Well OK, so neither walls nor plants have actual ears however it seems that somehow plants do sense and react to vibrations. Recent research at the University of Missouri has shown that plants react defensively to the munching of a caterpillar but not to the noise, to the vibration.
Using a small flowering plant called Arabidopsis Thaliana scientists placed caterpillars on the leaves. The plant’s defensive reaction was to release mustard oil which is a natural insect repellent. The caterpillars were then removed.
One set of plants was later left in silence whilst another was played a recording of caterpillars munching on a leaf. When the leaves were then analysed they found no change in the plants kept in silence however those exposed to the recording had released mustard oils as a chemical defence. So, they had reacted defensively to the vibrations caused by the sound of the caterpillars chewing.
Further studies showed that the plants could discriminate between the vibrations made by an insect chewing on a leaf and that made by wind or other insect noise. They simply did not react to the latter.
Earlier research has already shown that predatory insects use chewing vibrations to locate their prey. A clover worm snacking on soybean can be detected by a hungry stinkbug at quite some distance. And of course there has long been debate regarding the effect music and other noise can have on growing plants. This is however the first time that evidence has suggested that plants react to vibration in a defensive way.
Chewing vibrations are a source of acoustic energy for plants and they have evolved a way of increasing their defences to deal with the threat. Plants growing closely together can also “eavesdrop” on each other. Vibrations caused by an insect chewing on one plant can travel to neighbouring plants by over-lapping roots, stems or leaves.
Studies continue to discover by what method the plants actually manage to sense the vibrations. After all, they really do not have ears!