3. Explain the process of secondary growth in the stems of woody angiosperms with the help of schematic diagrams. What is its significance?
The increase in girth of a plant body is called secondary growth. The tissues involved in secondary growth are vascular cambium and cork cambium. In a young stem, vascular cambium is found in the form of patches as a single layer between the xylem and phloem. During the later stage, it forms a complete ring. Soon, the cambial ring becomes active and begins to cut off new cells towards the inner and the outer sides. The cells which are cut off towards pith mature into secondary xylem while those which are cut off towards the periphery after maturation develops into secondary phloem. The cambium is most of the times more active on the inner side than on the outer. This is the reason, the amount of secondary xylem produced is more than secondary phloem. The primary and secondary phloems get gradually crushed due to the continued formation and accumulation of secondary xylem. Then at some places, the cambium forms a narrow band of parenchyma, which passes through the secondary xylem and the secondary phloem in the radial directions. These are the secondary medullary rays