Insectivorous Plants

by Charles Darwin

On the other hand, another plant was exposed under the same vessel for a much longer time, viz. 20 m., to twice as much chloroform. Bits of meat were then placed on the glands of many tentacles, and all of them, with a single exception, reached the centre in from 13 m. to 14 m. In this case, little or no anaesthetic effect had been produced; and how to reconcile these discordant results, I know not.

Vapour of Sulphuric Ether.—A plant was exposed for 30 m. to thirty minims of this ether in a vessel holding 19 oz.; and bits of raw meat were afterwards placed on many glands which had become pale-coloured; but none of the tentacles moved. After 6 hrs. 30 m. the leaves appeared sickly, and the discal glands were almost dry. By the next morning many of the tentacles were dead, as were all those on which meat had been placed; showing that matter had been absorbed from the meat which had increased the evil effects of the vapour. After four days the plant itself died. Another plant was exposed in the same vessel for 15 m. to forty minims. One young, small, and tender leaf had all its tentacles inflected, and seemed much injured. Bits of raw meat were placed on several glands on two other and older leaves. These glands became dry after 6 hrs.; and seemed injured; the tentacles never moved, excepting one which was ultimately a little inflected. The glands of the other tentacles continued to secrete, and appeared uninjured, but the whole plant after three days became very sickly. [page 220]

In the two foregoing experiments the doses were evidently too large and poisonous. With weaker doses, the anaesthetic effect was variable, as in the case of chloroform. A plant was exposed for 5 m. to ten drops under a 12-oz. vessel, and bits of meat were then placed on many glands. None of the tentacles thus treated began to move in a decided manner until 40 m. had elapsed; but then some of them moved very quickly, so that two reached the centre after an additional interval of only 10 m. In 2 hrs. 12 m. from the time when the meat was given, all the tentacles reached the centre. Another plant, with two leaves, was exposed in the same vessel for 5 m. to a rather larger dose of ether, and bits of meat were placed on several glands. In this case one tentacle on each leaf began to bend in 5 m.; and after 12 m. two tentacles on one leaf, and one on the second leaf, reached the centre. In 30 m. after the meat had been given, all the tentacles, both those with and without meat, were closely inflected; so that the ether apparently had stimulated these leaves, causing all the tentacles to bend.

Vapour of Nitric Ether.—This vapour seems more injurious than that of sulphuric ether. A plant was exposed for 5 m. in a 12-oz. vessel to eight drops in a watch-glass, and I distinctly saw a few tentacles curling inwards before the glass was removed. Immediately afterwards bits of meat were placed on three glands, but no movement ensued in the course of 18 m. The same plant was placed again under the same vessel for 16 m. with ten drops of the ether. None of the tentacles moved, and next morning those with the meat were still in the same position. After 48 hrs. one leaf seemed healthy, but the others were much injured.

Another plant, having two good leaves, was exposed for 6 m. under a 19-oz. vessel to the vapour from ten minims of the ether, and bits of meat were then placed on the glands of many tentacles on both leaves. After 36 m. several of them on one leaf became inflected, and after 1 hr. almost all the tentacles, those with and without meat, nearly reached the centre. On the other leaf the glands began to dry in 1 hr. 40 m., and after several hours not a single tentacle was inflected; but by the next morning, after 21 hrs., many were inflected, though they seemed much injured. In this and the previous experiment, it is doubtful, owing to the injury which the leaves had suffered, whether any anaesthetic effect had been produced.

A third plant, having two good leaves, was exposed for only 4 m. in the 19-oz. vessel to the vapour from six drops. Bits of meat were then placed on the glands of seven tentacles on the [page 221] same leaf. A single tentacle moved after 1 hr. 23 m.; after 2 hrs. 3 m. several were inflected; and after 3 hrs. 3 m. all the seven tentacles with meat were well inflected. From the slowness of these movements it is clear that this leaf had been rendered insensible for a time to the action of the meat. A second leaf was rather differently affected; bits of meat were placed on the glands of five tentacles, three of which were slightly inflected in 28 m.; after 1 hr. 21 m. one reached the centre, but the other two were still only slightly inflected; after 3 hrs. they were much more inflected; but even after 5 hrs. 16 m. all five had not reached the centre. Although some of the tentacles began to move moderately soon, they afterwards moved with extreme slowness. By next morning, after 20 hrs., most of the tentacles on both leaves were closely inflected, but not quite regularly. After 48 hrs. neither leaf appeared injured, though the tentacles were still inflected; after 72 hrs. one was almost dead, whilst the other was re-expanding and recovering.

Carbonic Acid.—A plant was placed under a 122-oz. bell-glass filled with this gas and standing over water; but I did not make sufficient allowance for the absorption of the gas by the water, so that towards the latter part of the experiment some air was drawn in. After an exposure of 2 hrs. the plant was removed, and bits of raw meat placed on the glands of three leaves. One of these leaves hung a little down, and was at first partly and soon afterwards completely covered by the water, which rose within the vessel as the gas was absorbed. On this latter leaf the tentacles, to which meat had been given, became well inflected in 2 m. 30 s., that is, at about the normal rate; so that until I remembered that the leaf had been protected from the gas, and might perhaps have absorbed oxygen from the water which was continually drawn inwards, I falsely concluded that the carbonic acid had produced no effect. On the other two leaves, the tentacles with meat behaved very differently from those on the first leaf; two of them first began to move slightly in 1 hr. 50 m., always reckoning from the time when the meat was placed on the glands—were plainly inflected in 2 hrs. 22 m.—and in 3 hrs 22 m. reached the centre. Three other tentacles did not begin to move until 2 hrs. 20 m. had elapsed, but reached the centre at about the same time with the others, viz. in 3 hrs. 22 m.

This experiment was repeated several times with nearly the same results, excepting that the interval before the tentacles began to move varied a little. I will give only one other case. [page 222] A plant was exposed in the same vessel to the gas for 45 m., and bits of meat were then placed on four glands. But the tentacles did not move for 1 hr. 40 m.; after 2 hrs. 30 m. all four were well inflected, and after 3 hrs. reached the centre.

The following singular phenomenon sometimes, but by no means always, occurred. A plant was immersed for 2 hrs., and bits of meat were then placed on several glands. In the course of 13 m. all the submarginal tentacles on one leaf became considerably inflected; those with the meat not in the least degree more than the others. On a second leaf, which was rather old, the tentacles with meat, as well as a few others, were moderately inflected. On a third leaf all the tentacles were closely inflected, though meat had not been placed on any of the glands. This movement, I presume, may be attributed to excitement from the absorption of oxygen. The last-mentioned leaf, to which no meat had been given, was fully re-expanded after 24 hrs.; whereas the two other leaves had all their tentacles closely inflected over the bits of meat which by this time had been carried to their centres. Thus these three leaves had perfectly recovered from the effects of the gas in the course of 24 hrs.

On another occasion some fine plants, after having been left for 2 hrs. in the gas, were immediately given bits of meat in the usual manner, and on their exposure to the air most of their tentacles became in 12 m. curved into a vertical or sub-vertical position, but in an extremely irregular manner; some only on one side of the leaf and some on the other. They remained in this position for some time; the tentacles with the bits of meat not having at first moved more quickly or farther inwards than the others without meat. But after 2 hrs. 20 m. the former began to move, and steadily went on bending until they reached the centre. Next morning, after 22 hrs., all the tentacles on these leaves were closely clasped over the meat which had been carried to their centres; whilst the vertical and sub-vertical tentacles on the other leaves to which no meat had been given had fully re-expanded. Judging, however, from the subsequent action of a weak solution of carbonate of ammonia on one of these latter leaves, it had not perfectly recovered its excitability and power of movement in 22 hrs.; but another leaf, after an additional 24 hrs., had completely recovered, judging from the manner in which it clasped a fly placed on its disc.