The intercalary meristem is responsible for the increase in girth of the plant.
Cambium
There are three kinds of meristems - apical, intercalary and lateral. Apical meristems are found in root and shoot tips and are responsible for the increase in the length of the plant. Intercalary meristems are found at the bases of young leaves and internodes. They are mainly responsible for further lengthening of stems and leaves. Lateral meristems are found along the sides of roots and stems. They play a vital role in increasing the width or diameter of stems and roots.