The
inferior surface of the rectangularly bent terminal portion (carrying
the terminal leaflet), which forms the inner side of the end of the
hook, is the most sensitive part; and this portion is manifestly best
adapted to catch a distant support. To show the difference in
sensibility, I gently placed loops of string of the same weight (in
one instance weighing only 0.82 of a grain or 53.14 mg.) on the
several lateral sub-petioles and on the terminal one; in a few hours
the latter was bent, but after 24 hrs. no effect was produced on the
other sub-petioles. Again, a terminal sub-petiole placed in contact
with a thin stick became sensibly curved in 45 m., and in 1 hr. 10m.
moved through ninety degrees; whilst a lateral sub-petiole did not
become sensibly curved until 3 hrs. 30 m. had elapsed. In all cases,
if the sticks are taken away, the petioles continue to move during
many hours afterwards; so they do after a slight rubbing; but they
become straight again, after about a day's interval, that is if the
flexure has not been very great or long continued.
The graduated difference in the extension of the sensitiveness in the
petioles of the above-described species deserves notice.
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