Xylem and Transpiration

Xylem conducts water and minerals from the roots to the top of the plant. It also mechanically supports the plant. Its water-conducting cells (vessel members and tracheids) connect as hollow pipelines. Water flows from one cell to another through pits in their lignified walls. The tracheids and vessel members of the xylem are dead at maturity and therefore, are not actively pulling the water to the top of the plant.

Water moves from roots to stems and then to leaves. Most absorbed water is lost to evaporation through stomata in leaves. Evaporation of water from plant parts is called transpiration.

The cohesion theory of water transport explains water movement in plants. Transpiration creates negative tensions in the xylem. Tensions extend downward from leaves to roots. Hydrogen-bonded water molecules are pulled upward through the xylem as continuous columns. Hydrogen bonds attract the hydrogen of one water molecule to the -OH group of another. Hydrogen bonds make water cohesive. Water molecules stick together inside the narrow xylem walls as the molecules are pulled upward. Water responds to solute concentrations. It moves osmotically into plant cells. When water loss is balanced by osmotically induced movement inward, the plant is erect. If water concentration of soil drops, inward movement stops, and the plant wilts.

The drying power of air causes transpiration. Transpiration puts the water in xylem in a state of tension from leaves to stems to roots. As long as water molecules escape from the plant, molecules are pulled up to replace them. Columns of water are pulled by the hydrogen bonds between water molecules that are confined to the tubular xylem cells. Hydrogen bonds break when water escapes from the leaves.

This animation (Audio - Important) describes the cohesion transport theory.

REVIEW: Cells that are the main water-conducting cells of a plant are

REVIEW: Which theory of water transport states that hydrogen bonding allows water molecules to maintain a continuous fluid column as water is pulled from roots to leaves?