22 PLANT PHYSIOLOGY 



the wall equals the osmotic pressure, and as a consequence 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, 

 otherwise, P — T = 0. The water will neither enter the cell 

 nor escape from it, no matter how great the sap concentration 

 mthin 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 transpiration, 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; 

 i.e., P is always slightly greater than T. When such a cell is 

 immersed in water, this additional osmotic pressure induces the 

 entry of water into the cell and consequently leads to an increase 

 in volume. This enlargement continues until the increased 

 turgor pressure equals the magnitude of the osmotic pressure. 

 For land plants, therefore, the formula reads as follows : 



P = T,orP = T -\- S, 



or, finally, 



P - T = S. 



The value >S is called ''suction tension." It is this value, and 

 not the absolute magnitude of osmotic pressure, 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. 



It can be found by experiment that the suction tension of each 

 cell is not constant. It depends on the degree of saturation of a 

 cell ; for the more water it has lost in the process of transpiration, 

 the higher will be its suction tension. Thus the plant cell 

 possesses a self-regulating osmotic mechanism that absorbs 

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

 condition of complete wilting or of complete loss of turgor, the 

 suction tension reaches its maximum and equals the magnitude 



