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Show 260 DROSERA ROTU:NDIFOLIA. · CHAI'. X. tentacles, which acts on the cells at the Lending place, but does not induce aggregation until it has reached the glands ; these then send back some other i~fluence, causing the protoplasm to aggregate, first In th upper and then in the lower cells. . . The Be-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 fro1n 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; w·hen they afterwards re-expand, they bend obliquely back, so as to recover their original positions. The tentacles farthest from an excited point, wherever that 1nay 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 xperiment. Meat was placed on a leaf, and after the tentacles were closely inflected and had quite ceased to 1nove, 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 recomnlenced, and the already greatly bent portion went on bending until it formed a perfect circle; the straight uistal 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 counterbalance that of the concave surface, which, when free, curled into a complete ring. The tentacles of an expanded and uuexcited leaf CHAP. X. HE-EXPANSION OF THE TENTACLES. 261 are moderately rigid and elastic; if bent by a needle, the upper end yields more easily tha~ the ba~al a~d 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 reflexecl, and this aprare~tly indicates that the tension of the outer surface IS mechanical whilst that of the inner surface is vital, and is insta~tly destroyed by the boiling water. We can thus also understand why the tentacles as they grow old and feeble slowly becon1e much reflexecl. If a leaf with its tentacles closely inflected is dipped into boiling water, these rise up a little, ?ut by no means fully re-expand. This mar: be owing t?. the heat quickly destroying the tension and elasticity of. the cells of the convex surface; but I can hardly beheve that their tension, at any one time, would suffice to carry baek the tentacles to their ori~inal ~osition, often through an angle of above 180 . It Is more probable that fluid, which we. know. trav.els along the tentacles during the act of Inflection, IS slowly r~attracted into the cells of the convex surface, theu tension being thus gradually and continually increased. A recapitulation of the chief facts and discussions in this chapter will be given at the close of the next chapter. |