The movement of molecules or ions in a fluid from areas of high concentration to areas of low concentration. In a closed system this will continue until the solution or gas mixture is evenly mixed. Diffusion is responsible for such processes as *transpiration
and the uptake of carbon dioxide. The rate of diffusion depends on concentration differences and on the nature of the pathway the molecules have to take. Thus transpiration is greater when the air is dry and when the stomata are fully open.