Transpiration is the evaporation of water / loss of water from plants, especially leaves. It is a type of translocation. The amount of water lost by a plant depends on its size, the surrounding light intensity, temperature, humidity, wind speed, and soil water supply.
It happens because the plant opens its pores in order to get carbon dioxide gas from the air for photosynthesis. Transpiration also cools plants and enables the flow of mineral nutrients from roots to shoots. This mass flow is caused by the low (hydrostatic) water pressure in the upper parts of the plants.
The reason for this low pressure is that water moves out of the leaves into the atmosphere. The low pressure exerts a pull on the water column which brings the water up against gravity. Water gets into the plant at the roots by osmosis, and it transports dissolved mineral nutrients to the upper parts of the plant through the xylem.
Leaf surfaces are dotted with openings called stomata. In most plants they are more numerous on the undersides of the leaves. The stoma are bordered by guard cells that open and close the pore. Transpiration also cools plants and enables mass flow of mineral nutrients and water from roots to shoots.
A fully grown tree may lose several hundred gallons of water through its leaves on a hot, dry day. About 90% of the water that enters a plant's roots is used for this process. The transpiration ratio is the ratio of the mass of water transpired to the mass of dry matter produced; the transpiration ratio of crops tends to fall between 200 and 1000 (i.e., crop plants transpire 200 to 1000 kg of water for every kg of dry matter produced).
Transpiration rate of plants can be measured by a number of techniques, including potometers, lysimeters, porometers, photosynthesis systems and heat balance sap flow gauges.
Desert plants and conifers have specially adapted structures, such as thick cuticles, reduced leaf areas, sunken stomata and hairs to reduce transpiration and conserve water. Many cacti conduct photosynthesis in succulent stems, rather than leaves, so the surface area of the shoot is very low. Many desert plants have a special type of photosynthesis in which the stomata are closed during the day and open at night when transpiration will be lower.