An introduction to physiological and systematical botany/Chapter 16
CHAPTER XVI.
OF THE FUNCTIONS OF LEAVES.
The knowledge of the functions of leaves, and their real use with regard to the plant, is a curious branch of vegetable physiology, which made but a slow progress long after the nature of many other parts had been deeply scrutinized and thoroughly explained.
Caesalpinus (De Plantis, p. 6.) thought leaves merely a clothing, or a protection against cold and heat. He conceived that the rays of the sun, being moderated in passing through them, were prevented from acting too violently on the fruit and young buds. "Accordingly," says he, "many trees lose their leaves in autumn, when their fruits are perfected, and their buds hardened, while such as retain the fruit long, keep also their leaves; even till a new crop is produced, and longer, as in the Fir, the Arbutus, and the Bay. It is reported that in hot climates, where there is almost perpetually a burning sun, scarcely any trees lose their leaves, because they require them for shade." Cacsalpinus goes on to show that leaves proceed from the bark, with some remarks on the pith, (in which we may trace the origin of the Linnæan hypothesis of vegetation,) but which are now superseded by more accurate inquiries.
The above is certainly a very small part of the use of leaves. Yet the observations of this writer, the father of botanical philosophy among the moderns, are so far correct, that if the leaves of a tree be stripped off, the fruit comes to nothing, which is exemplified every year in Gooseberry bushes devoured by caterpillars; and though the fruit-trees of warm climates, partly naturalized with us, Grapes and Peaches for instance, ripen their fruit sooner perhaps if partially deprived of their leaves, yet if that practice be carried too far, the fruit perishes, as gardeners who tried it soon discovered. The White Mulberry indeed, cultivated in the south of Europe for the food of silkworms only, bears wonderfully the loss of its foliage three or four times a year. How far the fruit is injured nobody thinks it worth while to inquire, as it is never eaten, but it certainly does not fall off prematurely.
That Leaves imbibe and give out moisture has been long known, this being one of the most obvious facts belonging to them. Dr. Hales thought they might probably imbibe air; but since his time more certain discoveries have been made concerning this point, as well as the effects of light upon leaves, which also did not escape the consideration of that great philosopher. All these subjects we shall mention in their turn.
That Leaves give out moisture, or are organs of insensible perspiration, is proved by the simple experiment of gathering the leafy branch of a tree, and immediately stopping the wound at its base with mastick, wax, or any other fit substance, to prevent the effusion of moisture in that direction. In a very short time the leaves droop, wither and are dried up. If the same branch, partly faded, though not dead, be placed in a very damp cellar, or immersed in water, the leaves revive, by which their power of absorption is also proved. Hence the use of a tin box to travelling botanists, for the purpose of restraining the evaporation of plants, and so preserving them fresh for some days till they can be examined, as well as of reviving faded plants, if the inside of the box be moistened before they are shut up in it.
Dr. Hales found that a plant of the Great Annual Sunflower, Helianthus annuus, lost 1lb. 14oz. weight in the course of twelve hours in a hot dry day. In a dry night it lost about 3 oz.; in a moist night scarcely any alteration was observable, but in a rainy night it gained 2 or 3 oz. The surface of the plant compared with that of its roots was, as nearly as could be calculated, in the proportion of five to two; therefore the roots must have imbibed moisture from the earth of the pot in which the plant grew, and which was all previously weighed, in the same proportion of five to two, otherwise the leaves would have faded. The same experiment was made on the vine, the Cabbage, &c., with various results as to the exact degree of perspiration, but all proving it to be considerable. Evergreens are found to perspire much less than other shrubs.
The state of the atmosphere has a great effect on the rapidity of this perspiration. Practical botanists know how much sooner plants fade, and haymakers experience how much faster their work is done, some days than others, and those days are by no means always the most sunny. In a hot dry day plants are often exhausted, so as to droop very much towards evening, especially in the dry unsheltered bed of a garden. Such as have fleshy roots, indeed, have a singular power of resisting drought, which has already been explained p. 113. Succulent plants, destined to inhabit sunny rocks, or sandy deserts, imbibe with the greatest facility, and perspire very sparingly. Evergreens are not generally very succulent, but their cuticle appears to be constructed like that of succulent plants, so as to allow of little evaporation. The Cornelian Cherry, whose immense perspiration we have recorded, p. 68, has a thin dry leaf, capable of holding very little moisture.
The nature of the liquor perspired has been already noticed, p. 68. In hot weather it has been observed by Hales, Du Hamel and Guettard to partake occasionally of the peculiar scent of the plant that yields it, but in general the odorous matter is of too oily a nature to be combined with it.
The sensible perspiration of plants is of various kinds. When watery, it can be considered only as a condensation of their insensible evaporation, perhaps from some sudden change in the atmosphere. Groves of Poplar or Willow exhibit this phenomenon, even in England, in hot calm weather, when drops of clear water trickle from their leaves like a slight shower of rain. Sometimes it is of a saccharine nature, as De la Hire observed in Orange trees; Du Hamel Arb. v. 1. 150. It is more glutinous in the Tilia or Lime-tree, more resinous in Poplars, as well as in Cistus creticus, from which last the resin called Labdanum is collected, by beating the shrub with leather thongs. See Tournefort's Voyage, 29. In the Fraxinella, Dictamnus albus, it is a highly inflammable vapour. Ovid has made an elegant use of the resinous exudation of Lombardy Poplars, Populus dilatata, Ait. Hort. Kew. v. 3. 406, which he supposes to be the tears of Phaëton's sisters, who were transformed into those trees. Such exudations must be considered as effusions of the peculiar secretions; for it has been observed that Manna may be scraped from the leaves of Fraxinus Ornus, Fl. Græc. t. 4, as well as procured by incision from its stem. They are often perhaps a sign of unhealthiness in the plant; at least such appears to be the nature of one kind of honeydew, to which the Beech in particular is subject, and which, in consequence of an unfavourable wind, covers its leaves in the form of a sweet exudation, similar in flavour to the liquor obtained from its trunk. So likewise the Hop, according to Linnæus, Faun. Suec. 305, is affected with the honey-dew, and its flowers rendered abortive, in consequence of the attacks of the caterpillar of the Ghost Moth, Phalæna Humuli, upon its roots. In such case the saccharine exudation must decidedly be of a morbid nature[1]. That wax is also an exudation from the leaves of plants, appears from the experiments recorded by Dr. Thomson in his Chemistry, v. 4, 298, and it has been long ago asserted that wax may easily be gathered from the leaves of Rosemary. On this subject I have not made any experiments to satisfy myself.
With respect to the absorbing power of leaves, the best observations that have been made are those of Bonnet, recorded in the beginning of his Recherches sur l'Usage des Feuilles. His aim was, by laying leaves of various plants upon the top of a jar of water, some with their upper, and others of the same species with their under, surfaces applied to the water, to discover in which situation the leaves of each plant continued longest in health and vigour, and also how far different species differed from each other in this respect. The results were in many instances highly curious.
Of fourteen herbaceous plants tried by this philosopher, six lived nearly as long with one surface applied to the water as with the other; these were the common Arum maculatum, the French Bean, the Sun-flower, Cabbage, Spinach and the Small Mallow. By the last I presume is meant Malva rotundifolia, Engl. Bot. t. 3092. Six others, Plantain, White Mullein, the Great Mallow (probably M. sylvestris, t. 671), the Nettle, Cock's-comb, and Purple-leaved Amaranth (probably Amaranthus hypochondriacus), lived longest with their upper surface laid upon the water. The Nettle lived but three weeks with its under surface on the water, and about two months in a contrary position. The Mullein scarcely survived five or six days, and the Amaranth not a week, in the first-mentioned posture, while the leaves of the former remained in vigour about five weeks, and of the latter three months, when their upper surfaces imbibed the water. Marvel of Peru and Balm, the two remaining plants of the fourteen on which the experiment was made, had also an evident advantage in receiving that fluid by their upper surfaces. The leaves of some of the above species were found to thrive better when their stalks only were immersed in water, than when either of their sides was supplied with it, and the reverse was observable in several others; but the White Mullein, the Plantain and the Amaranth survived longer when they received the water by their stalk than by their under surface, though not so long as when it was applied to their upper sides.
Of sixteen trees tried by Bonnet, the Lilac and the Aspen, Populus tremula, were the only leaves that seemed to imbibe water equally well by either surface, whilst all the others evidently succeeded best with their under sides laid upon the water, being in that respect the reverse of herbaceous plants. Of these the White Mulberry leaf was the most remarkable, not living more than five days when supplied by the upper surface, while such as floated on their backs continued in perfection near six months. The Vine, the Poplar (probably Populus nigra), and the Walnut, were no less remarkable, for fading almost as soon, when fed by their upper surface, as when left without any water at all. Many of the other trees imbibed water as well, or better, by their foot-stalks as by their upper surfaces. Hazel-nut and Rose leaves, when laid with their backs upon the water, imbibe sufficiently to nourish other leaves on the same branch; so will one leaflet of a French bean supply its neighbour that does not touch the water.
Those who wish to repeat these experiments should be careful to choose full-grown healthy leaves, all as nearly as possible of the same age and vigour. It is also desirable that the precise species of plant should be recorded by its scientific name. For want of this, Bonnet, who despised method and nomenclature, has left us in uncertainty concerning several of the plants he examined. We ought to have been accurately informed what species of Poplar differed so remarkably in its power of absorption from the Aspen, another of the same genus. We ought likewise to have been told what Sun-flower, what Nettle, Amaranth and Mallows were examined; for want of which information the authority of such experiments is much impaired.
From the foregoing observations we learn the importance of shading and watering plants newly removed, cuttings, grafts, &c. and on the other hand the benefit of heat and air to promote due perspiration and evaporation.
The perspiration of aquatic plants seems to be remarkably copious. Of these some grow constantly immersed in the water, as most species of Potamogeton, Pond-weed, Engl. Bot. t. 168, 297, 376, &c. Their leaves are peculiarly vascular, and dry very quickly in the air, withering in a very few minutes after exposure to it. Their absorbing power seems equally great, so that they appear to be continually, in their natural situation, imbibing and giving out a quantity of water much greater than has been observed in land plants. Other aquatics, as the Nymphææ, Engl. Bot. t. 159, 160, float with only the upper surface of their leaves exposed to the air, which surface is so contrived that water will scarcely remain upon it. These leaves, though extremely juicy, dry with great rapidity, as does every part of the plants when gathered. It is probable that they imbibe copiously by their under sides, and perspire by the upper.
The œconomy of the Sarracenia, an American genus of which we now know four species, and of the East Indian Nepenthes distillatoria, deserves particular mention. Both grow in bogs, though not absolutely in the water. The former genus has tubular leaves which catch the rain like a funnel and retain it; at least such is the nature of S. purpurea, Curt. Mag. t. 849, whose margin seems dilated expressly for this purpose, while the orifice of the tubular part just below is contracted to restrain evaporation. Linnæus conceived this plant to be allied in constitution to Nymphæa, and consequently to require a more than ordinary supply of water, which its leaves were calculated to catch and to retain, so as to enable it to live without being immersed in a river or pond. But the consideration of some other species renders this hypothesis very doubtful. S. flava, t. 780, and more especially S. adunca, Exot. Bot. t. 53, are so constructed that rain is nearly excluded from the hollow of their leaves, and yet that part contains water, which seems to be secreted by the base of each leaf. What then is the purpose of this unusual contrivance? An observation communicated to me two years ago, in the botanic garden at Liverpool, seems to unravel the mystery. An insect of the Sphex or Ichneumon kind, as far as I could learn from description, was seen by one of the gardeners to drag several large flies to the Sarracenia adunca, and, with some difficulty forcing them under the lid or cover of its leaf, to deposit them in the tubular part, which was half filled with water. All the leaves, on being examined, were found crammed with dead or drowning flies. The S. purpurea is usually observed to be stored with putrefying insects, whose scent is perceptible as we pass the plant in a garden; for the margin of its leaves is beset with inverted hairs, which, like the wires of a mouse-trap, render it very difficult for any unfortunate fly, that has fallen into the watery tube, to crawl out again. Probably the air evolved by these dead flies may be beneficial to vegetation, and, as far as the plant is concerned, its curious construction may be designed to entrap them, while the water is provided to tempt as well as to retain them. The Sphex or Ichneumon, an insect of prey, stores them up unquestionably for the food of itself or its progeny, probably depositing its eggs in their carcases, as others of the same tribe lay their eggs in various caterpillars, which they sometimes bury afterwards in the ground. Thus a double purpose is answered; nor is it the least curious circumstance of the whole, that an European insect should find out an American plant in a hot-house, in order to fulfil that purpose.
If the above explanation of the Sarracenia be admitted, that of the Nepenthes will not be difficult. Each leaf of this plant terminates in a sort of close-shut tube, like a tankard, holding an ounce or two of water, certainly secreted through the footstalk of the leaf, whose spiral-coated vessels are uncommonly large and numerous. The lid of this tube either opens spontaneously, or is easily lifted up by insects and small worms, who are supposed to resort to these leaves in search of a purer beverage than the surrounding swamps afford. Rumphius, who has described and figured the plant, says "various little worms and insects crawl into the orifice, and die in the tube, except a certain small squilla or shrimp, with a protuberant back, sometimes met with, which lives there."—I have no doubt that this shrimp feeds on the other insects and worms, and that the same purposes are answered in this instance as in the Sarraceniæ. Probably the leaves of Dionæa muscipula, as well as of the Droseræ, Engl. Bot. t. 867—869, catch insects for a similar reason.
I proceed to consider the effects of Air and Light upon vegetables.
Dr. Grew, by the assistance of the microscope, detected a quantity of vesicles full of air in the leaves of plants, as also the spiral-coated vessels of their stems, which last he and all other physiologists, till very lately, considered as air-vessels likewise. Malpighi made the same observations about the same time; and as these two acute and laborious philosophers pursued their inquiries without any mutual communication, their discoveries strengthen and confirm each other. Their books have long served as magazines of facts for less original writers to work with. From their remarks physiologists have theoretically supposed that leaves imbibed air, which the spiral vessels were believed to convey all through the plant, in order that it might act on the sap as it does on the animal blood. The analogy thus understood was not correct, because air is conveyed no further than the lungs of animals; but without this hypothesis no use could be found for the supposed longitudinal air-vessels.
The observations of Dr. Hales come next in order to those of Grew and Malpighi. By means of the air-pump, an instrument much in use in his time, Hales obtained abundance of air from every part of the vegetable body, as well as from recently extracted sap. Plants were found to perish very soon in an exhausted receiver. Some of this great man's experiments, however, require to be received with caution. He rightly remarked that air was not only taken in by plants very copiously along with their food, but also imbibed by their bark; see Veg. Staticks, chap. 5. But when, from observing that it would freely from the bark pervade the longitudinal vessels of a branch, he concluded that Malpighi and Grew were right in their ideas of longitudinal air-vessels, he was misled by appearances. We cannot but be aware that, when a branch is gathered, the sap must soon flow out of those spiral-coated tubes, which are large, elastic, and, no doubt, irritable. After they are emptied, air may unquestionably pass through them, especially when the whole weight of the atmosphere is acting, as in Dr Hales's experiments with the air-pump, upon so delicate a fabric as the internal vascular structure of a plant, forcing its way through pores or membranes not naturally designed to admit it. We must also recollect that a plant, cut even for a short time, begins to lose its vital principle, after which no just judgment can be formed, by any experiments, concerning the movements of its fluids in life and vigour. See Chapter 1. These experiments of Dr. Hales therefore prove no more than that the vegetable body is pervious in various directions; and perhaps the only point they correctly establish is, that air is imbibed through the bark, a part known to be full of air-vessels. But the seventh chapter of the Vegetable Staticks contains some remarks much more to our purpose. Dr. Hales there clearly anticipates by conjecture, what succeeding philosophers, more enlightened chemists, have ascertained. His words are remarkable:
"We may therefore reasonably conclude, that one great use of leaves is what has been long suspected by many, viz. to perform in some measure the same office for the support of the vegetable life, that the lungs of animals do, for the support of the animal life; plants very probably drawing through their leaves some part of their nourishment from the air." p. 326. A little further on he adds, "And may not light also, by freely entering the expanded surfaces of leaves and flowers, contribute much to the ennobling the principles of vegetables?" p. 328.
Next in order of time to those of Hales follow the experiments of Bonnet. We have already detailed his observations on the power of leaves to imbibe moisture; whence it is ascertained that plants are furnished with a system of cuticular absorbents, which carry fluids into their sap-vessels, so as to enable them in some degree to dispense with supplies from the root. With respect to the effects of air upon leaves, this ingenious philosopher has not been equally successful. He is recorded as the discoverer of the expiration of plants, but it appears from his work that he merely observed the bubbles of air which cling to leaves, dead as well as living, and indeed to any other body, when immersed in water and exposed to the light of the sun. He found these bubbles disappeared in the evening, and returned again when the sun shone, and he faithfully reports that by their attachment to the surfaces of leaves, the latter were rendered more buoyant, and rose in the water; a sure proof that the air had not previously existed, in the same volume at least, in the substance of those leaves. Accordingly, Bonnet concluded that the latter, in imbibing the surrounding water, left the air which had been contained in the water, and that this liberated air became visible from being warmed and rarefied by the sun. This was as near the truth as Bonnet could come, it not being then known that light has a power of separating air of a peculiar kind, carbonic acid gas, from water. I find no indications in his work of his having had any idea of leaves absorbing air and giving it out again; still less of their affecting any change in its properties.
Dr. Priestley was the first who suggested this last-mentioned quality in vegetables. He ascertained their power of absorbing carbonic acid gas, denominated by him fixed air, and giving out oxygen gas, or pure respirable air. It was also his opinion that leaves imbibed the former by their upper, and gave out the latter by their under surface. He found some aquatic or marsh plants extremely powerful in this respect, especially the Willow-herb or Epilobium, and the Conferva, a minute branching cotton-like vegetable which grows in putrid water, and the production of which, in water become foul from long keeping on ship-board, Dr. Priestley judged to operate principally in restoring that fluid to a state fit for use.
Dr. Ingenhousz, pursuing Dr. Priestley's inquiries, found light to be necessary to these functions, and that in the dark leaves gave out a bad air. He observed moreover that fruits and flowers almost invariably gave out a bad, or carbonic, air, but more especially in the dark. He probably carries his ideas, of the deleterious effects of this air on animal life, too far; for no mischief has ever happened, as far as common experience goes, to persons sleeping in apple or olive chambers, neither do the inhabitants of the confined huts in Covent-garden market apparently suffer, from living day and night among heaps of drying herbs. Mischiefs have unquestionably arisen from flowers in a bed-room, or any other confined apartment, but that is to be attributed to their perfumed effluvia. So the bad effects, observed by Jacquin, of Lobelia longiflora on the air of a hot-house, the danger incurred by those who sleep under the Manchineel-tree, Hippomane Mancinella, or, as it is commonly believed, under a Walnut-tree, are probably to be attributed as much to poisonous secretions as to the air those plants evolve.
Dr. Ingenhousz introduced leaves into glass jars filled with water, which he inverted in a tub of the same water, and placed the whole together in the sun-shine. From their under sides came streams or bubbles of air, which collected in the inverted bottom of each jar. The air thus procured proved oxygen gas, more or less pure. The Nymphæa alba, Engl. Bot. t. 160, affords an extraordinary abundance of it. Dr. Ingenhousz observed plants to be very various in their mode of emitting these bubbles, but it was always uniform in the same species. Air collected from water placed in similar circumstances without plants, proved not oxygen, but much worse than common air, viz. carbonic acid gas, which following chemists have confirmed, and which we have already mentioned. Ingenhousz also found the air collected from plants under water in the dark worse than common air, especially that from walnut-leaves; which confirms the common opinion, above alluded to, respecting this tree.
Plants purify air very quickly. A vine-leaf in an ounce phial of carbonic acid gas, that immediately extinguished a candle, placed in the sun, without water, changed it to pure respirable air in an hour and half. Dr. Priestley found plants to alter even unmixed inflammable air, or hydrogen, especially the Epilobium hirsutum, if I mistake not, and Polygonum Hydropiper.
Succulent plants are found to afford most air, in consequence of the abundance of their Cellular Integument, or Parenchyma, in which, as I have hinted in the fourth chapter, the chemical operations of the leaves are performed.
That Light has a very powerful effect upon plants has long been known, independent of the remarks of Hales or Ingenhousz. The green colour of the leaves is owing to it, insomuch that plants raised in darkness are of a sickly white. It has even been observed that when light is admitted to the leaves through different glasses, each tinged of a different prismatic colour, the plant is paler in proportion as the glass approaches nearer to violet. The common practice of blanching Celery in gardens, by covering it up from the light, is an experiment under the eyes of every one. This blanching of plants is called by the French étiolation, and our chemists have adopted the term, though I think they err in deriving it from étoile, a star. When blanched plants are brought into the light, they soon acquire their natural green colour, and even in the dark they are green, if exposed to the action of hydrogen gas. Tulip and Crocus flowers have long ago been observed by Sennebier to be coloured even in the dark, apparently because their colour depends on a different principle from the green of leaves.
Light acts beneficially upon the upper surface of leaves, and hurtfully upon the under side; hence the former is always turned towards the light, in whatever situation the plant may happen to be placed. Trees nailed against a north wall turn their leaves from the wall, though it be towards the north, and in direct opposition to those on a southern wall over against them. Plants in a hot-house all present the fronts of their leaves, and this influences even the posture of the branches, to the side where there is most light, but neither to the quarter where most air is admitted, nor to the flue in search of heat. If the branches of a trained fruit-tree in full leaf be disturbed in their position, the leaves resume their original direction in the course of a day or two. The brighter the day, the more quickly is this accomplished. If the experiment be often repeated, they continue to turn, but more weakly, and are much injured by the exertion. Black spots appear about the veins on their under sides, and the cuticle scales off. Succulent leaves, though so thick and firm as many of them are, have been observed to be peculiarly sensible to light, while other plants, as Mallows, according to Bonnet, are much less so. The Miseltoe, Viscum album, Engl. Bot. t. 1470, the two sides of whose leaves are alike in appearance, and both equally, in general, presented to the light, are not found to turn upon any change in the posture of the branch. Neither do upright sword-shaped leaves alter their position, because in them both sides must be presumed to perform the same functions with respect to light as well as air.
Mr. Calandrini found vine-leaves turned to the light when separated from the stem and suspended by a thread. Of this any one may be easily satisfied, provided the experiment be made with sufficient care and delicacy. It is important, as demonstrating the turning to be accomplished by an impression made on the leaf itself, and not upon its footstalk.
Nor is this effect of light peculiar to leaves alone. Many flowers are equally sensible to it, especially the compound radiated ones, as the Daisy, Sun-flower, Marigold, &c. In their forms Nature seems to have delighted to imitate the radiant luminary to which they are apparently dedicated, and in the absence of whose beams many of them do not expand their blossoms at all. The stately Annual Sun-flower, Helianthus annuus, displays this phænomenon more conspicuously on account of its size, but many of the tribe have greater sensibility to light. Its stem is compressed in some degree, to facilitate the movement of the flower, which, after following the sun all day, returns after sun-set to the east, by its natural elasticity, to meet his beams in the morning. Dr. Hales thought the heat of the sun, by contracting the stem on one side, occasioned the flower to incline that way; but if so, it would scarcely return completely at night. There can be no doubt, from the observation of other similar flowers, that the impression is made on their radiated florets, which act as wings, and seem contrived chiefly for that purpose, being frequently destitute of any other use. A great number of leaves likewise follow the sun in its course; a clover-field is a familiar instance of this.
Of all leaves those of pinnated leguminous plants are found most affected by light, insomuch that it appears, in several cases, the sole cause of their expansion, for when it is withdrawn they fold over each other, or droop, as if dying; and this is called by Linnæus the Sleep of Plants, who has a dissertation on the subject in his Amænitates Academicæ. The term Sleep may not really be so hyperbolical as at first sight it seems, for the cessation of the stimulus of light, and of the consequent restrained position of the leaves, may be useful to the vegetable constitution, as real sleep is to the animal. Another purpose is answered by the nocturnal folding of some leaves, that they shelter their flowers from the dew, the advantage of which we shall explain hereafter.
Some pinnated leaves display a more extraordinary sensibility, not merely to light, but to the touch of any extraneous body, or to any sudden concussion, as those of Mimosa sensitiva, and pudica, Oxalis sensitiva, and Smithia sensitiva, Ait. Hort. Kew v. 3, t. 13. An impression made even in the most gentle manner, upon one of their leaflets, is communicated in succession to all of them, evincing an exquisite irritability, for it is in vain to attempt any mechanical solution of this phenomenon. One of this tribe, Hedysarum gyrans, has a spontaneous motion in its leaves, independent of any external stimulus, even of light, and only requiring a very warm still atmosphere to be performed in perfection. Each leaf is ternate, and the small lateral leaflets are frequently moving up and down, either equably or by jerks, without any uniformity or cooperation among themselves. It is difficult to guess at the purpose which this singular action is designed to answer to the plant itself; its effect on a rational beholder cannot be indifferent.
The chemical actions of light, heat, and the component parts of the atmospheric air, upon leaves, and, where the latter are wanting, on the green stems of plants, are now, as far as concerns all plants in common, tolerably well understood. The observations and experiments of Priestley and Ingenhousz have been confirmed, extended in a variety of ways, or explained on the principles of improved chemistry, by Dr. Percival and Mr. Henry in England, Dr. Woodhouse in America, and M. Sennebier and M. Théodore de Saussure, as well as various other philosophers, on the continent of Europe. It is agreed that in the day-time plants imbibe from the atmosphere carbonic acid gas, (which was formerly called fixed air, and is an union of oxygen and carbon), that they decompose it, absorb the carbon as matter of nourishment which is added to the sap, and emit the oxygen. So they absorb the same gas from water, when it is separated from that fluid by the action of light. The burning of a candle, or the breathing of animals, in confined air, produces so much of this gas, that neither of these operations can go on beyond a certain time, but the air so contaminated serves as food for vegetables, whose leaves, assisted by light, soon restore the oxygen, or, in other words, purify the air again. This beautiful discovery, for the main principles of which we are indebted to the celebrated Dr. Priestley, shows a mutual dependance of the animal and vegetable kingdoms on each other, which had never been suspected before his time. Comparative experiments upon the lower tribes of these kingdoms have not yet been made, but they would probably afford us a new test for distinguishing them. The air so copiously purified by a Conferva, one of the most inferior in the scale of plants, may be very extensively useful to the innumerable tribes of animated beings which inhabit the same waters. The abundant air-bubbles which have long ago given even a botanical name to one supposed species, Conferva bullosa, are probably a source of life and health to whole nations of aquatic insects, worms and polypes, whenever the sun shines.
In the dark, plants give out carbon and absorb oxygen: but the proportion of the latter is small, compared to what they exhale by day, as must likewise be the proportion of carbon given out; else the quantity of the latter added to their substance would be but trifling, especially in those climates, where the proportion of day to night is nearly equal, and which, notwithstanding, we know to be excessively luxuriant in vegetation. Plants also give out azotic gas: but M. de Saussure is of opinion that this proceeds from their internal substance; and it appears by his experiments to be rather a sign of disease or approaching decay, than a regular chemical production of their constitution when in health; for Sennebier found the quantity of oxygen emitted was in proportion to the thickness of the leaf, or quantity of parenchyma. Yet the parenchyma must be in its original organized state, for when bruised its functions are destroyed.
Possibly such an alternation in the functions of vegetables between day and night may afford a necessary repose to their vital principle, whose share in them we know to be of primary importance. Whatever may happen to plants in the dark, there can be no doubt of their principal business in the œconomy of nature being what we have described. The most luminous and compendious view of the whole subject is given by Dr. Thomson of Edinburgh in the fourth vol. of his Chemistry, which is well worth the attention of those who wish to enter more deeply into all the various chemical examinations respecting it than suits our purpose. It is only necessary to add a short view of Dr. Darwin's hypothesis which Dr. Thomson has not mentioned, probably on account of its insufficiency. That lively writer thought the watery perspiration of leaves, acted upon by light, gave out oxygen for the use of the plant itself, such oxygen being immediately absorbed by the air-vessels. This is by no means adequate to explain any of the phenomena, but rather contradictory to most of them, and is totally superseded by the observations and experiments of other writers.
There can be no question of the general purpose answered to the vegetable constitution by these functions of their leaves. They confirm Mr. Knight's theory of vegetation, who has proved that very little alburnum or new wood is secreted when light is kept from the leaves. They also help us to understand how essential oils may be produced, which are known, as well as sugar, to be composed of oxygen, hydrogen and carbon in different p oportions. We can now have a general idea how the nutritious sap, acted upon by all the agents above mentioned during its stay in the cellular substance of the leaf, and returned from thence impregnated with them into the bark, may prove the source of increase, and of peculiar secretions, in the vegetable frame. That portion of sap sent to the flower and fruit undergoes no less remarkable changes, for purposes to which those curious organs are devoted; nor is it returned from thence, as from the leaves, to answer any further end. The existence of those organs is still more temporary, and more absolutely limited to their own purposes, than even that of the leaves, from whose secretions theirs are very distinct.
But when we attempt to consider how the particular secretions of different species and tribes of plants are formed; how the same soil, the same atmosphere, should in a leaf of the vine or sorrel produce a wholesome acid, and in that of a spurge or manchineel a most virulent poison; how sweet and nutritious herbage should grow among the acrid crowfoot and aconite, we find ourselves totally unable to comprehend the existence of such wonderful powers in so small and seemingly simple an organ as the leaf of a plant. The agency of the vital principle alone can account for these wonders, though it cannot, to our understanding, explain them. "The thickest veil," says Dr. Thomson at the end of his chapter on vegetation, "covers the whole of these processes; and so far have philosophers hitherto been from removing this veil, that they have not even been able to approach it. All these operations, indeed, are evidently chemical decompositions and combinations; but we neither know what these decompositions and combinations are, nor the instruments in which they take place, nor the agents by which they are regulated."
The vain Buffon caused his own statue to be inscribed "a genius equal to the majesty of nature," but a blade of grass was sufficient to confound his pretensions.
- ↑ I do not mean to dispute the accuracy of Mr. Curtis's excellent paper, Tr. of Linn. Soc. v. 6, written to prove honey-dew lo be the dung of Aphides. I only contend that there are more than one kind of honey-dew.