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. ]