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neither can we imagine how any dilatation of the cells can arise. We consider his attempt to explain the progression of the sap by the action of these vesicles, as the weakest part of the whole work. To say the least, it is purely hypothetical, and we also think it contradictory to known physical principles. On a subject of such extreme difficulty we ought not to speak very positively, but it does appear to us that M. De Candolle has treated somewhat too slightingly the recently discovered property of Endosmose, and has shown too great an attachment to an early theory of his own, which perhaps served very well as a conjecture at the time it was first proposed, but which he ought to have given up after a cause had been discovered, which is apparently sufficient to account for the general diffusion and progression of the fluids introduced into the vegetable tissue. We shall presently revert to this subject when we come to consider the progression of the sap. In whatever way, however, the cellular tissue may be supposed to perform its functions, we believe it to be the chief instrument through which the only vital power allowed to plants produces its effects. Light and heat appear to be the two great causes by which its energies are stimulated; and to these we must probably add electricity, and the more remote excitations induced by various chemical and physical actions.
Nutrition and Reproduction have been called “ organic, or vegetative" functions, from belonging equally to the animal and to the vegetable kingdom. Notwithstanding the great differences observable in the external forms of organized bodies, the function of nutrition possesses certain general traits of resemblance in both kingdoms. In both, its operations may conveniently be separated into seven periods, and we shall attempt a brief sketch of the phenomena exhibited during each of these.
First period of nutrition.—This comprises the introduction of the food into the plant. As vegetables do not possess the power of locomotion, it is necessary that their food should be so generally diffused in nature, that they shall run no risk of perishing from their inability to search for it. Now it is a fundamental principle in vegetable physiology, that every thing capable of being imbibed into the tissue must previously be brought to a state of solution; but water is present every where in the earth, and in the atmosphere, and the material which constitutes the chief food of plants is carbonic acid, which is almost .constantly to be found dissolved in all water. The root is the only true absorbing organ of this nourishment; for although, under certain circumstances, the leaf and some other organs may be made to absorb moisture, these are not to be considered as
the organs originally destined for the introduction of food, any more than the pores in the skin of animals, which possess a like property, may be considered as their mouths. Neither is it by the whole surface of the root that this absorption takes place, but only through the “ spongioles," which are small expansions of cellular tissue situated at their extremities. It is not clearly ascertained, whether the force which regulates the absorption of the spongioles is wholly vital, or whether it is the result of a vital action, in combination with the hygroscopic property of the cellular tissue. De Candolle had formerly attributed this action of the spongioles to their hygroscopicity alone; but he is now disposed to consider it dependant, to some extent at least, on the vital force. It does not, however, appear, that because there is a cessation of this absorption in the dead plant, and a continuance of it only in the living one, that we must therefore conclude it to be the result of a vital action in the spongioles themselves; for if it were really due to their hygroscopic property only, still the immediate removal of the absorbed Auid, by the organs which cause its progression through the plant, would continually renew the conditions necessary to secure a momentary repetition of its action. This absorption also has more of the aspect of a mechanical than of a vital operation, from the circumstance of all plants being equally indifferent as to the quality of the solutions which they imbibe, the quantity being regulated chiefly by the state of liquidity in which they occur: a more liquid solution of some substances, deleterious to the health of the individual, being more readily imbibed by it than others which are more viscous, though composed entirely of materials which are highly nutritious.
From the great uniformity in the means employed for absorbing them, and the general similarity of the matters absorbed, arises a great resemblance between the nutritive apparatus of all vegetables; which makes these organs ill adapted to the purposes of classification, and compels us to search among the reproductive organs for the characteristics necessary to establish a scientific arrangement of plants.
The usual aliment of plants we have stated to be water, containing carbonic acid in solution, or, we inay add, at least containing some proportion of animal or vegetable matter capable of being converted into carbonic acid. But, besides this, air and various salts and other matters are absorbed in solution. Where, however, more substances have been found in the ashes of plants, than were supposed to have composed the materials by which they had been nourished, we must not conclude that the plants have created these substances, as some have imagined, but must consider them to have been extracted little by little, froin some me
VOL. XI. NO, XXII.
dium in which they really existed, though in such minute quantity as to be incapable of being detected by chemical tests. We may easily allow that plants surpass us in their powers of abstracting the minutest portions of any material disseminated through a given menstruum.
Second period of nutrition.—The water introduced by the absorption of the spongioles is called “ sap, or lymph.” It is then conveyed directly to the leaves, without sustaining any appreciable change in its progress, otherwise than by mixing with the vegetable juices it meets with in its course. The particular route which the ascending sap takes has often been a matter of discussion and dispute; but it has been clearly ascertained, by repeated experiments, that it ascends along that portion of the cellular tissue which constitutes the woody fibre, and not through the vascular tissue, which is intended primarily for the conveyance of air, though its tubes may occasionally be found filled with Auid. With respect, however, to the mode in which the sap is conducted along this cellular tissue, there is still much uncertainty. De Candolle favours the hypothesis of its passing along the intercellular passages, as we before mentioned, by means of the successive contractions and dilatations of the cells themselves. But we decidedly consider the newly-established principle of endosmose to afford a much better prospect of accounting for the fact. Endosmose is that property of all membranaceous substances, by which two opposite currents are established through their texture, whenever two fluids of unequal densities are placed on opposite sides of them. That the vital force must also be employed in securing any lengthened continuance of this action is evident; for in every apparatus constructed for the purposes of experiment, the effect must cease as soon as the two fuids have acquired the same density. Now, without attempting to account for the manner in which the healthy condition of the membrane is secured, we may easily imagine the constant development of fresh vesicles, and the continued secretion of fresh materials, to be sufficient to maintain the conditions necessary for the establishment of a perpetual endosmose during the lifetime of the plant, without considering this property itself to be directly dependant on the vital force. This property also explains the prodigious force with which the sap rises in certain seasons of the year; a force sufficient, as Hales determined, to support a pressure equal to two atmospheres and a half, and five times that by which the blood is. propelled in the crural artery of the horse. The ascent of the sap is the result of a compound action, partly depending on a force" à tergo" propelling it forward, and partly on a force attracting it towards the foliaceous parts of the leaf; each, however, resulting from the endosmose carried on by all parts of the cellu, lar texture.
Third period of nutrition. When the sap has arrived at the leaves, and at the other green parts on the surface of the plant, a considerable portion of its aqueous particles is transpired. A cabbage, for example, transpires from a given surface seventeen times as much as a man by his insensible perspiration. A small portion, indeed, of this effect must be ascribed to the process of evaporation; but this is comparatively very trifling, and the greater part must be attributed to the action of a vital function. This is so decidedly remarkable in the vegetable kingdom, that De Can, dolle proposes for it the name of " exhalation,” in order to distinguish it from the less conspicuous effects of the insensible perspiration of animals. It is manifestly produced by the instru. mentality of the stomata, or glandular pores, seated on the green parts of plants, and which are more especially abundant on the under surface of the leaf. Heat exerts a trifling influence in producing an increase of exhalation, but light is the chief stimu. lant which determines its extent. Plants do not exhale moisture in the dark, and as they still continue to absorb a little, they soon become dropsical. The fluid exhaled is nearly pure water, and consequently the sap must become considerably altered by this circumstance alone, as the materials introduced in solution will now bear a higher proportion to the whole quantity of water retained in the plant. This great exhalation of the superabundant fluid may be considered analogous to the combined effects pro. duced both by the insensible perspiration and excrementitial rejections of animals.
Fourth period of nutrition.-We have now arrived at the complicated phenomenon of vegetable “ respiration,” the most important of all the processes which together constitute the function of nutrition. One circumstance in this process is strictly in accordance with what takes place in the respiration of animals : the presence of oxygen being equally essential to the life of the individuals of each kingdom, though the ultimate results are diametrically opposite in each. In animals, the oxygen inhaled unites with the superfluous carbon in the blood, and the carbonic acid thus formed is exhaled into the atmosphere. In plants, a similar effect takes place by night, when the leaves and other green parts inhale the atmosphere, whose oxygen unites with the carbonaceous matters dissolved in the sap; but the carbonic acid thus generated is, for the most part, retained in solution within the plant, and not exhaled again. All the coloured parts of plants perform this function as well by day as by night; but the green parts always decompose carbonic acid by day, from whatever source they may be able to derive it; and the result of this decomposition is to tix the whole of the carbon, and a small portion of the oxygen, in the substance of the plant, and to exhale the rest of the oxygen into the air. The chief supply of carbonic acid provided for this purpose is that which is introduced by the roots in a state of so!ution, but the small quantity universally present in the atmosphere is also inhaled and decomposed by the leaves. That which is formed within the plant itself is either retained in solution, or it is exhaled and disseminated in the atmosphere, whence it may re-enter the plant by one or other of the two methods just specified. Although the decomposition of carbonic acid is always proceeding during the day, yet it is never so rapid, nor so decidedly appreciable, as when the plant is exposed to the direct rays of the sun. It is independent of the presence of the stomata, which we have already described as being the true exhaling organs of the green parts; and it is certainly effected by means of the cellular texture of these same parts, whose green tint must be ascribed to the result of this very action. There can be no question that the “ decomposition” of the carbonic acid is the direct operation of a vital function; but the “ formation" of this gas in the coloured parts of plants, at all times, and in the green parts by night, appears to be the result of a chemical action similar to the ordinary process of decomposition in all dead organized matter. If a plant be exposed to the light in an atmosphere deprived of oxygen, it soon dies, unless (which is very remarkable) it be enabled first to form a little oxygen by decomposing some portion of the carbonic acid within it, and thus to impregnate the atmosphere with a sufficient quantity to enable it to act as a further resource and constant stimulus for the formation and decomposition of fresh supplies of carbonic acid.
The result of all these compositions and decompositions of carbonic acid in the living plant, is the fixation of the whole of that carbon which is found in the entire mass of vegetation on the surface of the earth. Thus the atmosphere is continually being purified of every fresh addition of this material with which combustion, respiration, and putrefaction are perpetually adulterating it. For though all living plants do themselves generate carbonic acid in the way which we have specified, they also decompose much more than they form, and this excess is considered to be sufficient to counterbalance the quantity introduced into the atmosphere by other causes. In this single function, then, of vegetable life, we see an efficient yet simple means of restoring to the atmosphere that proportion of oxygen which is necessary to the health and existence of organized beings.