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Show 318 DION lEA MUSCIPULA. CHAP. XIII. dots now stood about 1 0 5 0 0 of an inch ('127 n1m.) nearer together than before. If we suppose the whole upper surface of the lobe, which was 1 4 0°0°0 of an inch in breadth, to have contracted in the same proportion, the total contraction will have amounted to about 32_ or _ 1_ of an inch ('635 mm.); but whether this TOOO 40 • • is sufficient to account for the shght Inward curvature of the whole lobe, I am unable to say. Finally, with respect to the movement of the leaves, the wonderful discovery made by Dr. Burdon Sanderson* is now universally known ; namely that there exists a normal electrical current in the blade and footstalk; and that when the leaves are irritated, the current is disturbed in the same manner as takes place during the contraction of the muscle of an animal. The Be-expansion of the Leaves.-This is effected at an insensibly slow rate, whether or not any object is enclosed. t One lobe can re-expand by itself, as occurred with the torpid leaf of which one lobe alone bad closed. We have also seen in the experiments with cheese and albumen that the two ends of the same lobe can re-expand to a certain extent independently of each other. But in all ordinary cases both lobes open at the same time. There-expansion is not determined by the sensitive filaments; all three filaments on one lobe were cut off close to their bases ; and the three * 'Proc. Royal Soc.' vol. xxi. p. 495 ; and lecture at the Royal In. titution, June 5, 1874, given in 'Nature,' 1874, pp. 105 and 127. t Nuttall, in his 'Gen. American Plants,' p. 277 (note), says that, whilst collecting this plant in its native home, "I had occasion to observe that a detached leaf would make repeated efforts towards disclosing itself to the influence of tho sun ; these attempts consi. ·ted in an undulatory motion of the marginal ciliro, accompanied by a partial opening and succeeding collapse of the lamina, which at length terminated in a complete expansion and in tho destruction of sensibility." I am indebted to Prof. Oliver for this reference ; but I do not understand what took place. CHAP. XIII. RE-EXPANSION. 319 leaves thus treated re-expanded,-one to a partial extent in 24 hrs.,-a second to the same extent in 48 hrs.,-and the third, which had been previously injured, not until the sixth day. These leaves after their re-expansion closed quickly when the filaments on the other lobe were irritated. These were then cut off one of the leaves, so that none were left. This mutilated leaf, notwithstanding the loss of all its filaments, re-expanded in two days in the usual manner. When the filaments have been excited by immersion in a solution of sugar, the lobes do not expand so soon as when the filaments have been merely touched; and this, I presume, is due to their having been strongly affected through exosmose, so that they continue for some time to transmit a motor impulse to the upper surface of the leaf. The following facts make me believe that the several layers of cells forming the lower surface of the leaf are always in a state of tension ; and that it is owing to this mechanical state, aided probably Ly fresh fluid being attracted into the cells, that the lobes begin to separate or expand as soon as the contraction of the upper surface diminishes. A leaf was cut off and suddenly plunged perpendicularly into boiling water: I expected that the lobes would have closed, but instead of doing so, they diverged a little. I then took another fine leaf, with the lobes standing at an angle of nearly 80° to each other ; and on immersing it as before, the angle suddenly increased to 90°. .A. third leaf was torpid from having recently re-expanded after having caught a fly, so that repeated touche~ of the filaments caused not the least movement; nevertheless, when similar I y immersed, the lobes separated a little. As these leaves were inserted perpendicularly into the boiling water, both surfaces and the filaments |
