Portrait of the German botanist Jakob Mathias Schleiden (1804-1881), a co-founder of the cell theory with the German physiologist and friend Theodor Schwann. Schleiden studied botany in three universities and graduated at Jena in 1831. Plant cells had been observed two centuries earlier by the English physicist Robert Hooke and others. Schleiden's studies convinced him that all plant or animal structures are composed of cells or their derivatives. He recognised the importance of the nucleus in cell division but believed wrongly that new cells were formed by budding from the surface of the nucleus.