Final answer:
Secondary growth in plants, which refers to an increase in plant thickness or girth, is facilitated by the activity of lateral meristems, specifically the vascular cambium and cork cambium.
Step-by-step explanation:
The type of meristematic tissue that produces secondary growth, which is an increase in the thickness or girth of the plant, is known as the lateral meristem. This growth is characteristic of woody plants and is achieved through the combined activities of two types of cambium. The vascular cambium is responsible for producing secondary xylem and phloem that add to the diameter of the plant, contributing to the formation of wood and the sturdy trunks in trees. The cork cambium, on the other hand, lies between the epidermis and the phloem. It replaces the epidermis with a new protective layer, the periderm, one component of which is cork. These two cambia work together to allow the plant to expand in width without rupturing the outer tissues, thereby enabling secondary growth.