The Power of Movement in Plants Part 16

circ.u.mnutate over the same small s.p.a.ce, which was only about the 1/26th of an inch (.97 mm.) in diameter. Although this branch circ.u.mnutated to a very small extent, yet it changed its course frequently. The movements ought to have been more magnified.

(15.) Aloysia citriodora (Verbenaceae, Fam. 173).--The following figure (Fig. 81) gives the movements of a shoot during [page 210]

31 h. 40 m., and shows that it circ.u.mnutated. The bush was 15 inches in height.

Fig. 81. Aloysia citriodora: circ.u.mnutation of stem, traced from 8.20 A.M.

on March 22nd to 4 P.M. on 23rd. Plant kept in darkness. Movement magnified about 40 times.

(16.) Verbena melindres (?) (a scarlet-flowered herbaceous var.) (Verbenaceae).--A shoot 8 inches in height had been laid horizontally, for the sake of observing its apogeotropism, and the terminal portion had grown vertically upwards for a length of 1 inch. A gla.s.s filament, with a bead at the end, was fixed

Fig. 82. Verbena melindres: circ.u.mnutation of stem in darkness, traced on vertical gla.s.s, from 5.30 P.M. on June 5th to 11 A.M. June 7th. Movement of bead magnified 9 times.

upright to the tip, and its movements were traced during 41 h. 30 m. on a vertical gla.s.s (Fig. 82). Under these circ.u.mstances the lateral movements were chiefly shown; but as the lines from side to side are not on the same level, the shoot [page 211]

must have moved in a plane at right angles to that of the lateral movement, that is, it must have circ.u.mnutated. On the next day (6th) the shoot moved in the course of 16 h. four times to the right, and four times to the left; and this apparently represents the formation of four ellipses, so that each was completed in 4 h.

(17.) Ceratophyllum demersum (Ceratophylleae, Fam. 220).--An interesting account of the movements of the stem of this water-plant has been published by M. E. Rodier.* The movements are confined to the young internodes, becoming less and less lower down the stem; and they are extraordinary from their amplitude. The stems sometimes moved through an angle of above 200o in 6 h., and in one instance through 220o in 3 h. They generally bent from right to left in the morning, and in an opposite direction in the afternoon; but the movement was sometimes temporarily reversed or quite arrested. It was not affected by light. It does not appear that M. Rodier made any diagram on a horizontal plane representing the actual course pursued by the apex, but he speaks of the "branches executing round their axes of growth a movement of torsion." From the particulars above given, and remembering in the case of twining plants and of tendrils, how difficult it is not to mistake their bending to all points of the compa.s.s for true torsion, we are led to believe that the stems of this Ceratophyllum circ.u.mnutate, probably in the shape of narrow ellipses, each completed in about 26 h. The following statement, however, seems to indicate something different from ordinary circ.u.mnutation, but we cannot fully understand it. M. Rodier says: "Il est alors facile de voir que le mouvement de flexion se produit d'abord dans les merithalles superieurs, qu'il se propage ensuite, en s'amoindrissant du haut en bas; tandis qu'au contraire le movement de redress.e.m.e.nt commence par la partie inferieur pour se terminer a la partie superieure qui, quelquefois, peu de temps avant de se relever tout a fait, forme avec l'axe un angle tres aigu."

(18.) Coniferae.--Dr. Maxwell Masters states ('Journal Linn. Soc.,' Dec.

2nd, 1879) that the leading shoots of many Coniferae during the season of their active growth exhibit very remarkable movements of revolving nutation, that is, they circ.u.mnutate. We may feel sure that the lateral shoots whilst growing would exhibit the same movement if carefully observed.

* 'Comptes Rendus,' April 30th, 1877. Also a second notice published separately in Bourdeaux, Nov. 12th, 1877.

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(19.) Lilium auratum (Fam. Liliaceae).--The circ.u.mnutation

Fig. 83. Lilium auratum: circ.u.mnutation of a stem in darkness, traced on a horizontal gla.s.s, from 8 A.M. on March 14th to 8.35 A.M. on 16th. But it should be noted that our observations were interrupted between 6 P.M. on the 14th and 12.15 P.M. on the 15th, and the movements during this interval of 18 h. 15 m. are represented by a long broken line. Diagram reduced to half original scale.

of the stem of a plant 24 inches in height is represented in the above figure (Fig. 83).

Fig. 84. Cyperus alternifolius: circ.u.mnutation of stem, illuminated from above, traced on horizontal gla.s.s, from 9.45 A.M. March 9th to 9 P.M. on 10th. The stem grew so rapidly whilst being observed, that it was not possible to estimate how much its movements were magnified in the tracing.

(20.) Cyperus alternifolius (Fam. Cyperaceae.)--A gla.s.s [page 213]

filament, with a bead at the end, was fixed across the summit of a young stem 10 inches in height, close beneath the crown of elongated leaves. On March 8th, between 12.20 and 7.20 P.M. the stem described an ellipse, open at one end. On the following day a new tracing was begun (Fig. 84), which plainly shows that the stem completed three irregular figures in the course of 35 h. 15 m.]

Concluding Remarks on the Circ.u.mnutation of Stems.--Any one who will inspect the diagrams now given, and will bear in mind the widely separated position of the plants described in the series,--remembering that we have good grounds for the belief that the hypocotyls and epicotyls of all seedlings circ.u.mnutate,--not forgetting the number of plants distributed in the most distinct families which climb by a similar movement,--will probably admit that the growing stems of all plants, if carefully observed, would be found to circ.u.mnutate to a greater or less extent. When we treat of the sleep and other movements of plants, many other cases of circ.u.mnutating stems will be incidentally given. In looking at the diagrams, we should remember that the stems were always growing, so that in each case the circ.u.mnutating apex as it rose will have described a spire of some kind. The dots were made on the gla.s.ses generally at intervals of an hour, or hour and a half, and were then joined by straight lines. If they had been made at intervals of 2 or 3 minutes, the lines would have been more curvilinear, as in the case of the tracks left on the smoked gla.s.s-plates by the tips of the circ.u.mnutating radicles of seedling plants.

The diagrams generally approach in form to a succession of more or less irregular ellipses or ovals, with their longer axes directed to different points of the compa.s.s during the same day or on succeeding days. The stems there- [page 214]

fore, sooner or later, bend to all sides; but after a stem has bent in any one direction, it commonly bends back at first in nearly, though not quite, the opposite direction; and this gives the tendency to the formation of ellipses, which are generally narrow, but not so narrow as those described by stolons and leaves. On the other hand, the figures sometimes approach in shape to circles. Whatever the figure may be, the course pursued is often interrupted by zigzags, small triangles, loops, or ellipses. A stem may describe a single large ellipse one day, and two on the next. With different plants the complexity, rate, and amount of movement differ much.

The stems, for instance, of Iberis and Azalea described only a single large ellipse in 24 h.; whereas those of the Deutzia made four or five deep zigzags or narrow ellipses in 11 h., and those of the Trifolium three triangular or quadrilateral figures in 7 h.

CIRc.u.mNUTATION OF STOLONS OR RUNNERS.

Stolons consist of much elongated, flexible branches, which run along the surface of the ground and form roots at a distance from the parent-plant.

They are therefore of the same h.o.m.ological nature as stems; and the three following cases may be added to the twenty previously given cases.

[Fragaria (cultivated garden var.): Rosaceae.--A plant growing in a pot had emitted a long stolon; this was supported by a stick, so that it projected for the length of several inches horizontally. A gla.s.s filament bearing two minute triangles of paper was affixed to the terminal bud, which was a little upturned; and its movements were traced during 21 h., as shown in Fig. 85. In the course of the first 12 h. it moved twice up and twice down in somewhat zigzag lines, and no doubt travelled in the same manner during the night. On the following [page 215]

morning after an interval of 20 h. the apex stood a little higher than it did at first, and this shows that the stolon had not been Fig. 85. Fragaria: circ.u.mnutation of stolon, kept in darkness, traced on vertical gla.s.s, from 10.45 A.M. May 18th to 7.45 A.M. on 19th.

acted on within this time by geotropism;* nor had its own weight caused it to bend downwards.

On the following morning (19th) the gla.s.s filament was detached and refixed close behind the bud, as it appeared possible that the circ.u.mnutation of the terminal bud and of the adjoining part of the stolon might be different. The movement was now traced during two consecutive days (Fig.

86). During the first day the filament travelled in the course of 14 h. 30 m. five times up and four times down, besides some lateral movement. On the 20th the course was even more complicated, and can hardly be followed in the figure; but the filament moved in 16 h. at least five times up and five times down, with very little

* Dr. A. B. Frank states ('Die Naturliche wagerechte Richtung von Pflanzentheilen,' 1870, p. 20) that the stolons of this plant are acted on by geotropism, but only after a considerable interval of time.

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lateral deflection. The first and last dots made on this second day, viz., at 7 A.M. and 11 P.M., were close together, showing that the stolon had not fallen or risen. Nevertheless, by comparing its position on the morning of the 19th and 21st, it is obvious that the stolon had sunk; and this may be attributed to slow bending down either from its own weight or from geotropism.

Fig. 86. Fragaria: circ.u.mnutation of the same stolon as in the last figure, observed in the same manner, and traced from 8 A.M. May 19th to 8 A.M.

21st.

During a part of the 20th an orthogonal tracing was made by applying a cube of wood to the vertical gla.s.s and bringing the apex of the stolon at successive periods into a line with one edge; a dot being made each time on the gla.s.s. This tracing therefore represented very nearly the actual amount of movement of the apex; and in the course of 9 h. the distance of the extreme dots from one another was .45 inch. By the same method it was ascertained that the apex moved between 7 A.M. on the 20th and 8 A.M. on the 21st a distance of .82 inch.

A younger and shorter stolon was supported so that it projected at about 45o above the horizon, and its movement was traced by the same orthogonal method. On the first day the apex soon rose above the field of vision. By the next morning it had sunk, and the course pursued was now traced during 14 h. 30 m. (Fig. 87). The amount of movement was almost the same, [page 217]

from side to side as up and down; and differed in this respect remarkably from the movement in the previous cases. During the latter part of the day, viz., between 3 and 10.30 P.M., the

Fig. 87. Fragaria: circ.u.mnutation of another and younger stolon, traced from 8 A.M. to 10.30 P.M. Figure reduced to one-half of original scale.

actual distance travelled by the apex amounted to 1.15 inch; and in the course of the whole day to at least 2.67 inches. This is an amount of movement almost comparable with that of some climbing plants. The same stolon was observed on the following day, and now it moved in a somewhat less complex manner, in a plane not far from vertical. The extreme amount of actual movement was 1.55 inch in one direction, and .6 inch in another direction at right angles. During neither of these days did the stolon bend downwards through geotropism or its own weight.

Four stolons still attached to the plant were laid on damp sand in the back of a room, with their tips facing the north-east windows. They were thus placed because De Vries says* that they are apheliotropic when exposed to the light of the sun; but we could not perceive any effect from the above feeble degree of illumination. We may add that on another occasion, late in the summer, some stolons, placed upright before a south-west window

* 'Arbeiten Bot Inst., Wurzburg,' 1872, p. 434.

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on a cloudy day, became distinctly curved towards the light, and were therefore heliotropic. Close in front of the tips of the prostrate stolons, a crowd of very thin sticks and the dried haulms of gra.s.ses were driven into the sand, to represent the crowded stems of surrounding plants in a state of nature. This was done for the sake of observing how the growing stolons would pa.s.s through them. They did so easily in the course of 6 days, and their circ.u.mnutation apparently facilitated their pa.s.sage. When the tips encountered sticks so close together that they could not pa.s.s between them, they rose up and pa.s.sed over them. The sticks and haulms were removed after the pa.s.sage of the four stolons, two of which were found to have a.s.sumed a permanently sinuous shape, and two were still straight. But to this subject we shall recur under Saxifraga.

Saxifraga sarmentosa (Saxifrageae).--A plant in a suspended pot had emitted long branched stolons, which depended like

Fig. 88. Saxifraga sarmentosa: circ.u.mnutation of an inclined stolon, traced in darkness on a horizontal gla.s.s, from 7.45 A.M. April 18th to 9 A.M. on 19th. Movement of end of stolon magnified 2.2 times.

threads on all sides. Two were tied up so as to stand vertically, and their upper ends became gradually bent downwards, but so slowly in the course of several days, that the bending was probably due to their weight and not to geotropism. A gla.s.s filament with little triangles of paper was fixed to the end of one of these stolons, which was 17 inches in length, and had already become much bent down, but still projected at a considerable angle above the horizon. It moved only slightly three times from side to side and then upwards; on the following day [page 219]

the movement was even less. As this stolon was so long we thought that its growth was nearly completed, so we tried another which was thicker and shorter, viz., 10 1/4 inches in length. It moved greatly, chiefly upwards, and changed its course five times in the course of the day. During the night it curved so much upwards in opposition to gravity, that the movement could no longer be traced on the vertical gla.s.s, and a horizontal one had to be used. The movement was followed during the next 25 h., as shown in Fig. 88. Three irregular ellipses, with their longer axes somewhat differently directed, were almost completed in the first 15 h. The extreme actual amount of movement of the tip during the 25 h. was .75 inch.

Several stolons were laid on a flat surface of damp sand, in the same manner as with those of the strawberry. The friction of the sand did not interfere with their circ.u.mnutation; nor could we detect any evidence of their being sensitive to contact. In order to see how in a state of nature they would act, when encountering a stone or other obstacle on the ground, short pieces of smoked gla.s.s, an inch in height, were stuck upright into the sand in front of two thin lateral branches. Their tips scratched the smoked surface in various directions; one made three upward and two downward lines, besides a nearly horizontal one; the other curled quite away from the gla.s.s; but ultimately both surmounted the gla.s.s and pursued their original course. The apex of a third thick stolon swept up the gla.s.s in a curved line, recoiled and again came into contact with it; it then moved to the right, and after ascending, descended vertically; ultimately it pa.s.sed round one end of the gla.s.s instead of over it.

Many long pins were next driven rather close together into the sand, so as to form a crowd in front of the same two thin lateral branches; but these easily wound their way through the crowd. A thick stolon was much delayed in its pa.s.sage; at one place it was forced to turn at right angles to its former course; at another place it could not pa.s.s through the pins, and the hinder part became bowed; it then curved upwards and pa.s.sed through an opening between the upper part of some pins which happened to diverge; it then descended and finally emerged through the crowd. This stolon was rendered permanently sinuous to a slight degree, and was thicker where sinuous than elsewhere, apparently from its longitudinal growth having been checked.

Cotyledon umbilicus (Cra.s.sulaceae).--A plant growing in a pan [page 220]

of damp moss had emitted 2 stolons, 22 and 20 inches in length. One of these was supported, so that a length of 4 inches projected in a straight and horizontal line, and the movement of the apex was traced. The first dot was made at 9.10 A.M.;

Fig. 89. Cotyledon umbilicus: circ.u.mnutation of stolon, traced from 11.15 A.M. Aug. 25th to 11 A.M. 27th. Plant illuminated from above. The terminal internode was .25 inch in length, the penultimate 2.25 and the third 3.0 inches in length. Apex of stolon stood at a distance of 5.75 inches from the vertical gla.s.s; but it was not possible to ascertain how much the tracing was magnified, as it was not known how great a length of the internode circ.u.mnutated.

the terminal portion soon began to bend downwards and continued to do so until noon. Therefore a straight line, very nearly as long as the whole figure here given (Fig. 89), was first traced on the gla.s.s; but the upper part of this line has not been copied in the diagram. The curvature occurred in the middle [page 221]

of the penultimate internode; and its chief seat was at the distance of 1 1/4 inch from the apex; it appeared due to the weight of the terminal portion, acting on the more flexible part of the internode, and not to geotropism. The apex after thus sinking down from 9.10 A.M. to noon, moved a little to the left; it then rose up and circ.u.mnutated in a nearly vertical plane until 10.35 P.M. On the following day (26th) it was ob-

Fig. 90. Cotyledon umbilicus: circ.u.mnutation and downward movement of another stolon, traced on vertical gla.s.s, from 9.11 A.M. Aug. 25th to 11 A.M. 27th. Apex close to gla.s.s, so that figure but little magnified, and here reduced to two-thirds of original size.

served from 6.40 A.M. to 5.20 P.M., and within this time it moved twice up and twice down. On the morning of the 27th the apex stood as high as it did at 11.30 A.M. on the 25th. Nor did it sink down during the 28th, but continued to circ.u.mnutate about the same place.

Another stolon, which resembled the last in almost every [page 222]

respect, was observed during the same two days, but only two inches of the terminal portion was allowed to project freely and horizontally. On the 25th it continued from 9.10 A.M. to 1.30 P.M. to bend straight downwards, apparently owing to its weight (Fig. 90); but after this hour until 10.35 P.M. it zigzagged. This fact deserves notice, for we here probably see the combined effects of the bending down from weight and of circ.u.mnutation. The stolon, however, did not circ.u.mnutate when it first began to bend down, as may be observed in the present diagram, and as was still more evident in the last case, when a longer portion of the stolon was left unsupported. On the following day (26th) the stolon moved twice up and twice down, but still continued to fall; in the evening and during the night it travelled from some unknown cause in an oblique direction.]