DISCUSSION QUESTIONS i53 



condition will determinations of its osmotic pressure and its diffusion pressure 

 deficit result in the same values. 



All of the methods of measuring the diffusion pressure deficits of plant 

 cells are subject to most of the flaws which are also inherent in plasmoljtic 

 determinations of osmotic pressure, as well as some other serious errors 

 (Ernest, 1931, 1934a). Except possibly for a few types of cells which are 

 especially well adapted to such measurements, it is doubtful if the values ob- 

 tained in determinations of diffusion pressure deficits are ever more than fair 

 approximations of the true values which obtain in plant cells. 



Neither the turgor pressure nor the wall pressure of a cell can be deter- 

 mined directly. However, if the osmotic pressure of a cell or group of cells 

 has been determined, and likewise the diffusion pressure deficit, the turgor 

 pressure or wall pressure can be calculated from the equations given earlier 

 in the chapter. 



Discussion Questions 



Note: In all questions on diffusion pressure deficit ("suction tension") assume 

 that membranes are permeable to water only, and that no changes occur in the 

 osmotic pressure of the cells as a result of volume changes or other causes. 



1. Can a cell with an osmotic pressure of 10 atmos. ever obtain water from a 



cell with an osmotic pressure of 12 atmos.? Explain. 



2. Cell A has an osmotic pressure of I2 atmos. and is immersed in a solution 



with an osmotic pressure of 6 atmos. Cell B has an osmotic pressure of 

 10 atmos. and is immersed in a solution with an osmotic pressure of 



8 atmos. Assume both cells are first allowed to comiC to equilibrium 

 with the solution in which each is immersed, and that they are then re- 

 moved and brought into contact. Which direction will water move? Why? 



3. When placed in a solution of 6 atmos. osmotic pressure, cell A decreased in 



size, while cell B increased. Cell A just plasmolyzed in a solution of 

 12 atmos. osmotic pressure while B just plasmolyzed in a solution of 



9 atmos. osmotic pressure. Cell B regained its original volume in a solu- 

 tion of 8 atmos. osmotic pressure. If in their original condition the cells had 

 been placed side by side, which would gain water from the other? If placed 

 in distilled water while in their original condition which cell would gain 

 water more rapidly? If placed in a solution of 7 atmos. osmotic pressure 

 which way would water move with respect to each cell? 



4. Suppose you were assigned the problem of determining the osmotic pressure 



and diffusion pressure deficit of the leaves on an oak tree at different 

 distances from the ground. Describe exactly how you would proceed. 



5. A cell with an osmotic pressure of 12 atmos. manifests three-fourths of its 



maximum turgidity. What is the diffusion pressure deficit of the cell? 



6. Cells A, B, and C, having osmotic pressures of 6, 8, and 3 atmos. respectively, 



constitute a chain of three cells in the order named. A part of the lowest 

 cell, C, dips into a solution with an osmotic pressure of 2 atmos. None 

 of the other cells is in contact with the solution, which is large in volume 

 in comparison with the cells. Evaporation from the cells is prevented. 

 What will be the diffusion pressure deficit and turgor pressure of each 

 cell at equilibrium? 



