Water moves up a tree through the xylem primarily by the cohesion-tension mechanism. Water evaporates from the mesophyll cells in the leaves—a process called transpiration—which creates a negative pressure (tension) in the leaf xylem vessels. This tension pulls water upward in a continuous column through the xylem, which is formed by vessels and tracheids extending from roots to leaves. The cohesion of water molecules due to hydrogen bonding allows them to stick together, forming an unbroken column of water. Additionally, adhesion between water molecules and the xylem vessel walls helps water climb upwards against gravity by capillary action. Root pressure, which results from osmotic water absorption in roots, can also push water up, but it generally moves water only a few meters and is less significant in tall trees. The process is passive and does not require energy from the plant cells themselves since the xylem elements are dead at maturity. The continuous transpiration pull, cohesion, and adhesion together facilitate the movement of water from the roots through the xylem to the leaves, enabling essential nutrient transport and photosynthesis.
