PHENOMENA OF VEGETABLE LIFE.

Psychology of Flowers—Progress of Matter towards Organization—Vital Force—Spontaneous Generation—The Vegetable Cell—Simplest Development of Organization—The Crystal and the Cell—Primitive Germ—Progress of Vegetation—Influence of Light—Morphology—Germination—Production of Woody Fibre—Leaves—Chlorophylle—Decomposition of Carbonic Acid—Influence of Light, Heat, and Actinism on the Phenomena of Vegetable Life—Flowers and Fruits—Etiolation—Changes in the Sun’s Rays with the Seasons—Distribution of Plants—Electrical and Combined Physical Powers

The variety of beautiful forms which cover the surface of this sphere, serve, beyond the physical purposes to which we have already alluded, to influence the mind, and give character to the inhabitants of every locality. There are men who appear to be dead to the mild influences of flowers; but these sweet blossoms—the stars of our earth—exert a power as diffusive as their pervading odours.

The poet tells us of a man to whom

But it was something more. He, perhaps, attended not to the eloquent teaching of its pure, pale leaves: he might not have been conscious of the mysterious singing of that lowly flower: he might, perchance, have crushed it beneath his rude foot rather than quaff the draught of wisdom which it secreted in its cell; but the flower still ministered to that mere sensualist, and in its strange, tongueless manner, reproved his passions, and kept him “a wiser and a better man,“ than if it had pleased God to have left the world without the lovely primrose.

The psychology of flowers has found many students—than whom not one read them more deeply than that mild spirit who sang of the Sensitive Plant, and in wondrous music foreshadowed his own melancholy fate.[248] That martyr to sensibility, Keats, who longed to feel the flowers growing above him, drew the strong inspiration of his volant muse from those delicate creations which exhibit the passage of inorganic matter into life; and other poets will have their sensibilities awakened by the Æsthetics of flowers, and find a mirror of truth in the crystal dew-drop which clings so lovingly to the purple violet, and draws fresh beauties from its coloured petals.

If we examine carefully all the conditions of matter which we have made the subject of our studies, we cannot but perceive how gradual is the progress of the involved action of the physical forces, as we advance from the molecule—the mere particle of matter—up to the organic combination. At first we detect only the action of cohesion in forming the rude mass; then we have the influence of the crystallogenic powers giving a remarkable regularity to bodies; we next discover the influences of heat and electrical force in determining condition, and of chemical action as controlled by them. Yet, still we have a body without organization. Light exerts its mysterious powers, and the same elements assume an organized form; and, in addition to the recognized agencies, we dimly perceive others on which vitality evidently depends. These empyreal influences become more and more complicated to us: ascending in the scale, they rise beyond our science; and, at length, we find them guiding the power of intelligence, while instinct and reason are exhibited in immediate dependence upon them.

Let it not be imagined that this view has any tendency to materialism. The vital energy is regarded as a spiritualization, and reason as a divine emanation; but they are connected with materialities, on which they act, and by which they are themselves controlled. The organic combinations, and the physical powers by which these unions of matter are effected and retained, have a direct action over that ethereality which is life, and the powers of life again control these more material forces. The spirit, in whatever state, when connected with matter, is, like Prometheus chained to his rock, in a constant struggle to escape from its shackles, and assert the full power of its divine strength.

We have seen variety enough in the substances which make up the inorganic part of creation; but infinitely more varied are the forms of organization. In the vegetable world which is immediately around us, from the green slime of our marshes to the lustrous flowers of our gardens and the lordly trees of our forests, what an extraordinary diversity of form is apparent! From the infusoria of an hour, to the gigantic elephant roaming in his greatness in the forests of Siam—the noble lion of the caves of Senegal—the mighty condor of the Andes—and onward to man, the monarch of them all, how vast are the differences, and yet how complete are they in their respective conditions! In the creation we have examined, we have had conclusive evidence, that from the combination of atoms every peculiar form has been produced. In the creation we are about to examine, we shall discover that all the immense diversity of form, of colour, and condition which is spread over the world in the vegetable and animal kingdoms, results from the combination of cells. The atom of inorganic nature becomes a cell in organic creation. This cell must be regarded as the compound radical of the chemist, and by decomposing it, we destroy the essential element of organization.

With the mysterious process by which the atom is converted into a cell, or a compound radical, we are unacquainted; but we must regard the cell as the organic atom. It is in vain that the chemist or the physiologist attempts to examine this change of the inorganic elements to an organized state; it is one of the mysteries of creation, which is to be, in all probability, hid from our eyes, until this “mortal coil” is shaken off, and we enjoy the full powers of that intelligence which we are promised we shall enjoy in an immortal state.

Again and again has the attention of men been attracted to the generatio Æquivoca; they have sometimes thought they have discovered a generatio primitiva or spontanea; but a more careful examination of these organisms has shown that an embryo existed—a real germination has taken place.

Count Rumford[249] stated that threads of silk and wool had the power of decomposing carbonic acid in water in the sunshine; and hence some have referred organization to a mere chemical change produced by luminous excitation; and we have heard of animal life resulting from pounded siliceous matter. All such statements must be regarded as evidences of imperfect investigation.

Dr. Carus, alluding to the experiments of Gruithuisen, Priestley, and Ingenhousz,[250] says:—“These show, more than any other experiments, that, in the purest water, under the influence of air, light, and heat, beings are formed, which, oscillating as it were between the animal and the plant, exhibit the primitive germs of both kingdoms.”[251] Treviranus[252] repeated, and appeared to confirm these results; but in these experiments we have no evidence that the germ did not previously exist in the spring-water which was employed.

Some have regarded the cell as a crystal; they see the crystal forming, by the accumulation of atoms, into a fixed form, under the influence of an “inner life;” and, advancing but a step, they regard the cell as the result of an increased exercise of the physical influences.[253] We have referred crystalline form to certain magnetic conditions; and it is evident that the atomic cell is influenced by similar forces; but if we place a crystal in its natural fluid, though it increases in size, it never alters in form: whereas, if we examine a cell in its natural position, it gives indications of motion, it produces other cells, and we have a development of organs which are in no respect the same in form as the original. From a vesicle floating invisible to the unaided human sense in its womb of fluid, is produced a plant possessing strange powers, or an animal gifted with volition. The idea, that two kinds of polarity—light on one side, and gravitation on the other—produce the two peculiar developments of roots and branches, can only be regarded as one of those fanciful analogies which prove more imagination than philosophy.[254]

The conditions are, however, most curious; they deserve very attentive study; but in examining the phenomena, the safest course is to allow the effects as they arise to interpret to us, and not admit the love of hypothesis to lead us into bewildering analogies; or uncertain phenomena to betray us to hasty inferences. It is of this evil that Bacon speaks, in his “Advancement of Learning.” He says:—

“The root of this error, as of all others, is this, that men, in their contemplations of nature, are accustomed to make too timely a departure, and too remote a recess from experience and particulars, and have yielded and resigned themselves over to the fumes of their own fancies and popular argumentations.”

Without venturing, therefore, to speculate on the origin of the primitive cell, or unit of organic life, which involves the problem of the metamorphosis of a rude mass—the primitive transformation of the rudimentary atoms into organic form,—we must admit that the highly organized plant or animal is but an aggregation of cells; their arrangement being dependent upon certain properties peculiar to them, and the exercise of forces such as we have been studying,—all of which appear to act externally to the plant or animal itself.

Experiments have been brought forward, in which it appeared that, after all organization which could by any possibility have existed, had been destroyed by the action of fire, solutions of flint and metallic salts, have, under the influence of electric currents, exhibited signs of organic formations, and that, indeed, insects—a species of acari—have been developed in them. The experiments were said to have been made with care, and many precautions taken to shut out all chances of any error, but not all the precautions required in a matter of such exceeding delicacy; and we are bound not to receive the evidence afforded as the true expression of a fact without much further investigation. All experience,—setting aside the experiment named,—is against the supposition that pounded or dissolved flint could by any artificial means be awakened into life. Ova may have been conveyed into the vessels which contained the solutions under experiment; and in due time, although possibly quickened by electric excitation, the animals—the most common of insects—came into existence.[255]

The rapid growth of confervÆ upon water has often been brought forward as evidence of a spontaneous generation, or the conversion of inorganic elements into organic forms; but it has been most satisfactorily proved that the germ must be present, otherwise no evidence of anything like organization will be developed. All the conditions required for the production of vegetable life appear to show, that it is quite impossible for any kind of plant, even the very lowest in the scale, to be formed in any other way than from an embryo in which are contained the elements necessary for it, and the arrangements required for the various processes which are connected with its vitality.

The earth is now covered with vegetable life, but there must have existed a time when “darkness was upon the face of the deep,” and organization had not yet commenced tracing its lovely net-work of cells upon the bare surface of the ocean-buried rock. At length the mystery of organic creation began: into this science dares not penetrate, but it is privileged to begin its search a little beyond this point, and we are enabled to trace the progress of organic development through a chain of interesting results which are constantly recurring.

If we take some water, rising from a subterranean spring, and expose it to sunshine, we shall see, after a few days, a curious formation of bubbles, and the gradual accumulation of green matter. At first we cannot detect any marks of organization—it appears a slimy cloud of an irregular and undetermined form. It slowly aggregates, and forms a sort of mat over the surface, which at the same time assumes a darker green colour. Careful examination will now show the original corpuscles involved in a net-work formed by slender threads, which are tubes of circulation, and may be traced from small points which we must regard as the compound atom, the vegetable unit. We must not forget, here, that we have to deal with four chemical elements,—oxygen, hydrogen, carbon, and nitrogen, which compose the world of organized forms, and that the water affords us the two first as its constituents, gives us carbon in the form of carbonic acid dissolved in it, and that nitrogen is in the air surrounding it, and frequently mixed with it also.

Under the influence of sunshine, we have now seen these elements uniting into a mysterious bond, and the result is the formation of a cellular tissue, which possesses many of the functions of the noblest specimens of vegetable growth. But let us examine the progress. The bare surface of a rock rises above the waters covered over with this green slime, a mere veil of delicate net-work, which, drying off, leaves no perceptible trace behind it; but the basis of a mighty growth is there, and under solar influence, in the process of time, other changes occur.

After a period, if we examine the rock, we shall find upon its face little coloured cups or lines with small hard discs. These, at first sight, would not be taken for plants, but on close examination they will be found to be lichens. These minute vegetables shed their seed and die, and from their own remains a more numerous crop springs into life. After a few of these changes, a sufficient depth of soil is formed, upon which mosses begin to develope themselves, and give to the stone a second time a faint tint of green, a mere film still, but indicating the presence of a beautiful class of plants, which, under the microscope, exhibit in their leaves and flowers many points of singular elegance. These mosses, like the lichens, decaying, increase the film of soil, and others of a larger growth supply their places, and run themselves the same round of growth and decay. By and by, funguses of various kinds mingle their little globes and umbrella-like forms. Season after season plants, perish and add to the soil, which is at the same time increased in depth by the disintegration of the rock over which it is laid, the cohesion of particles being broken up by the operations of vegetable life. The minute seeds of the ferns floating on the breeze, now find a sufficient depth of earth for germination, and their beautiful fronds, eventually, wave in loveliness to the passing winds.

Vegetable forms of a higher and a higher order gradually succeed each other, each series perishing in due season, and giving to the soil additional elements for the growth of plants of their own species or those of others. Flowering herbs find a genial home on the once bare rock; and the primrose pale, the purple foxglove, or the gaudy poppy, open their flowers to the joy of light. The shrub, with its hardy roots interlaced through the soil, and binding the very stones, grows rich in its bright greenery. Eventually the tree springs from the soil, and where once the tempest beat on the bare cold rock, is now the lordly and branching monarch of the forest, with its thousand leaves, affording shelter from the storm for bird and beast.

Such are the conditions which prevail throughout nature in the progress of vegetable growth; the green matter gathering on a pond, the mildew accumulating on a shaded wall, being the commencement of a process which is to end in the development of the giant trees of the forest, and the beautifully tinted flower of nature’s most chosen spot.

We must now consider closely the phenomena connected with the growth of an individual plant, which will illustrate the operation of physical influences throughout the vegetable world. The process by which the embryo, secured in the seed, is developed, is our first inquiry.

A seed is a highly carbonized body, consisting of integuments and embryo: between these, in most seeds, lies a substance called the albumen, or perisperm. The embryo contains the elements of the future plant—the cotyledons, the plumule, and the radicle; the former developing into stalk and leaves, the latter into roots. This embryo hides the living principle, for the development of which it is necessary that the starch and gluten undergo a chemical change, and that an elevation of temperature is produced. The vital power is dormant—it sleeps—in the seed until the proper conditions are produced. It has been proved, that the powers of maintaining life in the seed are very great; excessive cold, sufficiently intense to freeze mercury, will not kill seed, and they resist a comparatively high temperature. It is probable that heat only destroys seeds by drying them too completely. The temperature at which seeds germinate is exceedingly varied,—those belonging to our own clime will germinate when the thermometer rises above 40° F., but the seeds of tropical plants demand that a temperature of from 70° to 84°, or even to 90°, be steadily applied to them. In some cases it has been found that even boiling the seeds has been advantageous to the future process of germination in the soil. But let us take the seed of some ordinary plant, and trace its progress.

An apparently dead grain is placed in the soil. If the temperature is a few degrees above the freezing point, and the soil holds a due quantity of water, the integument of the seed imbibes moisture and swells; the tissue is softened, and the first effort of vital force begins. The seed has now the power of decomposing water, the oxygen combines with some of the carbon of the seed, and is expelled as carbonic acid. Saussure’s experiments prove this. The air above the soil in which a horse-bean was placed to germinate, gave, before the experiment, nitrogen 210·26, and oxygen 56·29, and after germination, nitrogen 209·41, oxygen 44·38, and carbonic acid 11·27. This part of the process is but little removed from the merely chemical changes which we have already considered. We find the starch of the seed changed into gum and sugar, which affords nutritive food for the developing embryo. The seed now lengthens downwards by the radicle, and upwards by the cotyledons, which, as they rise above the earth, acquire a green colour. Here the first stage of vegetable life ends, the chemically exciting process is at an end, and a new stimulus is required to continue in full activity the vital powers. Carbonic acid is no longer given off.

The cotyledons, which are two opposite roundish leaves, act as the lungs; by them carbonic acid taken from the atmosphere is absorbed and carried by a circulating process, now in full activity, through the young plant. The carbonic acid, a compound of carbon and oxygen, is decomposed; it is deprived of its carbon, which is retained by the plant, and oxygen is exhaled. The plant at this period is little more than an arrangement of cellular tissue, a very slight development of vascular and fibrous tissue appearing as a cylinder lying in the centre of the sheath. At this point, however, we begin more distinctly to trace the operations of the new power; the impulses of life are strikingly evident.

The young root is now lengthening, and absorbing from the moisture in the soil, which always contains some soluble salts, a portion of its nutriment, which is impelled upwards by a force—probably capillary attraction and endosmose action combined—to the point from which the plumule springs. Capillary force raises the fluids through the tubes in the stalk, and conveys them to the veins in the leaves, while the endosmose force diffuses them through the vegetable tissues. The plumule first ascends as a little twig, and, at the same time, by exerting a more energetic action on the carbonic acid than the cotyledons have done, the carbon retained by them being only so much as is necessary to form chlorophylle, or the green colouring matter of leaves, some wood is deposited in the centre of the radicle. From this time the process of lignification goes on through all the fabric,—the increase, and indeed the life, of the plant depending upon the development of a true leaf from the plumule.

It must not be imagined that the process consists, in the first place, of a mere oxidation of the carbon in the seed,—a slow combustion by which the spark of life is to be kindled;—the hydrogen of the water plays an important part, and, combining also with the carbon, forms necessary compounds, and by a secondary process gives rise again to water by combination with oxygen in the cells of the germinating grain. Nor must we regard the second class of phenomena as mere mechanical processes for decomposing carbonic acid, but the result of the combined influences of all the physical powers and life superadded.

This elongating little twig, the plumule, at length unfolds itself, and the branch is metamorphosed into a leaf. The leaf aËrates the sap it receives, effects the decomposition of the carbonic acid, the water, and in all probability the ammonia which it derives from the air, and thus returns to the pores, which communicate with the pneumatic arrangements of the plant, the necessary secretions for the formation of bark, wood, and the various proximate principles which it contains.

After the first formation of a leaf, others successively appear, all constructed alike, and performing similar functions. The leaf is the principal organ to the tree; and, indeed, LinnÆus divined, and Goethe demonstrated, the beautiful fact, that the tree was developed from this curiously-formed organ.

“Keeping in view,” says the poet-philosopher, “the observations that have been made, there will be no difficulty in discovering the leaf in the seed-vessel, notwithstanding the variable structure of that part and its peculiar combinations. Thus the pod is a leaf which is folded up and grown together at its edges, and the capsules consist of several leaves grown together, and the compound fruit is composed of several leaves united round a common centre, their sides being opened so as to form a communication between them, and their edges adhering together. This is obvious from capsules which, when ripe, split asunder, at which time each portion is a separate pod. It is also shown by different species of one genus, in which modifications exist of the principle on which their fruit is formed; for instance, the capsule of nigella orientalis consists of pods assembled round a centre, and partially united; in nigella damascena their union is complete.”[256]

Professor Lindley thus explains the same view:—“Every flower, with its peduncle and bracteolÆ, being the development of a flower-bud, and flower-buds being altogether analogous to leaf-buds, it follows as a corollary that every flower, with its peduncle and bracteolÆ, is a metamorphosed branch.

“And, further, the flowers being abortive branches, whatever the laws are of the arrangement of branches with respect to each other, the same will be the laws of the flowers with respect to each other.

“In consequence of a flower and its peduncle being a branch in a particular state, the rudimentary or metamorphosed leaves which constitute bracteÆ, floral envelopes, and sexes, are subject to exactly the same laws of arrangement as regularly-formed leaves.”[257]

The idea that the leaf is the principal organ of the plant, and that from it all the other organs are probably developed, is worthy the genius of the great German poet.

Every leaf, a mystery in itself, is an individual gifted with peculiar powers; they congregate in families, and each one ministers to the formation of the branch on

which it hangs, and to the main trunk of the tree of which it is a member. The tree represents a world, every part exhibiting a mutual dependence.

is influenced by, and influences, the lowest root which pierces the humid soil. Like whispering voices, the trembling leaves sing rejoicingly in the breeze and summer sunshine, and they tremble alike with agony when the equinoctial gale rends them from the parent stalk. The influences which pervade the whole, making up the sum of vital force, are disturbed by every movement throughout the system; a wound on a leaf is known to disturb the whole, and an injury inflicted on the trunk interferes with the processes which are the functions of every individual leaf.[258]

The consideration of the physical circumstances necessary to germination and vegetable growth, brings us acquainted with many remarkable facts. At a temperature below the freezing point, seeds will not germinate; at the boiling point of water, a chemical change is produced in the grain, and its power of germinating is destroyed. Heat, therefore, is necessary to the development of the embryo, but its power must only be exerted within certain prescribed limits: these limits are only constant for the same class of seeds, they vary with almost every plant. This is apparent to every one, in the different periods required for germination by the seeds of dissimilar vegetables.

The seed is placed in the soil; shade is always—absolute darkness sometimes—necessary for the success of the germinating process. We have seen that the first operation of nature is purely a chemical one, but this manifestation of affinity is due to an exertion of force, which is directly dependent upon solar power. The seed is buried in the soil, when the genial showers of spring, and the increasing temperature of the earth, furnish the required conditions for this chemistry of life, and the plant eventually springs into sunshine. Thus we obtain evidence that even through some depth of soil the solar power, whatever it may be, is efficient, and that under its excitement the first spring of life, in the germ, is effected.

The cotyledons and the plumule being formed, the plant undergoes a remarkable change. The seed, like an animal, absorbed oxygen and exhaled carbonic acid; the first leaves secrete carbon from carbonic acid inspired, and send forth, as useless to the plant, an excess of oxygen gas.

This power of decomposing carbonic acid is a vital function which belongs to the leaves and bark. It has been stated, on the authority of Liebig, that during the night the plant acts only as a mere bundle of fibres,—that it allows of the circulation of carbonic acid and its evaporation, unchanged. In his eagerness to support his chemical hypothesis of respiration, the able chemist neglected to inquire if this was absolutely correct. The healthy plant never ceases to decompose carbonic acid during one moment of its existence; but during the night, when the excitement of light is removed, and the plant reposes, its vital powers are at their minimum of action, and a much less quantity is decomposed than when a stimulating sun, by the action of its rays, is compelling the exertion of every vital function.

During this process, we have another example of natural organic chemistry. The four inorganic elements of which the vegetable kingdom is composed—oxygen, hydrogen, nitrogen, and carbon—are absorbed as air or moisture by the leaves and through the roots, and the great phenomenon of vegetable life is the conversion of these to an organic condition. Sugar and gum are constantly produced, and from these, by combination with atmospheric nitrogen, a proteine compound is formed, which is an essential element in the progress of development.[259]

Plants growing in the light are beautifully green, the intensity of colouring increasing with the brilliancy of the light. Those which are grown in the dark are etiolated, their tissues are weak and succulent, their leaves of a pale yellow. It is, therefore, evident that the formation of this chlorophylle—as the green colouring matter of leaves is called—results from some action determined by the sun’s rays.

Chlorophylle is a carbonaceous compound formed in the leaves, serving, it would appear, many purposes in the process of assimilation. In the dark the plant still requires carbon for its further development, and growing slowly, it removes it from the leaves, decomposing the chlorophylle, and supports its weak existence by preying on parts of its own structure, until at length, this being exhausted, it actually perishes of starvation.

Plants always turn towards the light: the guiding power we know not, but the evidence of some impulsive or attracting force is strong; and the purpose for which they are constituted to obey it, is proved to be the dependence of vegetable existence upon luminous power.

Light is not, however, alone sufficient to perfect the plant: another agent is required to aid in the production of flowers and fruits, and this power is proved to be heat—and heat, perhaps, in some peculiar condition. Having reached that point of development when the reproductive functions are, by another change in the chemical operations going on within the vegetable structure, to be called forth, it has been found that the heat rays become in a remarkable manner effective. It has also been observed that plants bend from the red, or calorific rays, instead of towards them, as they are found to do to every other ray of the spectrum. From this we may argue that the influence of these rays is to check the vegetation, and thus to ensure the perfection of the reproductive processes.

It has already been stated that we have the means of separating, to a considerable extent, the three principles which we discover in the sunbeam, from each other, by the use of absorbent media.

By a peculiar yellow glass we cut off the chemical principle of the sunbeam, and admit the passage of the luminous rays only—Light.

By a cobalt blue glass we obstruct the light, but allow the chemical agent to pass through freely, without, indeed, any loss—Actinism.

By a glass coloured deep blood-red by oxide of gold we obstruct the chemical principle and much of the light, but such a medium is perfectly transparent to Heat.

Therefore, this gives us the means of experimenting with either of these principles, and of examining the parts which they respectively play in the work of organization.

Some seeds being placed in the soil, in every respect in their natural conditions, duly supplied with moisture, and a uniform and proper temperature maintained, we place above the soil the three media above named, and allow one portion to be exposed to all the ordinary influences of the solar rays.

The result will be, that the seeds under the blue glass will germinate long before those which are exposed to the combined influences of the sunshine: a few of the seeds will struggle into day under the red glass, but the process of germination is entirely checked under the yellow glass. Here we see that the chemical radiations have quickened the chemical changes, and accelerated the process, under the red glass, through which rays having some peculiar chemical action pass; the germinating process, though checked, is not entirely stopped. Whereas, it would appear that under the influences of light which has been deprived of chemical power, this conversion of the starch into gum and sugar, which appears to be necessary, is entirely prevented.

If the experiment is continued, it will be found that under the blue glass the plants grow rapidly, but weakly; and that instead of producing leaves and wood they consist chiefly of stalk, upon which will be seen here and there some abortive attempts to form leaves. When the process of germination has terminated, if the young plant is brought under the yellow glass, it grows most healthfully, and forms an abundance of wood, the leaves having an unusually dark green colour, from the formation of a large quantity of chlorophylle. Plants do not, however, produce flowers with readiness under this medium; but if, at the proper period, they are brought under the red glass, the flowering and fruiting processes are most effectively completed.[260]

These experiments, simple as they are, prove to us the importance of light: the luminous principle of the sunbeam is exciting the vital powers of the plant to decompose carbonic acid and form wood; and the calorific agent, possibly under those modifications which have already been noticed as belonging to the parathermic rays, is essential to the production of flower and fruit.

Observations, which have been extended over many years, prove that with the seasons these solar powers are, relatively to each other, subject to an interesting change. In the spring, the actinic or chemical power prevails, and during this period its agency is required for the vitalization of the germ. As the summer comes on, the actinic rays diminish, and those of light increase. Perhaps it would be more strictly correct to say that the luminous intensity being increased, the chemical power was retarded; the former expression implies a variation in quantity, which may not be correct. We see the necessity for this, since luminous power is required for the secretion of the carbon, with which the woody fibre is formed, and also for the elaboration of the proximate principles of the plant. Autumn, the season of fruit, is characterized by an increase of the heat rays, and a diminution of the others: this change being necessary, as science now teaches us, for the due production of flower and fruit.

The calorific rays of the solar beam, to which the autumnal phenomena of vegetation appear particularly to belong, are of a peculiar character. They have been called, the Parathermic rays, and exhibit a curious compound nature. To these rays we may refer the ripening of fruit and grain, and the browning of the leaf before its fall. May not the rise of the sap in spring be traced to the excitement of either light or actinism, and its recession, in the autumn, to that power from which the plant is found to bend, and which appears to be their modified form of heat?

There can be no doubt that the varieties of climate and the peculiarities of countries, as it regards their animal and vegetable productions, are dependent on the same causes. The distribution of species has been referred by some to specific centres of creation around which the plants and animals have spread, without reference to physical conditions. Although centres of creation may be admitted, these centres themselves have been determined by the physical fitness of each centre to the conditions of the creation, and in like manner the migration of tribes is solely due to these physical forces we have been considering. In every zone we find that vegetable organization is peculiarly fitted for the considerations by which it is surrounded. Under the equator we have the spice-bearing trees, the nutmeg, the clove, the cinnamon, and the pepper-tree; there we have also the odoriferous sandal, the ebony, the banyan, and the teak: we have frankincense, and myrrh, and other incense-bearing plants; the coffee-tree, the tea-plant, and the tamarind.

A little further north we have the apricot, the citron, the peach, and the walnut. In Spain, Sicily, and Italy, we have the orange and lemon-tree blooming rich with perfume, and the pomegranate and the myrtle growing wild upon the rocks. Beyond the Alps the vegetation again changes; instead of the cypress, the chesnut, and the cork-tree, which prevail to the south of them, we have the hardier oak, the beech, and the elm. Still further north, we have the Scotch and spruce fir and larch. On the northern shores of the Baltic, and in that line of latitude, the hazel alone appears; and beyond this the hoary alder, the sycamore, and the mountain ash. Within the Arctic circle we find the mezereum, the water-lilies, and the globe-flowers; and, when the weakness of the solar radiations becomes too great even for these, the reindeer moss still lends an aspect of gladness to the otherwise sterile soil.

The cultivation of vegetables depends on the temperature of the clime. The vine flourishes where the mean annual temperature ranges between 50° and 73°, and it is only cultivated profitably within 30° S. and 50° N. of the equator. To the same limits is confined the cultivation of maize and of olives. Cotton is grown profitably up to latitude 46° in the Old World, but only up to 40° in the New. We have evidence derived from photographic phenomena, that the constitution of the solar rays varies with the latitude. The effects of the sun’s rays in France and England in producing chemical change are infinitely more decided than, with far greater splendour of light, they are found to be in the lands under or near the equator. Indeed, the remarks made on the variations in the character of the sunbeam with the changing seasons, may apply equally to the variations in latitude.

Fungals are among the lowest forms of vegetation, but in these we have peculiarities which appear to link them with the animal kingdom. Marcet found that mushrooms absorbed oxygen, and disengaged carbonic acid. In all probability this is only a chemical phenomenon of a precisely similar character to that which we know takes place with decaying wood. In the conversion of wood into humus, oxygen is absorbed, and combining with the carbon, it is evolved as carbonic acid. Of course we have the peculiar condition of vitality to modify the effect, and we have, too, in this class of plants, the existence of a larger quantity of nitrogen than is found in any other vegetating substance.

These few sketches of remarkable phenomena connected with vegetation are intended to show merely the operations of the physical powers of the universe, so far as we know them, upon these particular forms of organization. During the process of germination, electricity is, according to Pouillet, evolved; and again, in ripening fruits, there appears to be some evidence of electrical currents. Vegetables are, however, in the growing state, such good conductors of electricity, that it is not, according to the laws of this force, possible that they should accumulate it; so that the luminous phenomena stated to have been observed cannot be due to this agency. We know, however, that under every condition of change, whether induced by chemical or calorific action, electricity is set in motion; and we have reasons for believing that the excitation of light will also give rise to electrical circulation.

The question, whether plants possess sensation, whether they have any disposition of parts at all analogous to the nervous system of animals, has been often put forward, but as yet the answers have been unsatisfactory. The point is one well worthy all the attention of the vegetable physiologist; but regarding plants as the link between the animal and the mineral kingdom,—looking upon phyto-chemistry, as exhibited by them, as the means employed to produce those more complex organizations which exist in animals,—we necessarily consider plants as mere natural machines for effecting organic arrangements, and, as such, that they cannot possess any nervous sensibility. Muscular contraction may be represented in many of their marvellous arrangements; and any disturbance produced by natural or artificial means would consequently effect a change in the operations of those forces which combine to produce vegetable life. Indeed, the experiments of Carlo Matteucci, already referred to, prove that an incision across a leaf, the fracture of a branch, or the mere bruising of any part of the plant, interferes with the exercise of that power which, under the operation of luminous agency, decomposes carbonic acid, and effects the assimilation of the other elements.

To recapitulate. A plant is an organized creation; it is so in virtue of certain strange phyto-chemical operations, which are rendered active by the solar influences involved in the great phenomena of light, and by the excitation of caloric force mid electrical circulation. It is a striking exemplification of the united action of certain empyreal powers, which give rise to the combination, of inorganic principles under such forms that they become capable of obeying the mysterious impulses of life.

The poet has imaged the agency of external powers in various shapes of spiritualized beauty. From the goddess Flora, and her attendant nymphs, to the romantic enchantress who called up flowers by the light touch of her wand, we have, in all these creations, foreshadowings of the discovery of those powers which science has shown are essential to vegetable life. A power from without influences the plant; but the animal is dependent upon a higher agency which is potent within him.

The poet’s dream pleases the imaginative mind; and, associating in our ideas all that is graceful and loveable in the female form, with that diviner feeling which impresses the soul with the sense of some unseen spirituality, we perceive in the goddess, the enchantress, or the sylph, pure idealizations of the physical powers. The spirit floating over these forms of beauty, and adorning them with all the richness of colour—painting the rose, and giving perfume to the violet—is, in the poet’s mind, one which ascends to nearly the highest point of etherealization, and which becomes, indeed, to him a spirit of light; they ride upon the zephyrs, and they float, in all the luxury of an empyreal enjoyment, down to the earth upon a sunbeam. Such is the work of the imagination. What is the result of the search of plodding science after truth? The sunbeam has been torn into rays, and every ray tasked to tell of its ministry.

Nature has answered to some of the interrogations; and, passing over all the earth, echoed from plant to plant, we have one universal cry proclaiming that every function of vegetable life is due to the spirits of the sun.

The mighty Adansonia of Senegal, hoary with the mosses of five thousand years,—the Pohon upas in their deadly valleys,—the climbing lianas of the Guiana forests,—the contorted serpent-cactus on the burning hills,—the oaks, which spread their branches in our tempered climes,—the glorious flowers of the inter-tropical regions, and those which gem our virent plains,—the reindeer lichen of northern lands, and the confervÆ of the silent pool,—the greatest and humblest creations of the vegetable world,—all proclaim their direct dependence upon the mysterious forces which are bound together in the silver thread of Light.

These undulations, pulsations too refined for mortal ears, which quicken and guide these wonderful organisms, may be indeed regarded as sphered music for ever repeating the Divine command, “Let there be Light,” by the creation of which, a dark and dreary chaos was moulded into a star of beauty, capable of radiating brightness to other space-wandering worlds.