The Collaborative International Dictionary of English v.0.48:

Hydrotropism \Hy*drot"ro*pism\, n.
   1. (Bot.) A tendency towards moisture.
      [1913 Webster]

   2. (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.
         [Webster 1913 Suppl.]