Question

In a dicot stem, secondary growth occurs due to the activity of
A. Apical meristem
B. Lateral meristem
C. Cork
D. Bark

Answer 1

Hint:- In dicot plants, both primary growth and secondary growth occurs. Primary growth is the growth which increases the length of the plant and secondary growth increases the girth of the plant. Secondary growth occurs in both the stem and root of dicotyledonous plants. Two secondary meristems called vascular cambium and cork cambium are responsible for the secondary growth of plants.

Complete Answer:-Meristematic tissues are dividing tissues. Lateral meristems are seen at the lateral sides of roots and stem in a plant. It consists of two meristems called cork cambium and vascular cambium. Cambium cells are actively dividing cells which cause the secondary growth of stem and roots. Vascular cambium is responsible for the secondary growth of xylem and phloem which are the vascular tissues. In the Dicot stem, a layer of cambial cells is seen in between the primary xylem and primary phloem. This layer of cambium is called intrafascicular cambium. Intrafascicular cambium is combined together by interfascicular cambium which is formed in a later period, constituting the cambial ring. When the cambial ring is fully formed and becomes active, it starts to cut off cells. The cells are cut off towards the inside and outside. Cells cutting off towards the central part form the secondary xylem and the cells cutting off towards the outer part mature into the secondary phloem. The cambium actively forms the secondary xylem than the secondary phloem. The phloem cells are gradually crushed due to the continuous formation of secondary xylem. Secondary xylem becomes a compact mass. Cambium can also produce secondary medullary rays which are thin bands of parenchyma cells passing through the secondary xylem and secondary phloem radially.
Due to the continuous activity of vascular cambium, secondary xylem formation occurs, and primary cortical cells rupture. Another meristematic tissue called cork cambium or phellogen is developed. Phellogen cuts off more cells towards the outside and a few cells towards the inside. Due to continued external activity, the pressure is built in across the outer surface and the cells gradually become dead and undergo suberin deposition in the walls. This is the cork that sloughs off gradually.
Apical meristem is present at the shoot and root apex and is derived from the embryo. The apical meristem produces the primary plant body. They produce the ground tissue system, epidermal tissue system, and vascular tissue system. All the tissues seen exterior to the vascular cambium is termed as bark. It includes the periderm, secondary phloem, and primary tissues if present.

The correct answer is Option B.

Note:- In the dicot root, intrafascicular cambium cannot be seen in the early stages, it is completely secondary in origin because the vascular bundles of the roots are radial. The vascular bundles in the dicot stem are called conjoint open due to the presence of intrafascicular cambium in between xylem and phloem. Secondary growth also occurs in gymnosperms. The activity of cambium depends on several physiological and climatic conditions.