Imágenes de páginas

Its extremity, which is protrudcd by germination, is arranged in a ternary mode, simple or double, becomes longer. One end becomes thickened, or multiple. Dicotyledonous plants are much and buries itself in the soil, whence proceeds the less constant; nevertheless, the number five, simroot; the other end is elevated, and bears the ple or multiple, is more commonly peculiar to seed, like a little cap. Presently a green color them than any other number. It is extremely pervades it, and we can no longer doubt that it is difficult to assign a cause for this peculiarity; a genuine leaf. A little above the root is a perhaps it is to be sought in the manner in whick small lateral slit; from this a second leaf is pro- the fibres first proceed from the parent embryo. duced, a third follows, and so on. Each is en- 142. Having thus examined the progress of closed within the other, as in the palm ; and, development in the internal parts of a plant, like it, they all direct their efforts to produce a and considered them with relation to their funckind of base; by these means the dilatation of tions, we will conclude the subject of Vegetable the root takes place; and, the centre constantly Physiology by some remarks upon their variaforcing the interior outwards, a true onion is at tions. We have already seen, that the constitulast the consequence. The leaves, withering up ent parts of plants are, cellular tissue, woody as soon as they have performed their functions, fibre, and spiral vessels. perish, and leave behind them nothing but their 143. The Cellular Tissue consists of fine and fleshy sheaths, the most exterior of which wither membranous utricles. Individually, they reand perish also; the interior retain their fleshy semble oblong bladders inflated in the middle, and swollen habit. As soon as the period of as in the case of some plants; or circular or hexfructification has arrived, a simple leafless stipes agonal cells, as in the case of others. Collecis elevated from the centre of the root, and puts tively, they have been compared to an assemblage an end to the existence of the individual, except of threads of contiguous bladders or vesicles, or when buds exist among the leaves and give birth to the bubbles that are found on the surface or to what are called off-sets.

liquor in a state of fermentation. 140. The second example, which is equally 144. But this description is applicable to them familiar, shall be that of wheat. The valuable seed only as they occur in herbaceous plants; though which is borne by this herb is, like that of the onion in either case they are not always of the same and the palm, formed of albumen, which is what figure, in all the different parts of the same plant, we know under the name of flour, and of an em- In the leaf-stalk of the artichoke, for example, bryo, which reposes at its base. The latter is a their diversity of figure is very conspicuous, prelittle different in figure from the two others; senting, in their free and uncompressed state, but, like them, it gives rise to a sheath, out of whether on a horizontal or longitudinal slice, a which in succession scales and then leaves arise. beautiful assemblage of hexagonal cells ; but in At the base of each sheath or leaf, in the inside, their crowded and condensed state, as they apis found a bud, which is speedily developed, proximate the longitudinal fibres, an assemblage and contributes to form the tuft of herbage, under of tubular threads, successively inflated and which appearance the plant is seen in its earlier contracted. In woody plants their diversity of stages. But, as soon as the flowering season figure is still greater, as must appear evident if it arrives, a stem of a particular description is pro- is but recollected that they constitute not only duced. Each leaf becomes separated from that the bags or bladders of the cellular integument which is next it by a considerable space; these and pith, and of the pulp of the leaf and fruit, spaces are hollow, and partitioned off by a par- but also the very fabric of the divergent layers ticular kind of division. This kind of stem is themselves; assuming a peculiarity of aspect, called a culm. It is now obvious that there is according to the degree of compression they sussomething peculiar in the manner of growth of tain from other parts; or according to the degrce the mocotyledons, which distinguishes them from of induration they may have undergone, ascenddicotyledons. The leaves, however, offer marks ing progressively, from the succulent texture of of a more decisive kind. We have already seen, the pulp and pith to that of the firm and perfect that in dicotyledonous plants, the nerves of the wood. leaf resemble a sort of net-work, but in mono- 145. The structure of the utricles of the tree cotyledons have a parallel and rectilinear direc. is also said to be different from that of the utricles tion, passing without interruption from one end of the herb; the former being composed of a to the other; that is to say, those fibres which single membrane, and the latter of a double are nearest the principal rim, run alongside it as membrane.. Senebier is, however, of opinion, far as the tip, where they are lost in the margin; that they consist of a double membrane in both and all the fibres affect the same direction. cases, though not so conspicuous in the one case Hence the almost constant elongated form of as in the other, owing to the more compact and their leaves, which are in some sorts comparable condensed texture of the wood. However, they to the blade of a sword, being broadest at the are all mutually connected with one another, and base, and terminating in a point. You rarely also with the other vessels of the plant; which can perceive the crenatures, or denticulations, or double union is rendered evident by means of lobes, which are so common in the leaves of di- colored injections, or rather by means of the abcotyledons.

sorption of colored infusions, from which the 141. The flowers also offer some aid in dis- utricles, as well as the longitudinal tubes, always tinguishing these two great classes from each receive a tinge. But in the petals, stamens and other. The number of parts, which is so varia- pistils, they do not seem to be connected with ble in most plants, appears almost fixed in ihe longitudinal vessels, as in the other parts of monocotyledons. Every organ of fruitification the plant; and perhaps they are also somewhat Vol. IV.--Part 2.

2 C

peculiar in their organisation, as may be inferred detachable from one another by means of being from the following fact, namely, that the white steeped for a few days in spirit of turpentine, and milky juice with which they are filled in the when they become altogether colorless and stem and branch of the fig does not ascend above transparent, because the resinous matter which the peduncle. In the pith they are generally they contained has been dissolved. Senebier larger than in any of the other parts of the says they retain their cylindrical form even in plant; and in plants from which part of the their detached state; and if so, the membrane of trunk has been cut off, it has been remarked that which they are composed must be very strong. they become altogether larger and more inflated The porous tubes resemble the simple tubes in than in plants of the same species that have not their general aspect; but differ from them in been so treated; which enlargement is perhaps being pierced with small holes, or pores, which to be accounted for from the superabundance of are often distributed in regular and parallel rows. sap that now pervades them, in consequence of They are found in most abundance in woody the diminished bulk of the vegetable. Senebier plants, and particularly in wood that is firm and speaks of other utricles, distinct from those of compact, like that of the oak; but they do not, the parenchyma, by which he means the pulp or like the simple tubes, seem destined to contain pith, but without saying anything explicit on the any oily or resinous juice. subject, and without representing them as differ- 149. The spiral vessels are fine transparent and ent in form.

thrcad-like substances, occasionally interspersed 146. The woody fibre is made up of tubes of two with other tubes of the plant; but distinguished kinds, which have been distributed into large from them by being twisted from right to lest, or tubes and small tubes. The large tubes are from left to right, in the form of a cork-screw. distinguishable by the superior width of the They occur in most abundance in herbaceous diameter which they present, on the horizontal plants, particularly in aquatics; but they are also section of the several parts of the plant. to be met with in woody plants, whether shrubs

147. In herbaceous plants, they are represen- or trees. If the stalk of a plant of the liliaceous ted, by M. Mirbel, as being always found in the tribe, or a tender shoot of the elder, is taken and centre of the longitudinal fibres; while in woody partly cut across, and then gently broken or plants, they are often dispersed at random; though twisted asunder, the spiral tubes may be seen they occasionally form regular groups, which are with the naked eye, uncoiled somewhat, but sometimes concentric circles, constituting the remaining still entire, even after all the other principal mass of the ligneous layers. They parts have given way; and, if the inferior portion are generally to be found in great abundance of the stalk is not very large, it may be kept sussurrounding the medullary canal. They are pended for some considerable time merely by found also in the bark, and are capable of ihe strength of the tubes, which though now being traced from their origin in the extreme almost entirely uncoiled, by means of the weight fibres of the root, to their termination in the ex- they support, will, when they finally break, treme summit of the plant; uniting in the body suddenly wind up at each extremity, and again of the root, traversing the collar, penetrating and resume their spiral form. ascending the stem in a parallel direction, sepa- 150. Grew and Malpighi, who first discovered rating and entering the branches, buds, and foot- and described them, represented them as resemstalks; separating again, and distributing them- bling in their appearance the trachea, or windselves in smaller bundles, so as to form the nerves pipe of animals, and designated them by the and veins of the leaves and petals, the slender same term; an appellation by which they are fibres of the stamens and pistils, and the firm still very generally known.

Du Hamel enand woody fibres of the fruit

. In the lichens, deavoured to convey an idea of their form, by fuci, and fungi, no large tubes are discoverable, comparing it to that of a piece of riband rolled even with the aid of the microscope; though in round a small cylinder, and then gently pulled off the transverse section of most other plants they in the direction of its longitudinal axis. The figure are visible without a microscope.

of the riband becomes thus loosely spiral. This 148. The simple tubes, which are the largest is a very good illustration of the figure of the spiral of all the large tubes, are formed of a thin and tubes in their uncoiled state, but it does not repreentire membrane, without any perceptible des- sent them very correctly as they exist in the plant. cription of continuity, and are found chiefly in But the best illustration of this kind is perhaps the bark, though not confined to it, as they are that of Dr. Thomson's. Take a small cylinder to be met with also both in the alburnum and of wood, and wrap round it a piece of fine and matured wood, as well as in the fibres of herba- slender wire, so as that the successive rings may ceous plants. But they are particularly con- touch one another, and then pull out the cylinder. spicuous in the stem and other parts of the The wire, as it now stands, will represent the different species of euphorbia and periploca; spiral tubes as they exist in the plant. And if and in' all plants, in general, containing thick it is stretched, by pulling out the two extremities, and resinous juices, known by the name of the it will represent them in their uncoiled state also. proper juices, to the ready passage of which their But, although the spiral tubes are to be met great width of diameter is well adapted. Some- with in almost all plants, they are not yet to be times they are distinguishable by their color, met with in all the different organs of the plant; which is that of the juices contained in them, or, at least, there are organs in which they occur being white as in euphorbia ; or yellow, as in but rarely, or in very small numbers. celandine; or scarlet as in piscidia erythrina. In They do not seem to occur often in the root, this plant they are united'in bundles, but are or at least they are not easily detected in it.

9 FEB 1971

« AnteriorContinuar »