Explain the process of secondary growth in the stems of woody angiosperms with the help of schematic diagrams. What is its significance?

Secondary growth in the stems of woody angiosperms occurs due to the activity of two lateral meristems: the vascular cambium and the cork cambium (phellogen). These meristems add layers of secondary tissues, resulting in an increase in girth or diameter of the stem. Here's an explanation of the process of secondary growth along with schematic diagrams illustrating the key steps:

Process of Secondary Growth:

1. Vascular Cambium Activity:

   • The vascular cambium is a cylindrical layer of meristematic tissue located between the primary xylem and primary phloem in the vascular bundles.
   • It consists of actively dividing cells that give rise to secondary xylem (wood) towards the interior and secondary phloem towards the exterior.
   • As the vascular cambium divides, new secondary xylem cells are produced inwardly, forming wood, while new secondary phloem cells are produced outwardly, contributing to the bark.
   • This continuous activity of the vascular cambium results in the formation of annual rings in the stem, with each ring representing one year of growth.

2. Cork Cambium Activity:

   • The cork cambium (phellogen) is a lateral meristem located in the outer bark, just beneath the epidermis.
   • It produces cork cells (phellem) towards the exterior and phelloderm cells towards the interior.
   • Cork cells are dead at maturity and are filled with suberin, making them impermeable to water and gases.
   • The formation of cork cells by the cork cambium replaces the epidermis, contributing to the formation of the outer protective layer known as the cork or periderm.

Schematic Diagrams:
           Annual Rings Formation
   _______________________________________
   |                                      |
   |        Secondary Phloem (Bark)       |
   |______________________________________|
   |                                      |
   |        Secondary Xylem (Wood)        |
   |______________________________________|
   |        Vascular Cambium (Meristem)   |
   |______________________________________|
   |       Primary Phloem (Old Bark)      |
   |______________________________________|
   |       Primary Xylem (Old Wood)       |
   |______________________________________|
   |         Pith (Central Region)        |
   |______________________________________|

                Cork Cambium Activity
   _______________________________________
   |                                      |
   |              Cork (Phellem)          |
   |______________________________________|
   |             Cork Cambium             |
   |______________________________________|
   |            Phelloderm                |
   |______________________________________|
   |         Secondary Phloem             |
   |______________________________________|
   |         Secondary Xylem              |
   |______________________________________|
   |        Primary Phloem (Old Bark)     |
   |______________________________________|
   |       Primary Xylem (Old Wood)       |
   |______________________________________|
   |         Pith (Central Region)        |
   |______________________________________|