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. [page 230] It is probable that they were killed so instantly that they were not able to transmit any motor impulse; for in six observed cases (in two of which however the gland was quite pinched off) the protoplasm within the cells of the tentacles did not become aggregated; whereas in some adjoining tentacles, which were inflected from having been roughly touched by the pincers, it was well aggregated. In like manner the protoplasm does not become aggregated when a leaf is instantly killed by being dipped into boiling water. On the other hand, in several cases in which tentacles became inflected after their glands had been cut off with sharp scissors, a distinct though moderate degree of aggregation supervened.
The pedicels of the tentacles were roughly and repeatedly rubbed; raw meat or other exciting substances were placed on them, both on the upper surface near the base and elsewhere, but no distinct movement ensued. Some bits of meat, after being left for a considerable time on the pedicels, were pushed upwards, so as just to touch the glands, and in a minute the tentacles began to bend. I believe that the blade of the leaf is not sensitive to any stimulant. I drove the point of a lancet through the blades of several leaves, and a needle three or four times through nineteen leaves: in the former case no movement ensued; but about a dozen of the leaves which were repeatedly pricked had a few tentacles irregularly inflected. As, however, their backs had to be supported during the operation, some of the outer glands, as well as those on the disc, may have been touched; and this perhaps sufficed to cause the slight degree of movement observed. Nitschke*says
* ‘Bot. Zeitung,’ 1860, p. 234. [page 231]
that cutting and pricking the leaf does not excite movement. The petiole of the leaf is quite insensible.