life, by the ancients, as in the present day. The seed is sometimes naked, but more generally covered with a pericarp, whence plants become naturally divided into the two grand arrangements of gymnospermous and angiospermous. The pericarp is of various forms and structures; and of these the more common are the legume, silique, or silicle, being merely varieties of what, among ourselves, is denominated in popular language cod or pod; the loment which is a kind of pod not so frequent as either of the former, but of which we have an instance in the mimosas and the cassia fistula; the pome or core-apple, of which we have instances in the common apple and the pear; the drupe, or stone-apple, instances of which occur to us in the plum, cherry, and almond; the glume or chaff; the berry; the acinus or conglomerate berry, as in the rasp; the nut; and the capsule.† Stripping off this outer covering, we find the seed to consist internally of a corculum, or heartlet, and externally of a fleshy or parenchymatous substance, surrounded with a double integument, sometimes single, sometimes bifid, and sometimes more than bifid; and hence denominated monocotyledonous, dicotyledonous, polycotyledonous. In popular language these are ealled seed-lobes, or seed-leaves: and in the phaseolous vulgaris, or common kidney-bean, we have as striking an instance as in any plant, and which every one must have noticed, just peeping in two distinct segments above the ground, as soon as the seed has begun to germinate. It was very generally supposed formerly, and is still supposed by some botanists, that the seeds of various orders of plants, as the mosses, fungi, and algæ, are acotyledonous, or totally destitute of a cotyledon of any kind. But as many, perhaps most, plants of this kind have of late been found to possess some such parenchyma, we have great reason for believing that this organ is universal, and that there is no such thing as an acotyledonous seed in the whole vegetable kingdom. In reality, the cotyledon appears absolutely necessary for the germination and future growth of the seed, and may hence be denominated its lungs or plaeentule. Like the perfect plant, it possesses lymphatics and air-vessels. Through the former of these it absorbs the moisture of the soil into which it is plunged, decomposes a part of it into its elementary principles, and conducts those principles, together with the undecomposed water, to the corcle or heartlet, which becomes stimulated to the process of germination by the oxygen thus set at liberty. Mrs. Ibbetson has attempted to prove that the cotyledon is of no use whatever for the purpose of nourishment; which, according to her observations, is only conveyed to the corcle by what she calls a system of nourishing vessels, altogether distinct from the cotyledon. It is not very clear, however, what is here meant by nourishing vessels; nor can we for a moment admit that so large an organ as the cotyledon, and apparently so important, can be designed for no other office than merely, as this lady conjectures, to screen the primordial leaves from the light and air on their first formation." According to Mr. Mirbel's experiments, as detailed in the Memoirs of the National Institute, the soil and the albumen in the cotyledon are both coneerned in the developement of the germ; and both continue to contribute conjointly till the albumen is entirely absorbed: at which time the plant has strength enough to derive from the soil or the atmosphere the nourishment it requires from this period. In this respect the albumen of the cotyledon corresponds with the vitellus of the hen's egg. In marine plants that are destitute of a radicle, as the water caltrop (trapa And upon this beautiful verse, which he has preserved as a fragment, Aristotle remarks, tó тe yàp wdr κύημά ἐστι, καὶ ἔκ τινος αὐτοῦ γίγνεται τὸ ζῶον. "For the egg is the conception, and after the same manner the animal is created."-De Generat. Animal. i. 23. Compare Knight's General Theory of Vegetable Physiology, Horticultural Transactions, vol. i. p, 217, with Nicholson's Journal, vol. xxxii. p. 350. Nicholson's Journal, vol. xxvii. 9. natans), the germ must necessarily be supported in the first instance by means of the cotyledon. It is the corcle which is the true punctum saliens of vegetable life, and to this the cotyledon is subservient. The corcle consists of two parts, an ascending and a descending; the former called its plumule, which gives birth to the trunk and branches; the latter named its rostel, which gives birth to the root and radicles. The position of the corcle in the seed is always in the vicinity of the hilum or eye, which is a cicatrix or umbilicus remaining after the separation of the funis or umbilical cord from the pericarp, to which the seed has hereby been attached. The first radicle or germinating branch of the rostel uniformly elongates, and pushes into the earth, before the plumule evinces any change. Like the cotyledon, the radicles consist chiefly of lymphatics and air-vessels, which serve to separate the water from the soil, in order that the oxygen may be separated from the water. Hence originates the root, unquestionably the most important part of the plant, and which in some sense may be regarded as the plant itself: for if every other part of the plant be destroyed, and the root remain uninjured, this organ will regerminate and the whole plant be renewed; but if the root perish, the plant becomes lost irrecoverably. Yet there are various phenomena in vegetable life that manifest a smaller difference in the nature of the root and the trunk, than we should at first be induced to suppose; for Willoughby observed, more than a century and a half ago,* that in several species, and especially those of the prunus and salix, cherry and willow tribes, if the stem branches be bent down to the earth, plunged into it, and continued in this situation for a few months, these branches will throw forth radicles; and if, after this, the original root be dug up, and suffered to ascend into the air, so that the whole plant become completely inverted, the original root will throw forth stem-branches and bear the wild fruit peculiar to its tribe. The rhizophora Mangle, or mangrove-tree, grows naturally in this manner; for its stem-branches, having reached a certain perpendicular height, bend downwards of their own accord, and throw forth root-branches into the soil, from which new trunks arise, so that it is not uncommon, in some parts of Asia and Africa, to meet with a single tree of this species covering the oozy waters in which it grows with a forest of half a mile in length. The ficus Indica, or banyan, grows in the same manner, and often with enormous trunks, equally derived from a primary root. The largest tree of this kind known to Europeans, is on an island in the river Nerbedda in the Guzzerat, distinguished in honour of a Bramin, of high reputation, by the name of Cubbeer Bur. High floods have destroyed many of its incurved stems, yet its principal stems measure two thousand feet in circumference, the number of its larger trunks, each exceeding the bulk of our noblest oaks, amount to three hundred and fifty, while that of its smaller are more than three thousand; so that seven thousand persons may find ample room to repose under its enormous shade, and may at the same time be richly supplied from the vast abundance of fruit which it yields in its season. The solid parts of the trunk of the plant consist of CORTEX, cuticle, or outer bark; LIBER, CUTIS, or inner bark; ALBURNUM, or soft wood; LIGNUM, or Linnæus gave the name of medulla to hard wood;† and MEDULLA, or pith. the pith of plants, upon a supposition that it had a near resemblance to the medulla spinalis of quadrupeds. A closer investigation, however, has since proved that this resemblance is very faint, and that the pith or medulla of Phil. Trans. year 1669, iv. p. 963,-1670, v. p. 1165. 1168. 1199,-1671, vi. p. 2119, †There is a curious paper of Count Rumford's, mentioned among the labours of the French Imperial Institute for 1812, upon the chemical properties of the different parts entering into the composition of the trunk of trees; for an account of which see also Thomson's Annals of Philos, vol. i. p. 386. By a variety of experiments Count Rumford was led to this singular conclusion, that the specific gravity of the solid matter which constitutes the timber of wood is almost the same in all trees. By the same means he determined that the woody part of oak in full vegetation is only four-tenths of the whole. Air constitutes onefourth of it, and the rest consists in sap. Light woods have still a much less quantity of solid matter: but the season of the year and the age of the tree occasion considerable variations. Ordinary dry wood contains about one-fourth of its weight of water. Even the oldest wood, though in the state of timber for All absolutely dry woods give from 42 to ages, never contains less than one-sixth of its weight of water. 43 per cent. of charcoal: whence he concludes, that the ligneous matter is identic in all woods. vegetables consists of nothing more than a mere spongy cellular substance, forming, indeed, an admirable reservoir for moisture; and hence of the utmost importance to young plants, which, in consequence of their want of leaves and branches, whose surfaces are covered with the bibulous mouths of innumerable lymphatics, would otherwise be frequently in danger of perishing through absolute drought; but gradually of less use as the plant advances in age, and becomes possessed of these ornamental appendages; and hence, except in a few instances, annually encroached upon, and at length totally obliterated by the surrounding lignum. All these lie in concentric circles; and the trunk enlarges, by the formation of a new liber or inner bark every year; the whole of the liber of one year, excepting indeed its outermost layer, which is transformed into cortex, becoming the alburnum of the next, and the alburnum becoming the lignum. Such, at least, is the common theory, and which seems to be well supported by the experiments of Malpighi and Grew: but it has lately been controverted by Mr. Knight, who contends, that the liber has no concern in the formation of new wood, which proceeds from the alburnum alone, a new layer of alburnum being formed for this purpose annually. I cannot discuss the argument at present: nor is it of any great importance; since, under either system, it is obvious that a mark of any kind, which has penetrated through the outer into the inner bark, must in a long process of years be comparatively transferred to the central parts of the trunk. On which account we often find, in felling trees of great longevity, as an oak, for example, the date of very remote national eras, and the initials of monarchs, who flourished in very early periods of our national history, stamped in the very heart of the timber on its being subdivided. Some of these memorials are very curious, and M. Klein, the well-known Secretary of Dantzic, has given various examples in his letter to Sir Hans Sloane, bart., the President of the Royal Society. One of these consists of a long series of letters discovered, in 1727, in the trunk of a full-grown beech, near Dantzic, in land belonging to the family of Daniel Berckholtz. The letters D. B. were chiefly conspicuous in the solid wood; the wood towards the bark, and that towards the heart, that is, in each extremity, "bearing not the least trace of letters." M. Klein relates another example from the Ephemerides of Natural Curiosities,† recorded by Joannes Myerus. It consists of a thief hanging from a gibbet, apparently drawn by nature's own pencil in the timber of a beech-tree: as also the figure of a crucified man, found in a tree of the same kind; and that of a chalice with a sword, perpendicularly erect, sustaining a crown on its point; which was preserved at the Hague, and had been seen by himself. Such marks were formerly attributed to miraculous intervention, or regarded as marvellous sports of nature: but the hints now offered will easily explain their origin. Foreign substances have often been found imbedded in the same way, having at one time been sunk into the inner bark, or penetrated it by a wound or other excavation, and afterward covered over with new annual growths of liber and alburnum. Thus Sir John Clerk gives an account of a horn of a large deer which was found in the heart of an oak in Winfield Park, Cumber land, fixed in the timber with large iron cramps, with which, of course, it had been fastened on. And we are hence able to account for the occasional detection of a capricorn beetle, or other insect which has been found in the centre of a trunk, the animal having crept into an accidental cleft, and either died there naturally, or been arrested and imprisoned by the secretion of the matter of new inner bark while in the torpitude of its aurelian state. And hence, indeed, the cause of the very wonderful phenomena of toads or frogs Deing at times found in a like situation; having in the same way been mpacted in the hole or crack into which they had crept, by the glutinous fluid of the inner bark, during sickness or a protracted winter sleep. Some Phil. Trans. for 1739, vol. xli. p. 231. Ephem. Nat, Cur. decad, ili. an. v. obs. 29. of these are found alive when the tree is cut down, deriving both air and nutriment enough from the surrounding vessels of the tree during their imprisonment. In the Memoirs of the Paris Academy there is an example of a load found in a tree that was proved to be a century old.* As the series of concentric circles, produced in the trunk of a tree by the growth of every year, are still visible after the conversion of every other part into lignum, or hard wood, we can trace its age with a considerable degree of certainty, by allowing a year for every outer circle, and about two or three years for the complete lignification of the innermost.† Independently of these more solid parts of the trunk or stem, we generally meet with some portion of parenchyma and cellular substance, and always with the different systems of vegetable vessels disposed in one common and uniform arrangement. The lower orders of plants, indeed, such as the annuals and biennials, consist almost exclusively of parenchyma or cellular substance, with an inner and outer bark, and the respective vessels of the vegetable system, These vessels are adducent and reducent, or arteries and veins, lacteal or sap-vessels, and lymphatics. Many of these may be seen by the naked eye, and especially the sap-vessels: and the vascular structure of the whole has been sufficiently proved by Gessner, by means of the air-pump. The reducent or returning vessels are stated, by Sir E. Smith, to bring back the elaborated sap from the leaves to the liber for the new layer of the existing year.t The lymphatics lie immediately under the cuticle and in the cuticle. They anastomose in different ways through their minute intermediate branches, and, by surrounding the apertures of the cuticle, perform the alternating economy of inhalation and exhalation. Their direction varies in different species of plants, but is always uniform in the same species. Immediately below these lie the adducent vessels or arteries; they are the largest of all the vegetable vessels, rise immediately from the root, and communicate nutriment in a perpendicular direction: and, when the stem of a plant is cut horizontally, they instantly appear in circles. Interior to these lie the reducent vessels or veins; which are softer, more numerous, and more minute than the arteries; and in young shoots run down through the cellular texture and the pith. Between the arteries and veins are situated the airvessels, as they were formerly called; but which Dr. Darwin and Mr. Knight have sufficiently succeeded in proving to contain, not air in their natural state, but sap. They seem to be the true genuine lacteals issuing from the root, as, in animals, they issue from the villous coating of the intestinal canal. They are delicate membranous tubes, stretching in a spiral direction, the folds being sometimes close to each other, and sometimes more distant, but generally growing thicker towards the root, and especially in ligneous plants. These vessels also are very minute, and, according to numerous observations of Hedwig made with the microscope, seldom exceed a 290th part of a line, or a 3000th part of an inch in diameter. The lymphatics of a plant may be often seen with great ease by merely stripping off the cuticle with a delicate hand, and then subjecting it to a microscope; and in the course of the examination we are also frequently able to trace the existence of a great multitude of valves, by the action of which the apertures of the lymphatics are commonly found closed. Whether the other systems of vegetable vessels possess the same mechanism, we have not been able to determine decisively; the following experiment, however, should induce us to conclude that they do. If we take the stem of a com Mém. de l'Acad. Par. 1731, p. 24. The palms form an exception to this general rule, possessing neither proper bark, nor fascicles of vessels displayed in any circular form: the bark being produced by a remnant of the leaves, and the vessels running in a straight line without regular order, and surrounded by cellular substance. Introd. to Botany, p. 56. See also Willdenow's Introd. p. 236. Bee Smith's Introd. p. 47. This seems to acquire additional probability from Mr. Knight's experiments. See Phil. Trans. 1604, and Thomson's Chemistry, v. 385, Bee Willd. p. 236. mon balsamine, or of various other plants, and cut it horizontally at its lower end, and plunge it, so cut, into a decoction of Brazil wood, or any other coloured fluid, we shall perceive that the arteries or adducent vessels, as also the lacteals, will become filled or injected by an absorption of the coloured liquor; but that the veins, or reducent vessels, will not become filled; of course evincing an obstacle, in this direction, to the ascent of the coloured fluid. But if we invert the stem, and in like manner cut horizontally the extremity which till now was uppermost, and plunge it so cut into the same fluid, we shall then perceive that the veins will become injected, or suffer the fluid to ascend, but that the arteries will not: proving clearly the same kind of obstacle in the course of the arteries in this direction, which was proved to exist in the veins in the opposite direction; and which reverse obstacles we can scarcely ascribe to any other cause than the existence of valves.† By this double set of vessels, moreover, possessed of an opposite power, and acting in an opposite direction, the one to convey the sap or vegetable blood forwards, and the other to bring it backwards, we are able very sufficiently to establish the phenomenon of a circulatory system; and, according to several of the experiments of M. Willdenow, it seems probable that this circulatory system is maintained by the projectile force of a regular and alternate contraction and dilatation of the vegetable vessels. Yet the great minuteness of these vessels must ever render it extremely difficult to obtain any thing like absolute certainty upon this subject. Even in the most perfectly established circulatory systems of animals, in man himself, it is not once in five hundred instances that we are able to acquire any manifest proof of such a fact we are positive of the existence of an alternating systole and diastole in the heart, from the pulsation given to the larger arteries when pressed upon; but no degree of pressure produces any such pulsation in the minuter arteries, at least, in a healthy state; yet we have full reason to believe that the same action of the heart extends to the minutest as to the largest arteries. How much less, then, ought we to expect any full demonstration of this point in the vessels of vegetables, in every instance so much more minute than those of the more perfect animals, and seldom exceeding, as I have already observed, a three-thousandth part of an inch in diameter! It becomes me, however, to confess, that no experiments which have hitherto been made have detected the existence of either motific or sensific fibres themselves in vegetables, although very high degrees of galvanic electricity have for this purpose been applied to the most irritable of them, as the dionæa muscipula, or Venus fly-trap; oxalis sensitiva; different species of drosera, or sun-dew; acacias of various kinds, and other mimosas; and especially the mimosa pudica, and sensitiva, the common sensitive plants of our green-houses. Humboldt has uniformly failed; Rafn appears to have succeeded in one or two instances; but his general want of success prevents us from being able to lay any weight on the single case or two in which he seems to have been more fortunate. It should be observed, that the matter of fibrine, or the principle of the muscular fibre, formerly supposed to exist exclusively in animal substances, has lately been detected by M. Vauquelin in vegetables also. Dr. Hales cut off the stems of vines in the spring, and by fixing tubes on the stumps, found that the sap rose in many instances to the height of thirty-five feet. Tubes have been fixed to the large arteries of animals, as near as possible to the heart, in which the blood did not rise higher than nine feet. It has long been admitted by botanists in general, that the thorns of plants are abortive branches; the scales of buds have, in like manner, been regarded as transformed leaves; and it has lately been conjectured by M. de Candolle, Impatiens balsamina:-This is the plant recommended by M. Willdenow for this purpose, as afford ing the clearest results. Yet Hales and Duhamel seem to have shown, that in the sap-vessels no valves exist, and that branches imbibe moisture nearly equally at either end. See Thomson's Chemistry, v. 385; an assertion, however, opposed by various facts. See also Smith's Introd. p. 57. 60. |