PHYSIOLOGICAL PROCESSES IN PLANTS 83 



Note. Root tubercles are small, knotty, wartlike growths 

 that form on the roots of such plants as clover, vetch, alfalfa, 

 pea, "bean, cowpea, soja bean, etc. These tubercles are the 

 homes of germs or 

 bacteria. Their life 

 activities enrich the 

 soil by the addition of 

 much-needed nitro- 

 gen, which these 

 germs obtain from the 

 air and store in their 

 root homes. Plants 

 are unable to obtain 

 nitrogen from the air 

 without their aid, 

 although, as we have 

 learned, the air is 

 about four fifths ni- 

 trogen. Humus, ma- 

 nures, and decaying 

 organic matter con- 

 tain nitrogen in such 

 shape that plants may 

 use it. But these are \ 



sometimes difficult to FIG. 19. Roots of red clover with nodules which 

 get and are relatively are the homes of bacteria which enable the 

 expensive plant to use the nitr S en of the air - 



Conclusion. What should be done by farmers and gardeners 

 in order to increase the supply of available nitrogen in the 

 soil ? What are the advantages of this method ? 



g. Effect of Crops on Soil 



Observation. Copy Figure 20 in your notebook, using 

 different colored crayons, if possible, to represent the phos- 

 phoric acid, potash, and nitrogen. This and the two following 

 diagrams show the amounts of phosphoric acid, potash, and 



