Primary root is the direct prolongation of the radicle. When the primary root persists and continues to grow as in dicotyledons, it forms the main root of the plant and is called the tap root. Tap root produces lateral roots that further branches into finer roots. Lateral roots along with its branches together called as secondary roots.
Tap root modification
These are cone like, broad at the base and gradually tapering towards the apex. Example: Daucus carota.
These roots are swollen in the middle and tapering towards both ends. Example: Raphanus sativus
It is very broad and suddenly tapers like a tail at the apex. Example: Beta vulgaris
Some mangrove plants like Avicennia, Rhizophora, Bruguiera develop special kinds of roots (Negatively geotropic) for respiration because the soil becomes saturated with water and aeration is very poor. They have a large number of breathing pores or pneumatophores for exchange of gases.