WATER RELATIONS OF PLANTS 301 



night. The rate of water expenditure may fluctuate from 10 to 

 80 per cent; the efficiency of transpiration, from 1 to 8. Under 

 the conditions of a moderately moist cHmate, it is frequently 3. 

 The transpiration coefficient correspondingly varies from 1,000 

 to 125. Most often, it is 300. Relative transpiration is com- 

 monly expressed by values of 0.1 to 0.5, sometimes almost 

 attaining 1, and in other cases falhng to 0.01 or below, especially 

 in plants well protected from water loss. 



69. The Physical Aspects of Transpiration. Its Dependence 

 on Environmental Conditions and Its Daily Changes. — Essen- 

 tially, transpiration represents the physical process of evapora- 

 tion and as such may be expressed by Dalton's formula, 



V = K{F - /)^S, 



where K is the coefficient of diffusion or evaporation; F, the 

 saturation pressure of water vapor in air at the temperature of 

 the evaporating surface;/, the observed water vapor pressure in 

 the surrounding space; P, the barometric pressure; and S, the 

 area of the evaporating surface. From this formula, it follows 

 in the first place that transpiration is proportional to the satura- 

 tion deficit of the air, not to the relative humidity as is often 

 erroneously stated. Second, the formula shows that with a rise 

 in temperature the rate of transpiration rises ; for then the value 

 of F increases, and very rapidly. 



Evaporation from areas of small dimensions shows considerable 

 deviations from Dalton's rule. According to Stefan's law, 

 referred to in Art. 44, evaporation from a small surface is pro- 

 portional not to its area, but to its diameter. Other conditions 

 being equal, several smaU leaves lose^more water than a large 

 one of equal area. The greatest deviations from Dalton's law 

 may be observed when the air is perfectly still, which is rarely 

 reahzed under natural conditions. As soon as wind comes into 

 play, these differences become considerably smaller. In general, 

 wdnd markedly increases evaporation and consequently trans- 

 piration, by removing the moist air from the surface of the leaf 

 and replacing it with dryer air. Wind likewise decreases shghtly 

 the atmospheric pressure on the lee side of the leaves, thus draw- 

 ing the moist air from the intercellular spaces within the leaf. 



