Aggregation is excited by almost all the stimulants which induce
movement; such as the glands being touched two or three times, the
pressure of minute inorganic particles, the absorption of various
fluids, even long immersion in distilled water, exosmose, and heat. Of
the many stimulants tried, carbonate of ammonia is the most energetic
and acts the quickest: a dose of 1/134400 of a grain (.00048 mg.) given
to a single gland suffices to cause in one hour well-marked aggregation
in the upper cells of the tentacle. The process goes on only as long as
the protoplasm is in a living, vigorous, and oxygenated condition.
The result is in all respects exactly the same, whether a gland has
been excited directly, or has received an influence from other and
distant glands. But there is one important difference: when the central
glands are irritated, they transmit centrifugally an influence up the
pedicels of the exterior tentacles to their glands; but the actual
process of aggregation travels centripetally, from the glands of the
exterior tentacles down their pedicels. The exciting influence,
therefore, which is transmitted from [page 266] one part of the leaf to
another must be different from that which actually induces aggregation.
The process does not depend on the glands secreting more copiously than
they did before; and is independent of the inflection of the tentacles.
It continues as long as the tentacles remain inflected, and as soon as
these are fully re-expanded, the little masses of protoplasm are all
redissolved; the cells becoming filled with homogeneous purple fluid,
as they were before the leaf was excited.
As the process of aggregation can be excited by a few touches, or by
the pressure of insoluble particles, it is evidently independent of the
absorption of any matter, and must be of a molecular nature. Even when
caused by the absorption of the carbonate or other salt of ammonia, or
an infusion of meat, the process seems to be of exactly the same
nature. The protoplasmic fluid must, therefore, be in a singularly
unstable condition, to be acted on by such slight and varied causes.
Physiologists believe that when a nerve is touched, and it transmits an
influence to other parts of the nervous system, a molecular change is
induced in it, though not visible to us. Therefore it is a very
interesting spectacle to watch the effects on the cells of a gland, of
the pressure of a bit of hair, weighing only 1/78700 of a grain and
largely supported by the dense secretion, for this excessively slight
237