Schleiden, Matthias Jakob, 1804-81, German botanist. He was professor at the universities of Jena (1839-63) and Dorpat (1863-64). With Theodor Schwann, he is credited with establishing the foundations of the cell theory. Schleiden's paper Beiträge zur Phytogenesis (1838), although mistaken in some aspects, recognized the significance of the nucleus in the propagation of cells.