Moreover, he was able to rest on long-accepted facts in just
those points which were the most important, and on these he
had only to throw the light of his chemical knowledge to
dispel the previous darkness. In accordance with his main
purpose, which was to apply organic chemistry and vegetable
physiology to the service of agriculture, Liebig directed the
severity of his criticism first of all against the humus-theory
constructed by chemists and agriculturists and thoughtlessly
adopted by various physiologists ; this was the first thing that
must be got rid of, if the question was to be answered, of what
substances does the food of plants consist, for the humus-theory was at once incorrect, and the product of a want of
reflection which overlooked facts which lay before men's eyes.
Liebig showed that what was known as humus is not diminished
but constantly increased by vegetation, that the quantity in
existence would not suffice for any length of time for the
support of a vigorous vegetation, and that it is not taken up
by plants. This once established, and Liebig's calculations
left no doubt on the point, there remained one source only for
the carbon of the plant, namely, the carbon dioxide of the
atmosphere, with regard to which it was shown by a very
simple calculation resting on eudiometric results that its
quantity is sufficient to supply the vegetation of the whole
earth for countless generations. It is true that Liebig in his
zeal went much too far, when he found something contradictory
in the true respiration of plants, because it is connected with
the elimination of carbon dioxide, and simply denied its reality.
On the other hand the theoretical significance of the fact
established by de Saussure, that the elements of water are
assimilated at the same time as the carbon, was now for the
first time clearly explained. Liebig was better able to realise
the importance of this fact for the theory of nutrition than
de Saussure had been. But these weighty points were not the
ones which attracted most attention with the adherents and opponents of Liebig; the practical tendency of his book made
Page:History of botany (Sachs; Garnsey).djvu/547
Chap. ii.]
of Plants. Liebig.
527
![](http://upload.wikimedia.org/wikipedia/commons/thumb/5/55/History_of_botany_%28Sachs%3B_Garnsey%29.djvu/page547-1024px-History_of_botany_%28Sachs%3B_Garnsey%29.djvu.jpg)