THE ROOT AND ITS FUNCTIONS 59 



165. How does it happen that a plant can take up such large amounts 

 of salts when these salts occur in such very weak concentrations in the 

 soil? 



166. Many plants thrive for long periods without extending their 

 root-systems into fresh regions of soil. How are they able to obtain an 

 unfailing supply of nutrient salts under these conditions? 



167. A plant grown in "water-culture" (in a jar of water containing 

 the necessary nutrient salts in solution) will almost completely remove 

 these salts from the jar, even though its roots fill only a small part of the 

 jar. Explain. 



168. Iodine is much more abundant in the tissues of certain sea- 

 weeds than it is in the sea water. Explain how this can be. 



169. How is it possible for a group of cells in the middle of a tissue, 

 surrounded by other cells, to contain large amounts of a substance which 

 is rare or absent in the other cells? 



170. The text states that the cell-membrane of a root-hair is imperme- 

 able to sugar, and that sugar therefore cannot get out of the root-hair 

 into the soil. If this is true, how do you think the sugar was able to get 

 into the root-hair in the first place? 



171. A crop plant which removes a large amount of nutrient material 

 from the soil is known as a "heavy feeder" and one which removes 

 little as a "light feeder." What factors can you think of which would 

 cause plants to differ in this respect? 



172. The salts taken from the soil by one plant are often very differ- 

 ent in kind and amount from those taken in by another plant. To what 

 factors may these differences be due? 



173. One crop often needs a different fertiUzer from another. To 

 what physiological differences in the two crop-plants may this be due? 



174. How do submersed water-plants get their salts? 



175. Some fertilizers, when applied very abundantly, will often kill 

 plants. Why? 



176. A spray-solution which is strongly concentrated will often kill 

 plants to which it is applied. Why? 



177. A spray-solution which will kill one plant may not kill another. 

 Explain. 



178. Strong spray will often kill the young and growing parts of a 

 plant but not the older portions. Why? 



