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Show 228 DROSERA ROTUNDIFOLIA. CHAP. IX. becon1e inflected, are not excited to further movement by bits of meat placed on the glands, until some considerable time has elapsed. It is generally believed that with animals ·and plants these vapours act by arresting oxidation. Exposure to carbonic acid for 2 hrs., and in one case for only 45 m., likewise rendered the glands insensible for a time to the powerful stimulus of raw meat. The leaves, however, recovered their full powers, and did not seem in the least injured, on being left in the air for 24 or 48 hrs. We have seen in the third chapter that the process of aggregation in leaves subjected for two hours to this gas and then immersed in a solution of the carbonate of ammonia is much retarded, so that a considerable time elapses before the protoplasm in the lower cells of the tentacles becomes aggregated. In some cases, soon after the leaves were re1noved from the gas and brought into the air, the tentacles moved spontaneously; this being due, I presume, to the excitement from the access of oxygen. These inflected tentacles, however, could not be excited for some time afterwards to any further moveInent by their glands being stimulated. With other irritable plants it is known* that the exclusion of oxygen prevents their moving, and arrests the movements of the protoplasm within their cells, but this arrest is a different phenomenon from the retardation of the process of aggregation just alluded to. vVhether this latter fact ought to be attributed to the direct action of the carbonic acid, or to the exclusion of oxygen, I know not. * Sachs, ' Traitc de Bot.' 1874, pp. 846, 1037. CHAP. X. SENSITIVENESS OF THE LEA YES. 229 CHAPTER X. ON THE SENSITIVENESS OF THE LEAVES, AND ON THE LINES OF TRANSMISSION OF THE MOTOR IMPULSE. Glands and summits of the tentacles alone sensitive-Transmission of the motor impulse down the pedicels of the tentacles, and across the blade of the leaf- Aggregation of the protoplasm, a reflex action- First discharge of the motor impulse suddenDirection of the movements of the tentacles- Motor impulse transmitted through the cellular tissue- Mechanism of the mo\emonts- Nature of the motor impulse- Re-expansion of the tentacles. WE have seen in the previous chapters that many widely different stimulants, mechanical and chemical, excite the movement of the tentacles, as well as of the blade of the leaf; and we must now consider, firstly, what are the points which are irritable or sensitive, and secondly how the motor impulse is transmitted from one point to another. The glands are almost exclusively the seat of irritability, yet this irritability must extend for a very short distance below them; for when they were cut off with a sharp pair of scissors without being themselves touched, the tentacles often became inflected. These headless tentacles frequently re-expanded; and when afterwards drops of the two most powerful known stimulants were placed on the cut-off ends, no effect was produced. Nevertheless these headless tentacles are capable of subsequent inflection if excited by an impulse sent from the disc. I succeeded on several occasions in crushing glands between fine pincers, but this did not excite any movement; nor did raw meat and salts of ammonia, when placed on such crushed glands. |