FIXATION OF NITROGEN BY BACTERIA 191 



these conditions soon suffered from nitrogen hunger and 

 remained small and stunted in growth. It was not until 

 about 1886 that the question of the assimilation of free 

 nitrogen by leguminous plants was solved by the investi- 

 gations of Hellriegel and Wilfarth in Germany. Any 

 doubt about the problem was removed, and a satisfactory 

 explanation was furnished of the diametrically opposite 

 results obtained by Ville and Lawes and Gilbert. It 

 was discovered that peas grown in sterilized soil are 

 dependent on combined nitrogen and die of nitrogen 

 hunger when the amount supplied is small, just as cereals 

 do ; neither peas nor wheat are able to make use of 

 the free nitrogen of the air under these conditions. 

 They learnt, however, that after an addition to the 

 sterilized soil of a few cubic centimetres of a watery 

 extract of soil from a field which had grown peas, the 

 failing wheat plants were in no way benefited, but the 

 drooping pea plants recovered rapidly and increased 

 greatly in nitrogen-content, the free nitrogen of the 

 atmosphere supplying all their needs in respect of this 

 element. Upon the roots of the peas to which the 

 extract of the soil had been applied were seen a number 

 of warty excrescences termed nodules (Fig. 33), but on 

 those grown entirely in sterilized soil none were found. 

 A direct relationship was found to exist between the 

 possession of root nodules and the assimilation of free 

 nitrogen by leguminous plants. With them the plants 

 thrive luxuriantly, and nitrogenous compounds accumu- 

 late in all their tissues, even when no other source is 

 available for nitrogen than that present in a free state in 

 the atmosphere : without them growth is limited unless 

 combined nitrogen is supplied. The fact that steriliza- 

 tion of the soil or heating the watery soil extract to 



