The uptake or assimilation by living organisms of a particular isotope in preference to another isotope of the same element. A well-known example in nature is the preferential fixation by photosynthetic organisms of the lighter (and vastly more abundant) isotope carbon-12 compared with the heavier carbon-13. The relative abundance of these two stable isotopes in biological material, whether living or dead, differs from that in the atmosphere and according to various factors, including the type of ecosystem, season, and atmospheric conditions. A record of these changes, based on preserved organic material, can provide insights into historical climatic conditions. In plants isotopic discrimination occurs chiefly because carbon dioxide (CO₂) containing ¹³C diffuses more slowly than lighter CO₂ containing ¹²C and also because the enzymes involved in photosynthetic carbon fixation discriminate between the two isotopes, particularly rubisco, the first enzyme to encounter CO₂ in C₃ plants (see c₃ pathway). ¹²C/¹³C isotopic discrimination is less pronounced in C₄ plants, in which CO₂ is first incorporated by the C₄ pathway. This gives C₃ and C₄ plants characteristically different isotopic signatures. Determination of ¹²C/¹³C ratios in plant material using mass spectrometry is informative about plant physiology and atmospheric conditions. For example, it indicates whether the stomata of a plant were mainly open or mainly closed during its lifetime, and hence whether conditions were likely to have been moist or dry.