VEGETABLEPHYSIOLOGY

VEGETABLE PHYSIOLOGY.

In the Article on the ANATOMY of VEGETABLES, a view has been given of the organography of plants, or the structure and forms of vegetables through their several gradations from the seed to the perfect plant. We now proceed to consider the functions which the different organs of vegetables perform, the description of which constitutes what is termed the PHYSIOLOGY of Plants.

In a subject of such extent, and concerning which so much diversity of opinion prevails, we must bespeak the indulgence of our readers, not only for the imperfections in the errors into which we may fall. From the narrow limits within which we are necessarily circumscribed, we are constrained to give rather the results than the details of experiments; to avoid all discussion of disputable points; to reject many practical illustrations and much historical narration. Neither have we room to enlarge on the general distinctions between plants and animals; on the importance of vegetables in the scale of being; their geographical distribution; the nature of their living power or vitality; their sensibility, perceptivity, and many other properties which have been ascribed to them. The Articles ANIMAL and PHYSICAL GEOGRAPHY will however furnish the necessary information on these particulars. Our present aim will be confined to a general view of the leading and more important functions of the more perfect vegetables.

PART. I.—OF THE GENERAL FUNCTIONS OF VEGETABLES.

SECT. I.—Of the Germination of Seeds.

In the article referred to we have detailed pretty fully the anatomy of seeds, and described particularly their tunics and the nucleus contained within them. This nucleus consists of the organized parts, or embryo, and the inorganic matter destined to afford it nourishment. In its more complete forms, this embryo consists of the radicle, the stem, caulicle, and the plumule. The stem, however, is often not distinguishable. When present, it connects the radicle with the plumule; and the place of its junction with the radicle is denominated the neck of the embryo. In the progress of evolution, the radicle descends to form the root, and the plumule rises and constitutes the first bud of the new plant.

Besides the organised parts just mentioned, there are others called cotyledons, which derive their origin from the embryo. Many seeds have two cotyledons, and some more than two; others have only one, and some seeds have no cotyledon. When present, the cotyledons exhibit different forms; and between them and the embryo, a vascular communication is established, as may be seen in the dissection of a bean, represented in Plate XXXIX. fig. 12. Besides these, the cotyledons are partly made up of cells, within which the nutrient matter of the seed is contained. In some seeds, however, this matter is only partially contained in the cotyledons; in others, as that of wheat, it is wholly contained in a cellular tissue produced from the inner tunic. Lastly, the cotyledons of some seeds rise above the ground during germination, and perform the function of leaves; in others they continue beneath the soil. In all seeds their existence is temporary, for they perish after having yielded their nutrient matter to the embryo.

This matter, called albumen by Grew and Gærtner, is a secretion made by the vessels into the cells during the formation of the seed; and, though itself inorganic, is con-

tained in an organized tissue. It is of various colour and vegetable consistence in different seeds. Its bulk, compared with that of the organized parts, is, in some seeds, very small; in others very large. Vegetable Physiology

Such is a brief notice of the more important parts which construct the seed, and which it is necessary clearly to understand before we can properly appreciate the nature and effects of the actions that go on during its germination. In considering these actions, we have to inquire into the circumstances or conditions in which the seed requires to be placed—the agents which then act upon it—the change of quality and condition which these agents themselves suffer, and the effects which, in consequence, they produce in the seed—and, lastly, the physiological phenomena which thence arise, and terminate in those alterations of form and structure which constitute the evolution of the seed.

In general, seeds, when placed to grow, are buried more or less deeply in the earth, but this condition is not essential; for they readily shoot forth and display their forms, when confined in vessels of air. In whatever situation their germination is attempted, a certain temperature, and a certain portion of moisture, are necessary to its commencement; and the access of air is afterwards required to carry on the process. We have, therefore, to inquire into the operation of water, heat, and air, in commencing and carrying on the germination of the seed.

Water, in the first instance, penetrates the tunics of the seed apparently by simple attraction or imbibition; and the force with which this attraction is exerted is well illustrated in the experiments of Boyle and Hales. They filled strong bottles with dry beans and peas, over which water was poured, and the bottles were then closely stopped. As the seeds imbibed the water, they readily burst the bottles asunder; or, if small iron cylinders, closed by a plug, were employed, the plug was gradually raised by the expanding seeds, though pressed by a weight of nearly 200 lbs. It is by the exertion of such a force, that certain seeds, as those of the peach and apricot, are able, says Du Hamel, to burst open their stony envelope. This expansion from the imbibition of water occurs not only in seeds which retain the faculty of germinating, but in those also which have lost it. We must, therefore, regard this first step in germination as similar to the attraction of water by inanimate bodies. Accordingly, if, after water has been thus imbibed, air be excluded, the radicle never increases beyond a certain size; and, if the seed be kept wholly immersed in water at a temperature of 60°, decomposition of its substance ensues.

To this imbibition of water, a temperature above that of freezing is necessarily required; and, within a certain range, the rate of expansion will be more or less influenced by that of temperature. Cold, however, does not destroy, but only suspends, the germinating faculty in seeds. Seeds have germinated after having been exposed for a short time to a temperature of $-39^\{\backslash circ\}$ F.

When, by the combined operation of heat and moisture, the seed is brought into a condition fit for germination, then the presence of air is required. Many experiments were made by Boyle and others, to prove the necessity of air to germination; and since the composition of the atmosphere was made known, many more have been instituted to ascertain why the air is thus necessary, the nature of the changes it undergoes, the extent to which they proceed, and the manner in which they are accomplished. On all these points much information has been gained, and the results obtained are, in general, so

Vegetable precise, as to leave little doubt as to the nature and extent of the facts, whatever difference of opinion may exist as to the mode of their occurrence. We must refer those who desire full details on these subjects to the writings of Scheele, Cruickshank, Gough, De Saussure, Huber, and Senefier, &c. The results of their labours are given, more or less completely, in most of our chemical works, and are more fully detailed in Mr. Ellis's "Inquiries into the Changes induced on the Air by the Vegetation of Plants," &c. parts i. and ii.

From these results we learn, that atmospheric air is useful to germination, from containing oxygen gas; that by the germinating process, the oxygen gas of the air is changed into an equal bulk of carbonic acid gas; and that the azotic portion of the air remains unchanged in composition, and generally unaltered in volume.

The nature and extent of the change induced on the air being thus ascertained, we have next to inquire into the mode in which it is brought about, that is, how the carbonic acid is formed. Now, when the experiment is conducted in close vessels, no other substance, but the seed, is present that can afford carbon; and this fact, taken in connection with the circumstance that the seed actually contains carbon, and yields it, like other organized substances, to the atmosphere that surrounds it, authorises the conclusion, that, while the air supplies the oxygen, the seed yields the carbon by which the carbonic acid of germination is formed.

Granting, however, that carbon is afforded by the seed, and combines with the oxygen of the air, where, it may be asked, and in what manner, is this combination effected? A certain degree of moisture in the seed is necessary to enable it to yield its carbon; for, when perfectly dry, little or no reciprocal action goes on between the seed and the air. Neither does the living faculty of the seed seem necessary to this combination; for carbon is afforded by seeds when they are confined by vessels in azote or hydrogen gas, and even under actual decomposition. We may therefore regard the formation of carbonic acid, in the first stages of germination, as purely chemical, and as taking place either on the surface, or within the substance of the seed. Now, from the dense structure of the investing tunics, and the circumstance of the vessels of the seed being already filled with fluid, we see no way in which air can enter the seed, so as to act either on its organized or inorganic matter; and, consequently, we incline to the opinion, that the formation of carbonic acid takes place exterior to the tunics of the seed. Such, then, are the changes in composition which the air employed in germination suffers, and such appears to be the mode in which they are accomplished.

While these changes are produced in the air, others not less remarkable occur in the form and qualities of the seed itself; for not only are its organized parts gradually evolved, but its inorganic matter, besides being softened by the imbibed water, acquires, in many seeds, a sweetish taste. These facts, which had long been observed in the process of malting, were more distinctly ascertained by Cruickshank. He found that seeds of barley, when placed to grow in vessels, either of atmospheric air or of pure oxygen gas, acquired, in a few days, a sweetish taste, and were more or less completely converted into malt.

In what manner then, or by what agency must we suppose this change in the inorganic matter of the seed to be accomplished? This matter, though denominated albumen, does not resemble the albumen of chemists. In vegetable physiology, the term comprehends the whole inorganic matter of the seed, although that matter may contain no real albumen, but consist of several distinct substances, or "proximate principles," as they have been called. The principal ingredients of seeds, which afford nutrient matter

to the embryo, are mucilage, starch, and sugar. For a full account of the chemical properties of these substances, we must refer to the writings of chemists; our limits permit only a very slight notice of them.

Mucilage—the soft and liquid state of gum—is inodorous and insipid; soluble in hot or cold water, but insoluble in alcohol. Starch (fécule) is obtained from the flour (farina) of the more nutritive seeds. It is also insipid and inodorous; insoluble in alcohol, and even in water, unless raised to the temperature of 160°. If heated to 180°, the solution then jellies, and, by evaporation, may be reduced to a substance closely resembling gum. Sugar exists abundantly in the juice and fruits of plants; but in seeds, it is formed chiefly during their germination. It is soluble both in hot and cold water, and also in alcohol. Other ingredients, as gluten, albumen, and oil, are found in particular seeds.

Of the "proximate principles" which contribute to vegetable nutrition, chemists have attempted to ascertain not only the elements, but the proportions in which they enter into the several compounds. All agree in making these elements to consist chiefly of carbon, hydrogen, and oxygen, and in some instances also of nitrogen; but the proportions assigned by different analysts for the same substance differ scarcely less than those allotted for the composition of the different substances.

From the ultimate analysis of these substances, we derive little aid in explaining the chemical changes which they undergo; and we must therefore recur to other modes of accounting for those alterations in their sensible qualities which germinating seeds exhibit. In the germination then of many seeds, the hard and insipid albumen is gradually reduced to a milky form, and acquires a sweetish taste; while the organized parts become, at the same time, softened and expanded, and prepared to take on those actions, and exhibit those specific forms, which constitute the development of the embryo. Now, the only agents which act simultaneously on the several parts of the seed, when this development occurs, are water, heat, and air; and, as far as we have been able to trace their operation, the changes produced in the seed must therefore arise, more or less, from the action of heat and moisture; or from the loss of carbon; or from some specific agency exerted directly by the oxygen gas of the air; or arising indirectly out of its conversion into carbonic gas; or from the combined operation of these several agents. Let us then examine their operation, both separately and conjointly, and try to discover the share which each exerts in the production of these changes.

It has been shown that neither heat alone, nor moisture alone, nor both united, are able to produce the development of the seed; but that these agents contribute to bring it into a proper state for being acted on by the air. In what manner, then, does the air act on the germinating seed? No direct effect can be ascribed to its nitrogen; for that gas neither suffers nor produces change in germination, and the process goes on perfectly well either in pure oxygen gas, or in gaseous mixtures which contain no portion of azote.

But oxygen gas is essential to germination, and by that process is uniformly converted into carbonic acid gas. This disappearance of oxygen has led to the belief, that while a part of it was converted into carbonic gas, another portion actually combined with the seed, and contributed to its development. But it is well ascertained, that oxygen gas, by its conversion into carbonic acid, suffers no change of volume; and as the bulk of that acid gas, produced in germination, equals exactly that of the oxygen which has disappeared, it follows, that no portion of the oxygen, lost by the air, combines with the seed, but really exists exterior to it in the form of carbonic acid gas. The only other source from which the seed, in the experiments referred

able could derive oxygen, is from the decomposition of water; it germinating seeds, says M. de Saussure, emit neither hydrogen nor oxygen, but only convert the oxygen gas of the air into an equal bulk of carbonic acid gas. It seems therefore, to be quite certain that the changes produced in the germinating seed cannot arise from the combination of oxygen.

Neither, in conformity to the opinion held by some, that the mucilage of the seed becomes sugar by losing a part of its carbon, can we ascribe the change of quality in the seed to the loss of that substance; for the portion of the carbon given off by the germinating seed is exceedingly small, and we have no evidence that it is afforded by its mucilaginous matter. The chemical change, by which mucilage is converted into sugar, takes place also in circumstances where there is no reason to ascribe it to the loss of carbon.

That the combined action of heat and moisture will produce on the fecula of seeds changes analogous to those which occur in germination, has been long known. Dr. Irvine long since remarked, that the farinaceous matter of seeds could thus be rendered sweet. Hence, distillers, says he, do not mix not only grain imperfectly malted, but raw meal, with their malt; and the whole being then mixed with water, and submitted to distillation, becomes sweet, and forms wines and spirits. M. Fourcroy and M. Vauquelin obtained sugar and alcohol from bruised unmalted barley, by the combined use of water and heat; and Dr. Thomson has remarked, that the wort made from raw grain is nearly as sweet as from that which has been malted. In the experiments of M. Kirchoff and others, starch was converted into sugar by thirty-six hours boiling in four times its weight of water; and, from some later trials, it would seem that water-dust and other vegetable matters, as linen rags, may, by a similar process, be made to experience a like conversion. From these experiments we learn, that the conversion of the fecula of seeds into sugar is an operation purely chemical, effected by the combined and continued action of heat and water; and since, in germination, the albumen of the seed is made to undergo a similar chemical change, under the varied operation of the same agents, may we not presume that it is accomplished in a manner somewhat similar?

When, by germination, the albumen of the seed has been changed from a solid and tasteless, to a fluid and sweet substance, it is brought into a condition fit for the nutrition of the embryo. For this purpose, it is taken up, absorbed from the cells in which it had been deposited, and conveyed to the neck of the embryo, where a part of it is carried downward to feed the radicle, and another part upward to nourish the plumule.

Connected with the structure that determines the course which the nutrient matter takes on reaching the neck of the embryo, appears to be that tendency in the plumule and radicle to pursue opposite directions, in whatever position or circumstances the seed be placed to grow. These tendencies have been ascribed to the action of light on the same, and of earth on the radicle: but the radicle equally ascends, although no earth be present, and the plumule descends, although light be excluded. Others have attributed the descent of the radicle to the greater weight of its sap, and the ascent of the plumule to the lighter condition of that sap: but there is no evidence that, in these parts respectively, any such difference of sap exists. More lately, it has been supposed that gravitation acted in causing the descent of the radicle; and attempts have been made to counteract this by force, by keeping seeds, during their evolution, in continued motion on vertical or horizontal wheels: but the results obtained seem only to prove, that, in such circumstances, the radicle and plumule pursue, as usual, opposite directions, without affording any reason, why, in natural

growth, the one always rises and the other descends. It is worthy of remark, that this tendency to descend exists only in the primary radicle or tap-root: for the lateral shoots which it puts forth extend themselves, says Du Hamel, nearly horizontally. In like manner, the rootlets that spring from the extremity of a cutting descend perpendicularly, while those that issue from its sides proceed horizontally. We may observe a corresponding peculiarity in the plumule and its productions. It is very singular, continues this writer, that a tree which springs from a seed raises its stem very straight: it is the same with a cutting taken from a straight stem; but a cutting taken from a lateral branch, or the bent shoot of a tree, bends much in its growth, especially if its wood be of a hard nature.

Both the radicle and plumule, as they receive nutriment, increase in all their dimensions; that is, both in length and breadth. The elongation of the radicle, according to Du Hamel, is produced only by the addition of new matter to its extremity, an opinion which the observations of Mr. Knight confirm. In the more succulent plumule, Du Hamel has shown, by satisfactory experiments, that elongation is produced by an extension of parts already formed, as well as by the addition of new particles; but this extension is not observed when the new parts have acquired a certain degree of hardness. In their diametral growth, it is probable that both the radicle and plumule experience, in their tender state, some degree of expansion from the motion of their contained fluids, as well as from the addition of new matter to their exterior surface.

In this brief account of germination, we have supposed the process to be carried on in closed glass vessels, in which the progress of evolution can be observed, the agents concerned in carrying it on made known, and their action, to a certain extent, appreciated. In such vessels the development of the seed can be continued until all the nutrient matter is exhausted, and the organized parts assume their peculiar forms, and execute their appropriate functions. If indeed water and air be duly supplied, the seeds of various herbs will grow and produce flowers and fruits without coming in contact with earth, as M. Bonnet ascertained; and in other experiments of Du Hamel, the seeds of different trees, which had been made to germinate on wet sponges, and had their roots afterwards set in bottles so as to be in contact with water, continued to vegetate for several years, and produced annually new leaves, bark, and wood, by the aid of water alone; so that, "without attempting to explain how the parts of this fluid become solid, it is certain," says this excellent writer, "that water is able to furnish the nourishment necessary to plants." For a description of the daily appearances exhibited in the evolution of several kinds of seeds, we must refer to our former article.

In ordinary germination, however, by the time the nutrient matter of the seed is exhausted, the radicle has sent forth rootlets through the soil, which at once serve to fix the plant in its place, and to draw from the earth fresh materials to sustain its growth. These materials, as will afterwards be shown, undergo certain changes in the young leaves, which have now sprung forth from the plumule, and in part execute the function of cotyledons. The cotyledons, if they have risen above the surface, now fade and fall; if they have remained beneath it, they decay and perish. In addition to water, heat, and air, the only agents required to carry on the germination of the seed, light now becomes necessary to give perfection to the plant; and its operation in bestowing colour and other peculiar properties on plants will be more particularly noticed hereafter. In this account of germination, we have given attention chiefly to the physical phenomena which it exhibits, reserving what we have to say of the seed, as a living body, to another occasion.

The plant, like the seed from which it sprang, is constructed of two elementary systems, denominated vessels and cells. For a detailed account of these, and of the opinions held concerning them, we must refer to our former article. By these systems, variously blended and combined, the several textures, denominated Cuticle, Bark, Wood, and Pith, are composed. The structure of these textures, as they occur in different varieties of herbs and trees, has likewise been described in that article.

Though the vessels of plants differ in form and structure, yet, with regard to use, they appear to be but of two kinds; those, namely, which receive and convey the common sap or lymph, and are named, therefore, Sap or Lymphatic Vessels; and those which contain and convey the juices proper to each species of plant, and are therefore denominated Proper Vessels. In trees, the sap-vessels are found chiefly in the wood, and the proper vessels in the bark; but in many herbs and in palms, both kinds of vessels are associated together through the entire stem. Whether they occupy distinct places in the vegetable, or are associated together, their functions are respectively the same; the sap-vessels being always employed in raising the sap upward, and the proper vessels in conducting its descent.

In all parts of plants, the vessels are in contact with cells, which serve sometimes the purpose of a connecting medium; sometimes to fill up vacuities or augment the bulk of parts; and sometimes as receptacles for various secretions. Between the vessels and cells a vascular communication exists, so that matter deposited at one time in the cells of the plant, may, at another, be taken up and again mixed with the fluids, as occurs in the germinating seed; and these functions of internal secretion and absorption seem to be performed, in plants, as well as in seeds, by the alternate exercise of the same vessels, acting at different times and under different circumstances.

Although, as we have seen, plants not only grow, but produce flowers and fruits without the aid of soil, yet, in ordinary circumstances, they draw the materials of their food from the earth. In an inquiry, therefore, into the nutrition and growth of plants, we have to consider the nature and properties of soils, which afford them habitation and nutriment—the absorption of this nutriment and its conveyance through the vessels—the changes of quality which it experiences in its course, so as to fit it for nutrition—the agents required to effect these changes, and the mode in which they act—and, lastly, the manner in which this nutrient matter, after having undergone its destined changes, is applied to nourish and augment the plant.

The soils in which plants grow are composed of organized and inorganic matters in various proportions. Of inorganic substances, the earths which prevail most are silica, alumina, and lime. With these earths, magnesia and certain metallic oxides, particularly that of iron, are often to be found. To these we may add alkaline matter, and animal and vegetable substances in different stages of decomposition and mixture. According to the proportions in which these mineral and organized remains are present and blended together, the soil will vary greatly in texture, in its property of retaining heat and moisture, and in its degree of fertility. To say, however, what mixture of substances constitutes the most perfect soil would be very difficult; for not only does climate greatly modify the natural condition of soils, but plants themselves exhibit the greatest diversity of choice or liking in this respect. Hence it is, that the soil and climate best suited to one plant, are ill adapted or unsuited to another; and that every part of the earth's surface, in which heat and moisture sufficient to sustain vegetation

are present, is more or less clothed with its appropriate species of plants.

In considering physiologically the uses which the different ingredients of soils serve in vegetation, we must bear in mind that certain chemical elements seem essential to the constitution of vegetable matter; while others, though present and highly useful, are not so indispensably necessary. Thus, the vegetable substances, gum, starch, and sugar, are composed essentially of oxygen, hydrogen, and carbon; and woody fibre, when freed from all adventitious matter, is found to be composed of the same elements, united nearly in the same proportions. Now, considering woody fibre as the basis of the vegetable organs, and as formed, in germination, directly from the fecula of the seed, and probably the water in which it is dissolved, we may presume, that the elements which thus compose fecula, water and woody fibre, are the true constituents of vegetable matter.

In certain kinds of vegetable matter, which approximate to animal substance, nitrogen is a necessary ingredient. In other kinds, the vegetable substance partakes largely of the earthy materials of the soil. Hence lime and silica are abundant in certain plants; but as such substances can enter plants only in a state of solution, the earths met with in vegetables may not, says De Saussure, depend so much on those which constitute the basis of the soil, as on those held in solution by the water which it contains. Some have even supposed that the earths may be formed or generated in plants by the vegetative process; but the facts alleged in support of this opinion are not sufficiently precise. While, therefore, it is admitted, that earths are carried into plants, and, in certain tribes, enter largely into the composition of some of their textures, we have no evidence that they contribute directly to nutrition, or form an essential element in the composition of vegetable matter. Their use in affording station or habitation to vegetables is sufficiently obvious; and the temperature and moisture of the soil will also depend much on their kinds, proportions, and intermixture.

Together with the earths, chemical analysis shows, that sulphur, phosphorus, some metallic oxides, and particularly alkaline matter, exist in plants. Certain saline substances seem indeed necessary to vegetation. Marine plants languish in a soil destitute of common salt; and it is well known, that potash forms a large portion of the incombustible matter of land vegetables, and is especially abundant in the leaves. De Saussure found phosphate of lime in every plant he analysed. Certain plants thrive well only in soils containing nitrates of lime or potash; and sulphate of lime or gypsum accelerated much the growth of lucerne and trefoil. These saline ingredients are highly useful, and the alkalis, in particular, seem necessary to the due perfection of the vegetative process; but, as the elements of these substances do not form a necessary constituent of the vegetable fibre, they cannot be considered as an essential part of the food of plants. Perhaps they may be regarded as condiments which aid in the process of assimilation; and, as will appear, they are otherwise highly useful in the vegetable economy.

With regard to the organized remains, which form so large a portion of the most fertile soils, they are not only soluble in water, like the other ingredients, but are composed of the same elements as vegetable substance. M. de Saussure found pure vegetable mould to yield, by distillation, products similar to those of the undecayed wood from which it had been formed, differing chiefly from it by containing a larger proportion of charcoal and some azote. Water dissolved a portion of this mould; and when deprived of this soluble portion, the residue, though unaltered in appearance, did not support the growth of plants so well as before. The part thus dissolved by water exhibited the properties of extractive,—a principle found in the sap, and

especially in the bark of plants. Hence it appears, that decomposed vegetable matter, and the same may be said of animal remains, is not only conveyed into plants, but, being formed of the same elements as the living plant, may be received to furnish materials necessary to its growth.

But in all soils adapted to vegetation, water is a necessary ingredient, whether it be regarded merely as a vehicle for the conveyance of other substances, or as itself forming a portion of the food of plants. To the experiments of Van Elmont, Boyle, Bonnet, Du Hamel, and Braconnot, tending to prove that water alone affords nutriment to plants, it has been objected, that the water which they employed, though apparently pure, held in solution both earthy and saline matters; but, unless we suppose these matters convertible into oxygen, hydrogen, and carbon—the only essential elements of vegetables—the presence of these earths and salts in the water could not supply the elements required for the production of vegetable matter. If, on the other hand, with Du Hamel and De Saussure, we suppose water, though not decomposed, to lose its fluidity, and become fixed in vegetables, then we have at once two of the elements of woody fibre, combined in that proportion which the experiments of Gay, Lussac and Thenard exhibit them as holding in the composition of vegetable substance.

Of the source of the other element, carbon, it is not less difficult to speak. It is an ingredient of vegetable mould, and of the carbonic acid carried into plants with the sap, either in a free state, or in combination with alkaline matter; so that, by these means, it is pretty largely supplied to plants. On the other hand, M. Hassenfratz endeavoured to show, that plants which vegetated in the open air by the aid of pure water, yielded, on analysis, less carbon than the seeds or bulbs from which they had sprung. M. de Saussure obtained similar results when he analysed plants that had grown in pure water, and in a place weakly illuminated; but when they grew under the direct influence of light, the proportion of carbon in the plant nearly doubled that in the seed. This carbon he supposes to be derived from that decomposition of carbonic acid which is carried on by the leaves of plants growing in sunshine. Assuming this to be the fact, we may perhaps regard the carbon thus deposited in the leaf as contributing rather to the formation of the inflammable products of the plant, than undergoing assimilation, and being applied to the production of new vegetable substance. And hence, as will be shown, plants which grow in darkness not only contain less carbon than those which grow in light, but are at the same time destitute of those inflammable ingredients which plants growing in light possess.

Beside the earth and water, the air also has been supposed to furnish food to plants. That plants obtain moisture from the air will not be denied; and in as far as water concerned in vegetable nutrition, the moisture thus obtained may contribute to vegetable growth. But they have so been supposed to derive carbon from the atmosphere, by decomposing its carbonic acid. Since, however, the atmosphere contains less than $\backslash frac\{1\}\{1000\}$th part of carbonic acid, the portion of that gas decomposed by plants in the open air must, even in sunshine, be necessarily very small, and the quantity of carbon thus obtained is probably much exceeded by that continually given off by plants to unite with the oxygen gas of the atmosphere, through every period of active vegetation. M. de Crell, indeed, deeming the carbon that could be afforded by the atmosphere insufficient to account for the addition of that substance which plants receive during their growth, was led to suppose they possessed the power of forming carbon by the aid of water, and light; and M. Braconnot has maintained that vegetables find in pure water every thing necessary for them to assimilate; that mould and manures yield no nutriment; and that earths, alkalies, metals, sulphur, phosphorus, and

charcoal, are developed from water, by the organic powers of plants assisted by solar light. Vegetable Physiology

"But the experiments," says Sir Humphry Davy, "in which it is said that alkalies, metallic oxides, and earths, may be formed from air and water alone in processes of vegetation, have been always made in an inconclusive manner; for distilled water may contain both saline and metallic impregnations; and the free atmosphere almost constantly holds in mechanical suspension solid substances of various kinds. The conclusions of M. Braconnot," he adds, "are rendered of little avail in consequence of these circumstances. In the only case of vegetation in which the free atmosphere, in his experiments, was excluded, the seeds grew in white sand, which is stated to have been purified by washing in muriatic acid; but such a process was insufficient to deprive it of substances which might afford carbon, or various inflammable matters."

ART. II.—Of the Course of the Sap, and the Causes of its Motion.

In the warmer regions of the earth, the sap flows, in certain plants, through the whole year; but in more temperate climes the functions of vegetables are suspended during the winter season. Early in spring, however, it begins to rise in trees, and continues daily to ascend till it reaches the extremities of the branches. This sap is absorbed from the soil by the extremities of the capillary rootlets, or the spongioles, as they are called, and conveyed upwards through the vessels of the root to the trunk. In its ascent it rises only through the wood; for, at this early period, no sap is found in the bark, nor between it and the wood, nor in the pith. This rise of the sap occurs before the buds have shot forth into leaves; and as no outlet for its escape by transpiration then exists, it rises or falls in the vessels in which it is contained according to the temperature of the atmosphere. If at this period of its flow, its course, in certain trees, be intercepted, by piercing the vessels of the trunk in any part, it issues forth, and may be collected for examination. In this way the vine, the birch, and sugar-maple, yield sap, or bleed, as it is called, very abundantly. They bleed also from the extremity of a cut branch, if the experiment be made sufficiently late in the season, but still before the appearance of the leaves.

Early in February, before the sap began to flow, Dr. Walker made several incisions, at different heights, in a birch tree, in order to observe its motion. No sap was visible at the lowest incision in the trunk till the temperature of the atmosphere rose to $46^\{\backslash circ\}$ in the shade; after which, as the temperature augmented, the sap continued daily to rise. When the highest incision in the trunk, at the height of thirty feet, bled, the thermometer was at $52^\{\backslash circ\}$; and when the tree bled, not only from the incisions in its trunk, but from every cut extremity of its branches, it was at $56^\{\backslash circ\}$. During the whole experiment, when the temperature was nearly the same, the sap continued nearly stationary, rising again, as the temperature rose, just like the fluid in a thermometer. To the cut extremity of a vine branch, Dr. Hales, in the bleeding season, cemented long glass tubes, so that he could readily observe the movements of the sap. Into these tubes it would rise many feet through the morning after the sun was up; but while in this rising state, if there was a cold wind, or the sun was clouded, the sap would immediately subside, at the rate of an inch in a minute, for several inches; but as soon as the sunbeams broke out again, the sap would immediately return to its rising state, just as any liquor in a thermometer rises and falls with the alternations of heat and cold.

To ascertain the force and velocity of the sap's motion at this season, Dr. Hales made many experiments. He found it to rise in glass tubes, at the rate, sometimes, of an inch in three minutes, and to attain the height of more than

Vegetable twenty feet. In other experiments, it exerted a force sufficient to sustain a column of mercury at the height of thirty-eight inches; a force, says he, five times greater than that of the blood in the crural artery of a horse. In the chief bleeding season, the sap continued to rise by night and by day, but more in the day, and most of all in the greatest heat of the day; and when the sun shone hot upon the vine, a continued series of air-bubbles rose through the sap, so as to make a large froth on its top.

Such are the phenomena exhibited by the rising sap before the appearance of the leaves: when they have shot forth, a great change is observed in its movements. It still, however, continues to rise through the trunk; but if the wood be now pierced, none of it flows out, as it did in the bleeding season. In the excellent experiment of Dr. Walker, already referred to, the birch tree continued to bleed from the 5th of March to the 24th of April, on which day it bled from every incision in its trunk, and every cut extremity of its branches. On the 30th of April, vernal budding began, and the young leaves shot forth. As they advanced, the bleeding gradually lessened, till at length, on the 10th of May, when the leaves were fully expanded, all the incisions, says Dr. Walker, which had yielded sap so freely, were everywhere dry; and this, not from evaporation by the leaves, but from a general diffusion of the sap from the wood through the bark at that season. In conformity with these observations, Dr. Hales remarks, that, after the appearance of the leaves, the bark, which was before dry, and adhered to the wood, becomes lubricated with sap, and easily separates. Even after the bark has thus been brought to separate from the wood in a young tree full of sap, if all the leaves, says Du Hamel, be stripped off, the bark, in two days, will again adhere to the wood, and continue to do so through the winter. These facts distinctly prove, that after the leaves have sprung forth, the sap of plants is no longer confined to the wood, but finds its way into the bark; and we have next to trace its route into that texture.

M. De la Baisse and M. Bonnet traced the sap of plants from the extremities of the roots into the leaves and flowers; and when the plants were set in coloured liquors, the fluid was seen to pass from the vessels of the leaf into its cellular tissue, and the bark of the petiole afterwards to become tinged. The communication thus established between the wood and the bark, M. Bonnet considered to occur in the extreme ramifications of the leaf, where, as he supposed, the ligneous and cortical vessels mutually anastomose. In a plant of Euphorbia, set in a coloured liquor, Dr. Darwin observed the fluid to run along the inner ring of vessels in the petiole to the upper surface of the leaf; while on its under side, a white fluid was seen to return from the extremities of the same leaf, and to descend, by the exterior ring of vessels in the petiole, into the bark. In similar experiments on the apple branch, Mr. Knight followed the returning fluid through the bark, by the vessels of which it seemed to be conveyed to the roots. These facts show that the sap, which is observed in the bark, after the leaves have sprung forth, gets into that texture by passing through those organs.

The leaves, which thus form the organs of communication between the wood and the bark, not only vary the course of the sap, but greatly influence its motion. Before their appearance, no natural outlet for its escape existed, and it therefore rose, continued stationary, or fell in the same vessels, chiefly according to variations of temperature. But when the leaves are developed, a large portion of the sap, in its passage through them, is thrown off by transpiration, and the remainder is conveyed into the bark; so that, by these means, the vessels of the tree are emptied, and put into a condition to attract fresh portions of fluid. Hales accordingly found that amputated branches of trees, which

were furnished with leaves, and set in glass tubes of water, were attracted from fifteen to thirty ounces of water in the course of the day; while similar branches, from which the leaves had been stripped off, imbibed, in the same time and circumstances, not more than one ounce. In like manner, a growing vine, which was perspiring abundantly by its leaves, ceased at once to yield sap from its stem, when cut over below the leaves. He found also that amputated branches, when plunged in water, imbibed from the small end to the great end as well as in the opposite direction; that they imbibed also when deprived of their bark, but not when stripped of their leaves; and that they would imbibe water from their small cut extremity, while still attached to the trunk. Hence it appears, that, after the period of vernal, the flow of the sap is promoted chiefly by perspiration from the leaves; and, therefore, if the leaves be removed, or their perspiration counteracted by a cold and humid atmosphere, then, as Hales found by experiment, the attraction of fluid by the sap-vessels is proportionally diminished.

To find the force and velocity with which the sap moved in this more advanced stage of vegetation, Dr. Hales cemented branches of trees, furnished with leaves, in glass tubes filled with water, and then set the lower end of the tube in a vessel of mercury. As the water was attracted by the branch, the mercury rose into the tube, in one instance, to the height of twelve inches in seven minutes. When the mercury had reached its greatest height, it would hold to that height for several hours in a warm sunshine, which favoured perspiration from the leaves; but as the sun declined or set, perspiration decreased, and the mercury ceased to rise. So great at this season is the attractive force of the leaves, that if a notch be cut in the trunk of a tree through which the sap is rapidly flowing, yet will the notch remain dry; "because," says Hales, "the attraction of the perspiring leaves is much greater than the force of trusion from the column of water." By other experiments he ascertained, that, when once the motions of the sap have been brought under the dominion of the leaves, the sap-vessels of the root no longer possess the same power of forcing the sap upward, as they did in the bleeding season; but so long as the leaves throw off the sap, the roots more or less abundantly attract fresh supplies of it from the earth.

In these various motions of the sap, both before and after the bleeding season, Dr. Hales ascertained that the tree underwent no variation in its dimensions; yet, whenever it rained, the stem very sensibly dilated, and when the weather again became dry, it subsided as much. "This shows," he adds, "that the sap, in all stages of vegetation, is confined in its proper vessels, and does not confusedly pervade every interstice of the stem, as the rain does, and thereby dilate it." Du Hamel also noticed this alternate augmentation and diminution in the size of trees, under the different states of a humid and dry atmosphere.

Beside this perpendicular ascent, there is also, in certain circumstances, a lateral motion of the sap. Dr. Hales cut four large gaps in the branches of different trees, at several inches distance from each other. The cuts were carried down to the pith, and opposed, in position, to the four points of the compass. If the cut branches were then amputated and immersed in water, they imbibed that fluid by their extremities, but not so abundantly as before, and continued to give it off freely by their leaves; if they remained attached to the tree, after such gaps were made in them, both the leaves and fruit of the branch flourished as well as those on other branches of the same tree; proving, says Hales, a very free lateral passage of the sap, where the direct passage had been several times intercepted. In these gaps, no moisture could, at any time, be either seen or felt, notwithstanding much fluid was passing by, because the stems, above the gaps, was in a strongly attracting state to supply the great perspiration of the leaves. Mr. Knight made similar

incisions on the opposite sides of apple branches during the winter season; yet, through these branches, the sap flowed in spring, and pushed forth the buds as usual.

From the facts stated above, it appears, that, before the period of vernal, temperature is the chief agent in promoting the flow of the sap; and that, after that period, its progress is aided principally by perspiration from the leaves. There must, however, exist in the plant itself some condition or structure which favours the operation of these agents. At different periods, different causes have been assigned for the ascent of the sap. It has been supposed to exist in the state of vapour, and its ascent has been ascribed to its levity; others have attributed its rise to some imagined action of the spiral vessels; others to fermentation, or to the mechanism of valves; and others to a power of contraction and dilation in the vessels, or to capillary attraction. Of these alleged causes, the two last alone deserve particular notice. This contractile power, derived from a vital source, is not necessary to the motion of the sap in plants, seems certain from the fact of the ready transmission of fluids through the vegetables. Even the dissevered particles of vegetables, as the ashes of wood, were found by Hales capable of tracing water with a force nearly equal to living organic structures. From these and other facts, he considered the rise of the sap to be produced by capillary attraction, and by temperature, and especially by perspiration from the leaves.

The force of capillary attraction, when thus aided by evaporation, is strikingly illustrated by an experiment of Professor Leslie. He found that the attractive force, exerted by the very fine pores of a thin hollow ball of earthenware, from which water was continually evaporating, was more than sufficient to support a load of mercury, in a tube attached to the ball, equal to that of 400 inches of water, or a column of thirty-four feet of that fluid. He estimates the diameters of the pores in the ball at the 10,000th part of an inch, and supposes the pores in the leaves of plants to possess nearly the same dimensions.

Although capillary attraction, when aided by perspiration from the leaves, may exert great influence over the motion of the sap, it is yet probable that some power or property, inherent in the vessel as a living organ, assists its action. The direct effect of heat in promoting the flow of the sap in the bleeding season, and of cold in retarding it, seems to be more connected with some living property in the vessels, than with their powers as simple capillary tubes. If this heat be supposed to dilate the vessels, it ought to check capillary action, and cold, by diminishing their diameters, ought to increase it; but the results afforded are exactly the reverse of these. "If a capillary tube," says D. Thomson, "be taken of such a bore that a fluid will rise in it six inches; and if, after the fluid has risen to its greatest height, the tube be broken short three inches from the bottom, none of the liquid in the under half flows over.— If we cut a plant, the Euphorbia peplis, for instance, into places, so as to separate a portion of the stem from the rest, the milky juice of that plant flows out at both ends so completely, that if afterwards we cut the portion of the stem in the middle, no juice whatever appears. Now, the diameter of these vessels is so small, that, if it were to continue unaltered, the capillary attraction would be more than sufficient to retain their contents, and consequently not a drop would flow out. Since, however, the whole liquid escapes, it must be driven out forcibly, and consequently the vessels must contract." From similar experiments, Du Huel inferred that the proper juice is forced out by a contraction of the vessels that contain it.

Mr. Petit Thouars believes that the expansion of the buds at the extremities of the branches in spring is the first cause of the movement in the sap. He ascertained, by experiment, that the sap begins to move at the extremities

of the branches before it stirs at the roots. Dutrochet Vegetable Physiology brings forward his theory of galvanic currents to account for the flow of the sap. The fluids of plants, he says, are denser than those which surround them, and hence a galvanic action, or endosmosis, takes place, by means of which the water of the soil is propelled into their system. These two theories may be combined, and we may suppose that, when the buds are developed in spring, the fluid which they contain increases in density by evaporation; thus leading to a flow of the thinner fluids upwards by endosmosis, the motion being propagated through the whole plant down to the roots which attract the fluid from the soil.

From all that has been advanced relative to the flow of the sap, we may conclude that temperature, the development of buds and leaves, and the exhalation from them, endosmosis, and probably some vital actions, may all conspire to promote this important vegetable function.

In addition to the general circulation of sap, there are also special motions of it, confined to particular parts, and partaking more of a vital nature, to which the names of rotation and cyclosis have been given. The first of these motions is met with in plants of low organization, but is not confined entirely to flowerless or cellular plants; while the latter occurs only in plants of a higher organization, although not limited exclusively to flowering species.

Rotation has been noticed chiefly in the genera Chara and Nitella, and consists of currents setting from joint to joint, flowing down one side, generally that next the axis, and returning up the other, without any membrane intervening to separate the opposing currents, and the motion in each cell being independent of that in the cells next to it. The motion occasionally intermits, and is rendered obvious by green granules of starchy or oily matter, which float in the fluid of the cells, and are carried about by the currents. Some interesting observations have been made on these peculiar vital movements by Treviranus, Amici, Solty, Varley, and Slack, in the Annales des Sciences Naturelles, and the Transactions of the Society of Arts.

Cyclosis was discovered by Professor Schultz of Berlin in 1820, and consists of the motion of a fluid called latex, analogous, in many respects, to the blood of animals, and contained in peculiar vessels, to which the name of cinechyma or laticiferous vessels has been given. These vessels consist of branched anastomosing tubes, lying in no definite position with respect to the rest of the tissue, and so thin and capillary in the young state as to be scarcely visible. They are expanded and contracted at intervals, and their sides are not parallel as in other vessels. They exist often in connection with spiral vessels. The latex is sometimes transparent and colourless, but in many cases it is opaque, and either milk-white, yellow, red, orange, or brown. Its colour is due to the presence of numerous minute globules, which are constantly agitated as if by a spontaneous motion, and appear to be alternately attracted and repelled by each other. The latex is looked upon as the proper juice of the plant. On exposure to the air it separates into a tenacious coagulum and a thin serum. The motion of the latex, or cyclosis, consists of currents, which are observed traversing the laticiferous vessels, and passing through the lateral connecting tubes or branches into the principal channels. These currents, which are rendered visible by the movement of the globules in the latex, follow no determinate course, but seem to be very irregular in their direction, some proceeding up, others down, some to the right, and others to the left; the motion occasionally stopping suddenly and then recommencing. The motion is checked by diminution of temperature, or by a wound in the vessels, and it is completely arrested by a strong electric shock.

Cyclosis has been observed in Ficus elastica, or the In-

Vegetable dian rubber plant, Chelidonium majus or Celandine, Alisma Physiology Plantago, Tradescantia, &c.

For full particulars relative to these singular movements, the reader is referred to M. Schultz's paper, which will speedily be published in the Mémoires de l'Académie des Sciences.

ART. III.—Of the Qualities of the Sap, and the Changes which it undergoes in the Leaves from the agency of the Air.

Having said so much of the motion of the sap, and of the powers by which that motion is accomplished, let us next direct our attention to its qualities. If collected during the bleeding season, it is almost without taste, but sometimes a little sweet. It ordinarily yields, by evaporation, only a little mucilage; but that of the maple of Canada is said, by Du Hamel, to afford nearly 5 per cent. of sugar. The sap of the elm (Ulmus campestris) was examined by M. Vanquelin at successive periods of vegetation. In his first analysis he found 1039 parts of it to consist of

1027.904 water and volatile matter;
9.240 acetate of potass;
1.060 vegetable matter;
0.796 carbonate of lime.

At a later period, the vegetable matter was in greater quantity, and the saline ingredients had diminished; and, at a period still later, these changes were still more evident. Other saps were analysed, and found to possess similar ingredients; and some others in addition. The sap of the beech contained gallic acid and tannin; and that of the birch, sugar, and acetates of alumina, and lime. The sap of the vine, examined by Dr. Prout, resembled river water in appearance and specific gravity, but was sweetish to the taste. It yielded, by evaporation, a minute portion of residuum, consisting of a peculiar vegetable matter and carbonate of lime.

The increase of vegetable matter observed in the sap as the season advances, suggests the idea that it becomes mixed with matter previously deposited in the tree. This idea occurred to Malpighi, who considered the sap, as it rose through the vessels, to be partly deposited in the cells, where it underwent changes which fitted it for supplying the first nutriment to the young buds and tender leaves. A similar opinion was held by Darwin, who also supposed the sweet juices found in certain roots, and in the knots and stems of some of the grasses, to serve the same purpose. From finding the specific gravity of the sap to increase as it rose higher in the tree, and the albumen of a tree, felled in winter, to be heavier than in other seasons, Mr. Knight also supposed a deposition of nutrient matter to be made in the albumen, through the latter part of summer and autumn; so as to be ready to mix with the sap in the following spring, and afford nourishment to the buds and leaves. Hence, we may consider the young bud, like the embryo of the seed, to draw its first nutriment from matter previously secreted, and in part deposited in cells, and afterwards reabsorbed and applied to its destined use.

The bud, thus nourished in its early growth by the ascending sap, is more or less rapidly developed, and the tree soon becomes clothed with leaves. Of the changes in the motion of the sap when vernal occurs, we have already spoken, and have now to describe others which are effected in the qualities of the fluid. The first action of the leaves upon the sap is to throw off a large portion of it. The insensible perspiration of plants has been particularly investigated by Woodward and Hales, and by Bonnet and Guettard. Dr. Woodward found that a sprig of mint, weighing only 27 grains, imbibed, in seventy-seven days, 2558 grains of water: yet its weight was increased only 15 grains; and it must therefore have given off in that time 2543 grains of fluid. Hales calculated that a

sun flower, the area of the surface of which above ground was equal to 5616 square inches, gave off, by perspiration, in a warm dry day, about 20 ounces of fluid. In a warm dry night, without dew, it exhaled only three ounces. When the dew was sensible, there was no perspiration; and when the dew was abundant, or the night wet, then the weight of the plant was increased. The more succulent leaves perspired more than those of firmer texture, and deciduous more than those of evergreens. In plants of the same species, and placed in similar circumstances, perspiration is proportional to the extent of perspiring surface: but in all plants, cold and humidity more or less diminish or entirely suspend this function.

The fluid thus perspired was collected by Hales as it issued from the leaves of various herbs and trees. The liquor in all was very clear, nor could he distinguish any difference in its taste. It had nearly the specific gravity of water, but when exposed to a hot sun it began sooner to putrify. Senefier evaporated the perspired fluid of a vine: the residuum consisted of minute portions of resinous and gummy matter, and of carbonate and sulphate of lime, which ingredients seemed to augment as vegetation proceeded.

By the loss of its more aqueous parts, the proportions of the remaining ingredients of the sap must be much changed. Grew, Malpighi, Du Hamel, and others, have pointed out the great differences produced in the consistence, colour, odour, and taste of the sap during its transmission through the leaves; differences peculiar to each species of plants, and which have obtained for the sap, at this stage of its movement, the appellation of Proper Juice. It is in this proper juice, says Du Hamel, that the narcotic power of the poppy, the corrosive quality of the fig, the diuretic virtue of the fir, and the purgative property of jalap, resides: and even the peculiar products obtained from the sugar cane and maple arise probably from the intermixture of the proper juice with the common sap: whence we may infer that the virtue of plants resides principally in their proper juice.

It is difficult to collect these juices in a pure state. Those which have been examined differ much in their chemical properties. In some of them, mucus is the predominant ingredient, and such juices are generally mild and nutritious. Of the milky juices some are mild, others hot and acrid. From the proper juice of Euphorbia, M. Chaptal obtained, by the agency of chlorine, a white precipitate, consisting of two parts resinous matter and one part woody fibre. The juice of the carica papaya, a tree that grows in Peru, yielded M. Vanquelin a substance very like the fibrine of animal matter. From other juices, gums, resins, turpentine, balsams, tannin, sugar, and various other products, have been obtained: so that, in their sensible qualities, these juices differ as much from each other as they do from the common sap; but all of them appear to contain a substance resembling in character the woody fibre.

That the difference of quality observed in the common sap and proper juices is effected chiefly in the leaves, seems now to be generally admitted. Some part of this difference is doubtless attributable to the concentration which these juices experience from the exhalation of so much water; but a much larger part is to be ascribed to the effects which result from the combined agency of light and air. That air is essential to the vegetative process, and that the leaves of plants more especially act on the air, are positions long since established by decisive evidence; but physiologists are not yet agreed as to the nature and extent of this action, nor, consequently, as to the mode and degree in which it affects the vegetable fluids. This discordance appears to us to have arisen partly from imperfect experiment, and partly also from blending together two actions performed by the leaves, which, in their nature, are

que distinct, and which, though they commonly go on together, may be easily separated, and the peculiar and specific results of each better observed and estimated.

The ordinary circumstances of growth, plants are exposed at the same time to both light and air; and each exert on them peculiar and specific effects. If deprived of air, when in a state of active vegetation, plants not only cease to grow, but soon die; but the exclusion of light is followed by suspension of growth, and still less by death. Hence, in favourable circumstances as to heat and moisture, plants continue to vegetate even through the night; and are frequently seen to grow in situations from which light is wholly excluded. They then lose, however, many of their peculiar and more active properties; but they remain in size, and display their specific forms. Air, therefore, is essential to vegetation; but light, though necessary to the development of certain properties, is not essential to the growth of plants. It will be convenient to consider the functions of the leaves, first, in relation to air; and secondly, in relation to light.

The late Dr. Priestley led the way in pointing out the nature of the changes which the air suffers in vegetation; but from not clearly ascertaining, at that early period, the composition of the air he employed, nor the extent of change it suffered, nor being fully aware of all the circumstances which might vary his results, he arrived at contradictory conclusions as to the effects which vegetation ultimately produces in the air. On the whole, however, he considered the atmosphere, when vitiated by other processes, to be purified by the growth of plants. His illustrious contemporary, Schiele, by previously removing the carbonic acid from the air he employed, found that plants, by their vegetation, either in sunshine or in shade, constantly deteriorated the air. Similar results, as to the deterioration of the air, in ordinary vegetation, were obtained in many of the experiments of Ingenhousz, Senebier, Woodhouse, and De Saussure, and they have since been extended and confirmed by Mr. Ellis in the work already referred to. By these experiments, it is proved that plants, like the seeds from which they have sprung, require, in the atmosphere in which they are to grow, the presence of oxygen gas; that, by their vegetation, they convert this gas into an equal bulk of carbonic acid gas; and that the azotic portion of the air, as well in volume as in composition, remains unaltered.

The seed, in its germination, supplied carbon to unite with the oxygen of the atmosphere, so does the plant yield the element for the same purpose, during its vegetation. Most vegetable substances, either dead or living, solid or fluid when placed in suitable circumstances as to heat and moisture, deteriorate the atmosphere by forming carbonic acid gas; and the experiments of Huber, Senebier, and De Saussure, show, also, that, when all the oxygen of their has been consumed, they still yield carbon to unite with other ingredient, so as to form carburetted azotic gas; or, if hydrogen gas has been employed instead of azote, then carburetted hydrogen is formed. Neither the living state of a plant, nor the presence of oxygen gas, is essential to the separation of carbon from vegetable matter, and we may, therefore, presume that its separation is not so much to any attractive force exerted by the gas employed, as to some spontaneous change in the vegetable compound itself, whereby its carbon is enabled to combine with the gases that surround it, in the order of its affinity for them. In low temperatures, or when the plant is very dry, its functions are more or less completely suspended, and the formation of carbonic acid is then proportionately reduced; while, on the contrary, a vigorous exercise of the vegetative powers gives rise to a corresponding production of that gas. In what state or form the carbon exists, at the moment of its combination with elastic fluids, is not yet known; but it is probably held in solution by water,

and acquires and retains its elastic state, only while in union with a permanently elastic body. It may further be asked, in what part of the plant does this union of its carbon with the oxygen gas of the air take place, and in what manner is it accomplished? In the living plant, it is chiefly by the leaf that carbonic acid is formed. To discover the mode and place in which it occurs, we must take into view not merely the change produced in the air, but the structure of the living organ by which that change is effected; for it is only by combining a strict regard to anatomy with our chemical knowledge, that we can ever hope to arrive at true physiological conclusions, and avoid the crude and fanciful notions that have too often usurped their name.

The leaf, then, is formed of a vascular system, of cellular tissue, and of a cuticular covering that invests it on all sides. To this organ the sap is brought by vessels which spring from the albuminum or wood, and which, after forming several fasciculi in the petiole, proceed to the base of the leaf, and there, by their expansion and distribution, produce a minutely reticulated structure. With these sap-vessels, proper vessels are every where associated, which appear to communicate with the sap-vessels, and to convey into the inner bark the sap which they receive. Grew considered the vessels, which form the reticulations of the leaf, to be of the same size in every part, and never to inosculate or anastomose, except end to end, or mouth to mouth, after they have come to their final distribution. Malpighi, on the other hand, believed them to anastomose in every part; and, in regard to the minutely transverse fasciculi, given off from the longitudinal bundles, he has been followed by Dr. Todd Thomson, who, however, though he considers them as "distinct vessels, uniting with the longitudinal bundles in a singular manner," has never been able to determine "whether there is any opening directly into the longitudinal vessel on which the extremity of the transverse vessel is applied." It is during the course of the sap, through the two orders of vessels in the leaf, that it undergoes those changes in its properties which fit it for nutrition; but whether these changes are effected in the sap-vessels, or the proper vessels, or partly in both, has not yet been determined.

The minute network formed by the vessels is everywhere filled up with cellular tissue, constituting the parenchyma of the leaf. The cells of this tissue contain fluids derived from the neighbouring vessels, and are likewise the seat of the green colouring matter. There must, therefore, be a ready communication between the vessels and cells; and this many have supposed to be accomplished by the medium of pores in the sides both of the cells and vessels. But the best observers represent the cells as close cavities, having no visible communication with each other. Were we even to admit the existence of pores, we should find it difficult to conceive how the contents of the cells could be set in motion and transmitted through their own sides, and those of the vessels also, so as to be conveyed and deposited in the several parts where growth takes place, with all the regularity we actually observe. To us no other means occur of accomplishing these operations, consistently with the integrity of the cellular structure, than the exercise of those alternate functions of secretion and absorption, which, from so many other considerations, we have supposed to be carried on in every vegetating part of the plant.

The cuticular covering of the leaf is the organ through which, under different circumstances, the fluids that are exhaled and absorbed must pass; and through which also both light and air exert their peculiar action on the vegetable juices. The structure of this organ has excited particular attention, and the opinions held concerning it have been detailed in our former article. It is there described as being composed of a proper cuticle, beneath which is a vascular network, the vessels of which, springing from the

Vegetable Physiology larger fasciculi, form at various points an oval ring, from whence go off two or more radiating filaments, which terminate at the cuticle in an oval pore, more or less elongated. It is by the vascular filaments, which thus terminate at the pores of the cuticle, that the exhalation of fluids is held to be performed; and hence it is, that, in different plants, this function is more or less abundantly carried on, in proportion to the number of cuticular pores or stomata; and that plants, and parts of plants, which have but few pores, perspire little, and those that are destitute of pores not at all. Precisely the same coincidence between the number of stomata in leaves, and their power of absorbing fluids, has also been remarked; and M. Decandolle believes that the vascular pores, on the surfaces of the leaves and of porous stems, are the organs by which the functions both of exhalation and absorption are alternately carried on, according to the existing condition of dryness or humidity in the surrounding atmosphere.

Beside the function of exhalation, it has lately been maintained by Dr. Todd Thomson, who has bestowed great attention on the structure of the leaf, that the stomata are the organs by which the function of respiration in plants is performed. According to him, the pore is not a superficial aperture, but "a short cylindrical tube, penetrating completely through the cutis, and terminating in a cul de sac, which is impressed into a vesicle that appears to communicate with the oblong cells immediately beneath the cutis. But although the aperture penetrates the cutis, there is no opening through the epidermis, which, on the contrary, enters into the tubular part of the pore, and lines it throughout."¹ In different leaves, the form of the pore, of its short tube, and of the vesicle beneath, are said to vary; but the cuticle or epidermis dips down and lines the cavity in all. By the funnel-shaped pore above described, the air is said to be admitted into the vesicle situated beneath it; and as this vesicle probably communicates with the cuticular cells, which are in general filled with air, the aqueous contents of the cells that form the parenchyme of the leaf, are thus brought into immediate contact with the atmosphere.²

It can hardly, however, be admitted that the existence of stomata is indispensable to the production of that chemical change in the air which constitutes respiration; for, according to Decandolle and Rudolphi, the petals of flowers, the cuticle of fleshy fruits, the tunics of seeds, the stems and leaves of etiolated plants, most of the lower tribes of vegetables, and all roots and bulbs that grow beneath the soil, are alike destitute of stomata; and yet the united experience of Priestley, Scheele, Ingenhousz, and De Saussure, bears testimony that all these parts are capable of acting on the air like porous leaves. Dr. Thomson indeed states that some of the lower tribes of vegetables, and certain etiolated leaves, are really furnished with stomata: but even granting this to be the case, there still remain other parts of plants, which grow beneath the soil, and yet act upon the air, like porous parts that vegetate on the surface. Without denying, therefore, the probability that stomata may favour the exercise of the respiratory function, we are compelled to admit that surfaces, not yet discovered to have stomata, are capable of producing similar changes in the air.

But whether the air enter the stomata in order to be changed, or is merely changed at the surface of the plant; or whether, by some unknown means, it permeate the cuticle, cells, and vessels, so as to act directly on the fluids they contain; it seems certain that these fluids acquire the properties, which fit them for nutrition and growth, directly through the agency of the air. How, then, does the air act in producing such effects? No specific action can be ascribed to its azotic portion, since that gas is not essential in vegetable respiration; and when present, it neither suffers nor produces any known change. Neither can we suppose

the effects produced to arise from the loss of carbon, for if it were proved that this carbon is extracted directly from the juices contained in the vessels, rather than afforded by the fluids which they exhale, still the quantity given off is too minute to be considered as the cause of such remarkable changes; and it is more reasonable to believe that they proceed rather from something derived from the air, than from anything given off from the plant. They have accordingly been very generally ascribed to the combination of oxygen with the juices of the plant; this oxygen being partly supplied by the external air, and partly by the air contained in the spiral vessels of the plant.

ART. IV.—Of the mode in which the Proper Juice is applied to the Purposes of Growth.

It is only after the common sap has been duly changed in the leaf by the agency of the air that it is rendered fit for the formation of vegetable matter. For this purpose, it descends in the "proper vessels," which in trees are commonly situate in the bark. If at this period, therefore, a circular portion of that texture be cut away, the proper juice is seen to issue from the upper lip of the wound; but this soon ceases, and its accumulation in the vessels then forms an enlargement around that part. Sometimes the proper juices exude, and form concretions of a gummy, saccharine, or resinous nature on the surface of the bark, and sometimes they are effused into the sap-vessels or cells. Where the bark is young and succulent, the juices probably receive some further change in their descent; for such stems act on the air like leaves, and, in some species of plants, which are destitute of leaves, the functions of the leaves are performed by the stem alone. In ordinary trees, however, the bark is unable to form nutrient matter without the aid of the leaves, as an experiment of Hales distinctly shows. He removed circular portions of bark, half an inch in breadth, from a thriving branch of a pear tree, so as to leave several ringlets of bark, three quarters of an inch distant from each other. All these ringlets, except one, had a leaf-bearing bud on them, and all but this one swelled at their bottoms and grew; and the more leaves the bud produced, so much more did the bark on which it grew swell at the bottom, while the leafless ringlet did not swell or increase at all. From facts before stated, it also appears, that a portion of the nutrient matter that descends from the leaves is found in the albumen, as well as in the bark; and this will be still more clearly shown when we come to treat of the regenerating powers of the albumen.

In discoursing on the trunk of trees, it was stated that a layer of bark, called liber, and a circle of wood, named alburnum, are annually formed; and that between this liber and alburnum the new matter that adds to the bulk of the tree is deposited, and becomes organised. Malpighi believed the liber to form the new parts, and that these parts were afterwards converted into wood. Grew also considered the new wood to be formed by the bark, but not the liber to be converted into wood. Hales, on the other hand, supposed the new wood to be formed by the old; while others have held that both bark and wood contribute to form new matter. Are, then, the new layers which are annually added to the tree formed by the bark, by the wood, or by both? By various experiments, which we have not room to detail, M. du Hamel endeavoured to shew, first, that the bark is able to form new wood, and that this power resides not in its outer layers, but in the part called liber; secondly, that the alburnum is also able to produce new wood and bark, but that this power is not possessed by the older and more hardened vessels of the wood.

The process by which new vegetable matter is formed in trees is thus described by the same excellent writer. When a portion of bark has been removed from a tree, a glaze

¹ Thomson's Elements of Botany, vol. i. p. 614.

² Ibid. p. 622, 623.

The gelatinous fluid is first seen to flow from beneath the remaining bark, or, in certain circumstances, from the albumen. It differs in appearance from the proper juice, and was named cambium by Grew. To observe the process more completely, Du Hamel enclosed the stems of young elms and cherry trees, from which portions of bark had been removed, in glass tubes, closed at each end by cement. For a few days, the glass was obscured by vapour, which gradually disappeared; so that it was then easy to see what passed within. At first, a small tubercle appeared beneath the upper lip of the wound, and one still smaller at the lower lip. After this, granules of gelatinous matter issued from the albumen: they were isolated, and had no connection with the tubercles just mentioned; their colour was at first grayish, but, in about twelve days, passed to a greenish tint. These new parts continued to extend through the summit; the tubercle from the upper lip of the wound enlarged greatly, but that below, very little; so that it was principally by the growth from above that the wound was healed. The bark of the cicatrice having been formed by the union of new productions from the upper and middle part of the wound, was very rough, and in some places entirely wanting; but all the regenerated parts duly performed their appropriate functions, and the stems augmented so much as, in some instances, to burst asunder the glass tubes that enclosed them.

While Du Hamel's experiments showed that new wood was not produced from the old, they did not prove that it was produced by liber. Accordingly, with the view of clearing up this matter, Mr. Knight instituted new experiments, by which he was led to adopt the idea, that wood is not the produce of bark, but is formed by matter sent down from the leaves, this matter forming both new wood and new bark.

Du Petit Thouars, from observing, in the Isle of France, what takes place in the formation of the branches of Draecaria, and other monocotyledonous plants, has come to the conclusion, that woody fibres are the prolongation of the buds or germs which are developed. He looks upon buds as a fact fixed embryos, which send roots downwards to form new wood and bark, and shoots upwards to form new branches. According to this theory, the young shoot may be regarded as the plumule, the pith as the cotyledon, and the fibres of the wood and bark as the roots. The cambium he considers as useful in nourishing the bud roots. An important feature in this ingenious theory is, that it is applicable both to exogenous and endogenous plants. Various objections, however, have been urged against it by Decandolle and others, and farther observations are still required to confirm it.

Decandolle does not deny that buds, or the leaves which they produce, have some influence in the formation of wood, but the action, he says, is entirely physiological, and depends on their elaborating the descending sap by which the young wood is nourished. While Du Petit Thouars attributes to the buds the origin of woody fibres, and to albumen and leaves their nutrition, Decandolle thinks that leaves produce the nourishment, and that the fibres are developed by the liber and albumen. These opinions embrace the chief points under dispute, and further elucidation of them is required before we can come to a decided conclusion relative to the origin of wood. Physiologists seem at all events not to be agreed on this, that the formation of wood depends, in some measure, on the action of leaves freely exposed to the sun or bright light. We have before remarked that when the parts of plants have been once formed, they continue permanent, unless removed by accident or disease; that vegetables possess no power of removing decayed organs by internal absorption, as occurs in the animal system; but that their powers of regeneration are confined to the reproduction of parts or organs which have been removed by decay, by accident, or disease.

ART. V.—Of the Changes which the Sap undergoes in the Vegetable Leaves from the agency of Light.

In what has hitherto been said, we have supposed the functions of nutrition and growth in plants to be carried on, as in seeds, by the combined agency of water, heat, and air, without the access of light. Light is injurious to the growth of seeds, by impeding the change of focula into sugar, which is so favourable to germination; and that it also retards the formation of saccharine matter in plants, is proved by many facts familiar to every one in the cultivation of celery and other plants, which, when secluded from light, not only lose their colour, but acquire a mild, and even sweetish taste. If a plant, says M. Aclard, be covered with a glass vessel, it is observed sometimes to change from a sweet to a bitter taste; but if the vessel be opaque, the same plant, in its subsequent growth, will retain its sweetness. In the year 1774, the late Professor Robison observed, that tansy, mint, and other plants, which had grown in a dark coal mine, although they thrived well in darkness, lost their colour, their odour, and their taste. But when they were brought up and set to grow in day-light, their white parts died down, and the stocks then produced the proper plants in their usual dress, and having all their distinguishing properties. When deprived of light, says Dr. Irvine, all plants nearly agree in the qualities of their juices. The most pungent vegetables then grow insipid; the highest flavoured, inodorous; and those of the most variegated colours are of a uniform whiteness. Vegetables which grow in a natural situation, he adds, readily burn when dry; but a vegetable, bred in a dark box, contains nothing inflammable. The results of analysis perfectly accord with these observations; for etiolated plants are found to yield more saccharine matter, carbonic acid and water, and less inflammable matter than those which are green.

From the facts above stated, it appears, that, when deprived of light, plants continue to grow; that the juices which support this growth are then nearly alike in all; and that they acquire their peculiar properties as to colour, odour, taste, and inflammability, only when vegetation proceeds under the direct influence of light. To the agency of the air, therefore, we may ascribe those changes in the vegetable juices, which render them fit for nutrition and growth; while light bestows on them those more obvious and active qualities, on which their colour, odour, savour, and combustibility more immediately depend. The first series of effects resembles those which were exhibited in the germination of seeds, from which process light is excluded; the second series is superadded to the former, and is directly attributable to light. It is on the leaves and succulent stems of plants that light chiefly acts; and it is in those parts that the proper juices, which give colour and peculiar character to plants, more especially reside. Are we then able to trace out the mode in which it produces any of these singular effects?

In all periods of active vegetation, plants uniformly convert the oxygen gas of the atmosphere into an equal bulk of carbonic acid gas; and if, at the same time, they grow in obscurity, they soon lose their green colour and become white. On the other hand, if they be made to grow in impure air under exposure to sunshine, they not only resume their green colour, but occasion a production of oxygen gas. This curious discovery was first made by Priestley. It has since been extended and confirmed by the experiments of Ingenhousz, Senebier, Woodhouse, and Davy; and more lately the subject has been treated with still greater precision by De Saussure. From the united labour of these writers we learn, that air, which has been deprived by animal respiration or combustion, is again, in certain circumstances, rendered pure by the aid of plants; that the

Vegetable Physiology air, which experiences this purification, must contain a portion of carbonic acid, either in an elastic form, or held in solution by water, and that it must be exposed, with the plant confined in it, to the influence of the solar rays; that neither the plant alone, nor light alone, will effect the decomposition of carbonic acid gas, which is accomplished only by their united agency; that only the leaves and other green parts of plants are able, by this decomposition of carbonic acid, to produce oxygen gas; and that the bulk of oxygen produced in these circumstances is exactly equal to that of carbonic gas which has disappeared. Hence, if plants be made to vegetate in a given bulk of atmospheric air, and placed alternately in obscurity and sunshine, carbonic acid is successively formed and decomposed; so that, as De Saussure ascertained, the air suffers no permanent change either in its bulk or composition.

This operation of plants in purifying air was deemed by Priestley a vegetative function carried on by the aid of light; and by Ingenhousz it was ascribed not to vegetation, but to the influence of light combined with a certain state of vegetable structure. As plants grow in obscurity, where they produce no oxygen gas, this operation cannot be deemed essential to vegetation; and, on the other hand, the decomposition of carbonic acid is effected by the combined agency of plants and solar light, in situations and under circumstances where vegetation cannot exist. Thus, if plants be placed in sunshine, they produce oxygen gas either when immersed in vessels of water saturated with carbonic acid, or when confined in pure carbonic gas. So likewise a similar decomposition of this gas is accomplished by plants, with the aid of light, in temperatures many degrees below freezing, and in such a state of mutilation as is incompatible with the proper exercise of their vegetative powers. Hence, in these different instances, the decomposition of carbonic acid is effected by plants without the aid of that oxygen gas, or that degree of heat, or that condition of structure, which are essential to vegetation; while, on the contrary, it occurs only under exposure to light, which, as we have seen, is not necessary to vegetable nutrition and growth.

But the operation of light, which is thus necessary to the decomposition of carbonic acid, is required also to produce the green colour in plants; and exclusion of light, which prevents this decomposition, prevents also the appearance of that colour. The two effects, indeed, are not only accomplished by the same agents, acting in the same circumstances, but, as far as observation extends, are simultaneously performed; whence it may be inferred, that some necessary connection subsists between them; and, could we discover this, it might lead us to an explanation of the green colour of plants.

Senebier observed, with great care, the operation of light in rendering white leaves green. At first, in different places are seen yellow spots, which gradually become deeper, and at length green. These spots multiply, extend, and meet on the face of the leaf, till at last it is rendered entirely green. If part of a white leaf be secluded from light, by covering it with tin foil, that portion continues white, while the other parts become green; or, if a green portion be similarly covered, so as to exclude the light, it gradually loses its green colour, while the neighbouring parts retain it. From these and similar facts, Senebier considered the green colour in plants to be effected by the direct agency of light, independently of vegetation. He remarked, that a singular relation subsists between the parts of green leaves that furnish most oxygen, and those parts of white leaves which first become green; and from the circumstance of carbonic acid being decomposed and its oxygen expelled only when these parts became green, he was led to ascribe the green colour to the retention of the carbon of the decomposed gas, and its deposition in the cellular tissue of the leaf.

Mr. Ellis considers that the deposition of carbon in the manner above stated is not sufficient to change the colour. The less juice of the leaf to a green hue, and supposes that these colours are owing to the predominance of acid or alkaline matter. In support of this theory, he states that alkaline matter is an abundant ingredient in leaves; and the observations of Hales, Du Hamel, Coulomb, Knight, and others, show that carbonic acid is largely carried in with the sap, either in solution or in combination with alkaline matter. With the alkali already present in the leaf, the excess of acid in the sap will readily combine; and according to the proportion in which acid and alkali are present, the leaf will be variously coloured. If the acid abound, as in etiolated leaves, the leaf will be white; but if means can be found to withdraw this acid, and render the alkali predominant, then the green colour will appear. "Now, the decomposition of carbonic acid in plants, by the agency of solar light, seems to be the mean employed by nature to accomplish this purpose; for by this means," says Mr. Ellis, "the acid is not only withdrawn from its combination, and its oxygen expelled, but the alkali at the same instant becomes predominant, and exists, therefore, in a state fitted to exert its specific action on the colourless juices of the plant, which, in consequence, are rendered green. The coloration of the leaf, therefore, is not owing immediately to the decomposition of carbonic acid and expulsion of oxygen, but to the predominance of alkali which that decomposition occasions. Hence, in the earlier stages of the process, we cannot so properly say that the green leaf affords oxygen, as that it becomes green when that gas is expelled; and thus it is that the decomposition of carbonic acid in leaves, by the agency of solar light, gives rise at once to the production of oxygen gas, and the formation of the green colour in plants."

But though light thus appears to be the active cause of colour in plants, yet, as it acts only by rendering alkali predominant in their juices, it follows, that if, from the soil, or in any other manner, a similar predominance of alkali arise, these juices will be rendered green. Hence certain buds and the germs of some seeds, and parts which lie within the bark of certain herbs, exhibit a greenish hue, though perfectly secluded from light; so that it is not to be doubted, as Senebier remarks, that plants, and parts of plants, may become green, although light should not act immediately upon them.

If thus the predominance of alkali occasion a green colour, then deficiency of it, and, still more, excess of acid, should have a contrary effect. Accordingly, when light is excluded from plants, no carbonic acid is decomposed in the leaf, and then its retention and accumulation, by saturating the alkali, subdue the green colour, and give rise to the white appearance observed in etiolated plants. So, likewise, the changes of colour which leaves exhibit in autumn seem to arise from the abundance of acid matter, not, however, occasioned by the absence of light, but developed under the various conditions of decay and decomposition in which they are then placed.

From the facts stated above, with regard to the alternate consumption and production of oxygen gas by growing plants, a question has arisen, whether, on the whole, vegetables purify or deteriorate the atmosphere. We cannot enter further into this question than to observe, that it is not to be decided altogether by experiments made in close vessels of artificial mixtures of air, but demands a comprehensive survey of vegetation in different climes, and a knowledge of the times and circumstances in which the one or other process prevails, or is variously accelerated or retarded. Since the production of oxygen arises solely from the decomposition of carbonic acid in the plant, we must also know the modes and quantities in which that gas is supplied to plants; for the power of plants to produce a

large excess of oxygen, when confined in artificial atmospheres that contain from 7 to 10 per cent. of carbonic acid, does not at all apply to the natural condition of the atmosphere, which contains less than a thousandth part of that gas. We have already remarked, that carbonic acid is largely carried into plants with the rising sap; and various facts seem also to prove, that, when plants are confined in glass vessels containing carbonic acid, that gas enters the leaves in an elastic form. Other gases, as oxygen, hydrogen, and azote, under similar circumstances, also obtain admission and fleshy leaves, in particular, take up, through the high a volume of oxygen greater in bulk than themselves, while, according to De Saussure, they again give out when exposed to a bright sunshine. It is not yet determined in what way elastic fluids thus obtain admission into leaves, and are again expelled from them; but the indiscriminate loss of their entrance and expulsion, at times, too, and under circumstances, in which vegetation is completely suspended; their long retention in a bulk greater than the containing body, and other circumstances, which we have not room to detail, lead to the belief that the phenomena are quite distinct from those which properly constitute vegetation, and are attributable to the conjoined operation of mechanical and chemical causes, aided by a certain condition of structure in the vegetable organs.

Vien, in the experiments of M. de Saussure, plants were made to vegetate in atmospheric air, and placed alternate in sunshine and in darkness, the decomposition of carbonic acid in the former case so exactly balanced its previous formation in the latter, that the air suffered no permanent change either in purity or in volume. Even in sunshine, as the same author remarks, plants growing in close vessels continue to produce carbonic acid, and it is only because they then also decompose it, that they do not permanently vitiate the air: hence, if a substance that attracts the carbonic acid as fast as it is formed be placed in the vessel, the air of the vessel no longer preserves its volume, nor its proportion of oxygen gas.

Professor Burnett recently made some experiments on the changes which growing plants produce on the air, and came to the conclusion, that the production of oxygen and its converse, the formation of carbonic acid, are the unvarying results of two different functions; the former being the process of digestion, the latter of respiration, and both being vegetative functions dependent on vitality. The formation of carbonic acid, he states, is constant by day and by night during the life of the vegetable; it is equally carried on whether the plant is healthy or unhealthy; it is essential to its existence, and for the sustenance of its irritability; for, if deprived of oxygen, and confined in carbonic acid gas, plants, like animals, quickly die. This function, which is performed chiefly by the leaves and petals, though also in a less degree by the stems and roots, is, like the respiration of animals, attended with and marked by the conversion of oxygen into carbonic acid, and is in fact the respiration of plants.

Again, vegetables at certain times, and under certain circumstances, decompose carbonic acid, and renovate the atmosphere, by the restoration of its oxygen. This restoration, however, is dependent not on the respiratory, but on the digestive system. It in part arises from the decomposition of water, but chiefly from the decomposition of carbonic acid, absorbed either in the form of gas or in combination with water. And here again there is a marked analogy between the functions of respiration and digestion in plants and animals; for to both is carbonic acid deleted when breathed, and in both it is invigorating to the digestive system when absorbed as food.

The presence or absence of light appears to have little or no influence on vegetable respiration; but it produces very notable effects in the assimilating powers of vegetables,

by enabling them to separate from the air and water those principles which are necessary for their support, and to liberate such others as may be too abundant in the crude aliment which they absorb. Thus digestion or assimilation tends to increase, respiration to decrease the solid materials of plants.

Mr. Ellis, who has written most ably on the influence which plants exert on the air, agrees with Burnett in thinking that the function of nutrition is entirely distinct from that of respiration; the former process being primarily carried on by the roots, and the matters ultimately rendered fit for assimilation in the leaves. The leaves themselves, he says, apparently exert two opposite actions in the different circumstances of shade and sunshine, or night and day. In the shade and through the night they convert oxygen gas into carbonic acid gas, which is easily detected, because little or no purification of the air is then carried on. Even in sunshine, as De Saussure has shown, plants exert the same power in deteriorating the air; but it is not then so readily discoverable, because the acid gas which is formed is at once decomposed by the direct agency of solar light, and the purity of the air is restored. The former process, by which oxygen gas is converted into carbonic acid gas, he regards as the true respiratory process, essential to nutrition and growth; the latter, as a chemical operation effected by the agency of light, and contributing immediately to the colour, odour, and certain other sensible properties of the plant.

Professor Liebig, who has recently made some observations on this subject, maintains that plants derive their carbon exclusively from the decomposition of carbonic acid, chiefly, or often entirely, supplied by the atmosphere; that they at the same time restore oxygen to the atmosphere; that the decomposition of carbonic acid is arrested by the absence of the light, and then plants appear to produce and evolve carbonic acid; that at night a true chemical process commences in consequence of the action of the oxygen in the air upon the organic substances composing the leaves, blossoms, and fruit; that this is not connected with the life of the vegetable, but is in fact a process of oxidation. He also states, that the humus or fertile ingredient of the soil nourishes plants only by being a lasting source of carbonic acid, which it emits very slowly. The hydrogen which enters into the constitution of plants he thinks is derived from water, and the nitrogen from the decomposition of the ammoniacal salts contained in the soil.

Recent experiments have proved, that the quantity of carbon fixed by a plant exceeds that given off by the process of respiration; and Dr. Daubeny has shown, that if a plant with leaves and stem be confined in the same portion of air, day and night, and duly supplied with carbonic acid during sunshine, it will go on adding to the proportion of oxygen present, so long as it continues healthy; the slight diminution of oxygen and increase of carbonic acid during the night, bearing no considerable ratio to the degree in which the opposite effect occurs during the day. Plants, then, must be regarded as the chief provision employed by nature to maintain the purity of the atmosphere of our globe.

Beside contributing to their colour, light, as we have seen, exerts a direct action on the substances which impart odour, taste, and combustibility to vegetables. These several qualities depend immediately on the oils, volatile and fixed, the resins, gum-resins, balsams, and turpentine, the alkalis and acids, the earthy and saline compounds, and the tannin, extractive, and other principles met with in the proper juices. It is probable that these substances are formed in the leaves and other corresponding structures, chiefly by peculiar secreting organs; but either the functions of the organs, or the products which they yield, appear to experience great modifications and changes from the direct

Vegetable action of light; and hence, if this agent be wholly excluded, Physiology these vegetable products are either sparingly formed, or not at all produced. When formed, they become mixed with the sap in its course through the leaves, and variously change its sensible properties, so as to constitute juices proper to each species of plant. Thus blended with the sap, they are conveyed, more or less abundantly, through the organized parts, and impart to them those properties of colour, odour, taste, &c. observed in the several textures. Sometimes the gummy and resinous matters exude on the surface of the tree, or stagnate in the vessels or cells of the bark and wood, forming those collections and concretions to be found in different parts of those textures. All these ingredients of the proper juice serve, in vegetation, purposes different from those of the mucilage, starch, and sugar, from which the secretion called cambium is derived, and which is more immediately employed in the production or nourishment of new vegetable matter.

In these operations of light on plants, it is probable that the several species of rays that compose the solar beam exert specific but varied actions. The violet rays, or rather the invisible rays associated with them, were observed by Senecier to act most powerfully in producing the green colour of plants; and he likewise ascertained that they act by their own peculiar quality, and not by their heating or illuminating power. This agrees with the acknowledged power of this portion of the solar beam in producing decomposition, since it is through the decomposition of carbonic acid in plants that their green colour is obtained. The experiments of De Saussure also shew, that by this decomposition of that acid gas, not only is oxygen gas expelled from the plant, but the proportion of its carbon increased; but whether this carbon contributes to the formation of any of the more active ingredients of the proper juices, or of those which impart colour, or whether the calorific power of the solar beam acts in their production, nothing yet known enables us to determine.

CHAP. II.—OF THE FUNCTIONS OF THE INDIVIDUAL MEMBERS AND ORGANS OF VEGETABLES.

SECT. I.—Of Buds, and of the Members and Organs produced from them.

ART. I.—Of Buds.

Having thus exhibited a brief outline of the leading facts which constitute vegetation, as exemplified in the nutrition and growth of plants, we have now to notice, in a manner still more brief, the functions of certain parts and organs which serve different uses, and afford various products; and more especially those by which the continuance of their race is maintained. Some plants pass rapidly through the several stages of their existence, and having produced their seeds, fade and die; others continue for one or more years; and many prolong their existence to very distant periods. Even in these latter, the more active organs of vegetation, after producing their fruits and seeds, fade and fall like plants of shorter duration; and when the season adapted to the growth of annuals returns, then also perennials reproduce all the organs necessary to growth and fructification. In ordinary cases, reproduction in annuals is continued only by seeds; in perennials, both by seeds and buds. For an account of the species of buds, and of their formation and structure, we must refer to our former article: at present we can only afford space for a few observations on those varieties of buds which produce branches, leaves, and flowers.

During summer and autumn, when perennial plants add to their bulk by the formation of new layers, they also form new buds on the sides and at the extremities of their

branches. These buds continue to enlarge through the autumn, and in part through the winter; so as to be ready, on the return of spring, to shoot forth, and supply the place of those that annually decay. Some buds chiefly produce wood, others leaves, others flowers, and others both leaves and flowers; and this variety of production may be so modified by culture as to enable us often to substitute one species of bud for another. The wood and leaf buds are the result of vigorous growth, and are primarily of the same structure. As the plant approaches to maturity, or when the vigour of its growth abates, then flower-buds augment in number.

Buds naturally remain attached to the parent tree, and there execute their allotted functions; but they may also be made to grow as individuals, or be transferred, in various modes, to another stock, and perform the same functions on it, as on their proper parent. Though supplied then with sap from a different tree, they retain the power of effecting in that fluid the same changes, and forming with it the same products as they would have produced on their native stock. The bud of the tree, therefore, like the embryo of the seed, must be held to possess individuality of character, and to be capable of producing new individuals perfectly similar to itself. To the embryo of the seed, however, as to every organised body, is assigned certain periods of infancy, maturity, and decay, which may be varied in duration from accidental causes, but can never, beyond certain limits, be changed. What is true of the primary embryo of the seed, is true also of all the buds propagated from it, whether they remain on the parent stock, or are transferred to another. Hence, when the period arrives in which the function of reproduction naturally ceases in the buds of the parent tree, all the buds growing on foreign stocks, indicate the same character of age, and cease to bear fruit; and for the permanent continuance of the species recourse must then be had to a seminal progeny. Mr. Knight has very ingeniously applied these principles to account for that failure in bearing fruit which the oldest and best varieties of trees in the cider districts exhibit. Although grafts from these trees still grow on foreign stocks, yet they do not now yield fruit as formerly, because the trees, from which they have been taken, have outlived the fruit bearing period.

ART. II.—Functions of Branches, Thorns, and Tendrils.

In its progressive growth, the bud, under its various forms, gives origin to branches, with their various appendages of thorns and tendrils, of leaves and flowers. The functions of the branch may be considered as similar to those of the trunk; which indeed it so nearly resembles in structure and growth, that, if cut off and planted, it readily, in many species, puts forth rootlets and buds, and becomes a perfect tree.

The thorns of trees may be regarded generally as abortive branches, which assume their form chiefly from defective nutrition; and hence, as Malpighi observed, they often disappear under higher culture. Sometimes, however, they derive their origin from the degeneration of other organs, as from the stalks of leaves and flowers. They serve as a defence to plants, and protect them from the ravages of animals.

The several varieties of fulcra called claspers and tendrils have the same structure as branches; and, like thorns, sometimes originate from abortive leaves and flowers. Their obvious use is to connect the different parts of a plant with one another for mutual support, or to attach themselves, for the same purpose, to the bodies near them. Sometimes this purpose is assisted by means of a viscous secretion which they yield, and which glues them to neighbouring bodies with much force.

The leaves spring from the buds chiefly of young branches, and by their number, forms, and colour, constitute the chief ornament of trees. If they exhibit less splendour than flowers, they enjoy a longer existence. They furnish to animals a large part of their subsistence, afford them shade and shelter, and spread everywhere beneath us, that "all-refreshing green," on which the eye fatigued and distracted by the glare of other colours, always loves to repose.

We have already discoursed largely of the functions of the leaves as organs of respiration and of transpiration. Besides this transpiring power, the leaves exercise also an absorbent function. From the experiments of Bonnet it appears, that the leaves of many herbs, when laid upon water, absorb equally well by both surfaces, but those of trees only by the under surface: and these facts correspond with the observations of Decandolle and Rudolphi, as to the existence of stomata on these surfaces respectively. The power of the leaves to absorb moisture from the air is also abundantly proved in many of the experiments of Hes and Du Hamel, and more recently by those of Kight. This absorption appears to be carried on by the minute vascular terminations which open at the pores; and is apparently performed by the same vessels, which, at a different time, and under different circumstances of temperature and humidity, execute the function of exhalation.

Beside the aqueous fluids given off by leaves, others are sometimes afforded, which seem to be secreted by peculiar organs. On the leaves of different plants, mucilaginous, scharine, resinous, or oily fluids are sometimes seen; or invisible by the eye, they are often sensible to taste and smell. In some leaves, these secretions appear to proceed from minute glandular organs, seated in the cellular tissue; in others, small follicles in the cuticular texture seem to furnish them. Of the structure of these minute organs little is known, and still less of the mode in which they execute their functions. Of the influence of light on the secreted fluids of the leaf we have already spoken; and also of the mode in which it contributes to the production of its green colour. Bonnet has likewise shown that light exerts a great power over the motion or direction of leaves, of which some notice will be hereafter taken.

So nearly does the root agree in structure with the trunk, that, as Malpighi observes, we may consider it as a production of the trunk beneath the soil. From the principal root or stock proceed the buds that give origin to the primary rootlets; and these give off finer ramifications, at the extremities of which are formed the spongioles or sponges, which are the true absorbents of the root. These fine absorbents take up the nutrient matter from the earth, and convey it to the root, from whence it is sent to the trunk. Where these absorbents extend, the earth is exhausted of its nutrient matter, and not in the neighbourhood of the larger roots. In a severe winter, Du Hamel found these fine rootlets to die, and to be abundantly replaced by others, when the temperature became milder. Not only the root, but the branch of a tree, daily produces rootlets, if it be amputated, and set to grow in water or in soil. The form of the root is much influenced by the texture of the soil. If the soil be easily penetrable, the root descends in the form of a long taproot; but if it be hard and resisting, then the root remains shorter, and divides into lateral branches. Du Hamel remarks that roots extend into that portion of soil which is richest, while the barren parts are nearly destitute of them; so that the qualities of the soil, as well as its texture, exert the greatest influence on their direction and

growth. In regard, however, both to composition and texture, different soils are best suited to different kinds of roots. We have before remarked, that the elongation of roots and rootlets is made by the addition of new matter to their extremities, not, as in succulent branches, by the simple extension of parts already formed. The diametral growth of roots is effected, like that of the trunk, by the formation of new layers between the bark and wood.

Flowers may be regarded not only as the last, but the most elaborated organs of the vegetable system. Whether we contemplate the beauty of their forms, the splendour of their colours, or the delicious fragrance which they breathe around us; or whether, with a physiological eye, we survey the delicacy of their structure, and investigate the peculiar functions which they perform, we cannot but feel the greatest admiration of the skill with which, in a compass so small, and by means apparently so simple, such a series of actions, terminating in results so varied and important, can at once be combined and regulated.

The flower is attached to the plant that bears it by the peduncle, on the extremity of which is placed the cup or calyx, which, in its turn, supports the corolla, and the organs of reproduction. Of these organs, the male parts consist of one or more stamens, formed by the filament, bearing on its top the anther, which contains the fecundating particles, named pollen. The female organs consist of one or more pistils, the style of which bears on its top the stigma, and terminates below in the ovary that contains the rudiments of the seeds. For an account of the structure of these several parts, we refer to our former article.

Various as are the forms, colours, and functions of the several parts of the flower, yet, in structure, they are so similar, that, under a change of circumstances, almost any one part or organ can be made to assume the character of any other. Thus, not only are the petals of the corolla, or the stamens and pistils, sometimes abortive, but, at other times, the stamens become simple filaments or petals, or the petals take the form of stamens. Sometimes, again, the style of the pistil changes into a petal: in other instances, the petal becomes a floral leaf, or the calyx is changed into real leaves. In like manner, as leaves, or their petioles, are sometimes transformed into clasps and tendrils, so the peduncles and petals of the flower now and then exhibit similar transformations. Nor are these transformations confined to leaves and flowers; they extend to the more solid and permanent members of the plant. Thorns, we before remarked, were but abortive branches; and as a branch, by surrounding it with earth, may be made to throw out rootlets instead of buds, so a root, when brought into light and air, will, on the contrary, put forth buds. Even an entire tree may be inverted, and the roots and branches, by being placed in circumstances respectively opposite to their nature, may be made gradually to assume each other's character, and execute each other's functions. These facts demonstrate a great uniformity of structure in all parts of vegetables, and show with what facility modifications of form and of function are induced, by varying the application of those external agents and conditions, concerned in the development of vegetable organization.

1. Colours of Flowers.—The colours of flowers are not less diversified than their forms. They present every variety of tint and every shade of intermixture; and not unfrequently the same flower, at different times, or even at the same time, exhibits great diversity of colour. Grew remarked that no flower has its proper colour in the bud; that many of them are then pale or white; and that the full and proper colour is formed only when they expand. Even after their expansion, the colour of some flowers, as that of the rose, may be made to disappear by excluding it

Vegetable from light, as Sir Humphry Davy remarked; and the flower Physiology of honeysuckle, which continues white while the light is excluded, acquires its red hue on exposure to light and air. It has not however as yet been ascertained what influence air and light exert on the colour of flowers. Ellis believes that, as in the case of leaves, the predominance of air and alkali may be the cause of the different colours of the petals. It is, however, probable that the chemical condition of the juices themselves, or the textures by or through which their colours are reflected or transmitted, are modified by variations of structure in the organs, which altogether escape detection, and become known only in their effects. Du Hamel considered many varieties of colour in the flower to arise from the intermixture of different species and varieties of plants, at the period of fecundation. Poppies and primroses, which grow wild in our fields, are respectively red and yellow; but the same plants, transferred to our gardens, furnish a prodigious number of varieties. If the wild variety of primrose be removed from its natural place, and set to grow among cultivated varieties of the same kind which possess different colours, the seeds obtained will produce many yellow flowers, because the parent plant will have been fecundated by some of its own stamens; but it will also afford other varieties, because some of the seeds will have been produced by fecundation of neighbouring plants. Many of the fine varieties of flowers which florists procure by means of seeds, seem to be thus obtained. They indeed attribute them to some particular infusion with which they have watered their seeds, or to some colouring matter which they have mixed with their soil, or even to some differently coloured bodies which they have presented to their plants, or to a singular good fortune peculiar to themselves. "I have tried without success," says Du Hamel, "these infusions and these mixtures of colours; and I deem it unnecessary to resort to experiment to destroy the other two means." In the ultimate production of colour, modifications may also proceed not only from the texture of the parts, but the configuration of their external surface, as exemplified in the prismatic tints of "mother-of-pearl," which, according to the observations of Sir David Brewster, owe their existence to the configuration of the surface alone. Schubler and Funk, in a memoir published at Tübingen in 1825, have made some interesting observations on the colour of plants. They have divided flowers, in reference to their colours, into two series: 1st, the Xanthic series, or those having yellow for their type, or which are capable of passing into red or white, but never into blue; 2d, the Cyanic series, or those having blue for their type, and capable of passing into red and white, but never into yellow. For an account of these observations, as well as those of Köhler, on the colours prevalent in different natural orders, &c. we must refer the reader to the memoirs themselves, as it is impossible, in the space allotted to us, to give any thing like a correct view of the subject. From numerous experiments made on various leaves and flowers, Dr. Hope was led to the conclusion, that chronogen, or the colourable principle, is not an individual substance, as hitherto supposed, but that there are two distinct principles, one which forms a red compound with acids, which he names erythrogen, and another, which affords a yellow compound with alkalies, which he calls xanthogen. These principles exist sometimes separately, and sometimes together in different plants, or in different parts of the same plant. All green leaves, all white, and all yellow flowers, and white fruits, contain xanthogen alone; while in red and blue flowers, and in the leaves of a few plants which exhibit the former of these tints, the two principles occur together. Light, says Dr. Hope, is indispensable for the production of the green chromule of leaves, but not for the formation of some of the finest tints of flowers and fruits, if essential for any; differences probably connected

with the fact, that the formation of the green colour in leaves is always accompanied, or rather preceded, by the evolution of oxygen gas.

2. Odour of Flowers.—Next to colour, the property in flowers that most strikes our senses is their odour. Other parts of plants, indeed, possess odour; but the finer and more diffusible fragrance that emanates from them proceeds commonly from the flower. Of the peculiar organs in flowers which form and emit odorous particles, but little is known. Their ordinary seat is probably in the corolla, since many flowers, which are wholly destitute of sexual organs, emit their peculiar odours. Of the nature of the odorous matter, all we at present know is, that it is inflammable; and this property, M. de Saussure considers to depend rather on the presence of an essential oil, than on any variety of hydrogen gas. That light is more especially concerned in its production seems probable from the fact, already stated, that the most pungent odours cease to be formed in plants which are kept secluded from light; but are speedily produced in them when restored to its presence. In climates and situations where the sun exerts the greatest influence, plants possess the most exalted odours and the most active inflammable ingredients; but of the mode in which the solar rays act in thus contributing to produce odour and inflammability in plants, little is at present known. Schubler and Köhler have also made observations on the odour of plants, as connected with colour; for which we must refer to the memoirs already mentioned.

3. Savour of Flowers.—Connected with the odour of plants, at least in the mode of its production, is the property which they possess of imparting sensations of taste. This property is more generally distributed through the vegetable, and is of a much less fleeting and diffusive nature than that of odour. Tastes have been regarded either as simple or compound; and of each a great diversity is to be found in plants. Tastes differ also in quality, degree, and duration; are more or less fixed or diffusible; and, in some instances, affect variously the different parts of the organ which receives the impression. Of all these, and some other varieties, Grew has given examples, taken from the savours of the juices found in the wood, bark, and root, or in the leaves, flowers, and fruits of various plants.

In general, however, the roots, and all those parts that are secluded from light, have a taste milder and less intense than others; and, as we before remarked, plants, possessing naturally a hot or bitter taste, become mild, or even sweet, by the exclusion of light, and resume their pungent and acrid qualities if brought into day. Light, therefore, exerts a direct action in the formation of the savours as well as of the odours and colours of plants. The odour and savour are commonly more concentrated in some parts than in others; and when formed in the leaves, they are frequently mixed with the proper juices, and more or less pervade every part of the plant. Although, therefore, the action of light, in the production of colour, odour, and savour, be, in the first instance, local, or confined to those plants, and parts of plants, exposed directly to its influence, yet these properties may afterwards be diffused, by the motions of the fluids, through all parts of the vegetable, including even those buried beneath the soil, and thereby protected from the action of light. From the whole it appears probable, that all the effects, simultaneously produced in plants by the direct agency of light, are in position local, in operation chemical, and in nature entirely distinct from that series of actions accomplished by the air, and which contribute to their evolution, nutrition, and growth.

4. Fecundation of Flowers.—Although in appearance the flower differs so much from the leaf, yet in structure it is very similar; and, for the due performance of its proper functions, not only requires the presence of the same

agents, but acts in the same manner upon them. The necessity of air to the growth of plants, and the conversion of pure part into carbonic acid by the leaves, in every stage of that growth, have already been noticed; and P. Staley, Scheele, and Ingenhousz, showed that flowers, in like manner, require air, and convert its oxygen into carbonic acid. De Saussure has since ascertained that flowers do not develop in atmospheres destitute of oxygen gas; that, in proportion to their bulk, they consume more oxygen than leaves; and that the oxygen which disappears is replaced by an equal volume of carbonic acid gas; so that little or no variation of bulk occurs in the air employed. Like leaves, however, flowers do not produce oxygen gas in sunshine. Under such an increase of temperature, they consume even more oxygen than before; but no trace of this production, either of hydrogen or azote, is discoverable in the atmosphere in which flowers have been made to grow.

Connected immediately with the great consumption of oxygen by flowers, is the high temperature which some of them manifest at the period of fecundation. Lamarck and Senebier observed the flower of Arum cordifolium to impart the sensation of heat to the touch; and to possess, after midday, a temperature 12° higher than that of the surrounding atmosphere. In the Isle of Madagascar, M. Hubert found that the same plant raised the thermometer higher; that the male parts of the flower possessed, in this respect, greater power than the female, so that twelve stamens, placed round the bulb of a thermometer, raised it the moment of bursting, from 70° to 121°; that this power resided in the exterior surface, not within the substance of the organs; and, lastly, that air was necessary to the elevation of temperature, and was rapidly deprived in this process. In confirmation of these facts, De Saussure has since found that double or imperfect flowers consume less oxygen than those which are simple and perfect; that the greatest portion of this gas is consumed at the period of fecundation; and that the stamens, adhering by their base to the receptacle, consumed more than other parts. He further ascertained that the temperature of many flowers rises in proportion to the quantity of oxygen consumed; and, to the rapid combination of oxygen gas with the carbon of the flower, he ascribes, with Senebier, the great rise of temperature which occurs, in certain flowers, at the period of fecundation.

This necessity of air to the development of flowers, then in connection with the chemical changes which it suffers, and the high temperature thence arising at the period of fecundation, points to some peculiar action which it exerts in the exercise of the generative function. Not long after the true nature and use of the sexual organs had been made known by Grew, Dr. Blair, in a learned essay on the Generation of Plants, maintained, that, while the greater part of the ascending sap passed on to the leaf, a portion was also carried to the petals of the flower, and, in its course through them, underwent that change and elaboration which fitted it for forming the pollen, and rendering it the proper means of fecundation. A similar opinion of the use of the corolla was held by Du Hamel. The petals, says he, are organs necessary to fructification. They not only protect the stamens and pistils, but perform the office of leaves, in acting on the fluids of the sexual organs, and perhaps effect in them some important preparation. Dr. Darwin likewise considered the petals to act, by the agency of the air, in elaborating the juices destined to nourish and develop the sexual organs. These opinions ascribe to the less important parts of the flower functions essential to the perfection of the whole, and corresponding in nature with those executed by the leaves, only that "nature," as Grew observes, "hath lapped up the virtue in leaves as in brown paper, but in the flowers as in leaf-gold."

When the organs of reproduction have attained their perfect state, and a suitable condition of the atmosphere prevails, the process of fecundation is accomplished by various modes in different plants. By the agency of the solar rays, aided probably by that high temperature which at this period they derive from the decomposition of the air, the anthers burst and discharge the pollen in the form of a fine dust. This dust, in some instances, falls directly on the stigma of the pistil previously prepared, by the secretion of a viscid matter on its surface, to receive and detain it. In other instances, the pollen is conveyed to the stigma by insects, or by the wind; and in others, its conveyance is accomplished in different modes. When the pollen has been shed, the stamens and petals soon begin to fade and fall; the filament of the pistil likewise fades; but the ovary at its base augments in size, and the pulpy globules, or vesicles, previously formed within it, enlarge, and gradually assume the form and character of the perfect fruit or seed.

The pollen, which occasions these extraordinary changes in the ovarian vesicles, is composed of small particles, enclosed in two coverings, and possessing a different colour, size, and figure, in different plants. These particles are organized, and, when observed in a bright sunshine with a microscope, may, in some plants, be seen to burst; and then a liquor like saliva escapes, in which, says Du Hamel, small particles are obscurely visible. Of the chemical nature of the pollen little further is known, than that it yields a peculiar matter, called pollenin, which is described as being of a yellow colour, without taste or smell, insoluble in water and alcohol, but highly inflammable, and yielding, by distillation, a good deal of ammonia, which approximates, in composition, to animal matter; but this knowledge gives us no insight into the nature of its peculiar action.

When the grains of pollen have lain for a short time on the stigma, they open at their angles, or in certain definite points of their surface, and emit long slender tubes called pollen tubes, which, according to Brongniart and Brown, are formed of the inner lining of the pollen grains. These pollen tubes, which are not above the 1500th or 2000th part of an inch in diameter, pierce the stigmatic tissue, and find their way down to the region of the placenta, conveying the matter contained in the grains into the interior of the style. The active molecules of the pollen are thus brought into communication with the foramen of the ovule, and are the means by which impregnation is effected. Although tubes enter the ovarium, and run along the placenta, it has not been proved that the true pollen tubes come in contact with the ovule. What is the nature of the communication made by the pollen to the ovule, through the medium of the pollen tubes, and the result of which is fertilization, we have no means of knowing. Some consider it a peculiar stimulus, which is required to call into existence an embryo germ of life, residing in the point of the nucleus; others say that it is the act of conveying such a germ to the nucleus from the pollen grains. The latter idea has been supported by Schleiden, whose ingenious views on the subject are published in the Nova Acta Academiae Naturae Curiosorum, &c., vol. xix.

On this subject, Brown remarks, that our knowledge appears not yet sufficient to warrant even conjectures as to the form of the immediate agent derived from the male organ, and the manner of its application to the ovule, in the production of that series of changes constituting fecundation; and he at the same time remarks, that in investigating the obscure subject of generation, additional light is perhaps more likely to be derived from a further minute and patient examination of the structure and action of the several organs in asclepiadaceæ and orchideæ, than from that of any other department either of the vegetable or animal kingdom.

But whether the pollen act directly or indirectly on the

Vegetable ovarian vesicle, there is little doubt but its influence is necessary to the perfection of the seed: for though, as Ray observed, some fruits may be produced without the concurrence of the male parts of the plant, just as some birds will produce eggs without the concurrence of the cock, yet such fruits, like such eggs, are altogether barren and unproductive.

To the impregnation of one species of flower by another of a kindred nature, through the agency of winds and insects, Du Hamel ascribed most of the varieties of fruits denominated new. In some fruits, the species, in the hybrid production, are kept so distinct, that we are able to distinguish one part from another, with which it had been associated at the period of fecundation. Thus, there is a species of orange, which, on the same tree, says he, produces "des bigarades, des citrons, et des balotins séparés, ou même rassemblés par quartiers dans le même fruit." In like manner, a certain species of vine produces, on the same shoot, bunches both of red and white varieties; or on the same bunch both red and white grapes; or bunches on which the grapes are red and white by halves, or even by quarters. These diversities in fruits he attributed to the impregnation of one species by the pollen of another; and to a similar cause, as we before stated, he ascribed many of those diversities in the colours of flowers, where different varieties grow and blossom together. Others have made many direct experiments on the reproductive function in plants by crossing different species with each other; and, by a judicious extension of the same methods, Mr. Knight has been able to present us with several new and improved varieties both of seeds and fruits.

ART. VI.—Maturation of Fruits.

The period that intervenes between fecundation and that in which the ripening of the fruit or seed is completed, varies in different plants, and even in the same plant, is much modified by climate, season, habit, &c. Whatever, to a certain extent, diminishes the vigour of vegetation, favours the production and accelerates the maturation of the fruit. So long as trees continue to shoot and abound in sap, says Du Hamel, their fruits do not arrive at maturity. By stripping them of their leaves, we hasten this period, not so much, however, by exposing the fruit to the sun, as by weakening the flow of the sap. But if the tree be stripped before the fruit has reached its proper size, both its size and quality are bad. As the powers of vegetation decline, the fruit advances towards maturity; and then exposure to the sun, by promoting transpiration, and concentrating the juices of the fruit, hastens the ripening process. At an earlier period, however, the same degree of exposure, by exciting too great transpiration, might cause the fruit to languish and wither. When fruits are enclosed in bags to protect them from wasps, transpiration is checked, and the fruit enlarges; but has not so good a flavour as usual. The present taste for what are called "fine fruits," seems directed chiefly to size, and is content to resign the rich and racy flavour, found only in fruits of a moderate bulk, for the pampered and bloated produce of a too luxuriant vegetation.

Ingenhousz formerly maintained that fleshy fruits, whether ripe or unripe, and whether growing in sunshine or in shade, always vitiated the air in which they were confined: and in a late Memoir on the Maturation of Fruits, M. Berard has adopted the same opinion, and maintained that green fruits do not decompose carbonic acid and disengage oxygen gas in sunshine, but that, through every period of their growth, they uniformly convert the oxygen of the air into carbonic acid gas. To this Memoir, M. de Saussure, who had formerly combatted the opinion of Ingenhousz, has replied by new experiments; and has satisfactorily proved, that although, during the night, green fruits convert the oxygen gas of the air into carbonic acid gas, yet that,

when exposed to sunshine, they again reconvert this carbonic acid into oxygen gas; so that, if they be placed alternately in sunshine and in obscurity, for two entire days, the air of the vessel undergoes successive changes which nearly counterbalance each other, and at the close of the experiment no other degree of change exists in it, than may be attributed to errors of observation. Hence it is inferred, that, both in the shade and in sunshine, green fruits act upon the air like green leaves; but this action is carried on to a smaller extent, and diminishes as they approach maturity.

As thus the air, under similar circumstances, suffers the same changes from green fruits as from leaves, it may be presumed that the fruit owes its green colour to the same action of light upon it. Light seems also to act in the production of the other colours which fruits exhibit. M. Bonnet shut up, in cases of white tin, grapes of a black colour, which did not then acquire their natural hue. Pears, says Du Hamel, which grow in the shade, are often green, while others, exposed to the sun, are beautifully coloured; and the same things are observed in peaches. Neither peaches, pears, nor cherries, assume their proper colours, if, at the period of ripening, says Senebier, they are secluded from light; and if a portion of fruit be covered with tin-foil, that part will continue pale or yellow, while the uncovered portions of the same fruit become perfectly red. If the red juices of fruits be extracted by water or alcohol, they are affected by acids and alkalis like those of flowers; and similar changes are produced by these agents on the coloured infusions obtained from their skins. These facts show that the same chemical actions, which occasion the colour of leaves and flowers, are employed in the coloration of fruits: but in these latter, they are probably much modified by the chemical changes that go on in the fruit itself during the process of maturation.

To discover the chemical changes that take place in the fruit during its maturation, M. Berard analyzed several fleshy fruits at different periods of their growth. With this view, three apricots of the same size were selected, and being plucked in succession, one of them was analyzed at three different stages of growth, namely, in its green state, in a state more advanced, and in a ripe state. The several results are given in the following table.

In the interval between the first and last analysis, the fruit had so much increased in size, as nearly to double its weight. It will be seen that, with the exception of the green colouring matter, which had become yellow, all the ingredients found in the unripe fruit were present in the ripe one, but some were in greater proportion. Sugar in particular had greatly increased, and water had diminished. From these results it is inferred, that the different flavours of green and ripe fruits are not owing so much to the appearance of any primary ingredient, or its transformation into another substance, as to the production of new substances, and especially of sugar, made in the progressive stages of growth. Similar analyses of cherries, gooseberries, plums, and peaches, afforded the same results.

There are some fruits, however, as those of apples and pears, which ripen very well after they are detached from

there; and in these, sugar seems to be formed out of the other ingredients, as it is formed from fecula in the germination of the seed. M. Berard plucked a pear from the tree when firm and green, and shut it up in a close vessel of atmospheric air from the 12th to the 29th of August. Its color had then become yellow, and it was perfectly ripe. During this period, its total weight had diminished very little, and this was due to the loss of a little water, and a minute portion of carbon; but the proportions of its ingredients were much changed, for the quantity of sugar was nearly doubled, and that of gum, of water, and of woody fibre had decreased. The united loss of the gum and woody fibre was not, however, equal to more than half the gain of sugar; and therefore M. Berard supposes that water may have become fixed, and augmented the proportion of this latter substance. If water be thus held to have contributed to the production of sugar in the pear, we may suppose it to have served a similar purpose in the ripening of the apricot; for the loss of water in the ripe apricot, as compared with the green one, comes near to the gain of sugar; and no large in the relative proportions of the other ingredients can be deemed sufficient to account for the great increase of saccharine matter.

The external agents required to effect these chemical changes in the maturing fruit appear to be heat and air. In the above experiment with the pear, the vessel was kept in a temperature of about 82° Fahrenheit; and the air, as was the fruit, underwent a change of composition. It indeed remained unchanged in volume; but 100 parts of it yielded, on analysis, 13.52 carbonic acid, 7.51 oxygen, and 8.97 azote; so that, as in other cases, the loss of oxygen was supplied by an equal bulk of carbonic acid, since the united volumes of the acid gas and oxygen made together almost exactly the $\backslash frac\{1\}\{100\}$ of oxygen gas which the air at first contained. Hence no oxygen can have combined with the fruit; nor can the azote of the air be deemed to have undergone any necessary change, since the very minute portion of fat gas unaccounted for may fairly be set down to error in experiment. That this conversion of oxygen gas into carbonic acid is necessary to the maturation of fruits, M. Berard inferred from the fact that the process is arrested if fruit be kept in an atmosphere destitute of oxygen; yet, after being kept for some weeks in such an atmosphere, the process recommences if oxygen be supplied.

This process of maturation, light, though ordinarily present, and acting on the colours of fruits, does not seem necessary; for fruits will ripen in dark places; and, to hasten maturation, it is not uncommon to enclose bunches of grapes in black bags, which must, at the same time, exclude light and accumulate heat. Whether light be actually unfavourable to the formation of sugar in fruits, as it appears to be in seeds, remains to be ascertained; but certainly, though it should retard, it does not prevent maturation: and its presence is ordinarily accompanied with such an increase of temperature as may more than compensate for its supposed injurious operation. Since, also, the vegetating process gradually diminishes as the fruit approaches maturity, and ceases to act upon it when its growth is completed, we cannot describe the changes that constitute maturation to vegetation, but must regard it as a chemical change, effected by the reaction of the several ingredients of the fruit on each other, under the operation of those external agents necessary to its occurrence.

ART. VII.—Fecundity of Vegetables.

The period required for the accomplishment of those changes in the ovarian vesicles, which terminate in the formation of perfect seeds, varies much in different species of plants, and also in the same species, under different circumstances of climate, soil, habit, &c. When they are

completed, the ovary or pericarp, in which the seeds were contained, is opened, in various modes, for their discharge: or the fleshy pulp that invested them decays; or the stony covering in which they were imprisoned is rent asunder; so that, in one way or other, they are set free, and by various means are disseminated over the surface of the earth, destined either to reproduce new beings similar to themselves, or to minister to the gratification and sustenance of animal life.

Of the seeds thus produced, the number, size, figure, texture, and other properties, are infinitely diversified. With respect to their number, we have already, in our former article, given examples of the productiveness of wheat and barley, and described the peculiar structure by which, in the family of the grasses, this productive power may be almost indefinitely augmented. M. Dodart prosecuted the same inquiry on trees. He selected an elm, which, in the fifteenth year of its growth, he calculated to produce, in one season, 329,000 seeds. Supposing this tree to live 100 years, and its mean fecundity, for its whole life, to be taken at 329,000, this number, multiplied by 100, will give 32,900,000 as the number of seeds produced, through its whole life, by the single seed of an elm. But suppose further, says Du Hamel, all these seeds to be planted, and each to produce a tree as fruitful as its parent, and so on from generation to generation; then, calculating the produce of each of these trees during 100 years, we shall have an increasing geometrical progression, of which the first term will be one; the second, thirty-three millions; the third, the square of that number; the fourth, its cube; and so on to infinity; a fecundity which, in the revolution of ages, would be sufficient to cover the whole surface of the earth with one species of plant.

But propagation by seeds is not the only mode by which plants are multiplied. With the exception of some trees, as the pine and fir, which do not shoot afresh when they have been lopped, except when very young, most vegetables, continues the same author, contain in all parts of their branches, their trunk, and even roots, germs which do not develop unless rendered necessary by the retrenchment of their boughs. Thus, if an elm be headed, and its smaller branches removed, its trunk and larger branches will, in the following spring, be covered with new productions, which never would have appeared if the first branches had not been removed. At whatever part or height the tree is headed, new shoots spring forth. The whole tree, therefore, from the root to the extremities of the branches, is filled with germs (or rather, we would say, endowed with a capacity of producing them), when the parts previously existing receive such injury as to render these new productions necessary to supply the place of the former.

Roots also possess this capacity of producing shoots as well as the branches. If the root of the elm be exposed, with certain precautions, to the air, it puts forth young branches; and many creeping roots, when they come into light and air, produce branches, which, by transplantation, form individual trees. A sprig of willow, when both its ends are thrust into the earth, yields rootlets from both, while the intermediate portion pushes forth leaves into the air; and the leaves of certain vegetables, as those of Bryophyllum calycinum, are capable, in proper circumstances, of producing entire plants. We may therefore say, adds Du Hamel, that there is perhaps no point of the surface either of the branches, the stem, or the root, which does not contain a germ, ready to develop itself when circumstances shall arise wherein this development may be useful to the parent tree. Nay, more, continues the same author, there is not perhaps any point on the branches, the stem, or the root, from which rootlets may not spring, when the conditions required for their development shall be fulfilled. If the root of a species of campanula be cut into pieces, and these pieces

Vegetable be put into the earth, each piece will produce both roots and stems. Of these concealed germs, dissection indicates no trace, until they become sensible in the progress of their development. From whence do they proceed? From the vessels or the cells? Or are they formed by the sap? Do they exist in a form invisible to us before the tree was headed? This, says he, is pure conjecture, although it is true, that if this operation had not been performed, the sap would have continued its course in the parts already formed, and would not have aided in developing the germs of which we speak. But not to abandon ourselves to imagination, it is sufficient, he adds, to have shown the immense fertility of vegetables, first, by seeds, and, secondly, by invisible germs, of which but a small number of analogous facts are to be found in the animal kingdom. In these remarks, Du Hamel, the Haller of vegetable physiology, evidently leans to the doctrine of pre-existing germs, which at one time so much occupied the attention of naturalists; but of which he ultimately disposes, with that good sense and real candour, which are not less admirable in all his writings than the talent and information which they every where display.

CHAP. III.—OF THE VITAL POWERS, SECRETIONS, SPONTANEOUS MOTIONS, SLEEP, DECAY, AND DEATH OF PLANTS.

ART. I.—Of the Vital Powers.

Such is a brief outline of those vegetable functions which comprehend the evolution, growth, and reproduction of plants. In the description of these functions, we have endeavoured to keep within the limits of observation and experiment; and, in reasoning from the facts derived from these sources, we have adhered strictly to explanations, which apply only to the physical constitution of plants. But we are aware, that, to accomplish these physical changes, not only is a particular structure required, but that structure must be endowed with the property or principle that distinguishes living organized beings from dead and inorganic matter. Without embarrassing ourselves with inquiries into the nature and origin of life, we are content, on the present occasion, to seek it only in its effects; to regard it as a power or property not less essential to the constitution of living matter, than gravitation is to that of dead matter; and, rejecting all speculation about its nature, to study only the physical conditions required for the display of its operations, and, as far as we are able, trace the laws by which those operations are regulated. "It is not," says Dr. Franklin, "of much importance to us to know the manner in which Nature executes her laws; it is enough, if we know the laws themselves. It is of real use to us to know that china, left in the air unsupported, will fall and break; but how it comes to fall, and why it breaks, are matters of speculation. It is a pleasure indeed to know them, but we can preserve our china without it."

Beside the evidences of a living power in plants, derived from the ordinary phenomena of growth and reproduction, the function of secretion by which growth is sustained, and various new products formed, deserves more particular notice. Other evidences of this power have been drawn from the various motions exhibited by the roots, leaves, flowers, and fruits of plants; and also from the phenomena of infancy, maturity, and old age, which they exhibit in the successive periods of their existence. To enable them to execute these different functions, and exhibit these phenomena, some physiologists have pushed the analogies between plants and animals to an unwarrantable extent; and, in addition to all the attributes connected with growth and reproduction, have endowed plants not only with irritability but with sensibility, instinct, perception, and volition. In ascribing to them these attributes, more attention seems to

have been given to a supposed correspondence in effects, than to a real agreement in the structure and functions of the organs. Neither has any very nice distinction been taken between what may be due to physical agents, acting on vegetable organization, and what, from our present inability to explain on physical principles, we are too apt at once to attribute to what are called vital principles or causes. It is only, however, where physical explanations altogether fail, that it is allowable to resort to the mysterious aid of vital causes: and as the natural philosopher, in treating of inanimate matter, assumes gravitation as a fact, and, without investigating its nature, proceeds to describe the laws of its action; so the physiologist, in studying living bodies, may regard life, and direct his inquiries rather to the laws by which it acts, than to the nature or principle of its action.

ART. II.—Of Secretion.

By secretion is understood the separation of a peculiar matter from the general mass of fluids by some particular structure, and which may either retain its primary condition, or pass into a solid state. Though the mass of fluid from which secretions are produced be one and the same, and the secreting organ, as to external conditions, be often in the same circumstances, yet the matters secreted differ greatly from each other, which difference probably arises from variety of structure in the secreting organs. Thus, an essential oil is found only in the rind of the orange, a fat oil only in the kernel of the almond, and so with regard to other secretions which exist only in particular parts. Besides the acids, alkalies, earths, and metals, which, though of a mineral nature, are more or less constantly found in plants, chemists enumerate about forty products of vegetation, which possess distinct chemical characters; and of many of these products numerous varieties exist. As none of these substances can be detected in the common sap, they must have been elaborated by the specific organs of vegetables, under a process of secretion. By what peculiarity of structure, or of function, these organs are enabled to produce such remarkable chemical changes in the common sap, is quite unknown; neither do we know how much is to be attributed to the action of the organ itself, or to the reaction of the several ingredients on each other, or to the influence of external agents.

Of these secretions, the most important is the cambium, the fluid employed directly in vegetable nutrition and growth. By the changes which the common sap undergoes in the leaves, the proper juices of plants are formed. These juices differ greatly from each other both in their sensible and chemical qualities. It is from them that the cambium is directly formed by a process of secretion, and in all plants is said to possess nearly the same characters. It is a mucilaginous fluid, without colour, odour, or taste; while the proper juices themselves exhibit all these properties. The proper juices also are contained in the vessels, and flow out when they are divided; but the cambium transudes rather than flows, and that only in places where new parts are to be formed. Thus, in the pine, says Mirbel, while the proper or resinous juice flows in the large vessels, the cambium transudes beneath the liber, and similar observations on the fig show that the cambium is entirely distinct from the proper juice. The cambium then, we must regard as a secretion, separated from the proper juice by the vascular structure of the liber or alburnum, when and wheresoever it is required to support nutrition and growth. Hence, in an experiment of Du Hamel when a piece of the bark of a peach tree was engrafted on the wood of the plum, the new wood, formed beneath the bark, was white like that of the peach, not red like that of the plum. Of the other secretions of plants, which, for reasons already assigned, are found chiefly on the external parts, as the leaves, flowers, fruits, &c., the number and

diversity are very great. They are probably formed in each instance, by peculiar and appropriate organs, but in what manner is quite unknown.

ART. III.—Of the Spontaneous Motions of Plants.

Although plants, as is well known, possess no locomotive power, yet, in different parts, they often exhibit what are deemed spontaneous movements. In cloudy weather though the day, and always towards evening, many plants, at certain hours, close their leaves and flowers. Bonnet remarked, that the leaves of certain plants approached by the superior surface when the sun shone; but as he declined or set, they fell down, and, even in some instances, approached by their inferior surface. If heat was applied to leaves thus closed, they would open and fold back in a contrary direction: and, on the other hand, if moisture was applied to leaves that had been folded by the sun's heat, they also would open and fold back as from dew. In these examples, the different conditions of heat and moisture seem very much to influence the movements of the leaves. In like manner, the stalks of many fruits change their office under the different conditions, with respect to moisture, that accompany the ripening process: for under the desiccation which then occurs, many of them, says Du Hamel, exercise movements not unlike those of muscular action, and are thereby enabled, in some instances, to scatter their seeds.

We have already described many important effects produced in plants by the direct action of light, and have now to notice the influence which it exerts on their movements. If plants be confined in a chamber, where there is but one window, all the younger shoots will forsake their perpendicular direction and make towards the light. In like manner, a young tree, when growing in the midst of older ones, pushes rapidly upwards, till it reaches the height of those that surround it, when it ceases to grow in height, but augments in size: or, if a young plant be made to grow in an equine vessel pierced with holes which admit the light, the shoots which it puts forth will be directed towards the holes. Some plants appear to follow the sun more or less distinctly in its course from east to west. Over these movements, Bonnet ascertained that neither heat, nor humidity, nor air had much influence; so that the sun, says Du Hamel, in causing these motions, acts more by his light than by his heat.

But there are other movements, proper to certain plants, or to particular parts of them, over which light exerts little or no power. Many observations have been made on the motions of the sensitive plant by Hooke, Du Hamel, and others. The latter author remarks, that the movements of the plant do not depend essentially on light or heat; for if kept in a green-house, it closes its leaves early in the evening before the sun has withdrawn, and while the temperature is yet high; or if placed in perfect obscurity, it still continues to open in the morning, and close in the evening as before. If, during its expansion in the early part of the day, it be gently touched, its leaves partially close, but soon recover their former state. Mere touch, however, without agitation, does not produce motion; for the leaves may be pressed between the fingers, without causing motion, if no agitation be given. With proper address, it is possible, says Du Hamel, to divide the mid-rib of a leaflet, without exciting motion in the other leaflets, or even in its own foliage; nor does motion follow the puncture of a needle, if agitation be avoided. The time required for a branch, that has been touched, to resume its former state, depends on the vigour of the plant, the hour of the day, the season, &c; and the order in which the parts re-establish themselves likewise varies.

The motions of this plant seem to depend much on peculiarity of structure. From a branch proceed the branchlets that bear the leaves. These leaves are formed of a

common petiole, which at its extremity terminates in four conjugate leaflets, each of which has a mid-rib, furnished with a certain number of folioles. In the movements of this plant, the branchlets are so articulated with the branch, that they move on it in the manner of a hinge. The common petiole of the leaves has a like movement; and lastly, each foliole moves on its proper stalk to apply itself to the opposite foliole. This peculiarity of structure explains why agitation is so necessary to the movements of this plant; and why it bears such great violence without moving, if no agitation be employed to excite motion in its several articulations; so that it is principally in the articulations, says Du Hamel, that the sensibility of this plant resides. He adds, that, when this plant closes, it is not through weakness, but by a sensible contraction, which resists any attempt to replace it in its former state. Dutrochet accounts for the motions of the sensitive plant by the distension and contraction of the upper and lower portions of the swellings which are met with at the base of the petiole and of the leaflets.

In certain flowers spontaneous movements take place at the period of fecundation. The stamens of the barberry approach towards the pistil on the slightest irritation, as do those of the sun-flower and other plants. During the night, the petals of may-flowers close, and thereby protect the stamens and pistils; but they cease to do so after fecundation is effected. The water-lily is said to bear its flowers on a foot-stalk under water; and when the flowering season arrives, the stalk rises through the water, till the flowers reach above the surface. The flowers then expand, and the anthers burst and discharge their pollen on the stigma in the usual way. About four o'clock in the afternoon, the expanded flowers close, and the stalk then lies down either upon or under the water. The next day it rises, as before, and continues to do so daily until fecundation is completed, when it sinks beneath the surface, and there remains to ripen its seeds. Other spontaneous movements are exhibited by claspers and tendrils in seeking support from neighbouring bodies, and by roots in the directions which they take in search of food.

Unable to assign physical reasons for these and similar phenomena, some naturalists, guided by vague analogies drawn from the animal kingdom, ascribe such movements in vegetables to sensation and perception, by which they not only feel their wants, but perceive the best modes of gratifying them; and in the performance of the actions necessary to accomplish their objects, they are, according to some, directed by Instinct, and, according to others, by Volition. Such modes of reasoning not only afford no explanation of the phenomena described, but supersede all necessity for it; and are apt, therefore, to beget a conceit of knowledge where ignorance alone prevails. In reference to such attempts at explanation, Du Hamel well observes, that "every peasant has remarked the fact, that the radicle of the seed tends always towards the earth, and that the plume rises in the air. If we ask of them why one part thus strikes into the earth, and the other seeks the air, they give the fact for a reason, by replying that the one part strikes down because it is the root, and the other ascends because it is the stem. And let us not smile too complacently at these modes of expression; for we ourselves use them every day when we raise questions about things which are unknown to us. Do we not say that a stone falls because of its gravity? And those who give for a reason that it is attracted by the earth, do not satisfy the real philosopher, who never is content with simple terms void of meaning. To me it seems both more simple and more honest to make at once a confession of our ignorance."

ART. IV.—Of the supposed Sleep of Plants.

Some writers, deeming plants to possess voluntary power,

Vegetable have from thence inferred that they require sleep. We Physiology have no proof, however, that they possess any such power; nor that, in the exercise of their ordinary functions, they experience that fatigue and exhaustion which renders sleep necessary to their restoration. All the spontaneous movements of vegetables previously described, seem to arise from the operation of physical agents, conjoined with those inherent properties which belong to them as living beings. These agents act variously on different plants; and hence some close their leaves and flowers from the abstraction of heat or moisture, and others from the exclusion of light; and this at various periods of the day, as well as through the night. Other plants exhibit spontaneous movements only in the flower, and at the season of fecundation, when suitable conditions of the atmosphere prevail; and though, in some instances, these motions continue for a time after the conditions required for their display may have been withdrawn, yet we must ascribe such motions rather to habit than to any thing that partakes of the nature of volition.

The diminution or suspension of action through the night which occurs to plants that inhabit temperate climes, cannot be received as a proof of sleep, induced by the exhaustion of the vegetative powers; for even in such climes, vegetation, in favourable seasons, proceeds often by night as well as by day. In climates still more favourable, the same plants which with us produce fruits only once a year, yield two or more crops; and in Norway and Lapland, where the sun, at certain periods, continues almost constantly above the horizon, the whole period between seed-time and harvest sometimes occupies only about fifty days. In such cases, little or no suspension of the vegetative functions can have taken place; nor have we the smallest reason to believe that the continued exercise of them is followed by fatigue or exhaustion sufficient to require sleep. What, therefore, has commonly been denominated the "sleep of plants," we can regard only as a diminution or suspension of the vegetative functions, arising from the abstraction, more or less complete, of those external agents, whose presence is essential to their full operation and display.

ART. V.—Of the Decay and Death of Plants.

But whether the functions of vegetables unceasingly continue or be occasionally suspended by the abstraction of the conditions necessary to their exercise, all plants submit at length to the same general law, and die, either in whole or in part, when the great purposes of their existence—those, namely, of growth and reproduction—have been accomplished. Some plants speedily arrive at maturity, and having produced their seeds, die altogether; others flourish for one or two seasons, and then decay and perish; and others again die only in part, after having produced their seeds, and also a new series of buds to continue their growth and fructification. In the progress of our inquiry, we have seen that, in every stage of vegetation, certain organs fall into decay after having fulfilled their allotted functions. Thus the tunics of the seed perish beneath the soil, after having yielded their nutrient matter to carry on the evolution of the embryo; and those cotyledons which rise into the air, decay also, when the radicle has taken its proper hold of the soil, and the leaves are sufficiently developed to execute their appropriate functions. So likewise the petals, the stamens, and pistils of the flower, rapidly fade and fall as soon as the important function of fecundation is effected; the fruits next drop when they have reached maturity; and lastly, the leaves, even of perennials, when their allotted functions have ceased, decay and fall like those of annual plants.

To account for this fall of the leaves, many hypotheses have been proposed. Some have ascribed it to defective transpiration, and consequent accumulation of juices in the vessels; others to an inequality of growth between the stem

and the petiole of the leaf, during the progress of vegetation; others to the desiccation of cellular tissue, supposed to exist at the insertion of the petiole with the stem; others to a simple sloughing of worn-out parts; and by others, the fall of the old leaf has been attributed to the growth of the new bud. In all the examples enumerated above, of the decay and fall of cotyledons, flowers, and fruits, the organs ceased to execute their functions, when the purposes of their existence were accomplished; and such we must regard as the general law that determines the death of the leaves. In some instances, the death of parts seems to be hastened by the diversion of nutrient matter from the older organs to the new parts which are subsequently developed, as is exemplified in the decline and fall of the stamens and pistils from the growth of the ovary after fecundation; but, in other instances, as in the death of annual plants, no such acting cause is apparent, and nothing remains to account for the event that occurs, except the character of duration, more or less extended, which was impressed on the plant at the era of its formation.

But from whatever cause the deciduous organs of plants cease to perform their functions, the immediate cause of their fall seems to vary in different vegetables, and to depend often on accidental circumstances of climate, &c. In some instances, the growth of the young bud seems to occasion the fall of the leaf. Thus, though the leaves of the oak die and become dry in autumn, they do not, says Du Hamel, fall till spring, when the buds begin to open, and the new leaves to appear. In other instances, the fall of the leaf seems to be connected with the exercise of the transpiratory function; for plants which transpire largely, soonest lose their leaves, and hence evergreens, which transpire little, retain their leaves longest. Even if an evergreen be engrafted on a deciduous tree, it still retains its leaves after those of the stock have fallen. Sudden changes of temperature and humidity in the atmosphere, frequently promote the fall of leaves. Thus, in autumn, when rain succeeds to a white frost, the leaves sometimes rapidly fall. So, likewise, it sometimes happens, that the too great heats of summer dry up the leaves; and then also, if warm rains follow, the dried leaves fall and new ones succeed, which continue longer than those of spring. On the other hand, leaves equally fall, though not so speedily, when the winter is mild; and in conservatories, where a regular temperature is kept up, deciduous plants lose their leaves in spring, when the new ones shoot forth. Certain accidents or diseases, however, as lightning, or the eruption of the proper juice from its vessels, or a peculiar disease which separates the bark from the wood, sometimes kill a tree suddenly; and then, says Du Hamel, though the leaves become dry, they adhere strongly to the branches. These facts show, that, while the natural death of the leaf is to be sought in the specific nature and constitution of the plant to which it belongs, its fall sometimes depends on the growth of new buds; or on variations in the motion of its fluids; or on sudden changes in the temperature and humidity of the atmosphere; and sometimes, probably, the period of the fall is determined by a difference of texture in the fibre of the plant itself.

The duration of the stem or trunk, after the leaves have fallen, is very different in different plants. In many herbs the stem dies at the same time, or shortly after the leaf; but in some trees, the life of the trunk is prolonged through many ages. The Gentleman's Magazine for 1762 contains an account of the age of a chesnut tree, then growing at Tamworth in Staffordshire. This tree, it is said, was, at that period, probably the oldest, if not the largest, in England, being fifty-two feet in circumference. Its period of rising from the nut may be fixed at the year 800, in the reign of King Egbert. From that date to the reign of Stephen is 335 years, at which time it was fixed on as a