Discuss the different modifications of tap root for the storage of food, N 2 fixation and respiration.

The tap root system develops from the radicle of the embryo. These roots are changed in shape, structure and form to perform special functions such as support, storage of food and respiration in addition to normal function is known as tap root modifications. i) The storage of food: The tap roots are modified to store for materials. The primary root becomes swollen due to the presence of stored food materials. Depending upon the shape these can be following types: a) Conical roots: These are fleshy tap roots in the shape of a cone, i.e., broad at the base and gradually tapering towards the apex. Example: carrot. b) Fusiform roots: The primary root is spindle-shaped. It is swollen in the middle and gradually tapers at both the ends. Example: radish. c) Napiform: The primary root is almost spherical (pitcher-shaped) at the base and tapers abruptly at the lower end. Example: beetroot. d) Tuberous root: The primary root becomes thick and fleshy but do not attain any special shape. Example: 4 O'clock plant. ii) For nitrogen (N 2 ) fixation In some plants, the secondary and tertiary sometimes the primary roots bear many small swelling called root nodules. These roots are known as nodulated roots. These nodules contain many minutes symbiotic nitrogen-fixing bacteria, Rhizobium . Examples: pea, groundnut, etc. iii) For respiration In mangrove plants, growing in a marshy area, some roots grow vertically upward and are negatively geotropic. These roots are called pneumatophores or aerophores or respiratory roots. These roots have minute openings present on the tips that help in getting oxygen for respiration.