What is an orange-red crystalline substance, C15H10O5, obtained from the buckthorn, rhubarb, etc., and regarded as a derivative of anthraquinone; -- so called from a species of rhubarb (Rheum emodei) called?

emodin — An orange-red crystalline substance, C15H10O5, obtained from the buckthorn, rhubarb, etc., and regarded as a derivative of anthraquinone; -- so called from a species of rhubarb (Rheum emodei). (Webster's 1913.)

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