86 THE AMERICAN MONTHLY [March 



pierced the hair and grew down it into the tissues of the 

 root. Where it came into contact with the cells these 

 became curiously modified, the protoplasm becoming 

 denser and more granular. At the same time the cells 

 increased in size and divided rapidly, causing that por- 

 tion of the root to swell up and form the root tubercles 

 so characteristic of the leguminous plants. If older tub. 

 ercles are examined they will be seen to contain in tlieir 

 cells large numbers of curiously-sha})ed micro-organ- 

 isms, to which the name of bacteroids has been given. 

 (Fig. 7.) These bacteriods contain the spores, which are 

 liberated when the roots decay, and then the spore can 

 again infect the root hair. 



Of what benefit now are these small bacteriods to the 

 pea or bean which contains them in its roots? Well, it 

 has been found by experiments made both in this country 

 and abroad that the bacteriods are able to make use of 

 the nitrogen contained in the air, and to buildup with it 

 nitrogenous compounds wliicli become stored up in the 

 tubercles. Ordinary plants cannot make use of any of 

 the nitrogen of the atmospliere, but only of nitrates 

 contained in the soil ; hence farmers are constantly 

 adding nitrates in the form of manure to their fields. 



Leguminous crops, however, can flourish in a soil de- 

 void of nitrates, provided the bacteroids are present to 

 absorb and transform the nitrogen of the air. Hence in 

 the rotation of crops leguminous plants are exceedingly 

 important, for not only will they flourish on soil impov- 

 erished by former crops, but they enrich the soil they 

 grow in, for when the roots decay the nitrogenous com- 

 pounds contained in the tubercles are liberated, and serve 

 as food for the crop which is to follow. These bacteriods 

 are therefore useful in a high degree to the pea or bean, 

 and indirectly to the farmer if he knows his business. 

 The bacteriods, ou the other hand, may not only find a 



