Life in the Soil 123 



always on the roots certain little knots or nodules; and 

 it was also observed that when there were none of these 

 nodules on the roots the legume plants did not increase 

 in nitrogen any more than other plants. It was also found 

 that, when there was a great store of nitrogen in the soil, 

 the plants used this and formed no nodules. It was evi- 

 dent, then, that in some way these nodules were the agents 

 through which the nitrogen was acquired. Then scien- 

 tists began the study of the nodules, and found that they 

 were caused by parasitic bacteria which find a home on 

 the legume roots, and through their growth cause the 

 change in the tissues of the roots making the nodules. 

 The bacteria draw their nourishment from the legumes 

 in part, and hence are parasitic. But the parasitism has 

 been shown not to be harmful; for the bacteria enable 

 the legume to get from the air more than they take from 

 the plant otherwise; and it thus becomes a sort of sym- 

 biosis, or hving together, which is mutually beneficial. 



In the first study of these bacteria, they were found to 

 have so different a form from any other bacteria that they 

 were called bacteroids, or forms similar to a bacterium. 

 But it is now pretty well determined that they are true 

 bacteria, altered in form by their peculiar work. In fact, 

 it seems that they have been so altered that certain ones 

 Hve only on certain legumes and on no other, having be- 

 come specially adapted to certain host plants, so that we 

 have clover bacteria, pea bacteria and bacteria that live on 

 other legumes ; though it may be shown yet that any of them 

 in time can accommodate themselves to different hosts. 



While any of the legumes may be made to thrive and 

 grow on soil that abounds in abundant plant food, their 



