A little aquatic plant, called Aldrovanda vesiculosa is found in Europe, Australia, and India. Although inhabiting countries so remote from each other, this plant seems to be of one species in all. It has no roots, and floats like green stars in the water. The leaves are arranged in whorls in a stellate manner round the stem. Each leaf has two semicircular lobes, which are seated on broad foot-stalks. The lobes are generally found closed at the ordinary temperature in Europe, but they do separate, under favourable conditions, to about the same proportionate extent as a living mussel opens the valves of its shell.
The history and mystery of this little water-plant are very imperfectly known. Stein observed that water insects were sometimes caught by it. Professor Cohn has found crustaceans and larvæ within the leaves. Plants placed in water containing entomostraca were examined next morning, and found to enclose individuals of these minute crustaceans still alive. In one of the closed leaves of the Australian variety from Queensland a rather large beetle was found, with all the softer parts of the body dissolved.
The leaves evidently are well adapted for catching living creatures. There are long sensitive hairs which are probably sensitive. There are glands which, from analogy, may secrete a limpid fluid. Altogether, however, although kinship and analogy might point to this as another of the carnivorous plants of the Sundew family, a supposition which is supported by a sort of circumstantial evidence, still, so little is definitely known, that it is better to suspend the judgment than reach at too hasty a conclusion.
<!-- image -->Fig. 5.—Leaf of Aldrovanda, much enlarged.—COHN.
The Portuguese Fly-catcher is the name by which we may distinguish that rare little plant Drosophyllum lusitanicum, which hitherto has only been found in Portugal and Morocco. It is plentiful in the neighbourhood of Oporto, where the villagers call it the "fly-catcher," and hang it in their cottages for that purpose. The leaves are like slender filaments, of several inches in length, with the upper surface concave and channelled down the middle, and the under surface convex. Both surfaces are covered with tentacles of a pink or purplish colour, supported on peduncles of variable lengths, with a cap-like convex head. These tentacles secrete large drops of a viscid secretion (fig. 6).
Besides these tentacles are a number of very minute sessile glands, scarcely visible to the naked eye, colourless, but similar in structure to the tentacles; but with this difference in function, that they do not secrete spontaneously, but must be excited to do so. Both glands and tentacles speedily absorb nitrogenous matter. When an insect alights on a leaf of this fly-catcher, the drops of secretion, with which the tentacles are studded, at once, and readily, adhere to it; and as it moves other drops accumulate, until, at length, bathed with the viscid secretion, it becomes powerless, sinks down and dies, on the small sessile glands with which the leaves are covered. The tentacles have no power of motion, and are not consequently sensitive to the touch. The fly-catching operation is performed by the secretion alone. That the tentacles are capable of absorption is shown by the aggregation of the protoplasm after contact with nitrogenous substances. When the insect falls exhausted and dead, smothered with the viscid secretion of the tentacles, upon the small sessile glands, the contact stimulates the latter to secretion, and it is by their action that the prey is dissolved and assimilated. The process by which the insects are captured differs therefore from that of the Sundews; but after the insect is caught, and deposited upon the small sessile glands, the process of disintegration, and digestion, is evidently the same in all essential particulars.
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Fig. 6.—Glands on leaf of Drosophyllum, in the centre the sessile gland, on each side a tentacle.
An allied plant, at the Cape of Good Hope (Roridula dentata) probably acts in a similar manner, but no living specimens have been examined. The leaves are studded with glands, which secrete viscid matter, to which insects and other bodies adhere.
The same may be said of an Australian plant, belonging to another genus (Lyblis gigantea). These can only be named provisionally, as individuals concerning whom further information is desired.
The Sundew family (Droseraceæ) includes the six genera to which we have drawn attention, and of those the true Sundews (Drosera) and Venus's Fly-trap (Dionæa) are the most important. Of the true Sundews there are no less than one hundred species, "which range in the Old World from the Arctic regions to Southern India, to the Cape of Good Hope, Madagascar, and Australia; and in the New World from Canada to Terra del Fuego." There is every reason to suppose that the same habits, and carnivorous propensities, are common to all, and that, in all this wide range, these humble little bog plants are ever exposing their glittering tentacles to the sun, and luring myriads of insects to their destruction.
Bright and glorious is that revelation, Written all over this great world of ours; Making evident our own creation, In these stars of earth—these golden flowers.
CHAPTER IV
CARNIVOROUS PLANTS—SIDE-SADDLE FLOWERS.
THE Pitcher-plants, properly so called, are natives of the Old World, their representatives in the New World are called Side-saddle flowers, or Sarracenias. In the true Pitcher-plants the curious pitchers are suspended at the ends of the leaves, of which they are prolongations, but in the Sarracenias the entire leaf is folded and modified into a kind of pitcher. The eight North American species are found in the eastern States, in bogs, and in places covered with shallow water. Their leaves, which give them a character entirely their own, are pitcher-shaped, or rather they are trumpet-shaped, standing erect, collected in tufts, and springing immediately from the ground. They send up at the flowering season one or more slender stems, each of which bears a single flower, which is itself of a peculiar appearance and character, with a fancied resemblance to a side-saddle, and hence the popular name. It has been shown that there are at least two different kinds, or types, of pitcher in this group of plants. In one kind the mouth is open and the lid stands erect, so that the tube receives the rain-water in more or less abundance. In the other kind the mouth of the tube is closed with a lid, and into these the rain can hardly, if ever, find ingress.
As long ago as the year 1815 the fly-catching propensity of these plants was observed and commented upon, in a communication to the President of the Linnæan Society. Many of the assertions then made have since been verified; although at the time they excited but little notice, and perhaps did not receive implicit credence. "If," says the writer, "in the months of May, June, or July, when the leaves of these plants perform their extraordinary functions in the greatest perfection, some of them should be removed to a house and fixed in an erect position, it will soon be perceived that flies are attracted by them. These insects immediately approach the fauces of the leaves, and leaning over their edges appear to sip with eagerness something from their internal surface. In this position they linger, but, at length allured, as it would seem by the pleasures of taste, they enter the tubes. The fly which has thus changed its situation will be seen to stand unsteadily, it totters for a few seconds, slips and falls to the bottom of the tube, where it is either drowned, or attempts in vain to ascend against the points of the hairs. The fly seldom takes wing in its fall and escapes. In a house much infested with flies this entrapment goes on so rapidly that a tube is filled within a few hours, and it becomes necessary to add water, the natural quantity being insufficient to drown the imprisoned insects. The leaves of other species might well be employed as fly-catchers, indeed, I am credibly informed that they are in some neighbourhoods. The leaves of Sarracenia flava, although they are very capacious, and often grow to a height of three feet or more, are never found to contain so many insects as those of other species. The cause which attracts flies is evidently a sweet viscid substance resembling honey, secreted by, or exuding from, the internal surface of the tube. From the margin, where it commences, it does not extend lower than one fourth of an inch. The falling of the insect as soon as it enters the tube is wholly attributable to the downward or inverted position of the hairs of the internal surface of the leaf. At the bottom of a tube, split open, the hairs are plainly discernible pointing downwards; as the eye ranges upward they gradually become shorter and attenuated, till at, or just below the surface, covered by the bait, they are no longer perceptible to the naked eye, nor to the most delicate touch. It is here that the fly cannot take a hold sufficiently strong to support itself, but falls. The inability of insects to crawl up against the points of the hairs I have often tested in the most satisfactory manner." 1
The annexed figure represents the pitchers of the species to which these observations refer (fig. 7). It is also that on which many subsequent and confirmatory experiments were made.
The tissues of the internal, or lining, surfaces of the pitchers in Sarracenia are not identical in all the species. In some, and probably most, there are four kinds of surfaces, proceeding from the mouth downwards to the bottom of the tube. First, there is an attractive surface, often brightly coloured, which occupies the inner face of the lid, and this, in common with the mouth of the pitcher, is covered with minute honey-secreting glands. Then, secondly, there is a conducting surface of glassy cells, which are elongated into conical processes overlapping each other, like the tiles of a house, so as to afford no foothold for an insect attempting to crawl up again. This is succeeded by a large granular surface, which is smooth and polished so as to afford no foothold. And, finally, there is a detentive surface, which occupies the lower part of the pitcher. It is studded with deflexed rigid hairs, which converge towards the axis of the cavity; so that an insect, if once amongst them, is effectually detained, and its struggles have no other result than to wedge it lower and more firmly in the pitcher.
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Fig. 7.—Pitchers of Sarracenia variolaris, reduced.
A similar structure in Sarracenia purpurea (fig. 8) is thus described by Mr. W. H. Gilburt, in his memoir on "The Histology of Pitcher-Plants." He says:—"The interior surface of this pitcher is divided into four zones. On the first one, or that nearest the mouth of the pitcher, are numerous stomata, and also a large number of strongly developed rigid hairs, which point downward. The second zone is characterised by the fact that each cell of the surface is prolonged downward into a short mammillary process, its wall being striated longitudinally. We next come to a division which is smooth, hairs are entirely absent, and the cells are sinuous in outline. The fourth division is by far the longest, and is crowded with long hairs, the points of which are all directed towards the base, but they are not so stout or strong as those found near the mouth of the pitcher." In explanation the rigid hairs of the upper zone are shown to agree in all respects with an ordinary trichome, being simply the outgrowth of a single cell. These hairs (fig. 9) on their external surface show a few deeply-cut longitudinal striations, in fact, so well marked are they that the hair might almost be described as fluted. Yet are they in error who have described them as made up of a bundle of rod-like cells. Again, he says, "All these modifications of surface are, without doubt, of value to the plant, and in this direction, that while they will allow an insect to enter, and pass down the tube, it is almost impossible for it to return. Thus they become veritable insect-traps. The pitchers of many species contain fluid, but nothing corresponding to a digestive fluid has been detected in them."