Robert Brown (1773-1858), British botanist. Brown is most famous for his 1827 observation of erratic motion by pollen grains in water. This was named Brownian motion. In 1877, Desaulx recognised that the motion is caused by the pollen colliding with water molecules. This meant that Brownian motion was the first directly observable evidence for the existence of molecules. Brown spent years working on plant taxonomy, establishing the classification of two major divisions of plants, the gymnosperms and the angiosperms. He also observed an essential part of living cells, which he named the nucleus (1831).