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Fig. 10.—Pitchers of Darlingtonia.
The minute structure of the pitchers corresponds in some respects, but differs in others from that of Sarracenia. According to Mr. Gilburt, the inner surface of the pitcher is divided into two zones: the upper one is furnished with short, thick, spike-like trichomes, comparatively wide apart, while the lower one has the same kind and arrangement as exists in Sarracenia. "The glands are the simplest in structure of any found in the group. The epidermal cells of the upper zone of the pitchers have the common sinuous line, but scattered among them are a considerable number of large spherical cells, one portion of their wall being exposed at the surface, and the remainder dipping below the epidermis into the subjacent tissue. These glands are very inconspicuous when the tissue is in its natural condition, but if the colour is discharged from a portion of the plant by means of alcohol they are at once apparent, and their contents are seen to be different from that of the surrounding cells, the chlorophyll corpuscles being absent. These I take to be glands, but what their function may be, if any, is rather difficult to imagine." We have given these details thus minutely because everything connected with the structure of these plants is of interest so long as the mystery is unexplained wherefore they catch insects. The Darlingtonia, as the Sarracenia, gives no indication of possessing the faculty of digestion, and the remarks we have made on the latter will apply equally to the former. Whatever the future verdict may be, whether guilty or not guilty of being carnivorous, they still would claim a place in this work for the singularity of their appearance, the extraordinary form of their flowers, their peculiar trumpet-shaped receptacles, their fly-catching arrangements, and the mystery which enshrouds their domestic economy.
CHAPTER V
CARNIVOROUS PLANTS—PITCHER-PLANTS.
THERE are some plants which have commended themselves to notice either by their singular form, peculiar habit, showy flowers, or beautiful odour. Before carnivorous plants attracted any attention on account of their flesh-devouring proclivities, the Pitcher-plants had acquired notoriety, not on account of their showy flowers or beautiful odours—because these are attractions which they do not possess—but simply on account of their singular form. The pitchers, from whence the name is derived, hang suspended at the ends of the leaves, of which they are simply prolongations and modifications. Most Pitcher-plants consist of a clump of long, narrow green leaves. The extremities of the latter are attenuated down to the midrib, which becomes reduced to a cord, at the end of which hang suspended, one from each of many of the leaves, a curious bag or pouch, not unlike a small and delicate jug or pitcher, with a smaller leaf-like flap hanging over the mouth like a lid. These pitchers usually contain a little fluid, looking like water at the bottom, in which are drowned insects. Such were the Pitcher-plants to our forefathers, and they were regarded simply as "curiosities of vegetation." To us they are something more, now that their history is better known, and for reasons which it shall be our object to explain.
Botanically, the Pitcher-plants proper are known by the name of Nepenthes, an old classical name, the application of which to these plants is somewhat obscure. One writer has attempted an apology for it in the following manner:—"I have often wondered why Linnæus gave to this genus the name of Nepenthes. Every reader of classic story remembers that when Telemachus reached the court of Menelaus, tired and famished, the beautiful Helen gave him nepenthe to drink. No one has ever been able to say what this nepenthe was, though no doubt one of the 'drowsy syrups of the East.' Johnson defines nepenthe as an 'herb that drives away sadness.' Linnæus, perhaps, intended to refer to the tankard-like structure, so like also in the original species to a hot-water jug with its lid. Sometimes I am disposed to think that old Homer may have meant by nepenthe no physical beverage, but the sweet graces of Helen's queenly and consummate hospitality, and welcome, touching, as they did, her guest's inmost feelings of love and reverence. If so, Nepenthes is well applied to its present owner, for assuredly no plant appeals more strongly to our sense of the admirable and the unique."
These tropical plants can only be cultivated in hothouses in this country, and hence there are many persons to whom they are utter strangers. It may be true that all recent horticultural exhibitions have included specimens, but there are thousands of unfortunate individuals who can never visit "flower-shows," although there are but few in the neighbourhood of the metropolis who could not search out the Pitcher-plant in that favourite holiday resort—Kew Gardens. Travellers have described for us the appearance of these plants in their native homes, and especially those who have visited Borneo and the other islands of the Indian archipelago. Amongst others, Mr. Alfred Wallace thus alludes to them. He says:—"We had been told we should find water at Padang-batu, but we looked about for it in vain, as we were exceedingly thirsty. At last we turned to the Pitcher-plants, but the water contained in the pitchers (about half a pint in each) was full of insects, and otherwise uninviting. On tasting it, however, we found it very palatable, though rather warm, and we all quenched our thirst from these natural jugs." And again, when at Borneo, the same traveller writes:—"The wonderful Pitcher-plants, forming the genus Nepenthes of botanists, here reach their greatest development. Every mountain-top abounds with them, running along the ground or climbing over shrubs and stunted trees; their elegant pitchers hanging in every direction. Some of these are long and slender, resembling in form the beautiful Philippine lace-sponge, which has now become so common; others are broad and short; their colours are green, variously tinted, and mottled with red or purple. The finest yet known were obtained on the summit of Kini-balou, in north-west Borneo. One of the broad sort will hold two quarts of water in its pitcher. Another has a narrow pitcher 20 inches long, while the plant itself grows to the length of 20 feet." In 1847, when Lindley published the second edition of his "Vegetable Kingdom," he recorded, with somewhat of doubt, the number of different species as six, whereas, so many have been discovered since, that we may consider them equal to five times that number.
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Fig. 11.—Pitcher of Nepenthes bicalcarata.
There are, says Dr. Hooker, "upwards of thirty species of Nepenthes, natives of the hotter parts of the Asiatic archipelago, from Borneo to Ceylon, with a few outlying species in New Caledonia, in tropical Australia, and in the Seychelles Islands on the African coast. The pitchers are abundantly produced, especially during the younger state of the plants. They present very considerable modifications of form and external structure, and vary greatly in size, from little more than an inch to almost a foot in length; one species indeed, from the mountains of Borneo, has pitchers which, including the lid, measure a foot and a half, and the capacious bowl is large enough to drown a small animal or bird."
"In most species the pitchers are of two forms, one pertaining to the young, the other to the old state of the plant, the transition from one form to the other being gradual. Those of the young state are shorter and more inflated; they have broad fringed longitudinal wings on the outside, which are probably guides to lead insects to the mouth; the lid is smaller and more open, and the whole interior surface is covered with secreting glands. Being formed near the root of the plant, these pitchers often rest on the ground, and in species which do not form leaves near the root they are sometimes suspended from stalks which may be fully a yard long, and which bring them to the ground. In the older state of the plant the pitchers are usually much longer, narrower, and less inflated, trumpet-shaped; the wings also are narrower, less fringed, or almost absent. The lid is larger and slants over the mouth, and only the lower part of the pitcher is covered with secreting glands, the upper part presenting a tissue of different character. The difference of structure in these two forms of pitcher, considered in reference to their different positions on the plant, forces the conclusion on the mind that the one form is intended for ground game, the other for winged game. In all cases the mouth of the pitcher is furnished with a thickened corrugated rim, which serves three purposes: it strengthens the mouth, and keeps it distended; it secretes honey, and it is in various species developed into a funnel-shaped tube, that descends into the pitcher, and prevents the escape of insects, or into a row of incurved hooks, that are in some cases strong enough to retain a small bird, should it, when in search of water or of insects, thrust its body beyond a certain length into the pitcher. In one species (Nepenthes bicalcarata), there are also two strong pointed hooks, or teeth, which are directed downwards towards the mouth of the pitcher. Such appendages would doubtless be of service in preventing the free exit of any large insect after it had once entered the pitcher (see fig. 11).
The attractive surfaces of Nepenthes are two, those namely of the rim of the pitcher, and of the under surface of the lid, which is provided in almost every species with honey-secreting glands, often in great abundance. It is a singular fact that the only species known to the writer of these observations, in which the honey glands on the lid were absent, was a species in which the lid, unlike that of other species, is thrown back horizontally. The secretion of honey on a lid so placed would tend to lure insects away from the pitcher instead of into it.
From the mouth downwards, for a variable distance inside the pitchers, the glassy glaucous surface affords no foothold for insects. The rest is entirely occupied with the secretive surface, which consists of a cellular floor crowded with spherical glands in inconceivable numbers. Each gland resembles the honey-glands of the lid, semicircular, with the mouth downwards, so that the secretive fluid all falls to the bottom of the pitcher. In one species three thousand of these glands were ascertained by Dr. Hooker to occur on a square inch of the inner surface of the pitcher, and upwards of a million in an ordinary-sized pitcher. The glands secrete the fluid which is contained at the bottom of the pitchers previous to their opening, and this fluid is always acid. When the fluid is emptied out of a fully-formed pitcher, that has not received animal matter, it forms again, but in comparatively very small quantities, and the formation goes on for many days, even after the pitcher has been removed from the plant. "I do not find," says Dr. Hooker, "that placing inorganic substances in the fluid causes an increased secretion, but I have twice observed a considerable increase of fluid in pitchers after putting animal matter in the fluid."
A series of experiments performed with the pitchers of these Pitcher-plants, resembled those applied previously to the Sundews and Fly-trap, with similar results. White of egg, raw meat, fibrine, and cartilage were employed for feeding. In all cases the action was most evident, and in some surprising. After twenty-four hours' immersion, the edges of the cubes of white of egg were eaten away, and the surfaces gelatinised. Fragments of meat were rapidly reduced, and pieces of fibrine weighing several grains were dissolved, and had totally disappeared in two or three days. With cartilage the action was most remarkable. Lumps of this, weighing eight and ten grains, were half-gelatinised in twenty-four hours, and in three days the whole mass was greatly diminished, and reduced to a clear, transparent jelly.