Secondary growth takes place in the dicot plants where cambium is present in between the xylem and phloem. On examining the individual vascular bundle of a dicot stem you can clearly see primary xylem towards the pith, primary phloem towards the cortex and a layer of undifferentiated cells called vascular cambium or intrafascicular cambium in between the two.
The mass of cells present between the two vascular bundles constitute the medullary rays which on differentiation gives rise to interfascicular cambium. During secondary growth both interfascicular and intrafascicular cambia join to form a complete ring of cambium.
The cells of cambium ring become active and cut off new cells both towards outer side and inner side. Cambium ring cuts off secondary xylem towards inner side and secondary phloem towards outer side. Due to more activity on the inner side, the amount of secondary xylem is more than that the secondary phloem. The primary and secondary phloem gets crushed due to the continuous formation and accumulation of secondary xylem.
As the vascular cambium cells divide, the outermost stem tissues must expands to compensate for increased internal girth and to retain the continuous flow of food and water in the stem circumference.
For this reason another lateral meristematic tissue called cork cambium or phellogen differentiated into the cortex region.
Phallogen cut the cell inside as phelloderm or secondary cortex and outer side as phellem of cork. All the three phellem, phellogen and phelloderm constitute periderm. The dead tissues outside the vascular cambium constitutes the bark.
You can correlate it with the diagram provided in your NCERT book of XI class.
Hope you will find it useful.