The Collaborative International Dictionary of English v.0.48:
Aaron's rod \Aar"on's rod`\ ([^a]r"[u^]nz r[o^]d`). [See Exodus vii. 9 and Numbers xvii. 8] 1. (Arch.) A rod with one serpent twined around it, thus differing from the caduceus of Mercury, which has two. [1913 Webster] 2. (Bot.) A plant with a tall flowering stem; esp. the great mullein, or hag-taper, and the golden-rod. [1913 Webster]