Secondary growth in dicot stem: It is "permanent increase in thickness due to the activity of vascular cambium and cork cambium in stelar and extrastelar regions". In dicot stem intra fascicular cambium is present. The cells of 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 inner side form secondary xylem. The epidermis is replaced by a secondary protective tissue by increase in growth of the stem of the plant. It is made of phellogen (cork cambium). It arises from the peripheral cells of 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 girth or thickness of plant.
Annual rings of woody angiosperms are very distinct and thus helps in determining the age of plant.