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.