elapsed, instead of within half an hour, or even within a few minutes,
as usually occurred. All the glands on the same leaf are of nearly the
same size, as may best be seen by cutting off a narrow transverse
strip, and laying it on its side; hence their absorbing surfaces are
nearly equal. The long-headed glands on the extreme margin must be
excepted, as they are much longer than the others; but only the upper
surface is capable of absorption. Besides the glands, both surfaces of
the leaves and the pedicels of the tentacles bear numerous minute
papillae, which absorb carbonate of ammonia, an infusion of raw meat,
metallic salts, and probably many other substances, but the absorption
of matter by these papillae never induces inflection. We must remember
that the movement of each separate tentacle depends on its gland being
excited, except when a motor impulse is transmitted from the glands of
the disc, and then the movement, as just stated, does not take place
until some little time has elapsed. I have made these remarks because
they show us that when a leaf is immersed in a solution, and the
tentacles are inflected, we can judge with some accuracy how much of
the salt each gland has absorbed. For instance, if a leaf bearing 212
glands be immersed in a measured quantity of a solution, containing
1/10 of a grain of a salt, and all the exterior tentacles, except
twelve, are inflected, we may feel sure that each of the 200 glands can
on an average have absorbed at most 1/2000 of a grain of the salt. I
say at [page 139] most, for the papillae will have absorbed some small
amount, and so will perhaps the glands of the twelve excluded tentacles
which did not become inflected. The application of this principle leads
to remarkable conclusions with respect to the minuteness of the doses
causing inflection.
_On the Action of Distilled Water in Causing Inflection._
Although in all the more important experiments the difference between
the leaves simultaneously immersed in water and in the several
solutions will be described, nevertheless it may be well here to give a
summary of the effects of water. The fact, moreover, of pure water
acting on the glands deserves in itself some notice. Leaves to the
number of 141 were immersed in water at the same time with those in the
solutions, and their state recorded at short intervals of time.
Thirty-two other leaves were separately observed in water, making
altogether 173 experiments. Many scores of leaves were also immersed in
120