Final answer:
Secondary growth in roots and stems, which leads to an increase in diameter, involves the activities of lateral meristems, particularly the vascular cambium and the cork cambium.
Step-by-step explanation:
A root or stem increases in diameter through secondary growth, which occurs through a process driven by lateral meristems. Specifically, this growth is facilitated by the action of the vascular cambium and the cork cambium. The vascular cambium contributes to the increase in girth by adding layers of secondary xylem inwards and secondary phloem outwards. This activity results in the thickening of the stems and roots in woody plants, often observed as rings indicating annual growth. In contrast, the cork cambium replaces the outer epidermal layers with a tougher protective layer known as bark, which includes cork.
Secondary growth is responsible for the increase in thickness or girth of woody plants and is a critical characteristic of many tree species. It provides structural support and strength through the lignin found in the secondary xylem. However, it's important to note that secondary growth is typically absent in herbaceous plants, which do not develop woody tissue.