A model for carbon dioxide uptake by plant leaves, based on the partial differential equation for diffusion with chemical reaction, is used to simulate photosynthesis for various stomatal openings. The results show that not only the "stomatal resistance" but also the commonly calculated "mesophyll resistance" varies when nothing but stomatal aperture is changed. Simplistic uses of the resistance concept (or related conductances) may be more misleading than useful for modelling photosynthesis, because the various resistance components are neither independent nor additive.
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