For instance, the terminal leaflets of Cassia not
only move vertically downwards in the evening, but twist round, so that
their lower surfaces face outwards. Such movements are wholly, or almost
wholly, confined to leaflets provided with a pulvinus. But this torsion is
not a new kind of movement introduced solely for the purpose of sleep; for
it has been shown that some leaflets whilst describing their ordinary
ellipses during the daytime rotate slightly, causing their blades to face
first to one side and then to another. Although we can see how the slight
periodical movements of leaves in a vertical plane could be easily
converted into the greater yet simple nyctitropic movements, we do not at
present know by what graduated steps the more complex movements, effected
by the torsion of the pulvini, have been acquired. A probable explanation
could be given in each case only after a close investigation of the
movements in all the allied forms.
From the facts and considerations now advanced we may conclude that
nyctitropism, or the sleep of leaves
[page 413]
and cotyledons, is merely a modification of their ordinary circumnutating
movement, regulated in its period and amplitude by the alternations of
light and darkness.
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