A cut stalk of celery (Apium graveolens var. dulce) that has stood in water that had blue food colouring added to it. The leaves of the celery transpire, they allow water to evaporate as an outcome of the process of gaining carbon dioxide from the air. Water flows upward through the stem by a combination of capillary action and the difference in hydrostatic pressure through the xylem (seen as bright blue). Water enters the stem and roots by osmosis, bringing with it dissolved nutrients or, in this case, food colouring.