124 



The Living Plant 



of green-manuring, that is, plowing in Clover and other legumi- 

 nous crops to enrich the soil. It is from these same nodules, also, 

 that the Bacteria have been taken and grown for the commercial 

 enrichment of the soil, as mentioned on the preceding page. 

 In our consideration of these four natural methods of soil 



Fig. 'Sb. — The roots of several Bean plants, photographed about half the natural size, 

 showing the collections of wart-like nodules which contain the nitrifying Bacteria. 



nitrification we must not forget the artificial aid of man, who, 

 for his own purposes, adds to the soil both chemical fertilizers 

 and barnyard manures, with their rich supplies of nitrogenous 

 and other compounds. 



Finally it is important to note that the plants, for their part, 

 have a way of meeting the nitrogen scarcity of soils, — viz., they 



