WordNet (r) 3.0 (2006):

scammonyroot
    n 1: tropical American prostrate or climbing herbaceous
         perennial having an enormous starchy root; sometimes held
         to be source of the sweet potato [syn: wild potato vine,
         wild sweet potato vine, man-of-the-earth, manroot,
         scammonyroot, Ipomoea panurata, Ipomoea fastigiata]