At first a solicitor, Schleiden became later a botanist. He is the founder of the cell theory and the author of "Grundzüge der wissenschaftlichen Botanik" (Basic Scientific Botany), the definitive textbook on botany at the middle of the 19th century. He noticed that it is water and not carbon dioxide that is split during photosynthesis. It was he, who prompted CARL ZEISS (Jena) to produce microscopes on a commercial basis.