The seeds of this family are typically produced in pods (called legumes), and come in an astonishing array of shapes and sizes. Many legume species contain nitrogen-fixing bacteria inside root nodules which convert inert atmospheric nitrogen into ammonia, thereby making vital nitrogen available to other plants. Legumes include a wide range of species, from roadside weeds and valuable crop plants to ornamental shrubs, vines and giant canopy trees of the tropical rain forest. T he third largest family of flowering plants is the legume or pea family (Fabaceae), with more than 18,000 species.