Water is pulled up through the xylem to the leaves primarily by the process called transpirational pull. This occurs when water evaporates from the surfaces of mesophyll cells inside the leaves, creating a negative pressure (tension) at the top of the plant. The evaporation causes water to form concave menisci in the pores of the cell walls, and due to water's high surface tension, this generates a pulling force that draws water upward through the xylem vessels from the roots to the leaves
. This upward movement is facilitated by two key properties of water:
- Cohesion: Water molecules stick to each other through hydrogen bonding, maintaining an unbroken column of water within the xylem.
- Adhesion: Water molecules also adhere to the walls of the xylem vessels, helping resist gravity and supporting capillary action
As water exits the leaf during transpiration, the hydrogen bonding between water molecules pulls more water molecules up from below, effectively creating a continuous flow of water from roots to leaves
. In summary, water is pulled up through the xylem by a combination of negative pressure generated by transpiration at the leaf surface and the cohesive and adhesive properties of water molecules, which together enable the water column to be lifted against gravity through the plant. This mechanism is passive and does not require energy expenditure by the plant cells themselves
