THE ROOT AND ITS FUNCTIONS 61 



193. What is the fundamental difference between preserving food bj^ 

 salt and preserving it b}^ benzoate of soda or a similar substance? 



194. A little salt placed in the water in which cut flowers are standing 

 will often cause them to keep fresh longer than they otherwise would. . 

 Explain. Would a large amount of salt in the water have the same 

 result? Explain. 



195. What causes the root-hairs to force themselves in among the 

 soil particles? 



196. Growing roots and stems often exert tremendous pressures, 

 sometimes sufficient to split and lift heavy rocks. What causes this 

 expansive power? 



197. Rocks are sometimes split apart, in quarrying, by the insertion 

 of dry wooden wedges into drills or cracks. These wedges are then 

 wetted, and their swelling exerts a powerful pressure. How different 

 in its origin is this pressure from that exerted by a tree-root which splits 

 open a rock? 



198. Rapidly growing beets, turnips, and similar fleshy plant parts 

 sometimes crack open during growth. Why? 



199. Water is sometimes forced from the leaves of a plant in the form 

 of droplets. Why is this and under what conditions is it likely to take 

 place? 



REFERENCE PROBLEMS 



22. Are the "root-crops" (such as carrots, parsnips, turnips, and beets) 

 usually annual, biennial or perennial plants? Why? 



23. Are there any fleshy-rooted plants which do not have tap-roots? 



24. What important crop-plant is propagated by buds formed on its 

 roots? 



25. When and by whom was tlie Cell Theory first formulated? 



26. When and by wliom was the term "protoplasm" in its present sense 

 first used? 



27. Who discovered that every cell has a nucleus? 



28. What do we mean by saying that protoplasm is a colloidal substance? 



29. State two fundamental differences between the typical plant cell and 

 the typical animal cell. With what general differences between animals 

 and plants are these cell differences connected? 



30. What prevents the cells of a plant from falling apart? 



