58 



from the first period of nitrojjen starvation was observed to be closely 

 ■counected with the growth of the tubercles, and the infeience was 

 •drawn that the cause of the renewed vigor of the plants was to be (bund 

 in the root tubercles. Thinking that microorganisms in the soil might 

 in some way be connected with this process, Uellriegel cultivated peas 

 in soils which contained no nitrogen, but which were watered with a soil 

 infusion. This infusion was made by shaking in water soil from a fer- 

 tile field, and then allowing it to settle. The water thus treated, he 

 argued, would contain whatever organisms there might have been 

 in the soil, and if the production of the tubercles and the fixing of 

 nitrogen by plants iu nitrogen-free soil was due to the action of these 

 organisms, plants watered by such an infusion ought to indicate it. 

 The results were very striking. All of the peas watered with s(»il infusion 

 showed a vigorous growth, an increase in nitrogen, and numerous root 

 tubercles. Those not thus inoculated showed wide variations from vig- 

 orous growth to com[>lete failure to recover from the starvation period. 

 To make the result more positive, peas were cidtivated in nitrogen-free 

 soils which had been sterilized by heat. In this case no recovery from 

 the starvation period occurred uidess the plants were fust treated with 

 the soil infusion. P'inally it was provetl that the soil infusion was power- 

 less to produce any effect if it was first sterilized by heat. 



From these results of Hellriegel's investigations the conclusion was 

 warranted that leguminous jdants in some way ac(piire the power of 

 assimilating nitrogen fiom the atinosphere, but that this power <loes 

 not inhere in the phmts themselves. In onler that they may make 

 use of the nitrogen in the air, it is necessar}' that certain organisms in 

 the soil should penetiate into their roots. These organisms growing 

 in the roots of the legumes produce the root tubercles, an«l in some way 

 enable the plants to assimilate atmospheric nitrogen. Under onliiiary 

 conditions these microorganisms are freely sui)plied to the i)lant from 

 the soil, but in the culture experiments cited above in order to make 

 sure that they would be present, the plants were inoculated with an 

 aqueous infusion prepared from a rich soil. 



Two other points of signiticance were brought out by Uellriegel: 

 (1) If plants were grown in soils containingnitrogen, they could assini- 

 ilate this ecjually well whether root tubercles were present or not, thus 

 indicating that the tubercles are not necessary to the assimilation of 

 soil nitrogen. (-) Different species of legumes seemed to have different 

 species of tubercle microfirganisms associated with them, for it was 

 found that while soil in which peas had been growing might be well 

 adai)ted fc(o ])roduce the necessary tubercles in other pea ])lants, ir 

 might not produce them in other species of legumes. 



These suri)rising results of Hellriegel's inquiries led to other experi- 

 ments on the functions of the root tubercles and the power of plants to 

 assimilate atmospheric nitrogen. Hellriegel's general results have been 

 oonfirmed by several observers, among whom may be mentioned Frank, 

 Prazmowski, Atwater and Woods, Laucs and (jilbei't,,-Beverin(k ;ind 



