The Re-expansion of the Tentacles.—­This movement is always slow and gradual.  When the centre of the leaf is excited, or a leaf is immersed in a proper solution, all the tentacles bend directly towards the centre, and afterwards directly back from it.  But when the point of excitement is on one side of the disc, the surrounding tentacles bend towards it, and therefore obliquely with respect to their normal direction; when they afterwards re-expand, they bend obliquely back, so as to recover their original positions.  The tentacles farthest from an excited point, wherever that may be, are the last and the least affected, and probably in consequence of this they are the first to re-expand.  The bent portion of a closely inflected tentacle is in a state of active contraction, as shown by the following experiment.  Meat was placed on a leaf, and after the tentacles were closely inflected and had quite ceased to move, narrow strips of the disc, with a few of the outer tentacles attached to it, were cut off and laid on one side under the microscope.  After several failures, I succeeded in cutting off the convex surface of the bent portion of a tentacle.  Movement immediately recommenced, and the already greatly bent portion went on bending until it formed a perfect circle; the straight distal portion of the tentacle passing on one side of the strip.  The convex surface must therefore have previously been in a state of tension, sufficient to counter-balance that of the concave surface, which, when free, curled into a complete ring.

The tentacles of an expanded and unexcited leaf [page 261] are moderately rigid and elastic; if bent by a needle, the upper end yields more easily than the basal and thicker part, which alone is capable of becoming inflected.  The rigidity of this basal part seems due to the tension of the outer surface balancing a state of active and persistent contraction of the cells of the inner surface.  I believe that this is the case, because, when a leaf is dipped into boiling water, the tentacles suddenly become reflexed, and this apparently indicates that the tension of the outer surface is mechanical, whilst that of the inner surface is vital, and is instantly destroyed by the boiling water.  We can thus also understand why the tentacles as they grow old and feeble slowly become much reflexed.  If a leaf with its tentacles closely inflected is dipped into boiling water, these rise up a little, but by no means fully re-expand.  This may be owing to the heat quickly destroying the tension and elasticity of the cells of the convex surface; but I can hardly believe that their tension, at any one time, would suffice to carry back the tentacles to their original position, often through an angle of above 180o.  It is more probable that fluid, which we know travels along the tentacles during the act of inflection, is slowly re-attracted into the cells of the convex surface, their tension being thus gradually and continually increased.