(130.) Taxonomy. — We have no space to devote to any extended review of the various methods and systems which have been proposed for the classification of plants; and it is not necessary for us to explain the uses which a systematic arrangement of natural bodies is intended to serve. This subject has been thoroughly and sufficiently discussed by Mr. Swainson, in our sixty-sixth volume. We may just remark, that the number of species already named and classified in works of botany, amounts to about 60,000; and this fact alone must satisfy us, how necessary it is that botanists should possess those means of intercommunication, which a systematic classification alone can afford — whenever they wish to announce the discovery of a new species, or to refer, with certainty, to one which has been previously noticed. But, if we have the higher object in view, of searching after the laws and principles which regulate the structure and fix the properties of plants, then it is a necessary and immediate consequence of every discovery of this kind, that we thereby obtain a nearer conception of those affinities by which plants approach, and of those differences by which they recede from each other; and this, in fact, amounts to a closer insight into that hitherto undiscovered system, or plan, upon which we must feel satisfied that the Author of nature has proceeded in creating all natural objects.

(1.) Of the advantages which accrue from the cultivation of the natural sciences, sufficient has been said in the treatise of Sir J. Herschel, forming our fourteenth volume; and Mr. Swainson, in his discourse, which forms our fifty-ninth volume, has further exposed the importance of the study of Natural History in general, and more particularly of that department which he so successfully cultivates. In introducing the science of Botany to the general reader, for whom more especially this volume is designed, rather than for the scientific adept, it will be right that we should follow the example which has thus been set us, and say a few words by way of introduction to our present subject. Whenever we are about to enter upon any science which is new to us, it is always advantageous to take a general survey of the limits within which it is restricted, and to obtain some notions of the objects of which it professes to treat. We shall, therefore, offer a few remarks upon the position which Botany holds with respect to other kindred branches of Natural History; and point out the separate and subordinate departments into which it may be advantageously divided.

(2.) Botany. — In the most extended sense of the term, Botany may be considered as embracing every inquiry which can be made into the various phenomena connected with one of the three great departments into which the study of nature is divided, and which is familiarly styled the Vegetable Kingdom.

(38.) Fundamental Organs. — We may refer back to articles 8, 9, &c. for a general notice of the complex organs, which we are now about to describe more in detail, though we do not propose to enumerate all the varieties of form which these organs assume. There are certain appendages both to the stem and root, (or ascending and descending “axes” of vegetation); which are of very little importance in carrying on the function of nutrition. These appendages, as the thorns, scales, tendrils, &c. found on some stems, have without doubt their respective uses; but as the plant may be deprived of them, and still continue to vegetate as freely as when they were present, they are evidently not to be considered as fundamentally essential to the support of life. Moreover, they may in all cases be referred to certain modifications and metamorphoses, which have taken place in one or other of the three organs — the root, stem, and leaf,—which are more especially considered to be the “fundamental organs” of nutrition. The presence of neither of these can be dispensed with without injuring vegetation, and ultimately involving the destruction of the individual; unless where some means have been provided (as we shall see in the case of parasitic plants) to supply their deficiency, or where (as in the lowest tribes of cryptogamic plants) they are probably so blended and confounded together that we are not able to distinguish them.

SEVENTH PERIOD OF NUTRITION

(223.) Assimilation. — The chief end and object of the various processes which we have been describing, is the manufacture of the materials which are ultimately to be assimilated into the vegetable structure, and by which it is to be nourished and developed in all its parts. Of the precise manner in which the assimilation of this nutriment takes place we know nothing, and the first steps towards the formation and development of any organised being are entirely concealed from us. We may indeed observe when a gradual organisation of matter is taking place; but there is no stage in the process from whence we may not refer back to some previous state, out of which it appears to have emerged imperceptibly and inexplicably; and it is utterly impossible to note with any degree of accuracy, either the precise manner or exact time when the first traces of any new condition of organisation commenced. In other words, as soon as we can distinguish an organ it already exists in a developed form, however faintly its subordinate parts may be indicated.

(224.) Growth of the Tissues, — In dicotyledonous trees, as we have observed (art. 34. 2.), the new tissue makes its appearance between the old wood and old bark. In the earliest stage in which it is discoverable it appears as a thick clammy fluid termed the cambium, which gradually assumes the character of a newly formed cellular tissue intermixed with vessels which are disposed longitudinally through the stem.

(298.) Epirrheology. — This term has recently been proposed, to express that branch of our science which treats of the effects produced by external agents upon the living plant. It can only be considered as a subordinate department of vegetable physiology, and one indeed whose limits are not very strictly defined. For we have seen that life itself requires the stimulus of external agency, in order that its powers may be elicited, and produce the various phenomena of vegetation included under one or other of the two functions of nutrition and reproduction. But then these functions become variously modified, according as the external stimuli by which they are called into action are permitted to operate with greater or less intensity. In all cases, there is that happy mean which can so regulate the vital force as to produce a healthy and vigorous condition of existence; whilst every increase or diminution in the stimulus applied, only tends to injure or greatly to modify the individual subjected to its long-continued influence. Physiology might be considered as embracing the investigation only of such phenomena as resulted from the healthy condition of the vital functions; whilst epirrheology would take further cognisance of such as resulted from an unhealthy condition of vegetation. Hence this department would lay the foundations of another branch, termed the “nosology” of plants, or that science which treats of their diseases; and also of the extensive subject of “Botanical Geography,” which makes inquiry into those causes which limit the distribution of various species to certain spots upon the earth's surface.

(189.) Diffusion of proper Juice. — The crude sap having been subjected to the action of the atmosphere and the carbonic acid decomposed, the result is termed the “ proper juice” or elaborated sap of the plant. This liquid has now to find its way back again into the system for the purpose of nourishing and developing the various parts. There are three distinct kinds of movement to which the proper juices of plants are subjected. The first of these is its descent and transfusion; the second is a very singular rotation of the juices contained in the vesicles and short tubes of some plants; and the third is a sort of actual though local circulation more nearly resembling the circulation of blood in animals. We propose to describe each of these under the present period, though certainly they can hardly be all considered as subordinate processes of the same function.

(190.) Descent of Sap. — When a ring of bark is removed from a stem or branch of a dicotyledonous plant a tumour is formed at the upper edge of the ring, which indicates a stoppage to have taken place in the descent of the elaborated sap. This stoppage by causing an excess of nutriment to accumulate above the ring, operates in improving the size and quality of fruits, and will even occasion a tree to flower and produce fruit when it would otherwise have developed nothing but leaves.

(275.) Dissemination. — The manner in which the ripe seed is disseminated, forms a more important element in the history of the preservation of species than might at first be imagined. It may be considered analogous to the period of labour in the animal kingdom, and still more strictly to the laying of eggs among such as are oviparous. If the different modes of dissemination were not in harmony with the peculiar character of the species, we might expect in the lapse of ages that some combination of circumstances would arise which should so far interfere with the reproduction of a given species that it would disappear from the earth. This is guarded against by some peculiar adaptation of the mode in which the seed is disseminated to the conditions under which each species naturally thrives the best. In some cases, the seed falls immediately around the parent plant; and where many seeds are contained in the same seed-vessel, the young plants come up in a crowded manner and occupy the soil in society, to the exclusion even of more robust species. Other seeds and seed-vessels are furnished with the means of being transported by the influence of the wind or by some other cause to a considerable distance. The great diversity in the means by which the dissemination of the seed is naturally secured forms one important inquiry to the botanical geographer; and a complete description of the various appendages by which their dispersion is assisted would form an interesting topic of inquiry.

(243.) Propagation. — There are two distinct modes, according to which the propagation of the vegetable species is naturally secured, viz. “subdivision” and “reproduction.” In the first the individual plant maybe subdivided into several parts, each of which when detached from the parent stock is capable of existing as a separate individual. A familiar example of this mode of propagation may be seen in the common strawberry, to which we have alluded in art. 237. It is very common to find elms, poplars, and other trees throwing up suckers from their roots at a distance from the trunk, all of which are capable of becoming so many distinct trees, under favourable circumstances. Man has availed himself of this property, to extend the means which nature has provided for the propagation of the species; and by placing cuttings, slips, and buds under proper treatment, he forces them to throw out roots; or he grafts them on other stems, where they adhere and develop as so many separate and independent individuals. The process by which any detached portion of a plant becomes a distinct individual, similar to that from which it was derived, depends upon the power it possesses of reproducing those organs or parts in which it may be defective. Thus the ascending organs develop roots; and these again, produce buds from which the ascending organs proceed.

(8.) Organs. — The various parts of which a plant is composed have been called its “organs;” and this term is equally applied to those external portions, which may readily be recognised as being subordinate to the whole, such as its leaves, roots, flowers, &c, as to certain minute cells and vessels, of which its internal structure consists. De Candolle has included every inquiry, both into the external and internal organisation of plants, under the title of “Organography;” although such details as belong to their external characters have a more exclusive reference to our descriptive department, whilst those which relate to their internal organisation are more especially introductory to our physiological.

(9.) External Organs.— The principal external organs of which a plant is composed are familiar to every one. They are, the root, stem, branches, leaves, flowers, &c. These organs may be conveniently grouped under two heads, characterised by the nature of the functions which they are severally destined to perform. The root, stem, branches, leaves, and some other appendages to each of these, are concerned in carrying on the function of nutrition, or that act by which the life of every separate individual is maintained; and these are, in consequence, styled the “Conservative” organs. The flower and fruit, with their various appendages, are connected with the function of reproduction, by which the continuance of the species is provided for; and these are, therefore, named the “Reproductive” organs.

(159) Function of Nutrition. — The first of the two general functions (art. 152.), that of nutrition, may be conveniently subdivided into about seven distinct processes or subordinate functions, which are all carried on simultaneously in different parts of the vegetable structure, more especially during those seasons of the year in which the powers of vegetation are the most active. Sometimes, only one of them is in activity, whilst the rest are either partially or entirely suspended. But as the whole of the materials which serve to nourish the plant must have been subjected to these several processes in succession, we may consider the function of nutrition to be carried on during as many successive periods, before it is completed. We will briefly mention what these successive processes are, before we enter upon the details necessary for the more accurate description of each of them. In the first place, plants absorb their nutriment by the roots; this nutriment is then conveyed through the stem into the leaves; there it is subjected to a process by which a large proportion of water is discharged; the rest is submitted to the action of the atmosphere, and carbonic acid is first generated, and then decomposed by the action of light: carbon is now fixed under the form of a nutritive material, which is conveyed back into the system; and this material is further elaborated for the development of all parts of the structure, and for the preparation of certain secreted matters, which are either retained within or ejected from the plant.

(85.) Flower Buds. — Numerous examples are perpetually occurring, in which the attentive observer of nature may catch a glimpse of the mysterious connection which subsists between the organs of nutrition and reproduction, in plants. Instances continually present themselves, of flowers whose separate portions are singularly characterised, by possessing an intermediate condition, partly leaf-like, and partly like those variously coloured appendages which constitute the blossom. By an accurate examination of these and other “monstrosities” as all deviations from the ordinary conditions of vegetation are termed, it has been clearly ascertained, that the organs of reproduction and nutrition are merely modifications of some one common germ, which may be developed according to circumstances, either in the form of a flower-bud, or of a leaf-bud. In the latter case we have shown, how this body becomes a branch and leaves; and we have now to explain the conditions and characters of those several organs which are developed from the flower-bud, and collectively termed the “inflorescence.” It would be equally erroneous for us to call the flowerbud a metamorphosed state of the leaf-bud, as to say the leaf-bud was an altered condition of the flower-bud; and we are nearer the truth, when we consider each of them to be a peculiar modification of the same kind of germ, adapted in the one case to perform the functions of nutrition, and in the other, those of reproduction. Flower-buds ought consequently to make their appearance on similar parts of the stem and branches with the leaf-buds, viz.