Scientific American 12, 11.12.1847
The warmth of the sun has comparatively little to do with the specific action of his rays on the chemical functions of the plant, which is illustrated by the experiments at Mr. Hunt of the Royal Agricultural Society of England on the effect of the rays of light of different colors on the growing plant. He sowed cress seed, and exposed different portions of the soil in which the seeds were germinating, to the action of the red, yellow, green, and blue rays, which were transmitted by equal thicknesses of solutions of these different colors. "After ten days there was, under the blue fluid, a crop of cress of as bright a green, as any which grew in full light. and far more abundant. The crop was scanty under the green fluid, and of a Pale green unhealthy color. Under the yellow solution only two or three plants appeared, but less pale than those under the green; while beneath the red a few more plants came up than under the yellow, though they were also of an unhealthy color. The red and blue colors being now mutually transferred, the crop formerly beneath the blue in a few days appeared blighted, while on the patch previously exposed to the red some additional plants sprung up."
Besides the rays of heat and of light, the sunbeam contains what have been called chemical rays, not distinguishable to our senses, but capable of 'tieing recognized by the chemical effects they produce. These rays appear to differ in kind, as the rays of different colored lights do. It is to the action of these chemical rays on the leaf, associated with the blue light on the solar beam, that the chemical influence of the sun on the growth of the plant is to be ascribed, by the decomposition of the carbonic acid absorbed from the air by the leaf of the plant on the interior of the leaf, the retention of the carbon, and the rejection or onimission of the oxygen contained in the carbonic acid of the plant, which is returned to the atmosphere, which carbon retained uniting with the elements of water (hydrogen or oxygen,) absorbed at the same time by the roots, give rise to and furnish the elements for the formation of woody, cellular fibre, &c., and for which cause it is that "if light be excluded, vegatation never produces a leaf or a stock."
The decomposition of the carbonic acid contained in the atmosphere, and the emission of oxygen gas from plants, is determined by the solar light, pure oxygen gas is, therefore, separated by the action of light, and the operation is stronger as the light is more vivid. By this continued emission of vital air the Almighty thus incessantly purifies the air, and repairs the loss of oxygen occasioned by respiration, combustion, fermentation, putrefaction, and numerous other processes which have a tendency to contaminate this fluid, so essential to the vigor and comfort of animal life; so that, in this way, by the agency of light, a due equilibrium is always maintained between the constituent parts of the atmosphere.
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