The following observations are more precise. After trying heavier
pieces of string and thread, I placed a loop of fine string, weighing
1.04 gr. (67.4 mg.) on a terminal sub-petiole: in 6 hrs. 40 m. a
curvature could be seen; in 24 hrs. the petiole formed an open ring
round the string; in 48 hrs. the ring had almost closed on the
string, and in 72 hrs. seized it so firmly, that some force was
necessary for its withdrawal. A loop weighing 0.52 of a grain (33.7
mg.) caused in 14 hrs. a lateral sub-petiole just perceptibly to
curve, and in 24 hrs. it moved through ninety degrees. These
observations were made during the summer: the following were made in
the spring, when the petioles apparently are more sensitive:- A loop
of thread, weighing one-eighth of a grain (8.1 mg.), produced no
effect on the lateral sub-petioles, but placed on a terminal one,
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.
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