210 



Nitrogen Fixation— Symbiotic 



ing yields. Much higher yields were obtained when cereals followed 

 lupines than when the cereals were grown on the same soil not 

 preceded by a leguminous crop; the nitrogen content of the soil 

 was thereby found to increase. 



Role of Bacteria in Fixation Process 



The presence of nodules on the roots of leguminous plants was 

 known long before their significance was recognized. At first they 



S\ 



.v 



J ^'.....^ 





FiG. 87. Influence of the legume organisms on growth of peas. All are growing 

 in sterilized sand, but in the four pots placed alternately, which show good 

 growth, extract of garden soil has been added (from Hellriegel and Wilfarth, 



after Russell). 



were looked iii)()n as root galls. Althougli Lachmanu observed in 

 1858 that motile ])acteria cause the formation of tlie nodules and that 



