Secondary Growth

This results in an increase in width or girth due to activity of the cambium.

New tissues are formed by vascular cambium or cork cambium.

There are no cambium cells in the vascular bundles of monocotyledonous plants. The growth in diameter is due to the enlargement of the primary cells.

Secondary growth begins with the division of vascular cambium to produce new cambium cells between the vascular bundles. This forms a continuous cambium ring. These cambium cells divide to form the new cells that are added to the older ones. The cambium cells have now become meristematic.

The new cells produced to the outer side of cambium differentiate to become secondary phloem and those to the inner side differentiate to become the secondary xylem.

More secondary xylem is formed than secondary phloem. The intervascular cambium also cuts of parenchymatous which form secondary medullary rays.

Because of the increase in the volume of the secondary tissues, pressure is exerted on the outer cells of the stem. This results in stretching and rupturing of the epidermal cells. In order to replace the protective outer layer of the stem, a new band of cambium cells are formed in the cortex. These cells, called cork cambium or phellogen originate from the cortical cells. The cork cambium divides to produce new cells on either side.

 The cells on inner side of the cork cambium differentiate into secondary cortex and those produced on the outer side become cork cells.

Cork cells are dead with thickened walls. Their walls become coated with a waterproof substance called suberin. The cork cells increase in number and become the bark of the stem. This protects against water loss, mechanical damage and infection. The corky bark acts as an insulator layer hence it is resistant to fire.

Periodically the cork cells, instead of being tightly packed, they form a loose mass. The mass is known as lenticel. The lenticels allow gaseous exchange.

The rate of secondary growth in a stem varies with seasonal changes. During rainy season, xylem vessels and tracheids are formed in large numbers. These cells are large, have thin walls and the wood has a tight texture.

In the dry season, the xylem vessels and tracheids formed are few in number. They are small, thick-walled and their wood has a dark texture.

These leads to the development of two distinctive layers within the secondary xylem formed in a year, called annual rings.

It is possible to determine the age of a tree by counting the number of annual rings.

Furthermore, climatic changes of the past years can be interfered from the size of the ring.