One of the characteristics of life is responding to the surroundings. Plants cannot walk away from shade, but they respond by growing in a direction.
Key idea
Plants grow toward light (phototropism), roots grow down with gravity while stems grow up (gravitropism), and vines and tendrils curl around what they touch (thigmotropism).
Why it works
A plant hormone called auxin collects on the shaded side of a stem. It makes the cells there grow longer than the cells on the lit side, so the stem bends toward the light. That puts the leaves where they can catch more light for photosynthesis. Roots growing down find water; tendrils that wrap around a support hold a vine up toward the sun.
Worked examples
Worked example: Name the response
A pea plant's tendril wraps around a fence wire. Which response is this?
- The tendril responds to touch.
- "Thigmo" means touch.
Answer: Thigmotropism.
Worked example: A pot on its side
A potted bean seedling is laid on its side. What happens over a few days?
- Roots grow with gravity: they curve down.
- The stem grows against gravity: it curves up.
Answer: Roots turn down, stem turns up.
Common mistake
Thinking plants cannot respond because they do not move from place to place. They respond by growing, and some move fast: a sensitive plant folds its leaves within seconds of a touch, and a Venus flytrap snaps shut. Try a window plant at home: turn the pot and watch it lean back toward the light over a few days (more ideas in Nature Study).