124 PRINCIPLES OF SOIL MICROBIOLOGY 



sidered them as root galls. Lachmann 100 observed, in 1858, that motile 

 bacteria cause the formation of the nodules and he suggested that the 

 nodules are the organs of nitrogen fixation. In 1866 Woronin 101 found 

 the nodules to consist of bacteria, but even he considered these nodules 

 as pathological outgrowths. Frank 102 demonstrated in 1879 that the 

 formation of nodules can be prevented by the sterilization of the soil. 

 Frank's view as well as that of other investigators 103 was that the 

 nodules are caused by outside infection. Hellriegel and Wilfarth 104 and 

 Atwater 105 • 106 finally demonstrated in 1884-1886 that the nodules on 

 the roots of leguminous plants are due to bacterial infection, that this is 

 beneficial, since it is within these nodules where the bacteria fix the 

 atmospheric nitrogen. When nodules were formed, the plants could 

 be grown on artificial soils containing but traces of combined nitrogen, 

 provided the mineral elements necessary for the nutrition of the 

 plant were present. In the absence of nodules, the plants were unable 

 to utilize the atmospheric nitrogen for its growth. When sterilized 

 soil was treated with fresh soil infusion, nodule formation took place 

 and the plants grew normally. The growth of the Gramineae de- 

 pended, however, on the nitrate content of the soil. These results 

 were soon confirmed by Lawes and Gilbert 107 and others. 



100 Lachmann. tlber Knollchen der Leguminosen. Landw. Mitt. Zeitschr* 

 K. Lehranstalt. u. Vers. Sta. 1858, p. 37. 



101 Woronin, M. Observations sur certaines excroissances que presentent les 

 racines de l'aune et du lupin des jardins. Ann. Sci. Nat. Bot., ser. 5, 7: 73-86. 

 1867; also Mem. Acad. Imp. Sci. St. Petersberg, 7 ser., 10: 1-13. 1866. 



102 Frank, B. Uber die Parasiten in der Wurzelanschwellung der Papiliona- 

 ceen. Bot. Ztg. 37: 377-388, 393-400. 1879. 



103 Ward, M. On the tubercular swellings on the roots of Vicia faba. Phil. 

 Trans. Roy. Soc. London 178, 1887. 



104 Hellriegel, H. Welche Stickstoffquellen stehen der Pflanze zu Gebote? 

 Tagebl. Natforsch. Vers. Berlin, 1886, p. 290; Chem. Centrbl. 1886, 871; Landw. 

 Vers. Sta. 33: 464-465. 1886. Hellriegel, H., and Wilfarth, H. Untersuchungen 

 uber die Stickstoff-Nahrung der Gramineen und Leguminosen. Beilageheft 

 Ztschr. Ver. Riibenzuckerind. 1888, 1-234. 



105 Atwater, W. O. On the assimilation of atmospheric nitrogen by plants. 

 Rpt. Brit. Assn. Adv. Sci. 54: 685. 1884. 



106 Atwater, W. O., and Woods, C. D. The acquisition of atmospheric nitrogen 

 by plants. Amer. Chem. Jour. 6: 365. 1885; also 8: 398-420. 1886; Conn. 

 (Storrs) Agr. Exp. Sta. Bui. 5, 1889; Conn. (Storrs) Agr. Exp. Sta. Ann. Rpt. 1889. 

 11-51. 



107 Lawes, J., and Gilbert, J. New experiments on the question of fixation of 

 free nitrogen. Proc. Roy. Soc. London 47: 85-118. 1890. 



