The Collaborative International Dictionary of English v.0.48:

Xanthorhiza \Xan`tho*rhi"za\, n. [NL., fr. Gr. xanqo`s yellow +
   "ri`za root.] (Bot.)
   A genus of shrubby ranunculaceous plants of North America,
   including only the species Xanthorhiza apiifolia, which has
   roots of a deep yellow color; yellowroot. The bark is
   intensely bitter, and is sometimes used as a tonic.
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