Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth and metabolism. The majority of water that a plant takes in is lost through transpiration, which also helps to regulate a plant's water status.
When plants take in water through their roots, the water moves up the stem to the leaves where it evaporates from tiny pores called stomata. The stomata are located on the underside of leaves and are usually more numerous on sun-exposed surfaces than on shady ones. Transpiration also cools plants by evaporative cooling; when water evaporates from the surface of leaves it removes heat energy from the leaves.
The rate of transpiration is affected by many environmental factors such as wind speed, humidity, air temperature and light intensity. Plants can also adjust their stomatal opening in response to changes in these conditions. For example, if conditions are dry then plants will close their stomata to reduce water loss; if conditions are hot then they may open their stomata more to increase evaporative cooling.