Which type of growth is associated with an increase in stem girth?

The type of growth that is associated with an increase in stem girth is secondary growth. This process primarily occurs in woody plants and involves the activity of the vascular cambium, a type of lateral meristem. As the vascular cambium divides, it produces new layers of vascular tissue—xylem towards the inside and phloem towards the outside. This accumulation results in a thicker stem and increased girth as the plant ages.

In contrast, primary growth is responsible for the elongation of the plant, allowing roots and shoots to grow in length. Apical growth refers to the growth that occurs at the tips of roots and shoots and is also focused on lengthening. Meristematic growth encompasses all types of growth occurring in meristems, which are regions of undifferentiated cells that can divide and differentiate into various tissues. While meristematic cells are involved in both primary and secondary growth, it is specifically secondary growth that leads to increases in girth.