PLANT FOOD 61 



Since all animals live on plants, either directly or in- 

 directly, they are composed of these same elements. The 

 salt and water that an animal uses only adds to the amount 

 of sodium, chlorin, hydrogen, and oxygen that the plants 

 furnish. 



If any one of the first ten elements is lacking, the plant 

 will die. Of some, very small quantities are required, 

 but this small amount is necessary. Many experiments 

 have been performed to test this. Plants have been grown 

 in distilled water to which all these elements but one 

 have been added. Fig. 51 shows such a wheat plant 

 which had all the elements of plant food except nitrogen. 

 The same results are obtained when any nine are fur- 

 nished, but the tenth omitted. The carbon is furnished 

 by the air, so that is not put into the water. The legumes 

 are also able to take nitrogen from the air under certain 

 conditions. 



57. Sources of Plant Food. For a long time no one knew 

 where the plant got its food. Some argued that its food 

 came from the air, and others thought it came from the 

 soil. Only within the last fifty years has the question been 

 entirely answered. We now know that a plant secures 

 its food from both the soil and the air, the larger part 

 coming from the air. 



Oxygen and hydrogen, chemically united in the form 

 of water, are taken up by the roots from the soil, but all 

 water comes indirectly from the air. 



The carbon is obtained from the air by the leaves in 

 the form of carbon dioxid. 



The nitrogen comes from the soil, except in the case 

 of legumes, which are able to take nitrogen from both 



