The Power of Movement in Plants, Charles Darwin [the reading list book TXT] 📗

Fig. 84. Cyperus alternifolius: circumnutation of stem, illuminated from above, traced on horizontal glass, 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 glass [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 Circumnutation 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 circumnutate,—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 circumnutate to a greater or less extent. When we treat of the sleep and other movements of plants, many other cases of circumnutating 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 circumnutating apex as it rose will have described a spire of some kind. The dots were made on the glasses 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 glass-plates by the tips of the circumnutating 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 compass during the same day or on succeeding days. The stems there-

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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.

CIRCUMNUTATION 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 homological 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 glass 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

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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: circumnutation of stolon, kept in darkness, traced on vertical glass, 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 glass filament was detached and refixed close behind the bud, as it appeared possible that the circumnutation 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: circumnutation 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 glass and bringing the apex of the stolon at successive periods into a line with one edge; a dot being made each time on the glass. 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: circumnutation 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., W�rzburg,’ 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 grasses 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 pass through them. They did so easily in the course of 6

days, and their circumnutation apparently facilitated their passage. When the tips encountered sticks so close together that they could not pass between them, they rose up and passed over them. The sticks and haulms were removed after the passage of the four stolons, two of which were found to have assumed 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: circumnutation of an inclined stolon, traced in darkness on a horizontal glass, 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 glass 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

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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 glass, 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 circumnutation; 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 glass, 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