Describe vascular cambium responsible for secondary growth in detail.

$\rightarrow$ Vascular Cambium : Meristematic layer that is responsible for formation of two conducting tissues - xylem and phloem. It is called vascular cambium.

$\rightarrow$ In mature/young stem it is present in patches as a single layer between the xylem and phloem. Later it forms a complete ring.

$(a)$ Formation of Cambial Ring : In dicot stems, the cells of cambium present between primary xylem and primary phloem is the intrafascicular cambium. The cell of medullary rays adjoining these intrafascicular cambium become meristematic and form the interfascicular cambium. Thus continuous ring of cambium is formed.

$(b)$ Activity of Cambial Ring : The cambial ring becomes active and begins to cut off new cells, both, towards the inner and the outer sides.

$\rightarrow$ The cells cut off towards pith, mature into secondary xylem and the cells cut off towards periphery mature into secondary phloem.

$\rightarrow$ The cambium is generally more active on the inner side then on the outer. As a result, the amount of secondary xylem produced is more than secondary phloem and soon forms a compact mass. At this point secondary xylem becomes the main part of the stem.

$\rightarrow$ The primary and secondary phloem get gradually crushed due to the continued formation and accumulation of secondary xylem.

$\rightarrow$ The primary xylem however remains more or less intact in or around the centre.