Secondary Growth in Dicot Roots

The roots grow in length with the help of apical meristem. It is called primary growth. Apart from primary growth, roots grow in width i.e., they increase in girth. This increase is called secondary growth. It is found only in dicot roots.

The tissues involved in secondary growth are lateral meristems i.e., vascular cambium and cork cambium. Vascular cambium and cork cambium are secondary in origin and arise from the pericycle.

Secondary growth is as follows:

• Pericycle cells outside the protoxylem divide to form a strip of cambium.
• Another strip of vascular cambium appears in the conjunctive tissue on the inner side of phloem bundle.
• These two vascular cambium strips join laterally to form a ring which may initially be wavy but later becomes circular due to over production of secondary xylem tissue inner to primary phloem.
• Cambium cells consist of brick shaped cells which divide and add cells on its either side i.e. towards periphery and towards center. Those added towards the periphery differentiate into secondary phloem and the ones formed towards the center differentiate into secondary xylem.
• Secondary tissue formed outer to the protoxylem bundle differentiates into prominent primary medullary ray thus, protoxylem does not get crushed.
• Later, cork cambium (Phellogen) also differentiates in the pericycle.
• The cork cambium divides and gives rise to cork (Phellem) towards outside and secondary cortex (Phelloderm) towards inside.
• All the three layers i.e. Phellogen, Phellem and Phelloderm together form the Periderm of the root and have protective function.
• Finally all the primary tissues outside the developing cork (i.e. endodermis, cortex and epiblema) are sloughed off.