154 THE OSMOTIC QUANTITIES OF PLANT CELLS 



7. Suppose that evaporation was occurring from the surface of cell A (ques- 



tion 6). How would your answer to the question differ? 



8. If all three of the cells (question 6) were completely immersed in the solu- 



tion, what would be the diffusion pressure deficit and turgor pressure of 

 each at equilibrium? 



9. If a small block of tissue cut from a potato tuber or any other bulky tissue 



be immersed in a solution with an osmotic pressure of 8 atmos. what will 

 be the diffusion pressure deficit of the cells in the tissue after a dynamic 

 equilibrium has been established? Any exceptions? 



10. A chain of cells, each of which has an osmotic pressure of 8 atmos. is 



arranged so that one terminal cell dips in a solution with an osmotic pres- 

 sure of 3 atmos. and the other in a solution with an osmotic pressure of 

 6 atmos. The volume of both of these solutions is very large in comparison 

 to the size of the cells. Evaporation is prevented. Will any movement 

 of water occur? Explain. 



11. What effect will a change of starch to sugar in a cell have upon its diffusion 



pressure deficit? an increase in the permeability of the cell membranes to 

 solutes? to water? 



12. A cell has an osmotic pressure of 15 atmos. and water evaporates from it 



until the cell walls are pulled inwards enough that the enclosed water is 

 subjected to a tension of 12 atmos. What is the diffusion pressure deficit 

 and wall pressure of the cell? 



13. Measurements of the diffusion pressure deficit of the epidermis of a leaf 



sometimes show it to be lower than the diffusion pressure deficit of 

 adjacent mesophyll cells from which it must obtain water. What is a 

 possible explanation of this finding? 



14. The cells of a parasitic fungus were found to have a lower osmotic pres- 



sure than the cells of the host plant upon which it was growing. How 

 could the fungus obtain water from the host? 



15. Pollen grains of cotton germinate readily upon the stigma but burst rapidly 



if floated upon water or a dilute sugar solution. Explain. 



Suggested for Collateral Reading 



Harris, J. A. The physico-chemical properties of plant saps in relation to 



phytogcography. Univ. ^Minnesota Press. Minneapolis. I934- 

 Walter, H. Die Hydratur dcr Pflanze. Gustav Fischer. Jena. 1 93 1. 



Selected Bibliography 



Beck, W. A. Osmotic pressure, os?notic value, and suction tension. Plant 

 Physiol. 3: 413-440. 1928. 



Bennet-Clark, T. A., A. D. Greenwood, and J. W. Barker. Water rela- 

 tions and osmotic pressures of plant cells. New Phytol. 35: 277-291. 



1936. 

 Buhmann, A. Kritische Untersuchungcn ilher Vergleichende Plasmolytische 



und Kryoscopische Bestimmungcn des Osmotischen Wertes bei Pflanzen. 



Protoplasma 23: 579-612. 1935. 

 DeVries, H. Eine Methode zur Analyze dcr Turgorkraft. Jahrb. Wiss. 



Bot. 14: 427-601. 1884. 



