THE WORK OF ROOTS 161 



each plant selects from the soil certain elements which 

 it absorbs and uses for its growth, to the partial or 

 total exclusion of others. Thus, when we grow plants 

 in a solution of Chili saltpeter (sodium nitrate) the 

 plant takes up all the nitrogen and only a trace of the 

 sodium, leaving the rest in solution. From ammonium 

 sulphate the plant takes the ammonia, leaving most of 

 the sulphuric acid. 



This selective action depends on the fact that more 

 nitrogen is used or combined within the plant than 

 sodium. This action of the plant may be imitated 

 artificially by closing the end of a tube or lamp-chimney 

 with a piece of bladder, as described on page 60, plac- 

 ing within it water and a few pieces of zinc and setting 

 it in a glass containing a solution of copper sulphate 

 (blue stone) in water. The copper sulphate diffuses 

 through the membrane and the copper is deposited ,on 

 the metal and so removed from the solution, while 

 the sulphuric acid is left behind: another substance, 

 e.g., eosin in solution, will not be taken up by the zinc 

 but will be taken up by the membrane, which becomes 

 strongly colored: another substance, e. g., common 

 salt, will not be taken up by either glass, membrane 

 or metal, but will remain in solution in its original 

 strength. 



Inasmuch as the root is the most tender and succu- 

 lent part of the plant, it is liable to attacks by insects 

 and animals. Some roots, such as Monkshood, Yellow 



