Abstract: In this paper stomatal pore and substomatal cavity are considered to be elliptic cylinders, A three-dimensional diffusion model is presented, which describes the diffusion of vapour from the surfaces of the cells surrounding the cavity to the outer end of the pore Equations describing vapour diffusion in the model are set up, based on Fick's law and the law of conservation of mass, and are solved by using computer. Quantitative relation between the cavity resistance to water vapour diffusion and: stomatal aperture is obtained and is given more general theoretical explanation. Comparing the formula obtained in this paper with those of Brown and Escombe and of Cooke et al., it is found that the cavity resistance calculated by the latter two formulas are 0.5 to 1 times higher in a large rankle of stomatal aperture values. Besides, it is shown by calculating that the rates of loss from guard cells and subsidiary ceils account for 88%– 93% and 7%–12% respectively of that from epidermic cells, and the litter amounts to 86%–96% of that from all the cells in the cavity in the large range of stomatal change.

Key words: Stomata, Substomatal cavity, Resistance, Transpiration