i6o THE LOSS OF WATER FROM PLANTS 



become part of the liquid water. "High speed" molecules, on the other hand, 

 are much less likely to be captured by the attractive forces exerted by the 

 molecules at the surface of the body of liquid. They impinge upon the bound- 

 ing film of the liquid with such velocity that unless they hit that surface at 

 right angles or nearly so, they usually glance off along a new pathway. 



This picture of the kinetics of the evaporation process holds for any 

 evaporating surface whether it be the exposed surface of a body of water, 

 a moist piece of cloth, paper, or porous clay, or the mesophyll cell walls of 

 a leaf. Ordinarily the term evaporation is used to refer only to conditions 

 in which the rate of escape of molecules exceeds their rate of return, and its 

 use will be restricted to this sense in this discussion. 



If the air above the water surface is confined, as for example, when a dish 

 of water is covered by a bell jar, the number of water-vapor molecules in the 

 confined space will gradually increase due to evaporation from the surface 

 of the water. Since their movement is a random one the water-vapor mole- 

 cules will be continually colliding with walls of the container, each other, 

 the molecules of other gases present in the air, and surface of the liquid water. 

 Some of those which strike the surface of the liquid will be held there by 

 intermolecular attractive forces. As the concentration of v/ater-vapor mole- 

 cules in the air increases, the number which plunge back into the water in any 

 unit interval of time will increase. Eventually a dynamic equilibrium will 

 be attained at which the number of molecules leaving the surface and the 

 number returning to it in a unit time will be equal. At this point the air 

 will be saturated with water-vapor and evaporation will no longer be 

 occurring. 



The water-vapor molecules exert a definite pressure against the walls of 

 the container and the surface of the water. This is known as vapor pressure. 

 \n the illustration given in the preceding paragraph, the vapor pressure of 

 the water increases progressively until the saturation point is reached. The 

 vapor pressure of water under the conditions of such a dynamic equilibrium 

 may conveniently be termed the saturation vapor pressure. The vapor pres- 

 sure of a liquid is usually expressed in terms of millimeters of mercury. The 

 saturation vapor pressure of water at 20° C, for example, is 17.54 "im. Hg. 

 This means that at this temperature the water-vapor exerts a pressure equal 

 to the pressure exerted by a column of mercury 17.54 mm. high. The satura- 

 tion vapor pressure of any liquid is independent of the area of the evaporat- 

 ing surface, but increases with increase in temperature (Table 23). 



Botanists are often interested in measuring the rate of evaporation under 

 a given set of conditions in connection with studies of transpiration and of 

 plant distribution. The difficulties involved in making this apparently simple 



