CELL THEORY By N., Pam M.S. - April 7, 2013 Principle that all organisms are composed of one or more cells. Proposed in 1839 by Theodor Schwann for animals and by Matthias Jakob Schleiden for plants. CELL THEORY: "Cell theory indicates that living things have one or more cells." Related Psychology Terms CELL THEORY METABOLISM LIPPS ILLUSION THEORY NEUROFIBROMA ZOOPHARMACOGNOSY GERM LINE MULLERIAN MIMICRY EPIDERMIS PHOTOPERIODISM MYELIN SHEATH Cite this page: N., Pam M.S., "CELL THEORY," in PsychologyDictionary.org, April 7, 2013, https://psychologydictionary.org/cell-theory-2/ (accessed April 19, 2021).