Phototonus(Bot) A motile condition in plants resulting from exposure to light.
Phototonus(Physiol) An irritable condition of protoplasm, resulting in movement, due to a certain intensity of light.
Century Dictionary and Cyclopedia
nphototonusIn botany, a term proposed by Sachs for the peculiar condition in which the protoplasm is capable of exhibiting irritability induced in certain plant-organs by exposure to light of a certain intensity. This tonic influence of light is exhibited in the restoration of irritability in organs that have been kept for some days in continuous darkness.