plants to physico-mechanical principles; de Saussure and others
showed that the heat of plants is a product of respiration, and
by 1840 the earlier theory of a vital force might be looked
upon as antiquated and obsolete. It remained to restore to
their rights the observations of Ingen-Houss and de Saussure,
which under the influence of that theory and of the notions
respecting the humus had been so utterly misconstrued.
Liebig set aside the humus-theory in 1840, and referred the
carbon of plants entirely to the carbon dioxide of the atmosphere,
and their nitrogenous contents to ammonia and its derivatives;
he claimed the components of the ash as essential factors in
the nutrition, and taking his stand on the general laws of chemistry endeavoured to obtain chiefly by the method of
deduction an insight into the chemical processes of assimilation
and metabolism. The whole theoretical value of the facts
discovered by Ingen-Houss, Senebier and de Saussure was
first made apparent by the connection which Liebig succeeded
in establishing between the phenomena of nutrition. The
doctrine of nutrition burst suddenly into new life; firm
ground was gained, and the botanist, no longer distracted by
the difficulties raised by the vital force but resting on physical
and chemical principles, might now resume the task of investigation. Oxygen-respiration denied by Liebig was first
of all re-established by von Mohl and others. Liebig's views
on the source of nitrogen in plants and on the importance of
the ash-constituents rested chiefly on general considerations
and observations and on calculation, and had now to be tested
by systematic investigation and especially by experiments on
vegetation in individual plants. And here the place of honour
must be assigned to Boussingault, who pursued the path of
pure induction as contrasted with Liebig's deductive mode of
proceeding, gradually improved the methods for experimenting
on vegetation, and soon succeeded in so producing plants in a
purely mineral soil free from all humus, that he finally settled
the question of the derivation of the carbon from the atmosphere
Page:History of botany (Sachs; Garnsey).djvu/469
Chap. ii.]
the Nutrition of Plants.
449
