Source : Webster's Revised Unabridged Dictionary (1913)

Stonewort \Stone"wort`\, n. (Bot.)
   Any plant of the genus Chara; -- so called because they are
   often incrusted with carbonate of lime. See {Chara}.

Source : WordNet®

stonewort
     n : any of various submerged aquatic algae of the genus Chara
         having nodes with whorled filamentlike branches; usually
         encrusted with calcium carbonate deposits