A naturally optimized mass transfer process: The stomatal transpiration of plant leaves

Stomatal transpiration of leaves is a dominant pathway of plant physiological water loss. The leaf transpiration rate when stomata are fully open is commonly at the same level as the evaporation rate of a wet surface of the same area as that of the leaf area, although the cumulative area of the stomatal pores is typically less than 3% of the leaf area. To elucidate the highly efficient diffusion of the stomatal array from the perspective of mass transfer theory, stomatal distribution characteristics of various kinds of leaves were obtained with optical microscope, and steady diffusions of water vapor from isolated zero-depth circular stomata, elliptical stomata, and distributed stomatal arrays without airflow parallel to the surface were simulated with the finite element method. It was found that the long perimeter of the elliptical stomata and the specific distribution characteristics of the stomatal array are the dominant reasons for the highly efficient diffusion of the stomatal array on the leaves. Furthermore, the simulation results reveal that extremal transpiration rates exist for the stomatal arrays with different distribution characteristics. It was found that the transpiration rates of the vegetation tend to approach the extremal values for flourishing development in the process of natural optimization.