TRANSPIRATION (from Lat. trans, across, through spirare, to breathe). The tion of water from the aerial surfaces of plants. The term is used instead of evaporation, first, because the evaporation is modified by the action of the living organisms; and second, because the development of heat within this organism pre vents the complete stoppage of evaporation wben the plant is surrounded with a saturated atmos phere. In most land plants, transpiration from the epidermal cells is inconsiderable because such become nearly waterproof by eutinization of the outer walls. The cells bordering the interior air chambers (see AERATION) are saturated with water, and from them it evaporates readily into the air occupying these spaces, whence it finds its way by diffusion through the stomata (q.v.). The rate of transpiration will be determined by the relative humidity of the outer air, tempera ture, wind, light, etc. Transpiration is unavoid able, because gases must be absorbed from the air (see ABSORPTION) ; it is necessary, at least to some plants, for cooling; and it is advantageous for the movement of dissolved salts. See CON DUCTION.
Since excessive loss of water is one of the greatest dangers to which plants are subject, plants growing in dry regions show a great variety of adaptations to reduce the rate of transpiration and to conserve the moisture which reaches them. (See XEROPHYTES.) Tile amount
of water transpired varies greatly on account of variation in external conditions and internal structure. A few ekamples of transpiration under normal conditions will illustrate. In 12 hours on a hot, dry day a sunflower 3.5 feet high, having a leaf area of 5316 square inches, lost 30 ounces of water. The loss of water from 100 square centimeters of leaf surface in 24 hours for the pea was 2.51 grams, for the bop 4.3 grams, for the hemp 9.3 grams. Estimates as to the loss of water during the growing season by the plants of wheat grown on one hectare (about 2.5 acres) equal 1,179.920 liters; and by oats 2, 277,760 liters. If all this water could he caught and condensed ou the same area it would reach in the first case a depth of nearly 118 millimeters inches), and in the second 228 millimeters (9 inches). A heed) tree having 200,000 leaves was estimated to lose between 300 and 400 liters (about 2 barrels) on a hot day.