Although plants obtain their carbon from the atmosphere, the actual carbon-14 activity in them is lower by 3 or 4% (equivalent to an apparent excess age of 240-320 years). This is because in the process of photosynthesis carbon-14 is not taken up as readily as carbon-12. There are small variations from species to species in the degree of this isotopic fractionation, and it is now common practice to evaluate the effect for each sample that is dated.