When the sap is completely organized by respiration, evaporation, and the chemico-vital agency of light, it descends chiefly through the cambium, lying between the liber and the wood. From this layer the sap distributes to each organ capable of increase, the requisite nutritious liquids, deposits various organic compounds, and annually renews the cambium. Part of the sap in its descent runs into the wood through the horizontal medullary rays, in the cells of which it deposits starch. The descent of the sap is no doubt due to gravitation.
The latex is a general name for those white or coloured juices peculiar to some plants. It is separated in the leaves from the descending sap, which is always colourless, and consists of a clear liquid, thickened and coloured by white, yellow, reddish-brown, or green globules floating in it; it does not turn blue under the action of iodine, therefore it does not contain starch. These proper juices differ as much in quality as in colour; some contain fatty matters, others substances of a totally different nature, as caoutchouc; a few are bland and nutritious, many acrid and poisonous; some contain alkaloids, others have none. These juices are by no means essential to the life of the plant, for sometimes they are wanting in their most essential parts, and they are found in certain species and not in others most nearly allied. Certain it is, that tropical lactescent plants which do not produce their proper juices when brought to a cold climate, still produce their milk vessels.
These vessels follow the ramifications of the veins of the leaves in the highest class, and also in some of the monocotyledons. In the stem the milk vessels belong especially to the layers of the bark, where they take the form of long reticulated perpendicular ducts, through which the proper juices descend towards the roots.
Each plant has its own system of milk vessels, and M. Lestiboudois has found that the coloured liquids have a rapid motion; the movements are very complicated, not from point to point, but in such a manner that the granules are carried by the liquid into all the ramifications of a complicated network.
The septa, or divisions between the primordial cells, exert a powerful influence upon the substances contained in the sap as it permeates through them, no doubt acting as a dialysing membrane, which separates the gelatinous from the crystalloid matter. The latex is probably separated from the sap by the septa in the cells of the leaves, and sent into the vessels peculiar to it, and then, while the sap is descending and passing through the cambium, it is likely to be dialysed by the septa between the cells of that layer, arresting the protein, and other gelatinous substances, and allowing sugar, starch, and other crystalloid matter to pass freely, and form deposits of organic compounds for the following year. For perennial plants in extra-tropical countries remain in a dormant state during the winter; their cells are then full of organic compounds under the form of protein, as well as sugar, gum, &c., but especially starch, which is converted into sugar or dextrine, when spring awakens the plants to renewed life and activity.
The composition of inorganic matter is very simple; there are comparatively few radicals, and the substances are compounded of few equivalent atoms, at most eight or ten, sometimes only two or three. Carbonic oxide is formed of one atom of carbon and one of oxygen; carbonic acid is formed of one atom of carbon and two of oxygen; and acetylene, M. Bertholet’s base of synthetic compounds, contains two atoms of carbon and two of hydrogen, chemically united; but no organic compound contains less than three equivalent atoms, generally a great many more. For example, citric acid, which is lemon juice, contains 12 atoms of carbon, 5 of hydrogen, and 11 of oxygen; while strychnine contains 44 atoms of carbon, 23 of hydrogen, 4 of oxygen, and 2 of nitrogen. Experiment has proved that the powers which maintain stability among the numerous and complex constituents of organic substances decrease in energy as the number of the equivalent atoms augments; hence such compounds are in less stable equilibrium than those of inorganic bodies, and are more liable to be disturbed and changed into new and more stable forms.
As the chemical functions are not the same in all the cells, situated as they are in different parts of a plant, they elaborate different substances from the same materials. Besides, new substances are introduced with the growth of the plant, to be acted upon by the light and heat of the different seasons, so that numerous compounds may be formed out of a given number of the primary elements. For example, the ultimate elements of wheaten flour, or a grain of ripe wheat, are carbon, the three elementary gases, sulphur, phosphorus, calcium, magnesium, and silex; but during the germination and growth of the plant, its flowering, forming the seed, and ripening the grain, certain portions of these elements chemically combine in definite proportions to form cellulose, starch, sugar, gum, gluten, fibrin, albumen, casein, and fat, all of which are found in wheaten flour.
However much plants may differ in their organic products, they all agree in producing protein, which takes an active part in the formation of cells; and all produce neutral hydrates of carbon, such as cellulose, starch, sugar, gum, &c., which consist of carbon, combined with hydrogen and oxygen in the exact proportion that forms water. Many of them have precisely the same quantity of carbon, and only differ in the quantity of the aquatic element, as for example, lignin, starch, and cane-sugar, which consist of 12 parts of carbon, in a state of combination with 8, 10, and 11 parts of water respectively; indeed the affinity between many of these neutral hydrates is of a most intimate character. Some of their varieties are isomeric, that is to say, they contain the same ingredients in the same proportions, and yet they differ essentially in regard to their properties.
Next to cellulose, starch is the most universal and distinctive of vegetable productions, being a constituent of all plants, except the fungi. It abounds in the grains and other seeds, and supplies the young plant with food till it can feed itself. In both of the flowering classes it occurs in small colourless transparent grains, either floating in the sap, attached to the walls of the cells, or accumulated within them. Starch globules of very small size are imbedded, either singly or in groups, in the granules of chlorophyll, or leaf green; the manner in which the green coating takes place is unknown. Starch is an organic substance, varying from grains of inappreciable minuteness to such as are visible to the naked eye, and of such a variety of forms that it can be ascertained with tolerable certainty by what plant a grain of starch has been produced. The small grains are generally globular, but whatever the form may be, each consists of a series of superimposed layers of different densities, which exhibit coloured rings and a black cross in polarized light.
Starch is an early and transient product of young plants, which is destined to be changed into nutritious substances at a later period, but being insoluble in cold water it is unfit to travel with the sap. However a ferment called diastase produced during the incipient germination of the grains and seeds, in the tubers of potatoes, &c., being in a state of change, imparts that state to the starch, and converts it into a sweet soluble matter known as dextrine or starch-gum which is capable of being carried throughout the plant with the sap, and which is itself ultimately changed into sugar. Dextrine is an ingredient in the primordial cell. Starch, dextrine, and cellulose are isomeric: consisting of the same elements with different characters.