Do you like to touch the leaves of your house plants? Plants notice when they are touched. In a small lab plant, a light touch turned on many genes within 10 to 30 minutes. These genes help the plant get ready for danger.

If a plant is touched every day, it grows shorter and thicker. Trees in very windy places grow short and strong for the same reason.

Touching a plant may seem harmless, but plants react quickly. In 1990, American researchers found that when the small lab plant Arabidopsis was touched, sprayed with water or blown by wind, certain genes became up to 100 times more active within 10 to 30 minutes.

Later work showed that over 2.5 percent of its genes, which means several hundred, respond quickly to touch. If plants are touched again and again, they grow shorter and thicker. A 2012 study found that such plants flowered later but became more resistant to insects and a fungus. Trees in windy places also grow short and sturdy, because wind works like constant touching.

Stroking a houseplant may feel like a kind gesture, but to the plant it can be a warning. In 1990, Janet Braam and colleagues reported in Cell that rain, wind or touch quickly switched on a set of TCH genes in Arabidopsis, raising their activity by up to about 100 times within 10 to 30 minutes.

A 2004 review estimated that more than 2.5 percent of the plant's genes, several hundred in all, respond rapidly to mechanical stimulation. One touch causes only a temporary change. Repeated stimulation reshapes the whole plant, a process called thigmomorphogenesis, and the plants grow shorter and stockier. In 2012, Chehab and colleagues showed that repeatedly touched plants flowered later but resisted insects and a fungal disease better, an effect that depended on the hormone jasmonate.

A 2015 study added that touch triggers the breakdown of gibberellin, a hormone that promotes growth, as if the plant were easing off the accelerator. Most of these data come from one model species grown in the laboratory. "Stress" is also a metaphor here, and no one is claiming that a touched plant suffers. These responses help explain why trees on windy ridges stay short and sturdy.