Question

Select the correct option:
Increase in girth of the plant as a result of the activities of primary and secondary lateral meristems is called
A. primary growth
B. lateral growth
C. secondary growth
D. intercalary growth

Answer 1

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

Complete answer:
The secondary growth (increase in thickness) in plants is brought about by two lateral meristems. These are vascular cambium and cork Cambium. Vascular cambium is active in stelar region and cork cambium is active in the extra-stelar (cortex) region.
The vascular cambium is present outside the primary xylem and to the interior of the primary phloem. The cells of the vascular cambium continue to divide periclinal. The daughter cells that form as a result of divisions, 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 bring thickening of the stem.
The action of cork cambium is also necessary for secondary growth. The cork cambium or phellogen arises from the outer layer of cortex as the cells of cortex become meristematic during secondary growth.
Cork cambium also divides periclinal in the same way as vascular cambium. It forms cells towards the outside and becomes epidermis. It cuts some cells towards the inside which become the secondary cortex. The cells of epidermis become dead due to deposition of suberin. These cells are known as Cork or Phellem, Those cells formed towards the inside are differentiated into the parenchyma and may contain chloroplasts. These are called secondary cortex or Phelloderm.

Thus, option C. secondary growth is the correct answer.

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