Question

Which of the following is related to increase in the girth of plants?
A. Shoot apex
B. Root apex
C. Lateral meristem
D. Intercalary meristem of stem

Answer 1

Hint: The plant grows by the activity of meristems. Increase in plant organs length is primary growth and increase in the thickness in plant organs is secondary growth.

Complete answer:
The girth of an object is the width or thickness which is measured around its circumstance. The secondary growth (increase in thickness) is brought about by the two lateral meristem. These are vascular cambium and cork cambium. Vascular cambium is active in the stellar region and cork cambium is active in the extra stelar (cortex) cortex.

The vascular cambium is present outside the primary xylem and the interior of primary phloem. The cells of the vascular cambium continue to divide periclinal. The daughter cells that form as a result of division, if cells differentiate into secondary xylem towards the inside of vascular cambium and form secondary phloem to the outside of vascular cambium. The formation of secondary xylem and secondary phloem brings thickening of the stem. The action of the cork cambium is also important for secondary growth. The cork cambium arises from the outer layer of cortex as the cells of cortex become meristematic during secondary growth.

Cork cambium divides in the similar manner as vascular cambium. It makes cells outside and forms the epidermis. It cuts some cells towards the inside which becomes the secondary cortex. The cells of the epidermis become dead due to deposition of suberin. These cells are known as cork or phellem. Those cells forward towards the inside are differentiated into parenchyma and may contain chloroplast. These are called secondary phelloderms.

Hence, the correct answer is option (C).

Note: Increase in the thickness of plant organs due to formation of secondary tissues in steer and extra stellar regions is called secondary growth. Normally it takes place in the roots and stem of dicotyledons and gymnosperms. In monocotyledons secondary growth is absent due to absence of cambium.