On June 16, 2011, the U.S. Postal Service issued Forever® stamps celebrating the achievements of four American scientists: chemist Melvin Calvin, botanist Asa Gray, physicist Maria Goeppert Mayer, and biochemist Severo Ochoa. Calvin’s stamp art includes a photograph of him taken by Yousuf Karsh and the background shows chemical symbols and structures he used to represent the carbon cycle in photosynthesis.