小孔律（law of small pore）

rate at which gas molecules diffuse through the surface of the small hole is not directly related to the area of the small hole but to the perm of the small hole. This is because there is an edge effect when the gas molecules spread outward from the hole surface. That is, the diffusion molecules at the edges interfere with each other less than the middle part, so the diffusion rate of the edges is faster than the middle part. The ratio of the edge of the large hole to the area is small, and the ratio increases when the large hole shrinks (because the area shrinks by the square of the radius). Therefore, when large holes are dispersed into a number of small holes of equal area, the amount of diffusion through the edge will account for a larger proportion. The edge effect can be expressed, and there must be a certain distance between the holes so that the diffusers do not interfere with each other. The total area of the pores is only about
1
% of the leaf area, and the amount of water it steams is
50
% or more of the evaporation of the leaves as free water surface, because the pores are scattered on the leaves and there is a certain distance between them.