Emodin
Emodin is an anthraquinone found in cascara sagrada, rhubarb, and other plants. Emodin is cathartic; it stimulates the cathartic action by irritation of the intestinal walls. This differs from anthraquinone glycosides, which are absorbed into the bloodstream and stimulate the appropriate nerves.
References
Weiner, Michael and A. Janet Weiner. 1980. Weiner's Herbal The Guide to Herb Medicine. Stein and Day, New York. 224 pp.
Windholz, Martha, Susan Budavani, et. al. 1976. The Merck Index. Merck and Co., Inc. New Jersey.