CHAPTER III 



THE ABSORPTION OF NITROGEN AND THE CON- 

 STRUCTION OF NITROGENOUS SUBSTANCE 



The final abandonment of the theory of the humus as 

 the source of the nitrogen of plants was brought about 

 during the years 1840-60 ; the idea that the nitrates and 

 the ammonia compounds of the soil only serve to dissolve 

 it and enable absorption to take place having been advanced 

 and again set aside during that interval. The older writers, 

 to whose advocacy the theory was due, were gradually 

 retiring from the field and new workers were beginning to 

 make their influence felt. Of the latter, the man who stood 

 out most prominently in the field of vegetable physiology 

 was Boussingault, to whose work upon the methods of 

 carbon dioxide absorption reference was made in the last 

 chapter, and to whom we owe most important contribu- 

 tions to the solution of the problems associated with the 

 appropriation of nitrogen. Even before 1840 he had carried 

 out investigations on vegetation, but it was not till towards 

 the end of the twenty years that he succeeded in estab- 

 lishing the important fact that the normal green plant can 

 make no use of the free nitrogen of the atmosphere, but 

 that it obtains its supplies from the soil, and that these 

 supplies are furnished not in the shape of the humus, an 

 ill-defined accumulation of heterogeneous materials, but in 

 that of the nitrates and ammonia compounds it contains. 



In his History of Botany Sachs bore eloquent testimony 

 to the services rendered by him in this field of research, 

 attributing to him the credit of being the founder of modern 

 methods of conducting experiments on vegetation. 



Boussingault was engaged on these researches at the 



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