Transpiration and the Ascent of Sap. 



51 



will be sufficient to cause condensation to take place on the surface 

 of the lower pot while evaporation proceeds from the surface of the 

 upper one. Motion of the water upwards in the 

 tube may be demonstrated b}^ the introduction 

 of a mercurial index into the tube. If eva- 

 poration from the upper pot eliminates more 

 water than condensation on the lower pot 

 supplies, and if the liquid in the apparatus 

 is in a state capable of standing- tension, the 

 stress developed by the reduction of the volume 

 of water will drag in the menisci in the 

 interstices of the walls of both porous pots and 

 make them more concave. This will have a two- 

 fold effect. The rendering- of the upper menisci 

 more concave tends to bring- them into equi- 

 librium with a lower vapour pressure and 

 consequently reduces evaporation, while the 

 increased concavity of the lower menisci for 

 the same reason renders them more ready to 

 condense water vapour. 



When this model has been in action for 

 some time it will be noticed that the lower pot 

 is at a hig-her temperature than the upper one. 

 This difference of temperature is to be attributed 

 to the 'sorting demon' action progressing at the 

 two surfaces. The lower menisci form a trap 

 for more energetic water vapour molecules in 

 the soil spaces, the upper retain the less 

 energetic while those which are more energetic 

 escape into the surrounding space. Hence there 

 is a gain of heat to the lower menisci and a 

 loss of heat by molecular convection from the 

 upper menisci. This cooling of the upper menisci 

 maintains a constant flow of heat into the eva- 

 porating surfaces, which is constantly being 

 abstracted again by the escaping molecules. As 

 they escape fresh ones are drawn into their 

 places from the water beneath by the attractions 

 of those remaining in the meniscus, and these 

 mutual attractions find expression in the tensile 

 strength of the liquid which joins the whole 

 column to the evaporating menisci. Thus the 

 loss of molecules from the menisci, kept up by the inflow of heat^ 

 is able, by calling into play the mutual attractions of the water 



4* 



Fig. 6. Model repre- 

 senting the energy re- 

 lations of the plant to 

 its surroundings, two 

 porous vessels connected 

 by a glass tube. The 

 lower vessel represent- 

 ing the root is pluuged 

 in damp earth, the upper 

 one is exposed to the air. 

 The difference in the 

 state of saturation of 

 the surroundings of each 

 causes evaporation fiom 

 the surface of the upper 

 vessel and condensation 

 on the lower. 



