308 A TEXT-BOOK OF BOTANY. 



not known. The bacteria seem to be localized in the nodules and 

 are not found in any other part of the plant. 



It has been shown that when the roots of leguminous plants 

 are free from nodules they do not have the power of assimilating 

 free nitrogen. On the other hand, when the nodules produced by 

 the bacteria are developed, the plants will grow in soil practically 

 free from nitrogen salts. Because of this power the plants of this 

 family are useful in restoring worn-out land, i.e., land in which 



FIG. 168. Transverse section of a root bearing root hairs; the latter are thin walled, 

 irregularly bent, and attached at various places to small particles of soil. The hairs secrete 

 an acid, rendering the inorganic substances soluble, which are then diffused through the 

 walls of the hairs, transmitted to the cortical parenchyma and distributed through the 

 conducting cells of the xylem to the shoot. After Frank. 



the supply of nitrogen is exhausted, and they thus play an impor- 

 tant role in agricultural pursuits. 



The enriching of the soil is accomplished by ploughing under 

 the leguminous crops, as of clover or alfalfa, or allowing the 

 nodule-producing roots to decay, when the nitrogen compounds 

 are distributed in the soil. 



(Consult Bulletins on " Soil Inoculation for Legumes," issued 

 by the Bureau of Plant Industry, U. S. Department of Agri- 

 culture.) 



