Mimosa pudica (Leguminosae).--The cotyledons rise up vertically at night, so as to close together. Two seedlings were observed in the greenhouse (temp. 16o to 17o C. or 63o to 65o F.). Their hypocotyls were secured to sticks, and glass filaments bearing little triangles of paper were affixed to the cotyledons of both. Their movements were traced on a vertical glass during 24 h. on November 13th. The pot had stood for some time in the same position, and they were chiefly illuminated through the glass-roof. The cotyledons of one of these seedlings moved downward in the morning till 11.30 A.M., and then rose, moving rapidly in the evening until they stood vertically, so that in this case there was simply a single great daily fall and rise. The other seedling behaved rather differently, for it fell in the morning until 11.30 A.M., and then rose, but after 12.10 P.M. again fell; and the great evening rise did not begin until 1.22 P.M. On the following morning this cotyledon had fallen greatly from its vertical position by 8.15 A.M. Two other seedlings (one seven and the other eight days old) had been previously observed under unfavourable circumstances, for they had been brought into a room and placed before a north-east window, where the temperature was between only 56o and 57o F. They had, moreover, to be protected from lateral light, and perhaps were not sufficiently illuminated. Under these circumstances the cotyledons moved simply downwards from 7 A.M. till 2 P.M., after which hour and during a large part of the night they continued to rise. Between 7 and 8 A.M. on the following morning they fell again; but on this second and likewise on the third day the movements became irregular, and between 3 and 10.30 P.M. they circumnutated to a small extent about the same spot; but they did not rise at night. Nevertheless, on the following night they rose as usual.

Cytisus fragrans (Leguminosae).--Only a few observations were made on this plant. The hypocotyl circumnutated to a considerable extent, but in a simple manner--namely, for two hours in one direction, and then much more slowly back again in a zigzag course, almost parallel to the first line, and beyond the starting-point. It moved in the same direction all night, but next morning began to return. The cotyledons continually [page 38] move both up and down and laterally; but they do not rise up at night in a conspicuous manner.

Lupinus luteus (Leguminosae).--Seedlings of this plant were observed because the cotyledons are so thick (about .08 of an inch) that it seemed unlikely that they would move. Our observations were not very successful, as the seedlings are strongly heliotropic, and their circumnutation could not be accurately observed near a north-east window, although they had been kept during the previous day in the same position. A seedling was then placed in darkness with the hypocotyl secured to a stick; both cotyledons rose a little at first, and then fell during the rest of the day; in the evening between 5 and 6 P.M. they moved very slowly; during the night one continued to fall and the other rose, though only a little. The tracing was not much magnified, and as the lines were plainly zigzag, the cotyledons must have moved a little laterally, that is, they must have circumnutated.

The hypocotyl is rather thick, about .12 of inch; nevertheless it circumnutated in a complex course, though to a small extent. The movement of an old seedling with two true leaves partially developed, was observed in the dark. As the movement was magnified about 100 times it is not trustworthy and is not given; but there could be no doubt that the hypocotyl moved in all directions during the day, changing its course 19 times. The extreme actual distance from side to side through which the upper part of the hypocotyl passed in the course of 14 hours was only 1/60 of an inch; it sometimes travelled at the rate of 1/50 of an inch in an hour.

Cucurbita ovifera (Cucurbitaceae).--Radicle: a seed which had

Fig. 26. Cucurbita ovifera: course followed by a radicle in bending geotropically downwards, traced on a horizontal glass, between 11.25 A.M. and 10.25 P.M.; the direction during the night is indicated by the broken line.

The Power of Movement in Plants Page 20

19th Century English Literature

Charles Darwin

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