8.5 Secondary growth in plants :

The vertical growth of the roots and stems in length with the help of apical meristem is called as primary growth.

Dicotyledonous plants and gymnosperms exhibit increase in girth of root and stem.

In dicot stem, secondary growth begins with the formation of a continuous cambium ring.

The cambium present between the primary xylem and primary phloem of a vascular bundle is called intrafasicular cambium.

The cells of medullary rays adjoining these intrafascicular cambium strips become meristematic (regain the capacity to divide) and form the interfascicular cambium. Thus a complete and continous ring of vascular cambium is formed.

The cambium ring cuts off new cells, towards both the sides, inner and outer.

The cells that are cut-off towards pith (inner side) mature into secondary xylem and cells that are cut-off towards periphery mature into secondary phloem.

Generally, amount of secondary xylem is more than the secondary phloem.

In woody plants, secondary tissues constitute the bulk of the body. This provides support, conduction of water and minerals and protection. Lateral meristems play a major role in development of secondary tissues.

Formation of cambial ring :

With the onset of favourable season, meristematic cellsof intrafascicular cambium become active.

Simultaneously, the ray parenchyma cells, both fusiform initials and ray initials become meristematic. This is known asdedifferentiation.

These form patch of cambial cells (meristematic cells) in between the adjacent bundles and produce interfascicular cambium.

Now both intrafascicular and interfascicular cambium join and form a complete ring. This is known as cambial ring. This is possible because they lie in one plane.

 

 

Secondary growth in roots :

It is also observed in most of the dicot and gymnospermic roots by producing secondary vascular tissue and periderm.

Secondary growth is produced by vascular cambium and cork cambium respectively.

Conjuctive parenchyma cells present on the inner edges of primary phloem bundles become meristematic. These cells add secondary xylem and secondary phloem on the inner and outer side respectively.

These events are similar to secondary growth in stems.