What is the process by which plants lose water vapor through their leaves?

The process by which plants lose water vapor through their leaves is called transpiration.

Water absorbed by roots travels upward through the xylem to the leaves. Inside a leaf, some of that water evaporates from moist cell surfaces and then exits through tiny pores called stomata. These pores are controlled by guard cells, which open and close them in response to light, water availability, carbon dioxide, humidity, and other conditions.

Transpiration is more than simple water loss. It helps cool plants, supports the upward movement of water and dissolved minerals, and contributes to the flow known as the transpiration stream. The process is largely passive: evaporation at the leaf creates a pull that helps draw water upward through the plant.

Transpiration is often confused with guttation. Guttation releases liquid water, usually in droplets at leaf edges, whereas transpiration releases water vapor. Evaporation can occur from many wet surfaces, but transpiration specifically refers to water movement and vapor loss from living plants. Desert plants reduce it with thick cuticles, small leaves, sunken stomata, or nighttime gas exchange.