Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as from leaves but also from stems andflowers. Leaf surfaces are dotted with pores which are called stomata, and in most plants they are more numerous on the undersides of the foliage. The stomata are bordered by guard cells and their stomatal accessory cells (together known as stomatal complex) that open and close the pore.^[1]Transpiration occurs through the stomatal apertures, and can be thought of as a necessary "cost" associated with the opening of the stomata to allow the diffusion of carbon dioxide gas from the air for photosynthesis. Transpiration also cools plants, changes osmotic pressure of cells, and enables mass flow of mineral nutrients and water from roots to shoots.

Mass flow of liquid water from the roots to the leaves is driven in part by capillary action, but primarily driven by water potential differences. In taller plants and trees, the force of gravity can only be overcome by the decrease in hydrostatic (water) pressure in the upper parts of the plants due to the diffusion of water out of stomata into the atmosphere. Water is absorbed at the roots by osmosis, and any dissolved mineral nutrients travel with it through the xylem.

Regulation

Plants regulate the rate of transpiration by the degree of stomatal opening. The rate of transpiration is also influenced by the evaporative demand of the atmosphere surrounding the leaf such as humidity, temperature, wind and incident sunlight. Soil water supply and soil temperature can influence stomatal opening, and thus transpiration rate. The amount of water lost by a plant also depends on its size and the amount of water absorbed at the roots. Transpiration accounts for most of the water loss by a plant, but some direct evaporation also takes place through the cuticle[citation needed] of the leaves and young stems. Transpiration serves to evaporatively cool plants as the escaping water vapor carries away heat energy.

During a growing season, a leaf will transpire many times more water than its own weight. An acre of corn gives off about 3,000-4,000 gallons (11,400-15,100 liters) of water each day, and a large oak tree can transpire 40,000 gallons (151,000 liters) per year. 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 rates of plants can be measured by a number of techniques, including potometers, lysimeters, porometers, photosynthesis systems and heat balance sap flow gauges. Isotope measurements indicate transpiration is the larger component of evapotranspiration.[3]

Desert plants 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, termed crassulacean acid metabolism or CAM photosynthesis, in which the stomata are closed during the day and open at night when transpiration will be lower.