Sesbania flower

Sesbania is a genus of flowering plants in the pea family, Fabaceae and the only genus found in Tribe Sesbanieae. Notable species include the Rattlebox (Sesbania drummondii), Spiny Sesbania (Sesbania bispinosa), and Sesbania sesban, which is used in cooking. Plants of this genus, some of which are aquatic, can be used in alley cropping to increase the soil's nitrogen content. The species of Rhizobia responsible for Nitrogen fixation in Sesbania rostrata is Azorhizobium caulinodans.
Source: http://en.wikipedia.org/wiki/Sesbania