Insectivorous Plants Part 22

These were made sometimes on the inner and sometimes on the outer sides of the filaments; and after several days, when the leaves had reopened, these filaments were touched roughly (for they were always rendered in some degree torpid by the operation), and the lobes then closed in the ordinary manner, though slowly, and sometimes not until after a considerable interval of time. These cases show that the motor impulse is not transmitted along the vessels, and they further show that there is no necessity for a direct line of communication from the filament which is [page 315] touched towards the midrib and opposite lobe, or towards the outer parts of the same lobe.

Two slits near each other, both parallel to the midrib, were next made in the same manner as before, one on each side of the base of a filament, on five distinct leaves, so that a little slip bearing a filament was connected with the rest of the leaf only at its two ends.

These slips were nearly of the same size; one was carefully measured; it was .12 of an inch (3.048 mm.) in length, and .08 of an inch (2.032 mm.) in breadth; and in the middle stood the filament. Only one of these slips withered and perished. After the leaf had recovered from the operation, though the slits were still open, the filaments thus circ.u.mstanced were roughly touched, and both lobes, or one alone, slowly closed. In two instances touching the filament produced no effect; but when the point of a needle was driven into the slip at the base of the filament, the lobes slowly closed. Now in these cases the impulse must have proceeded along the slip in a line parallel to the midrib, and then have radiated forth, either from both ends or from one end alone of the slip, over the whole surface of the two lobes.

Again, two parallel slits, like the former ones, were made, one on each side of the base of a filament, at right angles to the midrib. After the leaves (two in number) had recovered, the filaments were roughly touched, and the lobes slowly closed; and here the impulse must have travelled for a short distance in a line at right angles to the midrib, and then have radiated forth on all sides over both lobes. These several cases prove that the motor impulse travels in all directions through the cellular tissue, independently of the course of the vessels.

With Drosera we have seen that the motor impulse [page 316] is transmitted in like manner in all directions through the cellular tissue; but that its rate is largely governed by the length of the cells and the direction of their longer axes. Thin sections of a leaf of Dionaea were made by my son, and the cells, both those of the central and of the more superficial layers, were found much elongated, with their longer axes directed towards the midrib; and it is in this direction that the motor impulse must be sent with great rapidity from one lobe to the other, as both close simultaneously. The central parenchymatous cells are larger, more loosely attached together, and have more delicate walls than the more superficial cells. A thick ma.s.s of cellular tissue forms the upper surface of the midrib over the great central bundle of vessels.

When the filaments were roughly touched, at the bases of which slits had been made, either on both sides or on one side, parallel to the midrib or at right angles to it, the two lobes, or only one, moved. In one of these cases, the lobe on the side which bore the filament that was touched moved, but in three other cases the opposite lobe alone moved; so that an injury which was sufficient to prevent a lobe moving did not prevent the transmission from it of a stimulus which excited the opposite lobe to move. We thus also learn that, although normally both lobes move together, each has the power of independent movement. A case, indeed, has already been given of a torpid leaf that had lately re-opened after catching an insect, of which one lobe alone moved when irritated. Moreover, one end of the same lobe can close and re- expand, independently of the other end, as was seen in some of the foregoing experiments.

When the lobes, which are rather thick, close, no trace of wrinkling can be seen on any part of their upper [page 317] surfaces, It appears therefore that the cells must contract. The chief seat of the movement is evidently in the thick ma.s.s of cells which overlies the central bundle of vessels in the midrib. To ascertain whether this part contracts, a leaf was fastened on the stage of the microscope in such a manner that the two lobes could not become quite shut, and having made two minute black dots on the midrib, in a transverse line and a little towards one side, they were found by the micrometer to be 17/1000 of an inch apart. One of the filaments was then touched and the lobes closed; but as they were prevented from meeting, I could still see the two dots, which now were 15/1000 of an inch apart, so that a small portion of the upper surface of the midrib had contracted in a transverse line 2/1000 of an inch (.0508 mm.).

We know that the lobes, whilst closing, become slightly incurved throughout their whole breadth. This movement appears to be due to the contraction of the superficial layers of cells over the whole upper surface. In order to observe their contraction, a narrow strip was cut out of one lobe at right angles to the midrib, so that the surface of the opposite lobe could be seen in this part when the leaf was shut.

After the leaf had recovered from the operation and had re-expanded, three minute black dots were made on the surface opposite to the slit or window, in a line at right angles to the midrib. The distance between the dots was found to be 40/1000 of an inch, so that the two extreme dots were 80/1000 of an inch apart. One of the filaments was now touched and the leaf closed. On again measuring the distances between the dots, the two next to the midrib were nearer together by 1 to 2/1000 of an inch, and the two further dots by 3 to 4/1000 of an inch, than they were before; so that the two extreme [page 318] dots now stood about 5/1000 of an inch (.127 mm.) nearer together than before. If we suppose the whole upper surface of the lobe, which was 400/1000 of an inch in breadth, to have contracted in the same proportion, the total contraction will have amounted to about 25/1000 or 1/40 of an inch (.635 mm.); but whether this is sufficient to account for the slight inward curvature of the whole lobe, I am unable to say.

Finally, with respect to the movement of the leaves, the wonderful discovery made by Dr. Burdon Sanderson* is now universally known; namely that there exists a normal electrical current in the blade and footstalk; and that when the leaves are irritated, the current is disturbed in the same manner as takes place during the contraction of the muscle of an animal.

The Re-expansion of the Leaves.--This is effected at an insensibly slow rate, whether or not any object is enclosed. One lobe can re-expand by itself, as occurred with the torpid leaf of which one lobe alone had closed. We have also seen in the experiments with cheese and alb.u.men that the two ends of the same lobe can re-expand to a certain extent independently of each other. But in all ordinary cases both lobes open at the same time. The re-expansion is not determined by the sensitive filaments; all three filaments on one lobe were cut off close to their bases; and the three

* Proc. Royal Soc.' vol. xxi. p. 495; and lecture at the Royal Inst.i.tution, June 5, 1874, given in 'Nature,' 1874, pp. 105 and 127.

Nuttall, in his 'Gen. American Plants,' p. 277 (note), says that, whilst collecting this plant in its native home, "I had occasion to observe that a detached leaf would make repeated efforts towards disclosing itself to the influence of the sun; these attempts consisted in an undulatory motion of the marginal ciliae, accompanied by a partial opening and succeeding collapse of the lamina, which at length terminated in a complete expansion and in the destruction of sensibility." I am indebted to Prof. Oliver for this reference; but I do not understand what took place. [page 319]

leaves thus treated re-expanded,--one to a partial extent in 24 hrs.,--a second to the same extent in 48 hrs., and the third, which had been previously injured, not until the sixth day. These leaves after their re-expansion closed quickly when the filaments on the other lobe were irritated. These were then cut off one of the leaves, so that none were left. This mutilated leaf, notwithstanding the loss of all its filaments, re-expanded in two days in the usual manner. When the filaments have been excited by immersion in a solution of sugar, the lobes do not expand so soon as when the filaments have been merely touched; and this, I presume, is due to their having been strongly affected through exosmose, so that they continue for some time to transmit a motor impulse to the upper surface of the leaf.

The following facts make me believe that the several layers of cells forming the lower surface of the leaf are always in a state of tension; and that it is owing to this mechanical state, aided probably by fresh fluid being attracted into the cells, that the lobes begin to separate or expand as soon as the contraction of the upper surface diminishes. A leaf was cut off and suddenly plunged perpendicularly into boiling water: I expected that the lobes would have closed, but instead of doing so, they diverged a little. I then took another fine leaf, with the lobes standing at an angle of nearly 80o to each other; and on immersing it as before, the angle suddenly increased to 90o. A third leaf was torpid from having recently re-expanded after having caught a fly, so that repeated touches of the filaments caused not the least movement; nevertheless, when similarly immersed, the lobes separated a little. As these leaves were inserted perpendicularly into the boiling water, both surfaces and the filaments [page 320] must have been equally affected; and I can understand the divergence of the lobes only by supposing that the cells on the lower side, owing to their state of tension, acted mechanically and thus suddenly drew the lobes a little apart, as soon as the cells on the upper surface were killed and lost their contractile power. We have seen that boiling water in like manner causes the tentacles of Drosera to curve backwards; and this is an a.n.a.logous movement to the divergence of the lobes of Dionaea.

In some concluding remarks in the fifteenth chapter on the Droseraceae, the different kinds of irritability possessed by the several genera, and the different manner in which they capture insects, will be compared. [page 321]

CHAPTER XIV.

ALDROVANDA VESICULOSA.

Captures crustaceans--Structure of the leaves in comparison with those of Dionaea-- Absorption by the glands, by the quadrifid processes, and points on the infolded margins-- Aldrovanda vesiculosa, var.

australis--Captures prey--Absorption of animal matter-- Aldrovanda vesiculosa, var. verticillata--Concluding remarks.

THIS plant may be called a miniature aquatic Dionaea. Stein discovered in 1873 that the bilobed leaves, which are generally found closed in Europe, open under a sufficiently high temperature, and, when touched, suddenly close.* They re-expand in from 24 to 36 hours, but only, as it appears, when inorganic objects are enclosed. The leaves sometimes contain bubbles of air, and were formerly supposed to be bladders; hence the specific name of vesiculosa. Stein observed that water-insects were sometimes caught, and Prof. Cohn has recently found within the leaves of naturally growing plants many kinds of crustaceans and larvae. Plants which had been kept in filtered water were placed by him in a vessel con-

* Since his original publication, Stein has found out that the irritability of the leaves was observed by De Sa.s.sus, as recorded in 'Bull. Bot. Soc. de France,' in 1861. Delpino states in a paper published in 1871 ('Nuovo Giornale Bot. Ital.' vol. iii. p. 174) that "una quant.i.t di chioccioline e di altri animalcoli acquatici" are caught and suffocated by the leaves. I presume that chioccioline are fresh-water molluscs. It would be interesting to know whether their sh.e.l.ls are at all corroded by the acid of the digestive secretion.

I am greatly indebted to this distinguished naturalist for having sent me a copy of his memoir on Aldrovanda, before its publication in his 'Beitrge zur Biologie der Pflanzen,' drittes Heft, 1875, page 71.

[page 322]

taining numerous crustaceans of the genus Cypris, and next morning many were found imprisoned and alive, still swimming about within the closed leaves, but doomed to certain death.

Directly after reading Prof. Cohn's memoir, I received through the kindness of Dr. Hooker living plants from Germany. As I can add nothing to Prof. Cohn's excellent description, I will give only two ill.u.s.trations, one of a whorl of leaves copied from his work, and the other of a leaf pressed flat open, drawn by my son Francis. I will, however, append a few remarks on the differences between this plant and Dionaea.

Aldrovanda is dest.i.tute of roots and floats freely in the water. The leaves are arranged in whorls round the stem. Their broad petioles terminate in from four to six rigid projections,* each tipped with a stiff, short bristle. The bilobed leaf, with the midrib likewise tipped with a bristle, stands in the midst of these projections, and is evidently defended by them. The lobes are formed of very delicate tissue, so as to be translucent; they open, according to Cohn, about as much as the two valves of a living mussel-sh.e.l.l, therefore even less than the lobes of Dionaea; and this must make the capture of aquatic animals more easy. The outside of the leaves and the petioles are covered with minute two-armed papillae, evidently answering to the eight-rayed papillae of Dionaea.

Each lobe rather exceeds a semi-circle in convexity, and consists of two very different concentric portions; the inner and lesser portion, or that next to the midrib,

*There has been much discussion by botanists on the h.o.m.ological nature of these projections. Dr. Nitschke ('Bot. Zeitung,' 1861, p. 146) believes that they correspond with the fimbriated scale-like bodies found at the bases of the petioles of Drosera. [page 323]

is slightly concave, and is formed, according to Cohn, of three layers of cells. Its upper surface is studded with colourless glands like, but more simple than, those of Dionaea; they are supported on distinct footstalks, consisting of two rows of cells. The outer

FIG. 13. (Aldrovanda vesiculosa.) Upper figure, whorl of leaves (from Prof. Cohn). Lower figure, leaf pressed flat open and greatly enlarged.

and broader portion of the lobe is flat and very thin, being formed of only two layers of cells. Its upper surface does not bear any glands, but, in their place, small quadrifid processes, each consisting of four tapering projections, which rise from a common [page 324] prominence.

These processes are formed of very delicate membrane lined with a layer of protoplasm; and they sometimes contain aggregated globules of hyaline matter. Two of the slightly diverging arms are directed towards the circ.u.mference, and two towards the midrib, forming together a sort of Greek cross. Occasionally two of the arms are replaced by one, and then the projection is trifid. We shall see in a future chapter that these projections curiously resemble those found within the bladders of Utricularia, more especially of Utricularia montana, although this genus is not related to Aldrovanda.

A narrow rim of the broad flat exterior part of each lobe is turned inwards, so that, when the lobes are closed, the exterior surfaces of the infolded portions come into contact. The edge itself bears a row of conical, flattened, transparent points with broad bases, like the p.r.i.c.kles on the stem of a bramble or Rubus. As the rim is infolded, these points are directed towards the midrib, and they appear at first as if they were adapted to prevent the escape of prey; but this can hardly be their chief function, for they are composed of very delicate and highly flexible membrane, which can be easily bent or quite doubled back without being cracked. Nevertheless, the infolded rims, together with the points, must somewhat interfere with the retrograde movement of any small creature, as soon as the lobes begin to close. The circ.u.mferential part of the leaf of Aldrovanda thus differs greatly from that of Dionaea; nor can the points on the rim be considered as h.o.m.ologous with the spikes round the leaves of Dionaea, as these latter are prolongations of the blade, and not mere epidermic productions.

They appear also to serve for a widely different purpose. [page 325]

On the concave gland-bearing portion of the lobes, and especially on the midrib, there are numerous, long, finely pointed hairs, which, as Prof. Cohn remarks, there can be little doubt are sensitive to a touch, and, when touched, cause the leaf to close. They are formed of two rows of cells, or, according to Cohn, sometimes of four, and do not include any vascular tissue. They differ also from the six sensitive filaments of Dionaea in being colourless, and in having a medial as well as a basal articulation. No doubt it is owing to these two articulations that, notwithstanding their length, they escape being broken when the lobes close.

The plants which I received during the early part of October from Kew never opened their leaves, though subjected to a high temperature.

After examining the structure of some of them, I experimented on only two, as I hoped that the plants would grow; and I now regret that I did not sacrifice a greater number.

A leaf was cut open along the midrib, and the glands examined under a high power. It was then placed in a few drops of an infusion of raw meat. After 3 hrs. 20 m. there was no change, but when next examined after 23 hrs. 20 m., the outer cells of the glands contained, instead of limpid fluid, spherical ma.s.ses of a granular substance, showing that matter had been absorbed from the infusion. That these glands secrete a fluid which dissolves or digests animal matter out of the bodies of the creatures which the leaves capture, is also highly probable from the a.n.a.logy of Dionaea. If we may trust to the same a.n.a.logy, the concave and inner portions of the two lobes probably close together by a slow movement, as soon as the glands have absorbed a slight amount of [page 326] already soluble animal matter. The included water would thus be pressed out, and the secretion consequently not be too much diluted to act. With respect to the quadrifid processes on the outer parts of the lobes, I was not able to decide whether they had been acted on by the infusion; for the lining of protoplasm was somewhat shrunk before they were immersed. Many of the points on the infolded rims also had their lining of protoplasm similarly shrunk, and contained spherical granules of hyaline matter.

A solution of urea was next employed. This substance was chosen partly because it is absorbed by the quadrifid processes and more especially by the glands of Utricularia--a plant which, as we shall hereafter see, feeds on decayed animal matter. As urea is one of the last products of the chemical changes going on in the living body, it seems fitted to represent the early stages of the decay of the dead body. I was also led to try urea from a curious little fact mentioned by Prof. Cohn, namely that when rather large crustaceans are caught between the closing lobes, they are pressed so hard whilst making their escape that they often void their sausage-shaped ma.s.ses of excrement, which were found within most of the leaves. These ma.s.ses, no doubt, contain urea.

They would be left either on the broad outer surfaces of the lobes where the quadrifids are situated, or within the closed concavity. In the latter case, water charged with excrement.i.tious and decaying matter would be slowly forced outwards, and would bathe the quadrifids, if I am right in believing that the concave lobes contract after a time like those of Dionaea. Foul water would also be apt to ooze out at all times, especially when bubbles of air were generated within the concavity.

A leaf was cut open and examined, and the outer [page 327] cells of the glands were found to contain only limpid fluid. Some of the quadrifids included a few spherical granules, but several were transparent and empty, and their positions were marked. This leaf was now immersed in a little solution of one part of urea to 146 of water, or three grains to the ounce. After 3 hrs. 40 m. there was no change either in the glands or quadrifids; nor was there any certain change in the glands after 24 hrs.; so that, as far as one trial goes, urea does not act on them in the same manner as an infusion of raw meat. It was different with the quadrifids; for the lining of protoplasm, instead of presenting a uniform texture, was now slightly shrunk, and exhibited in many places minute, thickened, irregular, yellowish specks and ridges, exactly like those which appear within the quadrifids of Utricularia when treated with this same solution. Moreover, several of the quadrifids, which were before empty, now contained moderately sized or very small, more or less aggregated, globules of yellowish matter, as likewise occurs under the same circ.u.mstances with Utricularia. Some of the points on the infolded margins of the lobes were similarly affected; for their lining of protoplasm was a little shrunk and included yellowish specks; and those which were before empty now contained small spheres and irregular ma.s.ses of hyaline matter, more or less aggregated; so that both the points on the margins and the quadrifids had absorbed matter from the solution in the course of 24 hrs.; but to this subject I shall recur. In another rather old leaf, to which nothing had been given, but which had been kept in foul water, some of the quadrifids contained aggregated translucent globules. These were not acted on by a solution of one part of carbonate of ammonia to 218 of water; and this negative result [page 328] agrees with what I have observed under similar circ.u.mstances with Utricularia.

Aldrovanda vesiculosa, var. australis.--Dried leaves of this plant from Queensland in Australia were sent me by Prof. Oliver from the herbarium at Kew. Whether it ought to be considered as a distinct species or a variety, cannot be told until the flowers are examined by a botanist.

The projections at the upper end of the petiole (from four to six in number) are considerably longer relatively to the blade, and much more attenuated than those of the European form. They are thickly covered for a considerable s.p.a.ce near their extremities with the upcurved p.r.i.c.kles, which are quite absent in the latter form; and they generally bear on their tips two or three straight p.r.i.c.kles instead of one. The bilobed leaf appears also to be rather larger and somewhat broader, with the pedicel by which it is attached to the upper end of the petiole a little longer. The points on the infolded margins likewise differ; they have narrower bases, and are more pointed; long and short points also alternate with much more regularity than in the European form. The glands and sensitive hairs are similar in the two forms. No quadrifid processes could be seen on several of the leaves, but I do not doubt that they were present, though indistinguishable from their delicacy and from having shrivelled; for they were quite distinct on one leaf under circ.u.mstances presently to be mentioned.

Some of the closed leaves contained no prey, but in one there was a rather large beetle, which from its flattened tibiae I suppose was an aquatic species, but was not allied to Colymbetes. All the softer tissues of this beetle were completely dissolved, and its chitinous integuments were as clean as if they had been [page 329] boiled in caustic potash; so that it must have been enclosed for a considerable time. The glands were browner and more opaque than those on other leaves which had caught nothing; and the quadrifid processes, from being partly filled with brown granular matter, could be plainly distinguished, which was not the case, as already stated, on the other leaves. Some of the points on the infolded margins likewise contained brownish granular matter. We thus gain additional evidence that the glands, the quadrifid processes, and the marginal points, all have the power of absorbing matter, though probably of a different nature.

Within another leaf disintegrated remnants of a rather small animal, not a crustacean, which had simple, strong, opaque mandibles, and a large unarticulated chitinous coat, were present. Lumps of black organic matter, possibly of a vegetable nature, were enclosed in two other leaves; but in one of these there was also a small worm much decayed. But the nature of partially digested and decayed bodies, which have been pressed flat, long dried, and then soaked in water, cannot be recognised easily. All the leaves contained unicellular and other Algae, still of a greenish colour, which had evidently lived as intruders, in the same manner as occurs, according to Cohn, within the leaves of this plant in Germany.

Aldrovanda vesiculosa, var. verticillata.--Dr. King, Superintendent of the Botanic Gardens, kindly sent me dried specimens collected near Calcutta. This form was, I believe, considered by Wallich as a distinct species, under the name of verticillata. It resembles the Australian form much more nearly than the European; namely in the projections at the upper end of the petiole being much attenuated and covered with [page 330] upcurved p.r.i.c.kles; they terminate also in two straight little p.r.i.c.kles. The bilobed leaves are, I believe, larger and certainly broader even than those of the Australian form; so that the greater convexity of their margins was conspicuous. The length of an open leaf being taken at 100, the breadth of the Bengal form is nearly 173, of the Australian form 147, and of the German 134. The points on the infolded margins are like those in the Australian form. Of the few leaves which were examined, three contained entomostracan crustaceans.

Concluding Remarks.--The leaves of the three foregoing closely allied species or varieties are manifestly adapted for catching living creatures. With respect to the functions of the several parts, there can be little doubt that the long jointed hairs are sensitive, like those of Dionaea, and that, when touched, they cause the lobes to close. That the glands secrete a true digestive fluid and afterwards absorb the digested matter, is highly probable from the a.n.a.logy of Dionaea,--from the limpid fluid within their cells being aggregated into spherical ma.s.ses, after they had absorbed an infusion of raw meat,--from their opaque and granular condition in the leaf, which had enclosed a beetle for a long time,--and from the clean condition of the integuments of this insect, as well as of crustaceans (as described by Cohn), which have been long captured. Again, from the effect produced on the quadrifid processes by an immersion for 24 hrs. in a solution of urea,--from the presence of brown granular matter within the quadrifids of the leaf in which the beetle had been caught,--and from the a.n.a.logy of Utricularia,--it is probable that these processes absorb excrement.i.tious and decaying animal matter. It is a more curious fact that the points on [page 331] the infolded margins apparently serve to absorb decayed animal matter in the same manner as the quadrifids. We can thus understand the meaning of the infolded margins of the lobes furnished with delicate points directed inwards, and of the broad, flat, outer portions, bearing quadrifid processes; for these surfaces must be liable to be irrigated by foul water flowing from the concavity of the leaf when it contains dead animals. This would follow from various causes,--from the gradual contraction of the concavity,--from fluid in excess being secreted,- -and from the generation of bubbles of air. More observations are requisite on this head; but if this view is correct, we have the remarkable case of different parts of the same leaf serving for very different purposes--one part for true digestion, and another for the absorption of decayed animal matter. We can thus also understand how, by the gradual loss of either power, a plant might be gradually adapted for the one function to the exclusion of the other; and it will hereafter be shown that two genera, namely Pinguicula and Utricularia, belonging to the same family, have been adapted for these two different functions. [page 332]

CHAPTER XV.

DROSOPHYLLUM--RORIDULA--BYBLIS--GLANDULAR HAIRS OF OTHER PLANTS-- CONCLUDING REMARKS ON THE DROSERACEAE.

Drosophyllum--Structure of leaves--Nature of the secretion--Manner of catching insects-- Power of absorption--Digestion of animal substances--Summary on Drosophyllum--Roridula- -Byblis--Glandular hairs of other plants, their power of absorption--Saxifraga--Primula-- Pelargonium--Erica--Mirabilis--Nicotiana--Summary on glandular hairs--Concluding remarks on the Droseraceae.

DROSOPHYLLUM LUSITANIc.u.m.--This rare plant has been found only in Portugal, and, as I hear from Dr. Hooker, in Morocco. I obtained living specimens through the great kindness of Mr. W.C. Tait, and afterwards from Mr. G. Maw and Dr. Moore. Mr. Tait informs me that it grows plentifully on the sides of dry hills near Oporto, and that vast numbers of flies adhere to the leaves. This latter fact is well-known to the villagers, who call the plant the "fly-catcher, " and hang it up in their cottages for this purpose. A plant in my hot-house caught so many insects during the early part of April, although the weather was cold and insects scarce, that it must have been in some manner strongly attractive to them. On four leaves of a young and small plant, 8, 10, 14, and 16 minute insects, chiefly Diptera, were found in the autumn adhering to them. I neglected to examine the roots, but I hear from Dr.

Hooker that they are very small, as in the case of the previously mentioned members of the same family of the Droseraceae.