Imagini ale paginilor
PDF
ePub

studying the beautiful science of vegetable physiology. It is in vain for the closet philosopher to insist that no conclusion can be drawn from analogies such as these, or that no analogies exist. Our knowledge will eventually amount to this—that gases have been selected, by the wisdom of the Almighty, as the propelling power which is to circulate all elementary matter. When bodies are to be formed, or when injuries are to be repaired, the process is effected through the medium of a fluid. All equable and slow depositions of matter have been transferred to their resting place by the instrumentality of gases aided by a fluid menstruum. But when a rupture or a dismemberment, or any convulsion of nature, is to take place, gases are the sole agents; they then want no fluid medium—they are sufficient of themselves to produce all the pains incident to the human body, and all the violent phenomena which occur on earth. They are as necessary to the existence of a plant as to an animal; and it is only when there is an excess, or too small a portion of them, that organized systems perish.

Nerves seem only necessary to a system where sensation is to be conveyed, and sensation, or a consciousness of it, appears only to belong to animal life. Pliancy and contractibility are all that the organs of a plant possess, and all that vegetation requires, if we except the power of absorption; and yet that power, on reflection, is a mere contractile force. But, in which ever way we view the subject, we still find that gases are the propelling power.

The vitality of plants and of the inferior order of animals, particularly those which have the power of reproducing a limb, or an outer case or skin, continues long after sensation has ceased-for we must insist on calling that violent action which the headless body of a chicken exhibits, the effect of the vital principle. It is of precisely the same nature as it was when it pervaded the whole system, during the time when the animal was completely imbued with it, and when the will could control the movements of each organ.

It may be urged that galvanism can produce similar movements; but, in the case of the headless chicken, the agency of that fluid is not perceptible, although we grant that the time is fast approaching when the propelling power of the galvanic and electric fluids will be identified with that which animates or renders active the principle of life itself. Of this we are certain, that electricity has a powerful effect on vegetationnot only blasting and shattering the largest trees, but ruptiving the sap vessels-thus producing congestion and death. But still, even in this, the propelling power, that which transports the electric fluid, is gas.

Although it is needless to enquire into the nature of that principle called life, yet we should certainly endeavour to comprehend what it is that sustains it and gives it facilities when it is present in organized life. Both in plants and animals, life is sustained by a due attention to the changes of the atmosphere, and to the judicious application of nutriment. We should recollect, however, that although large masses of food are taken into the stomachs of animals, and are applied likewise to the roots of plants, yet, while in its crude state, the solid parts are not forced into the absorbent vessels as the proper food for organized bodies. The stomach to animals is what the stercoraceous or compost heap is to plants--a place where food is elaborated. We apply this fermented mass to the roots or spongelets of a plant; but, separated as the particles are, only such portions of it as can be combined with or elevated by gases can ever find their way through the minute pores of the spongelets.

Yet whatever be the quality of the gaseous compound, when presented to the pores for absorption, it is to water that they are indebted for the equal and healthy admixture of the nutritive particles. In like manner, the whole system of an animal is indebted to a fluid menstruum for the equable diffusion of the proper juices which are to sustain life. In this respect the analogy is perfect.

We learn, therefore, that by the agency of gases in a fluid medium, nutriment is conveyed from without to the pores of the spongelets of a plant, and from within to the ducts of an animal. This is for the purpose of enlarging the size, repairing the waste and continuing the life of both, for it is by these disunited nutritive particles, that the interstices of the germ of every organized body are enlarged. Here nature operates alike and to the same end; the only difference of the economy is, that in an animal, the labour of preparing the food for the above purpose is performed in the centre of the system, whereas, in a plant, the food is prepared for it from without, at a distance, there being no such viscus in vegetable life as a stomach, strictly so called. Yet an elaboration of the ascending particles is effected by some means within the plant, for that which is admitted in the first instance becomes very different in its nature and character after it is deposited around the annular swellings that lie at the base of each limb or twig. Let us understand how this elaboration is effected.

In a healthy plant each articulated section appropriates to itself a sufficient quantity of this nutritive matter, for the purpose of enlarging and sustaining its own individual parts, having no other connection with the rest of the plant than by a slight ligament which unites it at the base. It may be said that it is united also by the bark ; but this is error, for the bark vessels are likewise connected by this slight ligament, the loss

[ocr errors]

of which ligament is of no disadvantage or injury to either the twig, the tree, or the tubular action, for the action is confined to short limits.

On a close inspection, it will be seen that at those places of deposit, the annular swellings, there is a concentration of this vegetable chyle, or elaborated sap, and it is there held in reserve during the winter, so that these parts are not dependent on the parent stem for their support. The ascending sap in the spring excites the vessels to make use of this conserve, and to distribute it to the leaf and flower buds which exist on its own section. This rise of sap takes place as soon as the air is sufficiently heated by the sun's rays in the spring to excite the extremities of each individual terminal shoot to action.

With respect to the long agitated question of the circulation of sap, taking into consideration the physical organisation of both plants and animals—the one rigid and immovable, the other flexible and capable of voluntary motion--we think the rise of sap in one continued stream is equivalent to the circulation in animals. Nor need we wonder at this, and object to the term circulation, because it is not like that in animals, for, even with them, the process varies as much, and is as diversified, as the different orders themselves. In our apprehension,

. there is not only a circulation in plants, but one of a more peculiar and simple character than would be inferred from an inspection of their organs. It does not alter the fact of positive circulation, because it is different in plants.

The leaves of plants, delicate and transient as they are, perform the office of lungs, or, rather, are the outlets and inlets of gaseous matter, and yet no one doubts that they are the respiratory organs, although so different from those of animals. They not only supply the system with new matter, but they submit, likewise, a portion of that which ascends to their surface to the action of atmospheric gases, and other portions of it they reject.

The upper surfaces of leaves, as well as the bark of the tree, imbibe through their pores a great deal of the humidity of the atmosphere, and allow of the evaporation of ascending sap. Knowing this, and that the edges of all leaves throw off the fluid and gaseous secretions which have been rejected by the circulation, we may with safety infer that some portions of the cuticle, even of the spongelets, minute as they are, have likewise the power of discarding the residuum of elaborated sap. The spongelets—the name given to these absorbents by Dr. Dutrochet, and a very appropriate one-have the same office assigned them as the leaves. In fact, they should be considered as subterraneous leaves or lungs, and notwithstanding that pores have never yet been detected in the bark of a tree, yet if fluids can VOL. XXI.--N0. 41.

16

[ocr errors]

permeate from without, they may be rejected from within. There can be no question of the fact, that the bark absorbs moisture, for experiment proves it; the bark of the tree is, for all similar purposes, exactly like the skin of an animal, which discharges from the pores that portion of the oily and fluid secretions, the retention of which would produce fevers.

That leaves may have the full benefit of light, heat, and moisture, the upper surfaces assume, as nearly as possible, a horizontal position, or else hang obliquely to receive the slanting rays. Not, as has been conjectured, in consequence of their green colour, but because the pores of the leaf, being vertical, or nearly so, they are the better enabled to submit ascending matter to the action of atmospheric gases. The green colour, common to all leaves, and generally to the bark of all stems, is produced by some chemical change which takes place in the secretions that lie in the cellular tissues of the bark, leaf, and bud. This secretive matter is at first of a pale yellow colour, which, in fact, is the colour of the leaf when it first protrudes from the bud in the spring. It is the colour, too, of all plants when they first emerge from the ground, changing to green when the cuticle of the leaf and bark are sufficiently prepared to admit the action of heat and light. It is well known that when plants are reared in darkness the fluids are white, which is likewise the colour of fibrils and tender rootlets, the colouring matter of fluids being uniformly white at the roots. Light, therefore, excepting in a solitary case or two, is necessary to the production of the green colour; light, heat, and moisture, to the continuation of it in the leaves; and light and moisture to the continuance of it in the bark.

The sap, in the first place, acquires a yellow tinge while passing up, which arises from its coming within the power of atmospheric gases. It is a fact well known, that if the bark of a tree be lifted up it will be seen that the sap, almost colourless at first, changes immediately to a deep orange tint, the particles of the atmosphere producing this change. The secretions, therefore, when they reach the cellular tissues of each leaf and bud, must receive a blue colouring matter from without, and the union of the blue and yellow produces green.

M. de Candolle observes that the oxygen of carbonic air is exhaled by the leaves, and that carbon is deposited there, which deposition produces the green colour. What evidence have we that carbon will change the yellow colouring matter to green? In putrefactive processes the green colour is destroyed by the presence of finely pulverized charcoal.

We cannot suppose that the powers of an agent are so unlimited as to make a fixed principle, like carbon, add a blue colouring matter to one substance and abstract it from another, particularly as it forms so conspicuous a part in the building up of organic bodies. It may, and no doubt is, present in the cellular tissues, for the same purpose that it is present elsewhere--as a counteracting power to the one which causes decomposition, or acetous fermentation. It is urged that the green part of wood, when burnt, exhibits a greater proportion of carbon than either the albumen or woody fibre. This is owing to the abundance of mucilage and gluten which that part contains, and not to the mere colouring matter. The quantity of carbon extracted from the burnt green wood might, with greater propriety, be suspected of having formed the principal part of the organic structure; for if it was so plainly recognized, there was more than enough to unite with the yellow fluid, as a quantity scarcely perceptible would be sufficient for the purpose. In our opinion, the matter which forms the blue tint is yet unknown to us.

A very curious phenomenon has recently come within our own knowledge, which will throw some light on the subject. We discovered that snow which fell on tanner's spent bark, when the ground underneath was not frozen, became immediately tinged with a beautiful green colour. This green snow lay in patches on a tan walk, and on closely investigating this singular circumstance, we observed that those parts alone of the snow were coloured which lay on pieces of bark that were covered with a dingy, yellow dust. In sprinkling dust of this kind on a board, and covering it with a slice of newly fallen snow, the colouring matter of the dust rose slowly through the pores of the snow, and tinged it throughout with a beautiful, bright green colour. Both the snow and the dust were perfectly dry at the time. This phenomenon did not appear when the dust was laid on ice, or on frozen ground, and then covered with snow.

If carbon alone were the cause of the green colour, or of the change in the yellow sap, then this green tint would remain fixed, as it were, even when the vitality of the plant had ceased. The very character of carbon consists in its preserving its identity; that is, the perceptible part of carbon remains fixed, and it is only in its perceptible state, as a substance, that we can ever know it. But this is not the character of the matter which produces the green tint in leaves, for let the blue tint come from either a mineral or vegetable pigment, for both can furnish it, its evanescent nature is the same. Be it what it may, light and a slight degree of heat seem necessary to its development. And, as regards its formation in snow, the blue tint must exist in the interstices of snow; and we recollect seeing a remark in corroboration of this, " that snow, when presented to certain rays of light, had a bluish cast throughout.” A writer, whose name is not now recollected, attributes the

« ÎnapoiContinuați »