Plants can only perceive a change in atmospheric [CO2] through tissues that are exposed to the open air. The protective cuticle of higher-plant leaves and other photosynthetic organs means that only the inner surfaces of the guard cells of stomata and the mesophyll can directly sense a change in atmospheric [CO2]. While many steps in metabolism use or respond to CO2, the only sites where there is convincing evidence for a response in the concentration range of relevance (240–1000 μmol mol−1) are ribulose 1 : 5 bisphosphate carboxylase/oxygenase (Rubisco) and a yet undefined metabolic step affecting stomatal aperture, that may also involve Rubisco