INSECTIVOROUS PLANTS
Venus fly trap is a relative of Darwin’s beloved sundews (see p. 129) that grows in coastal wetlands of the Carolinas in the United States. First brought to the attention of naturalists in 1759 by North Carolina colonial governor Arthur Dobbs, this species has long been an object of fascination for botanists and non-botanists alike, owing to its remarkable snap-trap leaves. When Carl Linnaeus, father of modern taxonomy, first heard about this plant, he reportedly exclaimed “miraculum naturae!” and American naturalist William Bartram, who had made some of the earliest collections, declared it an “astonishing production!”54
Triggered by unwary insects, the paired lobes of the leaves snap shut in a flash—some 100 milliseconds, one of the fastest recorded movements in the plant kingdom. With rows of spiky processes along the outer margins of the leaf lobes, the leaves look for all the world like a botanical version of a toothed steel trap, misleading early observers into thinking that depredating insects were pierced or crushed to death when the lobes snapped shut. In fact, those spikes inter-digitate, rather than pierce, making a more effective trap for holding insects—but just why they are held was unknown to early naturalists and it took another century to figure out, something Darwin played a role in.
Despite its deadly attributes, eighteenth century naturalist Daniel Solander, who accompanied Joseph Banks on Captain James Cook’s first voyage to the South Seas, thought that it “well deserved one of the names of the Goddess of Beauty,” dubbing it Dionaea “from the beautiful Appearance of its Milk-white flowers, and the Elegance of its Leaves,” according to London naturalist John Ellis in 1768. It was Ellis who added the specific epithet muscipula, the Latin word for mousetrap or flytrap.
Darwin’s interest in Venus fly trap was two-fold. In the 1860s, he was riding his new “hobby horse” of plant movement hard, sure that the animal-like attribute of plant “irritability”—touch-sensitivity and movement—was more than analogical, pointing to an essential connection between plants and animals. To Darwin, the connection was evolutionary, and the fly trap’s rapid movement (echoing the reflexive action of animals) and their carnivorous habit confirmed this view. One could hardly find a better plant-animal link than a green plant with a nervous system and stomach!1
He longed to study fly traps and compare them with sundews, but fly traps were still something of a rarity in Britain, and he was hard pressed to get his hands on specimens. Eventually, cajoling letters to Daniel Oliver, Keeper of the Herbarium at the Royal Botanic Gardens, Kew yielded a few traps and whole plants.55 Asa Gray put him in touch with William Marriot Canby, in Wilmington, Delaware, and Mary Treat in Vineland, New Jersey, both of whom obligingly sent Darwin their own observations along with leaves holding trapped insects. Darwin eventually performed over a dozen digestion experiments, feeding various bits of albumen, gelatin, roasted meat, and cheese to determine the power of the leaves to dissolve the matter with the aid of their enzymatic secretions. He also localized the specialized hairs that trigger the traps, and teamed up with plant physiologist John Burdon-Sanderson, who devised a modified galvanometer to measure the electrical potential of the traps as they snap shut. Darwin was deeply impressed by this most animal-like plant—it’s no surprise that he considered Dionaea “one of the most wonderful in the world.”56
This plant, commonly called Venus’ fly-trap, from the rapidity and force of its movements, is one of the most wonderful in the world. It is a member of the small family of the Droseraceae and is found only in the eastern part of North Carolina, growing in damp situations. The roots are small; those of a moderately fine plant which I examined consisted of two branches about 1 inch in length, springing from a bulbous enlargement. They probably serve, as in the case of Drosera, solely for the absorption of water; for a gardener who has been very successful in the cultivation of this plant grows it like an epiphytic orchid, in well-drained damp moss without any soil. The form of the bilobed leaf, with its foliaceous footstalk, is shown in the accompanying drawing.
Dionaea muscipula. Leaf viewed laterally in its expanded state.
The two lobes stand at rather less than a right angle to each other. Three minute pointed processes or filaments, placed triangularly, project from the upper surfaces of both; but I have seen two leaves with four filaments on each side, and another with only two. These filaments are remarkable from their extreme sensitiveness to a touch, as shown not by their own movement, but by that of the lobes. The margins of the leaf are prolonged into sharp rigid projections which I will call spikes, into each of which a bundle of spiral vessels enters. The spikes stand in such a position that, when the lobes close, they inter-lock like the teeth of a rat-trap. …
The sensitive filaments are formed of several rows of elongated cells, filled with purplish fluid. They are a little above the 1\20 of an inch in length; are thin and delicate, and taper to a point. Towards the base there is constriction, formed of broader cells, beneath which there is an articulation, supported on an enlarged base, consisting of differently shaped polygonal cells. As the filaments project at right angles to the surface of the leaf, they would have been liable to be broken whenever the lobes closed together, had it not been for the articulation which allows them to bend flat down.
These filaments, from their tips to their bases, are exquisitely sensitive to a momentary touch. It is scarcely possible to touch them ever so lightly or quickly with any hard object without causing the lobes to close. A piece of very delicate human hair, 2½ inches in length, held dangling over a filament, and swayed to and fro so as to touch it, did not excite any movement. But when a rather thick cotton thread of the same length was similarly swayed, the lobes closed. The sensitive filaments of Dionaea are not viscid, and the capture of insects can be assured only by their sensitiveness to a momentary touch, followed by the rapid closure of the lobes. …
The upper surface of the lobes is thickly covered with small purplish, almost sessile glands. These have the power both of secretion and absorption; but unlike those of Drosera, they do not secrete until excited by the absorption of nitrogenous matter. …
We will now consider the action of the leaves when insects happen to touch one of the sensitive filaments. This often occurred in my greenhouse, but I do not know whether insects are attracted in any special way by the leaves. They are caught in large numbers by the plant in its native country. As soon as a filament is touched, both lobes close with astonishing quickness; and as they stand at less than a right angle to each other, they have a good chance of catching any intruder. The angle between the blade and footstalk does not change when the lobes close. The chief seat of movement is near the midrib, but is not confined to this part; for, as the lobes come together, each curves inwards across its whole breadth; the marginal spikes however, not becoming curved. This movement of the whole lobe was well seen in a leaf to which a large fly had been given, and from which a large portion had been cut off the end of one lobe; so that the opposite lobe, meeting with no resistance in this part, went on curving inwards much beyond the medial line.
From the curving inwards of the two lobes, as they move towards each other, the straight marginal spikes intercross by their tips at first, and ultimately by their bases. The leaf is then completely shut and encloses a shallow cavity. If it has been made to shut merely by one of the sensitive filaments having been touched, or if it includes an object not yielding soluble nitrogenous matter, the two lobes retain their inwardly concave form until they re-expand. The re-expansion under these circumstances—that is when no organic matter is enclosed—was observed in ten cases. In all of these, the leaves re-expanded to about two-thirds of the full extent in 24 hrs. from the time of closure. Even the leaf from which a portion of one lobe had been cut off opened to a slight degree within this same time. How many times a leaf is capable of shutting and opening if no animal matter is left enclosed, I do not know; but one leaf was made to close four times, reopening afterwards, within six days, On the last occasion it caught a fly, and then remained closed for many days.
This power of reopening quickly after the filaments have been accidentally touched by blades of grass, or by objects blown on the leaf by the wind, as occasionally happens in its native place, must be of some importance to the plant; for as long as a leaf remains closed, it cannot of course capture an insect.
Dr. Canby, who observed in the United States a large number of plants which, although not in their native site, were probably more vigorous than my plants, informs me that he has “several times known vigorous leaves to devour their prey several times; but ordinarily twice, or, quite often, once was enough to render them unserviceable.” Mrs. Treat, who cultivated many plants in New Jersey, also informs me that “several leaves caught successively three insects each, but most of them were not able to digest the third fly, but died in the attempt. Five leaves, however, digested each three flies, and closed over the fourth, but died soon after the fourth capture. Many leaves did not digest even one large insect.” It thus appears that the power of digestion is somewhat limited, and it is certain that leaves always remain clasped for many days over an insect, and do not recover their power of closing again for many subsequent days. In this respect Dionaea differs from Drosera, which catches and digests many insects after shorter intervals of time.
We are now prepared to understand the use of the marginal spikes, which form so conspicuous a feature in the appearance of the plant and which at first seemed to me in my ignorance useless appendages. From the inward curvature of the lobes as they approach each other, the tips of the marginal spikes first intercross, and ultimately their bases. Until the edges of the lobes come into contact, elongated spaces between the spikes, varying from the 1/15 to the 1/10 of an inch (1.693 to 2.54 mm.) in breadth, according to the size of the leaf, are left open. Thus an insect, if its body is not thicker than these measurements, can easily escape between the crossed spikes, when disturbed by the closing lobes and in-creasing darkness; and one of my sons actually saw a small insect thus escaping. A moderately large insect, on the other hand, if it tries to escape between the bars will surely be pushed back again into its horrid prison with closing walls, for the spikes continue to cross more and more until the edges of the lobes come into contact. A very strong insect, however, would be able to free itself, and Mrs. Treat saw this effected by a rose-chafer (Macrodactylus subspinosus) in the United States. Now it would manifestly be a great disadvantage to the plant to waste many days in remaining clasped over a minute insect, and several additional days or weeks in afterwards recovering its sensibility; inasmuch as a minute insect would afford but little nutriment. It would be far better for the plant to wait for a time until a moderately large insect was captured, and to allow all the little ones to escape; and this advantage is secured by the slowly intercrossing marginal spikes, which act like the large meshes of a fishing-net, allowing the small and useless fry to escape.
As I was anxious to know whether this view was correct—and as it seems a good illustration of how cautious we ought to be in assuming, as I had done with respect to the marginal spikes, that any fully developed structure is useless—I applied to Dr. Canby. He visited the native site of the plant, early in the season, before the leaves had grown to their full size, and sent me fourteen leaves, containing naturally captured insects. Four of these had caught rather small insects, viz. three of them ants, and the fourth a rather small fly, but the other ten had all caught large insects, namely, five elaters, two chrysomelas, a curculio, a thick and broad spider, and a scolopendra. Out of these ten insects, no less than eight were beetles, and out of the whole fourteen there was only one, viz. a dipterous insect, which could readily take flight. Drosera, on the other hand, lives chiefly on insects which are good flyers, especially Diptera, caught by the aid of its viscid secretion. But what most concerns us is the size of the ten larger insects. Their average length from head to tail was .256 of an inch, the lobes of the leaves being on an average .53 of an inch in length, so that the insects were very nearly half as long as the leaves within which they were enclosed. Only a few of these leaves, therefore, had wasted their powers by capturing small prey, though it is probable that many small insects had crawled over them and been caught, but had then escaped through the bars.
Drosera rotundifolia. Watercolor by Lady Francis Howard, A Catalogue of English Plants.