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Page:Popular Science Monthly Volume 18.djvu/522

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THE POPULAR SCIENCE MONTHLY.

bend up so much that they press against the stem; and, on looking down on one of these young plants from vertically above, the lower surfaces of the leaflets are visible: and thus their upper surfaces, in accordance with the general rule, are best protected from radiation. While the leaves on this young plant were thus behaving, those on an old bush in full flower did not sleep at night."

Again, "the species of Melilotus sleep in a remarkable manner. The three leaflets of each leaf twist through an angle of 90°, so that their blades stand vertically at night, with one lateral edge presented to the zenith" (Fig. 8). We have no room for the description of the

PSM V18 D522 Melilotus officinalis in durnal and nocturnal state.jpg

Fig. 8.—Melilotus officinalis: A, leaf during the daytime; B, another leaf asleep; C. a leaf asleep as viewed from vertically above, but in this case the terminal leaflet did not happen to be in such close contact with the lateral one as is usual.

complicated movements performed by these plants. Their petioles and sub-petioles are continually circumnutating during the whole twenty-four hours. Their cotyledons do not sleep.

The nyctitropic movements of eleven species of Trifolium were observed and were found to be closely similar. If we select a leaf of Trifolium repens having an upright petiole and with the three leaflets expanded horizontally, the two lateral leaflets will be seen in the evening to twist and approach each other until their upper surfaces come into contact. At the same time they bend downward in a plane at right angles to their former position, until their midribs form an angle of about 45° with the upper part of the petiole. The terminal leaflet merely rises up without any twisting, and bends over until it rests on and forms a roof over the edges of the now vertical and united lateral