by Plants also make sounds when they are stressed, which is inaudible to the human ear because of the frequency of 20-100 kHz. But some animals might. Bats, mice and moths could potentially live in a world full of plant sounds, after previous research found that plants also respond to sounds made by animals. Researchers at Tel Aviv University in Israel placed tobacco and tomatoes in small boxes fitted with microphones. The microphones pick up any sounds the plants make, even if the researchers can’t hear them. The noise is especially noticeable on plants that are stressed from lack of water or recent mowing. Plants have no vocal cords or lungs. Current theories about how plants make sounds focus on their xylem, the conduit that transports water and nutrients from roots to stems and leaves. The water in the xylem is held together by surface tension, like being sucked up through a straw. When a bubble forms or bursts in the xylem, it may make a little popping sound; bubbles form more easily under drought stress. But the exact mechanism needs further study.
