128 



GROWTH AND WORK OF PLANTS 



ever, 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, and bear a good 

 crop in soil very poor in nitrogenous plant food provided the 

 other conditions are favorable. 



204. Root tubercle bacteria. Certain bacteria (see Chapter 

 XXVII) present in the tubercles on the roots of leguminous plants 

 fix free nitrogen. These bacteria* are widely distributed over 

 the earth in nearly all soils, especially in 

 regions where leguminous 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 forma- 

 tion 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 

 attached, so that they are easily seen 

 when the clover, pea or other legume is 

 dug up and the soil carefully washed from the roots (fig. 92). 



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 substances 

 which they have formed by assimilating (or "fixing") free 

 nitrogen which they have absorbed from the air in the soil 

 with carbohydrates and other organic substances which they 

 have absorbed from the cells of the clover or other legume. The 

 bacteria thus obtain their carbohydrate food from their "host," 



* Also called microbes. The name now generally used for this particular 

 microorganism is Pseudomonas radicicola. Earlier names are Phytomyxa 

 leguminosarum, Rhizobium leguminosarum, etc. 



Fig. 92. 



Root of the common vetch 

 showing root tubercles. 



