6 



HOW PLANTS GROW 



From the water may be taken : 

 13. Hydrogen. 



Oxygen may also come from water, and probably both hydrogen 

 and oxygen may be taken up from soil compounds. 



There are ten essential elements found in all plants. All the ten 



essential elements must be present 

 or the plant will not grow. This 

 is often tested in laboratories by 

 growing plants in water and pro- 

 viding only nine of the elements, 

 leaving out one. No matter which 

 is left out, no growth will take 

 place. In the case of iron, only 

 the smallest trace is required, per- 

 haps an ounce would be sufficient 

 for an acre of wheat, but the 

 plants will not grow without it. 

 Plant Food Sources. Most 

 of the soil is an inert mass that 

 plants could not live in, but 

 throughout this mass are small 

 quantities of the essential ele- 

 ments in the form of compounds. 

 By natural decay, these com- 

 pounds slowly become soluble in 

 water, just as salt will dissolve, 

 and then in turn, the water is 

 taken up by plant roots, the min- 

 erals thus being carried up to the 

 leaves. 



Carbon comes entirely from 

 the air in the form of a gas. All 



burning or decaying materials give up carbon dioxide gas to the air. 



The plant in turn is able to extract this carbon from the air, to build 



up new plants. 



Nitrogen is taken from the soil by most plants, but all nitrogen 



must first come from the air. Certain bacteria living in the soil can 



FIG. 1*. Diagram illustrating the relative 

 proportion of dry matter and water in 

 a green plant. 



