as applied to Fick's Law for photosynthesis or transpiration, this describes the passage of gasses through the stomatal pore. When open, the stomatal pore provides little resistance to movement, when closed, the stomatal resistance increases. This is a variable resistance and the most important of the resistances found in leaves.

The opposition to transport of quantities such as water vapor and carbon dioxide to or from the stomata (pores) on the leaves of plants. For water vapor, stomatal resistance is defined as where and are the specific humidities in the interior of the stomate cavity and at the exterior surface of the leaf, is air density, and is the moisture flux. The dimension of is time per distance, that is, inverse velocity. This equation is analogous to that for an electrical resistance, proportional to the ratio of voltage potential divided by current flow. See transpiration. Oke, T. R., 1987: Boundary Layer Climates, chap. 4.