The most obvious effect of light on vegetation is the production of colors, and this it effects by decomposing carbonic acid, and depositing the carbonaceous matter. In most cases, certainly, light is absolutely necessary for the deposition of the green coloring matter, since most plants become perfectly colorless by growing in situations in which they are deprived of light. There are cases, however, in which plants deposit the green chromule, when excluded from the light Green vegetables have been found in caves of the earth, from which the light of day was excluded, and we have seen the cotyledons of the Mustard and the Impatiens balsamina green, when the seeds have germinated within the perfectly closed pericarp; and I have now before me a large onion in which several of the central layers are as green as the leaves, while the parts above and around them are perfectly white. That these are exceptions to a general rule is manifest from innumerable examples to the contrary, constantly occurring within the observation of every one.

If a board lies upon the grass for a short time, the grass becomes blanched; plants growing in a dark cellar are colorless; the interior of the cabbage is white, while the other leaves are green, and if these are removed, those that are exposed soon become green. Plants, which in their natural situation are white, by accidental exposure become green; the side of a potato from which the soil has been by chance removed, soon changes its color from white to green. It may then be laid down as a general principle, that light is the great agent in the production of vegetable colors. All parts of the solar spectrum are not equally efficient in the production of vegetable colors. The yellow rays, according to Draper and Hunt, are the most powerful in the production of colors. Light, Raspail says, influences plants to produce vascular tissue, and to make them combine with earthy bases; while in darkness they produce the cellular tissue, and combine with ammoniacal bases. That light exercises an important agency over the growth of vegetables and their secretions, cannot be doubted. An equal amount of light and darkness seems to be the proportion in which the greatest amount of vegetable vigor is attained.

This is seen exhibited in the equatorial regions, where the days are uniformly twelve hours long, and the nights of equal length, and there we find the most luxuriant vegetation.

If, according to the hypothesis, light acts in producing the firmer and more compact parts of vegetables, and in its absence the more yielding and succulent parts are generated, we should be led to suppose, that where these periods were equal, the perfection of vegetable products would be found; and if the light is in much greater proportion than that of equality, just in the same proportion should we expect to find the products of such regions harder, smaller, and less symmetrical This is the exact state of vegetable products in high latitudes. Trees become harder, smaller, and less luxuriant the higher the latitude, for during the period of their growth, the sun is a great part of the time above the horizon. That this is owing to the action of light, is proved by the fact, that by transporting vegetables into higher latitudes, from equatorial regions, and keep them in an atmosphere, at the temperature of their natural situations, by means of the hot-house, they flourish during the summer; but during the short days, and long nights of winter, they droop, exhibiting their suffering from the due influence of the solar rays.

Raspail's theory above noticed receives confirmation from the fact, that those vegetables which consist entirely of cellular substance, are produced only in the absence of the light of the sun, such as mushrooms, Ac, their growth ceasing at the coming of light. And it is a common notion among gardeners that melons, cucumbers, and like pulpy fruits, increase much more at night than during the day. Although Fungi grow only in darkness, they will never produce spores capable of germination, without the action of the sun's light, and in cases where the light of day never enters, there may be Fungi, but they never increase or perpetuate themselves by the production of spores.

It is during the direct action of the sun's rays, and by their agency, that the most important vegetable products are generated. It is by their influence that water and carbonic acid are decomposed, the oxygen being mostly liberated, and the elements combining in other proportions, for the formation of the various oils, resins, etc, including the most important and abundant of the vegetable products. What is generally termed the sleep of plants, that is, the folding up of compound leaves, and the closing of flowers, is, no doubt, in most instances, occasioned by the want of the stimulating action of the solar rays; for we see leaves and flowers, that were folded up during the night, expand with the first rays of the morning sun.

We have upon record many instances of the singular phenomena of flowers during twilight emitting flashes of light. It is said the daughter of Linnaeus first observed this emission, exhibited by the Tropaeolum Majus or Garden Nasturtium. The flashes occur only during twilight, in the morning or evening; those of the evening being much the most brilliant. The plants, from whose flowers these flashes have been observed to issue most frequently, are the Marigold, Calendula offlcinalist Change lily, Lilium bulbiferum African marigold, Tagetee patula, and Sunflower, Helianthus annuus; but Mr. Trimmer, in an article in the 2d vol. of " Paxton's Magazine of Botany/' p. 193, observes that he had observed it in many other flowers.

The cause is supposed to be electrical, as the flashes are more brilliant, when the atmosphere is the most highly charged with electricity. " In walking in my garden," says Mr. Trimmer, " in which was a considerable quantity of Nasturtium in bloom, not at a'l thinking of the flashing of plants, I was struck with the very vivid flashes that proceeded from them ; the scintillations were the most brilliant that I had ever observed, at the same time the sky was overcast with a thunder-cloud;" and he further remarks, that be always found them most brilliant under such circumstances. The lower orders of plants, as the fungi, have long been noticed as giving light under particular circumstances. Some in New Holland, species of Agaric, are said to produce light enough to read by. In the mines of Germany certain fungi have been long celebrated for the light they emit.