buck bean

(Bot.) A perennial plant (Menyanthes trifoliata) of Europe and America which grows in moist and boggy places, having racemes of white, reddish, or purplish flowers and intensely bitter trifoliate leaves, sometimes used in medicine; marsh trefoil; -- called also bog bean. It often roots at the water margin and spreads across the surface.
Syn. -- water shamrock, bogbean, bog myrtle, marsh trefoil, Menyanthes trifoliata. [ 1913 Webster + WordNet 1.5 ]