Born in Hamburg, Schleiden was educated at Heidelberg, then practiced law in Hamburg, but soon developed his love for the botany into a full-time pursuit. Schleiden preferred to study plant structure under the microscope. While a professor of botany at the University of Jena, he wrote Contributions to Phytogenesis (1838), in which he stated that the different parts of the plant organism are composed of cells. Thus, Schleiden and Schwann became the first to formulate what was then an informal belief as a principle of biology equal in importance to the atomic theory of chemistry. He also recognized the importance of the cell nucleus, discovered in 1831 by the Scottish botanist Robert Brown, and sensed its connection with cell division.
Die Entwickelung der Meduse ("The Development of the Medusæ"), in Schleiden's Das Meer