EXPEEIMENTS OF HELLRIEGEL AND WILFAKTH 11 



Lawes and Gilbert themselves were disposed to look to the 

 subsoil as the source of this excessive amount of nitrogen, and 

 were conducting experiments to ascertain whether the widely 

 ranging roots of the leguminous plants, in virtue of their highly 

 acid sap, did not possess some special power of attacking the 

 dormant nitrogenous compounds in the subsoil, when the 

 clearing up of the whole subject came with the publication, in 

 1886, of the researches of Hellriegel and Wilfarth. These 

 investigators found that when plants were grown in sand and 

 were fed with nutrient solutions, the Graminese, the Cruciferse, 

 the Chenopodiacese, the Polygonese, grew almost proportionally 

 to the amount of combined nitrogen supplied ; and, if this were 

 absent, nitrogen starvation set in as soon as the nitrogen of the 

 seed was exhausted. With the Leguminosse, however, a plant 

 was observed sometimes to recover from the stage of nitrogen 

 starvation and begin a luxurious growth which lasted until 

 maturity, though no combined nitrogen was supplied. In such 

 cases the root of the plant was always found to be set with the 

 little nodules characteristic of the roots of leguminous plants 

 when growing under natural conditions. Further experiments 

 were made in which the plants were grown in sterile sand, but 

 as soon as the stage of nitrogen hunger was reached, a small 

 portion of a watery extract of ordinary cultivated soil was 

 added ; whereupon the plants receiving the extract recovered 

 from their nitrogen starvation and grew to maturity, assimilat- 

 ing considerable quantities of nitrogen. The renewed growth and 

 the assimilation of nitrogen were always found to be attendant 

 upon the production of nodules on the roots. The nodules 

 were found to be full of bacteria, to which the name of 

 Bacillus radicicola has been given ; they could only be pro- 

 duced by previous infection, either by an extract of the 

 crushed nodules or of a cultivated soil, in some cases 

 (lupins, serradella) only by soil which had previously carried 

 the same crop. 



Gilbert had been present at the meeting of the Naturforscher 

 Versammlung at Halle when Hellriegel and Wilfarth read their 



