Transpiration

Transpiration is the loss of water vapor from a plant, chiefly through stomata in its leaves. It drives water and dissolved minerals upward from roots through the plant’s vascular tissue.

Stomata: Adjustable pores in plant surfaces that allow gas exchange and water vapor to escape. Most leaf water vapor exits through these pores, whose opening regulates transpiration.
Leaf: A plant organ specialized mainly for capturing light and exchanging gases. Its internal surfaces and outer pores provide the main route for water vapor loss.
Stomatal conductance: A measure of how readily gases pass through plant stomata. It quantifies the pore-mediated pathway that governs much transpiration.
Guttation: The exudation of liquid water droplets from leaf margins through hydathodes. Unlike transpiration, it releases water as liquid rather than vapor.
Guard cell: A specialized plant cell that changes shape to open or close a stoma. Guard cells control the pore openings that set much of a leaf’s water loss.
Mesophyll: The photosynthetic tissue between the upper and lower epidermis of a leaf. Water evaporates from moist mesophyll cell walls into the leaf’s air spaces.
Water-use efficiency: The amount of carbon gained or biomass produced per unit of water used. Plants balance carbon uptake against water lost through transpiration.
Hydathode: A leaf structure that releases liquid water, often at a margin or tip. Hydathodes provide the outlet for guttation, a distinct form of plant water loss.
Cohesion-tension theory: The explanation that evaporation from leaves pulls a continuous column of water upward through xylem. It explains how transpiration generates the force that lifts water from roots.
Relative humidity: The ratio of air’s water-vapor content to the maximum it can hold at that temperature. Dry air usually steepens the vapor gradient from leaf interior to atmosphere.

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