Schematic diagram of woody angiosperms secondary …

The increase in girth of stems of woody angiosperms is called secondary growth. The tissue involved in secondary growth are the two lateral meristems?vascular cambium and cork cambium.
(i) Vascular cambium is a vascular layer present between xylem and phloem (vascular tissues). It is responsible for the cutting off vascular tissues, later it forms a complete ring.
(ii) The cambial ring becomes active and begins to cut off new cells, both towards the inner and the outer sides.
(iii) The cells cut off towards pith, mature into secondary xylem and the cell cut of towards periphery, mature into secondary phloem.
(iv) The cambium is generally more active on the inner side than the outer. As a result, the amount of secondary xylem produced is more than the secondary phloem and soon forms a compact mass.
(v) The primary and secondary phloem get gradually crushed due to the continued formation and accumulation of secondary xylem.
(vi) The primary xylem however, remains more or less intact, in or around the centre.
(vii) At some places, the cambium forms a narrow band of parenchyma, which passes through the secondary xylem and the secondary phloem in the radial directions. These are the secondary medullary rays.
(viii) The stem continues to increase in girth due to the activity of vascular cambium.