Light has a powerful influence over the health and life of a plant, from the moment its leaves pierce through the surface of the soil. If absent, they become yellow, or even white, unless uncombined hydrogen be present, in which case they retain their verdure.

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.

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 colour 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 which then remain, as nature dictates, unexcited by light.

A deficiency of light decreases the decomposing power of the leaves. For this reason the best glass should always be employed in the sashes of the hot-house, conservatory, and other structures of the forcing department. But the benefit sought for is frustrated, if that glass be not constantly well cleansed. The best glass, if dirty, allows fewer rays of light to pass through than inferior glass if kept bright.

Solar light is essential to the ripening of all fruit; it will not ripen in the dark; and the greater the light's intensity and the longer its daily endurance, the sweeter and the higher is the fruit's flavour. No fruits are so luscious as those grown within the tropics, and the fruits of the temperate zone are excellent in proportion to the brightness of its seasons. That light is essential in causing the colour of the leaves and other parts of plants, has been noticed already; and it aids the ripening process of fruit in a similar manner, to convert their acid and mucilaginous constituents into sugar: much carbon and hydrogen have to be got rid of; and this is effected, if light be admitted, by the evolution of carbonic acid and watery vapour. How light operates in promoting this and other decompositions, which are effected by the vegetable organs, is at present a mystery; but so it is; and the gardener promotes its access as much as lies within his power, by removing overshadowing leaves, by employing the best glass in his hot-houses, and by having their interior whitened; for white surfaces reflect all the rays of light back upon the objects those surfaces inclose.

Almost every flower has a particular degree of light requisite for its full expansion. The blossoms of the pea, and other papilionaceous plants, spread out their wings in fine weather to admit the solar rays, and again close them at the approach of night. Plants requiring powerful stimulants, do not expand their flowers until noon; whilst some would be destroyed if compelled to open in the meridian sun. Of such is the night-blooming cereus, the flowers of which speedily droop, even if exposed to the blaze of light attendant on Indian festivities. - Princ. of Gardening.