The Collaborative International Dictionary of English v.0.48:

Madderwort \Mad"der*wort`\, n. (Bot.)
   A name proposed for any plant of the same natural order
   (Rubiaceae) as the madder.
   [1913 Webster]

WordNet (r) 3.0 (2006):

madderwort
    n 1: any of numerous trees or shrubs or vines of the family
         Rubiaceae [syn: madderwort, rubiaceous plant]