Echinacea species (Echinacea angustifolia (DC.) Hell., Echinacea pallida (Nutt.) Nutt., Echinacea purpurea (L.) Moench): a review of their chemistry, pharmacology and clinical properties. - Barnes,J.; Anderson,L.A.; Gibbons,S.; Phillipson,J.D. (J Pharm Pharmacol, 2005)