The woody part of trees and shrubs, the fibres of hemp, flax, of the Agave, and many other plants, are formed of cellulose, the purest form of that substance being bleached flax and linen. During the progress of vegetation, the cells of the ligneous tissue of trees, also those of woody and fibrous plants, which are transparent and colourless when young, become internally coated or filled with sclerogen, the colouring matter of wood, a substance of various hues. In extra-tropical countries it is generally some shade of brown, sometimes dark, sometimes so pale as to be almost white with a yellowish or reddish tinge; and occasionally it is beautifully marked as in the wood of the olive. In tropical countries the colours are more vivid and varied, deeper and even black, as in ebony. This colouring matter has the same quantity of oxygen as cellulose, but it contains hydrogen and more carbon, hence wood is combustible in proportion to the quantity of sclerogen it contains. In beech it forms half of the wood, in oak two thirds, and in ebony nine tenths, so it is the most highly combustible of the three. The additional carbon is obtained by increased respiration, the hydrogen by decomposition of water in the sap.
Sugar is almost as universal a constituent of the higher classes of plants as cellulose and starch, for besides the saccharine juice of innumerable plants, starch, the acids of unripe plants, and even the acrid juice of the fig and other plants, is turned into sugar as the plant advances to maturity, and the fruits ripen. Manna and other saccharine exudations from the leaves or stems of trees, as the lime tree, are probably intercepted by the dialysing septa of the cells, and exude to the exterior through the pores of the skin. The sweet juice found in the nectaries of flowers is formed in other parts of the plant, and rarely flows to the flower before it is full blown; the quantity is at its maximum during the emission of the pollen, and ceases when the fruit is formed. In diœcious plants and that singular and beautiful race the Orchideæ, it is evidently intended to attract insects for their aid in fertilization.
Vegetable oils, resins, and wax, consisting of the same simple elements as the hydrates, form a large class of inflammable organic substances in which hydrogen predominates. Olive oil is a rare instance of a fixed oil being obtained from a fruit; some laurels have that property also, but the fixed oils are chiefly found in seeds, as the walnut, hazel nut, and the almond, in which the principle of oil is in its greatest purity. It is particularly abundant in hemp seed, and in a great variety of plants the starch in the seed is changed into oil to nourish the embryo, till the seed lobes are above the ground, and the true lobes appear.
Resins, gums, and wax, being colloid substances, are dialysed and ejected from the system either through the fissures in the bark, or by pores in the leaves. The resins exude through the bark from canals that run between the cells of the plant, in solution, and are consolidated by the oxygen on coming into the air. The herbaceous zone in the bark of the fir and pine family furnish an abundant supply of resins and balsams; the camphor tribe and the Amyrids are rich in them, as frankincense, myrrh, balm of Mecca, and the Olibanum, supposed to be the frankincense of scripture.
Wax is a frequent vegetable production, especially in the torrid zone, where many of the wax-bearing plants supply the natives with light. An exudation through the pores of many plants coats their surfaces with resin or wax. Young buds are often covered with resin to protect them from cold and wet during the winter and early spring, as those of the horse-chestnut and balsam poplar. It is wax that gives the bloom to the plum, cherry, and grape, and the rain drops lie on the waxy surface of the cabbage leaf, like balls of diamond, from the total reflection of light at their point of contact. Wax protects plants from damp in a rainy climate, and prevents too strong perspiration from the fleshy leaves of the aloe, cactus, and other inhabitants of the parched and hot regions in the tropics.
The vegetable substances hitherto under consideration are neutral, but the remarkable compounds albumen, fibrin, and casein, already mentioned as constituents of wheaten flour, not only contain carbon and hydrogen with a little oxygen, but azote and small quantities of sulphur and phosphorus. Each of these three organic compounds is the same, whether derived from animal or vegetable matter. Thus albumen is chemically the same, whether obtained from wheat and other grains, from arrowroot, dahlia roots, the serum of blood, or the white of an egg. As it constitutes the film or thin coating of the primordial cell, and combines with dextrine in its internal viscid lining, it not only forms an ingredient in all vegetable organisms, but plays an important part in the growth of the whole vegetable world. Fibrin is chemically the same in the juice of plants and in blood, in which it exists as a liquid during the life of the animal, and as a fibre after death. It forms the basis of the muscular system in animals, and that extracted from the juice of plants coagulates spontaneously like blood. Casein is chemically identical, whether derived from the curd of milk, or from peas and beans. Azote is a very important principle in these substances as well as in the gelatinous substance gluten. It forms an essential part of the animal structure, and is either highly nutritious or deleterious in the vegetable, being at once one of the most valuable, contradictory and powerful agents in nature.
Chemists have formed by synthesis compounds identical with all the fixed and essential oils, for confectioners can now give the flavour of the pear, orange, quince, pine apple and other fruits by means of artificial chemical compounds. All the saccharine substances have not yet been artificially obtained, nor the albuminous substances, albumen, fibrin, and casein.
It cannot be a matter of surprise, when chemists form organic substances out of inorganic elements, that they should succeed in transforming compounds produced by living plants into new compounds, as that of changing the vegetable acids into alcohols, which is now done. But some of the acids themselves are synthetically formed out of inorganic elements; as for example the oxalic, the most common of all the vegetable acids, which is found most abundantly in the Oxalis or wood sorrel, and is a frequent constituent of the highest and lowest plants. The formic acid, which is the acrid stinging principle in ants, is also synthetically formed; it is found in the juice of the stinging nettle and in decaying pine leaves, and contains hydrogen like all the other vegetable acids. These acids result from an augmentation of oxygen during nocturnal respiration, which penetrates deeply into the vegetable structure.
Octahedral, prismatic, and stellar microscopic crystals formed by the chemical combination of the natural acids with bases imbibed by the roots, are deposited in the cells under the skin, and in all parts of plants. However, they appear most frequently as bundles of needle-shaped crystals of carbonate of lime, lying side by side in the hollow of a cell. They are known as raphides, from raphis a needle, and may be easily seen under the skin of the medicinal squill. Large single crystals of oxalate of lime, octahedral or prismatic, are found in the cells under the skin of the onion and other plants; and stellar crystals of the same substance abound so much in the common rhubarb that the best specimens of the dry medicinal root contain as much as thirty-five per cent. of them; while certain aged plants of the cactus tribe have their tissues so loaded with them as to become quite brittle. The calcareous base in some instances is combined with tartaric, citric, or malic acid. The crystals of some raphides are 1⁄40th of an inch long, others are not more than the hundredth; they are brought into view by polarized light.[[80]] Spherical raphides between the 1⁄2000th and 1⁄4000th of an inch in diameter have been discovered scattered profusely through the tissues of the leaves, and those parts of plants which are modifications of the leaves; they may be seen under the skin of Pelargoniums and other plants, and it is supposed that few if any orders of plants are without them.[[81]]
Although azote forms 788 thousandth parts of the atmosphere, none, or at least no appreciable quantity of it, is absorbed by the vegetable world; that great principle of nourishment is entirely supplied by ammonia and nitric acid, imbibed by the roots, and decomposed by the chemico-vital power. Here it shows its capricious character by combining with other simple elements in the bark, to produce the most precious medicines in some plants, and in others the most deadly poisons, while no vegetable substance is perfectly nutritious without it.