rheum emodei

from The Collaborative International Dictionary of English v.0.48

Emodin \Em"o*din\, n. (Chem.)
   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}).
   [1913 Webster]