Question

what are nodular roots..​

Answer 1

Root nodules are organs induced on most species of legume plants by symbiotic, N2-fixing bacteria of the genera Azorhizobium, Bradyrhizobium, Mesorhizobium, Rhizobium, and Sinorhizobium (Martinez-Romero and Caballero-Mellado, 1996), collectively called rhizobia.

or in simple terms

In general terms, nodules are formed as a result of infection of the roots by soil bacteria. In the case of forage crops, the bacteria most often infecting the roots are bacteria in the genus Rhizobium. The complex process by which plant roots are infected by rhizobia is known as infection.