The elaborated sap is formed out of the ascending sap. The place where this change takes place is in the leaves and green parts of vegetables; it is generally in the spring season that the ascending sap pushes out the buds into branches, and developes the little scales which had surrounded these organs into leaves; but when these leaves are formed, the sap continues to ascend into them, and there undergoes those alterations from whence the elaborated sap results. Now, these alterations consist in the getting rid of all superfluous water and carbonic acid, which, originally absorbed as aliment, had not undergone the conversion into gum and sugar during the ascent of the sap; secondly, in the acquisition of additional nutriment from the atmosphere; and, thirdly, in the conversion of these substances into a variety of new compounds.
Let us examine each of those changes to which the ascending sap is subjected, in succession; and, first, with respect to the disengagement of superfluous water and carbonic acid, every one must have observed drops of water collected on the leaves of cabbages and other vegetables, when examined early in the morning. These are commonly supposed to be dew-drops, but are truly in great part the result of a kind of perspiration which is always taking place from the surface of plants. That this is the case, can be proved by covering a cabbage-plant with a bell-glass, and placing it in a room sufficiently heated to prevent the deposition of dew, when drops of water will be found equally to collect upon its leaves. These drops are not observed during the day, because the temperature is then commonly so high as to evaporate them as fast as they are transuded; but the fact is, that plants actually give off much more water during the day than night. The escape of carbonic acid is not so easily detected as that of water; it can, however, be proved, through the resources of chemistry. Unlike water, which is liberated both night and day, and indeed in greatest quantity during the latter period of time, carbonic acid is found to be disengaged during the night only. As long as plants are exposed to the light of the sun, their green parts liberate none of this gas.
We have mentioned that when the ascending sap arrives into the leaves, it not only throws off superfluous water and carbonic acid, but likewise derives an additional quantity of nutriment from the atmosphere. The presence of light is necessary for this latter circumstance to take place. The nutriment which, under the influence of sunlight, it acquires from this source, is a substance named “carbon;” this substance is a constituent of carbonic acid, which is indeed composed of carbon and oxygen; carbonic acid is contained in the atmosphere in the proportion of one part in a thousand; the green parts of plants absorb it, and under the influence of light decompose it; the carbon is retained, but the oxygen is again liberated. We now may perceive the reason of the fact mentioned in the preceding paragraph: plants give out no carbonic acid during the day, because the superfluous carbonic acid of the ascending sap becomes decomposed under the influence of light, in the same way as that which has been absorbed from the atmosphere.
A great many compound products are obtained from the vegetable kingdom. We need merely recall to the reader’s recollection starch, resin, camphor, bland and aromatic oils, bitter principles, colouring matters, the acids of the grape, the lemon, and the apple, &c. to assure him of this truth. All these different substances form themselves out of the sugar and gum of the ascending sap, together with the carbon absorbed under the influence of light.
When the ascending sap has parted with its superfluous water and carbonic acid, when under the influence of light it has absorbed carbon from the atmosphere, and when its constituents arrange themselves anew, so as to produce some or all of the substances above enumerated, its name as well as its functions cease: it has now become the descending or elaborated sap.
Let us now inquire the course which the descending sap pursues. We have stated in our last article, that if a ligature be twisted tightly round a branch of one of our common trees, the portion immediately above the ligature will become swollen, while that beneath it will retain its former thickness. If instead of a ligature we remove a circular ring of bark, the same phenomenon will take place: the part above this annular incision will swell out on every side. From this experiment we derive several important inductions. We learn from hence that this kind of sap descends, and moreover that the channel which conveys it is the bark.
Having ascertained the course which the elaborated sap pursues, let us now turn our attention to its composition. This is found to vary in different plants: thus in some, bitter principles are the chief constituents; in others, aromatic substances; in others it is principally resinous; but whatever may be the principal components, they may always be divided into two groups—namely, those which are subservient to the growth of the vegetable, and those which, becoming deposited in the different organs, confer on them those properties which entitle them to be employed as articles of medicine or aliment for animals, and by means of which different plants are in this respect distinguished from each other. The portion of the descending sap which serves for the growth of the vegetable, exudes in ordinary trees between the bark and the wood, forming a glutinous layer which separates these organs, and is the cause of the facility with which in autumn the bark can be detached from the stem: this portion is called cambium. In palms, and other trees of warm climates, there is no bark, and in such vegetables the nutritive part of the descending sap passes down through the centre of the stem.
The portion of elaborated sap which becomes deposited in the organs, and which varies more or less in every plant, is called the proper juice: proper vessels is the name given to the reservoirs which contain the proper juices; and according to the nature of their contents, the proper vessels are called milk-vessels, turpentine-vessels, vesicles of essential oil, &c.
In the foregoing paragraphs we have somewhat anticipated the uses of the descending sap: we have found that one portion of it is destined for the nutrition of the vegetable. Now, the same means which revealed to us the uses of the ascending sap, will also tell us how far the elaborated sap is concerned in vegetable nutrition. In the dark no sap is elaborated, and no vegetable fibre is developed. Are we not therefore justified in supposing that vegetable fibre is formed out of this elaborated sap? Again, let our readers call to their remembrance the experiment of tying a ligature around a branch: in that experiment not only does a considerable swelling take place above the ligature, but from this swollen portion cereal roots frequently protrude. These facts afford us a clue to the uses of the descending sap, for by developing vegetable fibre, it increases the thickness of the stem and the length of the roots, just as the ascending sap, by developing vegetable flesh, lengthens the stem, and enlarges the root in diameter.