60 BOTANY: PRINCIPLES AND PROBLEMS 



179. If a very strong spray (such as lime-sulphur, used against cer- 

 tain bark-insects of fruit trees) is to be applied to a tree, why must this 

 be done only wlien the tree is leafless? 



180. In such places as gravel walks and tennis courts it is often 

 customary to kill weeds by sprinkling salt upon them. Why is this 

 practice effective? Why is it not widely used in killing farm weeds? 



181. How is it possible for some plants to live on salt-marshes and 

 sea-beaches while others cannot? 



182. Desert plants and salt-marsh or sea-beach plants frequently 

 show similar modifications in structure. Explain. 



183. Why do salt-marsh or sea-beach plants usually die if subjected 

 to fresh water? 



184. The sap concentration in the cells of parasitic plants has been 

 found to be higher than in the cells of the plants upon which they are 

 parasitic. Explain. 



185. Why do dried currants, raisins, and prunes swell so much when 

 placed in water? 



186. When berries are cooked with little sugar they are apt to burst. 

 When cooked with much sugar they are apt to collapse. Explain. 



187. Vegetables usually cook more quickly if they are not salted 

 while cooking. Explain. 



188. Celery, sliced cucumbers and similar vegetables are often 

 placed in water for a while before they are served. What effect does 

 this produce and why? 



189. Why are we thirsty after eating much salt or sugar? 



Note. — In the five following questions, remember that decay is due 

 to the activity of bacteria, which are merely very small plant cells 

 (p. 285). 



190. Why is salt sugh a good perservative of vegetables, meat, fish, 

 and other foods? 



' 191. Which will "keep" better if exposed freely to the air, grape 

 juice or grape jelly? Why? 



192. Old-fashioned preserving of fruit was done by the "pound for 

 pound" method, a pound of sugar being used for every pound of fruit. 

 Why was this method successful even in the absence of boiling or any 

 other means of sterilization? 



