126 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



In root-pressure the water driven up at the cut 

 stock contains a smaller percentage of soluble sub- 

 stances than does the sap in the living cells : this 

 means that energy has been employed in separating 

 the very dilute solution in the vessels. Moreover, 

 energy is necessary to overcome the resistance to 

 filtration of the cell-membranes, and this often under 

 the pressure of a column of water. Still more will 

 energy be employed if the cells draw their supply of 

 water from vessels, and drive water into vessels again 

 at a higher level. 



Hence no mere mechanism of turgescence by 

 osmosis will account for the phenomena, unless we 

 can show that the sap in the cells becomes more 

 concentrated each time, and denser as we ascend the 

 tree : this is not the case. 



We must call in the supply of energy set free by 

 the respiration of the protoplasm, as well as tur- 

 gescence, as furnishing the forces which overcome 

 resistance to filtration, and which separate water again 

 from the dense sap in the cell. In the turgescence of 

 a cell we have, first, water being absorbed owing to 

 the attraction for it exercised by substances in the 

 sap-vacuole ; this continues until the tension of the 

 elastic cell wall and the hydrostatic pressure exerted 

 by the water inside are equal, when the cell is turgid. 



