Girth of stem of a plant increases due to division of cells in

Plant growth occurs through specialized tissues called meristems. The increase in the length of the shoot and root is known as primary growth, which results from cell division in the apical meristems. In contrast, the increase in the thickness or girth of the plant stem and root is referred to as secondary growth [1]. This thickening is caused specifically by the division of cells in the lateral meristems. Lateral meristems include the vascular cambium, which produces secondary xylem and phloem, and the cork cambium, which forms the protective bark layer. While apical meristems facilitate vertical elongation and intercalary meristems assist in localized growth (such as in grass internodes), only the lateral meristems are responsible for the radial expansion or girth increase of the plant body.

Sources

1. [1] https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/secondary-growth