They are soluble in water and produce foam. Most are toxic to animals. There are two types - triterpenoid and steroidal saponins. Believed to have a function in defense and as an attractant to pollinators. Also known as a secondary metabolite.
Most steroids are alcohols.
Others include some pseudoalkaloids and the aglycone portions of the saponins and cardiac glycosides. Other uses include dyes, stains and inks. They have a harsh astringent taste and are believed to be used by the plant as a defense against insects. These definitions have been adapted from references 27,28 and 29.