270 



TYPICAL FLOWERING PLANTS 



Rootlets are protected on the end by a structure called 

 a root cap (Figure 262). This cap is made up of loose cells 

 which are constantly formed from the inside. As fast as 

 the outer cells are destroyed by the pushing of the root 

 through the soil, new cells are ready to take their place. 



Small bunches, called 

 tubercles (Figure 263) are 

 found on the rootlets 

 of plants of the bean 

 family. The tubercles 

 are filled with bacteria 

 which gather nitrogen 

 from the air, use what 

 they need, and leave the 

 surplus in the roots. 

 Some of this nitrogen 

 is used by the growing 

 plants themselves, and 

 any that they do not use 

 is left in the soil for the 

 use of other plants. 

 Most plants take from 

 the soil more nitrogen 

 than they add to it, but 

 the opposite is the case 

 with beans and their 

 relatives. Thus clover and other relatives of the bean 

 are used by farmers as a cover crop or for "green 

 manure," so called, for the sake of replacing in the 

 soil the nitrogen which other crops have used up. The 

 practice of rotating crops depends on the fact that dif- 

 ferent kinds of plants use different material in the soil. In 

 successive years crops of different kinds will grow better 

 than crops of the same kind, unless the soil has been sup- 



Figure 263. — Bean Roots. 

 Showing tubercles. 



