Question

Explain the process of secondary growth in the stems of woody angiosperms.

Answer 1

In angiosperms, secondary growth is seen in the stems of dicots but not in monocots except few species.

In a typical dicot stem, secondary growth takes place in two different regions; stelar and extrastelar region.

• Secondary growth in stelar region: The vascular cambium becomes meristematic and it forms new strips of secondary meristem which are called interfascicular cambium. These strips of interfascicular cambium join the strips of fascicular cambium and a complete ring is formed which is called cambium ring.

The cells of cambium undergo periclinal divisions and new cells are formed both outside and inside. The cambial cells adjacent to ray initials cut off parenchymal cells that extend radially through the new secondary xylem and phloem. These are called medullary rays which are responsible for radial conduction of water, minerals and food.

• Secondary growth in extrastelar region: This is the cortical secondary growth and results in rupture of epidermis.

The permanent living cells in the outer cortical region, which is sometimes called pericycle, become meristematic and give rise to a secondary lateral meristem which is called as cork cambium or phellogen.

The cork cambium cut off cells towards outside which are called phellem and the cells cut off towards inside add to the cortex and are called secondary cortex cells or phelloderm cells. Phellem, phellogen and phelloderm are together called as periderm.