Insectivorous Plants

that pieces of raw meat stuc'k on have been left close to, but not in needles were fixed as close as contact with, sulphate of quinine, possible, without actual contact, The action of salts of quinine will to several leaves, but produced be described in a future chapter, no effect whatever. On the other Since the appearance of the paper hand, as we shall hereafter see, above referred to, M. Ziegler haa the vapours of certain volatile published a book on the same

substances and fluius, such as of subject, entitled, ' Atouieite ot carbouate of ammonia, chloro- Zoicite,' 187-i. form, certain essential oib, &c., than 17 m. 30 s. In the course of some hours this minute bit of meat, from having been brought into contact with some of the glands of the central disc, acted centrifugally on the outer tentacles, which all be- came closely inflected. Fragments of flies were placed on the glands of four of the outer tentacles, ex- tended in the same plane with that of the blade, and three of these fragments were carried in 35 m. through an angle of 180° to the centre. The fragment on the fourth tentacle was very minute, and it was not carried to the centre until 3 hrs. had elapsed. In three other cases minute flies or portions of larger ones were carried to the centre in 1 hr. 30 s. In these seven cases, the fragments or small flies, which had been carried by a single tentacle to the central glands, were well embraced by the other tentacles, after an interval of from 4 to 10 hrs.

I also placed in the manner just described six small balls of writing-paper (rolled up by the aid of pincers., so that they were not touched by my fingers) on the glands of six exterior tentacles on distinct leaves; three of these were carried to the centre in about 1 hr., and the other three in rather more than 4 hrs. ; but after 24 hrs. only two of the six balls were well em- braced by the other tentacles. It is possible that the secretion may have dissolved a trace of glue or animalised matter from the balls of paper. Four par- ticles of coal-cinder were then placed on the glands of four exterior tentacles ; one of these reached the centre in 3 hrs. 40 m. ; the second, in 9 hrs. ; the third within 24 hrs., but had moved only part of the way in 9 hrs. ; whilst the fourth moved only a very short distance in 24 hrs., and never moved any farther. Of the above three bits of cinder which were ultimately carried to the centre, one alone was well embraced by

many of the other tentacles. We here see clearly that such bodies as particles of cinder or little balls oi paper, after being carried by the tentacles to the central glands, act very differently from fragments of flies, in causing the movement of the surrounding tentacles. I made, without carefully recording the times of movement, many similar trials with other substances, such as splinters of white and blue glass, particles of cork, minute bits of gold-leaf, &c. ; and the propor- tional number of cases varied much in which the tentacles reached the centre, or moved only slightly, or not at all. One evening, particles of glass and cork, rather larger than those usually employed, were placed on about a dozen glands, and next morning, after 13 hrs., every single tentacle had carried its little load to the centre ; but the unusually large size of the particles will account for this result. In another case •f- of the particles of cinder, glass, and thread, placed on separate glands, were carried towards, or actually to, the centre ; in another case -£-, in another -jV, and in the last case only -^ were thus carried inwards, the small proportion being here clue, at least in part, to the leaves being rather old and inactive. Occasionally a gland, with its light load, could be seen through a strong lens to move an extremely short distance and then stop ; this was especially apt to occur when ex- cessively minute particles, much less than those of which the measurements will be immediately given, were placed on glands ; so that we here have nearly the limit of any action.

I was so much surprised at the smallness of the par- ticles which caused the tentacles to become greatly inflected that it seemed worth while carefully to ascertain how minute a particle would plainly act. Accordingly measured lengths of a narrow strip of blotting paper, of fine cotton-thread, and of a woman's hair, were carefully weighed for me by Mr. Trenham Reeks, in an excellent balance, in the laboratory in Jernayn Street. Short bits of the paper, thread, and hair were then cut off and measured by a micrometer, so that their weights could be easily calculated. The bits were placed on the viscid secretion surrounding the glands of the exterior tentacles, with the precautions already stated, and I am certain that the gland itself was never touched ; nor indeed would a single touch have produced any effect. A bit of the blotting-paper, weighing -5-^- of a grain, was placed so as to rest on three glands together, and all three tentacles slowly curved inwards ; each gland, therefore, supposing the weight to be distributed equally, could have been pressed on by only -j-g-Vr of a grain, or -0464 of a milli- gramme. Five nearly equal bits of cotton-thread were tried, and all acted. The shortest of these was -^ of an inch in length, and weighed 8 -^ , of a grain. The tentacle in this case was considerably inflected in 1 hr. 30 m., and the bit of thread was carried to the centre of the leaf in 1 hr. 40 m. Again, two particles of the thinner end of a woman's hair, one of these being -t j,"0 of an inch in length, and weighing -g-^i-rr °f a grain, the other -j-^hj- of an inch in length, and weigh- ing of course a little more, were placed on two glands on opposite sides of the same leaf, and these two tentacles were inflected halfway towards the centre in 1 hr. 10 m. ; all the many other tentacles round the same leaf re- maining motionless. The appearance of this one leaf showed in an unequivocal manner that these minute particles sufficed to cause the tentacles to bend.

Alto- gether, ten such particles of hair were placed on ten glands on several leaves, and seven of them caused the tentacles to move in a conspicuous manner. The smallest particle which was tried, and which acted plainly, was only T—TT of an inch (-203 millimetre) in length, and weighed the y-s-fr-o of a grain, or '000822 milligramme. In these several cases, not only was the inflection of the tentacles conspicuous, but the purple fluid within their cells became aggregated into little masses of protoplasm, in the manner to be described in the next chapter ; and the aggregation was so plain that I could, by this clue alone, have readily picked out under the microscope all the tentacles which had carried their light loads towards the centre, from the hundreds of other tentacles on the same leaves which had not thus acted.

My surprise was greatly excited, not only by the minuteness of the particles which caused movement, but how they could possibly act on the glands ; for it must be remembered that they were laid with the greatest care on the convex surface of the secretion. At first I thought — but, as I now know, erroneously — that particles of such low specific gravity as those of cork, thread, and paper, would never come into contact with the surfaces of the glands. The particles cannot act simply by their weight being added to that of the secretion, for small drops of water, many times heavier than the particles, were repeatedly added, and never produced any effect. Nor does the disturbance of the secretion produce any effect, for long threads were drawn out by a needle, and affixed to some adjoining object, and thus left for hours; but the tentacles remained motionless.

I also carefully removed the secretion from four glands with a sharply pointed piece of blotting-paper, so that they were exposed for a time naked to the air, but this caused no movement ; yet these glands were in an efficient state, for after 24 hrs. had elapsed, they were tried with bits of meat, and all became quickly inflected. It then occurred to me that particles float- ing on the secretion would cast shadows on the glands, which might be sensitive to the interception of the light. Although this seemed highly improbable, as minute and thin splinters of colourless glass acted powerfully, nevertheless, after it was dark, I put on, by the aid of a single tallow candle, as quickly as possible, particles of cork and glass on the glands of a dozen tentacles, as well as some of meat on other glands, and covered them up so that not a ray of light could enter ; but by the next morning, after an interval of 13 hrs., all the particles were carried to the centres of the leaves.

These negative results led me to try many more experiments, by placing particles on the surface of the drops of secretion, observing, as carefully as I could, whether they penetrated it and touched the surface of the glands. The secretion, from its weight, generally forms a thicker layer on the under than on the upper sides of the glands, whatever may be the position of the tentacles. Minute bits of dry cork, thread, blotting paper, and coal cinders were tried, such as those pre- viously employed ; and I now observed that they absorbed much more of the secretion, in the course of a few minutes, than I should have thought possible ; and as they had been laid on the upper surface of the secre- tion, where it is thinnest, they were often drawn down, after a time, into contact with at least some one point of the gland. With respect to the minute splinters of glass and particles of hair, I observed that the secretion slowly spread itself a little over their sur- faces, by which means they were likewise drawn down- wards or sideways, and thus one end, or some minute

prominence, often came to touch, sooner or later, the gland. In the foregoing and following cases, it is probable that the vibrations, to which the furniture in every room is continually liable, aids in bringing the par- ticles into contact with the glands. But as it was sometimes difficult, owing to the refraction of the secre- tion, to feel sure whether the particles were in contact, I tried the following experiment. Unusually minute particles of glass, hair, and cork, were gently placed 05 the drops round several glands, and very few of the tentacles moved. Those which were not affected were left for about half an hour, and the particles were then disturbed or tilted up several times with a fine needle under the microscope, the glands not being touched. And now in the course of a few minutes almost all the hitherto motionless tentacles began to move ; and this, no doubt, was caused by one end or some prominence of the particles having come into contact with the surface of the glands. But as the particles were unusually minute, the movement was small.

Lastly, some dark blue glass pounded into fine splinters was used, in order that the points of the par- ticles might be better distinguished when immersed in the secretion ; and thirteen such particles were placed in contact with the depending and therefore thicker part of the drops round so many glands. Five of the tentacles began moving after an interval of a few minutes, and in these cases I clearly saw that the par- ticles touched the lower surface of the gland. A sixth tentacle moved after 1 hr. 45 m., and the particle was DOW in contact with the gland, which was not the case at first. So it was with the seventh tentacle, but its movement did not begin until 3 hrs. 45 m. had

elapsed. The remaining six tentacles never moved as long as they were observed ; and the particles apparently never came into contact with the surfaces of the glands. From these experiments we learn that particles not containing soluble matter, when placed on glands, often cause the tentacles to begin bending in the course of from one to five minutes ; and that in such cases the particles have been from the first in contact with the surfaces of the glands. When the tentacles do not begin moving for a much longer time, namely, from half an hour to three or four hours, the particles have been slowly brought into contact with the glands, either by the secretion being absorbed by the particles or by its gradual spreading over them, to- gether with its consequent quicker evaporation. When the tentacles do not move at all, the particles have never come into contact with the glands, or in some cases the tentacles may not have been in an active condition. In order to excite movement, it is indispensable that the particles should actually rest on the glands ; for a touch once, twice, or even thrice repeated by any hard body is not sufficient to excite movement.

Another experiment, showing that extremely mi- nute particles act on the glands when immersed in water, may here be given. A grain of sulphate of quinine was added to an ounce of water, which was not afterwards filtered ; and on placing three leaves in ninety minims of this fluid, I was much surprised to find that all three leaves were greatly inflected in 15 m. ; for I knew from previous trials that the solution does not act so quickly as this. It immediately occurred to me that the particles of the undissolved salt, which were so light as to float about, might have come

into contact with the glands, and caused this rapid movement. Accordingly I added to some distilled water a pinch of a quite innocent substance, namely, precipitated carbonate of lime, which consists of an impalpable powder ; I shook the mixture, and thus got a fluid like thin milk. Two leaves were immeised in it, and in 6 m. almost every tentacle was much inflected. I placed one of these leaves under the microscope, and saw innumerable atoms of lime ad- hering to the external surface of the secretion. Some, however, had penetrated it, and were lying on the sur- faces of the glands ; and no doubt it was these particles which caused the tentacles to bend. When a leaf is im- mersed in water, the secretion instantly swells much ; and I presume that it is ruptured here and there, so that little eddies of water rush in. If so, we can under- stand how the atoms of chalk, which rested on the surfaces of the glands, had penetrated the secretion. Anyone who has rubbed precipitated chalk between his fingers will have perceived how excessively fine the powder is. No doubt there must be a limit, beyond which a particle would be too small to act on a gland ; but what this limit is, I know not. I have often seen fibres and dust, which had fallen from the air, on the glands of plants kept in my room, and these never induced any movement ; but then such particles lay on the surface of the secretion and never reached the gland itself.

Finally, it is an extraordinary fact that a little bit of soft thread, -J-0- of an inch in length and weigh- ing -a-Vy °f a grain, or of a human hair, —£7-0 of an inch in length and weighing only -.-g-'-r-g- of a grain (•000822 milligramme), or particles of precipitated chalk, after resting for a short time on a gland, should induce some change in its cells, exciting them 10 transmit a motor impulse throughout the whole length of the pedicel, consisting of about twenty cells, to near its base, causing this part to bend, and the tentacle to sweep through an angle of above 180°. That the contents of the cells of the glands, and after- wards those of the pedicels, are affected in a plainly visible manner by the pressure of minute particles, we shall have abundant evidence when we treat of the aggregation of protoplasm. But the case is much more remarkable than as yet stated ; for the particles are sup- ported by the viscid and dense secretion ; nevertheless, even smaller ones than those of which the measure- ments have been given, when brought by an insensibly slow movement, through the means above specified, into contact with the surface of a gland, act on it, and the tentacle bends. The pressure exerted by the particle of hair, weighing only y-g-i^ro of a grain and supported by a dense fluid, must have been inconceivably slight. We may conjecture that it could hardly have equalled the millionth of a grain ; and we shall hereafter see that far less than the millionth of a grain of phos- phate of ammonia in solution, when absorbed by a gland, acts on it and induces movement. A bit of hair, ^ of an inch in length, and therefore much larger than those used in the above experiments, was not perceived when placed on my tongue ; and it is extremely doubtful whether any nerve in the human body, even if in an inflamed condition, would be in any way affected by such a particle supported in a dense fluid, and slowly brought into contact with the nerve. Yet the cells of the glands of Drosera are thus excited to transmit a motor impulse to a distant point, inducing movement. It appears to me that hardly any more remarkable fact than this has been observed tn the vegetable kingdom.

The Inflection of the Exterior Tentacles, when their Glands are excited by Repeated Touches. We have already seen, that, if the central glands are excited by being gently brushed, they trans- mit a motor impulse to the exterior tentacles, causing them to bend ; and we have now to con- sider the effects which follow from the glands of the exterior tentacles being themselves touched. On several occasions, a large number of glands were touched only once with a needle or fine brush, hard enough to bend the whole flexible tentacle ; and though this must have caused a thousand- fold greater pressure than the weight of the above described particles, not a tentacle moved. On another occasion forty-five glands on eleven leaves were touched once, twice, or even thrice, with a needle or stiff bristle. This was done as quickly as possible, but with force sufficient to bend the ten- tacles ; yet only six of them became inflected, — three plainly, and three in a slight degree. In order to ascertain whether these tentacles which were not affected were in an efficient state, bits of meat were placed on ten of them, and they all soon became greatly incurved. On the other hand, when a large number of glands were struck four, five, or six times with the same force as before, a needle or sharp splinter of glass being used, a much larger proportion of tentacles became inflected ; but the result was so uncertain as to seem capricious. For instance, I struck in the above manner three glands, which happened to be extremely sensitive, and all three were inflected almost as quickly as if bits of meat had been placed on them. On another occasion I gave a single for-

cible touch to a considerable number of glands, and not one moved ; but these same glands, after an inter- val of some hours, being touched four or five times with a needle, several of the tentacles soon became inflected. The fact of a single touch or even of two or three touches not causing inflection must be of some service to the plant ; as during stormy weather, the glands cannot fail to be occasionally touched by the tall blades of grass, or by other plants growing near ; and it would be a great evil if the tentacles were thus brought into action, for the act of re-expansion takes a considerable time, and until the tentacles are re- expanded they cannot catch prey. On the other hand, extreme sensitiveness to slight pressure is of the highest service to the plant ; for, as we have seen, if the delicate feet of a minute struggling insect press ever so lightly on the surfaces of two or three glands, the tentacles bearing these glands soon curl inwards and carry the insect with them to the centre, causing, after a time, all the circumferential tentacles to embrace it. Nevertheless, the movements of the plant are not perfectly adapted to its requirements; for if a bit of dry moss, peat, or other rubbish, is blown on to the disc, as often happens, the tentacles clasp it in a useless manner. They soon, however, discover their mistake and release such innutritions objects.

It is also a remarkable fact, that drops of water fall- ing from a height, whether under the form of natural or artificial rain, do not cause the tentacles to move ; yet the drops must strike the glands with considerable force, more especially after the secretion has been all washed away by heavy rain; and this often occurs, though the secretion is so viscid that it can be re- moved with difficulty merely by waving the leaves in water. If the falling drops of water are small, they adhere to the secretion, the weight of which must be increased in a much greater degree, as before re- marked, than by the addition of minute particles of solid matter ; yet the drops never cause the tentacles to become inflected. It would obviously have been a great evil to the plant (as in the case of occasional touches) if the tentacles were excited to bend by every shower of rain ; but this evil has been avoided by the glands either having become through habit insensible to the blows and prolonged pressure of drops of water, or to their having been originally rendered sensitive solely to the contact of solid bodies. We shall hereafter see that the filaments on the leaves of Dionsea are likewise insensible to the impact of fluids, though exquisitely sensitive to momentary touches from any solid body.

When the pedicel of a tentacle is cut off by a sharp pair of scissors quite close beneath the gland, the tentacle generally becomes inflected. I tried this experiment repeatedly, as I was much surprised at the fact, for all other parts of the pedicels are insensible to any stimulus. These headless tentacles after a time re-expand ; but I shall return to this subject. On the other hand, I occasionally succeeded in crushing a gland between a pair of pincers, but this caused no inflection. In this latter case the tentacles seem paralysed, as likewise follows from the action of tou strong solutions of certain salts, and by too great heat, whilst weaker solutions of the same salts and a more gentle heat cause movement. We shall also see in future chapters that various other fluids, some

vapours, and oxygen (after the plant has been for some time excluded from its action), all induce inflection, and this likewise results from an induced galvanic current.* the observations of Dr. Burden inductive apparatus are inserted, Sanderson on Dionsea, finds that the tentacles curve inwards iti thf if two needles are inserted into course of a few minutes. My son the blade of a leaf of Drosera, the hopes soon to publish an tentacles do not move ; but that if of his observations, needles in connection with

future of the contents of the cells before aggregation — Various causes which excite aggregation — The process commences within the glands and travels down the tentacles — Description of the aggregated masses and of their spontaneous movements — Currents of protoplasm along the walls of the cells — Action of carbonate of ammonia — The granules in the protoplasm which flows along the walls coalesce with the central masses — Minuteness of the quantity of carbonate of ammonia causing aggregation — Action of other salts of ammonia — Of other substances, organic fluids, &c. — Of water — Of heat — Redissolution of the aggregated masses — Proximate causes of the aggregation of the protoplasm — Summary and concluding remarks — Supplementary observations on aggregation in the roots of plants.

I WILL here interrupt my account of the movements of the leaves, and describe the phenomenon of aggre- gation, to which subject I have already alluded. If the tentacles of a young, yet fully matured leaf, that has never been excited or become inflected, be ex- amined, the cells forming the pedicels are seen to be filled with homogeneous, purple fluid. The walls are lined by a layer of colourless, circulating protoplasm ; but this can be seen with much greater distinctness after the process of aggregation has been partly effected than before. The purple fluid which exudes from a crushed tentacle is somewhat coherent, and does not mingle with the surrounding water; it con- tains much flocculeiit or granular matter. But thia nmtter may have been generated by the cells having been crushed ; some degree of aggregation having been thus almost instantly caused.

If a tentacle is examined some hours after the gland has been excited by repeated touches, or by an in- organic or organic particle placed on it, or by the absorption of certain fluids, it presents a wholly changed appearance. The cells, instead of being filled with homogeneous purple fluid, now contain variously shaped masses of purple matter, suspended in a colour- less or almost colourless fluid. The change is so conspicuous that it is visible through a weak lens, and even sometimes by the naked eye ; the tentacles now have a mottled appearance, so that one thus affected can be picked out with ease from all the others. The same result follows if the glands on the disc are irritated in any manner, so that the exterior tentacles become inflected ; for their contents will then be found in an aggregated condition, although their glands have not as yet touched any object. But aggregation may occur independently of inflection, as we shall presently see. By whatever cause the process may have been excited, it commences within the glands, and then travels down the tentacles. It can be observed much more distinctly in the upper cells of the pedicels than within the glands, as these are somewhat opaque. Shortly after the tentacles have re-expanded, the little masses of protoplasm are all redissolved, and the purple fluid within the cells be- comes as homogeneous and transparent as it was at first. The process of redissolution travels upwards from the bases of the tentacles to the glands, and therefore in a reversed direction to that of aggre- gation. Tentacles in an aggregated condition were shown to Prof. Huxley, Dr. Hooker, and Dr. Burdon Sanderson, who observed the changes under the microscope, and were much struck with the whole phenomenon.

The little masses of aggregated matter are of the most diversified shapes, often spherical or oval, some- times much elongated, or quite irregular with thread- or necklace-like or club-formed projections. They consist of thick, apparently viscid matter, which in the exterior tentacles is of a purplish, and in the short discal tentacles of a greenish, colour. These little masses incessantly change their forms and posi- tions, being never at rest. A single mass will often separate into two, which afterwards reunite. Their movements are rather slow, and resemble those of Amoeba? or of the white corpuscles of the blood. We

Diagram of the same cell of a tentacle, showing the various forms successively assumed by the aggregated masses of protoplasm. may, therefore, conclude that they consist of proto- plasm. If their shapes are sketched at intervals of a few minutes, they are invariably seen to have undergone great changes of form ; and the same cell has been observed for several hours. Eight rude, though accurate sketches of the same cell, made at intervals of between 2 m. or 3 m., are hero given

(fig. 7), and illustrate some of the simpler and com- monest changes. The cell A, when first sketched, included two oval masses of purple protoplasm touch- ing each other. These became separate, as shown at B, and then reunited, as at C. After the next interval a very common appearance was presented— D, namely, the formation of an extremely minute sphere at one end of an elongated mass. This rapidly increased in size, as shown in E, and was then re- absorbed, as at F, by which time another sphere had been formed at the opposite end.

The cell above figured was from a tentacle of a dark red leaf, which had caught a small moth, and was examined under water. As I at first thought that the movements of the masses might be due to the absorp- tion of water, I placed a fly on a leaf, and when after 18 hrs. all the tentacles were well inflected, these were examined without being immersed in water. The cell Diagram of the same cell of a tentacle, showing the various forms successively assumed by the aggregated masses of protoplasm.

here represented (fig. 8) was from this leaf, being sketched eight times in the course of 15 m. These sketches exhibit some of the more remarkable changes which the protoplasm undergoes. At first, there was at the base of the cell 1, a little mass on a short footstalk, and a larger mass near the upper end, and these seemed quite separate. Nevertheless, they may have been connected by a fine and invisible thread of protoplasm, for on two other occasions, whilst one mass was rapidly increasing, and another in the same cell rapidly decreasing, I was able by varying the light and using a high power, to detect a connecting thread of extreme tenuity, which evidently served as

the channel of communication between the two. On the other hand, such connecting threads are some- times seen to break, and their extremities then quickly become club-headed. The other sketches in fig. 8 show the forms successively assumed. Shortly after the purple fluid within the cells lias become aggregated, the little masses float about in a colourless or almost colourless fluid ; and the layer of white granular protoplasm which flows along the walls can now be seen much more distinctly. The stream flows at an irregular rate, up one wall and down the opposite one, generally at a slower rate across the narrow ends of the elongated cells, and so round and round. But the current sometimes ceases. The movement is often in wraves, and their crests sometimes stretch almost across the whole width of the cell, and then sink down again. Small spheres of protoplasm, apparently quite free, are often driven by the current round the cells ; and filaments attached to the central masses are swayed to and fro, as if struggling to escape. Altogether, one of these cells with the ever changing central masses, and with the layer of protoplasm flowing round the walls, presents a wonderful scene of vital activity.

Many observations were made on the contents of the cells whilst imdergoing the process of aggregation, but I shall detail only a few cases under different heads. A small portion of a leaf was cut off, placed under a high power, and the glands very gently pressed under a compressor. In 15 m. I distinctly saw extremely minute spheres of protoplasm aggregating them- selves in the purple fluid ; these rapidly increased in size, both within the cells of the glands and of the upper ends of the pedicels. Particles of glass, cork, and cinders were also placed on the glands of many tentacles ; in 1 hr. several of them were inflected, but after 1 hr. 35 in. there was no aggregation. Other tentacles with these particles were examined after 8 hrs., and