Transpiration is the loss of water from parts of the plant above the ground due to evaporation. This process is the equivalent to sweating in animals. Hence, it helps to cool down the plant. Transpiration takes place primarily through the stomata (pore on the underside of leaves). The rate of transpiration is affected by many factors such as: temperature, wind-intensity, humidity, and light-intensity. The rate of photosynthesis is measured using an instrument called a potometer.
Because water is constantly being taken from the top of the xylem vessels, the effective pressure at that point is reduced. This is known as the transpiration stream.