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Longman Dictionary of Contemporary English
▪ Apart from mysterious perfumes, the transpiration of this unusual liquid is the most frequently reported phenomenon.
▪ More than half of this disappears through evaporation or transpiration through plants.
▪ Once in the xylem, the salts are carried passively in the transpiration stream to the upper parts of the plant.
▪ Plant roots in the peats and estuarine sediments concentrate uranium in cell walls, especially in regions associated with transpiration processes.
▪ They have abandoned leaves to reduce loss of water by transpiration, and their stems are swollen with stored water.
The Collaborative International Dictionary

Transpiration \Tran`spi*ra"tion\, n. [F. transpiration.]

  1. (Physiol.) The act or process of transpiring or excreting in the form of vapor; exhalation, as through the skin or other membranes of the body; as, pulmonary transpiration, or the excretion of aqueous vapor from the lungs. Perspiration is a form of transpiration.

  2. (bot.) The evaporation of water, or exhalation of aqueous vapor, from cells and masses of tissue.

  3. (Physics) The passing of gases through fine tubes, porous substances, or the like; as, transpiration through membranes.

Douglas Harper's Etymology Dictionary

early 15c., from Medieval Latin transpirationem (nominative transpiratio), noun of action from transpirare (see transpire).


n. 1 (context botany English) The loss of water by evaporation in terrestrial plants, especially through the stomata; accompanied by a corresponding uptake from the roots. 2 (context physiology English) The process of giving off water vapour through the skin or mucous membranes. 3 The passage of gases through fine tubes.

  1. n. the passage of gases through fine tubes because of differences in pressure or temperature

  2. the process of givng off or exhaling water vapor through the skin or mucous membranes

  3. the emission of water vapor from the leaves of plants


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 remaining 97–99.5% is lost by transpiration and guttation. Leaf surfaces are dotted with pores 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. 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. Two major factors influence the rate of water flow from the soil to the roots: the hydraulic conductivity of the soil and the magnitude of the pressure gradient through the soil. Both of these factors influence the rate of bulk flow of water moving from the roots to the stomatal pores in the leaves via the xylem.

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. If the water potential in the ambient air is lower than the water potential in the leaf airspace of the stomatal pore, water vapor will travel down the gradient and move from the leaf airspace to the atmosphere. This movement lowers the water potential in the leaf airspace and causes evaporation from the mesophyll cell wall menisci of liquid water. This evaporation increases the tension on the menisci surface and increases its radius. Because of the cohesive properties of water, the tension travels through the leaf cells to the leaf and stem xylem where a momentary negative pressure is created as water is pulled up the xylem from the roots. 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.

The Cohesion-tension theory explains how leaves pull water through the xylem. Water molecules stick together, or exhibit cohesion. As a water molecule evaporates from the surface of the leaf, it pulls on the adjacent water molecule, creating a continuous flow of water through the plant.

Usage examples of "transpiration".

The dorsiventral thallus is constructed on the same plan throughout the group, and shows a lower region composed of cells containing little chlorophyll and an upper stratum specialized for assimilation and transpiration.

In the Marchantiales the chief supply is obtained from the soil by the rhizoids, and its loss in transpiration is regulated and controlled.

It is essential that the cuttings be placed in water as soon as they are removed or a bubble of air (embolism) may enter the cut end and block the transpiration stream in the cutting, causing it to wilt.