"Nothing yet in the history of carnivorous plants come so near to the animal as this. I was forced to the conclusion that these little bladders are in truth like so many stomachs digesting and assimilating animal food.
"I have frequently trapped the snake larvæ and seen them enter the bladders. They seem to be wholly vegetable feeders, and specially to have a liking for the long hairs at the entrance to the bladders. When a larva is feeding near the entrance it is pretty certain to run its head into the net, whence there is no retreat. A large larva is sometimes three or four hours in being swallowed, the process bringing to mind what I have witnessed when a small snake makes a large frog its victim."
The trap-like structure of the bladders, with entrance inwards and no exit; the constant presence of insects in the interior, and quadrifid processes lining the walls, indicating some special function to be performed, would naturally, after the experience of the mode of action in other plants, such as sundews and pitcher-plants, lead to the conclusion not only that insects are caught but also that they are digested. There is, however, no proof that this is the case. The probability even is against it, because there are no secreting glands in the interior of the bladders, and without the outpouring of an acidulated juice we have no experience of a process of digestion taking place. On the other hand, fragments of meat, white of egg, and other substances were inserted in healthy and vigorous bladders, and after two or three days the bladders were cut open and the substances found just in their original condition, without the slightest trace of any digestive process having commenced.
Although there is no proof of digestion there is evidence furnished of absorption by the four-pronged processes. In bladders in which the animal contents were broken up and decayed, the processes exhibited the phenomenon of aggregation, and this we have been led by preceding experience to regard as an evidence of the absorption of soluble animal matter. The same kind of processes in bladders which contained no insects, or in which they were still fresh and unchanged, exhibited no signs of aggregation. As in previous experiments, fluids of a nitrogenous character were applied to the quadrifid processes, and, although without previous sign of aggregation, yet aggregation subsequently took place. The conclusion to be derived from these experiments is, that although the bladders do not digest animal food, yet, after such substances have decayed and become soluble, they are absorbed by the four-pronged processes, causing in them the characteristic aggregation.
There are many bladderworts besides the British species. The Rev. Charles Kingsley recognised them amongst the vegetation of the West Indies. "Our English bladderworts, as everybody knows, float in stagnant water on tangles of hair-like leaves, something like those of the water ranunculus, but furnished with innumerable tiny bladders; and this raft supports the little scape of yellow snapdragon-like flowers. There are in Trinidad and other parts of South America bladderworts of this type, but those which we found to-day growing out of the damp clay were more like in habit to a delicate stalk of flax, or even a bent of grass, upright, leafless, or all but leafless, with heads of small blue or yellow flowers, and carrying, in one species, a few very minute bladders about the roots; in another, none at all. A strange variation from the normal type of the family, yet not so strange after all as that of another variety in the high mountain-woods, which, finding neither ponds to float in nor swamp to root in, has taken to lodging as a parasite among the wet moss on tree-trunks; not so strange, either, as that of yet another, which floats, but in the most unexpected spots—namely, in the water which lodges between the leaf-sheaths of the wild pines perched on the tree-boughs, a parasite on parasites, and sends out long runners as it grows along the bough in search of the next wild pine and its tiny reservoirs."
Similar curious species of Utricularia were also found by Dr. Gardner in Brazil. One of these especially deserves notice (Utricularia nelumbifolia). "Like most of its congeners it is aquatic; but what is most curious is, that it is only to be found growing in the water which collects in the bottom of the leaves of a large Tillandsia that inhabits abundantly an arid, rocky part of the mountain at an elevation of about 5,000 feet above the level of the sea. Besides the ordinary method by seed, it propagates itself by runners which it throws out from the base of the flower-stem. This runner is always found directing itself towards the nearest Tillandsia, when it inserts its point into the water and gives origin to a new plant, which, in its turn, sends out another shoot; in this manner I have seen not less than six plants united. The leaves, which are peltate, measure upwards of three inches across, and the flowering stem, which is upwards of two feet long, bears numerous large purple flowers."
From this description may be gathered the fact that the species of bladderworts are at least of two kinds, if grouped in accordance with their habits. One group would consist of those which float freely in water, and are truly aquatic; the other of those species which, like the Brazilian one, are epiphytal or terrestrial, though loving moist places. A third group might almost be constituted of species which live in the crannies of rocks and bear bladders attached to their root-like underground stems. Yet, whatever their peculiar habit may be, the bladders in such as have been examined, even in the dried state, have been found to contain insects. It matters not, even should the bladders be subterranean, their function in all cases is evidently the same, and clearly not to cause the plant to float freely in water when so generally present, even in terrestrial species.
As long ago as the year 1858, when examining the species of Utricularia systematically, Professor Oliver remarked: "I may be allowed to express my conviction that in the investigation of the development and general morphology of the bladderworts there is a wide field for extended observation." This was followed by an enumeration of no less than twenty-seven species of Indian Utricularia. Subsequently, the same accomplished botanist published notes upon a number of South American species, two of which are figured with their bladders. There are, indeed, a great number of bladderworts known belonging to Utricularia and allied genera, widely distributed over the globe, and of these nothing is absolutely known of by far the greater number, either of the structure or contents of the bladders or the special habits of the plants themselves.
Mr. Darwin has examined, under the most unfavourable conditions, the bladders of a few species which had been dried and preserved in herbaria. In one of these (Utricularia montana) he found thirty-two bladders on one small branch and seventy-three on another, about two inches in length. In some of the bladders of this species he found animal remains. "The first contained a hairy Acarus, so much decayed that nothing was left except its transparent coat; also a yellow chitinous head of some animal with an internal fork, to which the œsophagus was suspended; also the double hook of the tarsus of some animal; also an elongated, greatly decayed animal; and, lastly, a curious flask-shaped organism having the walls formed of rounded cells" (perhaps the shell of a Rhizopod).
In the Brazilian species, above alluded to by Gardner, he found within one bladder the remains of the abdomen of some larva or crustacean of large size. In a Malayan species (Utricularia griffithii), in one bladder there was a minute aquatic larva; and, in another, the remains of some articulate animal. In the bladders of an Indian species (Utricularia cærulea) were the remnants of Entomostraca; and the bladders of another Indian species (Utricularia orbiculata) contained similar remains. In like manner, in other species from different parts of the world, the bladders enclosed the remains of minute animals. It is a fair inference, therefore, that these bladders, wherever found, are traps to catch unwary insects and minute animals; and as an examination of internal structure shows the presence of similar curious processes to those of the British species having the power of absorption, it may be concluded that decaying animal matter in the bladders is absorbed, and is one natural source of sustenance to the plants.
Very recently Dr. Maxwell Masters has made known the result of some investigations on the Christmas Rose (Helleborus niger), a common garden flower, in January, in which he thinks he has discovered an indication, if not of persistent fly-catching, at least of a capacity to assimilate animal food. His remarks are certainly of interest in connexion with the present subject.
"The true petals, formerly called nectaries, of the Hellebore are those peculiar green horns or tubes met with in one or two rows surrounding the stamens, and which secrete a honeyed juice. We suppose that the main object of this secretion is to serve as an attraction to insects to visit the flowers, and so transfer the pollen from one flower to another. We infer this from the fact that the maturity of the anthers and of the stigma is not simultaneous in the same flower, and hence transfer of the pollen to another flower, whose stigma is mature, is a necessity. If this be correct, it would, of course, be of no advantage to the plant to immolate its insect visitor, as what it would gain in one way it would lose in another. Indeed, we have not found any dead bodies of insects in the tubes of the Hellebore, such as one may find so frequently in the pitchers of Nepenthes or Sarracenia. Hence, then, as a rule it would be of no advantage to the plant to indulge in animal food. But it does not necessarily follow that the plant in question has therefore no such power, or that it does not exert it on occasion.
In the case of the Hellebore, the tubular petals were filled with very finely-chopped cooked meat, leaving some of the tubes unfilled for contrast-sake. The microscopic appearance of the normal petals was noticed, and the reaction of the juice with litmus paper tried.
In the normal tube there are certain cells filled with yellow juice, which is diffused throughout the whole interior of the cell. But after the insertion of the meat, and its retention for some days, the yellow-cell contents were found to be compacted together into a globose ball; at least, in the case of those cells nearest to the meat, those at a distance showed the contents diffused. Moreover, the fibre of the meat may be seen reduced to a pulp, and under the microscope its constituent fibres may be seen disintegrated, and the peculiar striations characteristic of striped muscular fibre even more conspicuous than ordinary. Granular matter and oil globules exist in abundance, and give evidence of partial solution. Some of the same meat kept moist under a bell-glass, side by side with the flowers that formed the subject of experiment, showed little or no trace of disorganisation or putrefaction.
The action of litmus paper is rather puzzling; at first, the juice of the tubes was neutral, or only faintly acid, but after the meat had been allowed to remain for some days, an alkaline reaction was evidenced by the appearance of a blue tinge on previously reddened litmus. For the present, then, we state merely that the muscular fibre was partially dissolved, and that certain changes in the appearance of the cell-contents took place. More than this it would be rash to affirm.