• (n.) An irritable condition of protoplasm, resulting in movement, due to a certain intensity of light. • (n.) A motile condition in plants resulting from exposure to light. Found on http://thinkexist.com/dictionary/meaning/phototonus/
Pho·tot'o·nus noun [ New Latin See Photo- , and Tone .] (Botany) A motile condition in plants resulting from exposure to light. -- Pho`to*ton'ic adjective Found on http://www.encyclo.co.uk/webster/P/78