Transpiration
Transpiration is the release of water vapor from plants, mainly through tiny pores in their leaves, returning water absorbed by roots to the atmosphere.
Definition
Transpiration is the release of water vapor from plants, mainly through stomata, tiny pores in leaves that open to let in carbon dioxide for photosynthesis, with the lost water replaced by more pulled up from the roots through the xylem. This movement is driven by a water potential gradient from soil, near $0$ MPa, through the plant to the atmosphere, often below $-50$ MPa, described by the cohesion-tension theory, in which evaporation from leaf cell walls creates tension that pulls a continuous column of water up the stem, held together by hydrogen bonding. Stomatal conductance is regulated by guard cells responding to light, $CO_2$, humidity, and drought signals, and transpiration accounts for roughly $60$ to $90\%$ of total evapotranspiration from land, moving about $60{,}000\ \text{km}^3$ of water to the atmosphere worldwide each year.
Example
Tying a clear plastic bag around a leafy branch on a sunny day shows droplets collecting inside within an hour, water that transpired straight out of the leaves, and a large oak tree can release over $150$ liters of water into the air on a hot day, which is one reason forests feel cooler and more humid than a parking lot. Eddy covariance towers over Duke Forest near Durham measure this directly and show mature pine stands returning most summer rainfall to the atmosphere, and the Amazon generates much of its own rainfall through transpiration, so deforestation there measurably reduces regional precipitation.
Key Insight
Plants do not transpire on purpose; it is simply the cost of keeping stomata open long enough to absorb carbon dioxide, so every pore that opens for $CO_2$ also leaks water, which is why plants in dry places evolve fewer or smaller pores. Rising atmospheric $CO_2$ now lets plants partially close their stomata while still capturing enough carbon, reducing transpiration and producing a physiological effect on the water cycle, more runoff and a drier lower atmosphere over some land regions, that is independent of temperature alone.