Some of the genuine botanical realities can be made to seem, on their own modest scale, almost as chilling. The American pitcher plants feed not just on insects but also on small lizards and frogs. A large pitcher plant called Nepenthes, native to Borneo, has been caught in the act of digesting mice. The biggest of the sundews, an Australian species named Drosera gigantea, grows into a three-foot-high bush of sticky, grabby paws. Then there’s the gaping red maw of the Venus’s flytrap, armed along each lip with a row of needle-like spines that were once thought to be capable of impaling victims. My own favorite bit of lore, though, involves the collective accomplishment of a whole field full of common British sundews.
On August 4, 1911, in the county of Norfolk on the east coast of England, a scientist named F. W. Oliver came across a two-acre meadow carpeted solid with sundews. These pretty little plants consist of a rosette of club-shaped leaves radiating from a central stem, each leaf covered with small tentacles at the end of which is a knobby gland, each gland wrapped in a drop of glistening mucilage. In the meadow when Oliver found it, every individual plant had recently captured between four and seven specimens of Pieris rapae, a small white butterfly. Evidently the butterflies were a migrating flock that had flown across from the Continent, settling on this flowery field for a rest and a snack. They had chosen badly, and the flock would be going no farther. The sundews were in the act of digesting them.
By Oliver’s estimate, that field of plants had just eaten six million butterflies.
Now the Pieris rapae butterfly, in its larval stage, is itself a notoriously voracious plant-eater. For six million of them to gobble away two acres of vegetables would be a routine agricultural annoyance. So why should it seem more macabre when the tables are turned?
• • •
Evolutionary biologists have been intrigued by the varieties of flesh-eating flora ever since Charles Darwin, who wrote an entire book titled Insectivorous Plants. Darwin himself had gotten interested during the summer of 1860, just after publication of The Origin of Species, when (on a heath in Sussex) he stumbled across a large insect-kill like the one later described by Oliver. The perpetrator in Darwin’s case was also the common sundew, Drosera rotundifolia, and because that species was locally plentiful and could be cultivated at his home for use in experiments, D. rotundifolia became the main focus of Darwin’s book. “I care more about Drosera than [about] the origin of all the species in the world,” he confessed intemperately in one letter. He also harbored a special fond fascination for the Venus’s flytrap, which is a purely American species that Darwin had never seen in the wild, and which he called “the most wonderful plant in the world.” Darwin begged samples of the flytrap from his American colleagues, tried to raise the species in his own greenhouse, and had to lament that “I cannot make the little creature grow well.”
He needn’t have been hard on himself about that, because the Venus’s flytrap is drastically finicky about its habitat. There is only one species, and that species confines itself to only one native range: a narrow strip of coastal plain in the Carolinas. The flytrap is so sensitive to its own habitat requirements that, even within such a small range, it can survive only in very particular types of terrain. More about this choosiness in a minute.
Despite its rarity, the Venus’s flytrap is the most famous of carnivorous plants, and we are all roughly familiar (we think) with its general anatomy and behavior. Each leaf is modified to the shape of a leg-hold trap, two semicircular lobes on a hinge, cocked open invitingly but ready to slam shut the instant a trigger is tripped. Right? Along the rim of each lobe protrude those needle-like spines. Maybe they can’t stab an escaping fly, as once thought, but they certainly add to the aura of implacable malice. Right? The inner surfaces of the lobes are cobbled with tiny liquid-filled glands, some of which show a mysterious red coloring, some of which exude a clear nectar. An insect is attracted by the color and smell, lands on or crawls into the open trap, and then—the heart-sinking snap. The insect’s demise is ugly, remorseless, and sudden. Right?
Well, yes and no. The reality is more complicated and more interesting.
The flytrap’s anatomy includes a few ingenious features that allow it to measure and taste its potential prey before committing itself to the meal. This plant is no heedless glutton. On the contrary, its behavior is forbearing and judiciously economical.
First the question of taste. Those reddish glands crowded onto the flytrap’s palate secrete a digestive fluid, a mixture containing weak acid and an enzyme called proteinase, which dismantles animal protein. Once the animal protein has been broken down into soluble fragments, that nutritious solution can be reabsorbed by the plant. But unlike the pitcher plants (which hold a permanent reservoir of digestive fluid, into which victims fall), the flytrap remains dry-mouthed until a morsel of prey has been caught. Furthermore, it isn’t to be fooled by poor substitutes. It responds only to real food. The lobes close on a cricket or a fly—and the proteinaceous saliva begins flowing. The lobes close on a small gobbet of raw beef—here also the saliva begins flowing. The lobes close in reaction to the touch of a glass rod, or the tip of a pencil, or the weight of a pebble fed to it just like the beef—and nothing at all happens. The plant does not waste its time or its juices.
It isn’t receiving the right signals of chemical feedback. In other words, the pebble tastes wrong. Still dry-mouthed, the flytrap opens its lobes again as soon as possible, resuming the wait for a genuine meal.
After a false alarm, the plant is ready again in less than twenty-four hours. If the chemical signals are positive and the chamber floods with digestive fluid, on the other hand, five to ten days will pass before the flytrap can reopen. That difference in the expenditure of resources (time and digestive fluid) seems to be why the plant also measures its prospective victims, and proceeds or refrains accordingly. It simply refuses to bother with insects that are too small to be worthwhile.
The measuring is done in two ways. One is inherent in the structure of the triggering mechanism. On the inner surface of each lobe, among the digestive glands, are three sensitive hairs that serve as trip wires for the trap. Merely touching one hair, though, is not sufficient to spring the trap. At least two distinct touches (upon the same hair or different ones) are required, and those touches must occur no less than about one second nor more than about twenty seconds apart. The hairs themselves are spaced just far enough from each other—as well as from the nectar glands that attract an insect’s attention—that a small insect cannot bump any two in close succession.
The second method of measuring was discovered by Charles Darwin himself. He had noticed an odd fact about how the flytrap closes: that the closing movement occurs in two discrete phases. Upon triggering, the lobes swing together quickly (in less than half a second) to a position where the long spines have crossed but the lobe edges haven’t quite met, leaving a row of narrow, short gaps like the spaces between bars in a jail window. For the lobes to close completely, sealing off that row of gaps, takes another half hour. Why the hesitation? wondered Darwin.
He guessed that the Venus’s flytrap was saving itself the trouble of digesting insignificant meals. “Now it would manifestly be a great disadvantage to the plant,” he wrote in Insectivorous Plants, “to waste many days in remaining clasped over a minute insect, and several additional days or weeks in afterwards recovering its sensitivity; 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.” So the Venus’s flytrap, terror of large insects, is benignly indifferent to little ones.
The most basic question remains: Why do they eat meat?
Not only the Venus’s flytrap but also the sundews, the pitcher plants, and a still more elaborate genus of a
nimal-trapping plants called the bladderworts—why do these species share a hunger for fresh flesh? Why must they feast on animal protein while other species of plant are content with sunshine, water, air, and a bit of decent soil? It seems not only presumptuous but greedy.
The truth is exactly opposite. Carnivorous plants have been driven to this extremity not by boldness and gluttony, but by shyness and starvation.
In the matter of habitat, evolution has awarded them hind tit. But like a determined runt that will grow into a proud hog, they make the best of it. They have developed strategies for collecting animal protein because, in the nutrient-poor habitats to which they are exiled, on soils so inhospitable that few other plants deign to invade, without some dietary supplement they could scarcely survive.
The floating islands of peat in the Okefenokee Swamp are a representative outpost, supporting carnivorous plants of three different genera (sundews, bladderworts, pitcher plants) within little more than a canoe’s length of one another. The Pine Barrens of New Jersey can claim the same distinction. What these spots have in common with that soggy meadow in Norfolk, where F. W. Oliver saw a million well-fed sundews, is a critical shortage of the basic soil nutrients (like nitrogen and phosphorus) that most flowering plants require. One study has shown that the average patch of bog inhabited by sundews has twenty-seven times less nitrogen than the average patch of pine forest. Around the world, habitats of carnivorous species tend to fit the same pattern—plenty of water, plenty of sun, terrible soil. These are unpromising corners of real estate where, if sundews or pitcher plants weren’t growing, almost nothing else would be.
Look at it this way: Meat-eating is the last resort of the shy, uncompetitive plant. Those carnivorous species have removed themselves evolutionarily from the ruthless competition of the thicket, the forest, from all those fecund and clamorous places where plants flourish in wild vigor and variety, battling each other upon nutritious substrata for position and water and sunlight. The Venus’s flytrap and those few others have taken a more gentle path.
In that sense they belong in company with certain other retiring creatures that go to great lengths to avoid gratuitous violence. I’m thinking especially of the rattlesnake and the black widow spider.
THE SELFHOOD OF A SPOON WORM
Sex Determination as a Mid-Life Experience
The study of biology is such a fine antidote to rigid, normative thinking that perhaps all our televised preachers and tin-whistle moralists should be required occasionally to take a dose of it. The experience couldn’t help but be broadening. No general truth emerges more clearly, from even a browser’s tour of the intricacies of the natural world, than this: Chances are, there is more than one right way to do it.
Flying is a good example. Birds and reptiles and insects and bats and seeds have all mastered that feat, at different times and in their utterly different ways. The arrangement of anatomical support is another. Who is to say that a skeleton should be worn inside the body (as by us vertebrates), when lobsters and other arthropods do so well with their skeletons on the outside, and jellyfish get by with none whatsoever? For a further instance, consider the matter of how gender is determined among those creatures showing two distinct sexes. Boy or girl, cow or bull, colt or mare, goose or gander: The interesting question, biologically, is not which but why. What dictates that a particular individual should turn out to be male or female?
In mammals the point is decided genetically at the moment of conception. That’s the most familiar sort of sex determination, and we humans are likely to think of it as the norm; but such genetic sex determination (GSD) is just a contingent fact, not a logical or biological necessity. Among certain other animals, known as “sequential hermaphrodites,” sexual identity can change as a stage of growth, as routinely as a human might pass through puberty or menopause. These sequential hermaphrodites, including a number of fish species, begin life in one sexual form (say, as males) and function reproductively in that role for a time; then as they grow older and bigger, they transform at some point to the opposite sex (female), in which role their large size may be more advantageous. If physical magnitude happens to be a more crucial advantage for males than for females, in any such species, then the sequence of sexual identities will be reversed, each individual making its transition from small female to big male.
There is also a third option for sex-differentiated species, one which has not gotten much scientific attention until the last few years. This option is called “environmental sex determination,” or ESD. The term means, simply, that in certain species the sex of the offspring is determined at a point sometime after conception, by some environmental influence acting upon those unhatched eggs or those sexless young. That environmental influence might be a matter of chemistry or sunlight or temperature or something else. Theoretical ecologists are still struggling to explain just how ESD might have evolved, and just why it might be useful, but in the meantime field and lab studies have shown that the phenomenon is more common than we might expect.
• • •
This ESD business was probably first recognized in a beast named Bonellia viridis, a benignly grotesque sea animal belonging to the phylum Echiurida, a group casually known as the “spoon worms” and not remotely related to anything you’ve ever heard of. Bonellia itself looks like some sort of bad party joke made out of latex. The adult female of the species consists of a bulbous body roughly the size of an avocado and with a similar dappled surface, from which extends a long tube-like proboscis ending in a pair of leafy lobes. It lives amid rocks on the bottom of the Mediterranean Sea, where the soft body can find safety by anchoring itself in a hole or a crevice, and the lobed proboscis can be extruded out, three feet or more, to grope for passing morsels of food. But the proboscis of Bonellia collects more than nourishment; it also collects mates.
When a tiny sexless larva of the same species comes into contact with this proboscis, the larva attaches itself there on the tube and (apparently in response to chemical signals) begins the process of turning into a dwarf Bonellia male. Eventually the mature male, still no larger than a caraway seed, will make his way up the proboscis and into the female’s gut, claiming a permanent home within the uterus. There he will live off her as a parasite, a feckless but useful gigolo, conveniently on hand to fertilize her eggs.
In finding its female host, the Bonellia larva has found also its own sexual identity, its own selfhood. If the same larva had not blundered upon a female proboscis, it would (in most cases, though there are exceptions) eventually have settled down in a rocky cleft and grown into a large bulb-and-tube female itself. The presence or absence of a female proboscis is the crucial environmental fact, in the life of each young Bonellia, that settles the matter of sex.
This is ESD at its most vivid, and back as early as 1920 Bonellia had already become quietly famous among zoologists as the leading exemplum of the phenomenon. But environmental sex determination seemed then just an oddity, an aberration, the kind of garish and mildly repugnant trick that one would expect from an obscure group of marine invertebrates like the spoon worms. Today we know better. ESD has been discovered also among orchids, nematode worms, crustaceans, lizards, at least one species of fish, four or five species of turtle, and the American alligator.
In the case of the alligator, an elaborate set of experiments and field observations has recently proved that sex determination for this species involves little or no genetic component. Instead, the sex ratio in a litter of hatchling alligators seems to be completely dependent upon the temperature at which those eggs were incubated. An alligator nest maintained at eighty-six degrees F. or cooler will produce nothing but females. The same batch of eggs, if kept at ninety-three degrees or warmer, will hatch out as all males. Alligator eggs have an incubation period of about sixty-five days, but sex determination seems to occur during just the second and third weeks. At temperatures between the range of eighty-six and ninety-three degrees, the nest will yield a mix of males and fe
males.
Still, the interesting issue is why. Why has the alligator come to depend upon thermal signals, rather than genetic coding, to set the sexual identities of its offspring? Why has Bonellia evolved a system using social contact (or the lack of it) for the same purpose?
And from that pair of questions derives another, even more puzzling: What could Bonellia and the American alligator have in common?
• • •
Writing in the journal Nature, Eric Charnov and James Bull have offered a conceptual model that might make evolutionary sense of the whole range of ESD cases. “We propose that labile sex determination (not fixed at conception) is favoured by natural selection when an individual’s fitness (as a male or female) is strongly influenced by environmental conditions and where the individual has little control over which environment it will experience.” Control is the key word. If an organism can’t completely control where it’s going (like a Bonellia larva, riding helplessly on the sea currents), then maybe there is compensatory value in retaining control over what sort of being (male or female) it will be when it gets there.
This might sound like something from Alice in Wonderland, but in truth there is nothing illogical about it. The apparent reversal of logic merely goes against our preconceptions. We think of sexual identity as virtually a prerequisite to existence. Under the Charnov-Bull model, by contrast, sex determination is just an intermediate step on the long path toward what might be called (in California, anyway) self-actualization.
The Flight of the Iguana Page 6
