150 TRANSPIRATION. 



from the surrounding earth. Suppose this evapo- 

 ration into the air to be continuous, and we have 

 a constant stream of water flowing from the 

 damp earth, through roots and rootlets, stem 

 and branches, to the leaves. This is what actu- 

 ally does take place, and this stream is known 

 as the transpiration current. 



But roots do not absorb water alone. The 

 delicate hairs, outgrowths of the skin, which 

 clothe thickly all their young parts, give out an 

 acid which can dissolve the mineral matters in 

 the soil. It is not certainly known what this acid 

 is, but, whatever it may be, by a most beautiful 

 arrangement it dissolves the food of the plant 

 just where the root-hairs are present to absorb 

 it. In this way roots are able to disintegrate 

 the soil, and to absorb its mineral matters in 

 weak solutions, which are carried all over the 

 plant. They are brought in the way we have 

 described into the cells communicating with the 

 external air, where they are greatly concentrated 

 by the escape of water, and are then ready to be 

 converted into food. This is done by the action 

 of carbonic acid gas, absorbed from the air. By 

 means of this gas the raw mineral material 

 brought from the ground is converted into or- 



