Insectivorous Plants · Charles Darwin
ACIDS, MUCH DILUTED, WHICH CAUSE INFLECTION.
A short immersion in water for 20 m. did not retard the subsequent action of the phosphate, or of splinters of glass placed on the glands; but in two instances an immersion for 50 m. prevented any effect from a solution of camphor. Several leaves which had been left for 20 m. in a solution of one part of white sugar to 218 of water were placed in the phosphate solution, the action of which was delayed; whereas a mixed solution of sugar and the phosphate did not in the least interfere with the effects of the latter. Three leaves, after being immersed for 20 m. in the sugar solution, were placed in a solution of carbonate of ammonia (one part to 218 of water); in 2 m. or 3 m. the glands were blackened, and after 7 m. the tentacles were considerably inflected, so that the solution of sugar, though it delayed the action of the phosphate, did not delay that of the carbonate. Immersion in a similar solution of gum arabic for 20 m. had no retarding action on the phosphate. Three leaves were left for 20 m. in a mixture of one part of alcohol to seven parts of water, [page 214] and then placed in the phosphate solution: in 2 hrs. 15 m. there was a trace of inflection in one leaf, and in 5 hrs. 30 m. a second was slightly affected; the inflection subsequently increased, though slowly. Hence diluted alcohol, which, as we shall see, is hardly at all poisonous, plainly retards the subsequent action of the phosphate.
It was shown in the last chapter that leaves which did not become inflected by nearly a day’s immersion in solutions of various salts and acids behaved very differently from one another when subsequently placed in the phosphate solution. I here give a table summing up the results. Column 1 : Name of the Salts and Acids in Solution. Column 2 : Period of Immersion of the Leaves in Solutions of one part to 437 of water. Column 3 : Effects produced on the Leaves by their subsequent Immersion for stated periods in a Solution of one part of phosphate of ammonia to 8750 of water, or 1 gr. to 20 oz. Rubidium chloride. : 22 hrs. : After 30 m. strong inflection of the tentacles. Potassium carbonate : 20 m. : Scarcely any inflection until 5 hrs. had elapsed. Calcium acetate. : 24 hrs. : After 24 hrs. very slight inflection. Calcium nitrate. : 24 hrs. : Do. do. Magnesium acetate. : 22 hrs. : Some slight inflection, which became well pronounced in 24 hrs. Magnesium nitrate. : 22 hrs. : After 4 hrs. 30 m. a fair amount of inflection, which never increased. Magnesium chloride : 22 hrs. : After a few minutes great inflection; after 4 hrs. all four leaves with almost every tentacle closely inflected. Barium acetate. : 22 hrs. : After 24 hrs. two leaves out of four slightly inflected.
Barium nitrate. : 22 hrs. : After 30 m. one leaf greatly, and two others moderately, inflected; they remained thus for 24 hrs. Strontium acetate. : 22 hrs. : After 25 m. two leaves greatly inflected; after 8 hrs. a third leaf moderately, and the fourth very slightly, inflected. All four thus remained for 24 hrs. Strontium nitrate. : 22 hrs. : After 8 hrs. three leaves out of five moderately inflected; after 24 hrs. all five in this state; but not one closely inflected. Aluminium chloride : 24 hrs. : Three leaves which had either been slightly or not at all affected by the chloride became after 7 hrs. 30 m. rather closely inflected. [page 215] Column 1 : Name of the Salts and Acids in Solution. Column 2 : Period of Immersion of the Leaves in Solutions of one part to 437 of water. Column 3 : Effects produced on the Leaves by their subsequent Immersion for stated periods in a Solution of one part of phosphate of ammonia to 8750 of water, or 1 gr. to 20 oz. Aluminium nitrate. : 24 hrs. : After 25 hrs. slight and doubtful effect. Lead chloride. : 23 hrs. : After 24 hrs. two leaves somewhat inflected, the third very little; and thus remained. Manganese chloride : 22 hrs. : After 48 hrs. not the least inflection.
Lactic acid. : 48 hrs. : After 24 hrs. a trace of inflection in a few tentacles, the glands of which had not been killed by the acid. Tannic acid. : 24 hrs. : After 24 hrs. no inflection. Tartaric acid. : 24 hrs. : Do. do. Citric acid. : 24 hrs. : After 50 m. tentacles decidedly inflected, and after 5 hrs. strongly inflected; so remained for the next 24 hrs. Formic acid. : 22 hrs. : Not observed until 24 hrs. had elapsed; tentacles considerably inflected, and protoplasm aggregated. In a large majority of these twenty cases, a varying degree of inflection was slowly caused by the phosphate. In four cases, however, the inflection was rapid, occurring in less than half an hour or at most in 50 m. In three cases the phosphate did not produce the least effect. Now what are we to infer from these facts? We know from ten trials that immersion in distilled water for 24 hrs. prevents the subsequent action of the phosphate solution. It would, therefore, appear as if the solutions of chloride of manganese, tannic and tartaric acids, which are not poisonous, acted exactly like water, for the phosphate produced no effect on the leaves which had been previously immersed in these three solutions. The majority of the other solutions behaved to a certain extent like water, for the phosphate produced, after a considerable interval of time, only a slight effect. On the other hand, the leaves which had been immersed in the solutions of the chloride of rubidium and magnesium, of acetate of strontium, nitrate of barium, and citric acid, were quickly acted on by the phosphate. Now was water absorbed from these five weak solutions, and yet, owing to the presence of the salts, did not prevent the subsequent action of the phosphate?
Or [page 216] may we not suppose* that the interstices of the walls of the glands were blocked up with the molecules of these five substances, so that they were rendered impermeable to water; for had water entered, we know from the ten trials that the phosphate would not afterwards have produced any effect? It further appears that the molecules of the carbonate of ammonia can quickly pass into glands which, from having been immersed for 20 m. in a weak solution of sugar, either absorb the phosphate very slowly or are acted on by it very slowly. On the other hand, glands, however they may have been treated, seem easily to permit the subsequent entrance of the molecules of carbonate of ammonia. Thus leaves which had been immersed in a solution (of one part to 437 of water) of nitrate of potassium for 48 hrs.—of sulphate of potassium for 24 hrs.—and of the chloride of potassium for 25 hrs.—on being placed in a solution of one part of carbonate of ammonia to 218 of water, had their glands immediately blackened, and after 1 hr. their tentacles somewhat inflected, and the protoplasm aggregated. But it would be an endless task to endeavour to ascertain the wonderfully diversified effects of various solutions on Drosera. Alcohol (one part to seven of water).—It has already been shown that half-minims of this strength placed on the discs of leaves do not cause any inflection; and that when two days afterwards the leaves were given bits of meat, they became strongly inflected. Four leaves were immersed in this mixture, and two of them after 30 m. were brushed with a camel-hair brush, like the leaves in the solution of camphor, but this produced no effect.
The vapour of alcohol acts differently. A plant having three good leaves was left for 25 m. under a receiver holding 19 oz. with sixty minims of alcohol in a watch-glass. No movement ensued, but some few of the glands were blackened and shrivelled, whilst many became quite pale. These were scattered over all the leaves in the most irregular manner, reminding me of the manner in which the glands were affected by the vapour of carbonate of ammonia. Immediately on the removal of the receiver particles of raw meat were placed on many of the glands, those which retained their proper colour being chiefly selected. But not a single tentacle was inflected during the next 4 hrs. After the first 2 hrs. the glands on all the tentacles began to dry; and next morning, after 22 hrs., all three leaves appeared almost dead, with their glands dry; the tentacles on one leaf alone being partially inflected.
A second plant was left for only 5 m. with some alcohol in a watch-glass, under a 12-oz. receiver, and particles of meat were then placed on the glands of several tentacles. After 10 m. some of them began to curve inwards, and after 55 m. nearly all were considerably inflected; but a few did not move. Some anaesthetic effect is here probable, but by no means certain. A third plant was also left for 5 m. under the same small vessel, with its whole inner surface wetted with about a dozen drops of alcohol. Particles of meat were now placed on the glands of several tentacles, some of which first began to move in 25 m.; after 40 m. most of them were somewhat inflected, and after 1 hr. 10 m. almost all were considerably inflected. From their slow rate of movement there can be no doubt that the glands of these tentacles had been rendered insensible for a time by exposure during 5 m. to the vapour of alcohol. Vapour of Chloroform.—The action of this vapour on Drosera is very variable, depending, I suppose, on the constitution or age of the plant, or on some unknown condition. It sometimes causes the tentacles to move with extraordinary rapidity, and sometimes produces no such effect. The glands are sometimes [page 218] rendered for a time insensible to the action of raw meat, but sometimes are not thus affected, or in a very slight degree. A plant recovers from a small dose, but is easily killed by a larger one.
A plant was left for 30 m. under a bell-glass holding 19 fluid oz. (539.6 ml.) with eight drops of chloroform, and before the cover was removed, most of the tentacles became much inflected, though they did not reach the centre. After the cover was removed, bits of meat were placed on the glands of several of the somewhat incurved tentacles; these glands were found much blackened after 6 hrs. 30 m., but no further movement ensued. After 24 hrs. the leaves appeared almost dead. A smaller bell-glass, holding 12 fluid oz. (340.8 ml.), was now employed, and a plant was left for 90 s. under it, with only two drops of chloroform. Immediately on the removal of the glass all the tentacles curved inwards so as to stand perpendicularly up; and some of them could actually be seen moving with extraordinary quickness by little starts, and therefore in an unnatural manner; but they never reached the centre. After 22 hrs. they fully re-expanded, and on meat being placed on their glands, or when roughly touched by a needle, they promptly became inflected; so that these leaves had not been in the least injured. Another plant was placed under the same small bell-glass with three drops of chloroform, and before two minutes had elapsed, the tentacles began to curl inwards with rapid little jerks. The glass was then removed, and in the course of two or three additional minutes almost every tentacle reached the centre. On several other occasions the vapour did not excite any movement of this kind.
There seems also to be great variability in the degree and manner in which chloroform renders the glands insensible to the subsequent action of meat. In the plant last referred to, which had been exposed for 2 m. to three drops of chloroform, some few tentacles curved up only to a perpendicular position, and particles of meat were placed on their glands; this caused them in 5 m. to begin moving, but they moved so slowly that they did not reach the centre until 1 hr. 30 m. had elapsed. Another plant was similarly exposed, that is, for 2 m. to three drops of chloroform, and on particles of meat being placed on the glands of several tentacles, which had curved up into a perpendicular position, one of these began to bend in 8 m., but afterwards moved very slowly; whilst none of the other tentacles [page 219] moved for the next 40 m. Nevertheless, in 1 hr. 45 m. from the time when the bits of meat had been given, all the tentacles reached the centre. In this case some slight anaesthetic effect apparently had been produced. On the following day the plant had perfectly recovered.
Another plant bearing two leaves was exposed for 2 m. under the 19-oz. vessel to two drops of chloroform; it was then taken out and examined; again exposed for 2 m. to two drops; taken out, and re-exposed for 3 m. to three drops; so that altogether it was exposed alternately to the air and during 7 m. to the vapour of seven drops of chloroform. Bits of meat were now placed on thirteen glands on the two leaves. On one of these leaves, a single tentacle first began moving in 40 m., and two others in 54 m. On the second leaf some tentacles first moved in 1 hr. 11 m. After 2 hrs. many tentacles on both leaves were inflected; but none had reached the centre within this time. In this case there could not be the least doubt that the chloroform had exerted an anaesthetic influence on the leaves. 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.
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.
I will give only one other experiment. After the exposure of a plant for 2 hrs. to the gas, one of its leaves was immersed in a rather strong solution of carbonate of ammonia, together with [page 223] a fresh leaf from another plant. The latter had most of its tentacles strongly inflected within 30 m.; whereas the leaf which had been exposed to the carbonic acid remained for 24 hrs. in the solution without undergoing any inflection, with the exception of two tentacles. This leaf had been almost completely paralysed, and was not able to recover its sensibility whilst still in the solution, which from having been made with distilled water probably contained little oxygen.] Concluding Remarks on the Effects of the foregoing Agents.—As the glands, when excited, transmit some influence to the surrounding tentacles, causing them to bend and their glands to pour forth an increased amount of modified secretion, I was anxious to ascertain whether the leaves included any element having the nature of nerve-tissue, which, though not continuous, served as the channel of transmission. This led me to try the several alkaloids and other substances which are known to exert a powerful influence on the nervous system of animals; I was at first encouraged in my trials by finding that strychnine, digitaline, and nicotine, which all act on the nervous system, were poisonous to Drosera, and caused a certain amount of inflection. Hydrocyanic acid, again, which is so deadly a poison to animals, caused rapid movement of the tentacles. But as several innocuous acids, though much diluted, such as benzoic, acetic, &c., as well as some essential oils, are extremely poisonous to Drosera, and quickly cause strong inflection, it seems probable that strychnine, nicotine, digitaline, and hydrocyanic acid, excite inflection by acting on elements in no way analogous to the nerve-cells of animals. If elements of this latter nature had been present in the leaves, it might have been expected that morphia, hyoscyamus, atropine, veratrine, colchicine, curare, and diluted alcohol would have produced some marked effect; whereas [page 224] these substances are not poisonous and have no power, or only a very slight one, of inducing inflection. It should, however, be observed that curare, colchicine, and veratrine are muscle-poisons—that is, act on nerves having some special relation with the muscles, and, therefore, could not be expected to act on Drosera. The poison of the cobra is most deadly to animals, by paralysing their nerve-centres,* yet is not in the least so to Drosera, though quickly causing strong inflection.
Text from Project Gutenberg, public domain in the United States.