in contact with the depending and therefore thicker part of the drops
round so many glands. Five of the tentacles began moving after an
interval of a few minutes, and in these cases I clearly saw that the
particles touched the lower surface of the gland. A sixth tentacle
moved after 1 hr. 45 m., and the particle was now in contact with the
gland, which was not the case at first. So it was with the seventh
tentacle, but its movement did not begin until 3 hrs. 45 m. had [page
31] elapsed. The remaining six tentacles never moved as long as they
were observed; and the particles apparently never came into contact
with the surfaces of the glands.
From these experiments we learn that particles not containing soluble
matter, when placed on glands, often cause the tentacles to begin
bending in the course of from one to five minutes; and that in such
cases the particles have been from the first in contact with the
surfaces of the glands. When the tentacles do not begin moving for a
much longer time, namely, from half an hour to three or four hours, the
particles have been slowly brought into contact with the glands, either
by the secretion being absorbed by the particles or by its gradual
spreading over them, together with its consequent quicker evaporation.
When the tentacles do not move at all, the particles have never come
into contact with the glands, or in some cases the tentacles may not
have been in an active condition. In order to excite movement, it is
indispensable that the particles should actually rest on the glands;
for a touch once, twice, or even thrice repeated by any hard body is
not sufficient to excite movement.
Another experiment, showing that extremely minute particles act on the
glands when immersed in water, may here be given. A grain of sulphate
of quinine was added to an ounce of water, which was not afterwards
filtered; and on placing three leaves in ninety minims of this fluid, I
was much surprised to find that all three leaves were greatly inflected
in 15 m.; for I knew from previous trials that the solution does not
act so quickly as this. It immediately occurred to me that the
particles of the undissolved salt, which were so light as to float
about, might have come [page 32] into contact with the glands, and
caused this rapid movement. Accordingly I added to some distilled water
a pinch of a quite innocent substance, namely, precipitated carbonate
of lime, which consists of an impalpable powder; I shook the mixture,
and thus got a fluid like thin milk. Two leaves were immersed in it,
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