Apical or primary growth increases the length of stems and roots. Secondary growth is the increase in diameter of stem and root due to activity of secondary meristem called cambium. Initially the cambium is restricted to a series of small group of cells. The first step in secondary growth involves the linking of these groups to form a ring of cambium, tissue between xylem and phloem. It is achieved by radical cambium, of cambium cells. Cells of cambium ring divide tangentially to form secondary xylem tissues on the inside and secondary phloem on the side. In the stem similar columns of parenchyma are formed within the secondary xylem and phloem forming secondary medullary rays.
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