Insectivorous Plants

Preliminary Sketch of the Action of the several Parts, and of the Manner in ivhich Insects are Captured. If a small organic or inorganic object be placed on the glands in the centre of a leaf, these transmit a motor impulse to the marginal tentacles. The nearer ones are first affected and slowly bend towards the centre, and then those farther off, until at last all become closely inflected over the object. This takes place in from one hour to four or five or more hours. The difference in the time required depends on many circumstances ; namely on the size of the object and on its nature, that is, whether it contains soluble matter of the proper kind ; on the vigour and age of the leaf ; whether it has lately been in action ; and, according to Nitschke,* on the temperature of the day, as likewise seemed to me to be the case. A living insect is a more efficient object than a dead one, as in struggling it presses against the glands of many tentacles. An insect, such as a fly, with thin integu- ments, through which animal matter in solution can readily pass into the surrounding dense secretion, is more efficient in causing prolonged inflection than an insect with a thick coat, such as a beetle. The inflec- tion of the tentacles takes place indifferently in the light and darkness; and the plant is not subject to any nocturnal movement of so-called sleep.

If the glands on the disc are repeatedly touched or bmshed, although no object is left on them, the marginal tentacles curve inwards. So again, if drops of various fluids, for instance of saliva or of a solu- tion of any salt of ammonia, are placed on the central glands, the same result quickly follows, sometimes in ander half an hour. The tentacles in the act of inflection sweep through a wide space ; thus a marginal tentacle, extended in the same plane with the blade, moves through an angle of 180° ; and I have seen the much reflected tentacles of a leaf which stood . upright move through an angle of not less than 270°. The bending part is almost confined to a short space near the base ; but a rather larger portion of the elongated exterior tentacles

Leaf (enlarged) with all the tentacles closely inflected, from immersion in a solution of phosphate of aannonla (one part to 87,500 of water). Leaf (enlarged) with the tentacles on one side inflected over a bit of meat placed on the disc. becomes slightly incurved ; the distal half in all cases remaining straight. The short tentacles in the centre of the disc when directly excited, do not become in- flected ; but they are capable of inflection if excited by a motor impulse received from other glands at a distance. Thus, if a leaf is immersed in an infusion of raw meat, or in a weak solution of ammonia (if the

solution is at all strong, the leaf is paralysed), all the exterior tentacles bend inwards (see fig. 4), excepting those near the centre, which remain upright ; but these bend towards any exciting object placed on one side of the disc, as shown in fig. 5. The glands in fig. 4 may be seen to form a dark ring round the centre ; and this follows from the exterior tentacles increasing in length in due proportion, as they stand nearer to the circumference.

The kind of inflection which the tentacles undergo is best shown when the gland of one of the long exterior Diagram showing one of the exterior tentacles closely inflected ; the two adjoining ones in their ordinary position. tentacles is in any way excited ; for the surrounding ones remain unaffected. In the accompanying outline (fig. 6) we see one tentacle, on which a particle of meat had been placed, thus bent towards the centre of the leaf, with two others retaining their original position. A gland may be excited by being simply touched three or four times, or by prolonged contact with organic or inorganic objects, and various fluids. I have distinctly seen, through a lens, a tentacle begin- ning to bend in ten seconds, after an object had been

placed on its gland ; and I have often seen strongly pronounced inflection in under one minute. It is sur- prising how minute a particle of any substance, such as a bit of thread or hair or splinter of glass, if placed in actual contact with the surface of a gland, suffices to cause the tentacle to bend. If the object, which has been carried by this movement to the centre, be not very small, or if it contains soluble nitrogenous matter, it acts on the central glands; and these transmit a motor impulse to the exterior tentacles, causing them to bend inwards.

Not only the tentacles, but the blade of the leaf often, but by no means always, becomes much in- curved, when any strongly exciting substance or fluid is placed on the disc. Drops of milk and of a solution of nitrate of ammonia or soda are particularly apt to produce this effect. The blade is thus converted into a little cup. The manner in which it bends varies greatly. Sometimes the apex alone, sometimes one side, and sometimes both sides, become incurved. For instance, I placed bits of hard-boiled egg on three leaves ; one had the apex bent towards the base ; the second had both distal margins much incurved, so that it became almost triangular in outline, and this perhaps is the commonest case ; whilst the third blade was not at all affected, though the tentacles were as closely inflected as in the two previous cases. The whole blade also generally rises or bends upwards, and thus forms a smaller angle with the footstalk than it did before. This appears at first sight a distinct kind of movement, but it results from the incurvation of that part of the margin which is attached to the footstalk, causing the blade, as a whole, to curve or move upwards.

well as the blade remain inflected over an object placed on the disc, depends on various circumstances ; namely on the vigour and age of the leaf, and, according to Dr. Nitschke, on the temperature, for during cold weather when the leaves are inactive, they re-expand at an earlier period than when the weather is warm. But the nature of the object is by far the most important circumstance ; I have repeatedly found that the tentacles remain clasped for a much longer average time over objects which yield soluble nitrogenous matter than over those, whether organic or inorganic, which yield no such matter. After a period varying from one to seven days, the tentacles and blade re- expand, and are then ready to act again. I have seen the same leaf inflected three successive times over insects placed on the disc ; and it would probably have acted a greater number of times.

The secretion from the glands is extremely viscid, so that it can be drawn out into long threads. It appears colourless, but stains little balls of paper pale pink. An object of any kind placed on a gland always causes it, as I believe, to secrete more freely ; but the mere presence of the object renders this difficult to ascertain. In some cases, however, the effect was strongly marked, as when particles of sugar were added ; but the result in this case is probably due merely to exosmose. Particles of carbonate and phos- phate of ammonia and of some other salts, for instance sulphate of zinc, likewise increase the secretion. Im- mersion in a solution of one part of chloride of gold, or of some other salts, to 437 of water, excites the glands to largely increased secretion; on the other hand, tartrate of antimony produces no such effect. Immersion in many acids (of the strength of one part to 437 of water) likewise causes a wonderful amount of

secretion, so that when the leaves are lifted out, long ropes of extremely viscid fluid hang from them. Somo acids, on the other hand, do not act in this manner. Increased secretion is not necessarily dependent on the inflection of the tentacle, for particles of sugar and of sulphate of zinc cause no movement. It is a much more remarkable fact that when an object, such as a bit of meat or an insect, is placed on the disc of a leaf, as soon as the surrounding tentacles become considerably inflected, their glands pour forth an increased amount of secretion. I ascertained this by selecting leaves with equal-sized drops on the two sides, and by placing bits of meat on one side of the disc ; and as soon as the tentacles on this side became much inflected, but before the glands touched the meat, the drops of secretion became larger. This was re- peatedly observed, but a record was kept of only thirteen cases, in nine of which increased secretion was plainly observed ; the four failures being due either to the leaves being rather torpid, or to the bits of meat being too small to cause much inflection. We must therefore conclude that the central glands, when strongly excited, transmit some influence to the glands of the circumferential tentacles, causing them to secrete more copiously.

It is a still more important fact (as we shall see more fully when we treat of the digestive power of the secretion) that when the tentacles become inflected, owing to the central glands having been stimulated mechanically, or by contact with animal matter, the secretion not only increases in quantity, but changes its nature and becomes acid ; and this occurs before the glands have touched the object on the centre of the leaf. This acid is of a different nature from that contained in the tissue of the leaves. As long as the

tentacles remain closely inflected, the glands continue to secrete, and the secretion is acid ; so that, if neu- tralised by carbonate of soda, it again becomes acid after a few hours. I have observed the same leaf with the tentacles closely inflected over rather indigestible substances, such as chemically prepared casein, pour- ing forth acid secretion for eight successive days, and over bits of bone for ten successive days. The secretion seems- to possess, like the gastric juice of the higher animals, some antiseptic power. During very warm weather I placed close together two equal- sized bits of raw meat, one on a leaf of the Drosera, and the other surrounded by wet moss. They were thus left for 48 hrs., and then examined. The bit on the moss swarmed with infusoria, and was so much decayed that the transverse striae on the muscular fibres could no longer be clearly distinguished ; whilst the bit on the leaf, which was bathed by the secretion, was free from infusoria, and its striae were perfectly distinct in the central and undissolved por- tion. In like manner small cubes of albumen and cheese placed on wet moss became threaded with filaments of mould, and had their surfaces slightly discoloured and disintegrated ; whilst those on the leaves of JJrosera remained clean, the albumen being changed into transparent fluid.

As soon as tentacles, which have remained closely inflected during several days over an object, begin to re-expand, their glands secrete less freely, or cease to secrete, and are left dry. In this state they are covered with a film of whitish, semi-fibrous matter, which was held in solution by the secretion. The drying of the glands during the act of re-expan sion is of some little service, to the plant ; for I have often observed that objects adhering to the leases

Xmld then be blown away by a breath of air; the leaves being thus left unencumbered and free for future action. Nevertheless, it often happens that all the glands do not become completely dry ; and in this case delicate objects, such as fragile insects, are some- times torn by the re-expansion of the tentacles into fragments, which remain scattered all over the leaf. After the re-expansion is complete, the glands quickly begin to re-secrete, and as soon as full-sized drops are formed, the tentacles are ready to clasp a new object.

When an insect alights on the central disc, it is instantly entangled by the viscid secretion, and the surrounding tentacles after a time begin to bend, and ultimately clasp it on all sides. Insects are generally killed, according to Dr. Nitschke, in about a quarter of an hour, owing to their tracheae being closed by the secretion. If an insect adheres to only a few of the glands of the exterior tentacles, these soon become inflected and carry their prey to the tentacles next succeeding them inwards ; these then bend in- wards, and so onwards, until the insect is ultimately carried by a curious sort of rolling movement to the centre of the leaf. Then, after an interval, the ten- tacles on all sides become inflected and bathe their prey with their secretion, in the same manner as if the insect had first alighted on the central disc. It is surprising how minute an insect suffices to cause this action : for instance, I have seen one of the smallest species of gnats (Culex), which had just settled with its excessively delicate feet on the glands of the outermost tentacles, and these were already beginning to curve inwards, though not a single gland had as yet touched the body of the insect. Had I not interfered, this minute gnat would

assuredly have been carried to the centre of the leaf and been securely clasped on all sides. We shall hereafter see what excessively small doses of certain organic fluids and saline solutions cause strongly marked inflection. Whether insects alight on the leaves by mere chance, as a resting-place, or are attracted by the odour of the secretion, I know not. I suspect from the number of insects caught by the English species of Drosera, and from what I have observed with some exotic species kept in my greenhouse, that the odour is attractive. In this latter case the leaves may be compared with a baited trap ; in the former case with a trap laid in a run frequented by game, but without any bait.

That the glands possess the power of absorption, is shown by their almost instantaneously becoming dark- . coloured when given a minute quantity of carbonate of ammonia ; the change of colour being chiefly or exclu- sively due to the rapid aggregation of their contents. When certain other fluids are added, they become pale- coloured. Their power of absorption is, however, best shown by the widely different results which follow, from placing drops of various nitrogenous and non- nitrogenous fluids of the same density on the glands of the disc, or on a single marginal gland ; and like- wise by the very different lengths of time during which the tentacles remain inflected over objects, which yield or do not yield soluble nitrogenous matter. This same conclusion might indeed have been inferred from the structure and movements of the leaves, which are so admirably adapted for capturing insects.

The absorption of animal matter from captured insects explains how Drosera can flourish in extremely poor peaty soil, — in some cases where nothing but sphagnum inoss grows, and mosses depend altogether on the atmosphere for their nourishment. Although the leaves at a hasty glance do not appear green, owing to the purple colour of the tentacles, yet the upper and lower surfaces of the blade, the pedicels of the central tentacles, and the petioles contain chlorophyll, so that, no doubt, the plant obtains and assimilates carbonic acid from the air. Nevertheless, considering the nature of the soil where it grows, the supply of nitrogen would be extremely limited, or quite deficient, unless the plant had the power of obtaining this important element from captured insects. We can thus under- stand how it is that the roots are so poorly developed. These usually consist of only two or three slightly divided branches, from half to one inch in length, furnished with absorbent hairs. It appears, therefore, that the roots serve only to imbibe water ; though, no doubt, they would absorb nutritious matter if present in the soil ; for as we shall hereafter see, they absorb a weak solution of carbonate of ammonia. A plant of Drosera, with the edges of its leaves curled in- wards, so as to form a temporary stomach, with the glands of the closely inflected tentacles pouring forth their acid secretion, which dissolves animal matter, afterwards to be absorbed, may be said to feed like an animal. But, differently from an animal, it drinks by means of its roots ; and it must drink largely, so as to retain many drops of viscid fluid round the glands, sometimes as many as 260, exposed during the whole day to a glaring sun.

Inflection of the exterior tentacles owing to the glands of the disc being excited by repeated touches, or by objects left in contact •with them — Difference in the action of bodies yielding and not yielding soluble nitrogenous matter — Inflection of the exterior tentacles directly caused by objects left in contact with their glands — Periods of commencing inflection and of subsequent re- expansion — Extreme minuteness of the particles causing inflection — Action under water — Inflection of the exterior tentacles when their glands are excited by repeated touches — Falling drops of water do not cause inflection.

I WILL give in this and the following chapters some of the many experiments made, which best illustrate the manner and rate of movement of the tentacles, when excited in various ways. The glands alone in all ordinary cases are susceptible to excitement. When excited, they do not themselves move or change form, but transmit a motor impulse to the bending part of their own and adjoining tentacles, and are thus carried towards the centre of the leaf. Strictly speaking, the glands ought to be called irritable, as the term sensi- tive generally implies consciousness ; but no one sup- poses that the Sensitive-plant is conscious, and as I have found the term convenient, I shall use it without scruple. I will commence with the movements of the exterior tentacles, when indirectly excited by stimulants applied to the glands of the short tentacles on the disc. The exterior tentacles may be said in this case to be indirectly excited, because their own glands are not directly acted on. The stimulus proceeding from the glands of the disc acts on the bending part of the

exterior tentacles, near their bases, and does not (as will hereafter be proved) first travel up the pedicels to the glands, to be then reflected back to the bending place. Nevertheless, some influence does travel up to the glands, causing them to secrete more copiously, and the secretion to become acid. This latter fact is, I believe, quite new in the physiology of plants ; it has indeed only recently been established that in the animal kingdom an influence can be transmitted along the nerves to glands, modifying their power of secretion, independently of the state of the blood- vessels.

The Inflection of the Exterior Tentacles from the Glands of the Disc being excited by Repeated Touches, or by Objects left in Contact with them. The central glands of a leaf were irritated with a small stiff camel-hair brush, and in 70 m. (minutes) several of the outer tentacles were inflected ; in 5 hrs. (hours) all the sub-marginal tentacles were inflected ; next morning after an interval of about 22 hrs. they were fully re-expanded. In all the following cases the period is reckoned from the time of first irritation. Another leaf treated in the same manner had a few tentacles inflected in 20 m. ; in 4 hrs. all the submarginal and some of the extreme marginal tentacles, -as well as the edge of the leaf itself, were inflected ; in 17 hrs. they had recovered their proper, expanded position. I then put a dead fly in the centre of the last-mentioned leaf, and next morning it was closely clasped ; five days afterwards the leaf re-expanded, and the tentacles, with their glands surrounded by secretion, were ready to act again.

Particles of meat, dead flies, bits of paper, wood, driod moss, sponge, cinders, glass. &c., were repeatedly placed on leaves, and these objects were well embraced in various periods from 1 hr. to as long as 24 hrs., and set free again, with the leaf fully re-expanded, in from one or two, to seven or even ten days, according to the nature of the object. On a leaf which had naturally caught two flies, and therefore had already closed and reopened either once or more probably twice, I put a fresh fly : in 7 hrs. it was moderately, and in 21 hrs. thoroughly well, clasped, with the edges of the leaf inflected. In two days and a half the leaf had nearly re-expanded ; as the exciting object was an insect, this unusually short period of in- flection was, no doubt, due to the leaf having recently been in action. Allowing this same leaf to rest for only a single day, I put on another fly, and it again closed, but now very slowly ; nevertheless, in less than two days it succeeded in thoroughly clasping the fly.

When a small object is placed on the glands of the disc, on one side of a leaf, as near as possible to its circumference, the tentacles on this side are first affected, those on the opposite side much later, or, as often occurred, not at all. This was repeatedly proved by trials with bits of meat ; but I will here give only the case of a minute fly, naturally caught and still alive, which I found adhering by its delicate feet to the glands on the extreme left side of the central disc. The marginal tentacles on this side closed inwards and killed the fly, and after a time the edge of the leaf on this side also became inflected, and thus remained for several days, whilst neither the tentacles nor the edge on the opposite side were in the least affected.

If young and active leaves are selected, inorganic particles not larger than the head of a small pin, placed on the central glands, sometimes cause the outer tentacles to bend inwards. But this follows much more surely and quickly, if the object contains nitrogenous matter which can be dissolved by the secretion. On one occasion I observed the follow- ing unusual circumstance. Small bits of raw meat (which acts more energetically than any other sub- stance), of paper, dried moss, and of the quill of a pen were placed on several leaves, and they were all embraced equally well in about 2 hrs. On other occasions the above-named substances, or more com- monly particles of glass, coal-cinder (taken from the fire), stone, gold-leaf, dried grass, cork, blotting-paper, cotton- wool, and hair rolled up into little balls, were used, and these substances, though they were some- times well embraced, often caused no movement what- ever in the outer tentacles, or an extremely slight and slow movement. Yet these same leaves were proved to be in an active condition, as they were excited to move by substances yielding soluble nitrogenous matter, such as bits of raw or roast meat, the yolk or white of boiled eggs, fragments of insects of all orders, spiders, &c. I will give only two instances. Minute flies were placed on the discs of several leaves, and on others balls of paper, bits of moss and quill of about the same size as the flies, and the latter were well embraced in a few hours ; whereas after 25 hrs. only a very few tentacles were inflected over the other objects. The bits of paper, moss, and quill were then removed from these leaves, and bits of raw meat placed on them ; and now all the tentacles were soon energetically inflected.

Again, particles of coal-cinder (weighing rather more than the flies used in the last experiment) were placed on the centres of three leaves : after an interval of 19 hrs. one of the particles was tolerably well embraced ; ft second by a very few tentacles ; and a third by none. 1 then removed the particles from the two latter leaves, and put on them recently killed flies. These were fairly well embraced in 7£ hrs. and thoroughly after 20^ hrs. ; the tentacles remaining inflected for many subsequent days. On the other hand, the one leaf which had in the course of 19 hrs. embraced the bit of cinder moderately well, and to which no fly was given, after an additional 33 hrs. (i. e. in 52 hrs. from the time when the cinder was put on) was completely re-expanded and ready to act again.

From these and numerous other experiments not worth giving, it is certain that inorganic substances, or such organic substances as are not attacked by the secretion, act much less quickly and efficiently than organic substances yielding soluble matter which is absorbed. Moreover, I have met with very few excep- tions to the rule, and these exceptions apparently depended on the leaf having been too recently in action, that the tentacles remain clasped for a much longer time over organic bodies of the nature just specified than over those which are not acted on by the secretion, or over inorganic objects.*

* Owing to the extraordinary and particles were then placed belief held bv M. Ziegler ('Comp- (every instrument with which tes rendus," May 1872, p. 122), they were touched having been that albuminous substances, if previously immersed in boiling neld for a moment between the water) on the glands of several fingers, acquire the property of leaves, and they acted in exactly making the tentacles of Drosera the same manner as other par- contract, whereas, if not thus held, tides, which had been purposely they have no such power, I tried handled for some time. Bits of gome experiments with great care, a boiled egg, cut with a knife but the results did not confirm \\hich had been washed in boiling this belief. Eed-hot cinders \vere water, also acted lik« any othei taken out of the fire, and bits animal substance. I breathed on of glass, cotton-thread, blotting some leaves for above a minute, paper and thin slices of cork and repeated the act two or three v, ere immersed in boiling water; times, with my month close to

The Inflection of ilie Exterior Tentacles as directlij caused ty Objects left in Contact with their Glands. I made a vast number of trials by placing, by means of a fine needle moistened with distilled water, and with the aid of a lens, particles of various substances on the viscid secretion surrounding the glands of the outer tentacles. I experimented on both the oval and long-headed glands. When a particle is thus placed on a single gland, the movement of the tentacle is particularly well seen in contrast with the stationary condition of the surrounding tentacles. (See previous fig. 6.) In four cases small particles of raw meat caused the tentacles to be greatly inflected in between 5 and 6 m. Another tentacle similarly treated, and observed with special care, distinctly, though slightly, changed its position in 10 s. (seconds) ; and this is the quickest movement seen by rne. In 2 m. 30 s. it had moved through an angle of about 45°.

The movement as seen through a lens resembled that of the hand of a large clock. In 5 rn. it had moved through 90°, and when I looked again after 10 m., the particle had reached the centre of the leaf; so that the whole movement was completed in less them, but this produced no effect. cause inflection. M. Ziegler I may here add, as showing that makes still more extraordinary the leaves are not acted on by the statements with respect to the .Ddour of nitrogenous substances, power of animal substances, which