16,19,[24][25][26][27]Isotope analysis of plant matter provides a powerful method of studying photosynthetic modes. In addition to the well known separation of C3 plants from C4 and CAM plants based on stable carbon isotope ratios (17), oxygen and hydrogen isotope ratios ofcellulose and cellulose nitrate, respectively, also are influenced by photosynthetic mode (20,(22)(23)(24)(25). Analysis of C3, C4, and CAM plants growing in the vicinity of each other show that cellulose nitrate from CAM plants is enriched in deuterium relative to C3 and C4 plants (20, 22, 25). Stemnberg et al. (20, 22-25) concluded that the difference in 6D values between CAM plants and C3 and C4 plants are due to fractionations occurring during biochemical reactions particular to CAM plants. C4 plants also tend to have higher abundances of deuterium and '80 than C3 plants (20,22,27). Stemnberg et al. (22)