Question

The youngest layer of secondary phloem in woody dicot stem is located ______.
A. Just outside the vascular cambium
B. Just on the inner side of the cambium
C. Between periderm and primary cortex
D. Just outside pith

Answer 1

The phloem is one of the two conducting vascular tissues of a plant. The phloem helps in food storage and transport. If a layer of cambium persists between the primary phloem and xylem. The cambium divides mitotically. It forms secondary xylem that pushes primary xylem towards the pith and secondary phloem that makes primary phloem invisible.

Complete answer: The two conducting tissues of the plant called the xylem and the phloem make up the vascular tissue system. Both of these tissues are present in layers. Sometimes a third layer forms between them called the cambium. Cambium tissue undergoes mitotic divisions. This results in the formation of xylem cells that form secondary xylem and phloem cells that form secondary phloem. As the xylem is present inwardly to the cambium, the secondary xylem pushes it towards the pith. On the other hand, the primary phloem tissue lies outwardly to the cambium. This results in secondary phloem that overtakes all of the primary phloem making it disappear. The secondary phloem masks the primary phloem.
This secondary phloem tissue is also referred to as the tree’s inner bark. Therefore, it lies just outside the vascular cambium of the woody dicot plants. The vascular cambium that results in this secondary growth consists of two types of cells called the fusiform cells and the ray initials. Phytohormones like auxins, ethylene, and abscisic acid regulate the secondary growth of the vascular cambium cells.
So, the correct answer is option A.

Note: The cambium that is present between the primary xylem and phloem is given a special term called intrafascicular cambium. This means inside the vascular bundles. Another layer of cambium forms during secondary growth between the vascular bundles called interfascicular cambium. This intrafascicular and interfascicular cambium forms a ring that results in the separation of primary xylem and primary phloem.