134 TEXTBOOK OF PLANT PHYSIOLOGY 



the osmotic pressure and tending to counterbalance it. With 

 further increase of volume, the pressure of the cell wall on the cell 

 contents is augmented. Finally, the moment comes when the 

 pressure of the wall equals the osmotic pressure and as a conse- 

 quence a further increase in volume ceases. This condition of 

 equilibrium may be called the state of complete saturation of the 

 cell with water. If P denotes the osmotic pressure of the cell sap, 

 and T the pressure of the wall on the cell's contents, as a result 

 of its strained condition which is called ''turgor pressure," it will 

 be in the state of complete saturation with water, P = T, or, other- 

 wise, P — T = 0. The water will neither enter the cell nor escape 

 from it, no matter how great the sap concentration within the cell 

 might be. This condition of perfect saturation is natural to the 

 cells of submerged plants, but it is hardly ever found in land 

 plants. Because of the loss of water in the process of transpira- 

 tion, their cells are never quite fully saturated with water and, 

 hence, do not attain perfect turgidity. Part of their osmotic 

 pressure remains unbalanced by the counterpressure of the cell 

 wall. When such a cell is immersed in water, this additional 

 internal pressure induces the entry of water into the cell, and con- 

 sequently leads to an increase in volume. This enlargement con- 

 tinues until the increased turgor pressure equals the magnitude of 

 the osmotic pressure. For land plants, therefore, our formula 



reads as follows : 



P> T or P = T + S 



or, finally, 



P-T = S. 



The value S has been called by Ursprung " suction tension." 

 It is this value, and not the absolute magnitude of osmotic pres- 

 sure, that determines the absorption of water by the cell. As 

 may be seen from the above formula, this value is the difference 

 between the full osmotic pressure of the cell sap and the turgor 

 pressure of the distended cell wall. 



Not every cell has the same suction tension because this 

 depends on its water saturation. The lower the state of satura- 

 tion of a cell or the more water it has lost in the process of trans- 

 piration, the higher will be its suction tension. Thus the plant 

 cell possesses a self-regulating osmotic mechanism which absorbs 

 water the more vigorously, the more it is in need of it. In the 



