The New International Encyclopædia/Transpiration

TRANSPIRATION (from Lat. trans, across, through + spirare, to breathe). The evaporation 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 prevents the complete stoppage of evaporation when the plant is surrounded with a saturated atmosphere. In most land plants, transpiration from the epidermal cells is inconsiderable because such become nearly waterproof by cutinization 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, temperature, wind, light, etc. Transpiration is unavoidable, 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 Conduction.

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.) The amount of water transpired varies greatly on account of variation in external conditions and internal structure. A few examples 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 hop 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 be caught and condensed on the same area it would reach in the first case a depth of nearly 118 millimeters (4⅝ inches), and in the second 228 millimeters (9 inches). A beech tree having 200,000 leaves was estimated to lose between 300 and 400 liters (about 2 barrels) on a hot day.