The Power of Movement in Plants

It is evident that a small object attached to the free point of a vertically suspended radicle can offer no mechanical resistance to its growth as a whole, for the object is carried downwards as the radicle elongates, ot upwards as the radicle curves upwards. Nor can the growth of the tip itself be mechanically checked by an object attached to it by gum-water, which remains all the time perfectly soft. The weight of: the object, though quite insignificant, is opposed to the upward curvature. We may therefore conclude that it is the irritation due to contact which excites the movement. The contact, however, must be pro- longed, for the tips of 15 radicles were rubbed for a «hort time, and this did not cause them to bend. Here then we have a case of specialised sensibility, like that of the glands of Drosera; for these are ex- quisitely sensitive to the slightest pressure if prolonged, but not to two or three rough touches.

When the tip of a radicle is lightly touched on one eide with dry nitrate of silver, the injury caused is yery slight, and the adjoining upper part bends away from the cauterised point, with more certainty in most cases than from an object attached on one side. Here it obviously is not the mere touch, but the effect produced by the caustic, which induces the tip to transmit some influence to the adjoining part, causing it to bend away. If one side of the tip is badly injured or killed by the caustic, it ceases to grow, whilst the opposite side continues growing ; and the result is that the tip itself bends towards the injured side and often becomes completely hooked ; and it is remarkable that in this case the adjoining upper part does not bend. The stimulus is too powerful or the shock too great for the proper influence to be trans- mitted from the tip. We have strictly analogous cases with Drosera, Dionea and Pinguicula, with which plants a too powerful stimulus does not excite the tentacles to become incurved, or the lobes to close, ot the margin to be folded inwards.

With respect to the degree of sensitiveness of the apex to contact under favourable conditions, we have seen that with Vicia faba a little square of writing- paper affixed with shellac sufficed to cause move- ment; as did on one occasion a square of merely damped goldbeaters’ skin, but it acted very slowly. Short bits of moderately thick bristle (of which mea- surements have been given) affixed with gum-water acted in only three out of eleven trials, and beads of dried shellac under 335th of a grain in weight acted only twice in nine cases; so that here we have nearly reached the minimum of necessary irrita- tion. The apex, therefore, is much less sensitive to pressure than the glands of Drosera, for these are afiected by far thinner objects than bits of bristle and by a very much less weight than sith of a grain.

But the most interesting evidence of the delicate sensitiveness of the tip of the radicle, was afforded by its power of discriminating between equal-sized squares of card-like and very thin paper, when these were attached on opposite sides, as was observed with the radicles of the bean and oak. When radicles of the bean are extended horizon- tally with squares of card attached to the lower sides ot their tips, the irritation thus caused was always con- quered by geotropism, which then acts under the most favourable conditions at right angles to the radicle. But when objects were attached to the radicles of and of the above-named genera, suspended vertically, the irritation conquered geotropism, which, latter power at first acted obliquely on the radicle; so that the immediate irritation from the attached object, aided by its after-effects, prevailed and caused the radicle to bend upwards, until sometimes the point was directed to the zenith. We must, however, assume that the after-effects of the irritation of the tip by an attached object come into play, only after movement has been excited. The tips of the radicles of the pea seem to be more sensitive to contact than those of the bean, for when they were extended horizontally with squares of card adhering to their lower sides, a most curious struggle occasionally arose, sometimes one and sometimes the other force prevailing, but uiti- mately geotropism was always victorious; neverthe- less, in two instances the terminal part became so much curved upwards that loops were subsequently formed. With the pea, therefore, the irritation from an attached object, and from geotropism when acting at right angles to the radicle, are nearly balanced forces. Closely similar results were observed with the horizontally extended radicles of Cucurbita ovifera,

Finally, the several co-ordinated movements by which radicles are enabled to perform their proper functions are admirably perfect. In whatever direc- tion the primary radicle first protrudes from the seed, geotropism guides it perpendicularly downwards ; and the capacity to be acted on by the attraction of gravity resides in the tip. But Sachs has proved * that the secondary radicles, or those emitted by the primary one, are acted on by geotropism in such a manner that they tend to bend only obliquely down- wards. If they had been acted on like the primary radicle, all the radicles would have penetrated the ground in a close bundle. We have seen that if the end of the primary radicle is cut off or in- jured, the adjoining secondary radicles become geo- tropic and grow vertically downwards. This power must often be of great service to the plant, when the primary radicle has been destroyed by the larvee of insects, burrowing animals, or any other accident. The tertiary radicles, or those emitted by the secondary ones, are not influenced, at least in the case of the bean, by geotropism; so they grow out freely in all directions. From this manner of growth of the various kinds of radicles, they are distributed, together with their absorbent hairs, throughout the surrounding soil. as Sachs has remarked, in the most advantageous manner ; for the whole soil is thus closely searched.

Geotropism, as was shown in the last chapter, excites the primary radicle to bend downwards with very little force, quite insufficient to penetrate the ground. Such penetration is effected by the pointed apex (protected by the root-cap) being pressed down by the longitudinal expansion or growth of the ter- minal rigid portion, aided by its transverse expan- sion, both of which forces act powerfully. It is, however, indispensable that the seeds should be at first held down in some manner. When they lie on the bare surface they are held down by the attach- ment of the root-hairs to any adjoining objects; and this apparently is effected by the conversion of their outer surfaces into a cement. But many seeds get covered up by various accidents, or they fall into crevices or holes. With some seeds their own weight suffices.

The circumnutating movement of the terminal grow- ing part both of the primary and secondary iadieles is so feeble that it can aid them very little in pene- trating the ground, excepting when the superficial layer is very soft and damp. But it must aid them materially when they happen to break obliquely into cracks, or into burrows made by earth-worms or larve. This movement, moreover, combined with the sen- sitiveness of the tip to contact, can hardly fail to be of the highest importance; for as the tip is always endeavouring to bend to all sides it will press on all sides, and will thus be able to discriminate between the harder and softer adjoining surfaces, in the same manner as it discriminated between the attached squares of card-like and,thin paper. Consequently it will tend to bend from the harder soil, and will thus follow the lines of least resistance. So it will be if it meets with a stone or the root of another plant in the soil, as must incessantly occur. If the tip were not sensitive, and if it did not excite the upper part of the root to bend away, whenever it encountered at right angles some obstacle in the ground, it would be hable

to be doubled up into a contorted mass. But we have seen with radicles growing down inclined plates of glass, that as soon as the tip merely touched a slip of wood cemented across the plate, the whole terminal growing part curved away, so that the tip soon stooa ut right angles to its former direction; and thus it would be with an obstacle encountered in the ground, as far as the pressure of the surrounding soil would permit. We can also understand why thick and strong radicles, like those of A#sculus, should be endowed with less sensitiveness than more delicate ones; for the former would be able by the force of their growth to overcome any slight obstacle.

After a radicle, which has been deflected by some stone or root from its natural downward course, reaches the edge of the obstacle, geotropism will direct it to grow again straight downward; but we know that geotropism acts with very little force, and here another excellent adaptation, as Sachs has remarked,* comes into play. For the upper part of the radicle, a little above the apex, is, as we have seen, likewise sensitive ; and this sensitiveness causes the radicle to bend like a tendril towards the touching object, so that as it rubs over the edge of an obstacle, it will bend downwards ; and the curvature thus induced is abrupt, in which respect it differs from that caused by the irritation of one side of the tip. This downward bending coincides with that due to geotropism, and both will cause the root to resume its original course.

As radicles perceive an excess of moisture in the air on one side and bend towards this side, we may infer that they will act in the same manner with respect to moisture in the earth. The sensitiveness to moisture resides in the tip, which determines the bending of the upper part. This capacity perhaps partly accounts for the extent to which drain-pipes often become choked with roots. Considering the several facts given in this chapter, we see that the course followed by a root through the soil is governed by extraordinarily complex and diversified agencies,—by geotropism acting in a different manner on the primary, secondary, and ter- tiary radicles,—by sensitiveness to contact, different in kind in the apex and in the part immediately above the apex, and apparently by sensitiveness to the varying dampness of different parts of the soil. These several stimuli to movement are all more powerful than geotropism, when this acts obliquely on a radicle, which has been deflected from its perpen- dicular downward course. The roots, moreover, of most plants are excited by light to bend either to or from it; but as roots are not naturally exposed to the light it is doubtful whether this sensitiveness, which is perhaps only the indirect result of the radicles being highly sensitive to other stimuli, is of any service to the plant. The direction which the apex takes at each successive period of the growth of a root, ultimately determines its whole course; it is therefore highly important that the apex should pursue from the first the most advantageous direction; and we can thus understand why sensitiveness to geotropism, to contact and to moisture, all reside in the tip, and why the tip determines the upper. growing part to bend either trom or to the exciting cause. <A radicle may be compared with a burrowing animal such as a mole, which wishes to penetrate perpendicularly down into the ground. By continually moving his head from side to side, or circumnutating, he will feel any stone

or other obstacle, as well as any difterence in the hardness of the soil, and he will turn from that side; if the earth is damper on one than on the other side he will turn thitherward as a better hunting-ground. Nevertheless, after each interruption, guided by the sense of gravity, he will be able to recover his down- ward course and to burrow to a greater depth. Tus CircumncTaTinc MovEMENTS OF THE SEVERA. PARTS OF Mature Puants. Circumnutation of stems: concluding remarks on—Circumnutation of stolons: aid thus affurded in winding amongst the stems of sur- rounding plants—Cirecumnutation of flower-stems—Circumnulation of Dicotyledonous leaves—Sinvular oscillatory movement of leaves of Dionzea—Leaves of Cannabis sink at night—Leaves of Gymno- sperms—Of Monocotvledons—Cryptogams—Concluding remarks on the circumnut itiun of leaves: generally rise in the evening and sink in the morning.

the cotyledons and the radicles, are continually cir- cumnutating—that is, they grow first on one side and then on another, such growth being probably preceded by increased turgescence of the cells. As it was unlikely that plants should change their manner of growth with advancing age, it seemed probable that the various organs of all plants at all ages, as long as they continued to grow, would be found to circum- nutate, though perhaps to an extremely small extent.

ously, and which were not known to move in any manner. We commenced with stems. Observations of this kind are tedious, and it appeared to us that it were sclected which, from being woody, or for other reasons, scemed the least likely to circumnutate. The observations and the diagrams were made in the manner described in the Introduction. Plants in pots were subjected to a proper temperature, and whilst being observed, were kept either in darkness or were feebly illuminated from above. They are arranged in the order adopted by Hooker in Le Maout and Decaisne’s ‘System of Botany.’ The number of the family to which each genus belongs is appended, as this serves to show the place of each in the series.

(1.) Mberis wmbellata (Crucifere, Fam. 14).—The movement of the stem of a young plant, 4 inches in height, consisting of four internodes (the hypozotyl included) besides a large bud Dheris umbellata: circumnutation of stem of young plant, traced from 8.30 a.m. Sept. 13th to same hour on following morning. Distance of summit of stem beneath the horizontal glass 7°6 inches. Diagram reduced to half of original size. Movement as here shown magnifie | between 4 and 5 times.

on the summit, was traced, as here shown, during 24 h. (Fig. 70). As far as we could judge the uppermost inch alone of the stem circumnutated, and this in a simple manner. The movement was slow, and the rate very unequal at different times. In part of its course an irregular ellipse, or rather triangle, was completed in 6 h. 30 m. (2) Brassica oleracea (Cruciferee).—A very young plant, beaving three leaves, of which the longest was only three-quarters of an inch in length, was placed under a microscope furnished with an eye-piece micrometer and the tip of the largest leaf was

found to be in constant movement. It crossed five divisions of the micrometer, that is, ;35th of an inch, in 6 m. 20s. There could hardly be a doubt that it was the stem which chiefly moved, for the tip did not get quickly out of focus; and this would have occurred had the movement been confined to the leaf, which moves up or down in nearly the same vertical plane. (3.) Linum usitatissimum (Liner, Fam. 39).—The stems of this plant, shortly before the flowering period, are stated by Fritz Miller (‘Jenaische Zeitschrift,’ B. v. p. 187) to revolve, or circumnutate.

(4.) Pelaryonium zonale (Geraniaces, Fam. 47).—A young plant, 73 inches in height, was observed in the usual manner; but, in order to see the bead at the end of the glass filament Pelargonium zonale: circumnutation of stem of young plant, feebly illu- minated from above. Movement of bead magnified about 11 times; traced on a horizontal glass from noon on March 9th to 8 A.M. oa the 11th. and at the same time the mark beneath, it was necessary to cut off three leaves on one side. We do not know whether it was owing to this cause, or to the plant having previously become bent to one side through heliotropism, but from the morning of the 7th of March to 10.380 p.m. on the 8th, the stem moved a considerable distance in a zigzag line in the same general direction. During the night of the 8th it moved to some distance at right angles to its former course, and next morning (9th) stood for a time almost still. At noon on the 9th a new tracing was begun (see Fig. 71), which was continued till 8 a.m. on the 11th. Between noon on the 9th and 5 p.m. on the 10th (i.e. in the course of 29 h.), the stem described a circle. This plant therefore circumnutates, but at a very slow rate, and toa small extent.

(5.) Tropwolum mujus (?) (dwarfed var. called Tom Thumb) ; (Geraniaces, Fam. 47).—The species of this genus climb by the aid of their sensitive petioles, but some of them also twine round supports; but even these latter species do not begin to circumnutate in a conspicuous manner whilst young. Che Tropeolum majus (?): circumnutation of stem of young plant, traced on a horizontal glass from 9 a.M. Dec. 26th to 10 A.M. on 27th. Movement of bead magnified about 5 times, and here reduced to half of original scale,

variety here treate1 of has a rather thick stem, and is so dwarf that apparently it does not climb in any manner. We there- fore wished to ascertain whether the stem of a young plant, consisting of two in- ternodes, together 3:2 inches in height, cir- cumnutated. It was observed during 25 h., and we see in Fig. 72 that the stem moved in a zigzag course, indicat- ing circumnutation. (6.) Trifolium vesupi- natum (Leguminose, Fam. 75).— When we treat of the sleep of plants, we shall see that the stems in several Trifolium resipinatun ; oircumnutation of aki pacwlaige even

stem, traced on vertical glass from 9.30 instunce, those of Hedy- AM. to 4.30 p.m. Nov. 3rd. Tracing not. saruin, Mimosa, Meli- greatly magnmifiel, reduced to half of Jotus, &e., which are not original size. Plant feebly illuminated li d ‘ aioe Foeabove: climbers, circumnutate i in aconspicuousmanner. We will here give only a single instance (Fig. 73), showing the circumnutation of the stem of a large plant of a clover, Trifolium resupinatum. In the course of 7 h. the stem changed

its course greatly eight times and completed three irregular circles or ellipses. It therefore circumnutated rapidly. Some of the lines run at right angles to one another. Rubus (hyboid) : circumnutation of stem, traced on horizontal glass, from 4 p.m. March 14th to 8.30 a.m. 16th. Tracing much magnified, re- (7.) Rubus idceus (hybrid) (Rosacez, pened to have a young plant, 11 inches in height and growing vigorously, which had been raised from a cross between the raspberry (Rubus idceus) and a North American Rubus, it was observed in the usual manner. During the morning of March 14th the stem almost completed a circle, and then moved far to the right. At 4 p.m. it reversed its course, and now a fresh tracing was begun, which was con- tinued during 40) h., and is given in Fig. 74. We here have weli-marked circumnutation.

(8.) Deutzia gracilis (Saxifrages, Fam. 77)—A shoot on a bush abont 18 inches in height was observed. The bead changed its course greatly cleven times in the course of 10h. 30 m. (Fig. 75), and there could be no doubt about the circumuutation of the stein. (9.) Fuchsia (grcenhouse van, with large flowers, probably a hybrid) (Ona- grariee, Fam. 100).—A young plant, Deutzia gracilis: circumna- tation of stem, kept in darkness, traced on hori- zontal glass, from 8.30 A.M. to 7 P.M. March 20th. Movement of bead origin- ally magnified about 20 times, here reduced to half scale.

accompanying figure (Fig. 76) gives the necessary particulars, ard: shows that the stem circumnutated, though rather Fuchsia (garden var.): circumputation of stem, kept in darkness, traced on horizontal glass, from 8.30 A.M. to 7 P.M. March 20th. Movement of bead originally magnified about 40 times, here reduced to half scale. (10.) Cereus sprciocissimus (garden var., sonictimes called Phyllocactus multiflorus) (Cactes, Fam. 109).— This plant which was growing vigorously from having been removed a few days before from the greenhouse to the hot-house, was observed with especial interest, as it seemed so little probable that the stem would circumnutate. The branches are flat, or flabelliform; but some of them are triangular in section, with the three sides hollowed out. A branch of this latter shape, 9 inches in length and 1) in diameter, was chosen for observa- tion, as less likely to circumnutate than a flabelliform branch. The movement of the bead at the end of the glass filament, affixed to the summit of the branch, was traced (A, Fig. 77) from 9.23 a.m. to 4.30 p.m. on Nov. 23rd, during which time it changed its course greatly six times. On the 24th another tracing was made (see B), and the bead on this day changed its course cftencr, making in 8h. what may be considered as four ellipses, with their longer axes differently directed. The position of the stem and its commencing course on the following morning are likewise shown. There can be no doubt that this branch, though appearing quite rigid, circumnutated; but the

extreme amount of movement during the time was very small, probably rather less than the j5th of an inch. Jerzus speciocissinus: circumnutation of stem, illuminated from above, traced on a horizontal glass, in A from 9 A.M. to 4.30 p.m. on Nov. 23rd; and in B from 8.30 a.m. on the 24th to 8 A.M. on the 25th. Movement of the bead in B magnified about 38 times. (11.) Hedera velix (Araliacere, Fam. 114).—The stem is known fo be apheliotropic, and several seedlings growing in a pot in the greenhouse became bent in the middle of the summer at right angles fiom the light. On Sept. 2nd some of these stems were tied up so as to stand vertically, and were placed before a north-east window; but to our surprise they were now decidedly heliotropic, for during 4 days they curved them- selves towards the light, and their course being traced on a horizontal glass, was strongly zigzag. During the 6 succeed- ing days they circumnutated over the same small space at a slow rate, but there could be no doubt about their circumnuta- tion. The plants were kept exactly in the same place before the window, aud after an interval of 15 days the stems were again observed during 2 days and their movements traced, aud

they were fuund to be still cirecumnutating, but on a yet smaller scale, (12.) Gazania ringens (Composite, Fam. 122).—The circum- nutation of the stem of a young plant, 7 inches in height, as measured to the tip of the highest leaf, was traced during 33 lh, and is shown in the accompanying figure (Fig. 78). Two Gazania rinjens: circunmutation of stem traced from 9 4.M. March 21st to 6 P.M. on 22nd; plant kept in darkness, Movement of bead at the close of the observations magnified 34 times, here reduced to half the original scale.

main lines may be observed running at nearly right angles to two other main lines; but these are interrupted by small loops. (18.) Azalea Indica (Ericinez, Fam. 128).—A bush 21 inches in height was selected for observation, and the circumuutation of its leading shoot was traced during 26 h. 40 m, as shown in the following figure (Fig. 79). (14.) Plumbago Cupensis (Plumbagines, Fam. 134).—A small lateral branch which projected from a tall freely growing bush, at an angle of 35° above the horizon, was selected for obser- vation. For the first 11h. it moved to a considerable distance in a nearly straight line to one side, owing probably to its having been previously deflected by the light whilst standing in the greenhouse. At 7.20 p.m. on March 7th a fresh tracing was begun and continued for the next 43 h, 40 m. (see Fig. 80). During the first 2 h. it followed nearly the same direction ag before. and then changed it a little; during the night it moved at nearly right angles to its previous course. Next

day (Sth) it zigzagged greatly, and on the 9th moved irregu- larly round and round a small circular space. By 3 P.M. on the 9th the figure had become so complicated that no more dots could be made; but the shoot continued during the evening of the 9th, the whole of the 10th, and the morning of the 11th to Azalea Indica: circumnutation of stem, illuminated from II above, traced on horizontal 1 glass, from 9.30 a.m. March 9th to 12.10 p.m. on the 10th.

But on the morning of the 10th only four dots were made between 8.30 A.M. and 12.10 p.M., both hours included, so that the circum- nutation is not fairly repre- sented in this part of the diagram. Movement of the bead here magnified about Plumbago Capensis; circumnu- tation of tip of a Jateral branch, traced on horizontal glass, from 7.20 P.M. on March 7th to 3 p.m. on the 9th. Movement of bead magnified 13 times. Plant feebly illuminated from above.

circumnutate over the same sinall.space, which was only about the ~;th of an inch (‘97 mm.) in diameter. Although this branch circumnutated to a very small extent, yet it changed its course frequently. The movements ought to have been more magnified. (15.) Aloysia citriodora (Verbenacce, Fam. 173).—The follow- ing figure (Fig. 81) gives the movements of a shoot during 31 h. 40 m., and shows that it cireumnutated. The bush waa 15 inches in height.

Aloysia citriodora: circumnutation of stem, traced from 8 20 a.m. on March 22nd to 4e.M.on 23rd. Plant kept in darkness. Movement magnified about 40 times, (16.) Verbena melindres (?) (a scarlet-flowered herbaceous var.) (Verbenaceze).—A shoot 8 inches in height had been laid hori- zontally, for the sake of observing its apogeotropism, and the terminal portion had grown vertically upwards for a length of 1} inches. A glass filament, with a bead at the end, was fixed