The power of movement which various plants possess, when irritated, has been attributed by high authorities to the rapid passage of fluid out of certain cells, which, from their previous state of tension, immediately contract.* Whether or not this is the primary cause of such movements, fluid must pass out of closed cells when they contract or are pressed together in one direction, unless they at the same time expand in some other direction.  For instance, fluid can be seen to ooze from the surface of any young and vigorous shoot if slowly bent into a semi-circle.  In the case of Drosera there is certainly much movement of the fluid throughout the tentacles whilst they are undergoing inflection.  Many leaves can be found in which the purple fluid within the cells is of an equally dark tint on the upper and lower sides of the tentacles, extending also downwards on both sides to equally near their bases.  If the tentacles of such a leaf are excited into movement, it will generally be found after some hours that the cells on the concave side are much paler than they were before, or are quite colourless, those on the convex side having become much darker.  In two instances, after particles of hair had been placed on glands, and when in the course of 1 hr. 10 m. the tentacles were incurved halfway towards the centre of the leaf, this change of colour in the two sides 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, ‘Trait 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