THE PLANT 



55 



air is enormous. Great as are plant demands upon it, there 

 is no danger that the supply will ever be exhausted. 



Nitrogen, which is necessary to all 

 living things, cannot be used by the 

 higher plants until it is prepared in 

 certain compounds. Most plants 

 get their nitrogen as nftrates, com- 

 pounds prepared by bacteria in the 

 soil. A class of plants, called leg'- 

 umes, can use the nitrogen of the 

 air indirectly by means of bacteria 

 which live in knots, called tu'ber- 

 cles, on their roots. 



In one way or another the nitro- 

 gen, without which all living things 

 would starve, is prepared by bac- 

 teria. Later on, you will learn more 

 about these bacteria. They are the 

 smallest known living organisms, 

 and are so tiny that they are visible 

 only under a powerful microscope; 

 their very existence was unsuspected 

 until recent years. 



Soil-derived Elements. The 

 plant derives from the soil only 

 about five per cent of its food ; but, small as this portion is, the plant 

 cannot live without it. The necessary soil-derived elements are 

 iron, sulphur, magnesium, calcium, phosphorus, and potassium. 

 Of these elements, four iron, sulphur, magnesium, and usually 

 calcium are abundant in most soils. As has been said, nitrogen, 

 phosphorus, and potash sometimes exist in soils in limited quan- 



Roots of sov bean, showing tubercles, 

 slightly reduced in size 



