This bending down
differs much in nature from that which occurs with the extremities of
the young leaves in many species of Clematis; for these, when thus
bent downwards or hooked, first acquire their full degree of
sensitiveness.
Dicentra thalictrifolia.--In this allied plant the metamorphosis of
the terminal leaflets is complete, and they are converted into
perfect tendrils. Whilst the plant is young, the tendrils appear
like modified branches, and a distinguished botanist thought that
they were of this nature; but in a full-grown plant there can be no
doubt, as I am assured by Dr. Hooker, that they are modified leaves.
When of full size, they are above 5 inches in length; they bifurcate
twice, thrice, or even four times; their extremities are hooked and
blunt. All the branches of the tendrils are sensitive on all sides,
but the basal portion of the main stem is only slightly so. The
terminal branches when lightly rubbed with a twig became curved in
the course of from 30 m. to 42 m., and straightened themselves in
between 10 hrs. and 20 hrs. A loop of thread weighing one-eighth of
a grain plainly caused the thinner branches to bend, as did
occasionally a loop weighing one-sixteenth of a grain; but this
latter weight, though left suspended, was not sufficient to cause a
permanent flexure.
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