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Its extremity, which is protruded by germination, becomes longer. One end becomes thickened, and buries itself in the soil, whence proceeds the root; the other end is elevated, and bears the seed, like a little cap. Presently a green color pervades it, and we can no longer doubt that it is a genuine leaf. A little above the root is a small lateral slit; from this a second leaf is produced, a third follows, and so on. Each is enclosed within the other, as in the palm; and, like it, they all direct their efforts to produce a kind of base; by these means the dilatation of the root takes place; and, the centre constantly forcing the interior outwards, a true onion is at last the consequence. The leaves, withering up as soon as they have performed their functions, perish, and leave behind them nothing but their fleshy sheaths, the most exterior of which wither and perish also; the interior retain their fleshy and swollen habit. As soon as the period of fructification has arrived, a simple leafless stipes is elevated from the centre of the root, and puts an end to the existence of the individual, except when buds exist among the leaves and give birth to what are called off-sets.

140. The second example, which is equally familiar, shall be that of wheat. The valuable seed which is borne by this herb is, like that of the onion and the palm, formed of albumen, which is what we know under the name of flour, and of an embryo, which reposes at its base. The latter is a little different in figure from the two others; but, like them, it gives rise to a sheath, out of which in succession scales and then leaves arise. At the base of each sheath or leaf, in the inside, is found a bud, which is speedily developed, and contributes to form the tuft of herbage, under which appearance the plant is seen in its earlier stages. But, as soon as the flowering season arrives, a stem of a particular description is produced. Each leaf becomes separated from that which is next it by a considerable space; these spaces are hollow, and partitioned off by a particular kind of division. This kind of stem is called a culm. It is now obvious that there is something peculiar in the manner of growth of the mocotyledons, which distinguishes them from dicotyledons. The leaves, however, offer marks of a more decisive kind. We have already seen, that in dicotyledonous plants, the nerves of the leaf resemble a sort of net-work, but in monocotyledons have a parallel and rectilinear direction, passing without interruption from one end to the other; that is to say, those fibres which are nearest the principal rim, run alongside it as far as the tip, where they are lost in the margin; and all the fibres affect the same direction. Hence the almost constant elongated form of their leaves, which are in some sorts comparable to the blade of a sword, being broadest at the base, and terminating in a point. You rarely can perceive the crenatures, or denticulations, or lobes, which are so common in the leaves of dicotyledons.

141. The flowers also offer some aid in distinguishing these two great classes from each other. The number of parts, which is so variable in most plants, appears almost fixed in monocotyledons. Every organ of fruitification VOL. IV.--PART 2.

is arranged in a ternary mode, simple or double, or multiple. Dicotyledonous plants are much less constant; nevertheless, the number five, simple or multiple, is more commonly peculiar to them than any other number. It is extremely difficult to assign a cause for this peculiarity; perhaps it is to be sought in the manner in which the fibres first proceed from the parent embryo.

142. Having thus examined the progress of development in the internal parts of a plant, and considered them with relation to their functions, we will conclude the subject of Vegetable Physiology by some remarks upon their variations. We have already seen, that the constituent parts of plants are, cellular tissue, woody fibre, and spiral vessels.

143. The Cellular Tissue consists of fine and membranous utricles. Individually, they resemble oblong bladders inflated in the middle, as in the case of some plants; or circular or hexagonal cells, as in the case of others. Collectively, they have been compared to an assemblage of threads of contiguous bladders or vesicles, or to the bubbles that are found on the surface or liquor in a state of fermentation.

144. But this description is applicable to them only as they occur in herbaceous plants; though in either case they are not always of the same figure, in all the different parts of the same plant, In the leaf-stalk of the artichoke, for example, their diversity of figure is very conspicuous, presenting, in their free and uncompressed state, whether on a horizontal or longitudinal slice, a beautiful assemblage of hexagonal cells; but in their crowded and condensed state, as they approximate the longitudinal fibres, an assemblage of tubular threads, successively inflated and contracted. In woody plants their diversity of figure is still greater, as must appear evident if it is but recollected that they constitute not only the bags or bladders of the cellular integument and pith, and of the pulp of the leaf and fruit, but also the very fabric of the divergent layers themselves; assuming a peculiarity of aspect, according to the degree of compression they sustain from other parts; or according to the degree of induration they may have undergone, ascending progressively, from the succulent texture of the pulp and pith to that of the firm and perfect wood.

145. The structure of the utricles of the tree is also said to be different from that of the utricles of the herb; the former being composed of a single membrane, and the latter of a double membrane.. Senebier is, however, of opinion, that they consist of a double membrane in both cases, though not so conspicuous in the one case as in the other, owing to the more compact and condensed texture of the wood. However, they are all mutually connected with one another, and also with the other vessels of the plant; which double union is rendered evident by means of colored injections, or rather by means of the absorption of colored infusions, from which the utricles, as well as the longitudinal tubes, always receive a tinge. But in the petals, stamens and pistils, they do not seem to be connected with the longitudinal vessels, as in the other parts of the plant; and perhaps they are also somewhat

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peculiar in their organisation, as may be inferred from the following fact, namely, that the white and milky juice with which they are filled in the stem and branch of the fig does not ascend above the peduncle. In the pith they are generally larger than in any of the other parts of the plant; and in plants from which part of the trunk has been cut off, it has been remarked that they become altogether larger and more inflated than in plants of the same species that have not been so treated; which enlargement is perhaps to be accounted for from the superabundance of sap that now pervades them, in consequence of the diminished bulk of the vegetable. Senebier speaks of other utricles, distinct from those of the parenchyma, by which he means the pulp or pith, but without saying anything explicit on the subject, and without representing them as different in form.

146. The woody fibre is made up of tubes of two kinds, which have been distributed into large tubes and small tubes. The large tubes are distinguishable by the superior width of the diameter which they present, on the horizontal section of the several parts of the plant.

147. In herbaceous plants, they are represented, by M. Mirbel, as being always found in the centre of the longitudinal fibres; while in woody plants, they are often dispersed at random; though they occasionally form regular groups, which are sometimes concentric circles, constituting the principal mass of the ligneous layers. They are generally to be found in great abundance surrounding the medullary canal. They are found also in the bark, and are capable of being traced from their origin in the extreme fibres of the root, to their termination in the extreme summit of the plant; uniting in the body of the root, traversing the collar, penetrating and ascending the stem in a parallel direction, separating and entering the branches, buds, and footstalks; separating again, and distributing themselves in smaller bundles, so as to form the nerves and veins of the leaves and petals, the slender fibres of the stamens and pistils, and the firm and woody fibres of the fruit. In the lichens, fuci, and fungi, no large tubes are discoverable, even with the aid of the microscope; though in the transverse section of most other plants they are visible without a microscope.

148. The simple tubes, which are the largest of all the large tubes, are formed of a thin and entire membrane, without any perceptible description of continuity, and are found chiefly in the bark, though not confined to it, as they are to be met with also both in the alburnum and matured wood, as well as in the fibres of herbaceous plants. But they are particularly conspicuous in the stem and other parts of the different species of euphorbia and periploca; and in all plants, in general, containing thick and resinous juices, known by the name of the proper juices, to the ready passage of which their great width of diameter is well adapted. Sometimes they are distinguishable by their color, which is that of the juices contained in them, being white as in euphorbia; or yellow, as in celandine; or scarlet as in piscidia erythrina. In this plant they are united in bundles, but are

detachable from one another by means of being steeped for a few days in spirit of turpentine, when they become altogether colorless and transparent, because the resinous matter which they contained has been dissolved. Senebier says they retain their cylindrical form even in their detached state; and if so, the membrane of which they are composed must be very strong. The porous tubes resemble the simple tubes in their general aspect; but differ from them in being pierced with small holes, or pores, which are often distributed in regular and parallel rows. They are found in most abundance in woody plants, and particularly in wood that is firm and compact, like that of the oak; but they do not, like the simple tubes, seem destined to contain any oily or resinous juice.

149. The spiral vessels are fine transparent and thread-like substances, occasionally interspersed with other tubes of the plant; but distinguished from them by being twisted from right to left, or from left to right, in the form of a cork-screw. They occur in most abundance in herbaceous plants, particularly in aquatics; but they are also to be met with in woody plants, whether shrubs or trees. If the stalk of a plant of the liliaceous tribe, or a tender shoot of the elder, is taken and partly cut across, and then gently broken or twisted asunder, the spiral tubes may be seen with the naked eye, uncoiled somewhat, but remaining still entire, even after all the other parts have given way; and, if the inferior portion of the stalk is not very large, it may be kept suspended for some considerable time merely by the strength of the tubes, which though now almost entirely uncoiled, by means of the weight they support, will, when they finally break, suddenly wind up at each extremity, and again resume their spiral form.

150. Grew and Malpighi, who first discovered and described them, represented them as resembling in their appearance the trachea, or windpipe of animals, and designated them by the same term; an appellation by which they are still very generally known. Du Hamel endeavoured to convey an idea of their form, by comparing it to that of a piece of riband rolled round a small cylinder, and then gently pulled off in the direction of its longitudinal axis. The figure of the riband becomes thus loosely spiral. This is a very good illustration of the figure of the spiral tubes in their uncoiled state, but it does not represent them very correctly as they exist in the plant. But the best illustration of this kind is perhaps that of Dr. Thomson's. Take a small cylinder of wood, and wrap round it a piece of fine and slender wire, so as that the successive rings may touch one another, and then pull out the cylinder. The wire, as it now stands, will represent the spiral tubes as they exist in the plant. And if it is stretched, by pulling out the two extremities, it will represent them in their uncoiled state also. But, although the spiral tubes are to be met with in almost all plants, they are not yet to be met with in all the different organs of the plant; or, at least, there are organs in which they occur but rarely, or in very small numbers.

They do not seem to occur often in the root, or at least they are not easily detected in it.

9 FEB 1971

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