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.