EXPERIMENTS OF HELLIUEGEI. AM ) \\ I l.I- Ainil 1 1 



Lawes and Gilbert themselves were disposed to look t.. tlir 

 subsoil as the source of this excessive amount of nitro^^en, and 

 were conducting experiments to ascertain whether the widely 

 ranging roots of the leguminous plants, in virtue of their hij^^hly 

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

 dormant nitrogenous compounds in the subsoil, wlicn the 

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

 1886, of the researches of Hellriegel and Wilfarth. 'IMnvsc; 

 investigators found that when plants w^ere grown in sand and 

 . were fed with nutrient solutions, the Graminea\ the Crucifera'. 

 the Chenopodiacea3, the Polygoneoe, 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 Leguminosai, however, a plant 

 was observed sometimes to recover from the stage of nitrogen 

 starvation and begin a luxurious grow^th 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 w^ere grown in sterile sand, Itut 

 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 consideraljle 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 

 Bacilhis radicicola has been given; they could only l)e pro- 

 duced ])y previous infection, either ])y an extract of the 

 crushed nodules or of a cultivated soil, in some cases 

 (lupins, serradella) only by soil which had i)reviously carried 

 the same crop. 



Gilbert had been present at themceting of th(> Naturforsclicr 

 Versamndung at Halle when Hellriegel and AVilfarlh read tlu-ir 



