Illustration of epidermal cells and tissues from plant seeds from Beitrage zur Botanik (Contributions to Botany) published in 1844 by the German botanist Jakob Matthias Schleiden (1804-1881). Schleiden was a co-founder of cell theory with the German physiologist Theodor Schwann. Schleidens studies convinced him that all plant or animal structures are composed of cells or their derivatives and that cells are the fundamental unit of life.