<sup>13</sup>C/<sup>12</sup>C ratios have been determined for plant tissue frin 104 species representing 60 families. Higher plants fall into two categories, those with low \[8,9]PDB 13C values (-24 to -34) and those with high \[8,9]13C values (-6 to -19). Algae have \[8,9] 13C values of -12 to -23. Photosynthetic fractionation leading to such values is discussed. 104 plant species representing 60 families were investigated carbon ratios within and among species. The highest level of the <sup>13</sup>C content was composed of aquatic, desert and salt marsh plants, and tropical grasses. The other class of the plants were low in <sup>13</sup>C and comprises the bulk of the plant kingdom. There was not overlap. The Park and Epstein Model (1960) The uptake of CO\-2\- into the cytoplasm involves isotopic fractionation due to the greater frequency of <sup>12</sup>CO\-2\- colliding with the cell membrane as compared with <sup>13</sup>CO\-2\-. After passing through the membrane, the dissolved CO\-2\- is partitioned into enzyme-catalyzed conversion to starch and into removal of some of the dissolved CO\-2\- through the vascular system resulting in excretion through the roots. The translocation step determines how rapidly CO\-2\- in the cytoplasm is removed from the plant system to avoid a build-up of <sup>13</sup>C in the cells. Any step in this whole process affects the final fractionation amoung that is associated with the fixation of CO\-2\- by plants. This explains why plants have such widely disparate ranges.