r ..(... 



204 TRANSPIRATION AND ASCENT OF SAP ch. 



water. The tension is transmitted downwards through 

 the roots to the absorbing cells. In these cells the actions 

 which occur must be the converse of those occurring in 

 the mesophyll. At the root the entry of water depends 

 on the gradient of pressure on passing from the outside 

 of the root to the inside of the tracheaa. The fall of pressure 

 due to the tension in the water is continuous all the way 

 up the stem to the leaf. Thus we may regard the flow of 

 water up the highest tree as due to the evaporation and 

 condensation produced by the difference between the 

 vapour pressure in the soil spaces and that obtaining round 

 the leaves. The column of tensile water flows 

 under the action of this difference from end 

 to end of the plant. 



Model. The relations in these respects 

 of the leaves to the roots may be illustrated 

 by two porous pots connected hermetically 

 by a glass tube about a metre long, the 

 pots and the tube being completely filled with 

 water (Fig. 29). If one is immersed in damp 

 earth and the other supported above it, the 

 difference in the state of saturation of the 

 spaces surrounding each will be sufficient to 

 cause condensation to take place on the 

 surface of the lower pot and evaporation 

 to proceed from the surface of the upper one. 

 Motion of the water upwards may be demon- 

 strated by the introduction of a mercurial 

 index into the tube. If evaporation 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 



Fi<;. 29. 



