Tran′spira′tion (in plants), the loss of water by evaporation from a plant. Since 50 to 98 per cent. of the plant-body is water, it loses water whenever the air contains less moisture in proportion to its capacity than the plant. Transpiration therefore is unavoidable under the conditions of most land-plants. To reduce this unavoidable loss, the larger land-plants make the outer tissues of the aërial parts nearly waterproof. But the air-passages (see Aëration) permit evaporation from the cells bounding them; hence the moisture diffuses through the stomata (q. v.) to the outer air. Were evaporation prevented by waterproofing these cells also, the absorption of gases for food-making and respiration would be nearly impossible. The amount of transpiration varies greatly in different plants according to their structure and contents; in the same plant it varies with age and the varying external conditions, as temperature, light, moisture etc. Therefore, only a very general idea can be given of the amount of water transpired. On a hot summer-day a plant with five square meters (about six square yards) of foliage (such as a thrifty sunflower as high as a man might have) will evaporate about 1.2 liters (2.5 pints) of water. A birch 40 feet high, with perhaps 200,000 leaves, will give off on a hot day 300 to 400 liters of water (about 2.5 barrels). But these rates would not be kept up as an average through the growing-season. The evaporation from a leaf-surface is 2 to 7 times less than that from an equal area of water.