XIII. On the action of light upon plants, and of plants upon the atmosphere

Abstract

The researches of Priestley, Ingenhousz, Senebier, Ellis, and above all of the younger Saussure, have long put us in possession of the leading facts appertaining to the influence of light upon the green parts of plants; and Professor Decandolle has embodied the substance of all that had been ascertained on this subject, up to the year 1831, in his admirable work on Vegetable Physiology. But there appear, by the confession of this latter naturalist, to remain certain subordinate questions respecting this same function, which, though perhaps occasionally touched upon by the above-cited experimentalists and by others, can scarcely be said to have as yet obtained a satisfactory reply. The first of these questions relates to the nature of the influence which, in the cases alluded to, is assignable to light. As this agent often produces chemical changes by its direct action upon inorganic bodies, decomposing saline solutions, discolouring oils, and reducing metallic oxides, so it may be supposed to operate directly upon the air, and to possess the power of decomposing carbonic acid, when this substance is presented to it within the pores of the vegetable tissue. And, on the other hand, as light appears to be a specific stimulus to the vital functions of animals, so it may be imagined to act in a similar manner on those of plants, thus enabling them to secrete from the carbonic acid presented to them the carbon required for their nutrition.