The 19th century German botanist Matthias Schleiden developed cell theory in cooperation with the German physiologist Theodor Schwann. This fundamental biological principle concludes that all living organisms, whether single-celled or multicellular, are made up of cells, the basic unit of life.
This cell theory also posits that all cells arise from pre-existing cells, establishing a hypothesis of cellular reproduction. Schleiden and Schwann worked together at the University of Jena. The former, as a botanist, wrote "Contributions to Phytogenesis" in 1838, essentially describing cell theory for plants. Schwann extended this theory with his 1839 work "Microscopical Researches into the Accordance in the Structure and Growth of Animals and Plants."