THE WORK OF ROOTS 141 



rising of bubbles) , and the milkiness will reappear as 

 the carbonate of lime separates out in solid particles. 



This experiment may also be performed by stirring 

 up fine marble dust or whiting (both of which consist 

 of carbonate of lime) in a little water, pouring off a 

 little of the milky fluid into two separate tumblers and 

 adding to one a considerable quantity of tap -water, to 

 the other the same quantity of soda-water. Cover each 

 tumbler with a piece of glass and set them aside, to 

 see in which the carbonate of lime will dissolve first. 



Since carbonate of lime is the principal constituent 

 of marble and limestone, we can readily understand how 

 these rocks are dissolved by water containing carbonic 

 acid and also why they are deposited on vessels in which 

 such water is boiled. The carbonic acid in soil -water 

 comes mostly from the decay of animal and plant re- 

 mains. It dissolves not only limestone, etc., but practi- 

 cally all other minerals which the plant uses as food. 



We have already learned (page 34) that seeds give 

 off carbonic acid; if the root also has this property it 

 must be of very great advantage to it in dissolving the 

 plant- food immediately around it. Does the root give 

 off carbonic acid ? We may test this by growing roots 

 in lime-water. Fill two similar bottles with lime-water 

 (filtered through filter paper or cotton wool), and cut 

 in the cork of each a notch large enough to receive 

 the stem of a seedling plant two or three inches long 

 (Peas, Beans, etc., grown in sawdust answer excel- 



