The Sap. The sap of trees maybe 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 vine, 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 annual sunflower is said to perspire about seventeen times as as fast 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, etc.
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 is a species of acid ; it may be derived from various sources - for instance the tanning from the oak, willow, etc. An acid is found united with even the sugar in the sugar cane.
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 mucilaginous: 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. We can but imperfectly account for the green, so universal in their herbage; but we may gratefully acknowledge the beneficence of the Creator, in clothing the earth with a colour the most pleasing, and the least fatiguing to the eye. We may be dazzled with the brilliancy of a flower garden, but our eyes repose at leisure on the verdure of a grove or meadow.