Saponins are substances found in plants which form stable foams with water. They normally occur combined with sugar in glycosides. The component sterols or triterpinoids are called sapogenins. Many are of considerable value in medicine in spite of being highly poisonous. The heart stimulant digitalis, is a saponin from the purple foxglove. The sapogenin, diosegnin, from Mexican yams (Dioscoria) is a cheap precursor to the steroid drugs.