caused it, after 24 hrs., to curve moderately; the curvature, though
the loop remained suspended, was after 48 hrs. diminished, but never
disappeared; showing that the petiole had become partially accustomed
to the insufficient stimulus. This experiment was twice repeated
with nearly the same result. Lastly, a loop of thread, weighing only
one-sixteenth of a grain (4.05 mg.) was twice gently placed by a
forceps on a terminal sub-petiole (the plant being, of course, in a
still and closed room), and this weight certainly caused a flexure,
which very slowly increased until the petiole moved through nearly
ninety degrees: beyond this it did not move; nor did the petiole,
the loop remaining suspended, ever become perfectly straight again.

When we consider, on the one hand, the thickness and stiffness of the
petioles, and, on the other hand, the thinness and softness of fine
cotton thread, and what an extremely small weight one-sixteenth of a
grain (4.05 mg.) is, these facts are remarkable. But I have reason
to believe that even a less weight excites curvature when pressing
over a broader surface than that acted on by a thread. Having
noticed that the end of a suspended string which accidentally touched
a petiole, caused it to bend, I took two pieces of thin twine, 10
inches in length (weighing 1.64 gr.), and, tying them to a stick, let
them hang as nearly perpendicularly downwards as their thinness and
flexuous form, after being stretched, would permit; I then quietly
placed their ends so as just to rest on two petioles, and these
certainly became curved in 36 hrs. One of the ends touched the angle
between a terminal and lateral sub-petiole, and it was in 48 hours
caught between them as by a forceps. In these cases the pressure,
though spread over a wider surface than that touched by the cotton
thread, must have been excessively slight.