CHKMICAL CHANGES 



225 



How the Nitrogen is Assimilated. There are three possibili- 

 ties : ( i ) The bacteria secrete an enzyme which causes the plant 

 to assimilate nitrogen through its leaves; (2) the bacteria 

 assimilate the nitrogen and are then consumed by the plant; (3) 

 the bacteria assimulate nitrogen and give it off as soluble com- 

 pounds, which are taken up by the plant. 



Fig. 52. Nodules on the roots of the soy bean. 



The following facts are related to these theories: (i) As 

 observed by Hellriegel, in a medium free from nitrogen the plant 

 ceases to grow while the nodules are developing, then suddenly 

 begins to grow vigorously as if it had a new source of nitrogen. 

 (2) Young nodules contain more nitrogen than old ones. 



