An introduction to physiological and systematical botany/Chapter 2
DEFINITION OF NATURAL HISTORY, AND PARTICULARLY BOTANY.—OF THE GENERAL TEXTURE OF PLANTS.
Natural History properly signifies that study by which we learn to distinguish from one another the natural bodies, whether Animal, Vegetable or Mineral, around us; to discover as much as possible their nature and properties, and especially their natural dependence on each other in the general scale of beings. In a more extensive sense it may be said to teach their secondary properties, or the various uses to which they have been, or may be, converted, in the service of mankind or of other animals; inasmuch as an acquaintance with their natural qualities is our only sure guide to a knowledge of their artificial uses. But as this definition would include many arts and sciences, each of them sufficient to occupy any common mind, as Agriculture, Dietetics, Medicine, and many others; it is sufficient for a philosophical naturalist to be acquainted with the general principles upon which such arts and sciences are founded.
That part of Natural History which concerns plants is called Botany, from βοτάνη, the Greek word for an herb or grass. It may be divided into three branches; 1st, The physiology of plants, or a knowledge of the structure and functions of their different parts; 2dly, The systematical arrangement and denomination of their several kinds; and 3dly, Their œconomical or medical properties. All these objects should be kept in view by an intelligent botanist. The two first are of essential service to each other, and the last is only to be pursued, with any certainty, by such as are versed in the other two. The present publication is intended to explain the fundamental principles of them all, with as much practical illustration as may be necessary for those who wish to become well acquainted with this delightful science. Botany has one advantage over many other useful and necessary studies, that even its first beginnings are pleasing and profitable, though pursued to ever so small an extent; the objects with which it is conversant are in themselves charming, and they become doubly so to those who contemplate them with the additional sense, as it were, which science gives; the pursuit of these objects is an exercise no less healthful to the body, than the observation of their laws and characters is to the mind.
In studying the functions of the Vegetable frame, we must constantly remember that it is not merely a collection of tubes or vessels holding different fluids, but that it is endowed with life, and consequently able not only to imbibe particular fluids, but to alter their nature according to certain laws, that is, to form peculiar secretions. This is the exclusive property of a living being. Animals secrete milk and fat from food which has no resemblance to those substances; so Vegetables secrete gum, sugar, and various resinous substances from the uniform juices of the earth, or perhaps from mere water and air. The most different and discordant fluids, separated only by the finest film or membrane, are, as we have already observed, kept perfectly distinct, while life remains; but no sooner does the vital principle depart, than secretion, as well as the due preservation of what has been secreted, are both at an end, and the principle of dissolution reigns absolute.
Before we can examine the physiology of vegetables, it is necessary to acquire some idea of their structure.
Much light has been thrown upon the general texture of Vegetables by the microscopic figures of Grew, Malpighi and others, repeated by Dr. Thornton in his Illustration of the Linnaean System, but more especially by the recent observations and highly magnified dissections of M. Mirbel. See his Table of Vegetable Anatomy in the work already mentioned. From preceding writers we had learned the general tubular or vascular structure of the vegetable body, and the existence of some peculiar spirally-coated vessels in many plants. On these slender foundations physiologists have at their pleasure, constructed various theories, relative to the motion of the sap, respiration and other functions, presumed to be analogous to those of animals. The anatomical observations of Mirbel go further than those of Grew, &c., and it is necessary to give a short account of his discoveries.
He finds, by the help of the highest magnifying powers, that the vegetable body is a continued mass of tubes and cells; the former extended indefinitely, the latter frequently and regularly interrupted by transverse partitions. These partitions being ranged alternately in the corresponding cells, and each cell increasing somewhat in diameter after its first formation, except where restrained by the transverse partitions, seems to account for their hexagonal figure. See Tab. I. f. a. The membranous sides of all these cells and tubes are very thin, more or less transparent, often porous, variously perforated or torn. Of the tubes, some are without any lateral perforations, f. b, at least for a considerable extent; others pierced with holes ranged in a close spiral line, f. c; in others several of these holes run together, as it were, into interrupted spiral clefts, f. d; and in some those clefts are continued, so that the whole tube, more or less, is cut into a spiral line, f. e; which, in some young branches and tender leaves, will unroll to a great extent, when they are gently torn asunder. The cellular texture especially is extended to every part of the vegetable body, even into the thin skin, called the cuticle, which covers every external part, and into the fine hairs or down which, in some instances, clothe the cuticle itself.
Before we offer any thing upon the supposed functions of these different organs, we shall take a general view of the Vegetable body, beginning at the external part and proceeding inwards.
- In microscopic figures they are generally drawn like circles intersecting each other.