Which type of plant growth involves the formation of tree rings?

The formation of tree rings is primarily due to secondary growth. This type of growth occurs in woody plants and involves the lateral or outward growth of the plant, mainly through the activity of the vascular cambium and cork cambium. The vascular cambium produces new layers of xylem (wood) and phloem (bark) each year, which results in a concentric pattern of growth that can be observed as rings when a tree is cross-sectioned.

Each ring typically corresponds to a single year of growth; thus, factors such as environmental conditions, availability of water, and nutrients can influence the width and density of the rings, providing valuable information about the plant's history and the climatic conditions over time.

In contrast, primary growth is associated with the elongation of plant stems and roots due to meristematic tissues at the tips, while apical growth refers to this same elongation process. Radial growth, though it might sound similar, is not a widely used term in the context of plant growth and does not specifically describe the processes leading to the formation of tree rings. Therefore, secondary growth is the correct answer as it directly encompasses the processes that lead to the annual formation of rings in trees.