'Salix pentandra. Bay-leaved Willow', 19th Century. Bay willow is a deciduous tree native to northern Europe and northern Asia. The leaves not only resemble those of the bay tree, but they produce a yellow gum, which smells like a bay leaf. Traditionally willows were used to relieve pain, and the painkiller Asprin is derived from salicin, a compound found in the bark of all Salix species.