THE LOSS OF WATER BY THE PLANT 161 



In order to compare the results obtained by different experi- 

 ments, it is necessary to reduce them to a common standard unit. 

 Most frequently the total quantity of water lost by a plant during 

 a certain period is expressed per unit of its leaf surface. The 

 magnitude thus obtained is called the '"intensity of transpiration" 

 and is usually expressed in grams per hour per square meter of leaf 

 area. Sometimes, instead of the use of the surface which in highly 

 dissected leaves of irregular shape is difficult to determine, the 

 green, or even the dry weight of the leaves is used in calculation. 



In comparing the amount of water lost per unit time with the 

 total amount of water contained in the plant, the rate of expendi- 

 ture of the water reserve, expressed in percentage, is obtained. 

 Comparing the amount of water lost by the plant during a rather 

 long period, several weeks, or even the entire growth period, with 

 the amount of dry matter accumulated in this time, the transpira- 

 tion efficiency, expressed in grams per kilogram of water lost, is 

 obtained. The reverse value, the number of grams of water used 

 in accumulating 1 g. of dry substance, is termed the transpiration 

 coefficient. Some authors, chiefly American, call this value the 

 "water requirement of the plant." 



A picture of the intensity of transpiration is obtained by 

 comparing the rate of transpiration from a unit leaf surface with 

 the rate of evaporation from a unit area of a free water surface. 

 The magnitude obtained is called relative transpiration. It shows 

 to what extent the transpiration of a leaf is slower when compared 

 to "free evaporation." 



All these magnitudes vary greatly with different plants and 

 under the influence of different environmental conditions. In most 

 plants, the intensity of transpiration ranges from 15 to 250 gm. 

 per square meter per hour in daytime, and from 1 to 20 g. at night. 

 The rate of water expenditure may fluctuate from 10 to 80 per 

 cent; the efficiency of transpiration from 1 to 8; under the con- 

 ditions of a moderately moist climate it is frequently 3. The 

 transpiration coefficient correspondingly varies from 1,000 to 125. 

 Most often, it is 300. Relative transpiration is commonly expressed 

 by values of 0.1 to 0.5, sometimes almost attaining 1, and in other 

 cases falling to 0.01 or below, especially in plants well protected 

 from water loss. 



51. The Physical Nature of Transpiration. Its Dependence 

 on Environmental Conditions and Its Daily March. — Essentially, 



