The Collaborative International Dictionary of English v.0.48:

Phototonus \Pho*tot"o*nus\, n. [NL. See Photo-, and Tone.]
   1. (Bot.) A motile condition in plants resulting from
      exposure to light. -- Pho`to*ton"ic, a.
      [1913 Webster]

   2. (Physiol.) An irritable condition of protoplasm, resulting
      in movement, due to a certain intensity of light. --
      Pho`to*ton"ic, a.
      [Webster 1913 Suppl.]