7.2.3—Transpiration from leaves

Transpiration is the loss of water vapour from a plant, mainly through stomata in the leaves. Water evaporates from moist mesophyll cell walls into the leaf air spaces, then the vapour diffuses out through open stomata.

• Leaf interface: Liquid water arrives in the leaf through xylem. It evaporates from mesophyll cell walls into the internal air spaces, creating water vapour there.
• Exit route: When stomata are open, water vapour diffuses from the humid internal air spaces to the atmosphere. Guard cells regulate the stomatal opening; loss of guard-cell water can close the stomata and reduce both transpiration and gaseous exchange.
• Supported influences: Lower external humidity and higher temperature can increase the water-vapour gradient between the leaf air spaces and the atmosphere. Stomatal state can override a simple factor prediction, so these are not guaranteed monotonic rate rules.
• Plant relevance: Evaporation provides evaporative cooling, and the associated transpiration stream helps mineral-ion uptake. The water-vapour loss itself is not the same thing as liquid-water movement through xylem.
• Process boundary: Transpiration is the vapour-loss process; the later water-pull objective explains how evaporation contributes to xylem transport without making transpiration alone equal to the whole cohesion-tension mechanism.

Path cue: mesophyll cell wall → leaf air space → stomatal pore → atmosphere.

Do not call transpiration photosynthesis, guttation or the whole xylem pathway. The observable process defined here is evaporation at mesophyll surfaces followed by vapour diffusion through stomata. Staff-only visual brief: show mesophyll wall → leaf air space → stomatal pore → atmosphere, with guard-cell control and a separate liquid-xylem arrow; do not generate or bind an image.