denser than the spring wood. The large and small cells juxtaposed in the trunk create a ringed look when the trunk is cross sectioned, with the number of annual rings in a trunk reflecting the age of the tree.