12 WALL'S MANUAL 



bustion; the "inorganic, or mineral elements," which 

 remain after the burning of the plant, and constitute 

 allies. These two classes seem equally necessary to 

 the healthy growth and full development of the plant. 

 They are the food of the plant, as they are taken up 

 by it while growing. 



Plants do not get ALL their food from the soil on which 

 they grow, as many suppose. The soil and the air 

 both furnish nourishment to growing crops. Through 

 its roots the plant is in constant contact with the soil, 

 and through its leaves it is in contact with the air. 

 The roots are so constructed as to be able to take up 

 from the soil such food as is required from that source, 

 whenever it is found there in proper condition. But 

 all substances absorbed by the roots must be rendered 

 soluble, as the organs can take up matter only in a 

 liquid form. 



The mineral elements, being non-volatile, are not 

 found in the air; therefore, they must be derived from 

 the soil alone. Besides these, the soil must have a 

 sufficient quantity of water or acids to dissolve what- 

 ever is required by the plant. The soil also, generally, 

 contains a considerable quantity of organic er veg- 

 etable matter. This vegetable matter, when 3 well rot- 

 ted, is called " HUMUS, or MOLD." 



Whence do plants obtain their organic elements ? 

 These, as we will show hereafter, are chiefly four 

 carbon, hydrogen, oxygen, and nitrogen. The carbon 

 of plants is derived chiefly (but not entirely) from 

 carbonic acid. This gas is one of the constituents of 

 the atmosphere, whether the air is collected on the 

 top of the highest mountain or in the lowest valley. 

 Plants have the power of absorbing carbonic acid gas 



