[seniour_baloch]

I read in my book that the Mesophyll cells in a plant leaf have a layer of moisture around them which facilitates the diffusion of Oxygen. It causes Oxygen to diffuse quickly. Why is this so? What is the relationship of diffusion with wet surfaces?

[Arkcon]

Diffusion is a process that happens because of molecular motion.  It can happen with a gas to a gas, when liquids mix, from gasses over the surface of a liquid, or even across certain solids to an extent.  As an example, gold diffuses into a copper surface when heated.  Not enough to leave a mark, but enough to weaken circuit traces in computer chips if there isn't another metal between them.  So moisture doesn't facilitate diffusion.  It does make the gasses more useful to living things to have them diffuse into water.