SECTION XIV.DICOTYLEDONOUS, OR EXOGENOUS PLANTS.
InEndogenous plants the seeds have but one lobe, and the growth is invariably from the interior. In the Exogenous class, on the contrary, the seeds have two lobes, and the increase in growth is external: hence the botanical distinction of Exogenous plants. Although the distinctive character of the highest class of vegetables is to have seeds with two lobes, yet the structure and position of the seeds are extremely diversified. Many have horny coats, such as the pips of apples and oranges; or hard ones, as nuts, and the stones of plums and cherries. They are sometimes on the outside of the fruit, as in strawberries, but oftener within it, as in the melon, the pear, and a variety of others. These succulent substances nourish the young seeds for a time, but, when they are matured, the light and heat which ripened the fruits now combine to accelerate their decay and decomposition, in order to set the seeds free.
Whatever be the size or form of the seeds, whether large or microscopic, they invariably contain two seed lobes or primary leaves, consisting of cellular tissue, between which the miniature plant, with its radicle, stem, and terminal bud lies concealed. At the end of the first year there is little difference in the structure of a young woody plant, whether from a one or two lobed seed; the distinction begins the second year. In herbaceous plants, the stem, which is in general annual, is of loose cellular tissue, with separate bundles of fibro-vasculartissue running from the roots upwards, and passing at last into the leaves, where they form the ribs or veins. Many of the higher forms of plants have fleshy underground stems, as instances of which may be named the Corm, as seen in the crocus and colchicum, the Tuber, as in the potato, the Rhizome, as in the fleshy rootstock of Iris florentina, which yields the violet powder of the shops, and indeed most bulbs may be considered as modifications of stems, though they are more strictly analogous to buds. The edible parts of the carrot, turnip, parsnip, and radish are not stems, but highly developed succulent roots, the unusual development of which is a direct result of cultivation.
A young tree, at the end of the first year of its growth, has subterranean roots, with their branches and fibrils, and an aërial stem, often more or less branched, formed of bark, wood, and pith, with a few leaves at its extremity, all exceedingly tender. Every succeeding year a new cylinder of woody fibre and vascular tissue is formed between the wood and the bark, both in the stem and branches. It follows from this manner of growth, that the stem of a tree, consisting of bark, wood, and pith, is formed of a series of cylinders or extremely elongated concentric cones closely united, so that a transverse section exhibits a series of concentric circles or zones from the surface of the bark to the central pith. The structure of the branches is similar, but the number of zones depends upon the age. Since all the tissues that have been described are combined to form the organs of nutrition and reproduction in a full-grown tree, it affords the best general illustration of the organization of the highest class of vegetables.
Every part of a tree or plant, except the top of the stigma and extremity of the roots, is covered by an extremely delicate film of cellulose, closely pressed down upon one, two, or three layers of transparent colourlesscellular tissue compressed into a tubular form, which constitutes the cuticle. These flattened cells, which are firmly united to one another by their edges, differ in shape in almost every tribe of plants. In the monocotyledons they are elongated in the direction of the parallel ribs of the leaves; in the highest class they differ little from circular discs, but they have large sinuosities in their edges, which make their junction very irregular. The upper cells of the cuticle are lined with a waxy substance, which protects the plant from damp; and in many cases it contains more or less silex.
This general covering, or cuticle, is perforated by numerous pores, especially on the under-side of the leaves, and the green tender parts of the branches; they are the organs of respiration. These pores, or stomata, are usually formed of two crescent-shaped cells, joined together by their points or horns, so as to leave an open space like a mouth, through which the plant breathes. These, however, are only the guardians: the opening between them leads into a cavity full of air, which is the antechamber to an interior cavity. The valves of the stomata open and shut according to the humidity or dryness of the atmosphere. All plants of both classes, woody or herbaceous, have stomates, except water plants, fungi, fuci, and others of the lowest class. They are generally very abundant on the under-surface of leaves. They are sometimes in vast numbers on both sides of the leaves, and are essential to the life of the plant. According to MM. Payen and Liepner, silex and azote, together with calcareous and alkaline salts, are invariably found in the cells forming the skin of the roots, stems, leaves, fruits, hairs, and spines. The strong cohesion of the skin, together with the presence of these substances, becomes a defence against the wearing effect of the weather, without diminishing the transparency of the tissues.
The bark is divided into three regions, or zones. The external coat, lying immediately under the skin, is formed of one or many layers of cubical or oblong cells, elongated horizontally. They are transparent and colourless at first, but become brown and opaque with the colouring matter of cork as they grow older. On that account it is called the suberous zone, and sometimes acquires great thickness, as in the Quercus Suber, or cork tree.
The green cellular envelope comes next to the corky layer, and consists of prismatic cells and laticiferous tubes, which form an irregular wide-meshed network, elongated in the direction of the axis of the tree, and sometimes constituting the chief thickness of the bark. This zone, as well as the succeeding, increases imperceptibly by new layers added to its interior, while the exterior coats of the bark perish annually. In some trees they are annually cast off in plates and large flakes, as in the Oriental plane, whose stem and branches look as if they had been peeled in autumn.
The liber, which is the third and innermost zone of the bark, generally consists of several layers of cellular tissue, traversed longitudinally by bundles of woody fibre and laticiferous tubes.
The generating layer of cambium, in which all the phenomena of growth takes place, is a semi-fluid mucilaginous substance, which comes between the liber and the wood. It is most abundant in the spring, and is the origin of all horizontal growth. This mucilage is really made up of a vast multitude of cells, with cell-walls as delicate as those of a soap bubble, which gradually undergo transformation into woody fibre, laticiferous ducts, spirals, &c., thence called the cambium zone. The whole of this matter spontaneously divides into two parts: one forms a new layer of liber on the interior of all those which precede it, and the other anew ring of young sap-wood, exterior to all its predecessors. A portion of the cambium, in its unchanged or liquid state, always remains between the wood and the bark, which are never in absolute contact.
As a new cylinder of wood enclosing all its predecessors is annually formed, the section of a stem perpendicular to its axis exhibits a ring of woody fibre, alternating with a ring of spotted and rayed vascular tubes, which constitute the silver grain of the wood. The rings are more and more crowded, and narrower towards the centre, and at last become impervious to the sap, which only rises through the younger part of the sap wood. In fact, a large portion of the solid fibres of most plants have ceased to take any active share in the performance of vital functions, and, like the solid heart of an oak, retain their integrity simply because they are not exposed to influences which would cause their decomposition. A vegetable tissue exposed to ordinary chemical action, can only remain entire so long as it is performing vital functions. The arrangement of the woody fibre and ducts in the different orders and genera is much varied. The breadth of the rings of wood shows the effect of good and bad seasons; and in extra-tropical latitudes, where there is alternately a period of growth and repose, their number frequently indicates the number of years’ growth; so that the age of a tree may be approximately, if not exactly, learned from a critical examination of a section of its stem.
The innermost cylinder of wood is lined by the medullary canal or tube containing the pith. It is a delicate membrane, entirely formed of hollow spiral tubes. The pith, which fills the canal, is of greenish cellular tissue when young, full of sap, and occasionally, though rarely, mixed with vascular and spiral tissue. It passes uninterruptedly to the end of every branch, leaf bud, andflower. Perpendicular plates, called medullary rays, radiate from the medullary sheath and end at the bark, dividing the whole mass of the wood into triangular or wedge-shaped sections. They are thin plates of cellular tissue, stretched horizontally between the central pith and the bark. In each family of trees and shrubs they have a different arrangement, but in all they keep up a horizontal communication between the centre and the circumference, though they do not all extend throughout the whole length of the stem; some do, others do not. Thus the cellular tissue forms a horizontal system, while the fibro-vascular ducts constitute a perpendicular system of tissues. In some trees the pith is scarcely perceptible, and in others it diminishes or vanishes with age, as in the oak. In the alder and other plants it dries up, breaks into pieces, and the canal is filled with air.
In the stem and branches of the Coniferæ, there is scarcely any mixture of vessels amongst the woody fibre, the vascular system generally consisting exclusively of glandular woody tissue, except in the medullary sheath, where spiral vessels are found in small numbers.
The subterranean growth, or descending axis of trees consists of large branches, sometimes tending downwards, but more frequently spreading in extensive ramifications, not far from the surface of the earth. Their growth and structure are similar to those of the stem, but the cylinders of wood are less apparent; they have medullary rays, but no pith; they merely connect the active roots with the stem, and fix the plant firmly in the ground, for they have few or no pores, and contribute little to the nourishment of the plant, except by conveying liquids from the fibrous roots to the upper growth. The active feeding roots spring from them in the form of bunches of white fibres, like cords or threads, which sink straight down into the ground. These real rootsare of cellular tissue enclosed in vascular tubes and spiral vessels, which terminate at a little distance from the extremity, leaving a point of loose spongy cellular tissue, called the spongiole, which absorbs from the ground the liquids that nourish the plant. These root fibrils are temporary organs; they die on the older parts of the subterranean branches, and are succeeded by others on the new.
The various tissues which form the stem of a tree form, in the same manner, though in diminished numbers, the complicated ramifications of the branches and the leaf-stalks, and terminate in the leaves themselves. Under the transparent film which forms the skin on the upper-surface of a leaf, there is a layer of soft thin-walled cylindrical or prismatic cells, closely pressed together, and full of green vegetable matter, or chlorophyll. Several layers of thick-walled cells follow, each more loosely aggregated than that which precedes it, and fuller of void spaces, till in the last green layer on the under-side of the leaf the cells are globular, with numerous large irregular void spaces, united in a reticulated system filled with air, and in direct communication with the atmosphere by means of the innumerable stomata, which are to be found in the under-surface of the leaves of all land plants of the higher classes, and which are their organs of respiration.
The form of the leaf is determined by the arrangement of the vascular bundles, which are in communication with those in the interior of the stem, and branch out in various directions through the green layers: these branches unite again, and form the skeleton of the leaf, which is often a delicate maze of the finest lacework of nerves. The vascular system is double, consisting of an ascending and descending portion. The ascending portion, which is continuous with the medullary sheath, becomes continuous at the apex of each nerve of theleaf with the descending portion, which is beneath and in contact with it throughout its ramifications. This descending portion at the base of the leaf-stalk, or petiole, becomes continuous with the bundles of the liber. In the upper part of the nerves of the leaf there are spotted vascular ducts, in the lower part there are laticiferous vessels. Those on the upper side carry the rising sap to the green matter, where it is elaborated and matured, and then it passes into the vessels on the under-side of the nerves or veins, which carry it down the liber.
Buds are generally formed of scales closely imbricated round the young leaves, which are variously folded and firmly packed; they contain the rudiments of the whole plant, and as in a large tree they are renewed every year, the sources of life are all but infinite.
The spines with which many plants are armed are of two kinds; one is permanent, being an excrescence from the wood, as in the blackthorn; the other proceeds from the bark, and may be stripped off, as in the rose; both contain silex, and are covered by the skin common to the whole plant.
Few plants of any kind are without hairs, which are chiefly found on the young shoots, and on the under surface of the leaves. They are either formed of a transparent elongated hollow cell, or consist of a number of transparent colourless superimposed cells, sometimes jointed, but more frequently rectilinear. When they sting, as in the nettle, they are set upon a kind of bulb composed of cells which secrete the acrid colourless liquid which causes the irritation, and when slightly pressed send it through the hair, the point of which breaks off as it enters the skin of the hand.
The hairs are so transparent that the gyration of the azotized liquid, called protoplasm they contain, has been distinctly traced, a motion so universal in some partof the structure of plants that according to the observations of Mr. Wenham, the difficulty is to find a plant, aquatic or terrestrial, in which it does not take place at some period of its growth. The gyration in any given cell preserves a uniform direction; in different cells the direction is different. It will persist in a detached part of a plant for several days or even several weeks. It is arrested by cold, and recommences its gyration when the temperature is raised.
It has been mentioned that in the primordial cell the solid coloured particles often form a nucleus in the centre of the viscid liquid called protoplasm, which is continually diminished by the increase of the watery vegetable sap. At length the protoplasm in the hairs is reduced to mere threads extending from the cell wall to the nucleus, so that the latter looks like a spider in the middle of its web. These threads are really streams of the viscid protoplasm flowing through the more liquid cell sap from the nucleus to the cell wall, where they turn and flow back again in another thread. When there are several currents in the same cell, the nucleus, which is the common point of departure and return, is the centre of the vital activity of the cell, though it does not always maintain a central position; in the cells of the leaves of the Vallisneria spiralis the nucleus even follows the protoplasm, which flows in a broad stream up one side of the cell and down the other, as in the Chara. In most plants the gyration is transitory, for the nucleus which always exists in young cells is dissolved as the cell advances in age, and the protoplasm is so much diminished in quantity, that its motion is imperceptible. There are exceptions, however, as in the hairs of the nettles and some other plants, where it is persistent.
The motion is in general very slow. The thinness and minuteness of the currents may be imagined, since theyand the cells containing them are microscopic objects, and the solid particles carried by the liquid, which afford the means of tracing its course, are not more than between the three and the five thousandth part of an inch in diameter. M. Schleiden ascribes the motion to changes in the form of the cells produced by an internal vital action, while Professor Karsten believes, from observations he made on the rotation of liquids in the hairs of the common nettle, that it is a phenomenon of diffusion, depending upon the chemical changes taking place in the cells of the hairs independent of any contractibility, not referable to them.
The whole of the tissues that exist in a well-grown tree are not to be met with in each of the numerous woody and herbaceous plants of the first class; some may be wanting, and those that do exist may be, and generally are, much modified both in form and size. All the trees in the temperate zone, and most of those in the tropics, belong to the class of Exogens; but the annual rings of wood are less distinct in the latter, the periods of repose and activity depending upon the dry and wet seasons not being so decided as our winter and summer. The leaves of tropical plants have a thicker skin than in colder climates, to defend them from an ardent sun. The structure of herbaceous plants in all countries is lax and juicy, they have abundance of pith, large medullary rays, and zones of fibro-vascular tubes, which separate the pith from the bark. In fact, each herbaceous and ligneous family has a structure and properties peculiar to itself; but although there is almost an infinite diversity of form and character, the general type of the class may be traced in all.
Vegetable matter consists of carbon, hydrogen, oxygen, and nitrogen, yet no plant can combine these simple elements into organic substances; they imbibe them by their roots and leaves under the form of carbonicacid, water and ammonia: these they have the power of decomposing, and recombining their simple elements into new compounds. Carbon forms the hard part of plants, and enters extensively into their most delicate structure; but it is never found free. Combined with hydrogen and oxygen it not only constitutes the cell wall cellulose, which may be regarded as the skeleton of the vegetable world, but hundreds of compounds differing decidedly in their properties, yet consisting only of these three elements united with one another in different quantities and proportions. Proteine, a compound of all the four simple elements, is a mucilaginous substance, which lines the primordial cell, is homogeneous at first, and afterwards more or less granulated. It is present wherever the vital energy is in activity.
Although these four primary elements form the basis of vegetation, plants require other substances which they absorb from the ground in a state of solution, such as silex, or rather silicious salts having a base of potash or soda, the carbonates, sulphates, and phosphates of lime, the phosphate of manganese, and the oxides of manganese and iron, with various other metals and substances in a state of combination and solution. A few are universal constituents, as the earths and alkalies; in general each race of plants only absorbs such as are peculiar to itself. Soda abounds in the Algæ and is found in the Liliaceæ, Cruciferæ, and other plants that are indigenous on the sea-coast, or in brackish marshes. Potash exists in land plants, and cannot be replaced by soda, for however rich the soil may be in soda, they do not thrive in it. The ashes of land plants consequently contain the metal potassium, while most of the Algæ yield sodium; they also yield chlorine, iodine, and bromine in a state of combination. It is proved by spectrum analysis, that every plant, with the exception ofthe very lowest, contains a variety of metals in infinitesimal quantities, as lithium, rhodium, and others; but they are not essential to the welfare of the plants. Iron is the most frequent constituent in very small quantities; there are also occasional deposits of soda, lime, and a little manganese. All the various substances which enter the vegetable system, are combined in definite proportions into an infinite variety of organic compounds in different plants, and in different parts of the same plant, for the decomposed matter is carried by the ascending sap to every part even of the highest trees. Throughout the whole process the law of the division of labour prevails; to each part of a plant, and to each group of cells, its own duty is allotted.
The vegetable sap, consisting of water, carbonic acid, ammonia, and other substances, which enter the spongy extremities of the roots in a liquid state, rises in the form of a crude fluid through the whole loose texture of herbaceous plants, through both the wood and pith of trees under two years’ growth, and in older trees and shrubs it rises through the sap-wood of the stem into the branches, and thence into the leaves, the limit of ascent in all plants, so that in spring, all the cells are full of sap. The vascular ducts are capillary tubes, and the cellular tissue is an assemblage of closed cells or sacs, whose wall or cell-membrane is permeable by liquids; hence the imbibition of the roots and the rise of sap in the plant are essentially due to capillary attraction acting contrary to gravitation. The ascension of the liquid is inversely as the diameter of the capillary tubes and cells in the stem and branches; the quantity raised is the same at all heights, and the velocity of ascent is inversely as the height.[79]As soon as the leaves are expanded, they evaporate a quantity of water through their stomata during the day, so that ina tree or any plant, an enormous extent of evaporating surface aids in raising the sap by creating a vacuum in all the upper cells and vessels, by which the force of suction and the rapidity of ascent are increased. It appears that the water evaporated by the leaves is in exact proportion to that taken up by the roots to supply its place; but as soon as the young branches are formed, the buds for the following year produced, and when the leaves are full of the chlorophyll which they have consolidated during the summer, the evaporation is less, the sap ceases to rise, the spirals and vascular ducts in the medullary canal and sap-wood are left dry, and fill with air, which they convey to every part of the plant except the bark, to assist in assimilation, that is, in the formation of organic compounds.
During the whole of this process the leaves and other green parts, which are the organs of vegetable respiration, are most active. They absorb carbonic acid gas from the atmosphere by day, and exhale oxygen. For by the direct action of solar light the carbonic acid gas and ammonia in the crude sap are decomposed, part of the oxygen is set free and exhaled, and the rest, with part of the remaining elements, combine to form chlorophyll, which is a compound of starch and a little nitrogen. The oxygen inhaled by plants during the night, combined with other elements in the sap, forms oxidized vegetable compounds.
M. Kosmann, of Strasburg, observed that both the leaves of plants and their corollas give out a ponderable quantity of ozonized oxygen, much more than that which exists in the air, and that the quantity is less in the night.
All parts of plants that arenot greenexhale carbonic acid gas, and inhale oxygen, like animals, night and day; if prevented from inhaling oxygen they lose theirvital power, are soon suffocated, and the plant dies. The expiration of oxygen by the leaves is connected with the nourishment of a plant, the inspiration of that gas is connected with its life.
When the sap is completely organized by respiration, evaporation, and the chemico-vital agency of light, it descends chiefly through the cambium, lying between the liber and the wood. From this layer the sap distributes to each organ capable of increase, the requisite nutritious liquids, deposits various organic compounds, and annually renews the cambium. Part of the sap in its descent runs into the wood through the horizontal medullary rays, in the cells of which it deposits starch. The descent of the sap is no doubt due to gravitation.
The latex is a general name for those white or coloured juices peculiar to some plants. It is separated in the leaves from the descending sap, which is always colourless, and consists of a clear liquid, thickened and coloured by white, yellow, reddish-brown, or green globules floating in it; it does not turn blue under the action of iodine, therefore it does not contain starch. These proper juices differ as much in quality as in colour; some contain fatty matters, others substances of a totally different nature, as caoutchouc; a few are bland and nutritious, many acrid and poisonous; some contain alkaloids, others have none. These juices are by no means essential to the life of the plant, for sometimes they are wanting in their most essential parts, and they are found in certain species and not in others most nearly allied. Certain it is, that tropical lactescent plants which do not produce their proper juices when brought to a cold climate, still produce their milk vessels.
These vessels follow the ramifications of the veins of the leaves in the highest class, and also in some of the monocotyledons. In the stem the milk vessels belongespecially to the layers of the bark, where they take the form of long reticulated perpendicular ducts, through which the proper juices descend towards the roots.
Each plant has its own system of milk vessels, and M. Lestiboudois has found that the coloured liquids have a rapid motion; the movements are very complicated, not from point to point, but in such a manner that the granules are carried by the liquid into all the ramifications of a complicated network.
The septa, or divisions between the primordial cells, exert a powerful influence upon the substances contained in the sap as it permeates through them, no doubt acting as a dialysing membrane, which separates the gelatinous from the crystalloid matter. The latex is probably separated from the sap by the septa in the cells of the leaves, and sent into the vessels peculiar to it, and then, while the sap is descending and passing through the cambium, it is likely to be dialysed by the septa between the cells of that layer, arresting the protein, and other gelatinous substances, and allowing sugar, starch, and other crystalloid matter to pass freely, and form deposits of organic compounds for the following year. For perennial plants in extra-tropical countries remain in a dormant state during the winter; their cells are then full of organic compounds under the form of protein, as well as sugar, gum, &c., but especially starch, which is converted into sugar or dextrine, when spring awakens the plants to renewed life and activity.
The composition of inorganic matter is very simple; there are comparatively few radicals, and the substances are compounded of few equivalent atoms, at most eight or ten, sometimes only two or three. Carbonic oxide is formed of one atom of carbon and one of oxygen; carbonic acid is formed of one atom of carbon and two of oxygen; and acetylene, M. Bertholet’s base of synthetic compounds, contains two atoms of carbon and two ofhydrogen, chemically united; but no organic compound contains less than three equivalent atoms, generally a great many more. For example, citric acid, which is lemon juice, contains 12 atoms of carbon, 5 of hydrogen, and 11 of oxygen; while strychnine contains 44 atoms of carbon, 23 of hydrogen, 4 of oxygen, and 2 of nitrogen. Experiment has proved that the powers which maintain stability among the numerous and complex constituents of organic substances decrease in energy as the number of the equivalent atoms augments; hence such compounds are in less stable equilibrium than those of inorganic bodies, and are more liable to be disturbed and changed into new and more stable forms.
As the chemical functions are not the same in all the cells, situated as they are in different parts of a plant, they elaborate different substances from the same materials. Besides, new substances are introduced with the growth of the plant, to be acted upon by the light and heat of the different seasons, so that numerous compounds may be formed out of a given number of the primary elements. For example, the ultimate elements of wheaten flour, or a grain of ripe wheat, are carbon, the three elementary gases, sulphur, phosphorus, calcium, magnesium, and silex; but during the germination and growth of the plant, its flowering, forming the seed, and ripening the grain, certain portions of these elements chemically combine in definite proportions to form cellulose, starch, sugar, gum, gluten, fibrin, albumen, casein, and fat, all of which are found in wheaten flour.
However much plants may differ in their organic products, they all agree in producing protein, which takes an active part in the formation of cells; and all produce neutral hydrates of carbon, such as cellulose, starch, sugar, gum, &c., which consist of carbon, combined with hydrogen and oxygen in the exact proportion thatforms water. Many of them have precisely the same quantity of carbon, and only differ in the quantity of the aquatic element, as for example, lignin, starch, and cane-sugar, which consist of 12 parts of carbon, in a state of combination with 8, 10, and 11 parts of water respectively; indeed the affinity between many of these neutral hydrates is of a most intimate character. Some of their varieties are isomeric, that is to say, they contain the same ingredients in the same proportions, and yet they differ essentially in regard to their properties.
Next to cellulose, starch is the most universal and distinctive of vegetable productions, being a constituent of all plants, except the fungi. It abounds in the grains and other seeds, and supplies the young plant with food till it can feed itself. In both of the flowering classes it occurs in small colourless transparent grains, either floating in the sap, attached to the walls of the cells, or accumulated within them. Starch globules of very small size are imbedded, either singly or in groups, in the granules of chlorophyll, or leaf green; the manner in which the green coating takes place is unknown. Starch is an organic substance, varying from grains of inappreciable minuteness to such as are visible to the naked eye, and of such a variety of forms that it can be ascertained with tolerable certainty by what plant a grain of starch has been produced. The small grains are generally globular, but whatever the form may be, each consists of a series of superimposed layers of different densities, which exhibit coloured rings and a black cross in polarized light.
Starch is an early and transient product of young plants, which is destined to be changed into nutritious substances at a later period, but being insoluble in cold water it is unfit to travel with the sap. However a ferment called diastase produced during the incipient germination of the grains and seeds, in the tubers ofpotatoes, &c., being in a state of change, imparts that state to the starch, and converts it into a sweet soluble matter known as dextrine or starch-gum which is capable of being carried throughout the plant with the sap, and which is itself ultimately changed into sugar. Dextrine is an ingredient in the primordial cell. Starch, dextrine, and cellulose are isomeric: consisting of the same elements with different characters.
The woody part of trees and shrubs, the fibres of hemp, flax, of the Agave, and many other plants, are formed of cellulose, the purest form of that substance being bleached flax and linen. During the progress of vegetation, the cells of the ligneous tissue of trees, also those of woody and fibrous plants, which are transparent and colourless when young, become internally coated or filled with sclerogen, the colouring matter of wood, a substance of various hues. In extra-tropical countries it is generally some shade of brown, sometimes dark, sometimes so pale as to be almost white with a yellowish or reddish tinge; and occasionally it is beautifully marked as in the wood of the olive. In tropical countries the colours are more vivid and varied, deeper and even black, as in ebony. This colouring matter has the same quantity of oxygen as cellulose, but it contains hydrogen and more carbon, hence wood is combustible in proportion to the quantity of sclerogen it contains. In beech it forms half of the wood, in oak two thirds, and in ebony nine tenths, so it is the most highly combustible of the three. The additional carbon is obtained by increased respiration, the hydrogen by decomposition of water in the sap.
Sugar is almost as universal a constituent of the higher classes of plants as cellulose and starch, for besides the saccharine juice of innumerable plants, starch, the acids of unripe plants, and even the acrid juice of the fig and other plants, is turned into sugar as the plant advances to maturity, and the fruits ripen. Manna and other saccharine exudations from the leaves or stemsof trees, as the lime tree, are probably intercepted by the dialysing septa of the cells, and exude to the exterior through the pores of the skin. The sweet juice found in the nectaries of flowers is formed in other parts of the plant, and rarely flows to the flower before it is full blown; the quantity is at its maximum during the emission of the pollen, and ceases when the fruit is formed. In diœcious plants and that singular and beautiful race the Orchideæ, it is evidently intended to attract insects for their aid in fertilization.
Vegetable oils, resins, and wax, consisting of the same simple elements as the hydrates, form a large class of inflammable organic substances in which hydrogen predominates. Olive oil is a rare instance of a fixed oil being obtained from a fruit; some laurels have that property also, but the fixed oils are chiefly found in seeds, as the walnut, hazel nut, and the almond, in which the principle of oil is in its greatest purity. It is particularly abundant in hemp seed, and in a great variety of plants the starch in the seed is changed into oil to nourish the embryo, till the seed lobes are above the ground, and the true lobes appear.
Resins, gums, and wax, being colloid substances, are dialysed and ejected from the system either through the fissures in the bark, or by pores in the leaves. The resins exude through the bark from canals that run between the cells of the plant, in solution, and are consolidated by the oxygen on coming into the air. The herbaceous zone in the bark of the fir and pine family furnish an abundant supply of resins and balsams; the camphor tribe and the Amyrids are rich in them, as frankincense, myrrh, balm of Mecca, and the Olibanum, supposed to be the frankincense of scripture.
Wax is a frequent vegetable production, especially in the torrid zone, where many of the wax-bearing plants supply the natives with light. An exudation through the pores of many plants coats their surfaces with resinor wax. Young buds are often covered with resin to protect them from cold and wet during the winter and early spring, as those of the horse-chestnut and balsam poplar. It is wax that gives the bloom to the plum, cherry, and grape, and the rain drops lie on the waxy surface of the cabbage leaf, like balls of diamond, from the total reflection of light at their point of contact. Wax protects plants from damp in a rainy climate, and prevents too strong perspiration from the fleshy leaves of the aloe, cactus, and other inhabitants of the parched and hot regions in the tropics.
The vegetable substances hitherto under consideration are neutral, but the remarkable compounds albumen, fibrin, and casein, already mentioned as constituents of wheaten flour, not only contain carbon and hydrogen with a little oxygen, but azote and small quantities of sulphur and phosphorus. Each of these three organic compounds is the same, whether derived from animal or vegetable matter. Thus albumen is chemically the same, whether obtained from wheat and other grains, from arrowroot, dahlia roots, the serum of blood, or the white of an egg. As it constitutes the film or thin coating of the primordial cell, and combines with dextrine in its internal viscid lining, it not only forms an ingredient in all vegetable organisms, but plays an important part in the growth of the whole vegetable world. Fibrin is chemically the same in the juice of plants and in blood, in which it exists as a liquid during the life of the animal, and as a fibre after death. It forms the basis of the muscular system in animals, and that extracted from the juice of plants coagulates spontaneously like blood. Casein is chemically identical, whether derived from the curd of milk, or from peas and beans. Azote is a very important principle in these substances as well as in the gelatinous substance gluten. It forms an essential part of the animal structure, and is either highly nutritious or deleterious in the vegetable, beingat once one of the most valuable, contradictory and powerful agents in nature.
Chemists have formed by synthesis compounds identical with all the fixed and essential oils, for confectioners can now give the flavour of the pear, orange, quince, pine apple and other fruits by means of artificial chemical compounds. All the saccharine substances have not yet been artificially obtained, nor the albuminous substances, albumen, fibrin, and casein.
It cannot be a matter of surprise, when chemists form organic substances out of inorganic elements, that they should succeed in transforming compounds produced by living plants into new compounds, as that of changing the vegetable acids into alcohols, which is now done. But some of the acids themselves are synthetically formed out of inorganic elements; as for example the oxalic, the most common of all the vegetable acids, which is found most abundantly in the Oxalis or wood sorrel, and is a frequent constituent of the highest and lowest plants. The formic acid, which is the acrid stinging principle in ants, is also synthetically formed; it is found in the juice of the stinging nettle and in decaying pine leaves, and contains hydrogen like all the other vegetable acids. These acids result from an augmentation of oxygen during nocturnal respiration, which penetrates deeply into the vegetable structure.
Octahedral, prismatic, and stellar microscopic crystals formed by the chemical combination of the natural acids with bases imbibed by the roots, are deposited in the cells under the skin, and in all parts of plants. However, they appear most frequently as bundles of needle-shaped crystals of carbonate of lime, lying side by side in the hollow of a cell. They are known as raphides, fromraphisa needle, and may be easily seen under the skin of the medicinal squill. Large single crystals of oxalate of lime, octahedral or prismatic, are found in the cells under the skin of the onion and other plants; and stellarcrystals of the same substance abound so much in the common rhubarb that the best specimens of the dry medicinal root contain as much as thirty-five per cent. of them; while certain aged plants of the cactus tribe have their tissues so loaded with them as to become quite brittle. The calcareous base in some instances is combined with tartaric, citric, or malic acid. The crystals of some raphides are1⁄40th of an inch long, others are not more than the hundredth; they are brought into view by polarized light.[80]Spherical raphides between the1⁄2000th and1⁄4000th of an inch in diameter have been discovered scattered profusely through the tissues of the leaves, and those parts of plants which are modifications of the leaves; they may be seen under the skin of Pelargoniums and other plants, and it is supposed that few if any orders of plants are without them.[81]
Although azote forms 788 thousandth parts of the atmosphere, none, or at least no appreciable quantity of it, is absorbed by the vegetable world; that great principle of nourishment is entirely supplied by ammonia and nitric acid, imbibed by the roots, and decomposed by the chemico-vital power. Here it shows its capricious character by combining with other simple elements in the bark, to produce the most precious medicines in some plants, and in others the most deadly poisons, while no vegetable substance is perfectly nutritious without it.
The milk sap, when exposed to the air, coagulates into a tenacious viscid solid. The white juice is generally acrid, or narcotic, or both, and for the most part extremely poisonous, though exhibiting strong contrasts even in nearly allied species. In the order Euphorbiaceæ or Spurgeworts, comprising nearly 1,500 species, a large proportion are hurtful; but there is a gradation from mere stimulants to the most formidable poisons.This order furnishes the Ethiope and the native Brazilian with poison for their arrows. It contains the Manchineel, and Excœcaria Agallocha, the most poisonous of plants; even the smoke from the burning branches of the Excœcaria affects the eyes with insufferable pain. The white juice of the Fig, one of the Morad order, is violently poisonous; in many, as in the common fig, it is acrid and irritating. The Antiaris toxicaria, the celebrated Upas-tree of Java, which is of the Artocarpeæ or Bread-fruit order, owes its virulence to its milky juice, which contains strychnia, the most fatal of drugs. Dangerous and acrid as these orders are, the Bread-fruit, abounding in starch, supplies the inhabitants of the East Indian islands with excellent food; the milky juice of the Cow-trees, chiefly of the Bread-fruit and Fig orders, furnishes a wholesome beverage to the South Americans; and the Manihot or Cassava, a poisonous spurgewort when raw, yields when roasted nutritious food to whole nations, the heat driving off the dangerous principle. Caoutchouc, a most harmless substance, is the solid produce of many of the most acrid and virulent juices of plants belonging to the preceding orders; the poison is probably left in the liquid. The chemico-vital power is strikingly illustrated by the number of safe and excellent fruits produced by trees full of the most deleterious juices, whether milky or not. Some of the finest fruits in the Indian Archipelago are products of eminently dangerous species of the Sapindaceæ or Soapworts. The acrid juices of the leaves and branches, are so much diluted with water in the fruits, that they become innocuous, or they may be changed into sugar, as in the common fig. Nothing can surpass the virulence of the juice of the Upas-tree, yet its nuts are eaten with impunity, and the pulpy contents of the fruit of the Strychnos nux vomica is food for birds. The leaves and berries of the potato are so strongly narcotic, that an extract from them isintermediate in power between that from deadly nightshade and hemlock, yet the potato itself, like the cassava, is rendered wholesome by being boiled or roasted.
The alkaloids are alkaline substances formed in the bark and milky juices of plants, always combined with an acid during the life of the plant. The chemical structure of this class of substances is very much alike, and chemists have succeeded in forming many of them synthetically; they all contain azote, and have a great affinity for acids. The bark of the different species of Cinchoneæ, especially the Cinchona cordifolia and C. Condaminea, yield three alkaloids—namely, cinchonine, quinine, and cusconine—they are all formed of carbon, hydrogen, and azote in the same proportions; but the first has one atom of oxygen in addition, the second has two atoms in addition, and the third has three; so that in these alkaloids the carbon, hydrogen, and azote combine to form an organic radical, which is oxidized in three different degrees. Six of the alkaloids have been obtained from opium, which is the solid portion of the milk juice of the poppy; of these, morphine seems to be the narcotic principle; and the orange-coloured milk sap of the Chelidonium, a very poisonous and acrid plant of the poppy order, has furnished chelidonine. The Colchicum order, containing the meadow saffron or autumnal crocus, and Veratrum album or white hellebore, as well as many other plants, yield alkaloids, all of which are medicinal or poisonous, according to the dose.
There is scarcely a people, however savage, that has not discovered some exciting narcotic. Opium is almost universally smoked or eaten among Eastern nations; and bhang, a strong narcotic, obtained from the leaves of Indian hemp, is in equally universal use among the Brazilian savages and Hottentots, but especially among the Malays, who are excited to madness when they smoke it too freely. The same intoxicating effect is produced by a strong liquor prepared from the Datura sanguinea,a species of stramonium; and its congener tobacco, now all but a necessary of life among civilized mankind, was smoked by the natives of the American continent, before the arrival of the Europeans, as a relief from hunger.
Coffee has been long in use on account of its stimulating principle caffeine, which is now discovered to be the same with theine, the latter, however, being less exciting, unless the tea plant grows in a very hot climate. In countries where nature furnishes few narcotic principles, wine, beer, and spirits supply their place, especially in the far north, where animal heat is rapidly carried off by the cold, and carbon must be furnished to satisfy the all-devouring oxygen which we draw in at every breath.
Caffeine, the highly azotized principle of coffee, obtained from tea leaves and coffee beans, is one of the substances known as neutral crystallisable principles. Similar substances are found in asparagus, pepper, almonds, the bark on the roots of the apple, pear, plum, and cherry trees, as well as in the bark of the willow. The two last are especially analogous, and contain no azote, as the others do.
The colouring matter of flowers is a fluid contained in cells, situated immediately under the skin, which itself is perfectly transparent and colourless. The whiteness of the white Camellia, rose, lily, and other flowers, is supposed to be owing to the total reflection of light from the cells immediately below the skin, which are either full of air, or of a colourless liquid. The predominating colours are yellow, red, and blue, with the various intermediate tints. Sometimes these colours are converted one into another in the petal after fertilization, at which period the colours are brightest. The chemical nature of these liquids, the cause of their variety, and their definite arrangement in one and the same petal, do not seem as yet to be ascertained.
The parts of plants that are not green inhale oxygenfrom the atmosphere, and exhale carbonic acid gas exactly like animals. During the chemical combinations of the oxygen with the carbon derived from the nutriment to form the carbonic acid gas, heat is necessarily evolved, especially in the flower, the point of maximum heat varying with its expansion. The blossoms of the Aroideæ, or Arums, are remarkable for the evolution of heat. According to Saussure, a blossom of the common Arum maculatum consumes five times its volume of oxygen in twenty-four hours previous to its evolution of fruit, so it is not wonderful that the chemical combination of such a quantity of oxygen should produce a strong development of specific temperature. By M. Dutrochet’s observations, the heat evolved by the Arum maculatum has a maximum in the day and a minimum in the night, and he found that it exceeded the heat of the surrounding air by between 25° and 27°. The heat of the Colocasia odorata, another Arad, was determined by several observers to be even 50° above the warmth of the air. The heat evolved by germinating seeds when in a heap is not from fermentation; it is owing to their consumption of oxygen and expiration of carbonic acid gas. The temperature of all vegetating parts of plants, the roots, leaves, young juicy shoots, &c., is far superior to that of their flowers. It arises from the nutritive process, and has a maximum at noon, and a minimum at midnight, like that of the flower. The growth of plants is most vigorous at noon; consequently there is then a greater evolution of heat.
Water in small quantities is secreted night and morning from the points of the leaves of many plants, probably to relieve them from a superabundance of liquid, which evaporation is insufficient to carry off. The arums are remarkable for the quantity they eject. It falls in drops from the points of the leaves. About half a pint is given out every night by the enormous leaves of the Caladium distillatorium, a species of Arad. Inthat plant, and in the Colocasia, the water flows in canals along each rib into a general duct, which runs along the border of the leaf, and terminates in an orifice upon the surface.
Since electricity is developed by chemical action in unorganized matter, it may be inferred that it is also developed within the vegetable cell where so many organic compounds are formed; but it is probably given off from the points of the leaves or by evaporation from their surfaces. Professor Fleming ascertained by actual experiment, that the sap of a leaf, and its surface, are in different electric states; he also found that the surface of the spongioles of the roots of plants and the ascending sap have opposite electricities. Both of the preceding cases the Professor ascribes, in part at least, to organic changes which take place during vegetation. Slight currents of electricity were obtained from the petioles of flowers, but fruits and tubers give powerful electrical currents due to the reaction of different vegetable juices upon one another. The tuberose is said to emit scintillations and dart small sparks of light in a hot electric evening, and gardeners have long been aware that mushroom spawn is most prolific in stormy weather.
The irritability of the tissues of plants which renders them liable to be acted upon by external causes, has occupied the attention of many celebrated botanists. From experiments by Professor Ferdinand Cohn and his pupil M. Krabsch upon the irritability of the stamens in the florets on the discs of composite flowers, more especially the Centaureas, they have come to the conclusion that susceptibility to the excitement of light, as well as to that of mechanical and probably electrical impulse, is possessed by all young vigorous tissues, and upon comparing the phenomena of these with those of animal irritability, they further conclude that the faculty of responding to external irritation by internal movements and change of form, belongs to cells, and holds good inthe vegetable as in the animal kingdom. To be irritable, to change its normal form as a vessel of excitation, and to revert to the normal form after a while by its internal elasticity, are characteristics of the living cell. In plants these properties are met with only when the vital processes are in full activity, and therefore are particularly noticed during the period of flowering, when the processes are at the maximum. And it may be remarked that the stamens, in which irritability is most frequently noticed, are the only organs in which an elevation of temperature measurable by the thermometer occurs, although a certain degree of heat is generated in all plant cells by the chemical process going on within them. It is to be supposed that irritable properties belong to all parts of plants, but that they exist in an intensified degree, and for a certain epoch, in those parts where their results arrest attention, as in the stamens of the Centaurea, berberry, cactus, Cistus, nettle, &c., and in the anthers of the Stylideæ, the leaves of Dionæa muscipula, and many others, all of which are more or less affected by the external action of mechanical force and electricity; for it is scarcely possible that plants should not be under the influence of atmospheric electricity, since every shower of rain forms a perfect conductor between the clouds and the earth. The motion does not always immediately follow the excitement; plants often require to be rudely shaken before the movement begins. M. Hofmeister has observed that all young shoots and leaves become curved by mechanical shaking.
Light is the most universal and important exciting cause in the vegetable world. The mouths of the stomata are opened by the influence of light. The leaves, young shoots, and tendrils turn to the light; it regulates the sleep of plants, as well as the diurnal motions of the daisy and sunflower. The opening of blossoms and of folded leaves which had been closed in sleep during the night, shows the susceptibility of their tissuesto the influence of light, an influence beautifully exhibited by the orange-coloured Eschscholtzia, which shuts its golden blossoms under every passing cloud.
All M. Cohn’s experiments prove that in the Mimosa pudica, which is highly sensible to the action of light, heat, electricity, and touch, ‘the propagation of the external excitement, proceeds in the same mode as in animals, and there is little doubt that the vascular tissue (which contain spiral vessels) constitute the special bundles adapted for the purpose, and that the phenomena of contractibility depend upon a muscular tissue.’[82]
From Professor Franklin’s experiments it appears that ‘the motions resulting from external causes are owing to vital contractibility, and that they are governed by the same laws which regulate similar action in the animal kingdom. Their energies vary with the vigour of the plant; they are exhausted by over exercise, and require rest; and like animals they are lulled and put to sleep by chloroform and narcotics.’