The C3 and C4 refer to how these classes of plants assimilate carbon dioxide into their systems. During the first steps in CO2 assimilation, C3 plants form a pair of three carbon-atom molecules.4 plants, on the other hand, initially form four carbon-atom molecules. In C3 plants CO2 enters the leaf through the stomata, which are microscopic pores found on the under-surface of leaves and on stems.