Several species of willow are used to produce willow bark herb, Salix alba, S. Fragilis, S. daphnoides and S. purpurea. All contain adequate levels of the important constituent, salicin, to allow their use as the herb. Salicin was isolated and synthesized into acetylsalicylic acid in the 1850's and eventually marketed as aspirin in the late 1890's by Frederich Bayer & Co.