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The Power of Movement in Plants, a non-fiction book by Charles Darwin |
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Chapter 12. Summary And Concluding Remarks |
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_ CHAPTER XII. SUMMARY AND CONCLUDING REMARKS Nature of the circumnutating movement--History of a germinating seed--The radicle first protrudes and circumnutates--Its tip highly sensitive-- Emergence of the hypocotyl or of the epicotyl from the ground under the form of an arch - Its circumnutation and that of the cotyledons--The seedling throws up a leaf-bearing stem--The circumnutation of all the parts or organs--Modified circumnutation--Epinasty and hyponasty--Movements of climbing plants--Nyctitropic movements--Movements excited by light and gravitation--Localised sensitiveness--Resemblance between the movements of plants and animals--The tip of the radicle acts like a brain. IT may be useful to the reader if we briefly sum up the chief conclusions, which, as far as we can judge, have been fairly well established by the observations given in this volume. All the parts or organs in every plant whilst they continue to grow, and some parts which are provided with pulvini after they have ceased to grow, are continually circumnutating. This movement commences even before the young seedling has broken through the ground. The nature of the movement and its causes, as far as ascertained, have been briefly described in the Introduction. Why every part of a plant whilst it is growing, and in some cases after growth has ceased, should have its cells rendered more turgescent and its cell-walls more extensile first on one side and then on another, thus inducing circumnutation is not known. It would appear as if the changes in the cells required periods of rest. In some cases, as with the hypocotyls of Brassica, the leaves of Dionaea and the joints of the Gramineae, the circumnutating movement when viewed under the microscope is seen to consist of innumerable small oscillations. The part under observation suddenly jerks forwards for a length of .002 to .001 of an inch, and then slowly retreats for a part of this distance; after a few seconds it again jerks forwards, but with many intermissions. The retreating movement apparently is due to the elasticity of the resisting tissues. How far this oscillatory movement is general we do not know, as not many circumnutating plants were observed by us under the microscope; but no such movement could be detected in the case of Drosera with a 2-inch object-glass which we used. The phenomenon is a remarkable one. The whole hypocotyl of a cabbage or the whole leaf of a Dionaea could not jerk forwards unless a very large number of cells on one side were simultaneously affected. Are we to suppose that these cells steadily become more and more turgescent on one side, until the part suddenly yields and bends, inducing what may be called a microscopically minute earthquake in the plant; or do the cells on one side suddenly become turgescent in an intermittent manner; each forward movement thus caused being opposed by the elasticity of the tissues? Circumnutation is of paramount importance in the life of every plant; for it is through its modification that many highly beneficial or necessary movements have been acquired. When light strikes one side of a plant, or light changes into darkness, or when gravitation acts on a displaced part, the plant is enabled in some unknown manner to increase the always varying turgescence of the cells on one side; so that the ordinary circumnutating movement is modified, and the part bends either to or from the exciting cause; or it may occupy a new position, as in the so-called sleep of leaves. The influence which modifies circumnutation may be transmitted from one part to another. Innate or constitutional changes, independently of any external agency, often modify the circumnutating movements at particular periods of the life of the plant. As circumnutation is universally present, we can understand how it is that movements of the same kind have been developed in the most distinct members of the vegetable series. But it must not be supposed that all the movements of plants arise from modified circumnutation; for, as we shall presently see, there is reason to believe that this is not the case. Having made these few preliminary remarks, we will in imagination take a germinating seed, and consider the part which the various movements play in the life-history of the plant. The first change is the protrusion of the radicle, which begins at once to circumnutate. This movement is immediately modified by the attraction of gravity and rendered geotropic. The radicle, therefore, supposing the seed to be lying on the surface, quickly bends downwards, following a more or less spiral course, as was seen on the smoked glass-plates. Sensitiveness to gravitation resides in the tip; and it is the tip which transmits some influence to the adjoining parts, causing them to bend. As soon as the tip, protected by the root-cap, reaches the ground, it penetrates the surface, if this be soft or friable; and the act of penetration is apparently aided by the rocking or circumnutating movement of the whole end of the radicle. If the surface is compact, and cannot easily be penetrated, then the seed itself, unless it be a heavy one, is displaced or lifted up by the continued growth and elongation of the radicle. But in a state of nature seeds often get covered with earth or other matter, or fall into crevices, etc., and thus a point of resistance is afforded, and the tip can more easily penetrate the ground. But even with seeds lying loose on the surface there is another aid: a multitude of excessively fine hairs are emitted from the upper part of the radicle, and these attach themselves firmly to stones or other objects lying on the surface, and can do so even to glass; and thus the upper part is held down whilst the tip presses against and penetrates the ground. The attachment of the root-hairs is effected by the liquefaction of the outer surface of the cellulose walls, and by the subsequent setting hard of the liquefied matter. This curious process probably takes place, not for the sake of the attachment of the radicles to superficial objects, but in order that the hairs may be brought into the closest contact with the particles in the soil, by which means they can absorb the layer of water surrounding them, together with any dissolved matter. After the tip has penetrated the ground to a little depth, the increasing thickness of the radicle, together with the root-hairs, hold it securely in its place; and now the force exerted by the longitudinal growth of the radicle drives the tip deeper into the ground. This force, combined with that due to transverse growth, gives to the radicle the power of a wedge. Even a growing root of moderate size, such as that of a seedling bean, can displace a weight of some pounds. It is not probable that the tip when buried in compact earth can actually circumnutate and thus aid its downward passage, but the circumnutating movement will facilitate the tip entering any lateral or oblique fissure in the earth, or a burrow made by an earth-worm or larva; and it is certain that roots often run down the old burrows of worms. The tip, however, in endeavouring to circumnutate, will continually press against the earth on all sides, and this can hardly fail to be of the highest importance to the plant; for we have seen that when little bits of card-like paper and of very thin paper were cemented on opposite sides of the tip, the whole growing part of the radicle was excited to bend away from the side bearing the card or more resisting substance, towards the side bearing the thin paper. We may therefore feel almost sure that when the tip encounters a stone or other obstacle in the ground, or even earth more compact on one side than the other, the root will bend away as much as it can from the obstacle or the more resisting earth, and will thus follow with unerring skill a line of least resistance. The tip is more sensitive to prolonged contact with an object than to gravitation when this acts obliquely on the radicle, and sometimes even when it acts in the most favourable direction at right angles to the radicle. The tip was excited by an attached bead of shellac weighing less than 1/200th of a grain (0.33 mg.); it is therefore more sensitive than the most delicate tendril, namely, that of Passiflora gracilis, which was barely acted on by a bit of wire weighing 1/50th of a grain. But this degree of sensitiveness is as nothing compared with that of the glands of Drosera, for these are excited by particles weighing only 1/78740 of a grain. The sensitiveness of the tip cannot be accounted for by its being covered by a thinner layer of tissue than the other parts, for it is protected by the relatively thick root-cap. It is remarkable that although the radicle bends away, when one side of the tip is slightly touched with caustic, yet if the side be much cauterised the injury is too great, and the power of transmitting some influence to the adjoining parts causing them to bend, is lost. Other analogous cases are known to occur. After a radicle has been deflected by some obstacle, geotropism directs the tip again to grow perpendicularly downwards; but geotropism is a feeble power, and here, as Sachs has shown, another interesting adaptive movement comes into play; for radicles at a distance of a few millimeters from the tip are sensitive to prolonged contact in such a manner that they bend towards the touching object, instead of from it as occurs when an object touches one side of the tip. Moreover, the curvature thus caused is abrupt; the pressed part alone bending. Even slight pressure suffices, such as a bit of card cemented to one side. therefore a radicle, as it passes over the edge of any obstacle in the ground, will through the action of geotropism press against it; and this pressure will cause the radicle to endeavour to bend abruptly over the edge. It will thus recover as quickly as possible its normal downward course. Radicles are also sensitive to air which contains more moisture on one side than the other, and they bend towards its source. It is therefore probable that they are in like manner sensitive to dampness in the soil. It was ascertained in several cases that this sensitiveness resides in the tip, which transmits an influence causing the adjoining upper part to bend in opposition to geotropism towards the moist object. We may therefore infer that roots will be deflected from their downward course towards any source of moisture in the soil. Again, most or all radicles are slightly sensitive to light, and according to Wiesner, generally bend a little from it. Whether this can be of any service to them is very doubtful, but with seeds germinating on the surface it will slightly aid geotropism in directing the radicles to the ground.* We ascertained in one instance that such sensitiveness resided in the tip, and caused the adjoining parts to bend from the light. The sub-aërial roots observed by Wiesner were all apheliotropic, and this, no doubt, is of use in bringing them into contact with trunks of trees or surfaces of rock, as is their habit. We thus see that with seedling plants the tip of the radicle is endowed with diverse kinds of sensitiveness; and that the tip directs the adjoining growing parts to bend to or from the exciting cause, according to the needs of the plant. The sides of the radicle are also sensitive to contact, but in a widely different manner. Gravitation, though a less powerful cause of movement than the other above specified stimuli, is ever present; so that it ultimately prevails and determines the downward growth of the root. The primary radicle emits secondary ones which project sub-horizontally; and these were observed in one case to circumnutate. Their tips are also sensitive to contact, and they are thus excited to bend away from any touching object; so that they resemble in these respects, as far as they were observed, the primary radicles. If displaced they resume, as Sachs has shown, their original sub-horizontal position; and this apparently is due to diageotropism. The secondary radicles emit tertiary ones, but these, in the case of the bean, are not affected by gravitation; consequently they protrude in all directions. Thus the general * Dr. Karl Richter, who has especially attended to this subject ('K. Akad. der Wissenschaften in Wien,' 1879, p. 149), states that apheliotropism does not aid radicles in penetrating the ground. arrangement of the three orders of roots is excellently adapted for searching the whole soil for nutriment. Sachs has shown that if the tip of the primary radicle is cut off (and the tip will occasionally be gnawed off with seedlings in a state of nature) one of the secondary radicles grows perpendicularly downwards, in a manner which is analogous to the upward growth of a lateral shoot after the amputation of the leading shoot. We have seen with radicles of the bean that if the primary radicle is merely compressed instead of being cut off, so that an excess of sap is directed into the secondary radicles, their natural condition is disturbed and they grow downwards. Other analogous facts have been given. As anything which disturbs the constitution is apt to lead to reversion, that is, to the resumption of a former character, it appears probable that when secondary radicles grow downwards or lateral shoots upwards, they revert to the primary manner of growth proper to radicles and shoots. With dicotyledonous seeds, after the protrusion of the radicle, the hypocotyl breaks through the seed-coats; but if the cotyledons are hypogean, it is the epicotyl which breaks forth. These organs are at first invariably arched, with the upper part bent back parallel to the lower; and they retain this form until they have risen above the ground. In some cases, however, it is the petioles of the cotyledons or of the first true leaves which break through the seed-coats as well as the ground, before any part of the stem protrudes; and then the petioles are almost invariably arched. We have met with only one exception, and that only a partial one, namely, with the petioles of the two first leaves of Acanthus candelabrum. With Delphinium nudicaule the petioles of the two cotyledons are com- pletely confluent, and they break through the ground as an arch; afterwards the petioles of the successively formed early leaves are arched, and they are thus enabled to break through the base of the confluent petioles of the cotyledons. In the case of Megarrhiza, it is the plumule which breaks as an arch through the tube formed by the confluence of the cotyledon-petioles. With mature plants, the flower-stems and the leaves of some few species, and the rachis of several ferns, as they emerge separately from the ground, are likewise arched. The fact of so many different organs in plants of many kinds breaking through the ground under the form of an arch, shows that this must be in some manner highly important to them. According to Haberlandt, the tender growing apex is thus saved from abrasion, and this is probably the true explanation. But as both legs of the arch grow, their power of breaking through the ground will be much increased as long as the tip remains within the seed-coats and has a point of support. In the case of monocotyledons the plumule or cotyledon is rarely arched, as far as we have seen; but this is the case with the leaf-like cotyledon of the onion; and the crown of the arch is here strengthened by a special protuberance. In the Gramineae the summit of the straight, sheath-like cotyledon is developed into a hard sharp crest, which evidently serves for breaking through the earth. With dicotyledons the arching of the epicotyl or hypocotyl often appears as if it merely resulted from the manner in which the parts are packed within the seed; but it is doubtful whether this is the whole of the truth in any case, and it certainly was not so in several cases, in which the arching was seen to commence after the parts had wholly escaped from the seed-coats. As the arching occurred in whatever position the seeds were placed, it is no doubt due to temporarily increased growth of the nature of epinasty or hyponasty along one side of the part. As this habit of the hypocotyl to arch itself appears to be universal, it is probably of very ancient origin. It is therefore not surprising that it should be inherited, at least to some extent, by plants having hypogean cotyledons, in which the hypocotyl is only slightly developed and never protrudes above the ground, and in which the arching is of course now quite useless. This tendency explains, as we have seen, the curvature of the hypocotyl (and the consequent movement of the radicle) which was first observed by Sachs, and which we have often had to refer to as Sachs' curvature. The several foregoing arched organs are continually circumnutating, or endeavouring to circumnutate, even before they break through the ground. As soon as any part of the arch protrudes from the seed-coats it is acted upon by apogeotropism, and both the legs bend upwards as quickly as the surrounding earth will permit, until the arch stands vertically. By continued growth it then forcibly breaks through the ground; but as it is continually striving to circumnutate this will aid its emergence in some slight degree, for we know that a circumnutating hypocotyl can push away damp sand on all sides. As soon as the faintest ray of light reaches a seedling, heliotropism will guide it through any crack in the soil, or through an entangled mass of overlying vegetation; for apogeotropism by itself can direct the seedling only blindly upwards. Hence probably it is that sensitiveness to light resides in the tip of the cotyledons of the Gramineae, and in the upper part of the hypocotyls of at least some plants. As the arch grows upwards the cotyledons are dragged out of the ground. The seed-coats are either left behind buried, or are retained for a time still enclosing the cotyledons. These are afterwards cast off merely by the swelling of the cotyledons. But with most of the Cucurbitaceae there is a curious special contrivance for bursting the seed-coats whilst beneath the ground, namely, a peg at the base of the hypocotyl, projecting at right angles, which holds down the lower half of the seed-coats, whilst the growth of the arched part of the hypocotyl lifts up the upper half, and thus splits them in twain. A somewhat analogous structure occurs in Mimosa pudica and some other plants. Before the cotyledons are fully expanded and have diverged, the hypocotyl generally straightens itself by increased growth along the concave side, thus reversing the process which caused the arching. Ultimately not a trace of the former curvature is left, except in the case of the leaf-like cotyledons of the onion. The cotyledons can now assume the function of leaves, and decompose carbonic acid; they also yield up to other parts of the plant the nutriment which they often contain. When they contain a large stock of nutriment they generally remain buried beneath the ground, owing to the small development of the hypocotyl; and thus they have a better chance of escaping destruction by animals. From unknown causes, nutriment is sometimes stored in the hypocotyl or in the radicle, and then one of the cotyledons or both become rudimentary, of which several instances have been given. It is probable that the extraordinary manner of germination of Megarrhiza Californica, Ipomoea leptophylla and pandurata, and of Quercus virens, is connected with the burying of the tuber-like roots, which at an early age are stocked with nutriment; for in these plants it is the petioles of the cotyledons which first protrude from the seeds, and they are then merely tipped with a minute radicle and hypocotyl. These petioles bend down geotropically like a root and penetrate the ground, so that the true root, which afterwards becomes greatly enlarged, is buried at some little depth beneath the surface. Gradations of structure are always interesting, and Asa Gray informs us that with Ipomoea Jalappa, which likewise forms huge tubers, the hypocotyl is still of considerable length, and the petioles of the cotyledons are only moderately elongated. But in addition to the advantage gained by the concealment of the nutritious matter stored within the tubers, the plumule, at least in the case of Megarrhiza, is protected from the frosts of winter by being buried. With many dicotyledonous seedlings, as has lately been described by De Vries, the contraction of the parenchyma of the upper part of the radicle drags the hypocotyl downwards into the earth; sometimes (it is said) until even the cotyledons are buried. The hypocotyl itself of some species contracts in a like manner. It is believed that this burying process serves to protect the seedlings against the frosts of winter. Our imaginary seedling is now mature as a seedling, for its hypocotyl is straight and its cotyledons are fully expanded. In this state the upper part of the hypocotyl and the cotyledons continue for some time to circumnutate, generally to a wide extent relatively to the size of the parts, and at a rapid rate. But seedlings profit by this power of movement only when it is modified, especially by the action of light and gravitation; for they are thus enabled to move more rapidly and to a greater extent than can most mature plants. Seedlings are subjected to a severe struggle for life, and it appears to be highly important to them that they should adapt themselves as quickly and as perfectly as possible to their conditions. Hence also it is that they are so extremely sensitive to light and gravitation. The cotyledons of some few species are sensitive to a touch; but it is probable that this is only an indirect result of the foregoing kinds of sensitiveness, for there is no reason to believe that they profit by moving when touched. Our seedling now throws up a stem bearing leaves, and often branches, all of which whilst young are continually circumnutating. If we look, for instance, at a great acacia tree, we may feel assured that every one of the innumerable growing shoots is constantly describing small ellipses; as is each petiole, sub-petiole, and leaflet. The latter, as well as ordinary leaves, generally move up and down in nearly the same vertical plane, so that they describe very narrow ellipses. The flower-peduncles are likewise continually circumnutating. If we could look beneath the ground, and our eyes had the power of a microscope, we should see the tip of each rootlet endeavouring to sweep small ellipses or circles, as far as the pressure of the surrounding earth permitted. All this astonishing amount of movement has been going on year after year since the time when, as a seedling, the tree first emerged from the ground. Stems are sometimes developed into long runners or stolons. These circumnutate in a conspicuous manner, and are thus aided in passing between and over surrounding obstacles. But whether the circumnutating movement has been increased for this special purpose is doubtful. We have now to consider circumnutation in a modified form, as the source of several great classes of movement. The modification may be determined by innate causes, or by external agencies. Under the first head we see leaves which, when first unfolded, stand in a vertical position, and gradually bend downwards as they grow older. We see flower-peduncles bending down after the flower has withered, and others rising up; or again, stems with their tips at first bowed downwards, so as to be hooked, afterwards straightening themselves; and many other such cases. These changes of position, which are due to epinasty or hyponasty, occur at certain periods of the life of the plant, and are independent of any external agency. They are effected not by a continuous upward or downward movement, but by a succession of small ellipses, or by zigzag lines,--that is, by a circumnutating movement which is preponderant in some one direction. Again, climbing plants whilst young circumnutate in the ordinary manner, but as soon as the stem has grown to a certain height, which is different for different species, it elongates rapidly, and now the amplitude of the circumnutating movement is immensely increased, evidently to favour the stem catching hold of a support. The stem also circumnutates rather more equally to all sides than in the case of non-climbing plants. This is conspicuously the case with those tendrils which consist of modified leaves, as these sweep wide circles; whilst ordinary leaves usually circumnutate nearly in the same vertical plane. Flower-peduncles when converted into tendrils have their circumnutating movement in like manner greatly increased. We now come to our second group of circumnutating movements--those modified through external agencies. The so-called sleep or nyctitropic movements of leaves are determined by the daily alternations of light and darkness. It is not the darkness which excites them to move, but the difference in the amount of light which they receive during the day and night; for with several species, if the leaves have not been brightly illuminated during the day, they do not sleep at night. They inherit, however, some tendency to move at the proper periods, independently of any change in the amount of light. The movements are in some cases extraordinarily complex, but as a full summary has been given in the chapter devoted to this subject, we will here say but little on this head. Leaves and cotyledons assume their nocturnal position by two means, by the aid of pulvini and without such aid. In the former case the movement continues as long as the leaf or cotyledon remains in full health; whilst in the latter case it continues only whilst the part is growing. Cotyledons appear to sleep in a larger proportional number of species than do leaves. In some species, the leaves sleep and not the cotyledons; in others, the cotyledons and not the leaves; or both may sleep, and yet assume widely different positions at night. Although the nyctitropic movements of leaves and cotyledons are wonderfully diversified, and sometimes differ much in the species of the same genus, yet the blade is always placed in such a position at night, that its upper surface is exposed as little as possible to full radiation. We cannot doubt that this is the object gained by these movements; and it has been proved that leaves exposed to a clear sky, with their blades compelled to remain horizontal, suffered much more from the cold than others which were allowed to assume their proper vertical position. Some curious facts have been given under this head, showing that horizontally extended leaves suffered more at night, when the air, which is not cooled by radiation, was prevented from freely circulating beneath their lower surfaces; and so it was, when the leaves were allowed to go to sleep on branches which had been rendered motionless. In some species the petioles rise up greatly at night, and the pinnae close together. The whole plant is thus rendered more compact, and a much smaller surface is exposed to radiation. That the various nyctitropic movements of leaves result from modified circumnutation has, we think, been clearly shown. In the simplest cases a leaf describes a single large ellipse during the 24 h.; and the movement is so arranged that the blade stands vertically during the night, and reassumes its former position on the following morning. The course pursued differs from ordinary circumnutation only in its greater amplitude, and in its greater rapidity late in the evening and early on the following morning. Unless this movement is admitted to be one of circumnutation, such leaves do not circumnutate at all, and this would be a monstrous anomaly. In other cases, leaves and cotyledons describe several vertical ellipses during the 24 h.; and in the evening one of them is increased greatly in amplitude until the blade stands vertically either upwards or downwards. In this position it continues to circumnutate until the following morning, when it reassumes its former position. These movements, when a pulvinus is present, are often complicated by the rotation of the leaf or leaflet; and such rotation on a small scale occurs during ordinary circumnutation. The many diagrams showing the movements of sleeping and non-sleeping leaves and cotyledons should be compared, and it will be seen that they are essentially alike. Ordinary circumnutation is converted into a nyctitropic movement, firstly by an increase in its amplitude, but not to so great a degree as in the case of climbing plants, and secondly by its being rendered periodic in relation to the alternations of day and night. But there is frequently a distinct trace of periodicity in the circumnutating movements of non-sleeping leaves and cotyledons. The fact that nyctitropic movements occur in species distributed in many families throughout the whole vascular series, is intelligible, if they result from the modification of the universally present movement of circumnutation; otherwise the fact is inexplicable. In the seventh chapter we have given the case of a Porlieria, the leaflets of which remained closed all day, as if asleep, when the plant was kept dry, apparently for the sake of checking evaporation. Something of the same kind occurs with certain Gramineae. At the close of this same chapter, a few observations were appended on what may be called the embryology of leaves. The leaves produced by young shoots on cut-down plants of Melilotus Taurica slept like those of a Trifolium, whilst the leaves on the older branches on the same plants slept in a very different manner, proper to the genus; and from the reasons assigned we are tempted to look at this case as one of reversion to a former nyctitropic habit. So again with Desmodium gyrans, the absence of small lateral leaflets on very young plants, makes us suspect that the immediate progenitor of this species did not possess lateral leaflets, and that their appearance in an almost rudimentary condition at a somewhat more advanced age is the result of reversion to a trifoliate predecessor. However this may be, the rapid circumnutating or gyrating movements of the little lateral leaflets, seem to be due proximately to the pulvinus, or organ of movement, not having been reduced nearly so much as the blade, during the successive modifications through which the species has passed. We now come to the highly important class of movements due to the action of a lateral light. When stems, leaves, or other organs are placed, so that one side is illuminated more brightly than the other, they bend towards the light. This heliotropic movement manifestly results from the modification of ordinary circumnutation; and every gradation between the two movements could be followed. When the light was dim, and only a very little brighter on one side than on the other, the movement consisted of a succession of ellipses, directed towards the light, each of which approached nearer to its source than the previous one. When the difference in the light on the two sides was somewhat greater, the ellipses were drawn out into a strongly-marked zigzag line, and when much greater the course became rectilinear. We have reason to believe that changes in the turgescence of the cells is the proximate cause of the movement of circumnutation; and it appears that when a plant is unequally illuminated on the two sides, the always changing turgescence is augmented along one side, and is weakened or quite arrested along the other sides. Increased turgescence is commonly followed by increased growth, so that a plant which has bent itself towards the light during the day would be fixed in this position were it not for apogeotropism acting during the night. But parts provided with pulvini bend, as Pfeffer has shown, towards the light; and here growth does not come into play any more than in the ordinary circumnutating movements of pulvini. Heliotropism prevails widely throughout the vegetable kingdom, but whenever, from the changed habits of life of any plant, such movements become injurious or useless, the tendency is easily eliminated, as we see with climbing and insectivorous plants. Apheliotropic movements are comparatively rare in a well-marked degree, excepting with sub-aërial roots. In the two cases investigated by us, the movement certainly consisted of modified circumnutation. The position which leaves and cotyledons occupy during the day, namely, more or less transversely to the direction of the light, is due, according to Frank, to what we call diaheliotropism. As all leaves and cotyledons are continually circumnutating, there can hardly be a doubt that diaheliotropism results from modified circumnutation. From the fact of leaves and cotyledons frequently rising a little in the evening, it appears as if diaheliotropism had to conquer during the middle of the day a widely prevalent tendency to apogeotropism. Lastly, the leaflets and cotyledons of some plants are known to be injured by too much light; and when the sun shines brightly on them, they move upwards or downwards, or twist laterally, so that they direct their edges towards the light, and thus they escape being injured. These paraheliotropic movements certainly consisted in one case of modified circumnutation; and so it probably is in all cases, for the leaves of all the species described circumnutate in a conspicuous manner. This movement has hitherto been observed only with leaflets provided with pulvini, in which the increased turgescence on opposite sides is not followed by growth; and we can understand why this should be so, as the movement is required only for a temporary purpose. It would manifestly be dis- advantageous for the leaf to be fixed by growth in its inclined position. For it has to assume its former horizontal position, as soon as possible after the sun has ceased shining too brightly on it. The extreme sensitiveness of certain seedlings to light, as shown in our ninth chapter, is highly remarkable. The cotyledons of Phalaris became curved towards a distant lamp, which emitted so little light, that a pencil held vertically close to the plants, did not cast any shadow which the eye could perceive on a white card. These cotyledons, therefore, were affected by a difference in the amount of light on their two sides, which the eye could not distinguish. The degree of their curvature within a given time towards a lateral light did not correspond at all strictly with the amount of light which they received; the light not being at any time in excess. They continued for nearly half an hour to bend towards a lateral light, after it had been extinguished. They bend with remarkable precision towards it, and this depends on the illumination of one whole side, or on the obscuration of the whole opposite side. The difference in the amount of light which plants at any time receive in comparison with what they have shortly before received, seems in all cases to be the chief exciting cause of those movements which are influenced by light. Thus seedlings brought out of darkness bend towards a dim lateral light, sooner than others which had previously been exposed to daylight. We have seen several analogous cases with the nyctitropic movements of leaves. A striking instance was observed in the case of the periodic movements of the cotyledons of a Cassia; in the morning a pot was placed in an obscure part of a room, and all the cotyledons rose up closed; another pot had stood in the sunlight, and the cotyledons of course remained expanded; both pots were now placed close together in the middle of the room, and the cotyledons which had been exposed to the sun, immediately began to close, while the others opened; so that the cotyledons in the two pots moved in exactly opposite directions whilst exposed to the same degree of light. We found that if seedlings, kept in a dark place, were laterally illuminated by a small wax taper for only two or three minutes at intervals of about three-quarters of an hour, they all became bowed to the point where the taper had been held. We felt much surprised at this fact, and until we had read Wiesner's observations, we attributed it to the after-effects of the light; but he has shown that the same degree of curvature in a plant may be induced in the course of an hour by several interrupted illuminations lasting altogether for 20 m., as by a continuous illumination of 60 m. We believe that this case, as well as our own, may be explained by the excitement from light being due not so much to its actual amount, as to the difference in amount from that previously received; and in our case there were repeated alternations from complete darkness to light. In this, and in several of the above specified respects, light seems to act on the tissues of plants, almost in the same manner as it does on the nervous system of animals. There is a much more striking analogy of the same kind, in the sensitiveness to light being localised in the tips of the cotyledons of Phalaris and Avena, and in the upper part of the hypocotyls of Brassica and Beta; and in the transmission of some influence from these upper to the lower parts, causing the latter to bend towards the light. This influence is also transmitted beneath the soil to a depth where no light enters. It follows from this localisation, that the lower parts of the cotyledons of Phalaris, etc., which normally become more bent towards a lateral light than the upper parts, may be brightly illuminated during many hours, and will not bend in the least, if all light be excluded from the tip. It is an interesting experiment to place caps over the tips of the cotyledons of Phalaris, and to allow a very little light to enter through minute orifices on one side of the caps, for the lower part of the cotyledons will then bend to this side, and not to the side which has been brightly illuminated during the whole time. In the case of the radicles of Sinapis alba, sensitiveness to light also resides in the tip, which, when laterally illuminated, causes the adjoining part of the root to bend apheliotropically. Gravitation excites plants to bend away from the centre of the earth, or towards it, or to place themselves in a transverse position with respect to it. Although it is impossible to modify in any direct manner the attraction of gravity, yet its influence could be moderated indirectly, in the several ways described in the tenth chapter; and under such circumstances the same kind of evidence as that given in the chapter on Heliotropism, showed in the plainest manner that apogeotropic and geotropic, and probably diageotropic movements, are all modified forms of circumnutation. Different parts of the same plant and different species are affected by gravitation in widely different degrees and manners. Some plants and organs exhibit hardly a trace of its action. Young seedlings which, as we know, circumnutate rapidly, are eminently sensitive; and we have seen the hypocotyl of Beta bending upwards through 109o in 3 h. 8 m. The after-effects of apogeotropism last for above half an hour; and horizontally-laid hypocotyls are sometimes thus carried temporarily beyond an upright position. The benefits derived from geotropism, apogeotropism, and diageotropism, are generally so manifest that they need not be specified. With the flower-peduncles of Oxalis, epinasty causes them to bend down, so that the ripening pods may be protected by the calyx from the rain. Afterwards they are carried upwards by apogeotropism in combination with hyponasty, and are thus enabled to scatter their seeds over a wider space. The capsules and flower-heads of some plants are bowed downwards through geotropism, and they then bury themselves in the earth for the protection and slow maturation of the seeds. This burying process is much facilitated by the rocking movement due to circumnutation. In the case of the radicles of several, probably of all seedling plants, sensitiveness to gravitation is confined to the tip, which transmits an influence to the adjoining upper part, causing it to bend towards the centre of the earth. That there is transmission of this kind was proved in an interesting manner when horizontally extended radicles of the bean were exposed to the attraction of gravity for 1 or 1 ½ h., and their tips were then amputated. Within this time no trace of curvature was exhibited, and the radicles were now placed pointing vertically downwards; but an influence had already been transmitted from the tip to the adjoining part, for it soon became bent to one side, in the same manner as would have occurred had the radicle remained horizontal and been still acted on by geotropism. Radicles thus treated continued to grow out horizontally for two or three days, until a new tip was re-formed; and this was then acted on by geotropism, and the radicle became curved perpendicularly downwards. It has now been shown that the following important classes of movement all arise from modified circumnutation, which is omnipresent whilst growth lasts, and after growth has ceased, whenever pulvini are present. These classes of movement consist of those due to epinasty and hyponasty,--those proper to climbing plants, commonly called revolving nutation,--the nyctitropic or sleep movements of leaves and cotyledons,--and the two immense classes of movement excited by light and gravitation. When we speak of modified circumnutation we mean that light, or the alternations of light and darkness, gravitation, slight pressure or other irritants, and certain innate or constitutional states of the plant, do not directly cause the movement; they merely lead to a temporary increase or diminution of those spontaneous changes in the turgescence of the cells which are already in progress. In what manner, light, gravitation, etc., act on the cells is not known; and we will here only remark that, if any stimulus affected the cells in such a manner as to cause some slight tendency in the affected part to bend in a beneficial manner, this tendency might easily be increased through the preservation of the more sensitive individuals. But if such bending were injurious, the tendency would be eliminated unless it was overpoweringly strong; for we know how commonly all characters in all organisms vary. Nor can we see any reason to doubt, that after the complete elimination of a tendency to bend in some one direction under a certain stimulus, the power to bend in a directly opposite direction might gradually be acquired through natural selection.* Although so many movements have arisen through modified circumnutation, there are others which appear to have had a quite independent origin; but they do not form such large and important classes. When a leaf of a Mimosa is touched it suddenly assumes the same position as when asleep, but Brucke has shown that this movement results from a different state of turgescence in the cells from that which occurs during sleep; and as sleep-movements are certainly due to modified circumnutation, those from a touch can hardly be thus due. The back of a leaf of Drosera rotundifolia was cemented to the summit of a stick driven into the ground, so that it could not move in the least, and a tentacle was observed during many hours under the microscope; but it exhibited no circumnutating movement, yet after being momentarily touched with a bit of raw meat, its basal part began to curve in 23 seconds. This curving movement therefore could not have resulted from modified circumnutation. But when a small object, such as a fragment of a bristle, was placed on one side of the tip of a radicle, which we know is continually circumnutating, the induced curvature was so similar to the movement caused by geotropism, that we can hardly doubt that it is due to modified circumnutation. A flower of a Mahonia was cemented to a stick, and the stamens exhibited no signs of circumnutation under the microscope, yet when they were lightly touched they suddenly moved towards the pistil. Lastly, the curling of the extremity of a tendril when * See the remarks in Frank's 'Die wagerechte Richtung von Pflanzentheilen' (1870, pp. 90, 91, etc.), on natural selection in connection with geotropism, heliotropism, etc. touched seems to be independent of its revolving or circumnutating movement. This is best shown by the part which is the most sensitive to contact, circumnutating much less than the lower parts, or apparently not at all.* Although in these cases we have no reason to believe that the movement depends on modified circumnutation, as with the several classes of movement described in this volume, yet the difference between the two sets of cases may not be so great as it at first appears. In the one set, an irritant causes an increase or diminution in the turgescence of the cells, which are already in a state of change; whilst in the other set, the irritant first starts a similar change in their state of turgescence. Why a touch, slight pressure or any other irritant, such as electricity, heat, or the absorption of animal matter, should modify the turgescence of the affected cells in such a manner as to cause movement, we do not know. But a touch acts in this manner so often, and on such widely distinct plants, that the tendency seems to be a very general one; and if beneficial, it might be increased to any extent. In other cases, a touch produces a very different effect, as with Nitella, in which the protoplasm may be seen to recede from the walls of the cell; in Lactuca, in which a milky fluid exudes; and in the tendrils of certain Vitaceae, Cucurbitaceae, and Bignoniaceae, in which slight pressure causes a cellular outgrowth. Finally it is impossible not to be struck with the resemblance between the foregoing movements of plants and many of the actions performed unconsciously by the lower animals.** With plants an * For the evidence on this head, see the 'Movements and Habits of Climbing Plants,' 1875, pp. 173, 174. astonishingly small stimulus suffices; and even with allied plants one may be highly sensitive to the slightest continued pressure, and another highly sensitive to a slight momentary touch. The habit of moving at certain periods is inherited both by plants and animals; and several other points of similitude have been specified. But the most striking resemblance is the localisation of their sensitiveness, and the transmission of an influence from the excited part to another which consequently moves. Yet plants do not of course possess nerves or a central nervous system; and we may infer that with animals such structures serve only for the more perfect transmission of impressions, and for the more complete intercommunication of the several parts. We believe that there is no structure in plants more wonderful, as far as its functions are concerned, than the tip of the radicle. If the tip be lightly pressed or burnt or cut, it transmits an influence to the upper adjoining part, causing it to bend away from the affected side; and, what is more surprising, the tip can distinguish between a slightly harder and softer object, by which it is simultaneously pressed on opposite sides. If, however, the radicle is pressed by a similar object a little above the tip, the pressed part does not transmit any influence to the more distant parts, but bends abruptly towards the object. If the tip perceives the air to be moister on one side than on the other, it likewise transmits an influence to the upper adjoining part, which bends towards the source of moisture. When the tip is excited by light (though in the case of radicles this was ascertained in only a single instance) the adjoining part bends from the light; but when excited by gravitation the same part bends towards the centre of gravity. In almost every case we can clearly perceive the final purpose or advantage of the several movements. Two, or perhaps more, of the exciting causes often act simultaneously on the tip, and one conquers the other, no doubt in accordance with its importance for the life of the plant. The course pursued by the radicle in penetrating the ground must be determined by the tip; hence it has acquired such diverse kinds of sensitiveness. It is hardly an exaggeration to say that the tip of the radicle thus endowed, and having the power of directing the movements of the adjoining parts, acts like the brain of one of the lower animals; the brain being seated within the anterior end of the body, receiving impressions from the sense-organs, and directing the several movements. [THE END] _ |
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