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 {{sc|Boussingault} took up the question. 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 observation 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
Page:History of botany (Sachs; Garnsey).djvu/551
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Chap. ii.]
of Plants. Boussingault
531
![](http://upload.wikimedia.org/wikipedia/commons/thumb/5/55/History_of_botany_%28Sachs%3B_Garnsey%29.djvu/page551-1024px-History_of_botany_%28Sachs%3B_Garnsey%29.djvu.jpg)