58 BOTANY: PRINCIPLES AND PROBLEMS 



153. If this same sac is filled with water, tied up and placed in a 

 vessel of molasses, what will happen? 



154. Under what conditions will water pass through a membrane 

 osmotically without any movement of dissolved substances taking place? 

 When would this be likely to happen in the root-hairs of plants? 

 Explain. 



155. Under what conditions will a dissolved substance pass through 

 a membrane without any movement of water taking place? When 

 would this be likely to happen in the root-hairs of plants? Explain. 



156. Under what conditions will water pass through a membrane in 

 one direction and a dissolved substance in the other? When would 

 this be likely to happen in the root-hairs of plants? Explain. 



Note. — Assume in the five following questions that the sac is made of 

 an elastic membrane which is semi-permeable in that it allows the free 

 diffusion of water through it but entirely prevents the passage of sugar. 

 Assume further that this membrane is readily permeable to dissolved 

 salt. Under the conditions set forth in each question, state clearly what 

 will happen, carrying the process through to its conclusion and noting 

 the differences, if any, between early and later stages. 



157. What will happen if the sac is filled with a solution of sugar and 

 placed in a vessel of pure water? 



158. What will happen if the sac is filled with a solution of sugar and 

 the liquid in the vessel is a solution of salt which is of lesser concentration 

 than the sugar solution in the sac? 



159. What will happen if the sac is filled with a solution of sugar and 

 the liquid in the vessel is a solution of salt which is equal in concentra- 

 tion to the sugar solution in the sac? 



160. What will happen if the sac is filled with a solution of sugar and 

 the liquid in the vessel is a solution of salt which is greater in concentra- 

 tion than the sugar solution in the sac? 



161. What will happen if the sac is filled with a mixture of sugar and 

 salt solutions of equal concentration and that the hquid in the vessel 

 is pure water? 



162. Can a plant take out all the water from the soil around its roots? 

 Explain. 



163. The stems of most woody plants will "bleed" if cut in the 

 spring, but will not do so if cut in the summer. Explain. 



164. Why do tomatoes and other soft and fleshy fruits tend to split 

 open in a wet season? 



