It is, too, a familiar fact to
students of vegetable physiology that the leaves of _Porleria
hygrometrica_ fold down or rise up in accordance with the state of the
atmosphere. In short, it was pointed out in the _Standard_, in
illustration of the extreme sensitiveness of certain plants to
surrounding influences, how the _Haedysarums_ have been well known ever
since the days of Linnseus to suddenly begin to quiver without any
apparent cause, and just as suddenly to stop. Force cannot initiate the
movement, though cold will stop it, and heat will set in motion again
the suspended animation of the leaves. If artificially kept from moving
they will, when released, instantly begin their task anew and with
redoubled energy. Similarly the leaves of the _Colocasia esculenta_--the
tara of the Sandwich Islands--will often shiver at irregular times of
the day and night, and with such energy that little bells hung on the
petals tinkle. And yet, curious to say, we are told that the keenest eye
has not yet been able to detect any peculiarity in these plants to
account for these strange motions. It has been suggested that they are
due to changes in the weather of such a slight character that, "our
nerves are incapable of appreciating them, or the mercury of recording
their accompanying oscillations.
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