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.
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.
Textbook Reference: Concept 24.3 Many Eudicot Stems and Roots Undergo Secondary Growth