If a sleeping-plant is placed in a dark room after it has
gone to sleep at night, it will be found next morning in the
light-position, and will again assume the nocturnal position
as evening comes. We have, in fact, what seems to be a habit
built by the alternation of day and night. The plant normally
drops its leaves at the stimulus of darkness and raises them at
the stimulus of light. But here we see the leaves rising and
falling in the absence of the accustomed stimulation. This is
the characteristic par excellence of habit. When a series of
actions are compelled to follow each other by applying a series
of stimuli they become organically tied together, or associated,
and follow each other automatically, even when the whole series
of stimuli are not acting.--_The Scientific American._
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