The Power of Movement in Plants

As the hypocotyls and epicotyls of some plants protrude from the seed-coats in an arched form, it is doubtful whether the arching of these parts, which is invariably present when they break through the ground, ought always to be attributed to epinasty ; but when they are at first straight and afterwards become arched, as often happens, the arching is certainly due to epinasty. As long as the arch is surrounded by compact earth it must retain its form; but as soon as it rises above the surface, or even before this period if artificially freed from the surrounding pressure, it begins to straighten itself, and this no doubt is mainly due to hyponasty. ‘The movement of the upper and lower half of the arch, and of the crown, was occa- sionally traced ; and the course was more or less zigzag, showing modified circumnutation.

With not a few plants, especially climbers, the suimit of the shoot is hooked, so that the apex points vertically downwards. In seven genera of twining plants * the hooking, or as it has been called by Sachs, the nutation of the tip, is mainly due to an exaggerated form of circumnutation. That is, the growth is so great along one side that it bends the shoot completely over to the opposite side, thus forming a hook; the longitudinal line or zone of growth then travels a little laterally round the shoot, and the hook points in a slightly different direction, and so onwards until the hook is completely reversed. Ultimately it

* «The Movements and Habits of Climbing Plants,’ 2nd edit. p. 13. comes back to the point whence it started. This was ascertained by painting narrow lines with Indian ink along the convex surface of several hooks, and the line was found slowly to be- come at first lateral, then to appear along the concave surface, and ultimately back again on the convex surface. In the case of Lonieera bruchypoda the hooked terminal part of the revolving shoot straightens itself periodically, but is never reversed ; that is, the periodically increased growth of the concave side of the hook is sufficient only to straighten it, and not to bend it over to the opposite side. The hooking of the tip is of service to twining plants by aiding them to catch hold of a support, and afterwards by enabling this part to embrace the support much more closely than it could otherwise have done at first, thus preventing it, as we often observed, from being blown away by a strong wind. Whether the advantage thus gained by twining plants accounts for their summits being so frequently hooked, we do not know, as this structure is not very rare with plants which do not climb, and with some climbers (for instance, Vitis, Ampelopsis, Cissus, &c.) to whom it does not afford any assist- ance in climbing.

With respect to those cases in which the tip remains always bent or hooked towards the same side, as in the genera just named, the most obvious explanation is that the bending is due to continued growth in excess along the convex side. Wiesner, however, maintains* that in all cases the hooking of the tip is the result of its plasticity and weight,—a conclusion which from what we have already seen with several climbing plants is certainly erroneous. Nevertheless, we fully admit that the weight of the part, as well as geotropism, &c., sometimes come into play.

Ampelopsis tricuspidata—This plant climbs by the aid of adhesive tendrils, and the hooked tips of the shoots do not appear to be of any service to it. The hooking depends chiefly, as far as we could ascertain, on the tip being affected by epinasty and geotropism; the lower and older parts continually straight- ening themselves through hyponasty and apogeotropism. We believe that the weight of the apex is an unimportant element, because on horizontal or inclined shoots the hook is often extended horizontally or even faces upwards. Moreover shoots frequently form loops instead of hooks; and in this case the

extreme part, instead of hang- ing vertically down as would follow if weight was the efficient cause, extends horizontally or even points upwards. A shoot, which terminated in a rather open hook, was fastened in a highly inclined downward position, so that the concave side faced upwards, and the result was that the apex at first curved upwards. This ap- parently was due to epinasty and not to apogeotropism, for the apex, soon after passing the perpendicular, curved so rapidly downwards that we could not doubt that the move- ment was at least aided by geotropism. In the course of a few hours the hook was thus converted into a loop with the apex of the shoot pointing straight downwards. The longer axis of the loop was at first horizontal, but after- wards became vertical. During this same time the basal part of the nook (and subsequently of the loop) curved itself slowly upwards; and this must have been wholly due to apogeo- tropism in opposition to hypo- nasty. The loop was then fastened upside down, so that its basal half would be simul- taneously acted on by hypo-

nasty (if present) and by apo- Ampelopsis tricuspidata : hyponastie geotropism; and now it curved itself so greatly upwards in the course of only 4h. that there could hardly be a doubt that both forces were acting movement of hooked tip of leading shoot, traced from 8.10 am. July 13th to 8a.m. 15th. Apex of shoot 53 inches from the vertical glass. Plant illuminated through a sky- light. Temp. 173°-19° C. Diagram reduced to one-third of origina‘ scale

Smithia Pfundir: hyponastic movement of the curved summit of astem, whilst straightening itself, traced from 9 A.M. July 10th to 3p.M. 13th. Apex 92 inches from the vertical glass. Diagram reduced to one-fitth of original scale, Plant illuminated through skylight ; temp. 1749-199 C. together. At the same time the loop became open and was thus reconverted into a hook, and this apparently was effected by the geotropic movement of the apex in opposition to epinasty. In the case of Ampelopsis hede- racea, Weight plays, as far as we could judge, a more im- portant part in the hooking of the tip.

In order to ascertain whether the shoots of A. tri- cuspiduta in straightening themselves under the com- bined action of hyponasty and apogeotropism moved in a simple straight course, or whether they circumnutated, glass filaments were fixed to the crowns of four hooked tips standing in their natural position ; and the movements of the filaments were traced on a vertical glass, All four tracings resembled each other in a general manner ; but we will give only one (see Fig. 122, p. 273). The filament rose at first, which shows that the hook was straighten- ing itself; it then zigzagged, moving a little to the left between 9.25 a.m. and 9 P.M. From this latter hour on the 13th to 10.50 a.m. on the fol- lowing morning (14th) the hook continued to straighten itself, and then zigzagged a short distance to the right. But from 1 p.m. to 10.40 p.m. on the 14th the movement

was reversed and the shoot became more hooked. During the night, after 10.40 p.m. to 8.15 a.m. on the 15th, the hook again opened or straightened itself. By this time the glass filament had become so highly inclined that its movements could. no longer be traced with accuracy; and by 1.30 p.m. on this same day, the crown of the former arch or hook had become perfectly straight and vertical. There can therefore be no doubt that the straightening of the hooked shoot of this plant is effected by the circumnutation of the arched portion—that is, by growth alternating between the upper and Jower surface, but prepon- derant on the lower surface, with some little lateral movement.

We were enabled to trace the movement of another straight- ening shoot for a longer period (owing to its slower growth and to its having been placed further from the vertical glass), namely, from the early morning on July 13th to late in the evening of the 16th. During the whole daytime of the 14th, the hook straight- ened itself very little, but zigzagged and plainly circumnutated about nearly the same spot. By the 16th it had become nearly straight, and the tracing was no longer accurate, yet it was manifest that there was still a considerable amount of movement both up and down and laterally; for the crown whilst con- tinuing to straighten itself occasionally became for a short time more curved, causing the filament to descend twice during thé day.

Smithia Pfundii.--The stiff terminal shoots of this Legu- minous water-plant from Africa project so as to make a rectangle with the stem below; but this occurs only when the plants are growing vigorously, for when kept in a cool place, the summits of the stems become straight, as they likewise did at the close of the growing season. ‘lhe direction of the rectangularly bent part is independent of the chief source of light. But from observing the effects of placing plants in the dark, in which case several shoots became in two or three days upright or nearly upright, and when brought back into the light again became rectangularly curved, we believe that the bending is in part due to apheliotropism, apparently somewhat opposed by apogeo- tropism. On the other hand, from observing the effects of tying a shoot downwards, so that the rectangle faced upwards, we are led to believe that the curvature is partly due to epinasty. As the rectangularly bent portion of an upright stem grows older, the lower part straightens itself; and this is effected through hyponasty. He who has read Sachs’ recent Essay on the vertical

and inclined positions of the parts of plants* will see how diffi- cult a subject this is, and will feel no surprise at our expressing ourselves doubtfully in this and other such cases. A plant, 20 inches in height, was secured to a stick close beneath the curved summit, which formed rather less than a rectangle with the stem below. The shoot pointed away from the observer ; and a glass filament pointing towards the vertical glass on which the tracing was made, was fixed to the convex surface of the curved portion. Therefore the descending lines in. the figure represent the straightening of the curved portion as it grew older. The tracing (Fig. 123, p. 274) was begun at 9 a.m. on July 10th; the filament at first moved but little in a zigzag line, but at 2 p.m. it began rising and continued to do so till 9 p.m.; and this proves that the terminal portion was being more bent downwards. After 9 p.m. on the 10th an opposite movement commenced, and the curved portion began to straighten itself, end this continued till 11.10 a.m. on the 12th, but was interrupted by some small oscillations and zigzags, showing movement in different directions. After 11.10 a.m. on the 12th this part of the stem, still considerably curved, cireumnutated in a con- spicuous manner until nearly 3 p.m. on the 13th; but during all this time a downward movement of the filament prevailed, caused by the continued straightening of the stem. By the afternoon of the 13th, the summit, which had originally been deflected more than a right angle from the perpendicular, had grown so nearly straight that the tracing could no longer be continued on the vertical glass. There can therefore be no doubt that the straightening of the abruptly curved portion of the growing stem of this plant, which appears to be wholly due to hyponasty, is the result of modified circumnutation. We will only add that a filament was fixed in a different manner across the curved summit of another plant, and the same general kind of movement was observed.

Lrifolium repens.—In many, but not in all the species of Tri- folium, as the separate little flowers wither, the sub-peduncles bend downwards, so as to depend parallel to the upper part of the main peduncle. In Zr. subterraneum the main peduncle curves downwards for the sake of burying its capsules, and in this species the sub-peduncles of the separate flowers bend Trifolium repens: circumnu- tating and epinastic move- ments of the sub-peduncle of a single flower, traced ou a vertical glass under a skylight, in A from 11.30 aM. Aug. 27th to 7 A.M. 30th; in B from 7 a.M. Aug. 30th to a little after 6 p.m. Sept. 8th.

upwards, so as to occupy the same position relatively to the upper part of the main peduncle as in 7. repens. This fact alone would render it probable that the movements of the sub- peduncles in Tr. repens were independent of geotropism. Never theless, to make sure, some flower-heads were tied to little sticka upside down and others in a horizontal position; their sub- peduncles, however, all quickly curved upwards through the action of heliotropism. We therefore protected some flower- heads, similarly secured to sticks, from the light, and although some of them rotted, many of their sub-peduncles turned very slowly from their reversed or from their horizontal positions, so as to stand in the normal manner parallel to the upper part of the main peduncle. These facts show that the movement is independent of geotropism or apheliotropism; it must there- be attributed to epinasty, which however is checked, at least as long as the flowers are young, by heliotropism. Most of the above flowers were never fertilised owing to the exclusion of bees ; they consequently withered very slowly, and the movements of the sub-peduncles were in like manner much retarded.

To ascertain the nature of the movement of the sub-peduncle, whilst bending downwards, a filament was fixed across the summit of the calyx of a not fully expanded and almost upright flower, nearly in the centre of the head. The main peduncle was secured to a stick close beneath the head. In order to see the marks on the glass filament, a few flowers had to be cut away on the lower side of the head. The flower under obscr- vation at first diverged a little from its upright position, so as to occupy the open space caused by the removal of the adjoining flowers. This required two days, after which time a new tracing was begun (Fig. 124). In A we see the complex circumnutating course pursued from 11.30 am. Aug. 26th to 7 a.m. on the 30th. The pot was then moved a very little to the right, and the tracing (B) was continued without interruption from 7 a.m. Aug. 30th to after 6 p.m. Sept. 8th. It should be observed that on most of these days, only a single dot was made each morning at the same hour. Whenever the flower was observed carefully, as on Aug. 30th and Sept. 5th and 6th, it was found to be cir- cumnutating over a small space. At last, on Sept. 7th, it began to bend downwards, and continued to do so until after 6 p.m. on the 8th, and indeed until the morning of the 9th, when its movements could no longer be traced on the vertical glass. Jt was carefully observed during the whole of the 8th, and by

10.30 p.m. it had descended to a point lower down by two-thirds of the length of the figure as here given: but from want of space the tracing has been copie in B, only to a little after 6p.m. On the morning of the 9th the flower was withered, and the sub- peduncle now stood at an angle of 57° beneath the horizon. If the flower had been fertilised it would have withered much sooner, and have moved much more quickly. We thus see that the sub-peduncle oscillated up and down, or circumnutated, during its whole downward epinastic course.

The sub-peduncles of the fertilised and withered flowers of Oxulis carnosa likewise bend downwards through epinasty, as will be shown in a future chapter; and thei. downward course is strongly zigzag, indicating cireumnutation. The number of instances in which various organs move through epinasty or hyponasty, often in com- bination with other forces, for the most diversified purposes, seems to be inexhaustibly great; and from the several cases which have been here given, we may safely infer that such movements are due to modified circumnutation.

Mopirtev CigcUsNUTATION: SLEEP OR Nyctirropic MovEmests, THEIR Use: SLEEP OF COTYLEDONS. Preliminary sketch of the sleep or nyctitropic movements of leaves— Presence of pulvini—The lessening of radiation the final cause of nyctitropic movements—Manner of trying experiments on leaves of Oxalis, Arachis, Cassia, Melilotus, Lotus and Marsilea, and on the cots ledons of Mimosa—Conucluding remarks on radiation from leaves —Small differences in the conditions make a great differe-ce in the result—Description of the nyctitropic position and movements of the cotyl.dons of various plants—List of species—Coxcluding rermarks—Indcpendence of the nyctitropic movements of the leaves aud cotyledons of the same species—Reasons for believing that the movements have been acquired fur a special purpose.

Tue so-called sleep of leaves is so conspicuous a phenomenon that it was observed as early as the time of Pliny ;* and since Linneus published his famous Essay, ‘Somnus Plantarum,’ it has been the subject of several memoirs. Many flowers close at night, and these are likewise said to sleep; but we are not here concerned with their movements, for although effected by the same mechanism as in the case of young leaves, namely, unequal growth on the opposite sides (as first proved by Pfeffer), yet they differ essentially in being excited chiefly by changes of temperature instead of light; and in being effected, as far as we can judge, for a different purpose. Hardly any one supposes that there is any real analogy

* Pfeffer has given aclearand — riodi-chen Bewegungen der Blat interesting sketch of the history — torgaue,’ 1875, p. 163 of this subject in his ‘Die Pe- between the sleep of animals and that of plants,” whether of leaves or flowers. It seems, therefore, advisable to give a distinct name to the so-called sleep-movements of plants. These have also generally been confounded, under the term “ periodic,” with the slight daily rise and fall of leaves, as described in the fourth chapter; and this makes it all the more desir- able to give some distinct name to sleep-movements. Nyctitropism and nyctitropic, ie. night-turning, may be applied both to leaves and flowers, and will be occasionally used by us; but it would be best to con- fine the term to leaves. The leaves of some few plants move either upwards or downwards when the sun shines intensely on them, and this movement has sometimes been called diurnal sleep; but we believe it to be of an .essentially different nature from the nocturnal movement, and it will be briefly considered in a future chapter.

The sleep or nyctitropism of leaves is a large subject, and we think that the most convenient plan will be first to give a brief account of the position which leaves assume at night, and of the advantages apparently thus gained. Afterwards the more re- markable cases will be described in detail, with respect to cotyledons in the present chapter, and to leaves in the next chapter. Finally, it will be shown that these movements result from circumnutation, much modified and regulated by the alternations of day and night, or light and darkness; but that they are also to a certain extent inherited.

Leaves, when they go to sleep, move either upwards or downwards, or in the case of the leaflets of com- pound leaves, forwards, that is, towards the apex of the leaf, or backwards, that is, towards its base; or, again, they may rotate on their own axes withott moving either upwards or downwards. But in almost every case the plane of the blade is so placed as to stand nearly or quite vertically at night. ‘Therefore the apex, or the base, or either lateral edge, may be directed towards the zenith. Moreover, the upper surface of each leaf, and more especially of each leaflet, is often brought into close contact with that of the opposite one; and this is sometimes effected by singulatly complicated movements. This fact suggests that the upper surface requires more protection than the lower one. For instance, the terminal leaflet in Trifolium, after turning up at night so as to stand vertically, often continues to bend over until the upper surface is directed downwards whilst the lower surface is fully exposed to the sky; and an arched roof is thus formed over the two lateral leaflets, which have their upper surfaces pressed closely together. Here we have the unusual case of one of the leaflets not standing vertically, or almost vertically, at night.

Considering that leaves in assuming their nycti- tropic positions often move through an angle of 90°; that the movement is rapid in the evening; that in some cases, as we shall see in the next chapter, it is extraordinarily complicated; that with certain seedlings, old enough to bear true leaves, the cotyledons move vertically upwards at night, whilst at the same time the leaflets move ver- tically downwards; and that in the same genus the leaves or cotyledons of some species move upwards, whilst those of other species move down- wards ;—from these and other such facts, it is hardly possible to doubt that plants must derive some

The uyctitropic movements of leaves and cotyledons are effected in two ways,” firstly, by means of pulvini which become, as Pfeffer has shown, alternately more turgescent on opposite sides; and secondly, by in- creased growth along one side of the petiole or midrib, and then on the opposite side, as was first proved by Batalin.t But as it has been shown by De Vries tf that in these latter cases increased growth is preceded by the increased turgescence of the cells, the difference between the above two means of move- ment is much diminished, and consists chiefly in the turgescence of the cells of a fully developed pulvinus, not being followed by growth. When the move- ments of leaves or cotyledons, furnished with a pul- vinus and destitute of one, are compared, they are seen to be closely similar, and are apparently effected for the same purpose. Therefore, with our object in view, it does not appear advisable to separate the above two sets of cases into two distinct classes. There is, how- ever, one important distinction between them, namely, that movements effected by growth on the alternate sides, are confined to young growing leaves, whilst those effected by means of a pulvinus last for a long time. We have already seen well-marked instances of this latter fact with cotyledons, and so it is with leaves, as has been observed by Pfeffer and by ourselves. The long endurance of the nyctitropic movements when effected by the aid of pulvini indicates, in addition tc the evidence already advanced, the functional imvort-

ance of such movements to the plant. There is another difference between the two sets of cases, namely, that there is never, or very rarely, any torsion of the leaves, excepting when a pulvinus is present ;* but this statement applies only to periodic and nyctitropic movements, as may be inferred from other cases given by Frank.t The fact that the leaves of many plants place themselves at night in widely different positions from what they hold during the day, but with the one point in common, that their upper surfaces avoid facing the zenith, often with the additional fact that they come into close contact with opposite leaves or leaflets, clearly indicates, as it seems to us, that the object gained is the protection of the upper sur- faces from being chilled at night by radiation. There is nothing improbable in the upper surface needing protection more than the lower, as the two differ in function and structure. All gardeners know that plants suffer from radiation. It is this and not cold winds which the peasants of Southern Europe fear for their olives.{ Seedlings are often protected from radiation by a very thin covering of straw; and fruit-trees on walls by a few fir-branches, or even by a fishing-net, suspended over them. There is a variety of the gooseberry,§ the flowers of which from being produced before the leaves, are not protected by them from radiation, and consequently often fail to yield fruit. An excellent observer || has remarked

} Martins in ‘Bull. Soc. Bot. de Frauce,’ tom. xix. 1872. Wells, in his fainous ‘ Essay on Dew,’ remarks that an exposed thermometer rises as soon as even w fleecy cluud, high in the sky, paszes over the zenith. 5 that one variety of the cherry has the petals of its flowers much curled backwards, and after a severe frost all the stigmas were killed; whilst at the same time, in another variety with incurved petals, the stigmas were not in the least injured.

This view that the sleep of leaves saves them from being chilled at night by radiation, would no doubt have occurred to Linnzus, had the principle of radia- tion been then discovered; for he suggests in many parts of his ‘Somnus Plantarum’ that the position of the leaves at night protects the young stems and buds, and often the young inflorescence, against cold winds. We are far from doubting that an additional advantage may be thus gained; and we have observed with several plants, for instance, Desmodium gyrans, that whilst the blade of the leaf sinks vertically down at night, the petiole rises, so that the blade has to move through a greater angle in order to assume its vertical position than would otherwise have been necessary ; but with the result that all the leaves on the same plant are crowded together as if for mutual protection.

We doubted at first whether radiation would affect in any important manner objects so thin as are many cotyledons and leaves, and more especially affect dif- ferently their upper and lower surfaces; for although the temperature of their upper surfaces would un- doubtedly fall when freely exposed to a clear sky, yet we. thought that they would so quickly acquire by conduction the temperature of the surrounding air, that it could hardly make any sensible difference to them, whether they stood horizontally and radiated into the open sky, or vertically and radiated chiefly in a lateral direction towards neighbouring plants and other objects. We endeavoured, therefore, to ascer- tain something on this head by preventing the leaves

of several plants from going to sleep, and by exposing to a clear sky when the temperature was beneath the freezing-point, these, as well as the other leaves on the same plants which had already assumed their nocturnal vertical position. Our experiments show that leaves thus compelled to reniain horizontal at night, suffered much more injury from frost than those which were allowed to assume their normal vertical position. It may, however, be said that conclusicns drawn from such observations are not applicable to sleeping plants, the inhabitants of countries where frosts do not occur. But in every sountry, and at all seasons, leaves must be exposed to nocturnal chills through radiation, which might be in some degree injurious to them, and which they would escape by assuming a vertical position.