Insectivorous Plants

Although these filaments are so sensitive to a momen- tary and delicate touch, they are far less sensitive than the glands of Drosera to prolonged pressure. Several times I succeeded in placing on the tip of a filament, by the aid of a needle moved with extreme slowness, bits of rather thick human hair, and these did not excite movement, although they were more than ten times as long as those which caused the tentacles of Drosera to bend ; and although in this latter case they were largely supported by the dense secretion. On the other hand, the glands of Drosera may be struck with a needle or any hard object, once, twice, or even thrice, with considerable force, and no movement ensues. This singular difference in the nature of the sensitiveness of the filaments of Diontea and of

the glands of Diosera evidently stands in relation to the habits of the two plants. If a minute insect alights with its delicate feet on the glands of Drosera, it is caught by the viscid secretion, and the slight, though prolonged pressure, gives notice of the presence of prey, which is secured by the slow bending of the tentacles. On the other hand, the sensitive filaments of Diomea are not viscid, and the capture of insects can be assured only by their sensitiveness to a momentary touch, followed by the rapid closure of the lobes.

As just stated, the filaments are not glandular, and do not secrete. Nor have they the power of absorption, as may be inferred from drops of a solution of car- bonate of ammonia (one part to 146 of water), placed on two filaments, not producing any effect on the contents of their cells, nor causing the lobes to close. When, however, a small portion of a leaf with an attached filament was cut off and immersed in the same solution, the fluid within the basal cells became almost instantly aggregated into purplish or colourless, irre- gularly shaped masses of matter. The process of aggregation gradually travelled up the filaments from cell to cell to their extremities, that is in a reverse course to what occurs in the tentacles of Drosera when their glands have been excited. Several other fila- ments were cut off close to their bases, and left for 1 hr. 30 m. in a weaker solution of one part of the carbonate to 218 of water, and this caused aggregation in all the cells, commencing as before at the bases of the filaments.

likewise causes aggregation. Nor is it rare to find the contents of a few of the terminal cells in a spontaneously aggregated condition. The aggregated masses undergo incessant slow changes of form, uniting and again separating ; and some of them apparently revolve round their own axes. A current of colourless granular protoplasm could also be seen travelling round the walls of the cells. This current ceases to be visible as soon as the contents are well aggregated ; but it probably still continues, though no longer visible, owing to all the granules in the flowing layer having become united with the central masses. In all these respects the filaments of Diontea behave exactly like the tentacles of Drosera.

Notwithstanding this similarity there is one re- markable difference. The tentacles of Drosera, after their glands have been repeatedly touched, or a particle of any kind has been placed on them, become inflected and strongly aggregated. No such effect is pro- duced by touching the filaments of Dionoea ; I com- pared, after an hour or two, some which had been touched and some which had not, and others after twenty-five hours, and there was no difference in the contents of the cells. The leaves were kept open all the time by clips ; so that the filaments were not pressed against the opposite lobe.

Drops of water, or a thin broken stream, falling from a height on the filaments, did not cause the blades to close ; though these filaments were afterwards proved to be highly sensitive. No doubt, as in the case of Drosera, the plant is indifferent to the heaviest shower of rain. Drops of a solution of a half an ounce of sugar to a fluid ounce of water were repeatedly allowed to fall from a height on the filaments, but produced no effect, unless they adhered to them. Again, I blew many times through a fine pointed tube with niy utmost force against the filaments without any effect; such blowing being received

with as much indifference as no doubt is a heavy gale of wind. We thus see that the sensitiveness of the filaments is of a specialised nature, being related to a momentary touch rather than to prolonged pressure ; and the touch must not be from fluids, such as air or water, but from some solid object. Although drops of water and of a moderately strong solution of sugar, falling on the filaments, does not excite them, yet the immersion of a leaf in pure water sometimes caused the lobes to close. One leaf was left immersed for 1 hr. 10 m., and three other leaves for some minutes, in water at temperatures varying be- tween 59° and 65° (15° to 18°-3 Cent.) without any effect. One, however, of these four leaves, on being gently withdrawn from the water, closed rather quickly. The three other leaves were proved to be in good condition, as they closed when their filaments were touched. Nevertheless two fresh leaves on being dipped into water at 75° and 62^° (23°-S and 16°-9 Cent.) instantly closed. These were then placed with their footstalks in water, and after 23 hrs. partially re-expanded ; on touching theii filaments one of them closed. This latter leaf after an additional 24 hrs. again re-expanded, and now, on the filaments of both leaves being touched, both closed. We thus see that a short immersion in water does not at all injure the leaves, but sometimes excites the lobes to close. The movement in the above cases was evidently not caused by the temperature of the water. It has been shown that long immersion causes the purple fluid within the cells of the sensitive filaments to become aggregated ; and the tentacles of Drosera are acted on in the same manner by long immersion, often being somewhat inflected. In both cases th& result is probably due to a slight degree of exosmose.

I am confirmed in this belief by the effects of immersing a leaf of Dionaea in a moderately strong solution of sugar ; the leaf having been previously left for 1 hr. 10 m. in water without any effect ; for now the lobes closed rather quickly, the tips of the marginal spikes crossing in 2 m. 30 s., and the leaf being com- pletely shut in 3 m. Three leaves were then immersed in a solution of half an ounce of sugar to a fluid ounce of water, and all three leaves closed quickly. As I was doubtful whether this was due to the cells on the upper surface of the lobes, or to the sensitive fila- ments, being acted on by exosmose, one leaf was first tried by pouring a little of the same solution in the furrow between the lobes over the midrib, which is the chief seat of movement. It was left there for some time, but no movement ensued. The whole upper surface of leaf was then painted (except close round the bases of the sensitive filaments, which I could not do without risk of touching them) with the same solution, but no effect was produced. So that the cells on the upper surface are not thus affected. But when, after many trials, I succeeded in getting a drop of the solution to cling to one of the filaments, the leaf quickly closed. Hence we may, I think, conclude that the solution causes fluid to pass out of the delicate cells of the filaments by exosmose ; and that this sets up some molecular change in their contents, analogous to that which must be produced by a touch.

The immersion of leaves in a solution of sugar affects them for a much longer time than does an immersion in water, or a touch on the filaments ; for in these latter cases the lobes begin to re-expand in less than a day. On the other hand, of the three leaves which were immersed for a short time in the solution, and were then washed by means of a svringe inserted between the lobes, one re-expanded after two days ; a second after seven days ; and the third after nine days. The leaf which closed, owing to a drop of the solution having adhered to one of the filaments, opened after two days.

I was surprised to find on two occasions that the heat from the rays of the sun, concentrated by a lens on the bases of several filaments, so that they were scorched and discoloured, did not cause any move- ment ; though the leaves were active, as they closed, though rather slowly, when a filament on the opposite side was touched. On a third trial, a fresh leaf closed after a time, though very slowly ; the rate not being increased by one of the filaments, which had not been injured, being touched. After a day these three leaves opened, and were fairly sensitive when the uninjured filaments were touched. The sudden immersion of a leaf into boiling water does not cause it to close. Judging from the analogy of Drosera, the heat in these several cases was too great and too suddenly applied. The surface of the blade is very slightly sensitive ; it may be freely and roughly handled, with- out any movement being caused. A leaf was scratched rather hard with a needle, but did not close ; but when the triangular space between the three filaments on another leaf was similarly scratched, the lobes closed. They always closed when the blade or midrib was deeply pricked or cut. Inorganic bodies, even of large size, such as bits of stone, glass, &c. — or organic bodies not containing soluble nitrogenous matter, such as bits of wood, cork, moss, — or bodies containing soluble nitrogenous matter, if perfectly dry, such as bits of meat, albumen, gelatine, &c., may be long left (and many were tried) on the lobes, and no movement is excited. The result, however, is widely different, as we

shall presently see, if nitrogenous organic bodies which are at all damp, are left on the lobes ; for these then close by a slow and gradual movement, very different from that caused by touching one of the sensitive fila- ments. The footstalk is not in the least sensitive ; a pin may be driven through it, or it may be cut off, and no movement follows. The upper surface of the lobes, as already stated, is thickly covered with small purplish, almost sessile glands. These have the power both of secretion and absorption ; but unlike those of Drosera, they do not secrete until excited by the absorption of nitrogenous matter. No other excitement, as far as I have seen, produces this effect. Objects, such as bits of wood, cork, moss, paper, stone, or glass, may be left for a length of time on the surface of a leaf, and it remains quite dry. Nor does it make any difference if the lobes close over such objects. For instance, some little balls of blotting paper were placed on a leaf, and a filament was touched ; and when after 24 hrs. the lobes began to re-open, the balls were removed by the aid of thin pincers, and were found perfectly dry. On the other hand, if a bit of damp meat or a crushed Hy is placed on the surface of an expanded leaf, the glands after a time secrete freely. In one such case there was a little secretion directly beneath the meat in 4 hrs. ; and after an additional 3 hrs. there was a considerable quantity both under and close round it. In another case, after 3 hrs. 40 m., the bit of meat was quite wet. But none of the glands secreted, except- ing those which actually touched the meat or the secretion containing dissolved animal matter.

If, however, the lobes are made to close over a bit of meat or an insect, the result is different, for the glands over the whole surface of the leaf now secrete copiously, As in this case the glands on both sides are pressed against the meat or insect, the secretion from the first is twice as great as when a bit of meat is laid on the surface of one lobe ; and as the two lobes corne into almost close contact, the secretion, containing dis- solved animal matter, spreads by capillary attraction, causing fresh glands on both sides to begin secreting in a continually widening circle. The secretion is almost colourless, slightly mucilaginous, and, judging by the manner in which it coloured litmus paper, more strongly acid than that of Drosera. It is so copious that on one occasion, when a leaf was cut open, on which a small cube of albumen had been placed 45 hrs. before, drops rolled off the leaf. On another occasion, in which a leaf with an enclosed bit of roast meat spontaneously opened after eight days, there was so much secretion in the furrow over the midrib that it trickled down. A large crushed fly (Tipula) was placed on a leaf from which a small portion at the base of one lobe had previously been cut away, so that an opening was left ; and through this, the secretion continued to run down the footstalk during nine days, — that is, for as long a time as it was observed. By forcing up one of the lobes, I was able to see some distance between them, and all the glands within sight were secreting freely.

We have seen that inorganic and non-nitrogenoua objects placed on the leaves do not excite any move- ment ; but nitrogenous bodies, if in the least degree damp, cause after several hours the lobes to close slowly. Thus bits of quite dry meat and gelatine were placed at opposite ends of the same leaf, and in the course of 24 hrs. excited neither secretion nor move- ment. They were then dipped in water, their sur- faces dried on blotting paper, and replaced on the samo

leaf, the plant being now covered with a bell-glass. After 24 hrs. the damp meat had excited some acid secretion, and the lobes at this end of the leaf were almost shut. At the other end, where the damp gela- tine lay, the leaf was still quite open, nor had any secretion been excited ; so that, as with Drosera, gela- tine is not nearly so exciting a substance as meat. The secretion beneath the meat was tested by push- ing a strip of litmus paper under it (the filaments not being touched), and this slight stimulus caused the leaf to shut. On the eleventh day it reopened ; but the end where the gelatine lay, expanded several hours before the opposite end with the meat.

A second bit of roast meat, which appeared dry, though it had not been purposely dried, was left for 24 hrs. on a leaf, caused neither movement nor secre- tion. The plant in its pot was now covered with a bell-glass, and the meat absorbed some moisture from the air ; this sufficed to excite acid secretion, and by the next morning the leaf was closely shut. A third bit of meat, dried so as to be quite brittle, was placed on a leaf under a bell-glass, and this also became in 24 hrs. slightly damp, and excited some acid secretion, but no movement.

A rather large piece of perfectly dry albumen was left at one end of a leaf for 24 hrs. without any effect. It was then soaked for a few minutes in water, rolled about on blotting paper, and replaced on the leaf; in 9 hrs. some slightly acid secretion was excited, and in 24 hrs. this end of the leaf was partially closed. The bit of albumen, which was now surrounded by much secretion, was gently removed, and although no filament was touched, the lobes closed. In this and the previous case, it appears that the absorption of animal matter by the glands renders

the surface of the leaf much more sensitive to a touch than it is in its ordinary state ; and this is a curious fact. Two days afterwards the end of the leaf where nothing had been placed began to open, and on the third day was much more open than the opposite end where the albumen had lain. Lastly, large drops of a solution of one part of car- bonate of ammonia to 146 of water were placed on some leaves, but no immediate movement ensued. I did not then know of the slow movement caused by animal matter, otherwise I should have observed the leaves for a longer time, and they would probably have been found closed, though the solution (judging from Drosera) was, perhaps, too strong.

From the foregoing cases it is certain that bits of meat and albumen, if at all damp, excite not only the glands to secrete, but the lobes to close. This move- ment is widely different from the rapid closure caused by one of the filaments being touched. We shall see its importance when we treat of the manner in which insects are captured. There is a great contrast be- tween Drosera and Dionoea in the effects produced by mechanical irritation on the one hand, and the absorp- tion of animal matter on the other. Particles of glass placed on the glands of the exterior tentacles of Dro- sera excite movement within nearly the same time, as do particles of meat, the latter being rather the most efficient ; but when the glands of the disc have bits of meat given them, they transmit a motor impulse to the exterior tentacles much more quickly than do these glands when bearing inorganic particles, or when irritated by repeated touches. On the other hand, with Dioneea, touching the filaments excites incomparably quicker movement than the absorption of animal matter by the glands. Nevertheless, in

certain cases, this latter stimulus is the more powerful of the two. Oil three occasions leaves were found which from some cause were torpid, so that their lobes closed only slightly, however much their filaments were irritated ; but on inserting crushed insects between the lobes, they became in a day closely shut. The facts just given plainly show that the glands have the power of absorption, for otherwise it is im- possible that the leaves should be so differently af- fected by non-nitrogenous and nitrogenous bodies, and between these latter in a dry and damp condition. It is surprising how slightly damp a bit of meat or albu- men need be in order to excite secretion and afterwards slow movement, and equally surprising how minute a quantity of animal matter, when absorbed, suffices to produce these two effects. It seems hardly credible, and yet it is certainly a fact, that a bit of hard-boiled white of egg, first thoroughly dried, then soaked for some minutes in water and rolled on blotting paper, should yield in a few hours enough animal matter to the glands to cause them to secrete, and afterwards the lobes to close. That the glands have the power of absorption is likewise shown by the very different lengths of time (as we shall presently see) during which the lobes remain closed over insects and other bodies yielding soluble nitrogenous matter, and over such as do not yield any. But there is direct evidence of absorption in the condition of the glands which have remained for some time in contact with animal matter. Thus bits of meat and crushed insects were several times placed on glands, and these were compared after some hours with other glands from distant parts of the same leaf. The latter showed not a trace of aggregation, whereas those which had been in contact with the animal matter wcro

well aggregated. Aggregation may be seen to occur very quickly if a piece of a leaf is immersed in a weak solution of carbonate of ammonia. Again, small cubes of albumen and gelatine were left for eight days on a leaf, which was then cut open. The whole surface was bathed with acid secretion, and every cell in the many glands which were examined had its contents aggregated in a beautiful manner into dark or pale purple, or colourless globular masses of protoplasm. These underwent incessant slow changes of forms ; sometimes separating from one another and then reuniting, exactly as in the cells of Drosera. Boiling water makes the contents of the gland-cells white and opaque, but not so purely white and porcelain-like as in the case of Drosera. How living insects, when naturally caught, excite the glands to secrete so quickly as they do, I know not; but I suppose that the great pressure to which they are subjected forces a little excretion from either extremity of their bodies, and we have seen that an extremely small amount of nitrogenous matter is sufficient to excite the glands.

Before passing on to the subject of digestion, I may state that I endeavoured to discover, with no success, the functions of the minute octofid processes with which the leaves are studded. From facts hereafter to be given in the chapters on Aldrovanda and Utricu- laria, it seemed probable that they served to absorb decayed matter left by the captured insects; but their position on the backs of the leaves and on the footstalks rendered this almost impossible. Never- theless, leaves were immersed in a solution of one part of urea to 437 of water, and after 24 hrs. the orange layer of protoplasm within the arms of these processes did not appear more aggregated than in other sped-

mens kept in water. I then tried suspending a leaf in a bottle over an excessively putrid infusion of raw meat, to see whether they absorbed the vapour, but their contents were not affected. Digestive Power of the Secretion* — When a leaf closes over any object, it may be said to form itself into a temporary stomach ; and if the object yields ever so little animal matter, this serves, to use Schiffs expres- sion, as a peptogene, and the glands on the surface pour forth their acid secretion, which acts like the gastric juice of animals. As so many experiments were tried on the digestive power of Drosera, only a few were made with Dionaja, but they were amply sufficient to prove that it digests. This plant, more- over, is not so well fitted as Drosera for observation, as the process goes on within the closed lobes. Insects, even beetles, after being subjected to the secretion for several days, are surprisingly softened, though their chitinous coats are not corroded.

Experiment 1. — A cube of albumen of ^ of an inch (2'540 mm.) was placed at one end of a leaf, and at the other end an oblong piece of gelatine, ± of an inch (5-OS mm.) long, and * Dr. W. M. Canby, of Wil- mington, to whom I am much indebted for information regard- ing Dionfea in its lative home, has published in the ' Gardener's Monthly,' Philadelphia, August 1868, some interesting observa- tions. He ascertained that the secretion digests animal matter, such as the contents of insects, bits of meat, &c. ; and that the secretion is reabsorbed. He was also well aware that the lobes remain closed for a much longer time when in contact with animal matter than when made to shut by a mere touch, or over objects

not yielding soluble nutriment; and that in these latter cases the glands do not secrete. The Rev. Dr. Curtis first observed (' Boston Journal Nat. Hist.' vol. i. p. 123) the secretion from the glands. I may here add that a gardener. Mr. Knight, is said (Kirby and Spencer's ' Introduction to Ento- mology,' 1818, vol. i. p. 295) to have found that a plant of the Dion?ea, on the leaves of which "he laid fine filaments of raw beef, was much more luxuriant in its growth than others not so treated."

^ broad; the leaf was then made to close. It was cut open after 45 hrs. The albumen was hard and compressed, with its angles only a little rounded ; the gelatine was corroded into an oval form ; and both were bathed in so much acid secretion that it dropped off the leaf. The digestive process apparently is rather slower than in Drosera, and this agrees with the length of time during which the leaves remain closed over digestible objects. Experiment 2. — A bit of albumen -^ of an inch square, but only -^5 in thickness, and a piece of gelatine of the same size as before, were placed on a leaf, which eight days afterwards was cut open. The surface was bathed with slightly adhesive, very acid secretion, and the glands were all in an aggregated condi- tion. Not a vestige of the albumen or gelatine was left. Simi- larly sized pieces were placed at the same time on wet moss on the same pot, so that they were subjected to nearly similar con- ditions ; after eight days these were brown, decayed, and matted with fibres of mould, but had not disappeared.

J:'.>-per iment 3. — A piece of albumen -^ of an inch (3'81 mm./ long, and -^ broad and thick, and a piece of gelatine of the same size as before, were placed on another leaf, which was cni open after seven days; not a vestige of either substance was left, and only a moderate amount of secretion on the surface. Experiment 4. — Pieces of albumen and gelatine, of the same size as in the last experiment, were placed on a leaf, which spontaneously opened after twelve days, and here again not a vestige of either was left, and only a little secretion at one end of the midrib.

Experiment 5. — Pieces of albumen and gelatine of the same size were placed on another leaf, which after twelve days was still firmly closed, but had begun to wither ; it was cut open, and contained nothing except a vestige of brown matter where the albumen had lain. Experiment 6. — A cube of albumen of -^ of an inch and a piece of gelatine of the same size as before were placed on a leaf, which opened spontaneously after thirteen days. The albumen, which was twice as thick as in the latter experiments, was too large ; for the glands in contact with it were injured and were dropping off; a film also of albumen of a brown colour, matted with mould, was left. All the gelatine was absorbed, and there was only a little acid secretion left on the midrib.

Experiment 7. — A bit of half roasted meat (not measured) and a bit of gelatine were placed on the two ends of a leaf, which opened spontaneously after eleven days ; a vestige ol the meat was left, and the surface of the leaf was here blackened ; the gelatine had all disappeared. Experiment 8.— A bit of half roasted meat (not measured) was placed on a leaf which was forcibly kept open by a clip, so that it was moistened with the secretion (very acid) only on its lower surface. Nevertheless, after only 22 i hrs. it was sur- prisingly softened, when compared with another bit of the same meat which had been kept damp.

Experiment 9. — A cube of T^ of an inch of very compact roasted beef was placed on a leaf, which opened spontaneously after twelve days ; so much feebly acid secretion was left on the leaf that it trickled off. The meat was completely disintegrated, but not all dissolved; there was no mould. The little mass was placed under the microscope ; some of the fibrillae in the middle Btill exhibited transverse strife ; others showed not a vestige of strire ; and every gradation could be traced between these two states. Globules, apparently of fat, and some undigested nbro-elastic tissue remained. The meat was thus in the same state as that formerly described, which was half di- gested by Drosera. Here, again, as in the case of albumen, the digestive process seems slower than in Drosera. At the opposite end of the same leaf, a firmly compressed pellet of bread had been placed ; this was completely disintegrated, I suppose, owing to the digestion of the gluten, but seemed very little reduced in bulk.

Experiment 10. — A cube of -^ of an inch of cheese and another of albumen were placed at opposite ends of the same leaf. After nine days the lobes opened spontaneously a little at the end enclosing the cheese, but hardly any or none was dissolved, though it was softened and surrounded by secre- tion. Two days subsequently the end with the albumen also opened spontaneously (i.e. eleven days after it was put on), a mere trace in a blackened and dry condition being left.