Matthias Schleiden and Theodor Schwann looked at plant and animal cells. Schwann wrote the first part of what is known as the cell theory. Which statement did he write? A. All organisms are composed of one or more cells. All cells come from existing cells. B. All organisms are composed of one or more cells. The cell is the basic unit of life in all living things. C. All cells come from existing cells. The cell is the basic unit of life in all living things. D. All cells are microscopic in size. All cells come from existing cells.