The Root Nodules 211 



In order to determine whether the formation of nodules 

 was due to the infection of the roots by microorganisms, 

 these investigators instituted an experiment with forty 

 pots of soil. Ten of these soils received each a very 

 small quantity of leachings from a fertile soil, while the 

 remaining thirty soils received none. It was noticed 

 that the plants in all of the pots that had received the 

 soil -leachings passed through the critical period and 

 turned dark green, whereas the plants in twenty-eight 

 out of the remaining thirty soils showed strong evidence 

 of nitrogen hunger. An examination of the roots showed 

 that the vigorous plants all possessed an abundance of 

 nodules. 



In further experiments, when everything was ster- 

 ilized, the peas germinated and grew vigorously for a 

 while, but finally turned yellow and died. No nodules 

 were found on the roots of these plants, and Hellriegel 

 and Wilfarth concluded, therefore, that the nourishment 

 of legumes, their power of acquiring free nitrogen, the 

 formation of nodules and the presence of microorganisms 

 stood in intimate relation to one another. Similar ex- 

 periments with crops belonging to other plant families 

 gave negative results. 



In commenting on these experiments, Gilbert, of 

 Rothamsted, said: "It must be admitted that Hell- 

 riegel's results, taken together with those of Berthelot 

 and others, do suggest the possibility that, although the 

 higher plants may not possess the power of directly 

 fixing the free nitrogen of the air, lower organisms, 

 which abound within the soil, may have that power, 

 and may thus bring free nitrogen into a state of com- 



