The existence of insectivorous plants has long stimulated the imagination with the possibility that somewhere, in some remote corner of the unmapped regions of the world, dwell plants large enough to feed on large, four-legged animals, even humans. The idea has been the inspiration of any amount of fiction - both straightforward fiction, and fiction disguised as travellers' tales.
If any of these stories were true, it is strange that they have never been authenticated by science. After all, unlike mystery animals, plants cannot run away and hide when you come looking for them. And these plants, by definition, must grow along animal trails. Their presence must be notorious to the inhabitants of the area. Yet, for some strange reason, every traveller describes a different type of plant to the others, and equally mysteriously, there is never any mention of smaller varieties in the neighbourhood.
Now, on earth, insectivory in plants is a response to low levels of nutrients, especially nitrogen, in the soil, and it has evolved at least six separate times. Essentially, the plants rely upon trapping insects and other small creepy-crawlies, whose decomposing bodies then provide the otherwise lacking nitrogen. Usually the decomposition is assisted by special digestive juices. Their methods are varied. Some are pit-fall traps, others force the victim into the digestive juices with downwardly directed hairs, some catch them on a sticky mucus, while others, such as the Venus fly-trap, snap shut on the victim, and some water plants suck the prey into a bladder. However, the travellers' tales frequently involve the monstrous plant engulfing its prey in a set of tentacles, often thereafter sucking its blood. It is hard to see how this could possibly benefit the plant, let alone evolve.
It is true that small rodents and lizards have occasionally fallen victim to the larger species of pitcher plants. However, these are essentially pit-fall traps; the prey is lured into a deep pool of digestive juices and drowned. The man-eating plants of fiction and legend almost invariably involve the movement of jaws or tendrils. In plants, such movement is produced by changes in hydrostatic pressure, and there will be a size limit at which such a mechanism becomes uncompetitive with the nerves and muscles of the prey. Furthermore, although the ability of higher animals to detect and avoid danger is not unlimited, it is bound to be greater than a plant's ability to choose the right ambush site on which to grow - especially since the advantage of the site will change with the growth of the plant.
Then again, it is far from obvious what advantage a plant might gain from such a lifestyle, or how it would survive in the absence of prey. One thing is certain: any advantage is likely to be greater for the smaller species - those closest in size to the known insectivorous plants. There should be no discontinuity in size; a plant big enough to eat a man must have evolved from one big enough to eat a dog, and the latter from something big enough to eat a rat. Indeed, the younger versions of the same plant must consume a set of smaller prey. And the smaller plants must greatly outnumber the larger ones, just as jackals outnumber lions.
You can understand, therefore, why tales of such aberrations existing on earth should be discounted. But what about outer space? The same problems should apply. However, there are a couple of additional issues to consider. On earth there are small marine animals which look like plants; the sea anemone is an obvious example.
Also, what exactly is a plant? A plant is an organism which gains it energy from the sun by protosynthesis, and its nutrients from the substrate, normally the soil. An animal gains both energy and nutrients from eating plants or other animals. Is it possible for an organism to do both? Yes. There are many single-celled creatures which fit the bill. Zoologists used to call them animals, while botanists called them plants. But it is unlikely any higher form of life could do both because, at the dawn of time plants and animals went their separate ways, and an organ which has been lost cannot be re-evolved. However, it is possible for animals and plants to exist in a commensal relationship. Coral polyps, for example, are tiny animals similar to sea anemones, which consume tiny prey caught in their stinging tentacles. But they also possess tiny algae called zooxanthellae within their tissues, which produce the oxygen the polyp needs by photosynthesis, while the algae benefit from the nutrients provided by the polyp. Could the same thing occur on a larger scale? I sometimes wonder whether John Wyndham's triffids were not an example of such a phenomenon.
In my next article, I shall tackle the idea of an intelligent being without emotions, such as Star Trek's Spock. Otherwise, click here to return to the Index.