Insectivorous Plants Part 25

PINGUICULA.

Pinguicula vulgaris--Structure of leaves--Number of insects and other objects caught-- Movement of the margins of the leaves--Uses of this movement--Secretion, digestion, and absorption--Action of the secretion on various animal and vegetable substances--The effects of substances not containing soluble nitrogenous matter on the glands--Pinguicula grandiflora--Pinguicula lusitanica, catches insects--Movement of the leaves, secretion and digestion.

PINGUICULA VULGARIS.--This plant grows in moist places, generally on mountains. It bears on an average eight, rather thick, oblong, light green leaves, having scarcely any footstalk. A full-sized leaf is about 1 1/2 inch in length and 3/4 inch in breadth. The young central leaves are deeply concave, and project upwards; the older ones towards the outside are flat or convex, and lie close to the ground, forming a rosette from 3 to 4 inches in diameter. The margins of the leaves are incurved. Their upper surfaces are thickly covered with two sets of glandular hairs, differing in the size of the glands and in the length of their pedicels. The larger glands have a circular outline as seen from above, and are of moderate thickness; they are divided by radiating part.i.tions into sixteen cells, containing light-green, h.o.m.ogeneous fluid. They are supported on elongated, unicellular pedicels (containing a nucleus with a nucleolus) which rest on slight prominences. The small glands differ only in being formed of about half the number of cells, containing much paler fluid, and supported on much shorter pedicels. Near the midrib, towards the base of the leaf, the [page 369] pedicels are multicellular, are longer than elsewhere, and bear smaller glands. All the glands secrete a colourless fluid, which is so viscid that I have seen a fine thread drawn out to a length of 18 inches; but the fluid in this case was secreted by a gland which had been excited. The edge of the leaf is translucent, and does not bear any glands; and here the spiral vessels, proceeding from the midrib, terminate in cells marked by a spiral line, somewhat like those within the glands of Drosera.

The roots are short. Three plants were dug up in North Wales on June 20, and carefully washed; each bore five or six unbranched roots, the longest of which was only 1.2 of an inch. Two rather young plants were examined on September 28; these had a greater number of roots, namely eight and eighteen, all under 1 inch in length, and very little branched.

I was led to investigate the habits of this plant by being told by Mr.

W. Marshall that on the mountains of c.u.mberland many insects adhere to the leaves.

[A friend sent me on June 23 thirty-nine leaves from North Wales, which were selected owing to objects of some kind adhering to them. Of these leaves, thirty-two had caught 142 insects, or on an average 4.4 per leaf, minute fragments of insects not being included. Besides the insects, small leaves belonging to four different kinds of plants, those of Erica tetralix being much the commonest, and three minute seedling plants, blown by the wind, adhered to nineteen of the leaves.

One had caught as many as ten leaves of the Erica. Seeds or fruits, commonly of Carex and one of Juncus, besides bits of moss and other rubbish, likewise adhered to six of the thirty-nine leaves. The same friend, on June 27, collected nine plants bearing seventy-four leaves, and all of these, with the exception of three young leaves, had caught insects; thirty insects were counted on one leaf, eighteen on a second, and sixteen on a third. Another friend examined on August 22 some plants in Donegal, Ireland, and found insects on 70 out of 157 leaves; fifteen of [page 370] these leaves were sent me, each having caught on an average 2.4 insects. To nine of them, leaves (mostly of Erica tetralix) adhered; but they had been specially selected on this latter account. I may add that early in August my son found leaves of this same Erica and the fruits of a Carex on the leaves of a Pinguicula in Switzerland, probably Pinguicula alpina; some insects, but no great number, also adhered to the leaves of this plant, which had much better developed roots than those of Pinguicula vulgaris. In c.u.mberland, Mr.

Marshall, on September 3, carefully examined for me ten plants bearing eighty leaves; and on sixty-three of these (i.e. on 79 per cent.) he found insects, 143 in number; so that each leaf had on an average 2.27 insects. A few days later he sent me some plants with sixteen seeds or fruits adhering to fourteen leaves. There was a seed on three leaves on the same plant. The sixteen seeds belonged to nine different kinds, which could not be recognised, excepting one of Ranunculus, and several belonging to three or four distinct species of Carex. It appears that fewer insects are caught late in the year than earlier; thus in c.u.mberland from twenty to twenty-four insects were observed in the middle of July on several leaves, whereas in the beginning of September the average number was only 2.27. Most of the insects, in all the foregoing cases, were Diptera, but with many minute Hymenoptera, including some ants, a few small Coleoptera, larvae, spiders, and even small moths.]

We thus see that numerous insects and other objects are caught by the viscid leaves; but we have no right to infer from this fact that the habit is beneficial to the plant, any more than in the before given case of the Mirabilis, or of the horse-chestnut. But it will presently be seen that dead insects and other nitrogenous bodies excite the glands to increased secretion; and that the secretion then becomes acid and has the power of digesting animal substances, such as alb.u.men, fibrin, &c. Moreover, the dissolved nitrogenous matter is absorbed by the glands, as shown by their limpid contents being aggregated into slowly moving granular ma.s.ses of protoplasm. The same results follow when insects are naturally captured, and as the plant lives in poor soil and has small roots, there can be no [page 371] doubt that it profits by its power of digesting and absorbing matter from the prey which it habitually captures in such large numbers. It will, however, be convenient first to describe the movements of the leaves.

Movements of the Leaves.--That such thick, large leaves as those of Pinguicula vulgarisshould have the power of curving inwards when excited has never even been suspected. It is necessary to select for experiment leaves with their glands secreting freely, and which have been prevented from capturing many insects; as old leaves, at least those growing in a state of nature, have their margins already curled so much inwards that they exhibit little power of movement, or move very slowly. I will first give in detail the more important experiments which were tried, and then make some concluding remarks.

[Experiment 1.--A young and almost upright leaf was selected, with its two lateral edges equally and very slightly incurved. A row of small flies was placed along one margin. When looked at next day, after 15 hrs., this margin, but not the other, was found folded inwards, like the helix of the human ear, to the breadth of 1/10 of an inch, so as to lie partly over the row of flies (fig. 15). The glands on which the flies rested, as well as those on the over-lapping margin which had been brought into contact with the flies, were all secreting copiously.

FIG. 15. (Pinguicula vulgaris.) Outline of leaf with left margin inflected over a row of small flies.

Experiment 2.--A row of flies was placed on one margin of a rather old leaf, which lay flat on the ground; and in this case the margin, after the same interval as before, namely 15 hrs., had only just begun to curl inwards; but so much secretion had been poured forth that the spoon-shaped tip of the leaf was filled with it.

Experiment 3.--Fragments of a large fly were placed close to the apex of a vigorous leaf, as well as along half one margin. [page 372] After 4 hrs. 20 m. there was decided incurvation, which increased a little during the afternoon, but was in the same state on the following morning. Near the apex both margins were inwardly curved. I have never seen a case of the apex itself being in the least curved towards the base of the leaf. After 48 hrs. (always reckoning from the time when the flies were placed on the leaf) the margin had everywhere begun to unfold.

Experiment 4.--A large fragment of a fly was placed on a leaf, in a medial line, a little beneath the apex. Both lateral margins were perceptibly incurved in 3 hrs., and after 4 hrs. 20 m. to such a degree that the fragment was clasped by both margins. After 24 hrs. the two infolded edges near the apex (for the lower part of the leaf was not at all affected) were measured and found to be .11 of an inch (2.795 mm.) apart. The fly was now removed, and a stream of water poured over the leaf so as to wash the surface; and after 24 hrs. the margins were .25 of an inch (6.349 mm.) apart, so that they were largely unfolded. After an additional 24 hrs. they were completely unfolded. Another fly was now put on the same spot to see whether this leaf, on which the first fly had been left 24 hrs., would move again; after 10 hrs. there was a trace of incurvation, but this did not increase during the next 24 hrs.

A bit of meat was also placed on the margin of a leaf, which four days previously had become strongly incurved over a fragment of a fly and had afterwards re-expanded; but the meat did not cause even a trace of incurvation. On the contrary, the margin became somewhat reflexed, as if injured, and so remained for the three following days, as long as it was observed.

Experiment 5.--A large fragment of a fly was placed halfway between the apex and base of a leaf and halfway between the midrib and one margin.

A short s.p.a.ce of this margin, opposite the fly, showed a trace of incurvation after 3 hrs., and this became strongly p.r.o.nounced in 7 hrs.

After 24 hrs. the infolded edge was only .16 of an inch (4.064 mm.) from the midrib. The margin now began to unfold, though the fly was left on the leaf; so that by the next morning (i.e. 48 hrs. from the time when the fly was first put on) the infolded edge had almost completely recovered its original position, being now .3 of an inch (7.62 mm.), instead of .16 of an inch, from the midrib. A trace of flexure was, however, still visible.

Experiment 6.--A young and concave leaf was selected with its margins slightly and naturally incurved. Two rather large, oblong, rectangular pieces of roast meat were placed with their ends touching the infolded edge, and .46 of an inch (11.68 mm.) [page 373] apart from one another.

After 24 hrs. the margin was greatly and equally incurved (see fig.

16) throughout this s.p.a.ce, and for a length of .12 or .13 of an inch (3.048 or 3.302 mm.) above and below each bit; so that the margin had been affected over a greater length between the two bits, owing to their conjoint action, than beyond them. The bits of meat were too large to be clasped by the margin, but they were tilted up, one of them so as to stand almost vertically. After 48 hrs. the margin was almost unfolded, and the bits had sunk down. When again examined after two days, the margin was quite unfolded, with the exception of the naturally inflected edge; and one of the bits of meat, the end of which had at first touched the edge, was now .067 of an inch (1.70 mm.) distant from it; so that this bit had been pushed thus far across the blade of the leaf.

FIG. 16. (Pinguicula vulgaris.) Outline of leaf, with right margin inflected against two square bits of meat.

Experiment 7.--A bit of meat was placed close to the incurved edge of a rather young leaf, and after it had re-expanded, the bit was left lying .11 of an inch (2.795 mm.) from the edge. The distance from the edge to the midrib of the fully expanded leaf was .35 of an inch (8.89 mm.); so that the bit had been pushed inwards and across nearly one-third of its semi-diameter.

Experiment 8.--Cubes of sponge, soaked in a strong infusion of raw meat, were placed in close contact with the incurved edges of two leaves,--an older and younger one. The distance from the edges to the midribs was carefully measured. After 1 hr. 17 m. there appeared to be a trace of incurvation. After 2 hrs. 17 m. both leaves were plainly inflected; the distance between the edges and midribs being now only half what it was at first. The incurvation increased slightly during the next 4 1/2 hrs., but remained nearly the same for the next 17 hrs.

30 m. In 35 hrs. from the time when the sponges were placed on the leaves, the margins were a little unfolded--to a greater degree in the younger than in the older leaf. The latter was not quite unfolded until the third day, and now both bits of sponge were left at the distance of .1 of an inch (2.54 mm.) from the edges; or about a quarter of the distance between the edge and midrib. A third bit of sponge adhered to the edge, and, as the margin unfolded, was dragged backwards, into its original position. [page 374]

Experiment 9.--A chain of fibres of roast meat, as thin as bristles and moistened with saliva, were placed down one whole side, close to the narrow, naturally incurved edge of a leaf. In 3 hrs. this side was greatly incurved along its whole length, and after 8 hrs. formed a cylinder, about 1/20 of an inch (1.27 mm) in diameter, quite concealing the meat. This cylinder remained closed for 32 hrs., but after 48 hrs.

was half unfolded, and in 72 hrs. was as open as the opposite margin where no meat had been placed. As the thin fibres of meat were completely overlapped by the margin, they were not pushed at all inwards, across the blade.

Experiment 10.--Six cabbage seeds, soaked for a night in water, were placed in a row close to the narrow incurved edge of a leaf. We shall hereafter see that these seeds yield soluble matter to the glands. In 2 hrs. 25 m. the margin was decidedly inflected; in 4 hrs. it extended over the seeds for about half their breadth, and in 7 hrs. over three-fourths of their breadth, forming a cylinder not quite closed along the inner side, and about .7 of an inch (1.778 mm.) in diameter.

After 24 hrs. the inflection had not increased, perhaps had decreased.

The glands which had been brought into contact with the upper surfaces of the seeds were now secreting freely. In 36 hrs. from the time when the seeds were put on the leaf the margin had greatly, and after 48 hrs. had completely, re-expanded. As the seeds were no longer held by the inflected margin, and as the secretion was beginning to fail, they rolled some way down the marginal channel.

Experiment 11.--Fragments of gla.s.s were placed on the margins of two fine young leaves. After 2 hrs. 30 m. the margin of one certainly became slightly incurved; but the inflection never increased, and disappeared in 16 hrs. 30 m. from the time when the fragments were first applied. With the second leaf there was a trace of incurvation in 2 hrs. 15 m., which became decided in 4 hrs. 30 m., and still more strongly p.r.o.nounced in 7 hrs., but after 19 hrs. 30 m. had plainly decreased. The fragments excited at most a slight and doubtful increase of the secretion; and in two other trials, no increase could be perceived. Bits of coal-cinders, placed on a leaf, produced no effect, either owing to their lightness or to the leaf being torpid.

Experiment 12.--We now turn to fluids. A row of drops of a strong infusion of raw meat were placed along the margins of two leaves; squares of sponge soaked in the same infusion being placed on the opposite margins. My object was to ascer- [page 375] tain whether a fluid would act as energetically as a substance yielding the same soluble matter to the glands. No distinct difference was perceptible; certainly none in the degree of incurvation; but the incurvation round the bits of sponge lasted rather longer, as might perhaps have been expected from the sponge remaining damp and supplying nitrogenous matter for a longer time. The margins, with the drops, became plainly incurved in 2 hrs. 17 m. The incurvation subsequently increased somewhat, but after 24 hrs. had greatly decreased.

Experiment 13.--Drops of the same strong infusion of raw meat were placed along the midrib of a young and rather deeply concave leaf. The distance across the broadest part of the leaf, between the naturally incurved edges, was .55 of an inch (13.97 mm.). In 3 hrs. 27 m. this distance was a trace less; in 6 hrs. 27 m. it was exactly .45 of an inch (11.43 mm.), and had therefore decreased by .1 of an inch (2.54 mm.). After only 10 hrs. 37 m. the margin began to re-expand, for the distance from edge to edge was now a trace wider, and after 24 hrs. 20 m. was as great, within a hair's breadth, as when the drops were first placed on the leaf. From this experiment we learn that the motor impulse can be transmitted to a distance of .22 of an inch (5.590 mm.) in a transverse direction from the midrib to both margins; but it would be safer to say .2 of an inch (5.08 mm.) as the drops spread a little beyond the midrib. The incurvation thus caused lasted for an unusually short time.

Experiment 14.--Three drops of a solution of one part of carbonate of ammonia to 218 of water (2 grs. to 1 oz.) were placed on the margin of a leaf. These excited so much secretion that in 1 h. 22 m. all three drops ran together; but although the leaf was observed for 24 hrs., there was no trace of inflection. We know that a rather strong solution of this salt, though it does not injure the leaves of Drosera, paralyses their power of movement, and I have no doubt, from the following case, that this holds good with Pinguicula.

Experiment 15.--A row of drops of a solution of one part of carbonate of ammonia to 875 of water (1 gr. to 2 oz.) was placed on the margin of a leaf. In 1 hr. there was apparently some slight incurvation, and this was well-marked in 3 hrs. 30 m. After 24 hrs. the margin was almost completely re-expanded.

Experiment 16.--A row of large drops of a solution of one part of phosphate of ammonia to 4375 of water (1 gr. to 10 oz.) was placed along the margin of a leaf. No effect was produced, and after 8 hrs.

fresh drops were added along the same margin without the least effect.

We know that a solution of this [page 376] strength acts powerfully on Drosera, and it is just possible that the solution was too strong. I regret that I did not try a weaker solution.

Experiment 17.--As the pressure from bits of gla.s.s causes incurvation, I scratched the margins of two leaves for some minutes with a blunt needle, but no effect was produced. The surface of a leaf beneath a drop of a strong infusion of raw meat was also rubbed for 10. m. with the end of a bristle, so as to imitate the struggles of a captured insect; but this part of the margin did not bend sooner than the other parts with undisturbed drops of the infusion.]

We learn from the foregoing experiments that the margins of the leaves curl inwards when excited by the mere pressure of objects not yielding any soluble matter, by objects yielding such matter, and by some fluids--namely an infusion of raw meat and a week solution of carbonate of ammonia. A stronger solution of two grains of this salt to an ounce of water, though exciting copious secretion, paralyses the leaf. Drops of water and of a solution of sugar or gum did not cause any movement.

Scratching the surface of the leaf for some minutes produced no effect.

Therefore, as far as we at present know, only two causes--namely slight continued pressure and the absorption of nitrogenous matter--excite movement. It is only the margins of the leaf which bend, for the apex never curves towards the base. The pedicels of the glandular hairs have no power of movement. I observed on several occasions that the surface of the leaf became slightly concave where bits of meat or large flies had long lain, but this may have been due to injury from over-stimulation.

The shortest time in which plainly marked movement was observed was 2 hrs. 17 m., and this occurred when either nitrogenous substances or fluids were placed on the leaves; but I believe that in some cases [page 377] there was a trace of movement in 1 hr. or 1 hr. 30 m. The pressure from fragments of gla.s.s excites movement almost as quickly as the absorption of nitrogenous matter, but the degree of incurvation thus caused is much less. After a leaf has become well incurved and has again expanded, it will not soon answer to a fresh stimulus. The margin was affected longitudinally, upwards or downwards, for a distance of .13 of an inch (3.302 mm.) from an excited point, but for a distance of .46 of an inch between two excited points, and transversely for a distance of .2 of an inch (5.08 mm.). The motor impulse is not accompanied, as in the case of Drosera, by any influence causing increased secretion; for when a single gland was strongly stimulated and secreted copiously, the surrounding glands were not in the least affected. The incurvation of the margin is independent of increased secretion, for fragments of gla.s.s cause little or no secretion, and yet excite movement; whereas a strong solution of carbonate of ammonia quickly excites copious secretion, but no movement.

One of the most curious facts with respect to the movement of the leaves is the short time during which they remain incurved, although the exciting object is left on them. In the majority of cases there was well-marked re-expansion within 24 hrs. from the time when even large pieces of meat, &c., were placed on the leaves, and in all cases within 48 hrs. In one instance the margin of a leaf remained for 32 hrs.

closely inflected round thin fibres of meat; in another instance, when a bit of sponge, soaked in a strong infusion of raw meat, had been applied to a leaf, the margin began to unfold in 35 hrs. Fragments of gla.s.s keep the margin incurved for a shorter time than do nitrogenous bodies; for in the former case there was [page 378] complete re-expansion in 16 hrs. 30 m. Nitrogenous fluids act for a shorter time than nitrogenous substances; thus, when drops of an infusion of raw meat were placed on the midrib of a leaf, the incurved margins began to unfold in only 10 hrs. 37 m., and this was the quickest act of re-expansion observed by me; but it may have been partly due to the distance of the margins from the midrib where the drops lay.

We are naturally led to inquire what is the use of this movement which lasts for so short a time? If very small objects, such as fibres of meat, or moderately small objects, such as little flies or cabbage-seeds, are placed close to the margin, they are either completely or partially embraced by it. The glands of the overlapping margin are thus brought into contact with such objects and pour forth their secretion, afterwards absorbing the digested matter. But as the incurvation lasts for so short a time, any such benefit can be of only slight importance, yet perhaps greater than at first appears. The plant lives in humid districts, and the insects which adhere to all parts of the leaf are washed by every heavy shower of rain into the narrow channel formed by the naturally incurved edges. For instance, my friend in North Wales placed several insects on some leaves, and two days afterwards (there having been heavy rain in the interval) found some of them quite washed away, and many others safely tucked under the now closely inflected margins, the glands of which all round the insects were no doubt secreting. We can thus, also, understand how it is that so many insects, and fragments of insects, are generally found lying within the incurved margins of the leaves.

The incurvation of the margin, due to the presence of an exciting object, must be serviceable in another [page 379] and probably more important way. We have seen that when large bits of meat, or of sponge soaked in the juice of meat, were placed on a leaf, the margin was not able to embrace them, but, as it became incurved, pushed them very slowly towards the middle of the leaf, to a distance from the outside of fully .1 of an inch (2.54 mm.), that is, across between one-third and one-fourth of the s.p.a.ce between the edge and midrib. Any object, such as a moderately sized insect, would thus be brought slowly into contact with a far larger number of glands, inducing much more secretion and absorption, than would otherwise have been the case.

That this would be highly serviceable to the plant, we may infer from the fact that Drosera has acquired highly developed powers of movement, merely for the sake of bringing all its glands into contact with captured insects. So again, after a leaf of Dionaea has caught an insect, the slow pressing together of the two lobes serves merely to bring the glands on both sides into contact with it, causing also the secretion charged with animal matter to spread by capillary attraction over the whole surface. In the case of Pinguicula, as soon as an insect has been pushed for some little distance towards the midrib, immediate re-expansion would be beneficial, as the margins could not capture fresh prey until they were unfolded. The service rendered by this pus.h.i.+ng action, as well as that from the marginal glands being brought into contact for a short time with the upper surfaces of minute captured insects, may perhaps account for the peculiar movements of the leaves; otherwise, we must look at these movements as a remnant of a more highly developed power formerly possessed by the progenitors of the genus.

In the four British species, and, as I hear from [page 380] Prof. Dyer, in most or all the species of the genus, the edges of the leaves are in some degree naturally and permanently incurved. This incurvation serves, as already shown, to prevent insects from being washed away by the rain; but it likewise serves for another end. When a number of glands have been powerfully excited by bits of meat, insects, or any other stimulus, the secretion often trickles down the leaf, and is caught by the incurved edges, instead of rolling off and being lost. As it runs down the channel, fresh glands are able to absorb the animal matter held in solution. Moreover, the secretion often collects in little pools within the channel, or in the spoon-like tips of the leaves; and I ascertained that bits of alb.u.men, fibrin, and gluten, are here dissolved more quickly and completely than on the surface of the leaf, where the secretion cannot acc.u.mulate; and so it would be with naturally caught insects. The secretion was repeatedly seen thus to collect on the leaves of plants protected from the rain; and with exposed plants there would be still greater need of some provision to prevent, as far as possible, the secretion, with its dissolved animal matter, being wholly lost.

It has already been remarked that plants growing in a state of nature have the margins of their leaves much more strongly incurved than those grown in pots and prevented from catching many insects. We have seen that insects washed down by the rain from all parts of the leaf often lodge within the margins, which are thus excited to curl farther inwards; and we may suspect that this action, many times repeated during the life of the plant, leads to their permanent and well-marked incurvation. I regret that this view did not occur to me in time to test its truth.

It may here be added, though not immediately [page 381] bearing on our subject, that when a plant is pulled up, the leaves immediately curl downwards so as almost to conceal the roots,--a fact which has been noticed by many persons. I suppose that this is due to the same tendency which causes the outer and older leaves to lie flat on the ground. It further appears that the flower-stalks are to a certain extent irritable, for Dr. Johnson states that they "bend backwards if rudely handled."*

Secretion, Absorption, and Digestion.--I will first give my observations and experiments, and then a summary of the results.

[The Effects of Objects containing Soluble Nitrogenous Matter.

(1) Flies were placed on many leaves, and excited the glands to secrete copiously; the secretion always becoming acid, though not so before.

After a time these insects were rendered so tender that their limbs and bodies could be separated by a mere touch, owing no doubt to the digestion and disintegration of their muscles. The glands in contact with a small fly continued to secrete for four days, and then became almost dry. A narrow strip of this leaf was cut off, and the glands of the longer and shorter hairs, which had lain in contact for the four days with the fly, and those which had not touched it, were compared under the microscope and presented a wonderful contrast. Those which had been in contact were filled with brownish granular matter, the others with h.o.m.ogeneous fluid. There could therefore be no doubt that the former had absorbed matter from the fly.

(2) Small bits of roast meat, placed on a leaf, always caused much acid secretion in the course of a few hours--in one case within 40 m. When thin fibres of meat were laid along the margin of a leaf which stood almost upright, the secretion ran down to the ground. Angular bits of meat, placed in little pools of the secretion near the margin, were in the course of

* 'English Botany,' by Sir J.E. Smith; with coloured figures by J.

Sowerby; edit. of 1832, tab. 24, 25, 26. [page 382]