The under
side seemed to be the most sensitive; but the sensitiveness or
irritability is slight compared to that which we shall meet with in
some of the following species; thus, a loop of string, weighing 1.64
grain (106.2 mg.) and hanging for some days on a young footstalk,
produced a scarcely perceptible effect. A sketch is here given of
two young leaves which had naturally caught hold of two thin
branches. A forked twig placed so as to press lightly on the under
side of a young footstalk caused it, in 12 hrs., to bend greatly, and
ultimately to such an extent that the leaf passed to the opposite
side of the stem; the forked stick having been removed, the leaf
slowly recovered its former position.
The young leaves spontaneously and gradually change their position:
when first developed the petioles are upturned and parallel to the
stem; they then slowly bend downwards, remaining for a short time at
right angles to the stem, and then become so much arched downwards
that the blade of the leaf points to the ground with its tip curled
inwards, so that the whole petiole and leaf together form a hook.
They are thus enabled to catch hold of any twig with which they may
be brought into contact by the revolving movement of the internodes.
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