Source : Webster's Revised Unabridged Dictionary (1913)

Hydrotropism \Hy*drot"ro*pism\, n. (Bot.)
   In a broader sense, any curvature or turning induced in
   certain growing plant organs under the influence of moisture.

   Note: When the movement is toward the moisture, as is the
         case in most roots, the phenomenon is called {positive
         hydrotropism}; when away from the moisture, as in the
         case of hyphae of certain fungi, {negative
         hydrotropism}.