OSMOSIS AND IMBIBITION 241 



3. Under what conditions will a dissolved substance pass through a mem- 

 brane without any movement of water taking place? 



4. Under what conditions will a dissolved substance pass through a mem- 

 brane in one direction and water in another? 



5. Which has the higher osmotic pressure, — a parasite or its host? Why? 



6. Why do fruits sometimes split open in very wet seasons? 



7. In plasmolysis, is the cytoplasm pushed or pulled from the cell wall? 



8. A plant grown in water-culture solutions will remove most of the salts 

 even when its roots fill only a small part of the bottle. Why? 



9. Why does a concentrated spraying mixture sometimes kill the leaves 

 of a plant? 



10. Why are salt solutions commonly used on gravel walks to kill weeds? 



11. Explain the action of salt and sugar as a preservative of meats and 

 vegetables. 



12. What are zoospores and how do they escape from the zoosporangium? 



13. What would you expect the isosmotic coefficient of magnesium sulphate 

 to be? 



14. Why does rain at pollination time commonly result in a small set of fruit? 



15. Seeds generally have an osmotic pressure of 20-45 atmospheres. What 

 "survival value" have these high pressures? 



16. Would seed germinate more or less readily in a salt solution? 



17. Epiphytes of the tropical rain forests often have a very low osmotic 

 pressure (3-6 atmospheres) in their cells. Explain. 



18. Harris and Lee (1930) found that the drouth resistance of sugar cane 

 varieties in Hawaii was directly proportional to their osmotic pressure. Ex- 

 plain. 



REFERENCES 



Beck, W. A. — Osmotic pressure, osmotic value, and suction tension. Plant 



Physiol, 3:413, 1928. 

 Gail, F. W. — Osmotic pressure of cell sap and its possible relation to winter 



killing and leaf fall. Bot. Gaz., 81 :434, 1926. 

 Harris, J. A. — Osmotic concentration of desert Loranthacese. Mem. Torr. 



Bot. Club, 17:307, 1918. 

 Shull, C. A. — Imbibition in relation to absorption and transportation of 



water in plants. Ecology, 5:230, 1924. 

 Stiles, W. — Osmotic pressure in plants. Sci. Prog., 16:554, 1922. 

 Thoday, D. — Turgescence and absorption. New Phyt., 17:108, 1918. 

 Wiggins, R. — Variation in the osmotic concentration of the guard cells during 



the opening and closing of stomata. Am. J. Bot., 8:30, 1921. 



