MORPHOLOGY OF ANGIOSPERMS. 



99 



LeguminosK, with possibly a few exceptions, are able to assimi- 

 late atmospheric nitrogen, and in this respect the majority of the 

 Leguminosas 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 



Fig. 64. 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. 



infection of the roots by a soil-bacterium (Pseudomonas radici- 

 cola), although the precise mode of fixing the nitrogen is 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 thev do not have the power of assimilating 

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

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



