Quite a number of plants, belonging to different orders, are provided with the means of capturing small animals, and of digesting their prey and absorbing the nutrient matter thus obtained into their own systems. In this way they are enabled to obtain nitrogenous material which, in the ordinary way, is absorbed in the form of mineral solutions, from the soil, by the agency of the roots. The greater number of these carnivorous plants are to be found in tropical lands; but a few are British, and are of such an interesting nature that we propose to devote a short chapter to a description of their peculiar structure and habits.

The plants to which we refer are often spoken of as insectivorous species; but although in nearly all cases the animal food consists almost entirely of insects, it is not entirely derived from this one group of animal life, and therefore the term carnivorous is rather more appropriate.

In pools we sometimes meet with floating plants that have no true roots, at least at the time of flowering, but consist of a tuft of long, rootlike, submerged branches, bearing much-divided leaves, and sending leafless stalks of yellow flowers above the surface of the water. These plants are the Bladderworts (Utricularia), of the order LentibulaceÆ, and are so called because they have little air-bladders either attached to the leaves or supported on leafless branches.

The leaves are divided into numerous very narrow segments, thus presenting a proportionately large amount of surface to the water for the absorption of dissolved gases required by the plant; and the flowers consist of a deeply two-lobed calyx; a spurred corolla, with its mouth closed or nearly closed by means of a convex 'palate'; two stamens; and a one-celled ovary that ripens into a globular fruit.

As to the little air-bladders mentioned above, they form, perhaps, the most interesting feature of the plant, for they are the traps by means of which small aquatic creatures are caught, and also the organs concerned in the absorption of nutritive products derived from the prey. Each bladder has an opening, guarded by a kind of valve which allows easy ingress, but no exit. It does not seem to produce any secretion which would hasten the death of the creatures entrapped, nor does it appear to produce any kind of digestive fluid, as is the case with other carnivorous plants; but small aquatic creatures, such as water-fleas, cyclops, very small larvÆ, &c., entering the bladders for shelter or some other purpose, are securely imprisoned until they die of starvation or suffocation; and their bodies then decay, giving rise to soluble gases and other products which are absorbed into the plant by special cells within the bladder.

There are three British species of these plants—the Greater, the Lesser, and the Intermediate Bladder-worts. The first of these—Utricularia vulgaris—is rather local in its distribution, and is easily distinguished from the other two by its superior size, having floating branches from a few inches to a foot in length. The second (U. minor) is much more common. Its floating branches are only two or three inches long at the time of flowering, but they grow longer after; and the flowers are pale yellow, with a short, broad spur. The third (U. intermedia), which is very local, has also pale yellow flowers, but with a much longer spur; and the bladders are at the ends of leafless branches.

In the preceding chapter we gave an account of certain plants which are parasitic on other plants and trees, deriving more or less of their nutriment from their vegetable hosts. One of these—the Tooth-wort (LathrÆa squamaria), of the order OrobanchaceÆ—is not only a parasite, deriving nourishment from the roots of trees, but is also a carnivorous species, feeding on minute animals which are captured and digested by its peculiar leaves; and therefore it may be conveniently considered here.

The whole plant is of a fleshy character, and lives entirely underground, attached to the roots of the Hazel, Elm, or other tree, except during April and May, when it sends up thick flowering stems, from four to ten inches high, bearing a few broad, fleshy scales which gradually pass into bracts, and a one-sided spike or raceme of flowers. The stem and scales above ground are of a pale rose colour, and the flowers are either brown, flesh-colour or slightly bluish. The latter are numerous, closely placed, and either sessile or shortly stalked. The calyx is bell-shaped, nearly half an inch long, with four broad lobes; and the corolla, which is about half as long again as the calyx, is distinctly lipped.

The whole plant is devoid of chlorophyll, and consequently has not the power of building up organic compounds after the manner of green plants; and, being parasitic on the roots of trees, it derives but little organic material from its host. To compensate for this the underground portion is so constructed that it can capture minute animals which exist in the soil, and has the power of digesting them and of absorbing the products of digestion.

The underground stems are quite white, and are thickly covered with broad, cordate, fleshy leaves that closely overlap one another. There appears to be nothing very remarkable in these underground leaves until one has been removed from the stem and closely examined; and then we find that what appears to be the apex of the leaf is really its middle; and that what seems to be, at first sight, the under surface, is really an extension of the upper side; for the leaf is bent backwards in such a manner as to bring its apex close to the stem, immediately below its base. This peculiar folding of the leaf results in the formation of an irregular cavity, and the tip of the leaf, brought close to its base, is curled upward, close to the stem, in such a way as to form a little canal, with several small openings by which the cavity may be reached. It will not be easy to make out this strange folding of the leaf by an examination of the exterior only, but a longitudinal section, made with a sharp knife or razor, will show it clearly.

When minute animals enter the cavity of the leaf through the little openings above mentioned, they are seized by means of small filaments that protrude from the lining cells; and although no special digestive secretion has been discovered in the leaves, it appears certain that the creatures entrapped are really dissolved, for nothing remains of them after a time except the harder, indigestible portions. Also, there is every reason to believe that the products of digestion are absorbed, probably by the same filaments that are concerned in the capture of the microscopic prey.

Perhaps the most interesting of the carnivorous plants are those which exhibit distinct movements in connexion with the capture of their prey, and among these are the British Butterworts and Sundews, which grow in bogs and other wet places.

There are three British species of Butterwort (Pinguicula), similar in structure and habit, all growing in bogs and on wet rocks. They have each a rosette of entire, radical leaves, the lowest of which lie close against the soil or rock on which the plant grows; and violet or yellow flowers on leafless peduncles. The calyx has four or five teeth, arranged in two lips; and the corolla, which is also lipped, has a broad, open throat, and a spur.

The commonest species is the Common Butterwort (P. vulgaris), which is found in bogs and wet places, principally in the hilly, humid districts of the West of Britain and Ireland, flowering from May to July. Its leaves are succulent and clammy, of a pale green colour, and covered all over with little glistening spots. The flower stems are three or four inches high, each bearing a single violet flower. In this species the throat of the corolla is bell-shaped, and the spur is as long as the rest of the corolla.

A second species—the Alpine Butterwort (P. alpina)—with smaller, pale yellow flowers appearing in June and July, is found only in Scotland; while a third, known as the Pale Butterwort (P. lusitanica), also with pale yellow flowers, and a curved spur, occurs in South-West England as well as in the boggy districts of Ireland and the West of Scotland, flowering from June to October.

The carnivorous habits of all species are the same. The horizontal leaves lie flat on the wet soil, with their margins turned upward forming a kind of shallow trough; and the upper surface of each is dotted with many hundreds of minute glands which secrete a colourless, sticky fluid, thus giving to the leaf its glistening and clammy appearance.

If any mineral or other non-nutritious substance be placed on a leaf, the contact stimulates the little glands, causing them to discharge a larger quantity of fluid, but no change seems to take place in the character or composition of the secretion. But if any nitrogenous organic substance, such as an insect or a small piece of meat, be brought in contact with the glands, not only will the secretion increase in quantity, but it will also assume an acid character, and contain a ferment which is capable of digesting the nitrogenous material. In fact, the secretion produced under these circumstances possesses the same properties as the gastric fluid of the stomachs of animals.

The animal food of the Butterworts consists of small insects and other little creatures. If an insect alights on the leaf, it is caught by the sticky secretion of the glands, and every effort to escape causes it to become more and more besmeared with the mucilage, till, at last, it is no longer able to move; and its death is probably hastened by the stoppage of its spiracles or breathing-holes.

If the insect is a small one, and it settles near the edge of the leaf, the curved margin slowly bends over it until it is more or less enclosed, and the larger number of glands thus brought in contact with its body pour out their digestive secretion, which slowly dissolves the nourishing portions, leaving nothing but the legs, wings, and other indigestible parts. A larger insect, alighting similarly near the edge of the leaf, could hardly be enclosed by the bending of the margin near it; but it is pushed towards the middle as the edge curls over, and then the opposite side also bends over it, till the insect is more or less enclosed, when it is digested as mentioned above.

The digestion of an insect and the absorption of nutrient matter by the cells of the leaf occupy from twenty to thirty hours, and when the whole is accomplished the leaf slowly expands, assuming its normal position, and exposing the indigestible residue of its prey to be blown away or washed off by the rain.

It has been observed that the Butterworts are not exclusively animal feeders, for their leaves readily digest any pollen cells or the spores of the lower plants that are carried to them by the wind.

Equally interesting are the habits of the Sundew (Drosera), of which there are three species, all readily distinguished from every other British plant by the glandular hairs that cover the long-stalked, radical leaves. They have leafless flower-stalks, each bearing a one-sided spike or raceme of white flowers. The sepals, petals, and stamens each number five; and the ovary, which ripens into a one-celled capsule of three or four valves, has three or four forked styles.

The commonest species—the Round-leaved Sundew (Drosera rotundifolia)—is abundant and widely distributed, and may be seen among the bog-mosses, sometimes almost completely covering rather large patches of marshland. Its leaves are round, from a quarter of an inch to near half an inch in diameter, spreading in such a manner that they lie close to or near the ground. The flower-stems are slender, erect, from three to six inches long; and the white flowers, which are in a one-sided raceme, bloom during July and August.

The Long-leaved Sundew (D. longifolia or D. intermedia) has oval leaves, tapering gradually into the stalk. They are more erect than the leaves of the last species, and are not half so broad as they are long. The plant flowers at the same time as the latter, but is not nearly so common.

The third species—the Great English Sundew (D. anglica)—is still rarer. Its leaves are still longer and narrower, being sometimes an inch or more in length, and more erect; and the flower-stalk sometimes attains a length of eight inches.

The carnivorous habits of these plants are very similar to those of the Butterworts, but the movements connected with the capture of the prey are more marked in the red filaments which cover the upper surface of the leaves than in the leaves themselves. Those filaments which are situated on the margin of the leaf are longest, and spread outwards, while the others are erect and decrease in length from the edge towards the middle.

Each filament is swollen at its extremity, and supports an enticing globule of glistening fluid which it secretes, for the enlarged extremity is really a minute gland. The fluid, though quite clear, is so viscid that it can be drawn out into threads, and it serves a purpose similar to that of the sticky globules on the spiral thread of a spider's web.

If some grains of sand or other inorganic material be sprinkled on the leaf, the sticky secretion of the glands is appreciably increased, and at the same time assumes an acid character; but it contains no digestive ferment, nor do the filaments change their position to any considerable extent. When, however, a small insect alights on the leaf, attracted by the glistening drops which are probably mistaken for nectar, the secretion not only increases and becomes acid, but a digestive ferment is produced, and the little creature is soon besmeared with the fluid, its condition becoming more and more hopeless through its struggles, till at last further movements are impossible and it dies of suffocation.

A few minutes later the filaments of the leaf immediately around the insect begin to bend towards it, and others a little farther off soon partake in the movement, which may finally extend more or less to all the filaments of the leaf, and thus a large number of glands are brought in contact with the prey. The process of digestion now goes on, and, in a day or two, all the digestible portions of the insect are dissolved and absorbed, and the filaments that were concerned in the work have resumed their original position, leaving the indigestible portions to dry and to be eventually blown away.

The principal food of the Sundews consists of small insects such as ants, midges, flies, small butterflies and moths, caddis-flies, and even small species of dragon-flies. Some of these, more particularly the long-bodied dragon-flies, the smallest of which are over an inch in length, are much too large to be caught and devoured by a single leaf; and in this case it is not at all uncommon for two or more leaves to be concerned in the capture and digestion of a single insect, each one converging its filaments towards the part of the body within its reach, and each one digesting and absorbing the portion against which it can apply its glands.

Insects, however, do not constitute the sole food of these plants, for small worms, spiders, centipedes, &c., are caught and digested in the manner described; and the plants may also be fed artificially on small pieces of meat or other nitrogenous substances, which give rise to the same processes and movements as we have observed in connection with the natural mode of feeding.