A plant genus of the family RHAMNACEAE. This genus is often called buckthorn but should not be confused with other plants called that such as HIPPOPHAE or KARWINSKIA. Some RHAMNUS species have been reclassified into this genus. F. purshiana bark is cascara sagrada (CASCARA). Members contain emodine, frangulanin, frangulin, and anthraquinones such as EMODIN.