We may suspect that we here see the energy which
is freed during the incessant chemical changes in progress in the tissues,
converted into motion. Finally, it should be noted that leaflets and
probably some leaves, whilst describing their ellipses, often rotate
slightly on their axes; so that the plane of the leaf is directed first to
one and then to another side. This was plainly seen to be the case with the
large terminal leaflets of Desmodium, Erythrina and Amphicarpaea, and is
probably common to all leaflets provided with a pulvinus.
With respect to the periodicity of the movements of sleeping leaves,
Pfeffer* has so clearly shown that this depends on the daily alternations
of light and darkness, that nothing farther need be said on this
* 'Die Periodischen Bewegungen der Blattorgane,' 1875, p. 30, et passim.
[page 407]
head. But we may recall the behaviour of Mimosa in the North, where the sun
does not set, and the complete inversion of the daily movements by
artificial light and darkness. It has also been shown by us, that although
leaves subjected to darkness for a moderately long time continue to
circumnutate, yet the periodicity of their movements is soon greatly
disturbed, or quite annulled.
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