From Agepedia

Jump to: navigation , search

PLANT. PLANTs are generally defined as being organized bodies without voluntary motion. (See Animal.) In this case, however, as in numberless others, it is much easier to understand the word than to find a definition sufficiently comprehensive and sufficiently exclusive. Plants consist, like all organized bodies, of solid and fluid parts. To the former belong the cellular substance, the various vessels, the fibres and the pith (see Medulla); to the latter belong the sap and the various juices, as well as the air contained in plants. The ah", the sap and the juices have appropriate vessels. The entire or proper vessels, so called, are intended to contain the proper juices of the PLANT, and are generally found filled with oils or resinous juices. They are generally in bundles in the cellular part of the bark, and are found in the young shoots of almost every plant. The spiral vessels, so called from their appearance, are the largest of the vegetable vessels, and in many plants their structure is visible to the naked eye. Their nature and their real economy are very obscure. They are situated round the medulla of the young shoots of trees and shrubs. The perforated vessels are cylindrical tubes, the sides of which are said to be pierced with minute perforations. They have, apparently, no office but that of air vessels. The fluid substances of plants move in the vessels just enumerated. The proper juices contain nourishment adapted for assimilation into the substance of the plant. They correspond in plants to what we call in animals blood, and may well be compared with it as to their functions. In a physiological respect, many points of correspondence between animals and plants are observable. Power of contraction, irritability, power of formation, power of reproduction, and other powers, are possessed VOL. x 15by plants as well as by animals, though in a lower degree. The vital power preserves in plants, as it does in animal bodies, in all the changes to which they are subject, the peculiar character of the individual; and by it the chemical affinity of the primitive substances of which organic bodies consist is modified, so as to be different from what it is in inorganic bodies. If this vital power ceases, the organic body dies, and its component parts become subject immediately to the universal laws of affinity prevailing in inanimate nature. Whether sensation is to be ascribed to plants is doubtful, because, as yet, no nerves have been discovered in them; and the phenomena connected with certain plants, which seem to indicate the existence of sensation in them, may, per haps, be reduced to simple irritability. Motion, as a consequence of vital power, is not to be denied to plants. Several of them, under certain circumstances, exhibit a motion in certain external parts, which is similar to that of animals. The motion of the juices in plants was known long before its cause was understood. Malpighi seems to have looked for the cause in a movement of the vessels; Hales in the warmth of the atmosphere ; later naturalists have referred it to mechanical causes, considering the vessels of plants as capillary tubes, (q. v.) But the insufficiency of these explanations is easily seen, and it appears more correct to consider the irritability of the vessels as the cause of the rising of their juices. This view is confirmed by the fact that the juice ceases to rise if the irritability of the vessels is deadened by electric shocks. But in what this irritability consists, and how it operates, has not, as yet, been demonstrated. Only its existence is known; and experiments have proved that, by certain artificial means, it may be increased as well as diminished,nay, entirely destroyed. Warmth, moreover, seems to influence the motion of the juice in plants; otherwise why should not the irritability produce motion in the juices in winter ? A certain degree of heat is necessary before the juices begin to rise and the growth to proceed. Cold weather immediately produces a check or suspension. The heat of summer appears to weaken this irritability by degrees, till at length the juices move more and more slowly, and begin to stand still in autumn. With the irritability of plants, too, their sleep and their turning towards the light is closely connected. The former seems to ensue after they have been in continued and violent activitv. Itis particularly observable in the corolla, but also, in a lower degree, in the leaves. The disposition of plants to turn towards the light is easily seen in such as have light from one side only, as all the stalks, branches, leaves and blossoms turn in that direction.Another important point in the physiology of plants is their breathing. This consists in an absorption and exhalation, especially observed in the case of the leaves. If a fresh leaf is put in a tumbler filled with springwater, and exposed to the rays of the sun, it soon appears covered with small airbubbles, which, by degrees, rise to the surface of the water, where they burst. If they are caught, it is found that they contain oxygen. The light of the sun is necessary to this phenomenon ; mere heat is insufficient to produce it. Experiments respecting the breathing of plants have led to very different opinions. Ingenhouss thinks that plants exhale oxygen only in the light of the sun, but during the night azote and carbonic acid gas. According to Senebier, healthy plants and their leaves do not exhale any air whatever during the night; the same was maintained by Spallanzani. Ackermann, on the other hand, maintains that plants, like animals, must continually inhale the basis of vital air (oxygen), and exhale carbonic acid. But plants exhale not only gaseous matter; fluids are evaporated from them, the amount of which is considerable. It is asserted that a tree of middling size evaporates daily about thirty pounds of moisture.As to the odor of plants, the recent progress of chemistry shows that the basis of it does not (as might have been supposed of so fleeting, diffusible, almost imponderable, entirely invisible a substance, affecting only the olfactory nerves) consist of a gaseous matter. Fourcroy showed that there does not exist a separate principle of scent. This property is as essential to bodies as gravity, but is proportionate to their volatility : the most volatile bodies have the strongest odor.The taste of plants seems to depend on the proportions of their elementary ingredients, and on the degree of heat to which the plant is exposed. The rays of the, sun, also, have a powerful influence on it. Of the colors of plants the same is true that has been said of their scent. Even Aristotle observed that plants are colored by the sun. Ray, Bonnet, Senebier, and others, made various experiments connected with this point. Senebier found that when plants were put in a dark place, their green leaves become first yellow on the surface and then white; whilst young plants which had grown up in the dark, when brought by him gradually to the light, exchanged their white color for yellow, which, after a while, became darker, and showed by degrees green spots, continually increasing in number and size, so that, after some time, the parts before white acquired a perfectly green color. With blossoms raised in th& dark the change of color is b it slight. Bonnet asserts the cooperation of heat in this process: but, according to the ex periments of Van Mons and Vasnlli, the light of lamps and of the moon onsrates in the same way. The cause of ta's remarkable phenomenon is at p. :sent known. Plants become lighter in consequence of combination with the oxygon which they inhale, darker if they lose it. The different proportion of oxygen to ;*s other component parts gives the various gradations and shades. Saturation with oxygen gives the yellow and white color. But if a plant saturated with oxygen is exposed to the rays of the sun, the substance of the light unites with the oxygen, the latter escapes, and the plant reassumes its green color. For the rest, the color seems to have its seat in the cellular substance ; the epidermis, however, is without color. The chemical analysis of plants shows that all vegetable matter consists chiefly of hydrogen, carbon and oxygen. Their different proportions produce the variety of vegetable substances. Of these substances chemistry has distinguished gum,fecula or starch, sugar, gluten, albumen, gelatin, caoutchouc or Indian rubber, wax, fixed oil, volatile oil, camphor, resin, gumresin, balsam, extract, tannin, acids, aroma, the bitter, the acrid and the narcotic principles, and ligneous fibre. Several of these substances are capable of transformation into each other. Thus the tasteless mucilage passes into sugar or acid. These changes are produced by heat, moisture, air, alkalies, which change more or less the proportion of the original constituents. The formation, therefore, of the various substances in vegetables is the consequence of truly chemical operations, which may be traced from the germ to the ripe fruit. To determine how the original constituents are absorbed by light and heat, and united to each other by the vegetable organization in such a manner that they produce the various substances of which plants are composed, and which again, in their last analysis, are resolved into those original constituentsthis is the problem of vegetation. The way in which plants grow, i. e. in which the nutritious their constituent parts, enter into new unions, and thus form the solid portions of plants. Hydrogen separates from the oxygen in order to unite with carbon, and thus oils, resin and the like, are formed. At the same time, oxygen is formed from the water and carbonic acid, and passes off, in union with caloric, as oxygen gas. By meaiis of these substances, the increase of the vegetable fibres, or the proper growth, is produced, though we are not able to^see clearly the way in which it is effected. As to the fructification ofplants, the ssjfne general theories exist as in regard to the fructification of animals ; i. e. thf? theory of evolution, which considers the germ of all creatures as already existing, and only waiting for the process wmch is to call them into life, and the more philosophical theory of actual generation by a wonderful cooperation in the two sexes. This process in plants takes place in the following way, very similar to that in the case of animals :Plants have male and female organs of generation, which may be observed by the naked eye; yet these parts are generally not permanent, as in the case of animals, but change after fructification has taken place. The pollen or farina is prepared and preserved in certain vessels destined for this purpose, called anthers. Its finest part penetrates through the stigma, an opening in the female part, through the pistil to the ovary,and fructifies the germs or ovules lying there. With most plants both sexes are united in one flower ; with a few they are separated. The former are called 'perfect flowers, the latter male or female. The two latter either stand on one stem or belong to different plants. With the (so called) perfect flowers fructification is effected most easily; and also, where the same stem has male and female blossoms, no particular difficulty exists; but where the two sexes are entirely separated, fructification takes place only when the two plants of different sexes stand near enough for the pollen of the male plant to be carried to the female by the wind or by insects. If this or an artificial fructification does not take place, the germ either falls off, or it forms a fruit, which, however, is incapable of germinating. Wonderful, indeed, are the means by which nature effects the fructification of these plants! Within the flower of the plants are generally glands, which exude a honey, by which insects are attracted ; but, in order to obtain this, they must powder themselves in the male view, they must deposit the pollen on the pistil. In some other plants, where the male and female parts in perfect flowers are placed so as not to be able to reach each other, little flies are attracted by the honey, but immediately upon their entrance the flower closes, and thus the insects, who crawl in all directions to find a way of escape, are forced to fructify it. Grasses are generally fructified by the wind. Linnoeus founded his system (sexual system) on the generating organs of plants. (See the article Botany, for other systems.) He divided the whole vegetable world into twentyfour classes. The twentythree first comprise the plants with visible blossoms, the phanerogamous. Of these, the thirteen first receive their names from the number of their stamens, or male organs of generation: their names are, 1. monandria, with one stamen; 2. diandria, with two; 3. triandria, with three; 4. tetrandria, with four; 5. pentandria, with five; 6. hexandria, with six; 7. heptandria, with seven; 8. octandria, with eight; 9. enneandria, with nine ; 10. decandria, with ten; 11. dodecandria, with twelve to nineteen ; 12. icosandria, with twenty; IS. polyayidria, with more than twenty stamens. In all these classes, the orders, or first divisions of classes, are determined by the number of female parts of fructification; i. e. the pistils; for instance, monogynia. with one pistil; digynia, with two; trigynia, tetragynia, &c. The fourteenth and fifteenth classes are determined rather by the situation of the filaments. They are called, 14. didynamia, in whose blossoms are always four stamens, of which two are longer than the resthence the name; 15. tetradynamia, in whose blossoms are always six stamens, of which four have longer filaments than the others. Each of these classes contains but two orders. Those in the fourteenth are determined by the circumstance of the seed lying naked in the calyx (gymnospermia\ or being covered (angiospermia). In the fifteenth class, the orders are determined by the comparative length of the pod or silique, the first being termed siliculosa, the second siliquosa. In the 16th, 17th and 18th classes, the number of bundles in which the filaments are united, determines the class; 16. monadelphia (one brotherhood), when the filaments are united in one bundle ; 17. diadelphia (two brotherhoods), when they are united in two; 18. polyadelphia (many brotherhoods;. The orders'in these classes are detevmined by the number of the separate stamens, as monandria, diandric/,, triandria, &c. 19. Syngenesia (grown together), or compound flowers. Almost all the flowers belonging to this class consist of a irumber of small flowers united. This class has six orders :(a.) polygamic/, aqualis, if the compound flower consists of perfect flowers only ; (b.) polygamia supeiflua, when in one compound flower there are fertile female flowers, styliferous as well as perfect flowers ; (c.) polygamiafrustranea, when there are perfect flowers, and female flowers, but the former only fertile and yielding seed ; (d.) polygamia necessaria, in which the reverse takes place, and the hermaphrodite flowers have no real stigmas ; (c.) polygamia segregata, in which there are two sets of calyces, the outer, or common involucrum, and an inner or included calyx containing one or more florets, and thus producing as it were a doubly compound flower; (f.) monogamia (an order now abolished) was so named because it had no compound flowers. 20. Gynandria, of which the character is, that the stamens, one oi aiore, are attached to the pistil or style. The orders are determined by the number of stamens, and are denominated monandria, diandria, &c. 21. Monozcia (onehoused plants), in which the sexes are separate, yet on one stem. The orders in this class are not only determined by the number of stamens, but there are also monadelphia, syngenesia and gynandria. In the last an imperfect pistil exists in the male flowers, on which stand the stamens. 22. Diazcia (twohoused plants), with entirely separate sexes, i. e. in which one plant produces only male, the other only female flowers. The orders are as in the 21st class. 23. Polygamia (a class now generally abolished and incorporated with diazcia)', plants with complete and incomplete flowers distributed on two or three different individuals of the same species. The three orders of this class are called monozcia, diozcia and triozcia, according to the mixture on one, two or three sterns. 24. Cryptogamia. To this class Linnaeus refers all plants in which he found no sexual parts; but in many they have been since discovered, and even in those in which they are not yet known, they certainly are not wanting. It contains four orders: 1. ferns; 2. mosses; 3. seaweeds, liverworts, lichens; and, 4. fungi. The palms, whose sexual parts Linnaeus was unable to determine, and which he therefore described in an appendix, are at present distriouted among the other classes. Later botanists have reduced the 24 classes to 20. This sexual system (so called) has been opposed by Schelver (Kritik der Lehi e von den Geschlechtern der Pflanzen (Heidelb., 1812), and Fortsetzung der Kritik (Carlsruhe, 1814), and particularly Henschel, Ueber die Sexualitat der Pflanzen (Bresl., 1820), whose views have attracted much attention. They start from the principle that the animal has the advantage of the plant in individuality, both in the general structure and in that of the various parts, and that the individuality which is the most prominent, is the animal generation; on the other hand, that with plants the similarity in the general structure, as well as in that of the single parts, is incompatible with diversity of sex, and that therefore all proofs alleged in support of the latter must undergo a reexamination. Henschel undertook this; but Treviranus, in his Die Lehre vom Geschlechte der Pflanzen (Brem., 1822), has contradicted most of his statements. Yet the famous K. Sprengel adheres fully to HenschePs views. To this artificial system is opposed the natural, which is founded on the presence or absence of the chief organs, because plants differ from each other chiefly in this way. Oken followed this system in his Natural History for Schools (Leipsic, 1821). And such an one only can give an insight into the great and beautiful order of this vast kingdom of nature. See Decandolle's Organogiaphie Vegetale (2 vols., 60 engrav.). As a convenient manual, we would refer the reader to NuttalPs Introduction to Systematic and Physiological Botany, 2d edit., Cambridge (Mass.), 1830. Respecting vegetable geography, see Schouw's Diss, de Sedibus Plantarum originariis (1816); his Grundziige einer Allg. Pflanzengeographie (Copenhagen, 1822; translated from the Danish into German, Berlin, 1823); Atlas of Veget. Geography (Berlin, 1824) ; Alexander von Humboldt's works; particularly the introduction to Bonpland's (q. v.) work, Nova Genera et Species Plantarum, by Kunth.Anatomy of Plants. A more accurate knowledge of the organization of plants has been obtained chiefly by the zealous and patient investigations of German and French naturalists, as Sprengel, Link, Treviranus, Mirbel, Richard, and many others. A short view of the organization of plants must suffice for our purpose. I. General Structure of Plants. The primitive form, which appears in the earliest stage even of the lowest PLANT, is the globule, which we may observe even in the nourishing juice, which exudes from lexiure 01 cenuies, wnicn is universally diffused through the vegetable world. The sides of these cells are entire, without any apertures, so that one cell has no communication with the others; but the juices contained in them perspire organically in the same way as those in the animal body. In those cases in which the globules do not touch' each other on all sides, they leave interstices, which serve as passages for the juices, particularly in trees with acicular leaves. Yet these passages are very often wanting in the cellular texture, because the little globules which form the latter are attracted so uniformly, that regular spaces are produced, the sides of which are perfect squares, pentagons or hexagons. The cellular texture serves for the preservation and preparation of the juices. Hence it is generally filled with mucilaginous, saccharine, oily or resinous substances. The cellular texture, in the more perfect plants, has a remarkable connexion with the air. From the ferns upward, it becomes more regular towards the surface of the PLANT, and full of spaces, which are filled with air, received through apertures of a peculiar organization. These apertures are found mostly where a green surface covers the plant, most frequently, however, on the lower surface of the leaves. They are more or less oval, generally surrounded by a glandulous ring, and have, sometimes, below them, small folds, which keep them open. They may be considered as destined to inhale and to exhale, but merely gases, not watery liquids. The second original formation is the rectilinear, fibrous, or, more properly, tubular structure. Powerful magnifiers show that the fibres are real tubes filled with juice, but not continuous, but here and there terminating in a point, e. g. in the liber of trees, also in the alburnum and in the (so called) nerves and ribs of leaves. Their first beginnings appear already in the mucilaginous nourishing juice, where they have the form of needles, and crystallize as it were in bundles. These tubes have the softest skin and the smallest diameter among all the original formations ; yet they are extremely extensible and tough. They form what is spun as flax, and what is obtained for useful purposes from hemp, from the papermulberry, &c. Their chief purpose seems to be the conducting of the ascending juices. The third original formation is called the spiral form, because it consists primitively of fibres spirally 15* irom me rems upward, in tne more perrect plants, surrounded by the vessels in bundles ^and single. In the trunk of common trees, it generally forms the alburnum and the wood. With the palms, the grasses, &c, the spiral bundles are distributed in the cellular texture. The spiral canals pass through all parts. Through the leafstalk they penetrate with the vessels that convey the juices into the nerves of the leaves, through the flower stalk into the corollse, into the filaments, the ovaries, the pistils, even into the seeds. As long as they remain original, they have no wall, but that which is formed by those winding fibres. But they are not always found in this original form. They appear often as annular vessels, often as stairshaped, or as perforated vessels, &c. At length there are transitions from them to the cellular form, particularly in the trees with acicular leaves. Here appear oblong cells perforated with regular holes provided with margins; nay, in the yew we even find cells with divisions winding spirally, which probably take the place of the spiral canals not existing here. The function of this third original formation seems to be the preparation and conducting of the gases, the moisture, &c, which proceed from the juices of the plants. II. Particular Structure of the single Parts of Plants, The root. The surface even of the firmest roots is suiTounded with fine hairs, and the points are covered with a spongy cap, by which and the hairs the absorption of the moisture in the ground is carried on. A bundle of tubes passes through the centre of the root, in which there is no pith. The stem consists in woody dicotyledonous plants of three distinct partsthe bark, the wood and the pith. The bark is composed of four parts, 1. a dry, leathery, tough membrane, the cuticle ; 2. a cellular layer adhering to the cuticle, and called the cellular integument; 3. a vascular layer; and, 4. a whitish layer, apparently of a fibrous texture, the inner bark, which is of a more complicated structure than the other layers. The wood is at first soft and vascular, and is then called alburnum ; but it afterwards becomes hard, and in some trees is of a density almost approaching that of metal. It is composed of concentric and divergent layers, the former consisting of longitudinal fibres and of vessels of various kinds, the latter of flattened masses of cellular substance, which cro&s the concentric layers. The individual cells are narrow and horizontal in their length, and extend in series from the centre to the circumference of the wood, so as to form .nearly right angles with the tubes of the concentric layers. Various opinions have been entertained respecting the origin of the wood or alburnum. Mr. Knight has proved that the alburnum is formed from the secretion deposited by the vessels of the liber. Wood, while in the state of alburnum, is endowed with nearly as much irritability as the liber, and performs functions of great importance in the vegetable system ; but when it is hardened, these functions cease, and in time it loses its vitality, not unfrequently decaying in the centre of the trunks of trees, which often, however, put out new shoots, as if no such decay existed. To carry on, therefore,, the. functions of the wood, a new circle is annually formed over the old. The hardness of these zones increases with the age of the tree, those in the centre being most dense. In the centre of the wood is the pith, enclosed by the medullary sheath. The pith or medulla in the succulent state of a stem or twig, is turgid with aqueous fluid, but, before the wood is perfected,it becomes dry and spongy, except near the terminal bud, or where branches are given off, in which places it long retains its moisture. In the majority of woody dicotyledons it is longitudinally entire. The color of the pith in the succulent shoot, or the young plant, is green, which, as the cells empty, changes to white; but to this there are some exceptions. In the greater number of plants no vessels are perceptible in the pith. Little is known as yet with certainty concerning its functions. The majority of leaves are composed of three distinct parts, one firm, and apparently ligneous, constituting the framework or skeleton of the leaf; another, succulent and pulpy, fills up the intermediate spaces: and a third, thin and expanded, encloses the other two, and forms the covering for both surfaces of the leaf. The first of these parts is vascular, the second cellular, and the third a transparent cuticular pellicle. The cellular substance becomes more compact towards the upper surface, and is here generally covered by a sort of varnish. Towards the lower surface it becomes looser, and receives those apertures which permit the entrance of air. In flowers the calyx is generally of the same construction with the leaves; but the corolla consists of the most delicate cellular substance, whose inner surface rises in the most delicate prominences. The spiral canals of a very, small diameter pass singly through the lower part of the leaves of the corolla?, and no trace of aper tures is to be discovered. The filaments have a similar construction; but the anthers differ in construction from all the other parts. Entirely cellular, they contain, from the beginning, a number of bodies peculiarly formed, called pollen. The surface of the female stigma is covered with the finest hairs, which, without a visible aperture, receive the fructifying mass in the same organic way as the hairs of the root receive the moisture of the earth. The ovary contains, before the fructification, merely little bladders, filled with the nourishing juice. After the fructification, the future plant shows itself first in a little point which floats in that juice. Nourished by the latter, the little plant either swells and developes its parts, the cotyledones particularly becoming visible ; or, if the juice is not entirely used up, it coagulates to a body like albumen, and the plant remains in the case of the (so called) monocotyledones, undeveloped. (For the pressing of plants, see Herbarium.)