FIXATION OF ATMOSPHERIC NITROGEN. 275 



may be brought about at will by placing the roots of young plants in con- 

 tact with pieces of old tubercles. Hellriegel in his preliminary communi- 

 cation of 1886 also showed that outside infection is necessary for the 

 production of tubercles, and called attention to the true function of the 

 latter as laboratories wherein nitrogen compounds are manufactured 

 out of elementary nitrogen. The true worth of HellriegePs investigations 

 was brought out more clearly in another paper that he published jointly 

 with Wilfarth in 1888. The authors showed that in sterilized soils 

 legumes behaved precisely like non-legumes, and died ultimately of 

 nitrogen hunger when not provided with nitrates or other suitable nitrogen 



FIG. 68. Ps. radicicola. i, From Melilotus alba; 2 and 3, from Medicago saliva; 4, 

 from Vicia v illosa. (After Harrison and Barlow from Lipman.) 



compounds. On the other hand, when the sterilized soil was later 

 infected with a few drops of leachings from fresh soil that had supported 

 a normal growth of legumes, the starving plants recovered and grew 

 vigorously. Under the same conditions non-legumes did not recover. 

 The recovery of the starving legumes was found to be coincident with 

 the formation of tubercles. 



Hellriegel and Wilfarth's studies were soon confirmed by the inves- 

 tigations of others. Wigand showed in 1887 that the tubercles contained 

 within them true bacteria. In the following year Beyerinck reported the 

 successful isolation of these bacteria on artificial media, and named the 

 organism B. radicicola (Fig. 68). Prazmowski also isolated pure cultures 

 of B. radicicola, and followed the entrance of the organisms into the root 



