New forever stamps commemorating American scientists! From left to right: Melvin Calvin, who traced the process of photosynthesis and researched plants as alternative energy source. Asa Gray, botast, who advocated evolutionary theory. Maria Goeppert Mayer, who developed a theoretical model that hleped explain the structure of the atomic nucleus. Severo Ochoa, biochemist, who synthesized ribonucleic acid and completed the race to decipher the genetic code. Calvin, Mayer and Ochoa won nobel prizes.