Animation of the transpiration of water in a plant leaf. Transpiration is the evaporation of water from a leaf, which helps create a pull that allows the roots to take up more water. Water (red and blue molecules) arrives at a leaf through a xylem vessel (brown tube). This leaf vein is lined by a sheath of close-fitting cells with large vacuoles (spaces). Water is drawn out of the vein by osmosis, passing from cell to cell along an osmotic gradient, before it reaches an air space in the leaf. This borders the lower surface (green) of the leaf, and the water is held behind a pore called a stoma formed of two guard cells (brown). Water evaporates through the open stoma, which helps to draw more water up from the ground. At the same time, carbon dioxide from the atmosphere diffuses in to the leaf, providing the source of the carbon used to make sugars in photosynthesis.