What type of growth increases the diameter of a plant?

Secondary growth is the process that increases the diameter of a plant, primarily occurring in woody plants. This growth arises from the activity of the vascular cambium, a lateral meristem that produces new layers of vascular tissue (xylem and phloem) in a radial manner. As the vascular cambium divides, it contributes to the thickening of the stem and roots, allowing the plant to gain structural support and increase its capacity for transporting water, nutrients, and photosynthates.

In contrast, primary growth, which occurs at the tips of roots and shoots, is responsible for the vertical increase in height rather than diameter. Apical growth refers specifically to the elongation at the tips of roots and shoots and does not contribute to the increase in stem or trunk thickness. Excurrent growth typically pertains to a specific growth habit in trees and is not a term used to describe a change in diameter.