This is the discharge of water vapor from the leaves of plants into the atmosphere. It is a process that the eye cannot see, even though the amounts of moisture involved it significant. It is believed that a large oak tree can transpire 40,000 gallons (151,000 liters) per year.
Transpiration is also the reason why there is higher humidity in places with lots of vegetation cover.
During transpiration, water absorbed from the soil by the roots of the plant is sent up into the leaves. There, it is released through small pores underside of the leaves. The amount of water that is transpired through this process depends on the plant itself, the moisture in the ground (soil), the surrounding temperature and the wind movement around the plant.
The term ‘evapotranspiration’ is used when there is also evaporation of moisture from the soil, together with that from the leaves of the plants.
Evapotranspiration accounts for about 10% of vapor in the atmosphere.