It occurs only in dicot stem a little away from the shoot apex and helps the plant to grow in girth (thickness) and makes it very strong to stand upright for many years. Growth in thickness in dicot stem becomes possible due to the formation of new tissues entirely by the activity of two lateral meristems - Vascular cambium and Cork cambium. These tissues thus formed are known as secondary tissues and growth in girth is referred as secondary growth.
It forms secondary vascular tissues as follows:
The strip of cambium present in the vascular bundle is called Fascicular Cambium.
The cells of medullary rays adjoining the strip of vascular (Fascicular) cambium become meristematic and form interfascicular cambium.
Both fascicular and inter-fascicular cambium join to form a continuous cambium ring.
Cambium divides and adds cells on internal side (towards pith) which mature into secondary xylem and cells added towards external side (periphery) mature into secondary phloem.
Amount of secondary xylem produced is comparatively comparatively more than secondary phloem.
It forms periderm as follows:
Cork cambium or phellogen develops in the cortex.
Phellogen divides and adds cells on both the inner and the outer side.
The inner cells differentiate into phelloderm or secondary cortex while outer cells into phellem or cork.
Cork cells are compactly arranged and become dead and suberized (deposition of suberin) except in regions of lenticels where cells are loosely arranged (complimentary cells) and non-suberized. It is through the lenticels that woody branches and tree trunks can undergo gaseous exchange.
Phellogen, phelloderm and phellem together constitute the periderm. Due to internal increase in thickness, periderm replaces the epidermis, becomes protective in function.
All the dead cells lying outside the active phellogen constitute the bark.