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in botany, the loss of water by evaporation in terrestrial plants. Some evaporation occurs directly through the exposed walls of surface cells, but the greatest amount takes place through the stomates, or intercellular spaces (see leafleaf,
chief food-manufacturing organ of a plant, a lateral outgrowth of the growing point of stem. The typical leaf consists of a stalk (the petiole) and a blade—the thin, flat, expanded portion (needlelike in most conifers) that is normally green in color because of the
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). Transpiration functions to effect the ascent of sapsap,
fluid in plants consisting of water and dissolved substances. Cell sap refers to this fluid present in the large vacuole, or cell cavity, that occupies most of the central portion of mature plant cells.
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 from the roots to the leaves (thus supplying the food-manufacturing cells with water needed for photosynthesis) and to provide the moisture necessary for the diffusion of carbon dioxide into and oxygen out of these cells. The rate of transpiration is almost always far greater than the above functions would seem to warrant; in most plants 200 to 1,000 lb (90–450 kg) of water are transpired for each pound of solid material added to the plant. Various factors influence the transpiration rate. Photosynthesis, induced by light, has the effect of increasing the water pressure in the guard cells that border each stomate and that, in expanding, pull apart to widen the stomate aperture and thereby increase water loss. Low humidity promotes the diffusion of water vapor from the air passages inside the leaf into the outside air. A lack of water in the soil cuts down the water supply to the cells, thus limiting expansion of the guard cells. Therefore the rate is highest on a bright, dry day and lowest at night or in drought conditions. Morphological factors such as reduced leaf surfaces, a heavy cuticle layer on the leaves, low numbers of stomates, and stomates recessed below the other epidermal cells also lower the rate; desert plants such as conifers and cacti conserve water in these ways. Plants also lose some water by guttation, a process whereby water is exuded directly through pores called hydathodes. The reaction of a plant to excessive water loss is wilting and, eventually, death.



the giving off of water into the surrounding atmosphere by plants. Transpiration in plants occurs according to the same laws as does evaporation from the surface of any moist body. However, the anatomical structure of the transpiring surface of plants experiences many changes during the process of evaporation. Thus, a purely physical process becomes a physiological process. The transpiring surface of plants is covered with a cuticle which is relatively waterproof. The cutinized surface is equipped with numerous stomata that open and close, thus regulating the water given off.



the exhalation of water vapor by a plant. The leaf is the principal organ of transpiration. The cells of its mesophyll constantly discharge water vapor into the intercellular spaces; the vapor is then discharged into the atmosphere through the stomata (stomatal transpiration) or through the cuticle (cuticular transpiration). In plants of the same species under similar conditions, the greater the leaf surface, the greater the amount of water vapor exhaled. Thus, 1 hectare (ha) of wheat plantings discharges about 2,000 tons of water, corn plantings 3,200 tons, and cabbage plantings 8,000 tons. Transpiration makes possible the flow of water and dissolved mineral salts (absorbed from the soil) through the plant. It prevents overheating of the leaves and maintains the leaf tissues in a state of undersaturation, thereby keeping the absorption power of the cells at a certain level.

The magnitude of transpiration depends on the number, distribution, and openness of the stomata. It also depends on the structure of the epidermis, the development of the conducting system, and the osmotic pressure of the cellular fluid. Also influential are the degree to which the protoplasm is saturated with water, the intensity of illumination, the temperature, the air humidity, the wind force, and amount of nitrogen and other nutrients in the soil.

The magnitude of transpiration may be expressed in several ways. The amount of water, in grams, transpired by the plant in one hour is calculated for a unit of weight of the plant, most often of the leaves. It is expressed as gm2/hr (sometimes the calculation is made for 1 gr of raw weight per hr). In determining the absolute magnitude of transpiration, the area of the leaf surface of plants per sq m of area is calculated. The ratio of the amount of water transpired from a unit of surface to the unit of free water surface is called the relative transpiration; under optimum water-supply conditions it equals 0.7–0.85.

The amount of water used by a plant during the entire vegetative period is related to the dry mass of the plant (transpiration coefficient). An important index of transpiration is the transpiration productivity—the reciprocal of the transpiration coefficient—which indicates the amount of dry matter formed in the plant with the use of a certain amount of water.


Timiriazev, K. A. Zemledelie ifiziologiia rastenii. Moscow, 1957.
Maksimov, N. A. Izbrannye raboty po zasukhoustoichivosti i zimostoikosti rastenii, vol. 1. Moscow, 1952.
Crafts, A., et al. Voda i ee znachenie v zhizni rastenii. Moscow, 1951. (Translated from English.)
Transpiratsiia i ee znachenie v zhizni rastenii (bibliographical index). Leningrad, 1962.
Slatyer, R. Vodnyi rezhim rastenii. Moscow, 1970. (Translated from English.)
Rubin, B. A. Kursfiziologii rastenii, 3rd ed. Moscow, 1971.
Genkel’, P. A. Fiziologiia rastenii, 4th ed. Moscow, 1975.



The passage of a gas or liquid (in the form of vapor) through the skin, a membrane, or other tissue.