Plants and their leaves, if excluded from light, become of a white or pale yellow color, in which state they are said to be blanched or etiolated. This is occasioned by their being neither able to decompose the water they imbibe, nor to inhale carbonic acid. In the dark, plants can only in-bale oxygen, and thus deprived of free hydrogen and carbon, on the due assimilation of which by the leaves all vegetable colors depend, and saturated with oxygen, they of necessity become white. An excess of oxygen has uniformly a tendency to whiten vegetable matters; and to impart that excess to them is the principle upon which all bleaching is conducted. An over-dose of oxygen causes in them a deficiency of alkaline, or an excess of acid matter, and light enables plants to decompose the acid matter, and to restore that predominance of alkalinity on which their green color depends. Sennebier and Davy found most carbonic acid in blanched leaves; and all green leaves contain more alkaline matter than the rest of the plant which bears them. Every cook knows that a little alkali, carbonate of soda, added to the water, improves the green hue of her boiled vegetables. That this is the cause of the phenomenon is testified by direct experiment.

Blanched Celery and Endive, and the white inner leaves of the Cos Lettuce, contain about one-third more water than the same parts when green; and if submitted to destructive distillation, do not yield more than half so much carbon. Then, again, if a plant of Celery is made to vegetate in the dark, under a receiver containing atmospheric air, with the addition of not more than one-twenty-fifth part of its bulk of a mixture of carburetted hydrogen, and hydrogen such as is afforded by the distillation of coal, that plant, though it becomes paler than when grown in the daylight, still retains a verdant color.

So effectual is the metamorphosis of plants effected by excluding them from the light, that Professor Robinson brought up from a coal mine, near Glasgow, some whitish-looking plants of which no one could detect the name or character. After exposure to the light the white leaves decayed and were succeeded by green ones, which speedily revealed that the plants were Tansy. They had found their way into the mine in some sods from a neighboring garden; but though they had retained life in its dark galleries, they had entirely lost their natural color, odor, combustibility. This is only in accordance with the gardener's yearly experience; for this blanched Sea-kale, Endive, and Lettuce are totally dissimilar in flavor and appearance to the plant left in its natural state.

Sir H. Davy excluded a Cos Lettuce from the light. In six days it was rendered very pale, and at the end of another week it was quite white; the growth of the plant was checked, and the analysis of its leaves showed that they contained more carbonic acid and water, but less hydrogen and residual carbon, than an equal weight of green leaves.

A Potato has been observed to grow up in quest of light from the bottom of a well twelve feet deep; and in a dark cellar a shoot of twenty feet in length has been met with, the extremity of which had reached and rested at an open window. In the leaves of blanched vegetables peculiar chemical compounds are formed. Thus in the blanched shoot of the Potato a poisonous substance called solanin is produced, which disappears again when the shoot is exposed to the light and becomes green (Otto). In Asparagus, in blanched Clover (Piria), and other plants grown in the dark, asparagin is formed, and no doubt other peculiar changes take place, which are not yet understood. - (Johnston's Lectures on Agricultural Chemistry).

It deserves notice, that it has been proved by the experiments of Dr. Hope and others, that light from artificial sources may be concentrated so as to enable plants to absorb oxygen and perfect those elaborations on which their green color depends; and the light of the moon has a similar influence. A similar concentrated light will make the Pimpernel, and other flowers which close until sunrise, open their petals and rouse from their rest; a fact which gives another reason why plants in rooms frequented at night become weak and exhausted sooner than those that remain, as Nature dictates, unexcitcd at night.

The yellow, red, and light brown tints which render the foliage of our plants so beautiful in autumn arise from the absorption of an excess of oxygen gas. When the reduced temperature of the season deprives a leaf of the power to elaborate the sap, and, indeed, stops the circulation to it of that fluid, the absorbent powers of the organ are reversed, and, instead of carbonic acid, it inhales oxygen. The effect is speedily perceptible. Gallic acid forms, and this, modified by the various saline constituents of different leaves, changes the hue of their green coloring matter, called chlorophyl or chromule, into various tints of yellow, red, and brown. This is the general effect of acids acting upon vegetable greens, and that it is the cause of the autumnal change of color in leaves is proved by the fact, that if a green leaf be dipped into an acid it assumes the same hue; and if some red or yellow leaves be dipped into an alkaline solution they are rendered green, the alkali evidently neutralizing the acid that had wrought the unnatural change of color.

Changes similar to those resulting from age may occur merely from accident, as from the puncture of insects, the growth of parasitic fungi, or the blighting influence of frost. First they change to yellow; then they change to red.

But some leaves present naturally a different color on each surface. The upper surface of the leaf of the Cyclamen is green; the under surface is red; yet the red chromule, in this case, exhibits the same chemical properties as the chromule that has been changed to red as the result of age. - (Macaire).

The hints and warnings which these facts suggest to the mind of every reflecting practitioner arc numerous. They explain and enforce the necessity of a regular, and by no means, as to quantity, indiscriminate supply of water to plants; the importance of shading after their transplanting, yet the evidence of unnecessary shading to those established; and of a free circulation of air, etc.; and the necessity of keeping the leaves as clean and as free from injury as possible. The leaves of plants must often be removed; and in some instances this is done with essential benefit; but the horticulturist should constantly keep in mind that, with every leaf that ho removes, he deprives the plant of a primary organ of its existence.

Light, it has just been stated, is the cause of the green color of plants; but it should be observed that its full power is only beneficial when directed upon their upper surface. This is evidenced by the position they always maintain. Trees, whether nailed to a north or south wall, or trained as espaliers, always turn the upper surfaces of their leaves outward to where there is most light. Plants in a hothouse, uninfluenced by the direction from whence proceeds the first supply of air, or the greatest degree of heat, turn not only their leaves, but their very branches, towards the source of brightest light, and, if not turned almost daily, entirely lose their symmetrical form.

If the branches of a tree trained against a wall, or other support, are so moved when their leaves are completely expanded, that the under side of the foliage is the most exposed to the light, they are always found to regain in their natural position in a day or two. If the experiment be often repeated on the same individual, the leaves to the last continue to revert, but become gradually weaker in the effort, partially decay, and their epidermis peels off. Succulent leaves are particularly sensitive of light, but those of pinnated, luguminous plants - as the Pea and Kidney Bean - are still more so. - J., in Cottage Gardener.