Schleiden, Matthias Jakob (1804–1881), a German botanist. He and Theodor Schwann established the fact that all living organisms are made up of cells. Schleiden published his conclusions, which applied only to plants, in 1838; Schwann extended Schleiden's concept to include animals. Schleiden was born in Hamburg. After a brief career as a lawyer he obtained a degree as a doctor of philosophy and medicine. He taught botany at the universities of Jena and Dorpat.