THE SOIL. 249 



it In the form of dust, and carried about by the 

 winds ; but the quantity which plants can derive from 

 this source is comparatively small. The air near the 

 sea-shore, and even to a great distance inland, is 

 frequently loaded with saline particles derived from 

 the sea : after a storm at sea a quantity of salts thus 

 suspended in the air is very considerable. 



645. The fact that some solid substances can be 

 thus carried along in the air in a state of very fine 

 powder is important, and well worthy of being re- 

 membered, as it explains many apparently mysteri- 

 ous phenomena. It is exceedingly diflBcult to detect 

 the very minute quantity of solid matter contained 

 in the air, but there is no doubt that it often does 

 exist, although we are not aware of its presence. 



646. Setting aside the small quantity of earthy 

 matters which plants may derive from the air, it is 

 evident that the great source of the lime, potash, and 

 other similar substances contained in plants can only 

 be the soil : hence, the chemical composition of the 

 soil must exert great influence on the plants which 

 are cultivated in it. 



647. The soil or earth is essential to the growth 

 of plants in several distinct ways. It enables them 

 to fix themselves firmly, as, from its open porous na- 

 ture, it allows the roots to extend in various directions, 

 and obtain a secure hold, so that the plant can grow 

 erect into the air, without danger of being blown 

 away by the winds. The soil likewise supplies them 

 with substances essential to their growth ; such as 



