The Collaborative International Dictionary of English v.0.48:

Lizard's tail \Liz"ard's tail`\n. (Bot.)
   A perennial plant of the genus Saururus (Saururus
   cernuus), growing in marshes, and having white flowers
   crowded in a slender terminal spike, somewhat resembling in
   form a lizard's tail; whence the name. --Gray.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Saururus cernuus
    n 1: North American herbaceous perennial of wet places having
         slender curled racemes of small white flowers [syn:
         lizard's-tail, swamp lily, water dragon, Saururus
         cernuus]