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Show 24~ DROSERA ROTUNDIFOLIA. CHAP. X. 10). The short tentacles within this ring still ;~tain their vertical position, as they likewise do when a large object is placed on thei~ glands, or when an insect is caught by them. In th1s latt~r case we can see that the inflection of the short central tentacles would be useless, as their glands are already in contact with their prey. The result is very different when a single gland on one side of the disc is excited, or a few in a group, These send an impulse to the surrounding tentacles, which do not now bend towards the centre of the leaf, but to the point of excitement. We owe this capital observation to Nitschke,* and since reading his paper a few years ago, I have repeatedly verified it. If a minute bit of meat be placed by the aid of a needle on a single gland, or on three or four together, halfway between • 1 the centre and the circum- Fw. 10 · ference of the disc, the (D1·osera 1·otundijolia.) . h Leaf (enlarged) with the tentacles in~ected. duected movement of t .e over~ bit of meat placed on one Slde of surrounding tentacles lS the dt::;c. well exhibited. An accurate drawing of a leaf with meat in this position is here reproduced (fig. 10), and we see the tentacles, including some of the exterior ones, accurately directed to the point where the meat lay. But a much better * 'Bot. Zeitung,' 1860, p. 2-10. CHAP. X. DIRECTION OF INFLECTED TENTACLES. 245 plan is to place a particle of the phosphate of lime moistened with saliva on a single gland on one side of the disc of a large leaf, and another particle on a single gland on the opposite side. In four such trials the excitement was not sufficient to affect the outer tentacles, but all those near the two points were directed to them, so that two wheels were formed on the disc of the same leaf ; the pedicels of the tentacles forming the spokes, and the glands united in a mass over the phosphate representing the axles. The precision with which each tentacle pointed to the particle was wonderful; so that in some cases I could detect no deviation from perfect accuracy. Thus, although the short tentacles in the middle of the disc do not bend when their glands are excited in a direct manner, yet if they receive a motor impulse from a point on one side, they direct themselves to the point equally well with the tentacles on the borders of the disc. In these experiments, some of the short tentacles on the disc, which would have been directed to the centre, had the leaf been immersed in an exciting fluid, were now inflected in an exactly opposite direction, viz. towards the circumference. These tentacles, therefore, had deviated as much as 180° from. the direction which they would have assumed if their own glands had been stimulated, and which may be considered as the normal one. Between this, the greatest possible and no deviation from the normal direction, every degree could be observed in the tentacles on these several leaves. Notwithstanding the precision with which the tentacles generally were directed, those near the circumference of one leaf were not accurately directed towards s01ne phosphate of lime at a rather distant point on the opposite side of the disc. It appeared as if the motor |
