Plants are a major source of complex and highly structurally diverse chemical compounds (phytochemicals), this structural diversity attributed in part to the natural selection of organisms producing potent compounds to deter herbivory (feeding deterrents). Major classes of phytochemical include phenols, polyphenols, tannins, terpenes, and alkaloids. Though the number of plants that have been extensively studied is relatively small, many pharmacologically active natural products have already been identified.  Clinically useful examples include the anticancer agents paclitaxel and omacetaxine mepesuccinate (from Taxus brevifolia and Cephalotaxus harringtonii, respectively), the antimalarial agent artemisinin (from Artemisia annua), and the acetylcholinesterase inhibitor galantamine (from Galanthus spp.), used to treat Alzheimer's disease. Other plant-derived drugs, used medicinally and/or recreationally include morphine, cocaine, quinine, tubocurarine, muscarine, and nicotine.