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Meaning of phototonus

pho•tot•o•nus

Pronunciation: (fō-tot'n-us), [key]
— n. Biol.
  1. the normal condition of sensitiveness to light in organisms or their organs.
  2. the irritability exhibited by cytoplasm when exposed to light of a certain intensity.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.