On the following morning the lobes were closing more quickly, and by 5 P.M. it was evident to the eye that they had closed considerably; by 8.48. P.M. this was still plainer, and by 10.45 P.M. the marginal spikes were interlocked. The leaf fell a little during the night, and next morning (25th) at 7 A.M. the lobes were completely shut. The course pursued, as may be seen in the figure, was

Fig. 107. Dionoea muscipula: closure of the lobes and circumnutation of a full-grown leaf, whilst absorbing an infusion of raw meat, traced in darkness, from 7.15 P.M. Sept. 24th to 9 A.M. 26th. Apex of leaf 8 ½ inches from the vertical glass. Figure here reduced to two-thirds of original scale.

strongly zigzag, and this indicates that the closing of the lobes was combined with the circumnutation of the whole leaf; and there cannot be much doubt, considering how motionless the leaf was during 2 h. 30 m. before it received the infusion, that the absorption of the animal matter had excited it to circumnutate. The leaf was occasionally observed for the next four days, but was kept in rather too cool a place; nevertheless, it continued to circumnutate to a small extent, and the lobes remained closed.

It is sometimes stated in botanical works that the lobes close or sleep at night; but this is an error. To test the statement, very long glass filaments were fixed inside the two lobes of three leaves, and the distances between their tips were measured in the middle of the day and at night; but no difference could be detected.

The previous observations relate to the movements of the whole leaf, but the lobes move independently of the petiole, and [page 242] seem to be continually opening and shutting to a very small extent. A nearly full-grown leaf (afterwards proved to be highly sensitive to contact) stood almost horizontally, so that by driving a long thin pin through the foliaceous petiole close to the blade, it was rendered motionless. The plant, with a little triangle of paper attached to one of the marginal spikes, was placed under a microscope with an eye-piece micrometer, each division of which equalled 1/500 of an inch. The apex of the paper-triangle was now seen to be in constant slight movement; for in 4 h. it crossed nine divisions, or 9/500 of an inch, and after ten additional hours it moved back and had crossed 5/500 in an opposite direction. The plant was kept in rather too cool a place, and on the following day it moved rather less, namely, 1/500 in 3 h., and 2/500 in an opposite direction during the next 6 h. The two lobes, therefore, seem to be constantly closing or opening, though to a very small distance; for we must remember that the little triangle of paper affixed to the marginal spike increased its length, and thus exaggerated somewhat the movement. Similar observations, with the important difference that the petiole was left free and the plant kept under a high temperature, were made on a leaf, which was healthy, but so old that it did not close when its sensitive hairs were repeatedly touched, though judging from other cases it would have slowly closed if it had been stimulated by animal matter. The apex of the triangle was in almost, though not quite, constant movement, sometimes in one direction and sometimes in an opposite one; and it thrice crossed five divisions of the micrometer (i.e. 1/100 of an inch) in 30 m. This movement on so small a scale is hardly comparable with ordinary circumnutation; but it may perhaps be compared with the zigzag lines and little loops, by which the larger ellipses made by other plants are often interrupted.

In the first chapter of this volume, the remarkable oscillatory movements of the circumnutating hypocotyl of the cabbage have been described. The leaves of Dionaea present the same phenomenon, which is a wonderful one, as viewed under a low power (2-inch object-glass), with an eye-piece micrometer of which each division (1/500 of an inch) appeared as a rather wide space. The young unexpanded leaf, of which the circumnutating movements were traced (Fig.

Charles Darwin

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