Activity of cambium in the secondary growth of the stem – 

1. F ormation of cambium ring – The vascular bundles of dicot stem have strips of cambium in between xylem and phloem. During secondary growth, the cells of medullary rays in a line with this cambium develop meristematic activity and forms strips of cambium. The intra and interfascicular cambium unites to form cambium ring or phellogen.

2. Formation of secondary tissue – The cambium ring becomes active as a whole and starts cutting off new cells. The cut off cells of the outer side get differentiated into phloem and are called secondary phloem. The cut off cells of the innerside are modified into secondary xylem. The activity of the cambium ring is more on the innerside than on the outerside which results in the increase of the xylem more rapidly than the phloem.

3. Secondary medullary ray – Certain cells of the cambium form some narrow bands of living parenchyma cells-passing through secondary xylem and secondary phloem and are called secondary medullary rays. This provides radial conduction of food from the phloem and water and mineral salts from the xylem.

4. Annual Ring – Activity of cambium is not uniform in those plants which grows in the regions where favourable climate conditions regularly alternates with the unfavourable conditions. The cambium is more active in spring and less active in winter. The wood formed in the spring is known as spring wood and that formed in summer and winter is known as autumn wood. The two woods appear together in concentric rings in the trunk and is known as annual ring. The age of the plant is approximately calculated by counting the number of annual rings.