---
title: "Plant Growth Responses: Tropisms and the Hormones Behind Them"
description: "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 pla"
canonical: https://lightmysky.com/learn/science/plant-growth-responses-tropisms-and-the-hormones-behind-them-mt_q1N7OG2_6j
source: https://lightmysky.com/learn/science/plant-growth-responses-tropisms-and-the-hormones-behind-them-mt_q1N7OG2_6j.md
retrieved: 2026-09-12
---

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# 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.

Subject: Science · Area: Organisms & Life Processes · Ages 17 to 18
Page: https://lightmysky.com/learn/science/plant-growth-responses-tropisms-and-the-hormones-behind-them-mt_q1N7OG2_6j

## Ready when they can

- 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

## Lesson: How shoots bend without moving

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.

**Example.** 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.

**Recap.** Plants answer light and gravity by growing unevenly under auxin from the tip.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Transpiration and Transport in Plants](https://lightmysky.com/learn/science/transpiration-and-transport-in-plants-mt_AqN0vL8QRu)
- [Plant Reproduction](https://lightmysky.com/learn/science/plant-reproduction-mt_ck57CDFGet)
- [Translocation and the Mass Flow Hypothesis](https://lightmysky.com/learn/science/translocation-and-the-mass-flow-hypothesis-mt_WZ8id0kou4)

## Opens up

- [Plant Hormone Signalling and How a Plant Decides](https://lightmysky.com/learn/science/plant-hormone-signalling-and-how-a-plant-decides-mt_Zg2xpsb-p_)