• Webster's Revised Unabridged Dictionary
    • 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.
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Century Dictionary and Cyclopedia
    • n phototonus In 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.
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Webster's Revised Unabridged Dictionary
NL. See Photo-, and Tone