The Power of Movement in Plants

(82.) Pistia stratiotes (Aroidee, Fam. 30). — Hofmeister remarks that the leaves of this floating water-plant are more highly inclined at night than by day.* We therefore fastened a fine glass filament to the midrib of a moderately young leaf, and on Sept. 19th measured the angle which it formed with the horizon 14 times between 9 a.m. and 11.50 p.m. The temperature of the hot-house varied during the two days of observation between 183° and 233°C. At 9 a.m. the filament stood at 32° above the horizon; at 3.34pm. at 10° and at 11.50 pm. at 55°; these two latter angles being the highest and the lowest observed during the day, showing a difference of 45°. The rising did not become strongly marked until between

5 and6p.m. On the next day the leaf stood at only 10° above the horizon at 8.25 a.m, and it remained at about 15° till past 3p.m.; at 5.40 p.m, it was 28°, and at 9.30 p.m. 58°; so that the rise was more sudden this evening than on the previous one, and the difference in the angle amounted to 48°. The movement is obviously periodical, and as the leaf stood on the first night at 55°, and on the second night at 58° above the horizon, it appeared very steeply inclined. This case, as we shall see in a future chapter, ought perhaps to have been included under the head of sleeping plants.

Pontedevia (sp. ?): circumnutatron of leaf, traced from 4.50 p.m. July 2nd to 10.15 a.m.4th. Apex of leaf 16} inches from the vertical glass, so tracing greatly magnified. Temp. about 17° C., and therefore rather too low. Brazil) (Pontederiace, Fam. 46).—A filament was fixed across the apex of a moderately young leaf, 7} inches in height, and its movements were traced during 423 h. (see Fig. 118). On the first evening, when the tracing was begun, and during the night, the leaf descended considerably. On the next morning it ascended in a strongly marked zigzag line, and descended again in the evening and during the night. The movement, therefore, seems to be periodic, but some doubt is thrown on this conclusion, because another leaf, 8 inches in height, appearing older and standing more highly inclined, behaved differently. During the first 12 h. it circumnutated over a

Cuar. IV. CIRCUMNUTATION OF CRYPTOGAMS. 257 small space, but during the night and the whole following day it ascended in the same general direction; the ascent being effected by repeated up and down well-pronounced oscillations. (34.) Nephrodium molle (Filices, Fam. 1).—A filament was fixed near the apex of a young frond of this Fern, 17 inches in height, which was not as yet fully uncurled; and its move- ments were traced during 24h. We see in Fig. 119 that it

Nephrodium molle: civcumnutation of rachis, traced from 9.15 A.M. May 28th to 9 a.m. 29th. Figure here given two-thirds of original scale. plainly cireumnutated. The movement was not greatly magnified as the frond was placed near to the vertical glass, and would probably have been greater and more rapid had the day been warmer, For the plant was brought out of a warm greenhouse and observed under a skylight, where the temperature was between 15° and 16°C. We have seen in Chap. I. that a frond of this Fern, as yet only slightly lobed and with a rachis only ‘23 inch in height, plainly cireumnutated.*

* Mr. Loomis and Prof. Asa Gray have described (‘ Botanical Gazette,’ 1880, pp. 27, 43), an extremely curious case of move- ment in the fronds, but only in the fruiting fronds, of Asplenium trachomanes. They move almost as rapidly as the little leaficts of Desmodium gyrans, alternately backwards and forwards through from 20 to 40 degrees, ina plane at right angles to that of the frond. The apex of the frond describes “a long and very narrow ellipse,” so that it circumnutates. But the movement differs from ordinary

In the chapter on the Sleep of Plants the conspicuous circum- nutation of Marsilea quadri/oliata (Marsileacee, Fam. 4) will be described. It has also been shown in Chap. I. that a very young Sela- ginella (Lycopodiacez, Fam. 6), only ‘4 inch in height, plainly circumnutated; we may therefore conclude that older plants, whilst growing, would do the same. The earth in an old flower-pot was Fig. 120, coated with this plant, bearing gemme. A highly inclined frond, which projected 3 inch above the soil and was ‘4 inch in breadth, was selected for observation. A glass hair of extreme tenuity, ‘75 inch in length, with its end whitened, was cemented with shellac to the frond at right angles to its breadth ; and a white stick with a minute black spot was driven into the soil close behind the end of the hair. The white end could be accurately brought into a line with the black spot, and dots could thus be suc- cessively made on the vertical glass-plate in front. Any move- ment of the frond would of course be exhibited and increased by the long glass hair; and the black spot was placed so close behind the end of the hair, relative'y to the dis- tance of the glass-plate in front, that the movement of the end was F Rete magnified about 40 times. Never- Cunularia vulgaris: cireemnuta- ‘

tion of a frond, traced from theless, we are convinced that ovr presentation of the movements of the frond. In the intervals between each observation, the plant was covered by a small bell-glass. The frond, as already stated, eirenmnutation as it occurs only sufficient to excite motion for » when the plant is exposed to the few minutes,” light; even artificial light “is was highly inclined, and the pot stood in front of a north-east window. During the five first days the frond moved downwards or became less inclined; and the long line which was traced was strongly zigzag, with loops occasionally formed or nearly formed; and this indicated circumnutation. Whether the sink- ing was due to epinastic growth, or apheliotropism, we do not kuow. As the sinking was slight on the fifth day, a new tracing was begun on the sixth day (Oct. 25th), and was continued for 47 h.; itis here given (Fig. 120). Another tracing was made on the next day (27th) and the frond was found to be still cir- cumnutating, for during 14h. 30 m. it changed its course com- pletely (besides minor changes) 10 times. It was casually observed for two more days, and was seen to be continually moving.

‘The lowest members of the vegetable series, the Thallogens, apparently circumnutate. If an Oscillaria be watched under the microscope, it may be seen to describe circles about every 40 seconds. After it has bent to one side, the tip first begins to bend back to the opposite side and then the whole filament curves over in the same direction. Hofmeister* has given a minute account of the curious, but less regular though constant, movements of Spirogyra: during 2} h. the filament moved 4 times to the left and 3 times to the right, and he refers to a movement at right angles to the above. The tip moved at the rate of about 0-1 mm. in five minutes. He compares the move- ment with the nutation of the higher plants.t We shall hereafter see that heliotropic movements result from modified circum- nutation, and as unicellular Moulds bend to the light we may infer that they also circumnutate.

The circumnutating movements of young leaves in 33 genera, belonging to 25 families, widely distributed * ‘Ueber die Bewegungen der Faden der Spirogyra princeps: Jahreshefte des Vereins fiir vater- lindische Naturkunde in Wiirt- temberg,’ 1874, p. 211. t+ Zukalalsoremarks (as quoted in ‘Journal R. Microscop. Soce.,’ 1880, vol. iii. p. 220) that the movements of Spirulina, a mem- ber of the Oscillatoriez, are closely analogous “to the well-known rotation of growing shoots and tendrils.”

amongst ordinary and gymnospermous Dicotyledons and amongst Monocotyledons, together with several Cryptogams, have now been described. It would, therefore, not be rash to assume that the growing leaves of all plants circumnutate, as we have seen reason to conclude is the case with cotyledons. The seat of movement generally lies in the petiole, but sometimes both in the petiole and blade, or in the blade alone. The extent of the movement differed much in different plants; but the distance passed over wag never great, except with Pistia, which ought perhaps to have been included amongst sleeping plants. The angular movement of the leaves was only occasionally measured ; it commonly varied from only 2° (and pro- bably even less in some instances) to about 10°; but it amounted to 25° in the common bean. The move- ment is chiefly in a vertical plane, but as the ascending and descending lines never coincided, there was always some lateral movement, and thus irregular ellipses were formed. The movement, therefore, deserves tc be called one of circumnutation; for all cireumnuta- ting organs tend to describe ellipses,—that is, growth on one side is succeeded by growth on nearly but not quite the opposite side. The ellipses, or the zigzag lines representing drawn-out ellipses, are generally very narrow; yet with the Camellia, their minor axes were half as long, and with the Eucalyptus more than half as long as their major axes. In the case of Cissus, parts of the figure more nearly represented circles than ellipses. The amount of lateral movement is therefore sometimes considerable. Moreover, the longer axes of the successively formed ellipses (as with the Bean, Cissus, and Sea-kale), and in several instances the zigzag lines representing ellipses, were extended in very different directions during the same day or on

the next day. The course followed was curvilinear or straight, or slightly or strongly zigzag, and little loops or triangles were often formed. A single large irregular ellipse may be described on one day, and two smaller ones by the same plant on the next day. With Drosera two, and with Lupinus, Eucalyptus and Pancratium, several were formed each day. The oscillatory and jerking movements of the leaves of Dionza, which resemble those of the hypocotyl of the cabbage, are highly remarkable, as seen under the microscope. ‘hey continue night and day for some months, and are displayed by young unexpanded leaves, and by old ones which have lost their sensibility to a touch, but which, after absorbing animal matter, close their lobes. We shall hereafter meet with the same kind of movement in the joints of certain Graminee, and it is probably common to many plants while cir- cumnutating. It is, therefore, a strange fact that no such movement could be detected in the tentacles of Drosera rotundifolia, though a member of the same family with Dionea ; yet the tentacle which was ob- served was so sensitive, that it began to curl inwards in 23 seconds after being touched by a bit of raw meat.

One of the most interesting facts with respect to the circumnutation of leaves is the periodicity of their movements; for they often, or even generally, rise a little in the evening and early part of the night, and sink again on the following morning. TExactly the same phenomenon was observed in the case of coty- ledons. Thé leaves in 16 genera out of the 33 which were observed behaved in this manner, as did probably 2 others. Nor must it be supposed that in the remain- img 15 geuera there was no periodicity in their move- ments; for 6 of them were observed during too short a period for any judgment to be formed on this head

and 3 were so young that their epinastic growth which serves to bring them down into a horizontal position, overpowered every other kind of movement, In only one genus, Cannabis, did the leaves sink in the evening, and Kraus attributes this movement to the prepotency of their epinastic growth. That the periodicity is determined by the daily alternations of light and darkness there can hardly be a doubt, as will hereafter be shown. Insectivorous plants are very little affected, as far as their movements are con- cerned, by light; and hence probably it is that their leaves, at least in the cases of Sarracenia, Drosera, and Dionza, do not move periodically. The upward move- ment in the evening is at first slow, and with different plants begins at very different hours ;—with Glaucium as early as 11 a.m., commonly between 3 and 5 p.m. but sometimes as late as 7 p.m. It should be observed that none of the leaves described in this chapter (except, as we believe, those of Lupinus speciosus) possess a pulvinus; for the periodical movements of leaves thus provided have generally been amplified into so-called sleep-movements, with which we are not here concerned. The fact of leaves and cotyledons frequently, or even generally, rising a little in the evening and sinking in the morning, is of interest as giving the foundation from which the specialised sleep- movements of many leaves and cotyledons, not pro- vided with a pulvinus, have been developed. The above periodicity should be kept in mind, by any one considering the problem of the horizontal position of leaves and cotyledons during the day, whilst illumi- nated from above.

Movirizp Circtunvuration: Ciimpinc PLAN’S; EPINAsTIC AND Hyponastic Movemunrs. Circumnutation modified through innate causes or through the action of external couditions—Innate causes— Climbing plauts; similarity of their movements with those of ordinary plants; increased ampli- tude; occasional points of difference—Epinastic growth of young leaves--Hyponastic growth of the hypovotyls and epicotyls of seed- lings—Hooked tips of climbing and other plants due to modified circumuutation — Ampelopsis tricuspiduta—Smithia Pfundii — Straightening of the tip due to hyponasty—Epinastic growth and circumnutation of the flower-peduucles of Trifulium repens and Oxalis carnosa,

Tue radicles, hypocotyls and epicotyls of seedling plants, even before they emerge from the ground, and afterwards the cotyledons, are all continually circum- nutating. So it is with the stems, stolons, flower- peduncles, and leaves of older plants. We may, there- fore, infer with a considerable degree of safety that all the growing parts of all plants circumnutate. Although this movement, in its ordinary or unmodified state, appears in some cases to be of service to plants, either directly or indirectly—for instance, the cireum- nutation of the radicle in penetrating the ground, or that of the arched hypocotyl and epicotyl in breaking through the surface—yet circumnutation is so general, or rather so universal a phenomenon, that we cannot suppose it to have been gained for any special pur- pose. We must believe that it follows in some un- known way from the manner in which vegetable tissues grow.

We shall now consider the many cases in whick sircumnutation has been modified for various special purposes; that is, a movement already in progress 1s temporarily increased in some one direction, and tem- porarily diminished or quite arrested in other direc- tions. These caves may be divided in two sub-classes ; in one of which the modification depends on innate or constitutional causes, and is independent of external conditions, excepting in so far that the proper ones for growth must be present. In the second sub-class the modification depends to a large extent on external agencies, such as the daily alternations of light and darkness, or light alone, temperature, or the attraction of gravity. The first small sub-class will be considered in the present chapter, and the second sub-class in the remainder of this volume.

The simplest case of modified circumnutation is that offered by climbing plants, with the exception of those which climb by the aid of motionless hooks or of rootlets; for the modification consists chiefly in the greatly increased amplitude of the movement. This would follow either from greatly increased growth over a small length, or more probably from moderately in- creased growth spread over a considerable length of the moving organ, preceded by turgescence, and acting suc- cessively on all sides. The circumnutation of climbers is more regular than that of ordinary plants; but in almost every other respect there is a close similarity between their movements, namely, in their tendency to describe ellipses directed successively to all points of the compass—in their courses being often inter- rupted py zigzag lines, triangles, loops, or small

ellipses—in the rate of movement, and in different species revolving once or several times within the same length of time. In the same internode, the move- ments cease first in the lower part and then slowly upwards. In both sets of cases the movement mav be modified in a closely analogous manner by geotropism and by heliotropism; though few climbing plants are aeliotropic. Other points of similarity might be pointed out. That the movements of climbing plants consist of ordinary circumnutation, modified by being increased in amplitude, is well exhibited whilst the plants are very young ; for at this early age they move like other seedlings, but as they grow older their movements gradually increase without undergoing any other change. That this power is innate, and is not excited by any external agencies, beyond those necessary for growth and vigour, is obvious. No one doubts that this power has been gained for the sake of enabling climbing plants to ascend to a height, and thus to reach the light. This is effected by two very different methods; first, by twining spirally round a support but to do so their stems must be long and flexible ; and, secondly, in the case of leaf-climbers and _ tendril- bearers, by bringing these organs into contact with a support, which is then seized by the aid of their sensitiveness. It may be here remarked that these latter movements have no relation, as far as we can judge, with circumnutation. In other cases the tips of tendrils, after having been brought into contact with a support, become developed into little discs which adhere firmly to it.

We have said that the circumnutation of climbing plants differs from that of ordinary plants chiefly by its greater amplitude. But most leaves circumnutate in an almost vertical plane, and therefore describe very narrow ellipses, whereas the many kinds of tendriis which consist of metamorphosed leaves, make much broader ellipses or nearly circular figures; and thus they have a far better chance of catching hold of a support on any side. The movements of climbing plants have also been modified in some few other special ways. Thus the circumnutating stems of Sol- nanum dulcamara can twine round a support only when this is as thin and flexible as a string or thread. The twining stems of several British plants cannot twine round a support when it is more than a few inches in thickness; whilst in tropical forests some can embrace thick trunks ;* and this great difference in power depends on some unknown difference in their manner of circumnutation. The most remarkable special modification of this movement which we have observed is in the tendrils of Hchinocystis lobuta ; these ure usually inclined at about 45° above the horizon, but they stiffen and straighten themselves so as to stand upright in a part of their circular course, namely, when they approach and have to pass over the summit of the shoot from which they arise. If they had not possessed and exercised this curious power, they would infallibly have struck against the suiminit of the shoot and been arrested in their course. As soon 23 one of these tendrils with its three branches begins to stiffen itself and rise up vertically, the 1evolvying motion becomes more rapid; and as soon as it has passed over the point of difficulty, its motion coinciding with that from its own weight, causes it to fall into its previously inclined position so quickly, that the apex can be seen travelling like the hand of a gigantic clock,

A large number of ordinary leaves and leaflets and a few flower-peduncles are provided with pulvini; but this is not the case with a single tendril at present known. The cause of this difference probably lies in the fact, that the chief service of a pulvinus is to prolong the movement of the part thus provided after growth has ceased; and as tendrils or other climbing- organs are of use only whilst the plant is increasing in height or growing, a pulvinus which served to prolong their movements would be useless.

It was shown in the last chapter that the stolons or runners of certain plants circumnutate largely, and that this movement apparently aids them in finding a passage between the crowded stems of adjoining plants. If it could be proved that their movements had been modified and increased for this special purpose, they ought to have been included in the present chapter; but as the amplitude of their revolutions is not so conspicuously different from that of ordinary plants, as in the case of climbers, we have no evidence on this head. We encounter the same doubt in the case of some plants which bury their pods in the ground. This burying process is certainly favoured by the circumnutation of the flower-peduncle; but we do not know whether it has been increased for this special purpose.

The term epinasty is used by De Vries* to express greater longitudinal growth along the upper than * ¢Arbciten des Bot. Inst. two terms as first used hy Schim- in Wiirzburg,’ Heftii. 1872, p. 223, per, and they have been adopted De Vries has slightly modified in this sense by Sachs. along the lower side of a part, which is thus caused to bend downwards; and hyponasty is used for the reversed process, by which the part is made to bend upwards. These actions come into play so frequently that the use of the above two terms is highly convenient. The movements thus induced result from a modified form of circumnutation; for, as we shall immediately see, an organ under the influence of epinasty does not. generally move in a straight line downwards, or under that of hyponasty upwards, but oscillates up and down with some lateral movement: it moves, however, in a preponderant manner in one direction. This shows that there is some growth on all sides of the part, but more on the upper side in the case of epinasty, and more on the lower side in that of hyponasty, than on the other sides. At the same time there may be in addition, as De Vries insists, increased growth on one side due to geotropism, and on another side due to heliotropism; and thus the effects of epinasty or of hyponasty may be either increased or lessened.

He who likes, may speak of ordinary cireumnutation as being combined with epinasty, hyponasty, the effects of gravitation, light, &c.; but it seems to us, from reasons hereafter to be given, to be more correct to say that circumnutation is modified by these several agencies. We will therefore speak of cireumnutation, which is always in progress, as modified by epinasty, hyponasty, geotropism, or other agencies, whether internal or external. One of the commonest and simplest cases of epinasty is that offered by leaves, which at an early age are crowded together round the buds, and diverge as they grow older. Sachs first remarked that this was due to increased growth along the uppe. side of the petiole and blade; and De Vries has now shown in tore detail that the movement is thus caused, aided slightiy by

the weight of the leaf, and resisted as he believes by apogeo- tropism, at least after the leaf has somewhat diverged. In our observations on the cirecumnutation of leaves, some were selected which were rather too young, so that they continued to diverge or sink downwards whilst their movements were being traced. This may be seen in the diagrams (Figs. 98 and 112, pp. 282 and 249) representing the circumnutation of the young leaves ot dcanthus mollis and I’elargonium zonale. Similar cases were ob- served with Drosera. The movements of a young leaf, only } inch in length, of Petunia violacea were traced during four days, and offers a betier instance (Fig. 111, p. 248), as it diverged during the whole of this time in a curiously zigzag line with some of the angles sharply acute, and during the latter days plainly circum- nutated. Some young leaves of about the same age on a plant of this Petunia, which had been laid horizontally, and on another plant which was left upright, both being kept in complete dark- ness, diverged in the same manner for 48 h., and apparently were not affected by apogeotropism ; though their stems were in a state of high tension, for when freed from the sticks to which they had been tied, they instantly curled upwards.

The leaves, whilst very young, on the leading shoots of the Carnation (Diauthus caryophyllus) are highly inclined or vertical ; and if the plant is growing vigorously they diverge so quickly that they become almost horizontal in a day. But they move downwards in a rather oblique line and continue for some time afterwards to move in the same direction, in connection, we pre- sume, with their spiral arrangement on the stem. The course pursued by a young leaf whilst thus obliquely descending was traced, and the line was distinctly yet not strongly zigzag ; the larger angles formed by the successive lines amounting only to 135°, 154°, and 163°. The subsequent lateral movement (shown in Fig. 96, p. 231) was strongly zigzag with occasional circum- nutations. The divergence and sinking of the young leaves of this plant seem to be very little affected by geotropism or heliotropism; for a plant, the leaves of which were growing rather slowly (as ascertained by measurement) was laid hori- zontally, and the opposite young leaves diverged from one another symmetrically in the usual manner, without any up- turning in the direction of gravitation or towards the light.

The needle-like leaves of Pinus pinaster form a bundle whilst young ; afterwards they slowly diverge, so that those on the up- right shoots become horizontal. The movements of one such young leaf was traced during 4} days, and the tracing here given (Fig. 121) shows that it descended at first in a nearly straight Pine, rvister : epinastic downward moversent of a young leaf, pro- duced by a young plant ina pot, traced on a vertical glass under a skylight, from U.45 a.m. June 2nd to 10.40 p.m. 6th.

line, but afterwards zigzagged, making one or two little loops. ‘the diverging and descend- ing movements of a rather older leaf were also traced (see former Fig. 113, p. 251): it descended during the first day and night in a some- what zigzag line; it then cir- cumnutated round a small space and again descended. By this time the leaf had nearly assumed its final posi- tion, and now plainly circum- nutated. Asin the case of the Carnation, the leaves, whilst very young, do not seem to be much affected by geotropism or heliotropism, for those on a young plant laid horizontally, and those on another plant left upright, both kept in the dark, continued to diverge in the usual manner without bending to either side.

With Cobea scandens, the young leaves, as they succes- sively diverge from the lead- ing shoot which is bent to one side, rise up so as to pro- ject vertically, and they retain this position for some time whilst the tendril is revolving. The diverging and ascending movements of the petiole of one such a leaf, were traced on a vertical glass under a sky- light; and the course pursued was in most parts nearly straight, but there were twc well-marked zigzags (one of them forming an angle of 112°), and this indicates circumnutation.

The still closed lobes of a young leat of Dionzea projected at right angles to the petiole, and were in the act of slowly rising. A glass filament was attached to the under side of the midrib, and its movements were traced on a vertical glass. It circum- uutated once in the evening, and on the next day rose, as already described (see Fig. 106, p. 240), by a number of acutely zigzag lines, closely approaching in character to ellipses. This move- ment no doubt was due to epinasty, aided by apogeotrvpism, for the closed lobes of a very young leaf on a plant which had been placed horizontally, moved into nearly the same line with the petiole, as if the plant had stood upright; but at the same time the lobes curved laterally upwards, and thus occupied an unnatural position, obliquely to the plane of the foliaceous petiole.