The Power of Movement in Plants Part 20

Fig. 118. Pontederia (sp.?): circ.u.mnutation of leaf, traced from 4.50 P.M.

July 2nd to 10.15 A.M. 4th. Apex of leaf 16 inches from the vertical gla.s.s, so tracing greatly magnified. Temp. about 17o C., and therefore rather too low.

Brazil) (Pontederiaceae, Fam. 46).--A filament was fixed across the apex of a moderately young leaf, 7 inches in height, and its movements were traced during 42 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 circ.u.mnutated over a [page 257]

small s.p.a.ce, 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-p.r.o.nounced oscillations.

CRYPTOGAMS.

(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 movements were traced during 24 h. We see in Fig. 119 that it

Fig. 119. Nephrodium molle: circ.u.mnutation of rachis, traced from 9.15 A.M.

May 28th to 9 A.M. 29th. Figure here given two-thirds of original scale.

plainly circ.u.mnutated. The movement was not greatly magnified as the frond was placed near to the vertical gla.s.s, 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 15o and 16o 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 circ.u.mnutated.*

* Mr. Loomis and Prof. Asa Gray have described ('Botanical Gazette,' 1880, pp. 27, 43), an extremely curious case of movement in the fronds, but only in the fruiting fronds, of Asplenium trichomanes. They move almost as rapidly as the little leaflets of Desmodium gyrans, alternately backwards and forwards through from 20 to 40 degrees, in a plane at right angles to that of the frond. The apex of the frond describes "a long and very narrow ellipse," so that it circ.u.mnutates. But the movement differs from ordinary [[page 258]]

circ.u.mnutation as it occurs only when the plant is exposed to the light; even artificial light "is sufficient to excite motion for a few minutes."

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In the chapter on the Sleep of Plants the conspicuous circ.u.mnutation of Marsilea quadrifoliata (Marsileaceae, Fam. 4) will be described.

It has also been shown in Chap. I. that a very young Selaginella (Lycopodiaceae, Fam. 6), only .4 inch in height, plainly circ.u.mnutated; we may therefore conclude that older plants, whilst growing, would do the same.

Fig. 120. Lunularia vulgaris: circ.u.mnutation of a frond, traced from 9 A.M.

Oct 25th to 8 A.M. 27th.

(35.) Lunularia vulgaris (Hepaticae, Fam. 11, Muscales).--The earth in an old flower-pot was coated with this plant, bearing gemmae. A highly inclined frond, which projected .3 inch above the soil and was .4 inch in breadth, was selected for observation. A gla.s.s hair of extreme tenuity, .75 inch in length, with its end whitened, was cemented with sh.e.l.lac 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 successively made on the vertical gla.s.s-plate in front. Any movement of the frond would of course be exhibited and increased by the long gla.s.s hair; and the black spot was placed so close behind the end of the hair, relatively to the distance of the gla.s.s-plate in front, that the movement of the end was magnified about 40 times. Nevertheless, we are convinced that our tracing gives a fairly faithful representation of the movements of the frond. In the intervals between each observation, the plant was covered by a small bell-gla.s.s. The frond, as already stated, [page 259]

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 circ.u.mnutation. Whether the sinking was due to epinastic growth, or apheliotropism, we do not know. 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.; it is here given (Fig. 120). Another tracing was made on the next day (27th) and the frond was found to be still circ.u.mnutating, for during 14 h. 30 m. it changed its course completely (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 circ.u.mnutate. 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 movement with the nutation of the higher plants.** We shall hereafter see that heliotropic movements result from modified circ.u.mnutation, and as unicellular Moulds bend to the light we may infer that they also circ.u.mnutate.]

CONCLUDING REMARKS ON THE CIRc.u.mNUTATION OF LEAVES.

The circ.u.mnutating movements of young leaves in 33 genera, belonging to 25 families, widely distributed

* 'Ueber die Bewegungen der Faden der Spirogyra princeps: Jahreshefte des Vereins fur vaterlandische Naturkunde in Wurttemberg,' 1874, p. 211.

** Zukal also remarks (as quoted in 'Journal R. Microscop. Soc.,' 1880, vol. iii. p. 320) that the movements of Spirulina, a member of the Oscillatorieae, are closely a.n.a.logous "to the well-known rotation of growing shoots and tendrils."

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amongst ordinary and gymnospermous Dicotyledons and amongst Monocotyledons, together with several Cryptogams, have now been described. It would, therefore, not be rash to a.s.sume that the growing leaves of all plants circ.u.mnutate, 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 pa.s.sed over was 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 2o (and probably even less in some instances) to about 10o; but it amounted to 23o in the common bean. The movement 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 to be called one of circ.u.mnutation; for all circ.u.mnutating 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 [page 261]

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 Dionaea, which resemble those of the hypocotyl of the cabbage, are highly remarkable, as seen under the microscope. They 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 Gramineae, and it is probably common to many plants while circ.u.mnutating. 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 Dionaea; yet the tentacle which was observed 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 circ.u.mnutation 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. Exactly the same phenomenon was observed in the case of cotyledons. The 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 remaining 15 genera there was no periodicity in their movements; for 6 of them were observed during too short a period for any judgment to be formed on this head, [page 262]

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 concerned, by light; and hence probably it is that their leaves, at least in the cases of Sarracenia, Drosera, and Dionaea, do not move periodically. The upward movement 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 provided 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 illuminated from above.

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CHAPTER V.

MODIFIED CIRc.u.mNUTATION: CLIMBING PLANTS; EPINASTIC AND HYPONASTIC MOVEMENTS.

Circ.u.mnutation modified through innate causes or through the action of external conditions--Innate causes--Climbing plants; similarity of their movements with those of ordinary plants; increased amplitude; occasional points of difference--Epinastic growth of young leaves--Hyponastic growth of the hypocotyls and epicotyls of seedlings--Hooked tips of climbing and other plants due to modified circ.u.mnutation--Ampelopsis tricuspidata-- Smithia Pfundii--Straightening of the tip due to hyponasty--Epinastic growth and circ.u.mnutation of the flower-peduncles of Trifolium repens and Oxalis carnosa.

THE radicles, hypocotyls and epicotyls of seedling plants, even before they emerge from the ground, and afterwards the cotyledons, are all continually circ.u.mnutating. So it is with the stems, stolons, flower-peduncles, and leaves of older plants. We may, therefore, infer with a considerable degree of safety that all the growing parts of all plants circ.u.mnutate. 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 circ.u.mnutation of the radicle in penetrating the ground, or that of the arched hypocotyl and epicotyl in breaking through the surface--yet circ.u.mnutation is so general, or rather so universal a phenomenon, that we cannot suppose it to have been gained for any special purpose. We must believe that it follows in some unknown way from the manner in which vegetable tissues grow.

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We shall now consider the many cases in which circ.u.mnutation has been modified for various special purposes; that is, a movement already in progress is temporarily increased in some one direction, and temporarily diminished or quite arrested in other directions. These cases may be divided in two sub-cla.s.ses; in one of which the modification depends on innate or const.i.tutional causes, and is independent of external conditions, excepting in so far that the proper ones for growth must be present. In the second sub-cla.s.s 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-cla.s.s will be considered in the present chapter, and the second sub-cla.s.s in the remainder of this volume.

THE CIRc.u.mNUTATION OF CLIMBING PLANTS.

The simplest case of modified circ.u.mnutation 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 increased growth spread over a considerable length of the moving organ, preceded by turgescence, and acting successively on all sides. The circ.u.mnutation 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 compa.s.s--in their courses being often interrupted by zigzag lines, triangles, loops, or small [page 265]

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 movements cease first in the lower part and then slowly upwards. In both sets of cases the movement may be modified in a closely a.n.a.logous manner by geotropism and by heliotropism; though few climbing plants are heliotropic.

Other points of similarity might be pointed out.

That the movements of climbing plants consist of ordinary circ.u.mnutation, 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 circ.u.mnutation. 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 circ.u.mnutation of climbing plants differs from that of ordinary plants chiefly by its greater amplitude. But most leaves circ.u.mnutate [page 266]

in an almost vertical plane, and therefore describe very narrow ellipses, whereas the many kinds of tendrils 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 circ.u.mnutating stems of Solnanum 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 circ.u.mnutation. The most remarkable special modification of this movement which we have observed is in the tendrils of Echinocystis lobata; these are usually inclined at about 45o 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 pa.s.s over the summit or the shoot from which they arise. If they had not possessed and exercised this curious power, they would infallibly have struck against the summit of the shoot and been arrested in their course. As soon as one of these tendrils with its three branches begins to stiffen itself and rise up vertically, the revolving motion becomes more rapid; and as soon as it has pa.s.sed 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.

* 'The Movements and Habits of Climbing Plants,' p. 36.

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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 circ.u.mnutate largely, and that this movement apparently aids them in finding a pa.s.sage 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 circ.u.mnutation of the flower-peduncle; but we do not know whether it has been increased for this special purpose.

EPINASTY--HYPONASTY.

The term epinasty is used by De Vries* to express greater longitudinal growth along the upper than

* 'Arbeiten des Bot. Inst., in Wurzburg,' Heft ii. 1872, p. 223. De Vries has slightly modified (p. 252) the meaning of the above two terms as first used by Schimper, and they have been adopted in this sense by Sachs.

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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 circ.u.mnutation; 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 circ.u.mnutation as being combined with epinasty, hyponasty, the effects of gravitation, light, etc.; but it seems to us, from reasons hereafter to be given, to be more correct to say that circ.u.mnutation is modified by these several agencies. We will therefore speak of circ.u.mnutation, 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 upper side of the petiole and blade; and De Vries has now shown in more detail that the movement is thus caused, aided slightly by [page 269]

the weight of the leaf, and resisted as he believes by apogeotropism, at least after the leaf has somewhat diverged. In our observations on the circ.u.mnutation 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.