SPECIAL ASPECTS OF NUTRITION OF PLANTS 121 



for man would be greatly reduced in price. In former years it 

 was held by some scientific men, that -the ordinary green plants 

 could use directly the free nitrogen of the air for food. Careful 

 experiments have demonstrated, however, that this is not the 

 case. Still it has been known for many years, that leguminous 

 plants (clover, peas, beans, alfalfa, vetches, honey locust, soja 

 beans, etc.) will grow,.. thrive, anjl bear a good crop in soil very 

 poor in m'trop^nous plant food provided the other conditions are 

 favorable. 



202. Root tubercle bacteria. Careful investigations have 

 shown that this is due to the work of microorganisms* in the 

 roots of these plants. These bacteria are 



widely distributed over the earth in nearly 

 all soils, especially in regions where legum- 

 inous plants grow. These bacteria enter 

 at the root hairs, extend by growth in the 

 form of a thread into the cortical region of 

 the root where they stimulate the root cells 

 to the formation of a gall or tubercle, 

 which is often of different form in different 

 species of legumes. These root tubercles 

 are short and thick, often oval in form, or 

 short, cylindrical and branched. They are 

 stouter than the roots to which they are Root of ^common vetch, 

 attached, so that they are easily seen when showing root tuber 

 the clover, pea or other legume is dug up and the soil carefully 

 washed from the roots (fig. 90). 



203. Within the root tubercle the bacteria spread by 

 means of branched threads or tubes. The cells of the tubercle 

 are rich in protoplasm. Within the cells great numbers of free 

 bacteria are formed which are oval or rod-like or Y or X shaped. 

 The bacteria in this condition are filled with nitrogenous sub- 

 stances which they have formed by assimilating (or ''fixing") 



* Certain bacteria, also called microbes. The name now generally used 

 for this particular microorganism is Pseudomonas radicicola. Earlier names 

 are Phytomyxa legutninosarum, Rhizobium leguminosarum, etc. 



