A new dot was now made on the glass, but in the
course of 14 h. 20 m. there was hardly any change in the position of the
filament. We may therefore infer that an old and only moderately sensitive
leaf does not circumnutate plainly; but we shall soon see that it by no
means follows that such a leaf is absolutely motionless. We may further
infer that the stimulus from a touch does not re-excite plain
circumnutation.
Another full-grown leaf had a filament attached externally along one side
of the midrib and parallel to it, so that the filament would move if the
lobes closed. It should be first stated that, although a touch on one of
the sensitive hairs of a vigorous leaf causes it to close quickly, often
almost instantly, yet when a bit of damp meat or some solution of carbonate
of ammonia is placed on the lobes, they close so slowly that generally 24
h. is required for the completion of the act. The above leaf was first
observed for 2 h. 30 m., and did not circumnutate, but it ought to have
been observed for a
[page 241]
longer period; although, as we have seen, a young leaf completed a fairly
large ellipse in 2 h.
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