Tap root
The root which develops from the radicle of an embryo during seed germination is known as tap root or true root. The main root is called as primary root; its branches of first order are called as secondary roots whereas branches of second order are called as tertiary roots e.g. Pea, Bean, Sunflower etc.The main root with all its branches is known as tap root system. Tap root system is commonly seen in dicotyledonous plants.
Adventitious roots
A root that develops from any part other than radicle is known as adventitious root. Such root may develop from the base of the stem, nodes or from leaves. In monocots, radicle is short lived and from the base of stem a thick cluster of equal sized roots arise. This is adventitious root system. eg. Maize, Wheat, Sugarcane etc. It is also known as fibrous root system as they look like fibre. The growth of roots is superficial. Adventitious root in some plants are used for vegetative oil particles and propagation. eg. Euphorbia, Carapichea ipecacuanha (Ipecac) etc.