84 TRANSPIRATION AND ASCENT OF SAP ch. 



draws out thinner and thinner until it leaves no water 

 on the surface. 



Theory of a tensile sap imbibed in the cell 

 walls.- A modification of this theory, combining it with 

 the Unger-Sachs imbibition theory, then occurred to us. 

 In order to escape the inevitable thinning out of the 

 unsupported water films, we assumed with Sachs that the 

 moving water is located in the substance of the walls, and 

 that the surface-tension forces developed at the surface of 

 the fine-textured substance of the wall prevent the water 

 column from becoming indefinitely attenuated. Thus 

 the tension generated at the leaves is transmitted 

 downwards through the imbibed water in the walls. 

 This theory has undoubted advantages over the 

 imbibition hypothesis. It replaces the diffusion flow 

 by a movement under great tensions, and so the rate 

 of transmission may be increased proportionately to 

 the increased tension. But it is open to many of the 

 objections which overthrew the imbibition hypothesis, viz., 

 the lumina are known to transmit the major part of the 

 current, and it seems improbable, even where we can invoke 

 forces only limited by the tensile strength of water, that 

 they could suffice to drag an adequate water supply through 

 the fine-grained cell-walls. 



When we found ourselves compelled to give up these 

 hypotheses, the one as assuming conditions inimical to 

 the transmission of tension in the water, and the other 

 because it did not agree with the ascertained fact that the 

 water moved in the lumina, it was an easy transition to 

 arrive at the conclusion that the water passed up in the 

 lumina in a state of tension. How, in the lumina of the 

 conducting wood, the necessary conditions for the pro- 

 duction of tension are fulfilled, we shall now proceed to 

 inquire. 



Cohesion of water. Perhaps the easiest method of 

 realising the cohesion of water and the conditions 



