162 TEXTBOOK OF PLANT PHYSIOLOGY 



transpiration represents the physical process of evaporation and as 

 such may be expressed by Dalton's formula. 



760 

 V = K(F - f)—S, 



where K is the coefficient of diffusion or evaporation, F the satura- 

 tion pressure of water vapor in air at the temperature of the evap- 

 orating surface, / the observed vapor pressure in the surrounding 

 space, P the barometric pressure, and £ the area of the evaporating 

 surface. From this formula it follows, in the first place, that 

 transpiration is proportional to the saturation deficit of the air; 

 not to the relative moisture, as is often erroneously considered. 

 Second, the formula shows that with a rise in temperature the rate 

 of transpiration goes up, for then the value of F increases very 

 rapidly. 



Evaporation from areas of small dimensions shows considerable 

 deviations from Dalton's rule. According to Stephan's law, referred 

 to in Art. 11, evaporation from a small surface is proportional not 

 to its area, but to its diameter. Other conditions being equal, 

 several small 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 realized under natural 

 conditions. As soon as wind comes into play, these differences 

 become considerably smaller. In general, wind markedly increases 

 evaporation and thus transpiration by drawing the moist air from 

 the surface of the leaf and as a consequence from the intercellular 

 spaces within the leaf. 



Light has a very important function in the process of trans- 

 piration. Since chlorophyll absorbs easily radiant energy, the 

 temperature of the leaf is increased perceptibly in light. This 

 immediately increases the difference between the factors F and / 

 and consequently intensifies transpiration, which in turn lowers the 

 temperature of the leaf; hence, plants that have a high power of 

 evaporation are hardly heated at all in light, but their transpira- 

 tion increases considerably. It was stated in the chapter on assim- 

 ilation (Art, 10) that usually only 1 to 3 per cent of the radiant 

 energy absorbed by the plant is used in photosynthesis, while over 

 80 per cent is spent in transpiration. Even diffuse light increases 

 transpiration by 30 to 40 per cent; while in direct sunlight it may 



