cell-walls of Drosera contracting, considering their high state of
organisation; as shown in the case of the glands by their power of
absorption and secretion, and by being exquisitely sensitive so as to
be affected by the pressure of the most minute particles. The
cell-walls of the pedicels also allow various impulses to pass through
them, inducing movement, increased secretion and aggregation. On the
whole the belief that the walls of certain cells contract, some of
their contained fluid being at the same time forced outwards, perhaps
accords best with the observed facts. If this view is rejected, the
next most probable one is that the fluid contents of the cells shrink,
owing to a change in their molecular state, with the consequent closing
in of the walls. Anyhow, the movement can hardly be attributed to the
elasticity of the walls, together with a previous state of tension.
With respect to the nature of the motor impulse which is transmitted
from the glands down the pedicels and across the disc, it seems not
improbable that it is closely allied to that influence which causes the
protoplasm within the cells of the glands and tentacles to aggregate.
We have seen that both forces originate in and proceed from the glands
within a few seconds of the same time, and are excited by the same
causes. The aggregation of the protoplasm lasts almost as long as the
tentacles remain inflected, even though this be for more than a week;
but the [page 259] protoplasm is redissolved at the bending place
shortly before the tentacles re-expand, showing that the exciting cause
of the aggregating process has then quite ceased. Exposure to carbonic
acid causes both the latter process and the motor impulse to travel
very slowly down the tentacles. We know that the aggregating process is
delayed in passing through the cell- walls, and we have good reason to
believe that this holds good with the motor impulse; for we can thus
understand the different rates of its transmission in a longitudinal
and transverse line across the disc. Under a high power the first sign
of aggregation is the appearance of a cloud, and soon afterwards of
extremely fine granules, in the homogeneous purple fluid within the
cells; and this apparently is due to the union of molecules of
protoplasm. Now it does not seem an improbable view that the same
tendency namely for the molecules to approach each other should be
communicated to the inner surfaces of the cell-walls which are in
contact with the protoplasm; and if so, their molecules would approach
each other, and the cell-wall would contract.
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