188 



THE PLANT WORLD 



wootule enlargecC 



causes soil to become more favorable for the growth of other 

 plants, but the reason for this has been discovered in late years. 

 On the roots of the plants mentioned are found little nodules 



or tubercles; in each 

 nodule exist millions 

 of bacteria, which 

 take nitrogen from 

 the air in the soil and 

 build it into nitrites 

 which are converted 

 by other bacteria 

 into nitrates. In this 

 form it can be used 

 by the plants. Only 

 these bacteria, of all 

 living plants, have 

 the power to take 

 free nitrogen from 

 the air and make it 

 over into a form that 

 can be absorbed by 

 the roots. They live in a symbiotic l relationship with the plants 

 on which they form tubercles, for the legumes provide them with 

 organic food. All the compounds of nitrogen are used over and 

 over again, first by plants, then as food by animals, eventually 

 returning to the soil again, or in part being released as free nitro- 

 gen ; but any new supply of usable nitrogen must come by means 

 of these nitrogen-fixing bacteria. 



Rotation of crops. The facts mentioned above are made use 

 of by progressive farmers who wish to produce as large crops as 

 possible from a given area of ground. Plants that are hosts 

 for the nitrogen-fixing bacteria are raised early in the season. 

 Later these plants are plowed in and a second crop of a different 

 kind is planted. The latter grows quickly and luxuriantly because 

 of the nitrates left in the soil by the bacteria which lived with the 



1 symbiotic (sim'biofik) : The living together in intimate association of two dis- 

 similar organisms. 



section, throurfk 

 ce nocUxle 



bacteria fcunct wi_- 

 Cells of a nodule/ 



Explain by use of this diagram where the nitrogen-fixing 

 bacteria live. 



