This was observed with Desmodium gyrans and Mimosa pudica. With this latter
plant, moreover, the pinnae converge in the evening by a steady movement,
whereas during the day they are continually converging and diverging to a
slight extent. In all such cases it was scarcely possible to observe the
difference in the movement during the day and evening, without being
convinced that in the evening the plant saves the expenditure of force by
not moving laterally, and that its whole energy is now expended
[page 412]
in gaining quickly its proper nocturnal position by a direct course. In
several other cases, for instance, when a leaf after describing during the
day one or more fairly regular ellipses, zigzags much in the evening, it
appears as if energy was being expended, so that the great evening rise or
fall might coincide with the period of the day proper for this movement.
The most complex of all the movements performed by sleeping plants, is that
when leaves or leaflets, after describing in the daytime several vertically
directed ellipses, rotate greatly on their axes in the evening, by which
twisting movement they occupy a wholly different position at night to what
they do during the day.
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