146 THE OSMOTIC QUANTITIES OF PLANT CELLS 



attain a turgor pressure equal to their osmotic pressure. Cells low or entirely 

 lacking in turgor pressure are usually referred to as flaccid. 



Cell C will remain, according to the conditions prescribed for this ex- 

 periment, in a state of incipient plasmolysis. The diffusion pressure deficit of 

 the water in the cell sap and in the solution are equal {i.e. their osmotic pres- 

 sures are equal and neither is under any pressure, except, of course, atmos- 

 pheric pressure) hence a dynamic equilibrium will be established as soon as the 

 cell is immersed in the solution. Since there is no net movement of water 

 into the cell no turgor pressure and hence no wall pressure will develop, a 

 condition which will obtain in all plant cells in the condition of incipient 

 plasmolysis. 



The diffusion pressure deficit of the water in the cell sap of cell B is 

 10 atmos., that of the water in the surrounding solution 6 atmos. Hence 

 the diffusion pressure of the water in the solution is 4 atmos. greater than 

 that of the water in the cell sap, and there will be a net inward movement 

 of water into the cell. A turgor pressure thus develops within the cell, but 

 at its maximum under these conditions it will attain a value of only 4 atmos. 

 Counterbalancing this turgor pressure will be a wall pressure of 4 atmos. 

 Since the initial diffusion pressure deficit of the water in the cell sap was lO 

 atmos., imposition of a wall pressure of 4 atmos. reduces this to 6 atmos., 

 which is the diffusion pressure deficit (osmotic pressure) of the water in the 

 surrounding solution. Hence the attainment of a turgor pressure of only 4 

 atmos. results under these conditions in an osmotic equilibrium. Since the 

 magnitude of the turgor pressure developed in cell B will be less than in cell 

 A, the latter cell will be more completely distended than the former. 



In all three of these cells a dynamic equilibrium has been attained by an 

 adjustment of the diffusion pressure deficit of the water in the cell sap until 

 it is equal to that of the water in the surrounding liquid. This adjustment 

 was attained in each of these cells by a shift in the magnitude of the wall 

 pressure, which is one of the two components determining the diffusion pres- 

 sure deficit of the water in the cell sap, the other being the osmotic pressure. 

 Unlike a solution exposed to the atmosphere, the diffusion pressure deficit of a 

 cell is not equal to its osmotic pressure except when the cell is flaccid, i.e., 

 possesses a zero wall pressure. The term "diffusion pressure deficit of a cell" 

 should be considered as an abbreviation for "diffusion pressure deficit of the 

 water in the cell sap." 



The inter-relationships among the osmotic pressure, turgor pressure, wall 

 pressure, and diffusion pressure deficit of a plant cell should be further clari- 

 fied by a study of Fig. 32. This diagram also takes into account the influence 

 of volume changes in the cell upon these physical quantities. In the interests 



