CHAP, ii.] of Plants. Ingcn-Houss and dc Sanssurc. 491
The theory of nutrition, like a forced plant, needed light that it might recover strength. This light came with the discoveries of Ingen-Houss, and with the mighty strides made by chemistry after 1760 in the hands of Lavoisier.
4. THE MODERN THEORY OF NUTRITION FOUNDED BY INGEN-
Houss AND THEODORE DE SAUSSURE. 1779-1804.
THE two cardinal points in the doctrine of the nutrition of plants, namely that the leaves are the organs which elaborate the food, and that a large part of the substance of the plant is derived from the atmosphere, were established, as we have seen, by Malpighi and Hales, and employed by them in framing their theory ; it remained to supply a direct and tangible proof of the fact that the green leaves take up a constituent of the atmosphere and apply it to purposes of nutrition. It was evi- dently the want of such direct proof which caused the suc- cessors of the first great physiologists to overlook the import- ance of the propositions thus obtained by deduction, and so to grope their way in the dark with no principle to guide them.
The discoveries of Priestley, Ingen-Houss and Senebier, and the quantitative determinations of de Saussure in the years be- tween 1774 and 1804, supplied the proof that the green parts of plants, and the leaves therefore especially, take up and decompose a constituent of the air, while they at the same time assimilate the constituents of water and increase in weight in a corresponding degree ; but that this process only goes on copiously and in the normal way, when small quantities of mineral matter are introduced at the same time into the plant through the roots. The discoveries and facts, from which this doctrine proceeded, were those which overthrew the theory of the phlogiston, and from which Lavoisier deduced the prin- ciples of modern chemistry ; the new theory of the nutrition of plants was indeed directly due to Lavoisier's doctrines, and it is necessary therefore to take at least a hasty glance at the