Insectivorous Plants

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 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.

But there is another action, namely that of aggregation, which in certain cases may be called reflex, and it is the only known instance in the vegetable kingdom. We should bear in mind that the process does not depend on the previous bending of the tentacles, as we clearly see when leaves are immersed in certain strong solutions. Nor does it depend on increased secretion from the glands, and this is shown by several facts, more especially by the papillae, which do not secrete, yet undergoing aggregation, if given carbonate of ammonia or an infusion of raw meat. When a gland is directly stimulated in any way, as by the pressure of a minute particle of glass, the protoplasm within the cells of the gland first becomes aggregated, then that in the cells immediately beneath the gland, and so lower and lower down the tentacles to their bases; – that is, if the stimulus has been sufficient and not injurious. Now, when the glands of the disc are excited, the exterior tentacles are affected in exactly the same manner: the aggregation always commences in their glands, though these have not been directly excited, but have only received some influence from the disc, as shown by their increased acid secretion. The protoplasm within the cells immediately beneath the glands are next affected, and so downwards from cell to cell to the bases of the tentacles. This process apparently deserves to be called a reflex action, in the same manner as when a sensory nerve is irritated, and carries an impression to a ganglion which sends back some influence to a muscle or gland, causing movement or increased secretion; but the action in the two cases is probably of a widely different nature. After the protoplasm in a tentacle has been aggregated, its redissolution always begins in the lower part, and slowly travels up the pedicel to the gland, so that the protoplasm last aggregated is first redissolved. This probably depends merely on the protoplasm being less and less aggregated, lower and lower down in the tentacles, as can be seen plainly when the excitement has been slight. As soon, therefore, as the aggregating action altogether ceases, redissolution naturally commences in the less strongly aggregated matter in the lowest part of the tentacle, and is there first completed.

Direction of the Inflected Tentacles. – When a particle of any kind is placed on the gland of one of the outer tentacles, this invariably moves towards the centre of the leaf; and so it is with all the tentacles of a leaf immersed in any exciting fluid. The glands of the exterior tentacles then form a ring round the middle part of the disc, as shown in a previous figure (fig. 4, p. 10). The short tentacles within this ring still retain their vertical position, as they likewise do when a large object is placed on their glands, or when an insect is caught by them. In this latter case we can see that the inflection of the short central tentacles would be useless, as their glands are already in contact with their prey.

The result is very different when a single gland on one side of the disc is excited, or a few in a group. These send an impulse to the surrounding tentacles, which do not now bend towards the centre of the leaf, but to the point of excitement. We owe this capital observation to Nitschke,[53 - 'Bot. Zeitung,' 1860, p. 240.] and since reading his paper a few years ago, I have repeatedly verified it. If a minute bit of meat be placed by the aid of a needle on a single gland, or on three or four together, halfway between the centre and the circumference of the disc, the directed movement of the surrounding tentacles is well exhibited. An accurate drawing of a leaf with meat in this position is here reproduced (fig. 10), and we see the tentacles, including some of the exterior ones, accurately directed to the point where the meat lay. But a much better plan is to place a particle of the phosphate of lime moistened with saliva on a single gland on one side of the disc of a large leaf, and another particle on a single gland on the opposite side. In four such trials the excitement was not sufficient to affect the outer tentacles, but all those near the two points were directed to them, so that two wheels were formed on the disc of the same leaf; the pedicels of the tentacles forming the spokes, and the glands united in a mass over the phosphate representing the axles. The precision with which each tentacle pointed to the particle was wonderful; so that in some cases I could detect no deviation from perfect accuracy. Thus, although the short tentacles in the middle of the disc do not bend when their glands are excited in a direct manner, yet if they receive a motor impulse from a point on one side, they direct themselves to the point equally well with the tentacles on the borders of the disc.

In these experiments, some of the short tentacles on the disc, which would have been directed to the centre, had the leaf been immersed in an exciting fluid, were now inflected in an exactly opposite direction, viz. towards the circumference. These tentacles, therefore, had deviated as much as 180o from the direction which they would have assumed if their own glands had been stimulated, and which may be considered as the normal one. Between this, the greatest possible and no deviation from the normal direction, every degree could be observed in the tentacles on these several leaves. Notwithstanding the precision with which the tentacles generally were directed, those near the circumference of one leaf were not accurately directed towards some phosphate of lime at a rather distant point on the opposite side of the disc. It appeared as if the motor impulse in passing transversely across nearly the whole width of the disc had departed somewhat from a true course. This accords with what we have already seen of the impulse travelling less readily in a transverse than in a longitudinal direction. In some other cases, the exterior tentacles did not seem capable of such accurate movement as the shorter and more central ones.

Nothing could be more striking than the appearance of the above four leaves, each with their tentacles pointing truly to the two little masses of the phosphate on their discs. We might imagine that we were looking at a lowly organised animal seizing prey with its arms. In the case of Drosera the explanation of this accurate power of movement, no doubt, lies in the motor impulse radiating in all directions, and whichever side of a tentacle it first strikes, that side contracts, and the tentacle consequently bends towards the point of excitement. The pedicels of the tentacles are flattened, or elliptic in section. Near the bases of the short central tentacles, the flattened or broad face is formed of about five longitudinal rows of cells; in the outer tentacles of the disc it consists of about six or seven rows; and in the extreme marginal tentacles of above a dozen rows. As the flattened bases are thus formed of only a few rows of cells, the precision of the movements of the tentacles is the more remarkable; for when the motor impulse strikes the base of a tentacle in a very oblique direction relatively to its broad face, scarcely more than one or two cells towards one end can be affected at first, and the contraction of these cells must draw the whole tentacle into the proper direction. It is, perhaps, owing to the exterior pedicels being much flattened that they do not bend quite so accurately to the point of excitement as the more central ones. The properly directed movement of the tentacles is not an unique case in the vegetable kingdom, for the tendrils of many plants curve towards the side which is touched; but the case of Drosera is far more interesting, as here the tentacles are not directly excited, but receive an impulse from a distant point; nevertheless, they bend accurately towards this point.

On the Nature of the Tissues through which the Motor Impulse is Transmitted. – It will be necessary first to describe briefly the course of the main fibro-vascular bundles. These are shown in the accompanying sketch (fig. 11) of a small leaf. Little vessels from the neighbouring bundles enter all the many tentacles with which the surface is studded; but these are not here represented. The central trunk, which runs up the footstalk, bifurcates near the centre of the leaf, each branch bifurcating again and again according to the size of the leaf. This central trunk sends off, low down on each side, a delicate branch, which may be called the sublateral branch. There is also, on each side, a main lateral branch or bundle, which bifurcates in the same manner as the others. Bifurcation does not imply that any single vessel divides, but that a bundle divides into two. By looking to either side of the leaf, it will be seen that a branch from the great central bifurcation inosculates with a branch from the lateral bundle, and that there is a smaller inosculation between the two chief branches of the lateral bundle. The course of the vessels is very complex at the larger inosculation; and here vessels, retaining the same diameter, are often formed by the union of the bluntly pointed ends of two vessels, but whether these points open into each other by their attached surfaces, I do not know. By means of the two inosculations all the vessels on the same side of the leaf are brought into some sort of connection. Near the circumference of the larger leaves the bifurcating branches also come into close union, and then separate again, forming a continuous zigzag line of vessels round the whole circumference. But the union of the vessels in this zigzag line seems to be much less intimate than at the main inosculation. It should be added that the course of the vessels differs somewhat in different leaves, and even on opposite sides of the same leaf, but the main inosculation is always present.

Now in my first experiments with bits of meat placed on one side of the disc, it so happened that not a single tentacle was inflected on the opposite side; and when I saw that the vessels on the same side were all connected together by the two inosculations, whilst not a vessel passed over to the opposite side, it seemed probable that the motor impulse was conducted exclusively along them.

In order to test this view, I divided transversely with the point of a lancet the central trunks of four leaves, just beneath the main bifurcation; and two days afterwards placed rather large bits of raw meat (a most powerful stimulant) near the centre of the disc above the incision – that is, a little towards the apex – with the following results: —

[(1) This leaf proved rather torpid: after 4 hrs. 40 m. (in all cases reckoning from the time when the meat was given) the tentacles at the distal end were a little inflected, but nowhere else; they remained so for three days, and re-expanded on the fourth day. The leaf was then dissected, and the trunk, as well as the two sublateral branches, were found divided.

(2) After 4 hrs. 30 m. many of the tentacles at the distal end were well inflected. Next day the blade and all the tentacles at this end were strongly inflected, and were separated by a distinct transverse line from the basal half of the leaf, which was not in the least affected. On the third day, however, some of the short tentacles on the disc near the base were very slightly inflected. The incision was found on dissection to extend across the leaf as in the last case.

(3) After 4 hrs. 30 m. strong inflection of the tentacles at the distal end, which during the next two days never extended in the least to the basal end. The incision as before.

(4) This leaf was not observed until 15 hrs. had elapsed, and then all the tentacles, except the extreme marginal ones, were found equally well inflected all round the leaf. On careful examination the spiral vessels of the central trunk were certainly divided; but the incision on one side had not passed through the fibrous tissue surrounding these vessels, though it had passed through the tissue on the other side.[54 - M. Ziegler made similar experiments by cutting the spiral vessels of Drosera intermedia('Comptes rendus,' 1874, p. 1417), but arrived at conclusions widely different from mine.]]

The appearance presented by the leaves (2) and (3) was very curious, and might be aptly compared with that of a man with his backbone broken and lower extremities paralysed. Excepting that the line between the two halves was here transverse instead of longitudinal, these leaves were in the same state as some of those in the former experiments, with bits of meat placed on one side of the disc. The case of leaf (4) proves that the spiral vessels of the central trunk may be divided, and yet the motor impulse be transmitted from the distal to the basal end; and this led me at first to suppose that the motor force was sent through the closely surrounding fibrous tissue; and that if one half of this tissue was left undivided, it sufficed for complete transmission. But opposed to this conclusion is the fact that no vessels pass directly from one side of the leaf to the other, and yet, as we have seen, if a rather large bit of meat is placed on one side, the motor impulse is sent, though slowly and imperfectly, in a transverse direction across the whole breadth of the leaf. Nor can this latter fact be accounted for by supposing that the transmission is effected through the two inosculations, or through the circumferential zigzag line of union, for had this been the case, the exterior tentacles on the opposite side of the disc would have been affected before the more central ones, which never occurred. We have also seen that the extreme marginal tentacles appear to have no power to transmit an impulse to the adjoining tentacles; yet the little bundle of vessels which enters each marginal tentacle sends off a minute branch to those on both sides, and this I have not observed in any other tentacles; so that the marginal ones are more closely connected together by spiral vessels than are the others, and yet have much less power of communicating a motor impulse to one another.

But besides these several facts and arguments we have conclusive evidence that the motor impulse is not sent, at least exclusively, through the spiral vessels, or through the tissue immediately surrounding them. We know that if a bit of meat is placed on a gland (the immediately adjoining ones having been removed) on any part of the disc, all the short sur- rounding tentacles bend almost simultaneously with great precision towards it. Now there are tentacles on the disc, for instance near the extremities of the sublateral bundles (fig. 11), which are supplied with vessels that do not come into contact with the branches that enter the surrounding tentacles, except by a very long and extremely circuitous course. Nevertheless, if a bit of meat is placed on the gland of a tentacle of this kind, all the surrounding ones are inflected towards it with great precision. It is, of course, possible that an impulse might be sent through a long and circuitous course, but it is obviously impossible that the direction of the movement could be thus communicated, so that all the surrounding tentacles should bend precisely to the point of excitement. The impulse no doubt is transmitted in straight radiating lines from the excited gland to the surrounding tentacles; it cannot, therefore, be sent along the fibro-vascular bundles. The effect of cutting the central vessels, in the above cases, in preventing the transmission of the motor impulse from the distal to the basal end of a leaf, may be attributed to a considerable space of the cellular tissue having been divided. We shall hereafter see, when we treat of Dionaea, that this same conclusion, namely that the motor impulse is not transmitted by the fibro-vascular bundles, is plainly confirmed; and Prof. Cohn has come to the same conclusion with respect to Aldrovanda – both members of the Droseraceae.

As the motor impulse is not transmitted along the vessels, there remains for its passage only the cellular tissue; and the structure of this tissue explains to a certain extent how it travels so quickly down the long exterior tentacles, and much more slowly across the blade of the leaf. We shall also see why it crosses the blade more quickly in a longitudinal than in a transverse direction; though with time it can pass in any direction. We know that the same stimulus causes movement of the tentacles and aggregation of the protoplasm, and that both influences originate in and proceed from the glands within the same brief space of time. It seems therefore probable that the motor impulse consists of the first commencement of a molecular change in the protoplasm, which, when well developed, is plainly visible, and has been designated aggregation; but to this subject I shall return. We further know that in the transmission of the aggregating process the chief delay is caused by the passage of the transverse cell-walls; for as the aggregation travels down the tentacles, the contents of each successive cell seem almost to flash into a cloudy mass. We may therefore infer that the motor impulse is in like manner delayed chiefly by passing through the cell-walls.

The greater celerity with which the impulse is transmitted down the long exterior tentacles than across the disc may be largely attributed to its being closely confined within the narrow pedicel, instead of radiating forth on all sides as on the disc. But besides this confinement, the exterior cells of the tentacles are fully twice as long as those of the disc; so that only half the number of transverse partitions have to be traversed in a given length of a tentacle, compared with an equal space on the disc; and there would be in the same proportion less retardation of the impulse. Moreover, in sections of the exterior tentacles given by Dr. Warming,[55 - 'Videnskabelige Meddelelser de la Soc. d'Hist. nat. de Copenhague,' Nos. 10-12, 1872, woodcuts iv. and v.] the parenchymatous cells are shown to be still more elongated; and these would form the most direct line of communication from the gland to the bending place of the tentacle. If the impulse travels down the exterior cells, it would have to cross from between twenty to thirty transverse partitions; but rather fewer if down the inner parenchymatous tissue. In either case it is remarkable that the impulse is able to pass through so many partitions down nearly the whole length of the pedicel, and to act on the bending place, in ten seconds. Why the impulse, after having passed so quickly down one of the extreme marginal tentacles (about 1/20 of an inch in length), should never, as far as I have seen, affect the adjoining tentacles, I do not understand. It may be in part accounted for by much energy being expended in the rapidity of the transmission.

Most of the cells of the disc, both the superficial ones and the larger cells which form the five or six underlying layers, are about four times as long as broad. They are arranged almost longitudinally, radiating from the footstalk. The motor impulse, therefore, when transmitted across the disc, has to cross nearly four times as many cell-walls as when transmitted in a longitudinal direction, and would consequently be much delayed in the former case. The cells of the disc converge towards the bases of the tentacles, and are thus fitted to convey the motor impulse to them from all sides. On the whole, the arrangement and shape of the cells, both those of the disc and tentacles, throw much light on the rate and manner of diffusion of the motor impulse. But why the impulse proceeding from the glands of the exterior rows of tentacles tends to travel laterally and towards the centre of the leaf, but not centrifugally, is by no means clear.

Mechanism of the Movements, and Nature of the Motor Impulse. – Whatever may be the means of movement, the exterior tentacles, considering their delicacy, are inflected with much force. A bristle, held so that a length of 1 inch projected from a handle, yielded when I tried to lift with it an inflected tentacle, which was somewhat thinner than the bristle. The amount or extent, also, of the movement is great. Fully expanded tentacles in becoming inflected sweep through an angle of 180o; and if they are beforehand reflexed, as often occurs, the angle is considerably greater. It is probably the superficial cells at the bending place which chiefly or exclusively contract; for the interior cells have very delicate walls, and are so few in number that they could hardly cause a tentacle to bend with precision to a definite point. Though I carefully looked, I could never detect any wrinkling of the surface at the bending place, even in the case of a tentacle abnormally curved into a complete circle, under circumstances hereafter to be mentioned.

All the cells are not acted on, though the motor impulse passes through them. When the gland of one of the long exterior tentacles is excited, the upper cells are not in the least affected; about halfway down there is a slight bending, but the chief movement is confined to a short space near the base; and no part of the inner tentacles bends except the basal portion. With respect to the blade of the leaf, the motor impulse may be transmitted through many cells, from the centre to the circumference, without their being in the least affected, or they may be strongly acted on and the blade greatly inflected. In the latter case the movement seems to depend partly on the strength of the stimulus, and partly on its nature, as when leaves are immersed in certain fluids.

The power of movement which various plants possess, when irritated, has been attributed by high authorities to the rapid passage of fluid out of certain cells, which, from their previous state of tension, immediately contract.[56 - Sachs, 'Trait de Bot.' 3rd edit. 1874, p. 1038. This view was, I believe, first suggested by Lamarck. Sachs, ibid. p. 919.] Whether or not this is the primary cause of such movements, fluid must pass out of closed cells when they contract or are pressed together in one direction, unless they at the same time expand in some other direction. For instance, fluid can be seen to ooze from the surface of any young and vigorous shoot if slowly bent into a semi-circle. In the case of Drosera there is certainly much movement of the fluid throughout the tentacles whilst they are undergoing inflection. Many leaves can be found in which the purple fluid within the cells is of an equally dark tint on the upper and lower sides of the tentacles, extending also downwards on both sides to equally near their bases. If the tentacles of such a leaf are excited into movement, it will generally be found after some hours that the cells on the concave side are much paler than they were before, or are quite colourless, those on the convex side having become much darker. In two instances, after particles of hair had been placed on glands, and when in the course of 1 hr. 10 m. the tentacles were incurved halfway towards the centre of the leaf, this change of colour in the two sides was conspicuously plain. In another case, after a bit of meat had been placed on a gland, the purple colour was observed at intervals to be slowly travelling from the upper to the lower part, down the convex side of the bending tentacle. But it does not follow from these observations that the cells on the convex side become filled with more fluid during the act of inflection than they contained before; for fluid may all the time be passing into the disc or into the glands which then secrete freely.

The bending of the tentacles, when leaves are immersed in a dense fluid, and their subsequent re-expansion in a less dense fluid, show that the passage of fluid from or into the cells can cause movements like the natural ones. But the inflection thus caused is often irregular; the exterior tentacles being sometimes spirally curved. Other unnatural movements are likewise caused by the application of dense fluids, as in the case of drops of syrup placed on the backs of leaves and tentacles. Such movements may be compared with the contortions which many vegetable tissues undergo when subjected to exosmose. It is therefore doubtful whether they throw any light on the natural movements.

If we admit that the outward passage of fluid is the cause of the bending of the tentacles, we must suppose that the cells, before the act of inflection, are in a high state of tension, and that they are elastic to an extraordinary degree; for otherwise their contraction could not cause the tentacles often to sweep through an angle of above 180o. Prof. Cohn, in his interesting paper[57 - 'Abhand. der Schles. Gesell. fr vaterl. Cultur,' 1861, Heft i. An excellent abstract of this paper is given in the 'Annals and Mag. of Nat. Hist.' 3rd series, 1863, vol. xi. pp. 188-197.] on the movements of the stamens of certain Compositae, states that these organs, when dead, are as elastic as threads of india-rubber, and are then only half as long as they were when alive. He believes that the living protoplasm within their cells is ordinarily in a state of expansion, but is paralysed by irritation, or may be said to suffer temporary death; the elasticity of the cell-walls then coming into play, and causing the contraction of the stamens. Now the cells on the upper or concave side of the bending part of the tentacles of Drosera do not appear to be in a state of tension, nor to be highly elastic; for when a leaf is suddenly killed, or dies slowly, it is not the upper but the lower sides of the tentacles which contract from elasticity. We may, therefore, conclude that their movements cannot be accounted for by the inherent elasticity of certain cells, opposed as long as they are alive and not irritated by the expanded state of their contents.

A somewhat different view has been advanced by other physiologists – namely that the protoplasm, when irritated, contracts like the soft sarcode of the muscles of animals. In Drosera the fluid within the cells of the tentacles at the bending place appears under the microscope thin and homogeneous, and after aggregation consists of small, soft masses of matter, undergoing incessant changes of form and floating in almost colourless fluid. These masses are completely redissolved when the tentacles re-expand. Now it seems scarcely possible that such matter should have any direct mechanical power; but if through some molecular change it were to occupy less space than it did before, no doubt the cell-walls would close up and contract. But in this case it might be expected that the walls would exhibit wrinkles, and none could ever be seen. Moreover, the contents of all the cells seem to be of exactly the same nature, both before and after aggregation; and yet only a few of the basal cells contract, the rest of the tentacle remaining straight.

A third view maintained by some physiologists, though rejected by most others, is that the whole cell, including the walls, actively contracts. If the walls are composed solely of non-nitrogenous cellulose, this view is highly improbable; but it can hardly be doubted that they must be permeated by proteid matter, at least whilst they are growing. Nor does there seem any inherent improbability in the cell-walls of Drosera contracting, considering their high state of organisation; as shown in the case of the glands by their power of absorption and secretion, and by being exquisitely sensitive so as to be affected by the pressure of the most minute particles. The cell-walls of the pedicels also allow various impulses to pass through them, inducing movement, increased secretion and aggregation. On the whole the belief that the walls of certain cells contract, some of their contained fluid being at the same time forced outwards, perhaps accords best with the observed facts. If this view is rejected, the next most probable one is that the fluid contents of the cells shrink, owing to a change in their molecular state, with the consequent closing in of the walls. Anyhow, the movement can hardly be attributed to the elasticity of the walls, together with a previous state of tension.

With respect to the nature of the motor impulse which is transmitted from the glands down the pedicels and across the disc, it seems not improbable that it is closely allied to that influence which causes the protoplasm within the cells of the glands and tentacles to aggregate. We have seen that both forces originate in and proceed from the glands within a few seconds of the same time, and are excited by the same causes. The aggregation of the protoplasm lasts almost as long as the tentacles remain inflected, even though this be for more than a week; but the protoplasm is redissolved at the bending place shortly before the tentacles re-expand, showing that the exciting cause of the aggregating process has then quite ceased. Exposure to carbonic acid causes both the latter process and the motor impulse to travel very slowly down the tentacles. We know that the aggregating process is delayed in passing through the cell- walls, and we have good reason to believe that this holds good with the motor impulse; for we can thus understand the different rates of its transmission in a longitudinal and transverse line across the disc. Under a high power the first sign of aggregation is the appearance of a cloud, and soon afterwards of extremely fine granules, in the homogeneous purple fluid within the cells; and this apparently is due to the union of molecules of protoplasm. Now it does not seem an improbable view that the same tendency – namely for the molecules to approach each other – should be communicated to the inner surfaces of the cell-walls which are in contact with the protoplasm; and if so, their molecules would approach each other, and the cell-wall would contract.

To this view it may with truth be objected that when leaves are immersed in various strong solutions, or are subjected to a heat of above 130o Fahr. (54o.4 Cent.), aggregation ensues, but there is no movement. Again, various acids and some other fluids cause rapid movement, but no aggregation, or only of an abnormal nature, or only after a long interval of time; but as most of these fluids are more or less injurious, they may check or prevent the aggregating process by injuring or killing the protoplasm. There is another and more important difference in the two processes: when the glands on the disc are excited, they transmit some influence up the surrounding tentacles, which acts on the cells at the bending place, but does not induce aggregation until it has reached the glands; these then send back some other influence, causing the protoplasm to aggregate, first in the upper and then in the lower cells.

The Re-expansion of the Tentacles. – This movement is always slow and gradual. When the centre of the leaf is excited, or a leaf is immersed in a proper solution, all the tentacles bend directly towards the centre, and afterwards directly back from it. But when the point of excitement is on one side of the disc, the surrounding tentacles bend towards it, and therefore obliquely with respect to their normal direction; when they afterwards re-expand, they bend obliquely back, so as to recover their original positions. The tentacles farthest from an excited point, wherever that may be, are the last and the least affected, and probably in consequence of this they are the first to re-expand. The bent portion of a closely inflected tentacle is in a state of active contraction, as shown by the following experiment. Meat was placed on a leaf, and after the tentacles were closely inflected and had quite ceased to move, narrow strips of the disc, with a few of the outer tentacles attached to it, were cut off and laid on one side under the microscope. After several failures, I succeeded in cutting off the convex surface of the bent portion of a tentacle. Movement immediately recommenced, and the already greatly bent portion went on bending until it formed a perfect circle; the straight distal portion of the tentacle passing on one side of the strip. The convex surface must therefore have previously been in a state of tension, sufficient to counter-balance that of the concave surface, which, when free, curled into a complete ring.

The tentacles of an expanded and unexcited leaf are moderately rigid and elastic; if bent by a needle, the upper end yields more easily than the basal and thicker part, which alone is capable of becoming inflected. The rigidity of this basal part seems due to the tension of the outer surface balancing a state of active and persistent contraction of the cells of the inner surface. I believe that this is the case, because, when a leaf is dipped into boiling water, the tentacles suddenly become reflexed, and this apparently indicates that the tension of the outer surface is mechanical, whilst that of the inner surface is vital, and is instantly destroyed by the boiling water. We can thus also understand why the tentacles as they grow old and feeble slowly become much reflexed. If a leaf with its tentacles closely inflected is dipped into boiling water, these rise up a little, but by no means fully re-expand. This may be owing to the heat quickly destroying the tension and elasticity of the cells of the convex surface; but I can hardly believe that their tension, at any one time, would suffice to carry back the tentacles to their original position, often through an angle of above 180o. It is more probable that fluid, which we know travels along the tentacles during the act of inflection, is slowly re-attracted into the cells of the convex surface, their tension being thus gradually and continually increased.

A recapitulation of the chief facts and discussions in this chapter will be given at the close of the next chapter.

CHAPTER XI

RECAPITULATION OF THE CHIEF OBSERVATIONS ON DROSERA ROTUNDIFOLIA

As summaries have been given to most of the chapters, it will be sufficient here to recapitulate, as briefly as I can, the chief points. In the first chapter a preliminary sketch was given of the structure of the leaves, and of the manner in which they capture insects. This is effected by drops of extremely viscid fluid surrounding the glands and by the inward movement of the tentacles. As the plants gain most of their nutriment by this means, their roots are very poorly developed; and they often grow in places where hardly any other plant except mosses can exist. The glands have the power of absorption, besides that of secretion. They are extremely sensitive to various stimulants, namely repeated touches, the pressure of minute particles, the absorption of animal matter and of various fluids, heat, and galvanic action. A tentacle with a bit of raw meat on the gland has been seen to begin bending in 10 s., to be strongly incurved in 5 m., and to reach the centre of the leaf in half an hour. The blade of the leaf often becomes so much inflected that it forms a cup, enclosing any object placed on it.

A gland, when excited, not only sends some influence down its own tentacle, causing it to bend, but likewise to the surrounding tentacles, which become incurved; so that the bending place can be acted on by an impulse received from opposite directions, namely from the gland on the summit of the same tentacle, and from one or more glands of the neighbouring tentacles. Tentacles, when inflected, re-expand after a time, and during this process the glands secrete less copiously, or become dry. As soon as they begin to secrete again, the tentacles are ready to re-act; and this may be repeated at least three, probably many more times.

It was shown in the second chapter that animal substances placed on the discs cause much more prompt and energetic inflection than do inorganic bodies of the same size, or mere mechanical irritation; but there is a still more marked difference in the greater length of time during which the tentacles remain inflected over bodies yielding soluble and nutritious matter, than over those which do not yield such matter. Extremely minute particles of glass, cinders, hair, thread, precipitated chalk, &c., when placed on the glands of the outer tentacles, cause them to bend. A particle, unless it sinks through the secretion and actually touches the surface of the gland with some one point, does not produce any effect. A little bit of thin human hair 8/1000 of an inch (.203 mm.) in length, and weighing only 1/78740 of a grain (.000822 mg.), though largely supported by the dense secretion, suffices to induce movement. It is not probable that the pressure in this case could have amounted to that from the millionth of a grain. Even smaller particles cause a slight movement, as could be seen through a lens. Larger particles than those of which the measurements have been given cause no sensation when placed on the tongue, one of the most sensitive parts of the human body.

Movement ensues if a gland is momentarily touched three or four times; but if touched only once or twice, though with considerable force and with a hard object, the tentacle does not bend. The plant is thus saved from much useless movement, as during a high wind the glands can hardly escape being occasionally brushed by the leaves of surrounding plants. Though insensible to a single touch, they are exquisitely sensitive, as just stated, to the slightest pressure if prolonged for a few seconds; and this capacity is manifestly of service to the plant in capturing small insects. Even gnats, if they rest on the glands with their delicate feet, are quickly and securely embraced. The glands are insensible to the weight and repeated blows of drops of heavy rain, and the plants are thus likewise saved from much useless movement.

The description of the movements of the tentacles was interrupted in the third chapter for the sake of describing the process of aggregation. This process always commences in the cells of the glands, the contents of which first become cloudy; and this has been observed within 10 s. after a gland has been excited. Granules just resolvable under a very high power soon appear, sometimes within a minute, in the cells beneath the glands; and these then aggregate into minute spheres. The process afterwards travels down the tentacles, being arrested for a short time at each transverse partition. The small spheres coalesce into larger spheres, or into oval, club-headed, thread- or necklace-like, or otherwise shaped masses of protoplasm, which, suspended in almost colourless fluid, exhibit incessant spontaneous changes of form. These frequently coalesce and again separate. If a gland has been powerfully excited, all the cells down to the base of the tentacle are affected. In cells, especially if filled with dark red fluid, the first step in the process often is the formation of a dark red, bag-like mass of protoplasm, which afterwards divides and undergoes the usual repeated changes of form. Before any aggregation has been excited, a sheet of colourless protoplasm, including granules (the primordial utricle of Mohl), flows round the walls of the cells; and this becomes more distinct after the contents have been partially aggregated into spheres or bag-like masses. But after a time the granules are drawn towards the central masses and unite with them; and then the circulating sheet can no longer be distinguished, but there is still a current of transparent fluid within the cells.

Aggregation is excited by almost all the stimulants which induce movement; such as the glands being touched two or three times, the pressure of minute inorganic particles, the absorption of various fluids, even long immersion in distilled water, exosmose, and heat. Of the many stimulants tried, carbonate of ammonia is the most energetic and acts the quickest: a dose of 1/134400 of a grain (.00048 mg.) given to a single gland suffices to cause in one hour well-marked aggregation in the upper cells of the tentacle. The process goes on only as long as the protoplasm is in a living, vigorous, and oxygenated condition.

The result is in all respects exactly the same, whether a gland has been excited directly, or has received an influence from other and distant glands. But there is one important difference: when the central glands are irritated, they transmit centrifugally an influence up the pedicels of the exterior tentacles to their glands; but the actual process of aggregation travels centripetally, from the glands of the exterior tentacles down their pedicels. The exciting influence, therefore, which is transmitted from one part of the leaf to another must be different from that which actually induces aggregation. The process does not depend on the glands secreting more copiously than they did before; and is independent of the inflection of the tentacles. It continues as long as the tentacles remain inflected, and as soon as these are fully re-expanded, the little masses of protoplasm are all redissolved; the cells becoming filled with homogeneous purple fluid, as they were before the leaf was excited.

As the process of aggregation can be excited by a few touches, or by the pressure of insoluble particles, it is evidently independent of the absorption of any matter, and must be of a molecular nature. Even when caused by the absorption of the carbonate or other salt of ammonia, or an infusion of meat, the process seems to be of exactly the same nature. The protoplasmic fluid must, therefore, be in a singularly unstable condition, to be acted on by such slight and varied causes. Physiologists believe that when a nerve is touched, and it transmits an influence to other parts of the nervous system, a molecular change is induced in it, though not visible to us. Therefore it is a very interesting spectacle to watch the effects on the cells of a gland, of the pressure of a bit of hair, weighing only 1/78700 of a grain and largely supported by the dense secretion, for this excessively slight pressure soon causes a visible change in the protoplasm, which change is transmitted down the whole length of the tentacle, giving it at last a mottled appearance, distinguishable even by the naked eye.

In the fourth chapter it was shown that leaves placed for a short time in water at a temperature of 110o Fahr. (43o.3 Cent.) become somewhat inflected; they are thus also rendered more sensitive to the action of meat than they were before. If exposed to a temperature of between 115o and 125o(46o.1-51o.6 Cent.), they are quickly inflected, and their protoplasm undergoes aggregation; when afterwards placed in cold water, they re-expand. Exposed to 130o (54o.4 Cent.), no inflection immediately occurs, but the leaves are only temporarily paralysed, for on being left in cold water, they often become inflected and afterwards re-expand. In one leaf thus treated, I distinctly saw the protoplasm in movement. In other leaves, treated in the same manner, and then immersed in a solution of carbonate of ammonia, strong aggregation ensued. Leaves placed in cold water, after an exposure to so high a temperature as 145o (62o.7 Cent.), sometimes become slightly, though slowly, inflected; and afterwards have the contents of their cells strongly aggregated by carbonate of ammonia. But the duration of the immersion is an important element, for if left in water at 145o (62o.7 Cent.), or only at 140o (60 °Cent.), until it becomes cool, they are killed, and the contents of the glands are rendered white and opaque. This latter result seems to be due to the coagulation of the albumen, and was almost always caused by even a short exposure to 150o (65o.5 Cent.); but different leaves, and even the separate cells in the same tentacle, differ considerably in their power of resisting heat. Unless the heat has been sufficient to coagulate the albumen, carbonate of ammonia subsequently induces aggregation.

In the fifth chapter, the results of placing drops of various nitrogenous and non-nitrogenous organic fluids on the discs of leaves were given, and it was shown that they detect with almost unerring certainty the presence of nitrogen. A decoction of green peas or of fresh cabbage-leaves acts almost as powerfully as an infusion of raw meat; whereas an infusion of cabbage- leaves made by keeping them for a long time in merely warm water is far less efficient. A decoction of grass-leaves is less powerful than one of green peas or cabbage-leaves.

These results led me to inquire whether Drosera possessed the power of dissolving solid animal matter. The experiments proving that the leaves are capable of true digestion, and that the glands absorb the digested matter, are given in detail in the sixth chapter. These are, perhaps, the most interesting of all my observations on Drosera, as no such power was before distinctly known to exist in the vegetable kingdom. It is likewise an interesting fact that the glands of the disc, when irritated, should transmit some influence to the glands of the exterior tentacles, causing them to secrete more copiously and the secretion to become acid, as if they had been directly excited by an object placed on them. The gastric juice of animals contains, as is well known, an acid and a ferment, both of which are indispensable for digestion, and so it is with the secretion of Drosera. When the stomach of an animal is mechanically irritated, it secretes an acid, and when particles of glass or other such objects were placed on the glands of Drosera, the secretion, and that of the surrounding and untouched glands, was increased in quantity and became acid. But, according to Schiff, the stomach of an animal does not secrete its proper ferment, pepsin, until certain substances, which he calls peptogenes, are absorbed; and it appears from my experiments that some matter must be absorbed by the glands of Drosera before they secrete their proper ferment. That the secretion does contain a ferment which acts only in the presence of an acid on solid animal matter, was clearly proved by adding minute doses of an alkali, which entirely arrested the process of digestion, this immediately recommencing as soon as the alkali was neutralised by a little weak hydrochloric acid. From trials made with a large number of substances, it was found that those which the secretion of Drosera dissolves completely, or partially, or not at all, are acted on in exactly the same manner by gastric juice. We may, therefore, conclude that the ferment of Drosera is closely analogous to, or identical with, the pepsin of animals.

The substances which are digested by Drosera act on the leaves very differently. Some cause much more energetic and rapid inflection of the tentacles, and keep them inflected for a much longer time, than do others. We are thus led to believe that the former are more nutritious than the latter, as is known to be the case with some of these same substances when given to animals; for instance, meat in comparison with gelatine. As cartilage is so tough a substance and is so little acted on by water, its prompt dissolution by the secretion of Drosera, and subsequent absorption is, perhaps, one of the most striking cases. But it is not really more remarkable than the digestion of meat, which is dissolved by this secretion in the same manner and by the same stages as by gastric juice. The secretion dissolves bone, and even the enamel of teeth, but this is simply due to the large quantity of acid secreted, owing, apparently, to the desire of the plant for phosphorus. In the case of bone, the ferment does not come into play until all the phosphate of lime has been decomposed and free acid is present, and then the fibrous basis is quickly dissolved. Lastly, the secretion attacks and dissolves matter out of living seeds, which it sometimes kills, or injures, as shown by the diseased state of the seedlings. It also absorbs matter from pollen, and from fragments of leaves.

The seventh chapter was devoted to the action of the salts of ammonia. These all cause the tentacles, and often the blade of the leaf, to be inflected, and the protoplasm to be aggregated. They act with very different power; the citrate being the least powerful, and the phosphate, owing, no doubt, to the presence of phosphorus and nitrogen, by far the most powerful. But the relative efficiency of only three salts of ammonia was carefully determined, namely the carbonate, nitrate, and phosphate. The experiments were made by placing half-minims (.0296 ml.) of solutions of different strengths on the discs of the leaves, – by applying a minute drop (about the 1/20 of a minim, or .00296 ml.) for a few seconds to three or four glands, – and by the immersion of whole leaves in a measured quantity. In relation to these experiments it was necessary first to ascertain the effects of distilled water, and it was found, as described in detail, that the more sensitive leaves are affected by it, but only in a slight degree.

A solution of the carbonate is absorbed by the roots and induces aggregation in their cells, but does not affect the leaves. The vapour is absorbed by the glands, and causes inflection as well as aggregation. A drop of a solution containing 1/960 of a grain (.0675 mg.) is the least quantity which, when placed on the glands of the disc, excites the exterior tentacles to bend inwards. But a minute drop, containing 1/14400 of a grain (.00445 mg.), if applied for a few seconds to the secretion surrounding a gland, causes the inflection of the same tentacle. When a highly sensitive leaf is immersed in a solution, and there is ample time for absorption, the 1/268800 of a grain (.00024 mg.) is sufficient to excite a single tentacle into movement.

The nitrate of ammonia induces aggregation of the protoplasm much less quickly than the carbonate, but is more potent in causing inflection. A drop containing 1/2400 of a grain (.027 mg.) placed on the disc acts powerfully on all the exterior tentacles, which have not themselves received any of the solution; whereas a drop with 1/2800 of a grain caused only a few of these tentacles to bend, but affected rather more plainly the blade. A minute drop applied as before, and containing 1/28800 of a grain (.0025 mg.), caused the tentacle bearing this gland to bend. By the immersion of whole leaves, it was proved that the absorption by a single gland of 1/691200 of a grain (.0000937 mg.) was sufficient to set the same tentacle into movement.

The phosphate of ammonia is much more powerful than the nitrate. A drop containing 1/3840 of a grain (.0169 mg.) placed on the disc of a sensitive leaf causes most of the exterior tentacles to be inflected, as well as the blade of the leaf. A minute drop containing 1/153600 of a grain (.000423 mg.), applied for a few seconds to a gland, acts, as shown by the movement of the tentacle. When a leaf is immersed in thirty minims (1.7748 ml.) of a solution of one part by weight of the salt to 21,875,000 of water, the absorption by a gland of only the 1/19760000 of a grain (.00000328 mg.), that is, about the one-twenty-millionth of a grain, is sufficient to cause the tentacle bearing this gland to bend to the centre of the leaf. In this experiment, owing to the presence of the water of crystallisation, less than the one-thirty-millionth of a grain of the efficient elements could have been absorbed. There is nothing remarkable in such minute quantities being absorbed by the glands, for all physiologists admit that the salts of ammonia, which must be brought in still smaller quantity by a single shower of rain to the roots, are absorbed by them. Nor is it surprising that Drosera should be enabled to profit by the absorption of these salts, for yeast and other low fungoid forms flourish in solutions of ammonia, if the other necessary elements are present. But it is an astonishing fact, on which I will not here again enlarge, that so inconceivably minute a quantity as the one-twenty-millionth of a grain of phosphate of ammonia should induce some change in a gland of Drosera, sufficient to cause a motor impulse to be sent down the whole length of the tentacle; this impulse exciting movement often through an angle of above 180o. I know not whether to be most astonished at this fact, or that the pressure of a minute bit of hair, supported by the dense secretion, should quickly cause conspicuous movement. Moreover, this extreme sensitiveness, exceeding that of the most delicate part of the human body, as well as the power of transmitting various impulses from one part of the leaf to another, have been acquired without the intervention of any nervous system.

As few plants are at present known to possess glands specially adapted for absorption, it seemed worth while to try the effects on Drosera of various other salts, besides those of ammonia, and of various acids. Their action, as described in the eighth chapter, does not correspond at all strictly with their chemical affinities, as inferred from the classification commonly followed. The nature of the base is far more influential than that of the acid; and this is known to hold good with animals. For instance, nine salts of sodium all caused well-marked inflection, and none of them were poisonous in small doses; whereas seven of the nine corre- sponding salts of potassium produced no effect, two causing slight inflection. Small doses, moreover, of some of the latter salts were poisonous. The salts of sodium and potassium, when injected into the veins of animals, likewise differ widely in their action. The so-called earthy salts produce hardly any effect on Drosera. On the other hand, most of the metallic salts cause rapid and strong inflection, and are highly poisonous; but there are some odd exceptions to this rule; thus chloride of lead and zinc, as well as two salts of barium, did not cause inflection, and were not poisonous.

Most of the acids which were tried, though much diluted (one part to 437 of water), and given in small doses, acted powerfully on Drosera; nineteen, out of the twenty-four, causing the tentacles to be more or less inflected. Most of them, even the organic acids, are poisonous, often highly so; and this is remarkable, as the juices of so many plants contain acids. Benzoic acid, which is innocuous to animals, seems to be as poisonous to Drosera as hydrocyanic. On the other hand, hydrochloric acid is not poisonous either to animals or to Drosera, and induces only a moderate amount of inflection. Many acids excite the glands to secrete an extraordinary quantity of mucus; and the protoplasm within their cells seems to be often killed, as may be inferred from the surrounding fluid soon becoming pink. It is strange that allied acids act very differently: formic acid induces very slight inflection, and is not poisonous; whereas acetic acid of the same strength acts most powerfully and is poisonous. Lactic acid is also poisonous, but causes inflection only after a considerable lapse of time. Malic acid acts slightly, whereas citric and tartaric acids produce no effect.