Most plants fall into three groups, based on their chemical makeup: C4, C3, and CAM. C4 plants are those that are subject to long growing seasons with lots of access to sun. The C designation represents carbon: the plants are differentiated on the basis of the photosynthesis process. C4 plants convert atmospheric carbon into a chemical compound with four carbon atoms; C3 plants one with three, etc. Biologists call these various conversion processes "pathways".
In general, plants which follow a C4 pathway originated in subtropical areas. C4 plants of special interest to archaeologists include maize, sorghum, sugarcane, millet, fonio, tef, and papyrus.
C4 plants absorb carbon 13 far faster than carbon 12, and so the total biomass of C4 plants ranges between -9 to -16%, compared to the agreed-upon standard VPDB measurement, with a mean of -12.5%.
C3 plants are those that convert carbon to a compound containing three carbon atoms. They are found in a far broader range of environments. C3 plants which are of particular interest to archaeologists include rice, wheat, rye, barley, cassava, potatoes, algae, spinach, and yams. C3 plants are slower to take in carbon 13, so their total biomass ranges from -22% to -35%, with a mean of -26.5%.
The third group of plants is called CAM for Crassulacean acid metabolism, which can use either pathway for photosynthesis, and so have biomass values that can look like C3 or C4 plants. To date, archaeologists have not used CAM plants for identification purposes.
There are other plant types, by the way; but they are not and never have been significant food resources for man or animal.
Michael DeNiro. 1987. Stable Isotopy and Archaeology. American Scientist 75: 182-191.
Nikolaas Van der Merwe. 1982. Carbon Isotopes, Photosynthesis and Archaeology. American Scientist 70:596-606.