Science: 'I need a drink' is the call of the wild: Botanists have found that plants have a thirst for communication. Paul Simons listens in
Monday 09 August 1993
Not that you can easily hear a cucumber's cry for help. Plants talk in an ultrasonic language whose frequencies are so high that a special microphone is needed. When they begin to run out of water, they grow noisy, somewhat like the slurping noise you get from sucking up the last dregs through a straw. In fact, plants slurp their drink through millions of microscopic 'straws' in their stems, but their suction power is so strong that the water inside suddenly breaks into air bubbles and makes ultrasonic snaps, crackles and pops.
Plant noises, which become noisier as the need for water increases, could have all sorts of pay-offs. John Milburn, of the University of New England, New South Wales, envisages plant breeders using ultrasound to measure the drought-resistance of new varieties. And microphones could be wired up to alarms to tell growers when watering is required.
Unfortunately, the plants' ultrasonic slurping also attracts some unwelcome visitors. Many wood-feeding insects can hear ultrasonic sound (as can bats, dogs and whales). These pests prefer to attack trees that are suffering from drought because they taste better and they are more vulnerable. The insects home in on the colour and smells of the distressed trees but, according to Robert Haack and Richard Blank at the US Agriculture Laboratory at Lansing, Michigan, they may use ultrasound to pinpoint the wood at close quarters. A tree's distress calls carry only a short way outside its trunk, but once inside the tree the insects hear the noises gradually becoming louder as they tunnel into the bark.
Noisy trees may also tell us when they are ready to come into leaf. It has long been a puzzle why the buds of different species break open at different times. Elm and red cherry are early leafers, whereas oak and ash follow about a month later.
By listening to the tree trunks during the winter, botanists can pick up tell-tale messages: the noisier the tree, the later it breaks into leaf. The reason is that air bubbles caught inside a trunk choke off its water supply, rather like an air-block in a waterpipe. The late-leafing ash and oak trees have large 'pipelines' prone to air- blocks and noises; only when these blocks are cleared can they come into leaf. But the early leafers, such as the elm and cherry, have much narrower pipes which hold their water better and are much quieter.
Plants even talk after death. Wood makes noises long after it has been cut down: if it dries out too fast, microscopic fractures open, well before any visible cracks appear. Tuning in to these messages could pay off: prevention of bad wood cracks during kiln drying has long been a problem for timber merchants. Now that scientists can eavesdrop on the warning noises, they can prevent serious damage. American researchers have actually made a kiln controlled by the timber noises. Computerised ultrasonic microphones can programme the kiln to recognise the danger signs and turn the heat down. Commercial savings could be high: kilns can render up to 70 per cent of certain timbers useless for furniture.
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