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two or three ounces. The cornelian cherry is most remarkable in this respect: 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 be 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 plants. The green colour of the leaves is owing to it, so that plants raised in the dark, are of a sickly white; and it is well known that the blanching of celery is effected by covering the plant, so as to exclude the light.
Light acts beneficially upon the upper surface of leaves, and hurtfully upon the under side; hence, the former is always turned towards the light, in whatever situation the plant may happen to be 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, as the daisy, sun-flower, marigold, &c. In their forms, Nature seems to have delighted to imitate the radiant luminary, in the absence 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 his sun→ beams in the morning. A great number of leaves, likewise, follow the sun in its course. A clover field is a familiar instance of this.
The chemical actions of light, heat, and the com→ ponent 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 wonderworking power of the Creator of all things.
In the dark, plants give out carbon, 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 carbon given out; else the quantity of the latter 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 manchineel, 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 understandings, 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.
The vain-glorious Buffon caused his own statue to be inscribed, "a genius equal to the majesty of nature;" but a blade of grass was sufficient to confound his pretensions. Sir J. E. Smith.
The sap of trees may be obtained, by wounding a branch, or stem, in spring, just before the buds open; or in the end of autumn, though less copiously, after a slight frost, yet not during the frost. It has always been observed to flow from the young wood, or alburnum, of our trees; not from the bark. A branch of the vines cut through, will yield about a pint of this fluid, in the course of twenty-four hours. The birch also affords much sap. It flows equally upward and downward, from a wound.
This great motion, called the flowing of the sap, which is to be detected principally in the spring, and slightly in the autumn, is, therefore, totally distinct from that constant propulsion of it going on in every growing plant.
This flowing of the sap has been thought to demonstrate a circulation; because, there being no leaves
at the time to carry it off by perspiration, it is evident that, if it were at these periods running up the sap vessels, it must run down again by other channels. But as soon as the leaves expand, its motion is no longer to be detected. The effusion of sap from plants, when cut or wounded, is, during the greater part of the year, comparatively very small. It is thought, therefore, that this flowing of the sap, is nothing more than a facility of the sap to run, owing to the peculiar irritability of the vegetable body, at that period; and that it runs only when a wound is made-being naturally at rest till the leaves open, and admit of its proper and regular conveyance.
As soon as the leaves expand, insensible perspiration takes place, very copiously, chiefly from those organs; but also, in some degree, from the bark of the young stem and branches. The perspiration of some plants is very great. The large annua sun-flower is said to perspire about seventeen times as fast as the ordinary perspiration of the human skin.
The sap, in its passage through the leaves and bark, becomes quite a new fluid, possessing the peculiar flavour and qualities of the plant; and not only yielding woody matter for the increase of the vegetable body, but furnishing various secreted substances more or less numerous and different among themselves. These, accordingly, are chiefly found in the bark. In herbaceous plants, the stems of which are only of annual duration, the perennial roots frequently contain these fluids, in the most perfect state; nor are they, in such, confined to the bark, but deposited throughout the substance, or wood, of the root, as in rhubarb and gentian.
Gum, or mucilage, a viscid substance, of little flavour or smell, and soluble in water, is a very common secretion. When superabundant, it exudes from many trees, in the form of large drops, as in the plum, cherry, and peach trees, and different species of the mimosa, or sensitive plants, one of which yields the gum arabic, others the gum senegal, &c.
Resin is a substance soluble in spirits, as the
turpentine of the fir and juniper. Most vegetable exudations partake of a nature between resin and mucilage, being partly soluble in water, partly in spirits; and are therefore called gum-resins. The more refined and volatile secretions, of a resinous nature, are called essential oils; and are often highly aromatic and odoriferous. One of the most exquisite of these is afforded by the cinnamon bark. They exist, in the highest perfection, in the perfumed effluvia of flowers, some of which, capable of combination with spirituous fluids, are obtainable by distillation, as those of the lavender and rose.
Acid secretions are well known to be very general in plants. The astringent principle seems to be a sort of acid, of which there are many different kinds; and among them, the tanning principle of the oak, willow, &c.
Sugar, more or less pure, is very generally found in plants. It abounds in various roots, as the carrot, beet, and parsnip; and in many plants of the grass or cane kind, besides the famous sugar cane.
It is curious to observe not only the various secretions of different plants, by which they differ from each other in taste, smell, qualities, and medical virtues, but also their great number, and striking difference, frequently in the same plant. Of this, the peach tree affords a familiar example. The gum of this tree is mild and mucilagenous: the bark, leaves, and flowers abound with a bitter secretion, than which, nothing can be more distinct from the gum. The fruit is replete, not only with acid, mucilage, and sugar, but with its own peculiar aromatic and highly volatile secretion, on which its fine flavour depends. How far are we yet from understanding the vegetable body, which can form, and keep separate, such distinct and discordant substances!
The odour of plants is, unquestionably, a volatile, essential oil. Its general nature is evinced by its ready union with spirits or oil, not with water.
To all the foregoing secretions of vegetables, may be added those, on which their various colours depend.