DISCUSSION OF DEPENDENT PLANTS 



375 



tubercle bacteria are known as the nitrogen-fixing bacteria, 

 since they fix free nitrogen from the air. Since the ordinary 

 grain-producing plants of the fields must have nitrogen in 

 order to grow, and since they can use it only in the nitrate 

 form, the significance to agricultural plants of the work of 

 these root-tubercle or nitrogen-fixing bacteria is evident. An 

 experiment may 

 readily be per- 

 formed (Fig. 306) 

 to determine the 

 relative influence 

 of these organisms 

 upon the rate and 

 amount of plant 

 growth. At the 

 death of the clo- 

 vers, peas, beans, 

 etc., upon which 

 the tubercles have 

 grown, those sur- 

 plus nitrates left 

 in the soil furnish 

 a most important 

 part of the food 

 of other agricul- 

 tural plants. 



We have, there- 

 fore, at least four 

 kinds of depend- 

 ent soil bacteria, each of which in its processes of nutrition 

 transforms nitrogen compounds in such a way that those com- 

 pounds are eventually available as food for higher plants. The 

 importance of this is further indicated in Sect. 37 (Partnership 

 of roots and bacteria) and in Chapter XXIV. There are yet 

 other kinds of bacteria which break up organic compounds so 

 as to release nitrogen into the air, the process being known as 



FIG. 305. Roots of red clover, with tubercles in which 

 are the bacteria that collect nitrogen from the air 



One half natural size 



