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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.*
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.+
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.
+ It must be remarked, however, that the existence of lym
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 air-vessels, 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 measures of other authors do not, however, very well accord with these.
The lymphatics of a plant may be often seen with great ease by merely stripping off the cuticle
phatic, arterial, and vinous vessels, is not recognised by those botanists who are regarded as the best authorities on the details of vegetable anatomy. - ED.
with a delicate hand, and then subjecting it to a microscope; and in the course of the examination some have thought themselves 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 common 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
*This seems to acquire additional probability from Mr. Knight's experiments. See Phil. Trans. 1804; and Thomson's Chemistry, v. 385. See Willd. p. 236. Some physiologists regard the existence of these valves as imaginary.
+Impatiens balsamina :- This is the plant recommended by M. Willdenow for this purpose, as affording the clearest results.
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 phænomenon of a circulatory system; and from several of the experiments of M. Willdenow, it has been inferred that this circulatory system is maintained by the projectile force of a regular and alternate contraction and dilatation of the vegetable vessels. But this alternate contraction and dilatation is a mere hypothesis: and, in truth, 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
* 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
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 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,