Water is transported round the plant using transpiration as the 'power': The air surrounding a stoma has a lower concentration of water than the cells behind the stoma so water moves to this area of lower concentration. The cells around the stoma now have a lower concentration than the other leaf cells so water moves into them via osmosis. In this way, diffusion causes water transport through the cells of the leaf from root cells (into which water diffuses from the soil) via the xylem.
