Transpiration(redirected from cutaneous transpiration)
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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.
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P. A. GENKEL’