was conspicuously plain. In another case, after a bit of meat had been
placed on a gland, the purple colour was observed at intervals to be
slowly travelling from the upper to the lower part, down the convex
side of
* Sachs, Traite de Bot. 3rd edit. 1874, p. 1038. This view was, I
believe, first suggested by Lamarck.
** Sachs, ibid. p. 919. [page 256]
the bending tentacle. But it does not follow from these observations
that the cells on the convex side become filled with more fluid during
the act of inflection than they contained before; for fluid may all the
time be passing into the disc or into the glands which then secrete
freely.
The bending of the tentacles, when leaves are immersed in a dense
fluid, and their subsequent re-expansion in a less dense fluid, show
that the passage of fluid from or into the cells can cause movements
like the natural ones. But the inflection thus caused is often
irregular; the exterior tentacles being sometimes spirally curved.
Other unnatural movements are likewise caused by the application of
dense fluids, as in the case of drops of syrup placed on the backs of
leaves and tentacles. Such movements may be compared with the
contortions which many vegetable tissues undergo when subjected to
exosmose. It is therefore doubtful whether they throw any light on the
natural movements.
If we admit that the outward passage of fluid is the cause of the
bending of the tentacles, we must suppose that the cells, before the
act of inflection, are in a high state of tension, and that they are
elastic to an extraordinary degree; for otherwise their contraction
could not cause the tentacles often to sweep through an angle of above
180o. Prof. Cohn, in his interesting paper* on the movements of the
stamens of certain Compositae, states that these organs, when dead, are
as elastic as threads of india-rubber, and are then only half as long
as they were when alive. He believes that the living protoplasm
230