Insectivorous Plants

by Charles Darwin

[(1) A minute fragment of a fly was placed on one side of the disc, and after 32 m. seven of the outer tentacles near the fragment were inflected; after 10 hrs. several more became so, and after 23 hrs. a still greater number; and now the blade of the leaf on this side was bent inwards so as to stand up at right angles to the other side. Neither the blade of the leaf nor a single tentacle on the opposite side was affected; the line of separation between the two halves extending from the footstalk to the apex. The leaf remained in this state for three days, and on the fourth day began to re-expand; not a single tentacle having been inflected on the opposite side.

(2) I will here give a case not included in the above thirty-five experiments. A small fly was found adhering by its feet to the left side of the disc. The tentacles on this side soon closed in and killed the fly; and owing probably to its struggle whilst alive, the leaf was so much excited that in about 24 hrs. all the tentacles on the opposite side became inflected; but as they found no prey, for their glands did not reach the fly, they re-expanded in the course of 15 hrs.; the tentacles on the left side remaining clasped for several days.

(3) A bit of meat, rather larger than those commonly used, [page 238] was placed in a medial line at the basal end of the disc, near the footstalk; after 2 hrs. 30 m. some neighbouring tentacles were inflected; after 6 hrs. the tentacles on both sides of the footstalk, and some way up both sides, were moderately inflected; after 8 hrs. the tentacles at the further or distal end were more inflected than those on either side; after 23 hrs. the meat was well clasped by all the tentacles, excepting by the exterior ones on the two sides.

(4) Another bit of meat was placed at the opposite or distal end of another leaf, with exactly the same relative results.

(5) A minute bit of meat was placed on one side of the disc; next day the neighbouring short tentacles were inflected, as well as in a slight degree three or four on the opposite side near the footstalk. On the second day these latter tentacles showed signs of re-expanding, so I added a fresh bit of meat at nearly the same spot, and after two days some of the short tentacles on the opposite side of the disc were inflected. As soon as these began to re-expand, I added another bit of meat, and next day all the tentacles on the opposite side of the disc were inflected towards the meat; whereas we have seen that those on the same side were affected by the first bit of meat which was given.]

Now for the general results. Of the eighteen leaves on which bits of meat were placed on the right or left sides of the disc, eight had a vast number of tentacles inflected on the same side, and in four of them the blade itself on this side was likewise inflected; whereas not a single tentacle nor the blade was affected on the opposite side. These leaves presented a very curious appearance, as if only the inflected side was active, and the other paralysed. In the remaining ten cases, a few tentacles became inflected beyond the medial line, on the side opposite to that where the meat lay; but, in some of these cases, only at the proximal or distal ends of the leaves. The inflection on the opposite side always occurred considerably after that on the same side, and in one instance not until the fourth day. We have also seen [page 239] with No. 5 that bits of meat had to be added thrice before all the short tentacles on the opposite side of the disc were inflected.

The result was widely different when bits of meat were placed in a medial line at the distal or proximal ends of the disc. In three of the seventeen experiments thus made, owing either to the state of the leaf or to the smallness of the bit of meat, only the immediately adjoining tentacles were affected; but in the other fourteen cases the tentacles at the opposite end of the leaf were inflected, though these were as distant from where the meat lay as were those on one side of the disc from the meat on the opposite side. In some of the present cases the tentacles on the sides were not at all affected, or in a less degree, or after a longer interval of time, than those at the opposite end. One set of experiments is worth giving in fuller detail. Cubes of meat, not quite so small as those usually employed, were placed on one side of the discs of four leaves, and cubes of the same size at the proximal or distal end of four other leaves. Now, when these two sets of leaves were compared after an interval of 24 hrs., they presented a striking difference. Those having the cubes on one side were very slightly affected on the opposite side; whereas those with the cubes at either end had almost every tentacle at the opposite end, even the marginal ones, closely inflected. After 48 hrs. the contrast in the state of the two sets was still great; yet those with the meat on one side now had their discal and submarginal tentacles on the opposite side somewhat inflected, this being due to the large size of the cubes. Finally we may conclude from these thirty-five experiments, not to mention the six or seven previous ones, that the motor impulse is transmitted from any single gland [page 240] or small group of glands through the blade to the other tentacles more readily and effectually in a longitudinal than in a transverse direction.

As long as the glands remain excited, and this may last for many days, even for eleven, as when in contact with phosphate of lime, they continue to transmit a motor impulse to the basal and bending parts of their own pedicels, for otherwise they would re-expand. The great difference in the length of time during which tentacles remain inflected over inorganic objects, and over objects of the same size containing soluble nitrogenous matter, proves the same fact. But the intensity of the impulse transmitted from an excited gland, which has begun to pour forth its acid secretion and is at the same time absorbing, seems to be very small compared with that which it transmits when first excited. Thus, when moderately large bits of meat were placed on one side of the disc, and the discal and sub-marginal tentacles on the opposite side became inflected, so that their glands at last touched the meat and absorbed matter from it, they did not transmit any motor influence to the exterior rows of tentacles on the same side, for these never became inflected. If, however, meat had been placed on the glands of these same tentacles before they had begun to secrete copiously and to absorb, they undoubtedly would have affected the exterior rows. Nevertheless, when I gave some phosphate of lime, which is a most powerful stimulant, to several submarginal tentacles already considerably inflected, but not yet in contact with some phosphate previously placed on two glands in the centre of the disc, the exterior tentacles on the same side were acted on.

When a gland is first excited, the motor impulse is discharged within a few seconds, as we know from the [page 241] bending of the tentacle; and it appears to be discharged at first with much greater force than afterwards. Thus, in the case above given of a small fly naturally caught by a few glands on one side of a leaf, an impulse was slowly transmitted from them across the whole breadth of the leaf, causing the opposite tentacles to be temporarily inflected, but the glands which remained in contact with the insect, though they continued for several days to send an impulse down their own pedicels to the bending place, did not prevent the tentacles on the opposite side from quickly re-expanding; so that the motor discharge must at first have been more powerful than afterwards.

When an object of any kind is placed on the disc, and the surrounding tentacles are inflected, their glands secrete more copiously and the secretion becomes acid, so that some influence is sent to them from the discal glands. This change in the nature and amount of the secretion cannot depend on the bending of the tentacles, as the glands of the short central tentacles secrete acid when an object is placed on them, though they do not themselves bend. Therefore I inferred that the glands of the disc sent some influence up the surrounding tentacles to their glands, and that these reflected back a motor impulse to their basal parts; but this view was soon proved erroneous. It was found by many trials that tentacles with their glands closely cut off by sharp scissors often become inflected and again re-expand, still appearing healthy. One which was observed continued healthy for ten days after the operation. I therefore cut the glands off twenty-five tentacles, at different times and on different leaves, and seventeen of these soon became inflected, and afterwards re-expanded. The re-expansion commenced in about [page 242] 8 hrs. or 9 hrs., and was completed in from 22 hrs. to 30 hrs. from the time of inflection. After an interval of a day or two, raw meat with saliva was placed on the discs of these seventeen leaves, and when observed next day, seven of the headless tentacles were inflected over the meat as closely as the uninjured ones on the same leaves; and an eighth headless tentacle became inflected after three additional days. The meat was removed from one of these leaves, and the surface washed with a little stream of water, and after three days the headless tentacle re-expanded for the second time. These tentacles without glands were, however, in a different state from those provided with glands and which had absorbed matter from the meat, for the protoplasm within the cells of the former had undergone far less aggregation. From these experiments with headless tentacles it is certain that the glands do not, as far as the motor impulse is concerned, act in a reflex manner like the nerve-ganglia of animals.