Explain the process of secondary growth in the stems of woody angiosperms with the help of schematic diagrams. What is its significance?
Secondary growth in dicot stem: It is a “permanent increase in thickness due to the activity of vascular cambium and cork cambium in stellar and extrasolar regions”. In dicot stem intra fascicular cambium is present. The cells of the medullary ray become meristematic and form interfascicular cambium. These two cambiums unite and make a complete cambial ring. The cells of it divide and produce new cells both on its outer and inner sides. The cells formed on the outer side differentiate into secondary phloem while the cells of the inner side form secondary xylem. The epidermis is replaced by a secondary protective tissue by an increase in the growth of the stem of the plant. It is made of phellogen (cork cambium). It arises from the peripheral cells of the cortex. The phellogen forms new cells on the outer side which make phellem (cork) and phelloderm on its inner side also. Significance: Secondary growth increases the girth or thickness of the plant. Annual rings of woody angiosperms are very distinct and thus helps in determining the age of the plant.
