The Collaborative International Dictionary
Phototonus
Phototonus \Pho*tot"o*nus\, n. [NL. See Photo-, and Tone.]
(Bot.) A motile condition in plants resulting from exposure to light. -- Pho`to*ton"ic, a.
(Physiol.) An irritable condition of protoplasm, resulting in movement, due to a certain intensity of light. -- Pho`to*ton"ic, a.
Wiktionary
phototonus
n. sensitivity to, or irritation caused by light