So you thought you knew everything about plants? Green leaves, pretty flowers and a, well, vegetable existence is what most of us think of the average plant. But we've got it very wrong: plants are sensitive, even 'irritable' creatures, with memories, muscles and movements - in fact, pretty much all the things you associate with animals.
€ Can Plants Feel?
€ Carnivorous Plants
€ Giant Flowers
€ Exploding Plants
CAN PLANTS FEEL?
How sensitive are plants? They often seem to behave like animals - excitable and often mobile. Is this behaviour just a freak of nature, or do plants really have 'nervous' feelings?
The cytoplasm and chloroplasts inside plant cells move around using muscle-like motor proteins. In some algae, if you touch their cells it triggers a nerve-like signal which stops their cytoplasm moving.
The most dramatic examples of 'nervousness' are the touch-sensitive leaf, flower and carnivorous trap movements, many of which also use electric signals.
Dozens of different flower species respond to the touch of a visiting insect by bending their sex organs, or flipping their petals. All these movements are designed to help cross-pollination.
Touch-sensitive leaves like Mimosa fold their leaflets up and leaf stalks collapse when touched, and they also react to shaking, electrocution, chilling, pressurising, cutting, or burning. The collapsed leaves expose thorns which are used in self-defence against animal attack. The Mimosa reflex relies for its speed on firing off electric signals, which turn on the motors of the leaf.
Touch-sensitive carnivorous plants rely on their speed to trap their prey. The Venus' flytrap and its underwater cousin the Waterwheel Plant both trap animal using touch-sensitive trigger hairs to trip their rapid movements. These reflexes work by electric signals too: as soon as the trigger hair is touched sufficiently hard it trips an electric signal across the trap telling it to move.
All plants, whether they move or not, are touch-sensitive. Touching or rubbing plants makes them grow shorter and thick stems - a good way of strengthening the plant against being buffetted by wind or animals. Plants also relies on touchiness for sex - pollen tubes feel their way by touch and taste through the female sex organ to her egg.
'Ordinary' plants also have their own electric signals for passing emergency messages about attacks from animals or whatever, and preparing the plant's defences against the intruder and the injury.
In short, then, plants have nearly all the senses of an animal, a primitive sort of 'nerve' system, and even rudimentary muscles. All of which can be traced back to the ancient ancestors of the plant and animal kingdoms - the one-celled creatures which swam around using their own sensitive 'nerve' and motor.
CARNIVOROUS PLANTS
Carnivorous plants are adapted to growing on poor soils by turning to a meat diet to supplement their nutrition. They use a variety of different traps, eg Pitchers plants lure insects to their traps by imitating flowers, then trap them using false windows, sliding cells, and by drugging them with mickey finns. Enzymes digest the bodies, helped by detergents which hasten the animal's death.
Some fungi also use carnivory, using explosive lassoos, sticky nets and knobs, all of which trap the prey before penetrating it with suckers.
There are probably many more carnivorous plants that have simply gone unrecognised, eg shepherds purse seeds which lure insect larvae into their sticky slime, or species of sticky-haired potatoes and tomatoes which kill and digest aphids and other small insects.
But there are also those 'carnivorous plants' that have turned away from meat-eating, and instead encourage animals to live in an 'aquarium' in their pitchers in return for feeding off their waste detritus.
GIANT FLOWERS
Amorphophallus titanum is one of the largest blooms in the world, standing up to ten feet tall and stinking of a rotting corpse when it first opens. It consists of a large poker, the spadix, wrapped in a bract, the spathe, which also hides the true flowers at the base of the spadix (see photo). It comes from Sumatra in Indonesia.
The largest single flower in the world, Rafflesia arnoldii, is 3 feet across (see photo). It imitates a rotting piece of meat with its colour and scent. It also comes from Sumatra and is becoming very rare because it has to grow on a jungle liana, and the jungles are being chopped down.
Rafflesia is so dependent on parasitism for its livelihood that evolution has stripped it of almost every feature we associate with plants - leaves, stem, roots and photosynthesis - and instead sucks out the juices of its host plant using fungi-like filaments. Eventually it bursts out into the open with a flower bud that grows into the largest bloom in the world.
EXPLODING PLANTS
Exploding plants rely on ballistics to launch their spores, seeds or fruits into orbit:
water guns - ballistic spores and fruits launched by water pressure, such as the dwarf mistletoes which explode their seeds as far as 15 metres , ejected at more than 100 kilometres an hour (the fastest projectiles in the plant kingdom);
electrostatic guns - spores launched by static electricity from fungi;
pop guns - spores shot off by air pressure created inside fungi;
catapults - spores and seeds flung out of their mother fungi or plants by spring-loaded explosions.
Some fungi even guide their ballistic spores towards a new home using an 'eye'. The spores are contained in a capsule in which a lens steers them towards daylight.
Some plants have evolved a two stage self-driven dispersal adventure. The seeds are first launched off their mother plant in a ballistic discharge. When they've landed on the ground they drive themselves along using a spring-powered engine, which also helps drill them down into a convenient crack in the soil.
The Action Plant, by Paul Simons, published by Blackwells, UK and USA, 1992.