Final answer:
Secondary growth in plants results from the activity of lateral meristems, which include the vascular cambium and cork cambium, leading to increased girth of roots and stems.
Step-by-step explanation:
Secondary growth refers to increases in the diameter or girth of roots and stems as a result of tissues produced in lateral meristems.
These meristems, specifically the vascular cambium and the cork cambium, play a crucial role in this process.
The vascular cambium is responsible for adding xylem inward and phloem outward, contributing to the increased thickness of the stem or root.
Secondary growth is visible as wood in woody plants and is commonly noted in some dicots and very rarely in monocots.
The cork cambium, which lies between the epidermis and the phloem, replaces the epidermis with bark, part of which includes cork.
This activity of the lateral meristems enables radial growth that contributes to the plant's structural strength and girth.