Which growth form is responsible for the increase in the girth of a tree?

The correct choice pertains to secondary growth, which is the process responsible for the increase in the girth of a tree. This growth occurs through the activity of the vascular cambium and the cork cambium. The vascular cambium produces new layers of xylem (wood) and phloem (the tissue that transports nutrients), contributing to the thickening of the trunk, branches, and roots.

Secondary growth is particularly significant in woody plants, as it enables them to support larger structures and withstand external forces, such as wind and weight from leaves. This process results in the formation of annual growth rings, which can be observed in the cross-section of a tree trunk, where each ring typically represents a year of growth.

Initial growth and primary growth primarily refer to the increase in height and length of the plant structures, such as roots and shoots, rather than the thickening of existing stem tissue. Lateral growth is a term that sometimes describes growth in width or girth, but it is more commonly associated with secondary growth in the context of trees. Therefore, secondary growth is distinctly responsible for the increase in girth.