geotropism
    n 1: an orienting response to gravity

Geotropism \Ge*ot"ro*pism\, n. [Gr. ge`a, gh^, the earth + ? to
   turn.] (Biol.)
   A disposition to turn or incline towards the earth; the
   influence of gravity in determining the direction of growth
   of an organ.
   [1913 Webster]

   Note: In plants, organs which grow towards the center of the
         earth are said to be positively geotropic, and those
         growing in the opposite direction negatively geotropic.
         In animals, geotropism is supposed by some to have an
         influence either direct or indirect on the plane of
         division of the ovum.
         [1913 Webster]