Matthias Jakob Schleiden was a German botanist who contributed to the development of the cell theory in the 19th century. In 1838, he published "Contributions to Phytogenesis," in which he stated that the different parts of the plant organism are composed of cells or derivatives of cells. He observed that the cell is the basic building block of all plant matter and that an embryonic plant arose from a single cell. Schleiden 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. Schleidens work with cells and cell theory as an organizing principle of biology contributed to the field of embryology.