cells are shown to be still more elongated; and these would form the
most direct line of communication from the gland to the bending place
of the tentacle. If the impulse travels down the exterior cells, it
would have to cross from between twenty to thirty transverse
partitions; but rather fewer if down the inner parenchymatous tissue.
In either case it is remarkable that the impulse is able to pass
through so many partitions down nearly the whole length of the pedicel,
and to act on the bending place, in ten seconds. Why the impulse, after
having passed so quickly down one of the extreme marginal tentacles
(about 1/20 of an inch in length), should never, as far as I have seen,
affect the adjoining tentacles, I do not understand. It may be in part
accounted for by much energy being expended in the rapidity of the
transmission.
Most of the cells of the disc, both the superficial ones and the larger
cells which form the five or six underlying layers, are about four
times as long as broad. They are arranged almost longitudinally,
radiating from the footstalk. The motor impulse, therefore, when
transmitted across the disc, has to cross nearly four times as many
cell-walls as when transmitted in a longitudinal direction, and would
consequently be much delayed in the former case. The cells of the disc
converge towards the bases of the tentacles, and are thus fitted to
convey the motor impulse to them from all sides. On the whole, the
arrangement and shape of the cells, both those of the disc and
tentacles, throw much light on the rate and manner of diffusion of the
motor impulse. But why the impulse proceeding from the glands of the
exterior rows of tentacles tends to travel laterally and towards the
centre of the leaf, but not centrifugally, is by no means clear. [page
254]
Mechanism of the Movements, and Nature of the Motor Impulse. Whatever
may be the means of movement, the exterior tentacles, considering their
delicacy, are inflected with much force. A bristle, held so that a
length of 1 inch projected from a handle, yielded when I tried to lift
with it an inflected tentacle, which was somewhat thinner than the
bristle. The amount or extent, also, of the movement is great. Fully
expanded tentacles in becoming inflected sweep through an angle of
180o; and if they are beforehand reflexed, as often occurs, the angle
is considerably greater. It is probably the superficial cells at the
bending place which chiefly or exclusively contract; for the interior
228