24 PLANT PHYSIOLOGY 



balanced by turgor pressure. The other unbalanced part 

 represents the magnitude of the suction tension of the cell. 



This diagram clearly demonstrates that both magnitudes, 

 the osmotic pressure of the cell sap and the suction tension, 

 are not constant but change according to surrounding conditions 

 and the internal state of the plant. However, osmotic pressure 

 is considerably the more stable. While the suction tension falls 

 with transition from wilting to complete saturation from its 

 maximum to zero, the osmotic pressure decreases only 10 to 

 20 per cent. That is why, in characterizing the general prop- 

 erties of the cell, it is customary to use the average value of 

 the osmotic pressure, but for stating the condition of the cell at 

 some definite moment, especially in connection with rapid 

 changes of the environmental concUtions, a more precise picture 

 is obtained from the data of suction-tension determinations. 

 There will be occa'sion to return to this question in discussing 

 the water relations of plants. 



Methods of determining the suction tension of plant cells 

 and tissues are mostly based on the same principle of the selection 

 of equivalent concentrations of the external solution, which is 

 applied in the plasmolytic method of determining the osmotic 

 pressure. But in this case, a solution is found in which the vol- 

 ume of the immersed cell remains unaltered. Most of these 

 methods have been devised by the Swiss plant physiologist 

 Ursprung, who has contributed much during the last 20 years 

 to clarification of the question of the suction tension of plants. 

 The method most widely used for estimating the changes in the 

 volume of cells is the measurement of the changes in the length 

 of narrow elongated strips cut from plant organs such as leaves 

 and immersed in solutions of neutral and harmless substances, 

 mostly sucrose, at various concentrations. The osmotic pressure 

 of the solution in which the length of the strip remains unaltered 

 will be equal to the suction tension of the cells forming it. The 

 suction tension is usually expressed in atmospheres of pressure. 

 Changes in volume of separate cells are sometimes determined, 

 but this can be accomplished only by difficult measurements 

 under the microscope. Furthermore, the preparation of micro- 

 scopic sections involves unavoidable injury to a part of the cells, 

 and the sap escaping from them increases the values for the 

 suction tension of the adjoining cells used for the measurements. 



