THE WORK OF ROOTS 139 



experiment further and add to the tap- water various 

 mineral substances, to stimulate the growth of the 

 plant. Saltpeter and bone superphosphate (one ounce 

 of each dissolved in three quarts of water) may be 

 used in this way. Experiments carried out in this 

 way have shown that the mineral matters indispensa- 

 ble to the plant consist of four bases and four acids. 



The bases : The acids : 

 Potash Nitric 



Lime Phosphoric 



Magnesium Sulphuric 



Iron Carbonic (from the air) 



If we dissolve in distilled water all the above sub- 

 stances (except carbonic acid), plants may be grown 

 in it until they flower and fruit and produce perfect 

 seeds; but if we omit any of the elements involved 

 (except carbonic acid), the plant soon stops growing 

 and fails to flower or fruit. (Carbonic acid is neces- 

 sary to the plant but is absorbed from the air, as we 

 shall see later.) Fig. 96 shows the result of such an ex- 

 periment made with the Wandering Jew or Inch Plant. 



All these substances must exist in the soil in order 

 that the plant may thrive, and they must not only 

 exist there but be soluble in the soil- water. The -car- 

 bonic acid of the soil is of great service to the plant, 

 since it dissolves many substances which would other- 

 wise remain undissolved. If, for instance, we add to 

 lime-water (which has been filtered clear) a little soda- 



