Admitting that the case is not sufficiently proved for us confidently to affirm that these carnivorous habits are conducive to the welfare of the plant, we cannot deny its probability, because otherwise we are placed in the dilemma of assuming, either that all this adaptation for catching and destroying animal life is wanton mischief, or that it is an expenditure of power without purpose. From experience of the operations of nature we are unwilling to recognise such a departure from the usual plan. We are accustomed to trace operations performed by an economy of force, and to believe that nothing is done in vain. Wanton destruction, or wasted energy, are not the probabilities which would suggest themselves to the mind of any one who has devoted himself to the study of the phenomena of life, nor would they elevate our conception of the All-wise Creator, of what in such a case would be undoubted failures.
CHAPTER III
CARNIVOROUS PLANTS—VENUS'S FLY-TRAP.
BELONGING to the same natural order of plants as the Sundews, Venus's Fly-trap, or, botanically, Dionæa muscipula, has recently been much harassed by experiments to test its flesh-eating capacity. It is not a British native, but an inhabitant of damp places in the eastern parts of North Carolina, so that its relationship to our Sundew may be described as that of an "American cousin." In like manner it will grow and flourish in wet moss, without any soil, and consists of a rosette of leaves, which radiate from a centre, but both leaves and tufts are larger, and more conspicuous, than in the Sundew. The foot-stalk of the leaves is flattened out, and leaf-like. The blade of the leaf is somewhat rounded in outline, and composed of two lobes, which are hinged down the centre, so that the lobes rise up, and apply themselves together face to face. Around the margin of the lobes stands a row of bristles, which will be more fully described shortly. With a coloured figure of this plant, published ninety years ago in "Shaw's Miscellany," is the following remark: "The surface of the leaves is irritable in the highest degree, and whatever insect is so unfortunate as to alight on it is caught as effectually as a mouse in a trap, and is even generally squeezed to death by the pressure. What particular purpose in the economy of nature is answered by the imprisoning power of this extraordinary vegetable, it is extremely difficult, and perhaps impossible, to determine."
<!-- image -->As long ago as 1768, a naturalist, named Ellis, called the attention of Linnæus to the peculiarities of the leaves of the Venus's Fly-trap, or Dionæa, by the following remarks: "The plant shows that Nature may have some views towards its nourishment, in forming the upper joint of its leaf like a machine to catch food; upon the middle of this lies the bait for the unhappy insect that becomes its prey. Many minute red glands that cover its surface, and which perhaps discharge sweet liquor, tempt the poor animal to taste them; and the instant these tender parts are irritated by its feet, the two lobes rise up, grasp it fast, lock the rows of spines together, and squeeze it to death. And, further, lest the strong efforts for life in the creature just taken should serve to disengage it, three small erect spines are fixed near the middle of each lobe, among the glands, that effectually put an end to all its struggles. Nor do the lobes ever open again while the dead animal continues there. But it is, nevertheless, certain that the plant cannot distinguish an animal from a vegetable or mineral substance; for if we introduce a straw or pin between the lobes it will grasp it full as fast as if it were an insect." Linnæus, however, only regarded these phenomena as illustrations of the extreme sensibility of the leaves. Sixty years subsequently, Dr. Curtis, of North Carolina, made further and more complete examination of these leaves. "Each half of the leaf," he says, "is a little concave on the inner side, where are placed three delicate hair-like organs, in such an order that an insect can hardly traverse it without interfering with one of them, when the two sides suddenly collapse and enclose the prey with a force surpassing an insect's efforts to escape. The fringe of hairs of the opposite sides of the leaves interlace, like the fingers of two hands clasped together. The sensitiveness resides only in these hair-like processes on the inside, as the leaf may be touched or pressed in any other part without sensible effects." After this, another American botanist, who was staying in the district where the "fly-trap" flourishes, resolved upon some experiments, and by feeding the leaves with small pieces of beef he found that these were completely dissolved and absorbed; the leaf opening again with a dry surface, and ready for another meal, though with an appetite somewhat jaded. He found that cheese disagreed horribly with the leaves, turning them black and finally killing them.
The insectivorous predilections of Dionæa have, therefore, been suspected, if not demonstrated, for more than a century. In the account of the plant given by Shaw, he commences by alluding to the different methods by which carnivorous animals catch their prey, and then he adds, "What is still more extraordinary, there are not wanting amongst vegetables some instances in which the smaller animals meet their fate by alighting on the flowers or leaves; being either held fast by a viscous exudation from the surface or confined by the pressure of the irritable parts of the plant." The date attached to the plate is 1790.
Sir Joseph Hooker has given a complete summary of the history of all the observations which have been made on the plant from the earliest times, which may be consulted by any who desire a more explicit narrative of the details than our space will enable us to furnish. We direct ourselves at once to the modus operandi by means of which the plant achieves its object. This mechanism has been compared by Dr. Burdon Sanderson to a rat-trap. "When it (the leaf) is open, the lobes are at right angles to each other. When an insect comes into contact with either, at once they approach each other, but this does not occur with the suddenness and completeness that it occurs in a rat-trap. The lobes begin to close sharply enough, but do not come quite together, remaining for some time entre-ouvert. When the leaf is in this state of half-closure, it is easy to see what is the significance of the two sets of prongs. You see that they are set on alternately, along the opposite edges of the lobes, so that, just like the teeth of a rat-trap, they fit into each other. It is not difficult to see why this is so, i.e., why the spikes are arranged alternately. The leaf, being a trap, is made like a trap. But I should not have been able to tell you why the leaf does not at once close on its prey had not Mr. Darwin told me. After having partially closed, as I have said, one of two things may happen. The insect, having been caught, at once begins to think of escaping, and makes efforts to do so, which may or may not be successful. If it is small, it easily finds its way out through this wonderful grating formed by the crossing of the teeth, and in this case the leaf soon recovers, expands again, and is ready for the capture of another victim. If it is large, all its efforts to regain its liberty are futile. Repelled by its prison-bars, it is driven back upon the sensitive hairs which stick into the interior of its cell, and again irritates them. By doing so it occasions a second and more vigorous contraction of the lobes. The result is, that the creature is not only captured, but crushed; not only swallowed, but digested."
The minute structure of the leaves differs in many respects from that of the Sundews. The rigid marginal spines are without glands on their tips, and are not irritable. The three minute filaments which project from the upper surface of both lobes, on the contrary, are remarkable for their extreme sensitiveness to the touch, but they also are pointed at their extremity. Besides these cuticular appendages the upper surface is thickly covered, except near the margin, with minute reddish or purish glands, but there are no glands on the leaf-like foot-stalk. The glands are elevated on short pedicels, and are convex above. Little stellate projections of an orange-brown colour, with eight radiating arms, are scattered over the foot-stalk, the back of the leaves, and the basal part of the marginal spikes, and a very few on the surface of the lobes. Here and there a few minute pointed hairs may be traced on the back of the leaves.
The functions of all these parts have been fairly ascertained. That of the marginal spines is of a mechanical nature, and perhaps entirely so, as they are neither sensitive nor glandular, and do not seem to possess any separate or spontaneous motion. The sensitive filaments, on the contrary, are eminently sensitive. Their apices are sometimes divided into two or three points, and, from apex to base, it is impossible to touch them, ever so lightly, without at once acting on the lobes of the leaf and causing them to close. These sentinel filaments, although so sensitive to a slight touch, are less sensitive to prolonged pressure. This difference between the filaments in Dionæa and the glands of Drosera relates to the different habits of the two plants. It has been seen how a slight prolonged pressure acts on the Sundew; but in the Dionæa there is no viscid secretion to detain the insect, which must be caught at once by the rapid closing of the lobes, simultaneously with the slightest touch; for the filaments neither secrete nor absorb, and are, in fact, purely sentinels. The tentacles of Drosera when excited become inflected and aggregated, but this property does not extend to the Dionæa filaments. Drops of water falling on them will not cause the lobes to close, nor blowing upon them strongly. Hence the sentinels are not likely to give a false alarm at a shower of rain or a gale of wind. Neither did the rays of the sun, when concentrated upon the filaments to such a degree as to cause them to be scorched and discoloured, produce any movement.
The minute glands with which the surface of the leaves is studded have the power of secretion and absorption, but they do not secrete until excited by the presence of animal matter. Other objects placed upon the glands will remain quite dry; but, if a fragment of meat, or a crushed fly, is placed on the surface of the expanded lobes after a time the glands will secrete freely. If the lobes are made to close over an insect, then the glands of the whole surface secrete copiously. Two or three instances are given by Mr. Darwin in proof of this: "On one occasion when a leaf was cut open, on which a small cube of albumen had been placed forty-five hours before, drops rolled off the leaf. On another occasion in which a leaf, with an enclosed bit of roast meat, spontaneously opened after eight days, there was so much secretion in the furrow over the midrib that it trickled down. A large crushed fly was placed on a leaf from which a small portion at the base of one lobe had previously been cut away so that an opening was left, and through this the secretion continued to run down the foot-stalk during nine days—that is, for as long a time as it was observed."