Acquisition of CO₂ by higher plants involves CO₂ diffusion from the air into leaves and, ultimately, into chloroplasts. There, fixation of CO₂ into organic compounds creates the concentration gradient which drives CO₂ diffusion. CO₂ encounters many obstructions along its diffusion path toward chloroplasts. The diffusion resistances attributable to boundary layer and stomata are shared with the opposing flux of water leaving the leaf. Once in the substomatal cavities, however, CO₂ faces additional resistances since it has to cross walls and membranes to reach the chloroplasts. We follow the CO₂ molecule from the air through the boundary layer, stomata and, finally, the mesophyll. After providing diverse anatomical examples at each level, we review the current understanding about the subtle balance which plants maintain between water loss and CO₂ acquisition.

Stomatal responses to many environmental variables are well known, despite our lack of understanding of the underlying mechanisms. Techniques areavailable that allow accurate measurements of conductances through the boundary layer and stomata. The estimation of conductance through the mesophyll, however, has not been feasible until the recent development of rapid measurements of isotopic discrimination and chlorophyll fluorescence. We discuss the principles which allow the estimation of internal conductance based on these techniques and the data available. Both stomatal conductance and internal conductance correlate strongly with photosynthetic capacity and are of similar magnitude. However, stomatal conductance can vary within minutes in response to changes in the environment. Internal conductance, on the other hand, seems to be stable over several days and depends on anatomical properties of the leaf. We close considering the special case of photosynthesis where spatial compartmentation and biochemical mechanisms of CO₂ concentration add complexity to the estimation of internal conductance.