ii6 THE LIFE OF THE PLANT 



give off carbonic acid ; and these acids must act as 

 solvents on the surrounding particles of the soil, the 

 more so because the hairs, as we have seen, come into 

 the very closest contact with, and almost grow into, these 

 particles (see fig. 33 D). But perhaps instead of all 

 these indirect considerations it would be well to prove 

 by experiment that roots produce such a solvent effect 

 on the solid particles of the soil. Let us take for this 

 purpose a piece of white marble carefully polished — 

 marble in its chemical composition is practically chalk — 

 and let us bury it at the bottom of a flat flower-pot. 

 Then let us plant something in the pot, say a bean. 

 The roots of the bean plant will soon reach the piece of 

 marble, and will spread over it, closely adhering to its 

 polished surface. If in a few days we take out the piece 

 of marble, wash it, dry it, and then examine it, holding 

 it to the light, we shall notice on its smooth surface, 

 which reflects the light, some dull worm-like traces. 

 These are the imprints of the roots, which in adhering 

 to the polished marble have eaten out their image 

 upon it by the action of their acid surface. These 

 impressions are certainly not very deep, but they are 

 nevertheless perfectly clear. ^ 



There can be no further doubt, after what has been 

 said, that a plant is able to obtain its food from the 

 solid particles of the soil as well as from solutions. 

 This fact can be made doubly sure by the following 

 curious experiment. The root of a plant of suitable 

 age was carefully washed and divided into two tufts, 

 one of which was sunk in water, while the other was 

 buried in soil, and given no water. Nevertheless, the 

 plant has continued to develop. It has absorbed 

 water with the one tuft of roots, while with the other 

 it has taken from the solid particles of the soil the sub- 

 stances- necessary for its nutrition. 



' These imprints can be made clearer by rubbing them with powdered 

 graphite. 



