of excitement.
The motor impulse, as it spreads from one or more glands across the
disc, enters the bases of the surrounding tentacles, and immediately
acts on the bending place. It does not in the first place proceed up
the tentacles to the glands, exciting them to reflect back an impulse
to their bases. Nevertheless, some influence is sent up to the glands,
as their secretion is soon increased and rendered acid; and then the
glands, being thus excited, send back some other influence (not
dependent on increased secretion, nor on the inflection of the
tentacles), causing the protoplasm to aggregate in cell beneath cell.
This may [page 277] be called a reflex action, though probably very
different from that proceeding from the nerve-ganglion of an animal;
and it is the only known case of reflex action in the vegetable
kingdom.
About the mechanism of the movements and the nature of the motor
impulse we know very little. During the act of inflection fluid
certainly travels from one part to another of the tentacles. But the
hypothesis which agrees best with the observed facts is that the motor
impulse is allied in nature to the aggregating process; and that this
causes the molecules of the cell-walls to approach each other, in the
same manner as do the molecules of the protoplasm within the cells; so
that the cell-walls contract. But some strong objections may be urged
against this view. The re-expansion of the tentacles is largely due to
the elasticity of their outer cells, which comes into play as soon as
those on the inner side cease contracting with prepotent force; but we
have reason to suspect that fluid is continually and slowly attracted
into the outer cells during the act of re-expansion, thus increasing
their tension.
I have now given a brief recapitulation of the chief points observed by
me, with respect to the structure, movements, constitution, and habits
of Drosera rotundifolia; and we see how little has been made out in
comparison with what remains unexplained and unknown. [page 278]
CHAPTER XII.
245