162 USES OF FORESTS AND OTHER 



the surface exposed to the water by an equal 

 amount; and when we come down to the fine 

 grains of mud which in time are made from the 

 larger fragments, we find that the surface of 

 the rock from which the waters may dissolve 

 plant-food is increased many thousand fold by 

 the process of division. The root-hairs, also, 

 secrete an acid, capable of dissolving mineral sub- 

 stances with which they are in contact, and this 

 acid fluid aids them to decompose the particles of 

 stone. In this way the rootlets of the plants serve 

 in part to create from the solid rocks the soil that 

 gives them support. 1 



Not only do the trees help to make the soil upon 

 which they dwell, but they also preserve it from 

 destruction. If the reader will notice any tilled 

 field in a time of heavy rain, he will perceive that 

 the soil is rapidly borne away in the form of mud 

 to the rivers and thence to the sea. If he will 

 observe such a piece of ground after a heavy rain, 



1 This influence of plants on their mineral substratum is clearly evi- 

 dent where Lichens and Mosses attach themselves to the free surfaces 

 of rocks, e.g., on high mountains. The solid crystalline surface of the 

 stone becomes gradually converted, by the activity of the roots of these 

 plants, into a friable, crumbling, loose substance. This decay continu- 

 ally penetrates deeper into the stone, and so affords a substratum in 

 which even the stronger roots of larger plants can then obtain a hold. 

 " Lectures on the Physiology of Plants." Sachs. 



