Diffusion is the net movement of molecules or ions from an area of higher concentration to an area of lower concentration. Think of it as a rebalancing. The molecules or ions are said to be moving along a diffusion gradient. If molecules or ions moving in the opposite direction are said to be moving against a diffusion gradient. Diffusion will continue until a state of equilibrium is reached. Rates of diffusion are affected by temperature and the density of the involved molecules among other things. In the leaves, water diffuses out via the stomata into the atmosphere.