FIXATION OF FREE NITROGEN. 139 



It was in the Agricultural Chemistry Section of the 

 " Naturforscher Versammlung," held in Berlin in 1886, when 

 one of us happened to be presiding, that Professor Hellriegel 

 first announced his new results. Quite consistently, not only 

 with common experience in agriculture, but also with the 

 direct experimental results of ourselves and others, Hell- 

 riegel found that plants of the Gramineous, the Cheno- 

 podiaceous, the Polygonaceous, and the Cruciferous Orders, 

 depended on combined nitrogen supplied within the soil. 

 On the other hand, he found that leguminous plants did not 

 depend entirely on such supplies. His results were, indeed, 

 not only very definite, but it is seen that they had a special 

 bearing on the admittedly unsolved problem of the source of 

 the whole of the nitrogen of leguminous crops. 



In the case of these plants — that of peas, for example — 

 it was observed that, in a series of pots to which no nitrogen 

 was added, most of the plants were apparently limited in 

 their growth by the amount of nitrogen which the seed 

 supplied. Here and there, however, a plant growing under 

 ostensibly the same conditions grew very luxuriantly ; and 

 on examination it was found that whilst no nodules were Root-nod- 

 developed on the roots of the plants of limited growth, ules - 

 they were abundant on those of the luxuriantly grown 

 plants. 



In view of this result Hellriegel, with his colleague Dr 

 Wilfarth, instituted experiments to determine whether, by 

 the infection of the soil with appropriate organisms, the 

 formation of the root-nodules, and luxuriant growth, could 

 be induced ; and whether, by the exclusion of such infection, 

 the result could be prevented. To this end, they added to 

 some of a series of experimental pots 25 or 50 cubic centi- 

 metres of the turbid watery extract of a fertile soil, made by 

 shaking a given quantity of it with five times its weight of 

 distilled water, and then allowing the solid matter to subside. 

 In some cases, however, the extract was sterilised. In those 

 in which it was not sterilised, there was almost always 

 luxuriant growth, and abundant formation of root-nodules ; 

 but with sterilisation there was no such result. Consistent 

 results were obtained with peas, vetches, and some other 

 Leguminosee ; but the same soil-extract had little or no effect 

 in the case of lupins, serradella, and some other plants of the 

 family which are known to grow more naturally on sandy 

 than on loamy or rich humus soils. Accordingly, they made 

 a similar extract from a diluvial sandy soil, where lupins 

 were growing well, in which, therefore, it might be supposed 

 that the organism peculiar to such a soil would be present ; 

 and, on the application of this to a nitrogen-free soil, lupins 



