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Show 310 DARWINIAN A. is full of interest as a physiological research, and is a model of its kind, as well for the simplicity and directness of the means employed as for the clearness with which the results are brought out-results which any one may verify now that the way to them is pointed out and which, surprising as they are, lose half their wo~der in the ease and sureness with which they seem to have been reached. Rather more than half the volume is devoted to one subject, the round-leaved sundew (Drosera rotundifolia), a rather common plant in the northern temperate zone. That flies stick fast to its leaves, being limed by the tenacious seeming dew-drops which stud its upper face and margins, had long been noticed in Europe and in this country. We have heard hunters and explorers in our Northern woods refer with satisfaction to the fate which in this way often befalls one of their plagues, the black fly of early summer. And it was known to some observant botanists in the last century, although forgotten or discredited in this, that an insect caught on the viseid glands it has happened to alight upon is soon fixed by many more-not merely in consequence of its struggles, but by the spontaneous incurvation of the stalks of surrounding and untouched glands; and even the body of the leaf had been observed to incurve or become cup-shaped so as partly to involve the captive insect. Mr. Darwin's peculiar investigations not. only confirm all this, but add greater wonders. They relate to the sensitiveness of these tentacles, as he prefers to call them and the mode in which it is manifested; their powe'r of absorption ; their astonishing discernment of lNSEOTIVOROUS AND CLIMBING PLANTS. 311 the presence of animal or other soluble azotized matter, even in quantities so minute as to rival the spectroscope- that most exquisite instrument of modern research-in delicacy; and, finally, they establish the fact of a true digestion, in all essential respects similar to that of the stomach of anima1s. First as to sensitiveness and movement. Sensitiveness is manifested by movement or change of form in response to an external impression. The sensitiveness in the sundew is all in the gland which surmounts the tentacle. To incite movement or other action it is necessary that the gland itself should be reache' d. Anything laid on the surface of the viscid drop, the spherule of clear, glairy liquid which it secretes, produces no effect unless it sinks through to the gland; or unless the substance is soluble and reaches it in solution, which, in the case of certain substances, has the same effect. But the glands themselves do not move, nor does any neighbori~g portion of the tentacle. The outer and longer tentacles bend inward (toward the centre of the leaf) promptly, when the gland is irritated or stimulated, sweeping through an arc of 180° or less, or more-the quickness and the extent of the inflection depending, in equa1ly vigorous leaves, upon the amount of irritation or stimulation, and also upon its kind. A tentacle with a particle of raw meat on its gland sometimes visibly begins to bend in ten seconds, becomes strongly incurved. in five minutes, and its tip reaches the centre of the leaf in half an ho~r; but this is a case of extreme rapidity. A particle of cinder, chalk, or sand, will also incite the bending, i£ actually brought in contact with the |
