Чарльз Дарвин
Insectivorous Plants
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 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,49 yet is not in the least so to Drosera, though quickly causing strong inflection.
Notwithstanding the foregoing facts, which show how widely different is the effect of certain substances on the health or life of animals and of Drosera, yet there exists a certain degree of parallelism in the action of certain other substances. We have seen that this holds good in a striking manner with the salts of sodium and potassium. Again, various metallic salts and acids, namely those of silver, mercury, gold, tin, arsenic, chromium, copper, and platina, most or all of which are highly poisonous to animals, are equally so to Drosera. But it is a singular fact that the chloride of lead and two salts of barium were not poisonous to this plant. It is an equally strange fact, that, though acetic and propionic acids are highly poisonous, their ally, formic acid, is not so; and that, whilst certain vegetable acids, namely oxalic, benzoic, &c., are poisonous in a high degree, gallic, tannic, tartaric, and malic (all diluted to an equal degree) are not so. Malic acid induces inflection, whilst the three other just named vegetable acids have no such power. But a pharmacopoeia would be requisite to describe the diversified effects of various substances on Drosera.
Seeing that acetic, hydrocyanic, and chromic acids, acetate of strychnine, and vapour of ether, are poisonous to Drosera, [] it is remarkable that Dr. Ransom (' Philosoph. Transact.' 1867, p. 480), who used much stronger solutions of these substances than I did, states "that the rhythmic contractility of the yolk (of the ova of the pike) is not materially influenced by any of the poisons used, which did not act chemically, with the exception of chloroform and carbonic acid." I find it stated by several writers that curare has no influence on sarcode or protoplasm, and we have seen that, though curare excites some degree of inflection, it causes very little aggregation of the protoplasm.)
Of the alkaloids and their salts which were tried, several had not the least power of inducing inflection; others, which were certainly absorbed, as shown by the changed colour of the glands, had but a very moderate power of this kind; others, again, such as the acetate of quinine and digitaline, caused strong inflection.
The several substances mentioned in this chapter affect the colour of the glands very differently. These often become dark at first, and then very pale or white, as was conspicuously the case with glands subjected to the poison of the cobra and citrate of strychnine. In other cases they are from the first rendered white, as with leaves placed in hot water and several acids; and this, I presume, is the result of the coagulation of the albumen. On the same leaf some glands become white and others dark-coloured, as occurred with leaves in a solution of the sulphate of quinine, and in the vapour of alcohol. Prolonged immersion in nicotine, curare, and even water, blackens the glands; and this, I believe, is due to the aggregation of the protoplasm within their cells. Yet curare caused very little aggregation in the cells of the tentacles, whereas nicotine and sulphate of quinine induced strongly marked aggregation down their bases. The aggregated masses in leaves which had been immersed for 3 hrs. 15 m. in a saturated solution of sulphate of quinine exhibited incessant changes of form, but after 24 hrs. were motionless; the leaf being flaccid and apparently dead. On the other hand, with leaves subjected for 48 hrs. to a strong solution of the poison of the cobra, the protoplasmic masses were unusually active, whilst with the higher animals the vibratile cilia and white corpuscles of the blood seem to be quickly paralysed by this substance.
With the salts of alkalies and earths, the nature of the base, and not that of the acid, determines their physiological action on Drosera, as is likewise the case with animals; but this rule hardly applies to the salts of quinine and strychnine, for the acetate of quinine causes much more inflection than the sulphate, and both are poisonous, whereas the nitrate of quinine is not poisonous, and induces inflection at a much slower rate than the acetate. The action of the citrate of strychnine is also somewhat different from that of the sulphate.
Leaves which have been immersed for 24 hrs. in water, and for only 20 m. in diluted alcohol, or in a weak solution of sugar, are afterwards acted on very slowly, or not at all, by the phosphate of ammonia, though they are quickly acted on by the carbonate. Immersion for 20 m. in a solution of gum arabic has no such inhibitory power. The solutions of certain salts and acids affect the leaves, with respect to the subsequent action of the phosphate, exactly like water, whilst others allow the phosphate afterwards to act quickly and energetically. In this latter case, the interstices of the cell-walls may have been blocked up by the molecules of the salts first given in solution, so that water could not afterwards enter, though the molecules of the phosphate could do so, and those of the carbonate still more easily.
The action of camphor dissolved in water is remarkable, for it not only soon induces inflection, but apparently renders the glands extremely sensitive to mechanical irritation; for if they are brushed with a soft brush, after being immersed in the solution for a short time, the tentacles begin to bend in about 2 m. It may, however, be that the brushing, though not a sufficient stimulus by itself, tends to excite movement merely by reinforcing the direct action of the camphor. The vapour of camphor, on the other hand, serves as a narcotic.
Some essential oils, both in solution and in vapour, cause rapid inflection, others have no such power; those which I tried were all poisonous.
Diluted alcohol (one part to seven of water) is not poisonous, does not induce inflection, nor increase the sensitiveness of the glands to mechanical irritation. The vapour acts as a narcotic or anaesthetic, and long exposure to it kills the leaves.
The vapours of chloroform, sulphuric and nitric ether, act in a singularly variable manner on different leaves, and on the several tentacles of the same leaf. This, I suppose, is owing to differences in the age or constitution of the leaves, and to whether certain tentacles have lately been in action. That these vapours are absorbed by the glands is shown by their changed colour; but as other plants not furnished with glands are affected by these vapours, it is probable that they are likewise absorbed by the stomata of Drosera. They sometimes excite extraordinarily rapid inflection, but this is not an invariable result. If allowed to act for even a moderately long time, they kill the leaves; whilst a small dose acting for only a short time serves as a narcotic or anaesthetic. In this case the tentacles, whether or not they have become 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 removed 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 movement by their glands being stimulated. With other irritable plants it is known50 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. Whether this latter fact ought to be attributed to the direct action of the carbonic acid, or to the exclusion of oxygen, I know not.
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 sudden – Direction of the movements of the tentacles – Motor impulse transmitted through the cellular tissue – Mechanism of the movements – 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. 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. Nitschke51says that cutting and pricking the leaf does not excite movement. The petiole of the leaf is quite insensible.
The backs of the leaves bear numerous minute papillae, which do not secrete, but have the power of absorption. These papillae are, I believe, rudiments of formerly existing tentacles together with their glands. Many experiments were made to ascertain whether the backs of the leaves could be irritated in any way, thirty-seven leaves being thus tried. Some were rubbed for a long time with a blunt needle, and drops of milk and other exciting fluids, raw meat, crushed flies, and various substances, placed on others. These substances were apt soon to become dry, showing that no secretion had been excited. Hence I moistened them with saliva, solutions of ammonia, weak hydrochloric acid, and frequently with the secretion from the glands of other leaves. I also kept some leaves, on the backs of which exciting objects had been placed, under a damp bell-glass; but with all my care I never saw any true movement. I was led to make so many trials because, contrary to my previous experience, Nitschke states52 that, after affixing objects to the backs of leaves by the aid of the viscid secretion, he repeatedly saw the tentacles (and in one instance the blade) become reflexed. This movement, if a true one, would be most anomalous; for it implies that the tentacles receive a motor impulse from an unnatural source, and have the power of bending in a direction exactly the reverse of that which is habitual to them; this power not being of the least use to the plant, as insects cannot adhere to the smooth backs of the leaves.
I have said that no effect was produced in the above cases; but this is not strictly true, for in three instances a little syrup was added to the bits of raw meat on the backs of leaves, in order to keep them damp for a time; and after 36 hrs. there was a trace of reflexion in the tentacles of one leaf, and certainly in the blade of another. After twelve additional hours, the glands began to dry, and all three leaves seemed much injured. Four leaves were then placed under a bell-glass, with their footstalks in water, with drops of syrup on their backs, but without any meat. Two of these leaves, after a day, had a few tentacles reflexed. The drops had now increased considerably in size, from having imbibed moisture, so as to trickle down the backs of the tentacles and footstalks. On the second day, one leaf had its blade much reflexed; on the third day the tentacles of two were much reflexed, as well as the blades of all four to a greater or less degree. The upper side of one leaf, instead of being, as at first, slightly concave, now presented a strong convexity upwards. Even on the fifth day the leaves did not appear dead. Now, as sugar does not in the least excite Drosera, we may safely attribute the reflexion of the blades and tentacles of the above leaves to exosmose from the cells which were in contact with the syrup, and their consequent contraction. When drops of syrup are placed on the leaves of plants with their roots still in damp earth, no inflection ensues, for the roots, no doubt, pump up water as quickly as it is lost by exosmose. But if cut-off leaves are immersed in syrup, or in any dense fluid, the tentacles are greatly, though irregularly, inflected, some of them assuming the shape of corkscrews; and the leaves soon become flaccid. If they are now immersed in a fluid of low specific gravity, the tentacles re-expand. From these facts we may conclude that drops of syrup placed on the backs of leaves do not act by exciting a motor impulse which is transmitted to the tentacles; but that they cause reflexion by inducing exosmose. Dr. Nitschke used the secretion for sticking insects to the backs of the leaves; and I suppose that he used a large quantity, which from being dense probably caused exosmose. Perhaps he experimented on cut-off leaves, or on plants with their roots not supplied with enough water.
As far, therefore, as our present knowledge serves, we may conclude that the glands, together with the immediately underlying cells of the tentacles, are the exclusive seats of that irritability or sensitiveness with which the leaves are endowed. The degree to which a gland is excited can be measured only by the number of the surrounding tentacles which are inflected, and by the amount and rate of their movement. Equally vigorous leaves, exposed to the same temperature (and this is an important condition), are excited in different degrees under the following circumstances. A minute quantity of a weak solution produces no effect; add more, or give a rather stronger solution, and the tentacles bend. Touch a gland once or twice, and no movement follows; touch it three or four times, and the tentacle becomes inflected. But the nature of the substance which is given is a very important element: if equal-sized particles of glass (which acts only mechanically), of gelatine, and raw meat, are placed on the discs of several leaves, the meat causes far more rapid, energetic, and widely extended movement than the two former substances. The number of glands which are excited also makes a great difference in the result: place a bit of meat on one or two of the discal glands, and only a few of the immediately surrounding short tentacles are inflected; place it on several glands, and many more are acted on; place it on thirty or forty, and all the tentacles, including the extreme marginal ones, become closely inflected. We thus see that the impulses proceeding from a number of glands strengthen one another, spread farther, and act on a larger number of tentacles, than the impulse from any single gland.
Transmission of the Motor Impulse. – In every case the impulse from a gland has to travel for at least a short distance to the basal part of the tentacle, the upper part and the gland itself being merely carried by the inflection of the lower part. The impulse is thus always transmitted down nearly the whole length of the pedicel. When the central glands are stimulated, and the extreme marginal tentacles become inflected, the impulse is transmitted across half the diameter of the disc; and when the glands on one side of the disc are stimulated, the impulse is transmitted across nearly the whole width of the disc. A gland transmits its motor impulse far more easily and quickly down its own tentacle to the bending place than across the disc to neighbouring tentacles. Thus a minute dose of a very weak solution of ammonia, if given to one of the glands of the exterior tentacles, causes it to bend and reach the centre; whereas a large drop of the same solution, given to a score of glands on the disc, will not cause through their combined influence the least inflection of the exterior tentacles. Again, when a bit of meat is placed on the gland of an exterior tentacle, I have seen movement in ten seconds, and repeatedly within a minute; but a much larger bit placed on several glands on the disc does not cause the exterior tentacles to bend until half an hour or even several hours have elapsed.
The motor impulse spreads gradually on all sides from one or more excited glands, so that the tentacles which stand nearest are always first affected. Hence, when the glands in the centre of the disc are excited, the extreme marginal tentacles are the last inflected. But the glands on different parts of the leaf transmit their motor power in a somewhat different manner. If a bit of meat be placed on the long-headed gland of a marginal tentacle, it quickly transmits an impulse to its own bending portion; but never, as far as I have observed, to the adjoining tentacles; for these are not affected until the meat has been carried to the central glands, which then radiate forth their conjoint impulse on all sides. On four occasions leaves were prepared by removing some days previously all the glands from the centre, so that these could not be excited by the bits of meat brought to them by the inflection of the marginal tentacles; and now these marginal tentacles re-expanded after a time without any other tentacle being affected. Other leaves were similarly prepared, and bits of meat were placed on the glands of two tentacles in the third row from the outside, and on the glands of two tentacles in the fifth row. In these four cases the impulse was sent in the first place laterally, that is, in the same concentric row of tentacles, and then towards the centre; but not centrifugally, or towards the exterior tentacles. In one of these cases only a single tentacle on each side of the one with meat was affected. In the three other cases, from half a dozen to a dozen tentacles, both laterally and towards the centre, were well inflected or sub-inflected. Lastly, in ten other experiments, minute bits of meat were placed on a single gland or on two glands in the centre of the disc. In order that no other glands should touch the meat, through the inflection of the closely adjoining short tentacles, about half a dozen glands had been previously removed round the selected ones. On eight of these leaves from sixteen to twenty-five of the short surrounding tentacles were inflected in the course of one or two days; so that the motor impulse radiating from one or two of the discal glands is able to produce this much effect. The tentacles which had been removed are included in the above numbers; for, from standing so close, they would certainly have been affected. On the two remaining leaves, almost all the short tentacles on the disc were inflected. With a more powerful stimulus than meat, namely a little phosphate of lime moistened with saliva, I have seen the inflection spread still farther from a single gland thus treated; but even in this case the three or four outer rows of tentacles were not affected. From these experiments it appears that the impulse from a single gland on the disc acts on a greater number of tentacles than that from a gland of one of the exterior elongated tentacles; and this probably follows, at least in part, from the impulse having to travel a very short distance down the pedicels of the central tentacles, so that it is able to spread to a considerable distance all round.
Whilst examining these leaves, I was struck with the fact that in six, perhaps seven, of them the tentacles were much more inflected at the distal and proximal ends of the leaf (i.e. towards the apex and base) than on either side; and yet the tentacles on the sides stood as near to the gland where the bit of meat lay as did those at the two ends. It thus appeared as if the motor impulse was transmitted from the centre across the disc more readily in a longitudinal than in a transverse direction; and as this appeared a new and interesting fact in the physiology of plants, thirty-five fresh experiments were made to test its truth. Minute bits of meat were placed on a single gland or on a few glands, on the right or left side of the discs of eighteen leaves; other bits of the same size being placed on the distal or proximal ends of seventeen other leaves. Now if the motor impulse were transmitted with equal force or at an equal rate through the blade in all directions, a bit of meat placed at one side or at one end of the disc ought to affect equally all the tentacles situated at an equal distance from it; but this certainly is not the case. Before giving the general results, it may be well to describe three or four rather unusual cases.
[(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, 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 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 or small group of glands through the blade to the other tentacles more readily and effectually in a longitudinal than in a transverse direction.
