The Collaborative International Dictionary of English v.0.48:

Heliotropism \He`li*ot"ro*pism\, n. [Helio- + Gr. ? to turn.]
   (Bot.)
   The phenomenon of turning toward the light, seen in many
   leaves and flowers.
   [1913 Webster]

WordNet (r) 3.0 (2006):

heliotropism
    n 1: an orienting response to the sun