Fierce Creepers

lants, it must be conceded, do not have an especially exciting image. They do not have a reputation for dynamism (although that is largely just a matter of timescale) and are ultimately at the bottom of every food chain. But there are exceptions. In certain environments, usually where the soil is very poor, some genera have evolved which defy these generalisations. They have climbed the Darwinian ladder, and have turned from prey to predator.

All in all there are nine families of carnivorous plants, between them comprising 17 genera and a total of around 550 known species.

Probably the most widespread are the pitchers. Sarracenia (of which there are nine species) has developed a long leaf joined at the edges to form an upward-pointing trumpet with a lid above the opening. The entrance and the top third of the pitcher bears nectar producing glands which encourages insects to visit. Once they land, a system of angled hairs directs them deeper into the trumpet towards the trapfeed. This zone, which forms roughly half the total length of the pitcher, has two deadly features: it has smooth, waxy walls on which crawling insects cannot keep their grip, and it tapers, denying flying insects enough space to use their wings. Once an insect has reached this depth, it is doomed - it descends inexorably towards the absorption zone. Here an array of glands generates enzymes, which break down the body, and absorbs the resulting soup. From here, the food canal transports the nutrients to the rhizome.

So effective is this system that after a while the pitcher has to use additional digestive glands in the trapfeed to cope with the volume of insect bodies packed into the pitcher.

Nepenthes, which takes its name from the deadly drug used by Helen in Homer's Odyssey, extends the pitcher principle. Its seventy species are essentially vines which bear pitchers on tendrils. The shapes are varied and specialised - small arial pitchers for flying insects and squat ones which rest on the floor to ensnare crawling insects. The plant's arsenal has also been enhanced. The smooth walls of the trapfeed flake off and coat the insect's feet making it impossible to grip. Like Sarracenia, the pitchers of Nepenthes fill with rain water, but here mixed with a digestive acid so powerful that a mosquito can be reduced to a hollow carapace inside a day, and insects can be overwhelmed by just the fumes from the fluid beneath them.

Nepenthes is so effective that it can consume large beetles and even scorpions. The largest of all, Nepenthes rajah, may hold more than six pints of liquid, and is capable of trapping mice.

Astonishingly, some species of pitcher attract permanent guests - tiny frogs, larvae and even crabs have been discovered which have developed a symbiotic relationship. They live in the entrance of the pitcher, feeding on some of the insects attracted to the plant. Their droppings are nutritious to the plant, and indirectly allow it to cope with larger insects which might otherwise prove uncatchable or indigestible. This is a risky strategy, though, as the guests can themselves fall prey to the pitcher's traps, providing a huge nutritional boost to the plant.

Impressive though these carnivores are, their developments seem tame compared with Drosera, the Sundew. Its 104 species are a diverse collection of rosettes and climbers. Their trap consists of clusters of hairs, each tipped with a sparkling bead of liquid. Any insect landing on one discovers that the liquid is sticky, but before it can struggle free the plant performs a remarkable feat - other hairs, which have not even been touched, lean in towards the victim to help secure it. They may even convey the creature towards the centre of the plant where the greatest number of hairs can be brought to bear, and sometimes the entire stem of hairs may fold over to smother a large insect. Once trapped, it will succumb to the digestive enzyme in the glue, and eventually be absorbed.

Other plants have also developed active traps. The Waterwheel plant Aldrovanda vesiculosa (a monotypic genus) is an aquatic carnivore with a spring trap.

But even this pales beside the most remarkable of them all - Dionaea muscipula, the Venus Flytrap, a monotypic genus which all but makes the transition from trapper to hunter. The twin, red-tinted lobes at the end of each stem, with their cruel spines at the outer edges, bear an uncanny resemblance to the now-outlawed steel mantraps of earlier days.

Set into the face of each lobe are three delicate hairs. It is these which spring the trap, but such is the sophistication of the mechanism that a single touch, which might be caused by an inanimate object, is not enough to set it off. If there is a another touch within 25 seconds the trap snaps shut.

The triggering process and the co-ordination of the two lobes are not fully understood, but are essentially based on electrical impulses similar to the action potential of animals. There is a cumulative memory effect, possibly using electrical build-up, so a larger number of touches at greater intervals will also spring the trap. As many as 14 touches may be combined over a 20 minute period to activate it. The sluggish reaction of specimens kept in cool climates may make one wonder how any insect could ever be caught, but in the warmth of its native southern US the response is startling, taking as little as 0.03 seconds to close completely.

The closing mechanism is also surprising - the trap does not shut at its hinge by flexing, but half way up the lobe by a process of extremely rapid growth. The inner surface contracts and the outer expands by up to 30% and becomes rigid.

The glands on the inside surface of the trap excrete a chemical cocktail which consists of water to drown the victim, chitinase enzyme to break down its exoskeleton and proteolytic enzyme to break down its proteins. The uric acid discharged by the insect keeps the trap closed and causes an increased release of digestive enzymes, so the dose is tailored to the size of the prey.

When the creature is fully digested and the fluids dry up, the plant slowly opens again and the dessicated remains are blown away by the wind. As it does so the inside expands by up to 15%, again by means of accelerated growth, whilst the outside remains contant. These expansions are irreversible, and for any trap can only be repeated a maximum three or four times.

But even this sophisticated system is not totally foolproof. Occasionally, insects have been known to escape by eating a hole to freedom. Truly, no force is as powerful as the will to live.

Which plant has the worlds largest flower?

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