Strephium floribundum* (Gramineae).--The oval leaves are provided with a
pulvinus, and are extended horizontally or declined a little beneath the
horizon during the day. Those on the upright culms simply rise up
vertically at night, so that their tips are directed towards the zenith.
(Fig. 164.)
Fig. 164. Strephium floribundum: culms with leaves during the day, and when
asleep at night. Figures reduced.
Horizontally extended leaves arising from much inclined or almost
horizontal culms, move at night so that their tips point towards the apex
of the culm, with one lateral margin directed towards the zenith; and in
order to assume this position the leaves have to twist on their own axes
through an angle of nearly 90o. Thus the surface of the blade always stands
vertically, whatever may be the position of the midrib or of the leaf as a
whole.
The circumnutation of a young leaf (2.3 inches in length) was traced during
48 h. (Fig. 165). The movement was remarkably simple; the leaf descended
from before 6.40 A.M. until 2 or 2.50 P.M., and then rose so as to stand
vertically at about 6 P.
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