Source : Webster's Revised Unabridged Dictionary (1913)

Phototropism \Pho*tot"ro*pism\, n. [Photo- + Gr. ? to turn.]
   (Plant Physiol.)
   The tendency of growing plant organs to move or curve under
   the influence of light. In ordinary use the term is
   practically synonymous with heliotropism.

Source : WordNet®

phototropism
     n : an orienting response to light