BACTERIA AND PLANT FOOD 



277 



find similar tubercles upon the roots of beans, peas, vetches, 

 sweet clover, alfalfa, and various other members of the great 

 family of legumes, or pod bearers. 



It took a long while for man to trace the connection 

 between these tubercles upon the roots of the pod-bearing 

 plants and the free nitrogen of the air. There is still much 

 mystery about it, but we now know that the tubercles or 

 nodules are composed chiefly of 

 colonies of bacteria, and that these 

 bacteria are able to convert the 

 free nitrogen in the air between 

 the soil particles into a form that 

 is utilized by the plants. They 

 may well be called the Nitrogen- 

 gathering Bacteria, and are to be 

 classed among the farmer's best 

 friends. 



We have seen that some bac- 

 teria may live as parasites upon 

 living plants or animals: these are 

 the Parasitic Bacteria. We have 

 also seen that some bacteria 

 including the nitrifying forms 

 may live as saprophytes upon dead 

 and decaying plants or animals: these are the Saprophytic 

 Bacteria. We have now to determine the relation of the 

 nitrogen-gathering bacteria to the leguminous plants. Evi- 

 dently they are not parasites, for they help rather than in- 

 jure the growth of the plant. Nor are they saprophytes, 

 for they develop upon living rather than dead plants. They 

 seem to form a sort of partnership with the plants. They 

 need a home to grow in and some help to get their growth ; 

 in return they gather nitrogen for the benefit of their hosts. 



ROOT OF SOY BEAN SHOW- 

 ING TL'BF.RCLES 



