A climbing vine can seem almost purposeful. In time-lapse footage, tendrils stretch outward, find a support, wrap around it, and pull the plant upward.

The secret is a touch-sensitive growth response called thigmotropism.

Many climbing plants continuously sweep their tendrils through the air in slow circles. When a tendril brushes a branch, fence, or trellis, specialized cells detect the contact. Growth then slows on the side touching the support while the opposite side continues to elongate. The result is a gentle bend that becomes a wrap, anchoring the plant in place.

Some climbing plants are even better at this than they first appear. Tendrils often grip slender branches and stems more successfully than very large objects because their coiling mechanism works best on supports they can encircle. A sweet pea may struggle to gain a foothold on something as wide as a telephone pole but readily wrap around a nearby twig.

What looks like a plant making a decision is actually a remarkably effective response to touch.