Primary growth produces growth in length and development of lateral appendages. Secondary growth is the formation of secondary tissues from lateral meristems. It increases the diameter of the stem. Secondary tissues are formed by two types of lateral meristems, fascicular cambium and cork cambium.
Fascicular cambium: The vascular bundles have intrafascicular cambium between xylem and phloem. Interfascicular cambium is formed from parenchyma cells between vascular bundles. Fascicular and interfascicular cambium join to form a complete ring of vascular cambium. The cells of cambium ring divide diagonally to form secondary phloem outside and secondary xylem towards the inner side. The amount of secondary xylem produced is always greater than that of secondary phloem. Due to formation of greater amount of secondary xylem, the cambium ring moves towards the periphery and the primary xylem and phloem which were near together are separated far apart.
Cork cambium: The outermost layer of cortex becomes meristematic and is called cork cambium. Sometimes hypodermis also become meristematic and forms cork cambium. The cells of cork cambium divide by periclinal division towards outer side to form cork. The cork has rectangular cells which become dead at maturity. These are highly thick walled due to the presence of a chemical called suberin. Thus cork is also called suberised tissue. The cork is impervious to water and thus outer layers like hypodermis and epidermis become dead.
The cells of the cork Cambium also divide towards their inner side by periclinal divisions to form secondary cortex. It has loosely packed parenchymatous cells which are thick walled. The phellem, phellogen and phelloderm are collectively called periderm. All dead cells outer to cork cambium form bark. It includes dead cells cork, hypodermis and epidermis.
