Our conclusion from various experiments is, that hydrogen is unfavorable to vegetation, and does not serve as the food of plants. But hydrogen is contained in plants as is evident from their analysis; and if they refuse it when presented to them in a gaseous state, in what state do they then acquire it? To this question it is sufficient for the present to reply, that if plants do not acquire their hydrogen in the state of gas, they may at least acquire it in the state of water, which is indisputably a vegetable food, and of which hydrogen constitutes one of the component parts.

1502. Vegetable extract. When it was found that atmospheric air and water are not, even conjointly, capable of furnishing the whole of the aliment necessary to the developement of the plant, it was then alleged that, with the exception of water, all substances constituting a vegetable food must at least be administered to the plant in a gaseous state. But this also is a conjecture unsupported by proof; for even with regard to such plants as grow upon a barren rock, or in pure sand, it cannot be said that they receive no nourishment whatever besides water, except in a gaseous state. Many of the particles of decayed animal and vegetable substances, which float in the atmosphere and attach themselves to the leaves, must be supposed to enter the plant in solution with the moisture which the leaves imbibe; and so also similar substances contained in the soil must be supposed to enter it by the root: but these substances may certainly contain vegetable nourishment; and they will perhaps be found to be taken up by the plant in proportion to their degree of solubility in water, and to the quantity in which they exist in the soil. Now one of the most important of these substances is vegetable extract. When plants have attained to the maturity of their species, the principles of decay begin gradually to operate upon them, till they at length die and are converted into dust or vegetable mould, which, as might be expected, constitutes a considerable proportion of the soil. The chance then is, that it is again converted into vegetable nourishment, and again enters the plant. But it cannot wholly enter the plant, because it is not wholly soluble in water. Part of it, however, is soluble, and consequently capable of being absorbed by the root, and that is the substance which has been denominated extract.

1503. Saussure filled a large vessel with pure mould of turf, and moistened it with distilled or rain water, till it was saturated. At the end of five days, when it was subjected to the action of the press, 10,000 parts in weight of the expressed and filtered fluid yielded, by evaporation to dryness, 26 parts of extract. In a similar experiment upon the mould of a kitchen-garden which had been manured with dung, 10,000 parts of a fluid yielded 10 of extract. And in a similar experiment upon mould taken from a wellcultivated corn-field, 10,000 parts of fluid yielded four parts of extract. Such was the result in these particular cases. But the quantity of extract that may be separated from the common soil is not in general very considerable. After twelve decoctions, all that could be separated was about one eleventh of its weight; and yet this seems to be more than sufficient for the purposes of vegetation: for a soil containing this quantity was found by experiment to be less fertile, at least for peas and beans, than a soil that contained only one half or two thirds the quantity. But if the quantity of extract must not be too much, neither must it be too little. Plants that were put to vegetate in soil deprived of its extract, as far as repeated decoctions could deprive it, were found to be much less vigorous and luxuriant than plants vegetating in soil not deprived of its extract; and yet the only perceptible difference between them is, that the former can imbibe and retain a much greater quantity of water than the latter. From this last experiment, as well as from the great proportion in which it exists in the living plant, it evidently follows that extract constitutes a vegetable food. But extract contains nitrogen; for it yields by distillation a fluid impregnated with ammonia. The difficulty, therefore, of accounting for the introduction of nitrogen into the vegetating plant, as well as for its existence in the mature vegetable substance, is done away; for, although the plant refuses it when presented in a gaseous state, it is plain that it must admit it along with the extract. It seems also probable that a small quantity of carbonic acid gas enters the plant along with the extractive principle, as it is known to contain this gas also.

The

1504. Salts, in a certain proportion, are found in most plants, such as nitrate, muriate, and sulphate of potass or soda, as has been already shown. These salts are known to exist in the soil, and the root is supposed to absorb them in solution with the water by which the plant is nourished. It is at least certain that plants may be made to take up by the roots a considerable proportion of salts in a state of artificial solution. But if salts are thus taken up by the root of the vegetating plant, does it appear that they are taken up as a food? Some plants, it must be confessed, are injured by the application of salts, as is evident from the experiments of Saussure; but others are as evidently benefited by it. Trefoil and lucerne have their growth much accelerated by the application of sulphate of lime, though many other plants are not at all influenced by its action. parietaria, nettle, and borage will not thrive, except in such soils as contain nitrate of lime, or nitrate of potass; and plants inhabiting the sea-coast, as was observed by Du Hamel, will not thrive in a soil that does not contain muriate of soda. It has been thought, however, that the salts are not actually taken up by the root, though converted to purposes of utility by acting as astringents or corrosives in stopping up the orifices of the vessels of the plant, and preventing the admission of too much water: but it is to be recollected that the salts in question are found by analysis in the very substance of the plant, and must consequently have entered in solution. It has been also thought that salts are favorable to vegetation only in proportion as they hasten the putrefaction of vegetable substances contained in the soil, or attract the humidity of the atmosphere. But sulphate of lime is not deliquescent; and if its action consist merely in accelerating putrefaction, why is its beneficial effect confined but to a small number of plants? Grisenthwaite (New Theory of Agriculture, 1819, p. 111.) answers this question by stating, that as in the principal grain-crops which interest the agriculturist, there exists a particular saline substance, pe

culiar to each, so, if we turn our attention to the clovers and turnips, we shall still find the same discrimination. Saintfoin, clover, and lucerne, have long been known to contain a notable quantity of gypsum (sulphate of lime); but such knowledge, very strange to relate, never led to the adoption of gypsum as a manure for those crops, any more than that of phosphate of lime for wheat, or nitrate of soda, or potassa for barley. It is true that gypsum has been long, and in various places, recommended as a manure, but its uses not being understood, it was recommended without any reference to crop, or indeed to the accomplishment of any fixed object. It is very well known that some particular ingredient may be essential to the composition of a body, and yet constitute but a very small proportion of its mass. Atmospheric air contains only about one part in the 100 of carbonic acid; and yet no one will venture to affirm that carbonic acid gas is merely an adventitious and accidental element existing by chance in the air of the atmosphere, and not an essential ingredient in its composition. Phosphate of lime constitutes but a very small proportion of animal bodies, perhaps not one part in 500; and yet no one doubts that it is essential to the composition of the bones. But the same salt is found in the ashes of all vegetables; and who will say that is not essential to their perfection?

1505. Earths. As most plants have been found by analysis to contain a portion of alkaline or earthy salts, so most plants have been found to contain also a portion of earths and as the two substances are so nearly related, and so foreign in their character to vegetable substances in general, the same enquiry has consequently been made with regard to their origin. Whence are the earths derived that have been found to exist in plants? Chiefly from the soil. But in what peculiar state of combination do they enter the vessels of the plant? The state most likely to facilitate their absorption is that of their solution in water, in which all the earths hitherto found in plants are known to be in a slight degree soluble. If it be said that the proportion in which they are soluble is so very small that it scarcely deserves to be taken into the account, it is to be recollected that the quantity of water absorbed by the plant is great, while that of the earth necessary to its health is but little, so that it may easily be acquired in the progress of vegetation. Such is the manner in which their absorption seems practicable: and Woodward's experiments afford a presumption that they are actually absorbed by the

root.

1506. The proportion of earths contained in the ashes of vegetables depends upon the nature of the soil in which they grow. The ashes of the leaves of the rhododendron ferrugineum, growing on Mount Jura, a calcareous mountain, yielded 45-25 parts of earthy carbonate, and only 075 of silica. But the ashes of leaves of the same plants, growing on Mount Breven, a granitic mountain, yielded two parts of silica, and only 1675 of earthy carbonate. It is probable however, that plants are not indebted merely to the soil for the earthy particles which they may contain. They may acquire them partly from the atmosphere. Margray has shown that rain-water contains silica in the proportion of a grain to a pound; which, if it should not reach the root, may possibly be absorbed along with the water that adheres to the leaves. But although the earths are thus to be regarded as constituting a small proportion of vegetable food, they are not of themselves sufficient to support the plant, even with the assistance of water. Giobert mixed together lime, alumine, silica, and magnesia, în such proportions as are generally to be met with in fertile soils, and moistened them with water. Several different grains were then sown in this artificial soil, which germinated indeed, but did not thrive; and perished when the nourishment of the cotyledons was exhausted. It is plain, therefore, that the earths, though beneficial to the growth of some vegetables, and perhaps necessary to the health of others, are by no means capable of affording any considerable degree of nourishment to the plant.

1507. Supply of food by manures and culture. With regard to the food of plants derived from the atmosphere, the supply is pretty regular, at least, in as far as the gases are concerned; for they are not found to vary materially in their proportions on any part of the surface of the globe; but the quantity of moisture contained in the atmosphere is continually varying, so that in the same season you have not always the same quantity, though in the course of the year the deficiency is perhaps made up. From the atmosphere, therefore, there is a regular supply of vegetable food kept up by nature for the support of vegetable life, independent of the aid of man: and if human aid were even wanted, it does not appear that it could be of much avail. But this is by no means the case with regard to soils; for if soils are less regular in their composition, they are at least more within the reach of human management. The supply of food may be increased by altering the mechanical or chemical constitution of soils; and by the addition of food in the form of manures. The mechanical constitution of soils may be altered by pulverisation, consolidation, draining, and watering; their chemical properties by aeration and torrification; both mechanical and chemical properties, by the addition of earths or other substances; and manures, either liquid or solid, are supplied by irrigation and distribution of dungs and other nourishing matters, with or without their interment. (See Book III.)

1508. Soils in a state of culture, though consisting originally of the due proportion of ingredients, may yet become exhausted of the principle of fertility by means of too frequent cropping; whether by repetition or rotation of the same, or of different crops. In this case, it should be the object of the phytologist, as well as of the practical cultivator, to ascertain by what means fertility is to be restored to an exhausted soil, or communicated to a new one. In the breaking up of new soils, if the ground has been wet or

marshy, as is frequently the case, it is often sufficient to prepare it merely by means of draining off the superfluous and stagnant water, and of paring and burning the turf upon the surface. If the soil has been exhausted by too frequent a repetition of the same crop, it often happens that a change of crop will answer the purpose of the cultivator; for although a soil may be exhausted for one sort of grain, it does not necessarily follow that it is also exhausted for another. And accordingly, the practice of the farmer is to sow his crops in rotation, having in the same field a crop, perhaps, of wheat, barley, beans, and tares in succession; each species selecting in its turn some peculiar nutriment, or requiring, perhaps, a smaller supply than the crop that has preceded it. But even upon the plan of rotation, the soil becomes at length exhausted, and the cultivator obliged to have recourse to other means of restoring its fertility. In this case, an interval of repose is considerably efficacious, as may be seen from the encreased fertility of fields that have not been ploughed up for many years, such as those used for pasture; or even from that of the walks and paths in gardens when they are again broken up. Hence also the practice of fallowing, and of trenching or deep ploughing, which in some cases has nearly the same effect.

1509. The fertility of a soil is restored, in the case of draining, by means of its carrying off all such superfluous moisture as may be lodged in the soil, which is well known to be prejudicial to plants not naturally aquatics, as well as by rendering the soil more firm and compact. In the case of burning, the amelioration is effected by means of the decomposition of the vegetable substances contained in the turf, and subjected to the action of the fire, which disperses part also of the superfluous moisture, but leaves a residue of ashes favorable to future vegetation. In the case of the rotation of crops, the fertility is not so much restored as more completely developed and brought into action; because the soil, though exhausted for one species of grain, is yet found to be sufficiently fertile for another, the food necessary to each being different, or required in less abundance. In the case of the repose of the soil, the restored fertility may be owing to the decay of vegetable substances that are not now carried off in the annual crop, but left to augment the proportion of vegetable mould; or to the accumulation of fertilising particles conveyed to the soil by rains; or to the continued abstraction of oxygen from the atmosphere. In the case of fallows, it is owing undoubtedly to the action of the atmospheric air upon the soil, whether in rendering it more friable, or in hastening the putrefaction of noxious plants; or it is owing to the abstraction and accumulation of oxygen. In the case of trenching, or deep ploughing, it is owing to the increased facility with which the roots can now penetrate to the proper depth, and thus their sphere of nourishment is increased. But it often happens that the soil can no longer be ameliorated by any of the foregoing means, or not at least with sufficient rapidity for the purposes of the cultivator; and in this case there must be a direct and actual application made to it of such substances as are fitted to restore its fertility. Hence the indispensable necessity of manures, which consists chiefly of animal and vegetable remains that are buried and finally decomposed in the soil, from which they are afterwards absorbed by the root of the plant, in a state of solution.

1510. But as carbon is the principal ingredient furnished by manures, as contributing to the nourishment of the plant, and is not itself soluble in water, nor even disengaged by fermentation in a state of purity; under what state of chemical combination is its solution effected? Is it effected in the state of charcoal? It has been thought, indeed, that carbon in the state of charcoal is soluble in water; because water from a dunghill, when evaporated, constantly leaves a residuum of charcoal, as was first ascertained by the experiments 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. Sencbier endeavors 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 favorable 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 Hassenfratz 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.

1511. Plants are nourished in a manner in some degree analogous to the animal economy. The food of plants, whether lodged in the soil, or wafted through the atmosphere, is taken up by intro-susception 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.

1512. Intro-susception. 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 intro-susception 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 upon the organisation. It has been a question, however, among phytologists, whether 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 and aged trunks, of which the original organisation is obliterated; nor by that of the larger and more aged branches. But it has been thought 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.

1513. Ascent of the sap. The means by which the plant effects the intro susception 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 favorable 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 rigor of the season. But all this could not possibly be accomplished, if the motion of the sap were wholly suspended.

1514. 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 syphon, 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 of 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.

1515. 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 that it ascended by the bark, wood, and pitli indiscriminately; and others thought it ascended between the bark and wood. The first opinion was maintained and supported by Malpighi; and Grew considers that the sap ascends by the bark, wood, and pith, indiscriminately. Du Hamel stript several trees of their bark entirely, which continued, notwithstanding, to live for many years, protruding new leaves and new branches as before. Knight stript the trunk 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