Question

Give an account of activity of cambium in the secondary growth of the stem.

Answer 1

On the basis of activities of cambium and cork cambium secondary growth occurs in two steps, activity of cambium and activity of cork cambium. In the activity of cambium, during secondary growth, the cells of medullary rays in line with the intrafascicular cambium develop meristematic activity and form strips of cambium called interfascicular cambium, which form a ring. This ring becomes active as a whole and starts forming new cells, in which the cells on the outer sides get differentiated into the phloem and inner side are modified into xylem and are called respectively secondary phloem and secondary xylem. The secondary phloem consists of sieve tubes, companion cells, phloem fibres and phloem parenchyma. The secondary xylem consists of pitted vessels, tracheids, xylem fibres and xylem parenchyma.
Certain cells of cambium form some narrow bands of living parenchyma cells passing through secondary xylem and phloem and are called secondary medullary rays. Whereas, in the activity of the cork cambium, the cork cambium forms secondary tissues but its activity is more on the outer side than on the inner side. The outermost layer of cortex becomes meristematic and begins to divide, this is known as cork cambium which protects the inner tissue from the attack of fungi and insects. The cells formed on the outer side constitute cork or periderm and those on the inner side of the secondary cortex are phelloderms.