Source : Webster's Revised Unabridged Dictionary (1913)

Arrow grass \Ar"row grass`\, n. (Bot.)
   An herbaceous grasslike plant ({Triglochin palustre}, and
   other species) with pods opening so as to suggest barbed
   arrowheads.

Source : WordNet®

arrow grass
     n : tufted perennial found in shallow water or marshland;
         sometimes poisons livestock [syn: {Triglochin maritima}]