Symbiosis is the phenomenon by which two organisms maintain relationship with each other to be mutually benefitted.
In symbiotic mode, organisms develop a special relationship with certain other organisms to obtain nourishment. Organisms involved in this type of relationship are called as symbionts. e.g. Rhizobium bacteria and Leguminous plants are symbionts exhibiting symbiosis. Plants cannot utilize atmospheric nitrogen directly. Hence, leguminous plants establish a symbiotic relationship with bacteria like Rhizobium. Both the organisms of symbiotic relationship are mutually benefited. Leguminous plant provides shelter and nourishment for the bacteria and in turn bacteria fix atmospheric nitrogen to the plant.