In the course of his experiments Mr. Darwin found that pollen from flowers, the leaves of other plants, and different small seeds, when they came in contact with the glands caused considerable acid secretion. He considers that albuminous matter would be dissolved out of them and absorbed by the glands. Hence, he says, "we may conclude that the Pinguicula, with its small roots, is not only supported to a large extent by the extraordinary number of insects which it habitually captures, but likewise draws some nourishment from the pollen, leaves, and seeds of other plants which often adhere to its leaves. It is therefore partly a vegetable as well as animal feeder."
Two other species, or reputed species, of Pinguicula which are indigenous to these islands were also examined with similar results. As for Pinguicula lusitanica, the chief difference appeared to be that in this plant the margins when excited were more strongly inflexed, and the inflection lasted for a longer period of time. The glands also apparently were more stimulated to increased secretion when in contact with bodies not yielding soluble nitrogenous matter, than was the case with the common butterwort. In other respects they all agreed, and all alike merit the appellation of insectivorous plants.
Apropos of this we may cite some remarks by one who did not believe or accept the carnivorous theory, the late Andrew Murray, who has given evidence so much the more valuable from his scepticism on the habits of Pinguicula. "I have been staying," he writes, "in Kinross-shire, where I had an abundance of material to observe, and a fair proportion of both dry and wet weather, so as to see the behaviour of the plant under both conditions. The first thing of which I convinced myself was that, whether it was carnivorous or not, Pinguicula was rightly regarded by Mr. Darwin as coming under the same category as Dionæa and Drosera: it was a fly-catcher and a fly-dissolver—whether it was a fly-digester is a different thing; but neither on that point any more than on the other can it be separated from them. If the one digests the other will no doubt do so likewise. If the one does not neither will the other." Thus far, then, there is the decided opinion of a very careful observer, that the phenomena exhibited by the Pinguicula are in entire harmony with those of the fly-traps, and that the conclusions arrived at with regard to the latter will also be applicable to the former. "When the insect alights, or is blown on to the leaf," he says, "it gets entangled in the sticky secretion, and it is killed, and speedily killed (long before the curving of the margin of the leaf could have any effect upon it), by the secretion adhering to and closing up the spiracles by which the insect breathes, just on the same principle that a drenching with oil is used in our hospitals, &c., to kill the vermin with which dirty patients or inmates may be swarming on their admission." He proceeds to argue that there is no irritability capable of being exerted by the plant to the injury of the insect, but this injury is confined to the secretion.
Although one could hardly doubt the power of the butterworts not only to catch, but also to digest and assimilate insects, such attributes cannot be claimed for the next group of plants to be noticed. It may be perfectly true that by means of a most elaborate contrivance they are enabled to capture living prey, yet there is no evidence forthcoming that they are able to digest the animals after having caught them. Wherefore, then, it may be asked, an elaborate trap, admirably adapted for the capture of minute organisms, such as are commonly found enclosed within them, if it is all of no use, and when the dainty morsels are captured they cannot be eaten? This question may partly be answered by a brief summary of the results of such investigations as have already been made by Professor Cohn and Mr. Darwin, and partly reserved as still requiring elucidation.
The bladderworts contain several species, found in different parts of the world, but the principal examinations have been made on two British species (Utricularia neglecta and Utricularia vulgaris), fig. 19. These are aquatic, commonly found in dirty ditches, or, as has been said, "remarkably foul ditches." They float freely, not being attached by roots at any period of their existence. The leaves are deeply divided into narrow filaments, each terminating in a short straight point, like a bristle. Small inflated vesicles, or bladders, seated on the leaves, originated the name of bladderwort. It is sometimes stated that these bladders are filled with air, and serve to buoy up the plant in the water. That such an assertion is erroneous may be inferred from the fact that although they often enclose a little bubble of air, they are usually filled with water; that branches float equally well without them; and that their elaborate construction indicates a much more complex function, whatever that function may be.
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Fig. 19.—Bladderwort (Utricularia vulgaris).
The chief point of interest is offered by these bladders, the minute structure of which is exhaustively treated in Mr. Darwin's work, but our purpose may be served by a meagre outline. When full grown the bladders are nearly one-tenth of an inch in length, nearly egg-shaped (fig. 20) with the smaller end upwards, and attached obliquely towards the base. The upper, or smaller, end is furnished with six or seven projecting bristles, not unlike antennæ, and, in fact, the bladder resembles some fixed aquatic insect, or some such crustacean as a water-flea anchored upon the leaf. The entrance to the bladder is at the apex, which is closed by a valve opening inwards. The surface of the valve is furnished with numerous glands, and four oblique bristles. The whole inner surface of the bladder is covered with projections, somewhat like stellate hairs, with four arms, two longer directed obliquely backwards, and two shorter ones directed forwards. Each arm encloses generally a minute brown particle in constant motion.
<!-- image -->Fig. 20.—Bladder of Utricularia vulgaris, enlarged.
That the use of these bladders is to capture insects may be inferred from the fact of their constant presence when the conditions are favourable. In seventeen bladders (of Utricularia neglecta) containing prey of some kind, eight of them contained entomostraca, those lively little crustaceans so common in stagnant water, three enclosed insects, and six the remains of decayed animals past identification. In five bladders, which appeared full, from four to ten crustaceans were found in each. Professor Cohn placed a plant of Utricularia in a vessel of water swarming with crustaceans, and in the morning some of these were observed entrapped in the bladders, in which prison they remained alive for several days.
The entrance to the bladders is effected by bending the valve inwards, which from its elasticity instantly rebounds and cuts off all chance of escape. Occasionally creatures are found fixed in the opening, held by the pressure of the valve, and unable to extricate themselves. No evidence could be obtained that the valves are at all sensitive, as they would not respond to pricking, scratching, or brushing. All that can be affirmed is, that aquatic insects, and like animals enter the bladders by forcing down the valve, and then passing through the slit, which closes after them, and prevents any return. Observations on this process were made by Mrs. Mary Treat, of New Jersey, as witnessed in an American species of Utricularia (Utricularia clandestina). She says: "The entrance into the bladder has the appearance of a tunnel net, always open at the large end but closed at the other extremity. The little animals seemed to be attracted into this inviting retreat. They would sometimes dally about the open entrance for a short time, but would sooner or later venture in, and easily open or push apart the closed entrance at the other extremity. As soon as the animal was fairly in, the forced entrance closed, making it a secure prisoner. I was very much amused in watching a water-bear (Tardigrada) entrapped. It went slowly walking round the bladder, as if reconnoitring, very much like its larger namesake; finally, it ventured in at the entrance, and easily opened the inner door, and walked in. The bladder was transparent and quite empty, so that I could see the movements of the little animal very distinctly, and it seemed to look around as if surprised to find itself in so elegant a chamber; but it was soon quiet, and on the morning following it was entirely motionless, with its little feet and claws standing out as if stiff and rigid. The wicked plant had killed it very much quicker than it kills the snake-like larva. Entomostraca, too, were often captured, Daphnia, Cyclops, and Cypris. These little animals are just visible to the naked eye, but under the microscope are beautiful and interesting objects. The lively little Cypris is encased in a bivalve shell, which it opens at pleasure, and thrusts out its feet, and two pairs of antennæ, with tufts of feather-like filaments. This little animal was quite wary, but, nevertheless, was often caught. Coming to the entrance of a bladder it would sometimes pause a moment and then dash away; at other times it would come close up, and even venture part of the way into the entrance, and back out as if afraid. Another, more heedless, would open the door and walk in, but it was no sooner in than it manifested alarm, drew in its feet and antennæ, and closed its shell. But after its death the shell unclosed again, displaying its feet and antennæ. I never saw even the smallest animalcule escape after it was once fairly inside the bladder.
"So these points were settled to my satisfaction—that the animals were entrapped, and killed, and slowly macerated. But how was I to know that these animals were made subservient to the plant? If I could only prove that the contents of the bladders were carried directly into the circulation, my point was gained. This now was my sole work for several days to examine closely the contents of the bladders. I found the fluid contents to vary considerably, from a dark muddy to a very light transparent colour. Hundreds of these bladders, one after another, were put to the test under the microscope, and I found that, to a greater or less extent, I could trace the same colour that I had found in the bladder into the stem on which the bladder grew, though the observation was not so clear and satisfactory as I could wish. After more critical examination I arrived at the conclusion that the cells themselves, and not their contents, change to a red colour; the stems also take on this colour, so as to make it appear as if a red fluid was carried from the bladders into the main stem, which is not specifically the fact, as far as the observations yet made determine, though the main point, that the contents of the bladders are carried into the circulation, does not seem open to question.
"The next step was to see how many of the bladders contained animals, and I found almost every one that was well developed contained one or more, or their remains, in various stages of digestion. The snake-like larva was the largest, and most constant animal found. On some of the stems that I examined fully nine out of every ten of the bladders contained this larva or its remains. When first caught it was fierce, thrusting out its horns and feet, and drawing them back, but otherwise it seemed partly paralysed, moving its body but very little; even small larvæ of this species, that had plenty of room to swim about, were very soon quiet, although they showed signs of life from 24 to 36 hours after they were imprisoned. In about 12 hours, as nearly as I could make out, they lost the power of drawing their feet back, and could only move the brush-like appendages. There was some variation with different bladders as to the time when maceration or digestion began to take place, but usually, on a growing spray, in less than two days after a large larva was captured the fluid contents of the bladders began to assume a cloudy or muddy appearance, and often became so dense that the outline of the animal was lost to view.