THE ASCENT OF SAP 193 



activities, if attended by no secondary changes in the 

 conducting tracts or transpiring leaves, will arrest the 

 transpiration stream. 



" While thus structural and physiological evidence 

 prevent us from accepting any of the previous physical or 

 vital theories, the same configuration, physical properties, 

 and structure of the wood compel us to admit that the 

 water in the conducting tracts, when not acted upon by 

 a vis a tergo, must pass into a state of tension. This 

 state is necessitated by the physical properties of water 

 when contained in a completely wetted, rigid, and permeable 

 substance which is divided into compartments. There- 

 fore when root pressure is not acting and when leaves 

 of trees are transpiring, the cohesion of their sap explains 

 fully the transmission of the tension downwards, and 

 consequently explains the rise of sap. 



" Resistance to a current of water moving through 

 wood at the velocity of the transpiration stream is 

 approximately equivalent to a head of water equal in 

 length to the wood traversed. Hence the tension applied 

 to the upper end of the water columns, which will be 

 able to raise the transpiration stream in a tree, must 

 equal the pressure produced by a head of water twice 

 the height of the tree. In a tree 100 m. high, therefore, 

 tension of 20 atm. must be produced. 



" The cohesion of sap amounting, as it does, to at 

 least 200 atm. is in no way taxed by this tension. 



" The transpiring cells of the mesophyll normally 

 remain turgid during transpiration ; accordingly we 

 would expect, if our line of reasoning is correct, that in 

 high trees the osmotic pressure keeping them distended 

 must correspond in magnitude to the tension necessary 

 to raise the sap. 



" This surmise has been confirmed by determinations 

 of the osmotic pressures of the saps of various leaves. 

 These pressures have always been found adequate to 

 resist the transpiration tension ; but in many cases other 



13 



