the movement of the sap, and generally into the whole internal
economy of plants, depends on a knowledge of the fact, that it
is only the cells which contain chlorophyll, and therefore in
the higher plants the leaves chiefly as consisting largely of
such cells, which have the power of converting the gaseous
food supplied by the atmosphere into the substance of the
plant with the aid of the materials taken up from the soil.
This fact is of fundamental importance to the whole theory of
the nutrition of plants; it is only by a knowledge of it that we
can explain the movement of material connected with nutrition
and growth, the dependence of vegetation on light, and to a
great extent also the function of the roots.
But this principle could not be discovered till the new chemical system founded by Lavoisier took the place of the old phlogistic chemistry, and it is remarkable that the discoveries, which laid the foundation of modern chemistry in the period between 1760 and 1780, contributed essentially to the establishment at the same time of the modern doctrine of the nutrition of plants. Ingen-Houss, in reliance on Lavoisier's antiphlogistic views on the composition of air, water, and the mineral acids, succeeded in proving that all parts of plants are continually absorbing oxygen and forming carbon dioxide, but that the green organs at the same time under the influence of light absorb carbon dioxide and exhale oxygen ; and as early as 1796 he considered it probable that plants obtain the whole mass of their carbon from the carbon dioxide of the atmosphere. Soon after (1804) de Saussure proved, that plants, while they decompose carbon dioxide, increase in weight by a greater amount than that of the carbon which they retain, and that this is to be explained by the fact that they at the same time fix the elements of water. He likewise showed that the small quantities of saline compounds, which plants take up from the soil, are a necessary part of their food, and that it was at least probable, that the nitrogen of the atmosphere does not contribute to the formation of nitrogenous substances in plants. Setiebier
