MORPHOLOGY OF HIGHER PLANTS. 



307 



present in various nitrogenous (protein) compounds which occur 

 as normal constituents of the plant. The nitrogen required by 

 plants is derived either from nitrogen salts contained in the soil, 

 as nitrates and ammonium salts, or from the free nitrogen of 

 the atmosphere. While most of the higher plants are able to 

 assimilate nitrogen compounds existing in the soil, only the 

 Leguminosae and Aristolochiaceae, with possibly a few exceptions, 

 are able to assimilate atmospheric nitrogen, and in this respect the 



FIG. 167. Root tubercles on Lupinus, one of the Leguminosce: A, roots with tubercles; 

 B, transverse section of root showing the cells (b) which contain the nitrogen bacteria. 

 A. after Taubert; B, after Frank. 



majority of the Leguminosae stand as a class by themselves. 

 Apparently in direct relation to this character stands the fact that 

 the seeds of these plants contain a high percentage of nitrogen. 

 This special ability of the Leguminosae to fix atmospheric nitrogen 

 in the plants depends upon the presence of the nodules, which are 

 due to the infection of the roots by a soil-bacterium (Pseudomonas 

 radicicola), although the precise mode of fixing the nitrogen is 



