The xylem helps in the movement of water from the root to the leaves. Two types of cells in the xylem, tracheids and vessels, form tubes that allow water to move up the plant. Tracheids are found in all vascular plants, but vessels are only found in flowering plants.
Water moves from the soil to the roots by osmosis and causes a positive pressure. This pressure pushes the water upward to the leaves. Root pressure is highest in the morning. The evaporation of water from the leaves to the atmosphere causes a negative pressure in the xylem, which pulls water up from the roots. This mechanism is called transpirational pull. The vessels transporting the water are small in diameter in order to prevent the water column from breaking.
Water molecules tend to form hydrogen bonds with each other. This intermolecular attraction helps water flow upward against the gravitational force. When water evaporates from the leaf, it pulls another water molecule into it. Water is required for photosynthesis, the process by which leaves produce their own food. The vascular system made up of the xylem helps carry the water and some nutrients to the leaves where it is utilized for photosynthesis.