These secretions never liar secretions as the result. form any part of the tissue of a plant, and are either excrementitious, i. e., those which are thrown off; or special secretions, which remain, in most cases, where they are formed, and are seldom removed from one organ to another: but in others pervade the whole plant, and, as in the case of Tannin, impregnate the soil around them. The excretions are extremely various, and are probably a provision for the removal of some material which is useless or injurious to the plant. One of the most singular is that of the fraxinella, though this is probably of the same class with the volatile oils to be mentioned presently. If at the close of a dry, hot day, a light be held near the top of that plant, the vapor which surrounds it takes fire, and burns with a lambent flame, without injury to the plant. This vapor appears to be of the nature of an extremely volatile oil, which escapes from the small glands that cover the surface of the plant, for the white fraxinella, which has fewer glands than the red, exhibits the phenomenon in a slighter degree. Other excretions are acid, some are caustic, some glutinous (such as the leaves of the gum cistus). Some plants secrete a waxy matter from their surface; others saline or saccharine particles. Manna is one of these excretions; it both exudes naturally, and is also obtained when artificial incisions have been made in the tree. It would be impossible to describe, or even enumerate, all the excretions of plants in this treatise; the above may convey an idea of their na ture. 51. The Special Secretions are liquids secreted in the bark, or some other organ. Their principal characters are, 1. That they are all composed of two or more principles, which can be separated, and are not homogeneous, like the nutritive juice, which, although it may of course, by chemical analysis, be resolved into its elementary constituents, presents no such peculiar principle as do the special secretions. 2. These latter contain (in addition to their carbon), oxygen and hydrogen, not in the proportion in which they combine to form water, but with a preponderance of one or the other of those gases, and some of them, and those the most important to man, also contain azote, i. e., nitrogen. 3. All these secretions, if they are absorbed by the roots of living plants, even by those which produced them, act on them as poisons-a sufficient proof of their not being intended to percolate the plant in the manner of the nutritive juices. They consist principally of four divisions:-1, Milky, and 2, Resinous Juices, 3, Volatile, and 4, Fixed Oils, and the local secretions, properly so called. The milky and resinous fluids, which form the first two classes, are sometimes expelled from the plant by accident or disease, and are almost always capable of removal from one portion of the plant to another. Professor Henslow gives among the milky juices the following curious instance of a tree called the Cow Tree, from Humboldt: "On the barren flank of a rock grows a tree with dry and leather-like leaves; its large woody roots can scarcely penetrate into the stony soil. For several months in the year not a single shower moistens its foliage. Its branches appear dead and dried; yet as soon as the trunk is pierced, there flows from it a sweet and nourishing milk. It is at sunrise that this vegetable With the fixed oils should perhaps be classed the vegetable tallows and butters. The The fountain is most abundant. The natives are then to be seen hastening from all quarters, furnished with large bowls to receive the milk, which grows yellow and thickens at the surface. Some empty their bowls under the tree, while others carry home the juice to their children. The milk, obtained by incisions made in the trunk, is glutinous, tolerably thick, free from all acrimony, and of an agreeable and balmy smell." The milky juices are contained in the bark and leaves, the volatile oils in closed cells, from which they are probably only exhaled in consequence of the permeability of the tissue, whence it happens that the organs which secrete these oils are in general strongly odorous. fixed, or fat oils, as they are called, are formed in cells, from which they never escape by any natural process, but must be artificially extracted. caoutchouc (India rubber) is an instance of a milky secretion, as are also our common spurge, and opium, the well-known product of the white poppy. Most of the juices to which the name of milky has been applied are white, but not all of them, for instance, the lactic secretion of our English celandine is of a brilliant orange color. Of the resinous juices one example, common resin, is familiar to every one. Of this class are the true balms, Gum Benzoin, &c. Examples of volatile or essential oils, as they are otherwise called, such as those of the rose, &c., will readily occur to every one's recollection; and the fixed oils, those, for instance, of the nut, the almond, linseed oil (the product of the seed of the flax), olive oil, so useful for both food and light to the inhabitants of the south of Europe, with many others, are too well known to need more particular notice here. The principal chemical distinction between the volatile and fixed oils is that the former are powerfully odorous, slightly soluble in water, with which they pass over in distillation, communicating their flavor to it; and that they are volatilized by heat without decomposition. The fixed oils, on the contrary, are inodorous and insipid, support two or three hundred degrees of heat without volatilizing, and are decomposed at a higher temperature. In a physiological point of view their difference is equally striking. The volatile oils are found in the leaves or in the cortical system, the usual place of the secretions; the fixed oils are either situated in the seeds themselves, or more rarely in the tissue of the pericarp. 52. There are many local secretions, of which a detailed account belongs more properly to a chemical treatise than to one that, like the present, is only physiological, and also too brief to do more than glance at the other sciences immediately connected with the subject: it will therefore only be possible to notice these secretions slightly here. They consist of acids, such as citric, malic, acetic, &c., prussic acid (remarkable by the absence of oxygen) which is found in peach and laurel leaves, &c., of Gluten, Albumen, Tannin and Coloring matter, of which indigo is one of the most important, and a variety of other secretions or principles, each confined to the particular vegetable in which it is found, such as Asparagin, whose name denotes its origin from the asparagus. 53. Besides the above, substances are found in plants which are purely mineral, and which are principally lime, magnesia, silica, alumina, and perhaps barytes. Potash and soda are found in very large quantities. Iron, manganese, and copper* have *Copper was found by M. Bischoff, Dr. Meissner and M. Sarzeau. See De Candolle, Phys. Veg., vol. i. p. 389. been observed, and besides the above there are occasionally found in plants chlorine, iodine, sulphur, and phosphorus. The reader is referred to No. 4 of these little treatises* for further particulars on the chemical part of the subject. 54. Those whose leisure permits, and whose inclination leads them closely to examine into the simple yet marvelous chemistry by which compounds, absolutely essential to the animal economy, but which it has no direct power of preparing for itself, are formed for it in the vegetable organism, will perceive how true it is that the more we search into those phenomena which we daily and hourly witness and experience, the more we shall see that nothing has been made in vain, and the more resistless will be the proof that such a chain of causes and effects as may be traced from one end of creation to the other, could only have had their origin in that One Mind to which everything is ever present, and who, in the very "constitution and course of nature," has stamped too deeply to be effaced, even amid the moral disorder man's folly has introduced, the "image" of his own perfection, and the " superscription" that the work of his hand is " very good." To God then let all "render the things that are God's," by a full acknowledgment of his wisdom and goodness in thus supplying what they need, and by making such a use of those gifts as may best prove their gratitude, and most tend to the glory of the Giver. * "Introduction to Organic Chemistry." |