Secondary Growth in Dicot Stem and Roots-Must Read

Secondary Growth in Dicot Stem and Roots

Description: The secondary growth of vascular bundles in dicot stem and root are made as the plant grows in order to incorporate changes for the required nutrition and mechanical support for the plant growth. The changes in secondary growth of stem include formation of bark and lenticels.
Vascular Cambium
The vascular cambium is present as patches between the vascular tissues xylem and phloem in younger stems which later develops into a complete cambial ring. The cells of cambium present between the xylem and phloem is called as intrafascicular cambium. The cells of medullary rays adjoining intrafascicular cambium are converted to meristematic cells to form the interfascicular cambium.
The cambial ring develops by cutting off new cells present in the inner and outer sides. The cells cut off towards pith later on mature to become secondary xylem and the cells cut off towards periphery mature into secondary phloem. The cambium is active on the inner side thus resulting in more production of secondary xylem thus forming a compact mass. As the secondary xylem gradually develops the primary and secondary phloem gets crashed. However, the primary xylem remains intact around the centre. At some places, the cambium forms a narrow band of parenchyma which passes through the secondary xylem and secondary phloem in radial directions. This cambium arranged in radial pattern is known as the secondary medullary rays.

Cork Cambium:
Once the cambium ring formation is active and spreading eventually in dicot stems, the cortical layers and epidermis are replaced by new protective layers of meristematic tissue called cork cambium or phellogen. The cork cambium develops in the cortex region and is couple of layers thick composed of narrow rectangular cells. The phellogen cuts off the outer cells to form cork or phellem and the inner cells to differentiate in to secondary cortex and phelloderm. The phellogen, phellem and phelloderm together constitute the periderm. As the cork cambium continues to expand, the pressure builds up in the periphery phellem resulting in death of the layers and sloughing off which is referred to as bark that is soft during younger stages of stem and woody later. 
At certain regions of cork cambium, the parenchymatous cells on the outer side are ruptured to form lens-shaped openings called lenticels. The lenticels permit the exchange of gases between the outer atmosphere and the internal tissue of the stem.

A. Lenticel B. Bark

Secondary growth in dicot roots:
In dicot roots, the events of secondary growth are quite similar to that of dicot stem. However, the vascular cambium is secondary in origin as it originated from the tissue located just below the phloem bundles, which is a portion of pericycle tissue thus resulting in formation of complete and continuous wavy ring.