Willows, also called sallows and osiers, form the genus Salix, around 400 species of deciduous trees and shrubs, found primarily on moist soils in cold and temperate regions of the Northern Hemisphere. Most species are known as willow, but some narrow-leaved shrub species are called osier, and some broader-leaved species are referred to as sallow (from Old English sealh, related to the Latin word salix, willow). Some willows are low-growing or procumbent shrubs, while others are trees up to 20 metres tall. Willows all have abundant watery bark sap, which is heavily charged with salicylic acid. This, together with the fact that willow bark was one of the first sources of salicylic acid for medicinal use, gives the genus its name. Willows are very cross-compatible, and numerous hybrids occur, both naturally and in cultivation.