Matthias Jakob Schleiden (5 April 1804 – 23 June 1881) was a German botanist and co-founder of the cell theory, along with Theodor Schwann and Rudolf Virchow.
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. He in 1838, studied plant tissues and stated that all plants are aggregates of individual cells which are fully independent. Thus he made the first statement of cell theory. 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. This made him the first to realize that the parts of the plants are made 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.
Schleiden was one of the first German biologists to accept Charles Darwin's theory of evolution. He became professor of botany at the University of Dorpat in 1863. He concluded that all plant parts are made of cells and that an embryonic plant organism arises from the one cell. He died in Frankfurt am Main on 23 June 1881.
The standard author abbreviation Schleid. is used to indicate this individual as the author when citing a botanical name.