castor bean, bean produced by Ricinus communis, a plant of the spurge family, widely cultivated as an ornamental. Moles die when they eat the roots. It has long been used as an ordeal poison in parts of Africa. Ricin, the toxic protein found in the bean seeds, can be extracted and used as a poison or chemical weapon, but it is not as poisonous or as readily absorbed as other such weapons. Castor oil is also extracted from the beans.