Give an illustrated account of both the external and internal structures of mustard (Brassica campestris) root.

EXTERNAL STRUCTURE OF BRASSICA ROOT:

The root arises from the radicle of the seed and grows downwards in the soil. The first root arising from the radicle is called the primary root. The primary root gives rise to secondary and tertiary roots. The primary root is thicker than any other root. The tips of all roots bear a root cap. It protects the root. Behind the root cap are present root hairs. The roots absorb water and mineral salts only through the root hairs. The roots fix the plants to the soil.

Internal Structure of Brassica Root: 

(1) Epidermis: It is the outer layer of the root. (epi=above, derma=skin). It is single layered. Root hairs are the outgrowths of epidermal cells, they are unicellular. There is no cuticle on the epidermis. The epidermis is also called the piliferous layer. 

Functions: 

• Absorption of water, salts and soil air through root hairs. 
• Protection of inner tissues. 

(2) Cortex: Lying inside the epidermis, is cortex. It is composed of many layers of thin walled living cells, parenchyma (packing tissue) cells.

There are intercellular spaces between parenchyma cells which are filled with water. 

Functions: 

• Transport of water, salts and air across root, either through or between cells. 
• Food is stored in the cortex. e.g. carrot.

(3) Vascular Bundles or Stele: The stele consists of central xylem, xylem is star shaped. 

In between the arms of the xylem is phloem. A cambium layer is present in the stele along with the xylem and phloem.

Functions: 

• Xylems transport water and salts upwards through transpiration. 
• Phloem transport organic food material up and down the plant through translocation. 
• Cambium layer is responsible for the formation of new phloem and xylem tissues.

(4) Endodermis: It is the innermost layer of the cortex with no intercellular spaces is There are thickening of special materials around cells. The endodermis surrounds the stele. 

Function: 

• It checks/stops the diffusion of water from xylem to cortex.

(5) Pericycle: The layer next to endodermis is called the pericycle There are no intercellular spaces in pericycle.

Function:

• All the branches of root originate from pericycle.