Leaves (Functions Of). If the leaves of a tree be stripped off, the fruit comes to nothing, which is exemplified every year in gooseberry bushes, the leaves of which have been devoured by caterpillars; and though the fruit trees of warm climates, partly naturalized with us, grapes and peaches, for instance, ripen their fruit sooner, perhaps, if partially deprived of their leaves ; yet if that practice be carried too far, the fruit perishes. The white mulberry, indeed, cultivated in the south of Europe, for the food of silk-worms only, bears wonderfully the loss of its foliage three or four times a year. These facts have led some to think, think, leaves were merely a clothing, or a protection against cold and beat. Though this is undoubtedly true, still it is a very small part Of the leaves.
That leaves give out moisture, or are organs of insensible perspiration, is proved by the simple experiment of gathering the leafy branch of a tree, and immediately stopping the wound at its huso, with wax, or any other tit substance, to prevent the effusion of moisture in that direction. In a very short time, the leaves droop, wither, and are dried up. If the same branch, partly faded, though not dead, be placed in a very damp cellar, or immersed in water, the leaves revive, by which their power of absorption is also proved.
The great annual sun-flower is said to have lost by perspiration 1 lb. 14 oz. weight, in the course of twelve hours, in a hot dry day. In a dry night, it lost about three ounces; in a moist night, scarcely any alteration was observable; but in a rainy night it gained two or three ounces. The cornelian cherry is most remarkable in this ect: the quantity of fluid which evaporates from its leaves, in the course of twenty-four hours, is said to be nearly equal to twice the weight of the whole shrub.
The perspiration of aquatic plants seems to he remarkably copious. Of these some grow constantly immersed in water. Their leaves are peculiarly vascular, and dry very quickly in the air, withering in a few minutes after exposure to it. Other aquatics float with only the upper surface of their leaves exposed to the air, which surface is so contrived that water will scarcely remain upon it. These leaves, though extremely juicy, dry with great rapidity, as does every part of the plant, when gathered. It is probable that they imbibe copiously by their under sides, and perspire by their upper.
Light has a very powerful effect upon plan's. The green colour of the leaves is owing to it, so that plants raised in the dark, ace of a sickly white; and it is well known that the blanching of celery is effected by covering the plant and excluding the light.
Light acts beneficially upon the up] surface of leaves, and hurtfully upon the under side ; hence the former is always turned towards the light, in whatever situa-tion the plant may happen to he placed. Plants, in a hot-house, present the fronts of their leaves to the side where there is most light, not to the quarter where most air is admitted, or to the flue, in search of heat It has been found, that vine leaves turn to the light, even when separated from the stem, if suspended by a thread.
Nor is this effect of light peculiar to leaves alone. Many flowers are equally sensible to it, especially the compound radiated ones, the daisy, sun-flower, marigold, etc. In their forms, Nature seems to have delighted to imitate the radiant luminary, in the ab-sence of whose beams many of them do not expand their blossoms at all. The stately annual sun-flower displays this phenomenon more conspicuously, on account of its size: the flower follows the sun all day, and returns, after sunset, to the east, to meet the sunbeams in the morning. A great number of leaves also follow the sun in his course. A clover field is a familiar instance of this.
The chemical actions of light, heat, and the component parts of the atmospheric air, upon leaves, are now tolerably well understood. It is agreed that in the day-time plants imbibe, from the atmosphere, carbonic acid gas (which is a compound of oxygen and carbon), that they decompose it, absorb the carbon as matter of nourishment, which is added to the sap, and emit the oxygen. The burning of a candle, or the breathing of animals, in a confined space, produces so much of this gas, that neither of these operations can go on beyond a certain time; but the air so contaminated serves as food for vegetables, the leaves of which, assisted by light, soon restore the oxygen, or, in other words, purify the air again. This beautiful discovery shows a mutual dependence of the animal and vegetable kingdoms, and adds another to the many proofs we have of the wisdom and wonder-working power of the Creator of all things.
In the dark, plants give out carbonic acid, and absorb oxygen; but the proportion of the latter is small, compared to what they exhale by day, as must likewise be the proportion of carbonic acid given out; else the quantity of carbon added to their substance would be but trifling, especially in those climates in which the proportion of day to night is nearly equal, and which, notwithstanding, we know to be excessively luxuriant in vegetation.
There can be no question of the general purpose answered to the vegetable constitution by these functions of leaves. But when we attempt to consider how the peculiar secretions of different species and tribes of plants are formed; how the same soil, the same atmosphere, should, in a leaf of the vine, or sorrel, produce a wholesome acid, and in that of a spurge, or manchineet. a most virulent poison - how sweet, and nutritious, herbage should grow among the acrid crow-foot, and aconite - we find ourselves totally unable to comprehend the existence of such wonderful powers in so small, and, seemingly, simple an organ, as the leaf of a plant. The agency of the vital principle alone can account for these wonders, though it cannot, to our understanding, explain them. The thickest veil covers the whole of these processes; and so far have philosophers hitherto been from removing this veil, that they have not even been able to approach it. All these operations, indeed, are evidently chemical decompositions and combinations: but we neither know what these decompositions and combinations are, nor the instruments in which they take place, nor the agents by which they are regulated.