30 



AGRICULTURE. 



tropical coasts break the force of the waves, 

 so that they cannot wash away the mud and 

 sand. 



(2) Chemical Effects. — The roots of living 

 plants, from their acid secretion, effect a chem- 

 ical action upon the insoluble substances with 



FIG. 10. — VEGETATION PROTECTING THE SOIL. 



which they come in contact, rendering them sol- 

 uble, and absorbing into themselves large quan- 

 tities of certain compounds as plant food, thus 

 depriving the soil of this material. That this 

 acid secretion actually corrodes or dissolves rock 

 material was proven by Sachs through actual 

 experiment, which any one may also prove for 

 himself. By the decomposition of vegetation 

 humic acids are formed, which have the power 



