CHAPTER XXIX 

 LEGUMmOS^ (Pea FamUy) 



The pea family is one of wide geographical distribution, 

 occurring both in temperate and warm climates. According 

 to Piper there are about 487 genera and 10,782 species in 

 the family. Of these, 3,846 species in 103 genera are 

 American. 



^^ Legume" is a popular name applied to members of the 

 Leguminosae. Probably no family is of greater agricultural 

 importance than this one, unless it is the Gramineas. Legu- 

 minous plants are comparatively rich in protein; this apphes 

 to all portions of the plant, and not to seeds alone. For this 

 reason they help to balance the food ration of man and of 

 domestic animals, which is quite largely made up of starchy 

 foods, such as are furnished by the cereal crops. Further- 

 more, the fact that legumes are rich in nitrogenous sub- 

 stances makes them of value as fertihzer crops. Moreover, 

 they leave a considerable quantity of vegetation behind them 

 when harvested, and thus add humus to the soil, which 

 improves both the chemical and physical properties of the 

 soil. 



Root Tubercles. — The roots of the legumes support the 

 growth of a bacterium {Pseudomonas radicicola) which forms 

 upon them abnormal growths called nodules or tubercles. 

 The tubercles are root colonies of the above organism, which 

 stimulates rapid growth of certain root cells and hence the 

 formation of swollen, gall-like structures. These organisms 

 have the power of fixing free nitrogen of the air. That is, 

 free nitrogen gas from the soil air is taken by the organism 



413 



