Chap, hi The Absorption of Nitrogen 331 



Beijerinck's work, which appeared at the end of the century, 

 is not so convincing as that of Winogradsky, inasmuch as 

 he did not support it by chemical analysis. 



The position at the end of the century, then, was that 

 there is no evidence of appropriation of atmospheric nitro- 

 gen by normal green plants, but that circumstances point 

 to a fixation in the soil by a kind of symbiotic association 

 of certain organisms. 



Certain other cases of a still more special symbiosis were 

 noted, to which attention will be drawn a little later. 



The exact nature of the fixation remained undetermined, 

 but facts pointed clearly to the soil as the chief source of 

 the combined nitrogen absorbed by the normal plant. 



During the period under discussion the combined nitrogen 

 of the soil, its sources, forms, and fate, were the subject 

 of much investigation, which proceeded side by side with 

 the study of the problems we have just examined. The 

 questions originated in the effort to determine the form 

 or forms in which the soil supplies combined nitrogen, 

 and the interactions in the soil by which those most advan- 

 tageous are secured. 



The sources which in i860 appeared possible were the 

 compounds of ammonia and of nitrous and nitric acid, 

 together with the heterogeneous complex generally referred 

 to as humus. 



The old doctrine of the humus had received its death- 

 blow at the hands of Liebig, but its disappearance from 

 the scene was very gradual. It was in a way resuscitated 

 by De Saussure in his old age and was put forward also 

 by Von Mohl and others, and not finally disposed of till 

 the researches of Boussingault, which were still incomplete 

 in i860. By these, however, we find established the part 

 played especially by the nitrates, which he showed to act 

 by furnishing their combined nitrogen to the plant directly, 

 and not indirectly by dissolving the humus. 



