Plant Growth Responses: Tropisms and the Hormones Behind Them
A plant that cannot move still responds to light and gravity, by growing unevenly. The uneven growth is caused by a hormone redistributing across the tip, which is one of the oldest experiments in plant biology and still the clearest.
What a learner can do afterwards
- Explain phototropism and gravitropism as differential growth, not movement
- Describe the classic shoot-tip experiments and say what each control ruled out
- Name a commercial use of a plant hormone and the response it exploits
1 · Read
You cannot chase the sun if you are a plant, so you grow toward it instead. When light strikes one side of a young shoot, the cells on the shaded side stretch longer than the cells on the lit side. The tip curves toward the light, and this bending by uneven growth is called phototropism.
The messenger behind the bend is auxin, a hormone made in the shoot tip. It spreads unevenly and collects on the side away from the light. Where auxin gathers, cells stretch more, so the shoot arcs toward the lamp or window.
Gravity gets an answer too. Roots grow down with gravity while shoots grow up against it, a response called gravitropism. Tip experiments proved the tip is in charge: removing it stopped the bending, replacing it restored bending, and covering only the tip blocked the response. The tip senses light and sends a chemical message downward.
Lay a seedling on its side and watch the stem curve back up within a day. Growers use the same machinery on purpose: auxin-like powder helps cuttings sprout roots, while other hormones ripen fruit or keep weeds in check. Each success comes from nudging a response the plant already owns.
Plants answer light and gravity by growing unevenly under auxin from the tip.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.