The Power of Movement in Plants Part 15

apex (protected by the root-cap) being pressed down by the longitudinal expansion or growth of the terminal rigid portion, aided by its transverse expansion, 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 attachment 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 circ.u.mnutating movement of the terminal growing part both of the primary and secondary radicles is so feeble that it can aid them very little in penetrating 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 larvae. This movement, moreover, combined with the sensitiveness 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 liable [page 198]

to be doubled up into a contorted ma.s.s. But we have seen with radicles growing down inclined plates of gla.s.s, 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 stood at 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 Aesculus, 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

* 'Arbeiten Bot. Inst., Wurzburg,' Heft iii. p. 456.

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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 tertiary 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 perpendicular 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 from 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 circ.u.mnutating, he will feel any stone [page 200]

or other obstacle, as well as any difference 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 downward course and to burrow to a greater depth.

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CHAPTER IV.

THE CIRc.u.mNUTATING MOVEMENTS OF THE SEVERAL PARTS OF MATURE PLANTS.

Circ.u.mnutation of stems: concluding remarks on--Circ.u.mnutation of stolons: aid thus afforded in winding amongst the stems of surrounding plants-- Circ.u.mnutation of flower-stems--Circ.u.mnutation of Dicotyledonous leaves-- Singular oscillatory movement of leaves of Dionaea--Leaves of Cannabis sink at night--Leaves of Gymnosperms--Of Monocotyledons--Cryptogams--Concluding remarks on the circ.u.mnutation of leaves; generally rise in the evening and sink in the morning.

WE have seen in the first chapter that the stems of all seedlings, whether hypocotyls or epicotyls, as well as the cotyledons and the radicles, are continually circ.u.mnutating--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 circ.u.mnutate, though perhaps to an extremely small extent. As it was important for us to discover whether this was the case, we determined to observe carefully a certain number of plants which were growing vigorously, 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 would be sufficient to observe the stems in about a score of genera, belonging to widely distinct families and inhabitants of various countries. Several plants [page 202]

were selected which, from being woody, or for other reasons, seemed the least likely to circ.u.mnutate. 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.) Iberis umbellata (Cruciferae, Fam. 14).--The movement of the stem of a young plant, 4 inches in height, consisting of four internodes (the hypocotyl included) besides a large bud

Fig. 70. Iberis umbellata: circ.u.mnutation 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 gla.s.s 7.6 inches. Diagram reduced to half of original size. Movement as here shown magnified 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 circ.u.mnutated, 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.) Bra.s.sica oleracea (Cruciferae).--A very young plant, bearing 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 [page 203]

found to be in constant movement. It crossed five divisions of the micrometer, that is, 1/100th of an inch, in 6 m. 20 s. 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 (Lineae, Fam. 39).--The stems of this plant, shortly before the flowering period, are stated by Fritz Muller ('Jenaische Zeitschrift,' B. v. p. 137) to revolve, or circ.u.mnutate.

(4.) Pelargonium zonale (Geraniaceae, Fam. 47).--A young plant, 7 inches in height, was observed in the usual manner; but, in order to see the bead at the end of the gla.s.s filament

Fig. 71. Pelargonium zonale: circ.u.mnutation of stem of young plant, feebly illuminated from above. Movement of bead magnified about 11 times; traced on a horizontal gla.s.s from noon on March 9th to 8 A.M. on 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 10.30 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 circ.u.mnutates, but at a very slow rate, and to a small extent.

(5.) Tropaeolum majus (?) (dwarfed var. called Tom Thumb); (Geraniaceae, Fam. 47).--The species of this genus climb by the [page 204]

aid of their sensitive petioles, but some of them also twine round supports; but even these latter species do not begin to circ.u.mnutate in a conspicuous manner whilst young. The

Fig. 72. Tropaeolum majus (?): circ.u.mnutation of stem of young plant, traced on a horizontal gla.s.s 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 treated of has a rather thick stem, and is so dwarf that apparently it does not climb in any manner. We therefore wished to ascertain whether the stem of a young plant, consisting of two internodes, together 3.2 inches in height, circ.u.mnutated. It was observed during 25 h., and we see in Fig. 72 that the stem moved in a zigzag course, indicating circ.u.mnutation.

Fig. 73. Trifolium resupinatum: circ.u.mnutation of stem, traced on vertical gla.s.s from 9.30 A.M. to 4.30 P.M. Nov. 3rd. Tracing not greatly magnified, reduced to half of original size. Plant feebly illuminated from above.

(6.) Trifolium resupinatum (Leguminosae, Fam. 75).--When we treat of the sleep of plants, we shall see that the stems in several Leguminous genera, for instance, those of Hedysarum, Mimosa, Melilotus, etc., which are not climbers, circ.u.mnutate in a conspicuous manner. We will here give only a single instance (Fig. 73), showing the circ.u.mnutation of the stem of a large plant of a clover, Trifolium resupinatum. In the course of 7 h. the stem changed [page 205]

its course greatly eight times and completed three irregular circles or ellipses. It therefore circ.u.mnutated rapidly. Some of the lines run at right angles to one another.

Fig. 74. Rubus (hybrid): circ.u.mnutation of stem, traced on horizontal gla.s.s, from 4 P.M. March 14th to 8.30 A.M. 16th. Tracing much magnified, reduced to half of original size. Plant illuminated feebly from above.

(7.) Rubus idaeus (hybrid) (Rosaceae, Fam. 76).--As we happened to have a young plant, 11 inches in height and growing vigorously, which had been raised from a cross between the raspberry (Rubus idaeus) 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 continued during 40 h., and is given in Fig. 74. We here have well-marked circ.u.mnutation.

(8.) Deutzia gracilis (Saxifrageae, Fam. 77).--A shoot on a bush about 18 inches in height was observed. The bead changed its course greatly eleven times in the course of 10 h. 30 m. (Fig. 75), and there could be no doubt about the circ.u.mnutation of the stem.

Fig. 75. Deutzia gracilis: circ.u.mnutation of stem, kept in darkness, traced on horizontal gla.s.s, from 8.30 A.M. to 7 P.M. March 20th. Movement of bead originally magnified about 20 times, here reduced to half scale.

(9.) Fuchsia (greenhouse var., with large flowers, probably a hybrid) (Onagrarieae, Fam. 100).--A young plant, 15 inches in height, was observed during nearly 48 h. The [page 206]

accompanying figure (Fig. 76) gives the necessary particulars, and shows that the stem circ.u.mnutated, though rather slowly.

Fig. 76. Fuchsia (garden var.): circ.u.mnutation of stem, kept in darkness, traced on horizontal gla.s.s, 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 speciocissimus (garden var., sometimes called Phyllocactus multiflorus) (Cacteae, 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 circ.u.mnutate. 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 observation, as less likely to circ.u.mnutate than a flabelliform branch. The movement of the bead at the end of the gla.s.s 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 oftener, making in 8 h. 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, circ.u.mnutated; but the [page 207]

extreme amount of movement during the time was very small, probably rather less than the 1/20th of an inch.

Fig 77. Cereus speciocissimus: circ.u.mnutation of stem, illuminated from above, traced on a horizontal gla.s.s, 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 helix (Araliaceae, Fam. 114).--The stem is known to be apheliotropic, and several seedlings growing in a pot in the greenhouse became bent in the middle of the summer at right angles from 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 themselves towards the light, and their course being traced on a horizontal gla.s.s, was strongly zigzag. During the 6 succeeding days they circ.u.mnutated over the same small s.p.a.ce at a slow rate, but there could be no doubt about their circ.u.mnutation. The plants were kept exactly in the same place before the window, and after an interval of 15 days the stems were again observed during 2 days and their movements traced, and [page 208]

they were found to be still circ.u.mnutating, but on a yet smaller scale.

(12.) Gazania ringens (Compositae, Fam. 122).--The circ.u.mnutation of the stem of a young plant, 7 inches in height, as measured to the tip of the highest leaf, was traced during 33 h., and is shown in the accompanying figure (Fig. 78). Two

Fig. 78. Gazania ringens: circ.u.mnutation of stem traced from 9 A.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.

(13.) Azalea Indica (Ericineae, Fam. 128).--A bush 21 inches in height was selected for observation, and the circ.u.mnutation of its leading shoot was traced during 26 h. 40 m., as shown in the following figure (Fig. 79).

(14.) Plumbago Capensis (Plumbagineae, Fam. 134).--A small lateral branch which projected from a tall freely growing bush, at an angle of 35o above the horizon, was selected for observation. For the first 11 h. 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 as before, and then changed it a little; during the night it moved at nearly right angles to its previous course. Next [page 209]

day (8th) it zigzagged greatly, and on the 9th moved irregularly round and round a small circular s.p.a.ce. 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

Fig. 79. Azalea Indica: circ.u.mnutation of stem, illuminated from above, traced on horizontal gla.s.s, 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 circ.u.mnutation is not fairly represented in this part of the diagram. Movement of the bead here magnified about 30 times.

Fig. 80. Plumbago Capensis: circ.u.mnutation of tip of a lateral branch, traced on horizontal gla.s.s, 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.