The Power of Movement in Plants

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, &c., 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 ob- jects 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 lique- faction 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 sur- rounding them, together with any dissolved matter. After the tip has penetrated the ground to a little depth, the increasing thickness of the radicle, togethe1 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 wili 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 con- tinually 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 245th of a grain (0°33 mg.) ; it is therefore more sensitive than the most delicate ten- dril, namely, that of Passiflora gracilis, which was barely acted on by a bit of wire weighing =1,th 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 7g+q5 of a grain. The sensi- tiveness 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 cauteriscd 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 perpendicu- larly 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 geo- tropism 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 ons instance that such sensitiveness resided in the tip, and caused the adjoining parts to bend from the light. The sub-aérial 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 alsc sensitive to contact, but in a widely different manner. Gravitation, though a less powerful cause of move- ment 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 in Wien, 1879, p. 149), states thad especially atten ed to this subject —apheliotropism docvs not aid re CK. Akad. der Wisseuschatten — dicles in penctrating the ground. arrangement of the three orders of roots is ex2ellently 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 down- wards, 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 con- dition 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 twe 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 explana- tion. But as both legs of the arch grow, their power of breaking through the ground will be much in- creased 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 Graminew 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 cer- tainly was not so in several cases, in which the arch- ing 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 in- herited, 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 conse- quent 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 Graminez, and in

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 Cucurbitacee 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 ex- panded and have diverged, the hypocotyl generally straightens itself by increased growth along the con- cave 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 extraordi- nary manner of germination of Megarrhiza Californica,

Ipomeea leptophylla and pandurata, ar.d of Quercus virens, is connected with the burying of the tuber-like roots, which at an early age are stocked with nutri- ment; 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. Gra- dations of structure are always interesting, and Asa Gray informs us that with Ipomaa 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 hypo- cotyl 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 relat vely 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 ag 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 sen- sitiveness, 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 con- tinually 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 describ- ing small ellipses; as is each petiole, sub-petiole, and leaflet. The latter, as well as ordinary leaves, gene- rally move up and down in nearly the same vertical plane, so that they describe very narrow ellipses. The flower-peduncles are likewise continually circum- nutating. 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 sur- rounding 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 circumnutateina conspicuousmanner,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 pcsition, 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 straighten- ing 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 circum- nutating movement which is preponderant in some one direction.

Again, climbing plants whilst young cireumnutate 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 im- mensely increased, evidently to favour the stem catch- ing 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 tendiils 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

tating movements—those modified through external agencies. The so-called sleep or nyctitropic move- ments of leaves are determined by the daily alterna- tions 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, indepen- dently 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 noc- turnal 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 nyetitropic 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 hori- zontally 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 pinne 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 circumnu- tation, 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 24h.; andin the evening one of them is increased greatly in amplitude until the. blade stands vertically either upwards or downwards. In this position it con- tinues to circumnutate until the following morning, when it reassumes its former position. These move- ments, when a pulvinus is present, are often compli- cated by the rotation of the leaf or leaflet; and such rotation on a small scale occurs during ordinary cir- cumnutation. The many diagrams showing the move- ments of sleeping and nun-sleeping leaves aud coty-

ledons 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 alterna- tions 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 haye given the case of a Porlieria, the leaflets of which remained closed all day, as if asleep, when the plant was kept dry, appa- rently for the sake of checking evaporation. Some- thing of the same kind occurs with certain Graminee. 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 Desmo- dium gyrans, the absence of small lateral leaflets on very young plants, makes us suspect that the imme- diate progenitor of this species did not possess lateral leaflets, and that their appearance in an almost rudi- mentary condition at a somewhat more advanced age is the result of reversion to a trifoliate predecessor. However this may be, the rapid cireumnutating 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 ot 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 vege- table 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-aérial roots. In the two cases investigated by us, the movement certainly consisted of modified circumnutation.