CHAP, ii.] of Plants. Boussingault. 531 



1860 that new paths were struck out on these subjects, and 

 important results achieved. More important at the time for 

 the advance of the science was the further examination of 

 the question respecting the source of the nitrogen which 

 plants assimilate ; it was the more necessary that this point 

 should be finally settled, because Liebig's deductions still 

 gave room for many doubts, and the first of vegetable 

 physiologists, de Saussure, in his later days made the mistake 

 of coming forward in opposition to Liebig as a defender of the 

 humus-theory, maintaining (1842) that ammonia or the nitrates 

 are not themselves the food-material of plants, but only serve 

 to dissolve the humus. Others also found it difficult to give 

 up entirely the old and favourite doctrine of the humus ; 

 though von Mohl and others acknowledged that the carbon of 

 plants is mainly derived from the atmosphere, yet they thought 

 themselves obliged to assign to the humus, on account of the 

 nitrogen which it contains, a very important share in promoting 

 vegetation. Under these circumstances it was extremely 

 fortunate for physiology that BOUSSINGAULT took up the ques- 

 tion. He had occupied himself before the appearance of Liebig's 

 book with experimental and analytical investigations into 

 germination and vegetation, and specially into the source of 

 nitrogen in plants. His experiments in vegetation in 1837 

 and 1838 produced no very decisive results; but he continued 

 them for some time longer, improving his methods of observa- 

 tion from year to year ; and between the years 1851 and 1855 

 he succeeded in establishing with all certainty as the result of 

 many repeated trials, that plants are not capable of assimilating 

 the free nitrogen of the atmosphere, but that a normal and 

 vigorous vegetation is produced, when they are supplied with 

 nitrogen from the nitrates in the soil. It appeared also that 

 plants will flourish in a soil from which all trace of organic 

 substance has been removed by heat, if a nitrate is added to 

 the constituents of the ash ; this proves at the same time that 

 the whole of the carbon in such plants is derived from the 

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