Popular Science Monthly/Volume 65/September 1904/Some Plants Which Entrap Insects
|SOME PLANTS WHICH ENTRAP INSECTS.|
By FORREST SHREVE,
THE JOHNS HOPKINS UNIVERSITY.
WE seldom give a thought to the fact that plants need food. They had such unite, motionless lives that it is difficult to believe that they have, like ourselves, a bread-and-butter problem staring them constantly in the face. We watch the geraniums and begonias of our window-garden grow and bloom with nothing to nourish them but a few handfuls of earth and a little water. Surely their food problem must be a simple one if the substances necessary to the formation of stem and leaves and blossom can all be got from so little earth. Wherein lies all the difficulty about poor soil which besets the farmer, and why must he buy tons of plant food and drill it carefully into the ground in order to get a remunerative crop of grain? The whole trouble for the farmer arises from the scarcity in the soil of two or three food elements which, although highly important, form I tut a small part of the total weight of growing plants. The foremost of these scarcer foods, nitrogen, has been a source of difficulty not only to man, but to a large number of plants as well, which have been forced to adopt a means of getting it which is radically different from all other methods of plant nutrition, so much so indeed that it was long looked upon as a mere meaningless 'freak of nature.' This method is the catching of insects.
Every one has heard of the pitcher-plant or perhaps seen its urnlike leaves half filled with water. Innocent looking as are these leaf-pitchers, and casual as may seem the presence of three or four drowned flies in the water which they contain, yet in truth each pitcher is a veritable trap, clever in design and effective in its purpose of alluring and drowning insects. Different in look, but to the same purpose, are the leaves of the sun-dew. Its bristling hairs bear beads of jelly, not for the mere splendor of their dazzling brilliance, but to allure, catch and hold fast gnats and mosquitoes, all to the same end as in the pitcher-plant,—supplying a shortage of nitrogen in the food. Not these two plants alone, but a large group of nearly four hundred species, are insect trappers, carnivorous or, as they are more commonly called, insectivorous plants. The varied devices which these plants possess for alluring insect prey, catching, holding and utilizing it furnish matter for one of the most interesting chapters in all botanical science. Perhaps no one of the insectivorous plants possesses what may be more truly called a trap than does the bladderwort (Utricularia). This is a floating aquatic plant without roots, confined to pools and quiet streams where it is in no danger of being washed away. Borne thickly upon the fine leaves, and like them entirely submerged in the
water, are the traps, minute hollow globular structures bristling with hairs at one end. Buried among the hairs is the entrance to the trap. Swimming about in search of food or in an attempt to escape from enemies, some minute crustacean or insect larva will push in among the hairs. Spying the entrance, it will dart forward and striking the almost transparent door it will unwittingly pass into the trap. But the door has instantly sprung shut again and vain will be all the efforts of the prisoner to make an escape. Starvation soon ends its struggles. death is followed by decomposition, and in the absorption of the products of this the plant accomplishes the end for which it possesses the traps—it gets its needed nitrogen.
In the pitcher-plants the urns of water are set and all else is left to the curiosity of the insects, the inner structure of the leaf being such—as we shall soon see—that a secure trap-door is not needed. The half dozen species of pitcher-plant (Sarracenia) exhibit considerable variety in the form of their leaves. In the purple species, the only one growing north of Virginia, the leaves form a rosette-like cluster procumbent on the mud or moss and bent upward so as to give the mouth a horizontal position. The stalk is exceedingly short and the leaves, heavy with water, never rise above the surface of the swamps and bogs which are the only home of the plant. At blooming time a stalk is sent upward for a foot or more, bearing the curious leathery flowers which nod to one side in a manner which has led some one of vivid imagination to call this the 'side-saddle plant.' The water which fills all but the youngest of the leaves is simply rain-water which has fallen in, and has nothing to do with the water supply of the plant in the ordinary sense. In the water are pretty sure to be some struggling insects trying to float or to crawl up the sides of the pitcher, some others whose struggles are over, and the remaining legs and wings of still earlier victims. All sorts of flying and creeping things seem to have a natural curiosity to examine hollow caverns such as the pitchered leaves appear to them. Perhaps, too, they are drawn to the plant by its rich red coloration or the striking veins which mark the mouth of the pitcher; indeed, one botanist has found drops of honey arranged in a row up the side of the pitcher, a lure to guide the steps of the insect directly to the interior. The wing which runs up one side has been thought, too, to serve a use in preventing insects from crawling round and round the pitcher, and to direct their steps upward to the slippery edge of the mouth. Once falling from the edge into the water, slim indeed, is the chance that any but the most active of insects will get out. The sides of the interior are not only steep, but are exceedingly smooth and offer no foothold by which to regain the top. And even if the bedraggled creature should succeed in crawling up beyond the slippery zone it would encounter an array of long stout hairs crowded close together and pointing downward. Over this ambush none but the most long-legged of insects can crawl, and they are the very ones least likely to have ever got up the slippery zone. Wet and exhausted, they fall again into the water, where they soon drown and yield up the substances of the body to be absorbed by fine hairs lining the bottom of the cup, and given over to the nourishment of their passive captor.
Some of the southern species of pitcher-plant have leaves standing erect as much as two or three feet in height and furnished with a lid-like
covering, not indeed a trap-door to shut down over the mouth, but serving to keep the rain from completely filling the pitchers and breaking them down with its weight, a provision which the purple pitcher-plant does not miss because of the prostrate position natural to its leaves. In other species the leaf is furnished with a hood arched over the mouth so as almost to conceal it, brightly colored with yellow or red, or marked with translucent spots. In the Californian pitcher-plant (Darlingtonia), a rare and local species, the leaf is hooded and the entrance small, hut accompanied by a hanging projection shaped like the tail of a fish. The hood is marked opposite the mouth by a number of translucent spots which some botanist has imagined are false windows, against which entrapped flies bump their heads in a wild effort to escape, being thus diverted from the real opening!
In the swamps and along the streams of the rich tropical forests of Borneo, Java and Ceylon are found the East Indian pitcher-plants (Nepenthes), a group in many ways more highly developed than their American relatives, for there are forty species of them, all plants growing to a considerable size and several of them forming important constituents of the vegetation. The stem is erect or half-climbing, with long narrow leaves tapering to slender ends, which will twine like a tendril about any supporting twig or stick and then give rise to a single pitcher, or failing to find a support, will fail too to produce the pitcher. In variety of design, brilliance of color and showy contrast of spots and stripes the East Indian pitchers far outdo those of the American plants. They are provided too with elaborate lids and covers which here have a double meaning, for the pitcher is not filled with rain-water, but with a secreted water of its own, which must not be diluted by the rain, as it contains precious substances given out with the water from glands in the bottom of the pitcher. The most important of these substances is a digestive principle much resembling the pancreatic juice of the human stomach; and scarcely less important is a faint trace of hydrochloric acid, without the presence of which the digestive juice can not work, for it is not here for any idle purpose, but for the business of digesting quickly the abundant prey which tropical insect life affords. A fly which falls into a glass of water is often able to escape because the close-set hairs covering its body do not permit it to become thoroughly wet. In the liquid of the pitchers is another substance known as azerin, the property of which is to cause any hairy surface to become quickly wet, which means for the fly sure drowning.
Here are pitfalls, then, not very unlike those of the American pitcher-plant in their mode of decoying victims and preventing their escape, but far advanced over them in their action, for the digestive juice yields up the nourishing substance of the insect much more rapidly than does the process of decay, and far more economically too, thus increasing the amount of the plant 's food, and thereby its abilities for growth and reproduction.
The closed trap and the pitchered leaf are not the only devices for insect capture which we find among plants; they also possess devices which man has closely paralleled in his invention of fly-paper. These
are, in general, sticky secretions borne either upon a flat leaf surface or on the ends of hairs arising from leaves. The plants possessing snares of this sort are more numerous than those with pitchers, and quite as successful as the latter in obtaining a generous supply of nitrogenous food.
Simplest of this class of the insectivorous plants is the butterwort (Pinguicula), a small annual, not unlike some of our commonest violets in appearance. A dweller in high mountains and the cold bogs of the north, it is particularly well known to every one who has climbed about in the Alps. The leaves are broad and undivided, slightly inrolled at the edges and coaled on the upper surface with a sticky slime exuded from minute groups of glandular cells. An insect alighting on the leaf is can-lit and held by the sticky secretion, and the more it struggles to escape the more firmly is it held and the more completely does its whole body become involved. If the insect has chanced to alight near the edge of the leaf the inrolled margin will roll still
further so as to cover it completely, but whether near the edge or not, there is something in the contact of this available food which causes the excretion of juice to be so abundantly renewed as to be sure to envelop the insect. In the renewed excretion there is to be found a digestive principle such as occurs in the pitchers of the East Indian pitcher-plant, so the captured prey, soon smothered to death, is also rapidly consumed and its essence carried into the leaves by means of the glandular secreting cells.
It is of interest that the only case in which any of the insectivorous plants have been found of practical use is in connection with the presence of the digestive juice in the butterwort. There is associated with it here, as in the stomachs of animals, a coagulating principle, a rennet. In some manner difficult to guess the shepherds of the Alps learned years ago that the leaf of the butterwort placed in fresh milk would cause it to thicken rapidly, and to this day they use these leaves rather than the animal extract in the making of curd for cheese.
The sun-dew (Drosera) is one of the best known of the insectivorous plants of this class, both because of the wide distribution of the ninety species over the world and on account of the detailed and patient study of it which was made by Darwin, whose Insectivorous Plants' is a rich mine of information for any one interested in (his subject. Our commonest sun-dew in the eastern Tinted States (Drosera rotundifolia) is a delicate little plant without stem—a mere rosette of long-stalked Leaves with rounded blades. Bristling from the upper surface of the leaf stand thirty or forty stout hairs nearly as long as the diameter of the Leaf. At the end of each hair is a swollen gland surrounded by a globule of viscid jelly, the whole scarcely as large as a pinhead. There are few more beautiful objects in the
plant world than a leaf of the sun-dew with its clear beads of jelly flashing in the sunlight against a rich setting of green and red. This splendid sight lures the small gnat or fly into contact with one or more of the beads and gives the jelly a tenacious hold on some particular leg or wing. The contact causes a disturbance to be set up in the leaf which brings the other hairs to bend towards the ones which have secured prey. If the insect caught is a minute one only a few of the hairs will be aggregated in this manner, but if a larger one has become entangled all the hairs will take part in the movement, and even the blade of the leaf may be bent together in such a way as to aid in the aggregation of the tips. The globules of jelly fuse into a mass about the insect and there is poured out from the glands a digestive juice such as that in the East Indian pitcher-plant and the butterwort. All the soft parts of the insect are digested and the nutritive juices, rich in nitrogen, are absorbed by the very same glands which secreted the digestive juice.
It is obvious that the aggregation of the hairs causes a more complete surrounding of the insect with jelly, increases the amount of digestive juice brought to act upon it, and also the number of channels for conducting the juices back into the leaf. The movements here involved are comparatively rapid—the hairs nearest the one which has made a capture begin to move in five seconds; if the capture is a big one all the hairs will be aggregated about it in half an hour. The time required for the digestion of prey depends entirely upon its size and nature; when completed the jelly dries off from the glands, the hard indigestible parts of the insect blow away, and the hairs resume their usual positions. The globules of jelly are then renewed and the leaf is ready for another capture. A single leaf may partake of above one hundred such meals, but more commonly its life is shorter, its place being rapidly taken by a younger leaf.
These are highly complex structures with which we meet in the sun-dew, and the united action of the hairs in aggregating themselves towards the spot where an insect has alighted is an example of coordinated activity such as is rarely met with in the plant world. There are, however, no unusual structures here, there is nothing in any way resembling a nervous system and nothing suggesting any similarity to the coordinated movements of animals which they so closely resemble. The most highly developed and remarkable of the insectivorous plants is the Venus' fly-trap (Dionæa), which will never cease to be the cardinal attraction with all florists so fortunate as to be able to humor it into healthy activity. It is one of our rarest American plants, being found only in the hogs of a restricted area on the coast of North Carolina.
In the Venus' fly-trap, as in the East Indian pitcher-plant, only a portion of the leaf has been modified for insect capture,—a rounded terminal portion cut off from the leaf proper by a constriction, and hinged along its midrib so as to be capable of closing like a book. On this portion, the 'trap,' are three sorts of hairs essential to its operation
—the first long and stout, fringing the edges of the two lobes, the second but six in number, three in the center of each lobe, and the third minute, innumerable ones covering the entire inner surface of each lobe.
If an insect alighting on a fly-trap leaf chances to touch any one of the six slender hairs or to alight on the triangular area on either lobe which is bounded by the three hairs, the lobes of the trap immediately begin to close together. In perhaps as short a time as five seconds they have shut on the luckless insect, the stout marginal hairs have formed an interlocking fringe about the edge of the trap, preventing its escape, and the small hairs have begun to pour out digestive juices upon the hitherto dry surfaces of the lobes. Any touch, pressure
or wound on the slender hairs or on the triangular areas will cause the lobes to close, whether the touch be that of an insect or that of an inanimate object. The other portions of the trap are so much less sensitive that it is necessary for them to he some time in contact with a nitrogenous substance in order to bring about closing of the trap. The contact of raindrops on any part of the leaf is without effect, and the other parts of the plant are entirely without sensibility. Still more effectually is the entrapped insect surrounded with the digestive juices than in the sun-dew, hut the complete consumption of its soft parts may take many days. On reopening after a capture the trap will he torpid and unresponsive for some time, hut if it has chanced to close without getting a meal it will open within twenty-four hours and will he at once ready for a capture. The number of times that a trap will close and digest nitrogenous fond is comparatively few; its delicate organization is soon worn out and ants may crawl over its face with impunity.
There is a wealth of interesting variety among other species and genera of the insectivorous plants, but the few described may well stand as examples of the whole number, for none depart very far from sonic one of these in the build and working of their pitfalls. While some of the traps are simple in structure and slow in action, and others are complicated and swift, yet there is a compensation in the fact that the more complicated devices are the most short lived.
The plants of insectivorous habit are not all members of a single family, and indeed are not all closely related—the group is a purely physiological one and owes its coherence to the fact that in all its members the meaning of the capture of insects is the same—the supplying of nitrogenous food. The very natural and pertinent question here arises: Why do these plants need such a remarkable means of getting nitrogenous food when other plants get on so well without this means? The insectivorous plants are obviously not able to subsist on insects alone, and—at least in the beginning of the habit—they were not different from other plants in their needs for nitrogenous food. So far as concerns the manufacture of starch and other non-nitrogenous foods the insectivorous plants are like all other green plants, deriving their carbon from the atmosphere, their hydrogen and oxygen from the water of the soil. It is in the elaboration of its more complex foods and the building up of new living substances in the process of growth, and particularly in the maturing of seed that a plant meets its need for nitrogen. In the ordinary plant this supply of nitrogen is derived from the nitrates which, together with several elements needed in small quantities, are brought up by the roots from the materials dissolved in the water of the soil.
Now, insectivorous plants in all parts of the world are found growing only in peat bogs, swamps, undrained pools and ditches—places in which it has been found that the water about their roots, from which they would be expected normally to draw their supply of nitrogen, is exceedingly poor in nitrates. Higher compounds of nitrogen exist in such places in plant and animal remains, but fail to be reduced to nitrates because of the absence of certain kinds of bacteria which carry on this work in all ordinary soils but are absent here because of the lack of drainage and consequent non-aeration of the water. It is this deficiency in available nitrogen, then, that is made up by a diet of insects, which are of course rich in nitrogenous substances.
How have such complex structures as those of pitcher-plant and sun-dew had their origin in the course of the evolution of the plant world? Such a question will ever remain a mystery, and on its solution we shall be able merely to throw an occasional ray of light. Very many plants have their stems or flower-stalks beset with glandular hairs secreting sticky substances; an example in point, the clammy cuphea of our fields which sticks to the fingers tenaciously if we attempt to pluck its flowers. So far as known, the sticky secretions serve the plant in no way other than making it unpleasant browsing for herbivorous animals and ridding it of marauding ants, which become stuck to the glands and seldom escape. Might not such a condition have existed in the ancestors of the sun-dew? Might not the accidental catching of insects in this manner have formed the starting-point for the habit which is now so essential to the plant? Pitchered leaves are found too in many plants, and in many of them catch insects by pure accident, for example, the Dischidia of the East Indies. It is quite possible that further study of such cases may reveal new examples of the insectivorous habit, or may discover plants which have an imperfect or partial dependence on insect food, and any such discoveries would throw light on the development of the habit in its full-fledged possessors.
Marvelous as are the adaptations of the insectivorous plants, they have not been all these years upon the earth without certain crafty insects having learned not only to escape falling a prey to them, but to use them to their own ends. The pitchers of the Californian pitcher-plant are the home of a small moth which is provided with sharp spurs on its middle legs, which enable it to crawl easily over the slippery surfaces of the interior. So fearless has the moth become that it even lays its eggs in the interior of the pitcher, and here, protected from all the manifold dangers of the outside world, they hatch out in security. The young caterpillars spin a web over the slippery surfaces and the projecting hairs, making a safe path for themselves to the outside. There is a blow-fly which is able, too, to crawl over the slippery surfaces by aid of peculiar claws which give it a good sure footing. The grubs of this fly hatch out in the water of the pitcher and, far from yielding themselves up as food, they live here their brief term of larval life, and escape by boring a hole in the side of the pitcher.
In the South African hills grows a sort of bushy sun-dew, known locally as the 'fly-bush' (Roridula). This plant is a large consumer of insect food, owing to its size and the completeness with which twigs and leaves and even parts of the flower are covered with glandular hairs. Among its branches a spider has been found to spin its web. Not content with the supply of flies from its web. however, the spider goes forth upon the twigs of the fly-bush, and walking about in safety among the glands, pulls oil' such insects as it desires and either devours them on the spot or returns with them to its web. Not less wonderful is the fly which pollinates the flowers of the fly-bush, ft, too, is enabled, by possessing long legs, to walk in safety among the glandular hairs, and it takes pay for the service of having pollinated its flowers by puncturing the leaves and with its long proboscis sucking the juices of the plant.
So far as may be judged from rather limited observations the spider and the fly of the fly-bush are not found on any other plants. The spider has probably been attracted from other homes by the rich feeding ground, and, as for the fly, no one knows how complex may have been the history of its evolution. What more complicated relation between plant and animal can be imagined than this of the fly and fly-bush? A plant in need of nitrogenous food possesses effective traps for insects, from which this food is got. The plant is in need, too, of the services of insects for the pollination of its flowers. How can this plant, a death trap to insects, secure their service in pollination? Its large showy flowers attract many insects, but they find no honey to reward them for their visit, and if they linger about the plant their doom is sealed. The flower is not without honey, but the cells containing it are covered by a layer of ordinary cells, and when the long-legged fly makes its visits to the petals its proboscis is brought into use, the layer of cells is punctured, and. the honey obtained. Insured against harm the fly may then visit other parts of the plant, and here it will use its proboscis again on leaves or stems to plunder the plant of its sap.
How much akin these phenomena are, you will say, to bribery, deceit and the taking of unfair advantage. True, there is no altruism among the lower forms of life; the benefit of self and of posterity is the supreme good of the animal and of the plant, and. necessarily so by reason of the multitude of competitors and the keenness of the strife with them. Yet the great mass of plants live independently and, so to speak, honestly; overcoming obstacles and withstanding reverses, doing no more than energetic men in jostling their neighbors in the winning of a livelihood, and being no more than normal in providing that their own offspring should have a good start in the world rather than the offspring of their neighbors.