Why are C4 and C3 plant called C4 and C3 plants?
During the first steps in CO2 assimilation, C3 plants form a pair of three carbon-atom molecules. C4 plants, on the other hand, initially form four carbon-atom molecules. It turns out that the important difference between C3 and C4 species for rising CO2 levels is that C3 species continue to increase photosynthesis with rising CO2 , while C4 species do not. So, C3 plants that can respond readily to higher CO2 levels, and C4 plants can make only limited responses.
