periments of Hassenfratz. But there seem to be reasons for doubting the legitimacy of the conclusion that has been drawn from it; for Senebier found that plants whose roots were immersed in water took up less of the fluid in proportion as it was mixed with water from a dunghill. Perhaps then the charcoal of water from a dunghill is held merely in suspension, and enters the plant under some other modification. But if carbon is not soluble in water in the state of charcoal, in what other state is it soluble? It is soluble in the state of carbonic acid gas. But is this the state in which it actually enters the root? On this subject phytologists have been somewhat divided in opinion. Senebier endeavours to prove that carbonic acid gas, dissolved in water, supplies the roots of plants with almost all their carbon, and founds his arguments upon the following facts:- In the first place, it is known that carbonic acid gas is soluble in water; in the second place, it is known to be contained in the soil, and generated by the fermentation of the materials composing manures; and, in the next place, it is known to be beneficial to vegetation when applied artificially to the roots, at least in a certain degree. This is evident from the following experiment of Ruckert, as well as from several experiments of Saussure's previously related. Ruckert planted two beans in pots of equal dimensions, filled with garden mould; the one was moistened with distilled water, and the other with water impregnated with carbonic acid gas. But the latter appeared above ground nine days sooner than the former, and produced twenty-five beans; while the former produced only fifteen. Now the result of this experiment, as well as the preceding facts, is evidently favourable to the presumption of Senebier, and shows that if carbonic acid is not the state in which carbon enters the plant, it is at least a state preparatory to it; and there are other circumstances tending to corroborate the opinion, resulting from the analysis of the ascending sap of plants. The tears of the vine, when analysed by Senebier, yielded a portion of carbonic acid and earth; and as the ascending sap could not be supposed to have yet undergone much alteration, the carbonic acid, like the earth, was probably taken up from the soil. But this opinion, which seems to be so firmly established upon the basis of experiment, Hassenfratz strenuously controverts. According to experiments which he had instituted with an express view to the investigation of this subject, plants which were raised in water impregnated with carbonic acid differed in no respect from such as grew in pure water, and contained no carbon that did not previously exist in the seed. Now if this were the fact, it would be decisive of the point in question. But it is plain from the experiments of Saussure, as related in the preceding section, that Has. senfratz must have been mistaken, both with regard to the utility of carbonic acid gas as furnishing a vegetable aliment, and with regard to the augmentation of carbon in the plant. The opinion of Senebier, therefore, may still be correct. It must be acknowledged, however, that the subject is not yet altogether satisfactorily cleared up; and that carbon may certainly enter the plant in some state different from that either of charcoal in solution, or of carbonic acid gas. Is not carbonic acid of the soil decomposed before entering the plant? This is a conjecture of Dr. Thomson's, founded upon the following facts:- The green oxide of iron is capable of decomposing carbonic acid; and many soils contain that oxide. Most soils, indeed, contain iron, either in the state of the brown or green oxide, and it has been found that oils convert the brown oxide into green. But dung and rich soils contain a quantity of oily substance. One effect of manures, therefore, may be that of reducing the brown oxide of iron to the green, thus rendering it capable of decomposing carbonic acid gas, so as to prepare it for some new combination, in which it may serve as an aliment for plants. All this, however, is but a conjecture; and it is more probable that the carbonic acid of the soil enters the root in combination with some other substance, and is afterwards decomposed within the plant itself.

SECT. III. Process of Vegetable Nutrition.

1537. Plants are nourished in a manner in some degree analogous to that in which animals are sustained. The food of plants, whether lodged in the soil, or wafted through the atmosphere, is taken up by introsusception in the form of gases or other fluids; it is then known as their sap: this sap ascends to the leaves, where it is elaborated as the blood of animals is in the lungs; it then enters into the general circulation of the plant, and promotes its growth.

1538. Introsusception. As plants have no organ analogous to the mouth of animals, they are enabled to take up the nourishment necessary to their support only by absorption or inhalation, as the chyle into the animal lacteals, or the air into the lungs. The former term is applied to the introsusception of non-elastic fluids; the latter to that of gaseous fluids. The absorption of non-elastic fluids by the epidermis of plants does not admit of a doubt. It is proved indisputably, that the leaves not only contain air, but do actually inhale it. It was the opinion of Priestley that they inhale it chiefly by the upper surface; and it has been shown by Saussure that their inhaling power depends entirely

it is not also effected by the epidermis of the other parts of the plant. We can scarcely suppose it to be effected by the dry and indurate epidermis of the bark of aged trunks, of which the original organisation is obliterated; nor by that of the larger and more aged branches. But it has been thought that there are even some of the soft and succulent parts of the plant by which it cannot be effected, because no pores are visible in their epidermis. Decandolle found no pores in the epidermis of fleshy fruits, such as pears, peaches, and gooseberries; nor in that of roots, or scales of bulbs; nor in any part not exposed to the influence of air and light. It is known, however, that fruits will not ripen, and that roots will not thrive, if wholly deprived of air; and hence it is probable that they inhale it by their epidermis, though the pores by which it enters should not be visible. In the root, indeed, it may possibly enter in combination with the moisture of the soil; but in the other parts of the plant it enters no doubt in the state of gas. Herbs, therefore, and the soft parts of woody plants, absorb moisture and inhale gases from the soil or atmosphere by means of the pores of their epidermis, and thus the plant effects the introsusception of its food.

1539. Ascent of the sap. The means by which the plant effects the introsusception of its food, is chiefly that of absorption by the root. But the fluids existing in the soil when absorbed by the root, are designated by the appellation of sap or lymph; which, before it can be rendered subservient to the purposes of vegetable nutrition, must either be intermediately conveyed to some viscus proper to give it elaboration, or immediately distributed throughout the whole body of the plant. Our present object, therefore, is that of tracing out the progress of its distribution or ascent. The sap is in motion in one direction or other, if not all the year, at least at occasional periods, as the bleeding of plants in spring and autumn sufficiently illustrates. The plant always bleeds most freely about the time of the opening of the bud; for in proportion as the leaves expand the sap flows less copiously, and when they are fully expanded it entirely ceases. But this suspension is only temporary, for the plant may be made to bleed again in the end of the autumn, at least under certain conditions. If an incision is now made into the body of the tree, after the occurrence of a short but sharp frost, when the heat of the sun or mildness of the air begins to produce a thaw, the sap will again flow. It will flow even where the tree has been but partially thawed, which sometimes happens on the south side of a tree, when the heat of the sun is strong and the wind northerly. At the seasons now specified, therefore, the sap is evidently in motion; but the plant will not bleed at any other season of the year. It has been the opinion of some phytologists, that the motion of the sap is wholly suspended during the winter. But though the great cold of winter, as well as the great heat of summer, is by no means so favourable to vegetation as the milder though more changeable temperature of spring and autumn, yet it does not wholly suspend the movement of the sap. Palms may be made to bleed at any season of the year; and although this is not the case with plants in general, yet there is proof sufficient that the colds of winter do not, even in this climate, entirely prevent the sap from flowing. Buds exhibit a gradual developement of parts throughout the whole of the winter, as may be seen by dissecting them at different periods. So also do roots. Evergreens retain their leaves; and many of them, such as the arbutus, laurustinus, and the beautiful tribe of the mosses, protrude also their blossoms, even in spite of the rigour of the season. But all this could not possibly be accomplished, if the motion of the sap were wholly suspended.

1540. Thus the sap is in perpetual motion, with a more accelerated or more diminished velocity, throughout the whole of the year; but still there is no decided indication exhibited in the mere circumstance of the plant's bleeding, of the direction in which the sap is moving at the time; for the result might be the same whether it was passing from the root to the branches, or from the branches to the root. But as the great influx of the sap is effected by means of the pores of the epidermis of the root, it follows that its motion must, at least in the first place, be that of ascent; and such is its direction at the season of the plant's bleeding, as may be proved by the following experiment: - If the bore or incision that has been made in the trunk is minutely inspected while the plant yet bleeds, the sap will be found to issue almost wholly from the inferior side. If several bores are made in the same trunk, one above another, the sap will begin to flow first from the lower bore, and then from those above it. If a branch of a vine be lopped, the sap will issue copiously from the section terminating the part that remains yet attached to the plant; but not from the section terminating the part that has been lopped off. This proves indubitably that the direction of the sap's motion, during the season of the plant's bleeding, is that of ascent. But if the sap flows so copiously during the season of bleeding, it follows that it must ascend with a very considerable force; which force has accordingly been made the subject of calculation. To the stem of a vine cut off about two feet and a half from the ground, Hales fixed a mercurial gauge which he luted with mastic; the gauge was in the form of a siphon, so contrived that the mercury might be made to rise in proportion to the pressure of the ascending sap. The mercury rose accordingly,

and reached, at its maximum, to a height of thirty-eight inches. But this was equivalent to a column of water to the height of forty-three feet three and one third inches; demonstrating a force in the motion of the sap that, without the evidence of experiment, would have seemed altogether incredible.

1541. Thus the sap, in ascending from the lower to the upper extremity of the plant, is propelled with a very considerable force, at least in the bleeding season. But is the ascending sap propelled indiscriminately throughout the whole of the tubular apparatus, or is it confined in its course to any particular channel? Before the anatomy of plants had been studied with much accuracy, there was a considerable diversity of opinion on the subject. Some thought it ascended by the bark; others thought it ascended by the bark, wood, and pith, indiscriminately; and others thought it ascended between the bark and wood. The first opinion was maintained and supported by Malpighi; and Grew considered that the sap ascends by the bark, wood, and pith, indiscriminately. Du Hamel stripped several trees of their bark entirely, which continued, notwithstanding, to live for many years, protruding new leaves and new branches as before. Knight stripped the trunks of a number of young crab trees of a ring of bark half an inch in breadth; but the leaves were protruded, and the branches elongated, as if the operation had not been performed. Du Petit Thouars removed the central wood and pith from the stems of several young sycamore trees, leaving the upper part to be supported only by four pillars of bark: in others he removed the bark, liber, and alburnum, leaving the upper part of the tree to be supported solely by the central wood. In each case the tree lived, so that he concludes that both the bark and wood are competent to act as conductors to the-sap. (Hist. d'un Marceau de Bois, Hort. Tour, 481.)

1542. That the sap does not ascend exclusively by the bark is thus rendered sufficiently evident. But it is equally evident that it does not ascend by the pith, at least after the first year; for then, even upon Grew's own supposition, it becomes either juiceless or wholly extinct and even during the first year it is not absolutely necessary, if at all subservient to the ascent of the sap, as is proved by an experiment of Knight's. Having contrived to abstract from some annual shoots a portion of their pith, so as to interrupt its continuity, but not otherwise materially to injure the fabric of the shoot, Knight found that the growth of the shoots which had been made the subject of experiment was not at all affected by it.

1543. The sap ascends neither by the bark nor pith, but by the wood only. But the whole mass of the wood throughout is not equally well adapted for the purpose of conveying it. The interior and central part, or that which has acquired its last degree of solidity, does not in general afford it a passage. This is proved by what is called the girdling of trees, which consists in making a circular gap or incision quite round the stem, and to the depth of two or three inches, so as to cut through both the bark and alburnum. An oak tree on which Knight had performed this operation, with a view to ascertain the channel of the sap's ascent, exhibited not the slightest mark of vegetation in the spring following. The sap then does not ascend through the channel of the matured wood. But if the sap ascends neither through the channel of the bark, nor pith, nor matured wood, through what other channel does it actually ascend? The only remaining channel through which it can possibly ascend is that of the alburnum. In passing through the channel of the alburnum, does the sap ascend promiscuously by the whole of the tubes composing it, or is it confined in its passage to any peculiar set? The earliest conjectures recorded on this subject are those of Grew and Malpighi, who, though they maintained that the sap ascends chiefly by the bark, did not yet deny that it ascends also partly by the alburnum or wood. It occurred to succeeding phytologists that the progress of the sap, and the vessels through which it passes, might be traced or ascertained by means of making plants vegetate in coloured infusions. Du Hamel steeped the extremities of branches of the fig, elder, honeysuckle, and filbert in common ink. In examining the two former, after being steeped for several days, the part immersed was found to be black throughout, but the upper part was tinged only in the wood, which was coloured for the length of a foot, but more faintly and partially in proportion to the height. The pith, indeed, exhibited some traces of ink, but the bark and buds none. In some other examples the external layers of the wood only were tinged. In the honeysuckle the deepest shade was about the middle of the woody layers; and in the filbert there was also observed a coloured circle surrounding the pith, but none in the pith itself, nor in the bark.

1544. Thus it is proved that the sap ascends through the vessels of the longitudinal fibre composing the alburnum of woody plants, and through the vessels of the several bundles of longitudinal fibre constituting the woody part of herbaceous plants. But it has been already shown that the vessels composing the woody fibre are not all of the same species. There are simple tubes, porous tubes, spiral tubes, mixed tubes, and interrupted tubes. Through which of these, therefore, does the sap pass in its ascent? The best reply to this enquiry

apple and horsechestnut, by means of circular incisions, so as to leave detached rings of bark with insulated leaves remaining on the stem. He then placed them in coloured infusions obtained by macerating the skins of very black grapes in water; and, on examining the transverse section at the end of the experiment, it was found that the infusion had ascended by the wood beyond his incisions, and also into the insulated leaves, but had not coloured the pith nor bark, nor the sap between the bark and wood. From the above experiment, Knight concludes that the sap ascends through what are called the common tubes of the wood and alburnum, at least till it reaches the leaves. Thus the sap is conveyed to the summit of the alburnum. But Knight's next object was to trace the vessels by which it is conveyed into the leaf. The apple tree and horsechestnut were still his subjects of experiment. In the former the leaves are attached to the plants by three strong fibres, or rather bundles of tubes, one in the middle of the leaf-stalk, and one on each side. In the latter they are attached by means of several such bundles. Now the coloured fluid was found in each case to have passed through the centre of the several bundles, and through the centre only, tinging the tubes throughout almost the whole length of the leaf-stalk. In tracing their direction from the leaf-stalk upwards, they were found to extend to the extremity of the leaves; and in tracing their direction from the leaf-stalk downwards, they were found to penetrate the bark and alburnum, the tubes of which they join, descending obliquely till they reach the pith which they surround. From their position Knight calls them central tubes, thus distinguishing them from the common tubes of the wood and alburnum, and from the spiral tubes with which they were every where accompanied as appendages, as well as from a set of other tubes which surrounded them, but were not coloured, and which he designates by the appellation of external tubes. The experiment was now transferred to the flower-stalk, and fruit-stalk, which was done by placing branches of the apple, pear, and vine, furnished with flowers not yet expanded, in a decoction of logwood. The central vessels were rendered apparent as in the leaf-stalk. When the fruit of the two former was fully formed, the experiment was then made upon the fruit-stalk, in which the central vessels were detected as before; but the colouring matter was found to have penetrated into the fruit also, diverging round the core, approaching again in the eye of the fruit, and terminating at last in the stamens. This was effected by means of a prolongation of the central vessels, which did not however appear to be accompanied by the spiral tubes beyond the fruit-stalk. Such then are the parts of the plant through which the sap ascends, and the vessels by which it is conveyed. Entering by the pores of the epidermis, it is received into the longitudinal vessels of the root by which it is conducted to the collar. Thence it is conveyed by the longitudinal vessels of the alburnum, to the base of the leaf-stalk, and peduncle; from which it is further transmitted to the extremity of the leaves, flower, and fruit. There remains a question to be asked intimately connected with the sap's ascent. Do the vessels conducting the sap communicate with one another by inosculation or otherwise, so as that a portion of their contents may be conveyed in a lateral direction, and, consequently, to any part of the plant; or do they form distinct channels throughout the whole of their extent, having no sort of communication with any other set of tubes, or with one another? Each of the two opinions implied in the question has had its advocates and defenders: but Du Hamel and Knight have shown that a branch will still continue to live, though the tubes leading directly to it are cut in the trunk; from which it follows that the sap, though flowing the most copiously in the direct line of ascent, is at the same time also diffused in a transverse direction.

1545. Causes of the sap's ascent. By what power is the sap propelled? Grew states two hypotheses: its volatile nature and magnetic tendency, aided by the agency of fermentation. Malpighi was of opinion that the sap ascends by means of the contraction and dilatation of the air contained in the air-vessels. M. De la Hire attempted to account for the phenomenon by combining together the theories of Grew and Malpighi; and Borelli, who endeavoured to render their theory more perfect, by bringing to its aid the influence of the condensation and rarefaction of the air and juices of the plant.

1546. Agency of heat. Du Hamel directed his efforts to the solution of the difficulty, by endeavouring to account for the phenomenon from the agency of heat, and chiefly on the following grounds: because the sap begins to flow more copiously as the warmth of spring returns; because the sap is sometimes found to flow on the south side of a tree before it flows on the north side, that is, on the side exposed to the influence of the sun's heat sooner than on the side deprived of it; because plants may be made to vegetate, even in the winter, by means of forcing them in a hot house; and because plants raised in a hot-house produce their fruit earlier than such as vegetate in the open air. There can be no doubt of the great utility of heat in forwarding the progress of vegetation; but it will not therefore follow that the motion and ascent of the sap are to be attributed to its agency. On the contrary, it is very well known that if the temperature exceeds a certain degree, it becomes then prejudicial both to the ascent of the sap and also to the growth of the plant. Hales found that the sap flows less rapidly at mid-day than in the morning; and every body knows that vegetation is less luxuriant at midsummer than in the spring. So also, in the case of forcing, it happens but too often that the produce of the hot-house is totally destroyed by the unskilful application of heat. If heat is actually the cause of the sap's ascent, how comes it that the degree necessary to produce the effect is so very variable, even in the same climate? For there are many plants, such as the arbutus, laurustinus, and the mosses, which will continue not only to vegetate,

but to protrude their blossoms and mature their fruit, even in the midst of winter, when the temperature is at the lowest; and, in the case of submarine plants, the temperature can never be very high: so that, although heat does no doubt facilitate the ascent of the sap by its tendency to make the vessels expand, yet it cannot be regarded as the efficient cause, since the sap is proved to be in motion even throughout the whole of the winter. Du Hamel endeavours, however, to strengthen the operation of best by means of the influence of humidity, as being also powerful in promoting the ascent of the sap, whether as relative to the season of the year or time of the day. The influence of the humidity of the atmosphere cannot be conceived to operate as a propelling cause, though it may easily be conceived to operate as affording a facility to the ascent of the sap in one way or other; which under certain circumstances is capable of most extraordinary acceleration, but particularly in that state of the atmosphere which forbodes or precedes a storm. In such a state a stalk of wheat was observed by Du Hamel to grow three inches in three days; a stalk of barley six inches, and a shoot of a vine almost two feet; but this is a state that occurs but seldom, and cannot be of much service in the general propulsion of the sap. On this intricate but important subject Linnæus appears to have embraced the opinion of Du Hamel, or an opinion very nearly allied to it; but does not seem to have strengthened it by any new accession of argument; so that none of the hitherto alleged causes can be regarded as adequate to the production of the effect.

1547. Irritability. Perhaps the only adequate cause ever suggested, prior to the hypothesis of Dutrochet, is that alleged by Saussure. According to Saussure the cause of the sap's ascent is to be found in a peculiar species of irritability inherent in the sap-vessels themselves, and dependent upon vegetable life; in consequence of which they are rendered capable of a certain degree of contraction, according to the affection of the internal surface by the application of stimuli, as well as of subsequent dilatation according to the subsidence of the action of the stimulus; thus admitting and propelling the sap by alternate dilatation and contraction. In order to give elucidation to the subject, let the tube be supposed to consist of an indefinite number of hollow cylinders united one to another, and let the sap be supposed to enter the first cylinder by capillary attraction, or by any other adequate means; then the first cylinder being excited by the stimulus of the sap, begins gradually to contract, and to propel the contained fluid into the cylinder immediately above it. "But the cylinder immediately above it, when acted on in the same manner, is affected in the same manner; and thus the fluid is propelled from cylinder to cylinder till it reaches the summit of the plant. So also when the first cylinder has discharged its contents into the second, and is no longer acted upon by the stimulus of the sap, it begins again to be dilated to its original capacity, and prepared for the introsusception of a new portion of fluid. Thus a supply is constantly kept up, and the sap continues to flow. The above is by far the simplest as well as most satisfactory of all theories accounting for the ascent of the sap.

1548. Contraction and dilatation. Knight has presented us with a theory which, whatever may be its real value, merits at least our particular notice, as coming from an author who stands deservedly high in the list of phytological writers. This theory rests upon the principle of the contraction and dilatation, not of the sap-vessels themselves, as in the theory of Saussure, but of what Knight denominates the silver grain, assisted perhaps by heat and humidity expanding or condensing the fluids. (Phil. Trans, 1801.) Keth considers this theory of Knight as beset with many difficulties, and the agency of the alleged cause as totally inadequate to the production of the effect to be accomplished.

1549 Necessity of an equilibrium in the plant. Du Petit Thouars attributes the motion of the sap to an inherent power, with which nature has been pleased to endow vegetables. But the cause of the renewal of its motion in the spring, after remaining in a quiescent state for several months, he ascribes to the necessity of maintaining a perfect equilibrium in the system of a plant. So that, if a consumption of sap is produced at any given point, the necessity of making good the space so occasioned consequently throws all the particles of sap into motion; and the same effect will continue to operate as long as any consumption of sap takes place. The first cause of this consumption of sap he declares to be the deve lopement of the buds, and already formed young leaves, by the stimulating action of light and heat, but particularly of the latter. As soon as this developement occurs, an assimilation and absorption of sap is occasioned for the support of the young leaves, a vacancy in the immediate vicinity of the leaves is produced, and a motion immediately takes place. (London Encyc., art. Bot.)

1560 Electricity. The most satisfactory hypothesis for the ascent of the sap is that of M. Dutrochet. This philosopher, by careful examination with a microscope, found that the minute conical termination of the radicle was furnished with other projecting bodies, like sponges, which perform the office of the piston of a syringe, and have the power of introducing into their cavity, and through their sides, the water which comes in contact with the exterior surface, and which spongioles oppose, at the same time, the exit of any fluid which they may imbibe. The motions of the sap and juice in plants take place, according to this author, in consequence of the operations of two distinct currents of electricity: the one negative, by which the vessels have the power of absorption, which M. Dutrochet calls endosmose, and by which the vessels become turgid; and the other positive, by which the vessels exude or secrete, which power M. Dutrochet calls exosmose. (Gardener's Mag., vol. iii. p. 78.; Dutrochet, Agent Immediat du mouvement vital, Paris, 8vo, 1826.)

1551. Elaboration of the sap. The moisture of the soil is no sooner absorbed into the plant than it begins to undergo a change. This is proved by the experiment of making a bore or incision in the trunk of a tree during the season of bleeding; the sap that issues from the wound possesses properties very different from the mere moisture of the soil, as is indicated by means of chemical analysis and sometimes also by means of a peculiar taste or flavour, as in the case of the birch tree. Hence the sap has already undergone a certain degree of elaboration; either in passing through the glands of the cellular tissue, which it reaches through the medium of a lateral communication, or in mingling with the juices contained in the cells, and thus carrying off a portion of them; in the same manner, we may suppose, that water, by filtering through a mineral vein, becomes impregnated with the mineral through which it passes. But this primary and incipient stage of the process of elaboration must always of necessity remain a mystery to the phytologist, as being wholly effected in the interior of the plant, and consequently beyond the reach of observation. All he can do, therefore, is to trace out its future progress, and to watch its succeeding changes, in which the rationale of the process of elaboration may be more evident.

1532. The process of elaboration is chiefly operated in the leaf: for the sap no sooner reaches the leaf, than part of it is immediately carried off by means of perspiration, perceptible or imperceptible; effecting a change in the proportion of its component parts, and by consequence a change in its properties.

1553. Hales reared a sun-flower in a pot of earth till it grew to the height of three feet and a half; he then covered the mouth of the pot with a plate of lead, which he cemented so as to prevent all evaporation from the earth contained in it. In this plate he fixed two tubes, the one nine inches in length and of but